Multiferroic BiFeO₃ (BFO) exhibits coupled ferroelectric, antiferromagnetic, and ferroelastic orders at room temperature, enabling unique optoelectronic properties such as switchable photovoltaic effects and polarization-driven carrier separation. The multi-valance of Fe element in BFO also implies its potential to engineer the bandgap and conductivity to enhance the photoelectric properties. Besides, the large remnant ferroelectric polarization in BFO offers the opportunity of bulk photovoltaic effect, promising to develop the optoelectronic devices based on the unique optical and multiferroic properties. While defects like oxygen vacancies are known to critically influence functional properties in oxides, especially the BFO films. The oxygen vacancies are proved to influence the electronic and magnetic properties, electromechanical response and resistive switching. Furthermore, the concentration of oxygen vacancies is clarified to be influenced by the annealed conditions, which in turn tune the photoresponse of BFO thin films. However, previous studies mainly focus on the influence of the oxygen vacancies as the point defects on properties and the atomic-resolved analyses of oxygen vacancies behavior are lacking. Here, by combining advanced STEM, UV-Vis, XRD and first-principles calculations, we systematically investigate oxygen-vacancy-engineered BFO thin films to reveal how ordered vacancy planes modulate lattice structure, create charged domain walls, reduce bandgaps and improve UV-visible absorption property, thereby demonstrating a defect-property correlation crucial for designing multifunctional devices.

BFO polycrystalline ceramic target (1% Bi-enriched), SrTiO₃ (STO) ceramic target, and commercial single-crystal substrates of NdGaO₃ (NGO) (110)_O and STO (001) were purchased from Hefei Kejing Materials Technology Co., Ltd. BFO and BFO/STO films were grown on NGO (110)_O substrates via pulsed laser deposition (PLD) using a Coherent ComPex PRO 201 F KrF excimer laser (λ = 248 nm). The substrates were preheated to 850 ℃ for 20 min and cooled to deposition temperature at 5 ℃/min. A 1% Bi-enriched BFO target was pre-sputtered for 20 min. The deposition parameters of 6 Hz repetition rate, 800 ℃ substrate temperature, 370 mJ laser energy were used to fabricate the films. Post-deposition annealing at 800 ℃ for 20 min was followed by cooling to 200 ℃ at a rate of 5 ℃/min and natural cooling to RT under 200 Torr O₂ partial pressures. Four film systems were prepared by varying O₂ partial pressures:1) BFO film: growing at 90 mTorr, annealing at 200 Torr; 2) BFO film: growing at 90 mTorr, annealing at 120 mTorr; 3) BFO/STO film: BFO layer growing at 90 mTorr, STO capping layer growing at 70 mTorr, both layers were annealed at 120 mTorr; 4) BFO/STO film: growing/annealing at 0.06 mTorr.

Cross-sectional TEM specimens were prepared using a ThermoFisher Helios 5UX FIB system with sequential steps: C/W protective layer deposition, trenching, lift-out, thinning (30 kV/90 pA → 8 kV/47 pA), and amorphous layer removal (5 kV/47 pA → 2 kV/47 pA). The atomic-resolved HAADF-STEM images were acquired on ThermoFisher Spectra 300 (300 kV, 25 mrad convergence, 71-200 mrad collection angles) with drift correction (DCFI in Velox). The strain analysis was performed using GPA via Gatan GMS3 software and the atomic positions were determined by 2D Gaussian fitting in MATLAB software. The UV-Vis measurements were performed in JASCO V-770. The RSMs were collected on Bruker D8 Advance (Cu K_α, λ = 1.5406 Å). First-principles calculations were based on the density functional theory (DFT) calculation and performed with the Vienna ab initio simulation package (VASP).

A series of BFO films were fabricated by modulating the oxygen environment to introduce the high-density oxygen vacancies. Using the Cs-STEM, the oxygen vacancy ordered plane was determined in BFO film annealed under the oxygen partial pressure of 120 mTorr, which displayed as the out-of-plane lattice expansion. Besides, the charged oxygen vacancy ordered plane would influence the surrounding polarization directions, stabilizing the tail-tail polarization distribution. By changing the annealed oxygen partial pressure and growing the capping layers, the concentration of oxygen vacancy ordered planes was evidently modulated, further extending the ultraviolet-visible absorption property of the BFO films. First-principles calculations revealed that the introduction of high-density oxygen vacancies could generate the vacancy-induced energy levels between the conduction and valance bands and thus reduce the bandgaps of BFO. The result was consistent with the UV-visible absorption measurement results.

The oxygen vacancy ordered planes were introduced in BiFeO₃ film systems by controllable growth methods. Its concentration was tunable by decreasing the annealed oxygen partial pressure and/or growing capping layers. Obvious lattice expansion was observed in BFO films by the existence of oxygen vacancy ordered planes. Besides, the polarization configuration in BFO film was modulated by the electrostatic potential of positively charged oxygen vacancy layer, accompanying the formation of tail-to-tail polarization configuration. The introduced oxygen vacancy ordered planes also had further improved the optical absorption properties of BFO films. Our results provide a further insight into the interaction between oxygen vacancy ordered planes with the ferroelectric polarization and the optical properties in multiferroic oxides and facilitate the design of nanoscale photoelectronic devices and ferroelectric solar cells.

Hafnium oxide-based ferroelectric memory has become a research hotspot due to its compatibility with CMOS processes, low power consumption, and inverse size effect. Memory technology has transitioned from 2D planar to 3D vertical stacking, enabling the stacking of multiple memory cells. The gate first process of 3D NAND architecture requires the ferroelectric layer to withstand high annealing temperature. Al-doped hafnium oxide films can endure annealing at 700-1000 ℃, meeting the process requirements. Florent et al. first demonstrated a 3D NAND device based on Al:HfO₂, achieving a 2P_r value of 17 μC/cm² after annealing at 850 ℃, showcasing its potential in non-volatile memory applications. As devices trend toward miniaturization and integration, reducing the thickness of the ferroelectric layer is key to achieving high-density stacking and lowering power consumption. When the thickness is scaled down to the nanoscale, interface effects become significant. Studies show that an interfacial oxide layer forms between the ferroelectric film and the metal electrode, leading to the accumulation of oxygen vacancies, the generation of a built-in electric field, and polarization asymmetry. To understand interface effects, it is essential to study them from the perspective of polarization switching. This work focuses on Hf_1-xAl_xO₂ films to investigate the influence of interfaces on polarization switching behavior. The study reveals that the interfacial layer between the ferroelectric layer and the metal electrode hinders polarization switching in the programmed state.

First, a 40 nm tungsten (W) bottom electrode was deposited on a p⁺-Si substrate at room temperature using magnetron sputtering. Subsequently, Al-doped HfO₂ (HAO) ferroelectric films with thicknesses of 6.8 nm, 15.8 nm, and 20.4 nm were prepared by atomic layer deposition (ALD) at a deposition temperature of 295 ℃. Tetrakis(ethylamido)hafnium (TEMA-Hf), trimethylaluminum (TMA-Al), and water (H₂O) were used as the hafnium source, aluminum source, and oxygen source precursors, respectively. The cycle ratio of aluminum to hafnium deposition was 1∶29. Total cycles for HAO films with thicknesses of 6.8, 15.8 nm and 20.4 nm are 90, 210 and 270, respectively. Then, a 40 nm W top electrode was deposited using magnetron sputtering, resulting in W/HAO/W capacitor structures. Finally, the two capacitor structures were subjected to rapid thermal annealing at 700 ℃ for 30 s in a nitrogen atmosphere.

The 6.8 nm, 15.8 nm, and 20.4 nm HAO films fabricated on W electrodes exhibited remanent polarization (2P_r) values of 38.8, 43.2 μC/cm² and 37.6 μC/cm² after waking up, demonstrating excellent ferroelectric properties. During the fabrication of 6.8 nm HAO films, interfacial WO_x layers with thicknesses of 0.5 nm and 1.8 nm were formed. Based on this observation, ERS and PRG polarization switching behaviors were designed to investigate interfacial effects on switching dynamics. Under an electric field of 1.8 MV/cm, the divergence in switching time between these two states amplifies with HAO film thickness scaling. Polarization switching in PRG state of 6.8 nm HAO film is hindered by the built-in electric field, exhibiting ΔP/2P_S significantly below 1, whereas ERS state achieves near-complete switching (ΔP/2P_S is about 1) with substantially faster kinetics. Comparative analysis of COD values from NLS model and KAI model fittings reveals: For ERS state, both models yield COD >0.9, indicating comparable timescales for domain nucleation and growth. In contrast, PRG state polarization switching behaviors at 1.8 MV/cm requires exclusive description by the NLS model, demonstrating nucleation-limited behavior under low fields. With increasing electric field, decreasing COD values for NLS fittings and increasing COD values for KAI fittings suggest a gradual transition from nucleation-dominated to growth-dominated switching mechanisms in ERS and PRG states.

This paper prepared HAO ferroelectric films with three thicknesses (6.8, 15.8 nm, and 20.4 nm) via ALD, and investigated the influence of interface effects on polarization switching characteristics by comparing differences in polarization switching behavior between ERS and PRG states. The results indicate that the divergence in polarization switching time between ERS and PRG states increases with HAO film thickness scaling, attributed to the thicker interfacial oxide layer at the bottom interface compared to the top interface. This asymmetry leads to higher oxygen vacancy concentration at the bottom interface, generating a built-in electric field directed from the film toward the top electrode. As HAO film thickness decreases, the proportion of interfacial oxide layers increases, amplifying the built-in electric field that hinders polarization switching in PRG state. For the 6.8 nm HAO film under ERS state, the COD values fitted by both NLS and KAI models exceed 0.9, indicating comparable timescales for domain nucleation and growth. However, in PRG state at 1.8 MV/cm, the ΔP/2P_S is significantly below 1., requiring description by the NLS model. With increasing electric field, the ΔP/2P_S progressively rises to 1 (Complete polarization reversal), accompanied by a transition from NLS to KAI model behavior. This arises because the built-in electric field impedes polarization switching in PRG state, necessitating additional energy to overcome nucleation barriers. Under high electric fields, sufficient energy is provided to surmount interfacial domain nucleation barriers, significantly enhancing PRG domain nucleation rate and accelerating polarization switching dynamics.

Conventional poling methods, such as contact poling and corona poling, are frequently hampered by susceptibility to dielectric breakdown and film failure. Achieving self-poling without external poling procedure required in lead-free ferroelectric thin films is critical for advancing flexible piezoelectric devices. Here, we introduce a LaNiO₃ seed layer on flexible mica substrates to induce (001)-textured BiFeO₃ films for self-poling, while systematically evaluating the impact of Mn doping on film properties. The resulting self-poled BiFeO₃ films achieve a piezoelectric coefficient d_{33, d}≈50 pC/N and demonstrate exceptional bending stability, retaining over 90% of their initial d_{33, d} after more than 5000 bending cycles. This work presents a compelling new pathway for the development of high-performance lead-free flexible ferroelectric devices.

Mica substrates were prepared via blade-cleaving, followed by sequential 3-minute ultrasonic cleaning in anhydrous ethanol, acetone, and anhydrous ethanol to remove contaminants. LaNiO₃ solution was synthesized by sol-gel processing: stoichiometric lanthanum nitrate and nickel acetate (La: Ni = 1∶1 mol ratio, 0.2 mol/L concentration) were dissolved in glacial acetic acid with stirring at 80 ℃ until complete dissolution, then stirred for 2 h at 25 ℃. The solution was aged for 48 h at 25 ℃. Subsequently, LaNiO₃ solution was spin-coated onto mica substrates at 5000 rpm, dried on a hotplate at 150 ℃, and thermally processed in a rapid thermal annealing furnace under air atmosphere. This involved a pyrolysis step at 400 ℃ for 5 min, followed by crystallization at 700 ℃ for 1 min. Layer-by-layer spin-coating with intermediate annealing achieved the target LaNiO₃ electrode thickness. Mn-doped BiFeO₃ solutions (0, 1%, 3%, 5% Mn-doping) were prepared by dissolving bismuth nitrate (10% excess), iron nitrate, and manganese acetate in ethylene glycol monomethyl ether (0.4 mol/L concentration), adding citric acid (1∶1 molar ratio to metal ions) as a chelating agent, magnetically stirring for 4 h at 25 ℃, and aging for 48 h. These sols were spin-coated onto LaNiO₃/mica substrates, followed by 10 iterative cycles of thermal treatment: drying at 200 ℃ (5 min), pyrolysis at 400 ℃ (5 min), and crystallization at 550 ℃ (5 min) per layer to attain the final film thickness.

The LaNiO₃ seed layer promotes a dominant (001) texture in BiFe_1-xMn_xO₃ (0%, 1%, 3%, 5% Mn) thin films. However, increasing Mn dopant concentration induces compressive stress relaxation, significantly degrading the film texture from 92.3% to 56.7%. Undoped and 0.01% Mn-doped films exhibit pronounced out-of-plane self-polarization, yielding piezoelectric coefficients d_{33, d} = 48.6 pC/N and 47.4 pC/N. Internal bias in PFM voltage-phase/amplitude curves and unipolar conduction in I-V characteristics unequivocally confirm a built-in electric field. This field weakens at higher Mn concentrations (x = 0.03 and 0.05), concurrently diminishing self-polarization—A causal relationship demonstrating that self-polarization originates from this internal field. Although increased Mn doping reduces self-polarization, it, however, significantly improves electrical performance: leakage current decreases by 78% at x = 0.05 compared to undoped films. Therefore, optimal Mn doping (x = 0.01) synergistically preserves self-polarization while enhancing electrical performance. As flexible films, BiFe_1-xMn_xO₃ demonstrates exceptional bending stability and mechanical fatigue resistance. P-E hysteresis loops remain invariant under compressive/tensile stresses compared to undeformed states. After 5000 bending cycles at tensile strain (r =10 mm), ferroelectric properties and piezoelectric coefficient d_{33, d} degrade by<10%, confirming outstanding operational durability.

This work demonstrates that LaNiO₃ seed layers deposited on mica substrates induce a dominant (001) texture in BiFeO₃-based thin films. Undoped and 0.01% Mn-doped compositions exhibit significant self-polarization driven by a built-in electric field, while increased Mn doping reduces texture degree from 92.3% to 56.7% and weakens self-polarization. The latter is evidenced by the decrease of piezoelectric coefficient, d_{33, d}, from 48.6 pC/N to 14.6 pC/N. Crucially, despite diminishing self-polarization, Mn doping substantially enhances electrical properties (78% leakage reduction at x=0.05). All films endow exceptional mechanical resilience: after 5,000 bending cycles (r = 10 mm), d_{33, d} degrades <10%, with P-E hysteresis loops remaining unaffected. By synergistically integrating texture engineering and doping optimization, we achieve self-poling with a d_{33, d}≈50 pC/N, establishing a novel technological pathway for lead-free flexible ferroelectric devices.

Bismuth ferrite (BiFeO₃, BFO) is a well-known ferroelectric material with excellent piezoelectric and ferroelectric properties at room and high temperatures, attributed to its high Curie temperature (1103 K). Its rich topological domain structures, such as vortex domains and skyrmions, show potential for high-density data storage due to their small size and stability. Recent studies show that ferroelectric materials near the morphotropic phase boundary (MPB) exhibit enhanced piezoelectric properties. Methods like applying strain, changing substrate orientation, and doping can create MPB structures in BFO films, improving their performance. Ferroelectric topological domains can be induced by engineering film boundaries, offering new opportunities for high-density information storage and logic devices. Pulsed Laser Deposition (PLD) was used to fabricate BFO films. By adjusting parameters, T-phase, R/T mixed-phase, and R-phase films were prepared. The effects of thickness and laser energy on crystal structure, growth mode, and domain structure were investigated. Finite element simulation and thermodynamic analysis revealed the influence of strain relaxation and deposition rate on film properties.

BFO films were fabricated on (001)-oriented LaAlO₃ (LAO) substrates using PLD with a KrF excimer laser (248 nm wavelength, 3 Hz frequency). By adjusting the number of pulses (2700, 3600, 5400) and laser energy density (0.90-1.50 J/cm²), various BFO films (T-phase, R/T mixed-phase, R-phase) were prepared. A Bi-rich B_1.1FeO₃ target was used to compensate for Bi evaporation. Growth conditions were 700 ℃ substrate temperature and 13 Pa oxygen pressure, followed by annealing at 650 ℃ in 20000 Pa oxygen for 30 min and cooling to room temperature at 5 ℃/min. The epitaxial strain on BFO from LAO was -4.5%. Characterization was performed using X-ray Diffraction (XRD) with a Rigaku Ultima IV, Atomic Force Microscopy (AFM) with a Bruker Dimension XR, and Piezoresponse Force Microscopy (PFM) with a Pt/Ir-coated Si cantilever. Finite element simulation was conducted to study strain distribution in BFO films on LAO, assuming a -4.5% strain boundary condition. The simulation, performed at 300 K with a 1 nm grid size, provided theoretical support for the experimental results.

In thin film systems, strain relaxation occurs as film thickness increases. Changing deposition cycles to increase thickness significantly impacts the crystal and domain structures of epitaxial films. With 2700 deposition cycles, the film exhibits a layer-by-layer step-flow growth mode with distinct in-plane polarization components in the T-phase BFO, forming regular stripe-like domains. Increasing cycles to 3600 results in an interwoven R/T mixed-phase stripe pattern and crescent-shaped domains, indicating a transition from T-phase to R/T mixed-phase structure. Further increasing cycles to 5400 leads to significant surface roughness, transitioning the film to R-phase BFO as the epitaxial relationship is disrupted. This suggests that excessive thickness causes strain relaxation, enhancing in-plane polarization signals in PFM images and increasing domain size.

Laser energy is another crucial factor affecting film growth. Higher laser energy increases the amount of evaporated target material and the plume size. At low energy (0.90 J/cm²), the film maintains layer-by-layer growth with impurity particles, showing in-plane polarizations in different orientations. Increasing energy to 1.35 J/cm² induces a transition to layer-island mixed growth, forming interconnected nanodots with ferroelectric characteristics. Further increasing energy to 1.50 J/cm² results in higher-density, smaller nanodots in an island growth mode, with fragmented domain structures.

Finite element simulation was used to analyze the effect of film thickness on strain relaxation in BFO/LAO films. Films with thicknesses of 10 nm and 50 nm showed minimal strain relaxation, maintaining -4.5% strain (T-phase). However, at 80 nm, strain relaxation began (-4.41%), and at 130 nm, it became more pronounced (-3.87%). This indicates that thicker films experience more significant strain relaxation, consistent with experimental observations. The concept of "evaporation depth" helps explain the impact of laser energy on growth modes. At low laser energy, fewer BFO cells are evaporated, allowing them to diffuse and form layer-by-layer structures. Conversely, high laser energy increases the number of evaporated cells and nucleation sites, promoting island growth. This study provides valuable insights into the effects of deposition cycles and laser energy on BFO film growth, offering guidance for optimizing film properties.

This study explores how epitaxial strain and growth mode affect the crystal structure and ferroelectric domains in BFO films. As film thickness increases, epitaxial strain relaxes, causing significant changes in high-strain BFO films. Films transition from T-phase to R/T mixed-phase and then R-phase BFO with increasing deposition cycles (2700 to 5400), highlighting thickness's role in strain relaxation and structure. Adjusting laser energy density (0.90 J/cm² to 1.50 J/cm²) shifts growth modes from T-phase to nanodot films, altering microstructure and domain distribution. Finite element simulations show that strain relaxation accelerates when film thickness exceeds 80 nm, with surface strain relaxing from -4.5% to -4.41%. Calculations of evaporated material per pulse reveal that low laser energy promotes layer-by-layer growth, while high energy induces island growth. These findings provide crucial insights into laser energy's role in controlling film growth and offer valuable guidance for developing high-performance piezoelectric materials and information storage devices.

Silicon photonics has emerged as a critical technology for high-performance communication and computing, with electro-optic modulators serving as essential components. Barium titanate (BaTiO₃, BTO) exhibits exceptional promise due to its outstanding electro-optic coefficient (r₄₂ ≈ 923 pm/V) and compatibility with silicon-based fabrication processes. However, achieving high-performance BTO devices requires overcoming key materials integration challenges. The primary obstacles include the formation of amorphous SiO₂ interfacial layer during high-temperature, oxygen-rich BTO growth, which prevents effective epitaxy, and the substantial lattice mismatch between BTO (a = b = 3.992 Å, c = 4.036 Å) and Si (5.431 Å), resulting in approximately 4.0% mismatch even with 45° rotational epitaxy. This mismatch induces high-density dislocations and strain relaxation, compromising film quality. Strontium titanate (SrTiO₃, STO) buffer layers (3.905 Å) can reduce lattice mismatch to 1.7%, providing stress relief and serving as an epitaxial template. The electro-optic effect in BTO strongly depends on ferroelectric domain structure and crystallographic orientation, with in-plane polarization achieving maximum coefficients. While domain structures can be controlled through strain engineering and growth conditions, current analyses rely on post-growth characterization, lacking real-time monitoring of domain evolution processes and critical transition mechanisms.

Oxide molecular beam epitaxy (MBE) combined with in situ reflection high-energy electron diffraction (RHEED) for real-time monitoring was employed to achieve controlled growth of high-quality BTO films at 2 in wafer scale. The systematic investigation focused on lattice relaxation behavior during the growth of STO buffer layers and BTO layers with varying thicknesses. Silicon substrates underwent hydrofluoric acid (HF, 10% mass fraction) treatment to remove surface SiO₂ amorphous layers, followed by rapid transfer to ultra-high vacuum chambers (background pressure ≤ 5×10^-8 Pa). STO epitaxial growth employed a two-stage process: first, 2.5 monolayers (ML) of Sr and Ti metals were deposited at a substrate temperature of 200 ℃ under ultra-high vacuum (< 5×10^-7 Pa) with precisely controlled oxygen partial pressure (< 5×10^-6 Pa). The second stage involved in situ annealing at 500 ℃ for 8 minutes under ultra-high vacuum conditions (< 1×10^-6 Pa). After three such cycles, additional STO layers were epitaxially grown using co-deposition at a thermocouple temperature of 600 ℃ under an oxygen partial pressure of 6×10^-5 Pa. BTO layers were continuously co-deposited at 700 ℃ thermocouple temperature under 6×10^-5 Pa oxygen partial pressure, ultimately obtaining 310 nm BTO/5 nm STO/Si heterostructures. Comprehensive characterization employed high-resolution four-circle X-ray diffraction (XRD) for 2θ-ω scanning and rocking curve analysis, spectroscopic ellipsometry for determining optical constants and thickness, atomic force microscopy (AFM) for surface morphology assessment, and piezoresponse force microscopy (PFM) for ferroelectric domain characterization.

Systematic RHEED diffraction pattern analysis during film growth enable quantitative analysis of lattice relaxation processes in both STO and BTO epitaxial films. STO buffer layers completed relaxation after 12.5 ML, reducing the lattice mismatch between BTO and Si from 4.0% to 2.2%. BTO films underwent progressive relaxation from the interface to a depth of 30 nm, with in-plane lattice constants evolving from 3.905 Å (STO-templated) to 4.013 Å at 30 nm, then stabilizing. During cooling from 700 ℃ to room temperature, thermal constraints from Si substrate significantly modified BTO behavior. The effective thermal expansion coefficient was ~1.5×10^-6 K^-1, much lower than that of bulk BTO (~14.6×10^-6 K^-1) but comparable to Si (2.6×10^-6 K^-1), indicating strong substrate constraint effects. The resulting 300 nm BTO films exhibited excellent quality: a rocking curve full width at half maximum (FWHM) of 0.45°, surface roughness < 0.2 nm, and critical lattice parameters a = 4.010 Å, c = 3.990 Å (c/a = 0.994< 1). This represents a 0.45% in-plane expansion and 1.2% out-of-plane contraction compared to bulk BTO, indicating a tetragonal phase reorientation with the long-axis in-plane. This anisotropic distortion creates residual tensile stress in-plane and compression stress out-of-plane, driving polarization reorientation from out-of-plane to in-plane throughout the entire film. PFM characterization confirmed this structure: lateral PFM (LPFM) signals significantly exceeded vertical PFM (VPFM) signals, demonstrating dominant in-plane polarization components. Optical characterization at 1550 nm revealed ideal performance with a refractive index n = 2.28 and an extinction coefficient k ~10^-6, indicating extremely low optical loss suitable for high-performance electro-optic modulators.

High-quality BTO epitaxial growth on 2-inch silicon wafers was achieved via molecular beam epitaxy. In situ RHEED monitoring enabled real-time observation of lattice relaxation processes during STO buffer layer and BTO film growth, revealing that STO lattice relaxation occurs within the first 12.5 ML from the interface, while BTO lattice relaxation occurs within the first 30 nm from the interface. Based on this relaxation behavior, the STO intermediate layer provides BTO with a low-mismatch template, while BTO lattice relaxation is critical for ultimately achieving in-plane polarized films. In situ RHEED data analysis demonstrated that constraints from low thermal expansion coefficient Si substrate enable BTO films to maintain larger in-plane lattice constants during cooling to room temperature, thereby stabilizing domain structures with predominantly in-plane polarization. This in situ monitoring technique provides crucial information for precise analysis of BTO film lattice relaxation processes and domain engineering, offering significant implications for developing silicon-based BTO electro-optic modulators, high-speed communication devices, and optical quantum computing chips.

As electronic technology and integrated circuits advance rapidly, the functional demands of devices are increasingly elevated. Ferroelectric/semiconductor heterostructures are significant in applications including memory, sensing, and optoelectronic devices because they combine the best qualities of both ferroelectric and semiconductor materials. In recent years, the variety and refinement of the preparation process have led to the successful direct integration of ferroelectric materials with semiconductor silicon. Nevertheless, constructing ferroelectric/semiconductor heterostructures devoid of any interface layers, particularly on silicon substrates, is exceedingly challenging. Due to thermal diffusion, there is usually a SiO_x layer at the interface, which subsequently affects device performance. However, systematic researches are urgently need regarding the effect of the interface layer on the potential barriers, ferroelectric polarization, and carrier transport. This work develops SiO_x layers of varying thicknesses and densities on the n-Si surface, followed by the fabrication of bismuth ferrite (BiFeO₃) films on the pretreated substrate. When the substrate pretreatment temperature rises, the SiO_x layer progressively thickens, thereby altering the interface barrier, polarization switching, and the transport of photogenerated carriers. This study presents a novel concept for the creation of ferroelectric/semiconductor heterostructures featuring several interface contact types and electrical transport pathways.

Oxygen was introduced into the tube furnace, which was later heated to 400-1000 ℃. Once the temperature stabilizes, the (100) oriented n-Si substrate (resistivity of approximately 10 Ω·cm) is put into the furnace. After heating for 1 h, the substrate is directly removed from the tube furnace and allowed to cool in a room temperature environment.

The raw materials Bi(NO₃)₃·5H₂O and Fe(NO₃)₃·9H₂O were dissolved in acetic acid and ethylene glycol methyl ether, respectively. Subsequently, these two solutions were mixed, stirred for 8 h, and filtered to obtain a BiFeO₃ precursor solution at a concentration of 0.1 mol/L. The precursors were spin-coated onto the pretreated n-Si (100) substrate at 3500 r/min for 30 s and heated at 180 ℃ for 5 min. The spin-coating process was done three times to attain a film thickness of approximately 60 nm, afterwards, the samples were annealed in a nitrogen environment at 380 ℃ for 15 min, followed by 550 ℃ for 50 min, with a heating rate of 10 ℃/min.

The n-Si substrate was pretreated at various temperatures in an oxygen environment. The cross-sectional high-resolution transmission electron microscopy indicates an increase in the thickness of the SiO_x layer with increasing substrate pretreatment temperature. The dielectric measurements show that when the pretreatment temperature is above 600 ℃, the activation energy of the dielectric relaxation begins to decline, signifying a decrease in the oxygen vacancy content while the transition of the oxygen vacancies from divalent to monovalent. Subsequently, BiFeO₃ films were fabricated on the pretreated substrate. X-ray diffraction studies demonstrated that all the BiFeO₃ films exhibit R3c polycrystalline structure.

After substrate pretreatment, the clamping phenomenon of the BiFeO₃/n-Si hysteresis loop disappears. As the pretreatment temperature increases, the loop becomes slimmer, and the coercive voltage decreases. The reason is that the increased thickness of the SiO_x layer hinders the carrier migration between BiFeO₃ and the n-Si substrate. At temperatures below and above 700 ℃, the SiO_x can be considered as a tunneling layer and a resistive layer, respectively. I-V curves show that the BiFeO₃/n-Si samples still exhibit rectification effects, but the cut-off voltage under negative bias and the conduction current under positive bias both decrease, which is consistent with the weakening of the interface barrier and the decrease in conductivity. Although the photocurrent and on-off ratio generally decline with the increasing pretreatment temperature, the on-off ratio of the sample on 700 ℃ pretreated substrate increases abnormally, attributed to the increased conductivity associated with the aggregation of monovalent oxygen vacancies.

BiFeO₃ films are grown on n-Si substrates pretreated at different temperatures. As the pretreatment temperature increases, the SiO_x layer on the n-Si surface became thicker and denser, accompanied by a reduction in oxygen vacancies and a transition from divalent to monovalent. The hysteresis loops are no longer clamped, with the coercive voltage deceased, and both the cut-off voltage of the I-V curves and the on-off ratio of the photocurrent reduce. The primary explanation is that the SiO_x layer reduces the pn junction barrier and the built-in electric field at the BiFeO₃/n-Si interface, facilitating the polarization switching of BiFeO₃ and the transport of photogenerated carriers. The accumulation of oxygen vacancies at a particular temperature can result in an anomalous rise in photocurrent.

Ferroelectric thin films have drawn growing interest for their potential in electro-optic modulation, nonlinear optics, and integrated photonics, owing to their intrinsic spontaneous polarization. Among them, BaTiO₃ (BTO) stands out for its strong ferroelectricity, piezoelectricity, and nonlinear optical properties, making it a promising material for high-performance optoelectronic devices. To realize these functionalities, it is essential to achieve high crystallinity and controlled orientation in BTO films—especially along the c-axis, which is critical for polarization-related responses. While techniques such as pulsed laser deposition (PLD) and atomic layer deposition (ALD) have been used for BTO growth, they face challenges in scalability and substrate compatibility. Radio frequency (RF) magnetron sputtering offers a scalable alternative with good control over film uniformity and composition. However, obtaining high-quality epitaxial BTO films via sputtering remains challenging due to the sensitivity of the process to parameters like substrate temperature, gas composition, and RF power. This work systematically investigates the effects of key sputtering parameters on the structural and optical properties of c-axis-oriented BTO thin films. The relationship between deposition conditions, domain structure, and optical performance is further explored using second harmonic generation (SHG), aiming to guide the fabrication of high-quality ferroelectric films for integrated photonic applications.

Epitaxial BaTiO₃ (BTO) thin films were deposited on (001) (La,Sr)(Al,Ta)O₃ (LSAT) and (110) GdScO₃(GSO)single-crystal substrates using RF magnetron sputtering with a 2 in ceramic BTO target. The chamber pressure was maintained at 1.5 Pa, with either pure Ar or Ar/O₂ gas mixtures as the sputtering atmosphere. Substrate temperatures ranged from 25 ℃ to 750 ℃, and RF power was set at 50 W or 75 W (2.47 W/cm² and 3.70 W/cm², respectively). A 10 min pre-sputtering step was performed before each run to stabilize film composition. Structural characterization was conducted using an X-ray diffractometer (SmartLab, Rigaku) with a Cu-Kα₁ source and Ge (220) monochromator. Measurements included θ-2θ scans, rocking curves, X-ray reflectivity (XRR), and reciprocal space mapping (RSM). Surface morphology and roughness were analyzed via atomic force microscopy (AFM, tapping mode), while domain structures were evaluated using second harmonic generation (SHG) with a Ti:sapphire femtosecond laser (800 nm, 35 fs). SHG measurements were performed in reflection geometry at a 45° incidence angle. Film composition was assessed using a Zeiss GeminiSEM 500 with a windowless EDS detector at 20 kV to determine the Ba/Ti ratio. Optical constants, including refractive index (n₀) and extinction coefficient (k₀), were measured by spectroscopic ellipsometry (UVISEL plus, Horiba) and fitted using the Lorentz model. Ellipsometric thickness values were validated by XRR.

The deposition temperature plays a critical role in determining the crystallinity, strain state, morphology, and polarization of BaTiO₃ (BTO) thin films grown on LSAT and GSO substrates. High-resolution X-ray diffraction (HRXRD) results show enhanced c-axis orientation and crystallinity with increasing temperature, peaking at 700 ℃ on LSAT. A further rise to 750 ℃ slightly reduces quality, likely due to defect formation. On GSO, improved Laue oscillations and narrower FWHM at 750 ℃ indicate better lattice matching and epitaxial growth. Scherrer-based estimates reveal that vertical coherence length peaks at 700 ℃ (about 32 nm) on LSAT and increases steadily on GSO, reaching about 60 nm at 750 ℃. This contrast reflects superior lattice compatibility between GSO and BTO. Reciprocal space mapping shows strain evolution with temperature. At 500 ℃, films exhibit strong in-plane tensile strain (c/a ratio < 1), which relaxes at higher temperatures as c/a exceeds 1. Calculated strain confirms this, with in-plane strain decreasing and out-of-plane strain increasing from 500 ℃ to 750 ℃. SHG measurements reveal that low-temperature films (500 ℃) favor in-plane polarization, while higher temperatures induce out-of-plane domains and complex multi-domain structures, indicating temperature-driven polarization reorientation. AFM results show the smoothest surface at 600 ℃ (RMS is 1.175 nm), with roughness slightly increasing at higher temperatures due to strain relaxation and defect formation. These findings align with XRD and XRR trends. Ellipsometry and XRR confirm that film thickness decreases from about 80 nm at 500 ℃ to about 50 nm at 600-650 ℃ due to enhanced lateral growth, then slightly increases at higher temperatures. Clearer XRR fringes at 700 ℃ indicate improved interface quality. Deposition optimization identified the best conditions as 30° angle, pure Ar atmosphere, 13.361 cm target-substrate distance, and 75 W RF power, achieving uniform and high-quality films. Finally, SEM and EDS analyses show denser, smoother films at elevated temperatures. However, Ba/Ti ratios deviated from stoichiometry (0.66-0.97), likely due to off-axis sputtering effects, contributing to strain and preferential c-axis domain formation.

Epitaxial BaTiO₃ (BTO) thin films were successfully deposited on (001)-oriented LSAT and (110)-oriented GSO substrates via radio-frequency (RF) magnetron sputtering. The structural, morphological, compositional, and optical properties of the films were systematically investigated. X-ray diffraction (XRD) analysis confirmed that the films predominantly crystallized in the tetragonal phase with a strong (001) preferred orientation, indicating high-quality epitaxial growth. Particularly, the film deposited on the GSO substrate at 750 ℃ exhibited superior crystallinity, as evidenced by the appearance of distinct Laue oscillations. Deposition parameters such as target-to-substrate distance, deposition angle, O₂/Ar gas volume flow ratio, and RF power were found to significantly influence the film's structure and performance. Optimal results were achieved using a target-to-substrate distance of 13.361 cm, a deposition angle of 30°, and an RF power of 75 W, with pure Ar atmosphere yielding the best film quality and highest deposition rate. The synergistic optimization of these parameters is essential for obtaining high-crystalline-quality BTO thin films and provides a solid foundation for their application in optoelectronic and ferroelectric devices.

Making use of erasable conductive channels of atomically thin domain walls, ferroelectric domain-wall memory devices achieve exceptionally high storage densities and ultrafast write/read speeds ranging from nanoseconds to picoseconds. These devices also exhibit outstanding performance of high fatigue endurance number and long-term data retention. While demonstrations of prototypical devices have validated the fundamental operation principle of domain-wall-based memories, several critical issues must be resolved before practical application. First, many novel transport phenomena observed at conductive domain walls remain inadequately understood, demanding thorough studies of domain-wall motion kinetics and the interplay between polarization, defects, and lattice coupling. Second, the domain nucleation and domain wall motion are correlated to the device reliability; therefore, precise control over domain-wall creation, propagation, and pinning is essential for ensuring fast operations and long-term data retention. Finally, in vertically stacked three-dimensional architectures, potential cross-talks between adjacent cells must be systematically evaluated especially for stable and reliable operations of multilevel memories. These challenges are crucial for advancing ferroelectric domain-wall memories toward large-scale integration and commercialization.

Although significant achievements have been made in hafnia-based ferroelectric materials and their device applications, several fundamental and practical challenges persist.

The preparation of thin-film devices based on HfO₂ faces significant challenges related to electric field cycling stability. Early cycling phases experience the wake-up effect, attributed to field-induced phase changes, charge detrapping and redistribution, domain wall depinning, and reduction of depolarization fields from breakdown in nonpolar layers near electrodes. Later cycling stages suffer fatigue, indicated by a reduction in remanent polarization (P_r), often ending in device breakdown.

Specifically, FeFET devices encounter reliability challenges related to their metal-ferroelectric-insulator-semiconductor (MFIS) gate stack; the interfacial layer (IL), due to high permittivity and coercive fields, undergoes increased stress during polarization switching, causing premature fatigue and short retention times driven by depolarization fields. Optimizing the IL is thus crucial for improving FeFET reliability. FeRAM, despite demonstrating promising retention stability, still faces limitations in commercialization due to polarization fatigue, imprint effects, wake-up phenomena, and substantial device-to-device variability, necessitating further optimization of electrode design, composition, interface treatments, and crystallization anneals. Meanwhile, FTJs rely on ferroelectric polarization-modulated tunneling current, with main challenges including low tunnel current due to relatively thick tunneling barriers, as well as endurance issues. Although FTJ technology holds significant potential for 3D stacking and in-memory computing, it remains at an early development stage, requiring further research to assess future applicability.

The discovery of ferroelectricity in hafnia oxide has opened numerous promising pathways in nanoelectronics, enabling significant advancements in non-volatile memories, sensors, actuators, and neuromorphic computing. Future research efforts should focus on overcoming current challenges such as material fatigue and reliability issues inherent in FeFET, FeRAM, and FTJ devices. Continuous advances in material science, device engineering, and integration technology will thus play a pivotal role in establishing hafnia-based ferroelectric materials as cornerstone components of next-generation computing platforms.

HfO₂-based materials have been successfully commercialized applications for DRAM capacitors and field-effect transistor gate dielectrics due to their excellent CMOS compatibility, superior thermochemical stability, wide bandgap, and high dielectric constant. The recent discovery of their ferroelectric properties has further expanded the application prospects of HfO₂-based thin films in information storage technologies.

1) Dielectric properties: HfO₂-based thin films with MPB structures located at the phase boundary between orthorhombic (o) and tetragonal (t) phases, exhibit high dielectric constants under relatively low electric fields. Precise control of phase composition is crucial for realizing MPB structures, with approaches including: superlattice design, elemental doping, oxygen vacancy and carbon defect engineering, annealing process optimization, grain size control, and electrode material selection, and so on.

2) Ferroelectric properties: HfO₂-based thin films demonstrate good ferroelectricity especially at a thickness below 10 nm. However, relatively poor endurance and high coercive field remain major bottlenecks for practical applications. Researchers have developed various improvement strategies. Introducing oxide interlayer, optimizing domain switching ratio, elemental doping, and adopting oxide electrode can significantly enhance endurance; While superlattice design, elemental doping, and new ferroelectric phase engineering can effectively reduce coercive fields.

3) Devices: HfO₂-based high-κ dielectrics have been utilized in commercial DRAM capacitors. However, with continuously shrinking feature size, further increasing dielectric constant while maintaining low leakage current require deeper investigation. Successful 3D trench deposition of HfO₂-based ferroelectric thin films has enabled high-density integration of FeRAMs with advantages including large polarization, fast switching, and good endurance. Novel device architectures like FeFETs and FTJs demonstrate great potential in neuromorphic computing through nonvolatile multi-state manipulation. However, challenges remain in FeFET retention characteristics and FTJ endurance. Future efforts should focus on improving thin-film quality and scaling up from single-cell to array-level applications, thereby fully realizing the potential of HfO₂-based FeFETs and FTJs.

Porous medium combustion has high efficiency, low pollutant emissions, and effective utilization of low-calorific-value fuels, which is crucial for energy conservation and carbon reduction in high-temperature industries. As a core component of porous medium burners, the porous medium material must withstand the high temperatures generated by fuel combustion and the corrosive effects of water vapor and excess oxygen in flue gases. Among various inorganic nonmetallic materials, Alumina porous ceramics are widely used as dielectric materials in high-temperature porous medium burners due to their excellent thermal stability and resistance to wet oxidation and corrosion. However, the relatively low thermal radiation coefficient (i.e., approximately 0.3) of alumina significantly reduces the heating efficiency of the burner. Therefore, enhancing the surface infrared emissivity of alumina porous ceramics is critical to improving the radiative heat transfer performance of alumina porous ceramic burners. While enhancing material radiation primarily involves constructing infrared radiation coatings on material surfaces, these methods often overlook the combustion stability of low-calorific-value fuels in porous medium burners. This study was to address this issue by firstly preparing flower-like CeO₂ catalysts via coprecipitation and subsequently developing alumina porous ceramics coated with flower-like CeO₂. The unique flower-like structure of the CeO₂ coating could enhance both radiative heat transfer and catalytic performance, thereby improving the overall radiation capability and heating efficiency of the porous medium burner.

To address the issue of low radiation efficiency on the surface of alumina porous ceramics used in high-temperature porous media combustion, this study adopted a systematic approach. NH₄HCO₃ and Ce(NO₃)₃·6H₂O were utilized as raw materials to synthesize flower-like CeO₂ precursor via a low-temperature co-precipitation method. These precursors were subsequently transformed into flower-like CeO₂ through heat treatment at 500 ℃. Alumina porous ceramics were fabricated by an organic foam impregnation technique. Alumina porous ceramic preforms were obtained after heat- treatment at 1300 ℃. Subsequently, α-Al₂O₃ powder and deionized water were mixed to prepare an impregnating slurry, which was applied to the preforms via vacuum impregnation. After drying, these preforms underwent further heat-treatment at 1500 ℃ to finalize the alumina porous ceramics. Commercially available ordinary CeO₂, synthesized flower-like CeO₂ powder, and flower-like CeO₂ precursor powder were sprayed onto the surfaces of the alumina porous ceramic skeletons. After heat-treatment at 500 ℃, three distinct CeO₂ samples CCA, FCA, and PFCA were produced. The effect of the flower-like CeO₂ coating on the combustion characteristics, lower flammability limit, and combustion temperature of alumina porous ceramics were evaluated by a porous media combustion platform, complemented by infrared thermal imaging (Fluke TiX 1060) and flue gas analysis (Testo 350). In addition, the surface element valence, specific surface area, and microstructure of the three types of CeO₂ were also characterized to elucidate the advantages of flower-like CeO₂ over ordinary CeO₂. Furthermore, the enhancement mechanism of infrared emissivity for flower-like CeO₂ was investigated.

Flower-like CeO₂ precursor and flower-like CeO₂ are synthesized with NH₄HCO₃ and Ce(NO₃)₃·6H₂O as raw materials through a coprecipitation method and a subsequent heat-treatment at 500 ℃. Compared to conventional CeO₂, flower-like CeO₂ exhibits a significantly larger specific surface area and a higher proportion of surface reactive oxygen species. The prepared flower-like CeO₂ coating enhances infrared emissivity from 0.52 to 0.71 due to its well-developed petal and microporous structure. When applied to porous ceramic burners, this coating increases the surface temperature to 1032.6 ℃ and the heating rate from 32.38 ℃/min to 41.81 ℃/min, and reduces CO emissions to only 4.58 mg/m³. In addition, NOx emissions are lowered to as low as 0.19 mg/m³, and the lean fuel limit is extended to 3.5% (in volume fraction).

The alumina porous ceramics with flower-like CeO₂ coatings were fabricated. The interface between the coating and the alumina skeleton exhibited the superior characteristics. The unique morphology of the flower-like CeO₂ coating, characterized by well-developed inter-petal pores and micropores formed during the decomposition of CeO₂ precursor, could enhance the infrared emissivity of the coating. This improvement in emissivity led to increased surface temperature and heating efficiency of the alumina porous media burner. The flower-like CeO₂ coating had a high specific surface area and a significant proportion of reactive oxygen species, which facilitated methane combustion within the alumina porous ceramics. Consequently, this could broaden the range for stable methane combustion and substantially reduce CO emissions in the flue gas.

Refractories are important supporting materials for high-temperature industries. During high-temperature service, refractories are subjected to severe thermal shock, complex chemical corrosion, and mechanical wear, and must balance superior properties like thermal shock resistance, corrosion resistance, and high strength. However, the key service properties of refractories are often interdependent, such as superior corrosion resistance requiring a dense structure, which sacrifices the thermal shock resistance of the material. The superior thermal shock resistance requires a certain amount of pores to relieve thermal stress, but this reduces the strength and corrosion resistance of the material. Developing novel refractories with gradient composition and structure based on the different service micro-environments of refractory products in different regions, such as cold and hot surfaces, to enhance key service performance in different zones is an important research direction. In this paper, a gradient functional material with "21R-SiAlON dense layer AlN-SiC solid solution reinforced corundum porous structure" was developed based on the sensitivity of metal Al and Si to oxygen partial pressure at high temperatures.

Tabular alumina (3-1 mm, 1-0 mm; w(Al₂O₃) > 99.31%; Anmai Aluminum Co., China), α-Al₂O₃ powder (≤5 μm; w(Al₂O₃) > 99.28%), active α-Al₂O₃ powder (≤ 5 μm; w(Al₂O₃)>99.28%; Anmai Aluminum Co., China), metal aluminum powder (≤ 45 μm; w(Al)>99.77%, Henan Yuayang Co., China) and silicon powder (≤ 45 μm; w(Si)>99.52%; Yingkou Xianren Island Taihe Silicon Co., China) were used as raw materials, with phenolic resin (residual carbon of about 56%, Shengquan Group, China) as a binder. In the preparation, the weighted raw materials were mixed in a planetary mixer for 40 min to obtain uniform mixed slip. Afterwards, Al-Si-Al₂O₃ composite green brick samples (with the sizes of 230 mm × 113 mm × 65 mm) were obtained by compacting the mixed slip under 200 MPa by a model 630 t friction press. After natural dehydration for 24 h, the sample bricks were dried in a tunnel kiln at 200 ℃ for 12 h. The dried samples were nitriding sintered in a high-temperature nitriding furnace at 1550 ℃ for 3 h.

The phase composition of the sintered samples was analyzed by X-ray diffraction (XRD; Ultima IV, Rigaku Co., Japan). The microstructures of the samples were analyzed by scanning electron microscopy (SEM) in a model NOVA NANOSEM 450 electron microscope (FEI Co., USA). The pore structure of the samples was determined by a model FF35 CT microfocus industrial computered tomography (CT, YXLON International GmbH, Germany).

Based on the sensitivity of metal Al and Si to oxygen partial pressure at high temperatures, Al-Si-Al₂O₃ composites are sintered in a nitrogen atmosphere at 1550 ℃ for 3 h to prepare a gradient functional material, which is composed of "21R-SiAlON dense layer-AlN-SiC solid solution reinforced corundum with a porous structure". The synthesis mechanism is analyzed by CT, SEM, and XRD combined with thermodynamic analysis. The results indicate that P(O₂) on the surface of the sample is higher, where the"active oxidation to indirect nitridation"mechanism of Al and Si is predominated. Al and Si on the surface layer preferentially undergo active oxidation, generating gaseous sub oxides such as Al₂O and SiO, and reducing the local P(O₂). 21R-SiAlON is formed via a reaction of metal Al, Si, active α-Al₂O₃ powder and N₂. Under the action of concentration gradient, the preformed gaseous sub-oxides diffuse into the surface layer and react with nitrogen to form 21R-SiAlON, thus promoting the staggered growth of sheet-like 21R-SiAlON and ultimately forming a dense layer (with porosity<4%), which is constructive to the corrosion resistance of the material. As the active oxidation of outer Al and Si consumes O₂ layer by layer, P(O₂) inside the brick becomes extremely low, and the "direct nitriding/carbonization" reaction mechanism of Al and Si becomes dominant. Al and Si inside the brick react directly with nitrogen and residual carbon of phenolic resin, generating granular AlN-SiC solid solution reinforcement phase in-situ, forming a high-strength porous structure (with a porosity of 15%), which can alleviate a stress concentration caused by temperature gradient and improve thermal shock resistance.

A gradient functional material with a "21R-SiAlON reinforced denselayer—AlN-SiC solid solution reinforced porous structure" was prepared via nitriding Al-Si-Al₂O₃ composite in a nitrogen atmosphere at 1550 ℃ for 3 h. During sintering, there were two reaction mechanisms for metal Al and Si in different regions of the outer and inner layers of the sample, i.e., the "active oxidation to indirect nitridation" mechanism and the "direct nitridation/carbonization" mechanism in the outer and inner layers resulting in a gradient change in the composition and structure of the material. In the outer layer of the sample, the oxygen partial pressure was higher, and metal Al and Si preferentially underwent an active oxidation, generating gaseous sub oxides (i.e., Al₂O, AlO, SiO, etc.), which reduced the oxygen partial pressure. As the oxygen partial pressure decreased, Al powder, Si powder, and α-Al₂O₃ micro powder reacted with nitrogen to form 21R-SiAlON. Gaseous metastable phases such as Al₂O and SiO diffused to the outer layer of the sample to deposition, further promoting the staggered growth and development of sheet-like 21R-SiAlON, forming a dense layer. The porosity of this dense layer was less than 4%, and it was mainly composed of closed micropores. With the consumption of O₂ by the outer metal Al and Si of the sample, the oxygen partial pressure inside the sample was reduced to an extremely low level. Metal Al and Si directly reacted with N₂ and residual carbon of phenolic resin, generating AlN-SiC solid solution reinforcement phase in-situ. The porosity inside the sample was approximately 15%.

Recent development on low-carbon/ultra-low carbon refractories with superior properties for the rapid development of green low-carbon economy and high-quality clean steel is a research hotspot. The structure and properties of low-carbon refractories in a variety of ways are optimized to solve the problem of deterioration of high- temperature properties caused by simple reduction of graphite content. The introduction of carbon nanomaterials can significantly reduce the overall carbon content of the material. Also, the mechanical strength and thermal shock resistance of carbon-containing refractories are improved simultaneously because the nanostructures absorb and buffer the thermal stress generated by a large number of thermal shocks. In addition, reinforcement phases such as carbides, borides and nitrides are also applied to improve the organizational structure and properties of low-carbon refractories to solve the long-standing problem of carbon source oxidation. However, the poor dispersion of nanomaterials in the matrix is a significant technical problem, and the synthesis process of composite powders is complex and challenging. High production costs and the difficulty of achieving large-scale application restrict the industrial development of these materials. The structural degradation at high temperatures leads to a decline in the strength of refractories and even deterioration of comprehensive performance, which cannot guarantee the stability of long-term service of refractories

The MAX phase materials were expected to achieve the balance between low carbonization and service performance of carbon-containing refractories due to their graphite-like layered structure, high thermal conductivity, high damage tolerance and good oxidation resistance and thermal shock resistance. In this work, in-situ Ti₃Si(Al)C₂ reaction bonded low carbon Al₂O₃-C refractory was prepared by a reactive melt infiltration method based on the preparation process of ceramic matrix composite materials. The effects of the amount of TiC/graphite in the prefabricated material and the type of Si-Al alloy as the infiltration medium on the phase composition, microstructure, physical properties, mechanical properties and thermal shock resistance of the refractories were investigated.

The phase composition and performance (i.e., the apparent porosity, bulk density and flexural strength) are different at different compositions of the preforms. Corundum, Ti₃Si(Al)C₂ and TiC phases appear in all the samples, and trace amounts of SiC, TiAl_xSi_y and residual Al also occur. When the preform does not contain graphite, the content of Ti₃Si(Al)C₂ in the sample is higher, and the apparent porosity is much lower. The flexural strength of the sample increases from 9.0 MPa and 23.4 MPa to 79.4 MPa and 82.2 MPa, respectively, when the graphite removes from the raw material. Also, the bulk density of refractories is higher. This is due to the poor wettability of graphite and alloy, which prevents the infiltration of alloy melt into the preform. Graphite reacts with Si to form SiC phase and consumes Si element, which affects the growth of subsequent Ti₃Si(Al)C₂ phase. The content of titanium carbide in the raw material also affects the generation of Ti₃Si(Al)C₂, increasing the content of titanium carbide is beneficial to generating more Ti₃Si(Al)C₂, which causes a lower porosity and a higher bulk density of Al₂O₃-Ti₃Si(Al)C₂-C efractories. The overall apparent porosity of samples Al₅₀Si₅₀ and Al₉₅Si₅ is lower than that of sample Al₅Si₉₅, and the bulk density is relatively high. This is related to a fact that the presence of Al promotes the infiltration of alloy melt into the precast and is conducive to the formation of Ti₃Si(Al)C₂. The flexural strength of the sample T25@Al₅₀Si₅₀ using Al₅₀Si₅₀ alloy as a penetration medium increases from 75.5 MPa to 110.5 MPa after thermal shock. The rich toughening mechanism of Ti₃Si(Al)C₂ helps to improve the thermal shock resistance of the refractory, and the oxidation products generated after the oxidation of Ti₃Si(Al)C₂ can make the structure of the refractory more dense. The presence of whiskers in the refractory is also beneficial for better thermal shock resistance.

Low carbon Al₂O₃-Ti₃Si(Al)C₂-C refractory could be prepared after the preform was infiltrated with Al₅₀Si₅₀ alloy. The presence of graphite in the preform obstructed the penetration of Si-Al alloy and ultimately affected the mechanical properties of the sample. The flexural strength of the preform without graphite was up to 82.2 MPa after the infiltration reaction at 1550 ℃. The content of Al in the infiltration medium had an effect on the production of Ti₃Si(Al)C₂ in the refractory. The presence of Al was conducive to the reduction of twinning grain boundary energy between TiC grains, effectively promoting the formation of TiCx twins. Ti₃Si(Al)C₂ phase was also easy to nucleate from the TiCx twin and grow in the Ti-Si-Al liquid phase. The Ti₃Si(Al)C₂ phase consumed energy through various deformation mechanisms, thus increasing the flexural strength of the refractory material at room temperature. Also, Ti₃Si(Al)C₂ could form a continuous and dense oxide layer during the thermal shock process, effectively preventing the Ti₃Si(Al)C₂ inside the sample from being further oxidized. The formation of the dense layer could restore or even strengthen the mechanical properties of the refractory after thermal shock. The flexural strength of sample T25@Al₅₀Si₅₀ increased from 75.5 MPa to 110.5 MPa after thermal shock.

To improve the quality and efficiency of the nickel metal melting process, the performance of oxygen-rich top-blown melting furnaces must be considered, particularly the furnace lining material used in the melt pool area. As a furnance lining, the refractory material is exposed to the mechanical and thermal stresses, and the corrosion of the refractories by the slag is an important cause of their failure. Investigating the corrosion resistance of refractories is thus a prerequisite to improve the service life of materials and ensure the product quality. Semi-rebonded magnesia-chrome bricks with a superior high-temperature performance is currently the first option for oxygen-rich top-blown furnace linings. However, the effect of Cr⁶⁺ after using magnesia-chrome bricks should not be ignored due to environmental protection requirements. The development of refractories with a superior high-temperature performance can replace the semi-rebonded magnesia-chrome bricks used directly in the oxygen-rich top-blown molten furnace bath, which is imperative. In this paper, different types of bricks (i.e., semi-rebonded magnesia-chrome bricks, high alumina bricks, and clay bricks) were used, and their phase compositions and microstructures were analyzed. In addition, the mechanism of high-temperature corrosion of nickel slag and refractories was also investigated via thermodynamic simulations, thus providing a theoretical reference for the design and manufacture of environmentally friendly furnace lining materials.

The corrosion resistances of semi-rebonded magnesia-chrome bricks (Beijing Yenai Jiye New Technology Co., China), high alumina bricks (Zhengzhou Rongsheng Kiln Refractories Co., China), and clay bricks (Zhengzhou Rongsheng Kiln Refractories Co., China) to nickel slag with the size of 325 mesh (Jinchuan Group Co., Ltd., China) from the top-blown furnaces were analyzed by a static crucible method. The crucible specimens used were cylindrical specimens with a diameter of φ50 mm×50 mm and holes with a diameter of φ 20 mm×20 mm in the center, which were drilled with a rock drilling and coring machine. The dried crucible was filled with 25 g of nickel slag powder and reacted at 1450 ℃ for 3, 7, and 11 h to obtain specimens after corrosion. The corroded specimens were then cut uniformly along the axial direction, and their cross-sections were analyzed. After fixing the corroded specimens with epoxy resin, the specimens were polished to obtain specimens for microstructure analysis.

The microstructures of the corroded specimens were determined by field-emission scanning electron microscopy in the back-scattered electron mode (FE-SEM, Gemini 500, ZEISS Co., Germany). The phase compositions of the corroded specimens were analyzed by X-ray diffraction (XRD; D/MAX 2000PC, Rigaku Co., Japan). The corrosion reaction was simulated by a software named Factsage (version 7.0) for thereaction, equilib, and viscosity modules of the thermodynamics simulation.

The semi-rebonded magnesia-chrome bricks, high alumina bricks, and clay bricks are exposed to nickel slag corrosion at high temperatures. The semi-rebonded magnesia-chrome bricks are found to be more resistant to nickel slag corrosion and penetration because of the formation of a spinel isolation layer at the interface between the slag and the refractory. The slag content in both the high alumina bricks and clay bricks during reaction with nickel slag corrosion is higher than that in the semi-rebonded magnesia-chrome bricks under the same conditions. Al₂O₃ in the clay bricks continuously dissolves in the slag due to the presence of more SiO₂ in the slag, resulting in a poor resistance of the clay brick to nickel slag corrosion. The viscosity of the slag gradually increases due to the grid structure formed by Si-O, weakening the penetration of the slag into the clay brick. Although high alumina bricks have some resistance to nickel slag corrosion, their resistance to nickel slag penetration is worse than that of clay bricks.

The spinel isolation layer formed during the corrosion reaction between the semi-rebonded magnesia-chrome bricks and nickel slag effectively blocked the penetration of slag into the interior of the material. High-alumina bricks could only form a corrosion isolation layer when corundum reached a saturation solubility in the slag. A corrosion isolation layer could not be formed during the corrosion reaction of the clay bricks. However, as mullite continued to dissolve in the slag, SiO₂ content in the molten slag increased and the network structure composed of Si-O enhanced the viscosity of the slag, thus weakening its ability to penetrate the interior of the clay bricks.

Silicon carbide porous ceramics are widely used in many fields because of their corrosion resistance, good thermal shock resistance, large specific surface area and superior mechanical properties. Silicon carbide porous ceramics are mainly prepared by different methods (i.e., pore-forming agent method, organic foam impregnation method, freeze-drying method, and foaming method). Among these, the foaming method with simple operation and controllable density and shape of the products is a common approach to prepare silicon carbide porous ceramics with a high porosity. However, silicon carbide porous ceramics still have the disadvantage of low mechanical properties. The In-situ generation of SiC whiskers can greatly improve the mechanical properties of the materials. Therefore, in this paper, silicon carbide whiskers self-reinforced porous ceramics were prepared by foaming method and reactive sintering with silicon carbide powder of 70% (in mass fraction) as a raw material, 30% silicon powder and carbon black as a self-bonding phase, AlF₃·3H₂O as an additive. In addition, the effect of AlF₃·3H₂O content on the phase composition, microstructure and mechanical properties of silicon carbide whiskers self-reinforced porous ceramics was also investigated.

In this work, silicon carbide powders were used as raw materials. Silicon powders and carbon black were used as self-bonding phase materials. AlF₃·3H₂O was selected as an additive. Triethanolamine dodecyl sulfate was used as a blowing agent. The ground powders were slowly added to a solution containing 0.5% Isobam 104 and 0.15% sodium carboxymethylcellulose, which was vigorously stirred in a mechanical stirrer to produce a slurry containing 50% (in volume fraction) of ground powders. Subsequently, 1.0% (in volume fraction) triethanolamine dodecyl sulphate was added to the obtained slurry and then vigorously stirred to prepare a homogeneous wet foam, which was rapidly poured into moulds with the dimensions of 160 mm × 80 mm × 40 mm and dried at ambient temperature for 48 h. Afterwards, the demoulded foam green bodies were dried at 40-100 ℃. Finally, the dried foam green bodies were fired in argon atmosphere furnace at 1600 ℃ for 2 h to obtain porous ceramics.

, The average large spherical pore size and annular pore size in as-prepared silicon carbide whiskers self-reinforced porous ceramics both firstly decrease and then increase with increasing the content of AlF₃·3H₂O from 0 to 4%. These results are due to the minimum surface free energy of the slurry with 2% AlF₃·3H₂O, leading to the stable bubbles existing in the slurry. Adding an appropriate amount of AlF₃·3H₂O can promote the formation of silicon carbide whiskers and improve the strength of as-prepared silicon carbide whiskers self-reinforced porous ceramics. At AlF₃·3H₂O content of 2%, the flexural and compressive strengths of porous ceramic samples with a porosity of 76.9% are 2.6 MPa and 3.7 MPa, respectively, which are significantly higher than those of the porous ceramic samples prepared without AlF₃·3H₂O (i.e., 1.1 MPa and 1.4 MPa). The enhanced mechanical properties can be related to the microstructure of the porous samples. The more uniform pore distribution and smallest average spherical pores and annular pores in porous sample with AlF₃·3H₂O addition of 2% are formed, resulting in improved mechanical properties of porous sample. Also, a large number of silicon carbide whiskers with the largest length are generated, which act as a reinforcing phase for further improving the mechanical properties of porous samples.

Adding an appropriate amount of AlF₃·3H₂O could promote the formation of silicon carbide whiskers and improve the strength of porous ceramic specimens. At AlF₃·3H₂O content of 2%, the flexural and compressive strengths of as-prepared silicon carbide whisker self-reinforced porous ceramic samples with a porosity of 76.9% were 2.6 MPa and 3.7 MPa, respectively. The length and the diameter of SiC_w whiskers generated in the samples were 4 μm and 62 nm, respectively, and their growth was controlled due to the gas-solid reaction mechanism.

The global imperative to reduce carbon emissions has catalyzed the development of hydrogen-based metallurgical technologies, with hydrogen shaft furnaces emerging as a promising low-carbon alternative for ironmaking. These reactors demand refractory linings with an exceptional stability to withstand synergistic thermal, chemical, and mechanical stresses when operating under extreme reducing environments (i.e., H₂/CO atmospheres at 1050 ℃, 0.5-1.0 MPa). Conventional aluminosilicate refractories, while widely used, suffer from deleterious reactions in H₂-rich atmospheres, including silica volatilization, carbon deposition via the Boudouard reaction, and structural degradation induced by iron oxide (Fe₂O₃) reduction. Recent studies highlight that co-doping Fe₂O₃ with TiO₂ can mitigate these issues, yet systematic investigations into the role of TiO₂/Fe₂O₃ phase chemistry and pretreatment strategies remain scarce. This study was to elucidate the impact of TiO₂/Fe₂O₃ composite oxides on the reduction stability of mullite-based refractories under simulated hydrogen shaft furnace conditions (i.e., H₂/CO = 5∶2). We were to investigate the phase evolution and microstructural engineering influence resistance to H₂/CO induced degradation via varying the TiO₂/Fe₂O₃ molar ratio (0.5-4.0) and employing pre-sintering treatments.

High purity electromelting mullite (Al₂O₃ >70% (in mass fraction), 3-1 mm, 1-0 mm and 200 mesh), plate corundum (Al₂O₃ >99.5%, 325 μm), α-Al₂O₃ (Al₂O₃ >99.5%, 2-5 μm), Fe₂O₃, TiO₂ powder (analytically pure), silicon powder (2 μm), and high viscosity yellow dextrin (200 μm) were used as raw materials. Fe₂O₃ and TiO₂ powder were proportioned according to the mass ratio m(TiO₂):m(Fe₂O₃) of 0.5, 1.0, 1.5, 2.0, 3.0, and 4.0, respectively, and were placed into a planetray ball mill according to a mass ratio of m(zirconium ball):m(water):m(mixed powder) of 2∶1∶1, The ball mill was operated at 300 r/min for 3 h. After milling, the slurry was taken out and put it in an oven for drying at 110 ℃ for 24 h. The ground sample was screened by a sieve with 200 mesh, thus obtaining the composite oxide of titanium oxide and iron oxide without pre-burning. The sample of φ50 mm×10 mm was prepared with the sifted powder at 100 MPa, and then heated in air at 1300 ℃ for 3 h. After cooling to room temperature, pre-burned titanium-iron oxide composite oxide was obtained.

The dextrin aqueous solution was prepared according to a ratio of m(dextrin):m(warm water) of 1∶1. The mixture was trapped at 25 ℃ for 12 h, and the cylindrical sample was shaped into φ50 mm×50 mm at 150 MPa, and dried in an oven at 110 ℃ for more than 24 h. Finally, the dried sample was heated in air to 1600 ℃ for 3 h, obtaining the titanium-iron oxide composite mullite refractory. The temperature increasing rates of this experiment were 5 ℃/min at 0-1000 ℃, 3 ℃/min at 1000-1400 ℃, and 2 ℃/min at 1400-1600 ℃, respectively.

A large amount of Al₂TiO₅ and a small amount of Fe₂TiO₅ are formed in mullite refractory via the introduction of non-pre-fired titanite composite powder, which is not a dense Mosaic structure. In particular, the porosity increases during the formation of Al₂TiO₅, weakening the protective effect of iron oxide under reduction conditions, and resulting in more pores and cracks formed after the reduction atmosphere treatment. The stability cannot improve as TiO₂/Fe₂O₃ ratio increases. After reduction, the compressive strength of the sample is only 32.2-38.6MPa, showing a significant downward trend, and the attenuation rate is as high as 17.3%-26.5%.

Fe₂TiO₅ solid solution is formed in the pre-fired powder at 1300 ℃, and a large number of titanium oxide is coated iron titanate complex, forming a dense Mosaic structure in the corundum mullite matrix. Fe₂TiO₅ is formed during a progressive reduction of ferrous compound FeTiO₃, greatly inhibiting the reduction of iron oxide. Thus, the anti-reduction stability of the material is significantly improved. When the mullite refractory with m(TiO₂):m(Fe₂O₃) > 2.0 is added, the reduction stability is improved. The compressive strength remains 74.3-84.5 MPa at the reduction temperature of 1050 ℃, and the compressive strength attenuation rate at room temperature is only 2.6%-5.8%.

The results indicated that non-pre-sintered titanium-iron oxide composite samples exhibited a significant structural degradation after being treated at 950 ℃. This degradation was characterized via the formation of numerous micron-sized pores in the matrix and a weakening of the aggregate-matrix interface, leading to a peak compressive strength of only 38.6 MPa and a strength decay rate of 17.3%-26.5%. In contrast, pre-fired titanium-iron oxide composite samples maintained a good structural integrity after reduction at 1050 ℃. When a molar ratio of TiO₂/Fe₂O₃ exceeded 2, the compressive strength remained at 71.9 MPa. The microstructural analysis revealed little cracks in the samples, and the strength attenuation rate was controlled within 10%. In the non-pre-sintered titanium-iron oxide composites, the composite phase of Al₂TiO₅ and Fe₂O₃, which was introduced without pre-firing, could not effectively inhibit the reduction reaction of Fe₂O₃. This resulted in the precipitation of metallic iron and subsequent volume shrinkage. However, in the pre-fired composites, Fe₂TiO₅ was preferentially transformed into the ferrous compound FeTiO₃ during the reduction process. The resistance of the material to H₂/CO reduction erosion was enhanced via constructing a cooperative stable structure of Fe—O—Ti. This finding could provide a theoretical basis for the development of high-stability refractories for hydrogen-based shaft furnaces.

The development of improving the thermal shock resistance of refractories materials is based on scientific and reliable thermal shock resistance theories combined with scientific and accurate testing methods, and thermal shock resistance enhancement methods. The research on the thermal shock resistance of carbon-containing refractories focuses on the composition/structure nano-scaling and microstructure design. However, single modification approach cannot fulfil the requirement imposed by the low-carbon containing refractories ascribe to their complex thermal shock behavior. Integration of matrix toughening, aggregates modification and aggregate/matrix interface design is assumed to exhibit some effects on improving the thermal shock resistance of refractories.

Macro-mechanical testing methods are not sensitive to the optimization of the microstructure of refractories. Nanoindentation method can test the mechanical properties of materials in micro-scales. The combination of macro-mechanical property test and micro-mechanical property test can deeply reveal the improving mechanism of the properties of refractories. There is still a long way for nanoindentation to be widely used in refractories with a high heterogeneity because of its nano-scale characteristics. It is of great importance to standardise nanoindentation test in refractories.

Numerical simulation and machine learning can verify the experimental results and predict the properties of refractories. Combining them with experiments can provide a further guidance for the microstructure design of refractories. Besides, the combination of numerical simulation and nanoindentation or fatigue test can accelerate the development of nanoindentation test standards and fatigue test standards for refractories.

(K, Na)NbO₃ (KNN)-based lead-free piezoelectric ceramics are considered to be one of the important candidates to replace lead-based piezoelectric ceramics due to their high Curie temperature and excellent comprehensive performance. To further improve its performance, the strategy of element doping is employed. The doping behavior of the Mn element in pressure-sintered KNN ceramics has been extensively studied. However, there are significant differences in the density of Mn-doped KNN ceramics in different works, which poses a challenge to the analysis of the underlying mechanism of Mn doping and its modification effect. Therefore, this study takes hot-pressed KNN ceramics as the research object due to their high densification, and systematically explores the impact of Mn doping on the microstructure and electrical properties of KNN ceramics.

In this study, KNN ceramics with MnO₂ doping were prepared by hot-pressed sintering. The specific composition is (K_0.5Na_0.5)NbO₃-x% MnO₂ (abbreviated as KNN-x% MnO₂, x = 0, 0.5, 1.0, 1.5). The raw materials include K₂CO₃ (99.0%), Na₂CO₃ (99.8%), Nb₂O₅ (99.99%), and MnO₂ (98.8%), all of which were purchased from Sinopharm Chemical Reagent Co., Ltd. First, carbonate and Nb₂O₅ were placed in a nylon ball mill with zirconia balls according to the formula ratio, and ball milled for 24 h using anhydrous ethanol as the medium; the ball-milled slurry was dried and calcined at 730 ℃ for 4 h, and then ball-milled again and calcined again at 930 ℃ for 4 hours. Subsequently, different amounts of MnO₂ were added to the calcined powder, mixed and ball-milled for 24 h, and then dried and ground to obtain Mn-doped KNN ceramic powder. Finally, the ceramic block was prepared by hot pressing and sintering at 950 ℃ and 30 MPa for 2 h under argon atmosphere. After cutting, grinding, and annealing at different temperatures, the sample surfaces were subsequently coated with silver electrode for electrical measurements. The poling process was performed at 3.5 kV/mm in 120 ℃ silicone oil for 30 min.

The surface micromorphology of the ceramics was characterized by Merlin scanning electron microscope (SEM). The crystal structure was analyzed by D/Max 2500 X-ray diffractometer (XRD). The density of the samples was measured using the Archimedean drainage method. The dielectric properties were tested using a TH2827 impedance analyzer. The ferroelectric properties were obtained using a TF Analyzer 2000 ferroelectric analyzer. The piezoelectric constant d₃₃ was measured using a ZJ-3A quasi-static d₃₃ tester. The mechanical quality factor Q_m was measured using a TH2839 impedance analyzer. The local domain structure and domain switching behavior were tested and analyzed using an MFP-3D atomic force microscope with piezoelectric force microscopy (PFM).

High-density (relative density > 98%) KNN-x% MnO₂ ceramics were successfully prepared by hot pressing sintering process. XRD results show that MnO₂ doping does not change the perovskite phase structure of KNN. With the increase of Mn doping amount, the P_r of the ceramic shows a trend of first increasing and then slightly decreasing. At x = 1.0-1.5%, P_r reaches 14.92-18.32 μC/cm², d₃₃ can reach up to 106 pC/N, and both positive and negative strains are improved, reflecting enhanced ferroelectric and piezoelectric responses after Mn doping. PFM test shows that the switching voltage of the local ferroelectric domain gradually increases from 5 V to 20 V, indicating that Mn doping enhances the domain wall pinning effect. At the same time, the mechanical quality factor Q_m of the ceramic is significantly improved, with the highest value reaching 175, reflecting the typical "hard" doping behavior.

This study found that Mn-doped KNN ceramics exhibit an atypical "hard" doping effect for the high-density hot-pressed ceramics that is different from the traditional doping mechanism in lead-based ceramics, in which Q_m and P_r are simultaneously improved and d₃₃ is maintained at a high value. This atypical "hard" doping effect induced by Mn doping provides a new perspective for understanding the mechanism of element doping in the KNN system, and also provides significant guidance for the optimized design of high-performance lead-free piezoelectric ceramics.

The rapid advancement of electronic technologies leads to a widespread electromagnetic interference (EMI), which disrupts device performance and poses health risks, including neurological disorders, due to the prolonged exposure. Some emerging fields such as 5G communications, IoT, aerospace, and defense demand high-performance EMI shielding materials capable of operating under extreme conditions (e.g., high temperatures, radiation, and humidity) to ensure device reliability and data security. Conventional shielding materials (i.e., metals, carbon-based materials, and MXenes) have some limitations. Metals suffer from high density and corrosion, while carbon-based materials and MXenes degrade in high-temperature or oxidation environments. In contrast, ceramic materials, particularly perovskite oxides, offer superior thermal stability, corrosion resistance, and mechanical strength, making them promising candidates for next-generation EMI shielding. Among perovskite oxides, SrTiO₃ ceramics exhibit a high dielectric constant and superior insulating properties, but their intrinsically low conductivity limits their EMI shielding performance. The high-entropy design strategy, which incorporates multiple elements to enhance synergistic effects, provides a novel approach to improve electrical conductivity and thermal stability. Applying this high-entropy design to perovskite oxide ceramics can optimize their electromagnetic shielding performance under high-temperature environment, providing new research perspectives and technical pathways for developing novel high-temperature EMI shielding materials. In addition, the effect of high-temperature annealing on perovskite oxide performance remains unclear. Annealing in air leads to oxygen vacancy annihilation, causing structural and electronic changes that degrade both conductivity and EMI shielding efficiency. It is thus crucial to optimizing material design and ensuring stable performance under harsh environmental conditions. This study was to use a high-entropy design approach to develop perovskite oxide ceramics, and the EMI shielding properties in the X-band and their stability under high-temperature annealing were investigated. This work was to advance the development of robust, high-temperature EMI shielding materials via elucidating the annealing-induced structural and performance evolution, thus providing both theoretical insights and practical solutions for next-generation electronic and aerospace applications.

The perovskite oxide ceramics were synthesized via solid-state reaction with high-purity raw materials (i.e., SrCO₃, BaCO₃, CaCO₃, La₂O₃, Na₂CO₃, K₂CO₃, and TiO₂, all >99% purity from Aladdin Co., China). The powders were initially calcined at 400 ℃ for 3 h to remove moisture, and then ground by stoichiometric ball-milling in ethanol (150 r/min, 6 h). After rotary evaporation drying, the mixture was sintered in air at 1500 ℃ for 5 h to obtain pre-sintered powders, which were further ground into fine particles. The dense disk-shaped samples (φ26 mm) were prepared via vacuum hot-pressing (i.e., 10 Pa, 1400 ℃, and a heating rate of 10 ℃/min) at 70 MPa for 30 min, and subsequently annealed in air at 1000 ℃ for 1 h.

Two A-site high-entropy perovskite ceramics, i.e., (SrBaCaLaNa)_0.2TiO₃ and (SrBaCaLaK)_0.2TiO₃, are prepared through high-entropy design strategy. The XRD patterns indicate that the both ceramics maintain a single-phase cubic perovskite structure before and after 1000 ℃ annealing, and a peak shifts corresponding to average A-site ionic radii. The SEM images/EDS mapping and TEM images/EDS mapping demonstrate excellent elemental homogeneity without segregation, showing an effective solid solution formation. The Grain e analysis reveals a good structure stability after annealing due to the lower treatment temperature, compared to sintering temperature. The electron paramagnetic resonance (EPR) measurements show an enhanced oxygen vacancy concentration in high-entropy ceramics, compared to SrTiO₃. The electrical characterization indicates a room-temperature conductivity of 21 S/cm for (SrBaCaLaNa)_0.2TiO₃ and 16 S/cm for (SrBaCaLaK)_0.2TiO₃ due to La³⁺ aliovalent doping and oxygen vacancies. However, annealing causes 4-order conductivity reduction due to oxygen vacancy annihilation via oxygen re-filling. The dielectric analysis demonstrates superior properties in high-entropy ceramics (ε′=50-70, ε″=40-60), which are 20% higher than those of STO, where the conduction loss dominates the dielectric loss mechanism. The enhanced dielectric properties originated from increased dipole polarization and conduction loss associate with a higher oxygen vacancy concentration.

EMI shielding evaluation shows the superior performance with SE_Total >25 dB (99.6% efficiency) in X-band for both high-entropy ceramics, where reflection dominates the shielding mechanism (i.e., SE_R>10 dB, >90% reflectivity) due to large impedance mismatch. (SrBaCaLaNa)_0.2TiO₃ ceramic exhibits a slightly better shielding performance due to its higher oxygen vacancy concentration and conductivity. After annealing, high-entropy ceramics still outperform STO, while shielding performance degraded (i.e., SE_Total<20 dB), maintaining SE_R and SE_A at 10 dB. This result demonstrates that high-entropy design effectively enhances EMI shielding properties through lattice distortion effects, i.e., 1) modified local dipole moments from atomic displacement, and 2) enhanced dipole polarization through electron cloud redistribution. However, the performance degradation after annealing indicates that high-entropy design cannot completely prevent property deterioration in high-temperature oxidation environment, primarily due to oxygen vacancy and conductivity elimination.

A-site high-entropy perovskite ceramics, i.e., (SrBaCaLaNa)_0.2TiO₃ and (SrBaCaLaK)_0.2TiO₃, prepared via a high-entropy design, exhibited a single-phase cubic perovskite structure with a homogeneous elemental distribution. Compared to single-component STO ceramics, these high-entropy ceramics demonstrated higher oxygen vacancy concentration and electrical conductivity, leading to enhanced electromagnetic wave reflection and absorption. In the X-band, their total EMI shielding effectiveness exceeded 25 dB, with shielding efficiency surpassing of 99.6%. However, after annealing in air at 1000 ℃, the elimination of oxygen vacancies caused a notable decrease in conductivity, severely weakening polarization and loss capabilities. The reduction in complex permittivity diminished electromagnetic wave reflection and impaired dissipation ability, ultimately deteriorating EMI shielding performance. These results indicated that high-entropy design could improve the EMI shielding properties of perovskite ceramics, but it could not fully prevent performance degradation under high-temperature oxidative conditions.

High piezoelectricity and optical transmittance are ordinarily difficult to simultaneously achieve in randomly oriented ceramics or textured ones, affecting their further applications in optoelectronic field. How to break this trade-off relationship becomes a challenge for realizing electrical-optical coupling. In this paper, Er³⁺-doped randomly oriented (K_0.5Na_0.5)NbO₃ (abbreviated as xEr-KNN-R) and textured (K_0.5Na_0.5)NbO₃ (abbreviated as xEr-KNN-T) ceramics were prepared via tape-casting. the results showed that the xEr-KNN-T ceramics could have a high degree of orientation and a high relative density (up to 98.8%). The piezoelectric coefficient d₃₃ of the textured ceramics substantially boosted to 121 pC/N compared to randomly-oriented ones (30 pC/N), without significant decrease of the Curie temperature. Also, refinement of grains with tiny pores and relaxor-like behavior could endow the textured ceramics with moderate optical transparency (about 30% in the visible region). Meanwhile, the xEr-KNN-T ceramics exhibited enhanced up-conversion green and red luminescence intensity. The translucent-piezoelectric ceramics could provide insight for further development of high-performance optoelectronic devices.

x% (x = 0, 0.50, 0.75, 1.00, 1.25) Er³⁺ doped KNN with a nominal composition of (K_0.5Na_0.5)_1-xEr_xNbO₃ (abbreviated as xEr-KNN) matrix powders were synthesized via conventional solid-state reaction with K₂CO₃ (99.99%), Na₂CO₃ (99.99%), Nb₂O₅ (99.99%) and Er₂O₃ (99.9%) as raw materials. Anisotropic plate-like NN templates (with about 20 µm in length and width and about 5µm in thickness) were synthesized by a two-step molten salt method. The textured ceramics (abbreviated as xEr-KNN-T) were prepared via reaction template grain growth. Meanwhile, the non-textured randomly oriented ceramics (abbreviated as xEr-KNN-R) prepared without NN templates were used for comparison purpose. The microstructures of samples were determined by scanning electron microscopy (SEM, SUPRA 55 SAPPHIRE, Carl Zeiss Co., Germany) on the surface parallel to the casting direction and the section perpendicular to the casting direction using the accelerating voltage in a range of 0.02-30 kV. To measure the electrical properties, silver paste was coated on both sides of the sintered samples, and then poled in a silicone oil under 3 kV/mm at 150 ℃ for 30 min. The temperature dependences of the relative dielectric permittivity (ε_r) and dielectric loss (tanδ) of the ceramics were recorded by an Impedance Analyzer (Agilent 4294, Agilent Technologies Inc., USA), with an oscillation voltage of 1 V, at a frequency range of 1 kHz-1 MHz and a temperature range of 25-525℃ at a heating rate of 2℃/min. The piezoelectric coefficients (d₃₃) were characterized by a quasi-static piezoelectric constant testing meter (ZJ-6A, Institute of Acoustics, Chinese Academy of Sciences, China). The polarization-electric field (P-E) loops of the ceramics were determined by a ferroelectric test system (TF Analyzer 2000, aixACCT Co., Germany) with the frequency of 200 Hz. The optical transmittance was measured by ultraviolet-visible-near infrared spectroscopy (Lambda 950, PerkinElmer Co., USA). The PL emission spectra were recorded by a spectro-fluorometer (USB4000, Ocean Optics Co., USA) equipped with a 980-nm diode laser (MDL-III, CNI Optoelectronics Tech. Co., China).

Based on the SEM image of NN template, the NN grains synthesized by the topochemical microcrystal conversion (TMC) well inherit the anisotropic morphology of the precursor BNN5, showing a typical flake morphology. The length and width of NN grains are approximately 10-20 µm and the thickness is approximately 0.5-1.0 µm, meeting the size requirement of appropriate NN template used for tape-casting. From the optical transmittance curve, the addition of large size NN template with anisotropy is unfavorable to the transmittance of higher isotropy, resulting in the reduced transmittance of xEr-KNN-T. The XRD patterns of xEr-KNN-T show sharp diffraction peaks with a good crystallinity, presenting a typical ABO₃-type perovskite structure. Er³⁺ diffuses into the KNN lattice, and the NN template has a solid solution with the KNN matrix. Based on the SEM images of all the ceramics, the conventional solid-state reaction, the tape-casting process alone cannot improve the microstructure of KNN, but leads to deterioration in density. The existence of NN template leads to relatively small-sized grains of textured KNN-T ceramics. From the up-conversion PL spectra, the introduced flake-shaped NN template can reduce the symmetry of local environment around the KNN matrix and then cause local heterogeneity and change of crystal field round Er³⁺, thus increasing the up-conversion PL intensity of the xEr-KNN-T ceramics. In electrical tests (i.e., dielectric temperature spectrum and P-E loops), compared with xEr-KNN-R, the xEr-KNN-T ceramics have an improved piezoelectric coefficient (i.e., d₃₃ = 121 pC/N), while maintaining moderate ferroelectricity and a high Curie temperature (i.e., 371℃).

Er³⁺-doped KNN ceramics with random orientation and texture were prepared by a tape-casting method. The xEr-KNN-T ceramics exhibited a high relative density (i.e., up to 98.8%) and a uniform microscopic morphology (with the grain size of 0.33 µm). The piezoelectric performance of xEr-KNN-T ceramics was optimized to a large extent (i.e., d₃₃ = 121 pC/N), which was higher than that of the xEr-KNN-R ceramics, without lowering T_C. Meanwhile, xEr-KNN-T ceramics had a moderate optical transmittance (approximately 30% in the visible region), realizing the coexistence of piezoelectricity and transparency. Enhanced up-conversion luminescence characteristics were displayed in xEr-KNN-T. The translucent-piezoelectric KNN textured ceramics could provide a possibility and an enlightenment for the research of other high-performance lead-free optoelectronic materials.

The Clausius-Mossotti (C-M) equation is one of the most fundamental theoretical models to calculate the dielectric constant of materials. As was proposed over a century ago, it has been widely applied in dielectric material research due to its relatively simple mathematical form and clear physical interpretation. The equation, derived based on the Lorentz effective field approximation and the assumption of constant polarization, establishes a relationship among the macroscopic dielectric constant and the microscopic polarizability and unit cell volume. However, with the growing understanding of dielectric materials, it becomes clear that dielectric constant is affected by a range of additional factors, such as bond length, packing density and symmetry. These factors are not captured by the conventional C-M equation, indicating the limitations of the equation. Recently, machine learning (ML) methods show a promise in improving the prediction accuracy of dielectric properties. The ML models can outperform classical equations in terms of accuracy, but they often lack interpretability, which hinders their widespread use in material design and optimization. To address these issues, this study proposed a novel bidirectional embedding approach with domain knowledge and machine learning. The ML-based correction term was integrated into the classical C-M equation, enhancing its predictive accuracy, while retaining physical interpretability. This work could explore a potential of combining physical formulas with ML methods to create more robust and explainable models for material design.

The data of single-phase microwave dielectric ceramics were collected from published literatures and the Materials Project database. The dataset included materials whose dielectric constants were measured by the Hakki-Coleman method in a frequencies range from 5 to 18 GHz. A machine learning model identified molecular dielectric polarizability per volume (ppv), average bond length (blm) and unit cell volume (va) as the most significant contributors to the dielectric constant in a previous study. To enhance the accuracy of the C-M equation, correction terms were introduced based on the three key features. Symbolic regression, combined with genetic algorithms, was used to derive mathematical expressions for the correction terms. The symbolic regression technique utilized evolutionary algorithms to generate and evolve potential expressions. The structures and parameters of expressions were optimized to minimize prediction errors. Hyperparameters were optimized by a grid search method to identify the best-performing models. The process was carried out iteratively. A total of 16,000 candidate mathematical expressions were generated and compared. Finally, the Pareto front analysis was used to select the optimal expression that balances accuracy and complexity.

The revised C-M equation results in significant improvements in its predictive performance. For the Shannon polarizability dataset, the R² value of the revised C-M equation is increased by 42.29%, and the RMSE value is decreased by 14.72%. The correction term compensates for the inaccuracies of the original equation, particularly in cases where the molecular dielectric polarizability per volume of Shannon value (ppv_s) is either overestimated or underestimated. For low-polarizability materials with ppv_s values less than 0.2087, the Shannon database tends to underestimate the dielectric constant, and the correction term compensates for this error. Conversely, for high-polarizability materials, the dielectric constant is often overestimated, and the correction term reduces the overestimation. The analysis of the relationship between the correction term (Δppv_s) and features reveals that the correction term is most sensitive to ppv_s, followed by blm, with va having the least impact. This finding indicates that the dielectric polarizability plays a dominant role in determining the accuracy of the revised C-M equation. The influence of the features on the correction term is further explored through partial derivatives. The first-order partial derivatives show that the correction term's contribution from ppv_s is much greater than that from blm or va. The second-order partial derivatives reveal a non-linear relationship between the correction term and ppv_s and blm, while the relationship with va is linear. The results indicate that the compensatory effect of the correction term gradually diminishes, and the suppression effect becomes more pronounced as ppv_s increases. This behavior is consistent with the correction mechanism of "elevating the underrated, reducing the overrated".

This study introduced a bidirectional embedding approach that could integrate machine learning with domain knowledge to enhance both the accuracy and interpretability of the Clausius-Mossotti equation. The revised equation achieved a higher prediction accuracy with a 42.29% increase in R² and a 14.72% decrease in RMSE via incorporating the correction term derived from symbolic regression. The revised equation retained the physical meaning of the original one. The research could highlight a potential of bidirectional embedding approach to create models that could balance both accuracy and interpretability, offering a promising perspective on optimizing classical equations and material design.

High-entropy oxides (HEOs) have attracted much attention in the field of electrochemistry due to their distinctive structural characteristics and unique properties. The multiple-principal elements in HEOs offer the multiple redox pairs and multiple possible active sites, which can enhance the energy storage capacity and the electrocatalytic performance. Although the notable progress is achieved in the development of HEOs electrodes, their electrochemical properties should be further improved to meet the requirements of high-performance supercapacitors and OER electrocatalysts. The abundant active sites for the Faradic redox reactions and short pathways for charge transportation could be constructed through the design of novel HEOs with advanced microstructures, thus improving the electrochemical properties. As advanced microstructures, a hollow structure has a great promise for energy storage and conversion because it can provide more accessible storage sites, more catalytic centers and a larger electrode/electrolyte contact area. It is thus expected that the construction of hollow structure is an alternative route to significantly promote the electrochemical properties of HEOs electrode materials. However, it is difficult to prepare the HEOs with a hollow structure due to the complexity of the high-entropy system.

In this work, a hollow spherical high-entropy perovskite oxide of La(Cr_0.2Mn_0.2Fe_0.2Ni_0.2Cu_0.2)O₃(HS-HEPs) was prepared by microwave solvothermal process and subsequent calcination treatment. The as-prepared HS-HEPs exhibited the excellent electrochemical performance when used as an electrode material for supercapacitors and OER electrocatalysts due to the advantages resulted from the combination of high-entropy perovskite and special hollow structure.

HS-HEPs were prepared by microwave solvothermal process and subsequent calcination treatment. Typically, 0.134 mmol Cr(NO₃)₃·6H₂O, 0.134 mmol Mn(NO₃)₂·4H₂O, 0.134 mmol Fe(NO₃)₃·9H₂O, 0.134 mmol Ni(NO₃)₂·6H₂O, 0.134 mmol Cu(NO₃)₂·3H₂O, and 0.5 mmol La(NO₃)₃·6H₂O were dissolved in 30 mL ethanol under stirring for 1 h to obtain the homogeneous solution. Afterwards, 60 mg of carbon spheres were added in the solution under ultrasonic treatment for 30 min. The resulting mixture was transferred to a 50 mL microwave digestion vessel. The vessel was heated in a microwave oven at a power of 210 W for 10 min. Subsequently, the obtained mixture was centrifuged, washed with deionized water, and dried in a vacuum drying oven at 70 ℃ for 12 h. Finally, the obtained precursor powder was calcinated in a tube furnace with a heating rate of 3 ℃/min at 650 ℃ for 2 h to acquire HS-HEPs.

The crystalline structure of the sample was determined by X-ray diffraction (XRD, D8 Davinci, Bruker Co., Germany). The morphology and microstructure of sample were characterized by field-emission scanning electron microscopy (FESEM, S-4800, Hitachi Co. Ltd., Japan) equipped with energy dispersive X-ray spectroscopy (EDS) and transmission electron microscopy (TEM, 2100F, JEOL Co., Japan). The X-ray photoelectron spectra were obtained by a X-ray photoelectron spectrometry (XPS, ESCALab 250, Thermo VG Co., USA). The supercapacitor and OER performance of the sample were measured on a CHI 660E electrochemical workstation (Shanghai Chenhua Instrument Co., China).

The as-prepared samples display a cubic perovskite crystalline structure and a hollow sphere morphology. According to the XPS analysis, the variable oxidation states of Cr, Fe and Mn present in the HS-HEPs, which benefits the Faradaic redox reactions and increases the capacitance. In addition, the existence of high concentration of oxygen vacancies in HS-HEPs is beneficial to enhancing the capacitance and OER activity. Based on the GCD curve, the specific capacitance of HS-HEPs is estimated to be 406 F/g at 1 A/g. After GCD cycles of 5000 at a current density of 5 A/g, 65% capacitance is retained, implying a good long-term electrochemical stability. An asymmetric supercapacitor device (HS-HEPs//AC) with a two electrode configuration is assembled. A maximum energy density of 39.4 W·h/kg is achieved at power density of 746 W/kg. The OER activity of HS-HEPs is evaluated by a linear sweep voltammetry (LSV) polarization curve in 1 mol/L KOH aqueous solution using a standard three-electrode system. The overpotential of HS-HEPs is identified as 347 mV versus RHE for achieving a current density of 10 mA/cm², which is smaller than that of commercial IrO₂ (372 mV). The HS-HEPs possess the excellent electrochemical performance, which can be ascribed to the high specific surface area, abundant active sites, and high oxygen vacancy content, resulting from the combination of high-entropy perovskite and special hollow structure.

High-entropy La(Cr_0.2Mn_0.2Fe_0.2Ni_0.2Cu_0.2)O₃ hollow spheres with a perovskite crystalline structure were prepared by microwave solvothermal process and subsequent calcination treatment. The HS-HEPs possessed the excellent electrochemical performance, which could be ascribed to the high specific surface area, abundant active sites, and high oxygen vacancy content, resulting from the combination of high-entropy perovskite and special hollow structure. Based on the electrochemical performance, HS-HEPs could be used as supercapacitor electrode material and OER electrocatalysts. This work could provide a strategy to design and prepare high-entropy oxides with a hollow sphere structure, having promising applications in energy storage and conversion.

Y₂O₃ transparent ceramics are increasingly demanded in advanced technological applications, particularly in high-power laser systems, infrared optical windows, and semiconductor devices. These applications require materials that maintain both exceptional optical transmittance (i.e., >80% at 1064 nm) and mechanical strength (i.e., flexural strength >200 MPa), even when fabricated into complex geometries. Gel casting offers a promising route for producing large-scale, complex-shaped Y₂O₃ components. However, process-induced defects such as delamination, microcracks, and density gradients often compromise the microstructural integrity of green bodies, leading to a reduced performance in the ceramics. These defects primarily stem from excessive internal stresses generated during gelation, thus highlighting a need for precise control over gelation kinetics. This study was to investigate the synergistic effects of catalyst concentration and reaction temperature on the microstructural homogeneity of Y₂O₃ green bodies. The highly uniform green bodies were obtained via optimizing the parameters, thus enabling the fabrication of Y₂O₃ transparent ceramics with a high optical performance through vacuum sintering.

A certain amount of commercial Y₂O₃ powder was mixed with ethanol as a dispersion medium. The slurry was ground in a ball mill with ZrO₂ beads of 2 mm in diameter for 8 h. Ammoniumpolymethacrylate (A40) was added as a dispersant, concentrated H₂SO₄ as an additive and ZrO(NO₃)₂ as a sintering aid. The ground slurry was placed in an oven to dry at 120 ℃ for 6 h. After drying, the powder was sifted through a 200-mesh screen. The sifted powder was then calcined in air at a certain temperature for 4 h to obtain a Y₂O₃ nano-powder.

Acrylamide (AM) and N,N'-Methylenebisacrylamide (MBAM) were dissolved in deionized water to prepare a 15% AM aqueous solution in a mass ratio of AM to MBAM of 88∶10. After the organic compounds were completely dissolved, 5 mm ZrO₂ beads were added in a mass ratio of ball-to-powder of 5∶2. Tetramethylammonium Hydroxide (TMAH), ammonium citrate (TAC), A40, and Y₂O₃ powder were then added sequentially to prepare Y₂O₃ slurry with a solid content of 50%. The suspension was ground at 170 r/min for 6 h, then degassed under a vacuum of 1×10^-1Pa for 30 min. Catalyst and initiator were added sequentially, and followed via degassing the slurry again under a vacuum of 1×10^-1Pa for 3 min. Finally, the well-mixed slurry was poured into polytetrafluoroethylene or silicone molds heated at different temperatures (i.e., 25, 40, 60, and 80 ℃). The obtained green bodies were dried in air.

The gelation rate is evaluated via monitoring temperature variations during the process. The results show that increasing catalyst content accelerates gelation, but exceeding 0.08% causes a rapid reaction, leading to a green body cracking. The SEM images reveal a stratified microstructure, with a dense surface layer and a porous interior. The dense layer thickness decreases as catalyst content increases. Elevated reaction temperature also enhances gelation rate, and, stratification disappears at 60 ℃, yielding uniformly porous structures.

The infrared spectra of green bodies (i.e., 0.02% catalyst, 25 ℃) indicate an incomplete AM polymerization on the surface, while a well-developed PAM network in the interior impeded particle movement during drying. This structural disparity results in a dense surface and porous interior.

Pre-sintered green bodies exhibit heterogeneous microstructures that persist during densification. Higher catalyst content and temperature reduce the surface dense layer thickness and internal porosity.

The final vacuum-sintered samples further demonstrate that lower gelation reaction temperatures correspond to thicker surface dense layers under identical vacuum sintering temperatures, significantly hindering the expulsion of internal pores in the ceramics. This results in the center of the ceramic exhibiting an opaque condition.

Y₂O₃ transparent ceramics with excellent properties were prepared via gel casting and vacuum sintering with Y₂O₃ nano-powder as a raw material. During the gel casting process, the rate of gelation significantly affected the uniformity of the green body microstructure. A complete and uniformly structured Y₂O₃ green body was obtained via controlling the factors influencing the gelation rate at a catalyst addition of 0.04% and a reaction temperature of 60 ℃, while ensuring that the slurry met the filling requirements. After vacuum sintering at 1850 ℃ for 6 h, the obtained Y₂O₃ transparent ceramic (with a thickness of 2 mm) achieved a transmittance of 82% at 800 nm. This study demonstrated that during the gel casting process, the rate of gelation could affect the uniformity of the internal microstructure of the green body, and this internal microstructural non-uniformity further influence the subsequent densification process of the ceramic during sintering.

To solve the problems that the porosity is difficult to accurately control and the pore structure uniformity is insufficient in the traditional process of replicating polyurethane foam with ceramic slurry, a biclinic body design method based on regular hexahedron cell structure was introduced to prepare porous ceramic parts for high-performance porous media burner. The influence of parameters (i.e., cell edge length, pore rib diameter and array layer number) on the strength and combustion characteristics of porous ceramic materials was investigated.

The model was firstly designed by a software named ANSYS Spaceclaim modeling, and then the samples were prepared by 3D printing. The compressive strength of porous ceramic structures was tested by a model TSE104B universal material testing machine at a loading rate of 0.5 mm/min. A circular loading plate with a diameter of 100 mm was used to apply pressure to the samples, and the maximum compressive strength σ_s of each sample was calculated by recording the failure load. Combustion performance testing was conducted in a porous media combustion platform, and the composition and temperature of the flue gas were monitored by a model testo 350 flue gas analyzer. The temperature changes were recorded by an infrared thermal imager.

The stress distribution and internal combustion mechanism of the porous ceramic under uniaxial compression were analyzed by numerical simulation methods. Among them, ANSYS Static Structural module was used for stress distribution analysis, and ANSYS Fluent module was used for finite element analysis of the flow field of porous media, thus evaluating the effect of porosity on gas disturbance, heat transfer behavior, fluid pressure drop, and heat exchange efficiency.

In a high porosity range (i.e., ε≥90%), extending the residence time of premixed gas and enhancing turbulence effects can effectively promote gas mixing and combustion, thereby improving the overall combustion performance. Moderately reducing porosity (i.e., from 93% to 86%) improves the compressive strength of the material (approximately 100%) although this leads to a decrease in the maximum surface temperature (about 18.4 ℃) and an increase in CO emissions (about 8.75 mg/m³). This indicates that a moderate reduction in porosity sacrifices combustion efficiency at a high porosity can enhance the strength. In contrast, the structure in a low porosity range (ε<90%) restricts the free flow of gas flow, resulting in uneven velocity distribution and reduced turbulence, hindering effective gas mixing and combustion, thereby reducing combustion efficiency. This indicates that it is difficult to simultaneously improve combustion efficiency and strength at a low porosity. It is thus possible to balance combustion efficiency and strength via finely adjusting the porosity in the high porosity range (i.e., ε≥90%), achieving a synergistic effect.

The strength of SiC porous ceramic structures could be effectively improved via reducing the cell edge length a, increasing the number of layers n, and pore reinforcement diameter d. The pore reinforcement diameter d had a more significant impact on the strength of SiC porous ceramic structures. There was a certain enhancement effect on the maximum surface temperature of SiC porous ceramic structure via increasing a and decreasing n and d. At a high porosity, a and n had a more significant impact on the maximum surface temperature of SiC porous ceramic structure. At a low porosity, d had a more significant impact on CO emissions from SiC porous ceramic structures.

Silicified graphite composites, as a type of ceramic-based solid lubrication material, possess the superior properties of silicon carbide ceramics, such as high strength and high hardness, along with high-temperature strength, and self-lubricating performance of graphite materials. It has extensive application prospects in aerospace, chemical engineering, and nuclear energy. The reactive melting infiltration (RMI) method features near-net shape forming, a short processing cycle, and low cost, as an efficient approach for fabricating silicified graphite composites. Nevertheless, the infiltration reaction of liquid silicon into the porous carbon preform is extremely rapid after driven by reactive wetting and capillary forces, resulting in a sharp consumption of carbon. Hence, controlling the graphite content emerges as a crucial issue for the development of silicified graphite composites with superior lubrication properties. In this paper, a carbon source was fabricated through carbon source modification to prepare composite graphite spheres and coat on the surface of the composite graphite spheres with nano-carbon black. Subsequently, silicified graphite composites were prepared via compression molding and reaction melt infiltration. The carbon content of the composites was regulated via altering the particle size of the composite graphite spheres. The influence of the carbon content on the mechanical and tribological properties of the silicified graphite composites was discussed, and the film formation and lubrication mechanisms of the silicified graphite composites with a high carbon content under reciprocating friction conditions were analyzed.

Composite graphite spheres were prepared by mixing spherical graphite with liquid phenolic resin. First, the liquid phenolic resin and spherical graphite were weighed and mixed according to a mass ratio of 8∶10, and mechanically stirred at 45 ℃ for 20-30 min. It was cured at 80 ℃ for 2 h and 150 ℃ for 10 h to obtain the cured product in a drying oven. The cured product was carbonized in a tubular furnace with nitrogen atmosphere. The temperature increased to 1000 ℃ at 1 ℃·min^-1, and the heat kept for 2 h. Finally, the bulk materials were crushed and screened, and five kinds of composite graphite particles with different particle sizes were obtained. The average particle sizes of the selected composite graphite spheres were 203 µm, 153 µm, 135 µm, 105 µm and 75µm, respectively.

The composite graphite particles and nano carbon black were evenly dispersed in alcohol, and the mixed particles were obtained under magnetic stirring in a water bath at a constant temperature of 70 ℃ until dry. Subsequently, prefabricated 5% PVA solution was added to the dry mixed particles as a binder, and after being mixed evenly, the porous carbon preform was formed at 10 MPa for 30 s. The porous carbon preforms reacted with liquid silicon at 1550 ℃ for 30 min to obtain the silicified graphite composites. According to the size of the composite graphite spheres from large to small, the silicified graphite composites were labeled as samples GS-NCB-1, GS-NCB-2, GS-NCB-3, GS-NCB-4 and GS-NCB-5, respectively.

The introduction of nano-sized carbon black facilitates to the protection of carbon phase. The carbon content of the composites fabricated with the composite graphite particles coated by nano-sized carbon black obviously increases, compared with the composites without nano-sized carbon black. This is because that nano-sized carbon black with a higher reactivity forms a continuous and dense silicon carbide layer on the surface of the composite graphite powders, which effectively inhibits the reaction between the composite graphite powders and silicon to form β-SiC. In addition, the increase of particle size of the composite graphite particles has a positive effect on the carbon content of the silicified graphite composites. The carbon content of silicified graphite composites increases from 20.08% to 43.03%, the mechanical properties gradually decreases from 151 MPa to 96 MPa and the friction coefficient and wear rate reduce (the minimum: 0.11, 5×10^-7 mm^3·N^-1·m^-1, 15 N) as the particle size of composite graphite powders increases, indicating superior self-lubricating properties. The results show that the improvement in the lubricating properties of silicified graphite composites is achieved due to the formation of lubricating films. The dense and continuous lubrication film avoids a direct contact with the abrasive material and reduces the coefficient of friction and wear rate. The decrease of the mechanical properties of the silicified graphite composites is due to the increase of the soft carbon content.

Nano-sized carbon black layer on the surface of the composite graphite particle first contacted with liquid silicon to form a silicon carbide layer during the reactive melting process, preventing the further infiltration and reaction of liquid silicon to the composite graphite particle, playing a protective role in the composite graphite particle, and leading to the high carbon content of the silicified graphite composites.

The increase in the particle size of the composite graphite particle could improve the carbon content of the silicified graphite composites because the thickness of the silicon carbide layer after the reaction of the composite graphite particle of different particle sizes was the same, and the increase in the particle size could reduce the specific surface area of the porous carbon preform, thereby reducing the reaction contact area with liquid silicon. The silicon carbide content reduced and the carbon content increased.

The improvement of lubrication performance of silicified graphite composites was achieved due to the formation of surface lubrication film. The dense and continuous lubrication film could avoid a direct contact between grinding materials, effectively reducing the coefficient of friction and wear rate.

3% yttria-stabilized tetragonal phase zirconia ceramics (3Y-TZP) show a significant potential in aerospace application due to their low thermal conductivity, excellent high-temperature stability, and mechanical strength. However, it is difficult for conventional forming methods of porous ceramics to meet the requirements for complex structural designs. Digital Light Processing (DLP) is an important additive manufacturing method to fabricate components with complex geometries and high-precision, which can balance micron-scale resolution and efficient production. The macro-microporous structure of multiscale porous ceramics can take full advantage of multilevel porosity advantages, balancing thermal insulation and mechanical properties. Therefore, in this paper, DLP technology was combined with the pore-forming agent method to achieve the controllable molding of zirconia ceramics with multilevel pores. The printability of high solid-loading porous ceramic slurry was optimized, and the effect of sintering temperature on the microstructural evolution and mechanical properties of ceramics was investigated.

The ceramic slurry was prepared through batched addition of ball-milled and dried 3Y-TZP powder into a photosensitive resin premix, followed by thorough homogenizationin a model DSG2300Z homogenizer at 1200 r/min. 3Y-TZP slurries with 40% solid loading were prepared with different dispersants. Their rheological behavior was analyzed by a model MCR302 rheometer to identify the optimal dispersant type and content. Subsequently, the slurries with different solid loadings were prepared, and the appropriate solid loading was selected. Based on the optimized parameters, liquid organic pore-forming agent (i.e., polyethylene glycol (PEG) 400) was added at different volume fractions of 5%, 10%, and 15% to replace of the equivalent resin volume fractions. The optimal slurry composition was determined by rheological property.

The green body was prepared using a DLP printer with 405 nm UV light to cure the resin. The argon debinding and air decarbonization-sintering combined processes were then determined by thermogravimetry-differential scanning calorimetry (TG-DSC) on the green body. The ceramic components were subsequently held at different sintering temperatures (i.e., 1400, 1450, 1500 ℃) for 2 h, respectively. The microstructures of 3Y-TZP powder, green body, and sintered ceramics were determined by field-emission scanning electron microscopy (MAIA3 LMH, Tescan Co., Czech Republic). The chemical composition of the ceramic powders and dispersant was characterized using Fourier infrared spectrometry (IRPrestige-21, Shimadzu Co., Japan). The phase composition was analyzed by X-ray diffraction (d8 advance, Bruker Co., Germany). Three-point bending tests were conducted on ceramic specimens (50 mm×5 mm×4 mm) to evaluate the mechanical strength. The span was 20 mm and the load loading rate was 0.5 mm·min^-1. The microstructural porosity was measured by Archimedes water displacement method.

In this study, the slurry formulation for porous zirconia ceramics is optimized. Compared with dispersants KH560 and Solsperse41000, a dispersant KOS110 effectively reduces slurry viscosity. The infrared spectroscopy analysis reveals that KOS110 undergoes esterification reactions with zirconia particles through hydroxyl groups(-OH), thereby enhancing the dispersion of ceramic particles. The slurry viscosity initially decreases and then increases as the content of KOS110 increases, and reaches the lowest value at 3% addition. The yield stress and K value of the slurry are lowest when 3% KOS110 is added, proving that the rheological behavior of the slurry is optimal at this time. Increasing solid loading reduces interparticle distances in the dispersed slurry. When solid content exceeds 50%, the viscosity increases dramatically. Therefore, 50% solid loading can ensure a printability and a high solid content.

The introduction of PEG into 3Y-TZP slurry can form lubricating layers between ceramic particles through free molecular movement, improving the dispersion of the slurry. The optimal rheological performance is achieved at 10% PEG addition. The optimized high-solid 3Y-TZP slurry demonstrates a superior formability, which can be used to print 3Y-TZP hierarchical porous ceramic green body. When the sintering temperature increases from 1400 ℃ to 1500 ℃, the ceramic particles gradually transition from loose to tightly packed arrangement, accompanied by porosity reduction. After sintering at 1500 ℃, the ceramic samples have both appropriate porosity and strength. The flexural strength reaches 717 MPa and the porosity is 4.03%. The XRD patterns show that the ceramic crystal phase is a stable tetragonal phase after sintering at this temperature.

3Y-TZP ceramics with a multidimensional pore structure were prepared by DLP technology combined with the pore-forming agent method. The effects of dispersant type and content on the rheological behavior of ceramic slurries with 40% solid content were investigated. The results indicated that 3% mass fraction of KOS110 could effectively improve the rheology of the slurry due to the chemical bonding and steric hindrance. The solid content and PEG content were synergistically optimized. The results showed that an effective lubrication layer could be formed between ceramic particles when 10% PEG was added to replace the photosensitive resin at 50% solid content. The viscosity of the slurry at this time met the requirements for DLP printing. After DLP printing and high temperature sintering at 1500 ℃, the 3Y-TZP ceramic was structurally stable and had both high appropriate porosity and mechanical strength.

It is necessary for an ideal bioceramic scaffold to have a suitable structure. The structure can affect the mechanical properties of the scaffold (i.e., elastic modulus and compressive strength) and the biological properties of the scaffold (i.e., degradability and cell growth rate). Lattice structure is a kind of periodic porous structure, which has some advantages of light weight and high strength, and is widely used in the preparation of bioceramic scaffolders. For the structure of the scaffold, high porosity and large pore size are important for bone growth, bone integration and promoting good mechanical interlocking between neighboring bones and the scaffold. However, scaffolds with a high porosity often lack mechanical strength. In addition, different parts of the bone have different structural requirements. In this paper, scaffolds with a non-uniform structure or a hierarchical structure were designed, with loose and porous exterior to facilitate cell adhesion, osteogenic differentiation and vascularization as well as relatively dense interior to provide sufficient mechanical support for bone repair.

In this work, composite ceramics scaffolds with 10% akermanite content were prepared by DLP technology. The scaffold had a high porosity outside to promote the growth of bone tissue, and a low porosity inside to withstand external forces. The compressive strength, fracture form, in-vitro degradation performance and bioactivity of graded bioceramic scaffolds were investigated. The models of scaffolds were imported into the DLP printer with a 405 nm light. The samples were printed with the intensity of 8 mJ/cm² and a layer thickness of 50 µm. Finally, the ceramic samples were sintered at 1100 ℃. The degradability of the hierarchical gyroid bioceramic scaffolds was evaluated through immersion in Tris-HCl solution and SBF solution at a ratio of 200 mL/g. The bioactivity of bioceramic was obtained via immersing them in SBF solution for two weeks. The concentrations of calcium, phosphate, silicon, and magnesium ions in the soaking solution were determined by an inductively coupled plasma optical emission spectrometer.

In this work, a hierarchical Gyroid structure HA-AK10 scaffold (sintered at 1100 ℃) with a radial internal porosity of 50% and an external porosity of 70% is prepared, and the influence of structural form on the compressive strength and degradation performance of the scaffold is investigated. The biological activity of the bioceramics in vitro is also verified. The mechanical simulation results show that the stress distribution corresponds to the porosity distribution of the structure, and the low porosity is larger and the overall stress concentration phenomenon does not appear. After soaking in SBF solution, Si—OH is firstly formed on the surface of bioceramics, and then silicon gel layer is produced due to the presence of calcium and silicon ions. The silicon gel layer is dissociated into negatively charged groups under alkaline environment secondary adsorption of calcium ions and phosphate ions, forming amorphous calcium phosphate, and finally amorphous calcium phosphate crystals and adsorption of carbonate ions, forming carbonate hydroxyapatite. This indicates that the composite bioceramics have a good biological activity in-vitro and can provide a good environment for the growth of bone cells. A hierarchical Gyroid ceramic scaffold with a bone geometry is prepared via applying the hierarchical structure to the bone contour scaffold. The maximum load capacity of the hierarchical Gyroid ceramic scaffold is 8 times that of the uniform structure.

The hierarchical structure scaffold designed had good overall compressive performance, good degradation performance, and still maintained a good mechanical stability during degradation. In addition, in-vitro biological experimental results showed that the surface graded composite scaffold could have a good in-vitro biological activity and provide a good environment for bone cells. Compared to the heterosexual structure, the graded scaffold had greater mechanical properties.

Ammonia is a toxic and harmful gas that affects the environment and human health. It is thus of great significance to identify and detect ammonia and its concentration rapidly and accurately for human health monitoring and ecological environment management. TiO₂ is widely used in the field of gas sensors due to its advantages of good stability, environmental tolerance, non-pollution, and outstanding contact reaction properties. However, it is gradually found that it has the disadvantages of long response/recovery time and low response value. MXene has a great potential in the field of gas sensing because of its large specific surface area and superior electron mobility rate, but its response value is rather low. In this work, TiO₂/Ti₃C₂ composites were prepared by an one-step hydrothermal method. The synergistic effect of semiconductor properties of TiO₂ and excellent electron transport performance of Ti₃C₂ was analyzed.

TiO₂/Ti₃C₂ composites at different hydrothermal temperatures were prepared by a hydrothermal method with MXene-Ti₃C₂ and butyl titanate as raw materials. The physical phase, surface morphology, specific surface area and pore size distribution, and structure of the samples were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM), surface area measurement based on BET, and Raman spectroscopy. In addition, the ammonia sensing properties of TiO₂/Ti₃C₂ composites at room temperature were also analyzed.

TiO₂/Ti₃C₂ composites can be prepared by a hydrothermal method. The XRD patterns indicate that the characteristic peaks of Ti₃C₂ and the diffraction peaks of anatase TiO₂ both appear in the composites. The Raman spectra show the generation of anatase TiO₂. The SEM images indicate that TiO₂ nanoparticles are uniformly loaded on the interlayer and surface of Ti₃C₂. The agglomeration of TiO₂ particles becomes more severe with the increase of hydrothermal temperature. The N₂ adsorption-desorption experiment shows that the specific surface area of prepared TiO₂/Ti₃C₂ is much larger than that of Ti₃C₂. Moreover, the specific surface area of TiO₂/Ti₃C₂-150 is the largest, but it gradually decreases with the increase of hydrothermal temperature possibly due to the agglomeration of the generated TiO₂ particles. The gas sensing test results show that TiO₂/Ti₃C₂ composites and pure TiO₂ have responses to 100 μL/L ammonia at room temperature, while Ti₃C₂ has little response. The response values of TiO₂/Ti₃C₂ composites are significantly better than that of pure TiO₂ obtained at the same temperature. Among all the samples, TiO₂/Ti₃C₂-150 has the maximum response value (i.e., 8.37). Also, the response value of the composites increases with the increase of ammonia concentration, showing a high linear correlation. After the stability test for one month, the response value of TiO₂/Ti₃C₂-150 to 100 μL/L ammonia is only decreased by 14.1%, indicating a long-term stability. Compared to TiO₂-150, the response time of TiO₂/Ti₃C₂-150 decreases from 43 s to 28 s, and the recovery time reduces from 49 s to 37 s, manifesting a high selectivity to ammonia. The stable gas absorption/desorption reaction of TiO₂ and thesuperior electron transport rate of Ti₃C₂ can enhance the response of TiO₂/Ti₃C₂ to ammonia.

The nanoparticles of anatase TiO₂ were supported on the interlayer and surface of Ti₃C₂ by a hydrothermal method. At the hydrothermal temperature of 150 ℃, TiO₂ particles were supported evenly and agglomerated weakly, with a greater specific surface area and smaller particle size. Compared to pure TiO₂ and Ti₃C₂, TiO₂/Ti₃C₂-150 had a good response to ammonia at room temperature. At an ammonia concentration of 300 μL/L, the gas sensing response value of TiO₂/Ti₃C₂-150 was 34.77, which was greater than that of pure TiO₂. In addition, TiO₂/Ti₃C₂-150 also showed a rapid response recovery rate, a good selectivity and a long-term response stability, having a promising application prospect in the field of rapid detection of ammonia at room temperature.

Two-step sintering offers unique advantages for fabricating ultrafine nanocrystalline ceramics due to its simplicity, low equipment requirements, and effective suppression of rapid grain growth during the final sintering stage. This review represents the research background of two-step sintering, thus clarifying its suitability towards various materials. The related mechanisms behind the suppression of rapid grain growth and the uniformity of the grain size distribution are descirbed. The grain boundary mobility transition at a low temperature is the most important mechanism of two-step sintering. In addition, the uniformization of the microstructural network during the initial and intermediate sintering stages at T₁ and the freezing of the microstructural network at T₂ are important reasons for the uniformity of two-step sintered samples. The two-step sintering needs a further research on its mechanisms, guidance on the selection of sintering parameters, development of more suitable molding techniques, combination with other sintering techniques and investigation on the macroscopic mechanical properties of large-sized samples.

Synthesis of organic hafnium polymers gradually tends to be non-oxidation, simplification, low cost, high conversion rate, and high performance. The pyrolysis of polymers containing Hf-O bonds needs to undergo the carbon-thermal reduction process at high temperatures, and the prepared ceramics have more pores and cracks and insufficient performance, so non-oxygen hafnium-containing polymers are an important direction for future research. The molding performance of solid precursors limits their application in preparing ultra-high temperature ceramic composites via the PIP method, and liquid precursors with a high ceramic yield becomes a hot direction for future development. In addition, polymer-derived continuous ultra-high temperature ceramic fibers and their composites is a research hotspot, and exploring a new paradigm of artificial intelligence to accelerate the research of ultra-high temperature ceramic materials is also an important direction for development.