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.

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.

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.

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.

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.

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.

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.

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.

(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.