The photoluminescence (PL) of rare-earth-doped materials is predominantly controlled by the local crystal field symmetry around rare-earth ions. Using electric fields (E) to modify the PL properties is particularly intriguing as it is compatible with modern electronics and is promising for advanced applications in optical information storage and communication. It is reported that the E-induced structural evolution in host ferroelectrics (FEs) can effectively manipulate the crystal field environment surrounding rare-earth ions, consequently enabling a dynamic modulation of PL intensity (i.e., the E-PL effect). However, the hysteresis behavior in FEs limits the PL modulation range. Antiferroelectrics (AFEs) show a reversible AFE-FE phase transform with zero remanent polarization, particularly promising for realizing reversible E-PL modulation with large tunability. In AFE thin films, the clamping effect from the substrate may exert a significant influence on the structural evolution during the E-induce phase transition. Therefore, compared with the ceramic forms, it is possible to analyze distinct E-PL behavior in rare-earth doped AFE thin films.

1% Er³⁺-dopedPb_0.98La_0.02Zr_0.95Ti_0.05O₃ (PLZT-Er) thin films were prepared on Pt/Ti/SiO₂/Si substrates by a sol-gel/spin-coating method with lead acetate trlhydrate (PbC₄H₆O₄·3H₂O, 99.99%(379.33), Aladdin Co., China), lanthanum nitrate hexahydrate (LaN₃O₉·6H₂O, 99%(433.03), Aladdin Co., China), erbium trinitrate pentahydrate (Er(NO₃)₃·5H₂O, 99.9%(443.37), Aladdin Co., China), zirconium propoxide (C₁₂H₂₈O₄Zr, 70%(327.56), Aladdin Co., China), and titanium butoxide (C₁₆H₃₆O₄Ti, ≥99.0%(340.32), Aladdin Co., China) as the raw materials. The crystal structure was characterized by X-ray diffraction (XRD, Ultima III, Rigaku Co., Japan). The surface morphology was observed using atomic force microscopy (AFM, ICON, Bruker Co., Germany). The polarization (P-E) and current (I-E) loops were measured by a ferroelectric analyzer (AIX ACT, TF2000) at 1 kHz. The down-conversion photoluminescence (DCPL) emission spectra were measured by fluorescence spectroscopy (ULS2048x64TEC, Avantes Co., the Netherlands) under 405 nm and 532 nm laser excitation. For the E-PL tests, 20 nm-thick transparent indium tin oxide (ITO) top electrodes were deposited on the surfaces of PLZT-Er thin film by magnetron sputtering in argon atmosphere (2.0 Pa) at 320 wd3.

The structural, PL, and E-PL properties of PLZT-Er AFE thin films are systematically investigated. The XRD patterns confirm a pure perovskite structure with a lattice shrinkage due to the substitution of Er³⁺ at A-sites, whereas the AFM analysis reveals a smooth surface with a roughness of approximately 3 nm. The double P-E loop and characteristic I-E peaks verify the AFE nature of PLZT-Er thin films, with reversible AFE→FE transitions at ±452 kV·cm^-1 and FE→AFE transitions at ±199 kV·cm^-1. Under 405 nm excitation, the PLZT-Er exhibits 487 nm (⁴F_7/2→⁴I_15/2) and 610 nm (⁴F_9/2→⁴I_15/2) emissions, whereas an excitation at 532 nm induces 560 nm (⁴S_3/2→⁴I_15/2), 660 nm (⁴F_9/2→⁴I_15/2), and 850 nm (⁴S_3/2→⁴I_13/2) emissions, all originating from efficient energy transfer and relaxation of Er³⁺ ions. The reults of E-PL tests demonstrate a remarkable reversible modulation. In 830 kV·cm^-1, the emission at 487 nm shows an 85.14% reduction, with stable switching over six cycles. This performance outperforms PLZT-Er ceramics (+30% modulation) and most reported FE/AFE materials, which is attributed to their higher breakdown strength. The opposite E-PL tuning compared to PLZT-Er ceramics is explained by increased local crystal field symmetry around Er³⁺ ions, arising from either substrate clamping-induced antiferroelectric orthorombic →ferroelectric orthorombic phase transition or incomplete ferrielectric→antiferroelectric transition in an insufficient electric field. These findings highlight the PLZT-Er thin films as promising candidates for optical information storage and communication devices.

Er³⁺-doped Pb_0.98La_0.02Zr_0.95Ti_0.05O₃ (PLZT-Er) thin films exhibited a reversible electric-field-modulated photoluminescence (E-PL) effect. Under 405 nm laser excitation, a PL reduction of -85% could be achieved in an applied electric field of 830 kV·cm^-1. This negative E-PL tuning trend was anomalously opposite to that in bulk PLZT-Er ceramics. Two potential mechanisms could be proposed to explain this anomalous phenomenon, i.e., either the antiferroelectric orthorombic (AFE_O) to ferroelectric orthorombic (FE_O) phase transition or the incomplete ferrielectric (FiE) to antiferroelectric (AFE) phase transition in PLZT-Er thin films could increase the local crystal field symmetry around Er³⁺ ions.

In this paper, 0.99(0.7BiFeO₃-0.3BaTiO₃)-0.01Bi_0.5Na_0.5TiO₃-xLi₂CO₃ (i.e., x = 0, 0.001, 0.003, 0.005; abbreviated as 0Li, 0.1Li, 0.3Li, 0.5Li, BFBTBNT-xLi) ceramics were prepared by a conventional sintering technique. The ceramics gradually changed from the R phase to PC phase structures, and the ceramic with 0.3Li showed obvious coexistences of R and PC phases as the addition of Li₂CO₃ increases. The Curie temperature of Li₂CO₃-doped BFBTBNT-xLi ceramics increases from 483 ℃ to 535 ℃, and the dielectric loss of the samples decreased. The remanent polarization of the ceramics increased with increasing the Li₂CO₃ addition, reaching the maximum value at 0.3Li. Subsequently, the polarization decreased due to the increased coercive field. Li₂CO₃ could enhance the insulation of the ceramics because of the enhanced migration barrier of oxygen vacancies. The ceramic with 0.3Li could be obtained with an optimal piezoelectric constant of 183 pC·N^-1 and a high depolarization temperature of 380 ℃. This material displayed a great potential in high-temperature piezoelectric application.

The 0.99(0.7BiFeO₃-0.3BaTiO₃)-0.01Bi_0.5Na_0.5TiO₃-xLi₂CO₃ + 1% MnO₂ (x = 0, 0.001, 0.003, 0.005; abbreviated as 0Li, 0.1Li, 0.3Li, 0.5Li, BFBTBNT-xLi) ceramics were prepared by a conventional sintering process. The raw materials were weighed according to the stoichiometric ratio and ground for 12 h. After grinding, the ground raw materials were firstly sintered at 720 ℃ for 5 h, and then the mixtures with MnO₂ were further ground for 12 h. After granulating and pressing, the samples were heated at 600 ℃ for 10 h. Afterwards, the samples were heated at 1000 ℃ for 3 h.

The phase structure of the ceramics was analyzed by X-ray diffraction (XRD, Bruker Co., Germany). The microstructures of the ceramics were determined by scanning electron microscopy (SEM, JSM Co., Japan). In order to test the electrical properties of the BFBTBNT-xLi ceramics, the silver paste was coated on the surface of the samples and fired at 800 ℃ for 10 min to form silver electrodes. Afterwards, the dielectric constant and loss as a function of temperature and impedance spectroscopy was determined by impedance analysis (Agilent E4980A). The ferroelectric performance and J-E curves was measured by ferroelectric analysis (Precision Premier II, Radiant Co., USA). The chemical binding state of O was tested by X-ray photoelectron spectroscopy (XPS, PHI Co., USA). The electron paramagnetic resonance measurements were performed by the EPR spectroscopy (EPR, Bruker Co., Germany). The d₃₃ values at room temperature were measured by a quasi-static piezoelectric constant apparatus (ZJ-3A, China). The in-situ d₃₃ values were collected by a quasi-static d₃₃ apparatus (YE2730A, Sinocera Co., China).

The content of the R phase continuously decreased with increasing Li₂CO₃ addition, especially in the sample with 0.3Li. The content of the R phase reduced to 36.28%, which was the MPB. The content of the R phase in the sample with 0.5Li further decreased to 18.76%, while the content of the PC phase increased to 81.24%, indicating that the sample is mainly composed of the PC phase. The microstructure showed that the surface of the sample was smooth without obvious holes, and the ceramic grains were full and the grain boundaries were clear. The addition of Li₂CO₃ could refine the grains, which was beneficial to improving the toughness against breakdown. The Curie temperatures (T_C) of the samples with 0Li, 0.1Li, 0.3Li and 0.5Li were 483, 502, 521 ℃ and 535 ℃, respectively. The dielectric loss of the BFBTBNT-xLi showd a significant increase with increasing temperature, especially when the temperature exceeded 150 ℃. The dielectric loss of the ceramic samples modified by Li₂CO₃ was significantly reduced, especially more significantly in the high-temperature environment. The residual polarization of the ceramics with the addition of Li₂CO₃ increased, reaching a peak value for the ceramic with 0.3Li. The samples exhibited semi-circular complex impedance characteristics, and the resistance of the ceramics decreased with increasing temperature, which was in line with the characteristics of the semiconductor conduction mechanism. The activation energies (E_a) of the ceramics with 0Li, 0.1Li, 0.3Li and 0.5Li were 1.196, 1.218, 1.251 eV and 1.263 eV, respectively. The oxygen vacancies were inhibited with the gradual addition of Li₂CO₃, thereby enhancing the insulating properties of the ceramics. The leakage current of the samples with 0.1Li, 0.3Li and 0.5Li modified by Li₂CO₃ was improved; especially the leakage current of the sample with 0.5Li reduces to 10^-6 A·cm^-2, which was reduced by two orders of magnitude. The d₃₃ values of the ceramics with 0Li, 0.1Li, 0.3Li and 0.5Li were 149, 161, 183 pC·N^-1 and 152 pC·N^-1, respectively, showing a consistent with the residual polarization. The ceramic with 0.3Li exhibited the maximum piezoelectric constant because of the higher residual polarization, fewer oxygen vacancies and leakage current, thus enabling a more complete polarization and demonstrating the optimum piezoelectric performance. The ceramics modified by Li₂CO₃ showed a high-temperature stability, and the de-polarization temperatures (T_d) of the ceramics with 0Li, 0.1Li, 0.3Li and 0.5Li were 350, 370, 380 ℃ and 390 ℃, respectively.

The 0.99(0.7BiFeO₃-0.3BaTiO₃)-0.01Bi_0.5Na_0.5TiO₃-xLi₂CO₃ (x = 0, 0.001, 0.003, 0.005; abbreviated as 0Li, 0.1Li, 0.3Li, 0.5Li) ceramics were prepared by a conventional sintering process. The ceramics gradually changed from the R to PC phase structure as the addition of Li₂CO₃ increased. The ceramic with 0.3Li showed obvious coexistences of the R and PC phases. The content of R phase in the ceramics BFBTBNT-xLi decreased from 78.02% to 18.76%, while the content of PC phase increased from 21.98% to 81.24%. The addition of Li₂CO₃ could refine the grains, which was beneficial to improving the toughness against breakdown. The Curie temperature increased in the ceramics BFBTBNT-xLi doped with Li₂CO₃, and the dielectric loss of the ceramics was significantly reduced, especially at high temperatures. The remanent polarization of the ceramics increased gradually with increasing the Li₂CO₃ addition, reaching the maximum value for the ceramic with 0.3Li. The oxygen vacancies in the ceramics were suppressed as the Li₂CO₃ addition increased, thus enhancing the insulation performances. The piezoelectric constant of the ceramic with 0.3Li was 183 pC·N^-1, and the depolarization temperature was as high as 380 ℃, which could be promising for high-temperature piezoelectric applications.

Dielectric capacitors are widely used in pulsed power and electronic systems due to their high power density, long lifespan, and low cost. However, their energy density is considerably lower than that of batteries. Dielectric materials are the core components for enhancing capacitor performance. Although conventional lead-based ceramics (e.g., PZT) exhibit the superior properties, their toxicity drives the development of environmentally friendly lead-free alternatives as a major research focus. Bismuth-based perovskite materials, due to the similar electronic configurations of Bi³⁺ and Pb²⁺, demonstrate ferroelectric properties comparable to those of lead-based materials. Among them, bismuth potassium titanate (i.e., K_0.5Bi_0.5TiO₃, KBT) stands out as a highly promising lead-free energy storage candidate due to its combination of high theoretical polarization and a wide bandgap. However, a pure KBT is a typical ferroelectric with a high remnant polarization, which limits its energy storage performance. In general, introducing a paraelectric material into a ferroelectric to form a relaxor ferroelectric can disrupt a long-range order and refine ferroelectric domains, thereby improving energy storage characteristics. This study selected quantum paraelectric SrTiO₃ (ST) to form a solid solution with KBT. The effect of ST content on the crystal structure, microstructure, dielectric, and ferroelectric properties was investigated. The ceramics maintained a pure perovskite phase with increasing ST content, showing a shift of the dielectric peak to lower temperatures with an enhanced frequency dispersion and gradually slimmer polarization-electric field hysteresis loops. The optimal composition (i.e., 0.6KBT-0.4ST) achieved a recoverable energy storage density of 3.64 J/cm³ and an energy efficiency of 70% in an electric field of 210 kV/cm, along with good temperature and frequency stability. The results indicated that KBT-ST is a promising class of high-performance lead-free dielectric materials for energy storage applications.

In this study, (1-x)KBT-xST ceramics (where x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by a conventional solid-state sintering method. The starting raw materials were Bi₂O₃ (≥99%), K₂CO₃ (≥99%), TiO₂ (≥99%), and SrCO₃ (≥99%), from Aladdin Co., China. Prior to weighing, K₂CO₃ powder was dried in a muffle furnace at 200 ℃ for 2 h to remove absorbed moisture. The raw materials were then weighed according to the stoichiometric ratio and placed in a nylon milling jar with zirconia grinding balls. Isopropanol was added as a medium in grinding, and the raw materials were wet ground for 12 h. After grinding, the slurry was poured out, dried, and calcined at 850 ℃ for 2 h. The calcined powder with isopropanol was further ground for 22 h. Afterwards, the slurry with 0.2% of binder was ground for additional 2 h. The slurry obtained after further grinding was dried, sieved, and pressed into green ceramic pellets with a diameter of 10 mm and a thickness of 1-2 mm. The green pellets were placed in a muffle furnace, heated at a rate of 2 ℃/min to 600 ℃, and held for 6 h to allow binder burnout. After debinding, the pellets were embedded in an alumina crucible with calcined powder of the same composition (without binder) and sintered at 1050-1400 ℃ (with higher sintering temperatures corresponding to higher ST content) for 2 h. The sintered ceramics were then ground and polished. Silver paste was applied on the both sides of the ceramics and heat-treated at 850 ℃ for 20 min to form silver electrodes. Alternatively, gold electrodes were deposited on the both sides by an ion sputtering coater. The phase composition of the ceramic powders was analyzed by a model Smartlab X-ray diffractometer (XRD) (Rigaku Co., Japan). The Raman spectra were determined on well-polished ceramic samples by a model Alpha 300R Raman imaging microscope (WITec Co., Germany). The microstructure of the materials was characterized by a model Regulus 8230 high-resolution cold-field emission scanning electron microscope (Hitachi Co., Japan). The temperature-dependent dielectric properties were measured by a model IM3536 LCR meter (HIOKI Co., Japan) equipped with a model HCP421V-MP heating stage (INSTEC Co., USA). The polarization-electric field (P-E) hysteresis loops of the ceramics were tested by a model Multiferroic II ferroelectric analyzer (Radiant Co., USA).

The results show that the (1-x)K_0.5Bi_0.5TiO₃-xSrTiO₃(KBT-ST) lead-free dielectric ceramic system is synthesized by a conventional solid-state reaction method. The effect of SrTiO₃ incorporation on the microstructure, phase evolution, and electrical properties of the materials is systematically investigated.

The XRD patterns and SEM images confirm the formation of a single perovskite phase and a dense microstructure for all the compositions. The temperature stability of the dielectric constant is significantly enhanced, accompanied by a shift of the dielectric peak towards lower temperatures, as SrTiO₃ content increases. The gradual slimming of polarization-electric field hysteresis loops indicates a transformation from a typical ferroelectric to a relaxor ferroelectric state. The optimal composition (i.e., 0.6KBT-0.4ST) exhibits a high recoverable energy storage density of 3.64 J/cm³ and an energy efficiency of 70 % in an electric field of 210 kV/cm. This work demonstrates that KBT-ST ceramics constitute a highly promising lead-free dielectric system for high-performance energy storage applications.

In this work, KBT-ST lead-free ceramics were prepared by a solid-state sintering method. Their structural evolution, properties, and potential for dielectric energy storage applications were systematically investigated. Single-phase

perovskite-structured KBT-ST solid solutions were successfully synthesized. The lattice contraction occurred, and the incorporation of Sr²⁺ effectively introduced A-site compositional disorder as the ST content increased. The addition of ST effectively suppressed a long-range ferroelectric order, thus inducing a transition from a ferroelectric to a relaxor ferroelectric state. This transition was evidenced by enhanced frequency dispersion of the dielectric peak, slimmer polarization-electric field hysteresis loops, and improved temperature stability of the dielectric constant. The optimal energy storage performance was achieved in the composition with x of 0.4, which exhibited the optimal overall energy storage properties. The introduction of Sr²⁺ produced a synergistic effect. It enhanced a local polarization, and suppressed a long-range ferroelectric order, promoting the formation of an ergodic relaxor state. The effects both contributed to a reduction in (P_max-P_r). The high breakdown strength of 0.6KBT-0.4ST, combined with the high polarization contributed by the hybridization between Bi 6s and O 2p orbitals, collectively yielding a high recoverable energy density of 3.64 J/cm³ and a high energy efficiency of 70 %. Furthermore, this composition demonstrated superior cycling stability and temperature stability in its energy storage performance. In summary, A-site modification of KBT ceramics through SrTiO₃ solid solution was an effective strategy for optimizing their energy storage properties. This study could present a promising lead-free energy storage ceramic system and provide a valuable reference for designing high-performance relaxor ferroelectrics via tuning local structural disorder.

With the rapid development of new energy, pulsed power systems and other fields, there is an urgent demand for energy storage devices with a high power density, a fast charge-discharge rate and a long service life. Inorganic thin-film dielectric capacitors, featuring microsecond-level discharge speed and high safety, face a challenge of synergistically improving energy density and efficiency (e.g., the existing PZT thin films have an efficiency of <50% due to the large remanent polarization). As a widely studied material, PZT has an excellent polarization but a large leakage current. The existing optimizations include doping and heterostructure construction. This paper was to fabricate a "PZT/MgO" multilayer heterostructure by a sol-gel method. MgO was introduced to obtain a slim hysteresis loop, and the effect of annealing temperature on the phase structure, microstructure, dielectric and energy storage properties was investigated.

0.5 mol/L PbZr_0.52Ti_0.48O₃ (PZT) and MgO thin film precursor solutions were prepared by a sol-gel method, aged at room temperature in dry darkness for 24 h, and then spin-coated on Pt/Ti/SiO₂/Si substrates at 4000 r/min for 30 s. The wet films were dried at 200 ℃ for 5 min and pyrolyzed at 450 ℃ for 10 min. The process was repeated to obtain "PZT/MgO" multilayer films, which were annealed in O₂ at 500-600 ℃ for 10 min.

The crystal structure was analyzed by X-ray diffraction (XRD, Rigaku Co., Japan). The surface/cross-sectional morphologies were determined by scanning electron microscopy (SEM, Thermo Scientific Co., USA). The leakage current was characterized by a Keithley 2400 analyzer (Keithley Co., USA). Au electrodes with the diameter of 200 μm were sputtered, and the P-E loops, breakdown strength (E_b), dielectric properties, and stability were determined by an aixACCT TF Analyzer 3000 (TF Co., Germany). Results and discussion The results show that the characteristic peaks of the perovskite phase of PZT and MgO phase are intensified, the full width at half maximum (FWHM) is narrowed, and the crystallinity is improved with the increase of temperature when the PZT/MgO multilayer heterostructure thin films are annealed at 500-600 ℃ for 10 min. The SEM images show that the thin film annealed at 550 ℃ for 10 min has a dense and defect-free surface, a tight cross-sectional structure without delamination, and a total thickness of approximately 250 nm. In terms of dielectric properties, the dielectric constant and dielectric loss change slightly when annealed at 500-550 ℃, but the both increases when the temperature exceeds 550 ℃. For energy storage performance, the optimal annealing temperature is 550 ℃. The recoverable energy density is 66.0 J/cm³ and the energy storage efficiency is 68.9% in an electric field of 6.77 MV/cm. At 550 ℃, the leakage current of the thin film is ≤ 10^-7 A/cm² in an electric field of 0-2.4 MV/cm, with the Weibull coefficient β of 19.93, indicating a high breakdown stability. Regarding the stability, the energy storage performance fluctuates slightly at 1-30 kHz in an electric field of 4 MV/cm. After 10⁸ charge-discharge cycles, the attenuation of W_rec (recoverable energy density) and η (energy storage efficiency) is extremely small. W_rec increases with the electric field ranging from 1 MV/cm to 6 MV/cm, and the fitting result shows that n ≈ 1.86, presenting the quasi-linear polarization characteristics.

The surface of PZT/MgO multilayer film annealed at 550 ℃ for 10 min was dense without obvious delamination, and the thickness was approximately 250 nm, exhibiting good dielectric properties. In an electric field of 6.77 MV/cm, this film had a recoverable energy density of 66.0 J/cm³ and an energy storage efficiency of 68.9%, with a leakage current ≤ 10^-7 A/cm² (the Weibull coefficient β of 19.93). In addition, the film also had excellent frequency and cycling stability. W_rec increased with the electric field, and the fitting result showed that n ≈ 1.86, presenting the quasi-linear polarization characteristics.

With the evolution of optical communication from 5G to 6G, core optical devices face the increasing performance demands. Electro-optic (E-O) materials have attracted much attention due to their fast modulation speed, wide bandwidth, and good stability. Conventional materials such as KDP single crystals are hygroscopic and possess a low E-O coefficient, while LN single crystals have a limited damage threshold, thus hindering their suitability for high-power applications. Binary PMN-PT relaxor ferroelectrics exhibit excellent E-O properties, but suffer from a poor temperature stability. In contrast, the ternary PIN-PMN-PT system combines a high Curie temperature with outstanding ferroelectric and piezoelectric performance. However, the related research on its E-O properties remains limited. Previous studies mostly focused on strong ferroelectric materials exhibiting the primary E-O (Pockels) effect, which typically show low extinction ratios (<30 dB), insufficient for optical communication devices that require high extinction ratios (>30 dB). This work fabricated a series of transparent E-O ceramics with a weakened ferroelectricity in the xPIN-(85-x)PMN-15PT ceramics (i.e., x=0, 5, 10, 15, 20), to achieve both a high extinction ratio and good E-O performance through composition design. The results demonstrated that the composition with 5% PIN could exhibit optimal overall performance (i.e., a quadratic EO coefficient of 51.0×10^-16 m²/V², an extinction ratio of 31.87 dB, and a transmittance of 66.87% at 1550 nm). A modulation system based on this material was constructed, verifying its potential for applications such as optical switch. This study was to systematically investigate the influence of PIN content on the phase structure, transmittance, ferroelectric properties, and E-O performance, providing valuable insights for the future design of ternary PIN-PMN-PT E-O materials.

The ternary PIN-PMN-PT transparent electro-optic ceramics were prepared by a columbite precursor method with high-purity oxide raw materials (MgO, Nb₂O₅, In₂O₃, PbO, TiO₂, and La₂O₃, purity ≥ 99.9%). First, MgO, In₂O₃, and Nb₂O₅ were mixed by grinding in a high-energy ball mill and then calcined at 1100 ℃ to synthesize MgNb₂O₆ and InNbO₄. Subsequently, these precursors were mixed with La₂O₃, PbO and TiO₂ by milling and calcined at 850 ℃ to obtain PIN-PMN-PT powder. After dry pressing into green bodies, sintering was carried out in oxygen atmosphere at 1235 ℃ for 10 h to achieve densification. This was followed by hot-press sintering at 1200 ℃ and 100 MPa for 3 h to further eliminate pores, ultimately yielding highly dense transparent ceramics.

A systematic characterization was performed on the 0.5La-xPIN-(85-x)PMN-15PT (x=0, 5, 10, 15, 20) transparent electro-optic ceramics. After cutting and polishing, the transmittance, refractive index were measured by ultraviolet-visible-near infrared spectroscopy and spectroscopic ellipsometry, respectively. The quadratic electro-optic coefficient of the material was determined by a single-beam compensation method with a 632.8 nm He-Ne laser. The phase structure and microstructure were analyzed by X-ray diffraction and scanning electron microscopy. Finally, the ferroelectric properties were investigated via measuring the polarization-electric field hysteresis loops by a ferroelectric test system after depositing gold electrodes on the samples.

A series of 0.5La-xPIN-(85-x)PMN-15PT (x=0, 5, 10, 15, 20) transparent ceramics are fabricated. The effect of PIN solid-solution ratio on the transmittance, phase structure, microstructure, ferroelectricity, and electro-optic properties of the samples is systematically investigated. The transmittance of the ceramic sintered at 1235 ℃ gradually increases at a higher PIN content. The 0.5La-20PIN-65PMN-15PT transparent ceramic exhibits the optimum transmittance of 69.11% at 1550 nm, approaching a theoretical upper limit of 69.38%. All the compositions display a pseudocubic phase structure, allowing optical losses due to the birefringence to be neglected. The samples show dense microstructures with relative densities of 99.8%, 99.84%, 99.85%, 99.88%, and 99.95%, respectively, nearly eliminating scattering losses caused by internal pores. The average grain sizes are 6.63, 5.98, 6.41, 7.33 μm and 7.54 μm, respectively. The coercive field and remnant polarization of the samples both increase with increasing PIN content, indicating an enhanced ferroelectricity, which is attributed to the stronger ferroelectric nature of PIN, compared to PMN. The T_m also gradually increases, benefiting from a higher Curie temperature of PIN (90 ℃) relative to PMN (-10 ℃). The maximum and room-temperature dielectric constants both show a monotonic decreasing trend, which is related to a reduction in the relaxor behavior of the samples. The measured electro-optic coefficients at room temperature first increase and then decrease with PIN content, reaching a maximum value of 51.0×10^-16 m²/V² for the 0.5La-5PIN-80PMN-15PT ceramic. This is likely associated with domain sizes constrained by grain dimensions. The extinction ratios of the samples are 33.47, 31.87, 28.77, 23.87 dB and 22.08 dB, respectively, showing a gradual decrease influenced by the variation in ferroelectricity.

Based on the PIN-PMN-PT transparent electro-optic ceramics, an electro-optic modulation system is designed and constructed. Modulation experiments are conducted using a 250 kHz high-frequency AC signal and an audio signal. The modulated optical signal closely matches the original input signal, confirming a feasibility of applying PIN-PMN-PT transparent electro-optic ceramics in the field of optical communications.

La-doped PIN-PMN-PT transparent electro-optic ceramics were fabricated by a two-step sintering method, and the influence of PIN content on the transmittance and electro-optic coefficient was investigated. The 0.5La-20PIN-65PMN-15PT composition exhibited the optimum transmittance of 69.11% at 1550 nm, approaching the theoretical limit, which could be attributed to its pure perovskite phase, pseudocubic structure, and dense microstructure. The 0.5La-5PIN-80PMN-15PT composition demonstrated an optimal electro-optic performance with a transmittance of 66.87% and a high extinction ratio (31.87 dB), while exhibiting a significantly high electro-optic coefficient of 51×10^-16 m²/V², surpassing most reported PMN-PT-based ceramics. Its fine-grained structure facilitated domain switching. An electro-optic modulator based on this ceramic system achieved 200 kHz sinusoidal signal modulation and radio-frequency signal transmission, confirming its promising potential for optical communication applications.

(1-x%)(K_0.5Na_0.5NbO₃)-x%(Sr_0.7Bi_0.2TiO₃) (x = 1, 4, 7, 10, and 15 molar fraction) lead-free ferroelectric ceramics, denoted as xSBT, were synthesized by a conventional solid-state reaction method and a subsequent pressureless sintering. The effect of SBT incorporation on the microstructure, phase evolution, optical properties, and energy storage performance was investigated. The introduction of SBT effectively refined the grain size and promoted densification, resulting in a high optical transmittance of 72% and 58% in the visible region for the 7SBT and 10SBT ceramics, respectively. SBT doping induced a transition from a normal ferroelectric to a relaxor ferroelectric state in the KNN-based matrix, with the 7SBT and 10SBT compositions exhibiting pronounced relaxor characteristics. The results of ferroelectric performance tests revealed recoverable energy storage densities of approximately 1.05 J/cm³ at 150 kV/cm for 7SBT and 3.61 J/cm³ at 300 kV/cm for 10SBT, accompanied by an energy storage efficiency of 52%. This outstanding comprehensive performance (energy storage density and optical transmittance) demonstrated significant advantages over other reported lead-free ferroelectric ceramic systems, thereby providing both a material system and theoretical foundation for developing high-performance lead-free ferroelectric transparent dielectrics for optoelectronic integrated applications.

(1-x%)(K_0.5Na_0.5NbO₃) -x%(Sr_0.7Bi_0.2TiO₃) (x = 1, 4, 7, 10, and 15) ceramics, denoted as xSBT, were fabricated with high-purity oxides and carbonates as starting materials by a conventional solid-state reaction method and a subsequnt pressureless sintering. The phase structure was analyzed by X-ray diffraction (XRD). The microstructure and elemental distribution were examined by scanning electron microscopy (SEM) coupled with energy-dispersive X-ray spectroscopy (EDS). The structural evolution was determined by Raman spectroscopy. The optical transmittance was measured by ultraviolet-visible-near infrared spectrophotometry. The temperature-dependent relative dielectric permittivity (ε_r) was evaluated by an impedance analyzer and the polarization-electric field (P-E) hysteresis loops were measured by a ferroelectric test system to assess energy storage performance. Results and discussion The SEM images demonstrate that the introduction of xSBT effectively refines the grain size and promotes the microstructural densification. The grain size is significantly reduced from approximately 290 nm (as x = 4) to below 150 nm (as x = 7 and 10), effectively minimizing a light scattering. Consequently, the 7SBT and 10SBT ceramics achieve a high optical transmittance of approximately 72% and 58% at 800 nm in the visible light region, respectively.The XRD patterns and XRD Rietveld refinement results reveal a continuous phase structural evolution with increasing x content, i.e., from an orthorhombic phase (x = 1) to a tetragonal phase (x = 4), then to a coexistence of tetragonal and cubic phases (x = 7, 10), and finally to a pure cubic phase (x = 15). This progression signifies a structural transition from low-symmetry orthorhombic to high-symmetry cubic phases. The enhanced structural symmetry, along with chemical disorder induced by heterovalent substitutions of Sr²⁺/Bi³⁺/Ti⁴⁺, effectively can disrupte the long-range ferroelectric order, facilitating the formation of polar nanoregions (PNRs) and leading to a pronounced relaxor behavior. This is quantitatively supported by the significant increase in the diffuseness coefficient (γ) to 1.49 and 1.56 for x = 7 and 10, respectively. Correspondingly, the P-E loops evolve from a saturated square shape for 1SBT to a slim shape for 10SBT. As a result, the 7SBT and 10SBT compositions deliver recoverable energy storage densities (W_rec) of 1.05 J/cm³ in 150 kV/cm and 3.61 J/cm³ in 300 kV/cm, respectively, with an efficiency (η) of 52%. The combination of high optical transmittance and superior energy storage performance makes the xSBT ceramics a promising lead-free material system for integrated optoelectronic applications.

This study achieved the grain refinement and structural densification in KNN-based ceramics via doping SBT components, resulting in lead-free ceramics with a high optical transparency. The optical transmittance of the 7SBT and 10SBT ceramics in the visible light region reached 72% and 58%, respectively. Furthermore, the increased crystal symmetry and introduced chemical disorder by SBT doping disrupted the long-range ferroelectric order of the KNN matrix, thereby inducing pronounced relaxor characteristics. These modifications contributed to excellent ferroelectric energy storage properties in the 7SBT and 10SBT transparent ceramics. The 7SBT and 10SBT ceramics maintained an energy storage efficiency of 52% in electric fields of 150 kV/cm and 300 kV/cm, respectively, while achieving recoverable energy storage densities of approximately 1.05 J/cm³ and 3.61 J/cm³. Their outstanding overall performance (i.e., energy storage density and optical transparency) demonstrated significant advantages among reported lead-free ferroelectric transparent ceramics. This work could provide important theoretical and experimental foundations for advancing key materials in next-generation optoelectronic integrated systems and open pathways for designing novel multifunctional lead-free ferroelectric ceramics. The main limitation could lie in the substantial room for improvement in the energy storage efficiency of the current system. Future work could involve more precise composition control and interfacial engineering to reduce dielectric loss and leakage. In addition, the stability of electrical performance across wide temperature ranges and high frequencies could also require a systematic evaluation.

Piezoelectric ceramics are critical components in sensors, actuators, energy converters, and micro-electromechanical systems. The lead-based compositions dominate in the existing market. However, the release of lead during their lifecycle poses severe environmental and human-being health risks, which contradicts global sustainable development goals. Therefore, developing high-performance lead-free alternatives becomes a paramount research priority. Among lead-free alternatives, potassium sodium niobate ((K,Na)NbO₃, KNN)-based ceramics stand out due to their low driving voltage, large electrostrain, and minimal hysteresis, making them performance-competitive with conventional PZT for precision actuators. Their performance is typically enhanced by constructing polymorphic phase boundaries and regulating microstructure. However, the existing doping strategies inadvertently degrade thermal stability, which remains a key bottleneck for practical applications. Rare-earth ions act as effective modifiers in KNN-based piezoceramics. They enhance a relaxor behavior, and improve thermal stability by shifting rhombohedral-tetragonal (R-T) phase transition below room temperature, and introduce photoluminescence due to their abundant energy levels. The integration of electrostrain and luminescence in KNN-based systems is thus significant, paving a way for developing novel bimodal functional materials for next-generation optoelectronic devices. In this work, the 0.96(K_0.48Na_0.52)(Nb_0.95Sb_0.05)O₃-0.04Bi_0.5(Na_0.82K_0.18)_0.5ZrO₃(0.96KNNS-0.04BNKZ) composition with a polymorphic phase transition characteristic was chosen as a matrix, and rare-earth oxides (Dy₂O₃ and Eu₂O₃) served as modifiers. The effect of rare-earth oxides doping on the microstructure, ferroelectric/electrostrain properties, and photoluminescence performance of the 0.96KNNS-0.04BNKZ ceramics was investigated.

0.96KNNS-0.04BNKZ-x%Dy/Eu (x = 0, 0.05, 0.10, 0.20, 0.40) lead-free ferroelectric ceramics were synthesized by a solid-state reaction method. The stoichiometric raw materials were weighed according to the formula. They were then ground in ethanol in a ball mill with zirconia balls for 24 h. The powders were calcined at 850 ℃ for 4 h, then ground again for 24 h. After drying, the powders were mixed with 10% polyvinyl alcohol (PVA) as a binder and then granulated. Subsequently, disc-shaped green compacts (with the sizes of 10 mm in diameter and 0.5 mm in thickness) were pressed and subjected to a debinding process at 550 ℃. The ceramics were obtained after sintering at 1130-1200 ℃ for 2 h. After polishing, silver electrodes were fired at 650 ℃ for electrical testing. The ceramics were sintered at at 1130 ℃ to 1200 ℃ for 2 h, depending on the doping levels. After polishing, silver electrodes were fired on the surfaces at 650 ℃ for electrical characterization. The phase structure was analyzed by an X-ray diffractometer (D8 advance). The surface morphology was determined by a field emission scanning electron microscope (Merlin compact, Carl Zeiss Co., Germany). The temperature-dependent dielectric constant was measured by a broadband dielectric spectrometer (Novocontrol concept 40). The ferroelectric and electric-field-induced strain were characterized by a ferroelectric analyzer (TF 2000) coupled with a laser interferometer (SP-S 120/500). The photoluminescence properties were evaluated by a fluorescence spectrometer (FLS920).

Dy/Eu-doped 0.96KNNS-0.04BNKZ ceramics exhibit a single perovskite structure. At low doping levels (i.e., x ≤ 0.10%), the rhombohedral-tetragonal (R-T) coexisting phase maintains, while a higher doping (i.e., x ≥ 0.20%) induces a transition to the pseudocubic (P_c) phase, with no secondary phases appears in all the samples. All the ceramics present dense and uniform microstructures. The average grain size gradually decreases with increasing Dy/Eu doping, indicating that rare-earth ions effectively inhibit grain growth. In terms of electrical properties, a moderate Dy/Eu doping (i.e., Dy: x=0.10%; Eu: x=0.05%) enhances ferroelectricity, as reflected by improved P-E loop squareness and increased saturated polarization (P_max). The 0.10%Dy and 0.05%Eu samples achieve an optimal electrostrain performance, delivering unipolar strains (S_uni) of 0.257% and 0.241% at 70 kV/cm, which are accompanied by large piezoelectric strain coefficients (d₃₃^*) of 367 pm/V and 344 pm/V, respectively. An appropriate doping optimizes the domain structure and facilitates domain switching in an external electric field, thereby improving both ferroelectric and electrostrain properties. In contrast, excessive doping leads to a severe lattice distortion, notably octahedral tilting, which pins domain walls and hinders their reorientation in response to the electric field, ultimately compromising ferroelectric and electrostrain performance. Furthermore, the 0.05% Dy- and 0.05% Eu-doped ceramics exhibit excellent thermal stability, showing less than 10% variation in electrostrain at 25-100 ℃, which demonstrates their great potential for use in piezoelectric actuators. In addition, Dy/Eu doping introduces a pronounced photoluminescence to the ceramics. Under blue-light excitation, Dy³⁺-doped samples yield an intense yellow emission, whereas Eu³⁺-doped samples produce a bright orange-red emission.

The introduction of Dy/Eu could promote a phase transition from a mixed R-T phase to a P_c phase. An appropriate doping effectively enhanced the ferroelectric and electrostrain properties of the materials. Specifically, the 0.10% Dy- and 0.05% Eu-doped ceramics achieved unipolar strains (S_uni) of 0.257% and 0.241% at an electric field of 70 kV/cm, with corresponding piezoelectric strain coefficients (d₃₃^*) of 367 pm/V and 344 pm/V, respectively. Furthermore, the 0.05% Dy- and 0.05% Eu-doped samples exhibited excellent thermal stability, with a variation in electrostrain of less than 10% at 25-100 ℃, demonstrating a great potential for piezoelectric actuator applications. In addition, Dy/Eu doping imparted outstanding photoluminescence properties to the ceramics. Under blue-light excitation, Dy³⁺-doped samples exhibited an intense yellow emission, while Eu³⁺-doped samples displayed a bright orange-red emission. The combination of electrical and luminescent characteristics could make this material a promising candidate for optoelectronic coupling devices.

Dielectric capacitors are indispensable for pulsed power systems due to their high power density, ultrafast charge/discharge capability (>µs), and excellent fatigue resistance. However, their widespread adoption is fundamentally limited by poor energy storage performance, characterized by inadequate recoverable energy density (W_rec) and efficiency (η). To meet the demands of high-performance applications, it is imperative to simultaneously enhance W_rec, η, and breakdown strength (E_b). Among environmentally friendly candidates, BiFeO₃-BaTiO₃ (BF-BT) relaxor ferroelectric ceramics stand out due to their high Curie temperature (T_C ≈ 830 ℃) and large maximum polarization (P_max). However, the intrinsic low E_b of BF-BT ceramics caused from the volatilization of Bi³⁺ and the change in the valence state of Fe³⁺ hinder their energy storage potential. To address these challenges, this work was to use a dual-strategy approach with compositional tailoring and two-step sintering for the preparation of (0.7-x)BF-0.3BT-xSAN relaxor ferroelectric ceramics. The introduction of Sr(Nb_0.5Al_0.5)O₃ into the 0.7BF-0.3BT system was to enhance relaxor behavior and refine the grain size, synergistically boosting both E_b and polarization response. Furthermore, the effect of SAN on the microstructure, dielectric, and energy storage properties was systematically investigated. This could provide research ideas and theoretical basis for improving the energy storage performance of BF-BT ceramics based relaxor ferroelectric ceramics.

The raw materials powders of Bi₂O₃, Fe₂O₃, BaCO₃, TiO₂, SrCO₃, MnO₂, Nb₂O₅ and Al₂O₃ were analytical purity. These powders were precisely weighed according to a stoichiometric ratio, then ground with ethanol in a ball mill at 400 r/min for 12 h prior to drying. The mixed powder with a small amount of sintering aids (MnO₂) was subsequently calcined at 850 ℃. The mixture was further ground for 12 h and dried. The dried powder was compacted by cold isostatic pressing (200 MPa, 2 min) into circular discs (Ф10 mm×1 mm). These discs were placed in covered alumina crucibles to minimize Bi volatilization and thermally treated at 1050-1110 ℃ for 2 min, and then sintered at 1020-1050 ℃. The ceramic sheet was polished for microstructure characterization. After polishing for microstructural analysis, silver electrodes were screen-printed on the both surfaces of the 0.7 mm thick sintered ceramics and fired at 700 ℃ for 20 min for electrical characterization.

The crystal structure of the ceramics was analyzed by X-ray diffraction (XRD). The surface morphology and grain distribution were examined by field emission scanning electron microscopy (FE-SEM) and performed by a software named Nano Measure. The microscopic domain structure of the ceramics was characterized by piezoelectric force microscopy (PFM). The Raman spectra in the range of 90-900 cm^-1 were determined by Raman spectroscopy. The polarization-electric field hysteresis loops (P-E) were measured by a ferroelectric tester on disk-shaped samples (with the sizes of Ф1.5 mm × 0.08 mm).

The XRD patterns reveal that all the (0.7-x)BF-0.3BT-xSAN ceramics exhibit an asymmetric singlet pattern, indicating that the crystal structure is a pseudo-cubic structure. The content of the second phase gradually increases with the increase of x due to the solid solubility limitation of SAN in BF-BT ceramics. The addition of SAN causes the XRD diffraction peaks shifting to higher angles, indicating a significant contraction of the lattice volume. The Raman peaks of ceramics do not show a significant shift in the Raman full spectrum, but the peak half width gradually increases, which is attributed to the increase in structural disorder. The Raman fitting results further indicate that increasing SAN weakens both A—O and B—O bonds, thereby reducing unit cell polarity and increasing local disorder in the BO₆ octahedra. This enhanced disorder facilitates the establishment of local random electric fields, which in turn promotes the formation of polar nanoregions. The SEM images manifest that the SAN results in a significant grain size reduction, especially as the x exceeds 0.14. The PFM results demonstrate that ceramics of matrix and 0.54BF-.3BT-0.16SAN exhibit both strong and weak piezoelectric response nanodomains, and the nanodomains in 0.54BF-0.3BT-0.16SAN exhibit a weaker piezoelectric response. All the ceramics exhibit significant frequency dispersion and diffusion phase transition in their dielectric results, which imply a strong relaxation behavior. Polar, which can be attributed to a synergistic effect from the fine grain size, moderates a dielectric constant, and the presence of highly active nanodomains. Moreover, the measurements of P-E loops under varying freqization-electric field (P-E) hysteresis loops show a significant decrease in the polarization strength of the ceramic with the increase of SAN, which is attributed to the increase of relaxor characteristics. The Weibull distribution fitting results reveal that the breakdown strength (E_b) initially increases and then decreases with increasing x. Specifically, the 0.54BF-0.3BT-0.16SAN ceramic achieves the maximum E_b of 370 kV/cm. The results of cycling tests reveal that the ceramic exhibits robust frequency, thermal, and fatigue stability.

The (0.7-x)BF-0.3BT-xSAN relaxor ferroelectric ceramics with a pseudo-cubic structure were prepared by a two-step sintering method. The addition of SAN led to a contraction of unit cell and a concurrent weakening of both A—O and B—O bonds, consequently reducing unit cell polarity and enhancing local disorder within the BO₆ octahedra. The results of dielectric test indicated that (0.7-x)BF-0.3BT-xSAN ceramics had strong relaxation properties. The 0.54BF-0.3BT-0.16SAN ceramic simultaneously achieved the superior energy storage performance (i.e., W_rec of 3.90 J/cm³ and η of 85.5%) at E_b of 370 kV/cm. Furthermore, the 0.54BF-0.3BT-0.16SAN ceramic exhibited the excellent frequency (i.e., 1-1000 Hz), temperature (i.e., -20 ℃ to100 ℃), and cycling stability (i.e., 10⁵ cycles).

Compared with dense ceramics, porous ceramics exhibit lower density and acoustic impedance, as well as excellent thermal resistance, enabling their broader application in hydrophones, high-temperature ultrasonic transducer backing layers, biopiezoelectric devices, and related fields. However, conventional fabrication methods for porous ceramics, such as freeze-casting and gelcasting, are often associated with complex processing procedures. The digital light processing (DLP) additive manufacturing technique offers several advantages, i.e., high resolution, rapid curing, uniform surface exposure, and low production cost, making it a promising alternative for fabricating complex porous ceramic structures. Lead-free barium titanate piezoelectric ceramics have attracted much attention due to their favorable piezoelectric performance. In this study, porous barium titanate ceramics with varying porosity levels were fabricated via DLP 3D printing by adjusting the content of polymethyl methacrylate (PMMA) from 0 to 20%. The influence of porosity on the phase structure, microstructure, and electrical properties of the ceramics was systematically investigated.

A ceramic slurry containing 82% barium titanate (BTO), composed of BTO powder (500 nm), photosensitive resin, and dispersant, was mixed with PMMA microspheres (15 μm) at different mass fractions of 5%, 10%, 15% and 20%, respectively. The mixture was homogenized at 2000 r/min by a homogenizer (ZYE, ZYMC-180V) for 2 min. Subsequently, the designed 3D model in STL format was imported into the control computer for DLP printing. The printed samples had dimensions of Ф12mm×2 mm, with a layer thickness of 50 μm. After printing, the green bodies were subjected to a two-step debinding process (i.e., firstly under vacuum and then in an air atmosphere) and then sintered. The sintered samples were ground, polished, and coated with gold electrodes. The poling was performed in an electric field of 3 kV/mm in silicone oil at 60 ℃ for 15 min. The microstructure of the samples was determined by field emission scanning electron microscopy (LYRA 3 XMU, Tescan Co., CZ). The phase composition of the BTO ceramics was determined by X-ray diffraction (X'Pert PRO, PANalytical B.V., the Netherlands). The sample density was determined based on the Archimedes principle. The dielectric properties were measured by precision impedance analysis (6500B, Wayne Kerr Co., UK), while the piezoelectric coefficient (d₃₃) was evaluated by a d₃₃ meter (ZJ-3A, Institute of Acoustics, Chinese Academy of Sciences, China).

The results show that the porosity of barium titanate ceramics gradually increases as the PMMA content increases from 0 to 20%. The porous ceramics exhibit closed, spherical pores, with pore sizes closely matching the particle size of the employed PMMA (approximately 15 μm). The axial shrinkage ratio of the ceramics increases from 11% to 29%, and the radial shrinkage increases from 14% to 26%. This anisotropic shrinkage behavior is attributed to the layer-by-layer printing process, resulting in a greater axial shrinkage relative to the radial direction. Besides, after the introduction of the air phase, the bulk density decreases from 5.7 g/cm³ to 4.5 g/cm³, the porosity increases from 6.0% to 25.7%, and the acoustic impedance reduces from 23.12 MRayls to 15.25 MRayls. Regarding the electrical properties, the piezoelectric coefficient d₃₃ gradually decreases from 223 pC/N to 135 pC/N, and the dielectric constant ε_r declines from 1255 to 407. This is primarily attributed to the reduction in the effective volume fraction of the piezoelectric phase caused via increasing porosity, as well as the non-uniform electric field distribution within the material, which impairs a polarization efficiency. Meanwhile, the dielectric loss tanδ remains in the range of 0.02-0.04, exhibiting minor fluctuations with an increasing porosity and maintaining a low value.

The effect of PMMA content on the microstructure and electrical properties of BTO ceramics was investigated. The piezoelectric constant and dielectric constant of porous barium titanate ceramics both decreased, while the dielectric loss remained a low range as the PMMA content increased, indicating a good stability. An increase in porosity reduced the bulk density of the BTO ceramics, thereby lowering the acoustic impedance and enhancing acoustic impedance matching. DLP-based 3D printing, by combining high precision with rapid prototyping capability, could provide an efficient and cost-effective route for the fabrication of porous BTO ceramics in acoustic device applications.

Piezoelectric ceramics are widely used in the fields such as electronics, telecommunications, sensing, and acoustic due to their capability for bidirectional conversion between electrical and mechanical energy. The existing lead zirconate titanate ceramics dominate a wide range of practical applications due to their excellent piezoelectric properties and good manufacturability. However, lead is toxic to human-being health and environment during the processing, application, and disposal. Furthermore, their low Curie temperature (T_C<400 ℃) limits their application potential in the field of high temperature environment. CaBi₄Ti₄O₁₅ (CBT) is one of the piezoceramics with the T_C of < 790 ℃ and high thermal stability, which makes it a leading candidate for high-temperature applications. The potential ultrahigh temperature employment of CBT-based ceramics is, nevertheless, constrained by a low piezoelectric coefficient (d₃₃ < 10 pC/N), small dielectric constant, and poor poling capability. It is difficult to fully pole these ceramics in high electric fields because the volatilization of Bi during high-temperature sintering results in a stoichiometric deviation, a higher density of lattice defects, and reduced electrical resistivity. Recently, numerous researchers use chemical doping strategy to tailor the crystal structure and modify microdefects, yielding an initial success. The A-site doping of rare-earth ions (e.g., Er³⁺, Nd³⁺, and Ce³⁺) or alkali metal ions (e.g., Na⁺, K⁺, and Li⁺) is a practical way of improving the piezoelectric property via suppressing Bi volatilization, inducing lattice distortion and tailoring densification. According to some previous reports, rare-earth element doping promotes the d₃₃ of CBT-based ceramics to 20 pC/N. It is reported that Nd/Ce co-doping substantially elevates the d₃₃ to 28 pC/N in Na_0.5Bi_4.5Ti₄O₁₅, demonstrating that synergistic rare-earth element co-doping plays a critical role in enhancing crystal structure stability and defect control. Despite the promising progress of rare-earth element doping in bismuth layer-structured ferroelectrics, there is still little systematic research regarding the synergistic mechanisms of La/Ce co-doping in regulating the microstructure and piezoelectric properties of CBT ceramics. La³⁺ can incorporate into the A-site in CBT-based ceramics due to the similar ionic radius of La³⁺ and Bi³⁺, enhancing the lattice stability. Meanwhile, Ce can reduce oxygen vacancy concentration by charge compensation mechanism and improve densification. The synergistic interaction between them can boost polarization and piezoelectric property. In this work, CBT-based ceramics with La/Ce co-doping were prepared via conventional solid-state reaction. Moreover, the effect of doping concentration on the crystal structure, microstructure, and various electrical properties, including dielectric, ferroelectric, and piezoelectric characteristics was systematically investigated.

Ca(La_1/2Ce_1/2)_xBi_4-xTi₄O₁₅(CLaCe-x, x = 0.000, 0.025, 0.050, 0.075, 0.100) ceramics were synthesized via solid-state reaction. The raw materials were CaCO₃(99.99%)、Bi₂O₃(99.99%)、TiO₂(99.8%)、CeO₂(99.99%) and La₂O₃(99.99%). First, all the starting materials were weighed according to different stoichiometric ratios and ground in a ball mill with alcohol for 12 h. Then, the wet slurries were dried at 80 ℃ and calcined at 82 ℃ for 4 h. Afterwards, the powder was further ground. Consecutively, the dried powder with 0.5% polyvinyl alcohol was pressed into discs with a thickness of 0.7 mm and a diameter of 12 mm. The pellets were sintered at 1025-1075 ℃ for 1 h.

An improved piezoelectric coefficient of 19 pC/N is obtained, along with an outstanding Curie temperature of 791 ℃ can be achieved in the ceramic. The XRD patterns and SEM images confirm that all the ceramics show a pure single phase, indicating that La/Ce ions are dissolved into the original crystal lattice. Furthermore, the influence of doping ions on the grain size is negligible. The co-doping bigger radius ions at the A-site induce the expansion of lattice, which can improve the crystal structure. The dielectric loss of the ceramic is reduced, and its impedance is increased by over one order of magnitude at a high temperature due to this structure modification. The activation energy is calculated with the Arrhenius relation, manifesting that the oxygen vacancy concentration can reduce. Consequently, the ceramic resistivity is a boosted result from a lower concentration of hole carriers. This improved resistivity facilitates the poling process in high electric fields, leading to a sharp increase in the piezoelectric coefficient. Subsequently, P-E hysteresis loops are measured at 100 ℃. The CLaCe-0.075 ceramic exhibits a larger remanent polarization (P_r) than pure CBT, which is consistent with the enhancement in piezoelectric property. In summary, the La/Ce co-doping ceramics demonstrate the superior d₃₃, T_C, resistivity, and thermal stability, compared to most bismuth-layered piezoelectric ceramics. This work confirms that the rare-earth element co-doping is an efficient strategy for elevating the electrical performance of CBT-based ferroelectrics.

La/Ce co-doping CBT ceramics were fabricated via a solid-state reaction. The effect of rare-earth element doping on the structure and properties of bismuth-layered ceramics was systematically investigated. The results revealed that the ceramics remained phase-pure Aurivillius structures with a high relative density and a negligible variation in grain size. The high d₃₃ of 19 pC/N and T_C of 791 ℃ of the CLaCe-0.075 ceramic were achieved. In addition, the dielectric loss reduced to 1.62%, and the resistivity increased to 2.4 × 10⁸ Ω·cm at 400 ℃. The impedance spectroscopy analysis revealed that La/Ce co-doping effectively inhibited Bi volatilization, leading to a reduced oxygen vacancy concentration, which was responsible for the enhanced piezoelectric property. This work demonstrated that La/Ce co-doping could be an effective strategy for enhancing piezoelectric property of bismuth layer-structured ferroelectrics, providing valuable insights for the development of novel high-temperature piezoelectric materials.

The Aurivillius type layered perovskite structure compound can be represented by a chemical formula (Bi₂O₂)²⁺(A_n-1B_nO_3n+1)^2-. Its crystal structure can be seen as a unique "natural superlattice" structure composed of alternating layers of perovskite-like structure and fluorite-like structure along the c-axis direction. Among them, n represents the number of layers of BO₆ octahedra in the c-axis direction in the perovskite structure layer (i.e., n = 1, 2, 3, 4, 5, ...). The value of n determines the thickness of the perovskite layer and physical properties of the material. The A-site ion is located at the octahedral apex of the perovskite like layer, with 12 coordination. Usually, it can be larger cations such as Ba²⁺, Sr²⁺, Ca²⁺, Pb²⁺, K¹⁺, Na¹⁺, Bi³⁺, and Ln³⁺(lanthanide elements). Multiple ion substitutions can be achieved to regulate the performance. The B-site ion is located at the center of the oxygen octahedron and coordinated with 6. Usually, it can be small high valence transition metal ions such as Fe³⁺, Ti⁴⁺, Nb⁵⁺, Ta⁵⁺, W⁶⁺, and Mo⁶⁺). The displacement of B-site ions is one of the main sources of ferroelectric polarization. The stereochemical activity of the 6s² lone pair electrons of Bi³⁺ ions also has a significant impact on their structure and polarization. From the chemical formula and crystal structure above, it can be inferred that this type of compound can have a broad spectrum of physical and chemical properties as well as tunability, while the physical and chemical properties can be accompanied by significant anisotropic characteristics.

Acetate and tetrabutyl titanate [Ti(OC₄H₉)₄] were used as starting materials solvent in acidic solutions. A 10% excess amount of Bi(CH₃COO)₃ was used to compensate the Bi volatilization in the process. Before spin coating, the substrates were cleaned ultrasonically in acetone, alcohol and deionized water, sequently. The spin coating and baking processing were repeated to enhance the thickness. Finally, the baked thin films were crystallized in ambient atmosphere at 700 ℃ for 10 min by rapid thermal annealing.

The crystal structure for the derived thin films was determined by X-ray diffraction (XRD, Philips Co., the Netherlands) with Cu k_α radiation. The thickness and the surface morphologies were characterized by field-emission scanning electron microscopy (FE-SEM, FEI Co., Japan). Au electrodes of 0.2 mm in diameter were deposited by a sputter (China) onto the surfaces of thin films through a shadow mask. The ferroelectric and leakage properties were measured by a standard ferroelectric test system (Radiant Technologies Co., USA).

For the sample with x of 0, its main diffraction peaks correspond one-to-one with the six-layered perovskite structure, which can be classified as the A2₁am space group, belonging to a typical even layered Aurivillius type compound. There are no diffraction peaks of other impurities except for the substrate diffraction peak. After introducing element Mn doping, the doped film still maintains the layered structure characteristic peak position as a whole, and there is no significant shift. In general, doping often leads to a shift in diffraction peak position due to the difference in ionic radius, which is usually one of the reference evidences for doping elements entering the main lattice. There is no significant shift in the diffraction peak position of the Mn ion doped film possibly due to the unsuccessful entry of Mn ions into the main lattice or the fact that the radius of Mn ion (0.064 5 nm) is the same as the radius of Fe ion (0.064 5 nm). As x increases, the most intense diffraction peak of the sample gradually shifts from (1113) to (0014), and the (00l) series diffraction peaks gradually increase in intensity, indicating that the c-axis orientation of the thin film is strengthened. This change is not significant in ceramic samples, but it is also evident. The change in relative peak intensity also confirms the doping of Mn ions into the main lattice, indicating that the diffraction peak position is not significantly shifted due to the same ion radius. For the structural characteristics of Aurivillius type compounds, especially their polarization anisotropy, this preferred orientation can inevitably have a profound impact on the electrical properties of thin films.

The BFTM thin films doped with manganese ions were prepared by a chemical solution method. The preferred orientation of the film's c-axis enhanced, and the orientation factor increased from 0.08 as x=0 to 0.89 as x=1.00 with the increase of manganese ion doping content. The ferroelectric properties of the thin film were enhanced with doping, and the effective residual polarization reached its maximum as x=0.50. This work could provide an experimental reference for the preparation and ferroelectric properties of Aurivillius type layered perovskite structure thin films. These results could indicate the anisotropic characteristics of the crystal structure polarization of this type of compound, and provide feasible strategies for the preparation of preferred orientation thin films and performance control.

As an important branch of lead-free piezoelectric materials, ceramics with a tetragonal tungsten bronze (TTB) structure have a high spontaneous polarization strength, high Curie temperature (T_C), and beneficial dielectric properties, making them particularly useful in ferroelectric applications. Depending on which sites are filled, the TTB structure can be divided into three categories, i.e., the "stuffed" structure, the "filled" structure and the "unfilled" structure. As a typical "filled" TTB compound, the Sr₂NaNb₅O₁₅ (SNN) ceramics can be regulated through compositional design to exhibit a wide range of structures and functional characteristics, which have a broad development potential in the fields of dielectric energy storage, multiferroic materials, nonlinear optics, and pyroelectric sensors. However, SNN ceramics require relatively high sintering temperatures, and the secondary phase Na_0.5Sr_0.25NbO₃ (NSN) readily forms during the sintering process, which adversely affects the electrical properties. Although some previous studies used the methods of eliminating the impurity phase, there could be few studies on the relationship between impurities and the properties of SNN ceramics. In this paper, SNN ceramics (i.e., Sr_1-xNa_xNb₅O₁₅, 0.56 ≤ x ≤ 1.06) were prepared with the SrNb₂O₆ (SN)-NaNbO₃ (NN) binary solid solution system, to examine the relationship among the presence of the secondary phases and the ceramic composition, as well as the sintering temperature.

SN and NN powders were synthesized by a solid-state method with reagent-grade SrCO₃ (99.99%), Na₂CO₃ (99.99%), and Nb₂O₅ (99.99%) as raw materials. The starting raw materials were pre-dried at 120 ℃ for 24 h, and then weighed according to the target stoichiometric ratios. The powders were thoroughly mixed in ethanol in a planetary ball mill with ZrO₂ balls at 401 r/min for 20 h. The mixtures were dried at 80 ℃ for 24 h, and then calcined at their respective suitable temperatures. Subsequently, the SN and NN powders were obtained by separately grinding the calcined products in a mortar after natural cooling. Afterwards, SNN ceramics (Sr_1-xNa_xNb₅O₁₅, 0.56 ≤ x ≤ 1.06) were synthesized with polycrystalline SN and NN according to the desired SNN compositions, calcined at 1220 ℃ for 6 h, and uniaxially compacted into pellets with a binder (namely 5% polyvinyl alcohol). The binder was removed at a high temperature. Finally, SNN ceramics were prepared by sintering at 1280-1340 ℃ for 4 h.

The effect of the secondary phases on the structure of SNN ceramics was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and field emission transmission electron microscopy (FETEM). Furthermore, the ceramics were ground and polished down to a thickness of about 0.6 mm, coated with silver electrodes, and fired at 840 ℃ for 30 min. Their dielectric characteristics and piezoelectric coefficients d₃₃ were measured by the LCR meter and quasi-static piezoelectric meter. The ceramics were also ground and polished down to a thickness of about 0.2 mm and coated with gold electrodes to record the room-temperature ferroelectric hysteresis loops by a Radiant Precision Workstation.

The required composition and sintering temperature ranges of the SNN pure phase are determined. For Na⁺contents (x) of 0.56-0.68, SN-NN can be solidly dissolved to obtain a pure phase at high temperatures (i.e., > 1280 ℃). As x = 0.71 and 0.80, SN-NN can be solidly dissolved at 1200-1360 ℃. At the Na⁺ content of 0.88, the SN-NN solid solution can be obtained, but it decomposes at higher temperatures (> 1320 ℃), accompanying the formation of the secondary NN phase. As the Na⁺ content further increases, it cannot be solidly dissolved at 1200-1360 ℃.

The maximum density is obtained for SNN ceramics sintered at 1340 ℃, and the change in Na⁺ content has little effect on the microstructure. The T_C of SNN ceramics increases from 272 ℃ as x = 0.56 to 323 ℃ as x = 0.88 because the crystal structure changes from "unfilled" to "filled" as the Na⁺ content increases, thus affecting the degree of crystal distortion. T_C gradually decreases because a minor amount of NN appears at x of 0.88. The dielectric constant (ε_r) of SNN ceramics initially increases and then decreases, exhibiting a maximum of 1664 as x = 0.80. The dielectric loss (tanδ) remains at a low value at this point. The remnant polarization P_r (11.49 μC/cm²) and coercive field E_c (20.99 kV/cm) are both nearly maximized as x = 0.80 for SNN ceramics due to the "filled" TTB structure, and the maximum d₃₃ value of 34 pC/N is obtained for SNN ceramics as x = 0.80, which is consistent with the optimal composition for ferroelectric properties.

Based on the XRD patterns, the ceramics with a stable, pure TTB phase could be obtained by sintering at 1200-1360 ℃ when x = 0.71-0.80, and the existence of this phase depended on the ceramic composition and the sintering temperature. As the Na⁺ content increased, T_C initially increased because the filling degree of the TTB structure increased, and then gradually decreasesd when x > 0.88 because the increase in Na⁺ content induced the formation of the secondary NN phase. When x=0.71-0.88, the electrical properties could be optimal. The ceramic with x of 0.80 demonstrated a high ferroelectric performance (i.e., P_r = 11.49 μC/cm², and E_c = 20.99 kV/cm). This study could provide certain theoretical basis and technical support for the subsequent studies of SNN ceramics.

Lead-based relaxor ferroelectric ceramics exhibit relatively low strain hysteresis (mostly below 20%) and superior strain temperature stability, demonstrating significant advantages in electrostrain properties, which are highly favorable for applications in high-precision piezoelectric actuators. The existing lead-based relaxor ferroelectric ceramics still dominate the high-end piezoelectric actuator market. However, lead is extremely harmful to human health and the environment, and numerous domestic and international policies have explicitly restricted the usage of lead. Consequently, reducing the lead content or even achieving lead-free compositions in ferroelectric ceramics becomes an inevitable trend. Although lead-free relaxor ferroelectric ceramics, which avoid the environmental toxicity of lead, become a research hotspot in the piezoelectric materials field, their comprehensive electrostrain properties still cannot compete with those of lead-based ferroelectric ceramics for piezoelectric actuator applications. Lead-free relaxor ferroelectric ceramics can achieve high electrostrain responses through reversible phase transitions between relaxor and ferroelectric phase, they suffer from excessively large strain hysteresis and poor strain temperature stability. How to realize a balanced electrostrain properties remains a core challenge for lead-free relaxor ferroelectric ceramics. Despite these challenges, lead-free relaxor ferroelectric ceramics are expected to replace lead-based ferroelectric ceramics in mid-to-low-end applications through component design, process optimization, and reliability improvements. Nevertheless, in high precision or extreme environment scenarios, lead-free relaxor ferroelectric ceramics still struggle to fully substitute lead-based ceramics, necessitating long-term technological breakthroughs.

In recent years, lead titanate (PZT) based ceramics are used in the electronic ceramics due to their excellent piezoelectric and dielectric properties. However, PZT contains a high proportion of toxic heavy metal lead, which poses a serious threat to the environment and human-being health during production, use, and disposal. With increasingly strict global environmental regulations, the development of high-performance lead-free piezoelectric-based materials becomes an urgent task and consensus in the field of materials science. Among these ceramic materials, K_0.5Na_0.5NbO₃ (KNN) ceramics have attracted much attention as a lead-free alternative to conventional piezoelectric materials due to their excellent electromechanical properties and environmental compatibility. This study was to address these limitations via implementing a manganese dioxide (MnO₂) doping strategy. The primary objectives were to establish structure-property relationships and optimize dielectric performance for high-temperature/high-frequency electronic applications, while elucidating the underlying defect-mediated mechanisms governing the performance enhancement.

The KNN ceramics with controlled MnO₂ doping concentrations (i.e., x = 0, 0.010%, 0.020%, 0.035%, 0.060%, and 0.095%) were synthesized via conventional solid-state reaction. High-purity precursors (i.e., Na₂CO₃, K₂CO₃, Nb₂O₅, MnO₂) were stoichiometrically weighed, ground in a planetary mill with ethanol, calcined (at 800-900 ℃), re-ground, and pressed into pellets with a polyvinyl alcohol binder. The sintering was conducted at 1050-1070 ℃ for 3 h. The poling was performed at 2 kV/mm for electrical characterization. The phase purity and crystal structure were analyzed by X-ray diffraction (XRD). The microstructure and elemental distribution were characterized by scanning electron microscopy and energy-dispersive spectroscopy (SEM-EDS). The dielectric permittivity and loss tangent were determined at 40 Hz-10 MHz and varied temperatures using an LCR meter. The electrical conductivity, nonlinear "J-E" characteristics, and impedance spectroscopy were systematically evaluated.

The XRD patterns confirm that all the compositions retain a pure tetragonal perovskite structures (space group P4mm). As 0.060% as an optimal doping concentration, Mn²⁺/Mn³⁺/Mn⁴⁺ substitution for Nb⁵⁺ induces lattice expansion and forms defect dipoles. The SEM images reveal a substantial grain growth from 1.2 μm (undoped) to 3.5 μm (doped at 0.060%), due to liquid-phase sintering and lattice strain. The doping concentration of 0.060% can achieve a synergistic enhancement of dielectrical property, i.e., ε_r increases significantly, while tanδ decreases dramatically at low frequencies, governed by grain homogenization suppressing electron-hopping conduction. Conversely, the doping concentration of 0.095% has anomalous high-frequency losses due to the intensified grain-boundary polarization and skin effects. The temperature-dependent studies demonstrate a reduced dielectric maximum temperature from 240 ℃ (undoped) to 200 ℃ (doped), with a superior stability in tan δ at10⁴-10⁶ Hz. The impedance spectra reveal an accelerated ion mobility at > 200 ℃. The optimal operational window is identified at 200-240 ℃ and 10⁴-10⁶ Hz. The excessive doping concentration (i.e., 0.095%) degrades the performance due to the secondary phase formation, reduces relative density, and increases porosity. The moderate improvements in DC conductivity (attributed to defect-assisted hopping) and nonlinear coefficient α are obtained.

In this study, a series of K_0.5Na_0.5NbO₃ (KNN) ceramics with different manganese oxide (MnO₂) concentrations were synthesized. The results showed that the optimal doping concentration of MnO₂ was 0.060%. At this optimal concentration, manganese ions in multiple oxidation states replaced Nb⁵⁺, causing the lattice expansion and the formation of beneficial defect dipoles. This doping strategy simultaneously improved the microstructure, promoted a significant grain growth from 1.2 µm to 3.5 µm, and enhanced dielectric properties, achieving a significant synergistic enhancement effect. This improvement could be mainly attributed to the uniformity of the microstructure, which inhibited a harmful electron hopping conduction. The study also clarified that the optimal window for stability performance could be 200 -240 ℃ and 10⁴-10⁶ Hz. Beyond the optimal doping level, the excessive doping concentration (i.e., 0.095%) led to a performance degradation due to the formation of secondary phases, increased porosity, and increased losses at high frequencies due to grain boundary polarization and surface effects. This study could provide a basic framework for developing manganese-doped potassium sodium nickel oxide ceramics for specific electronic applications.

Piezoelectric ceramics like PZT are essential for energy harvesting and sensing, yet conventional bulk materials face limitations. Introducing ceramics with a porosity decouples the piezoelectric constant (d₃₃) from the dielectric constant, significantly enhancing the piezoelectric voltage constant (g₃₃) and hydrostatic figure of merit (g_h) for hydrophone applications. However, conventional fabrication methods, such as pore-forming agent burnout, result in random, irregular pores that cause uneven polarization, mechanical instability, and performance degradation. Digital light processing (DLP) offers an effective solution of enabling the precise fabrication of complex geometries. This study was to utilize DLP to construct the Gyroid-structured porous PZT ceramics based on triply periodic minimal surfaces (TPMS). These structures could provide superior connectivity and mechanical stability, compared to random pores. The study was to investigate the influence of porosity on electro-mechanical properties and validate the material application in high-sensitivity piezoelectric hydrophones.

A photosensitive ceramic slurry was prepared via mixing PZT-5H powder with oligomers (HDDA, TMPTA) and photoinitiators. Gyroid-structured green bodies with the porosities ranging from 55% to 70% were fabricated by digital light processing (DLP) and subsequently sintered at 1200 ℃. The microstructure was analyzed by scanning electron microscopy (SEM), and the ferroelectric, dielectric, and piezoelectric properties were measured based on standard analysis. For application testing, the piezoelectric hydrophones were assembled. The influence of encapsulation was evaluated via comparing surface sealing against complete epoxy filling. The hydroacoustic performance, including open-circuit output voltage, receiving sensitivity, and directivity, was tested in an anechoic tank by a comparison method against a standard hydrophone (GF-197) at 100-500 kHz.

The DLP-fabricated ceramics accurately reproduce the Gyroid designs. As a porosity increases to 70%, the relative dielectric constant and polarization decrease linearly due to the reduced ceramic volume. However, the piezoelectric constant (d₃₃) remains remarkably stable (i.e., 568 pC/N) rather than degrading, attributed to the Gyroid structure's uniform stress distribution and effective polarization. The sample with a porosity of 70% achieves an exceptional piezoelectric voltage constant (g₃₃) of 103.4 (mV·m)/N. In hydrophone assembly, a surface sealing preserves the electromechanical performance, whereas a complete filling significantly reduces d₃₃ due to the clamping effects. The resulting porous hydrophone demonstrates a maximum receiving sensitivity of -196.9 dB at 150 kHz, outperforming a standard hydrophone by 21.1 dB. Also, it exhibits a tunable directivity, with beam width narrowing as frequency increases, validating its suitability for high-sensitivity detection.

This study indicated that fabricating Gyroid-structured PZT ceramics via DLP could be an effective strategy to enhance piezoelectric performance. The unique topology of the Gyroid structure ensured that the electrical connectivity of the ceramic phase remained stable as porosity increased, maintaining consistent local electric fields and enabling full polarization. This structural advantage resulted in a stable d₃₃ (568 pC/N) even at a porosity of 70%, leading to a drastically improved piezoelectric voltage constant (g₃₃) of 103.4 mV·m/N, compared to random porous ceramics. The practical application demonstrated that a porous hydrophone achieved a receiving sensitivity of -196.9 dB to 21.1 dB. These findings could highlight a promising potential of TPMS-structured porous ceramics in overcoming the limitations of conventional materials for advanced hydroacoustic sensing.

Negative temperature coefficient (NTC) thermistors are widely applied in various industries such as automotive electronics, communications, military, and medical fields due to their high sensitivity and fast response. Conventional nonlinear thermistors exhibit a resistivity that varies as an exponential function of temperature, thereby necessitating the use of complex circuitry to achieve linearized output. However, such compensation circuits increase the complexity of use and result in a decrease in the accuracy, limiting the applications. It is thus of great significance for the practical application of the materials to investigate linear NTC thermistors. In this study, the effect of the component ratio on the material properties was investigated via introducing the high-resistance phase CdWO₄ with the low-resistance phase CdSnO₃ to form a three-dimensional conductive network and inducing the acquisition of linear electrical properties. The prepared xCdWO₄-(1-x)CdSnO₃-based linear NTC thermosensitive materials could exhibit good negative temperature coefficient properties at 300-450 K, i.e., the resistivity of the samples could decrease linearly with increasing temperature.

The linear thermistor material of xCdWO₄-(1-x)CdSnO₃ (0.1≤x≤0.35) was synthesized by a solid-phase reaction method. CdCO₃, WO₃ and SnO₂ were used as raw materials and weighed according to the stoichiometric ratio. After grinding, drying and calcination, CdWO₄ and CdSnO₃ powders were obtained. The two powders were weighed according to a certain ratio, ground and dried to obtain the precursor powder. Based on the precursor powders prepared above, the precursor powders were pressed at 200 MPa by a molding method to obtain plain discs with a diameter of 10 mm and a thickness of approximately 3 mm, and sintered in a muffle furnace at 1423 K for 2 h to obtain the target ceramic samples.

The phase structure of the sintered samples was determined by a model XRD-6000 X-ray diffractometer (Shimadzu Co., Japan). The microscopic morphology and grain size of the powders and ceramic block sections were characterized by a model SIGMA500 scanning electron microscope (SEM, Zeiss Co., Germany), and the elemental characterization and distribution of the sections were analyzed by an energy dispersive spectroscope (EDS). The elemental content and valence were determined by a model K-Alpha X-ray photoelectron spectroscope (XPS, Thermo Fisher Co., USA). After the silver electrodes were prepared via applying silver paste on the upper and lower surfaces of the ceramic disc specimens, the temperature characteristics of the resistance were measured by a model MS2008-10T thermistor R/T test system (Weihong Technology Co., Shenzhen, China) at 300-450 K. The impedance spectra of the ceramic disc specimens at 300-450 K and 100-20 MHz were determined by a model WK6500B impedance analyzer (Wayne Kerr Co., UK).

The thermosensitive ceramics of xCdWO₄-(1-x)CdSnO₃ (0.1≤x≤0.35) system are prepared by a solid-phase method. The XRD patterns show that the samples consist of two main crystalline phases in the precursor powders, and no other phases occur. The peak intensity of the crystalline plane CdSnO₃ (121) gradually decreases and shifts to a low angle as CdWO₄ addition increases, indicating that the cell parameters of the phase CdSnO₃ increase and the cell volume increases. The SEM images and EDS spectra show that all the samples have the coexistence of two phases CdWO₄ and CdSnO₃. The XPS analysis shows that the Sn peaks move toward a higher binding energy with the increase of CdWO₄ addition, and the value of Sn⁴⁺/Sn²⁺ decreases with the increase of x, which is due to the fact that there is more Sn⁴⁺ converted to Sn²⁺ in CdSnO₃ alone. According to the curves of resistivity ρ versus temperature (T), the resistivity of all the samples exhibits a linear decrease with increasing temperature at 300-450 K, showing the typical NTC characteristics. When x = 0.1, the linearity is the optimum, and R² reaches 0.996. The Cole-Cole plots show a single quarter-circle arc, and the diameter decreases with increasing temperature, indicating that it is dominated by the grain effect, and the relaxation of grain boundaries is not obvious. The R_g and R_gb in the equivalent circuits both decrease with increasing temperature, further confirming the NTC characteristics.

xCdWO₄-(1-x)CdSnO₃ (0.1≤x≤0.35) linear thermistors were prepared by a solid-phase reaction method, having good linear resistance-temperature characteristics at 300-450 K. The room temperature resistivity and resistance temperature coefficient of the composite thermosensitive ceramics decreased, while the material constant B and activation energy increased as CdWO₄ doping increased. When x = 0.1, the thermistor had the optimal resistance-temperature linearization with R² of 0.996. Based on the impedance spectroscopy analysis, the conductive behavior of the xCdWO₄-(1-x)CdSnO₃ system was mainly dominated by the mechanism of the valence jump of Sn ion. The resistance-temperature linearized conductive mechanism originated from the unique microstructure formed by the composite of the high-resistance phase CdWO₄ and the low-resistance phase CdSnO₃. The microstructural units consisting of grain resistance, grain boundary resistance, and grain boundary capacitance could function through synergistic effects.

Lithium orthosilicate (Li₄SiO₄) is considered as the most promising tritium breeder because of its high lithium density, good chemical stability and promising tritium release property. During the operation of a fusion reactor, the tritium breeders can be exposed to harsh condition for a long time, such as high temperature, thermal gradient and irradiation, which causes the degradation of the structure and property. In this work, Li₄SiO₄ pebbles were fabricated by a melt spraying method. The effect of annealing temperature on the microstructure and mechanical property of ceramic pebbles was investigated. The evolution of crush load at high temperatures was analyzed. In addition, the thermal stability of Li₄SiO₄ pebbles was also examined by long- time annealing experiments at high temperatures.

Li₄SiO₄ pebbles were fabricated by a melt spraying process. Firstly, Li₄SiO₄ powder was synthesized by a solid-state reaction. Lithium carbonate and silicon dioxide were mixed by grinding, and the ground powders were calcined at 850 ℃ for 4 h. Li₄SiO₄ ceramic powder was added into a platinum crucible, and heated until the powder melted to liquid. The melt liquid flowed out of the crucible, the droplet size was adjusted via spraying gas flowrate. During flying, the liquid droplets cooled down and formed spherical pebbles due to the surface tension. Finally, Li₄SiO₄ ceramic pebbles could be obtained by thermal annealing.

The microstructure of Li₄SiO₄ pebbles was determined by scanning electron microscopy (SEM). The crush load was measured by a universal testing machine, the pebble size was (1.00±0.05) mm. The chemical analysis was performed by inductively coupled plasma-optical emission spectrometry (ICP-OES).

Li₄SiO₄ pebbles with uniform microstructure and excellent properties are fabricated by a melt spraying process. The suitable annealing temperature is 600 ℃, and the grains grow up and the mechanical properties deteriorate significantly with further increasing the temperature. The fabricated pebbles annealed at 600 ℃ for 2 h display a dense and homogeneous structure with an average grain size of 3.88 μm, and the crush load is 20.7 N. The crush load at a high temperature decreases with the increase of temperature due to the thermal stress and plastic deformation, and the crush load at 700 ℃ is 8.7 N. According to the results of the long-time annealing experiments, the temperature is a key factor affecting the thermal stability of Li₄SiO₄ pebbles. The pebbles show a good mechanical property stability with moderate grain growth by annealing at 500-600 ℃. However, significant structure and property degradation occur due to rapid grain growth by annealing at 700 ℃. The microstructure degradation can be an inherent microstructural instability related to the competition between the densification and the grain growth kinetics, which is enhanced via increasing the annealing temperature.

Li₄SiO₄ pebbles with a fine structure and impressive property were fabricated by a melt spraying process. The pebbles displayed a dense and homogeneous structure with an average grain size of 3.88 μm, and the crush load was 20.7 N by annealing at 600 ℃ for 2 h. The crush load at high temperatures decreased with the increase of temperature. Besides, the Li₄SiO₄ pebbles could become unstable and deteriorate during the long-time annealing at high temperatures, accompanied by significant grain growth and pore evolution. The original isolated spherical pores could deteriorate to produce a crack-like interconnected pore network. Furthermore, the lithium evaporation was enhanced until the temperature reached 900 ℃. The degeneration of structure and properties of Li₄SiO₄ pebbles at > 700 ℃ could be considered in the design and operation of TBM.

Lithium-ion power batteries offer the advantage of high energy density but present a risk of thermal runaway. Under the conditions such as mechanical impact or overcharging, thermal runaway can initiate an explosive combustion, rapidly propagating throughout the entire battery pack and severely threatening life and property safety. Passive thermal protection systems represent one of the most economical and effective strategies for battery thermal protection. However, the surface temperature of ternary lithium batteries during thermal runaway exceeds 700 ℃, imposing severe challenges on the high-temperature stability of thermal insulation materials. Conventional materials such as mica and ceramic fiber wool are unable to meet these requirements. In contrast, fiber-reinforced aerogel composites emerge as an ideal choice due to their non-combustibility, excellent thermal insulation properties, and compressibility. Their core component like silica aerogel possesses a nanoporous structure (with the porosity of 99.8%) and employs multiple heat-blocking mechanisms. These characteristics render it one of the solid materials with the lowest thermal conductivity (approximately 0.014 W/(m·K)). There exist some critical challenges, i.e., organic fibers (e.g., aramid) exhibit an insufficient temperature resistance, high-performance ceramic fibers (e.g., mullite, silicon carbide) incur high costs and involve complex manufacturing processes, and lower-cost glass fiber-reinforced aerogel felts demonstrate an inadequate high-temperature performance. Basalt fiber becomes a preferred reinforcement material due to its high strength, high-temperature resistance, low cost, and environmentally friendly properties. Previous studies demonstrated that aerogel composites prepared via supercritical drying with basalt fibers could exhibit a superior thermal insulation performance, compared to glass fiber systems. Furthermore, industrialization requires overcoming limitations in the drying process. The predominant supercritical drying method utilizes expensive equipment and has operational hazards. Ambient pressure drying presents greater application prospects due to its advantages of low cost, safety, and operational simplicity. This study employed low-cost basalt fiber combined with ambient pressure drying to develop aerogel composites that integrated excellent thermal insulation, robust mechanical properties, and economic viability. This approach was to meet the thermal protection requirements for power batteries in new energy vehicles.

Basalt fiber needle-punched felt was employed as a reinforcement phase. Composites were prepared via regulating the concentrations of tetraethyl orthosilicate (TEOS) (0.67-1.17 mol/L) as well as by sol-gel process, hydrophobic modification using trimethylchlorosilane (TMCS), and ambient pressure drying. The surface morphology and structure of the composites were characterized by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, and Fourier transform infrared spectroscopy (FTIR). The thermal properties were characterized by HotDisk thermal conductivity analysis, thermogravimetric analysis-differential scanning calorimetry (TG-DSC), and a hot plate temperature rise test at 600 ℃, respectively. The compression testing was performed by a universal testing machine. The flame resistance was analyzed via exposing the composites into a butane torch flame at 1090 ℃ for 20 min.

As the TEOS concentration increases from 0.67 mol/L to 1.17 mol/L, the aerogel matrix becomes progressively denser and the cracks disappear. The sample with 1.00 mol/L TEOS has the lowest shrinkage, a density of 0.159 g/cm³, a specific surface area of 465.7 m²/g, and an average mesopore diameter of 12 nm. The enhanced structural integrity resulting from this density variation leads to a decrease in room-temperature thermal conductivity from 0.0274 W/(m·K) to 0.025 W/(m·K). After 10-min exposure to the hot surface at 600 ℃, the cold-side temperature is only 106 ℃, demonstrating the supervior thermal insulation performance. The sample with 1.17 mol/L TEOS displays the optimal compression performance, with a compressive modulus of 625 kPa and a compressive strength of 179 kPa. The fiber can effectively distribute the load, achieving a compression ratio of exceeding 60%, indicating the excellent cushioning and energy absorption capability. After annealing at 300 ℃, the removal of organic components increases the specific surface area to 581.5 m²/g and results in more ordered pore structures, while the compressive strength increases to 400 kPa. At 500 ℃, the thermal conductivity is 0.049 W/(m·K)), which is still lower than that of comparable ceramic fiber-reinforced aerogels. Beyond 700 ℃, the aerogel fragments, and the basalt fibers oxidize (turning red), while their skeleton dimensions remain stable. The total weight loss at 1000 ℃ is only 10%. Upon exposure to a butane flame at 1090 ℃ for 20 min, sintering occurs to a depth of only 2 mm in the cross-section, while the internal fibers remain intact. The mass loss is 3.8%, indicating the outstanding high-temperature service performance. These properties meet the requirements for passive thermal protection under extreme operating conditions in power batteries.

The TEOS concentration significantly affected the structural integrity of the material. At the TEOS concentration of 1.00 mol/L, the aerogel matrix achieved a density of 0.159 g/cm³ with minimal shrinkage, forming a continuous mesoporous structure (i.e., the specific surface area of 465.7 m²/g). This structure imparted the excellent thermal insulation performance (i.e., the thermal conductivity of 0.025 W/(m·K) at room temperature, with a cold-side temperature of only 106 ℃ under a hot surface at 600 ℃) and mechanical properties. The material maintained a functional stability under high-temperature conditions. After heat treatment at 500 ℃, its thermal conductivity remained as low as 0.049 W/(m·K). Moreover, the compressive strength increased due to the homogenization of the pore structure. Although particle formation occurred in the aerogel at 700 ℃, the basalt fiber framework remained structurally intact, and the weight loss was only 10% at 1000 ℃. The results of Butane flame exposure tests for 20 min demonstrated the outstanding flame resistance of the material. A sintered protective layer of aerogel and fiber could form on the surface, effectively inhibiting heat penetration. The sample showed no burn-through, with sintering confined to a shallow depth, while the internal structure remained intact.

Thermal barrier coatings (TBCs) are crucial for protecting aero-engine components against high-temperature and corrosive environments. Conventional yttria-stabilized zirconia (YSZ) coatings undergo phase transformation and degradation at high temperatures of > 1250 ℃, while accelerating their failure process particularly under Ca-Mg-Al-Si (CMAS) attack. The emergence of high-entropy ceramics can give an effective pathway for the development of advanced thermal barrier coating materials. Some studies demonstrate that the incorporation of multiple rare-earth elements endows high-entropy rare-earth zirconates with a lower thermal conductivity, a better phase stability, and a superior corrosion resistance, compared to YSZ. However, the existing research predominantly focuses on rare-earth zirconate materials or process optimization of conventional YSZ coatings, while studies on the atmospheric plasma spraying (APS) preparation process of high-entropy rare-earth zirconate coatings as well as their CMAS erosion behavior and dynamic reaction mechanisms remain insufficient. This study was to thus investigate the CMAS erosion behavior of atmospheric plasma-sprayed high-entropy rare-earth zirconate (Eu_0.2Gd_0.2Er_0.2Tm_0.2Yb_0.2)₂Zr₂O₇ thermal barrier coatings. In addition, the regulatory mechanism of high-entropy design on the CMAS erosion resistance of the coatings was also revealed via systematically analyzing the microstructural evolution, phase transformation, and chemical reaction kinetics under high-temperature CMAS environments.

High-entropy (Eu_0.2Gd_0.2Er_0.2Tm_0.2Yb_0.2)₂Zr₂O₇ (EGETYZ) powder was synthesized via solid-state reaction and subsequent spray granulation to produce spherical particles suitable for APS. Coatings were applied on nickel-based alloy substrates with varying spraying distances (i.e., 80, 100 mm and 120 mm) to optimize the microstructure. The CMAS corrosion tests were conducted at different temperatures (i.e., 1300, 1400 ℃ and 1500 ℃) with a synthetic CMAS composition (33CaO-9MgO-13AlO_1.5-45SiO₂). The phase composition was analyzed by X-ray diffraction (XRD), the surface roughness was determined by laser confocal microscopy (LCM), and the microstructure/elemental distribution was characterized by scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS).

The results show that the high-entropy zirconate coating exhibits a defect-fluorite structure with a minimal surface roughness (2.72 μm) and a low porosity (10.10%) at an 80 mm spraying distance. At 1300 ℃, the CMAS corrosion forms a dense reaction layer of apatite and fluorite, effectively limiting a penetration (i.e., 19.7 μm after 5 h). At 1400 ℃, a three-layer reaction structure emerges, with an increased penetration (i.e., 95.2 μm). At 1500 ℃, a penetration deepeneds significantly (i.e., 321.1 μm), with sparse apatite and prevalent gehlenite and transitional phases. Higher temperatures reduce the CMAS viscosity (by up to 77.57%), accelerating infiltration along coating defects. The dense reaction layer at lower temperatures is a key to corrosion resistance, while a rapid CMAS penetration at higher temperatures hinders its formation.

The EGETYZ coating exhibited the superior resistance to the CMAS corrosion. The corrosion temperature directly affected the viscosity change of CMAS, thereby exerting a significant influence on the corrosion behavior and reaction products. At 1300 ℃, the reaction layer was mainly composed of fluorite and apatite, forming a dense structure that effectively inhibited the CMAS infiltration. At 1400 ℃, the EGETYZ at the bottom of the reaction layer was not completely melted. The elements Al/Mg enriched in the gaps, in synergy with elements Ca/Si/RE, form a rare-earth gehlenite solid solution, resulting in a three-layer structure of the reaction layer (i.e., pure fluorite at the top, fluorite and apatite in the middle, and gehlenite-containing phase at the bottom). At 1500 ℃, only a minimal amount of apatite phase appeared, the degree of EGETYZ melting was relatively low, and it was not completely transformed into the fluorite phase, with the gehlenite phase distributing more extensively. To enhance the CMAS corrosion resistance of high-entropy rare-earth zirconate thermal barrier coatings, the key could lie in promoting the reaction with the CMAS to form a continuous and dense reaction layer. All the formed reaction products could effectively consume the CMAS components.

Exhaled breath analysis technology enables the preliminary diagnosis of certain diseases via detecting the concentration of volatile organic compounds (VOCs) in human breath. Some studies indicate that acetone can serve as a biomarker to assist in the diagnosis of diabetes. Developing acetone gas sensors with low detection limits and high selectivity is thus crucial for achieving early screening of diabetes. In recent years, spinel ferrites, as an important class of metal oxides, have attracted much attention in the gas sensing field due to their low cost and excellent stability. Among them, ZnFe₂O₄ can improve the number of active sites on the material surface via tailoring its microstructure, thereby exhibiting a high sensitivity to VOCs like acetone. However, single-component ZnFe₂O₄ gas sensors suffer from issues such as high operating temperatures and low gas-sensing selectivity in practical detection. The existing research shows that ion doping is a modification method that induces lattice distortion in metal oxides, thereby regulating surface defects on the material. The generation of surface defects can provide more active centers for gas-sensing reactions and promote the adsorption of oxygen molecules on the material surface, thus improving its gas-sensing properties. Furthermore, doping rare-earth elements into MOS materials can enhance their catalytic activity and increase electron migration rates, leading to an improved sensing performance. This study was to prepare a series of La³⁺-doped ZnFe₂O₄ composites via electrospinning. Their morphology, surface chemical composition, and elemental valence states were characterized, and the gas-sensing properties of the composites were analyzed.

0.808 g Fe(NO₃)₃·9H₂O (AR, Sinopharm Chemical Reagent Co., Ltd., China), 0.297 g Zn(NO₃)₂·6H₂O (AR, Sinopharm Chemical Reagent Co., Ltd., China), and varying mass ratios of La(NO₃)₃·nH₂O (AR, Sinopharm Chemical Reagent Co., Ltd., China) were accurately weighed. Each mixture was put into respective 10 mL beakers, add 5 mL of DMF (AR, Sinopharm Chemical Reagent Co., Ltd., China), and stir to dissolve. 0.85 g of PVP (AR, Shanghai Aladdin Biochemical Technology Co., Ltd., China) was added slowly to each solution and continue stirring for over 12 h until complete dissolution of PVP. After stirring no visible bubbles remained in the solution. The solutions were transferred into 10 mL syringes and mounted onto a horizontal electrospinning apparatus. The needle tip was set as an anode (11-13 kV) and the aluminum foil collector as a cathode, with an electrode distance of 15 cm. The ambient humidity was maintained at 30%-50% and the solution was fed at a rate of 3 μL/min. The electrospun fibers collected were calcined in a muffle furnace at 600 ℃ for 2 h (at a heating rate of 1 ℃/min). The phase composition was analyzed by X-ray diffraction (XRD, Bruker D8 Advance, 40 kV, 40 mA, Cu-K_α, 5(°)/min, 10°-80°). The microstructure of the samples was determined by scanning electron microscope (SEM) with an accelerating voltage of 15 kV. The elemental valence states of the materials were analyzed by a X-ray photoelectron spectroscopy (XPS, Al K_α, hν=1486 eV).

For the determination of the gas-sensitive properties of La doping ZnFe₂O₄ composite material, the sample was placed in a mortar, and 1-2 drops of turpentine penetration alcohol were added. The mixture was ground until a uniform paste was formed. The paste was then evenly applied onto the surface of an alumina ceramic tube using a small brush. Afterwards, a nickel-chromium heating wire was passed through the ceramic tube and welded to the base of the gas sensor. Finally, the gas sensor was aged at 200 ℃ for 12 h before conducting gas-sensing performance tests.

The XRD patterns show that the grain size is significantly reduced and the gas-sensing performance is improved, indicating that La³⁺ doping inhibits the crystallization process and hinders crystal growth. A reduction in grain size leads to an increase in the specific surface area of the sensor, providing more acetone active sites on the surface of the material. The SEM images show that the surface of La³⁺-doped ZnFe₂O₄ nanomaterials is relatively smooth and cross-linked to form a network structure. The EDS results indicate that the composite material is composed of elements La, Zn, Fe, and O. According to the XPS analysis, La³⁺doping causes a lattice distortion, resulting in an increase in adsorbed oxygen on the material surface. The more oxygen is adsorbed on the surface of the material, which in turn improves the gas-sensing performance. Based on the analysis of UV-visible absorption spectroscopy and the corresponding band gap, the sample LZF-8 has a narrower band gap, indicating that the energy required for electron transition is lower, which is beneficial to electron migration, thereby shortening the response/recovery time. The response value of sample LZF-8 to 0.1 g/L acetone at room temperature is 21.3, which is 11.5 times greater than that of pure ZnFe₂O₄. Meanwhile, the selectivity and sensitivity of the material to different gases at room temperature are investigated. The results show that the sample LZF-8 has good selectivity and detection limit for acetone, with the minimum detection limit of 0.00001 g/L, proving that the prepared sensor LZF-8 has a great application potential.

ZnFe₂O₄ composites doped with different proportions of La³⁺ were prepared via electrospinning. Based on the analysis of the XRD patterns, SEM images, and XPS spectra, La³⁺ was successfully doped into ZnFe₂O₄. The results of gas-sensing test showed that as 8% La³⁺ was doped, the response value (i.e., R_a/R_g = 21.3) to 0.1 g/L acetone at room temperature was approximately 11.5 times greater than that of pure ZnFe₂O₄ (R_a/R_g = 1.7), with the response/recovery time of 27.0 s/10.7 s, and the minimum detection limit of acetone concentration of 0.00001 g/L. This composite gas sensor could have a great potential for efficient detection of acetone gas under room temperature conditions.

Cutting-edge technologies such as hypersonic vehicles and reusable launch systems impose stringent requirements on thermal protection system, demanding for ultra-lightweight materials with extremely low thermal conductivity and high-temperature stability. Conventional SiO₂ aerogels suffer from rapid sintering and densification at elevated temperatures, leading to pore collapse and decline in thermal insulation performance. Although introducing high-melting-point oxides or increasing particle size can delay the sintering process, it often results in a trade-off between thermal stability and insulation performance. To overcome this bottleneck, a hollow-core-shell synergistic design strategy is proposed. This approach is expected to simultaneously maintain low density and low thermal conductivity, while significantly enhancing the material's thermal stability via coupling the effects of micro-nano pore regulation and interfacial phonon scattering. In this work, the influence of Al₂O₃ coating thickness on the density, thermal conductivity, and thermal stability of large hollow particles of SiO₂ coated with Al₂O₃ (SiO₂@Al₂O₃) was investigated. The evolution behavior of this composite structure at high temperatures was elucidated, and the mechanism by which the Al₂O₃ shell could suppress a liquid-like atomic diffusion was revealed, providing some insights and theoretical guidance for the development of next-generation high-performance thermal insulation materials.

A hard-template-assisted precipitation method was employed to synthesize SiO₂@Al₂O₃ hollow particles. Polystyrene microspheres were used as templates, on which SiO₂ was deposited to form large hollow structures. After calcination to remove the PS cores, SiO2 hollow particles were obtained. The SiO₂ particles were dispersed in an aqueous phase, and Al(OH)₃ was in-situ precipitated on their surfaces via the dropwise addition of AlCl₃ and NaOH solutions with cetyltrimethyl ammonium bromide (CTAB) as a dispersant. Subsequent high-temperature calcination converted the hydroxide layer into an Al₂O₃ coating, yielding SiO₂@Al₂O₃ core-shell hollow particles. A series of samples with different Al/Si mass ratios (i.e., 1%, 5%, and 10%) were prepared via adjusting the amount of AlCl₃. The morphological evolution of the materials at 25-1200 ℃ was monitored in real time by in-situ transmission electron microscopy. The effective thermal conductivity of the samples at 25-1100 ℃ was measured by a Hot Disk thermal constants analyzer and the laser flash method. Furthermore, the molecular dynamics simulations were performed by Large-scale Atomic/Molecular Massively Parallel Simulator (LAMMPS), and a "liquid-like atom" cluster identification algorithm was applied to reveal the mechanism by which the Al₂O₃ shell could suppress surface diffusion and particle sintering in an atomic scale.

The results of in-situ TEM sintering experiments reveal that uncoated SiO₂ hollow particles exhibit significant shrinkage and rupture at 1200 ℃ for only 2 min. In contrast, the sample coated with 3.9% Al₂O₃ (the sample with Al/Si of 5%) maintains its hollow structure after 30 min under the same conditions, with a linear shrinkage ratio of below 9%. However, the excessive coating (e.g., 10%) leads to the formation of a continuous Al₂O₃ layer between particles, increasing the effective contact area and promoting high-temperature mass transport and densification. The thermal property measurements further demonstrate that the 3.9% Al₂O₃-coated sample exhibits an ultralow effective thermal conductivity of 0.036 W·m^-1·K^-1 at 25 ℃, and a moderate increase to 0.074 W·m^-1·K^-1 at 1100 ℃, indicating the excellent high-temperature thermal insulation performance. The initial density of this sample is 0.268 6 g·cm^-3, which is 14% lower than that of SiO₂ solid nanoparticles with the size of 10 nm and shows a minimal variation (i.e., 0.268 6-0.298 8 g·cm^-3) at 25-1200 ℃. Its effective thermal conductivity remains below that of various fiber-reinforced aerogel systems in the literature. The molecular dynamics simulations based on LAMMPS reveal the atomic-scale mechanism underlying the high-temperature stability of the Al₂O₃ shell. During heating, the number of "liquid-like atom" clusters increases continuously, while effectively suppressing their size growthes. This behavior significantly delays the kinetics of cluster coalescence, elevating the surface fusion completion temperature from approximately 1000 K in the uncoated system to 1500 K. The Al₂O₃ shell effectively delays the transition from surface diffusion-dominated sintering to volume diffusion-dominated sintering, providing the related mechanism and theoretical foundation for overcoming the intrinsic trade-off between low thermal conductivity and high-temperature stability.

This study proposed a "large hollow structure with Al₂O₃ shell" synergistic design, enabling the controllable fabrication of SiO₂@Al₂O₃ particles that could combine a structural stability at 1200 ℃ with ultralow thermal conductivity and low density. At 3.9% Al₂O₃ coating, the particles maintained a shrinkage ratio of below 9% after 30-min high-temperature exposure. The effective thermal conductivity increased modestly from 0.036 W·m^-1·K^-1 at 25 ℃ to 0.074 W·m^-1·K^-1 at 1100 ℃, while the density was 14% lower than that of conventional SiO₂ solid nanoparticles with the size of 10 nm. The in-situ TEM spectra and molecular dynamics simulations confirmed that the Al₂O₃ shell significantly delayed sintering via suppressing cluster coalescence and the surface diffusion of liquid-like atoms, thus elucidating the high-temperature stabilization mechanism across multiple scales. This strategy could provide both experimental and theoretical foundations for controllable preparation of lightweight and high-temperature thermal insulating materials, offering a substantial engineering value for thermal protection in extreme environments and for additive manufacturing of complex lightweight thermal structures.

ZrB₂ is considered as a crucial candidate material for thermal protection systems in hypersonic vehicles due to its excellent high melting point and ablation resistance. However, the brittleness and poor thermal shock resistance severely limit practical application. To enhance the performance, the introduction of continuous carbon fiber-reinforced ZrB₂ (i.e., C_f/C-ZrB₂) composites becomes a research hotspot. Needle-punched carbon fiber preforms exhibit a promising reinforcement potential due to their flexible structural design and low preparation cost. To address the drawback of long processing cycles in conventional preparation methods, this study employed injection-assisted vacuum impregnation (IVI) to fabricate C_f/C-ZrB₂ composites. The as-fabricated composites subsequently underwent carbon densification treatment via chemical vapor infiltration (CVI). The anisotropic characteristics of mechanical and thermal conductivity properties were investigated from multiple fiber orientation angles before and after the CVI treatment.

ZrB₂ powder, phenolic resin, and acetone were mixed to prepare a ceramic-resin slurry of 20% solid content. This slurry was injected into a needle-punched carbon fiber preform with the sizes of 20 mm×20 mm×20 mm. Subsequently, two rounds of vacuum impregnation, drying, and pyrolysis at 800 ℃ were performed to obtain the C_f/C-ZrB₂ composite. Carbon densification treatment was then conducted via chemical vapor infiltration (CVI) with CH₄ as a carbon source and Ar as a carrier gas at 1000 ℃ for 144 h, yielding the CVI-C_f/C-ZrB₂ composite material. The macroscopic microstructure and 3D morphology of the composites were characterized by a model VHX-700 white light ultra-deep microscope (Keyence Co., Japan). The detailed microstructural features were analyzed by a model SU8230 cold field emission scanning electron microscope (Hitachi Co., Japan). To analyze the anisotropy of the material properties, the samples were cut at angles of 0°, 30°, 45°, 60° and 90° relative to the plane XY of the carbon fiber felt. The compressive strength was measured for the cuboid shaped samples with dimensions of 10 mm (width)×10 mm (length)×15 mm (height) by a model 34TM-30 screw-driven test frame (Instron Co., USA). The thermal diffusivity was determined by a model LFA 467 laser flash instrument (Netzsch Co., Germany) in a temperature range from room temperature to 900 ℃ on the samples with 10 mm (width)×10 mm (length)×1 mm (height).

The C_f/C-ZrB₂ composites prepared by IVI exhibit a non-uniform internal pore distribution, with a pore volume of (10⁴-10⁶ μm³), concentrated primarily in fiber net felt and interlaminar region. After the CVI treatment, approximately 17.16% of pores are filled with pyrolytic carbon, reducing pore volume and increasing material density from 1.8 g/cm³ to 2.1 g/cm³. The microstructural analysis reveals that pyrolytic carbon grows in spherical crown-shaped configurations, gradually forming continuous layered coatings that fill material pores. Regarding the compressive properties, the samples at 0° and 90° orientations exhibit higher Young's moduli ((2.52±0.99) GPa and (3.80±1.03) GPa, respectively) and compressive strengths ((41.62±9.60) MPa and (41.50±5.18) MPa). The samples at 60° demonstrate the lowest modulus ((0.41±0.08) GPa) and strength ((8.47±1.44) MPa). The intermediate-angle samples show a reduced performance due to shear stress concentration at inter-layer pores. For the CVI carbon densification, the mechanical properties significantly improve in all orientations. The samples at 90° achieve Young's modulus of (10.79±2.21) GPa and compressive strength of (155.98±39.22) MPa, while the samples at 0° reach (6.76±0.82) GPa and (130.09±13.19) MPa. Although the intermediate angles show a substantial improvement, they maintain relatively low values due to a incomplete densification of inter-layer pores. The results of thermal conductivity test indicate that prior to CVI, the samples at 90° display a thermal diffusivity of 1.40 mm²/s and a thermal conductivity of 1.57 W/(m·K) at 25 ℃, significantly exceeding those of the samples at 0° Post-CVI, pyrolytic carbon enhances thermal pathways. The conductivity of the samples at 90° increases to 3.48 W/(m·K) and 0° to 2.37 W/(m·K), with a reduced anisotropy ratio. The samples at 30°-60° orientations also show significantly an enhanced thermal diffusivity, with a heat transfer capability approaching that of the 90° direction.

The C_f/C-ZrB₂ composites fabricated via IVI exhibited a non-uniform pore distribution. After CVI, pores were refined and partially filled, increasing material density and structural densification. The compressive properties showed a significant anisotropy. The orientations at 0° and 90° demonstrated an optimal mechanical performance, while intermediate angles suffered a substantial degradation due to the shear stress dominance and pore concentration. The CVI treatment could enhance the properties across all the orientations, with a maximum improvement at 90°. The thermal conductivity also exhibited an anisotropy, revealing a superior heat transfer efficiency along fiber-dominant directions. For the CVI process, the thermal diffusivity and conductivity both increased, accompanied by a significant reduction in thermal anisotropy ratio, as pyrolytic carbon filling improved heat transfer pathways.

Al₂O₃-C refractories are used as essential structural materials in continuous casting processes, particularly in ladle shrouds that guide molten steel from the ladle to the tundish These components are subjected to extreme thermo-mechanical environments, including cyclic thermal shocks, severe oxidation, and aggressive slag/steel corrosion, inevitably leading to premature failure. Conventional studies mainly emphasized optimizing composition to improve oxidation resistance or thermal shock stability. The dynamic evolution of thermo-mechanical properties under realistic service conditions remains insufficiently understood. In specific, the time- and temperature-dependent performance degradation or strengthening, as well as fracture behaviours under repeated casting cycles, are rarely addressed systematically. Such a understanding is critical for extending shroud service life and improving casting stability and product quality. In this context, this study investigated the phase evolution, microstructural transformation, and fracture behaviours of Al₂O₃-C refractories extracted from post-mortem ladle shrouds that experienced different service cycles in both submerged and non-submerged regions. The work further validated the feasibility of using the single-edge notched bending (SENB) method for evaluating fracture toughness in refractories with limited sample sizes, in comparison with the wedge splitting test (WST) and conventional three-point bending test (3PBT).

Representative specimens were cut from both submerged (B) and non-submerged (N) regions of post-mortem ladle shrouds after 1, 5, and 10 service cycles. The compositions differed slightly, with Si powder incorporated into N regions to counter oxidation, while B regions contained trace silica additions to improve thermal shock resistance. The phase compositions of samples N and B were analyzed by X-ray diffraction (XRD). The microstructural features were determined by scanning electron microscopy equipped with an energy-dispersive spectroscopy (SEM-EDS). The mechanical performance was assessed through three complementary approaches, i.e., 1) three-point bending tests to evaluate flexural strength evolution; 2) wedge splitting tests for reference fracture parameters, and 3) single-edge notched bending tests with digital image correlation (DIC, a software named Match ID 2D) to monitor stable crack propagation and determine fracture energy, nominal tensile strength, and the critical stress intensity factor K_IC. The SENB geometry (125 mm×15 mm×15 mm, 3.5 mm notch, 80 mm span) was tailored for shroud walls with thickness limitations. The DIC provided full-field strain maps, enabling the visualization of crack initiation and propagation.

The results reveal a distinct dependence of phase evolution and fracture behaviours on service position and thermal cycles. In non-submerged specimens, the oxidation of metallic Si progressively generates dense SiC whiskers during thermal service, strengthening the microstructure through crack-bridging and densification. This process is accompanied via quartz crystallization at -1050 ℃, with further SiC formation at >1400 ℃. Consequently, the flexural strength nearly doubles, and SENB-derived fracture toughness (K_IC) increases from 8.48 to 15.04 MPa·mm½ with increasing service cycles (1T→10T). However, the ratio G/σ_t, representing brittleness, declines, indicating a trade-off between strength enhancement and increasing brittleness. In contrast, submerged specimens experience more severe thermal gradients (at 1500-1600 ℃) and cyclic shock from molten steel contact. A low silica content limits SiC formation, while the crystallization and phase transformation of fused silica (low- to high-cristobalite) induce microcracking and volume mismatch. This "roof-tile structure" generates stress concentrations and progressive damage. As a result, the flexural strength of submerged samples shows a non-linear trend. It initially increases after one thermal cycle due to sintering and slight densification, then declines during extended service. At the 10th cycle, the strength decreases below that of non-submerged specimens, while K_IC also decreases, highlighting a performance degradation under harsher conditions. The comparisons of fracture test methods indicate consistent fracture energy values between SENB and WST, confirming SENB as a reliable alternative when limiting the sample dimensions. Unlike 3PBT, which causes an abrupt brittle failure, SENB and WST both enable a stable post-peak crack propagation. The full-field DIC strain maps further confirm a gradual crack evolution, with non-submerged samples showing straighter, sharper cracks due to the higher brittleness, while submerged specimens display broader strain fields and more tortuous crack paths. This highlights the ability of SENB coupled with DIC to capture subtle fracture mechanisms in quasi-brittle refractories.

Overall, the findings indicate that thermo-mechanical property evolution is governed by a competition between strengthening mechanisms (i.e., SiC whisker formation, sintering densification) and damage mechanisms (i.e., silica crystallization, microcracking, oxidation). The relative dominance of these processes depends strongly on service position and thermal cycles. While non-submerged materials gain the strength over time, their rising brittleness concerns a catastrophic failure if used beyond safe limits. Submerged materials, despite initial improvements, exhibit a progressive weakening, underlining the necessity of microstructural design strategies tailored to each service environment.

The single-edge notched bending test, validated against wedge splitting and three-point bending methods, could be an effective approach for characterizing fracture toughness of post-mortem shrouds with a limited sample size, thus enabling stable crack growth monitoring. The thermo-mechanical performance of Al₂O₃-C refractories showed a pronounced dependence on service time, temperature, and position, with property evolution closely linked to oxidation-driven SiC formation and fused silica crystallization. In non-submerged regions, the flexural strength and fracture toughness increased with service cycles, and the brittleness increased as well, significantly elevating a risk of sudden fracture if service continued beyond the strength limit. In contrast, submerged regions exhibited a non-linear evolution of properties, i.e., the mechanical performance peaked during early service cycles due to densification, but then declined with increasing cycles, thus leading to silica crystallization and microcracking. These insights could provide a scientific basis for tailoring composition and microstructure to enhance the service life of Al₂O₃-C refractories in continuous casting operations.

The existing domestic research and applications of SiC_f/SiC composites on static parts of turbine engines is not mature yet, and that on rotor parts is still in a preliminary research stage. For instance, the working conditions for the rotor blade are extremely harsh and its configuration is complex, which make the component structure design a core and critical technology. The turbine rotor blade must ensure the structural integrity of the blade body and tenon, as well as the continuity of the fiber in the bearing direction due to the huge centrifugal mechanical load, thereby leveraging a high tensile strength of the fibers. The structural design of turbine rotor thus must consider the characteristics of composite materials, forming a structure/material integration design method.

The upper limit of long-term service temperature for SiC_f/SiC composites based on the third-generation SiC fibers is approximately 1400 ℃. However, the wall temperature requirements for the combustion chambers of the new generation of aero-engines exceed 1600 ℃. To ensure the long-term service reliability of SiC_f/SiC composite hot-end components in high-temperature gas and corrosive environments, the design and preparation of high-quality protective coatings become one of the key breakthroughs in the technological iteration of aero-engine research and development. The development of protective coatings for the hot-end components of the new generation of aero-engines must rely on advanced technologies such as efficient screening and design of new multi-functional coating materials, innovative coating structure design and precise control of coating microstructure, large-scale preparation of coating feedstock powders, and performance evaluation and failure control of coating systems under typical service environments.

In summary, there remains a significant gap between domestic SiC_f/SiC composite materials and protective coatings, as compared with those of the Western developed countries such as the United States. There is an urgent need to systematically and progressively develop the application technology for ceramic matrix composite materials suitable for aero engines, accelerate the establishment of technical reserves, and provide competitive hot-end components materials and technological support for the high-performance aero engines in China.

This review systematically represents recent advances on the sol-gel synthesis of high-performance continuous alumina fibers. The analysis reveals that a precise control of microstructural characteristics, such as grain size, phase composition, and grain boundary properties, can profoundly affects the mechanical properties and high-temperature performance of the fibers. However, some substantial challenges persist in understanding the mechanisms of alumina fiber formation, achieving a precise process control, and enhancing performance in extreme environments. Accordingly, future research should focus on the following directions:

1) Transitioning from empirical trial-and-error to theoretically guided design via developing multiscale simulation techniques to inform holistic process optimization during fiber fabrication. Computational simulations can uncover underlying mechanisms to enable theory-driven fiber design. In an atomic scale, first-principles calculations and molecular dynamics simulations can reveal grain boundary segregation behavior of sintering aids such as rare-earth oxides. molecular dynamics can also simulate atomic rearrangement during α-Al₂O₃ phase transformation, clarifying heterogeneous nucleation mechanisms induced by iron sol additions. In a mesoscale, phase-field modeling can dynamically simulate densification-grain growth competition, grain boundary pinning by secondary phases, and defect evolution. In a macroscale, the fiber service failure can be simulated to correlate predicted microstructures to mechanical properties.

2) Advancing from static cross-sectional observation to dynamic in-situ characterization via developing advanced in-situ techniques to directly elucidate fiber microstructural evolution mechanisms. The existing understanding of microstructural evolution relies largely on static examination of samples from different processing stages, rendering direct validation difficult and many conclusions speculative. Future work should emphasize in-situ experimental methods applicable to both the preparation and service of alumina fibers, enabling the real-time observation of microstructural evolution.

3) Bridging a gap between laboratory samples and industrial products via addressing challenges in continuity, stability, and cost efficiency in fiber manufacturing. Future efforts should focus on developing online monitoring and feedback systems for precursor sols to maintain viscosity and spinnability, advancing intelligent temperature and tension control to minimize fiber bending damage while collecting real-time data on the effects of thermal and tensile variations and exploring low-cost alternative aluminium sources paired with optimized sintering protocols to reduce energy consumption.

4) Addressing next-generation application requirements via investigating fiber performance limits under ultra-high-temperature, long-duration service conditions and developing novel reinforcement strategies. Systematic studies should be conducted on creep mechanism transitions, grain boundary evolution, and the performance degradation mechanisms in high-temperature environments such as steam and air.

This review systematically compares the advantages and disadvantages of various synthetic methods. The existing challenges such as complex synthesis processes, high production costs, significant impurity content in products, non-uniform morphology, and a tendency toward particle agglomeration remain prominent. Exploring low-cost silicon and organic carbon sources with effective impurity removal capabilities and tunable morphological properties thus represents a key direction for future research and development. Furthermore, the construction of core-shell and other structures can solve a problem of agglomeration. Consequently, advancing composite phase preparation technologies and designing sophisticated nanostructures, including core-shell configurations, are anticipated to be critical future developments. Elucidating the influence of key parameters (i.e., gas-phase partial pressure ratios) on the growth behavior of one-dimensional SiC materials, and establishing a process-morphology correlation model through molecular dynamics simulations can enable a precise control over product characteristics and hold a substantial potential for future applications.