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.

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.