Journal of the American Ceramic Society最新文献

Tungsten disilicide (WSi₂) is a promising material for high-temperature applications due to its excellent mechanical properties and superior thermal stability. This study aims to investigate the mechanisms underlying the performance enhancement of polycrystalline WSi₂ and tungsten silicide (W-Si) composites synthesized by high-temperature and high-pressure (HPHT) strategies, focusing on their phase composition and microstructure. The results show that high pressure can effectively reduce the synthesis temperature of the tungsten silicide system, with the phase composition and microstructure of the products being dominated by the treatment temperature. The high intergranular strain resulting from the reactive process and the HPHT environment facilitated the formation of high-density dislocations, including dislocation arrays and networks with multiple orientations, especially near the grain boundaries. Additionally, a 3-nm thick amorphous ribbons were identified between these boundaries. This microstructure, characterized by high-density dislocations and amorphous ribbons, confers exceptional mechanical properties and oxidation resistance. The differences in oxidation behavior between powder and bulk WSi₂ samples underscore the complex relationship between phase stability and material properties under varying processing conditions. These findings indicate that dislocations and amorphous ribbons can achieve in high-temperature ceramics with strong covalent bonding, which is essential for the design and synthesis of advanced W-Si-based ceramics.

Sintering and phase transitions of materials in current large-area thermal insulation systems are critical problems for hypersonic vehicles. Searching for thermal insulation materials with good structure stability, thermal stability, and superior insulation performance is urgent for the development of these vehicles. Herein, we reported a novel porous BaZrO₃ ceramic fabricated by a particle-stabilized foam method. By controlling the sintering temperature, the porosity (92.96%–94.62%), thermal conductivity (0.088–0.193 W/(m·K)), and strength (0.30–1.31 MPa) of the porous ceramics are tunable. Most importantly, these porous BaZrO₃ ceramics exhibit excellent sintering resistance. After being heat treated at 1300°C for 6 h, the shrinkage of porous BaZrO₃ ceramics is nearly 0. The shrinkage of the samples heat-treated even at 1600°C is still lower than 3%, demonstrating outstanding high-temperature structure stability. Our results demonstrate that porous BaZrO₃ ceramics are promising candidates for high-temperature thermal insulation applications.

To enhance the ablation resistance of porous C_sf/MA–SiBCN composites, the polymer infiltration and pyrolysis (PIP) process is adopted to obtain layered C_sf/SiBCN composites containing MA–SiBCN and PDCs–SiBCN. After ablation for 10 s, the ablative layer fails to remain on the C_sf/MA–SiBCN surface with mass ablation rate of 0.062 g/s and linear ablation rate of 0.208 mm/s, while the dense C_sf/SiBCN surface is covered by a dense and stable ablative layer mainly consisting of amorphous SiO₂ glassy phase due to the synergistic ablation effect of MA–SiBCN and PDCs–SiBCN, which could resist the intense scour by ablation flame and prevent internal corrosion, and thus the ablation resistance improves with a 37% decrease in mass ablation rate (0.039 g/s) and an 18% decrease in linear ablation rate (0.171 mm/s), respectively.

Metal vanadates have garnered widespread attention across various scientific fields due to their exceptional performance. In this work, we investigate the lattice dynamics and thermophysical properties of barium orthovanadate (Ba₃V₂O₈) using first-principles calculations. We provide reliable thermodynamic data, including entropy and heat capacity across a broad temperature range. The complex crystal structure of Ba₃V₂O₈ leads to strong phonon anharmonicity, which greatly suppresses the contribution of propagative phonons to the lattice thermal conductivity (κ_L). Moreover, utilizing the unified theory, we find that the significant contribution of coherent phonon transport makes κ_L exhibit low-temperature dependence. Notably, coherent phonons along the a-axis begin to exceed the contribution of propagative phonons at around 400 K and reach a high proportion of 71.9% at 800 K, leading to an anomalous change in κ_L. This research emphasizes the significant impact of strong anharmonicity and coherent phonon channels on the thermal transport process in Ba₃V₂O₈, providing a crucial perspective for predicting and understanding the thermophysical properties of metal vanadates and contributing to the further regulation of the thermal transport characteristics.

Utilizing γ-Al₂O₃, MgO, and black carbon powders as starting materials, single-phase magnesium aluminum oxynitride (MgAlON) powders with excellent sinterability were successfully synthesized by carbothermal reduction and nitridation (CRN) method, and the CRN synthesis mechanism was investigated through analyzing phase assemblage and morphology of the powders during heating. As heating to 1700°C, single-phase MgAlON is obtained, while its O/N ratio can be adjusted by controlling the composition of starting mixture and the CRN process. The MgAlON powders with O/N ratio of 9.52–11.39 (using the mixture with 8.6 wt.% MgO and 5.2 wt.% carbon black, and prepared at 1700°C for 90–150 min or 1720°C for 120 min) exhibit high sinterability. These powders were successfully fast pressureless sintered into ceramics with high transmittance (>80%@3750 nm) at 1880°C for 2.5 h. During the heating process for preparing MgAlON powder, magnesium aluminate spinel (MAS) is first formed at <1500°C, then at 1500–1600°C, along with formation of AlN via CRN and its instantaneous solid solution with MAS, MgAlON is obtained. The phase assemblage and morphology comparison between the Al₂O₃–MgO–C, Al₂O₃–MgO, and Al₂O₃–C powders during heating indicates that carbon and MAS act in concert to inhibit the growth of Al₂O₃, which contribute to the rapid formation MgAlON and high sinterability of prepared MgAlON powders.

Germanate ceramics Li₂ZnGeO₄:Nd³⁺ and Li₂MgGeO₄:Nd³⁺ have been studied for near-infrared (IR) luminescence applications. The phase identification showed that both ceramic compounds crystallized in a monoclinic lattice, characteristic of a broad family of germanate olivines. Near-IR luminescence spectra have been analyzed for Li₂AGeO₄:Nd³⁺ (A = Zn, Mg) under different excitation wavelengths. Near-IR emission bands observed in the second biological window correspond to the ceramic host as well as ⁴F_3/2 → ⁴I_11/2 and ⁴F_3/2 → ⁴I_13/2 transitions of Nd³⁺ ions, respectively. Based on luminescence spectra and their decays, several spectroscopic parameters for un-doped and Nd³⁺-doped ceramic samples were determined. The experimental results for germanate olivines containing Nd³⁺ ions suggest that ceramic host gives important contribution to near-IR luminescence at second biological window.

Geopolymers (GP) represent a promising class of inorganic materials with diverse applications due to their properties, including high temperature resistance and strong interfacial bonding ability. They are produced through alkali activation of aluminosilicate sources, such as metakaolin or fly ashes. Despite their attractive characteristics, conventional casting methods for GP production often result in prolonged curing times and inferior mechanical properties to OPC or other benchmark materials. In this study, we investigated the feasibility of rapidly densifying GP matrices using cold sintering technology (CSP), a novel approach previously employed in ceramic systems. Through CSP, it was possible to obtain a dense body starting from GP sodium-based powder with optimal moisture content (10% wt.) under mild isostatic pressure (70 MPa) and moderate temperature (150°C) conditions, with a short duration process (10 min). The resulting products exhibited chemical stability (high resistance to boiling test), high density (> 90% theoretical density) and good mechanical properties (flexural strength equal to 30 MPa and compressive strength over 200 MPa) without requiring additional thermal treatments. SEM, EDS and NMR studies indicated that the predominant densification mechanism was likely to be homogeneous dissolutions and precipitation of the material, consistent with pressure solution creep. Dilatometric tests were performed to track the densification process in real-time and to determine the activation energy, which revealed an exceptionally low value for the system (21.7 kJ/mol). Our results demonstrate the potential of CSP as a rapid and efficient method for producing high-quality GP-based components, paving the way for their broader application in various fields.

A uniform, dense, and well-bonded mullite/Y₂Si₂O₇/ZrO₂(MYZ) composite barrier coating was synthesized using a spraying technique followed by a two-step sintering process. The microstructure, phase composition, and thermal shock resistance of the MYZ coating were systematically investigated. The sintering process comprised two stages. In the initial stage (25–1350°C), granular mullite grains transformed into columnar crystals with progressive growth. Concurrently, a eutectic composite of mullite and Y₂Si₂O₇ formed and dispersed within the grain boundaries, significantly enhancing the coating's density and adhesion. In the subsequent stage (∼1400°C), ZrO₂ reacted with the eutectic composite to produce yttria-stabilized zirconia (YSZ) in situ. The YSZ phase exhibited excellent interfacial compatibility with the substrate, markedly improving the coating's high-temperature stability. As a result, the coating achieved high hardness and a coefficient of thermal expansion matching that of the SiC substrate. Remarkably, the MYZ coating withstood 20 thermal cycles between 25°C and 1200°C without noticeable interfacial damage.

The structural integrity of continuous carbon fiber-reinforced silicon carbide (C_f/SiC) composites is often compromised by carbon fiber damage during high-temperature sintering, nonuniform fiber distribution, and the brittleness of the ceramic matrix, which limits their broader application. This study reported an innovative approach of vacuum bagging combined with thermal press curing, followed by precursor infiltration and pyrolysis (PIP) and reaction melt infiltration (RMI), densification to fabricate a C_f/SiC composite with remarkable properties. The pyrolytic carbon (PyC)/boron nitride (BN) double interface layers were deposited to the C_f via chemical vapor deposition (CVD). Results revealed that vacuum bagging combined with thermal press curing effectively eliminated bond-line defects, while the PyC/BN interface layer further improved the toughening of C_f. The combination of vacuum bagging and thermal press curing showed optimal strength of 330 ± 30 MPa and 7.4 ± 0.5 MPa m^1/2, representing increases of 50% and 47%, respectively. The failure mechanisms responsible for the mechanical behavior are gained through macro and microanalysis. This work provides a high-performance, economical, and convenient process for enhancing the mechanical performance of C_f/SiC composites, providing valuable insights for the development of ceramic matrix composites.

Relaxor films constructed by doping point defects are widely applied in various fields, including nanoelectromechanical systems, capacitive energy storage, and pyroelectric energy conversion. Despite their broad utility, the underlying mechanisms by which point defects affect the dielectric properties of these films under varying substrate strains remain insufficiently understood. This work employs a phase–field model to explore the influence of point defects on the domain structure and dielectric properties of BaTiO₃ and Pb(Zr,Ti)O₃ films, with a comparative analysis of their respective responses to different substrate strains. Our results reveal that the domain sizes in both BaTiO₃ and Pb(Zr,Ti)O₃ films decrease with doping, leading to a transition into a relaxor state. Notably, Pb(Zr,Ti)O₃ exhibits a dielectric peak at a lower doping concentration and a more pronounced reduction in dielectric constant, which can be attributed to its smaller domain size and greater susceptibility to phase transitions. As substrate strain increases from −4% to 4%, the dielectric constant initially rises, peaking at zero strain. Moreover, compared with Pb(Zr,Ti)O₃, the BaTiO₃ relaxor films display a higher dielectric constant, due to a larger proportion of noninitial phases and a more uniform phase structure. These findings provide valuable theoretical insights into the manipulation of substrate strain as a strategy to tailor the dielectric properties of relaxor films.