Journal of the American Ceramic Society最新文献

Lu_0.2Sc_0.8Fe_1‒xMn_xO₃ (LSFMO) ceramics were synthesized via aerodynamic levitation method, and their magnetic and dielectric properties were systematically characterized. All samples are crystallized in a hexagonal structure and exhibit a dense, layered morphology with no detectable pores and a uniform elemental distribution. X-ray photoelectron spectroscopy analysis revealed that the lattice oxygen (O_L) and non-lattice oxygen (O_V) contents in the LSFMO ceramic were 71% and 29%, respectively, indicating a lower concentration of oxygen vacancies. Iron (Fe) primarily existed as Fe²⁺ and Fe³⁺, while manganese (Mn) was mainly present as Mn²⁺ and Mn³⁺, with a minor amount of Mn⁴⁺. Magnetic measurements indicate that with Mn³⁺ doping, the magnetic transition temperature of LSFMO is initially increased and subsequently decreased, reaching a maximum of 350 K, which exceeds the room-temperature limit. A moderate enhancement in the remanent magnetization is also observed. Dielectric tests reveal that with Mn³⁺ doping, the dielectric constant increases while the dielectric loss decreases, accompanied by a reduction in conductivity, effectively minimizing the leakage current of the material. The concomitant enhancement of room-temperature magnetism and low-loss dielectric behavior positions these ceramics as promising candidates for integrated multiferroic devices operating at ambient conditions.

The microstructure refinement and crystallographic orientation control in directionally solidified eutectic ceramics are critical for achieving stable and reliable material performance. This study investigates the evolution of microstructure and crystallographic texture throughout the entire directional solidification process to elucidate the seed-induced mechanism in high-entropy (Y_0.2Gd_0.2Ho_0.2Er_0.2Yb_0.2)₃Al₅O₁₂/Al₂O₃ ((5RE_0.2)AG/Al₂O₃) eutectic ceramics fabricated via optical floating zone melting method with four different seed crystals. Within the stable growth zone, (5RE_0.2)AG/Al₂O₃ ceramics exhibit an irregular “Chinese Script” morphology composed of (5RE_0.2)AG and Al₂O₃ phases. Quantitative evaluation demonstrates that the average eutectic interspacing of four high-entropy (5RE_0.2)AG/Al₂O₃ DSE ceramics in the stable growth zone ranges narrowly from 26.65 to 34.34 µm, indicating that seed crystal selection has a minimal impact on eutectic interspacing. However, the strategic implementation of seed crystal enables selective control over (5RE_0.2)AG phase orientation (<111> and <110>). The distribution of rare earth elements is critically influenced by seed-induced solidification dynamics, which are governed by distinct mechanisms dominated by nucleation processes and grain growth competition. The findings demonstrate that seed crystal selection serves as an effective approach for controlling the orientation of Al₂O₃-based eutectic ceramics, thereby establishing a robust foundation and feasibility for the design of high-performance eutectic ceramics.

Although ZrB₂‒SiC (ZS) ceramics have been extensively researched for reusable space vehicles, the gaseous Si-bearing oxides are actively yielded, which limits their service for ultra-high-temperature applications (>2000°C). In this work, medium-entropy oxide spiral fibers (MEO_sf) with a composition of Zr_0.68Y_0.07Ce_0.2Ti_0.05O_2‒δ (Zr_0.68) were developed to promote the generation of oxide scale. The MEO_sf-ZS composites demonstrated a low ablation rate of 0.27 µm/s when exposed to ten 60 s cycles of oxyacetylene flame at 2150°C. The first-principle calculations revealed that the MEO-Zr_0.68 exhibited minimum lattice distortion in the ferroelastic tetragonal (t) phase and a highly integrated crystal orbital overlap of the Ti–O bond, which contributed the good phase stability at high temperature. The enhanced ablation resistance of MEO_sf-ZS was ascribed to the outstanding t-phase stability and structural integrity of the oxide scale. The intact MEO-glassy SiO₂ oxide scale formed by capillary resistance effect of spiral geometry finally retarded the active oxidation of SiC grains.

Zirconolite-based glass-ceramics offered promising potential for immobilizing actinides due to their chemical durability and structural adaptability. In this study, Ca_1‒xCe_xZrTi_2‒2xFe_xCr_xO₇‒Na₂Al₂Si₆O₁₆ (x = 0‒0.2) glass-ceramics were synthesized at 1250°C under air atmosphere to investigate the effects of dopant concentration on zirconolite polytypism. Phase identification confirmed the direct formation of the predominant zirconolite phase in all samples, with the observed transformation from the monoclinic 2M to the trigonal 3T polytype as Ce‒Fe‒Cr content increased to x = 0.2. Detailed selected area electron diffraction and high-resolution transmission electron microscopy analysis validated the presence of zirconolite-3T polytype through characteristic d-spacings and interplanar angles, alongside the presence of stacking faults. Microstructural observations revealed homogeneous distribution of zirconolite grains within dense glass matrix, and energy dispersive X-ray spectrometry and X-ray photoelectron spectroscopy proved successful dopant incorporation and the near-equimolar Ce³⁺/Ce⁴⁺ ratio. These results indicated that the glass matrix enhanced densification at moderate temperature and facilitated polytype transformation of zirconolite, where the 2M-to-3T transition occurred at lower doping-level and temperature than in ceramic-only systems, indicative of the catalytic influence of the glass phase on zirconolite polytypism.

The internal oxidation behavior of unidirectional SiC/BN/SiC minicomposites was investigated to clarify the mechanisms governing BN interphase recession, silica scale growth, and oxidation-induced closure under dry and wet oxidizing environments at 1000°C. Using high-resolution scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses of both pristine and precracked specimens, the evolution of interfacial oxidation was examined as a function of BN coating thickness, crack opening, and environmental water content. Oxidation-induced closure occurs through multiple mechanisms, governed by crack width and environmental conditions. Four distinct closure pathways were identified: (i) silica scale formation on matrix crack surfaces leading to closure of narrow cracks, (ii) borosilicate glass accumulation filling recession gaps and matrix cracks with minimal volatilization loss, (iii) recession gap filling by silica-enriched glass under conditions of enhanced boria volatilization, and (iv) oxide buildup on the fibers and matrix at the base of recession channels in the absence of liquid boria. These mechanisms are sensitive to the oxidation environment, the geometry of the crack or recession gap, and the presence of fiber-coating debonds. In contrast to prior models that assume gas transport-limited BN recession, current results demonstrate that recession may be reaction limited in some operational domains and may terminate at the reaction front when silica saturation is achieved in the borosilicate glass. Dimensionless closure times for each mechanism are derived, enabling direct comparison under varying crack geometries and environmental conditions. These findings provide new insight into the governing processes of interfacial degradation and help establish a framework for lifetime prediction in SiC/SiC composites.

Ferroelectric energy storage research is a hot topic of today. Based on the requirements of microdevice development and environmental protection, it has become a challenge to develop high-performance lead-free ferroelectric films for energy storage. In this study, a lead-free 0.95(K_0.49Na_0.49Li_0.02)(Nb_0.8Ta_0.2)O₃–0.05CaZrO₃ (KNNLT-CZ) film was epitaxially grown on Nb:SrTiO₃ (001) substrate by pulsed laser deposition. The film exhibits relaxor ferroelectric characteristics and possesses an excellent energy storage density (W_tot) of 33.26 J/cm³ and recoverable energy storage density (W_rec) of 25.13 J/cm³; the energy storage efficiency exceeds 75% with a maximum of 93.8%. Furthermore, the film maintains stable performance within a temperature range of 25°C–175°C and a frequency range of 1–10 kHz, and shows a high fatigue resistance after 10⁶ cycles. Transmission electron microscopy (TEM) image proves the existence of polar nano-domains in the film, which are unique to relaxor ferroelectrics. This study presents that KNN-based films are expected to be candidates for future lead-free energy storage.

The complexity of manufacturing defects imposes constraints on the precise prediction of elastic modulus in needled-ceramic matrix composites. This study presented a novel computational framework that synergistically combined a multiscale homogenization method with an improved particle swarm optimization neural network to predict the stiffness of the composites. The framework developed has successfully captured the inherent nonlinear correlation between the multiscale microstructural characteristics of the composites and their macroscopic stiffness. A fully connected neural network was employed, and hyperparameters were optimized by using an improved particle swarm optimization algorithm. Microstructure parameters obtained from microscopy were utilized to develop the multiscale generalized method of cells. Through the derivation of the strain transformation matrix at different scales, a database comprising manufacturing defect parameters and elastic properties was established. Structural parameters and operating temperature were identified as the input parameters for the neural network, whereas the predicted elastic properties were utilized as the corresponding outputs. Implementation of the neural network training and validation procedures was executed through MATLAB, with mechanical tests conducted to validate the accuracy of the simulated and predicted elastic properties. The results demonstrated that this framework could effectively predict the stiffness of the composites, achieving high accuracy and computational efficiency.

This study investigates the effect of SiC pre-oxidation on the air reactive wetting and brazing behavior of Ag–Nb₂O₅ fillers. An obvious SiO₂ layer induced by the pre-oxidation can improve the wettability of the Ag–Nb₂O₅/SiC system as the contact angle decreases from ∼75° to ∼25° with increasing Nb₂O₅ content, accompanied by localized AgNbO₃ formation at the interface and the triple line. Moreover, the Nb₂O₅ addition promotes a progressive growth of AgNbO₃, transforming the interfacial structure from Ag/SiO₂/SiC to Ag/AgNbO₃/SiO₂/SiC. An optimal Nb₂O₅ content effectively reduces the direct Ag/SiO₂ contact while avoiding excessive AgNbO₃ formation and associated thermal stresses, resulting in a maximum shear strength at 4% Nb₂O₅. First-principles calculations demonstrate that the AgNbO₃/SiO₂ interface exhibits a higher bonding strength than the Ag/SiO₂ interface, showing a strong interfacial charge transfer between AgNbO₃ and SiO₂, which is consistent with enhanced mechanical performance. The insights gained from SiC pre-oxidation offer a potential pathway for advancing the air brazing of other non-oxide ceramic systems.

The nanoporous skeleton structure of alumina aerogel gives it a series of excellent properties, but it also greatly reduces its mechanical properties and thermal stability, which severely restricts its practical application. To address this challenge, herein we propose to simultaneously improve the thermal and mechanical properties of alumina aerogel by reinforcing the matrix with bimodal-scale whiskers, namely microscale SiC whisker (SiC_MW) and nanoscale mullite whisker (Mullite_NW), through an efficient vacuum freeze-drying process. The prepared SiC_MW/Mullite_NW/Al₂O₃ aerogel composites are crack-free, with a uniform microstructure, a well-developed mesoporous network featuring ink-bottle-type pores, low density, and high specific surface area. The whiskers are well-bonded with the alumina matrix. At room temperature, the SiC_MW/Mullite_NW/Al₂O₃ composite has low thermal conductivity, which increases with the SiC_MW content. At high temperatures (1400°C), Mullite_NW inhibits the transformation of the alumina matrix into dense α-Al₂O₃, thus helping retain the nanoporous skeleton. Combined with the infrared shielding effect of SiC_MW, the composites exhibit superior thermal insulation and stability. Moreover, due to the effective inhibition of SiC_MW and Mullite_NW on microcrack initiation and propagation, the mechanical properties are significantly enhanced. This study presents an effective strategy to prepare high-performance alumina aerogel composites for potential practical applications and commercialization.

This study investigates the damage behavior of He-irradiated amorphous SiC at 750°C. Using transmission electron microscopy and high-resolution transmission electron microscopy, He bubble formation, lattice defects, and recrystallization phenomena in amorphous SiC were investigated. Results show that He bubbles in amorphous SiC are smaller and more numerous than those in crystalline SiC. Empirical potential molecular dynamics simulations were carried out. It found that small He bubbles with the mean size of about 0.3 nm were easily formed in amorphous SiC. The study also reveals irradiation-accelerated recrystallization of amorphous SiC at 750°C, driven by defect dynamics and atomic mobility. These findings highlight the role of structural disorder in mitigating He-induced embrittlement and provide insights for designing SiC-based materials for nuclear applications.