Journal of the American Ceramic Society最新文献

This study investigates the effect of Ce/Gd co-doping on the work function and thermionic emission properties of lanthanum hexaboride (LaB₆). Density functional theory calculations demonstrate that the Ce/Gd co-doping effectively reduces the work function of LaB₆, with the value decreasing from 2.11 eV for the LaB₆ (100) surface to 2.09 eV for the La_0.5Ce_0.25Gd_0.25B₆ (100) surface. Using spark plasma sintering, Ce/Gd co-doped LaB₆ bulks with a CsCl-type single-phase substitutional solid solution structure were fabricated, achieving relative densities exceeding 96.2%. The dense SPSed specimens exhibit no noticeable texture, comparable average grain size, and a homogeneous distribution of rare-earth metal cations. The H_v value of 21.2 ± 0.54 GPa for La_0.5Ce_0.25Gd_0.25B₆ is higher than that of 18.5 ± 0.60 GPa for LaB₆, which can be ascribed to the solid solution strengthening effect induced by Ce/Gd co-doping; however, the Ce/Gd co-doping is incapable of improving fracture toughness. Ultraviolet photoelectron spectroscopy analysis further confirms that the work function of Ce/Gd co-doped LaB₆ (2.58 eV for La_0.5Ce_0.25Gd_0.25B₆) is lower than that of pure LaB₆ (2.68 eV). Importantly, under identical operating conditions, the dense La_0.5Ce_0.25Gd_0.25B₆ bulk demonstrates higher thermionic emission current densities than LaB₆, highlighting its promising potential as a high-performance thermionic cathode material.

Fully ceramic microencapsulated (FCM) fuel, as a key category of accident-tolerant fuel, relies critically on the mechanical integrity of the silicon carbide (SiC) layer with the tristructural-isotropic (TRISO) fuel particles for its service reliability. The introduction of stacking faults (SFs) in SiC has been demonstrated to significantly enhance its mechanical performance. Building on this principle, we propose an innovative flash spark plasma heat treatment strategy that tailors electrical parameters to induce the formation of SFs within the SiC layer. A modified spark plasma sintering system, equipped with a boron nitride-insulated graphite mold, enabled direct current flow through TRISO particles, allowing thermal processing under varied power conditions. Under low-voltage (3.46 V) and moderate-current conditions, the SiC layer exhibited substantial improvements in mechanical properties, with nanohardness and elastic modulus reaching 35.3 and 328.6 GPa, corresponding to increases of approximately 50.2% and 43.6%, respectively, relative to the as-deposited state. This remarkable strengthening effect arises from current-induced high-density SFs and Lomer–Cottrell lock networks, which effectively impede dislocation motion and enhance mechanical stability. This work elucidates the microstructural and mechanical evolution of the SiC under current-assisted conditions, establishing a new paradigm for current-mediated property tuning in ceramics and providing theoretical and practical guidance for designing a strengthened SiC layer in FCM fuels.

Bi₂Te₃-based materials are the best commercial thermoelectric (TE) materials for applications near room temperature. However, zone-melted (ZM) materials suffer from poor mechanical properties, while traditional powder metallurgy-derived Bi₂Te₃ exhibits a strong donor-like effect when refined into fine grains, creating a fundamental barrier to the simultaneous enhancement of mechanical and TE performance. Herein, high-performance n-type Bi₂Te₃-based materials were fabricated via a hybrid process combining melt spinning (MS) and hot extrusion (HE). The MS-derived foils exhibit a strong (110) orientation, nanocrystalline structure, and no significant donor-like effect despite air exposure. These characteristics of these foils are maintained in the precursor for HE via rapid sintering by directly laying the foils flat without grinding. This strongly oriented, fine-grained characteristic of the ribbon precursor is inherited and further intensified via subsequent HE, which yields fine-grained, highly textured bulk materials with an orientation factor F₍₁₁₀₎ of 0.54. The enhanced texture and microstructure result in a high carrier mobility of 348 cm²·V⁻¹·s⁻¹ and a power factor of 52.20 µW·cm⁻¹·K⁻², while intensifying phonon scattering and reducing lattice thermal conductivity to 0.5 W·m⁻¹·K⁻¹. Consequently, a peak ZT of 1.25 at 345 K and a room temperature ZT of 1.12 are achieved. This material also demonstrates exceptional mechanical properties, with a record-high compressive strength of 338 MPa and flexural strength of 153.8 MPa. This work resolves the longstanding trade-off between mechanical robustness and TE efficiency, enabling the fabrication of TE legs (<100 µm) for large-scale applications. It paves the way for the fabrication of micro-TE devices.

We present new evidence for the generation of mammoth concentrations of defects produced in flash experiments, postulated to be vacancies (and interstitials). We show that the defects become so ubiquitous that the metal loses cohesion because of the absence of nearest-neighbor bonds. The defects have been shown to form their own crystalline phase that is congruent, like a child within the host crystal. This defect-phase spells a new state of vacancies that amass into large colonies; we call “anti-mass”. The phase continues to expand with current density, eventually causing the metal to lose cohesion, without evidence of melting. The experiments are performed simply by injecting current. Relationships between the current density, the specimen temperature, and the defect concentrations are developed. Despite deploying a wide range of current densities, varying from 20 to 1000 A·mm⁻²·min⁻¹, the loss of cohesion occurs, in all instances, when the current rises to within a narrow range of current density (150–200 A·mm⁻²). The specimen temperatures at this point are far below melting, also falling within a narrow range. The experiments are carried out without furnace heating. The defects impart unusual properties to the “mother” crystal, including enhanced electronic conductivity, electroluminescence, and super-rates of solid-state diffusion. Please note that the words “flash general” in the title are meant to highlight that flash phenomena are not just about sintering: they have a widespread relevance in materials science.

This study develops a moderate-temperature (<1100°C) glass filler in the Al₂O₃-B₂O₃-SiO₂ system, modified with K₂O/Li₂O, for joining reaction-bonded silicon carbide (RB-SiC). The optimized composition (2.5 wt% Al₂O₃, B/Si = 1:2) achieves a coefficient of thermal expansion (CTE) of 3.25×10⁻⁶/K, closely matching that of RB-SiC (ΔCTE < 0.05×10⁻⁶/K), and yields a superior shear strength of 44 MPa when joined at 900°C. Contrary to conventional wisdom, maximum joint strength was achieved not under conditions of optimal wettability (which occurred at higher temperatures) but where interfacial reactions were optimally controlled to prevent defect formation. Comprehensive characterization reveals the bonding mechanism: the in-situ formation of a nanoscale interfacial layer comprising a silica-rich region and alkali carbonates, facilitating strong covalent bonding between the glass and the substrate. This work demonstrates that controlling interfacial chemistry, rather than solely pursuing optimal wettability, is the key to developing robust glass-to-ceramic joints, offering a promising joining solution for high-performance RB-SiC components.

Graded multilayer zirconias exhibit a microstructural gradient based on yttria content, but the transition zone between layers remains poorly characterized. This study evaluated the fracture energy required to create new surfaces in multilayer zirconias. Brazil-nut specimens were tested under different loading angles to induce tensile, shear, or mixed failure modes, using 3Y-TZP and 5Y-PSZ as controls. Groups were defined by zirconia type, loading angle, and hydrothermal aging. Fractured specimens underwent fractographic analysis, failure classification, scanning electron microscopy, and energy-dispersive X-ray spectroscopy characterization. Two-way analysis of variance revealed significant differences between loading angles but not aging. At 25°, where shear forces predominated, fracture energy was significantly higher [baseline: 964.74 (± 202.43); aged: 1389.12 (± 978.47) N/m] compared with most groups, except 15°. Multilayer zirconia showed intermediate fracture energy values between 5Y-PSZ and 3Y-TZP. Importantly, the transition zone presented a heterogeneous interphase rather than a smoothly graded structure. Shear stresses required higher energy release than tensile stresses. These results reveal the microstructural discontinuity and distinct fracture behavior of multilayer zirconias, providing new insights into the structure–property relationships of this class of ceramics.

Aluminum oxynitride (AlON) ceramics, valued for their superior mechanical properties and optical transparency, are promising candidates for advanced armor protection. However, their mechanical response under varying loading rates remains insufficiently understood, constraining reliability-based design and performance optimization for impact conditions. In this study, quasi-static and dynamic indentation tests were conducted to elucidate the strain rate–dependent response of AlON ceramics. The results reveal a pronounced strain-rate hardening effect, with dynamic Vickers hardness markedly exceeding quasi-static values. Correspondingly, crack morphology transitions from simple radial cracks to complex crack networks with secondary branching. Microstructural analysis further shows that dynamic loading activates multiple plastic-related deformation processes within the indentation region, including high-density dislocation structures, slip-band interactions, dislocation-free zones, and deformation twins. These features help explain both the enhanced hardness and the evolution of crack patterns and demonstrate that AlON ceramics exhibit a brittle–plastic coexisting failure mode at high strain rates. This work provides new insight into the strain rate–dependent indentation response and microstructural evolution of AlON ceramics, offering valuable guidance for the design of high-performance transparent armor materials.

A glass series with constant ∼30 mol% BaO in the BaO–B₂O₃–SiO₂ system was studied as a function of SiO₂/B₂O₃ ratio. This system has been studied in-depth to reveal the structure, Part 1, and its relationship to the mechanical properties, Part 2. Both the B coordination and network polymerization are quantified both experimentally, using Raman, IR, and ¹¹B NMR spectroscopies, and theoretically, using classical molecular dynamics (MD) simulations with effective partial charge potentials with composition dependent boron parameters. These results show that ^IIIB, threefold-coordinated boron, increases linearly with increasing boron, at the expense of ^IVB. The Q³ equilibrium constant decreases slightly with boron addition up to 37 mol%, whereas at greater B₂O₃ contents, the silica tetrahedra become more polymerized. These trends are reinforced by MD simulation results, which show that the average connectivity, polymerization, and ring size are directly related. The glass transition temperature increases with increasing silica content, where the range of temperatures follows the packing density. The BaO–B₂O₃–SiO₂ system shows systematic trends from high to low oxygen packing between the binary borate glasses to the binary silicate glasses, indicating a high degree of predictability for properties controlled by density.

Resolving the inherent conflict between high porosity and superior mechanical properties in porous silicon nitride (Si₃N₄) ceramics is critically important for their structural applications. This study presents a novel strategy to simultaneously enhance flexural strength and fracture toughness without compromising porosity. By introducing a small amount of silicon (Si) powder into the starting composition (α-Si₃N₄ and Y₂O₃), a network of in situ Si₃N₄ nanowires was formed via a gas-phase reaction and vapor–solid mechanism under a nitrogen atmosphere. These nanowires eventually transform into nanoscale crystal nuclei, providing abundant heterogeneous nucleation sites for β-Si₃N₄ crystal grains. This process resulted in a refined and uniform microstructure characterized by significantly reduced grain size and pore diameter, while maintaining a high aspect ratio (∼10) of the β-Si₃N₄ grains. Consequently, at a constant high porosity of ∼56%, the flexural strength and fracture toughness were remarkably improved from 171.2 to 234.9 MPa and from 2.41 to 3.33 MPa·m¹/², respectively. The evolution of nanowires and the phase transformation were systematically characterized, and the underlying strengthening and toughening mechanism involving microstructure refinement and Hall–Petch strengthening is thoroughly discussed.

Reactive sintering offers an advantageous route to produce high-performance ceramics from coarse and cheap precursor powders; however, impurities in these sources often prevent the desired product composition from being achieved even when the raw materials are proportioned according to the reaction stoichiometry. This study employed a thermodynamics-guided approach to predict phase evolutions during the consolidation of TiB₂-AlN ceramics (TA) within the TiN–Al–B system. Although oxygen impurities (B₂O₃, Al₂O₃) exist in the raw powders, the formation of undesirable phases in TA could be highly suppressed through a proper composition design. Thermodynamic calculations revealed that excess Al compensates for the reduction of B₂O₃, while a TiN deficiency below a critical threshold is essential to inhibit the formation of hBN. Guided by these insights, stoichiometric, Al-excess (TA5: 5 mol% excess Al) and dual-optimized (T5A5: 5 mol% excess Al and 5 mol% deficient TiN) were consolidated by spark plasma sintering at 1800°C/60 MPa for 5 min. Whereas TA and TA5 contained residual TiN and hBN, T5A5 achieved near-phase purity with refined microstructures and exhibited better mechanical properties.