Journal of the American Ceramic Society最新文献

A high-entropy dodecaboride composite (HEDC), i.e., (Dy_1/6Ho_1/6Er_1/6Tm_1/6Lu_1/6Hf_x)B₁₂ matrix reinforced by HfB₂ particles, was prepared by spark plasma sintering technique to explore its friction and wear behaviors under various conditions. The wear measurements indicate that its volume wear rates are not only far smaller than those of widely used abrasive materials, including SiO₂, but also lower than the typical values reported recently for high-entropy ceramics, making the HEDC the most wear-resistant known high-entropy conductor with metallic electrical conductivity. We demonstrate that the high-entropy effect significantly improves the inherent wear resistance of the dodecaboride matrix, and the introduction of HfB₂ reinforcement grains suppresses the initiation and propagation of microcracks to further enhance the wear-resisting performance. The ball-on-disc tests show that the average friction coefficients of the HEDC increase with rising sliding speeds and applied loads, completely opposite to the variation trend of the single-phase high-entropy dodecaboride. Through detailed microstructure analysis, it is found that the wear behaviors of the HEDC ceramics comply well with the characteristics of fatigue wear. This study promises to open a new avenue toward super wear-resistant materials by forming particle-reinforced high-entropy composites.

The CaAl₂O₄–CaAl₄O₇ (CA–CA₂) tailing slag was adopted to replace CaCO₃, synthesizing calcium hexaluminate (CA₆) ceramics through reactive sintering with Al₂O₃. Digital image correlation (DIC) technology was employed to monitor volumetric effects in real time, while phases and morphology were combined to elucidate the densification mechanism. The results demonstrate that reducing intermediate reactions while modifying the crystal structure of CA₆ through doping can significantly enhance densification. Thermal expansion, in situ CA, CA₂, and CA₆ formation sequentially dominated the volumetric expansion. Using CA–CA₂ tailing slag as the calcium source reduced the expansion caused by CA and CA₂ formation. The trace TiO₂ and MgO impurities inherently present in the CA–CA₂ tailing slag dissolved into CA₆ lattice, promoting a morphological transition from plate-like to equiaxed grains, thereby further enhancing densification. Dense CA₆ ceramics with an apparent porosity of 2.4% and bulk density of 3.39 g·cm⁻³ were successfully prepared via one-step sintering at 1700°C and performed good alkali corrosion resistance.

This paper focuses on the spectroscopic properties of Cr⁴⁺:YAG transparent ceramic. Absorption, excitation, and emission spectra were measured over a temperature range from 5 to 300 K. Low-temperature absorption spectra reveal sharp and narrow lines corresponding to partially allowed transitions from the ground state to the crystal-field splitting components of the ⁴T₂ energy level. The shape of the excitation spectra was found to be independent of the monitored emission wavelength, indicating that Cr⁴⁺ emission originates from the lowest excited state. Low temperature emission spectra exhibit a sharp and narrow ZPL, accompanied by the vibronic sidebands extending up to ∼2000 cm⁻¹. Both absorption and emission spectra of the lowest excited state at low temperature consist of a doublet, with a splitting of 28 cm⁻¹. The temperature dependence of the spectroscopic parameters of this doublet is reported. Based on the obtained results, possible explanations of their origin are proposed.

CaZrO₃ is a chemically robust perovskite dielectric that is attractive for multilayer ceramic capacitors co-fired with base-metal electrodes, but its properties under reducing conditions are governed by oxygen-vacancy–related defects. In this work, CaZrO₃ ceramics with 0, 0.2, and 0.5 mol% MnO were prepared by solid-state reaction and sintered in N₂. x-Ray diffraction and Rietveld refinement show single-phase orthorhombic CaZrO₃ for all compositions, with a slight decrease in lattice parameters and B–O bond lengths consistent with Mn²⁺ substitution on the Zr⁴⁺ (B) site. Dilatometry, density, and SEM reveal that Mn strongly promotes densification: nearly full density (∼99%) and modest grain growth are obtained at 1400°C for 0.5 mol% MnO. Photoluminescence and EPR show that Mn doping introduces a g ≈ 2 resonance whose intensity and linewidth increase with MnO content, confirming the formation of Mn²⁺ and associated oxygen-vacancy–related centers. The disappearance of the V_O^•-like shoulder at higher Mn levels indicates that oxygen vacancies increasingly form strongly coupled or EPR-silent complexes, reflecting a systematic evolution of the defect landscape with Mn addition. Low-temperature dielectric spectroscopy and Arrhenius analysis of the AC conductivity yield activation energies of ∼0.49 eV for undoped CaZrO₃ and 0.18 eV for 0.2 mol% MnO, attributed to electron emission from vacancy traps of V_O^•• and V_O^•, respectively. Increasing Mn content suppresses the low-temperature relaxation and reduces low-frequency dielectric loss (tanδ at 20 Hz from 1.80 to 0.68), while the intrinsic high-frequency permittivity remains almost unchanged (ε′ ≈ 24, tanδ ≈ 0.06 at 1 MHz). The results are explained by a grain-boundary space-charge model with a vacancy-rich, positively charged boundary core and Mn_Zr″-enriched space-charge layers, providing guidelines for tailoring CaZrO₃-based dielectrics for base-metal electrode applications.

The development of transparent glass-ceramics that simultaneously exhibit high fracture toughness, hardness, and optical transparency poses a significant scientific challenge, since enhancement of mechanical properties often compromises visible light transmission via crystallinity-induced scattering. Here, we report novel MgO-Al₂O₃-SiO₂-ZnO-B₂O₃ glass-ceramics that overcome this trade-off, attaining high indentation fracture toughness (K_IC(ind) = 2.1 MPa·m^1/2) and Vickers hardness (H_V = 9.2 GPa) while simultaneously achieving good visible light transmittance (∼74% at 550 nm). The crystallization behavior and residual stress distribution are governed by the interplay between ZnO and B₂O₃, where ZnO acts as a network modifier to enhance Mg²⁺ mobility and α-cordierite crystallization, while B₂O₃ stabilizes the glass matrix through tetrahedral BO₄ linkages, suppressing interfacial stress gradients. Molecular dynamics simulations reveal that thermal expansion mismatch between α-cordierite and MgAl₂Si₃O₁₀ with the glass matrix generates residual compressive stresses, enhancing crack deflection and interfacial cohesion. A dual-phase microstructure comprising α-cordierite (rigid hexagonal framework) and MgAl₂Si₃O₁₀ (energy-dissipating interfaces) enables mechanical resilience, while controlled crystal size minimizes light scattering. Surface crystallization induces compressive stress (ZC-6, -368.4 MPa) analogous to that produced via ion-exchange strengthening, effectively counteracting tensile stresses at crack initiation sites. The optimized ZC-6 composition exemplifies this balance between mechanical and optical performance, surpassing many conventional glass-ceramics in both fracture toughness and transparency. This work establishes a novel paradigm for designing high-performance transparent materials, bridging the gap between optical clarity and mechanical resilience.

CaBi₂Nb₂O₉ (CBN) is a promising layered perovskite piezoelectric ceramic with high Curie temperature (T_C) for vibration sensor applications. However, its relatively low resistivity and piezoelectric response limit its application in high-temperature environments. In this work, A-site doped Ca_0.6(Na_0.5Bi_0.5)_0.4-x(Li_0.5Nd_0.5)_xBi₂Nb₂O₉ (CBN-NBN-xLiNd) ceramics were synthesized using a conventional solid-state method. The piezoelectric and electrical properties of CBN-NBN-xLiNd ceramics simultaneously improved. Notably, the CBN-NBN-0.12LiNd ceramic exhibited excellent overall performance, including a high T_C value of 904°C, an enhanced piezoelectric constant (d₃₃∼16.5 pC/N), and excellent high-temperature resistivity (1.37 × 10⁶ Ω·cm at 600°C). Nanoindentation and Vickers hardness tests were performed to evaluate the effect of doping on the mechanical properties of CBN-NBN-xLiNd ceramics, revealing a hardness of 3.56 GPa for CBN-NBN-0.12LiNd. These findings provide valuable insights into the development of CBN-based ceramics for high-temperature piezoelectric applications.

Ultrafine equiaxed α-Si₃N₄ powder is a promising raw material for next-generation high-performance substrates. However, conventional industrial methods are often hampered by high energy consumption, prolonged production cycle, and the use of polluting additives. Thus, the low-cost production of high-purity α-Si₃N₄ powder remains a persistent technical challenge. In this study, equiaxed α- Si₃N₄ particles were synthesized via an additive-free combustion synthesis approach. The influences of N₂ pressure and Si/Si₃N₄ molar ratio on the combustion process, phase composition, and microstructure of the synthesized powders were systematically investigated. Under optimized conditions—specifically, a N₂ pressure of 3 MPa and a Si to Si₃N₄ molar ratio of 2.5:1—equiaxed α-Si₃N₄ particles with an average size of 0.35 µm were obtained. Furthermore, the growth mechanism of α-Si₃N₄ particles was elucidated through heat absorption–assisted quenching and thermo-kinetic analysis. The resulting sintered Si₃N₄ ceramic exhibited a bending strength of 767 MPa and a thermal conductivity of 94.5 W·m⁻¹·K⁻¹, respectively. This work provides a cost-effective and efficient strategy for synthesizing high-quality α-Si₃N₄ powder, demonstrating its potential application as a substrate raw powder for microelectronic applications.

Dual-phase carbide-boride ceramics in different vanadium-Me (Me = Cr, Hf, Ti, and Zr) binary systems were synthesized by boro/carbothermal reduction under stoichiometric and carbon-deficient conditions and densified by spark plasma sintering. Thermodynamic analysis was used to evaluate the influence of composition on the phase stability and metal segregation between phases, along with the resulting effects on microstructure and hardness. Pairing vanadium with Group IV elements (Hf, Ti, and Zr) consistently formed one boride and one carbide phase, while the Cr-V system formed a monoboride phase and a carbon-deficient carbide. Metal segregation trends depended on composition. Vanadium preferentially segregated to the carbide phase in the Ti-V and Cr-V systems, while it was enriched in the boride phase in Hf-V and Zr-V systems. Hardness measurements showed a higher hardness for the Ti-V sub-stoichiometric system, reaching 27.7 ± 0.9 GPa at 9.8 N, while the Cr-V sub-stoichiometric system presented the lowest hardness at 18.8 ± 0.3 GPa at the same load. Notably, in the Zr-V system, thermodynamic predictions based solely on standard Gibbs energy deviated from experimental observations. However, when combined with first-principles calculations that accounted for non-stoichiometry in vanadium carbide, the predictions aligned more closely with experimental observations, qualitatively indicating a preference for vanadium segregation to the boride phase and zirconium to the carbide phase. These results suggest that elemental distribution between phases results from complex interactions beyond simple Gibbs free energy minimization, especially in systems like Zr-V with strong carbide-to-boride ratio dependence. The optimized systems were characterized with nominal compositions of (Ti_0.61,V_0.39)B₂-(Ti_0.39,V_0.61)C_0.9, (Hf_0.12,V_0.88)B₂-(Hf_0.88,V_0.12)C_0.9, (Zr_0.29,V_0.71)B₂-(Ti_0.71,V_0.29)C_0.9 and (Cr_0.60,V_0.40)B-(Cr_0.40,V_0.60)C_0.8. Their thermodynamic interactions provided insights into metal segregation in dual-phase ceramics, demonstrating its strong composition dependence.

A systematic investigation was conducted on the oxidation behavior of silicon-bond coats within environmental barrier coating (EBC) systems applied to Si-carbide (SiC) substrates, aiming to understand how different underlying SiC substrates influence the bond coat's thermally grown oxide (TGO) and its properties. The study examined (Y/Yb)₂Si₂O₇/Si coatings on three cost-effective surrogate SiC substrates (chemical vapor deposition [CVD]-grown β-SiC, sintered α-SiC, and reaction-bonded [RB] SiC) for SiC_fiber/SiC_matrix ceramic matrix composites (CMCs). Discrepancies in TGO growth were observed, with noticeably higher growth rates reported for the coated CMC specimens than for the four monolithic SiC specimens. The CMC samples produce an amorphous TGO, whereas the other monolithic substrates formed a crystalline TGO, which lowered the oxygen permeability through the SiO₂ scale. The vitrification of the TGO in the (Y/Yb)₂Si₂O₇/Si/CMC system resulted from the migration of boron species from the CMC substrate to the SiO₂ scale, leading to network modification via boron doping.