Journal of the American Ceramic Society最新文献

Synthesis and optimization of tantalum monoboride (TaB) ceramics were investigated to address a gap in the literature where TaB-based bulk ceramics are underexplored despite their predicted excellent mechanical properties. The main focus was put on evaluating influence of extra boron (0.85–1.5 at%), silicon carbide (3–15 wt%), and silicon nitride (1–15 wt%) additions on densification, phase purity, microstructure, and mechanical performance of the TaB ceramics. In the study, a Ta+B powder mixture for stoichiometric TaB was consolidated via reaction reaction hot pressing (RHP) at 1800°C and 1900°C under 30 MPa for 1 h. Because it yielded poor densification (∼62% relative density) and undesirable impurity phases, various processing routes were tried to obtain the desired TaB ceramic, the methods including (i) using B additive and C (introduced as impurity by wear of milling ball coating) to eliminate the minor oxygen-related impurities TaC and Ta₂O, (ii) using SiC/Si₃N₄ to deoxidize the reaction system and reinforce the TaB composites, and (iii) using RHP versus spark plasma sintering (SPS) to compare effects of sintering temperatures and time. Thermodynamic analysis was done to facilitate understanding phases formation. The results showed that densification was improved by additions of 0.85–1.5 at% extra B in combination with SPS at 2100°C under 40 MPa for 10–30 min, but the minor impurity phase TaC or Ta₂O remained. Additions of Si₃N₄ at 3 wt% and above emerged as effective, achieving near-full density (>97% relative density) and good mechanical properties, with Vickers hardness, flexural strength, and fracture toughness reaching 26.1 ± 1.3 GPa, 705 ± 45 MPa, and 4.40 ± 0.19 MPa·m^1/2, respectively. Residual oxygen seemed to have been collected in Si₃N₄-Si₂N₂O aggregates due to the formation of minor Si₂N₂O, a beneficial situation, leaving clean TaB grain boundaries for possible better mechanical performance of the TaB-Si₃N₄ composites at high temperatures. In contrast, SiC addition cannot be recommended because it promoted impurity phases (e.g., TaC, TaSi₂, Ta₅B₆, and Ta₃B₄) and failed to enhance densification (RHP at 1800°C under 30 MPa for 1 h) at low content of SiC additions <5 wt%, while the aimed TaB was completely missed at 5–15 wt% SiC additions.

Achieving the simultaneous optimization of high hardness and fracture toughness remains a critical challenge for protective coatings operating under extreme conditions. This study systematically investigates the effect of Nb alloying on the microstructure and properties of AlCrSiN coatings. It was demonstrated that the addition of an appropriate amount of Nb significantly enhances fracture toughness while maintaining high hardness, overcoming the inherent strength-toughness trade-off in hard coatings. Density functional theory calculations revealed that Nb solid solution enhanced metallic bonding characteristics by broadening the spatial distribution of delocalized electrons, elucidating the fundamental mechanism for the toughness improvement. Nb alloying also improved the high-temperature performance of the coatings. By suppressing the precipitation of the soft w-AlN phase, the thermal stability was elevated to 1100°C, enabling excellent wear resistance to be retained even at 800°C. The Nb-content-modulated hardness–toughness synergy was identified as the determining factor for the erosion resistance, driving a transition in the erosion failure mode from brittle fracture to a ductile wear-dominated process. This research provides a crucial foundation for designing and fabricating tough–hard integrated physical vapor deposition coatings with superior wear and erosion resistance.

The rapid advancement of modern electromechanical applications has imposed urgent demands for piezoelectric ceramics featuring both superior piezoelectric performance and a broad operational temperature range. Nevertheless, concurrently achieving high piezoelectricity and reliable thermal stability in lead zirconate titanate (PZT)-based piezoceramics remains a herculean challenge. To address this issue, we designed a [Pb_0.99Sm_0.01][(Zr_0.54Ti_0.46)_1−xTa_x]O₃ (PSZT-xTa) ceramic system. By introducing defect dipoles via heterovalent ion doping in the perovskite ABO₃ lattice to suppress oxygen vacancies, we realized exceptional properties in the PSZT-0.03Ta ceramics, including a Curie temperature (T_C) of 334°C, a piezoelectric coefficient (d₃₃) of 539 pC/N, and an electromechanical coupling factor (k_p) of 0.63. These materials also exhibited excellent thermal stability, with d₃₃ varying by less than 10% from 18 to 188°C and k_p and resonance frequency (f_r) changing by only 3.5% and 2%, respectively, from 30 to 200°C. The synergism between high piezoelectricity and thermal stability arises from the synergistic interplay of multiple factors: The stable coexistence of tetragonal (T) and rhombohedral (R) phases at the morphotropic phase boundary, where the R phase contributes significant lattice distortion and volume expansion while the T phase ensures structural stability; A hierarchical domain structure comprising micrometer-scale domains and nano-stripe domains; Defect dipoles that effectively mitigate oxygen vacancy formation.

Organic-inorganic halide perovskites (OIHPs) are promising optoelectronic materials, but their instability in radiation environments restricts their durability and practical applications. Here, electron and synchrotron X-ray beams are employed with energies in the tens of keV range, individually, to investigate the radiation-induced instability of two types of OIHP single crystals (FAPbBr₃ and MAPbBr₃). Under the electron beam, the FAPbBr₃ sample exhibits 3-point star-style cracks spanning a few micrometers on its surface, and the MAPbBr₃ sample shows bricklayer-style cracks extending over tens of micrometers on its surface. Under the X-ray beam, a new composition, PbBr₂, is identified in both single crystals. Such cracking phenomena and composition evolutions are attributed to the volatilization of organic components according to energy dispersive X-ray spectroscopy and XRD results. Nanoindentation measurements reveal that beam radiation reduces the Young's modulus of FAPbBr₃ (2%) and MAPbBr₃ (16%) and increases the hardness of both crystals over 10%. A volume-strain-based mechanism is proposed in which the energy conversion results from the organic cation loss. The difference between volume strain energy and crack formation energy leads to the variation in crack patterns. This study provides insights into the structural and mechanical instability of OIHP single crystals in high-energy beam radiation environments.

Femtosecond direct laser writing (DLW) was employed to control the formation of silver nanostructures in ZnO–P₂O₅ glass co-doped with Ag⁺ and Nd³⁺ ions, aiming to establish the relationship between local structure, processing parameters, and optical functionality. By varying the writing speed between 1 and 10 µm/s, the silver species evolved from sub-nanometer nanoclusters to small (≤2 nm) nanoparticles. Ag K-edge X-ray absorption fine structure (XAFS) analysis revealed the coexistence of Ag⁺–O complexes and metallic Ag atoms, quantifying the structural transformation induced by DLW. The microstructural evolution was correlated with pronounced photoluminescence (PL) enhancement of Nd³⁺ ions: tracks written at 2–4 µm/s exhibited up to a 24-fold increase in PL intensity at 875 nm. This enhancement originated from local electric-field amplification and energy transfer processes mediated by the laser-induced Ag nanostructures. The relative contributions of Ag nanoclusters and nanoparticles to the observed plasmon-assisted PL were analyzed. These results demonstrate DLW as a powerful tool for spatially engineering and studying structure–property relationships in rare-earth-doped glass systems with embedded plasmonic nanostructures.

Theoretical quantitative prediction of strain rate-dependent fracture toughness and fracture strength is crucial for evaluating the service performance of ceramic protective materials. Based on Li's Principle of Energy Equivalence, considering the equivalence between strain energy and work done by external pressure, theoretical models of strain rate-dependent mode I fracture toughness and fracture strength have been developed, respectively. These models quantitatively characterize the effects of strain rate, Young's modulus, and Poisson's ratio on the mode I fracture toughness/fracture strength. The predictions of these models were validated by three groups of all available strain rate-dependent mode I fracture toughness experimental data and 18 groups of all available strain rate-dependent fracture strength experimental data, demonstrating that these models have good predictive capability. These models require one mode I fracture toughness/fracture strength data at quasistatic and one mode I fracture toughness/fracture strength data at high strain rate to conveniently and efficiently evaluate the mode I fracture toughness/fracture strength at different strain rates, especially at intermediate strain rates. This work provides an effective method for quantitatively evaluating the mode I fracture toughness/fracture strength of ceramic materials under different strain rates and a convenient theoretical tool for investigating and applying ceramic materials across a wide strain-rate range.

A key challenge in high-expansion Li₂O–ZnO–SiO₂ glass–ceramics is the sudden change in the thermal expansion curve caused by the α–β phase transformation of cristobalite near ∼220°C, which results in a nonlinear thermal strain response. This abrupt thermal strain can cause a transient mismatch between the glass–ceramic and the metal during sealing, ultimately compromising interfacial reliability. In this study, a novel and efficient regulation method was found that the near-linear thermal expansion of Li₂O–ZnO–SiO₂ (LZS) glass–ceramics was obtained by adjusting the precipitation of quartz and cristobalite with the addition of Sm₂O₃. FTIR and Raman results showed that the addition of Sm₂O₃ increased the content of bridging oxygen in the LZS glass network structure, enhancing the glass structure and increases the crystallization activation energy. Correspondingly, the increase of bridging oxygen content could inhibit the precipitation of cristobalite in LSZ glass–ceramics, which reduced the abrupt change in the thermal expansion curve around 220°C. When the addition of Sm₂O₃ exceeds 6 wt%, the thermal expansion curve with 15.2 × 10⁻⁶ K⁻¹ (30–300°C) becomes near-linear, with a Pearson correlation coefficient of .9951. At 500°C, the electrical resistivity of LSZ glass–ceramics reaches as high as 10⁸ Ω cm, demonstrating excellent high-temperature electrical insulation. Additionally, an in-situ Zn–Si–Cu–O transition layer with a thickness of ∼2 µm was observed at the sealing interface between high Sm₂O₃ content LSZ glass–ceramics and copper. This study was useful for controlling optimal the near-linear thermal expansion and in situ interfacial regulation of Li₂O–ZnO–SiO₂-based glass–ceramic sealant for efficient and stable glass–ceramic to metal seal.

The role of nano-Al₂O₃ content on the phase composition, morphology, microstructure, mechanical properties, and purification performance for molten steel of Al₂O₃‒MgAl₂O₄‒C filters was investigated through X-ray diffraction, scanning electron microscopy, energy dispersive spectrometry, and immersion test with molten steel. Results indicate that nano-Al₂O₃ improves slurry retention performance, increases filter strut thickness, and ensures uniform distribution without cracks. During the sintering process, nano-Al₂O₃ accelerates the mass transfer (Mg²⁺ and Al³⁺) between microporous Al₂O₃‒MgAl₂O₄ particle and nano-Al₂O₃ due to its high reactivity, thereby greatly enhancing the sintering driving force, which promotes the development and growth of neck connections between the microporous Al₂O₃‒MgAl₂O₄ particles, resulting in a substantial improvement in the mechanical properties. The filter N5, with 5 wt% nano-Al₂O₃ exhibits excellent mechanical properties with a cold compressive strength of 1.41 and 1.21 MPa after three thermal shock tests. Regarding molten steel purification, on the one hand, nano-Al₂O₃ enhanced the filter's resistance to molten steel erosion, protecting the internal graphene nanosheets from dissolving into the molten steel. On the other hand, its high reactivity significantly promoted the carbothermal reduction reaction, generating reductive gases that interact with and adsorb [Al], [O], and Al₂O₃ inclusions in the molten steel, quickly forming a continuous and uniform Al₂O₃‒MgAl₂O₄ reaction layer at the interface, which contributed to the high molten steel purification efficiency, reducing inclusions by 44% and lowering the total oxygen content from 56.3 to 26.1 ppm. Finally, the role of nano-Al₂O₃ in sintering behavior and purification function as well as the comprehensive molten steel purification mechanism were proposed.

In this work, porous Al₂O₃–MgAl₂O₄ foam ceramics with ultra-high porosities (92.8%–95.7%), high strength, and excellent thermal shock resistance were fabricated by particle-stabilized foaming method. The effects of theoretical content of in situ formed spinel (0, 25, 50, and 75 wt.%) on the microstructure, pore characteristics, and mechanical properties of the foam ceramics were investigated. The introduction of nano-Mg(OH)₂ powder remarkably promoted the development and growth of spinel grains in the foam ceramics, owing to the porous MgO pseudomorph from the decomposition of Mg(OH)₂. A large number of nanopores (i.e., secondary pore structures) can be constructed on the cell wall surfaces of the foam ceramics, due to the generation of the spinel phase accompanied by the volume expansion and Kirkendall effect. The appropriate addition of nano-Mg(OH)₂ powder led to the formation of spinel neck connections between Al₂O₃ grains, which significantly enhanced the strength and thermal shock resistance of the foam ceramics. Overall, when the theoretical content of in situ formed spinel was 25 wt.%, the porous Al₂O₃–MgAl₂O₄ foam ceramics exhibited an ultra-high porosity of 93.50%, a high cold compressive strength of 0.67 MPa and an excellent thermal shock resistance. The cell walls were characterized by single-layer grains (thickness of about 1 µm) with nanoscale secondary pore structures on their surfaces.