Journal of the American Ceramic Society最新文献

The K₂O-Al₂O₃-SiO₂ system is an important component of microcrystalline glass. The K₂O-Al₂O₃ and K₂O-SiO₂ binary systems and the K₂O-Al₂O₃-SiO₂ ternary system were thermodynamically evaluated and optimized using the CALculation of PHAse Diagram (CALPHAD) method. The liquid phase is described by using the ionic two-sublattice model, and the solid solutions involved in the ternary system are all described using the compound energy formalism (CEF) model. The new parameters obtained from the optimization are capable of describing the equilibrium phase relations of the ternary system and its subsystems, and the calculated thermodynamic properties of the ternary compounds are in good agreement with the experimental data.

In this study, the influence of foreign and native point defects in magnesium oxide on the sintering process is examined. We have introduced dopants into magnesium oxide, with cations that share similar sizes but possess varying charges to minimize the direct impact of strain. The selected foreign cations were as follows: (1) Li¹⁺, an acceptor that enhances the concentration of oxygen vacancies; (2) Sc³⁺, a donor that enhances the concentration of magnesium (metal) vacancies; and (3) Zn²⁺, an isovalent dopant. The results reveal that oxygen vacancies introduced by lithium doping greatly decrease the sintering temperature of magnesium oxide compared to the magnesium vacancies’ effect (scandium doping). Zinc doping was found to increase the surface oxygen vacancies with only a minor effect on the sintering temperature. Enhancing the oxygen vacancy concentration by lithium doping creates an additional mechanism for sintering because the anion sublattice is the backbone of the material, and oxygen ion diffusion is the rate-limiting step. Scandium doping also has a sintering–promoting effect, yet a minor one. The doping factor analysis is considered and implies that aliovalent dopants do not affect the concentration of the fast-diffusing species, which are native vacancy associates.

High-performance MAX phase-based composites were developed to overcome the inherent low hardness and low strength of MAX phases by combining lattice distortion-induced strengthening, texture strengthening, and second-phase particle strengthening. Textured high-entropy M₄AlC₃/Al₂O₃ (M = Ti, V, Mo, Nb, Ta) composites with different Al₂O₃ contents were prepared using spark plasma sintering at 1350°C for 70 min. The microstructures of all samples with different compositions were characterized in detail. It was found that as the Al₂O₃ content increased, the grain size of the high-entropy M₄AlC₃ phase gradually decreased, and the aggregation of Al₂O₃ became more severe. Based on this, the density, hardness, strength, and fracture toughness of all composites were tested. The results indicate that all textured composites exhibit significant anisotropy in their properties, with the high-entropy M₄AlC₃/15 vol%Al₂O₃ composite showing the best overall performance. Additionally, the mechanism of performance improvement was systematically discussed. This work provides an important reference for the subsequent preparation of high-performance MAX phase-based composites.

Lead-based piezo-ceramics like lead zirconate titanate (PZT) are a mainstay for many piezoelectric applications. However, lead oxide (PbO) evaporation during sintering poses a significant environmental challenge. Flash sintering (FS) is a novel technique that can densify ceramics in seconds and at a much lower furnace temperature. The liquid-phase FS (LPFS) of PZT (Pb (Zr_0.5Ti_0.5) O₃, with 3 wt.% Cu₂O and PbO in the molar ratio of 1:4) is investigated in this work. Further, a comparison has been made among the lead loss, dielectric, and piezoelectric properties of flash-sintered and conventionally liquid-phase-sintered PZT. It has been observed that the evaporation of PbO has been brought down 3–5 times by FS. The dielectric constant of LPFS PZT is significantly higher, especially at higher frequencies with lower dielectric loss. An enhanced piezoelectric coefficient in flash-sintered PZT has also been observed. The LPFS of PZT shows that the lead loss can be brought down significantly with the added benefit of enhanced dielectric and piezoelectric properties. XRD and Rietveld analysis show an increase in tetragonality after FS in comparison with conventional sintering. XPS and ESR studies show a difference in defect concentration after FS in comparison with conventional sintering that is likely responsible for the enhanced dielectric and piezoelectric properties.

The common feature of non-ergodic systems is an internal timescale that greatly exceeds the external observational timescale $�t_{�o�b�s�}$. This kinetic state of broken ergodicity occurs in many systems, with profound thermodynamic implications. In this work, we present a review of non-ergodic physical systems focused on the common origins of non-ergodic behavior across diverse material systems. We begin with a theoretical discussion of energy landscapes and two treatments of thermodynamics in broken ergodic systems. We then discuss several characteristic material classes that exhibit non-ergodic behavior, describing the process of ergodic breakdown and its signatures for each. The disordered nature and frustration of different energetic interactions in the example systems are discussed as the possible origin of non-ergodic dynamics. We conclude with several considerations that can assist in the identification of non-ergodic behavior. This review intends to unify the behavior of a diverse group of physical systems with a common description to aid future discussions between these fields of study.

Delayed fracture occurs when cracks grow subcritically prior to achieving K_IC. Glass science research in this area has focused almost exclusively on non ion-exchanged glasses. In this study, an ion-exchange strengthened aluminosilicate glass was intentionally damaged to measure delayed fracture behavior. Damage was applied both pre and post ion-exchange and the role of heat-treatments was investigated. It was found that the delayed fracture likelihood was reduced for samples that were heat-treated in an environment containing water vapor. An increase in the static fatigue limit was measured for increasing temperature and it was hypothesized that crack toughening takes place during the heat-treatment due to a water-assisted stress relaxation mechanism.

Zirconia/alumina composites are a family of structural ceramics with excellent mechanical properties. In order to improve the engineering reliability of this kind of composites, the uniform spatial distribution of the two phases is necessary. One of the effective methods to achieve this goal is to synthesize uniform zirconia–alumina composite nano powders. Here, we report mechanochemical synthesis of zirconia–alumina composite nano powders, in which dehydration and crystallization of Zr–Y–Al hydroxide can be achieved under milling conditions, leading to the formation of the tetragonal zirconia phase with supersaturated Al³⁺. The mechanochemically synthesized powders have the uniform element distribution and a fine primary crystallite size of around 11 nm. After sintering at 1400°C, t-ZrO₂/α-Al₂O₃ composite with the uniform phase distribution and grain size distribution could be obtained. To uncover the full benefits of these supersaturated composite powders in ceramic processing, future work could be done to break up the deagglomerations and improve the sinterability of the mechanochemically synthesized zirconia/alumina powders.

This investigation explores the impact of incorporating graphene (Gr) reinforcement on the microstructure, mechanical properties, corrosion behavior, and biocompatibility of a composite derived from a magnesium–calcium (Mg–Ca) alloy. Two concentrations of Gr (0.1% and 0.2%) were introduced to an Mg–Ca alloy. The addition of 0.1% Gr resulted in a refined grain structure, enhancing both tensile and compression strength. However, electrochemical analysis and immersion testing revealed an increase in corrosion rate ($�R_w$) with the incorporation of Gr. Although the corrosion rate of the Mg–Ca-0.1%Gr composite was comparable to that of Mg–Ca, the Mg–Ca–0.2%Gr exhibited higher corrosion rates attributed to the enhancement of micro-galvanic corrosion. Interestingly, cell survival rate tests demonstrated improved biocompatibility for the Mg–Ca–0.1%Gr sample, emphasizing its potential for applications in the biomedical domain, which requires enhanced mechanical strength, corrosion resistance, and biocompatibility.

On the rising demand for eco-friendly refractories, reducing the use of likely toxic magnesia-chromium aggregates remains a challenge. Previous studies by some of the authors have proposed Cr-free alternative compositions, although the morphology of the spinel precipitates has varied across the different suggested systems. The mechanisms involved in the formation of these distinct morphologies were unclear and, therefore, are the focus of this work. In all compositions, SEM/electron backscatter diffraction revealed cube–cube orientation relationships between matrix and precipitates, indicating that their formation is influenced by the lattice parameter misfit (δ), which was measured using synchrotron X-ray diffraction. It could be concluded that coarser and spherical precipitates form to minimize their surface-to-volume ratio in compositions with high absolute δ-values. Conversely, low-misfit systems enable the spinel to form a 3D-network. The potential use of this knowledge to tailor the microstructure of novel compositions was demonstrated by a small Nb₂O₅ addition into one of the proposed compositions.

Due to the depletion of fossil fuels, extensive CO₂ emissions, and growing demand for electrochemical energy, it is important to develop further utilization of renewable energy, as well as advanced energy storage techniques. To this end, the optically chargeable supercapacitor, which can convert the photon energy of solar light into electrochemical energy and store it for long-term usage, attracts extensive investigations. Among these, heterojunction construction was revealed as an effective method to enhance capacity. Recent works have reported the implementations of precipitation in ceramics, where the introduced matrix-precipitate two-phase structure is inspiring for constructing heterojunction, whereas it was seldom reported. In this work, Li-doped NaNbO₃ electrode material was prepared by adopting a multi-step solid reaction to precipitate LiNbO₃ and construct LiNbO₃/NaNbO₃ heterojunctions. The electrochemical measurement demonstrates an enhanced optically chargeable capacity of roughly 85% increase at a scan rate of 1 mV/s and a four-fold increase at a charging–discharging current density of 1 A/g, as well as outstanding stability by introducing precipitates and constructing heterojunctions. This finding proposes a novel methodology design of heterojunction construction via precipitation to enhance the optically chargeable capacity of supercapacitor electrode materials and, therefore, may open up a potentially new application field for precipitate-tuned functionality in functional ceramics.