Journal of the American Ceramic Society最新文献

A series of glass ceramic samples precipitated with Ca₂(Mg_0.5Al_0.5)(Si_1.5Al_0.5O₇): Yb³⁺, Er³⁺ nanoparticles were synthesized by the melt quenching method to effectively recycle blast furnace slag. Their structures, morphologies, and luminescent properties were comprehensively investigated using X-ray diffraction, transmission electron microscopy, and photoluminescence, respectively. The impact of adding Al₂O₃/SiO₂ to the blast furnace slag raw material on the transmittance, hardness, and luminescent properties of the glass-ceramics was analyzed. Upon excitation with a 980 nm laser, all glass-ceramic samples emitted strong Er³⁺ upconversion luminescence at 526, 549, and 650 nm. Moreover, the addition of Yb³⁺ ions significantly enhanced the luminescent intensity. A dual-mode temperature sensor based on fluorescence intensity ratios was realized using a single luminescent center. All results indicate that the glass ceramic samples prepared from blast furnace slag are potentially promising for self-referential optical temperature measurement.

The thermal barrier coatings (TBCs) consisting of Sr_x(Zr_0.9Y_0.05Yb_0.05)O_1.95+x were prepared using the suspension plasma spraying (SPS) method. Precursor suspensions were prepared via co-precipitation for this purpose. Detailed microstructural, phase composition, and phase content analyses were conducted on these coatings, designated as SZYY-1 (x = 1.0), SZYY-2 (x = 0.9), and SZYY-3 (x = 0.8). Additionally, the coatings underwent a thermal cycling test at 1121°C for 1 h. To assess the morphology of thermally grown oxide (TGO) in a real coating, a finite element model was employed. SPS-SZYY-2 with columnar crystal structure was found to have the highest number of thermal cycling, up to 345 times. The thermophysical high-temperature properties of the coating can be effectively improved through suitable second-phase content. Microstructural and finite element analyses revealed that stress, primarily caused by the continuous growth of the Co₃O₄, NiO and Spinel (CNS) layer within TGO, was the predominant factor leading to coating failure. At elevated temperatures, transverse cracks formed at the interface between the bond coating and ceramic top coating due to mismatched thermal expansion and sintering effects, ultimately leading to coating degradation. Therefore, reducing the generation rate of large stresses in the coating, especially shear stresses, can effectively prevent the generation and propagation of transverse cracks and increase the thermal cycling life of TBCs.

Defect chemistry that results in the thermal processing of dielectric and piezoelectric films, crystals and ceramics ultimately controls the properties and long-term performance of materials and devices. This paper reviews several thermochemical defect reactions using important perovskite base composition dielectrics including Pb(Zr,Ti)O₃, (Na,K)NbO₃, (Bi_0.5Na_0.5)TiO₃‒BaTiO₃, and Ca(Hf,Ti,Mn)O₃. Within this group of perovskite-based functional materials, we note ways the point defects can be formed to create non-stoichiometric compositions changing the overall cation-to-anion ratios during the synthesis process. These reactions can be developed with the loss of volatile species such as metal and oxygen ions. The relative concentrations of these can impact the over conductions in terms of the mixed contributions of ionic conductivity from the oxygen vacancies and the electronic conductivity, along with microstructure and properties in some cases.

Thermal barrier coatings (TBCs) require both a porous structure to effectively prevent heat flux and a considerably dense structure to resist cracking for long-term protection. These opposite requirements are difficult to achieve in conventional TBCs, which often exhibit uniform structures across their thickness. In fact, the main requirements of a coating vary with thickness owing to differential service conditions. In this study, the structure of a TBC is locally tailored to meet regional performance requirements. First, the load-bearing conditions across the thickness are investigated in a simulation study. Resulting from multiple causes, the bottom region must bear a larger stress than the top region, which is directly exposed to heat flux. Therefore, the structure should be crack-resistant in its bottom region and thermally insulating in its top region. Second, region-function-matching TBCs were prepared, and their performances were evaluated through isothermal cycling and thermal exposure tests. Results show that the TBCs with matching design exhibited double the lifespan of the conventional samples, whereas the thermal insulation was comparable. Finally, the structural evolutions were examined in different regions to analyze the failure behaviors of the TBCs. Healing of the intrinsic two-dimensional pores and formation of the new large pores mainly account for the changes in thermal and mechanical properties of the TBCs. Overall, this region-function-matching design is expected to balance the tradeoff between high thermal insulation and long lifespan.

Glasses formed from transition metal oxides have shown tailorable electrical and optical properties depending on the valence state and individual element. Vanadate glasses have received specific attention for their high conductivities as compared to most glass families. In this study, the frequency-dependent capacitance and direct current (dc) conductivity properties of alkaline earth vanadate glasses were investigated. Glasses in the xSrO–(100 − x)V₂O₅ and xBaO–(100 − x)V₂O₅ systems, where x = 30, 40, and 50 in mol%, were prepared via melt-quench synthesis. Capacitance measurements were used to calculate dielectric constants, dielectric loss, and alternating current (ac) conductivity for each sample. Dielectric constants varied between 10–13 and 14–16 for SrO–V₂O₅ and BaO–V₂O₅ glasses, respectively, at 1 MHz. Current measurements were made as a function of temperature and voltage for each glass sample. A strong dependence on vanadate content was noted where temperature had a less strong effect. Activation energies were calculated to describe electrical transport mechanisms. All samples showed activation energies governed by electron hopping mechanisms. Such vanadate glasses have properties suitable for applications as cathode materials for batteries, solid state electrolytes, and conductive glass paste with potential for electro-optic effects involving nonlinear processes.

The first demonstration of plasma-flash sintering (PFS) is presented in this work. PFS is performed under a low-pressure atmosphere that consecutively generates plasma and flash events. It is shown, by using several combined characterization techniques, that PFS stabilizes metastable phases on the surface of the material, which may be partially, but not solely, attributed to the generation of oxygen vacancies, and induces the absorption of ionized species, if a reactive atmosphere is employed. Even though additional research is required to understand the fundamentals of PFS, it is evidenced its potential to be used as a material surface engineering tool, which may widen the technological capabilities of flash sintering.

Magnesium silicate hydrate (M-S-H) represents a promising alternative to traditional cement, particularly for low-pH construction applications such as nuclear waste encapsulation and carbon dioxide injection. The durability of construction materials, a critical aspect of their suitability for various purposes, is primarily governed by the kinetics of dissolution of the binder phase under service conditions. In this study, we employed in situ atomic force microscopy to assess the dissolution rates of M-S-H in water equilibrated with air. Quantitative analysis based on changes in volume and height revealed dissolution rates ranging from 0.18 to 3.09 × 10⁻¹² mol/cm²/s depending on the precipitate Mg/Si ratio and morphology. This rate surpasses its crystalline analogs, talc (Mg₃Si₄O₁₀(OH)₂) and serpentine (Mg₃(Si₂O₅)(OH)₄), by about three to five orders of magnitude. Interestingly, oriented M-S-H dissolved faster than non-oriented M-S-H. Spatially resolved assessments of dissolution rates facilitated a direct correlation between rates and morphology, showing that edges and smaller crystallites dissolve at a faster pace compared to facets and larger crystallites. The outcomes of this study provide insights into the mechanisms governing the dissolution of M-S-H and the factors dictating its durability. These findings hold implications for the strategic design and optimization of M-S-H for various applications.

In this paper, a new micromechanical hysteresis loop constitutive model of C/SiC composites with different interphases was developed considering the probabilistic-statistical matrix fragmentation process. The lengths of matrix fragmentation were divided into three types, that is, long matrix fragments (LMFs), medium matrix fragments (MMFs), and short matrix fragments (SMFs). The distributions of the LMFs, MMFs, and SMFs with increasing tensile stress were determined using the probabilistic-stochastic model by assuming the two-parameter matrix strength distribution. The micro stress field of the LMFs, MMFs, and SMFs upon unloading and reloading was obtained and adopted to determine the corresponding stress-strain relations. The interaction of matrix fragmentation lengths, especially for the LMFs with large debonding energy (LDE) and SMFs, was considered in the closed-form constitutive model and hysteresis-based inverse tangent modulus (ITMs) damage parameter. Synergistic effects of the fiber volumes, peak stresses, and interface debonding energy on the interface damage state, mechanical hysteresis loops, and related ITMs with small debonding energy and LDE were also analyzed. Comparisons of the mechanical hysteresis loops using the new hysteresis models considering matrix stochastic fragmentation and hysteresis models considering constant matrix fragmentation were also discussed. Experimental cyclic tensile hysteresis loops and unloading/reloading ITMs of C/(PyC)/SiC and C/(PyC+SiC)/SiC composites with different interphase thickness (i.e., t = 300, 600, 1000, and 2000 nm) were predicted using the developed constitutive model. Evolution of the unloading/reloading interface slip ratio was analyzed for different tensile peak stresses.

Ultra-low temperature sintered ceramics, Li₂(Mg_1−xCa_x)₂(MoO₄)₃ (x = 0.025–0.100), were synthesized using the solid-phase method. XRD analysis revealed that adding Ca²⁺ increased the second-phase CaMoO₄ content. Particularly, Li₂(Mg_0.975Ca_0.025)₂(MoO₄)₃ ceramics exhibited excellent properties: ε_r = 9.31, Q × f = 55,400 GHz, and τ_f = −38.9 ppm/°C. Incorporating Ca²⁺ significantly improved the performance of ceramics. The microwave dielectric properties of Li₂(Mg_0.9Ca_0.1)₂(MoO₄)₃ ceramics sintered at 650°C showed enhancements: ε_r of 9.55, Q × f of 63,500 GHz, and τ_f of −10.8 ppm/°C. The rise in ε_r can be attributed to polarizability, while a higher packing fraction led to a larger Q × f value, and the reduced distortion of the [Li/Mg/CaO₆] octahedra enhanced the τ_f value. Moreover, the increased of CaMoO₄ also positively influenced the ceramic's properties. Furthermore, in accordance with the principles of complex chemical bonding, the elevation of ionicity, lattice energy, and bond energy of the Mo–O bond contributed to the enhancement of ε_r, Q × f, and τ_f, respectively. Raman spectroscopy showed a positive correlation between dielectric loss and full width at half maximum of the Raman peak. Good chemical compatibility between silver and Li₂(Mg_0.9Ca_0.1)₂(MoO₄)₃ ceramic was demonstrated through our co-firing experiment.

Knowledge of the thermodynamic equilibria and domain structures of ferroelectrics is critical to establishing their structure–property relationships that underpin their applications from piezoelectric devices to nonlinear optics. Here, we establish the strain condition for strain phase separation and polydomain formation and analytically predict the corresponding domain volume fractions and wall orientations of, relatively low symmetry and theoretically more challenging, monoclinic ferroelectric thin films by integrating thermodynamics of ferroelectrics, strain phase equilibria theory, microelasticity, and phase-field method. Using monoclinic K_xNa_1 − xNbO₃(0.5 < x < 1.0) thin films as a model system, we establish the polydomain strain–strain phase diagrams, from which we identify two types of monoclinic polydomain structures. The analytically predicted strain conditions of formation, domain volume fractions, and domain wall orientations for the two polydomain structures are consistent with phase-field simulations and in good agreement with experimental results in the literature. The present study demonstrates a general, powerful analytical theoretical framework to predict the strain phase equilibria and domain wall orientations of polydomain structures applicable to both high- and low-symmetry ferroelectrics and provide fundamental insights into the equilibrium domain structures of ferroelectric K_xNa_1 − xNbO₃ thin films that are of technology relevance for lead-free dielectric and piezoelectric applications.