Journal of the American Ceramic Society最新文献

The increase in size and efficiency of gas turbines leads to higher temperature in the combustion chamber, putting greater demands on the performance of the thermal insulation tiles. Corundum–mullite has been used because of its high-temperature resistance, thermal shock resistance, and low thermal conductivity. However, how to inhibit heat transfer through composition regulation while ensuring the safe use of high-temperature insulation tiles is the key to improving the heat conversion efficiency of gas turbines. In this study, thermal insulation tiles were prepared by casting molding. On the basis of determining the optimal corundum/mullite ratio (22:45 wt%) in the aggregate, the thermal conductivity of the sample was reduced by adding MgO (2 wt%). The results show that phonon intrinsic and defect scattering, caused by changes in phase composition, effectively reduce the thermal conductivity of the insulation tile sample to 2.05 W·m⁻¹·K⁻¹, which is 34.71 % lower than the maximum value before regulation. During 30 cycles of thermal shock (air-cooling at 1000°C), the residual strength gradually decreased and tended to be stable, with a minimum of 8.6 MPa, indicating that the thermal insulation tile can provide better thermal insulation without affecting the safety of gas turbines, providing new ideas and methods for improving the thermal insulation performance of high-temperature thermal insulation materials.

Graphene nanoplatelet (GNP)–reinforced TiB₂ + SiC composites were fabricated by reactive spark plasma sintering (SPS) from a TiC–B₄C–SiC powder mixture with equal-volume percentages, optimizing their SPS temperature and evaluating their unlubricated sliding wear against diamond. First, it is shown that during the heating ramp of the SPS cycle, TiC and B₄C react according to the chemical reaction 2TiC + B₄C → 2TiB₂ + 3C, and that the thus-formed C is graphenized as GNPs, leading to composites with microstructures consisting of a ceramic matrix of fine TiB₂ and SiC grains with abundant randomly oriented GNPs at grain boundaries. It is also shown that this reactive SPS is optimal at 2000°C (under 75 MPa pressure and 5 min soaking), resulting in a very hard (∼28.5–29.9 GPa) and very tough (∼6.7(3) MPa m^1/2) composite. And second, it is shown that these two properties and its proneness to develop an oxide tribolayer make this composite very resistant to unlubricated sliding wear against diamond (∼2.8(1)·10⁸ (N m)/mm³), undergoing only very mild two-body abrasion. Finally, opportunities for the fabrication of toughened and very hard ceramic composites for contact-mechanical and tribological applications are discussed.

When a small atom is induced into a larger lattice, new phonon scattering modes are aroused and thermal conductivity is significantly reduced by increased anharmonicity. This effect, called “rattling,” has been reported in pyrochlores and some cage compounds, but not in α-SiAlON. In this study, we reveal that in (Yb+Ca) co-doped α-SiAlON ceramics (Yb_aCa_bSi_7.5Al_4.5O_1.5N_14.5, x = a/[a+b]), the rattling effect plays a crucial role in reducing thermal conductivity. Samples with different Yb/Ca ratios (x = 0, 0.1, 0.2, 0.3…, 1.0) were sintered by spark plasma sintering (SPS) at 1600°C and their thermal diffusivity/conductivity were measured by the laser-flash method. We found that substituting Ca²⁺ with smaller Yb³⁺ cations in Ca-α-SiAlON caused significant reduction in thermal conductivity. On the contrary, substituting Yb³⁺ with larger Ca²⁺ cations in Yb-α-SiAlON caused only slight reduction in thermal conductivity. A lowest thermal conductivity of 3.3 W/(m·K) was achieved, when x = 0.3 rather than x = 0.5 or 0.7. Further analysis of phonon scattering intensity confirmed that both intrinsic and extrinsic scatterings are enhanced in the sample of x = 0.3. This study demonstrates that by utilization of the rattling effect, α-SiAlON, well-known for its excellent mechanical properties, can be tuned to exhibit low thermal conductivity comparable to that of La₂Zr₂O₇ and 8YSZ.

The recent realization of ferroelectricity in scandium- and boron-substituted AlN thin films has spurred tremendous research interests. Here we established a molecular dynamics simulation framework to model the ferroelectricity of AlN thin films. Through reparameterization of Vashishta potential for AlN, the coercive field strength and the AlN polarization were found to be close to experimental values. Furthermore, we examined the effects of film thickness, temperature, in-plane strain on polarization-electric field hysteresis loop, and the thickness-dependent Curie temperature. Lastly, we incorporated electrodes towards atomic-level modeling of ferroelectric device, by considering the induced charge at the interface between electrodes and ferroelectric film. We found that low dielectric contrast significantly lowers the coercive field for switching AlN.

Thermally robust and highly stable bulk luminescent materials are essential for advancing high-power laser-driven lighting. In this study, we report a yellow–green LuAG:Ce–Al₂O₃ eutectic, synthesized using the laser-heated pedestal growth (LHPG) technique. The emission intensity of the eutectics reaches a maximum at an Al₂O₃ content of 20% due to the enhanced light scattering. Additionally, owing to the high thermal conductivity of Al₂O₃, the prepared LuAG:Ce–Al₂O₃ eutectic exhibits low thermal quenching, with only a 5% loss in luminescence observed at 150°C, along with a high luminance saturation threshold of approximately 15.8 W·mm⁻². When irradiated under blue laser excitation at 7.9 W, the prepared LuAG:Ce–Al₂O₃ eutectic demonstrates a luminous flux of 1917 lm and a luminous efficacy of 242.4 lm·W⁻¹. These results highlight that the potential of LuAG:Ce–Al₂O₃ eutectics as luminescent materials for high-power laser-driven lighting applications.

Borosilicate glasses are the primary waste forms for the industrial immobilization of high-level liquid waste. Yet, the compositional variation of waste glasses can form the diopside phase, which can be detrimental to the melting process and the properties of the final glass products. This study prepared simulated waste glasses with variable contents of alkaline earth metals, boron, transition metals, and rare earth metal oxide and subjected them to heat treatments. The effect of the compositional variation on the diopside crystallization behavior was explored using differential scanning calorimetry, X-ray diffraction, optical microscopy, and scanning electron microscopy-energy dispersive spectroscopy. The results revealed that the average size of diopside crystals was proportional to the square root of the heat treatment durations. Increased contents of alkaline earth and transition metal oxides could contribute to the growth of diopside crystals, while boron oxide tended to inhibit it. Finally, a prediction model correlating the average crystal size, compositional variation, and heat treatment durations was discussed.

This study explored the impact of microstructure and residual stresses on the fracture behavior of as-deposited thermal barrier coatings (TBCs). Two distinct air plasma sprayed TBCs, Coating A (conventional lamellar porous) and Coating B (dense vertically cracked), were investigated. Coating A involved coarser but less dense powders as feedstock and a lower substrate temperature during deposition. Further, Coating A had $��\sim �2.5�$ times higher randomly oriented porosities, finer grains, lower hardness, and elastic stiffness. Strikingly, however, the fracture strength was higher for the porous as-deposited Coating A. The answer to this apparent contradiction emerged from the intergranular residual stresses. These were measured using both X-ray diffraction and high-resolution-electron backscattered diffraction. Coating B, deposited at a higher substrate temperature, had clear growth selection of $��\left(��1��0��2��\right)�$ oriented grains. These also had more out-of-plane normal and shear residual stresses. The growth selection induced residual stresses appeared responsible for the decohesion of Coating B from the substrate and, correspondingly, lower fracture strength.

Tailoring of the stoichiometry, crystallinity, and microstructure of manganese oxides (MnO_x) is of utmost importance for technological applications in the field of catalysis, energy storage, and water splitting. In this work, α-Mn₂O₃, α-Mn₃O₄, and MnO thin films with defined stoichiometric compositions and crystal structures were prepared by calcination of an anodically electrodeposited Mn-oxyhydroxide precursor film in different gas atmospheres (air, inert, or reducing gas). The crystal structure and composition of the precursor and product films were determined by combining X-ray diffraction, transmission electron microscopy, Raman spectroscopy, Rutherford backscattering spectrometry, and elastic recoil detection analysis. The anodically electrodeposited precursor film consists of nanocrystals of α-Mn₃O₄ dispersed in an amorphous MnOOH matrix phase, and can be fully transformed into either crystalline α-Mn₂O₃, α-Mn₃O₄, or MnO upon calcination in an oxidizing, inert or reducing atmosphere, respectively. In situ high-temperature X-ray diffraction was applied to derive the phase transformation kinetics, resulting in a corresponding activation energy which decreases in the order α-Mn₂O₃ (268 kJ/mole) > MnO (102 kJ/mole) > α-Mn₃O₄ (60 kJ/mole). The disclosed synthesis routes for the preparation of single-phase MnO_x films with a defined crystal structure and stoichiometry can be exploited for a wealth of applications.

Based on the Matusita–Sakka equation and the exothermic peaks in the differential thermal analysis (DTA) curves, in silicate glasses with complex crystallization processes containing the precipitation of two or more crystal phases, the effects of particle sizes on the calculations of crystal growth dimensionality and activation energy of crystal growth have been studied in depth. In crystallization processes with two or more crystal phases but one exothermic peak, 0.3 mm is considered as the boundary between the fine and the coarse particles in this type of glass. For glass samples with a particle size less than 0.3 mm, the crystal growth dimensionality is a three-dimensional mechanism, and E_G increases slightly with decreasing particle size. For glass samples with a particle size greater than 0.3 mm, the crystal growth dimensionality is a two-dimensional mechanism, and E_G increases with decreasing particle size. The E_G for the fine-particle starting material is much lower than that of the coarse-particle starting material. In crystallization processes with two or more crystal phases and two exothermic peaks, 0.104 mm is considered as the boundary between the fine and the coarse particles in this type of glass raw material. For the first peak, in glass samples with a particle size less than 0.104 mm, the crystal growth mechanism is mainly one-dimensional growth, and E_G increases slightly with decreasing particle size. And for glass samples with particle size greater than 0.104 mm, the crystal growth mechanism is mainly two-dimensional growth, and E_G decreases with decreasing particle size. For the second peak, the crystal growth mechanism is mainly a three-dimensional growth, and E_G increases with decreasing particle size.

Using the method of non-reactive sintering of nanocrystalline powders, highly transparent samples of optical ceramics based on solid solutions of yttrium-scandium aluminum garnets containing 50 at.% scandium in the dodecahedral position and doped with erbium (1 at.%) and ytterbium (0–49 at.%) cations (YSAG:Yb:Er) were produced. The influence of Yb³⁺ concentration on the crystal lattice parameter, density, refractive index, stability of YSAG:Yb:Er solid solutions during high-temperature sintering, thermal conductivity, optical transmittance and absorption coefficients in the wavelength range from 200 to 1100 nm, as well as Stokes and anti-Stokes luminescence, was determined. The revealed patterns make it possible to fine-tune the physical and chemical characteristics of ceramics for the design of composite laser materials.