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0.75BiFeO 3 -0.25BaTiO 3 (0.75BF-0.25BT) ceramics have been widely studied because of their high Curie temperature, good thermal stability and lead-free trait. The evolution of domain motion during electric poling and thermal depoling processes is closely related to the piezoelectric properties and thermal stability of ferroelectric ceramics. Herein, the non-180° domain reversals of 0.75BF-0.25BT ceramics in these two processes were investigated by an in situ XRD technique. The non-180° domain switching fraction during the poling and depoling processes was calculated quantitatively based on the (111) and (11 1 ‾) diffraction peaks. During the poling process, the non-180° domain reversal percentage N 111 and piezoelectric properties increase abruptly when the poling field exceeds 35 kV/cm and then saturate at the electric field of 50 kV/cm (N 111 reaches the maximum value of 59%). The domains switched under the poling electric field of 50 kV/cm are stable below 500 °C. The depoling temperature is sensitive to the domain switching fraction under different poling electric fields, and the ceramics under a strong poling field possess excellent thermal stability and high depolarization temperatures. This work revealed the relationship between the electric properties, domain switching, poling electric field, and depoling temperature in the 0.75BF-0.25BT ceramics, which may pave the way to enhancing the room temperature piezoelectric properties and their thermal stability. [ABSTRACT FROM AUTHOR]

A series of three-dimensional molecular dynamics (MD) simulations was performed to investigate the effect of temperature on the nanoscale deformation behaviour and mechanical properties of c -plane monocrystalline gallium nitride (GaN) under nanoindentation. Simulation results showed that the atomic displacement and strain increased with increasing temperature. Thus, the nucleation and propagation of dislocations in the main slip systems were promoted to intensify the plastic deformation of GaN crystal. Increasing the temperature also contributed to the increase in local phase transition from the wurtzite crystal structure (B4-GaN) to the zinc-blende crystal structure (B3-GaN) and a very evident amorphization of GaN in the indentation zone. Based on the analysis of the load-depth curves, it was found that both the hardness and Young's modulus decreased as the temperature increased. This work can enrich the atomic-level understanding of the effect of temperature on the mechanical response of monocrystalline GaN when subjected to the external loads. [ABSTRACT FROM AUTHOR]

The demand for separation and purification applications under harsh conditions has grown strongly in recent years. Silicon carbide (SiC) ceramic membranes have broad prospects in this aspect due to their unique characteristics, but its pore size control is a crucial problem. Therefore, it is of great significance to develop simple and feasible methods for precise control of the pore size of SiC membranes to improve membrane selectivity and expand their application range. This review describes the pore formation process in the preparation of SiC membranes, focusing on the selection of SiC particles, sintering temperature, sacrificial template, sintering aids, oxidation process and other factors affecting the pore size and analysis. Finally, the control of SiC membrane pore size is summarized and the outlook is proposed. [ABSTRACT FROM AUTHOR]

In fusion reactors, tritium permeation barrier (TPB) technology is one of the key scientific technologies. α-Al 2 O 3 has been considered an ideal candidate material for TPBs. In this work, a series of first-principles calculations have been performed to investigate the dissolution, clustering and diffusion behavior of hydrogen (H) in bulk α-Al 2 O 3. The calculation results show that the most energetically stable form of hydrogen in a perfect α-Al 2 O 3 crystal under H 2 gas annealing treatment is the H 2 molecule. This can also be confirmed by the electron localization function results. The attraction between two H atoms located in first and second nearest octahedral interstitial sites (OISs) is so strong that if multiple H atoms are dissolved in α-Al 2 O 3 , these atoms can migrate toward their adjacent H atoms and form clusters, which will prevent further diffusion of H. The most stable configuration of H cluster in α-Al 2 O 3 is 2H i , with the smallest formation energy and the largest average binding energy. The formation energy and binding energy are similar to those of the gaseous H 2 molecule. We have derived the temperature-dependent diffusivity of the H 2 molecule in α-Al 2 O 3 as D (T) = (3.65 × 10 − 7 m 2 / s) e x p (− 2.27 e V / K T) , which is in good agreement with the experimental values. In addition, both the dissolution energy and migration barrier of the H 2 molecule are so high that dissolution and diffusion of the H 2 molecule in α-Al 2 O 3 are very difficult, resulting in low hydrogen permeability. [ABSTRACT FROM AUTHOR]

Abstract: The reaction process between MgSiN2 SiO2 and Si3N4 was investigated by analyzing the composition change of the powder mixture of 61wt% MgSiN2, 34wt% SiO2 and 5wt% α-Si3N4 after heat treatment at different temperatures. The phase and chemical compositions of the grain boundary phase in the silicon nitride ceramic was analyzed by x-ray diffraction, transmission electron microscope, and energy-dispersive x-ray spectroscopy. The results demonstrated that MgSiN2 reacted with the surface silica and Si3N4 to form Mg–Si–O–N liquid phase, which promoted the consolidation densification of silicon nitride powders through liquid-phase sintering mechanism. The amount of Mg–Si–O–N glass boundary phase using MgSiN2 as additives is much less than that using the same amount of MgO additive, owing to the lower oxygen concentration and higher nitrogen content. [Copyright &y& Elsevier]

Abstract: α/β Si3N4 composites with β-Si3N4 content ranging from 26% to 100% were hot-pressed with or without β-Si3N4 seeds, using MgSiN2 as additives, and their mechanical properties were investigated. When the α-Si3N4 content was over 58%, the microhardness of α/β Si3N4 composites was in the range of 23–24GPa, and then the indentation hardness decreases with decreasing the content of α-Si3N4, whether with β-Si3N4 seeds or not. The toughness increased with increasing elongated β-Si3N4 grains, which improved fracture resistance by crack bridging, pull out or the crack deflection mechanism, and reached the maximum value of 7.0MPam1/2 with 1wt% β-seeds. In comparison with α/β Si3N4 composite with a similar phase composition, the fracture strength was improved by adding β-Si3N4 seeds because of the relatively smaller grain sizes and higher toughness. The α/β Si3N4 composite with 5wt% β-seeds showed a high strength of 1253MPa, a high hardness of 20.9GPa and a toughness of 6.9MPam1/2. [Copyright &y& Elsevier]

Abstract: The present work reports the development of aqueous gelcasting of ZTA ceramic with a low-toxicity monomer DMAA. The rheological properties and the gelation behaviors of the slurries for gelcasting were investigated. It was proved that the time available for casting the slurry (idle time) can be controlled by the amounts of initiator. The ZTA green bodies exhibited a mechanical strength as high as 21MPa. After sintered at 1600°C for 2h, the highest bending strength and fracture toughness of the sintered ZTA samples were as high as 643.3±75MPa and 6.3±0.3MPam1/2, respectively. SEM photographs revealed that the green bodies and sintered part had a uniform microstructure. The volume fraction of tetragonal phase zirconia was as high as 90%. Dense ZTA green bodies and ceramic parts with complex shaped were produced through the new gelcasting system. [Copyright &y& Elsevier]