Advanced Materials Research Vols. 105-106

Abstract: GaN nanopowders were synthesized by ammoniating pure Ga2O3 powders at 1000°C for different soaking time. The results show that the growth of GaN at this temperature is based on vapor-solid mechanism. The ammoniating reaction finished and the rearrangement process started when the soaking time is 0.5 h. When the soaking time is 1.5 h, the process of high-temperature aggradation transformed to thermal decomposition-re-crystallization process. Further increasing of soaking time, the fabricated GaN particles with large size aggregated by directional fixed nanorods are substituted by self-governed particles, which agglomerate as honeycomb. Photolumine- scence measurement (PL) test reveals that as prepared sample with 1.5 h possesses the best luminescent property.

Abstract: Nano-size BaM hexaferrites (BaLa0.5Fe11.5O19) were synthesized in the presence of SiO2 nano- particles by sol-gel method. The X-ray diffraction (XRD) patterns of ferrites annealed at above 900°C manifested that all the species had hexagonal crystal structure and no diffraction peaks of SiO2 found. Compared with BaLa0.5Fe11.5O19 powders, those synthesized in the presence of SiO2 nano-particles presented in smaller powder size, as proved by scanning electron microscopy (SEM). As a result, SiO2 nanoparticles promoted the formation of more homogeneous and finer ferrite crystal while the structure of hexaferrites consisted with that synthesized without SiO2 nano-particles. Microwave absorbing properties were evaluated. As a result, BaLa0.5Fe11.5O19/SiO2 showed good microwave absorbing properties with the maximum absorption more than 20dB.

Abstract: The forming mechanism of Dy2Ce2O7 prepared by solid reaction using CeO2 and Dy2O3 as reactants was analyzed by X-ray diffraction method. It is indicated that there is almost no solid solution reaction between Dy2O3 and CeO2 before 1300°C, the Dy3+ cations enter the crystal lattice of CeO2 and substitute sites of Ce4+ cations in the range between 1300°C and 1400°C which resulted in the formation of Dy2O3-CeO2 solid solution. The Dy2O3-CeO2 solid solution transformed gradually into Dy2Ce2O7 after 1400°C and pure Dy2Ce2O7 can be obtained at 1500°C. No other intermediate phases were found in the whole process.

Abstract: Ca0.6Mg0.4Zr4(PO4)6 coating was used as environmental barrier coatings (EBC) to improve the alkali corrosion resistance of SiC ceramic at high temperature. A new sol-gel based process has been developed for preparing high quality and thick C0.6M0.4ZP coating on SiC ceramic with once deposition. The prepared C0.6M0.4ZP coating on SiC ceramic is homogeneous and dense, and shows a thickness of about 200 μm when sintered at 1400 °C for 3h. The effectiveness of C0.6M0.4ZP coating in improving the alkali corrosion resistance of SiC ceramic was evaluated based on the SEM microstructural analysis, mass change and flexural strength measurements after being subjected to alkali Na-salt attack at 1000 °C for 96 h. Significantly improved alkali corrosion resistance was confirmed for SiC ceramic using C0.6M0.4ZP coating as EBC.

Abstract: Numerical simulation the temperature and stress of the SHS reactive flame spray coatings which were cooled in air and sand from a high temperature to room temperature (25°C). The distribution of temperature and stress of the coatings were discussed. The results show that the cooling velocity of air-cooled product is fast than that of sand-cooled product; temperature distribution of sand-cooled product is more uniform than that of air-cooled product in the process of cooling; the stress state of air-cooled coating is tensile stress state, while the stress state of sand-cooled coating is compressive stress state; the stress distribution and strain situation are close relative to temperature distribution, heat emission speed and geometric shape of products in process of cooling; the smooth geometric shape of the product, uniform temperature distribution and heat emission speed are beneficial to improve the state of residual thermal stress; the stress and strain state of sand-cooled coating is better than that of air-cooled coating.

Abstract: Complex rare-earth zirconates (Gd0.4Sm0.5Yb0.1)2(Zr1-xCex)2O7 (x=0, 0.3, 0.4) powders were synthesized by co-precipitation method. XRD and SEM results revealed that the complex rare-earth zirconates with single pyrochlore structure were prepared and no other phases existed among the grains. The dielectric properties of these compounds were investigated at low and microwave frequencies. It was found that they have intermediate dielectric constants and low dielectric loss in the whole frequency region, and they show the dielectric permittivity with weak frequency dependence. Substituting Zr4+ by Ce4+ leaded to smaller permittivity. Meanwhile, the dielectric contant values increase with increasing in the r(A3+)/r(B4+) value in the pyrochlore-type phase region, which can be explained by the increase in the degree of ordering of oxygen vacancy. This new series of pyrochlore type oxides would be potential candidates for electronic ceramic applications and microwave frequency components.

Abstract: CNT (carbon nanotube) was purified by acid washing method and then dispersed in alcohol. Sm2Zr2O7-CNT composite powders containing different amounts of CNT were synthesized by corprecipitation method and hot-press sintering were selected to prepare the composites. The density and thermal conductivity of the samples were tested. The micro-morphology and phase structure of composites were characterized by field emission scanning electron microscope and X-ray diffraction, respectively. Based on these experiments and theoretical analyses, the influences of carbon nanotube on the micro-morphology, phase structure and thermal conductivity of Sm2Zr2O7-CNT composite were revealed. Results showed that the carbon nantube uniformly dispersed in Sm2Zr2O7 matrix and the thermal conductivity of composites varied with the different amounts of CNT. In this article, the variation of thermal conductivity can be explained by the comprehensive evaluation of Sm2Zr2O7, CNT, pore and the heterogeneous interfaces thermal resistance between matrix and additives.

Abstract: In order to further improve the thermal expansion coefficient of LaMgAl11O19, LaMgAl11O19 with Sr2+ doped is designed to increase the density of oxygen vacancies, and the effect of doping Sr2+ to hardness and toughness was studied. SrxLa1-xMgAl11O19-0.5x pellets were synthesized by hot-pressing (1500°C, 40MPa). The relative density of LaMgAl11O19 reaches 97.68%. The hardness of LaMgAl11O19 is about 25.47GPa and the fracture toughness is 11.3 MPa•m1/2. The thermal expansion coefficient of LaMgAl11O19 was 8.71×10-6K-1, and Sr0.6La0.4MgAl11O18.7 was 8.95×10-6K-1. The results showed hot-pressure sintering could synthesize dense SrxLa1-xMgAl11O19-0.5x with magnetoplumbite structure. Sr2+ doping could increase the thermal expansion coefficient of LaMgAl11O19.

Abstract: Oxidation-resistant coatings with gradient structures on C/SiC composites were fabricated. The gradient oxidation-resistant coatings were composed of three layers. The inner layer was reaction-bonded SiC which was fabricated by silicon infiltration. The middle layer was dense CVD SiC with the gradient component which was produced by the pyrolysis of methyltrichlorosilan (MTS) in excess hydrogen. The outer layer was a sealed layer with B4C and MoSi2 powders which was fabricated by slurry process. Optical microscope and SEM were used to observe the cross-section morphology and microstructure of coatings. XRD and EDS were used to characterize the phase composition and component distribution of coatings. EDS analysis showed the component distribution of CVD SiC coating changed significantly with the change of ratio of MTS to H2. The result indicated that C/SiC composite had good oxidation resistance when it was used at high temperature in the environment of high-speed combustion gas.

Abstract: Intermetallic compound MoSi2 is useful for protective coatings against high-temperature oxidation. In this study, MoSi2-based coatings were prepared using electrothermal explosion ultra-high speed spraying methods. The spraying distance from electric exploding chamber to substrate are 5mm, 10mm, 15mm and 20mm, respectively. The microstructures, phase structures and hardness of the coatings were investigated by scanning electron microscope (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and Vickers hardness tester, respectively. The result indicates that MoSi2 primary phase is synthesized. When spraying distance is 10mm, the coatings have density microstructure, submicron grains and high microhardness. The bonding of coating-substrate is metallurgical. The oxidation resistance of Mo-Si-Al coating is higher than that of Mo-Si coating.