Advanced Materials Research Vol. 66

Abstract: Polymethylsilsesquioxane (PMSQ) fiber was exposed to metal chloride vapors in a controlled atmosphere or electron beam irradiation in air to promote the curing process. The cured fibers were pyrolyzed at 1273K to compare the efficiency of individual curing method. The cured fibers were investigated by FT-IR, an optical microscope and TG analysis. In the case of successful curing, averaged diameter and tensile strength were analyzed on the obtained Si-O-C fibers.

Abstract: Silicon carbide (SiC) based fibers with continuous pore structures were synthesized by the precursor method using a polycarbosilane (PCS) and polymethylhydrosiloxane (PMHS) polymer blends. The pore formation process can be explained by hydrogen gas dissolution in the polymer melt and desaturation process of the dissolved gas during the fiber spinning. We investigated the effect of PMHS additives with different chemical and physical natures on the obtained pore structures, because PMHS decomposition process played a role of hydrogen gas source. The individual polymer melts were characterized by viscosity measurement, gas chromatograph analysis and thermogravimetric (TG) analysis in order to obtain details of pore structure control.

Abstract: The effect of Nd2O3 and Sm2O3 on the microstructure, nonlinear electrical properties, and dielectric properties of WO3-based ceramics was investigated. The dopant Nd2O3 and Sm2O3 can promote the grain growth of WO3 grains. It was also found that Nd2O3 and Sm2O3 can reduce the breakdown voltages values of WO3-based ceramics effectively, but do not strongly influence the nonlinear values. The nonlinear coefficient of doped samples was 2-3 and the barrier voltage was very low with the value of 0.04-0.08 V. Through impedance analysis, it can be found that those rare earth ions have a great influence on the dielectric properties. The dielectric constant of doped samples was higher than that of undoped samples, and the high dielectric constant makes them suitable as capacitor-varistor materials. The theory of defects in the crystal lattice was introduced to explain the nonlinear electrical behavior of the WO3-based varistor ceramics.

Abstract: The 1.2µm and 100µm silicon carbide were prepared by the process of reshaping and classification. The effect of particle morphology and particle size distribution of silicon carbide on tap density, green body density and apparent viscosity of slurry were analysized. The result showed that the particle size distribution closed to normal distribution and the particle morphology closed to spherical were the main cause that leads to the lower apparent viscosity and the higher density of the green body.

Abstract: Nanocomposites and heavy doping both are regarded as effective way to improve materials’ thermoelectric properties. 0.7at% Bi-doped Mg2Si nanocomposites were prepared by spark plasma sintering. Results of thermoelectric properties tests show that the doping of Bi atom effectively improves the electrical conductivity of Mg2Si，and the nanocomposite structures are helpful to reduce thermal conductivity and increase Seebeck coefficient, hence improving the thermoelectric performance. A maximum dimensionless figure of merit of 0.8 is obtained for the Bi-doped Mg2Si nanocomposite with 50 wt % nanopowder inclusions at 823K, about 63% higher than that of Bi-doped Mg2Si sample without nanopowder inclusions and 119% higher than that of microsized Mg2Si sample without Bi-doped, respectively.

Abstract: In hard plastic extruding course, the frictional resistance among particles and between the particles and mould make the extruding pressure 4 to 20 times larger than soft-plastic extruding. This paper study the plasticizer mechanism of water-soluble plasticizer in hard plastic extruding course, and emphasize that in this course the plasticizer liquid film not only have good combination with ceramic particle, but also has high yield strength so that the plasticizer liquid film is difficult to slide and be pulled thin. The compound usage of different plasticizers which can improve the effect of plasticizer enhances the quality of the final molding body.

Abstract: A group of boron-carbon ceramic material was in-situ synthesized and densified simultaneously via Spark Plasma Sintering (SPS) technique from carbon and boron element powders with different molar ratio. The phase structures of samples with different B/C molar ratio were characterized by X-ray Diffraction (XRD). The B/C atomic ratio of the sintered materials was calculated from X-ray photoelectron spectroscopy (XPS) measurement data. Meanwhile, the chemical analysis (CA) method had also been taken to verify the B/C atomic ratio. Finally, the experience equation had been obtained to control the B/C atomic ratio of sintered samples.

Abstract: The radome performance is evaluated by means of a computer aided design (CAD) for the wall structure of multilayer ceramic radome in accordance with the microwave propagation theory. The calculation model for the ceramic radome with A-sandwich structure is built. By optimizing the thickness of the whole wall and the thickness and dielectric constant of skins and core layer, the power transmission efficiency at a broadband frequency of A-sandwich structure ceramic radome is calculated especially. The calculation results suggest that when the wall thickness is 6 mm, the thickness ratio of surface layer to core layer is 1:15 and dielectric constant of core layer is less than 2.5, the maximal broadband transmission efficiency is obtained.

Abstract: The single phase of Bi-doped Mg2Si0.5Sn0.5 compounds have been successfully fabricated by solid state reaction-spark plasma sintering (SPS). The effect of Bi doping concentration on the thermoelectric properties of Mg2Si0.5Sn0.5 is mainly investigated. The doping of Bi atom introduces impurity energy to Mg2Si0.5Sn0.5 compounds, which results in the increase of carrier concentration ( ), meanwhile it causes the increase of crystal distortion, enhancing the scatter of phonon. The results show that with the increasing of Bi doping content, the electrical conductivity (σ) increase, the absolute Seebeck coefficient ( ) and thermal conductivity ( ) decrease slightly in the measuring temperature range between 300 K and 800K. When the doping concentration of Bi is up to 2.5at% (nominal molar percent), the sample shows a maximum value of the figure of merit, ZT, is 0.78 at 800K.