Papers by Author: Xin Xing

Abstract: SiC ceramics were synthesized by polymer derived method and microwave sintering processes. The as-produced SiC composites were developed by oxidizing the amorphous Si-C raw materials derived from a polymer precursor polycarbosilane. The products analyzed by X-ray diffraction (XRD), Fourier transmittance infrared spectroscopy (FTIR), and electronic microscopye consist of excess free carbon, β-SiC and SiO₂ produced by the partial oxidation of amorphous Si-C on the surface of the starting particles. The thermophysical properties of the partially oxidized SiC composites were investigated by measuring the thermal diffusivity, the specific heat and thermal conductivity. The amount of SiO₂ increased as the sintering temperature is increased, resulting in a reduction of thermal diffusivity and an increase of density due to improved liquid phase sintering of SiC particles. The thermal conductivity of samples decreased, instead of increased, as the sintering temperature was increased from 1100 °C to 1200 °C. The sample dwelled at 1200 °C, in this study, has highest density and a relatively low thermal conductivity, slightly over 0.7 W/(m·K) at room temperature.

Abstract: The thermoelectric SiC structural materials for supersonic vehicles can convert intense aerodynamic heat to electricity simply by temperature difference. A general model of nose tip is developed to predict the thermal-electrical energy con-version performance of the thermoelectric SiC materials. The temperature distributions of model was obtained by solving the Navier-Stokes (N-S) equations and the heat conduction equation. The largest temperature difference between the hot side and cold side of the hypothetical thermoelectric module is about 275 K. With the thermoelectric properties assumed constant in the presence of temperature gradient, the output power and thermoelectric efficiency of the model are calculated. The maximum of thermoelectric efficiency and output power of the model are 0.4×10^-3 W and 1.6×10^-4 %, respectively, at a current of 0.014 A. The thermoelectric performance of the model shows great potential for the application of SiC ceramic structures to thermoelectric generation from aerodynamic heat on supersonic vehicles.

Abstract: Generally, high thermal conductivity of SiC ceramics is required. However, in some circumstances, low thermal conductivity SiC is also needed, for example, for good thermoelectricity or heat insulation. In this work, the product of pyrolysis of polycarbosilane (PCS) is chosen as raw materials. The microwave sintering process was applied to obtain low thermal conductivity at a relatively low sintering temperature. The microwave sintering processes were performed at 1200 °C under Ar atmosphere with dwelling time of 10, 20 and 30 min. The morphology and phases were analyzed by electric microscopy and X-ray diffraction (XRD), respectively. The thermal diffusivities, specific heat and thermal conductivities were measured by laser-flashed method using a thermal analyzer. The thermal conductivity of samples is within the range from 0.81 to 1.37 W/m•K. The sample with dwelling time of 30 min shows potential for multiple applications, due to its good crystallization in XRD spectra and relatively low thermal conductivity in the considered temperature range. The results also suggest a phase transformation at a dwelling time near 20 min that may have occurred.

Abstract: Polymer derived SiC ceramics usually present a relatively low thermal conductivity for the large porosity and complex phases. In order to obtain condensed SiC ceramic with low thermal conductivity, a preceramic precursor polycarbosilane (PCS) was selected as the raw materials. And hot press sintering processes were performed at 1600 °C under Ar atmosphere with the holding time in the range of 20 to 40 min for consolidation. The microstructures and phases were analyzed by scanning electron microscopy (SEM) with energy disperse X-ray spectroscopy (EDS), X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM). The specific heat, thermal diffusivities and thermal conductivities were measured and investigated from room temperature to 650 °C. A minimum thermal conductivity of 4.13 W•m-1•K-1 was obtained at 650 °C with a holding time of 30 min.

Abstract: The mesoporous tungsten oxide (WO₃) films were derived from the peroxotungstic acid (PTA) sol with templates through sol-gel method. Polyethylene glycol (PEG) 400 and tri-block polymer P123 (HO(CH₂CH_2O)₂₀(CH₂CH(CH₃)O)₇₀(CH₂CH_2O)₂₀H) were chosen as templates. The structural, morphological, optical and electrochromic properties of the WO₃ thin films derived from different sols were studied. The composition and crystal phase of the films change at different annealing temperatures. The films derived from the sols containing templates have higher crystallization temperature than those without templates. And the morphologies are distinctly different from different sols. The addition of the templates can improve the electrochromic properties of the WO3 films, and those prepared from the 3% of P123 sol show the best electrochromic properties. The highest transmittance modulation is near 60%, and the largest ion inserted and deinserted diffusion coefficient can reach 5.706×10^-12 cm²/s and 1.271×10^-11 cm²/s, respectively.

Abstract: A melt-spinnable precursor for SiC based fibers was prepared from blend polymers of polycarbosilane (PCS) and modified polymethylsilane (M-PMS). The blend polymers cured at 320°C are different from M-PMS and PCS. The ceramic yield of these blend polymers is about 83%. The C/Si ratio of M-PMS/PCS derived ceramics (pyrolyzed at 1250°C) is linear to the content of MPMS in M-PMS/PCS. After melt spinning, thermal oxidation curing, and pyrolysis, Si-C-O fibers were obtained. The diameter and the tensile strength of the resulted fibers are 16.5μm and 1.62GPa, respectively.

Abstract: Silicon carbide coatings on graphite were prepared through polymer vapor pyrolysis deposition process (PVPD) under N2 atmosphere. During this process, some low molecular weight substances that polycarbosilane (PCS) pyrolyzed can be deposited on graphite, and they can convert into SiC in high temperature. The results of XRD showed that amorphous SiC coatings were formed on graphite when the pyrolysis temperature was 1000°C, andβ-SiC phase formed in the coatings when the temperature up to 1250°C. Effects of the coatings on the microstructure and properties were investigated. It was shown that the uniform dense SiC coatings could be obtained by carefully controlling the pyrolysis temperature and ramping rate when the number molecular weight of PCS was in the range of 1,000~1,500.

Abstract: A novel SiC precursor antimony-substituted polymethylsilane (APS) is present in this paper. First, pre-APS was synthesized from polymethylsilane (PMS) and SbCl3 at room temperature. Pre-APS was a liquid mixture and could be used in PIP process without solvent. After the elemination of HCl and the crosslinking of pre-APS, the novel precursor APS with the ceramic yield 91% was prepared.

Abstract: By means of high temperature solid-state reaction process, tetra-pod ZnO whisker (T-ZnOw) was doped with Fe(NO3)3. The doping induced the lattice distortion of the T-ZnOw, and improved its electrical properties significantly. After doping, the volume resistivity of the T-ZnOw decreased from 1010 &·cm to 103 &·cm, and its dielectric constant changed correspondingly.