Xin Xing

Reducing Thermal Conductivity of Polymer Derived SiC Ceramics via Microwave Sintering Processing

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.

Numerical Investigations on Thermoelectric Recovery of SiC Ceramics for Supersonic Vehicles Structures

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.

Study on Thermophysical Properties of Partially Oxidized Polymer Derived SiC Composites

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.

Theoretical Study on Photochromism and Spectrum Properties of Spiropyran

In order to elucidate the mechanism of photochromism and spectrum properties observed for spiropyran, the model complexes of spiropyran, in which different substituent and structure are key factors, have been optimized by using ab initio and hybrid DFT programs. The primary relationship between the substituent species and structure and photochromism was given by simulation calculation.