Papers by Author: Wei Min Gao

Abstract: To improve AO resistance of polyimide, a type of polyimide/silica (PI/SiO2) hybrid film was prepared by the sol-gel process. The coupling agent p-aminophenyltrimeth- oxysilane (APTMOS) was chosen to enhance the compatibility between the polyimide (PI) and silica (SiO2). AO resistance of the PI/SiO2 hybrid films were tested in the ground-based simulation AO facility. The erosion yield of the films was 4.7×10-26 cm3/atom, decreased by two orders of magnitude compared with the value of 3.0×10-24 cm3/atom of the polyimide film. Results from FTIR, XPS, AFM on AO treated polyimide/silica hybrid films indicate the formation of a passivating inorganic SiO2 layer. The layer significantly retards the penetration of oxygen atoms, preventing further degradation of the polymer in the bulk. The addition of SiO2 in polyimide does not significantly alter the optical properties of polyimide during AO exposure.

Abstract: Raman scattering spectroscopy, ultraviolet and visible absorption spectroscopy and Rutherford backscattering spectrometry were employed to analyze the annealing behavior of defects and the optimistic effect of cavities to oxygen. It was found that the cavities had strong getting effect to oxygen and captured its neighboring implanted oxygen atom, and enhanced the formation of SixOy compound, thus helped shaping the buried oxide in a well defined region. In addition, it also minished the damage level in lattice. The interface between damage layer and crystalline layer was estimated to be 198 nm below surface of 6H-SiC. The implanted oxygen was pegged in the compressed and serried cavity layer, making the amorphous layer narrower than that of reference samples

Abstract: Hydroxyapatite nanoparticles were synthesized in the presence of two kinds of acidic amino acids in order to investigate the effects of acidic amino acids on the morphology of hydroxyapatite obtained by homogeneous precipitation and hydrothermal treating. The results of X-ray diffraction analysis and microphotographs of transmission electron microscope showed that the morphology of hydroxyapatite nanoparticles changed significantly only with aspartic acid. Hydroxyapatite synthesized with aspartic acid showed smooth flake-like morphology at pH= 11 and flake-like morphology with stripes at pH= 8. However, hydroxyapatite synthesized in the present of glutamic acid showed rod-like morphology at both pH values. Variation in the morphology of nanoparticles could be consistent with the difference in the affinity of amino acids binding at the hydroxyapatite crystal surface. The adsorption process was revealed by computer simulation of molecular mechanics using energy minimizing method.

Abstract: To avoid microcrack formation caused by the addition of heterogeneous sintering aids, nano hydroxyapatite powder was prepared by precipitation method and doped into commercial hydroxyapatite powders as homogeneous sintering aid. Gelcasting method was used to prepare these hydroxyapatite green bodies. Densification of the blended powder was performed by pressureless sintering in air atmosphere at the range 1100°C to 1400°C for 2h, with a step size of 50°C. The results showed that the nano particle can greatly improve the densification process of the green bodies, and at 1300°C, which is the highest sintering temperature for hydroxyapatite without decomposition in the air atmosphere, the relative sintering densities of the doped green bodies and the ones without doping were 93.99% and 83.13% respectively.

Abstract: Hydroxyapatite (HAp) was synthesized in the presence of a variety of amino acids in order to investigate the effect of amino acid on the morphology of HAp obtained by homogeneous precipitation and hydrothermal treating. In the results of X-ray diffraction analysis, HAp synthesized in the presence of some amino acids showed different crystallinity compared with HAp synthesized in the absence of amino acid. The results of Fourier transform infrared spectroscopy suggested the adsorption of these amino acids on HAp. Microphotographs of transmission electron microscope showed that the size and morphology of HAp adsorbed amino acids changed significantly. Collectively, this study suggests that the morphology and the crystallinity of synthesized HAp are different owing to the variation of amino acids in the synthesizing condition.

Abstract: The silica colloidal crystal multilayers were deposited from ethanol suspensions with different particle concentrations by vertical deposition method. The microstructures and thicknesses of silica colloidal multilayers were characterized by scanning electron micrography. The thicknesses of colloidal crystal multilayers increased with the particle concentration increasing within a certain range of particles concentrations. When the particle concentration exceeded 2.48wt%, the thicknesses of colloidal crystal multilayers didn’t increased with the concentration increasing. Colloidal crystal multilayers with few defects could be deposited from the suspensions with the particle concentration 2.48wt%. The optical properties of the silica colloidal multilayers were investigated by a UV-Vis scanning spectrophotometer in normal incidence. They corresponded well to the microstructure obtained by SEM images.