Electromagnetic Field Chemical Vapor Deposition Isotropic Pyrolytic Carbon And Microstructure Research

The micro-positive pressure Chemical Vapor Deposition (CVD) and negative pressure CVD methods were proposed and employed for preparation of high density and bulk isotropic pyrocarbon on the basis of the principle of CVD in electromagnetic field which was proposed in the early research by our lab for the first time. With self-designed CVD furnace, using graphite as substrate, propylene as carbon source gas, nitrogen and hydrogen as carrier gas and dilution gas, the isotropic pyrocarbon products with average density from 1.03～1.76 g/cm3 and thickness from 5～22mm were fabricated, and the deposition conditions are as following:deposition temperature 1000～1400℃for 20～40h, propylene flow from 0.1～1.0 L/min, hydrogen flow from 0～0.6 L/min, and nitrogen flow from 0～15 L/min.The degree of graphitization, microstructure and surface morphology as well as fracture characteristics of pyrocarbon were studied by X-ray diffraction (XRD), polarized light microscopy (PLM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM), respectively. The results reveal that as-prepared pyrocarbon is optically isotropic and has no growth charaterization when its degree of graphitization is less than 20%. Pores with a variety of pore size distribute in the isotropic pyrocarbon. Moreover, the pore morphology gets more well-distributed and the pore diameter get smaller with the density of materials increasing. The fracture characteristics of isotropic pyrocarbon show that the isotropic pyrocarbon are mainly composed of the spherical pyrocarbon granules with diameter from 100nm～3μm, and spherical pyrocarbon granules more or less conglutinated with each other. With the increase of material's density, the conglutinated area gets larger. The microhardness, flexural strength and compression strength of materials were also researched. It shows that the microhardness, flexural strength and compression strength of the products all get huger with the increase of material's density.Deposition temperature and carbon source gas concentration which strongly influence the deposition of isotropic pyrocarbon were studied. And the optimization parameters for CVD isotropic pyrocarbon were obtained. While the deposition temperature was 1300℃, propylene flow 0.2L/min, hydrogen flow 0.6L/min and nitrogen flow 4L/min, the density of isotropic pyrocarbon fabricated by micro-positive pressure CVD method was 1.76 g/cm3. In contrast to micro-positive pressure CVD method, the isotropic pyrocarbon with the density 1.72 g/cm3 and the lowest porosity was obtained by negative CVD method under the conditions of the deposition temperature 1300℃, propylene flow 0.2 L/min, hydrogen flow 0.2 L/min and nitrogen flow 0.3 L/min.Combining the experimental results, a new deposition mechanism of isotropic pyrocarbon -electromagnetic field fluidization deposition mechanism was discussed. The presence of electromagnetic fields is considered to promote the deposition of isotropic pyrocarbon.