Papers by Author: Hong Jie Wang

Abstract: SiOC ceramic as anode material for lithium ion batteries has received extensive attention recently. In this work, polycarbosilane and polymethylhydrosiloxane were used as precursor polymers through a curing, pyrolysis and ball-milling progress to synthesize three kinds of ceramic powders with different components noted as SCO1, SCO2 and SCO3, respectively. The pyrolysis process of precursor polymers were investigated by the thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). And the effect of ball milling progress on the particle size of SiOC ceramic powders was researched. The microstructure of ceramic powders was investigated by scanning electron microscopy (SEM), phase and element composition was analyzed by X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS), and the tap density of ceramic powders was measured. Results show that when the curing temperature was 190 °C and the pyrolysis temperature was 900 ^oC , the ceramic powders with the particle size between 70 nm-2 μm can be obtained through the ball-milling method at the milling speed of 350 r∙min^-1 and ball-milling time of 25 h. The SiOC ceramic powders pyrolyzed at 900°C were amorphous, containing Si, C and O elements. The addition of polymethylhydrosiloxane can adjust contents of C and O in SiOC ceramics. The tap density of the powders was increased from 0.65g∙cm^-3 to 0.76 g∙cm^-3 with the addition of polymethylhydrosiloxane.

Abstract: Porous wave transmitting Si₃N₄ ceramics have been of great interest and technological importance in aerospace area. In this paper, the high performance wave transmitting porous Si₃N₄ ceramics was prepared by gel-casting method, without any pore-former. The porosity was controlled by the solid content of the slurry and the sintering temperature. The flexural strength and dielectric properties of the material were determined by three bending method and a resonant cavity method. The relationship of the dielectric constant versus the sintering temperature was discussed, and the effect of porosity on dielectric properties was investigated also. As a result, the flexural strength of the porous Si3N4 is 40-200MPa with porosity of 67-45%, dielectric constant of 2.2-3.4 in the frequency of 8GHz at room temperature. With the increase of porosity, both the dielectric constant and dielectric loss decreased. This technique is considered to be a potentially useful method to fabricate the porous ceramics with complex shape.

Abstract: The green bulk amorphous silicon nitride was prepared by cold isostatic pressing without the presence of sintering additives, using an amorphous silicon nitride as raw powder. The crystallization behavior of amorphous silicon nitride was investigated under N2, Ar atmosphere and vacuum using X-ray diffractometry (XRD). The effect of atmosphere on the process and the end products of crystallization of amorphous silicon nitride was studied. The single phase beta-Si3N4 and Si2N2O could be prepared in the different sintering atmosphere as the end product. The possible reaction process was discussed also.

Abstract: The porous SiC ceramics were prepared by two methods of pore-forming. The gas permeability and flexural strength of the porous SiC ceramics prepared by different ways were investigated. The porous SiC ceramics prepared by particle stacking have evenly pore distrubution; the gas permeability was greatly increased with the particle size increasing while the porous SiC ceramics were kept a higher flexural strength. The pore-former also can be used to form large pores to increase the gas permieability. However, these unevenly distribution large pores lead to a deterioration in flexural strength.

Abstract: Sm2O3 and MgO as a sintering additives to fabricated porous silicon nitride by reaction-bonded. The phase of as-produced silicon nitride characterized by XRD-diffraction. The microstructure of product was investigated by SEM. The samples were machined into test bar for flexural strength testing. Using Archimedes theory testing the porosity of porous silicon nitride. MgO have restrain effect on the growth of rod-like silicon nitride, because of produce a restrain layer which is MgO reacted with SiO2 on the surface of silicon., the microstructure of crystal is particle like, the maximum flexural strength is 48MPa with porosity of 35%. Sm2O3 can assistant the growth of high aspect ratio Si3N4 crystal at beginning temperature of 1300°C for it’s low melt point and low viscidity in liquid state, the as-product’s morphology is rod-like and the flexural strength is as high as 300 MPa when the porosity is 30%, high than the sample that of low rod-like crystal content.

Abstract: In order to obtain dense-porous laminated structure in green bodies of SiC ceramics, rapid aqueous electrophoretic deposition (EPD) was introduced. The suspension for the electrophoretic deposition was prepared using silicon carbide, silicon and carbon powders as the starting materials. During the electrophoretic deposition process, the intending dense and porous layers were deposited alternately to form the green body. After drying, the green bodies were reaction-bonded at 1550°C in vacuum atmosphere. Pore fraction of the porous layers could be adjusted by changing process preferences of EPD and suspension composition. Pore size and size distribution could be controlled by using different sized starting powders. Using this process, no additional substance is necessary to generate the pores via burnt-off, and the dense/porous laminated structure can be obtained by one-step sintering process.

Abstract: In this paper, highly porous Si3N4 ceramics with high strength, homogeneous microstructure were fabricated by introducing a proper amount of nanometer carbon in the Si3N4 slurry by gel-casting. Scanning electron microscopy, X-ray diffraction, Archimedes water-displacement method and three-point bending tests were employed to analyze the microstructures and mechanical properties of the sintered bodies. It was shown by the XRD analysis that SiC particles were formed in the sintered bodies. The pillar β- Si3N4 morphology, homogeneous microstructure and the SiC particles as a reinforcement phase are the contributing factors for high porosity and good mechanical behavior.

Abstract: Polyacrylonitrile (PAN) based carbon fiber felt which contains abundant various SiC microthreads and some other microstructures was prepared through sintering the pretreated felt at high temperature at low nitrogen pressure. XRD, SEM, TEM, HRTEM analyses for the sintered felt were carried out to study its components and microstructures. There are SiC nanothreads, SiC submicron threads, SiC micron threads and a few SiO2 two- or three-dimensional microstructures (possibly intermix with the similar form of SiC) existed within the inner hollow spaces of the felt. The complex permittivity, complex permeability of the sample in the X-band frequency range were obtained.

Abstract: We used ethylene glycol as pore-forming agent to prepare porous carbon with interconnected pores derived from phenol–formaldehyde resin. The mixture of resins and glycol was by polymerization and pyrolysis monolithic material of porous carbon with interconnected mesopores and a narrow pore size range. The average pore size of the porous carbon obtained was 28.2 nm. The nitrogen adsorption isotherm for the porous carbon exhibited type IV isotherm, which corresponded to mesoporous adsorption. The method could endow porous carbon with BET surface area and pore volume about 500 m2/g and 0.607 cm3/g, respectively. The mesopores in porous carbon formed as a result of phase separation of resin-rich phase and glycol-rich phase at polymerization and remove of glycol-rich at subsequent pyrolysis.