Papers by Author: Si Qing Wang

Abstract: Ammonia borane (AB) hydrides have been employed as disposable hydrogen (H2) sources for fuel cell applications, due to their high hydrogen capacity. In this paper, ammonia borane (AB) complex with high purity was synthesized by chemical method, using the low cost raw materials of NaBH4, CO2, and NH3. The thermal dynamic for the synthesis process is analyzed. The phase composition for the obtained ammonia borane (AB) complex powders was detected by X-ray diffraction (XRD) characterization. The results suggest that, very high purity ammonia borane (AB) complex powders were obtained, which was quite in agreement with the standard index of ammonia borane.

Abstract: Sino Fibers Reinforced BN Wave-Transparent Composites (SiNO_f/BN) Were Fabricated through Precursor Infiltration and Pyrolysis (PIP) Method Using Borazine as Precursor. The Effect of Pyrolysis Temperature on the Densification Behavior, Microstructures, Mechanical Properties and Dielectric Properties of the Composites Was Investigated. The Results Suggest that with the Increase of the Pyrolysis Temperature from 800 °C to 1000 °C, the Density, Mechanical Properties and Dielectric Constant of the Composites Are Increased, but the Infiltration Efficiency Varies Little. At the Pyrolysis Temperature of 1000 °C, the Density of SiNO_f/BN Composites is 1.84 g∙cm^-3 and the Flexural Strength and Elastic Modulus Are 148.2 MPa and 26.2 GPa Respectively. The Dielectric Properties, Including Dielectric Constant of 3-4 and Dielectric Loss Angle Tangent of below 7×10^-3, Obtained at Three Different Temperatures Are Excellent for the SiNO_f/BN Composites Applied as Wave-Transparent Materials.

Abstract: Boron nitride coatings have been prepared by chemical vapor deposition using borazine as single precursor at 900 °C. The effect of the total pressure on the surface morphologies of the coatings was investigated. For low total pressures (≤ 3 kPa), the deposits presents a compact pebble-like surface structure. However, when high total pressures (> 3 kPa) were used, the surface of the coatings presents a loose grain-like feature. When the total pressure increases up to 12 kPa, the coatings shows a porous surface structure. The composition and structure of the deposited coatings were investigated by means of FTIR and XRD analysis. It shows that the coatings have a structure of turbostratic boron nitride.

Abstract: Braided silica fibers reinforced nitride composite (SFRN), which was prepared by the polymeric precursor infiltration and pyrolysis (PIP) process with the precursor polyborosilazane (PSBZ), was a new typed microwave transparent material with high mechanical and ablation resistance performance for high-temperature application. The thermal ablation performance of the SFRN was evaluated by the ablation equipment with the kerosene and liquid oxygen as the heating source. The ablation surface texture of the SFRN including macrostructure and roughness were measured by Three-dimensional Macrostructure and Contour Scale System (TMCSS). Results showed that there are no concurrent observation of thermal delaminations or cracks and the specimen remains intact. The SFRN has an excellent thermal shock resistance and good ablation resistance with the linear recession rate of 0.038mm/s. The ablation surface texture of the SFRN can be well illuminated by the TMCSS. And the ablation performance will be improved by enhancing material density and homogeneous intertextures.

Abstract: Toray T300 PAN-based carbon fibers were surface oxidized in air at 300, 400 and 500 °C. The composition of surface was determined by X-ray photoelectron spectrometry (XPS), and the monofilaments of original carbon fiber and surface oxidized carbon fibers were tensile tested at room temperature. Three-dimensional carbon fiber reinforced BN-Si3N4 matrix composites were prepared by precursor infiltration and pyrolysis using a hybrid precursor mixed by borazine and perhydropolysilazane. With the increase of the oxidation temperature, the content of size on the surface of fiber reduces, and the tensile strength of carbon fiber declines. Carbon fiber oxidized at 400 °C has a 93% residual strength and the fiber oxidized at 500 °C is seriously decayed. The composite reinforced by original carbon fibers exhibits excellent mechanical properties, including high flexural strength (182.3 MPa) and good toughness; while the composite reinforced by 400 °C oxidized carbon fibers is weak (only 102.4 MPa) and brittle. The distinct difference of mechanical properties between the two composite is attributed to the change of the interfaces between carbon fibers and nitride matrices.