Papers by Author: Bin Li

Abstract: The high dielectric constant of Si₃N₄ ceramic limited its application as wave-transparent materials, thus Si₃N₄ ceramics always been prepared as porous ceramics to enhance the properties of wave transparent. While the mechanical properties would be declined in this way, so the BNNTs were used to improve the properties of the composites in this paper. The porous BNNTs/ Si₃N₄ composites were prepared by normal pressure sintering in nitrogen atmosphere. Then the effects of sintering temperature and contents of BNNTs to Si₃N₄ porous ceramics and composites were investigated. The results show that the Si₃N₄ phase was transformed to β-Si₃N₄ completely when the sintering temperature was raised to 1750°C. The BNNTs and rod-like β-Si₃N₄ guaranteed the considerable mechanical properties of the composites, and the mechanical properties increased with the increase of the sintering temperature and the addition of the BNNTs. When the sintering temperature was 1750°C and the content of BNNTs was 0.5wt.%, the porosity and density of the composite are 35% and 2.0g/cm³, respectively. While the flexural strength and the elastic modulus of the composite are 231.8MPa and 62.04GPa, respectively.

Abstract: Si₃N₄-BN composites were prepared by die-pressing and precursor infiltration and pyrolysis (PIP) route using borazine as the precursor. The composition, microstructure, mechanical, and dielectric properties of the composites with different porosities were investigated. With the adoption of starch as pore forming substance, drawn the Si₃N₄ preform from the liquid precursor borazine and decrease the pressure during curing, the porosity of the Si₃N₄-BN composites were effectively increased. Along with the increase of the porosity of the composites, the mechanical properties were decreased and the dielectric properties were improved. With 20 wt.% starch and drawn Si₃N₄ preform from borazine before curing, the density, porosity, flexural strength and elastic modulus of the composites were 1.70 g·cm^-3, 29.78%, 48.05MPa and 32.45GPa, respectively. The dielectric constants and loss tangents were 4.20~4.44 and 0.48~3.42×10^-3 at the frequency 7~ 18GHz. Composites with various dielectric and mechanical properties can be designed and prepared according to the application needs.

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.