Papers by Author: Feng Cao

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: A melt-spinnable precursor for SiC based fibers was prepared from blend polymers of polycarbosilane (PCS) and modified polymethylsilane (M-PMS). The blend polymers cured at 320°C are different from M-PMS and PCS. The ceramic yield of these blend polymers is about 83%. The C/Si ratio of M-PMS/PCS derived ceramics (pyrolyzed at 1250°C) is linear to the content of MPMS in M-PMS/PCS. After melt spinning, thermal oxidation curing, and pyrolysis, Si-C-O fibers were obtained. The diameter and the tensile strength of the resulted fibers are 16.5μm and 1.62GPa, respectively.

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.

Abstract: Due to the introduction of oxygen that works to keep the shape of the fibers and enhance the ceramic yield during high temperature pyrolysis, air-curing is a critical step during the preparation of the polymer-derived Si-Al-C-O fibers (KD-A). In this work, to investigate the evolvement of oxygen in the fibers and the influence of oxygen on the mechanical properties of the resulted KD-A fibers, FT-IR, TGA, XRD, SEM and element analysis were performed on the air-curing process of polyaluminocarbosilane (PACS) fibers and the pyrolysis process of the cured PACS fibers. The results showed that the oxygen in the cured PACS fibers was originated from aluminum aletylacetanate (Al(AcAc)3) and the air-curing process, which could be regarded as a constant mass during the pyrolysis. In addition, it was found that the mechanical properties of the achieved KD-A fibers were greatly effected by the amount of oxygen in the fibers due to its inhabiting the crystallization of silicon carbide. And the oxygen content which leaded to the KD-A fibers with the highest tensile strength was found to be 8~10%.

Abstract: Perhydropolysilazane was synthesized by the ammonolysis of dichlorosilane-pyridine adduct, and used as precursor to prepare three-dimensional silica fiber reinforced silicon nitride matrix composites via preceramic polymer infiltration and pyrolysis at 1073K
in anhydrous ammonia atmosphere. The polymer-derived ceramic matrix was amorphous near-stoichiometric silicon nitride with an empirical formula of SiN1.34O0.02C0.01H1.21. The as-received composites were amorphous with a high density of 1.96g/cm3 after five infiltration-pyrolysis cycles. The process was successful to fabricate dense silicon nitride matrix composites reinforced by three-dimensional silica fiber due to the low viscosity, good wettability and high ceramic yield of perhydropolysilazane.

Abstract: The rapid development of magnetic materials has witnessed a sustained consumption increase in corundum-mullite kiln furniture application, yet a comparatively short duration of them. In this view, the thermal shock resistance of sagger plate in floppy magnets sintering turns out a factor of critical importance. This paper makes a study concerning the influence of factors including the size of critical electro-melted mullite particles, the electro-melted corundum or mullite as medium particles and the addition of upon the thermal shock resistance. The result shows that, with critical particles of 2 mm, the sheet of about 10mm thickness is characterized by better performance in modulus of rapture and thermal shock resistance. The latter proves to be more outstanding in samples rather than mullite. When zircon powder is added in a two-hour sintering process at the temperature of 1550°C, a remarkable improvement is made by the sample in terms of its thermal shock resistance. SEM analysis shows the existence of micropores in the section of samples with corundum as medium particles, whereas the glasses phase in that of mullite sample, resulting in a dense structure. Finally, zircon powder added makes for the improvement of thermal shock resistance due to the formation of baddeleyite on the particle surface of the corundum.