Papers by Author: Xin Bo He

Abstract: Chemical liquid-vapor deposition (CLVD) process is a new style of fast densification, which combines the advantages of PIP process and CVI process. 2D C/SiC composites were prepared at 800~1200°C for 3~4 hours with liquid polycarbosilane and carbon fiber cloth by CLVD process with induction heating, and had the density of 1.7 g/cm3, the flexural strength of 84.6MPa, and the flexural modulus of 20GPa. XRD pattern of the sample proved that the matrix was β-SiC. It was found that SiC deposited mainly around single fiber instead of among fiber bundles and layers.

Abstract: Stitched carbon fiber cloth reinforced zirconium carbide composite (C/ZrC) was prepared by polymer infiltration and pyrolysis (PIP) process. C/ZrC-SiC composite was obtained by further introduction of SiC with PIP process in order to improve anti-oxidation property. The results show that 1.9vol% SiC addition improves the mechanical and anti-oxidation properties of C/ZrC composite. The flexural strength of C/ZrC is 247.9MPa, while that of C/ZrC-SiC is 273.1MPa. After oxidation in a muffle furnace at 1200°C for 30 minutes, the mass loss rate was reduced from 30.6% (C/ZrC) to 20.1% (C/ZrC-SiC), and the flexural strength and elastic modulus of C/ZrC were 56.7MPa and 5.7GPa, respectively, while those of C/ZrC-SiC were 122.9MPa and 17.2GPa, respectively.

Abstract: A binder comprising of 60% PW (paraffin wax), 10% HDPE (high-density polyethylene), 12% PS (polystyrene), 13% PPR (random-polypropylene), and 5% SA (stearic acid) has been selected and successfully employed to fabricate the diamond preforms possessing high strength and well shape retention capability. Various effects of processing methods on the properties of the diamond preforms have been investigated. The results indicate that the binder consisting of PW-HDPE-PS-PPR-SA possesses excellent comprehensive rheological properties and is more appropriate as the organic vehicle for fabricating porous diamond preform. It has been evaluated that the mixing of 140m size diamond particles with 20m size diamond particles in the weight proportion of 3:1 provides the best particle size distribution for preparation of diamond preforms. It is also observed that impregnation of preform with 1.7wt (SiO2)% water glass and subsequently heat-treated in vacuum at 800°C for 40min yields in high strength (with the strength of 7.1 MPa) diamond preform for infiltration. The strengthened preforms have been observed to possess an excellent shape maintaining capability. It is concluded that the diamond-copper composites fabricated by utilizing this high strength preform through hot-press infiltration method have high relative density of 96.4% and thermal conductivity of 300W.m-1.K-1.

Abstract: A novel Sn-2.5Ag-2.0Ni alloy has been developed for soldering of SiCp/Al composites substrate with various types of Ni coatings. An investigation about electroplated Ni layer, electroless Ni-5 wt.% P, Ni-10wt.% P and Ni-B layers has been carried out. It is found that the solder joints possess a single intermetallic compound (IMC) Ni3Sn4, which coarsens with an increase in aging time. The formation of Ni2SnP has been observed to significantly affect the reliability of the solder joints. But the formation of Ni2SnP can be suppressed by lowering the P contents in as-deposited Ni coatings. It has been also found that the thermal stresses generated in solder joint increases with the decrease of P contents in Ni-P layer. Furthermore, the concentration of thermal stresses in the electroplated Ni solder joint is found to be higher than that in other three electroless Ni layers. Out of four as-deposited Ni coatings, the Ni-B layer exhibits good wettability with solder and low IMC growth rate during aging. Also, the shear strength of solder joint decreases with an increase in aging time and Ni-B solder joint demonstrates the highest shear strength after long term aging.

Abstract: In this studies, Mg(Al)-50 vol.% SiC particle composites were fabricated by pressure infiltration with the two different matrix base materials of Mg and Mg-6Al alloy. Thermal conductivities of the two composites were compared and the effect of addition was studied using X-ray diffraction and scanning electron microscopy (SEM). The analysis of the X-ray diffraction shows that the Al-addition to the Mg causes a contraction of the Mg lattice parameters, which brings a better matching between the matrix and SiC particle. The microstructure and the fracture surface of the composite were characterized by using SEM. The alloying element Al exists mainly in the form of α solid solution, which has a uniform distribution in matrix. From calculation, the interfacial thermal barrier resistance of the Mg-6Al/SiCp composite is about one order of magnitude lower than that of the Mg/SiCp composite.

Abstract: Carbosilanes synthesized from high-pressure treatment of polysilane, were fractional distilled and separated. Carbosilanes, characterized by IR and 1HNMR measurement, are composed of linear and cyclic carbosilanes with some Si-H end groups. Crosslinking occurred between carbosilanes and divinyl benzene and gave gel content of 52%, and further ceramic yield of 36% upon pyrolysis at 1200°C. The pyrolyzates contain microcrystal SiC and free carbon, which was approved by XRD measurement.

Abstract: In this study, the effect of SiC powder characteristic on the rheological properties of the feedstock was investigated. It was found that the viscosity of the feedstock decreased with the increase of the particle size. For the binary powder mixtures, the viscosity of the feedstock decreased with the increase of the particle size ratio of the larger particle to finer particle, which is attributed to the increase of the tap density of the powder.

Abstract: CF/SiC and Hi-Nicalon/SiC composites were prepared by precursor pyrolysis-hot pressing, and the microstructure and fracture behavior of the composites were investigated. Because of a strongly bonded fiber/matrix interface primarily resulting from the direct reactions between the fibers and matrix, Hi-Nicalon/SiC composite exhibited a typical brittle fracture behavior. However, CF/SiC composite displayed a tough fracture behavior with extensive fiber pullout, which was primarily attributed to a weakly bonded fiber/matrix interface as well as higher strength retention of the fibers. As a result, CF/SiC composite achieved better mechanical properties of 691.6 MPa in strength and 20.7 MPa•m1/2 in toughness, which were much higher than those of Hi-Nicalon/SiC composite.

Abstract: Mechanical properties of tungsten heavy alloys are dependent on many factors including the purity of the raw materials, their tungsten content, manufacturing parameters and the microstructure of the final compact. The main object of this research was to examine the effect of sintering conditions (temperature and time) on the microstructure of tungsten heavy alloys and how the resulting modification of the microstructure can be used to optimize their mechanical properties. Alloys composed of 88%, 93% and 95% wt. of tungsten with the balance of Ni: Fe in the ratio of 7:3 were consolidated into green compacts. Samples of each of the three resulting alloys were sintered at different temperatures (1350°C,1450°C and 1500 0C) for different sintering holding times (3 and 30 minutes) in hydrogen atmosphere. Standard metallographic procedures were used to obtain SEM micrographs. The mechanical properties of tungsten heavy alloys were found to be dependent on the microstructural parameters such as W particle size, solid volume fraction, connectivity and w-w contiguity. It was shown that the mechanical properties of the alloys, and especially their ductility, are harmed when tungsten grains are contiguous.