Papers by Author: Zhi Qiang Cheng

Abstract: ZrB2-SiC and ZrB2-SiC-C ultra-high temperature ceramics (UHTCs) were fabricated by pressureless sintering under an argon atmosphere. The mass and linear ablation rates were tested in an oxyacetylene flame with high velocity. The microstructure and phase transformation of the ZrB2-based UHTCs were characterized by scanning electron microscopy along with energy dispersive spectrometry. Results show that the UHTCs have excellent properties of ablation resistance at ultra-high temperature. The values of mass and linear ablation rates were lower in the ZrB2-SiC UHTCs than those measured for ZrB2-SiC-C. The effect of C addition on the ablation resistant was not obvious but it influenced the microstructure of the ZrB2-SiC UHTCs. And the ablation resistant mechanisms of ZrB2-based UHTCs were discussed according to microstructure analysis.

Abstract: In this paper, XRD was engaged in studying phase transformation of quartz fibers, SEM was engaged in studying the surface micromorphology of quartz fibers heat treated at different temperatures, and the tensile strength was measured by a single fiber strength electronics instrument. The results indicate that surface infiltration agent have been iliminated after heat treatment at 500°C, and the tensile strength decreaced significantly. The higher the temperature was, the more the tensile strength decreaced. There were no significant phase transformation and no crystallization heat treatmented at 500~800°C. But there were some round and strip bulges, and scap defects on the surface. With temperature increasing,some scab defects and bulges began to flake off, and some new rifts and cracks were formed. This was one of the important factors that decreaced tensile strength markedly.

Abstract: In this study, boron nitride (BN) fibers were prepared via chemical method using boric acid as the raw material, under the atmosphere of NH3 and N2, respectively. Firstly, the boric acid was heated to melt in order to acquire B2O3 fibers. Secondly, the B2O3 fibers were pre-azotized under a NH3 atmosphere and held for 1 h at 970°C, and the sample was then sintered in N2 to the temperature of 2000°C and held for 1 h. The obtained samples were texted to analyze nitrogen content, morphology, tensile strength, and elastic modulus by neutralization titration, SEM, XRD and so on. The results show that the diameter of the BN fibers is ~ 4-6 μm with the nitrogen content of 54%, tensile strength of 1400 MPa, and elastic modulus of 120 GPa.

Abstract: An oxidation kinetics model of hot-pressed ZrB2-SiC composites was established and then proved by experiments. The relationship between oxidation rate and temperature was attained by reaction activation energy and frequency factor at the different oxidation stages. Results showed that the relations between the oxidation gain of unit area and the oxidation time are linear, conic and parabolic, respectively, during the stage of the oxidation prophase, midterm and anaphase.