Papers by Author: Zhi Qiang Fu

Abstract: The stability of SiC and SiO₂ in helium with little water vapor is studied in this paper in order to confirm whether they can service as anti-oxidation layers on the surface of fuel elements of high temperature gas-cooled reactor (HTR) under water-ingress accident. Through thermodynamic analysis, it is found that the influence factor controlling the critical temperature of passive oxidation for SiC in He-H₂O is the partial pressure of H₂O; the critical temperature of passive oxidation for SiC increases with the partial pressure of H₂O. The influence factors controlling the critical temperature of decomposition for SiO₂ are the partial pressure of H₂O and the amount of H₂O in gas for 1 mol SiO₂, the critical temperature of decomposition for SiO₂ increases with the partial pressure of H₂O while it decreases with the amount of H₂O in gas for 1 mol SiO₂. SiO₂/SiC composite coating is more suitable than SiC coating in He-H₂O with little H₂O.

Abstract: The effect of the chemical vapor deposition (CVD) process parameters on the structure of graded SiC-SiO₂ coating was studied through thermodynamic computation. The addition of enough hydrogen into the carrier gas is necessary for the synthesis of the graded SiC-SiO₂ coating. Both high deposition temperature and low deposition temperature make the change of the composition of the coating abrupt with the change of the composition of CVD atmosphere, which is harmful to the process control of the coating. A low concentration of reactants is preferred according to the thermodynamic computation but the deposition rate is too low at a low concentration of reactants. When hydrogen is the carrier gas and the concentration of SiCl₄ is between 1 – 2 vol%, the graded SiC-SiO₂ coating with a suitably graded distribution of SiC and SiO₂ can be easily obtained through gradually changing the flow rate ratio of methane and water vapor at 1100 - 1200 °C.

Abstract: The influence of chemical vapor deposition process parameters including carrier gas composition, deposition temperature, and content of reactants on the structure of graded SiC-C coating is discussed on the basis of thermodynamic calculation in this paper. The addition of enough hydrogen into carrier gas is necessary for the fabrication graded SiC-C coating. The increase of deposition temperature benefits the control of composition in graded coating but the concentration of free Si and free C becomes high at a too high deposition temperature. A high concentration of reactants is preferred while more defects are apt to exist in coatings if the concentration of reactants is too high. The optimum CVD process parameters for graded SiC-C coating are: gradually changing the molar ratio of SiCl₄ and CH₄ from 0 to 1 when the concentration of CH₄ in hydrogen is 1-2 vol%, and the deposition temperature is 1200-1500 °C.

Abstract: Mn-Zn ferrites with different doping contents of W⁶⁺ ions were prepared by using standard ceramic technique. The microstructure and magnetic properties of the as-prepared Mn-Zn ferrites were investigated. It was found that all the samples with different contents of W⁶⁺ ions consisted of ferrite phase of typical spinel cubic structure. With increasing doping content of W⁶⁺ ions, the lattice constant of the ferrites decreased but the grain size increased. Through the measurement of magnetic properties, it was revealed that the saturation magnetization and initial permeability of the samples increased with small doping content of W⁶⁺ ions but decreased with additional doping, and the Curie temperature decreased monotonously with W6+ ion doping.

Abstract: B2O3-doped ZnO-Pr6O11 based varistor ceramics were fabricated by sintering samples at 1350 °Cfor 2h with conventional ceramic processing method. The microstructure and electrical properties of the as-prepared samples were investigated. X-ray diffraction analysis showed that after the addition of B2O3 with the amount designed in this study, no new phase was examined in the detection limit. Through scanning electron microscopy it was found that the doping of B2O3 can promote the growth of ZnO grains, and the sintering of the samples. The result of electrical properties indicated that the nonlinear coefficient and varistor voltage of the samples could be improved to some extent with appropriate doping amount of B2O3, resulting in the highest nonlinear coefficient 6.7, lowest leakage current 329 μA/cm2, and highest varistor voltage 92.4 V/mm, respectively.

Abstract: Ultrafine WC-Ni cemented carbides with addition of SiC whisker (SiCw) were fabricated by spark plasma sintering. The microstructure and mechanical properties of the fabricated cemented carbides were investigated. It was found that the addition of SiC whisker had no obvious influence on the phase compositions of the cemented carbides, but the mean grain size of the cemented carbides decreased as the addition fraction of SiC whisker increased. The fabricated WC-Ni cemented carbides presented the highest hardness when 0.75 wt% SiC whisker was added. However, the addition of SiC whisker was detrimental to the flexural strength of the cemented carbides because of the formation of inhomogeneous microstructure in the WC-Ni cemented carbides.

Abstract: WC-Ni-SiCw alloys were prepared through hot-pressed sintering by using WC, Ni, VC, TaC and SiC nano-whisker as raw materials. The influence of SiC whisker addition on the microstructure and mechanical properties of the as-prepared WC-Ni alloys was investigated. Except WC hard phase and Ni binder phase, no other phase was detected by X-ray diffraction. The observations by scanning electron microscope demonstrated that there were some micropores in the as-prepared materials, and their size and quantity increased with increasing amount of SiCw. Vickers hardness and flexural strength tests indicated that the bulk hardness of WC-Ni-SiCw alloys slightly decreased with increasing amount of SiCw; and the flexural strength increased at first and then decreased with increasing amount of SiCw. The alloys with 0.53 wt% SiCw presented the highest flexural strength of 1700 MPa.

Abstract: TiCN-based cermets with different amounts of SiC nano-whiskers were prepared by spark plasma sintering. The microstructure and mechanical properties of the as-prepared cermets were investigated. X-ray diffraction revealed that there were no SiC peaks detected, turning out some peaks of new carbide and silicate hard phases. Scanning electron microscopy indicated that there were more and more pores in the cermets with increasing amount of SiC whisker added, and the fracture mechanism of the cermets was mainly inter-granular fracture. With increasing addition amount of nano-SiC whisker, the hardness and flexural strength of the cermets increased first and decreased then, presenting the highest hardness (2170 HV) and flexural strength (750 MPa), respectively, when the addition content of nano-whiskers is 2.5 wt%.

Abstract: The combination of diamond-like carbon (DLC) coatings and fluid lubrication is an efficacious method to improve the performance and service life of the friction-pairs working under the severe conditions, but the synergistic effect of DLC coatings and lubricant additives have not been clearly unveiled. The synergistic effect of Cr-doped DLC coatings with different Cr content and the lubricant additives of ZDDP, MoDTC, and T307 on the wear of DLC-coated stainless steel sample was studied, and it was found that the wear resistance of DLC-coated stainless steel sample can be further improve by the combination of DLC coatings and fluid lubrication; doping DLC coatings with Cr at an optimum level is beneficial for the wear resistance of DLC-coated stainless steel sample lubricated by PAO, PAO+ZDDP or PAO + MoDTC, but DLC coatings with a moderate Cr content is harmful to the wear resistance of DLC-coated stainless steel sample lubricated by PAO+T307. The synergistic effect of T307 and undoped DLC coatings or Cr-doped DLC coatings with a Cr content of 23.3 % is the best, while the three studied lubricant additives are harmful for the wear resistance of DLC coatings with a Cr content of 2.4 %.

Abstract: Si-C-N-O amorphous films were fabricated at about 800°C by pyrolysis of poly- vinylsilazane precursor films in a vacuum oven under N₂ flow. The polymer precursor films were prepared onto Si wafers by spinning from a solution of the polymer. The compositions of the as-prepared films were confirmed by X-ray diffraction, Fourier transformation infrared spectroscopy, and x-ray photoelectron spectroscopy. SEM analysis indicated that the films can be formed with thickness more than several micrometers, and very smooth surfaces. Such films will be promising in application of protecting equipments in high-temperature, corrosive and wear environments.