Papers by Author: Yue Zhang

Abstract: An oxidation protective coating of SiBCN(O) is prepared by slurry method, using polyborosilazane (PBSZ)-ethanol solution as the precursor. The dense and uniform coatings are obtained by pyrolysis and sintering at 1573K. The sintering temperature is significantly reduced and the densification is improved by introducing oxygen. The coating shows high oxidation protecting performance during the thermal shock between 1773K and room temperature for 10 cycles. The oxidation mechanism of the coating is studied and the results show that two stages are included: the relatively stable stage and the weight loss stage.

Abstract: In order to investigate the density anomaly of vitreous silica in the medium-or long-range order, different models were made to study the atomic configuration revolution in thermal history by molecular dynamics. The void structures have been studied through analyzing the best model that is carefully selected. The principle of void size distribution revolution at elevated temperatures was used to explain the density anomaly of the vitreous silica. The simulation results showed that when the temperature is low, the void radius increases with the temperature rising. After 2000 K, large void structures are destroyed, filled, or separated into small radius voids. In the range from 2000 K to 2400 K, large void structures decrease faster exceeding the bond extension on thermal expansion contribution, this should be the root cause of negative thermal expansion behavior for vitreous silica. When the temperature is greater than 2400 K, with the temperature rising, the normal thermal expansion is recovered gradually because number of large voids has been reduced and their destroying cannot eliminate the contribution to expansion of bond extension. Therefore, the negative thermal expansion of vitreous silica could be described by the revolution of void structures in the medium-or long-range clearly, and is mainly influenced by the existence and change of larger voids.

Abstract: Molecular dynamics simulations are performed to research the diffusion behavior of amorphous silica with hydroxyl group. Muliken analysis is employed for the determination of initial charge status of simulated systems with various hydroxyl contents. Modified BKS potentials for the interactions between introduced hydroxyl groups and other atoms, are adopted in the present molecular dynamics simulations. Short-range atomic arrangement and self diffusion coefficients of hydroxyl-doped amorphous silica systems are calculated and hereafter compared with those of pure amorphous silica. The calculation results suggest that the doped hydroxyl groups play an important role for the mobility of atoms within the system, which can be employed to the theoretical interpretation of the oxidation process of the ceramics such as silicon nitride.

Abstract: Based on the Potts model, the grain growth of Si3N4 in liquid phase sintering process was simulated by Monte-Carlo method. A two-dimensional, square lattice is used to digitize the microstructure and the components and grain orientation are distributed randomly. The periodical boundary condition is applied. In the initial simulation, the grain growth and coarsening process driven by the reduction in interfacial free energy within a complex system involving a liquid phase were investigated with 32 orientations and different fraction of a liquid phase has been considered. Simulation was carried out with 200×200 lattice. The effects of the liquid amount on the grain growth mechanism and microstructures were discussed.

Abstract: Improved genetic algorithm, combined with artificial neural network, is present for the optimal design of 2.5D braided composite. Dispersal simulation data, including maximal stresses and elastics properties, are adopted by artificial neural network for the calculation of strength property. Based on calculation method of strength mentioned above and other calculation models for other mechanical properties, genetic algorithm is employed for the design of structure parameters of 2.5D braided composite, such as wrap fiber density, fill fiber density and interface strength. These structure optimal parameters are finally optimized for practical application.

Abstract: SrZrO3@SiO2 composite materials, in which perovskite polycrystalline SrZrO3 was wrapped by amorphous SiO2, were prepared. The corrosion behavior of the samples was studied under hydrothermal conditions in 40% NaOH solution. The composite materials were easily corroded in the sodium hydroxide solution by reason of the presence of a continuous amorphous SiO2 phase. The corrosion rate increased with the increasing of the porosity because the pores provided channels, via which corrosion solution could enter into the sample, and increased the contact area between corrosion solution and the sample. There was a nonlinear relation between the corrosion and time.

Abstract: Carbon doped TiO2 and pure TiO2 thin films were prepared by atmospheric metal organic chemical vapor deposition (MOCVD) method. Both pure TiO2 and carbon doped TiO2 films are in anatase structure. X-ray photoelectron spectra (XPS) of the carbon doped TiO2 film indicate carbon atoms occupy oxygen sites and form Ti-C bonds in TiO2 lattice. In UV-VIS absorption spectra, it can be found that the carbon doped TiO2 film has a red-shifted absorption edge compared with the pure TiO2 film. In the photocatalytic decomposition experiment of Rh.B, carbon doped TiO2 exhibited remarkably visible light activity.

Abstract: The structure and magnetic properties of fcc-Fe/Cu (100) superlattices have been investigated by the first-principles total energy calculation based on density functional theory (DFT). Through the optimization of the structure of Fe/Cu superlattices, it has been found that the interlayer spacing of Cu layers is contracted while the interlayer spacing of Fe layers is expanded. There are no obviously changes of Fe/Cu interfaces for Fe3Cu3 and Fe3Cu5 models. The layer spacing for Fe3Cu5 changes larger than that of Fe3Cu3 model, which results to a slightly larger magnetic moment of FeCu5 than that of Fe3Cu3 model. We also analyze the density of state near the Fermi surface and calculate spin asymmetry factor of each layer in Fe/Cu systems. Based on the two-current model, we evaluate the magnetoresistance ratio 21.8% for Fe3Cu3 and 22.8 % for Fe3Cu5 system.

Abstract: Vitreous silica, as high temperature resistant material, has not been completely studied with the influence of extreme working conditions due to experimental limitations. In this work, the structure correlations of vitreous silica were investigated by molecular dynamics method at elevated temperatures from 0 K to 4000 K. COMPASS force field was firstly used in simulating vitreous silica. The temperature dependence of volume for vitreous silica was studied and a maximum of volume was found. The calculated density and the thermal expansion coefficient are close to experimental results. The evolutions of structure in thermal history were discussed in detail. The correlations between the average Si-O bond length and the Si-O-Si bond angle is shown in agreement with the studies used other potentials in literatures. It is proved that the COMPASS force field is appropriate for simulating vitreous silica in some extent, especially in depicting the Si-O interaction and the [SiO4] tetrahedron. Finally, the origin of the volume maxima was discussed based on the analysis of the structure.

Abstract: In this paper the composite fracture process has been simulated via the finite element method. A micromechanics model was developed to predict the stress-strain response of a SiO2f/ SiO2 composite explicitly accounting for the local damage mechanisms such as fiber fracture and interfacial debonding. The effects of interfacial strength and fiber volume fraction on the toughness of fiber-reinforced ceramic matrix composites were investigated. The results showed that the composite failure behaviors correlated with the interface strength, which could achieve an optimum value for the elevation of the composite toughness. The increase of fiber volume fraction can make more toughening contributions.