Key Engineering Materials Vols. 368-372

Abstract: Based on experiments, micro-mechanics and finite element theory, nanocomposite ceramics is simplified as representative volume cell in the paper, and in order to study the interaction of the particles, each cell concludes two nano-ceramic particles. We use ANSYS program to construct the cell model and simulate the stress field. The stress distribution and the material properties are obtained. The results show that the stress is uneven in the cell and the elements near the nano-ceramic particle with high stiffness will damage first. Finely, the stress change and damage evolution route are simulated by using the element birth and death technology. The computational results are accordance with experiments and theories.

Abstract: Rod-shaped oxide eutectic ceramics fabricated through SHS process can acquire outstanding mechanical properties. First, Eshebly-Mori-Tanaka method was used to study Thermal Stress Field of rod-shaped oxide eutectic ceramics, and then average strain of rod-shaped oxide eutectic ceramics aroused thermal inconsistency is obtained. The effective thermal expansion longitudinal strain and transverse strain of rod-shaped oxide eutectic ceramics are determined by the average strain. Rod-shaped oxide eutectic ceramics is transverse isotropy and has two independent thermal expansion coefficients. The results show that the thermal expansion coefficients of rod shaped oxide eutectic ceramics are dependent on the diameter of nano-fiber. The thermal expansion coefficients will decrease when the diameter of nano-fiber decrease. When the diameter of nano-fiber is bigger than 300nm, the thermal expansion coefficients of rod shaped oxide eutectic ceramics is constants.

Abstract: Compounds Yb2Mo3O12 and Lu2Mo3O12 were prepared by conventional solid-state reaction. Their crystal structures and thermal expansion properties were investigated. It was found that Yb2Mo3O12 and Lu2Mo3O12 adopt orthorhombic structure and show negative thermal expansion (NTE) in the temperature range of 200-800 °C. Their a-axis and c-axis exhibit stronger contraction in the temperature range of 200-800 °C, while b-axis slightly expands in the temperature range of 200-300 °C and then contracts in the temperature range of 300-800 °C. The linear thermal expansion coefficients al of Yb2Mo3O12 and Lu2Mo3O12 are −5.17 × 10−6 °C−1 and −5.67 × 10−6 °C−1, respectively.

Abstract: A new series of solid solutions Dy2-xGdxMo4O15 (x = 0.0-0.9) were prepared. These compounds all crystallize in monoclinic structure with space group P21/c. The lattice parameters a, b, c and unit cell volumes V increase almost linearly with increasing gadolinium content. The intrinsic thermal expansion coefficients of Dy2-xGdxMo4O15 (x = 0.0 and 0.25) were obtained in the temperature range of 25 to 500°C with high-temperature X-ray diffraction. The correlation between thermal expansion and crystal structure was discussed.

Abstract: Sintering models at the atomistic, particle and component scales are briefly reviewed and problems in the current generation of sintering models are highlighted. A strategy of multi-scale modelling is proposed and our progress in the multiscale modelling is reported. Mathematical details of the models are left out and the paper is set out to demonstrate what is possible in the next generation of sintering models.

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: 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: The three-layer structure back-propagation network model based on the non-linear relationship between the break percentage elongation of the Mg,Al-hydrotalcite/PE nanocomposites and the technological factors was established. And in order to accelerate the converging rate and avoid the local minimum, dimensionality reduction and pre-whitening methods were used. Moreover, the optimum technological process parameters were optimized with genetic algorithm. And the results show that using both the back propagation neural networks and genetic algorithm is very efficient for the prediction of the break percentage elongation of the Mg,Al-hydrotalcite/PE nanocomposite.

Abstract: Numerical modeling on falling of sodiumtetraborate aqueous solution drops as the initiator before the gelation of PVA-TiO2 suspensions was conducted. Effect of time and elevation angle of the PVA-TiO2 suspensions on the falling velocity of the sodiumtetraborate aqueous solution drops was analyzed. An ordinary differential equation was given. Integration of the ordinary differential equation was fulfilled using the fourth-order Runge-Kutta method in Matlab 6.5. From the model, a two-order nonlinear effect of time on the velocity of the drops during falling is determined and the quadratic term -3.408t2 serves as the time dependent air resistance. The component of the falling velocity along the suspensions increases with the increasing of the elevation angle. However, for the component vertical to the suspensions, with elevation angle increasing, it decreases.

Abstract: With Ti-B4C-C as self-reactive spray forming system, the flying combustion process of the sprayed particles was studied by means of water-quenching experiments and numerical simulation. It was found that after the particles have been heated in the oxyacetylene flame for a short time, Ti in the particles melts first and then infiltrates B4C and C. The SHS reaction of the sprayed particles takes place subsequently. Then the liquid ceramic beads appear and crystallize into ceramic grains finally. By the ANSYS finite element analysis, it can be known that the SHS reaction of the sprayed particles starts after they have left the muzzle for about 9.5×10-4s and lasts about 1.45×10-3s before the ceramic beads solidify. The calculated optimal melting distance for the spray particles is about 116mm, which is consistent with the experimental results on the whole.