Engineering Materials III

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Authors: Bin Xu, Li Li, Mu Sen Li, Cai Gao, Ren Hong Guo
Abstract: Despite many studies have been carried, there is no clear understanding of the growth mechanism involved in the high-pressure and high-temperature (HPHT) diamond synthesis with metal catalyst, especially the problem about carbon source. In this paper, the lattice constants of diamond, graphite and Fe3C at HPHT were calculated with the linear expansion coefficient and elastic constant. Then based on the empirical electron theory of solids and molecules (EET), the valence electron structures of them and their common crystal planes were calculated, and the boundary condition of electron movement in the Thomas-Fermi-Dirac theory modified by Cheng (TFDC) was applied to analyzing the electron density continuity of the interface. It was found that the relative electron density differences across graphite/diamond interfaces are great and discontinuous at the first order of approximation, while the relative electron density differences across Fe3C/diamond interfaces were continuous. The results show that the carbon atom cluster is easier to decompose from Fe3C than from graphite and to transform into diamond structure, so the carbon source for diamond crystal growth may come from the decomposition of Fe3C instead of graphite. Accordingly, the diamond growth mechanism was analyzed from the viewpoint of valence electron structure.
Authors: Han Ning Xiao, Tao Sun, Hua Bin Liu, Yin Cheng
Abstract: The influences of B2O3/SiO2 ratio and different alkali earth metal oxides MO (M= Ba, Mg, Ca) on the crystallization behavior of B2O3-Al2O3-SiO2 (BAS) glass were investigated by means of DSC, XRD and SEM. With the reduction of B2O3/SiO2 ratio, the crystallization activation energy increases at first and then decreases, reaching the minimum value of 375.4 kJ·mol-1 when the B2O3/SiO2 ratio is 2.2. The crystalline indices (n) are all more than 4, which indicates that the glass is in bulk crystallization. When the glass was heated to 800°C, the primary precipitated crystalline phase was Al4B2O9. With the increase of temperature up to 1100°C, Al18B4O33 and Al5BO9 appeared and became the major crystalline phases in BAS glass-ceramics.

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