Ab initio calculations of mechanical, thermodynamic and electronic structure properties of mullite, Iota-alumina and boron carbide
Aryal, Sita Ram
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The alumino-silicate solid solution series (Al₄₊₂xSi₂-2xO₁₀-x) is an important class of ceramics. Except for the end member (x=0), Al₂SiO₅ the crystal structures of the other phases, called mullite, have partially occupied sites. Stoichiometric supercell models for the four mullite phases 3Al₂O₃ • 2SiO₂, 2Al₂O₃ • SiO₂ , 4Al₂O₃ • SiO₂ , 9Al₂O₃ • SiO₂ , and ι-Al₂O₃ (iota-alumina) are constructed starting from experimentally reported crystal structures. A large number of models were built for each phase and relaxed using the Vienna ab initio simulation package (VASP) program. The model with the lowest total energy for a given x was chosen as the representative structure for that phase. Electronic structure and mechanical properties of mullite phases were studied via first-principles calculations. Of the various phases of transition alumina, ι-Al₂O₃ is the least well known. In addition structural details have not, until now, been available. It is the end member of the aluminosilicate solid solution series with x=1. Based on a high alumina content mullite phase, a structural model for ι-Al₂O₃ is constructed. The simulated x-ray diffraction (XRD) pattern of this model agrees well with a measured XRD pattern. The ι-Al₂O₃ is a highly disordered ultra-low-density phase of alumina with a theoretical density of 2854kg=m3 . Using this theoretically constructed model, elastic, thermodynamic, electronic, and spectroscopic properties of ι-Al₂O₃ have been calculated and compared it with those of α-Al₂O₃ and γ-Al₂O₃. Boron carbide (B₄C) undergoes an amorphization under high velocity impacts. The mechanism of amorphization is not clear. Ab initio methods are used to carry out largescale uniaxial compression simulations on two polytypes of stoichiometric boron carbide (B₄C), B₁₁C-CBC, and B₁₂-CCC where B₁₁C or B₁₂ is the 12-atom icosahedron and CBC or CCC is the three-atom chain. The simulations were performed on large supercells of 180 atoms. Simulated results indicate bending of the three-atom chain leads to the amorphization of the B₁₁C-CBC(B₁₂-CCC) at a uniaxial strain s=0.23 (0.22) and with a maximum stress of 168 (151) GPa. The mechanism of amorphization is analyzed with radial pair distribution function (RPDF), total density of states (TDOS), and the distribution of effective charges on atoms.
Table of Contents
Introduction -- Theoretical background -- Simulation packages and methods used -- Mechanical and electronic structure properties of mullite -- Structure and properties of iota-alumina (ι-Al₂O₃) -- Amorphisation of boron carbide (B₄C) -- Future works -- Appendix