Molecular-dynamics and first-principles study of point defects on static and thermal properties of mullite
Date Issued
2007
Date
2007
Author(s)
Chen, Jen-Chang
DOI
en-US
Abstract
Mullite has extraordinary thermo-mechanical properties and durability; therefore, it is a strong candidate for high-temperature structural applications. From the structural viewpoints, such properties are strongly interwoven with the oxygen vacancies and Si atoms replaced by Al atoms. Based on experimental observations, several mechanisms have been proposed to explain mullite properties. However, some of them are controversial in which further insights are much needed. The objective of this work is to fulfill the needs through atomistic simulations. We applied classical molecular dynamics and first-principles simulations to study the effect of point defects on static and thermal properties of mullite.
Due to the existence of partial atom occupancy of oxygen, aluminum, and silicon atoms in mullite, we first proposed suitable atomistic models of mullite for computer simulations. We then utilized classical molecular dynamics to calculate the lattice constants with different mullite compositions, the change of lattice constants with temperatures, and self-diffusivity of each species at high-temperature. In order to clarify the effects of the single point defect and defect-pair, the ab initio total-energy calculation was used to compute the lattice constants of different defective structures and defect formation energy for each single point defect and the defect pair. We also applied the third Birch-Murnaghan equation-of-state fitting to calculate bulk modulus of mullite with different external pressures. Furthermore, ab- initio molecular dynamics simulation was applied to study the structural change of mullite at high temperature. Finally, we used the climb-image nudged elastic band method to evaluate the activation energies of two migration paths for oxygen atoms.
The simulation results show that the average structure model of mullite is suitable for the computer simulations. The increase of oxygen vacancies and Si atoms replaced by Al atoms in mullite causes the decrease of lattice constant b. The single oxygen vacancy reduces the lattice constants, and the single Si atom replaced by Al atom expands the lattices. The special defect pair (one oxygen vacancy with two Si atoms replaced by Al atoms) for mullite causes the increase of lattice constant a but decrease of lattice constant b. External pressure on all defective structures shortens all lattice constants. The lattice constants of mullite expand while temperature is increasing from 0℃ to 1200℃. At 1200℃-1400℃, lattice constants a and b increase rapidly, but lattice constant c decreases slightly. The bond lengths in AlO6 units affect lattice constants at room temperature, but at high temperature the bond lengths of SiO4 mainly influence lattice constants of mullite. We assert that such phenomenological change is related to the glass transition. This assertion is further reinforced by analysis of energy landscape. Oxygen vacancy at O(C) site and the Si atom replaced by Al atom have the lowest formation energy; therefore, these might explain why the two defects occur normally in mullite. The calculated self-diffusivity values for each species using the mean-square-displacement method do not match the experimental ones. This indicates that the mean-square-displacement method might not appropriate to study the diffusion of mullite. From nudged elastic band analysis, oxygen diffusion in mullite is the migration of oxygen atom at O(C) site. The activation energy for the oxygen to be fully hopping into the existed vacancy is around 15 eV. The value is quite high and thus such migration path is not likely happen during the diffusion. According to the experimental values and the nudged elastic band calculation, the oxygen at O(C) site moves slightly toward the existed structural vacancy only. We also confirmed such small movement in the ab-initio molecular dynamics simulation.
We have created a research process for conducting the atomistic simulation of ceramics. The anomalies of lattice constants and thermal expansion and oxygen diffusion have been revealed in this study. Moreover, the effects of point defects within the structure and equation of state of mullite with different pressure have also been studied. Despite significant progress has been made in this study on mullite simulation, much work remains to be done in the future. For example, it is stall need to study the thermal equation-of-state, to find a proper methodology for calculating self-diffusivity, to propose a better potential model for mullite and sillimanite, to calculate mechanical properties at high temperature, and to analyze the structural transformation at high temperature.
Subjects
莫來石
點缺陷
傳統分子動力學
第一原理計算
晶格常數
熱膨脹
擴散
mullite
point defect
classical molecular dynamics
first-principles simulation
lattice constants
thermal expansion
diffusion
Type
thesis