Atomistic simulation of defects and thermal conductivity in oxides
Author(s)
Lu, Haiming
Type
Thesis
Abstract
Atomic scale computer simulation techniques are used to predict the thermal conductivity
and defect properties in Li2O and Bi4Ti3O12 ceramics. Li2O represents
a simple model system while Bi4Ti3O12 is a complex layered oxide ceramic with
potential engineering applications due to its highly anisotropic thermal conductivity.
The efficacy of the available pair potential models for Li2O are evaluated by comparing
with experimental properties such as lattice parameters, elastic constants,
thermal expansion and bulk modulus. Other properties, such as the Li ion superionic
transition temperature, activation energy and diffusion coefficients for
lithium diffusion are also compared with experimental data. The empirical potential
set of Chroneos et al. [1] was found to best replicate the experimental
diffusion data; therefore this potential was used to calculate the reaction energies
of the intrinsic disorder properties and then used in the investigation of the
thermal properties of Li2O.
The thermal conductivity of Li2O was calculated using the velocity exchange
methodology within molecular dynamic simulations. The calculated thermal conductivity
was compared to experimental data. A number of corrections have been
suggested to improve the agreement between the simulated thermal conductivity
and the equivalent experimental value. These corrections and the underlying
physics are also discussed within the context of Li2O.
The extent of the isotope effect on thermal conductivity was investigated by
comparing a supercell, in which there is a random distribution of 6Li and 7Li
atoms, to a supercell where all the atoms are given a fractional average mass. The
results show that the effect of a non-homogeneous distribution of Li mass can lead
to a significant decrease in the thermal conductivity at low temperatures but this
effect gradually decreased up to the superionic transition temperatures, where it
disappears altogether.
Two different structures have been proposed for low temperature Bi4Ti3O12, one
with a monoclinic space group (B1a1 ) and the other with a very similar structure but an more symmetric orthorhombic space group (B2cb). A combination of
empirical pair potential and first principles density functional simulations are
employed to compare the two structures. Both techniques suggest the monoclinic
structure has the lower energy of the two candidate structures. The enthalpy
of formation of Bi4Ti3O12 is calculated using first principles simulation. The
macroscopic properties of a material are determined by the concentration and
behaviour of point defects such as vacancies and interstitials. Therefore, density
functional theory (DFT) simulations were performed to investigate the formation
energies and defect volume of vacancy defect in Bi4Ti3O12.
The pair potential set of Islam et al. [2] and Snedden et al. [3] which replicate
the structural properties of Bi4Ti3O12 were used to predict the anisotropic thermal
conductivities of Bi4Ti3O12 in x, y and z direction. The sound velocity and mean
free paths were calculated and used to explain the temperature independent
thermal conductivity in the z direction and the anisotropic thermal conductivity.
Finally, the Debye temperature as a function of temperature was computed.
and defect properties in Li2O and Bi4Ti3O12 ceramics. Li2O represents
a simple model system while Bi4Ti3O12 is a complex layered oxide ceramic with
potential engineering applications due to its highly anisotropic thermal conductivity.
The efficacy of the available pair potential models for Li2O are evaluated by comparing
with experimental properties such as lattice parameters, elastic constants,
thermal expansion and bulk modulus. Other properties, such as the Li ion superionic
transition temperature, activation energy and diffusion coefficients for
lithium diffusion are also compared with experimental data. The empirical potential
set of Chroneos et al. [1] was found to best replicate the experimental
diffusion data; therefore this potential was used to calculate the reaction energies
of the intrinsic disorder properties and then used in the investigation of the
thermal properties of Li2O.
The thermal conductivity of Li2O was calculated using the velocity exchange
methodology within molecular dynamic simulations. The calculated thermal conductivity
was compared to experimental data. A number of corrections have been
suggested to improve the agreement between the simulated thermal conductivity
and the equivalent experimental value. These corrections and the underlying
physics are also discussed within the context of Li2O.
The extent of the isotope effect on thermal conductivity was investigated by
comparing a supercell, in which there is a random distribution of 6Li and 7Li
atoms, to a supercell where all the atoms are given a fractional average mass. The
results show that the effect of a non-homogeneous distribution of Li mass can lead
to a significant decrease in the thermal conductivity at low temperatures but this
effect gradually decreased up to the superionic transition temperatures, where it
disappears altogether.
Two different structures have been proposed for low temperature Bi4Ti3O12, one
with a monoclinic space group (B1a1 ) and the other with a very similar structure but an more symmetric orthorhombic space group (B2cb). A combination of
empirical pair potential and first principles density functional simulations are
employed to compare the two structures. Both techniques suggest the monoclinic
structure has the lower energy of the two candidate structures. The enthalpy
of formation of Bi4Ti3O12 is calculated using first principles simulation. The
macroscopic properties of a material are determined by the concentration and
behaviour of point defects such as vacancies and interstitials. Therefore, density
functional theory (DFT) simulations were performed to investigate the formation
energies and defect volume of vacancy defect in Bi4Ti3O12.
The pair potential set of Islam et al. [2] and Snedden et al. [3] which replicate
the structural properties of Bi4Ti3O12 were used to predict the anisotropic thermal
conductivities of Bi4Ti3O12 in x, y and z direction. The sound velocity and mean
free paths were calculated and used to explain the temperature independent
thermal conductivity in the z direction and the anisotropic thermal conductivity.
Finally, the Debye temperature as a function of temperature was computed.
Date Issued
2012-02
Date Awarded
2012-03
Copyright Statement
Attribution NoDerivatives 4.0 International Licence (CC BY-ND)
Advisor
Grimes, Robin
Sponsor
EPSRC
Creator
Lu, Haiming
Publisher Department
Materials
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)