Computational investigation into the properties of potentially ultra hard boride materials
Date
2011-02-25
Authors
Letsoalo, Thabo Ezekiel
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Abstract
Boron exists in many different structures with an important similarity that they all
contain connected boron icosahedron formed from twelve B atoms. Many have the
potential to be important hard materials. The icosahedra are arranged in planes and
are bonded to each other by three centered bonds within a plane and by two centered
bonds to icosahedra in adjacent planes. It is likely that the two center bonds are
stronger than any other bonds in the crystals and may contribute signi cantly to the
strength of these materials. Various structures that hold potential for super hard
material properties are examined in the present work using ab-initio computational
techniques. Systematic trends are established. The charge density between B-B bonds
in each structure are examined and it is suggested that hardness of the material, in
part, relate to the average charge density contained on these bonds. Atoms connecting
the B12 icosahedra can donate charge that enhance the strength of the B-B bonds.
The structural and thermodynamics properties of boron icosahedral materials are
also studied using molecular dynamics (MD)simulation with the use of bond order
Terso¤ potentials and are compared with ab-initio computational results and exper-iments. Various physical quantities including the elastic constants of boron carbide
(B4C), thermal expansion coe¢ cient, speci c heat are predicted at high temperatures.
The linear thermal expansion coe¢ cient for the a and c axis are examined. Predicted
speci c heats for B4C and boron suboxide (B6O) structures obey the classical Dulong-
Petit result which is obtained at high temperatures for all solids. Moreover, thermo-
dynamic properties obtained in this work are used to estimate Gr uneisen parameters
for these potentially ultra-hard boride materials.
Finally we examined the elastic constants of an ultra hard boride B6O and some
defects in the crystal structure using the rst principle calculations. The single crys-
tal elastic constants calculated were used to estimated polycrystalline properties and
thermodynamic properties such as the melting and Debye temperaturwell as
sound wave velocities and melting temperatures of B6O and defect structures. Single
crystal elastic constants are found to be comparable with that estimated previously
from theoretical calculations and polycrystalline elastic moduli were also calculated
and analyzed systematically in comparison with available theoretical and experimen-
tal data. We also estimated the formation energies of the various structures using
chemical potentials. Analysis of the computed results shows that the formation en-
ergies of substitutional defect, nitrogen to oxygen are smaller that those of carbon
and vacancies and the low values of nitrogen substitutional defect suggest a possible
great solubility of nitrogen in B6O.