Molecular Dynamics Study Of Mechanical Properties Of Bi₂Te₃ Thermoelectric Materials

Thermoelectric materials, which directly convert heat energy to electricity, can be used for cooling and power generation. Bi2Te3 based materials are the best thermoelectric material at room temperature and have been used in many practical applications due to its good thermoelectric performance. Over the past decades, nanostructured materials have attracted considerable attention. Theoretical predictions suggest that the thermoelectric materials with nanostructure show significantly enhanced thermoelectric properties. Bi2Te3 and its alloy have been given considerable attention due to its classical and quantum mechanical size effects, which enables additional ways to enhance energy conversion efficiency in nanostructured materials. However, so far there are no reports on the mechanical properties of Bi2Te3 by using molecular dynamics method. Many materials science and mechanical researcher are concerned about the issue. Using molecular dynamics, we can get the micro-mechanics information of Bi2Te3 material. The presented results may give reference for further study on Bi2Te3 nanostructured thermoelectric device.In this dissertation, the molecular dynamics simulations on the mechanical properties of Bi2Te3 bulk, nanofilm and nanowire have been performed by using the many-body potentials functions developed by Huang et.al. We studied the temperature, strain rate and the size factors on the mechanical properties and mechanical behavior of Bi2Te3.Firstly, we discussed the crystal structure and Bi2Te3 bond model, then analyzed the interactions between Bi2Te3 atoms. Through the discussions of the potential function, we choose the Bi2Te3 many-body potential function.In order to verify the reliability of present potentials, the structural properties, lattice constants, linear thermal expansion coefficients, independent elastic constants were calculated from 0 to 600 K. All the calculated results are in good agreement with previous experimental and theoretical results. These agreements confirm the reliability of the present potential functions. The simulation results enable us to predict the mechanical properties of Bi2Te3 as an effective thermoelectric material in the whole range of its working temperature.During the simulation of Bi2Te3 bulk, it is found that under uniaxial stretching, due to its marked anisotropy, the bulk shows quite unique failure behaviors in different directions. The fracture features conform to conventional brittle materials. From the evolution of atomic configurations, we analyzed their different failure mechanisms. Affected by the temperature effects, the elastic modulus, failure stress and failure strain of Bi2Te3 bulk decrease as the temperature rises.In the study of Bi2Te3 nanofilm, all the simulation results are compared with that of Bi2Te3 bulk. After the stable free-relaxation state has been obtained, the radial distribution function of Bi2Te3 nanofilm is computed to validate its crystal structure. According to the radial distribution function, it is found that the Bi2Te3 nanofilm has the same crystal structure as that of Bi2Te3 bulk material. The distributions of potential energy and stress along the thickness direction are obtained. It is found that the surface atoms possess high potential energy; the surface of the nanofilm undergoes tensile stress, while the inside undergoes compressive stress. Tension simulations have been conducted to evaluate the mechanical properties of the nanofilm. Compared with bulk, Bi2Te3 nanofilm has smaller Young's modulus, ultimate strength and failure strain. The effects of surface, size and temperature on the calculating results of Bi2Te3 nanofilm are discussed in detail.The mechanical behaviors of Bi2Te3 nanowire are investigated and the results are compared with that of bulk Bi2Te3. The simulation results show that due to its marked anisotropy, the nanowire shows quite unique failure behaviors in different directions. According to the radial distribution functions of nanowire, the peaks broaden and the intensity weakens. This indicates that the average lattice shape does not change much, but the nanowire possesses a more disordered crystal structure than the bulk. The stress-strain curves showed that the elastic modulus, ultimate strength and failure strain of nanowire are significantly reduced when compared with bulk. Similar to the bulk and nanofilm, the temperature will reduce the mechanical properties of nanowire. The study on strain rate effects shows that the failure stress and strain of nanowire are dependent on the strain-rates; high strain-rate can intensify the strength of the Bi2Te3 nanowire. This may be explained by the fact that, due to the high strain rate, the material does not have enough time to rearrange the configuration of the atomic system.