The Influence Of Impurities On The Thermal Transfer: Theoretical Simulation

Thermoelectric effect is an achievable energy conversion effect between thermal energy and electric energy, which can be used for thermoelectric power generation or thermoelectric cooling. Thermoelectric power generation and cooling technology are pollution-free and very reliable and so on. Traditionally, the term thermoelectric effect encompasses three separately identified effects:the Seebeck effect, the Peltier effect, and the Thomson effect. The application of thermoelectric materials is very extensive, not only can be used for thermoelectric power generation, refrigeration appliances, medical equipment, etc, but also can be expected to use vehicle exhaust for cogeneration. The dimensionless figure of merit ZT represents the performance of thermoelectric materials. And higher ZT values will make higher efficiency of the thermoelectric conversion. ZT=SS2σ/k, where S,σand k, are Seebeck coefficient, electrical conductivity and thermal conductivity, respectively. Currently, the low ZT values of available materials restrict the applications of this technology with large scales. Much research in thermoelectric materials has focused on increasing the ZT. One of the main methods to improve the ZT of thermoelectric materials is to reduce the thermal conductivity by adjusting the component. Today for power generation devices that operate near room temperature have a ZT of about 1. The thermoelectric material designed to replace a conventional Freon-gas refrigerator must have a ZT of equal to or larger than 3. With the development of computer technology, computer simulation has become one of the most important means to research the physical properties of materials. Molecular dynamics simulation is based on a small particle simulation to calculate the macroscopic properties.In order to investigate how the different doping methods, this thesis separately simulates a simple one-dimensional atomic chain model and two-dimensional atomic chain model through the programming. The simulation sets up the equations of motion between the grid points by using Newton's second law and the Verlet algorithm of molecular dynamics. If different impurities are doped into the system, giving an initial displacement to one end of the system, and then the lattice begins to vibrate. The other end of the system will have the end energy through the phonon transmission, by calculating the ratio of the end energy and the initial energy to determine the impact of the impurities on the thermal conduction. Through these researches mentioned above, this thesis has achieved the following major innovations:1. This thesis separately simulates a simple one-dimensional atomic chain model and two-dimensional atomic chain model's vibration through the programming, when two kinds of atomic chain are disturbed. Simulation results show that the lattice vibration will go through multiple scattering between both ends and eventually spread the energy throughout the crystal. The first energy peak which reaches the end is the maximum end energy. And the end energy will reduce after a few reflections. The results also show that any impurities will affect the thermal conduction.2. Through the programming, this thesis separately simulates the conditions that different of quantities, mass and elasticity coefficient of impurities are doped into one-dimensional atomic chain system, and get the law of obstructing energy transfer by impurities.3. Through the programming, this thesis separately simulates the conditions that different of quantities, mass and elasticity coefficient of impurities are doped into two-dimensional atomic chain system, and get the law of obstructing energy transfer by impurities.This thesis use programming to simulate the influence of impurities on thermal conductivity. The results show that:Doped within the system with the same quantity and the elasticity coefficient ratio of impurities, the mass ratio takes a set of values, both one-dimensional model and two-dimensional model will get the same result. The more similar of the mass of impurity atoms and the mass of the substrate materials, the less help to reduce the thermal conductivity. Doped within the system with the same quantity and the mass ratio of impurities, the elasticity coefficient ratio takes a set of values. The smaller of the elasticity of impurity atoms compared with the elasticity of the substrate materials, the more help to reduce the thermal conduction. Doped within the system with the same quantity and property of impurities, the more dispersed of impurities, the more help to reduce the thermal conduction.