Tuning The Thermoelectric Properties Of Films By Strain Effects

The rapid development of the economy and the severe destruction of the environment have made the world’s demand for energy production,protection and management more and more intense.This has also prompted mankind to find more effective means of generating electricity.A potential source of electricity is the use of thermoelectric materials to turn heat sources into electricity.Thermoelectric materials have the function of converting heat sources into electricity.Using this feature,People can recycle a lot of waste heat that is wasted in social production.But until now,the current thermoelectric materials have very low energy conversion efficiency,resulting in the inability to widely promote it in actual life and production.Therefore,finding new thermoelectric materials with novel physical properties and increasing the thermoelectric efficiency of the currently known materials are necessary activities for achieving high thermoelectric performance.In this dissertation,the electronic structure characteristics of penta-graphene and nitrogenated holey graphene（C₂N-h2D）are studied by using the first principle.The thermoelectric properties of the two materials are fully analyzed under the semi classical Boltzmann theory.The following is a summary of the content of the paper:（1）In this paper,the thermoelectric properties of penta-graphene were studied by biaxial tensile strain.We find out from the results of the calculation,the maximum energy valence band curve between（38）and（34）by tensile strain are more curved than that in the unstrained state.This indicates a reduction in effective mass,indicating that thermoelectric properties will be improved.We found that with the biaxial tensile strain of 0%,4%,8%,11%,and 12%,the n-type Seebeck coefficient increases first and then decreases,and reaches a maximum at tensile strain of 11%.The electrical conductivity and maximum power factor of the p-type penta-graphene also increases first and then decrease.Under the biaxial tensile strain of 11%,the maximum power factor is obtained.And it is 16.1 times that of the non strain case.Like the power factor,biaxial tensile strain can also significantly increase the zT value of p-type system first and then decrease.（2）The effects of biaxial tensile strain on thermoelectric properties of C₂N-h2D were studied.From the energy band results of the C₂N-h2D,it was observed that the band gap value of the C₂N-h2D without tensile strain was 1.665 eV.Under tensile strain,the bandgap of C₂N-h2D gradually shrinks.When the strain of the biaxial strain is 4%,the highest valence band near the Fermi surface is more than the unstrained bending,indicating that the decrease of the effective mass indicates that the thermoelectric properties will increase.On the other hand,we investigated the C₂N-h2D.For the unstrained C₂N-h2D,we explored and analyzed the temperature regulation in detail.It is observed from the results that the Seebeck coefficient of the unstrained C₂N-h2D is very sensitive to temperature.In the range of temperature studied,increasing the temperature causes the Seebeck coefficient to show a decrease.In addition,the electrical conductivity and the relaxation time ratio of the n-type unstrained C₂N-h2D are much larger than that of the p-type unstrained C₂N-h2D.We also found that the electron thermal conductivity and relaxation time have a positive correlation with temperature in a given temperature range.Study on the ratio of power factor to relaxation time,we noticed that the n-type ratio of power factor to relaxation time is almost four times the p-type ratio of power factor to relaxation time.And increasing the temperature can also increase the ratio of power factor to relaxation time.When tensile strain is applied to the C₂N-h2D,the p-type Seebeck coefficient is similar to the peak value of the n-type Seebeck coefficient in the chemical potential range of-0.5 eV¹.5 eV,which shows a gradual decline.The ratio of the electrical conductivity and the relaxation time of the C₂N-h2D increases with the increase of the tensile strain.Under 8%and 12%tensile strains,the ratio of power factor to relaxation time of the C₂N-h2D is reduced.However,the ratio of power factor to relaxation time is larger than when it is not at 4%biaxial tensile strain.These works provide a theoretical basis for further improving the thermoelectric performance of the two materials experimentally.These works provide a way for finding more effective methods for improving the thermoelectric properties of materials in experimental and theoretical aspects.