First-Principle Studies Of SnS And CrN As Thermoelectric Materials

Thermoelectric materials are developed to convert a thermal gradient into electricity and widely used in fields of the high-tech industry and the military defense.However,the commercially available thermoelectric material BiSb suffers from high toxicity,too much side reaction,and low thermoelectric efficiency,which seriously limit its extensive application.Hence,it is of great urgency to design and develop a novel environment-friendly thermoelectric material with excellent thermoelectric efficiency.With the development of modern science and technology,simulation calculation appeared to be a powerful tool in the structural analysis of thermoelectric material.Through the calculation of the atomic scale of material,we can accurately predict some properties of the material and explain the phenomena in the experiment,and then provide the theoretical basis for experimental research.In the thesis,we investigated two potential thermoelectric materials SnS and CrN,and their energy band structures and density of states results were discussed in detail.First,it tended out that SnS was nondirect-gap semiconductors and the energy gap equaled to 0.733 eV.Meanwhile,the experiment results revealed that SnS was more likely to obtain better thermoelectric properties at higher temperatures,which was consistent with the existing experimental results.By comparing the transport properties of the material at two different temperatures(500K and 700K),we found that SnS got much larger power factor at 700 K,which was similar to previous band structure analysis result.Furthermore,based on the analysis of the variation of the Seebeck coefficient,conductivity and power factor with the Fermi level,it was shown that the doping of P type was better than that of the N type doping for the tin sulfide system.Next,we selected six elements Na,Mg,Al,Si,Ge and Pb as doping elements to analyze the energy band structures and the density of states of each system.We found that Al and Pb doping were more likely to get a higher Seebeck coefficient and electrical conductivity then a higher ZT value.Another material system of chromium nitride was proved to have the Fermi level in the valence band and showed the characteristics of metal with high conductivity.By including spin polarization or not,a preliminary analysis of the Seebeck coefficient and the conductivity of the material was performed.It turned out that the Seebeck coefficients were similar,but the conductivity of the material significantly increased as well as the power factor by including spin polarization.Through the theoretical calculation,we can reveal the electronic structures and the semiconductor properties of the materials.Then,we can use the Seebeck coefficient,electrical conductivity and thermoelectric power factor to predict the thermoelectric properties of materials to provide guidance for the following experiments.