First-principles Study Of Thermoelectric Properties Of Bulk TcX₂?FeAsS And Two Dimensional ?-Te

With the development of society,the energy problem has become the most serious challenge for mankind in the new century.Research shows that almost 90%of the total energy consumption of mankind comes from fossil fuels,and the demand for energy is increasing year by year.But fossil fuels are non-renewable energy.If we only rely on fossil fuels,energy will be exhausted one day.At the same time,the process of using fossil fuels will be accompanied by high emissions.The amount of waste gas causes serious pollution to the environment.It is understood that the utilization rate of fossil fuels is only about 30%,so the waste of energy is also considerable.Therefore,it is very urgent to find new energy sources and improve energy utilization rate.It is found that thermoelectric materials can realize the mutual conversion of thermal energy and electrical energy.Thermoelectric materials have been well used in waste heat recovery,microelectronics,aerospace,biomedicine,and it has the advantages of small size,light weight,no vibration,no pollution,long life cycle and so on.However,because the conversion efficiency of traditional thermoelectric materials is not high,it is difficult to achieve large-scale application.It is very important to find thermoelectric materials with high thermoelectric conversion efficiency.By using the first principle and Boltzmann equation,we studied the thermal and electrical properties of TcX₂?X=S,Se,Te?,FeAsS and?-Te.Firstly,we have studied the thermoelectric properties of TcX₂?X=S,Se,Te?.The results show that TcS₂ and TcSe₂ have high Seebeck coefficients within a reasonable doping level,and their Seebeck coefficients are all at the magnitude of 1×10^-4VK^-1.However,the Seebeck coefficients of TcTe₂ are always smaller than 2×10⁴ VK^-1.The peak of Seebeck coefficient is uplifted as doping level decreases.The power factors have a promising value and can be greatly enhanced by P-type doping along all direction of TcS₂,along x-direction of TcSe₂,and along x and y-direction of TcTe₂,while by N-type doping for other directions.It is found that when the doping level is 1×10¹99 cm^-3,the peak of power factor appears along z-direction of TcS₂ and x-direction of TcSe₂.And when the doping level is 1x10²⁰cm^-3,the power factors along x and y-direction of TcS₂and x-direction of TcSe₂ reach its peak.Meanwhile,the peaks of power factor along the x and y-direction of TcTe₂ occur when the doping level is 1x10²¹cm^-3.These results mean that we can optimize the Seebeck coefficients and power factor by proper doping,and TcX₂?X=S,Se,Te?is candidate for thermoelectric application.Secondly,we study the electronic structure and thermoelectric properties of FeAsS.The results show that FeAsS is a semiconductor with an indirect band gap of 0.73eV.The dimensionless figure of merit?ZT?has obvious anisotropy,and P-type doping has better thermoelectric performance than N-type doping.The ZT of P-type doping along x,y and z-direction can reach 0.84,0.82,0.63,respectively.The ZT of N-type doping along x,y,z-direction can also reach 0.62,0.71,0.59,respectively.The lattice thermal conductivity is extremely low,which is smaller than 1Wm^-1K^-1.The results show that FeAsS is a promising candidate for thermoelectric applications.Finally,we also study the thermoelectric properties of?-Te,a single-layer two-dimensional material.Our calculations show that?-Te is an indirect bandgap semiconductor with a bandgap of 0.63 eV.Through the systematic analysis the results,we find that the average Seebeck coefficient of alpha-Te is always greater than 200 V K^-1under the studied temperature conditions and carrier concentration range.The conductivity is also in a favorable range of 1×10⁶?^-1m^-1,and the lattice thermal conductivity is always smaller than 4Wm^-1K^-11 in the temperature range of 50 to 1500,which is always lower than 4Wm^-1K^-1,that is to say,it is very small.It is exciting to note that the thermoelectric optimal value of alpha-Te is more than 4.5.The results imply that?-Te is a thermoelectric material with excellent thermoelectric conversion performance.