The Theoretical Study On The Mechanism Of The Pressure Effect On The Thermoelectric Properties Of CoSb₃System

Thermoelectric material （TE）, one of the functional materials, can convertbetween the heat and the electricity directly. It depends on the movement andinteraction of carriers （electronics and holes） in the solids. The advantages ofthermoelectric materials devices are as below: small、 light、 portable、affordable、clean and the ability for spot‘cooling system, and no pollutants are released to theenvironment. Thus the TE materials are not harmful to the environmentally and haveimmense values and application prospects. With the energy crisis and environmentalpollution become seriously all over the world, researchers pay much more attentionon the exploration of new TE materials.The efficiency of a TE material is determined by the dimensionless figure ofmerit, ZT=S2T/κ, S is the Seebeck coefficient （Thermoelectric voltage）, κ is thethermal conductivity, is the electrical conductivity, T is the absolute temperature,S2is the power factor （P）. Here, thermal conductivity is contributed from electronicconductivity eand thermal electronic conductivity L,=e+L. From thedefinition of ZT, a good thermoelectric material should have a larger power factor（S2） and low thermal conductivity, but it has a forbidden mission to improve ZT over2because of the conflicting between the S,, and which means improvement ofone parameter will usually adversely influence the others. Therefore, it is quite hardoptimize these variables simultaneously in conventional bulk materials since thesethree parameters as a function of carriers （electrons or holes） concentrations arealways restricted with each other. Therefore，up to now, energy converting efficiencyof several categories of thermoelectric materials are relative low, the development ofhigh performance novel thermoelectric materials should be the principal to beconcerned for the researchers. Recently, the filled skutterudites material have attract great interest by both theexperimentalist and the theorist for their high performance on ZT values since it is aneffective way of reducing the lattice thermal conductivity by filling the lattice voidswith impurity atoms. A lot of elements had been tried in the voids to enhance thethermoelectric properties, but normally these researches carried out in the ambientpressure or lower pressure. With the development of high-pressures technology, a lotof good CoSb₃and CoSb₃-based compounds skutterudites had been found, howevernot too much reports on a big range pressure induced properties of these compounds,especially on the main part, CoSb₃itself, which is really important for understandingthe mechanism of the skutterudites CoSb₃and the CoSb₃-based skutterudites indifferent pressures. In our studies, we present the result of theoretical investigation ofthe electronic structure of pure CoSb₃and transport properties of skutterudites CoSb₃under different pressure. The purpose of our work is to provide a trend of TEperformance under pressure for the pure skutterudites CoSb₃.Prediction anddevelopment of the novel thermoelectric materials also benefit from thefirst-principles method. We use first-principles electronic structure calculations in theframework combining with Boltzmann transport theory to investigate the electricaltransport properties.First part, we have carried out a theoretical calculation of electronic structures,and the transport properties as a function of chemical potential for CoSb₃underdifferent pressures from0to70GPa are discussed. Lattice constant at zero pressureagreed well with the experiment data for the CoSb₃lattice. CoSb₃has a large densityof states near the Fermi level which proves that this compound is a goodthermoelectric material from the DOS. P-type direct band gap semiconductor CoSb₃changes to indirect band gap with the pressure increasing and the band gap enlargedby the enhanced pressure. The electrical conductivity of undoped CoSb₃significantlydecrease with the pressure increasing from0to10GPa, the Seebeck coefficient ofundoped CoSb₃significantly enhanced with the pressure increasing. The n-typedoping Seebeck coefficient significantly enhanced in the low doping region, theextreme value of Seebeck coefficient is1193V/K for n-type doping at40GPa. Asfor the electric conductivity respect to, it could be only p-type doping system could be enhanced with pressure increasing in the high carrier concentration region. At last,the power factor for both n-type and p-type doping system have significant increasewith the increase of the carrier concentration in the low doping region. The maximumvalue reaches43.121014μW·cm1K·2s·1at20GPa which is much higher than thatof39.05μW·cm1K·2s·1at0GPa. The thermoelectric properties of CoSb₃could beeffectively improved under some certain pressures, not the higher pressure we put thelarger power factor we get, and n-type doping in CoSb₃would be more favorable forenhancing the thermoelectric properties than that of p-type doping under pressure.In the second part, the crystal structure, the electronic structure, effectivemass, and the influence of the vibration energy of different filling fraction ofsingle filled Ba and Ca in CoSb₃were calculated. For these different singlefilling, the lattice structure were expanded for the filling of the Ba and Caatom in the CoSb₃crystal structure which finally result in the linear band gapchange with the filling fraction of Ba and Ca atom. The Fermi level moveupwards to the conductive band with the growing of the filling fraction, the ptype CoSb₃become to n type with the growing of the filling fraction atom. Theeffective masses of the conductive band minimum and the valence bandmaximum for the single filling CoSb₃were also calculated. The effectivemasses were enlarged with the increasing of filling fraction of Ba and Ca atom.According to the results from the lattice vibration energy of the fillers in CoSb₃, wecould find the lattice vibration energy was induced by the filling of Ba and Ca atoms.It is also predictable that two filler of the CoSb₃that Ba and Ca atoms combining withthe rare earth elements such as La, Ce, In, Eu that would have much more significantdeduce of the lattice thermal conductivity, and further enhance the ZT value.