Theoretical Study On Thermoelectricproperties Of Al-Based Binary Semiconductors

High efficiency thermoelectric semiconductors are considered to be significant components for power generation devices that can convert waste heat into electric energy.And they are potential to address both the energy crisis and environmental pollution.Generally,the thermally-driven electrical performance of thermoelectric materials is measured by the power factor?PF?.The conversion efficiency of thermoelectric materials for a given temperature difference is gauged by the dimensionless figure of merit?zT?.A thermoelectric material with high conversion efficiency not only needs a high power factor,but also it has to possess a poor thermal conductivity.Improving the conversion efficiency of previous thermoelectric materials and identifying novel materials with intrinsically high thermoelectric performance are the major objective pursued by the thermoelectric material science.First-principles calculations in conjunction with the semi-classical Boltzmann transport theory and deformation potential theory have been adopted to study the effect of isoelectric substitution and low-dimensional structure on thermoelectric properties of D0₃ type gapless semiconductor V₃Al and Al X?X=S,Se,Te?monolayers.As a result,the thermoelectric properties of the gapless semiconductor V₃Al are optimized by substituting Al with isoelectric element Ga in the D0₃ structure.In particular,these two-dimensional semiconductors could alleviate the coupling between Seebeck coefficient and electronic conductivity due to the quantum confinement effects,consequently enhancing thermoelectric performance.The main contents and results are listed as follow:1.Thermoelectric properties of D0₃ V₃AlThe structural,electronic,and thermoelectric properties of D0₃ V₃Al in antiferromagnetic?AF?phase are investigated.The structural results are consistent with other theoretical and experimental data.AF-D0₃ V₃Al is verified to be a gapless semiconductor.Based on the semi-classical Boltzmann theory in combination with deformation potential theory,the thermoelectric properties of AF-D0₃ V₃Al have been predicted.V₃Al exhibits higher Seebeck coefficient,thus resulting in a better thermoelectric performance.The maximal zT value of p-type AF-D0₃ V₃Al reaches 0.32 at carrier concentration of 1.59?10²² cm^-3 and temperature of500 K.Further studies should be focused on reducing the thermal conductivity to enhance the TE performance of AF-D0₃ gapless semiconductor V₃Al.2.Effects of Ga substitution on electronic and thermoelectric properties of gapless semiconductor V₃AlStructural and mechanical stability,electronic structure and transport properties of V₃Al_1-xGa_x?x=0.25,0.5,0.75,1?compounds have been studied based on first-principles calculations with the combination of the semi-classical Boltzmann theory and deformation potential theory.All the compounds are structurally and mechanically stable gapless semiconductors.They show narrow energy gaps of 0.04 eV?x=0.25,0.5,0.75?and 0.06 eV?x=1?and exist as antiferromagnetic gapless semiconductors.Ga substitution in pure V₃Al makes the energy band near the CBM and VBM a little flatter,which results in an enhanced effective mass,but contributes more to the decreased thermal conductivity.Thus,the maximal zT is enhanced from0.242?n-type?and 0.259?p-type?to 0.244?x=0.25?and 0.243?x=1?for n-type systems and0.268?x=0.25?and 0.275?x=1?for p-type systems at room temperature.As a result,the changed band structure induced by Ga substitution leads to greatly decreased thermal conductivity and enhanced thermoelectric efficiency.3.High thermoelectric performance of AlX?X=S,Se,Te?monolayersBy using the first-principles calculations in combination with the Boltzmann trasport theory,we systematically study the thermoelectric properties of monolayer layered Al X?X=S,Se,Te?semiconductors.The unique electronic density of states,which consists of a rather sharp peak at the valence band maxima and an almost constant band at the conduction band minima,makes AlX?X=S,Se,Te?excellent thermoelectric materials.The optimized power factors at room temperature are 22.59,62.59,and 6.79 mW/mK² under reasonable electronic concentration for AlS,AlSe and AlTe monolayers,respectively.The optimized zT of 0.52,0.59,and 0.26 at room temperature are achieved under reasonable electronic concentration for AlS,AlSe and AlTe monolayers,respectively,indicating that two-dimensional layered AlX?X=S,Se,Te?semiconductors,especially AlSe can be potential candidate matrices for high-performance thermoelectric nanocomposites.