First-principles Study Of The Electronic And Thermoelectric Properties Of Mg₂X?X=Si,Ge,Sn,Pb?

With the accelerated process of global industrialization,the world energy crisis and environmental pollution has become a problem that can not be ignored in different country,which severely restricts the development of human.Research and development of new clean energy sources become the trend of global energy development.As a new type of renewable energy materials,thermoelectric materials have attracted great attention.Based on Seebeck effect and Peltier effect,thermoelectric materials can perform direct conversion between thermal energy and electrical energy.Due to the low thermoelectric conversion efficiency,the current application of thermoelectric technology still exists only in some special fields.Searching and exploring methods and means for enhancing the thermoelectric conversion efficiency of materials,has great significance for promoting the thermoelectric technology vigorously.The thermoelectric conversion efficiency of a thermoelectric material strongly depends on the dimensionless figure of merit ZT,and the greater the ZT value,the higher the thermoelectric conversion efficiency.In this dissertation,the lattice structure,electronic structure and the thermoelectric properties of Mg₂X?X=Si,Ge,Sn?are investigated by combining the first-principles method and the semiclassical Boltzmann transport theory,then we explore the methods for enhancing the ZT value.This dissertation is divided into two parts:?1?The electronic structure and the thermoelectric properties of Mg₂X?X=Si,Ge,Sn?are investigated by the density functional theory combined with the semiclassical Boltzmann transport theory.It is found that the transport properties of p-type Mg₂ Si,Mg₂Ge and Mg₂ Sn are better than that of n-type one at optimum carrier concentration.This is because the valence band are all comprised of three degenerate band at ? point along the ?-X direction,while the conduction band are not degenerate.In particular,a maximum ZT of 1.1 for p-type Mg₂ Sn can be achieved at 800 K with a carrier concentration of 9.8 × 1019 holes per cm3,which is higher than that of Mg₂Si?0.8?and Mg₂Ge?1.0?.The high ZT of Mg₂ Sn is mainly attributed to its low lattice thermal conductivity that is a consequence of the low velocity of the optical modes caused by the larger mass density.Most interestingly,comparised to Mg₂ Si and Mg₂ Ge,the two lowest conduction band of Mg₂ Sn at the ? point are reversed in energy because the unoccupied Mg bands lower in energy than the Sn bands.This phenomenon is very important for band engineering,as one can adjust the band gap between the first and the second conduction bands at CBM by alloying or doping.For some compersition x,Mg₂Si1-xSnx and Mg₂Ge1-xSnx solid solution or doping samples may display a convergence in energy of the two conduction bands.Thus,the band convergence stimulated by doping or alloying is a new promising approach for optimizing the properties of thermoelectric materials.?2?It is well known that a high valley degeneracy in the electronic bands is conducive to high power factor,and this in turn has stimulated attempts to engineer such degeneracy by temperature tuning?Pb Te?and heavy doping?Sn Se?.Here we present ab initio electronic structure calculations that give a new picture to direct the convergence of many valleys in a bulk material by engineering the degeneracy of the reversed conduction bands?L and ??.It exploits some orbitals to engineer the band structure and produce that the existence of the secondary?L?conduction band slightly below the principal???conduction band,More specically,their respective band edges??and L?have reversed.From the point of this view,Mg₂Si1-xSnx solid solutions must display a convergence in energy of the two conduction bands with increasing the alloying concentration x.More generally,the specific band structures of this finding on the TE properties is demonstrated in the class of the pseudobinary and pseudoternary systems: by engineering the degeneracy of the two low-lying reversed conduction bands,power factors 0.5 to 1 times larger than related single-crystal can be obtained.Hence,band engineering to converge the conduction bands originating from the reversed band edges to achieve high valley degeneracy should be a general strategy in the search for and improvement of bulk thermoelectric materials.