Thermoelectric Performance Optimization Of SiGe Alloys

Thermoelectric materials,as a kind of environmental friendly functional materials,can realize the directconversion between heat and electricity,which enable them to play an important role in thermoelectric power generation and solid-state refrigeration.The performance of thermoelectric materials is determined by a dimensionless factor ZT,ZT=S²σT/κ,where S is Seebeck coefficient,σis electrical conductivity,T is absolute temperature andκis thermal conductivity.The higher ZT value,the better thermoelectric performance of the material and the higher energy conversion efficiency.Due to the increasingly serious environmental pollution and energy crisis,it is of great practical significance to develop high-performance thermoelectric materials.Silicon-germanium（SiGe）alloy is a kind of thermoelectric material with excellent performance in high temperature region,which is stable at high temperature and environmental friendly.It has important applications in spacecraft,photoelectricity and other fields.However,SiGe alloy has a high thermal conductivity,which seriously restricts its improvement in thermoelectric properties.In order to meet the extensive demand for high performance thermoelectric application,it is of great significance to reduce the thermal conductivity of SiGe alloy and optimize its thermoelectric properties.In this paper,n-type SiGe alloy(Si₈₀Ge₂₀P₂)is taken as the research object to study the effects of different preparation methods on the microstructure and thermoelectric properties of n-type SiGe alloy.The regulatory mechanism of different doping components on the thermoelectric properties of n-type SiGe alloy is also explored.The specific research contents are as follows:1.The influence of different preparation methods on microstructure and thermoelectric properties of n-type SiGe alloy.Si₈₀Ge₂₀P₂ bulk samples were prepared by two different methods:Ball Milling（BM）+Spark Plasma Sintering（SPS）,BM+High Temperature and High Pressure synthesis（HTHP）,respectively.In terms of microstructure,the samples prepared by SPS have compact structure and neat small protrusion shape,the samples prepared by HTHP have obvious layered structure.As for thermoelectric properties,the samples prepared by BM+SPS have higher carrier concentration and mobility,thus the samples have higher conductivity.The nano-scale grains after ball milling are beneficial for the scattering of phonons,but the thermal conductivity of samples is still high（～4 W/m K at room temperature）due to the grain growth during SPS process.Finally,the Si₈₀Ge₂₀P₂ sample prepared by BM+SPS has the highest ZT value of 1.57 at 800°C.The samples prepared by HTHP retain the nanostructured feature produced by ball milling.Because they have a large number of unique layered structure inside,these layered structures further enhance the scattering of phonons,which significantly reduces the thermal conductivity（～2.5 W/m K at room temperature）of Si₈₀Ge₂₀P₂ sample.However,these layered structures not only scatter phonons,but also scatter carriers,which leads to the decrease of mobility.Therefore,the Si₈₀Ge₂₀P₂ samples prepared by BM+HTHP have low thermal conductivity and low electrical conductivity simultaneously.Finally,the Si₈₀Ge₂₀P₂ sample prepared by BM+HTHP has the highest ZT value of 1.34 at 800°C.2.Influence of Sn on thermoelectric properties of n-type SiGe alloys.In this work,Si₈₀Ge₂₀P₂Sn_xbulk alloys were synthesized using SPS method without ball milling process.It is found that the addition of Sn can increase the carrier concentration of Si₈₀Ge₂₀P₂ sample,promote the formation of SiGe alloy and improve the crystal quality,thus improving the carrier mobility of the sample.Compared with the undoped Si₈₀Ge₂₀P₂ sample and the nanostructured SiGe alloy reported in the literature,the power factor of Si₈₀Ge₂₀P₂Sn₂ sample is obviously improved in the whole test temperature range due to the simultaneous increase of carrier concentration and mobility,and the Si₈₀Ge₂₀P₂Sn₂ sample has the highest power factor（～3711μW/m K²）at 800℃.Sn doping will not only improve the conductivity of n-type SiGe alloy,but also reduce the thermal conductivity.Due to the difference of atomic radius between Si and Sn,stress field fluctuation scattering and mass fluctuation scattering effects are introduced to enhance the scattering of phonons.In addition,Sn particles as the second phase in SiGe alloy matrix introduce more grain boundaries,which also contributes to phonon scattering and reduces lattice thermal conductivity.Therefore,compared with the Si₈₀Ge₂₀P₂ sample without Sn incorporating,the thermal conductivity of the Si₈₀Ge₂₀P₂Sn_x sample is obviously reduced.Time-consuming ball milling process was not used in this work,but the ZT value（～1.26）of Si₈₀Ge₂₀P₂Sn₂ sample is comparable with that of the nanostructured n-type SiGe alloy in the literature,which provides a new way to efficiently prepare high-performance SiGe alloy.3.Influence of Mo on thermoelectric properties of n-type SiGe alloys.In this work,nanostructured Mo was added into nanostructured Si₈₀Ge₂₀P₂ alloy by BM+SPS in order to research the effect of Mo on thermoelectric properties of n-type SiGe alloys.The composition analysis shows that all the synthesized samples contain Mo Si₂ phase,which is uniformly distributed in SiGe alloy matrix.The addition of Mo Si₂ improves the carrier mobility and optimizes the electrical properties of SiGe alloys.Compared with the Si₈₀Ge₂₀P₂ sample without Mo incorporating,the power factor of SiGe alloy samples is significantly improved in the whole test temperature range when the Mo content is 0.3 vol.%and 1.5 vol.%.In addition,proper Mo doping can also reduce the thermal conductivity of Si₈₀Ge₂₀P₂ samples.Compared with the Si₈₀Ge₂₀P₂ sample without Mo incorporating,it not only improves the power factor but also reduces the thermal conductivity simultaneously of the SiGe alloy samples with Mo incorporating when Mo content is 0.3 vol.%,this sample has the highest ZT value（～1.3）at 800℃and average ZT value（～0.73）in the test temperature range.Therefore,appropriate Mo doping can effectively increase the average ZT value of n-type SiGe alloy.