The Preparation And Size Optimization Of Silicon Germanium Alloy Thermoelectric Unicouple

Silicon germanium (SiGe) alloys are suitable candidates for high-temperaturethermoelectric applications because of their reasonably good thermoelectricproperties and superior long term reliability at elevated temperatures. Consequently,SiGe thermoelectric modules have been used in space radioisotope thermoelectricgenerator (RTG). However, the ZT value is so low that the conversion efficiency ofthermoelectric generator is not high. This article adopts nanocomposite method toimprove the ZT value of SiGe alloy. The ideal nanocomposite technique requiresthat the main phase and the doping phase have the similar electrical properties;meanwhile the doping particles are dispersed in the matrix. So the electricalproperties change little and the nanoparticles would scatter on phonon transportprocess. The result is that the ZT value is improved.Nano-Si95Ge5powder and nano-Si70Ge30B5doping powder were fabricated byhigh temperature melting and mechanical alloying technology, respectively. Thepowders were mixed according to the doping ratio (15%,25%, and35%). P-typethermoelectric bulk alloys would be prepared by spark plasma sintering technology.The X-ray diffraction characterization (XRD) indicates that the bulk samples are theSiGe alloy with diamond structure. The scanning electron microscope (SEM)indicates that the thermoelectric bulk alloys have the uniform microstructure andhigh density. Thermoelectric performance test shows that the electrical conductivityand Seebeck coefficient of P-type thermoelectric bulk alloys change little with theincreasing of Si70Ge30B5doping content, while the thermal conductivity decreasesgreatly. At last, P-type thermoelectric bulk SiGe alloy which Si70Ge30B5dopingcontent is25%has a larger ZT value compared with other P-type thermoelectricbulk SiGe alloys, which can be0.47at710K.Three kinds of N-type thermoelectric bulk alloys have been prepared with thesame process, which matrix particle is nano-Si95Ge5and the doping particle isnano-Si70Ge30P3(the doping ratio is15%,25%, and35%, respectively).Thermoelectric performance test shows that the electrical conductivity and Seebeckcoefficient of N-type thermoelectric bulk alloy change little with the increasing ofSi70Ge30P3doping content, while the thermal conductivity decreases greatly. At last,N-type thermoelectric bulk SiGe alloy which Si70Ge30P3doping content is35%has alarger ZT value compared with other N-type thermoelectric bulk SiGe alloys, whichcan be0.58at710K.At last, this article adopts P-type SiGe thermoelectric alloy which Si70Ge30B5doping content is25%and N-type SiGe thermoelectric alloy which Si70Ge30P3 doping content is35%as the galvanic arm of thermoelectric generators. Consideringthe variable physical parameters and the contact effect, the output power andconversion efficiency are simulated by the finite element software ANSYS tooptimize the size parameters of the galvanic arm. The conclusion shows that whenthe cross section side length W of the thermocouple is4.5mm, the arm length L is2.5mm and the external resistance R is5.0mΩ,0.18W output power and4.8%conversion efficiency can be obtained.