Study On Thermoelectric And Mechanical Properties Of Multiple Doped Mg₂Si-based Thermoelectric Materials Synthesized Under High Pressure

Thermoelectric conversion technology is a green energy technology that uses thermoelectric material's Seebeck effect,Peltier effect and Thomson effect to transform thermal energy and electric energy.It has many notable advantages,such as no mechanical movement,no noise and no emissions,which made it become a research hotspot in recent decades.Mg2Si has the advantages of low density,high melting point,low price of raw materials,non-toxic and non polluting,which make it one of the most valuable thermoelectric materials.High pressure high temperature?HPHT?technology has been used in the synthesis of thermoelectric materials more and more in recent years,and it has become a way to explore new structure and unique properties of materials.In this paper,Mg₂Si based thermoelectric materials are prepared by HPHT.The thermoelectric properties of Al,Bi and Sb multi-doped Mg₂Si thermoelectric materials are studied.The effects of SiC whiskers and K₂Ti₆O₁₃ whiskers on the thermoelectric properties and mechanical properties of Mg₂Si-based thermoelectric materials are investigated.The main results are as follows:?1?The thermoelectric properties of Al,Bi and Sb multi-doped Mg₂Si thermoelectric materials Mg_2.02-xAl_xSi_1-3yBi_2ySb_y?x=0.03,0.06,y=0.01,0.02?prepared by HPHT are studied.In addition to the main phase Mg₂Si,there are some undetermined phases in the sample.The annealing process can not only eliminate most impurities but also increase the solid solubility of Bi element.The sample with 6 at.%Al has a lower lattice thermal conductivity compared with that of the sample with 3 at.%Al,indicating that Al can aggravate the phonon scattering caused by Bi and Sb doping.Mg_1.96Al_0.06Si_0.85Bi_0.01Sb_0.005 sample got maximum ZT=0.9 at 773 K,obviously higher than that of single doped Mg₂Si materials.?2?The Mg₂Si Mg_1.99Al_0.03Si_1-3yBi_2ySb_y thermoelectric materials with better surface integrity and thermal stability are prepared by secondary HPHT process.The influence of Bi and Sb content on the thermoelectric properties of materials is studied.The doping of Bi and Sb can significantly improve the electrical conductivity and reduce the lattice thermal conductivity of the material when the dosage is not greater than 3 at.%.Mg_1.99Al_0.03Si_0.955Bi_0.03Sb_0.015.015 samples got maximum ZT=0.71 at 773 K.The optimum content of Bi element in this experiment was y=0.03,which is higher than 1.3 at.%reported in the literature.It suggests that the HPHT process and doping with Al can increase the solid solubility of Bi.?3?The effects of introduction of SiC whiskers and K₂Ti₆O₁₃ whiskers on the thermoelectric properties and mechanical properties of Mg₂Si based thermoelectric materials are studied.Both of the whiskers significantly improved the bending strength,bending modulus,hardness and fracture toughness of the materials,and the enhancement effect of K₂Ti₆O₁₃ whisker was more obvious.The optimal dosage of K₂Ti₆O₁₃ whisker is 1 wt.%,and the greater dosage will slightly reduce the mechanical properties of the material.The doping of the whisker blocks the synthesis of Mg₂Si,and the incomplete reaction of Mg makes the material exhibit metallic characteristics,therefore the thermoelectric performances are poor.?4?The problem of incomplete reaction is solved by secondary HPHT.The mechanical properties of samples after secondary HPHT process are relatively lower,which may due to the residual stress caused by HPHT sintering.The strengthening effect of two kinds of whiskers on mechanical properties are outstanding,which is consistent with one step HPHT.The introduction of whisker reduces the electrical conductivity of the material and the lattice thermal conductivity.Because of the reduction of electrical conductivity is greater than that of the thermal conductivity,the dimensionless figure of merit value decreased.In general,the mechanical properties and thermoelectric properties of the samples doped with K₂Ti₆O₁₃ whisker are better than that with SiC whiskers.