Microstructure And Thermoelectric Properties Of Mg₃Bi₂ Based Thin Films Deposited By Magnetron Sputtering

Thermoelectric material is a kind of functional material that can directly carry out the mutual conversion of thermal energy and electrical energy.Thermoelectric devices can be used as waste heat power generation or electronic refrigeration.Most of the waste heat is low temperature,the effective use of low temperature waste heat has become an important direction in the field of thermoelectric materials.Mg₃Bi₂,as a semi-metallic Zintl phase thermoelectric material,has high electrical conductivity and low thermal conductivity and its constituent elements are low price and abundant reserves.It is an excellent medium and low temperature(<500K)thermoelectric material.In this paper,the Mg₃Bi₂based thin films were prepared on high-purity Si(100)wafer using metal Mg,Mg₃Bi₂and Mg₃Bi_1.9Sb_0.1targets by high vacuum magnetron sputtering equipment.The deposited Mg₃Bi₂based thin films were analyzed by X-ray diffractometer,field emission scanning electron microscope and thermoelectric performance testing system.The influence of the Mg content,sputtering power and doped Sb element on the phase composition,surface morphology and thermoelectric properties of the deposited Mg₃Bi₂based thin films were investigated.The results show that the Mg content in the deposited Mg-Bi thin films has a significant influence on thermoelectric properties.When the Mg content is 41.50%and 73.01at%,the deposited Mg-Bi thin films are composed of the metallic Bi phase and the amorphous Mg₃Bi₂phase.Due to the high resistivity and poor thermoelectric performance,it is not suitable to use as thermoelectric materials.When the Mg content exceeds 79.89at%,the metal Bi completely reacts with Mg and forms the Mg₃Bi₂phase.The deposited Mg-Bi thin films are composed of the amorphous Mg₃Bi₂phase and the nano-crystalline metal Mg phase.As the Mg content in the deposited Mg-Bi thin films increase,the quantity of nano-crystalline metal Mg phase increase.With the increase of the content of the nano-crystalline metal Mg phase in the deposited Mg₃Bi₂thin films,the Seebeck coefficient value first increase and then decrease.It shows that the thermoelectric performance of the Mg₃Bi₂thin films containing an appropriate amount of nano-crystalline metal Mg phase has obviously been improved.The direct current sputtering power of the Mg₃Bi₂target has a certain influence on thermoelectric properties of the Mg₃Bi₂thin films.Keeping the 40W sputtering power of the Mg target,the deposited films under the Mg₃Bi₂target sputtering power of 50W,60W and70W were studied.The results show that the Seebeck coefficient value of the deposited films with the Mg₃Bi₂target sputtering 50W have hardly difference with other deposited films.However,the resistivity is relatively small so that the power factor is the largest.The experiment of Sb-doping shows that the resistivity of the Sb-doped Mg₃Bi₂thin film is higher than that of the without Sb-doped thin films.Thus,the Seebeck coefficient value and the power factor is higher.The Seebeck coefficient value of the Mg₃Bi₂thin film with about1.10at%Sb and 83.65at%Mg is relatively higher and the power factor is the largest.In the range of whole testing temperature,the average power factor can reach 4m W·m^-1·K^-2and thermoelectric performance is excellent.