Effect Of Doping And Nanocomposite On The Thermoelectric Properties Of Sn Te-Based Semiconductor Materials

Thermoelectric materials,which can directly convert thermal energy to electrical energy,have versatile applications in many fields such as waste heat recovery and utilization.As an important type of medium temperature thermoelectric material,SnTe attracts much attention in recent years because of its low cost and non-toxic components.However,its thermoelectric properties are inferior due to the extremely high carrier concentration and large lattice thermal conductivity.To overcome the above problems,Sb and I elements were co-doped to compensate the surplus hole concentration.In addition,Ag₂O nanoparticles have also been added into the SnTe samples to regulate its thermal transport properties in this thesis.Assisted with variety of analysis and testing technologies,the effect of Sb/I co-doping amount and Ag₂O addition amount on the microstructure and thermoelectric parameters of SnTe has been studied,and the main research results are as follows:1.The effect of Sb/I co-doping amounts on the thermoelectric properties of SnTe materials were studied,and the results show that Sb and I enter the Sn and Te sites respectively,therefore they bothcontribute electrons and decrease the surplus hole density.With the limit of solubility,the hole concentration decreases with the increase of doping content.First-principles calculations confirm that the co-doping of Sb/I can introduce a donor level,which contributes a large number of electrons to compensate for excessive holes,at the bottom of the SnTe conduction band.Therefore,the resistivity and Seebeck coefficient of the sample increase accordingly.When the doping content is beyond the solubility moderately,Sb₂Te secondary phase appears and multiscale microstructure has been introduced,which contribute to significant reduction in the lattice thermal conductivity and thus increase in the thermoelectric figure of merit ZT of the material.Finally,the Sn_0.88Sb_0.12Te_0.97I_0.03sample achieved the largest ZT value of 0.97 at 873K.2.The effect of Ag₂O addition on the thermoelectric properties of SnTe materials have been studied.The results show that Ag₂O decomposes to form Ag and O₂during the SPS sintering process.Subsequently,the resulted O₂reacts with the SnTe matrix,while Ag enters the Sn vacancy and replaces the Sn site.When the amount of Ag₂O is over 2 wt%,a Ag₂Te phase is formed in the sample.Due to the above-mentioned chemical reactions,a variety of nanophases are formed in the sample,which scatter carrier and thereby increase the resistivity of sample.On the other hand,the two valence bands of SnTe converge by the dissolvement of Ag in the SnTe matrix.Moreover,the nanophases can filter low energy carriers in the material,therefore the Seebeck coefficient also increases.In addition,the lattice thermal conductivity has been greatly reduced due to the extra phonon scattering by the nanoinclusions introduced by addition of Ag₂O.As a result,a maximum thermoelectric figure of merit ZT 1.15 is obtained at 873K in the sample with 4 wt%Ag₂O addition.