Preparation And Thermoelectric Properties Of Type-Ⅲ Ge Based Clathrates

Clathrate compounds are considered to behave as “phonon glass-electron crystal(PGEC)” materials, with polyhedral cages and guest atoms encapsulated in the cages.Polyhedral cage generally consists of IV and Ⅲ elements, while alkali metal andalkaline earth metal atoms are generally the guest atoms. There are several types ofclathrate compounds, type-I, type-Ⅷ and type-Ⅲ clathrates. Type-Ⅲ Ba24Ge100clathrates possess low thermal conductivity and high electrical conductivity at roomtemperature, it is of great potential for high temperature thermoelectric materials. Butthe high intrinsic carrier concentration causes very low Seebeck coefficient. Inaddition, the synthesis of type-Ⅲ Ba24Ge100clathrates very difficult to control andneeds long time. Therefore, exploring a new preparation method, control andoptimize the carrier concentration of Ba24Ge100are among the most important issuesfor the research and application of the Ba24Ge100compounds.Considering the issues above, this study mainly aims to explore new preparationmethods for type-Ⅲ Ba24Ge100clathrates compound, and find out new dopingelements to reduce the carrier concentration and thermal conductivity, so as tooptimize the dimensionless thermoelectric figure of merit. Pure phase of Ba24Ge100isdifficult to synthesis, and this study tries to use melt method and subsequent sparkplasma sintering(SPS) method to synthesis Ba24Ge100. In order to reduce the carrierconcentration, Cu, Ag and Ga has been adopted as doping elements, respectively toreplace the framework element Ge, in order to reduce excess electron number in thestructure, and the effects of the single and double doping on the thermoelectricproperties of Ba24Ge100have been studied. In order to further reducing the carrierconcentration, Li is used as doping element to replace the guest atom Ba, and also Geatom together, the overall effect on the thermoelectric properties of Ba24Ge100hasbeen discussed. The main research conclusions are as follows:This work investigated the effects of the single doping of Cu, Ag and Ga whichreplace the framework element Ge on electrical and thermal transport properties of type-Ⅲ Ba24Ge100compounds. There are lagre difference on the radius and outershellelectron of those atoms Cu, Ag compared with Ge, with the increase of dopingamount, type-I clathrates phase increases, reducing the excess electron number in thestructure, and thus lowering the carrier concentration, the electrical conductivity, andthe thermal conductivity. The Seebeck coefficient increases with the doping amountof Cu or Ag. As a result, the maximum dimensionless figure of merit ZT of0.1isobtained at873K for Ba24Ge100, and0.28for Ba24Cu8Ge92, and0.35forBa24Ag10Ge90, which is improved by~180%and~260%compared to the undopedsample. Single phase of Ba24GaxGe100-xcan be obtain for x＜8，with the increase ofGa content, the carrier concentration and thermal conductivity decreases, Seebeckcoefficient increases. The maximum dimensionless figure of merit ZT of0.32isobtained at873K for Ba24Ga10Ge90, which is~220%improvement over the undopedsample.This work investigated the effects of the double doping of Ag/Ga replace theframework element Ge on electrical and thermal transport properties of type-ⅢBa24Ge100.compounds. Ga/Ag doping together significantly reduce the carrierconcentration, electrical conductivity and thermal conductivity. For Ba24Ga4AgxGe96-x,single phase is obtained when x is less than2. The maximum dimensionless figure ofmerit ZT of0.22is obtained at873K for Ba24Ga4Ag2Ge94. Compared with that ofternary alloy Ba24Ga4Ge96,it increases by80%at the same temperature, which is~120%improvement over the undoped sample. Increasing Ga and Ag content forBa24Ga8Ag2Ge90and Ba24Ga8Ag4Ge88compounds,there appears type-I clathratephase in type-Ⅲ matrix, and the thermal conductivity decreases significantly. Themaximum dimensionless figure of merit ZT of0.35and0.45is obtained at873K forBa24Ga8Ag2Ge90and Ba24Ga8Ag4Ge88. Compared with that of Ba24Ge100,it increasesby250%and350%, respectively.On the basis of single doping and double doping to replace Ge atoms, Li asdoping element to replace the guest atom Ba is conducted to investigate the overalleffects of doping elements to replace Ge and Ba on electrical and thermal transportproperties of type-Ⅲ Ba24Ge100compounds. Ba24-xLixGe100and Ba24-xLixGa4Ge100(x=1~4) are all single phase by XRD examination, but the back scattered electronimages show the BaGe2phase, the carrier concentration increase, possibly due to the fact Li does not dop on the position of Ba, for Li is small and thus enter the interspace,so the thermoelectric performance does not improve. Ba20Li4Ga4Ag2Ge96,Ba20Li4Ga8Ag2Ge90exhibit multinary phases, with the amount of Li is increased andthe amount of Ba decreased, the amount of type-I clathrate phase increases. Themaximum ZT of0.24is obtained at873K for Ba20Li4Ga4Ag2Ge96, a little higher thanthe sample of Ba24Ga4Ag2Ge94, because Ba20Li4Ga4Ag2Ge96shows has multinaryphases and the thermal conductivity is lower than Ba24Ga4Ag2Ge94. The maximumdimensionless figure of merit ZT of0.35is obtained at873K for Ba20Li4Ga8Ag2Ge90.