Preparation And Thermoelectric Properties Of ?-? High Entropy Thermoelectric Materials

Compound A^?B^??A^? = Ge,Sn,Pb;B? = S,Se,Te?based on elements of ? and ? group is thermoelectric materials which have promising prospect to be applied in power generation from waste heat in the medium temperture?400 K 900 K?.In this thesis,?Sn,Ge,Pb,Ag,Bi??Te,Se,S?were chosen to prepare different series of alloys and to investigate their thermoelectric properties because these elements can form multiple elements solid solution,so that they can rise high entropy effect?these compounds were named as high entropy alloy?and result in low thermal conductivity.The four series alloys:(Sn_0.5Ge0.4875)_1-x Pbx Te,Pb_0.33Sn_0.33Ge_0.33Te_1-x Sex,Pb_0.33Sn_0.33Ge_0.33Te_1-x Sx and(Pb_0.5Sn_0.25Ge_0.25Te)_1-x?Ag Bi Te2?x were prepared by high frequency melting,qunching and spark plasma sintering?SPS?techniques.Based on the first series alloys,the other three series alloys were prepares with doping method,and the influence of different doping elements on their phases,microstructures and thermoelectric properties were studied.Achievements can be summarized as follow:?1?The investigation of(Sn_0.5Ge_0.4875)_1-x Pb_x Te(x = 0.2,_0.33,_0.5,0.6,0.8)alloys shows that the high entropy solid solution?Sn,Ge,Pb?Te single phase with Na Cl-type structure forms in the alloys with x = 0.2 and 0.8,while the minor cubic Ge Te-based phase arises in the?Sn,Ge,Pb?Te matrix in the alloys with x = _0.33,_0.5 and 0.6.All alloys show p-type conduction.Pb doping in Sn_0.5Ge0.4875 Te decreases its high carrier concentration and thus reduces its carrier thermal conductivity.The synergistic effects on the enhancement of scattering of phonons in the alloys by the?Sn,Ge,Pb?Te high entropy alloy main phase and lamellae microstructures produced from the spinodal decomposition reduce effectively the lattice thermal conductity.As a result,the figure of merit ZT in the alloys were enhanced.The maximum ZT of 1.61 at 773 K was eventually obtained in the alloy with x = _0.5.?2?The investigation of Pb_0.33Sn_0.33Ge0.322Te_1-x Sex(x = 0.05,0.10,0.15,0.20,_0.25,0.30)alloys shows that the substitution of Se for Te in the alloys result in the formation of the?Sn,Ge,Pb?Te solid solution high entropy alloy single phase from its mulitple phases.Two Na Cl-structure phases,named?Pb,Sn,Ge?Te and?Sn,Ge,Pb?Te,with close lattice parameters,together with minor Ge Te phase were formed in the alloy with x = 0.05,while oneNa Cl-structure solid solution dissappeared in the alloy with x = 0.10.However,only?Sn,Ge,Pb?Te solid solution high entropy alloy single phase in the alloy with x = 0.15.All alloys keep p-type conduction.A small amount of Se doping decreses the electric resitivity slightly and slightly increases the Seebeck coeffienct,leading to the increase of power factor.The synergistic effects on the reduction of the thermal conductivity by scattering of phonons from minor Ge Te phase and?Sn,Ge,Pb?Te solid solution high entropy alloy can improve the figure of merit ZT of the alloys.The maximum ZT of 1.36 at 773 K was obtained in the x = 0.1 alloy.?3?The investigation of Pb_0.33Sn_0.33Ge0.322Te_1-x Sx(x = 0.05,0.10,0.15,0.20,_0.25,0.30)alloys shows that the?Sn,Ge,Pb?Te solid solution high entropy alloy single phase with Na Cltype structure can be formed in the alloys with a small amount of S doping.All alloys keep p-type conduction.The electric resitivity,carrier mobility and Seebeck coeffienct were increased,and the thermal conductivity was reduced with S substitution.The thermoelectric figure of merit was enhanced.The ZTmax of 1.07 was obtained at 773 K in the x = 0.1 alloy.?4?The investigation of(Pb_0.5Sn_0.25Ge_0.25Te)_1-x?Ag Bi Te2?x?x = 1/26,1/21,1/16,1/11?alloys shows that the?Sn,Ge,Pb,Ag,Bi?Te solid solution high entropy alloy single phase with Na Cl-type structure can be formed in the alloys with Ag Bi Te2 doping,which contain Sn,Ge,Pb,Ag,Bi and Te multiple elements,showing low thermal conductivity.The lattice parameter decreases with the increase of Ag Bi Te2 content x.All alloys show p-type conduction.The carrier concentration decreases and carrier mobility increases with low Ag Bi Te2 content,but change conversely with the further increasing Ag Bi Te2 content.Ag Bi Te2 doping increases the electric resitivity and Seebeck coefficient,and thus reduces the carrier thermal conductivity.The figure of merit ZT and the average ZT was enhanced.Although the maximum ZT of 1.6 at 673 K obtained in the alloy with x = 1/21 is closed to that of parent alloy,its average ZT of 1.03 is much higher than 0.68 of its parent alloy.