Preparation Of Gete-agsbte₂ Based Thermoelectric Materials And Control Of Their Properties

Thermoelectric(TE)materials are used in thermoelectric conversion systems.Due the advantages of long service life,no moving component,no noise,and green environmental protection,TE material are widely used in the field of aerospace,biology,and electronics,etc.(GeTe)100-x(AgSbTe2)x material,namely TAGS-x,is considered as one of candidates for future mass-market commercial applicaion,because of its high Seebeck coefficient,low thermal conduvtivity and excellent TE performance.Unfortunately,its TE performance is hardly enhancd by traditional methods in previous researches.In this paper,Pb2+is substituted for part of the cation Ge2+ in the lattice of TAGS in order to optimize the carrier concentration,the dissolving and precipitating of PbTe in the GeTe-rich matrix of the GeTe-PbTe pseudo-binary alloy is used to reduce the thermal conductivity,the increasing ratio of GeTe is used to improve its electrical performance,and finally the purpose of improving the thermoelectric performance is achieved.The Ge1-xPbxTe-AgSbTe2 based TE materials were prepared by a method combined of vacuum melting,quenching and spark plasma sintering(SPS).Phase components analysis,microstructure and micro-morphology,electrical and thermal properties,and TE performance of those samples were studied systemically.The main studies and results of this paper are as follows:(1)In(Ge1-xPbxTe)90(AgSbTe2)10(x=0,0.1,0.15,0.20,0.25,0.30)samples,the introduction of Ag and Sb can increase the maximal solid solubility of PbTe in GeTe-rich matrix.These samples with x<0.15 are single phase,but in the samples of x?0.15,PbTe-rich precipitations in the shape of graininess,block,and modulated stripe begin to appear and are embed in the GeTe-rich matrix.Its Seebeck coefficient arises and the electrical conductivity gradually decreases as the Pb content x increases.Finally,a high power factor of 2942.06?W/mK2 is achieved for x=0.15 sample at 753 K,and the thermal conductivity is reduced to 1.34W*m-1K-1.The maximum value of TE figure of merit ZT is 1.65.The TE performance of x=0.15 sample has been enhanced by 1.26 times,compared with that of x=0 sample.(2)(Ge1-xPbxTe)90(AgSbTe2)10 samples(x=0,0.13,0.15,0.17,0.20)with high density are made by a SPS process with optimized sintering parameters.The size of PbTe-rich precipitations decrease in all samples.The electrical conductivity decreases and the Seebeck coefficient effectively arises due to the reduction of carrier concentration and mobility,as the Pb content x increases.There are some significant changes in the micro structure of those materials,results from the Pb substitution.One is a lot of lattice defects brought by substituting Pb for Ge,such as atomic mass fluctuation,strain field and stress filed;the other is abundance of PbTe-rich precipitations with different sizes,which is embedded in the GeTe-rich matrix.All of those behave as phonon scattering centers,which can reduce the mean free path of phonon and lattice thermal conductivity remarkably.At 753K,the thermal conductivity of the x=0.17 sample is decreased to 1.19 W*m-1K-1 and its power factor can reach 2642.18?W/mK2.Finally,a maximal ZT value of 1.8 is obtained,which is an increasing of 25.7%in comparison with that of x=0 sample.(3)The limit of solid solubility of Pb in(Ge1-xPbxTe)95(AgSbTe2)5 samples(x=0,0.13,0.15,0.17,0.20)is cut down,responding to the increase of proportion of GeTe in TAGS.As results,the percentage and size of PbTe-rich precipitations is over than that of previous(Ge1-xPbxTe)10(AgSbTe2)10 materials.Therefore,the overall Seebeck coefficient is reduced by about 80?V/K,and the electrical conductivity increases with the increase of carrier concentration.At 753K,the power factor of x=0.13 sample can reach 3104.04?W/mK2,and its thermal conductivity is as low as 1.38 Wm-1K-1.The figure of merit ZT is 1.69,which is larger than that of x=0 sample by 21.6%.