Preparation And Properties Of GeTe Based Thermoelectric Materials

Thermoelectric materials,which convert energy between heat and electricity without pollution and noise,and are widely used in power generation and cooling.GeTe,as a substitution for Pb Te in the intermediate temperature region,has drawn widely attentions to survey.The large number of Ge vacancies results in the deviation of stoichiometry in the GeTe matrix,and the carrier concentration is as high as 10²¹cm^-3.Besides,the rhombohedral phase convert to cubic phase occurs at about 700 K.However,phase transition leads to drastic volume variation,which cause mechanical failure and service decline.Doping can tune the phase transition temperature,reduce thermal conductivity and enhance Seebeck coefficient simultaneously.In our work,a melting-quenching-annealing process combined with spark plasma sintering method was used.The effects of melting,annealing and sintering on the thermoelectric properties of GeTe matrix were investigated.Then,the optimized Seebeck coefficients and carrier density By In-doping were studied.After that,the effect of Mn-doping on lattice thermal conductivity and phase transition were also explored.The main conclusions are as follows:?1?High performance matrix was prepared by optimizing the process parameters?melting temperature,annealing temperature,sintering temperature and holding time?.The results show that GeTe matrix were obtained in all progress.When the melting temperature arrives 950°C,?is the lowest,so the ZT value of 0.4 obtained at 600 K;When the annealing temperature reaches 600?,the S²sis the largest,accordingly,ZT value can be increased to 0.5;The longer retain time and higher sintering temperature promote the electric conducticity,but decrease the Seebeck coefficient,because of the increased density and carrier density.When the sintering temperature reaches 550°C and hold for 3 minutes,the?obtains the lowest,so ZT could be increased to 0.52.Therefore,melting at 950°C,annealing at 600°C,and sintering at 550°C for 3 min can obtain the highest thermoelectric performance of GeTe.?2?The thermoelectric properties of Ge_1-xIn_xTe?x=0?0.10?were studied by adjusting the In-doping.Research shows that with the increase of In content,the lattice constant increased along the ab aixs,while decreases along c axis,resulting in lattice distortions.Moreover,the fractured surface tended to be disordered and inapparent,thus reducing?_L by enchancing the phonon scattering.The second phase appeares at x=0.10.The valence state of Ge,In are both+2,indicating the In atom substitute for the Ge vacance.With the increase of In content,S rised gradually,owning to the decreased carrier density to 9.82×10²⁰cm^-3.When the content reaches 0.03,the S²sof 36?W/cm¹K²and?of 2.17 W/m K were obtained at 600 K,the highest ZT value acquires 1.15 eventually,26%higher than of GeTe.Consequently,In can effectively promote S and reduce?,so as to improve the thermoelectric properties of Gete matrix.?3?The thermoelectric properties of Ge_1-yIn_0.03Mn_yTe?y=0?0.18?were studied by doping Mn.With the increase of Mn content,the?024?and?220?diffraction peaks move to a higher angles,indicating that the interplane spacing increases,and then gradually merged into?220?when y?0.12,demonstrating that rhombohedral structure transforms to cubic structure;The?200?lattice fringes of cubic structure were observed by TEM,which confirmed this results.However,the change of enthalpy shows that the phase transition is in the range of 450?500 K,consequently,the phase transition temperature can be reduced to 450 K;The S andsexist a extremum in all samples.This phenomenon disappear in S²s,the values are lower than that of the undoped sample.The ultralow?_Lof0.24 W/m K obtained in y=0.12 at 700K,as a result,the lowest?of 1.43 W/m K was obtained,as well as the maximum figure of merit 1.4 was obtained,which is 23%higher than that of Ge_0.97In_0.03Te.Therefore,Mn can reduce the phase transition temperature and?_L effectively,which further improve the thermoelectric performance of GeTe.