The Structure And Thermoelectric Performance Of Germanium Telluride Based Materials

In the past ten years,the rapid growth of the world population and the development of the global economy have brought about serious energy crisis and environmental pollution problems.Therefore,exploring the eco-friendly new energy materials and related conversion technologies have become frontier issues.Thermoelectric conversion technology has attracted tremendous attention because of no transmission parts,small size,free of noise and pollution,as well as high reliability,etc.It shows great wide application prospects in the application of waste heat recycling,power generation and other fields,which has generated extensive attention worldwide from researchers.The core for thermoelectric conversion technology is the thermoelectric materials.However,the performance of thermoelectric materials is still relatively poor,and the ZT value is around 1 for most usable materials.Therefore,exploring new material systems and preparation processes,optimizing the composition and microstructure and then further improving the thermoelectric properties of the materials is a common challenge for all researchers.In our study,we mainly focused on the GeTe-based compounds with high phase transition temperature,high carrier concentration and high thermal conductivity.The influence of alloying with MnTe and Sb doping in GeTe on phase composition,phase transition and thermoelectric properties was systematically investigated.Furthermore,the thermoelectric properties of the material were optimized by fixing the MnTe content in the GeTe compound and adjusting the Sb content,the effects of MnTe and Sb content on the phase composition,phase transition and thermoelectric properties of the GeTe compound were studied.The main research contents and results of this study are as follows:Alloying MnTe into GeTe gradually changed the structure from the rhombohedral phase to the cubic phase,and the corresponding phase transition temperature gradually decreases to near room temperature.Due to the magnetic impurities introduced by MnTe,the carrier mobility of the samples decreases dramatically.Alloying with MnTe promotes the electronical band convergence,accompanied with a reduced energy difference between the light and heavy valence bands.Aditionally,alloying with MnTe gives rise to extra states in the gap in both the low temperature rhombohedral phase and the high temperature cubic phase.At the same time,additional vacancy defects introduced by MnTe also enhances the phonon scattering,which greatly reduces the lattice thermal conductivity.Although the thermal conductivity decreases significantly,due to the apparent increase of the carrier concentration and the decrease of the carrier mobility,the Seebeck coefficient decreases because of the increasing carrier concentration,resulting in a decrease in the power factor,so that the ZT value is not improved.For Sb doped Ge_1-xSb_xTe,we find that the phase transition temperature from the rhombohedral phase to the cubic phase gradually decreases.Sb as a donor impurity reduces the domiant hole carrier concentration effectively in the samples.In addition,Sb doping decreases the lattice thermal conductivity dramatically due to the intensified alloying phonon scatterig.Finally,Ge_0.90Sb_0.10Te sample achieves a maximum ZT value of 1.65 at 773 K,an increase of 51%compared to the intrinsic GeTe sample.In order to further optimize the thermoelectric properties of GeTe,we fixed the MnTe content in the GeTe compound and adjusted the Sb content.When the content of MnTe is constant,doping Sb reduces the carrier concentration and improves the electrical properties.However,the increase of MnTe content is not conducive to improving the electrical transport properties of the samples.At the same time,alloying with MnTe and doping with Sb in GeTe significantly reduce the lattice thermal conductivity.Finally,the maximum ZT value of the sample Ge_0.86Mn_0.10Sb_0.04Te at 823 K is 1.61,which is 47%higher than that of the intrinsic sample.The Ge_0.72Mn_0.20Sb_0.08Te compound reaches the highest ZT value of 1.48 at823 K,about 36%increase compared with the intrinsic GeTe compound.The Ge_0.64Mn_0.30Sb_0.06Te compound reaches values as high as 1.30 at 823 K,which is about 19%higher than that of the intrinsic GeTe compound.Due to the enhancement in the ZT value,in the temperature range of 400 K-800 K,the average ZT value of Ge_0.86Mn_0.10Sb_0.04Te is 1.09,which is higher than other GeTe-based compounds reported in the literature,which indicates that MnTe alloying and Sb doping can increase GeTe over the entire temperature range and lays the foundation for the wide application of GeTe materials.