Research On Phase Transition Control And Thermoelectric Performance Optimization Of GeTe-Based Materials

Energy issues and environmental issues have always been closelylinked.How to solve the contradiction between energy supply and environmental protection becomes a hot issue that people pay more and more attention to.Therefore,thermoelectric materials,which are clean and green energy materials,have gained more attention.The role of thermoelectric materials is to convert electrical and thermal energy into each other.PbTe,with its high efficiency,is already in practical use in some fields.But the problem of lead pollution caused by PbTe is a major concern.GeTe,a medium-temperature thermoelectric material with excellent thermoelectric properties,has therefore been extensively studied as an alternative to PbTe.However,there are several problems with GeTe that inhibit its thermoelectric properties and hinder its application.First,due to the low formation energy of Ge vacancies,the inherent presence of Ge vacancies in the material leads to a high carrier concentration,which results in a low Seebeck coefficient and a high electronic thermal conductivity.Secondly,the material will undergo a phase transition at 700 K,while at lower temperature region the energy band structure of the material is affected due to the low symmetry,which hinders the thermoelectric properties of the material.To address the above shortcomings,in this chapter we adjust the carrier concentration and phase transition temperature of the material by doping with rare-earth singlet elements,alloys,and inexpensive metallic elements in order to enhance the performance of GeTe-based thermoelectric materials,and the specific research work is as follows.1.The effects of the co-doping of Er-Bi elements on the phase transition temperature and thermoelectric properties performance of GeTe based materials were investigated.The samples were prepared according to the stoichiometric ratios of Ge_1.03-xEr_xTe（x=0,0.01,0.03,0.05,0.07,0.09）and Ge_0.98-yEr_0.05Bi_yTe（x=0.03,0.05,0.07）.It was found that the DSC thermal absorption peaks of the materials shifted towards lower temperatures,indicating that the Er and Bi elements could push the phase transition temperature of the GeTe materials to lower temperature.The doping concentration results in a phase transition temperature of sample Ge_0.93Er_0.05Bi_0.05Te to be 610 K,which is 90 K lower than that of undoped GeTe.The SEM and TEM images show that Er elements can form secondary phases of Er,Ge and ErTe₃ in GeTe,which hinders the conduction of high frequency phonons and medium frequency phonons in the material,and reduces the lattice thermal conductivity of the material from 1.7 Wm^-1K^-1 to 0.88 Wm^-1K^-1.The Er and Bi elements also can fill the Ge vacancies in GeTe,reducing the carrier concentration to 1.8×10²⁰ cm^-3and giving the material a high Seebeck coefficient and a low electronic thermal conductivity.This can result in a comprehensive improvement of the thermoelectric properties of the material.Finally,Ge_0.93Er_0.05Bi_0.05Te,the sample with the best thermoelectric performance,was synthesized,reaching a maximum ZT value of 1.44 at 700 K.2.In this chapter we investigate the alloying of BiSbSe₃ into GeTe to reduce the carrier concentration of GeTe-based materials and thereby optimize their thermoelectric properties.We prepared a series of samples according to the stoichiometric ratio of（GeTe）_1-x（BiSbSe₃）_x（x=0,0.01,0.02,0.03,0.04）.Firstly,the XRD and SEM images showed that no secondary phase of Ge was formed in the samples,indicating that the alloying of BiSbSe₃ suppressed the formation of Ge vacancies and reduced the carrier concentration.BiSbSe₃ and GeTe are also medium temperature thermoelectric materials,and the carriers of BiSbSe₃ are mainly electrons,which is the opposite of the carrier type of GeTe.Therefore,the carrier concentration can be effectively regulated by dispersing BiSbSe₃ into the matrix of GeTe.The Bi and Sb elements diffusing out of BiSbSe₃ can fill the Ge vacancies and reduce the carrier concentration.The combined effects of these three elements reduce the carrier concentration to 1.1×10²⁰ cm^-3.The alloying of BiSbSe₃ also reduces the phase transition temperature of GeTe to 603 K,as revealed by neutron diffraction.produced by the decomposition of BiSbSe₃ during the alloying process.It scatters the propagation of high frequency phonons,reducing the lattice thermal conductivity to 0.81 Wm^-1K^-1.Finally,the as-synthesized（GeTe）_0.96（BiSbSe₃）_0.04 is the best thermoelectric sample,reaching a maximum ZT value of 1.73 at 750 K.3.In this section,we investigate the effect of Mn-In co-doping on the properties of GeTe-based thermoelectric materials.A series of samples were prepared according to the stoichiometric ratio of Ge_1-x-yIn_xMn_yTe（x=0.02,0.04,y=0.01,0.02）.It is found that doping Mn in the GeTe matrix can optimize the energy band structure of GeTe.At the same time,it is found through SEM images that the second phase of Mn Te₂ is formed by Mn doping in the matrix,and the lattice thermal conductivity reaches 0.71 Wm^-1K^-1.The In element can be used as an electron donor in the GeTe matrix,and the carrier concentration can reach 6.4×10²⁰ cm^-3.The final experimental results are in line with expectations and the best doping concentration is Ge_0.96In_0.02Mn_0.02Te at 750 K,a high ZT value of 1.26 is obtained.