Preparation And Properties Of N-type Cubic AgBiSe₂ Thermoelectric Materials

Thermoelectric materials are a new type of functional materials.Thermoelectric devices prepared by using the thermoelectric conversion effect can convert waste heat into electric energy.They can replace some non-renewable energy in many fields,so they are a kind of energy materials that have attracted wide attention.N-type AgBiSe₂-based thermoelectric compounds are considered as potential high performance thermoelectric materials due to their inherent low lattice thermal conductivity.However,the intrinsic AgBiSe₂ compound will undergo two reversible phase transitions in the range of 300K-700K,which is hexagonal at room temperature,rhombohedral at 460K,cubic at 580K,and hexagonal at room temperature.The phase transition process can cause a sudden change in the lattice constant of the intrinsic AgBiSe₂ compound,resulting in internal thermal stress,which limits the practical application of the compound in thermoelectric devices.It has been found that high temperature cubic phase AgBiSe₂ has better thermoelectric performance potential,so how to prepare AgBiSe₂-based thermoelectric materials with stable cubic phase structure and excellent performance over a wide temperature range（300-700K）is more valuable for application.The research objective of this paper is to prepare AgBiSe₂-based thermoelectric compounds with stable cubic phase structure over a wide temperature range（300-700K）,and further optimize its thermoelectric properties,so as to explore a new research route for the crystal structure and thermoelectric properties regulation of AgBiSe₂.The main research contents are as follows:1.The（AgBiSe₂）_1-y（MnTe）_y compound and（AgBiSe₂）_1-x（SnTe）_x compound were prepared by alloying MnTe and SnTe with AgBiSe₂,respectively.The results showed that the introduction of MnTe did not significantly inhibit the phase transition.Not only does（AgBiSe₂）_1-y（MnTe）_y fail to maintain a stable cubic phase structure at room temperature,but MnSe heterophase is also formed.When SnTe is alloyed with AgBiSe₂,XRD results show that（AgBiSe₂）_1-x（SnTe）_x is cubic phase structure when the content of SnTe is 25%.DSC and variable temperature XRD tests showed that with the increase of SnTe content,the endothermal peak corresponding to the（AgBiSe₂）_1-x（SnTe）_xphase transition moved to low temperature,and the（AgBiSe₂）_0.75（SnTe）_0.25 sample phase changed into a cubic phase structure at 300K.And keep the cubic phase constant in the test temperature range.This indicates that the introduction of SnTe alloying not only reduces the cubic phase transition temperature,but also effectively inhibits the reversible phase transition,and the stable cubic phase（AgBiSe₂）_0.75（SnTe）_0.25 thermoelectric material was prepared.The XRD results show that the lattice constant of AgBiSe₂ gradually increases with the increase of SnTe content.The thermoelectric performance test results show that the electrical performance decreases with the increase of SnTe content,because SnTe is p-type conduction.The absolute value ofSeebeck coefficient decreases obviously,which is due to the highly disordered cation and the decrease of carrier effective mass at high temperature.Due to the co-action of n-type and p-type carriers inside the material,the bipolar diffusion thermal conductivity increases significantly in the high temperature range,resulting in the thermoelectric value z T of the material first rising and then decreasing with the increase of temperature.Therefore,the maximum z T value of（AgBiSe₂）_0.75（SnTe）_0.25 is～0.3 at 500K.On the basis of the above work,ball milling was used to refine the raw material powder to reduce the grain size and lattice thermal conductivity,and further optimize the thermoelectric performance of（AgBiSe₂）_0.75（SnTe）_0.25.It is found that with the increase of ball milling time,the powder particles decrease gradually,the grain size of sintered bulk sample decreases,and the grain boundary increases.However,the thermal conductivity of the bulk samples did not change much,while the electrical conductivity decreased significantly,which indicated that the effect of ball milling treatment powder on the reduction of lattice thermal conductivity was not obvious.2.In order to optimize the thermoelectric properties of the stable cubic phase structure（AgBiSe₂）_0.75（SnTe）_0.25,the Ag site was doped with Nb and In,respectively.The results showed that doping did not affect the stability of the phase structure,but also improved the room temperature conductivity of the material,which increased to 151.4 S·cm^-1（Nb doping 0.02）and144.7 S·cm^-1（In doping 0.03）,and due to the combined effects of increasing point defects,enhanced phonon scattering and alloying scattering,At 700K,the lattice thermal conductivity decreases to 0.39 W·m^-1·K^-1（Nb doping 0.05）and 0.45 W·m^-1·K^-1（In doping 0.03）,respectively.The(Ag_0.98Nb_0.02BiSe₂)_0.75（SnTe）_0.25 and(Ag_0.99In_0.01BiSe₂)_0.75（SnTe）_0.25 samples of cubic phase structure were obtained with z T peaks of～0.32（650K）and～0.34（600K）,respectively.3.Based on the above research results,different lead-free Ⅳ-Ⅵ compounds（X=SnTe,SnSe,GeSe,GeTe）were alloyed with AgBiSe₂ respectively,and Nb doped(Ag_0.99Nb_0.01BiSe₂)_0.75（X）_0.25compounds were obtained.The better performance of GeSe alloyed samples is that the power factor can reach 4.2μW·cm^-1·K^-2 and the highest z T value can reach～0.4 at 700K.When the GeSe content is 50%,the（AgBiSe₂）_0.5（GeSe）_0.5 sample is still cubic phase structure.The Ag position of the sample is doped with Nb element to obtain(Ag_0.98Nb_0.02BiSe₂)_0.5（GeSe）_0.5 sample.The power factor reaches the maximum value of 4.8μW·cm^-1·K^-2 at 700K,and the thermoelectric value reaches 0.36.