Preparation And Properties Of AgInSe₂-Based Thermoelectric Materials

Energy crisis and environmental issues are the main challenges in current world.In order to face the challenges,it is necessary to update the energy system.Efficient,eco-friendly,and clean energy conversion technology provides a new way for improving energy efficiency.Thermoelectric material is a functional material that can realizes the direct conversion between thermal and electrical energy via using the carrier transport in solid,which provides a safe,reliable,pollution-free,noise-free all-solid-state power generation,and cooling method,and thus it has broad application prospects.Recent advances in thermoelectric materials and devices are motivating thermoelectric technology into an important application in advanced systems of energy conversion.In the 21^st century,with the continuous development and improvement of thermoelectric transport mechanism,many high-performance thermoelectric materials have been discovered and developed rapidly.Diamond-like structural compounds are derived from diamond structures such as silicon,which have typical tetragonal structures with diamond structures.In recent years,researchers paid more attention to diamond-like thermoelectric materials because of the stable tetrahedral structure and excellent thermoelectric performance.The ternary chalcopyrite,AgInSe₂material,is considered as a potential high-performance thermoelectric material due to its low lattice thermal conductivity(0.86 Wm^-1K^-1,323 K).However,there are few studies on the thermoelectric properties of AgInSe₂ compounds.It has been found that the dominant factor to restrict the improvement in thermoelectric performance of AgInSe_2,which is attributed to the low intrinsic carrier concentration originated from a broad band gap（E_g=1.2 e V）,leading to poor electrical transport properties.Fortunately,the tunability of band structure makes it possible for us to study the structure and properties of AgInSe₂ materials.Lots of researches showed that the high-presssure and high-temperature（HPHT）method,as a static high-pressure technique,can manipulate the crystal structure,band structure,and microstructure,etc.,realizing the optimization of electro-thermal transport performance.Accordingly,the preparation and properties of AgInSe₂ material would be studied by the HPHT method in this paper.The main contents of the dissertation are shown as below:（1）AgInSe₂（I-42d）was prepared by mechanical alloying method,and then AgInSe₂bulk material was prepared by HPHT.The results indicated that AgInSe₂ underwent a structural transformation.The crystal structure transfered from a tetragonal crystal system（I-42d）to a rhombohedral crystal system（R-3m）.The high-pressure phase of AgInSe₂（R-3m）can be retained stably to atmospheric condition.By modulating the synthesis pressure,the initial temperature of phase transition would decrease with increasing pressure,the mixed region of I-42d and R-3m exhibited a reduction with pressure simultaneously.Besides,the grain size of the samples decreased with the increase of the synthesis pressure,in contrast,the grain size increased with sintering temperature under the same pressure.In addition,the electrical transport properties of AgInSe₂（R-3m）prepared under different pressures were studied at room temperature.The results showed that the pressure reduced the electrical resistivity and thus improving the power factor.Finally,the maximum power factor(PF=5.85506μWm^-1K^-2)was obtained in the sample prepared at 2.5 GPa.（2）Doping,as an effective means to optimize carrier concentration,is employed to study the electrical transport performance of AgInSe₂ materials.In this work,the electric transport performance of AgInSe₂ is studied systematically through Te doping and intrinsic defect regulation.Lots of researches suggested that AgInSe₂ had abundant intrinsic defects,such as vacancy and antisite defects.Ag doping could compensate for Ag vacancy,improving the electrical conductivity and power factor.This has been confirmed that it is an effective means to optimize the electrical properties of AgInSe₂.In our work,the results indicated that the electrical conductivity was increased significantly by compensating for the intrinsic Ag vacancy.It is obvious that the electrical conductivity of Ag_1.1In Se₂ is higher than that of other doping contents at 816 K,that is 4291 Sm^-1,which is 49 times higher than that of pristine AgInSe₂.The enhancement in electrical conductivity results in a significant increase in the power factor.The studies showed that Te doping had a positive effect on the electrical conductivity and power factor of AgInSe₂（R-3m）samples,and the Ag_1.1In Se_1.9Te_0.1 sample prepared at 2.5 GPa obtained a maximum electrical conductivity of 10748 Sm^-1 at 327 K.