Preparation And Performance Optimization Of N-Type Ag₂Se-Based Bulk Thermoelectric Materials And Specialshaped Devices

As a functional material that can realize the direct conversion of heat and electricity,thermoelectric materials have the advantages of small size,no pollution,light weight,and simple structural units,and can be widely used in the fields of power generation and refrigeration.Especially in the past ten years,wearable smart devices have developed rapidly,and it is necessary to develop a device that can provide uninterrupted power supply.High-performance thermoelectric materials near room temperature can directly convert human body heat into electricity to effectively solve the problem of poor battery life of smart wearable devices.At present,the only thermoelectric materials suitable for applications near room temperature is Bi₂Te₂-based alloy,but its poor mechanical properties limit practical applications.Therefore,it is urgent to develop alternative materials with excellent thermoelectric and mechanical properties.Ag₂Se-based thermoelectric material is a typical"phononic liquid-electronic crystal"thermoelectric material,which has the potential of high thermoelectric figure of merit at room temperature and good mechanical properties.That means it is a suitable candidate material for flexible thermoelectric devices.In this thesis,Ag₂Se as the main research object,optimizing the material properties by doping heteroatoms and adjusting the ratio of silver to selenium,and optimizing the thermoelectric devices with excellent performance by device shape design and hot-press treatment.The main research methods of this paper are as follows:（1）Ag₂Se powder is synthesized by mechanical alloying method,and the lattice distortion occurs inside the material by doping the transition metal atom Zn at the Ag site,which enhances the internal phonon scattering and reduces the thermal conductivity of Ag₂Se.At385 K,the z T peak of Ag_1.995Zn_0.005Se reaches 0.69,which is about 2 times higher than the z T value of the original Ag₂Se sample,but Zn doping sharply reduces the carrier mobility of Ag₂Se,so it is not the preferred method to optimize the Ag₂Se based material.（2）On the basis of the above research experience,the self-defect concentration of Ag₂Se is controlled by introducing excess Se by adjusting the stoichiometric ratio of silver to selenium.Due to the increase of Ag vacancies,the carrier concentration of the material is significantly reduced,and the carrier mobility is significantly increased.The power factor of Ag_1.92Se is increased from 1250μW m^-1 K^-2 to 2750μW m^-1 K^-2.In addition,the increase of defect concentration further affects the phonon transport,and the lattice thermal conductivity of the material is greatly reduced,and the lowest 0.25 W m^-1 K^-1.Finally,at 385 K,the z T peak of Ag_1.92Se reaches 0.87.（3）A flexible special-shaped thick-film device is prepared by hot-press the above-mentioned bulk Ag₂Se-based material with optimized thermoelectric properties and combining with p-type silver-copper-tellurium thermoelectric material.Experiments show that when the temperature difference between the hot and cold ends of a pair of devices prepared by the thermoelectric arm thickness is 0.1 mm and the bottom width is 6 mm,the maximum power is 21.4 m W,and the maximum power density is 23.80 W m^-2.Maximum bending radius up to 12 mm.