Properties Control Of Ge₁₇Sb₂Te₂₀ Thermoelectric Materials

Owing to the directional migration of the internal carriers of materials driven by the temperature gradient,thermoelectric materials,which can directly convert thermal into electrical energy,can be utilized to solve the current increasingly serious energy crisis.Recently,Ge-Sb-Te based thermoelectric materials have become one of the research focuses in the field of mid-temperature thermoelectric materials due to their excellent mid temperature thermoelectric properties and adjustable band structures.However,the core performance index thermoelectric merit ZTis still low,which leads to the low thermoelectric conversion efficiency and significant limitation on its application.Therefore,it plays an critical issue to improve and optimize ZT value in this field.The purpose of this paper is to optimize the thermoelectric properties of Ge₁₇Sb₂Te₂₀,by improving the intrinsic high hole carrier concentration and large thermal conductivity.Through structural regulation,element doping,and other means,the band structure and thermal conductivity were regulated successfully to improve the thermoelectric merit of the material.The main research contents of this paper were listed as follows:（1）The influence of particle sizes on the thermoelectric properties of Ge₁₇Sb₂Te₂₀ was studied by changing the time of ball milling.The results showed that the lattice thermal conductivity of the system decreased significantly after ball milling,and the average particle size of the sample milled for 3 h was the smallest,obviously,indicating that refining the grains could effectively reduce the thermal conductivity of our samples.It can effectively reduce the thermal conductivity of materials.（2）To further reduce the thermal conductivity of the system,nanoscale composite nanostructures were formed by introducing nano Si combining with ball milling process.The research showed that the introduction of Si particles,effectively increased the phonon scattering center,while the thermal conductivity of the material further decreased.The thermal conductivity of the ball milled 3 h+Si sample decreased to 2.12 W·m^-1·K^-1 at 530 K,which is 39%lower than that of pure Ge₁₇Sb₂Te₂₀.But the decrease in thermal conductivity cannot compensate for the negative impact of Te precipitation on electrical transport performance,and the ultimate improvement in ZT value was also limited.（3）The incorporation of Ca into Ge sites in Ge₁₇Sb₂Te₂₀ was performed to increase the formation energy of Ge vacancies.The carrier concentration of the system was successfully reduced to 3.913 ×10²⁰ cm^-3.At the same time,due to the large difference between the radius of Ca atoms and the matrix element,significant lattice distortion was caused.When x=0.05,the formation of micro and nano CaTe precipitates at the interface constituted a multiscale scattering center,resulting in a decrease in the thermal conductivity of the system to 1.81 W·m^-1·K^-1.Finally,the ZT value of the x=0.05 sample reached 1.25,which was 46%higher than that of the undoped sample.（4）A rhombohedral to cubic structural phase transition was observed at room temperature in Ge₁₇Sb₂Te₂₀ by doping Fe_1.1Se.The effective mass of doped samples increased to 2.8 m₀.The Seebeck coefficient significantly increased,and the thermoelectric performance at low temperature of the samples were significantly improved.In addition,the lattice distortion of the system caused by the incorporation of Fe²⁺ and Se^2-into the lattice generated to a significant decrease in the thermal conductivity of the system.The ZT value of the final x=0.05 sample raised to 1.08 at 723 K,which was 32%higher than that of the undoped sample.