P-Type?Bi,Sb?₂?Te,Se?₃ Based Thermoelectric Materials And Performance Optimization

(Bi,Sb)₂?Te,Se?₃ based alloys are studied earliest thermoelectric?TE?material with best performance near room temperature.The zT values of the p-type and n-type components are both around 1.0,which are widely applied for commercial refrigeration.However,serious intrinsic excitation caused by narrow band gap of common components at higher temperature,limits the applications of?Bi,Sb?₂?Te,Se?₃alloys in mid-temperature power generation.Meanwhile,the poor mechanical properties of commercial zone-melted alloys also restrict its further practical development.We prepared p-type Sb₂?Te,Se?₃ polycrystalline alloys,by doping Sb₂TeSe₂polycrystalline alloy to increase the carrier concentration,the TE properties are optimized in mid-temperature region.Then Se-alloying induced changes in crystal structure and TE properties of the alloys were systematically investigated.Meanwhile,we explored the TE properties of p-type Bi₂?Sb,Te?₃ based nanocomposites.The main conclusions are listed as below:1.We improved the TE performance of Sb₂TeSe₂ alloy through thermal conductivity reduction and further doping.The lattice thermal conductivity of polycrystalline sample is lower²0%than zone-melted sample,but carrier mobility also decreases.An appropriate amount of Ag was doped to Sb₂TeSe₂ alloy,then the intrinsic excitation is inhibited by increased hole concentration and the zT value of Sb_1.98Ag_0.02TeSe₂ is improved to 0.4 at 680 K.Sn dopant can obtain higher hole concentration in Sb₂TeSe₂ alloy,and the density-of-state effective mass is greatly increased by increased N_v or resonance doping,leading to larger Seebeck coefficient.However,Sn doping significantly deteriorates mobility,finally the zT value of Sb_1.92Sn_0.08TeSe₂ at 680 K is around 0.5.Combining two dopants,Ag-Sn co-doping Ag_0.02Sb_1.92Sn_0.06TeSe₂ sample shows a highest zT of 0.55 at 680 K.Although the hole concentration of Sb₂TeSe₂ alloy has been greatly increased by doping,the zT value cannot be effectively improved due to the existence of impurity phases.2.The TE properties of p-type Sb₂?Te,Se?₃ alloy were optimized by adjusting Se content.The Sb₂Te_3-xSe_x alloys show the structural transition from rhombohedral?R?phase to mixed phases at x=1.8 composition.The carrier concentration and density-of-state effective mass decrease with increasing Se contents x,and the existence of mixed phase structure does not significantly improve thermal performance.The R-Sb₂Te_1.3Se_1.7 shows preferable TE performance due to the higher carrier concentration and carrier mobility.The zT peak of the Sb₂Te_1.3Se_1.7.7 matrix is further improved and shifted to higher temperature by optimizing carrier concentration via Sn-doping and Ag-doping.As a consequence,a maximum zT of⁰.63 is obtained at 680 K in Sb_1.92Sn_0.08Te_1.3Se_1.7.7 alloy,20%higher than Sb_1.92Sn_0.08TeSe₂?3.We explored the TE performance of p-type Bi₂?Sb,Te?₃ based nanocomposites.The Ag₂Te is precipitated in the Bi_0.3Sb_1.7Te₃ matrix with a mass of Ag atoms entering the lattice,which greatly increases the carrier concentration and inhibites the intrinsic excitation,contributing to the mid-temperature TE performance.The zT maximum of Ag_0.01Bi_0.3Sb_1.7Te_3.005 samples is around 0.83 at 575 K.Besides,Bi_0.3Sb_1.7Te₃ based nanocomposites dispersed with SiO₂ nanoparticles?¹00 nm?are fabricated by mechanical ball-milling and hot-pressing.The decreased carrier concentrations of 0.5wt.%and 1 wt.%samples leads to larger Seebeck coefficient aroud room temperature,and the lattice thermal conductivity is slightly improved.The Bi_0.3Sb_1.7Te₃+1 wt.%SiO₂ sample shows a preferable zT values of¹.1 at 300 K,¹.2 at 350 K.