Preparation And Thermoelectric Properties Of Ag₉GaTe₆ Base Compound

In the 21st century,energy shortage,which is one of the three major problems for human society,has severely restricted the economic development and is an important issue that should be urgently resolved for both social progress and scientific development.Considering that thermoelectric materials can achieve direct conversion between thermal energy and electrical energy with nearly no noise or pollution.It is believed that thermoelectric materials are one of the most promising energy materials and have good development prospects.Conventional thermoelectric materials such as Bi2Te3 and its alloys have been extensively studied because of their high thermoelectric properties until now.At the same time,many new thermoelectric materials such as Zn₄Sb₃,Ag₂Te and BiCuSeO have attracted the attention of researchers due to their special properties,and become a new hot spot in thermoelectric materials research.Among them,the Ag-Ga-Te-based compounds have very low thermal conductivity,and the argyrodite-type material has a structure of“phonon liquid-electron crystal”.So as a combination of the Ag-Ga-Te-base and argyrodite-type compounds,Ag9GaTe6 is believed to have low intrinsic thermal conductivity as well as the crystal structure beneficial to obtaining high thermoelectric performance.However,there are quite a lot problems should be solved in the research of Ag9GaTe6.The preparation method needs to be optimized,the crystal structure has yet to be resolved,and there is a lack of systematic regulation and optimization research for its low conductivity,.Based on the above issues,in this study,we focus on Ag₉GaTe₆,explored the parameters for the preparation process,analyzed the crystal structure by single crystal sample.In addition,Cd doping,Se solid solution and off-stoichiometric ratio are used to increase the carrier concentration.The effects of doping solid solution and stoichiometric deviation on Ag₉GaTe₆ were systematically studied.The main contents and conclusions are as follows:The crystal structure analysis results show that?-Ag₉GaTe₆ has a hexagonal structure,and the[GaTe₄]^5-tetrahedron forms an anion framework with Te^2-anions,while the Ag⁺cations in the framework are disorderly arranged,which makes a same composition as the argyrodite-type materials.The nature of liquid-like materials leads to a lower thermal conductivity of Ag9GaTe6.The preparation process of pure phase Ag₉GaTe₆ was systematically studied.The method of melting-quenching-annealing combined with plasma activation sintering process,which improved the density of the samples.The abnormal change of electrical conductivity of Ag₉GaTe₆ with the rising temperature can be well explained by the results of the high temperature Hall test.The phenomenon that conductivity increases first and then decreases with temperature is due to the rapid decrease in mobility,but the increases in carrier concentration of Ag₉GaTe₆ with increasing temperature.The pristine Ag9GaTe6 has very low thermal conductivity by melting-quenching-annealing combined with PAS.It maintains the value of thermal conductivity between 0.2-0.4 Wm^-1K^-1 during the test temperature.Moderate power factor combined with the extremely low thermal conductivity makes the pristine Ag₉GaTe₆ reaches a maximum value ZT_max=0.80@673 K,which is 122%higher than the maximum value ZT_max=0.36@723 K reported in the earlier literature.The effects of Cd doping at the Ga site and the solid solution at the Te site on the thermoelectric properties of Ag₉GaTe₆ were systematically investigated.The impure Ag₉Cd_xGa_1-xTe₆ compounds lead to the increase of the thermal conductivity due to the presence of the high thermal conductivity second phase.And Seebeck coefficient increases slightly and the conductivity decreases,which makes the ZT value of Cd doped sample is not developed.For Se-solid solution samples,Ag₉Ga(Te_1-xSe_x)₆compounds all turned to be pure phase.And their conductivity decreased compared with the pristine sample during the whole test range,and the power factor of Ag₉Ga(Te_1-xSe_x)₆ was slightly enhanced.Se solid solution improved the point defect scattering and reduces the thermal conductivity of the material.Finally,the highest ZT value of Ag₉Ga(Te_1-xSe_x)₆ compound is 0.78,and the performance is not improved compared with the pristine Ag₉GaTe₆ maximum ZT value of 0.80..The influence of stoichiometric deviation on the thermoelectric properties of Ag₉GaTe₆ was systematically studied.Ag deficiency can play a role in optimizing the Seebeck coefficient,but the conductivity and thermal conductivity of the Ag-deficiency samples are degraded,so that the ZT value of the Ag_9-xGaT_e6 compound is not significantly improved.For Ga deficiency and Te excess samples,the high temperature Hall test showed that the proper amount of Ga deficiency and Te excess increased the carrier concentration of Ag9GaTe6,but its electrical conductivity decreased due to the decrease of mobility.The Seebeck coefficient is also improved by Ga deficiency and Te excess,and finally a power factor higher than the pristine sample is obtained.The relationship between C_p/T² versus T² and C_p/T³ versus T of Ag₉GaTe₆ base compounds can be well fitted by Debye combined with double Einstein model,in addtion with the Boson Peak appearing in the relationship between C_p/T³ versus T.These proof indicate that the origin for the low intrinsic thermal conductivity of Ag₉GaTe₆ base compounds is the coupling of low-frequency optical phonon mode with acoustic phonon mode.The decrease of Debye temperature happened in both Ga deficiency and Te excess samples,show that the lattice becomes soft and the thermal conductivity is reduced.Finally,the component with Ga deficiency x=0.05 achieved ZTmax=1.13@623 K,which was 41%higher than the pristine sample of 0.80,and Ag₉GaTe_6.06.06 obtained ZT_max=0.97@673 K,which was 21%higher than the pristine Ag₉GaTe₆.