Thermoelectric Properties Of Sn-based Type-Ⅷ Clathrates By Theoretical And Experimental Research

Sn-based type-Ⅷ clathrates have become one of the most promising thermoelectric materials because of their high Seebeck coefficient and low thermal conductivity.The energy band structure of materials can be adjusted by doping and changing the compositions of the material and the ratio of the framework atoms,and the thermoelectric properties can get optimized.To understand the effect of these factors on the electronic structure and heat transport and electrical transport of Sn-based type-Ⅷ clathrates,the thermoelectric properties of Sn-based type-Ⅷ clathrates are investigated through the theoretical calculations and experimental results.It is of great significance for research and development of high-performance materials.In this paper,Yb and Ba,Sm and Ba double-atom filled Sn-based type-Ⅷ Ba₈Ga₁₆Sn₃₀ clathrates were synthesized through the Sn flux method,and the thermoelectric transport properties were studied;in addition,based on the higher thermoelectric properties of n type type-Ⅷ Sn-based clathrate Ba₈Ga₁₀Al_<sub>Sn₃₀,the effect of Ga content and Sn flux content on the thermoelectric transport properties were studied,respectively,and the structural properties of Ba₈Ga₁₀Al_<sub>Sn₃₀ were further studied based on the first principles of density functional theory;at the same time,Ba8Ga16 Znx Sn30 clathrates were prepared using a-Sn as flux and the thermoelectric properties were studied;finally,the structural stability and electronic properties of Ba₈Ga₁₆Sn₃₀-x Gex for different Sn/Ge ratio were investigated,and the phase transition under high pressure and the electronic structure properties of Sr₈Al₁₆Sn₃₀ under static pressure modulation were studied through the theoretical calculation.The main conclusions obtained are as follows:1.Type-Ⅷ Ybx Ba_8-xGa₁₆Sn₃₀（x = 0,0.25,0.5,0.75,1,1.25,1.5,1.75,2）and Smx Ba_8-xGa₁₆Sn₃₀（x = 0,0.25,0.5,0.75,1,1.5,2）clathrates were synthesized by Sn flux method.Yb and Sm content in the samples is low.The lattice parameters of Ybx Ba_8-xGa₁₆Sn₃₀ decreased with the increase of Yb content;The lattice parameters of Smx Ba_8-xGa₁₆Sn₃₀ samples increased with the increase of Sm content,but they were lower than those of Ba₈Ga₁₆Sn₃₀;all Yb-doped and Sm-doped samples exhibit n-type conductivity behavior;The electrical conductivity for the sample of Yb initial content 1.5 was higher during the measuring temperature range,which increased by about 60% compared with that of Ba₈Ga₁₆Sn₃₀;during the test temperature range,the Seebeck coefficient of the sample of x = 0.75 was higher with the maximum value of-341mV/K at 483 K,coupled with the high electrical conductivity.For the sample of Yb initial content 1.5,the maximum of ZT 1.35 was obtained at 489 K,and the maximum of ZT 1.31 was obtained at 457 K for the sample of Sm initial content 0.75.Due to the ionic radius of Yb,Sm is smaller than that of Ba,it is difficult to replace the filled atoms,but after the introduction of Yb,Sm to adjust the framework atomic ratio and the band structure,the thermoelectric performance was improved.2.Type-Ⅷ Sn-based Ba₈Ga_xAl₆Sn₃₀ clathrates（x = 10,20,30;namely Ga10,Ga20,Ga30）with p-type and n-type conduction were successfully prepared using the Sn flux method,and the carrier of n-type type-Ⅷ Sn-based Ba₈Ga₁₀Al_<sub>Sn₃₀ clathrates got modulated by adjusting the starting Ga content.With the increase of initial Ga content,the solubility of Al in the compounds decreased and the lattice constants were also decreased;In the test temperature range,the sample of Ga10 is n-type conduction,while the sample of Ga30 behaves p-type conduction,and the Ga20 sample converted from p-type conduction to n-type near 550 K;this finding showed that the change of the initial Ga content could effectively adjust the carrier of the compound.In addition,the carrier effective mass of the samples which exhibit p-type conduction in the whole test temperature range is higher than that of the n-type sample at room temperature.The p-type samples of Ga30 obtained the maximum ZT value of 0.86 at 457 K.3.The influence of the starting content of Sn flux on the structural properties and thermoelectric transport properties of type-Ⅷ Sn-based Ba₈Ga₁₀Al₆ Snx clathrates（x = 40,50,60;Sn40,Sn50,Sn60）has been explored.The results showed that the initial Sn content has no significant effect on solid solubility of Al in the clathrate and lattice constant;All the samples showed n-type conductivity characteristics in the range of test temperature,the absolute value of the Seebeck coefficient and electrical resistivity increased gradually with the increase of temperature;the three samples of Sn40,Sn50 and Sn60 have similar content of each element,but the electrical properties varied a certain extent.Sn60 sample at room temperature has high carrier concentration.In the range of 300-600 K,the sample of Sn50 has a higher power factor,which reaches the maximum value of 1.82×10^-3 Wm^-1K^-2 at 488 K.Theoretical calculation showed that the energy band near Fermi level of the compound Ba₈Ga₁₀Al_<sub>Sn₃₀ is relatively convergent compared with Ba₈Ga₁₆Sn₃₀ and Ba₈Al₁₆Sn₃₀,thus it has good thermoelectric properties.4.Zn doped type-Ⅷ Sn-based clathrates were prepared using the grey Sn（a-Sn）flux method.The results showed that Zn tend to replace the Ga element,and the lattice constant of the compound decreased with the increase of the Zn content;The melting point of the samples is basically consistent with the Ba₈Ga₁₆Sn₃₀ prepared by b-Sn,and it can be stable in the range of test temperature;all samples are n-type semiconductors;The Seebeck coefficient of x=0 sample at room temperature is lower than that of the one prepared by b-Sn,but it gets improved obviously for the sample with the incorporation of Zn;As a result of the higher Seebeck coefficient,the sample of x=1.5 has a higher ZT value,which reaches the maximum of 0.71 in the vicinity of the 568 K.5.The calculation results of type-Ⅷ Ba₈Ga₁₆Sn₃₀-x Gex clathrates（0≤x≤30）showed that: Ge atoms preferentially occupy the 2a and 24 g position of Sn atoms in clathrates;all compounds are indirect bandgaps semiconductors;for the Ge substituted compounds,the lattice constant decreased,the band gap increased,and the formation energy and binding energy decreased with the increase of Ge content;all clathrates are indirect band gap semiconductors;the substitution of Ge has no significant effect on the n-type conductivity,while it has an important effect on the p-type conductivity;the change of potential energy for Ba atoms along the three coordinate axes has anisotropic properties,the vibration energy of Ba increased with the decrease of cage size;the Ba atoms of all the clathrates were deviated from the center of the cage,and the distance of Ba atom deviation from the center of the cage initially increases and subsequently decreases as the Ge content increases.6.The structural properties and electronic structure of type-Ⅰ and type-Ⅷ Sr₈Al₁₆Sn₃₀ under isostatic pressure were studied.The results shows that the type-Ⅷ structure is stable while the type-Ⅰ structure is a metastable phase.The stability of the two kinds of structures decreased with the increase of pressure,but at different pressures,the type-Ⅷ is more stable than the type-Ⅰ structure.there is no phase transition from type-Ⅷ to type-Ⅰ under isostatic pressure,but as the volume increase,type-Ⅷ has a trend of phase transition towards the type-Ⅰ.The electronic structure shows that type-Ⅰ and type-Ⅷ Sr₈Al₁₆Sn₃₀ are indirect band gap semiconductors with the band gaps of 0.041 e V and 0.124 e V,respectively;The electronic band structure of the type-Ⅷ near the Fermi level has a higher degree of dispersion,the type-Ⅰ structure has a higher asymmetry in the band edge,and the band gaps of the two structure varied with pressure,which shows that the pressure can regulate the band edge structure and band gap of type-Ⅰ and type-Ⅷ.