Preparation, Structure And Thermoelectric Properties Of P-type Ge Based Type-I Clathrates

In recent years, clathrates with cage structures have gained renewed attention due to their promising thermoelectric properties and potential application in thermoelectric generator and type-â… germanium clathrate is one of them. Electron density analysis has shown that inâ…¡₈â…¢₁₆â…£₃₀ type-â… clathrates, each guest atom (as donor) donates two electrons to the framework (acceptor) and the total electrons donated by the guest atom are accepted completely by the framework, and hence it should behave intrinsic semiconduction characteristic. However, most reports in literature show samples with metal-like and n-type properties. There are some p-type samples have been reported. However, the thermoelectric property of p-type clathrates is very low compared with that of n-type Ge-based clathrate with high ZT value, so it is very urgent and necessary to develop and investigate p-type clathrates with high performance.In this thesis, using Zn, Cd, Al and Mg to replace the framework atom Ge of type-â… Ba₈Ga₁₆Ge₃₀ clathrate which with n type conduction, the type-â… clathrates with p type conduction characteristics were synthesized successfully, and the main obtained results as follows.Zn-doping Ba₈Ga₁₆Zn_xGe_3o-x(x=2.4, 2.6, 2.8, 3.0, 3.2) p type clathrates with different Zn contents were synthesized by combing solid-state reaction with spark plasma sintering (SPS) method. The effects of Zn doping on the structure and on the atom displacement parameters (ADP) is not obviously due to difference of the atom mass and the covalence radius between the Zn and Ge is not obvious. The carrier concentration N_p increases while the electrical conductivity, and the carrier mobility and the thermal conductivity decrease with the increasing Zn content for Zn doped Ba₈Ga₁₆Zn_xGe_3o-x compounds. The carrier scattering mechanism is mainly ionized impurity mechanism in the low temperature range (7<200 K), and gradually change to an acoustical mechanism with the rising temperature. The maximum ZT value of 0.38 is obtained at 790 K for Ba₈Ga₁₆Zn_3.0Ge_27.0 compound. The effects on the thermoelectric properties of Ba₈Ga_16+xZn_3.0Ge_27.0 were studied by slightly adjusting the Ga content, The carrier concentration, the electrical conductivity and the thermal conductivity increase, while the Seebeck coefficient and the carrier mobility decrease with the increasing of Ga content. The highest ZT value of 0.43 is obtained for the specimen with x=16.2 at 720 K, which is increased by 13% compared with that of Ba₈Ga₁₆Zn_3.0Ge_27.0 compound.P-type Ba₈Ga₁₆Cd-xGe_30-x (x=0.95, 1.0, 1.05, 1.10) type-â… clathrates were synthesized using Cd as doping element. The ADP of the Cd atom is much larger than that of other framework atoms compared with that of Zn doped clathrate. The electrical conductivity increases while Seebeck coefficient decreases with the increasing Cd content, and the temperature which the Seebeck coefficient reaches its maximal value shift to the low temperature range with increasing Cd content. The maximum power factor value of 0.4×10^-3 Wm^-1K^-2 is obtained at 550 K for Ba₈Ga₁₆Cd_1.0Ge_29.0.The room temperature thermal conductivity for Ba₈Ga₁₆Cd_1.0Ge_29.0 is reduced by 6% compared with that of Ba₈Ga₁₆Zn₃Ge₂₇ compound. The largest ZT of 0.173 is obtained for Ba₈Ga₁₆Cd_1.0Ge_29.0 at 600 K.P type Ba₈Ga₁₆Al_xGe_30-x(x=1.0, 2.0, 3.0, 4.0, 5.0) type-â… clathrates were synthesized by excess group-â…¢Al element. The results reveal that the Al atom reference for the 6c site. The Al substitutions do not affect the ADP of framework atoms (Ge/Ga) and filled atoms (Ba) compared with that of Ba₈Ga₁₆Ge₃₀; and the ADP of Al are nearly equivalent to that of other framework Ge/Ga atoms. The carrier concentration and electrical conductivity increase while Seebeck coefficient decreases with the increasing Al content for the specimens with x≤4.0. But for the sample with x=5.0, the electrical conductivity increase distinctly due to the higher electrical conductivity of BaAl₄.The lattice thermal conductivity of Al doping compounds are relatively lower due to the mass fluctuation between Al atoms and other atoms in the framework; it is as low as 0.96 Wm^-1K^-1 at 300 K for Ba₈Ga₁₆Al_3.0Ge_27.0 compound. The maximum ZT value of 0.61 is obtained at 760 K for Ba₈Ga₁₆Al_3.0Ge_27.0. The electrical properties of the Al doping p type Ba₈Ga_xAl₃Ge_43-xclathrate are enhanced by slightly adjusting the Ga content, and the ZT value of 0.63 is obtained for the specimen with x=16.2 at 810 K.Using Mg atom to substitute for the framework atom Ge of type-â… clathrate Ba₈Ga₁₆Ge₃₀, p-type Ba₈Ga₁₆Mg_xGe_3o-x(x=1.0, 2.0, 3.0, 4.0) type-â… clathrates were synthesized. The analysis results indicate that the specimens with≤3.0 are single type-â… clathrates with cubic structure (space group pm (3|-) n). But, for the specimen with x=4.0, two different type-â… clathrates which with two different Mg content are detected: the main phase with higher Mg content, and the second phase with lower Mg content. The electrical conductivity decreases and Seebeck coefficient increases with the increasing of Mg content. The lattice thermoelectric is dominant part in the total thermoelectric conductivity, and it decrease with the increasing Mg content. The lattice thermal conductivity of Ba₈Ga₁₆Mg_3.0Ge_27.0 compound is 1.17 Wm^-1K^-1 and 0.81 Wm^-1K^-1 at the room temperature and 720 K respectively, which is correspond to that of Al doping p type Ba₈Ga₁₆Al_3.0Ge_27.0 at the same temperature and lower than that of Zn doping p-type clathrates. The dimensionless figure of merit ZT of 0.32 is obtained for the p type Ba₈Ga₁₆Mg_3.0Ge_27.0 clathrate.