The Study On Preparation, Structure And Thermoelectric Characteristics In Ge-and Sn-based Clathrate Materials

It is urgent to find new, environmentally-friendly renewable energy sources andenergy conversion technologies with the gradual reduction of fossil energy andenvironmental pollution worsening. Thermoelectric （TE） power generation technologywith some excellent properties such as small size, the inherent advantages of reliability,low operating costs, long life, environment-friendly features, directly converttemperature gradients and heat into electrical voltages and power by the Seebeck effectand cause attaches great importance to the world. The relatively low efficiency ofavailable material limits the use of the technology to niche commercial applications.Thus, the development of higher-performance TE materials is becoming more importantThe efficiency of TE depend on the figure of merit ZT defined as ZT=σS2/κ, whereS, T, σ and κ are the Seebeck coefficient, absolute temperature, electrical conductivity,and total thermal conductivity, respectively. one can clearly see that, in principle, thedirection of increasing the figure of merit （ZT） is to increase electrical conductivity andthe Seebeck coefficient and to decrease thermal conductivity. However, in reality, it isnot easy to improve ZT due to the fact that σ, S, κ are all coupled with each other. So itis very difficult to improve ZT value. Recently, One of the recent novel approaches tosearch for good thermoelectric materials is to find or design a new kind of materialbased on the phonon glass electron crystal （PGEC） concept which is to significantlyenhance phonon scattering （similar with glass） but at the same time keep good electricconductivity （electron crystal）. A number of studies have been carried out from the ideaand the considerable progress on this group of materials has been made.The clathrates, which has the cage-like structures, is typical of the phonon-glasselectron-crystal thermoelectric materials, so it are attracting considerable attention asnew high temperature thermoelectric materials. Among clathrates, Ge-and Sn-basedclathrates have been attracting much attention due to its superior thermoelectricproperties. The thermoelectric conversion efficiency needs to be improved further forcommercial applications. The electrical and thermal transport properties of the materialare strongly dependent on the electronic band structure. Studies showed that chemicalsubstitution, changes in the proportions of the material components, some physical waysuch as application of pressure are able to tune the electronic structure and latticevibration, thus improve thermoelectric efficiency. In this thesis, based on the first-principles density-functional calculations, we report on the evolvement of thestructure and the electronic structure by substitution of Sn, Cu, Ag within the framework.At the same, some structural and electronic properties of these materials were studiedby changing guest inside the cages and application of pressure. Then,I-Sr₈Ga₁₆Sn_xGe_30-xand VIII-Ba₈（Cu, Ag）_xGa_16-xSn₃₀ single crystal samples weresynthesized by the Sn-flux method and I-Ba₈Ga₁₆In_xGe_30-xpolycrystalline samplewere synthesized by melting method. XRD, XPS and Raman spectroscopy were used toanalyze the structure properties and chemical state of the materials, at the same,electrical conductivity and Seebeck coefficient have been measured. The effect of thedifferent dopant elements and concentration of the dopants on the structural andthermoelectric properties was investigated. The main obtained results as follows.1. The calculations showed that the valence band is primarily composed of the sp³bonding orbitals of the framework, and the hybridization of the guest orbitals with theconduction band of framework forms the conduction band of Clathrates. It shows thatthe conduction bands are influenced by the guest atoms, whereas the valence bands arenot much affected. We found that the calculated electronic structure for Yb8Ga16Ge30isdifferent from that of Ba8Ga16Ge30and Sr8Ga16Ge30. The results indicate that thedispersion relation near the top of the valence band is changed, but there is nosignificant change in the dispersion of the top of the valence band by Sn, Cu, Agdoping.2. The structural and the electronic properties of the clathrates Ba8Ga16SnxGe30-xare calculated. The results show that all materials are indirect band gap semiconductor,the stability and the band gap of the materials decreases with increase in Sn content.Raman and x ray photoelectron spectroscopy （XPS） measurement revealed that theframework compositional ratio and the average distribution of framework atoms on thevarious sites change with doping concentration. Single crystal samples of type-Iclathrates Sr₈Ga₁₆Sn_xGe_30-xwere synthesized and the specimens possess p-typeconduction properties. The electrical conductivity increases with increasing temperature,which is a typical behavior of heavily doped semiconductors. The Seebeck coefficientof Ge24（starting composition） compound is-89μVK^-1and-190μVK^-1at the roomtemperature and750K respectively. For samples Ge24and Ge28and Ge30（startingcomposition）, the largest power factor were obtained at about600K, but the the largestpower factor of Ge26（starting composition） did not reach its maximum value in thetemperature range of measurements. The maximum value of ZT were obtained at about 750K and the estimated maximum ZT value reaches0.79for Ge26（startingcomposition）, which is larger than the previously reported value of an optimizedpolycrystalline Sr8Ga16Ge30.3. There are two excess holes after Ga is replaced by Ag, so Ba₈Ga_16-xAg_xSn₃₀should be p-type semiconductors. But the Seebeck coefficients of all the specimens arenegative and the specimens possess n-type conduction properties, which indicates that（Ga+Ag）/Sn values may be smaller than the ideal ratio of16/30. At room temperature,the Seebeck coefficient absolute value decrease with the increase of Ag content. TheSeebeck coefficient was-239μVK^-1,-197μVK^-1 and-189μVK^-1 for the sample withx=0,0.5, and1, respectively. The dependence of the electrical conductivity on Tindicates that Ba₈Ga_16-xAg_xSn₃₀is heavily doped semiconductors. The Seebeckcoefficient absolute value with higher Ag content increase more rapidly and the Seebeckcoefficient value of-332μVK^-1is obtained at about600K for the sample with x=1. Ofall the Ba₈Ga_16-xAg_xSn₃₀compounds, Ba₈Ga₁₅AgSn₃₀compound has the largestestimated ZT, the figure of merit ZT of1.2at about600K.4. The p-type Ba₈Ga₁₆In_xGe_30-x（x=0.8,1.0,1.4） type-I clathrates were synthesizedusing In atom to substitute for the framework atom Ga. Substitution of Sn within theGa-Ge lattice framework of the type-I clathrate Ba8Ga16Ge30results in chemical shiftfor Ga3d, Ge3d and Ba3d and modulation in Raman spectra, which indicates Insubstation for Ga at a small level induces rearrangement of atoms on the cage. TheSeebeck coefficients of the all samples are positive and the specimens are p-typesemiconductors. We found that the Seebeck coefficients of all specimens increase withincreasing temperature and reach their maximum values at about702K, and thengradually decrease with the further increase of temperature. The specimen with x=1possesses the largest in the temperature range of measurement; it reaches180V/Kat300K, and is as large as256V/K at about550K. Of all the Ba8Ga16InxGe30xcompounds, the sample with x=0.8has the largest power factor and its maximal value is0.37×10^-3Wm^-1K^-2at about550K.5. The stability dependence on hydrostatic pressure of two kinds of configurationsof Ba₈Ga₁₆Sn₃₀was investigated by first-principle calculations, and the electronicstructures also were studied. The results that the α-phase is the stable ground state phasewhile the β-phase is considered to be metastable. The results indicate that it isimpossible for βâ†'α phase transition under hydrostatic pressure, but αâ†'β phasetransition may occur under high temperature. Calculations indicate that the type-I and type-VIII Ba₈Ga₁₆Sn₃₀are indirect semiconductors with band gaps of0.24eV and0.19eV, respectively. The type-I clathrate has a slight higher anti-symmetry near the Fermienergy compared to the type-VIII clathrates, which suggested that type-I clathrateintrinsic semiconductor Ba₈Ga₁₆Sn₃₀has a larger value of the TE power. It is revealedthat the positions of the conduction band minimum （CBM） and the valence band top（VBM） for the type-VIII phase remain at the same k-point under hydrostatic pressures,but for type-I phase the k-point of the VBM does not change but the k-point of the CBMchange. With the increase of pressure，the band gap in the type-I phase decrease butincrease in the type-VIII phase.