Investigation On Microstructure And Thermoelectric Properties Of P-type Zintl Phase YbZn₂Sb₂ Based Materials

Thermoelectric materials as a new type of green energy materials can convert heat to electricity directly and reversibly,providing an effective solution for recovering waste heat and conserving energy.Sb-based Zintl compounds AM₂Sb₂?A: Ca,Ba,Sr,Yb or Eu,M: Mn,Zn,Mg or Cd?with CaAl₂Si₂ structure have attracted extensive attention due to their unique "electronic-crystal phonon-glass" structure.This unique structure can meet the challenging requirements for ideal thermoelectric materials.The substitution of A-site or M-site has been widely used to improve the thermoelectric performance of AM₂Sb₂.The electrical property is enhanced through adjusting the carrier concentration and the lattice thermal conductivity is reduced by strengthening the phonon scattering.By this way,the thermoelectric performance of AM₂Sb₂ has been significantly improved.However,there are also some challenges for AM₂Sb₂ based thermoelectric materials,including the limited optimization strategy and regulation method.Therefore,this thesis aims to develope new optimizing methods and propose noval mechanisms to manipulate the thermo-electric transport,which is essential to promote the further development and application of AM₂Sb₂-based thermoelectric materials.Particularly,the effects of La-doping on the thermoelectric properties of La_xYb_1-xZn₂Sb₂ have been investigated,the synergistic regulation principle of in-situ formed InSb on the thermoelectric performances of Yb_1+y%Zn_1.90In_0.10Sb₂/y%?InSb?is proposed,and the effects of Sb-site substitution on regulating the electro-acoustic transport performances of YbZn₂Sb_2-xBi_x have been studied.The major results reported in this thesis are summarized as follows:?1?The thermoelectric properties of La_xYb_1-xZn₂Sb₂ systemsLa_xYb_1-xZn₂Sb₂ is a heavily doped p-type semiconductor with narrow band gap.With increasing the La-doping content,the electrical conductivity was reduced and the Seebeck coefficient wasenhanced.Furthermore,the point defects were induced by La-doping,whichstrengthened the phonon-scattering effect and reduced the lattice thermal conductivity from 3.4 Wm^-1K^-1 to 2.3 Wm^-1K^-1.With the help of almost unchanged power factor and significantly reduced thermal conductivity,La_0.01Yb_0.99Zn₂Sb₂ achieved a maximum ZT value of 0.4 and had an increase of 18% compared to the pure sample.Doping with donor impurities at the Yb site via replacing the bivalent Yb with trivalent La,the analysis found that the effect of La-doping on the carrier concentration is not significant.?2?Study on the spontaneously promoted carrier mobility and strengthened phonon scattering of YbZn₂Sb₂ via the in-situ formed InSb nanoinclusionsThe family of Yb_1+y%Zn_1.90In_0.10Sb₂/y%?InSb?compounds were characterized by scanning electron microscopy?SEM?,transmission electron microscopy?TEM?,high resolution transmission electron microscopy?HRTEM?,scanning transmission electron microscopy?STEM?and X-ray energy spectrum analysis?EDS?.The stacking sequence of the YbZn₂Sb₂ unit cell was proved to be -Yb-Sb-Zn-Zn-Sb in atomic scale directly.The in-situ formed InSb includes two types of structures: coherent and non-coherent with the matrix.And the in-situ formed InSb not only acted as“phonon barrier”to enhance phonon-scattering and reduce lattice thermal conductivity to 0.7 Wm^-1K^-1,but also as“electronic channel”to increase mobility from 56 cm² V^-1 s?-1? to 162 cm² V^-1s^-1 at room temperature and improve electrical performance.In other words,the interdependencies of electrical conductivity,Seebeck coefficient and lattice thermal conductivity,to some extent,were weakened due to the in-situ formed InSb.Therefore,the optimization of electrical and thermal performances were achieved simultaneously,which lead to the increase of ZT value by 200%.Based on the high temperature Hall data,the comparative experiments and the "parallel structure" model of mobility,and the STEM result,a conclusion was drawn that InSb can effectively enhance the mobility for the nanocomposite over the entire test temperature range.?3?Study on the enhanced thermoelectric properties of YbZn₂Sb₂ by regulating Fermi level via Bi-dopingThe YbZn₂Sb_2-xBi_x systems were comprehensively characterized by X-ray diffraction?XRD?,Raman spectroscopy and X-ray photoelectron spectroscopy?XPS?and the results showed that the Bi atom was successfully doped into the Sb-site in the covalent bonded?Zn₂Sb₂?^2-.The carrier concentration was linearly regulated from 12.7 × 10¹⁹ cm^-3 to 16.0 ×10¹⁹ cm^-3 by Bi-doping.The intrinsic reason was that the energy difference?Ev-EF?between the Fermi level and the top valence band was changed due to the substitution of Bi atom for Sb atom.Moreover,the effective mass was increased from m^*?0.62 m_e to m^*?0.85 m_e by theeffect of Sb-site substitution.Therefore,the electrical performance was significantly improved.In addition,Bi-doping also introduced a large number of point defects and enhanced phonon-scattering,resulting in the lattice thermal conductivity close to or lower than the theoretical limit(k_Llimit= 0.5 Wm^-1K^-1).The Callaway model was compatible well with the experiment results,and the synergistic regulation of electro-acoustic transport was realized,accompanied with a increase of ZT value by 32%.