Manipulating Transport Properties of Bi2Se3/In2Se3 Superlattices and Half-Heusler Thermoelectric Materials

Author:Ren Wu Yang

Supervisor:wang zhi ming


Degree Year:2019





Thermoelectric materials are capable of directly converting heat into electricity and vice versa,which involves the eletron and phonon transport.This solid-state energy conversion technique has the merits of simplicity,reliability,compactness,quietness and eco-friendliness,showing the potential application in power generation,waste heat recovery and solid-state cooling.However,the development of high-performance thermoelectric materials has been impeded due to the strong coupling between the Seebeck coefficient,electrical conductivity and thermal conductivity,which determine a primary measure for materials’performance--figure of merit,ZT.In this paper,the transport properties of Bi2Se3/In2Se3 supperlattices and half-Heusler thermoelectric materials were studied for the purpose of optimizing the electrical and thermal parameters,and finally improving the thermoelectric performance.The main results are summarized as below:(1)By means of molecular beam epitaxy,we fabricated low-dimensional thermoelectric material Bi2Se3/In2Se3 supperlattices on fluorophlogopite mica.According to the characterization of reflection high-energy electron diffraction(in situ)and high-resolution X-ray diffraction(ex situ),Bi2Se3/In2Se3 supperlattices with accurately controlled period thickness were realized.By investigating the thermal conductivity as a function of the hetero-interface density,a minimum value was observed.For long-period superlattices,the behaviour of the phonons is particle-like,where the thermal conductivity decreased monotonically due to the incoherent phonon scattering at the hetero-interfaces.For short-period superlattices,the phonons show wave-like transport property,which are coherently reflected at the hetero-interfaces,leading to an increased thermal conductivity.Thus,the minimal thermal conductivity is ascribed to the crossover from particle-like to wave-like transport of the phonons.Noticeably,the thermal-conductivity minimum of the Bi2Se3/In2Se3 superlattices was nearly an order of magnitude lower than that of an intrinsic Bi2Se3 film,demonstrating the great importance of phonon transport manipulation for thermal conductivity reduction.(2)By introducing impurity atoms Ta or V in ZrCoSb,strong mass fluctuation and strain field fluctuation were induced.Based on the Debye-Callaway model,which was employed to specify the phonon transport under various phonon scattering process,this kind of point defect can remove the contribution of high-frequency phonons.Consequently,a dramatical reduction of lattice thermal conductivity was observed in either Ta-doped or V-doped ZrCoSb.However,V-doped ZrCoSb shows quite inferior electrical properties.According to the first principle calculations,the vanadium impurity level locates deep in the band gap.Hence it is difficult to supply carriers and also adversely affects the mobility.On the contrary,Ta-doped ZrCoSb shows decent power factor.In conjunction with low lattice thermal conductivity,a peak ZT of0.8 at 973 K was achieved in Ta-doped ZrCoSb,showing an improvement more than twice in comparison to V-doped ZrCoSb.This result also emphasizes the significance of electron transport manipulation rather than only pursuing low thermal conductivity by phonon scattering.(3)A material system Nb0.95M0.05FeSb(M=Hf,Zr,Ti)with ultrahigh peak power factor100μW cm-1 K-2 was demonstrated.Such high power factor has not been reported in any other semiconductor-based thermoelectric materials.By increasing the temperature of the heat treatment,the grain size increased rapidly,resulting in few defects and less disorder in the materials,where the manipulation of carrier scattering mechanism can be achieved.Thus a significantly enhanced mobility was observed.Meanwhile,the carrier concentration of Nb0.95M0.05FeSb was optimized according to the single parabolic band model.As a result,the significantly enhanced mobility and optimized carrier concentration contribute to the ultrahigh power factor.More importantly,owing to the strong mass fluctuation and surrounding strain field fluctuation between Nb and Hf atoms,Nb0.95Hf0.05FeSb exhibits lower lattice thermal conductivity but with the ultrahigh power factor maintained,leading to a peak ZT of0.9 at 973 K,which is 22%and 37%higher than those of Nb0.95Ti0.05FeSb and Nb0.95Zr0.05FeSb.Additionally,a large output power density of21.6 W cm-2 was achieved based on a single-leg device under a temperature difference of560 K,showing the realistic prospect of Nb0.95Hf0.05FeSb for power generation.(4)The naturally formed Ni interstitials in TiNiSn compound can be effectively controlled by intentionally reducing the amount of Ni based on the evidence from differences of lattice parameter,bandgap,electrical and thermal properties between TiNiSn and TiNi0.92Sn.Benefiting from the manipulation of Ni interstitials where the carrier scattering from interstitial defects is weakened,Ta-doped TiNi0.92Sn shows greatly enhanced mobility and power factor in comparison to Ta-doped TiNiSn with similar carrier concentration.As a result,peak power factor of Ta-doped TiNi0.92Sn can reach to50μW cm-1 K-2,which is comparable to the best n-type half-Heusler compounds.Moreover,according to the disorder scattering parameter in Ta-doped TiNi0.92Sn,Ta doping can introduce strong mass fluctuation and strain field fluctuation,which account for scattering phonons and in turn reducing lattice thermal conductivity.Therefore the synergistic optimization of the electrical and thermal transport properties was realized by employing Ni-interstitial-manipulation for mobility enhancement and heavy-element-doping for phonon scattering,leading to a peak ZT of0.73 in Ta-doped TiNi0.92Sn,which is higher than that of all other reported TiNiSn-based materials.