Investigation On Electrical And Thermal Transport Properties Of InSb-based Thermoelectric Materials

In Sb is a direct bandgap semiconductor with a zinc blende structure,which possesses a narrow bandgap of 0.18 e V and electron mobility of 7800 cm²V^-1s^-1at room temperature.Such materials can be widely used in infrared detectors,thermal imaging,and Hall devices.In addition to the above applications,In Sb compound has also gained much attention in the thermoelectric field due to its relatively small bandgap and high mobility.Though,the inferior thermoelectric of In Sb-based materials owing to its low-value Seebeck coefficient(?90?VK^-1),and high thermal conductivity(14?18 Wm^-1K^-1).Thus,to resolve the above issues,the thermoelectric transport properties of In Sb compound have been optimized by the point-defect and nanostructure engineering such as the elemental-composition control,nano-compositing,and microstructure approaches.Consequently,the improvement in the thermoelectric performances of In Sb-based materials are detailed in this thesis.The main contents and conclusions of the employed approaches are as follows:1)The effect of extra In on the thermoelectric properties of the In Sb compound has been studied.The results show that a small amount of In second phase can effectly reduce the lattice thermal conductivity and finally a ZT value of 0.24 of In_1.01Sb sample has obtained.After that,the effect of different sintering processes on the electrical and thermal transports properties of In Sb has compared.Studies showing that,a higher Seebeck coefficient was observed due to the filtering of the low-energy carriers due to the smaller grain size and a weaker growth orientation by the RS-SPS process comparatively.Also,the calculated electronic band structure,phonon dispersion spectrum by the first-principles and two-kane-band model theory illustrates that the Seebeck coefficient and hence power factor can be improved by suppressing minority carriers.Besides,the lattice thermal conductivity of In Sb samples has been effectively reduced by the scattering of medium and long wave phonons by hierarchical architecture.Ultimately,through increasing the Seebeck coefficient and reducing the lattice thermal conductivity of In Sb,its power factor and thermoelectric performance can be fixed at a superior interval.2)The effect of in-situ precipitation on the thermoelectric properties of(In Sb)_1-x(In₄Se_2.5)_x,and(In Sb)_1-y(Sn Se₂)_y samples had been studied.The results reveal the improvement in the electrical conductivities of the doped samples due to the introduction of Sn _In⁺and Se⁺_Sb point defects,though a corresponding reduction in the Seebeck coefficient leads to the inferior power factor.Meanwhile,the microstructures such as saturated phase(In Se,In,Se)and point defects facilitated the reduced thermal conductivity of(In Sb)_1-x(In₄Se_2.5)_x and(In Sb)_1-y(Sn Se₂)_y samples,which provides a basis and guidance for designing low thermal conductivity of In Sb experiments.3)The effect of In Bi peritectic structure on the thermoelectric properties of In Sb_1-xBi_x has been studied.The experimental results indicate the formation of a second phase In Bi in the Bi-doped In Sb_1-xBi_x samples due to the peritectic reaction.With such reaction,the electrical conductivity and thus power factor increases with the introduction of metallic In Bi in In Sb_1-xBi_x compounds.At the same time,the dispersed Bi and In Bi secondary phases act as the additional phonon scattering centers to reduce the lattice thermal conductivity in In Sb_1-xBi_x samples.Finally,an enhanced ZT 0.5 at 733 K has been obtained in In Sb_0.97Bi_0.03 sample.4)The effect of Sn Se₂ and WSe₂nanocomposites on the electrical and thermal transport properties of In Sb has been studied.The experimental results reveal the increase of electrical conductivity due to the increase in carrier concentration of In Sb samples with the introduction of point defects(Sn⁺_In,W⁺_In,Se⁺_Sb).As WSe₂ has a suitable matching of energy level with In Sb as compared to the Sn Se₂,therefore the Seebeck coefficient and hence the power factor of y wt.%WSe₂ composites increases significantly due to the energy filtering of minority carriers as compared to the x wt.%Sn Se₂ composites.On the other hand,WSe₂ fibers and other nano phases(Sn Sb,Se)cooperate in the scattering of phonon to reduce the lattice thermal conductivity of the In Sb composites.By such an approach,an enhanced ZT value of 0.63 and 0.82 had been achieved in the 9 wt.%Sn Se₂and 3 wt.%WSe₂ samples respectively.In addition to the above work,In Bi nanophase has been introduced as a result of the peritectic reaction between In Sb and nano-Bi.The In Bi nanophase enhances the scattering of minority carriers,thereby improving the Seebeck coefficient and power factor of y at.%Bi sample.However,the In Bi nano-precipitates of y at.%Bi composites are more effective in scattering the medium-long wave phonons as compared to the In Bi micro-sized peritectic structure of In Sb_1-xBi_x which decreases the lattice thermal conductivity of the composite material.Eventually,an enhanced ZT 0.7 at733 K has been achieved in the 3 at.%Bi sample.5)The effect of in-situ reaction on the thermoelectric properties of In Sb has been studied.The results indicate the formation of Co,In₂O₃,and Co Sb₃ precipitation with the addition of Co₂O₃ nano-oxide due to the in-situ reaction between Co₂O₃ and In Sb.Thereby,the carrier concentration and hence the electrical transport properties increase due to the highly conductive nature of the N-type second phase.It is worth noting that the second phase of In₂O₃ and Co Sb₃ are distributed in the In Sb matrix as a type of double-core-shell structure(Co Sb₃@In₂O₃@Co Sb_x),which can effectively scatter the high-frequency phonons and thus greatly reducing the high-temperature lattice thermal conductivity of In Sb sample.Ultimately,a maximum ZT of 0.7 at 733 K was realized in the 1 wt.%Co₂O₃added sample.In addition to the above work,both the replacement and eutectic reactions had been introduced through the addition of Ti O₂nano inclusions.On one hand,the electrical conductivity and thus power factor has been significantly improved due to the introduction of large amounts of Ti⁺_In point defects.On the other hand,the thermal conductivity has been greatly reduced through the extra phonon scattering by dispersive In₂O₃ nanoparticles and stacking faults.Furthermore,a small amount of In Sb-Sb eutectic structures has also been introduced by the replacement reaction at 753 K.Such eutectic structures can filtrate the transverse acoustic phonons from the thermal phonons,and thus resulting in a further rapid reduction in the ?_L.Eventually,a relatively high ZT value?1.1has been obtained at 773 K for the 0.1 wt.%Ti O₂ added In Sb sample.