| With the deterioration of environment and energy problems,thermoelectric(TE)materials which can directly convert heat into electricity or vice versa,has attracted widespread attention.Bi2Te3 based alloys are one of the best TE materials for commercial application near room temperature,but the Achilles heel that limits a broad application of TE materials is its low thermoelectric properties and poor mechanical properties,therefore,it is important to improve the properties of Bi-Te alloys.Our previous works about the liquid state suggest that the transition of liquid states would result in the change solidification structure and properties.In this paper,p-type Bi0.4Sb1.6Te3+5%Te based alloys were selected as the research object,we explored the behavior of liquid state change in Bi0.4Sb1.6Te3+5%Te alloys,and then designed post process,such as free-solidification and ball milling combined with spark plasma sintering,to optimize the solidification structure and thermoelectric performance based on the liquid state.Whereby we have found:1.The curves of temperature-dependent liquid resistivity of Bi0.4Sb1.6Te3+5%Te alloys show abnormal behaviors during the heating process,within the temperature range of 850?1036 C,these abnormal behaviors suggest an irreversible temperature-induced liquid-liquid structural transition(TI-LLST)may exist during the heating and cooling process.2.Based on the investigated abnormal temperature regime,the raw materials were smelted at temperatures below and above TI-LLST,and then the melts were cooled down at the same temperature.Subsequently,the solidification curves were recorded and analyzed.The results show that,the alloys with liquid state manipulation(LSM)exhibits a much higher nucleation undercooling degree,nucleation rate,and a shorter solidification time,which result in a much refined grains,a larger amount of nanoparticles,higher density of lattice distortions and dislocations.3.The solidification microstructures of samples which under different cooling rates are refined and the Vickers hardness and electrical properties are increased by manipulating the parent liquid states,and the effects of structure refinement and hardness elevation are further improved with the increasing of cooling rates from sand-cooling to copper-cooling,but the electrical performance is declined;The solidified microstructures of sand cooling sample are evidently refined and the crystal defects are obviously increased with LSM,the phonon scattering is enhanced and the lattice thermal conductivity is greatly reduced,finally,the ZT peak value of cast sample increases from 0.5 to 0.7 at 70 C,with an enhancement of 40%.4.We also investigated the influence of BM+SPS on TE performance for cast alloys.The results show that:with increasing ball milling(BM)speed,the"donor like effect" is enhanced and results in an increased,meanwhile,the ?increased first and then decreased due to the change of particle size,as a result,the sample with BM speed of 500rpm has the best electrical properties;Moreover,the ? is also decreased first and then increased with the change of phonon scattering,finally,the sample with BM speed of 500rpm has the largest ZT of 0.85.With increasing spark plasma sintering(SPS)temperature,the"donor like effect" is suppressed,while Te atoms of Te rich phase back to the matrix phase,the n decreased first and then increased,as a result,the sample with SPS temperature of 400 C has the best electrical properties;In addition,with increasing SPS temperature,the grains grow which lead to a weakened phonon scattering;Finally,the sample with SPS temperature of 400 C has the largest ZT of 0.87.5.The results of LSM on the properties of the samples under ball milling combined with spark plasma sintering techniques show that:the sample with LSM retained an increased Te-rich phases and effective mass,results in an increased ? and slightly decreased a,as a result,the sample with LSM has a high peak PF of 33 ?W·cm-1 ·?-2 at room temperature;Meanwhile,the sample with LSM has more tinier nanoprecipitates and much more crystal defects,such microstructures would scatter more phonons and result in a low ?;All these alterations contribute to a significantly improved ZT and reaches a maximum value of 1.17 at 120 C. |
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