Exploring New Methods To Modulate The Microstructures And Thermoelectric Properties Of Bi_0.5Sb_1.5Te₃ Alloys Based On Melt States

Thermoelectric materials,which can realize the direct conversion between thermal energy and electricity based on the Seebeck and Peltier effects,has been considered as one of the best candidates to meet the demand for sustainable energy consumption and carbon-free emission.The Achilles heel that limits a broad application of thermoelectric technology is its low conversion efficiency,which is determined by the dimensionless figure of merit,ZT.The widely utilized methods to enhance ZT are adjusting the composition of existed alloys and developing novel processing methods to render band engineering and structural modulation.Bi₂Te₃-based alloys are one of the best thermoelectric materials for near room-temperature applications.After sustained research efforts,the optimal compositions for p-and n-type component are fixed to be BixSb2-xTe₃?x=0.4,0.5,0.6?and Bi₂Te₃-ySey?y=0.3-0.6?,respectively.Therefore,the main approach to improve the thermoelectric performance of Bi₂Te₃-based alloys is developing novel sample-preparation methods,for example,ball milling or melt spinning combined with powder sintering,and uniaxial deformation.These methods have been proven to be efficient in enhancing the thermoelectric properties,but a suitable way to achieve large-scale production remains by solidification.There has been a notable trend in engineering practices that the solidified structures of many materials are often dependent on the thermal history of the liquid they originate from.Our previous work suggested that the change of solidification microstructures was indeed induced by a transition of liquid states.Thus,it is of significance to modify the structural configurations of Bi₂Te₃-based alloys,and then to render the optimization of thermoelectric properties by manipulating their liquid states.In this work,we explored the behavior of liquid structural transition in Bi₂Te₃-based alloys and then designed the experiments to optimize the thermoelectric performance based on the liquid state manipulation.Whereby we have found:1)The curves of temperature-dependent liquid resistivity of Bi₂Te₃ and Bi_0.5Sb_1.5Te₃ alloys show abnormal behaviors during the first heating process,i.e.there is a "valley"within 672.7-695.5 0C at a heating rate of 5 ?/min for Bi₂Te₃ and a "hump" within 786.4-906.7 0C at a heating rate of 3 0C/min?803.5-937.1 ? at 5 ?/min?for Bi_0.5Sb_1.5Te₃,These abnormal behaviors suggest that a temperature-induced liquid-liquid structural transition?TI-LLST?occurs within the temperature ranges.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.The results of solidification curves show that the alloy melted at temperatures above TI-LLST?with liquid state manipulation?exhibits a lower solidification temperature,a larger nucleation rate,a higher recalescence temperature,and a shorter solidification time,compared with the ones without liquid state manipulation.3)The solidification microstructures of Bi₂Te₃ and Bi_0.5Sb_1.5Te₃ are refined and the Vickers hardness is increased with the increasing of cooling rates from air-cooling to iron-cooling to copper-cooling.Intriguingly,the effects of structure refinement and hardness elevation are further improved by manipulating the parent liquid states.4)The free solidified Bi_0.5Sb_1.5Te₃ ingots out of the melts treated with and without liquid state manipulation possess distinct solidification microstructures and thermoelectric properties.The solidified microstructures out of the melt with liquid state manipulation are evidently refined with more Te-rich eutectic strips precipitating on the matrix boundaries.More significantly,hierarchical crystal defects are obviously increased,with tinier nanoprecipitates,more subgrains and boundaries,and much more densified lattice distortions.Consequently,the thermal conductivity,especially the lattice thermal conductivity,is decreased notably with the power factor increased moderately at high temperatures.In addition,the ambipolar excitation temperature is shifted to a higher value and the slope of thermal conductivity vs temperature is also declined.All these alterations contribute to a synergistic enhancement of ZT and the maximum figure shifts to a higher temperature point,with ZTmax=0.78 at 442 K,which is comparable to the value of commercialized ingot prepared by the time-consuming zone melting method.5)We have proved that the microstructures and thermoelectric properties of as-cast Bi_0.5Sb_1.5Te₃ alloy can be modified by manipulating the parent liquid states.After that,we further explored whether the efficacy of liquid state manipulation can be retained after post processing,i.e.by ball milling,melt spinning,and spark plasma sintering.We find that the grain sizes of the ball milled powders and sintered bulks remain smaller for the sample treated with liquid state manipulation.More significantly,we obtained dedicated microstructures containing plenty of 60° twin boundaries.These twin boundaries firstly scatter the very low-energy carriers and serve as electron donors to compensate for the majority holes,which leads to an enhancement of the Seebeck coefficient.Secondly,they provide a considerable high carrier mobility due to the coherent interface of twin boundaries and the special natural superlattice formed by the extra bilayers,compensating for the negative effect of reduced hole concentration on the electrical conductivity.Thirdly,both experimental and calculated results demonstrate that the twin-boundary scattering dominates the conspicuous decrease of the lattice thermal conductivity.Consequently,the highest ZT value of 1,42 is achieved at 348 K,which is 27%higher than that of the sample with less twin boundary treated without liquid state manipulation.The average ZT value from 300 K to 400 K reaches 1.34.Our particular sample processing methods enabling the twin-dominant microstructure is an efficient avenue to simultaneously optimize the thermoelectric parameters.6)We also studied the influence of liquid state manipulation on n-type Bi₂Te₃-xSex?x=0.3,0.45,0.6?alloys.The solidification behaviors and corresponding effects of microstructure refinement and hardness enhancement are similar to the above-mentioned phenomena.Electron back-scatter diffraction results demonstrate that the samples treated with liquid state manipulation have finer and more homogeneous lamellar structures.The prevalence of low-angle grain boundaries between these lamellae is increased.Atom probe tomography analyses prove that there is no segregation or nanoprecipitation within the grains,but the Te-rich eutectic structure and the evolution of composition near the Te-matrix phase boundary are investigated in a sample which was treated with liquid state manipulation.The interface between the Te-rich eutectic phase and the matrix has a diffusive region with large variations of Te and Bi concentrations within-3 nm.It is reasonable to conjecture that these structural alterations might lead to an enhancement of the thermoelectric and mechanical properties of commercial-scale ingots,thus benefitting applications.This work implies the existence of temperature-induced liquid-liquid structural transition in Bi₂Te₃-based alloys well above their liquidus.The solidification behaviors and solidified microstructures are strongly related to their parent liquid states.The corresponding thermoelectric properties can be also improved by the liquid state manipulation method or in conjunction with post-processing methods,providing an alternative approach to tailor the thermoelectric properties.