Fabrication And Transport Properties Optimization Of P-type Bismuth Telluride Based Thermoelectric Materials And Devices With Multilayered Structure

Thermoelectric(TE)technology is a reliable and environmentally friendly new energy technology,which can directly convert electric energy to thermal energy and vice versa.It is mainly used in TE power generation and TE cooling.The performance of TE materials is quantified by the dimensionless figure of merit ZT.Although there is no theoretical upper limit on the ZT value of the TE material,it is difficult to achieve significant increases in the ZT value only through optimizing each TE parameter of the material.Simultaneously optimizing electrical and thermal transport properties of TE materials remains a key challenge.The multilayered structures provide new ideas and effective approaches to this challenge because it can provide the anisotropy of the electrical and thermal transport properties.In this work,p-type Bi0.5Sb1.5Te3(BST)with excellent TE properties near room temperature were studied.Four N/BST(N=YbAl3,Bi,Co and Ni)artificially tilted multilayer thermoelectric devices(ATMTDs)were quickly screened out by high-throughput calculation results.The YbAl3/BST ATMTDs and Bi/BST artificially tilted multilayer thermoelectric film devices(ATTFDs)were fabricated,and different directions of the current and heat flow have been achieved in these devices.The TE power generation and cooling performances of the ATMTDs and ATTFDs were also measured.A series of stoichiometric p-type BST thin films with multilayered structure has been fabricated and investigated,and the effect of multilayered structure on the preferential orientation and electrical transport properties was studied.The main contributions are as follows:A high-throughput calculation software has been developed and successfully used to establish the material genome databases for screening and designing N/BST ATMTDs(where N stands for metals or n-type semiconductors matched with BST).The relationship among the transport properties of material N,the geometrical structure parameters and the thermoelectric properties of the N/BST ATMTDs were investigated.The results indicate that the main factors of the ZTzx values of N/BST ATMTDs are the TE parameters of N material(electrical conductivity ?N,Seebeck coefficient ?N,and thermal conductivity ?N)and geometrical structure parameters of the device(thickness ratio ? and tilt angle ?).The performance indicator or material genome factor of N/BST ATMTDs can be expressed as ?N=?N·(170-?N0.42)and used to rapidly predict the peak ZTzx values of N/BST ATMTDs.The quantitative criterion of ?N>10-2 was set as the filter to rapidly screen the matching materials for N/BST ATMTDs with big peak ZTzx values,and four kinds of matching materials YbAl3,Bi,Co and Ni are eligible for the quantitative criterion,with the ZTzx values larger than 0.4.Selecting BST and YbAl3 as the constituent materials,the geometrical structure of YbAl3/BST ATMTD has been rapidly optimized by investigating the ? and ?dependences of ?xx,?zx,?zz and ZTzx from the high-throughput calculation.It is discovered that the highest ZTzx value of YbAl3/BST may theoretically reach 0.536 at room temperature when ?=16° and ?=0.46.Based on the optimized geometrical parameters,a YbAl3/BST ATMTD with 20 mm × 6 mm × 6 mm in sizes was fabricated by two-step sintering method following the sloping cutting process.The microstructure analysis reveals that the interface between YbAl3 and BST is compact while the mutual diffusion depth of the interface is very small and less than 3 ?m.The performance measurements show that the maximum output power and the transverse TE voltage for the ATMTD are 3.7 mW and 13.7 mV,respectively.The conversion efficiency(?)of YbAl3/BST ATMTD reached about 1.0%,increased by more than 3.3 times as compared with the highest data of 0.3%reported by other groups.This part of the work provides an effective strategy to design and fabricate high-performance ATMTDs via material genome engineering method,and proofs that the material genome engineering can greatly reduce the experimental works for material exploration.Selecting BST and Bi as the constituent materials,the geometrical parameters ? and? of YbAl3/BST ATTFDs has been rapidly optimized by investigating their dependences of transverse TE properties based on high-throughput calculations.The optimized geometrical parameters are ?=27° and ?=0.5 when the maximum ZTzx of Bi/BST ATTFDs is theoretically 0.48 at room temperature.A novel misaligned mask assembly has been successfully designed and developed to low-costly fabricate ATTFDs with excellent transverse power generation and cooling performances through accurately controlling the tilted arrangement of nanometer-scale multilayer films.A series of Bi/BST ATTFDs with optimized geometrical parameters have been fabricated by the layer-by-layer stacking method.It is discovered that the huge transverse output voltage of 300 ?V·K-1 and cooling temperature gradient of 11 K·mm-1 are obtained in the Bi/BST ATTFD with the thickness of 21.6 ?m.This demonstrates that ATTFDs can be promisingly applied to high-efficient film refrigerators in next-generation electronics and optoelectronics,high-sensitive thermal response sensors,and thermoelectric-driven inorganic LED film devices.A layer-by-layer in-situ growth method has been developed to fabricate a series of multilayered BST thin films with high-crystallinity,well-controlled preferential orientation,and minimized compositional deviation.The electrical performance of the as-prepared BST films is much higher than that of the corresponding bulk materials.The effect of substrate temperatures in the range of 473-723 K on the evolution of preferential orientation and electrical transport properties were systematically investigated.The results indicate that with increasing the substrate temperatures(Ts)in the range of 473-723 K,the crystallinity of the as-prepared BST film was gradually increased,the preferential orientation was transformed from the(0115)to(10110)to(000l)orientation,the hexagonal nanoflakes were gradually stacked into a layered structure parallel to the substrate,the average grain size was gradually increased,the surface roughness was gradually reduced,the electrical conductivity is increased from 3.2×104 S·m-1 to 5.4×104 S m-1,and the mobility is increased from 28.9 cm2·V-1·s-1 to 43.7 cm2·V-1·s-1.The layer-by-layer in-situ growth model is proposed to understand the formation mechanism of the multilayered films.The theoretical calculations show that the electrical transport properties can be greatly improved due to the increase of carrier mobility originated from the multilayered structures,because the layered structure stacking along the(000l)orientation provides the fastest channels for carriers transport.The performance measurements were repeated for five times to prove the good thermal stability of the multilayered BST thin films.The as-prepared(000l)-oriented multilayered BST film fabricated at Ts=723 K exhibits the best electrical properties due to the rapid transport of carriers along the(000l)orientation.The highest electrical conductivity reaches 8.0×104 S·m-1,the power factor reaches 3.8 mW·K-2·m-1,which are much higher than those of the BST bulk materials(2.7 mW·K-2·m-1).