Bismuth Doping And Antimony Vacancies Optimize The Thermoelectric Performance Of Polycrystalline Iron Antimonide

As a new type of functional material,thermoelectric material can realize the direct mutual conversion of heat and electrical energy,which has been developed rapidly in recent years and used in waste heat power generation,semiconductor refrigeration and other fields.Thermoelectric conversion technology possesses characteristics of green environmental protection,simple structure,high reliability and long service life.The dimensionless thermoelectric figure of merit ZT is usually used to measure thermoelectric conversion efficiency.ZT is defined as ZT=（S²/ρκ）T,where the three core parameters electrical resistivity（ρ）,Seebeck coefficient（S）and thermal conductivity（κ）have a strong coupling relationship.Electrical resistivity will be reduced if carrier concentration（n）is increased,but it will result in a decrease in Seebeck coefficient and an increase in carrier thermal conductivity.Therefore,it is a challenging task to adjust the size of each parameter to optimize the ZT value.Due to its own narrow band gap structure（0.1～0.3 e V）and strongly correlated physical characteristics,iron antimonide（Fe Sb₂）has a high density of electronic states,so that its single crystal possess an extremely excellent Seebeck coefficient（-45 m V/K）and power factor（PF）at 10 K,and is considered to be an important class of low temperature thermoelectric materials.However,the thermal conductivity of this temperature point is as high as 500 W/m K,resulting its ZT value being only about 0.005,which is far lower than the practical application（ZT>1）.Therefore,the key of improving thermoelectric performance of Fe Sb₂ is to reduce its thermal conductivity.Thermal conductivity of polycrystalline materials are generally lower than that of their single crystals.Taking Fe Sb₂ polycrystalline as research object here,new mechanisms are being found to maximally maintain its high Seebeck coefficient and reduce thermal conductivity,which is expected to further optimize its ZT value.Element doping,low-dimensional nanoscale,and vacancy defects are effective strategies to enhance ZT values of materials.They optimize Seebeck coefficient or electrical conductivity by changing carrier concentration and mobility,and enhance phonon scatterings to reduce lattice thermal conductivity.In this paper,element doping and vacancy defects are used to improve thermoelectric properties of nano-Fe Sb₂ polycrystalline bulks and nano-Fe Sb₂polycrystalline thin films,respectively.The specific research contents are as follows:1.Nano-polycrystalline Fe Sb₂ bulk materials doped with heavy metal main-group element Bi were prepared by powder melting and hot press sintering.And low temperature（5～320 K）thermoelectric properties of Fe_1-xBi_xSb₂（x=0,0.01,0.02,0.05,and 0.1）samples were measured.The results show that electrical resistivity and Seebeck coefficient of samples both decrease monotonically with the increase of Bi doping in all temperature regions,which is due to the increase of carrier concentration resulting from Bi doping.In addition,thermal transport capacity also decreases with the increase of Bi doping.For example,Fe_0.9Bi_0.1Sb₂ sample has the lowest thermal conductivity（1.4 W/m K）at 40 K,which is reduced 80%compared to undoped Fe Sb₂.This may be resulted from the differences in mass and size between doped Bi ions and substituted Fe ions,and the further introduction of large mass and strain field fluctuations lead to increased phonon scatterings and a significant decrease in lattice thermal conductivity.Although Fe_0.95Bi_0.05Sb₂ sample does’t possess the lowest thermal conductivity,it reaches the maximum ZT value（about 0.016）at 40 K due to its higher PF than Fe_0.9Bi_0.1Sb₂ sample.And this maximum ZT value is more four times than that of undoped Fe Sb₂ sample.These characteristics demonstrate that doping heavy metal element in intermetallic to occupy part of original atom sites is a promising method in optimizing thermoelectric performance.2.Defect engineering has recently proven to have tremendous potential for improving thermoelectric performance in topological materials with two-dimensional layered structures.In this paper,Fe Sb_2-x（x=0,0.1,0.2,and 0.3）thin films were prepared through magnetron sputtering and the influence of Sb vacancies on their power factor was investigated.The structure and morphological analysis of samples show that the crystallinity of two-dimensional polycrystalline thin films are excellent,and the grain sizes are nanometers.The low temperature electrical measurement results show that carrier concentration of Fe Sb_2-x thin films are two orders of magnitude higher than that of nano-Fe Sb₂ polycrystalline materials prepared by hot pressing sintering,and the PF at 45 K are four times higher than that of nano-Fe Sb₂ polycrystalline materials.For Fe Sb_2-x films itself,Sb vacancies don’t change electrical transport behavior of sample’s semiconductor properties.However,the presence of vacancies allows Seebeck coefficient under all temperature regions of film samples to be increased,and Fe Sb_1.8 film obtained a maximum value of 357μV/K at 45 K.The reason for this is that carrier concentration of thin films with Sb vacancies are lower than that of Fe Sb₂ film without vacancies.The final result proves that the existence of Sb vacancies improves PF of films.Fe Sb_1.8film obtains the maximum PF of 4.5 m W/m K² at 50 K,which is about four times than that of Fe Sb₂ film.