Preparation,Electrical Transport Properties And Oxidation Resistance Of Single Crystalline ?-Zn₄Sb₃

Thermoelectric material has become a promising functional material with its unique performance to achieve transformation between heat and electricity including thermoelectric generation and refrigeration.High cost and low thermoelectric conversion efficiency has been limiting the extensive application of thermoelectric materials for a long time in the past.The ?-Zn₄Sb₃ compound is considered to be a promising medium temperature p-type thermoelectric material with its low thermal conductivity and relatively inexpensive and non-toxic constituent elements.In this paper,?-Zn₄Sb₃ was prepared by the flux method combined with melting and slow cooling,the electrical transport properties and thermal stability of the samples were investigated by changing the flux or doping,and the oxidation resistance of ?-Zn₄Sb₃ were also studied.The main conclusions are as follows:A Zn-rich single crystal ?-Zn₄Sb₃ was prepared using a Sn flux method based on the stoichiometric ratios of Zn4.4Sb3Sn3.The oxidation resistance of the sample was determined by exploring the effects of heat treatment in air on electrical transport properties and thermal stability,which is of practical importance in the application of the material at high temperatures.Results showed that the prepared sample possessed high electrical transport properties,with a high power factor of 1.67× 10-3 W?m-1?K-2 at 563 K.The heat treatment in air weakened the electrical conductivity of the single crystalline ?-Zn₄Sb₃,whereas the Seebeck coefficients were rarely independent of the annealing condition.The power factor obtained after the first heating at 523 K for 4 h became comparable to that of the as-prepared sample,although it decreased after the second heating at 573 K for 5 h.Nevertheless,the single crystalline ?-Zn₄Sb₃ still possessed a good thermal stability after the heat treatment process.Single crystalline ?-Zn₄Sb₃ with Ge and Sn codoped were prepared by the Sn-flux method according to the nominal stoichiometric ratios of Zn4.4Sb3GexSn3?x= 0–0.15?.The prepared samples possess a metallic luster surface with perfect appearance and large crystal sizes.The microscopic cracks or defects are invisible in the samples from the back-scattered electron image.Except for the heavily Ge-doped sample of x= 0.15,all the samples are single phase with space group R 3 c.The thermal analysis results show that the samples doped with Ge exhibit an excellent thermal stability.Compared with the polycrystalline Ge-substituted ?-Zn4Sb3,the present single crystals have higher carrier mobility,and hence the electrical conductivity is improved,which reaches 7.48×104 S·m-1 at room temperature for the x= 0.1 sample.The change of Ge and Sn contents does not improve the Seebeck coefficient significantly.Benefiting from the increased electrical conductivity,the sample with x= 0.075 gets the highest power factor of 1.45×10-3 W·m-1·K-2 at 543 K.Single crystalline ?-Zn₄Sb₃ thermoelectric materials with space group R 3 c were successfully prepared using the ?-Sn flux method based on the stoichiometric ratios of Zn4+xSb3Sn3?x= 0,0.2,0.4,0.6?.All the crystal sizes were more than 5 mm.The prepared samples had high density and no microcracks.The powder x-ray diffraction patterns showed that all the samples were single phase.All single-crystal samples possessed good electrical transport performance.Compared with polycrystalline ?-Zn₄Sb₃,the carrier mobility of the single crystals were significantly improved.Moreover,excess Zn improved the Seebeck coefficient of the material.The electrical conductivities of the samples were comparable with those obtained by the b-Sn flux method,whereas the Seebeck coefficient declined on the whole and the intrinsic conduction temperature decreased.The power factor of the sample with x= 0.2 is 0.82 × 10-3 W·m-1·K-2 at 635 K,which exhibited the best electrical transport performance.A group of single-crystalline ?-Zn₄Sb₃ samples were prepared using the Zn–Sn mixed-flux method based on the stoichiometric ratios of Zn4+x Sb3 Sny.The effect of Zn-to-Sn proportion in the flux on the structure and electrical transport properties is investigated.All samples are strip-shaped single crystals of different sizes.The actual Zn content of the present samples is improved?>3.9?compared with that of the samples prepared through the Sn flux method.Larger lattice parameters are also obtained.The carrier concentration of all the samples is in the order of over 1019 cm-3.With increasing Sn rate in the flux,this carrier concentration decreases,whereas mobility is significantly enhanced.The electrical conductivity and Seebeck coefficients of all the samples exhibit a behavior that of a degenerate semiconductor transport.Electrical conductivity initially increases and then decreases as the Sn ratio in the flux increases.The electrical conductivity of the x:y= 5:1 sample reaches 6.45 × 104 S·m-1 at 300 K.The flux proportion of the x:y= 7:1 sample finally achieves the highest power factor value of 1.4 × 10^-3 W·m^-1·K^-2 at 598 K.