Synthesis And Characterization Of YbAl₃ Thermoelectric Thin Films

YbAl₃ has attracted considerable attention in recent years due to its excellent thermoelectric?TE?performance.However,the present studies on YbAl₃ focus mainly on how to optimize the TE performance of YbAl₃ bulk materials through doping and filling,and there are no reports on the preparation and thermoelectric properties of YbAl₃ thin films.In this thesis,a magnetron sputtering method to preparing YbAl₃ thin films by a double-target co-sputtering has been developed,and the sintering process of YbAl₃ target,magnetron sputtering process,and annealing treatment of YbAl₃ thin films were systematically explored.The main results are included as follows:?1?YbAl₃ powder was prepared by traditional melt-quenching-annealing processes.This powder was mixed with Al powder,and then used to prepare YbAl₃targets with plasma activated sintering.The investigation on the effect of holding temperature on the microstructure of YbAl₃ targets revealed that at 773 K the internal structure was dense without obvious holes.The composition of the target was uniformly distributed from center to edges.The highest electrical conductivity,Seebeck coefficient,and power factor of YbAl₃ target obtained at 300 K was 2.30×10⁶ S m^-1,-82.9?V K^-11 and 15.9 mW m^-11 K^-2,respectively.The thermal conductivity had the minimum value of only 17.0 W m^-11 K^-11 and the highest ZT was 0.28.?2?Magnetron sputtering control method for preparing YbAl₃ thin film by a co-sputtering technique has been established,and a series of YbAl₃ thin films were prepared by this method at different substrate temperature and voltage of Al target.The results show that Al/Yb atomic percentage,particle size and electrical conductivity gradually increased with the increase of substrate temperature,and Seebeck coefficient and power factor reached the maximum at 673 K.With the increase of voltage for Al target from 250 V to 450 V,the composition changed from single-phase YbAl₂ to single-phase YbAl₃,and the Al/Yb atomic percentage,particle size,and the values of electrical conductivity,Seebeck coefficient,and power factor were gradually increased.The YbAl₃ thin-film materials prepared under the conditions of substrate temperature 673 K and voltage of Al target 450 V exhibited the best electrical transport properties.The maximum values of electrical conductivity,Seebeck coefficient,and power factor at 300 K were 1.06×10⁶ S m^-1,-30.8?V K^-11 and0.94 mW m^-11 K^-2,respectively.?3?The effects of subsequent heat treatment processes on the phase composition,microstructure,and TE properties of YbAl₃ thin films prepared under the optimized magnetron sputtering conditions were studied.The results indicated that all the samples annealed at 723-823 K for 4 h were composed of main phase YbAl₃ and a small amount of Al.Numerous nanoparticles with diameter of around 30 nm were found inside the thin films,and Al particles deposited on the film surface and gradually grew up with the increase of annealed temperature.The crystallinity,electrical conductivity,Seebeck coefficient and ZT values first increased and then decreased with increasing the annealing temperature.The optimum annealing temperature was 773 K.Under the optimized annealing temperature,the thin films annealed for 0² h were composed of single-phase YbAl₃,and those annealed over 2h were composed of main phase YbAl₃ and a small amount of secondary phase Al.Abnormal gowth of grains appeared after annealing for 8 h.The crystallinity,electrical conductivity,Seebeck coefficient and ZT values first increased and then decreased with prolonging the annealing time.The optimum annealing time has been determined to be 4 h.The thin films,obtained under the optimized annealing conditions?773 K and 4h?,had the largest values of power factor and ZT reaching 7.4mW m^-11 K^-22 and 0.14 at 300 K.