Microstructure Controlled Growth And Thermoelectric Properties Of BI-TE Based Films

With the intensification of the energy crisis and the enhancement of people's environmental protection awareness,the development of human society urgently needs to accelerate the development of new energy,in recent years,Thermoelectric materials(TE)can convert thermal energy and electrical energy each other directly,which are widely used in thermoelectric power generation,thermoelectric refrigeration and waste heat recovery.Thermoelectric materials have the characteristics of long life,small volume,low cost and no pollution.It is of great significance to deepen energy conservation and emission reduction.The figure of merit is the key indicator to evaluate the thermoelectric properties of thermoelectric materials,which is decided by the Seebeck coefficient,electrical conductivity,thermal conductivity and absolute temperature.Currently,Bi2Te3 is one of the best thermoelectric materials at room temperature.In this paper,by adjusting the process parameters of Bi-Te based film fabricated by magnetron sputtering,the raw material films with smooth interface and compact structure was obtained,and then the rapid annealing process was used to obtain the high performance Bi-Te based thermoelectric thin films can be used for the fabrication of thermoelectric devices.A series of experimental schemes were arranged for the sputtering power,the substrate temperature,the argon flow rate and other process parameters of the magnetron sputtering by the central composite design method.Through the performance test of the prepared experimental samples,the surface roughness and the conductivity of the single-element Bi film and the single-element Te film corresponding to the experimental conditions were obtained.According to the results of variance analysis in the central composite design method,the relation calculation formula between performance parameters and sputtering conditions is fitted out.Subsequently,the fitting formula is further validated by SAS software to determine the true validity of the fitting method and results.Using the fitting formaula obtained above as a fitness function of non-dominated genetic algorithm ?(NSGA-?),Pareto optimal solution within a certain constrained range can be quickly obtained,that is,the optimal range of the single-element film sputtering process conditions.Subsequently,within the optimized process conditions,the single factor variation experiment was carried out to obtain the microstructure characteristics and the thermoelectric performance parameters under different process conditions.By controlling the growth of the microstructure and influencing factors and trend analysis on the thermoelectric properties,the relatively optimal sputtering conditions of the single-element film were further determined:RF sputtering of single-element Bi target to obtain the Bi film,sputtering power was 30 W.The substrate temperature was 125 ? and the argon flow rate 3.3 sccm(work pressure 0.4 Pa).The Te films were obtained by the proportional co-sputtering of single-element Te target and single-element Bi target.The sputtering power of Te and Bi were 23 W and 15 W,respectively,substrate temperature 125 ?,argon flow rate 5 sccm(work pressure 0.5 Pa).For the Bi-Te based thermoelectric thin films prepared under the optimal conditions,the microstructure and phase composition of the samples are improved by annealing,so the thermoelectric properties are improved.By adjusting the annealing temperature(300 ?,350 ?,400 ?)and annealing time(2-23 min,the time interval is 3 min):There have been big defects in the 400 ? annealing temperature.The highest Seebeck coefficient is 272 ?V·K-1 at the annealing temperature of 350?.At 300 ? annealing temperature,periodeic quantum well structures are formed between the inside layered structures.So the carrier concentration and the Seebeck coefficient appears oscillating phenomenon.At the same time,the highest power factor 28 ?W·K-2·cm-1 obtained at 300 ? annealing temperature.The results indicate that by increasing the substrate temperature during sputtering,sputtering of single-element Te film by co-sputtering of Bi and annealing at 300 ?can effectiviely improve the thermoelectric properties of the obtained Bi-Te based film material.