| Thermoelectric(TE)materials have the characteristics of no noise,easy maintenance,long life and fast response time.They can directly convert thermal energy into electrical energy and can be used in low-grade waste heat recovery.Among various TE materials,Bi2Te3 materials has stable chemical properties and excellent TE properties,and it is the most widely used low-temperature TE material for commercialization.However,the Bi2Te3 films still have disadvantages such as low TE properties,poor mechanical properties,and high cost.Therefore,it is crucial to carry out research on efficient and low-cost Bi2Te3 films.Nanocomposite is currently a common method to improve the TE properties of the Bi2Te3 films.But it is difficult to realize the trade-off of conductivity,Seebeck coefficient and thermal conductivity in a single nanocomposite,and the depletion layer generated at the interface of the nanocomposite will hinder the migration of carriers and limit the improvement of TE properties of the nanocomposite effect.Focusing on the problems of improving the TE properties of Bi2Te3 films by nanocomposite,the study explores a binary composite method with pulsed electric field(PEF),studies the effect of the binary composites of porous and carbon nanotubes(CNT)on the microstructure and TE properties of the Bi2Te3 films,elucidates the effect of the PEF on the depletion layer of the nanocomposite interface and the relationship with the film electroacoustic transport and TE properties,reveals the path and mechanism of trade-off of conductivity,Seebeck coefficient and thermal conductivity.First,it studies the influence of high magnetic field(HMF)and PEF on the microstructure and TE properties of pure Bi2Te3 film,reveals the adjustment mechanism of external field treatment on the film interface structure,and obtains the best external field treatment conditions to improve the TE properties of the Bi2Te3 film;Then it studies the effect of different nanocomposite methods on the microstructure and TE properties of the Bi2Te3 film,and determines the best nanocomposite method of the Bi2Te3 film and the effect of depletion layer on carrier migration.Next,it uses the pulsed electric field to process the composite films.The effects of the PEF and binary composite on the microstructure and TE properties of the Bi2Te3 films are studied,and the effect of the PEF on the depletion layer and the mechanism of improving film TE properties are revealed.The main conclusions are listed as follows:(1)The effects of HMF and PEF on the microstructure and TE properties of pure Bi2Te3 films were studied.The results show that the HMF can improve the power factor of the film.At 313 K,the power factor of the Bi2Te3 film is 2528μW·m-1·K-2.However,the HMF changes the phase composition of the film and at the same time changes the surface morphology from spherical to flat structure.However,the PEF has little effect on the microstructure phase composition and surface morphology of the Bi2Te3 film.When the pulse current density decreases,the defect content of the Bi2Te3 film decreases,and the carrier concentration decreases.As the pulse width increasing,the PEF will increase the carrier mobility.However,when the pulse width is large,the content of defects in the Bi2Te3 film increases,and the carrier concentration increases.When the pulse current density is 333.3 A·cm-2 and the pulse width is 1.0 s,the power factor of the Bi2Te3 film is the largest at 313 K(1168 μW·m-1·K-2).This provides a basis for the PEF treatment of binary composite Bi2Te3 films.(2)First,the effect of porous and CNT composite on the microstructure and TE properties of Bi2Te3 film was studied.The results show that the porosity and CNT binary composite has the highest power factor(845 μW·m-1·K-2)at 313 K,and revealed the inhibitory effect of the charge depletion layer generated at the composite interface on the improvement of carrier mobility.Furthermore,the binary composite Bi2Te3 films are treated with pulsed electric field.The effect of PEF on the microstructure and TE properties of binary composite Bi2Te3 film was studied.The results show that the binary composite can realize the decoupling of electrical conductivity,Seebeck coefficient and thermal conductivity.And the PEF can promote atomic diffusion at the composite interface,eliminate the effect of depletion layer on carriers,and make the PEF-composite-Bi2Te3 film with a mobility of 28.49 cm2·V-1·s-1,thereby improving the film conductivity and Seebeck coefficient,making the power factor research 4400 μW·m-1·K-2.In addition,the porosity structure enhances the phonon scattering of the thin film,so that the thermal conductivity is 0.752 W·m-1·K-1.The temperature difference between the hot and cold ends is increased,so that the output power is 951.20 pW.At the same time,the ZT value of the PEF-composite-Bi2Te3 film is 1.83. |
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