| Thermoelectric(TE)materials can generate electrical energy from waste heat or be used as solid-state coolers,which play an important role in the global sustainable energy solution.The energy conversion efficiency mainly depends on the performance of the TE material and the device,but has weak dependence on the the size of the energy conversion system.In industrial application,TE materials are often used for waste heat recovery.More importantly,TE materials have more advantages than other energy conversion systems in the milliwatt to microwatt power level for electronic devices.With many advantages compared with bulk TEs,such as higher integration density,heat pumping capability,faster response time,more compact size,thin film TE materials are more suitable for the development of modern electronic device systems and have attracted rapidly growing interest in the technologies of thermal energy conversion of waste into electricity and site-specific cooling around ambient temperature.Thin-film TE materials prepared by physical vapor deposition(PVD)can be used in commercial micro-thermoelectric generators and refrigeration chips,providing a key solution for micro-nano electronic equipment power supply and micro-system cooling.However,limited strategies can be utilized to tailor the performance of thin film through controlling the microstructure,as compared to the bulk TE materials.Therefore,how to design,prepare and tailor the properties of TE thin films is a crucial problem to promote their development.Furthermore,TEs should obtain close contact with the heat source surface with arbitrary geometries for minimizing heat loss and achieving a highly efficient energy conversion.However,the intrinsic bonding features of conventional bulk TE materials determine their native rigidity and brittleness which would lower the efficiency of energy conversion due to the poor thermal contact with the heat source surface.Thus,the development of flexible TE materials with good flexibility has become one of the key factors for the large-scale applications of TE technology.How to obtain thin film with high TE properties,machinability,and flexibility is an important research topic in this field.Based on the above challenges,this paper systematically studied the effects of Te concentration,microstructure,heat treatment conditions,substrate,etc.,on TE properties and flexibility of Bi2Te3 thin films.Based on these studies,nanopore structures with various pore characteristics were introduced into Bi2Te3 thin film to tailor the TE properties.A Bi2Te3-glass fiber hybrid with highly ordered microstructures was fabricated for flexible micro-TE generator.The main results achieved are as follows:1.In this work,Bi2Te3 thin film with(000l)texture and different Te concentration was fabricated by DC magnetron co-sputtering method to systematically study the effects of the two factors on the TE properties and the growth mechanism of thin films.As the Te atomic concentration increases from 59%to 78%in the thin film,the growth mechanism of the thin film was gradually inclined to island growth mechanism from layer growth mechanism.The enhancement of(000l)orientation factor in the thin film can lead to the effectively improvement of the in-plane electrical conductivity without lower the Seebeck coefficient,resulting in the decline of power factor.This work reveals the effect of Te concentration and(000l)orientation factor to TE property of thin film,providing important basic science for enhanced TE performance and further development of the TE device.2.This work firstly reports a novel method to prepare nanoporous Bi2Te3 thin films with various pore characters by magnetron sputtering and post annealing and reveal the effect of pore characters on in-plane TE properties.Through depositing over-stoichiometry Te in the thin film and then reversely diffusing Te atoms and vacancies by post annealing,over-stoichiometry Te atoms were evacuated out of samples through grain boundary diffusion and vacancies were aggregated into nanopores.Bi2Te3 thin films with different pore characteristic like porosities,pore sizes,distribution and intervals were successfully fabricated.By designing the pore intervals greater than the mean free path of carriers and less than the mean free path of phonons,the thermal conductivity was significantly reduced without significantly affecting electrical conductivity.Furthermore,the pore size and pore interval can be further tuned through designing the film with gradient Te concentration,in contrast to those of samples with uniform Te concentration to intensive study the effect of these pore characteristics on the thermal and electrical properties of the material.Tuning the pore size in a large distribution may provide more possibilities for phonon scattering in a wide range of wavelengths.This phonon-nanopore scattering effect in our Bi2Te3 film gives rise to a maximum power factor of?18 ?W cm-1 K-2 and a ZT value of?0.67 at room temperature,which is comparable to the values of commercial bulk TEs.In addition,the approach is film-thickness independent,which is beneficial to the preparation of device-level TE film materials with a certain thickness to reduce the internal resistance.And it can also be applicable to fabricate many other nanoporous chalcogenide TE films.The evacuated Te could be recycled to reduce the use of expensive Te elements.This work may provide a general approach to introduce tailorable nanoporous structures,opening a new window to engineer high-performance thin-film TE materials for practical applications.3.A flexible TE hybrid consisting of highly(000l)-textured Bi2Te3 thin film uniformly coated on glass fiber was prepared by magnetron sputtering.Highly ordered microstructure featuring(000l)-textured Bi2Te3 nanocrystals/glass fiber with perfect alignment between Bi2Te3(000l)plane and fiber axial is successfully fabricated.XRD analysis confirm that microscopic stresses exist in the Bi2Te3 thin film which are consistent with the highly curved(000l)atomic plane observed in the SEM results.The results of TE properties reveals that the hybrid has PFs of 14.85 to 17.65 ?W cm-1 K-2 from room temperature to 383 K.Meanwhile,owing to the strong phonon scattering effects of strain field in the thin film,the lattice thermal conductivity is remarkably reduced to 0.25 W m-1 K-1,which is close to the theoretical minimum lattice thermal conductivity of Bi2Te3,As a result,the TE figure of merit,ZT,is as high as 0.43 at room temperature.Moreover,the hybrid exhibits excellent flexibility resulting from the strong(000l)texture,size effect and flexible fiber substrate.In addition,a prototype of TE generator integrated with the hybrid was fabricated and evaluated for energy conversion.This work explores a new method to design and fabricate flexible thermoelectric materials and devices. |
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