| Thermoelectric materials can directly convert thermal energy and electrical energy,and are less affected by the scale of the energy conversion system and temperature difference,which have great potential applications in the cooling of small electronic equipment,wasted heat power generation and micro sensors.Especially for wearable and flexible electronic products that are developed rapidly,thermoelectric materials can use the temperature difference between the skin and the environment to generate electricity or to monitor temperature and other information in real time.New application demands have prompted the research of thermoelectric materials and devices to the trend towards flexibility,miniaturization and high-density integration gradually.Compared with traditional bulk thermoelectric materials,thermoelectric thin film materials can achieve flexibility and miniaturization much easier.The great potential of further improvement on their thermoelectric performance through nanostructure optimization make it a heated topic in this research field.This thesis systematically studied the performance characterization and processing technology platform of materials and devices,the performance optimization research of carbon nanotube-bismuth telluride composite film materials and the design and production of flexible micro-thermoelectric thin film devices.Recently,composite materials and devices have been widely studied.It solves many challenges in the research work of thin-film thermoelectric materials,such as the difficulty of characterizing the thermal conductivity,the difficulty of material processing,the optimization of interface performance,and the difficulty of manufacturing thin-film devices.It can be applied as a systematic reference research program for thin-film thermoelectric materials.The main results achieved are as follows:1.Based on the 3? transient heat flow method and the steady state silicon nitride cantilever method,two complete sets of thermal conductivity characterization platform have been well developed for the carbon nanotube-bismuth telluride composite thin film material system in this work.The material thermal conductivity characterization platform specifically solves the difficulties in the sample preparation process of the two methods.We have developed high-precision silicon nitride physical masks and self-supporting thin film sample transfer methods for the two sample preparation technical methods.Completed the construction of the basic platform for the research on the properties of the thermoelectric film material.Related work has obtained the authorization of one invention patent and four utility model patents.2.Set-up and optimized the femtosecond laser processing system.In the research process of thin-film thermoelectric materials,due to the intrinsic brittleness and easy oxidation characteristics of the materials,the processing and transfer process of the materials have increased a lot of difficulty.For this reason,we independently built a set of femtosecond laser processing and transfer system,using the advantages of femtosecond laser processing of less heat effect and high processing accuracy,to realize the micro-machining/cutting process and transportation of thermoelectric thin film materials.This work provides solid foundation for the research of material performance and the development of micro-devices.3.The carbon nanotube-bismuth telluride composite films with different two-phase volume ratios were prepared by magnetron sputtering,and the thermoelectric properties of the composite films were analyzed.It was found that at the beginning of the deposition process,bismuth telluride and carbon nanotubes were combined with each other,contributing the performance enhancement of the film.However,at this moment,because there are not enough electron and phonon transmission channels formed between the bismuth telluride grains,the contribution of the two relative composite membranes to the performance of the composite film is more like to the relationship of series.Carbon nanotubes are the main transmission body,and the effect of the carbon nanotube-bismuth telluride two-phase interface on the performance will be particularly prominent.With the continuous increase of the deposition amount,the bismuth telluride contribution to the performance dominates gradually.At this time,the contribution of the two relative composite membrane performance is closer to the parallel relationship.When the deposition time is longer(>1500s),the volume ratio of bismuth telluride can reach 96%,the Seebeck coefficient of the composite film is close to that of the bismuth telluride film.At this time,the performance of the composite film is mainly composed of bismuth telluride.4.Aiming at the influence of the two-phase interface on the performance of the composite film that has not been explored before,plasma was used to pre-treat the surface of the carbon nanotube film in this work,and then the two-phase interface of the composite film was modified.The XPS and HRTEM were applied to analyze the changes of the two-phase interface.It is proposed that after plasma treatment,the interface of the composite film becomes rougher,and the phonons will produce diffuse reflection at the rough interface,which makes the pretreated sample relative to the untreated sample.The phonon heat transfer efficiency of the processed sample was greatly reduced,while due to the mean free path of the carriers in the bismuth telluride is much smaller than that of the phonons,the change of the interface has little effect on the carrier transport.Thus,a general approach to the decoupling of phonon and carrier transport properties was developed,and the thermoelectric performance of the composite film has been improved by about 50%.5.Complete the investigation on the material properties,and further use the materials to design and manufacture micro-devices.Aiming at solving the contradiction between the best in-plane performance direction of thermoelectric film materials and the common out-of-plane temperature difference direction in actual application scenarios.Two schemes of silicon suspended chip bridge and Polydimethylsiloxane(PDMS)flexible deformable substrate is proposed.Two kinds of miniature thermoelectric thin film devices were designed and fabricated,along with the performance characterization.The silicon nitride floating chip thermoelectric device is a thin-film thermoelectric device with a planar structure.In terms of power generation,when ?Tg reaches 20 K,the open circuit voltage of the device is 15 mV,and is accompanied by short-circuit current of-60 ?A.Pgmax can reach up to 0.225 ?W.In terms of refrigeration,the thermoelectric device can obtain a cooling temperature difference of about 7.5 K at a room temperature.Qgmax can reach up to?89 ?W at 300 K.The PDMS flexible deformable substrate thermoelectric thin film device is a deformable flexible device.The two-dimensional flexible material is transformed into a three-dimensional arched thermoelectric device by pre-stretching the PDMS to realize the production of a flexible micro thermoelectric thin film device.In an atmospheric environment where the temperature of the hot stage is 50 degrees,the open circuit voltage of the device is 0.4 mV,which initially meets the requirements of the sensor.And the temperature difference and electrode connection scheme need to be further improved. |
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