Study of flexible thermoelectric composites and textile thermoelectric generator

Abstract

Thermoelectric technology enables the direct conversion between temperature difference and electrical voltage, which shows promising prospects in waste heat energy harvesting applications. By using body heat, flexible and wearable thermoelectric materials are expected to apply into wearable self-powered electronic systems. Compared with the rigid bulk inorganic thermoelectric materials that mainly used in large-scale power generations, organic thermoelectric materials with good flexibility and processability are more suitable for the wearable applications. However, the study of organic flexible thermoelectric material is still in infancy. The thermoelectric efficiency of organic materials is much lower than their inorganic counterparts, and the popular film generator structure cannot be applied into real wearable applications. These obstacles confined their development in wearable electronics and need to be overcome urgently. Herein, from the viewpoint of textiles, we proposed a new series of organic thermoelectric composites, and explored their potential application in wearable electronics. The design principle, mechanism analysis, fabrication process, structure characterization, and function evaluation for the flexible thermoelectric composites and textile generator were introduced in this thesis. In this study, a series of polyurethane based thermoelectric composites were synthesized and characterized. The thermoelectric performance of the composites was evaluated by measuring the electrical conductivity, Seebeck coefficient, and power factors. The initial strategy of introducing polyurethane into thermoelectric application is to make composite with polyurethane and multi-walled carbon nanotube (MWCNT). The thermoelectric properties and mechanical properties were investigated and a film generator prototype was fabricated to explore the feasibility of making flexible thermoelectric generator with polyurethane. To further improve the thermoelectric performance and better apply the composites into textile based thermoelectric generator, waterborne polyurethane was introduced as the polymer matrix, and MWCNT combined with highly conductive PEDOT;PSS were incorporated as conductive fillers. The resulted composites could be easily coated on textiles and yarns, which provide the foundations to the new structural design of textile thermoelectric generator. Eventually, to fabricate a textile based wearable thermoelectric generator, 3D fabric was employed as substrate, and yarns coated with the as-prepared composite with optimal thermoelectric performance were acted as legs, thus a flexible thermoelectric generator with sandwich structure can be obtained. This structure allows generating temperature difference along the thickness direction like the classic inorganic thermoelectric generator. In general, it's the first time that polyurethane based composites were introduced in thermoelectric application, and the novel 3D fabric based flexible thermoelectric generator structure was proposed. Three types of polyurethane based thermoelectric composites were synthesized, characterized, and analyzed. The optimal composite recipe was further utilized in textile coating. Finally, a 3D fabric thermoelectric generator prototype was fabricated. The sandwich structure is innovative that breaks the limitation of broadly used film structure in organic flexible thermoelectric generators, which could be applied into wearable application by using our body heat as heat source.