Structural Design And Applications Of Three-dimensional Thermoelectric Textiles

Thermoelectric(TE)materials can convert heat into electricity and vice versa based on Seebeck effect and Peltier effect.Compared with other kinds of energy conversion device,they have no noise,no moving parts,and can supply power continuously.Compared with traditional air conditioners,it is portable and refrigerant free.Furthermore,compared to passive radiative cooling,they can maintain the body temperature regardless of ambient temperature and humidity.The human body is a stable heat source,and textiles are the direct interacting medium between human body and the environment.Therefore,TE textiles(TETs)prepared by combining TE materials and textiles are expected to integrate a variety of functions of body heat harvesting and boy micro-climate thermoregulation.However,how to design and manufacture TETs with good wearable comfortability,durability and high TE performance still faces great challenges.Basic scientific questions in related fields remain to be resolved.In this paper,the TE properties of organic-based TE yarns were studied and regulated.The heat transport mechanism of 3D TETs woven by organic-based segmented TE yarns was explored by finite element simulation and experiments.A 3D organic-based TET was woven according to the simulation results and the difficulty of manufacturing.Its applications in wearable electronics and self-powered sensing were characterized.The strengthening and toughening mechanism of inorganic-based segmented flexible bead-like TE string were explored through finite element simulation and experiments.The heat transport mechanism in inorganic-based TETs and its effect on TE output performance were studied.The application in power generation through harvesting heat from human body and body temperature regulation were characterized.The contents and conclusions are as follows:(1)In order to prepare organic-based TE materials with excellent TE properties,the TE properties of carbon nanotube yarn(CNTY)were regulated,and the carrier transport mechanism was investigated in Chapter 2.First,the author used poly-3,4-ethylenedioxythiophene:polystyrene sulfonate(PEDOT:PSS)and CNTY to prepare p-type PEDOT:PSS/CNTY by dip coating method.The Seebeck coefficient,electrical conductivity and TE power factor of PEDOT:PSS/CNTY are70.1?V/K,1043.5 S/cm and 512.8?W/(m·K~2).The TE power factor is 108%higher than that of CNTY(246.1?W/(m·K~2)).The improvement of the TE power factor is attributed to two aspects.The enhanced electrical conductivity of the PEDOT:PSS/CNT composite yarn might be ascribed to the compacted structure of CNTY leading to smaller diameters and thus facilitated electron transport in CNTY.The enhanced Seebeck coefficient may arise from the interfacial effect between PEDOT:PSS and CNTs,where CNTs can align along the conductive PEDOT chains manifesting the template effect that can enhance the Seebeck coefficient.Furthermore,the authors systematically studied the effect of polyethyleneimine(PEI)concentration on the TE properties of CNTY(PEI/CNTY)using PEI as the n-type dopant.The PEI molecule is nitrogen-abundant and has strong electron donating ability.When CNTY is immersed in PEI solution,PEI molecules will be physically adsorbed to the surface of CNTY to inject electrons into CNTY.This causes the Fermi level(E_F)of CNTY to move toward the vacuum level,thus realizing n-type doping of CNTY.When the PEI concentration is 5.96 wt%,the PEI/CNTY composite yarn obtained the optimum TE performance.The Seebeck coefficient,electrical conductivity and TE power factor are-68.7?V/K,1408.3 S/cm and 667.8?W/(m·K~2).Based on the above investigation results,the author fabricated ten-meter-long flexible segmented organic-based TE yarn by dip coating method.(2)The effect of textile parameters(textile structure and thickness)on the heat transport mechanism of TETs was explored in Chapter 3.TETs with different textile structures and thicknesses were established by finite element simulation.Further,their output performance under natural convection was simulated.Subsequently,the accuracy of the simulation results was verified by experiments.It is shown that the textile structure has a significant effect on the TE output performance of TETs.The TE output performance of stacked knitted fabric is the largest,followed by warp-knitted spacer fabric,and stacked woven fabric is the worst when other parameters are the same.The yarns in the stacked knitted fabric are bent into loops and interlocked with each other,which reduces the heat conduction between the yarns.At the same time,it retains more static air.Hence,the thermal resistance and output voltage of stacked knitted fabric are the largest.Although there is a lot of static air inside the warp-knitted spacer fabric,the spacer yarns are arranged regularly and consistent with the heat transfer direction.Therefore,their thermal resistance and output voltage are smaller than those of stacked knitted fabrics.In contrast,the yarns in stacked woven fabric are regularly arranged and there is almost no static air inside it.Hence,the heat conduction of stacked woven fabric is more serious compared with stacked knitted and warp-knitted spacer fabric.Therefore,the thermal resistance and output voltage are the smallest.The thickness of textile substrate also has a significant effect on the TE properties of TETs.The TE output performance will be enhanced with the increased thickness of textile substrate when other parameters are the same.More static air will be locked in the TET if the thickness of the textile substrate is larger,which will lead to a larger thermal resistance of TET.Therefore,the established temperature difference and output voltage will become larger under the condition of natural convection.The investigation results of the heat transport mechanism in the TETs provides theoretical support for the design of TETs with high TE output performance.(3)According to the finite element simulation results,difficulty of manufacturing process and wearing comfort,the traditional embroidery method was used to weave the organic-based segmented TE yarn into a warp-knitted spacer fabric in Chapter 3 and 4.A 3D TET that can generate electricity in the thickness direction was developed.It can be bent,twisted and compressed,showing good flexibility and skin conformability.At temperature difference of 47.5 K,the organic-based TET can output short-circuit current of?402?A,voltage density of 520.9 V/m~2,and maximum output power density of 51.5 m W/m~2.The insertion of TE yarns has little effect on the thermal comfort of the warp-knitted spacer fabric.The changes in air permeability,thermal resistance,thermal transfer coefficient,Clo value and insulation rate are all within 10%.The as-prepared TET can successfully power a series of wearable electronics continuously.Subsequently,an industrial knitting process was used to preliminarily explore the preparation of TETs at a large scale.Furthermore,the applications of the organic-based TETs in flexible passive temperature and pressure sensing were demonstrated.In the pressure range of 8.8 k Pa,the temperature sensitivity is0.87 m V/K.When the temperature difference is 5.0 K,the pressure sensitivity is 0.312 k Pa^-1(0-1.2k Pa).(4)In order to improve the mechanical stability of brittle inorganic TE materials,the strengthening and toughening mechanism of inorganic bismuth telluride(Bi₂Te₃)based TE materials were explored in Chapter 5.Combining cold pressing and ultrafast high-temperature sintering process,Bi₂Te₃ based TE materials were compressed onto the surface of PI filaments,which were used as core materials,utilizing graphite mold.Subsequently,polydimethylsiloxane(PDMS)elastic layer was used to encapsulate the Bi₂Te₃ based TE materials and PI filaments.Finally,the ternary coaxial inorganic based p-n segmented flexible bead-like TE string were produced.At temperature of?360 K,the TE figure of merit(z T value)of the p-type and n-type TE units in the string is?1.0 and?1.2,respectively.Furthermore,finite element simulation and experiments were used to study the mechanical and electrical stability of the inorganic based segmented flexible bead-like TE string under three-point bending condition,in order to explore the strengthening and toughening effect of PI filaments and PDMS layer on the Bi₂Te₃ based TE materials.The experimental results indicate that the applied mechanical energy can be absorbed by the deformation of PDMS layer and TE materials,crack propagation and debonding between TE bulk and PI filaments.The finite element simulation results demonstrate that PI filaments and PDMS layer can hinder the crack propagation of Bi₂Te₃ based TE materials under three point bending load.The PI filaments mainly change the direction of crack propagation to achieve the toughening effect;while the PDMS achieves the strengthening effect by reducing the crack opening displacement.Therefore,the service life of the TE string is extended.(5)The heat transport in the inorganic-based TET and the effect on the TE output performance were investigated in Chapter 6.In inorganic-based TETs containing multiple components,each component and TE legs are in thermally parallel,and the heat transport is complicated.A TET containing multiple components was established by finite element simulation method.The voltage output and solid-state cooling effect of TET in the ambient air condition were simulated.Moreover,the accuracy of simulation results was verified by experiments.The simulation results indicate that increasing the equivalent thermal resistance of the non-TE components in the TET can improve the TE output performance and cooling effect when the TE materials are constant.According to the finite element simulation results,the as-prepared inorganic based segmented flexible beaded-like TE strings were woven into a flexible 3D inorganic-based TET.When the temperature difference and external resistance are 25 K and 30.1?,respectively,the maximum power density of the TET reaches up to 0.58 W/m~2(?T=80 K,the predicted power density is 6.06 W/m~2).For the first time,the reported TET can continuously and stably generate solid-state cooling effect.When the ambient temperature is 26.3^oC,and the input current is 140 m A,the cold side temperature of the TET is reduced by 3.1 K.