**2. Textile thermoelectric materials**

with conductors. When temperature difference exists between the two sides of thermoelectric materials, the charge carriers in the materials will be driven to flow from the hot side to the cold side, and thus a thermo-voltage can be formed. As key components in a thermoelectric system, the energy conversion efficiency of a thermoelectric material can be evaluated by a

In this equation, σ is electrical conductivity, S is Seebeck coefficient, *κ* is thermal conductivity, T is absolute temperature, and ZT is a dimensionless value without upper limit. The value of

σ is called power factor, which also can be used to represent the thermoelectric performance of materials where the impact of thermal conductivity is secondary. For example, in polymerbased thermoelectric materials, power factor is widely used, since most polymers have much lower thermal conductivity than inorganic semiconductors and then their thermoelectric per-

Now, two main strategies can be adopted to improve the efficiency of thermoelectric system. One is to increase the ZT value of the thermoelectric materials, and the other is to adjust the structure of TEG for better utilization of heat energy. In past few years, the thermoelectric efficiency of bulk inorganic metal alloys has been improved significantly. A record-breaking high ZT of 2.6 achieved by SnSe single crystals at 923 K was reported in 2014, which gives thermoelectric generators a better application prospect in future [10]. Meanwhile, a new solar heat TEG system has been developed. By combining solar heat collection and thermoelectric

conversion technologies, the new system could possess a peak efficiency of 7.4% [11].

Nowadays, with the rapid development of wearable and flexible electronics, the field of flexible thermoelectric materials and generators were growing dramatically. Various flexible thermoelectric materials were developed in form of films, papers, fabrics, and even fibers. Thermoelectric films are most studied among them, while the research about thermoelectric fabrics and fibers are just beginning very recently. Generally, these flexible thermoelectric materials are mostly made by organic polymers or polymer/inorganic semiconductor composites, so that they are easier to fabricate on large scales. However, the thermoelectric efficiency of these polymer-based flexible materials is much lower than the inorganic semiconductors. Hence, the methods to further improve the thermoelectric efficiency of these flexible materials

Although fabrics, fibers and polymer films are all flexible materials, textile thermoelectric materials have exhibited more competitive characteristics than simple films, especially on the wearable application. Textiles composed of different sets of fibers and yarns have excellent structure design flexibility, which allows them to meet the requirement of flexible, wearable, nontoxic, light weight, and even washable for wearable devices so that we can utilize our human body heat as power source. Until now, many pioneering research works have been conducted on this area and showed gratifying results. Therefore, in this chapter, the development of these textile thermoelectric materials and generators will be thoroughly

*<sup>κ</sup> T* (1)

thermoelectric figure of merit, also called ZT value [9]. As shown in Eq. (1).

ZT = *<sup>σ</sup> <sup>S</sup>*<sup>2</sup> \_\_\_

24 Bringing Thermoelectricity into Reality

formance will be mainly decided by the power factor.

will still be a hot research issue in future.

described.

S2

Textile is an ideal candidate to utilize our human body heat to generate power due to its excellent flexibility, wearability, comfort, and air-permeability properties, especially compared with those rigid bulk electronic materials and impermeable polymer-based flexible films. Thus, it is promising to develop textile material-based thermoelectric materials and generators to utilize our body heat. In the past few years, many research works have been done on both textile thermoelectric materials and fabric generators, which have paved the way and given us inspirations for future wearable thermoelectric systems.

#### **2.1. Fabric-based thermoelectric materials**

Fabric finishing and coating are mature technologies in textile industry that can be easily used for large-scale production. Thus, it is a cost-effective and practical way to added effective thermoelectric materials on fabrics for both organic and inorganic materials. In 2015, Yong Du et al. prepared a textile thermoelectric material by coating commercial polyester fabrics with PEDOT:PSS [12]. Their adopted a simple dip coating process to fabricate textiles with thermoelectric functions. **Figure 1** shows the dip coating process the prepared thermoelectric textile strip by Tong Lin. P-type PEDOT:PSS doped with DMSO was acted as effective thermoelectric materials. Commercial available polyester fabric was used as a flexible substrate. Both advantages of textiles and PEDOT:PSS were utilized. The prepared textile materials exhibit electrical conductivities ranged from 0.5–3 S/cm and Seebeck coefficient of 15.3–16.3 μV/K at 300 K. The highest power factor of 0.045 μWm−1 K−2 was achieved at 390 K. Although the thermoelectric efficiency of this flexible textile thermoelectric material is not quite satisfied, this is

**Figure 1.** (a) A dip coating process and (b) PEDOT:PSS coated polyester fabric by Yong Du et al. [12].

still an initial attempt to applying fabrics into thermoelectric application. The low efficiency may also attribute to the used organic polymer thermoelectric materials. Choosing high efficient inorganic semiconductors especially their nano-sized counterparts as fabric coating materials, the thermoelectric efficiency coated fabrics could be further improved.

Another example is to use nano inorganic materials to produce thermoelectric fabric. In 2015, Chongjian Zhou et al. prepared a thermoelectric fabric made by inorganic nanowires and organic polymer composite [13]. A simple five-step vacuum filtration process was employed. Cu1.75Te nanowires sheet was fabricated by filtration, hot press, and annealing first, and then PVDF solution was drop cast on the nanowire sheet. After a heating process, a flexible composite material was formed. The composite structure and prepared fabric film are shown in **Figure 2**. The prepared flexible fabric film has a high electrical conductivity of 2490 S/cm, and a Seebeck coefficient of 9.6 μV/K at room temperature, which resulted in a power factor of 23 μWm−1 K<sup>2</sup> . The performance of prepared fabric could keep steady after 300 cycles of bending tests. The results indicate that this easy to scale-up method is effective to fabricate flexible thermoelectric fabric and can be extended to other effective inorganic materials such as Bi<sup>2</sup> Te3 , Ag2 Te, or Ag2 Se nanowires.

In 2016, we proposed a water-processable thermoelectric coating material made of waterborne polyurethane, MWCNT, and PEDOT:PSS composite [15]. The optimal electrical conductivity and Seebeck coefficient could achieve ~13,826 S/m and ~10 μV/K at room temperature respectively, and the resulted power factor is about ~1.41 μWm−1 K−2. Compared with other organic-solventbased thermoelectric polymers, this water-based composite exhibits satisfactory thermoelectric performance and good processability. Then, we coated the prepared water-based composite on polyester and cotton yarns respectively, as shown in **Figure 4**. The results show that the fabricated thermoelectric composite can be successfully coated on textile yarns, and polyester filament is more suitable as coating substrate than staple cotton yarn. Besides, these coated yarns

**Figure 3.** (a) Dip coating process of fibers and (b) comparison of uncoated and coated glass fibers made by Daxin Liang

In 2016, Jae Ah Lee et al. using electro-spinning technology to fabricate thermoelectric nanofibers and then twisted into yarns [16]. Polyacrylonitrile (PAN) nanofiber sheets were elec-

thermoelectric materials, and deposited on two sides of the PAN sheets. After a twisting process, thermoelectric yarns can be formed, as shown in **Figure 5**. The highly porous structure of the electrospun yarns are quite resistant to mechanical damage, which enables the yarns to be knitted and woven into fabrics without the significant changes in their electrical conductivity

**Figure 4.** (a) Yarn coating process and (b) polyester yarn coated with waterborne polyurethane thermoelectric composites

and Sb<sup>2</sup>

Te3

were selected as n-type and p-type

Thermoelectric Textile Materials

27

http://dx.doi.org/10.5772/intechopen.75474

Te3

can be treated as thermoelectric legs and were further used to fabricate fabric TEG.

trospun on two parallel wire collectors. Bi2

and thermopowers.

made by Jinlian Hu [15].

et al. [14].

The preparation of fabric thermoelectric materials is simple and very easy to fabricate on large scale. The integration of inorganic high performance thermoelectric materials would give fabrics better thermoelectric performance than organic polymers. Although fabrics usually have the same 2D flat structure as polymer films, porous fabric structures would have better airpermeability and wearable comfort than polymer films, which is more appropriate for wearable devices.

#### **2.2. Fiber/yarn-based thermoelectric materials**

In addition to fabrics, fibers and/or yarns can be also fabricated into thermoelectric materials. Since fibers and yarns are the structural basis of fabrics, to offer fibers and yarns thermoelectric properties other than fabric itself will give more design flexibility for flexible textile TEGs. In 2012, Daxin Liang et al. reported a fiber thermoelectric material. PbTe nanocrystals were coated onto glass fibers via a dip coating process [14], as shown in **Figure 3**. The PbTe nanocrystal coated fibers have an electrical conductivity of 104.4 S/m, Seebeck Coefficient 1201.7 μV·K−1, and thermal conductivity of 0.228 W·m−1·K−1 at 300 K. The resulted power factor is 0.15 mW·m−1·K−2 at 300 K. It should be noted that the thin coating layer would lower the electrical conductivity of PbTe nanocrystals compared with PbTe bulk crystals.

**Figure 2.** (a) The composite film structure and (b) the picture of flexible fabric film prepared by Chongjian Zhou et al. [13].

still an initial attempt to applying fabrics into thermoelectric application. The low efficiency may also attribute to the used organic polymer thermoelectric materials. Choosing high efficient inorganic semiconductors especially their nano-sized counterparts as fabric coating

Another example is to use nano inorganic materials to produce thermoelectric fabric. In 2015, Chongjian Zhou et al. prepared a thermoelectric fabric made by inorganic nanowires and organic polymer composite [13]. A simple five-step vacuum filtration process was employed. Cu1.75Te nanowires sheet was fabricated by filtration, hot press, and annealing first, and then PVDF solution was drop cast on the nanowire sheet. After a heating process, a flexible composite material was formed. The composite structure and prepared fabric film are shown in **Figure 2**. The prepared flexible fabric film has a high electrical conductivity of 2490 S/cm, and a Seebeck coefficient of 9.6 μV/K at room temperature, which resulted in a power factor of 23 μWm−1 K<sup>2</sup>

The performance of prepared fabric could keep steady after 300 cycles of bending tests. The results indicate that this easy to scale-up method is effective to fabricate flexible thermoelectric

The preparation of fabric thermoelectric materials is simple and very easy to fabricate on large scale. The integration of inorganic high performance thermoelectric materials would give fabrics better thermoelectric performance than organic polymers. Although fabrics usually have the same 2D flat structure as polymer films, porous fabric structures would have better airpermeability and wearable comfort than polymer films, which is more appropriate for wearable

In addition to fabrics, fibers and/or yarns can be also fabricated into thermoelectric materials. Since fibers and yarns are the structural basis of fabrics, to offer fibers and yarns thermoelectric properties other than fabric itself will give more design flexibility for flexible textile TEGs. In 2012, Daxin Liang et al. reported a fiber thermoelectric material. PbTe nanocrystals were coated onto glass fibers via a dip coating process [14], as shown in **Figure 3**. The PbTe nanocrystal coated fibers have an electrical conductivity of 104.4 S/m, Seebeck Coefficient 1201.7 μV·K−1, and thermal conductivity of 0.228 W·m−1·K−1 at 300 K. The resulted power factor is 0.15 mW·m−1·K−2 at 300 K. It should be noted that the thin coating layer would lower the

**Figure 2.** (a) The composite film structure and (b) the picture of flexible fabric film prepared by Chongjian Zhou et al. [13].

electrical conductivity of PbTe nanocrystals compared with PbTe bulk crystals.

.

Se

Te, or Ag2

Te3 , Ag2

materials, the thermoelectric efficiency coated fabrics could be further improved.

fabric and can be extended to other effective inorganic materials such as Bi<sup>2</sup>

**2.2. Fiber/yarn-based thermoelectric materials**

nanowires.

26 Bringing Thermoelectricity into Reality

devices.

**Figure 3.** (a) Dip coating process of fibers and (b) comparison of uncoated and coated glass fibers made by Daxin Liang et al. [14].

In 2016, we proposed a water-processable thermoelectric coating material made of waterborne polyurethane, MWCNT, and PEDOT:PSS composite [15]. The optimal electrical conductivity and Seebeck coefficient could achieve ~13,826 S/m and ~10 μV/K at room temperature respectively, and the resulted power factor is about ~1.41 μWm−1 K−2. Compared with other organic-solventbased thermoelectric polymers, this water-based composite exhibits satisfactory thermoelectric performance and good processability. Then, we coated the prepared water-based composite on polyester and cotton yarns respectively, as shown in **Figure 4**. The results show that the fabricated thermoelectric composite can be successfully coated on textile yarns, and polyester filament is more suitable as coating substrate than staple cotton yarn. Besides, these coated yarns can be treated as thermoelectric legs and were further used to fabricate fabric TEG.

In 2016, Jae Ah Lee et al. using electro-spinning technology to fabricate thermoelectric nanofibers and then twisted into yarns [16]. Polyacrylonitrile (PAN) nanofiber sheets were electrospun on two parallel wire collectors. Bi2 Te3 and Sb<sup>2</sup> Te3 were selected as n-type and p-type thermoelectric materials, and deposited on two sides of the PAN sheets. After a twisting process, thermoelectric yarns can be formed, as shown in **Figure 5**. The highly porous structure of the electrospun yarns are quite resistant to mechanical damage, which enables the yarns to be knitted and woven into fabrics without the significant changes in their electrical conductivity and thermopowers.

**Figure 4.** (a) Yarn coating process and (b) polyester yarn coated with waterborne polyurethane thermoelectric composites made by Jinlian Hu [15].

**3.1. Two-dimensional textile TEGs**

with an internal resistance of 1270 Ω.

output Pmax of ~12 nW.

maximum output power Pmax could achieve 12.29 nW.

A 2D flat TEG structure is also adopted in the first generation of textile TEGs. In 2012, C. A. Hewitt et al. developed a PVDF/CNT composite-based fabric TEG. Several n-type and p-type CNT composite films were alternatively arranged between the insulated PVDF films. The PVDF films are shorter than the CNT composite films. Thus, n-type and p-type CNT composites could form p-n junctions to connect the generator legs by hot press the stacked films, and resemble a felt fabric TEG [17]. The schematic structure of they prepared fabric TEG is shown in **Figure 6**. The prepared TEG composed of 72 layers fabric could generate power about 137 nW

Thermoelectric Textile Materials

29

http://dx.doi.org/10.5772/intechopen.75474

In 2015, Yong Du et al. prepared a 2D fabric TEG by using PEDOT:PSS coated polyester fabric strips [12]. To fabricate the TEG, a whole PEDOT:PSS coated polyester fabric was cut into several strips first. Then, these strips were further adhered on a polyester fabric substrate by silver paint and connected in series with silver wires. Thus, a fabric TEG only composed of p-type materials can be prepared. The TEG arrangement is shown in **Figure 7**. Temperature difference will generate along the fabric length direction. The fabric TEG prototype consisting of five fabric strips could generate 4.3 mV when temperature difference ΔT is 75.2 K. The

The same arrangement can also be achieved by thermoelectric yarns. In 2017, Ryan et al. reported a highly durable thermoelectric silk yarn made by dyeing with PEDOT:PSS, which could be produced with a length of up to 40 m and keep steady after repeated machine washing and drying [18]. Then, they embroidered these yarns into a felted wool fabric substrate and connected them end-to-end with silver wires to form a fabric TEG. The structure is illustrated in **Figure 8**. In an in-plane fabric TEG prototype composed of 26 yarn legs, the internal resistance is about 8.7 KΩ, and the output voltage of Vout/ΔT is about 313 μV K−1 when temperature difference ΔT is about 120°C. An output current of 1.25 μA can be observed when ΔT is 66°C, and resulted a maximum power

**Figure 6.** (a) Alternative arranged multilayer fabric TEG structure and (b) fabric film TEG prepared by Hewitt et al. [17].

**Figure 5.** Schematic illustration of the conversion of thermoelectric sheet into a yarn.

The studies of yarn and fiber thermoelectric materials are just beginning. The same coating process of fabrics can be further used to prepare fiber and yarn thermoelectric materials. Additionally, more complicated fabrication technique such as electrospinning and twisting process can also be adopted to prepare fiber and yarn materials with better performance. It can be imagined that there will be more ingenious methods in future to fabricate high performance thermoelectric fiber or yarn materials, so that a fully textile-based thermoelectric generator can be achieved.
