**3.2. Three-dimensional textile TEGs**

In general, 2D fabric TEG is a first attempt to apply textile structures to the design of flexible TEGs. Although these initial 2D fabric TEGs have almost the same in-plane structure with film TEGs, fabric TEGs can be easily rolled up, bent, twisted, and are permeable to air and

moisture, making them more flexible and comfort to wear.

**Figure 8.** Thermoelectric yarn arrangement in a fabric substrate.

**Figure 7.** Arrangement of 2D fabric TEG composed of only p-type materials.

30 Bringing Thermoelectricity into Reality

In addition to the initial 2D textile TEG structures, researchers also developed several 3D textile TEGs similar to the classical sandwich bulk TEG. In 2014, Kim et al. fabricated a wearable TEG on a glass fabric by screen-printing technology [19]. The device structure and is shown in **Figure 9**. The inorganic Bi2 Te3 and Sb<sup>2</sup> Te3 thermoelectric materials were printed on a glass fabric substrate first. Then, the fabric containing an array of eight thermocouples was connected by several cooper foils, and finally encapsulated with PDMS. The fabricated TEG exhibits a high output power density of 3.8 mWcm−2 and 28 mWg−1 at ΔT = 50 K. Besides, the TEG could endure repeated bending for 120 cycles.

Another research reported a silk fabric-based TEG similar to the Kim's structure in 2016. Zhisong Lu et al. deposited nanostructured Bi<sup>2</sup> Te3 and Sb<sup>2</sup> Te3 synthesized by hydrothermal method on two sides of a commercial available silk fabric [20]. The deposited p-type and n-type thermoelectric materials were further connected with silver foils to form a flexible TEG using a similar arrangement of Kim et al. [19]. The prototype containing 12 thermocouples could generate a maximum thermos-voltage of ~10 mV and output power of ~15 nw under a temperature difference of 35 K. The power output performance can sustain stable even during 100 cycles of bending and twisting.

Jae Ah Lee et al. using textile structure designed a new type of fabric TEG, which can utilize thermal energy along fabric thickness direction. Both knitting and weaving technology can be employed to fabricate this fabric TEG [16]. Several p-type and n-type yarns prepared by electro-spinning were arranged in the fabric according to the predesigned patterns. In knitted structure, p-type and n-type yarns were alternatively arranged and connected in series. In a plain weave structure, several single yarns containing metallic connected n-type and p-type components were woven into fabrics with insulating yarns. In addition, these single yarns should be carefully placed in correct way to ensure the right contact between p-n junctions and hot/cold surfaces. **Figure 10** illustrates the fabric TEG in knitted and woven structures respectively. The best output power of the prepared fabric TEG could achieve 8.56 Wm−2 at a temperature difference of 200°C. This study is the first attempt to utilize fabric structures to realize textile TEG that suitable for fabric thickness power generation.

**Figure 9.** Arrangement of fabric TEG that allowing generate temperature difference along fabric thickness direction.

used as cladding materials to form a core-shell structure fiber. The fabricated thermoelectric fibers are long, flexible, intrinsically crystalline, and mechanically stable, which can be further applied in flexible TEGs. The two pairs of p-n fibers composed TEG has an internal resistance of 410 Ω, and could generate an open-circuit voltage of 24.2 mV, a short-circuit current of 59.1 μA, and a maximum output power of 0.36 μW, when applying a temperature difference of 50 K. To demonstrate the advantages of fiber's length and flexibility, a thermoelectric cup and a thermoelectric pipe were fabricated by wrapping several pairs of p-n thermoelectric

In 2016, Abu Raihan Mohammad Siddique et al. employed a manual dispenser printing tech-

containing 12 pairs of p-n thermoelectric materials were fabricated by printing the selected thermoelectric materials on polyester fabric. The fabricated prototypes consisting 12 pairs of n-type and p-type legs and connected in series with silver wires. The best open circuit voltage and output power were 23.9 mV and 3.107 nW, respectively, under a temperature difference of 22.5°C. The wearing test on human body proves that the fabricated prototypes are very flexible, twistable, and durable with the substrate as well as conforming well to the human

In 2017, Jaeyoo Choi et al. reported a flexible and ultralight TEG-based on carbon nanotube yarn (CNTY) with excellent thermoelectric performance [24]. A high electrical conductivity of 3147 S/cm was achieved by a highly aligned CNTY, which has increased longitudinal carrier mobility. In their work, the CNTY exhibits multifunction in the TEG. N- and p-types dop-

p-type legs in the CNTY device respectively. Between the doped regions, highly conductive undoped CNTY regions were acted as electrodes to connect and minimize the circuit resistance. Thus an all-carbon TEG without additional metal deposition process can be formed. A prepared TEG prototype containing 60 pairs of n- and p-type doped CNTY exhibits the

Screen printing technology is a low-cost fabrication technique that is applicable to large scale production. Zhuo Cao et al. demonstrated that screen printing technology can be used to fabricate BiTe/SbTe-based TEGs for room temperature energy harvesting applications on flexible substrate [25]. Although the fabricated TEG contained only eight pairs of thermoelectric legs, the screen printing process and materials flexibility allow longer strips with larger numbers of thermocouples to be connected in series and rolled up to form a coil TEG and enabling the power output to be increased. Thus, it is also a practical fabrication method to prepare fabric-

Besides the thermoelectric performance of thermoelectric materials, temperature difference is also an important impact factor on the thermoelectric efficiency of TEGs. A higher △T would increase the output power of TEGs. However, the temperature difference between human body and environment are usually in the range of 1.5–4.1°C, which is unfavorable for body heat utilization. It is necessary to find ways to enlarge the temperature difference for wearable TEGs. In 2017, Yeon Soo Jung et al. proposed a novel approach to increase the △T

maximum power density of 10.85 and 697 μW/g at △T of 5 and 40 K, respectively.

(0.9Te,0.1Se)<sup>3</sup>

was used as p-type material. Two TEG prototypes

were alternatively doped into CNTY to generate n- and

was used as n-type

Thermoelectric Textile Materials

33

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

nique to fabricate a flexible fabric TEG [23]. (0.98Bi,0.02Sb)<sup>2</sup>

(0.95Te,0.05Se)<sup>3</sup>

fibers on the curved surface.

material, (0.25Bi,0.75Sb)<sup>2</sup>

ants of polyethylenimine and FeCl3

based TEGs on large scale.

body.

**Figure 10.** (a) Thermoelectric yarn arrangement in knitted fabric structure and (b) thermoelectric yarn arrangement in woven fabric.

In the following 2017, we designed a new flexible 3D fabric TEG which have a sandwich structure, so that the temperature difference could be generated along fabric thickness direction [21]. A 3D spacer fabric is composed of two separate fabric layers on the top and bottom of the fabric, and a set of pile yarns in the middle used to connected the top and bottom fabric layers. Compared with 2D materials, the sandwich 3D structure has more dimensions in thickness direction, so that the temperature difference can be sustained along the fabric thickness direction, and the flexibility of the TEG can be retained due to its fabric nature. Therefore, 3D fabric would be a good candidate as substrate for flexible and wearable TEG. **Figures 3**–**5** show the designed 3D fabric TEG.A prototype was fabricated by embroidered p-type and n-type coated polyester yarns into a 3D spacer fabric. The 3D fabric TEG consisting of 10 couples of thermoelectric yarns would generate a thermo-voltage of ~800 μV and output power of ~2.6 nW at a temperature difference of 66 K.

#### **3.3. Other textile TEGs**

There are also some new creative textile TEGs developed very recently. In 2017, Ting Zhang et al. demonstrated a novel crystalline ultralong thermoelectric fiber and two TEG prototypes [22]. Thermal drawing technology was employed to integrate high performance thermoelectric materials in a fiber-like carrier, which is also a physical thermal size-reduction strategy. P-type Bi0.5Sb1.5Te3 and n-type Bi<sup>2</sup> Se<sup>3</sup> nanowires were used as core thermoelectric materials, and borosilicate glass tube with a Tg slightly higher than the thermoelectric nanowires were used as cladding materials to form a core-shell structure fiber. The fabricated thermoelectric fibers are long, flexible, intrinsically crystalline, and mechanically stable, which can be further applied in flexible TEGs. The two pairs of p-n fibers composed TEG has an internal resistance of 410 Ω, and could generate an open-circuit voltage of 24.2 mV, a short-circuit current of 59.1 μA, and a maximum output power of 0.36 μW, when applying a temperature difference of 50 K. To demonstrate the advantages of fiber's length and flexibility, a thermoelectric cup and a thermoelectric pipe were fabricated by wrapping several pairs of p-n thermoelectric fibers on the curved surface.

In 2016, Abu Raihan Mohammad Siddique et al. employed a manual dispenser printing technique to fabricate a flexible fabric TEG [23]. (0.98Bi,0.02Sb)<sup>2</sup> (0.9Te,0.1Se)<sup>3</sup> was used as n-type material, (0.25Bi,0.75Sb)<sup>2</sup> (0.95Te,0.05Se)<sup>3</sup> was used as p-type material. Two TEG prototypes containing 12 pairs of p-n thermoelectric materials were fabricated by printing the selected thermoelectric materials on polyester fabric. The fabricated prototypes consisting 12 pairs of n-type and p-type legs and connected in series with silver wires. The best open circuit voltage and output power were 23.9 mV and 3.107 nW, respectively, under a temperature difference of 22.5°C. The wearing test on human body proves that the fabricated prototypes are very flexible, twistable, and durable with the substrate as well as conforming well to the human body.

In 2017, Jaeyoo Choi et al. reported a flexible and ultralight TEG-based on carbon nanotube yarn (CNTY) with excellent thermoelectric performance [24]. A high electrical conductivity of 3147 S/cm was achieved by a highly aligned CNTY, which has increased longitudinal carrier mobility. In their work, the CNTY exhibits multifunction in the TEG. N- and p-types dopants of polyethylenimine and FeCl3 were alternatively doped into CNTY to generate n- and p-type legs in the CNTY device respectively. Between the doped regions, highly conductive undoped CNTY regions were acted as electrodes to connect and minimize the circuit resistance. Thus an all-carbon TEG without additional metal deposition process can be formed. A prepared TEG prototype containing 60 pairs of n- and p-type doped CNTY exhibits the maximum power density of 10.85 and 697 μW/g at △T of 5 and 40 K, respectively.

In the following 2017, we designed a new flexible 3D fabric TEG which have a sandwich structure, so that the temperature difference could be generated along fabric thickness direction [21]. A 3D spacer fabric is composed of two separate fabric layers on the top and bottom of the fabric, and a set of pile yarns in the middle used to connected the top and bottom fabric layers. Compared with 2D materials, the sandwich 3D structure has more dimensions in thickness direction, so that the temperature difference can be sustained along the fabric thickness direction, and the flexibility of the TEG can be retained due to its fabric nature. Therefore, 3D fabric would be a good candidate as substrate for flexible and wearable TEG. **Figures 3**–**5** show the designed 3D fabric TEG.A prototype was fabricated by embroidered p-type and n-type coated polyester yarns into a 3D spacer fabric. The 3D fabric TEG consisting of 10 couples of thermoelectric yarns would generate a thermo-voltage of ~800 μV and output power of ~2.6 nW

**Figure 10.** (a) Thermoelectric yarn arrangement in knitted fabric structure and (b) thermoelectric yarn arrangement in

There are also some new creative textile TEGs developed very recently. In 2017, Ting Zhang et al. demonstrated a novel crystalline ultralong thermoelectric fiber and two TEG prototypes [22]. Thermal drawing technology was employed to integrate high performance thermoelectric materials in a fiber-like carrier, which is also a physical thermal size-reduction strategy.

nanowires were used as core thermoelectric materials,

slightly higher than the thermoelectric nanowires were

at a temperature difference of 66 K.

and borosilicate glass tube with a Tg

and n-type Bi<sup>2</sup>

Se<sup>3</sup>

**3.3. Other textile TEGs**

woven fabric.

32 Bringing Thermoelectricity into Reality

P-type Bi0.5Sb1.5Te3

Screen printing technology is a low-cost fabrication technique that is applicable to large scale production. Zhuo Cao et al. demonstrated that screen printing technology can be used to fabricate BiTe/SbTe-based TEGs for room temperature energy harvesting applications on flexible substrate [25]. Although the fabricated TEG contained only eight pairs of thermoelectric legs, the screen printing process and materials flexibility allow longer strips with larger numbers of thermocouples to be connected in series and rolled up to form a coil TEG and enabling the power output to be increased. Thus, it is also a practical fabrication method to prepare fabricbased TEGs on large scale.

Besides the thermoelectric performance of thermoelectric materials, temperature difference is also an important impact factor on the thermoelectric efficiency of TEGs. A higher △T would increase the output power of TEGs. However, the temperature difference between human body and environment are usually in the range of 1.5–4.1°C, which is unfavorable for body heat utilization. It is necessary to find ways to enlarge the temperature difference for wearable TEGs. In 2017, Yeon Soo Jung et al. proposed a novel approach to increase the △T of conventional wearable TEGs, they present a wearable solar TEG possessing a high △T value of ~20.9°C by introducing a local solar absorber and thermoelectric legs on a polyimide substrate [26]. The prepared solar absorber is composed of a five-period Ti/MgF<sup>2</sup> superlattice. The structure and thickness of each layer was carefully designed to absorb sunlight at maximum extent. A dispenser printing technique was employed to prepare thermoelectric legs on the substrate. The n- and p-type ink were made by alloyed BiTe-based powders and Sb<sup>2</sup> Te3 based sintering additive dispersed in glycerol. A wearable TEG prototype consisting of 10 pairs of thermoelectric legs exhibits an open circuit voltage of 55.15 mV and an output power of 4.44 μW when exposed to sunlight. The generated high △T of ~20.9°C between the hot solar absorber and cold edges is also the highest △T value of all wearable TEGs reported to date. In 2016, Melissa Hyland et al. also reported a wearable TEG device with optimized heat spreaders to increase the △T of wearable TEG [27]. The integration of heat spreader would improve the dissipation of heat and cooling throughout the wearable TEG. In their design, a three-layered device structure were used. Two flat heat spreaders were equipped on the top and bottom of TEG respectively. The sandwich structure was chosen as the final design due to its high efficiency and low form factor.

**Acknowledgements**

**Conflict of interest**

**Author details**

Qian Wu1

China

Hong Kong

**References**

14/7/11957/

1041-1054

SR-05-2014-652

ieee.org/document/6974987/

National Natural Science Foundation of China, 51673162.

There are no conflicts of interest to declare.

\*

\*Address all correspondence to: jin-lian.hu@polyu.edu.hk

and Jinlian Hu2

This work was supported by University Research Grants Council, PolyU 5162/12E, Shen Zhen Government Key Incubation Fund, JC201104210132A, National Key Technology R & D Program Project, The Ministry of Science and Technology of P.R.C, 2012BAI17B06, and

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

1 School of Textile Science and Engineering, Xi'an Polytechnic University, Xi'an, Shaanxi,

2 Institute of Textiles and Clothing, The Hong Kong Polytechnic University, Kowloon,

[1] Stoppa M, Chiolerio A. Wearable electronics and smart textiles: A critical review. Sensors [Internet]. Jul 2014;**14**(7):11957-11992. Available from: http://www.mdpi.com/1424-8220/

[2] Mukhopadhyay SC. Wearable sensors for human activity monitoring: A review. IEEE Sensors Journal [Internet]. Dec 9, 2014;**15**(3):1321-1330. Available from: http://ieeexplore.

[3] Shaikh FK, Zeadally S. Energy harvesting in wireless sensor networks: A comprehensive review. Renewable and Sustainable Energy Reviews (Elsevier). Mar 1, 2016;**55**(C):

[4] Cha Y, Hong S. Energy harvesting from walking motion of a humanoid robot using a piezoelectric composite. Smart Materials and Structures (IOP Publishing). Sep 3, 2016;**25**(10):1-5

[5] Bogue R. Energy harvesting: A review of recent developments. Sensor Review [Internet]. Jan 19, 2015;**35**(1):1-5. Available from: http://www.emeraldinsight.com/doi/abs/10.1108/

### **4. Conclusion**

With the rapid growth in wearable and flexible electronics, the demand in flexible self-power technologies are also increased. Thermoelectric energy conversion technology shows great potential in make use of our human body heat to generate power, which would be an ideal power source candidate for wearable electronic systems. In most of the developed flexible thermoelectric materials and generators, textile-based thermoelectric materials and TEG have unique advantage in body heat energy conversion due to their excellent air permeability, flexibility, and wearing comfort, especially for natural materials such as silk and cotton, which shows good biocompatibility. Therefore, a fabric-based thermoelectric generator with great wearability would overcome the wearable difficulty of existing organic film generators, which shows promising application in flexible and wearable self-powered electronic systems by harvesting body heat to generate electricity. In the past years of study, many textile thermoelectric materials were prepared in forms of fibers and fabrics including some traditional inorganic semiconductors and the newly developed organic polymers and composites. Compared with 2D fabrics, fiber-based thermoelectric legs would give more design flexibility for the flexible TEGs. Besides, some pioneering researches also designed and fabricated several novel textile TEGs with excellent flexibility and thermoelectric performance. Various TEG structures such as 2D generator making use of in-plane temperature difference and 3D generators generating temperature difference along fabric thickness directions are included. These textile-based TEGs with multiple structures are more practical and suitable for wearing than the previous widely studied film TEG, which shows promising application in future self-powered wearable system driven by body heat. The study of flexible and wearable thermoelectric materials and generators is just beginning. The efficiency of textile thermoelectric materials and generator need to be further improved for real application. These creative works bring many inspirations for the future explorations in thermoelectric field.
