Conductive textiles losses

Figure 10.16 (a) E-fiber TL with polymer encapsulation to prevent weathering and corrosion of conductive textile surface (b) increased loss in a 5-cm E-fiber TL after a year with no protection of textile surface and (c) comparison of S21 of unprotected and encapsulated 5-cm E-fiber TLs. 

A microstrip patch is a representative candidate for a wearable integration, because it can be thin, lightweight, low maintenance, robust, and easily integrated into a garment and coupled with RF circuits (Wang et al., 2012). Moreover, the conductive textile used for antenna purposes has to have a low and stable electrical resistivity (<10/sq.) to minimize losses (Locher et al., 2006). Several properties of the materials can influence the behavior of the antenna properties. For instance, the permittivity and the thickness of the substrate change the bandwidth and the efficiency of a planar microstrip antenna (Liu et al., 2011). In general, fabrics present a complex structure, in term of density of fibers and hence air volume and size of the pores, which allow a very low dielectric constant with a reduction of the surface wave losses and an increase of the impedance bandwidth. 

The guarded hot plate is a standard instrument for measuring the relative thermal resistance of textiles as heat flows from a heated plate in contact with the textile and dissipates into still air at a lower ambient temperature via radiation, conduction, and convection. By design, it minimizes errors due to edge heat losses and validates the total quantity of heat flowing through the specimens. Convection and surface radiation can be controlled by use of a hood (2j+). Simpler devices such as the Reeves warmth tester and a chamois-covered copper cylinder also measure thermal... 

In order to determine sensitivity of the textile heat fluxmeters, the conductive heat flux, which is constant at a steady state through aU of the elements of the unidirectional (z) thermodynamic system, was used (Eq. [19.9]). It was considered that the loss of the energy to the axes x and y is insignificant. 

Efficiency to maximize the radiated power, the antenna designer wiU aim at a large radiation efficiency. As previously discussed, the total efficiency is the product of two terms. The first one, the impedance mismatch factor M, can he maximized by minimizing the reflection coefficient. The second term, the conductive-dielectric efficiency Ced can be maximized by using textile materials with low ohmic and dielectric losses, ie, electrotextiles with high conductivity and textile dielectric substrates with low tan 6. 

The above results demonstrate the remarkably high conductivity of the flexible embroidered textiles and low loss of the flexible PDMS substrate at RF. As expected, loss increases with frequency. But of more importance is the fact that the overall loss of E-fibers was very low. We remark that the embroidered E-fiber textiles sustained their RF performance and structural integrity after repetitive flexing (Wang et al., 2012b). These characteristics are highly attractive for wearable RF applications. 

Generally speaking, denser embroidery is preferred, as it reduces potential physical discontinuities in the textile surfaces. Concurrently, conductivity is increased. However, in practice, denser embroidery is more challenging to carry out and may result in sewing needle breakage. Therefore, the embroidery density is optimized to achieve a compromise between high-quality embroidery and feasible fabrication. As shown in Table 10.2, a low loss of 0.2 dB in insertion loss was observed at 3 GHz for a 664-thread E-fiber TL (sample E) when the embroidery density increased from 1.6... 

Moisture accumulation. Some textile fibers (eg, cotton, wool) are hydrophilic in nature. Due to this, a fabric may absorb moisture (sweat) generated from wearers bodies and accumulate it in its structure. The thermal conductivity of moisture is much higher than the thermal conductivity of air or fibers. This may cause rapid transmission of metabolic heat from wearers bodies to the ambient environment that can result in better thermal comfort for wearers under some conditions (Barker et al., 2006 Song et al, 2011). In this context, it is necessary to remember that absorbed moisture in cotton and wool may not always increase the amount of interstitial free water inside the fabric and this situation may not cause the rapid transmission of metabolic heat from wearers bodies. Additionally, rapid loss of metabolic heat may not always lead to thermal comfort for wearers. 

They also tested the textile supercapacitor under 50% stretching and the result was a small loss in capacitance due to the breaking of the conductive network between carbon particles when changing the dimensions of the device. 

Heat transfer through textile structures comprises of (a) conduction, (b) convection, (c) radiation and, (d) evaporation [4-8], The focus of this section is on dry heat transfer. The term dry heat is used to distinguish it from heat loss by evaporation of moisture, which takes place either within the skin, or at the surface of the skin. The moisture transfer and moisture based heat transfer will be discussed in latter sections. 

In case of steady state heat conduction, the material property is the conductivity which can be calculated once the heat loss from the body is known and the boundary temperature is measured. In case of transient heat flow, the main factor is the diffusivity a which is equal to the ratio of the conductivity and the heat content of the body. Hence equation 10.8 is the basic governing equation of transient heat transfer with boundary conditions relevant for textile materials. Transient state heat conduction is related to instantaneous conduction of heat fi-om the surface of the body to the clothing. Instantaneous heat transfer can be related to the warmth or coolness to touch and the warm-cool feeling of any clothing can be quantified. 

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