Highly efficient CNT functionalized cotton fabrics for flexible/wearable heating applications

Abstract

In this work, a highly efficient, flexible electro thermal heater based on highly conductive carbon nanotube functionalized cotton fabrics has been studied. Cotton fabrics were functionalized with single walled carbon nanotubes through a simple dip coating method. To explore their potential as heaters, electrothermal performances of the devices were studied in terms of applied voltage, heating rate and input power density. The highly flexible heater is constructed based on uniformly interconnected CNT networks, which yields an effective and rapid heating of the heater at low input power. The investigation results suggest promising applications of these devices in wearable electronics and beyond and they could also be woven into textile materials.

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Introduction

Nowadays, textile based wearable devices have started to receive significant interest in smart energy applications. The growing interest in textile materials could be rooted to their flexibility and light weight features, which projects them as ideal candidates for wearable applications. Among the several textile materials studied so far, cotton (natural cellulose) appears to be the most commonly preferred material for electronic applications, due to its potential advantages like economic processability, and mechanical and wearable properties.^1–3 Recently, several groups have started to work on the fabrication of conductive cotton yarns for a wide range of electronic applications. The interesting aspect in conductive yarns is that they are said to preserve their original physical and comfort properties. This aspect could be capitalized for the realization of textile-based applications in healthcare, sports, military and fashion related sectors, wearable displays, bio-monitoring and power storage devices.^4–8 In this regard, strenuous efforts are also being made to integrate functional nanomaterials with conventional textiles to improve their electrical characteristics.

Carbon nanotubes (CNT) appear to be the most promising nanomaterial in textile based wearable devices, with remarkable flexibility, physical, chemical and electrical properties.^9,10 The incorporation of CNT into everyday fabrics might also yield improved opportunities for innovations in wearable electronic devices, supercapacitor and sensors.^11–16 In this regard, conductive fabrics made by incorporating CNT through a dipping-drying and spinning process have been discussed by several groups.^17–21 Additionally due its extraordinary thermal and electrical properties CNT functionalized cotton fabrics allows the design of light weight flexible heaters at low operating voltage. Also, CNT based transparent film heaters are being investigated for their applications in defrosters automobile defogging/deicing systems and heatable smart windows.^22–26 Though several researchers have examined the electrical behavior of CNT and CNT composites device under heating or cooling conditions for the transparent and conducting heating applications, limited works have been made on the electrical heating behavior of CNT functionalized cotton fabrics.^27–29

From the aforementioned perspectives, we have established a simple and facile strategy in the present study for the development of CNT functionalized cotton fabrics for flexible heating applications. Here, the fiber provides a low thermal conductivity (0.026–0.065 W mK⁻¹, superior to other synthetic and natural fibers), strong sweat/moisture absorption ability and open texture structure to enhance the convection in the present application.^30,31 The rapid thermal response in CNT is one of their unique characteristics, which we believe could aid in efficiently heating up surfaces of any size. The significance of our heaters lies with their engineering, which allows them to be designed over wide dimensions. The overall analysis on the CNT functionalized cotton fabrics strongly suggested the fabrics to inherit the intrinsic conductivity and mechanical flexibility of CNT for wearable heating needs explicitly. The constructed devices were also studied to offer the prospect for potential applications in flexible heaters, bullet-proof vests and spacesuits, if the procedures were applied selectively in a versatile manner.

Experiment

The wearable and flexible heaters were constructed through integrating the CNT with the cotton fabrics, using SWCNT dispersion. The colloid was prepared through dispersing 0.5 mg mL⁻¹ of SWCNT (prepared by arc discharge technique) in water containing 10 mg mL⁻¹ of sodium dodecyl sulfide (SDS) as the surfactant. The prepared dispersion was subjected to sonication (prior to dip coating) for a period of 1 h to procure a homogeneous dispersion. The cotton fabrics were then dip coated in the aforementioned dispersions and dried on a hot plate for 10 min at 120 °C. Similarly, ten cycles were carried out to improve the CNT adherence to the cotton fabrics. The CNT are established on the cotton fabrics through self-interconnecting mechanisms during each cycle. The simplicity and scalability of the adopted experimental procedure ensures the fabrication of CNT based heaters, without any complicated setups. The morphologies of CNT functionalized cotton fabrics were investigated using scanning electron microscopy (SEM XL-30 Philips). The CNT based cotton heaters with short and long dimensions (1 × 2 and 2 × 4 cm) were made in two terminal side contact configuration. The resistance and heating performance of the heaters was evaluated through I–V measurements (Keithley 617 semiconductor parameter analyzer) and a thermocouple (heating performance was studied under various applied potentials).

Results and discussions

Fig. 1a shows the schematic illustration of the fabrication procedures involved in the functionalization of cotton fabrics using CNT. A true representation of the CNT functionalized single cotton thread and large area bundles are also additionally shown in the Fig. 1b, through the photographic images. The SEM images revealing the morphological evolution of the CNT structures on the surface of cotton fabrics are shown in Fig. 1c and d. The images suggest the CNT structures to be continuously interconnected with spatial uniformity over large areas. The continuous network provides the required electrical interconnects throughout the entire network with the formation of an effective percolative network. Fig. 2a and b illustrates the outstanding flexibility observed in the CNT functionalized fabrics. The image reveals the structure to remain intact without breaking and reflects the strong binding of CNT to the fabric. This could be reasoned with the strong hydrogen bonding established between the fabrics and CNT, due to van der Waals interactions.^32,33

Fig. 1 (a) Schematic illustration showing the fabrication process for preparing flexible CNT functionalized fabrics with cotton, CNT dip coated and CNT functionalized cotton fabrics. (b) Photographs of bare cotton and CNT functionalized cotton fabrics. (c and d) SEM image of CNT functionalized cotton fabric.

Fig. 2 (a and b) shows the flexibility of the CNT functionalized cotton fabrics with single thread and large area bundles.

Fig. 3a shows the variation in resistance values across the CNT functionalized fabrics, as a function of dipping cycles. The noted significant decrease in the resistance values as a function of dipping cycle could be correlated with the increase in density of CNT structures (which keeps growing as per the number of dipping cycles). Likewise, the surface resistance of the CNT functionalized fabrics was also noted to shift dramatically from 190 to 5 kΩ on increasing the dipping cycles from one to ten, respectively. The improved conductivity values could be inferred through the enhanced interlinking of the nanotubes that could actually facilitate the generation of an excellent electrically conductive path.³⁴ Here, we would like to emphasize that the quantity of nanotubes that gets attached to the fabric is in analogous with the increasing diameter of the conducting structures and only imposes a limited effect in promoting the conductance values.

Fig. 3 (a) Resistance as a function of the number of dipping cycles. (b) (I–V) measurements of CNT functionalized cotton fabrics with short (D_S) and long (D_L) lengths.

Fig. 3b shows the current–voltage (I–V) characteristics studied for two lengths of CNT functionalized fabrics, namely short (D_S) and long (D_L) of dimensions 1 × 2 and 2 × 4 cm, respectively. Here, a linear relationship between the current and voltage values could be observed across the fabrics, with a significant variation in the resistance values (corresponding to their lengths). This suggests the electric resistance of the fabric to be directly proportional to its length. Additionally, the value of sheet resistance was found to be around 2.5 and 5 kΩ from the plots, for device D_S and D_L, correspondingly. These values clearly signify the improvements in electrical and thermally conductive pathways available for the charge flow. The heating performance of the fabrics D_S and D_L was then studied through measuring the change in temperature at the surface of heater, as a function of time. To demonstrate the potential of CNT functionalized cotton fabrics for flexible heating applications, we have constructed the heaters in D_S and D_L configurations. A schematic sketch of the two-terminal side-contact configuration is shown in Fig. 4a. The time-dependent temperature profiles were initially studied as a function of surface resistance for the constructed heaters (ESI†), under a constant applied voltage. Here, the devices possessing minimum surface resistance values were found to exhibit a maximum steady state temperature.

Fig. 4 (a) Schematic diagram of the CNT functionalized cotton heaters. (b and c) Heating experiments of CNT functionalized cotton fabrics, temperature as a function of time for long (D_L) and short (D_S) length. The inset shows the optical image of the heaters.

Fig. 4b and c show the temperature plots for the D_S and D_L configurations of CNT functionalized cotton based heaters. The plots were made as a function of the applied DC input voltages of 10, 20, 30 and 40 V, respectively. The voltages were sequentially applied to the heaters, during which the steady-state temperature was noted to increase linearly with the applied voltage. Here, the increase in temperature was noted to be very fast, irrespective of the applied voltage. Likewise, a similarity in heating rates and steady-state temperatures was also noted, irrespective of the dimension of the heaters. It takes approximately 40 s for the heater to reach the steady-state temperature in both the configurations. A saturation temperature of 25 °C was obtained for the device D_L in 10 V. This was increased to 45 °C by increasing the applied voltage to 40 V. With regard to device D_S, the steady-state temperature at 40 V was up to 96 °C. However, the two heaters revealed maximum heating rates under an applied voltage of 40 V. From the results it could be inferred that lower the resistance, higher the steady state temperature obtained. The functionalization of CNT on cotton fabrics must have reinforced and protected the fabric, to result with the generation of more heat from their overall surface. This could also be reasoned for the faster heating rates observed with homogeneous temperature distribution.³⁵

The heating mechanism taking place in the aforementioned systems could be comparatively studied with the concept of electrothermal heaters. Initially, the migration of charge carriers in the system might have got accelerated due to the application of an external electric potential. These accelerated electrons might collide in-elastically with phonons, impurities or defects present in the CNT walls, to release the observed heat.^36,37 The mean free path available for the electrons starts to reduce on increasing the applied potentials which results with increased scattering effects that are responsible for observed rise in temperature.³⁸ The observed trend additionally signifies the excellent bonding between the CNT and fabric, so that when voltage is applied the current gets distributed over the conductive layer and heat is generated uniformly. This proves that the thermal and the electrical conductivity of CNT films is a crucial factor for the present heating applications.

Fig. 5a shows the average steady-state temperature of the two heaters with respect to the applied voltage. Here, a higher steady-state temperature was observed for a fixed applied voltage. The power consumption of the CNT based fabric was also calculated using eqn (1),

P = I²R (1)

where P denotes the total power consumption, R the resistance of the operating CNT heater and I is the current passing through the CNT cotton heater. The visualization of high power at low input voltage implies the efficient transduction of electrical energy into Joule heating, which may be attributed to the excellent conductivity of the CNTs. Here, we would like to emphasize that as the heaters are not limited to their size, it is possible to realize large area heating applications also. Fig. 5b shows the heating performance of the two CNT functionalized fabrics, which reveals that low power is enough for D_S devices. This might be due to the difference in thermal diffusivity of the cotton fabric. Generally, heat disperses more rapidly in a material that possesses high thermal diffusivity value. However, in the present case, though cotton possess a lower thermal conductivity, the homogeneous integration of CNT as shell like structures (improved surface area) on the fibers might help to improve the same. The small surface area helps to dissipate the heat rapidly and leads to a higher steady-state temperature, where the heater loses its heat to the air by convection and to the cotton by conduction.

Fig. 5 (a) Steady-state temperature with respect to the applied voltage for long (D_L) and short (D_S) length. (b) Input power as a function of the steady state temperature for long and short length heaters.

The operational stability and reliability of the CNT based fabric heaters was studied as a function of time, through the aid of heat cycle test. Fig. 6 shows the temperature response of the device D_S, under a heat cycle with an on/off-ratio of 200 s. The faster heating and cooling response and the maximum steady-state temperature were found to be preserved during the test. The absence of any significant variation in temperature or decrease in heating performances illustrates the high stability of the fabric heater. Such heaters could be operated with same performance even after long storage without any precautionary measures. Finally, the excellent conductivity, lightweight, flexibility and stability related features witnessed in the present CNT based fabric heaters strongly suggest their potential applications in wearable heating/electronic devices.

Fig. 6 Temperature response of CNT functionalized fabrics under a heat cycle.

Conclusions

In summary, a CNT functionalized cotton fabrics based heater was studied for wearable, flexible heating applications. The electro thermal response of the cotton fabric heaters was verified in terms of response time and input power. The observed higher steady-state temperature at low input power (large rise in temperatures even under small voltages) could be attributed to the excellent electrical and thermal properties of CNT. The obtained results clearly demonstrates the efficiency of CNT functionalized cotton fabrics as promising candidate for low-cost wearable fabrics, flexible heaters, bullet-proof vests, radiation protection suits, and spacesuits.

Acknowledgements

This research was supported by Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (no. 2014-039452).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10667a

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