WO2018146592A1

WO2018146592A1 - Reduced grapheneoxide nanomaterial coated cotton fabric as a heating device and method therefore

Info

Publication number: WO2018146592A1
Application number: PCT/IB2018/050721
Authority: WO
Inventors: Nagarjuna NEELLA; Vaishakh KEDAMBAIMOOLE; Rajanna K; Nayak M. M; Dinesh N. S.
Original assignee: Indian Institute Of Science
Priority date: 2017-02-09
Filing date: 2018-02-06
Publication date: 2018-08-16

Abstract

The invention relates to a heating device comprising a fabric impregnated or coated with reduced graphene oxide nanosheets films and method of fabrication thereof.

Description

TITLE: REDUCED GRAPHENEOXIDE NANOMATERIAL COATED COTTON FABRIC AS A HEATING DEVICE AND METHOD THEREFORE

Field of invention

The present invention relates to the field of heating devices. In particular the invention relates to reduced graphene oxide nanosheets based heating devices. More specifically, the invention relates to reduced graphene oxide nanosheets based heating device which can be used as disposable thermal body warmers, wherein film of reduced graphene oxide nanosheets are coated on fabrics to fabricate thermal body warmer for heat generating applications and a method of fabrication thereof.

Background of invention

Body warmers are generally used to combat cold weather conditions in chilly areas. They also find use in biomedical fields such as in electrotherapy treatments, medical blankets for patients in order to maintain their body temperatures; also in military jackets worn by soldiers in the defence forces and the like. However, these products are bulky, expensive, with low flexibility and huge power consumptions. There is a constant lookout for devices with effective body warming functionality, compatible size as per requirement, flexible, less energy consumption along with low manufacturing costs. In the recent past, micro and nano systems are known to play a key role in the development of miniaturized electronic devices in various applications such as chemical sensing, health care, wearable electronics, biomedical applications and the like. They also find use in heating elements designed for creating flexible electronics, flexible heaters and nanomaterials for garments. Currently textiles are found to be flexible platforms for the electronic devices in the area of stretchable or bendable and wearable electronics due to their unique properties such as large surface to volume ratio and low dimensionality. These materials are aimed to overcome the drawbacks of opacity, bulkiness, rigidity, low heating efficiency and the like. US 4722860 describes a flexible, electrically conducting cloth comprising a plurality of intermingled or interwoven fibres of a refractory material and a conducting coating encapsulating a majority of the fibres; US6501056 describes carbon heating elements comprising carbon material wherein the carbon materia! is selected from the group consisting of carbon fiber, carbon fiber cloth, a wood carbon material, a carbon rod and a shaped article of carbon powder and methods of manufacturing the elements; and US 13/355,212 provides information related to the electrically conductive and radio frequency (RF) transparent films that include a graphene layer (or multilayer) and a substrate associated with the graphene layer. The products of said disclosure are not user friendly due to heavy weight and rigidity.

Lin X, Qin.Z, Dou. Z, Liu. N, Chen. Land Zhu in RSC Advances, 2014; 4(45):23869-75. M, titled "Fabricating conductive poly (ethylene terephthalate) nonwoven fabrics using an aqueous dispersion of reduced graphene oxide as a sheet dye stuff". The method is tedious and involve multiple steps and reagents rendering the invention expensive for adoption.

Commercial film heaters made from stripes of Fe-Cr-Al (Kanthal) and CuNi (Cupronickel) based alloy, ITO and Ga doped Zinc Oxide have many disadvantages, as its fabrication technology process is complicated, associated withopacity, heavy weight, rigidity, intolerance to acid or base, fragile under mechanical deformation and lower heating efficiency.

The present invention benefits over the conventional heating systems, where the invention provides for a heating device that is corrosion free, flexible in terms of bendability, that can be formed into required shapes for minimizing the device size, biocompatible, with minimum energy consumption and low manufacturing cost.

Summary of invention

Accordingly, the present invention relates to a disposable thermal body warmer comprising a fabric integrated with a film of reduced graphene oxide nanosheets, encapsulated with parylene coatingwherein the thickness of film ranging about 200 + 10 μιη. The invention is also in relation to a method of fabrication of film of reduced graphene oxide nanosheets on the fabric to obtain thermal body warmer.

Brief description of figures

The features of the present invention can be understood in detail with the aid of appended figures. It is to be noted however, that the appended figures illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope for the invention.

Figure 1 : provides a schematic view of reduced Graphene oxide nanosheets coated cotton cloth based thermal body warmer.

Figure 2: provides a photograph of the fabricated reduced Graphene oxide coated cotton cloth based thermal body warmer.

Figure 3: illustrates XRD phases of Graphene oxide and reduced Graphene oxide nanosheets coated cotton cloth based films. Figure 4: shows FE-SEM images: (a) Bare textile cotton cloth with pore sizes of vertical and horizontal dimensions at lower magnification, (b) the same dimensions at higher magnification.

Figure 5: shows FE-SEM images of cross sectional view of reduced Graphene oxide coated cotton cloth films:(a) the thickness of singlemulti -layered reduced Graphene oxide nanosheets, (b) surface view in the reduced Graphene oxide nanosheets coated on the textile cotton cloth, (c) the textile cotton cloth fibres diameter with pores filled with reduced Graphene oxide nanosheets, (d) the thickness of reduced Graphene oxide nanosheets coated on the textile cotton cloth based filmsfor heating element.

Figure 6 shows FE-SEM images of cross sectional view of parylene coated complete heating element device: (a) the thickness of single multi-layered reduced Graphene oxide nanosheets decorated polymer parylene, (b) side view of thickness of reduced Graphene oxide nanosheets coated on the textile cotton cloth after parylene coating.

Figure 7: shows Raman Spectroscopy of (a) Graphene oxide sheets, (b) reduced Graphene oxide nanosheets.

Figure 8: discloses the complete experimental set up for testing of reduced Graphene oxide nanosheets coated on the cotton cloth based films for heating element.

Figure 9: provides for Temperature versus applied Voltage plot, (a) One minute stabilization for five times repeatability, (b) Five minutes stabilization for five times repeatability, (c) Under Vacuum five minutes stabilization for four time repeatability.

Figure 10:shows Resistance profiles versus the applied Voltage, (a) One minute stabilization for five times repeatability, (b) Five minutes stabilization for five times repeatability, (c) Under Vacuum five minutes stabilization for four time repeatability. Figure 11: shows the average normalized resistance change of reduced Graphene oxide versus temperature generation with the applied voltage, (a) One minute stabilization time, (b) Five minutes stabilization time and (c) Under Vacuum five minutes stabilization time.

Figure 12: provides for Temperature vs time plot for the aqueous reduced Graphene oxide nanosheets coated cotton cloth based films due to different applied voltages under atmosphere condition, (a) Temperature rise due to applied voltages 30V and 40V in heating condition, (b) Temperature drop when applied voltages are switched off (30V and 40V in cooling condition).

Figure 13: illustrates temperature verses Voltage performance of cotton cloth heating element at different ambient temperatures.

Detailed description of invention

The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the figures, description and claims. It may further be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art.

The present invention is in relation to a heating device (A); comprising a fabric (1) integrated with a film of reduced graphene oxide nanosheets (2), encapsulated in parylene coating (3) ; wherein the thickness of film is ranging from about 190 μιη to about 210 μιη and size of nanosheets is ranging from about 250 nm to about 550 nm; wherein the films are connected to electrical leads (4) through metal electrodes (5); and a monitor (6) of heat comprising standard thin film based RTD Pt 100 attached to bottom of the fabric (1).

In an embodiment of the present invention the heating device is a disposable thermal body warmer.

In still another embodiment of the present invention, the fabric is selected from a range of natural and synthetic fibres such as cotton, jute flax, polyester, acrylic, nylon and spandex; preferably cotton.

In still another embodiment of the present invention, the electrical leads is of a conductor selected from a group comprising conductors of metallic conductor comprising copper, aluminium, silver and gold; organic conductor, Tetrathiafulvalene (TTF), tetramethyltetrathiafulvalene (TMTSF) and bisethylenedithio-tetrathiafulvalene (BEDT- TTF); preferably copper.

In still another embodiment of the present invention, the metal for the electrode is selected from a group comprising silver, gold and platinum; preferably silver.

In still another embodiment of the present invention, the device provides heat to raise the temperature from about -35°C to about 20°C by input voltage of about 60V.

The present invention is also in relation to a method of fabrication heating device (A) of present invention, said method comprising acts of i) preparing solution of reduced graphene oxide nanosheets, comprising acts of a) oxidising graphite powder using potassium permanganate, hydrogen peroxide and deionized water in presence of sulphuric acid to obtain graphite oxide;

purifying the graphite oxide using hydrochloric acid and deionized water; c) exfoliating the graphite oxide to obtain graphene oxide sheets; d) grinding the graphene oxide sheets to a powder , diluting it with de -ionized water and ultra- sonicating the diluted mixture to obtain mixture containing nano sheets; and

e) reducing the mixture containing nanosheets with hydrazine hydrate solution to obtain reduced graphene oxide nanosheets;

ii) preparing and coating the reduced graphene oxide nanosheets solution on a patterned substrate integrated on fabric by dip coating method;

iii) annealing the coating of reduced graphene oxide solution at a temperature ranging from about 75°C to about 85°C, preferably 80°C to obtain a film wherein thickness of film is about 190μιη to about 210 μιη; preferably 200μιη on the fabric;

iv) encapsulating the fabric coated with reduced graphene oxide in Parylene of about thickness of ~2 μιη and annealing at about 80°C for about 1 hr.

v) providing electrical leads through an electrode to obtain the heating device.

In still another embodiment of the present invention, the reduced graphene oxide solution is prepared in an organic solvent selected from a group comprising N- Methyl-2-pyrrolidone (NMP), Dimethyl formamide (DMF), Tetrahydrofuran (THF), acetone and water , preferably N-Methyl-2-pyrrolidone.

The present invention is also in relation to heating device (B) comprising one or more heating device (A) of present invention.

The present invention relates to reduced graphene oxide nanosheets coated as film on a fabric; to be used as disposable thermal body warmers for heating applications . The film is encapsulated in Parylene (thickness of ~ 2 μιη) coating as it provides complete protection from moisture, dust, protects films from peel off while handling the device. Parylene coating gives complete conformal, uniform thickness as well as makes the film pinhole free.

The reduced grapheme oxide is obtained by modified hummer's method as described below, typically the thickness of reduced graphene oxide is ranging about 300 + 50 nm to about 500 + 50 nm ( the dimensions of the reduced grapheme oxide, nano particle size/dimensions before dispersion in NMP and coating).

The present invention, heating device (A); comprising a fabric (1) integrated with a film of reduced graphene oxide nanosheets (2), encapsulated in parylene coating (3); wherein the thickness of film ranging about 200 + 10 μιη and the size of nanosheets ranging from about 300 + 50 nm to about 500 + 50 nm were formed. Nanosheets based films connected to electrical leads (4) through metal electrodes (5) is schematically represented in figure 1 and 2. The temperature generation has been monitored by using standard thin film based RTD Pt 100 mounted or attached on the bottom of the cotton cloth (6).

The fabric is selected from a range of natural and synthetic fibres such as cotton, jute flax, polyester, acrylic, nylon and spandex; preferably cotton (Figure 4). The method of preparation of the film typically involve steps of synthesis of graphene oxide sheets; reduction of graphene oxide nanosheets using hydrazine hydrate solution; preparing the reduced graphene oxide nanosheets solution by dispersing reduced graphene oxide nanosheets and N-Methyl-2-Pyrrolidone solution. Dip coating a patterned substrate integrated in a fabric using the graphene oxide nanosheets solution for use as body warmer or any other heating applications.

Figure 1(a) & 1(b) shows the schematic representation and photograph image of fabricated reduced graphene oxide nanosheets (RGO) based film coated on a fabric of body warmers. After the fabrication of the RGO nanosheets based cotton coated fabric, its response to various parameters such as temperature, resistance change with respect to the applied voltage variations are studied and the typical responses of saturation temperature obtained are shown in figures 9 to 12.

Experimental

A] The process of synthesis of the film of reduced graphene oxide nanosheets asfollows- i) Synthesis of Graphene Oxide (GO):

2g of the graphite powder is oxidized in a strong acidic environment using 54ml of H₂SO₄ through constant magnetic stirring, at a temperature maintained about 25 ± 10°C. About 6g of KMn0₄ is slowly added to the solution over a period of about 20 minutes. The concentrated solution is then rigorously stirred for 40 min. 100 ml of deionized water is added to the solution in a dropwise manner for a period of about 10-15 min. As fumes evolve from the solution, stirring is continued for another 90 minutes. 200ml of deionized water is then added to the solution. The oxidation process is completed upon adding 30ml of hydrogen peroxide (H₂O₂) to the solution. After few minutes, it is observed that the colour of the solution mixture changes from black to reddish brown indicating the end of the reaction process. The mixture is washed two to three times with Hydrochloric acid (HC1) solution and deionized water, in order to remove metal ions and un- oxidized graphite from the reaction mixture. The mixture is filtered through Whatman filter paper by using vacuum filtration method to separate the unreacted components from the mixture and the filtration cake residue is collected at the end of the process. Finally the thus obtained Graphene oxide (GO) powder is annealed at 90°C for 8 hours. ii) Synthesis of RGO nanosheets:

The Graphene oxide powder is diluted with de-ionized water taken in a weight ratio of 1:1. 0.3g (1 mgmr¹) of Graphene oxide is dissolved in 300 ml of deionized water. The diluted suspension is ultra- sonicated (the operating frequency is about 33 K Hz + 3 %) for 90 min. The flakes of Graphene oxide are split into the individual nano sheets. The entire mixture is reduced using reducing agent, that is, O. lg (3ml) Hydrazine hydrate solution is added to the mixture under constant stirring at 95°C for 4 hours. The mixture is filtered through Whatman filter paper by using vacuum filtration method to separate the unreacted components from the mixture and the filtration cake residue is collected at the end of the process. Finally reduced graphene oxide nanosheets are annealed at 80°C for 2 hours.

Hi) Preparation of RGO nanosheets solution:

The RGO nanosheets solution is prepared by dispersing RGO nanosheets and N-Methyl-2- Pyrrolidone (NMP) solution. The mixed solution is ultra-sonicatedin order to achieve a homogenous exfoliationof the RGO nanosheets. Subsequently, this composition is used for the fabrication of resistive heating elements for wearable thermal body warmers.

B) XRD studies of GO and RGO nanosheets (Figure 3)

X-ray diffraction (XRD) patterns are recorded on the synthesized GO nanosheets and RGOnanosheetsfor their structural analysis. The existence of XRD peak at 2Θ =25.61°, corresponding to the (002) plane of crystalline nature of GO nanosheets, which arises from stack of graphene layers before exfoliation of graphite is observed. The same peak is shifted to 11.2°, corresponding to the (001) plane after oxidation treatment, indicating that the interlayer spacing in the Graphite Oxide increases. The interlayer distance is increased in the GO nanosheets due to the presence of the epoxide, carboxyl groups and water molecules between the graphene oxides layers. The diffraction peak at 42.82° corresponding to the (100) plane, represents the reformation of graphite microcrystals nature in the GO system. In the RGO, after chemical reduction, nearly all peaks disappears indicating exfoliation of multilayers during the chemical reduction process of GO. However, towards the end of the reaction a wide diffraction peak appears at 23.5° which corresponds to an interlayer spacing of 0.38 nm as shown in Fig (c). The RGO nanosheets self-assemble due to the reduction reaction. The shoulder peak appearing at 42.82° which is fingerprint peak for graphite due to (100) diffraction indicates that the reformation of graphite microcrystals on reduced graphene oxide plane is because of chemical reduction of the Graphene Oxide.

C) FE-SEM Surface Morphology studies of bare Cotton fabric, RGO coated cotton cloth and Parylene coated RGO nanosheets-cloth system (Figure 4, Figure 5 and Figure 6)

The advanced characterized tools are used for surface morphology analysis of textile cotton cloth pieces, as-synthesized GO sheets and RGO nanosheets coated on the cotton cloth based films by field emission- scanning electron microscopy (FE-SEM (Carl Zeiss), ULTRA 55). Fig.4 (a) & (b) shows the pores nature of bare textile cotton cloth having dimensions around 159 μηι &115 μιη for vertical and horizontal positions separated with cotton fibers are arranged in the bundle structure. Fig.5 (a) - (d) shows the RGO nanosheets were used to fill the porous nature of the cotton cloth films for making functional conduction structure. The single RGO nanosheets having the thickness 300 nm to 500 nm as can be observed on the RGO coated cotton cloth as shown in fig.5 (a) - (d). From the cross sectional images , thickness of each cotton fiber in the bundle structure is around 13 to 15 μιη and the complete RGO coated on the cotton cloth based films thickness is around 202 μιη. Also, the prepared RGO nanosheets surface morphology confirms the loosely bound sheet like structure. The RGO nanosheets are homogeneously / uniformly coated onto the cotton cloth and intercalated RGO nanosheets are expected for the insulating cloth becomes electrically conductive. In the textile cotton cloth system, RGO nanosheets form conductive path between adjacent bundles of cotton fibre structure and plays an important role in thermal heat generation.

In order to avoid the RGO nanosheets peel off from the cotton cloth, the electrical leads taken cotton cloth was completely packaged with parylene coating thickness of around 2 μιη. From fig. 6 (a) & (b) shows the cross sectional view of the parylene coated RGO nanosheets are having dimensions of around 2.6 to 2.7 μιη .The completely packaged RGO coated cotton cloth based films have thickness around 204 to 206 μιη. Therefore, the fully packaged RGO coated cotton cloth based films are used as heating elements. D] Spectroscopic studies to depict the structural characteristics and properties of graphene based materials (Figure 7)

The spectrum of GO and RGO nanosheets are shown in Fig.7, which depicts the existence of the D, G and 2D bands. In the GO, the demonstration of two sharp peaks corresponding to D- band (1347.22 cm^"1) and G- band (1585.42 cm^"1), whichindicate the presence of structural defects in GO as shown in Fig.7.The G line is usually assigned to the first order scattering of the E_2g phonon vibration mode of sp bonded C atoms and the D line is the breathing mode of the K-point phonons of A_lg Symmetry. The 2D band (2717.17 cm^"1) originates from second order double resonant Raman Scattering. The peak position of 2D band is similar to the monolayer graphene prepared from the mechanical cleavage method. The intensity of 2D- band is sensitive to doping of graphene by either holes or electrons. In GO, G-band is located at (1585.42 cm^"1), while forreduced graphene oxide (RGO), the G-band moves (1581.66 cm^" ^l) which is closer to the value of the pristine graphite and confirms the reduction of the GO during chemical treatment. However, the existence of the D band at (1347.22 cm^"1) and (1340.28 cm^"1) corresponding to the GO and RGO also predict the defects are presented in the sample. The peak position of 2D (2703.33 cm^"1) represents graphitic nature in the RGO nanosheets system.

E] Method of fabrication of the film of reduced graphene oxide nanosheets on the fabric to obtain disposable body warmer (Figure 5 and Figure 6)

An active area of the structural device of dimensions 18 mm x 6 mm, is integrated on the textile cotton cloth measuring 30 mm x 6 mm x0.175 mm. The structural pattern is fabricated from aluminium thick sheet / block of dimensions 30 mm x 17 mm x 28 mm. The RGO nanocomposite solution is used to achieve the patterns of the developed micro mold structure on textile cotton cloth substrate by dip coating method. The RGO nanosheets coated on cotton cloth based film is kept at 80°C temperature for 60 min. The thickness of active area of RGO coated cotton cloth varies depending on the number of dipping times. The electrical leads are taken out with thin double enamelled copper wires (70 μιη) using silver paste on the top side at different respective locations of the nanosheets patterned/coated on cotton cloth films. Also, the fully fabricated sensing film is annealed at 90° C in 30 min for the purpose of curing of the electrical contacts and making the device robust.

Furthermore, the fabricated device is finally encapsulated with Parylene coating of thickness measuring 2 μιη in order to protect from the environmental conditions and also peel off. The purpose of Parylene coating onto the device; it provides complete protection from the moisture, dust, to avoid the films from peel off while handling and scratches onto the device. Moreover, the Parylene coated devices are annealed at 80°C for 1 hr in order to obtain a uniform film by rearrangement of atoms for good stability. F] The heating performance studies of RGO nanosheets coated on cotton cloth based films or body warmer applications is explained below:

The schematic view of the complete experimental set up is shown in Fig.8. The electrical leads of the RGO nanosheets coated cotton cloth based films for heating elements are connected to a 6 ½ digitalmultimeter (Gwinstek GDM-8261), power supply (Gwinstek GPD- 23038) and resistance temperature detector (RTD) to digital multimeter. DC power is applied to the conductive RGO films deposited on cotton cloth for temperature / heat generation. The electro thermal performance investigated under ambient conditions under three different cases are demonstrated here. The conductive cotton cloth is subjected to electrical power supply for one minute, five minutes and under vacuum five minutes. The surface temperature of the cotton cloth increases over time until a steady temperature is reached. The increase in temperature with applied voltage is repeated cycles as shown in Fig 9. In the first case, the surface heating performance of the cotton cloth in one minute duration shows steady temperature of 52°C with supply voltage of 40 V as shown in Fig. 9 (a). Fig.9 (b) shows second case for five minutes, a saturation temperature of 56° C is reached with a shift of 4°C from the first case. In the final case, in vacuum, a saturation temperature of 62° C is reached at the same operating power of 40V for five minutes and there is a shift of 6° C as shown in figure 9 (c). The RGO nanosheets coated cotton cloth based films consumes less power around 476 mW, 498 mW, 575 mW in one minute, five minutes and five minutes in vacuum at an operating voltage of 40V. Higher temperature generation (due to the no convention losses) can be obtained in vacuum condition. It is observed that heating limit for the device is 120° C, that is, when the cloth starts burning.

As shown in figure 10, similar results are seen in the resistance versus applied voltage. The surface temperature of the film, when charged with the electric power, causes a monotonic decrease in the film resistance with respect to applied voltage. In the similar way, we can see the response of the film resistance in three cases such as for one minute, five minute with five times repeatability and under vacuum conditions with a four times repeatability profiles as shown in figure 10 (a)- (c).

The relative resistance change remains same with respect to the heat generation of the surface in ambient for one minute, five minutes and under vacuum conditions as shown in figure 11 (a)- (c).

It is observed that, sufficient heat is generated to warm up the body under relatively low applied voltages. The electro thermal performance for RGO coated cotton cloth based films with a valid heating area of 18 mm x 6 mm under ambient conditions are as follows. When a voltage is applied, temperature rises exponentially over time around at maximum 3 minutes reaching a steady value. Similarly, when voltage supply is turned off, temperature drops exponentially around at maximum 3 minutes to reach a steady value. Change in temperature is recorded using a digital oscilloscope in terms of voltage reading. The temperature rise and drop are plotted with respect to time for two different voltages 30V and 40V as shown in Fig.l2 (a) and Fig. l2 (b).

When heating element device ambient goes down to the environmental conditions (around 20 °C) to lower side, say for example between, - 15 °C to -35 °C, higher voltage is required to maintain at 20 °C. The typical performance of the fabricated cloth heater at different ambient temperatures is shown in Figure 13. It clearly shows that when the ambient temperature is at 0 °C, in order to maintain the body/heater temperature at 25 °C, it requires 30 V as input supply. It consumes around 198 mW of power to generate the required temperature / heat. In the same way, when the heating element ambient temperature is -25 °C, it requires an input supply of 60 V (around 737 mW) to maintain sufficient required warming. By comparing the above conditions of the heating element performance, it is evident that a moderate input voltage supply will be sufficient to maintain the needed comfortable temperature of 20 °C.

The detailed performance data of RGO on cotton cloth at different ambient temperatures are shown in the following table. Power consumption at different ambient temperatures to maintain the heating element temperature at 20 °C is shown in the last column of the table 1.

Table 1: Power consumption at different temperatures

It has been observed that, the distribution of heat may be attributed to the thermal and electrical conductivity properties of the reduced graphene oxide, dip coated on a fabric as a uniform film. It is also noted that, the size of the films fabricated using solution based dip coating process is not limited to the small areas of fabric material, but is applicable to large active surface areas as well.

Due to electro thermal properties, RGO coated cotton cloth based films have profound applications. The tests conducted demonstrate that reduced graphene oxide nanosheets based films coated on fabric function as electrically conductive layers which exhibit properties of adhesion, robustness, flexibility and durability for next generation's heating elements using textiles and electronic devices. The table 2 below provides the characteristics of a typical example of a disposable body warmer of the present invention.

Table 2: Characteristics of a typical example of a body warmer.

A heating device (B) can be fabricated appropriately with one or more heating devices typically explained in Table 2 to obtain heating device of higher capacity and larger area. Although the present invention is aimed at providing a heating device which can be specifically used as a disposable thermal body warmer, it is evident that the invention can be used where heating is required by customizing the flexible heating device for example as liquid, food warmer bags, electrotherapy treatment, medical blankets for patients to maintain their body temperature and also jackets for soldiers in the defence applications and the like. The aforesaid description is enabled to capture the nature of the invention. It is to be noted however that the aforesaid description and the appended figures illustrate only a typical embodiment of the invention and therefore not to be considered limiting of its scope for the invention may admit other equally effective embodiments.

It is an object of the appended claims to cover all such variations and modifications as can come within the true spirit and scope of the invention.

Claims

We Claim:

1. A heating device (A); comprising a fabric (1) integrated with a film of reduced graphene oxide nanosheets (2), encapsulated in parylene coating (3) ; wherein the thickness of film is ranging from about 190 μπι to about 210 μπι and size of nanosheets is ranging from about 250 nm to about 550 nm; wherein the films are connected to electrical leads (4) through metal electrodes (5); and a monitor(6) of heat comprising standard thin film based RTD Pt 100 attached to bottom of the fabric (1).

2. The heating device as claimed in claim 1, wherein the heating device is a disposable thermal body warmer.

3. The heating device as claimed in claim 1, wherein the fabric is selected from a range of natural and synthetic fibres such as cotton, jute flax, polyester, acrylic, nylon and spandex; preferably cotton.

4. The heating device as claimed in claim 1, wherein the electrical leads is of a conductor selected from a group comprising conductors of metallic conductor comprising copper, aluminium, silver and gold; organic conductor, Tetrathiafulvalene (TTF), tetramethyltetrathiafulvalene (TMTSF) and bisethylenedithio-tetrathiafulvalene (BEDT- TTF); preferably copper.

5. The heating device as claimed in claim 1, wherein the metal for the electrode is selected from a group comprising silver, gold and platinum; preferably silver.

6. The heating device as claimed in claim 1, wherein the device provides heat to raise the temperature from about -35°C to about 20°C by input voltage of about 60V.

7. A method of fabrication heating device (A) of claim 1, said method comprising acts of i) preparing solution of reduced graphene oxide nanosheets, comprising acts of a) oxidising graphite powder using potassium permanganate, hydrogen peroxide and deionized water in presence of sulphuric acid to obtain graphite oxide;

b) purifying the graphite oxide using hydrochloric acid and deionized water;

c) exfoliating the graphite oxide to obtain graphene oxide sheets;

d) grinding the graphene oxide sheets to a powder , diluting it with de -ionized water and ultra-sonicating the diluted mixture to obtain mixture containing nanosheets ; and e) reducing the mixture containing nanosheets with hydrazine hydrate solution to obtain reduced graphene oxide nanosheets;

iii) annealing the coating of reduced graphene oxide solution at a temperature ranging from about 75 °C to about 85 °C, preferably 80°C to obtain a film wherein thickness of film is about 190μπι to about 210 μπι; preferably 200μπι on the fabric;

iv) encapsulating the fabric coated with reduced graphene oxide in Parylene of about thickness of ~2 μπι and annealing at about 80°C for about 1 hr.

The method as claimed in claim 7 wherein the reduced graphene oxide solution is prepared in an organic solvent selected from a group comprising N- Methyl-2-pyrrolidone (NMP), Dimethyl formamide (DMF), Tetrahydrofuran (THF), acetone and water , preferably N-Methyl-2-pyrrolidone.

A heating device (B) comprising one or more heating device (A) of claim 1.