Claims:

I Claim:

1. A transparent electrothermal device comprising of:
coating a substrate with both electrically and thermally conducting carbon nanotubes having atleast one or plurality of layers;
immersing the said film in a mixture of ethanol:sulfuric acid in the ratio of 10:1
for 1 hour and washing thereof;
depositing a pair of conducting contact electrodes spaced apart, along opposite edges of top surface of the carbon nanotube film as a means to supply electrical input allowing the said carbon nanotube film to conduct current and facilitate uniform heating.

2. The transparent heater as claimed in claim 1, wherein the substrate is selected from a group consisting of glass, quartz and polymer.

3. The transparent heater as claimed in claim 1, wherein the carbon nanotube film is formed through spray coating, spin coating, brush coating, inkjet printing and dip coating, preferably spray coating.

4. The transparent heater as claimed in claim 1, wherein the conducting contact electrodes is silver.

5. The transparent heater as claimed in claim 1, wherein the carbon nanotube is selected from either single walled, double walled or multi walled carbon nanotube.

6. The transparent heater as claimed in claim 1, characterized in that the said carbon nanotube film exhibiting a reduced sheet resistance.

7. The transparent heater as claimed in claim 1, characterized in that the said carbon nanotube film showing a transmittance of 81%.

8. The transparent heater as claimed in claim 1, characterized in that the said heater showing an increase in temperature of 420 – 763 K at an applied voltage of 60V, preferably 424 K.

9. A transparent defogger including a substrate having carbon nanotube coating treated in a mixture of ethanol:sulfuric acid in the ratio of 10:1 for 1 hour and conductive electrodes thereon; said defogger capable of removing frost formation at an applied voltage of 30V in 52s.

, Description:Field of invention

This invention relates to transparent electrothermal conducting films containing carbon nanotubes. This invention also relates to a room temperature treatment process to enhance the heating performance of theconducting films.

Background

Joule heaters or transparent heaters gained more interest due to their wide range of applications in displays, defoggers, temperature controllers, microchannel chips, biochips, microelectromechanical systems. Transparent heaters are made by coating the electrically conducting and optically transparent materials on the optically transparent substrates. Indium tin oxide (ITO) based transparent conducting films are mostly used for the transparent conducting films (TCF) because of its high performance (transmittance on glass >90% with sheet resistance 10 O/?), but ITO is brittle, easily cracks due to its fragile ceramic nature and require vapor phase deposition techniques. Potential alternatives for ITO in transparent heating application are silver nanowires, graphene, solution processed carbon nanotubes (CNT) and hybrid systems. Among these CNT possess excellent electrical, optical and thermal properties, which influence the electrothermal performance of the transparent heaters. Although the transmittance of the CNT based heaters can be high, the sheet resistance values are moderate in the heaters fabricated through solution process. According to the Joule’s and Ohms law, the power dissipated through the heater is inversely proportional to resistance of the heater.

Studies reporting use of CNT films in heaters have reduced sheet resistance through chemical doping. But the effect is temporary and degrades the performance of the film during aging. Nevertheless, doping of the metal structures though increases the conductivity of the films has the disadvantages such as metal degradation when exposed to corroding vapors like sulfur and self-heating effect producing discontinuity in the network.

US8426776 discloses carbon nanotube defrost windows which includes a transparent substrate, and carbon nanotube film and a protective layer.

US8581158 discloses a composition for electrically conductive coating for use in aircraft and other substrates to prevent the formation of ice or to melt ice. The conductive coating composition includes carbon nanotubes dispersed in a solvent and applied to a substrate to a thin film which is resistively heatable.

US 20090277897A1 discloses fabrication of heaters based on composite of carbon graphite, CNT and graphite epoxy material. Many prior art discloses use of acid, specifically nitric acid, to purify carbon nanotubes before using it in their inventions. However treatment of CNT with acid after anchoring on a substrate to reduce sheet resistance is scarcely reported. Both non-patent document (Manivannan et al., J Mater Sci: Mater Electron (2010) 21:72–77) and patent document KR101197336B1 describes method of fabricating transparent conducting film from carbon nanotube by performing acid alcohol treatment using propanol and nitric acid in the ratio of 3:1 to reduce sheet resistance. Nevertheless these reports focuses on the utilization of the transparent conducting film in optoelectronics such as in touch screens and liquid crystal display. Hence it would be desirable to fabricate a heater including a CNT film characterized by reduced sheet resistance which does not involve complex steps. The present invention thus discloses a CNT based heater exhibiting enhanced heating efficiency by reducing sheet resistance of CNT film through simple and quick steps and providing stability.Due to the stability and high transparency of SWCNT heaters, these transparent conducting films adapted in defoggers are feasible for outdoor displays, the back and side windows of vehicles, etc. for both defrosting and temperature maintenance.

Summary

This invention discloses transparent electrothermal conducting films containing carbon nanotubes. This invention also discloses a room temperature acid-alcohol treatment process to enhance the heating performance of the films. The transparent electrothermal heater comprises coating a substrate with thermally conducting CNT film having atleast one or plurality of layers;immersing the said film in a mixture of ethanol:sulfuric acid in the ratio of 10:1 for one hour and washing thereof;depositing a pair of conducting electrodes spaced apart, along opposite edges of top surface of the CNT film as a means to supply electrical input allowing the said CNT film to conduct current and facilitate uniform heating.

Brief description of figures

Figure 1: Schematic diagram of fabrication and acid-alcohol treatment process of SWCNT films
Figure 2:Schematic diagram of electrothermal measurement of SWCNT film heater
Figure 3A:TGA of as purchased and purified SWCNT
Figure3B: UV-Vis-NIR spectra of fabricated films.
Figure3C: UV-Vis-NIR spectra of acid-alcohol treated films
Figure 4A: Overlay of FT-IR spectra of as coated and acid-alcohol treated SWCNT films
Figure 4B:Overlay of micro Raman spectra of as coated and acid alcohol treated SWCNT films
Figure 5: FE-SEM imagesof SWCNT films
Figure 6: Resistance of the fabricated heater before and after acid-alcohol treatment process
Figure 7A: Thermal images of the TH1
Figure 7B: Current–Voltage (I–V) curves of transparent heaters with silver contact electrodes
Figure 8: Electrothermal performance of SWCNT films deposited on glass (a), (b), (c), (d), (e) and (f)
Figure 9A: Average steady state temperature with respect to the applied voltage
Figure 9B: Heating performance of fabricated heaters
Figure 10: Infrared thermal images at 60 V (a), (b), (c), (d), (e) and (f)
Figure 11: Photograph of TH3-P (a) before frost formation (b) after frost formation and (c) after defrosting test operated at 30 V

Objectives

The main objective of the present invention is to provide transparent electrothermal conducting films containing carbon nanotubes.
Another objective is to develop a transparent electrothermal conducting film having enhanced heating performance by subjecting it to a room temperature acid-alcohol treatment process.

Detailed description

The SWCNT used in this invention were bought commercially from Iljin Nanotechnology Inc., Korea(Arc discharged, 60–70 wt% purity, average diameter ~1.3 nm).

Any person skilled in the art will appreciate that there are other methods available to carry out commonly known processes such as dispersion and coating of nanoparticles.Therefore 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 description and claims.

The main embodiment of the invention is a transparent electrothermal CNT based heater having enhanced heating performance through alcohol-acid treatment. In one embodiment the nanomaterial may be selected from either SWCNT or double-walled carbon nanotubes (DWCNT) or multi-walled carbon nanotubes (MWCNT). However SWCNT are selected for further study in the present invention due to the excellent electrical, optical and thermal properties, which are the important factors influence the electrothermal performance of the transparent heaters. Hence it is to be construed by any person of ordinary skill in the art that the disclosed embodiments are not limited to the use of SWCNT.

In one embodiment the SWCNT is purified by dry oxidation followed by acid treatment to enhance its purity. The commercially available CNT contain many impurities which affects their electrical and thermal conducting properties. Hence they are subjected to acid treatment to effectively remove the impurities and increase the conducting properties and thus heating efficiency. In an exemplary embodiment, the CNT are initially dry oxidized at 623K for 10 hours followed by sonication using 4M nitric acid for 2 hours.

In an embodiment the purified SWCNT (001) are sonicated with a suitable dispersant at a desired concentration for an optimum time duration and temperature in order to effect uniform dispersion. In an exemplary embodiment the dispersant used is N-methyl-2-pyrrolidone (NMP) (002) though many alternatives such as dichlorobenzene, iso propyl alcohol, ethanol, methanol and their combinations are available.

In another embodiment the dispersed SWCNT solution is coated on a suitable substrate to fabricate the transparent conducting film. In a preferred embodiment the CNT is coated on glass substrate (003) though other substrates such as quartz, silicon and polymer are also suitable. For the purpose of coating methods such as spray coating, rod coating, spin coating, brush coating, inkjet printing, dip coating are possible though in the present invention spray coating (004) is adopted. It should also be noted that the size of the films fabricated using the solution-based process outlined here is not limited to small sizes. Thus, it is possible to produce large active area films from spray coating process.

In yet another embodiment the treatment of transparent conducting film involves immersion in a mixed solution of ethanol and sulfuric acid in the volume ratio of 10:1 (005) for an optimum time duration followed by rinsing in double distilled water and drying. The treatment process recommended is highly specific to the ratio mentioned and the type of acid and alcohol. In order to obtain clarity, other types of acid – alcohol mixture in different ratios studied did not provide results as efficient as ethanol and sulfuric acid in the volume ratio of 10:1.

In one embodiment the treated film is uniformly coated on opposite edges with metal as contact electrodes selected from a group consisting of silver, gold, copper, platinum, aluminium to conduct current and facilitate uniform heating.

In one embodiment the contact electrodes are connected to an external means of current supply (006) through a circuit to facilitate the heating of transparent conducting film.

In an embodiment of the invention the transparent heater is characterized by an increase in temperature of 420 – 763 K at an applied voltage of 60V, preferably 424 K. However the voltage in the range of 20 to 60V produces uniform heating. Heat loss in a transparent heater occurs due to heat transfer in the form of conduction to the glass substrate, convection to the surrounding medium and radiation from the hot surface of the heater. It is reasonable to expect that the heat loss in a conventional film heater with a smooth surface is not dependent on the change in film thickness. However, the dominant heat transfer losses in the SWCNT film may be affected by the porous morphology of the network.

In an embodiment the acid-alcohol treated transparent conducting film is incorporated in a transparent defogger (007) which is capable of removing frost formation at an applied voltage of 30V in 52s.

Examples

Example 1

Fabrication of SWCNT thin films and post treatment

Commercial SWCNT is purified by dry oxidation followed by acid treatment to enhance the purity of SWCNT. Purified SWCNT are sonicated with N-methyl-2-pyrrolidone (NMP) at a concentration of 2 mg/100 mL for 4 h at 303 K. Dispersed SWCNT solution is spray coated on glass substrates (5 cm × 5 cm) to fabricate the heating film. During the fabrication, concentration, solution spray rate, nozzle to substrate distance, carrier gas flow rate are kept in constant and the number of coatings are varied to fabricate TCF. TCF are further immersed in ethanol and sulfuric acid mixture (10:1 volume ratio) for 1 h followed by rinsing in double distilled water and dried at 303 K. Figure 1 shows the schematic diagram of fabrication of TCF and acid-alcohol treatment process. As coated and acid-alcohol treated films are coated with silver paste at the two edges of the films. Silver paste coated areas are used as contact electrodes for the supply of current. Heaters realized from 5, 10 and 15 times coated films are named as TH1, TH2 and TH3 respectively. Heaters realized from 5, 10 and 15 times coated films after the acid-alcohol treatment are named as TH1-P, TH2-P and TH3-P respectively.

Example 2

Electrothermal measurements

The schematic of experimental set up used for the electrothermal studies is shown in Figure 2. Electrothermal studies are performed by applying DC voltage at 300 K. The surface temperatures of heaters are measured using an IR camera kept at a distance of 8 cm from the heaters. The current passing through the heaters is measured using a multimeter.

Example 3

Characterization by TGA

The purity of commercial and purified SWCNT are determined by the thermogravimetry analysis (TGA). TGA is carried out in the temperature range 303-1200 K at the heating rate of 10 K/min under air atmosphere. Figure 3A provides the TGA profile of as purchased and purified SWCNT. The SWCNT purity is reached up to 94 wt.% after the two steps purification processes. The purified SWCNT started to decompose at 763 K. But, the as purchased SWCNT started to burn at 713 K, this proves the higher stability of the purified SWCNT.

Example 4

Characterization by optical measurements

The optical measurements are carried out using UV-Vis-NIR spectrophotometer in the wavelength range 350–1100 nm. Figure 3B and Figure 3C provides the UV-vis-NIR spectra of fabricated and acid-alcohol treated films. Optical transmittance of 5, 10 and 15 times as coated SWCNT films are measured as 92, 87 and 81% at 550 nm respectively. Transmittance of the films is not affected by the acid-alcohol treatment process.

Example 5

Characterization by FT-IR

FT-IR spectra of as coated and acid-alcohol treated films are collected in the range 4000-450
cm-1. Figure 4A provides overlay of FT-IR spectra of as coated and acid-alcohol treated SWCNT films. FT-IR spectra showed no new peaks after the acid-alcohol treatment for surface functionalization in the observed region.

Example 6

Characterization by micro-Raman

Micro-Raman measurements are performed at room temperature using a Raman spectrometer at 514.5 nm excitation. Figure 4B gives the overlay of micro Raman spectra of as coated and acid-alcohol treated SWCNT films. Micro Raman spectra of acid-alcohol treated films show no additional peak. Therefore, the pristine structure of the nanotubes is not disturbed by the acid-alcohol treatment process. Further, no appreciable peak shift in the RBM and G-band and no obvious development in the D-band is observed. Thus, the reduction of resistance after the acid-alcohol treatment arises from the digestion of impurities and densification of nanotubes by effective wetting.

Example 7

Characterization by SEM

Scanning electron microscopy (SEM) images (Figure 5) show clearly that SWCNT in the as coated films are protruded forming a wave like structure on the substrate (Figure 5a). This results in poor contact between the neighboring SWCNT resulting in high sheet resistance (Rs) in the network. Even though, the nanotubes are purified by oxidation and acid treatment processes, impurities are found (marked with white circles) in the films (Figure 5b) which act as a scattering sites and increase the Rs of films. The acid-alcohol treatment resulted in digestion of impurities and a dense SWCNT network producing smooth and debris free SWCNT network as shown in Figure 5c and Figure 5d.

Example 8

Sheet Resistance of SWCNT films
Rs of as coated and acid-alcohol treated SWCNT films are measured by using the four-point probe technique (Figure 6). Sheet resistance of 5, 10 and 15 times as coated films are measured as 2620 ?/?, 1630 ?/? and 790 ?/? respectively. After the acid-alcohol treatment, sheet resistance of 5, 10 and 15 times coated films are reduced to 1250 ?/?, 720 ?/? and 350 ?/?.
Example 9

Thermal image studies

Figure 7A shows thermal images of the TH1 when applied 60 V (a) without contact electrodes and (b) with contact electrodes. Without the silver electrodes, heater TH1 is directly connected with the alligator clips and heated by applying 60 V. While connecting with alligator clips without the silver electrodes, the thermal distribution is not uniform and the pressure points create high temperature regions in the film near to the alligator clips. This is because of the fact that saw tooth of the alligator clips create more pressure on the film, hence, more flow of current in that region at where they are in contact with the SWCNT network. This creates localized heating at the contact points and the alligator clips started damaging the films during repeated usage. After the formation of silver electrodes on the films, the temperature distribution is found to be uniform witnessed the uniform current flow throughout the film. Hence, it is clear that the formation of good electrical contact to supply the current is necessary to obtain uniform heating.

Example 10

I-V studies

I-V studies are performed in the voltage ranging from -20 to +20 V in steps of 0.1 V using the keithley source measurement unit (2450).Figure 7B provides Current–Voltage (I–V) curves of transparent heaters with silver contact electrodes. Current passing through the heaters is measured as 9.8 and 20 mA, with and without the silver electrodes coating respectively (TS1). It shows that the contact electrodes are important for the uniform and more current distribution in the heating films. The contact behavior of the silver paste over the SWCNT network is analyzed using the I-V studies. I-V curves are linear in the measured range shows that the contact established between the silver electrode and the heating film is Ohmic in nature.

Example 11

Electrothermal performance of SWCNT films
The electrothermal performance of SWCNT films deposited on glass and time versus temperature profiles with respect to different applied voltages. The time versus temperature profiles of all the fabricated heaters are shown in Figure 8. Among all the films, TH1 has a relatively higher Rs (2620 ?/?), displays the lowest steady state temperature and the lowest heating rate at the same applied voltage. TH1 show a steady state temperature of 326 K with a maximum heating rate of 0.6 K/s at a driving voltage of 60. Higher heating rate and higher steady state temperature observed for TH1-P is attributed to lower Rs (1250 ?/?). A steady state temperature of 332 K is reached for TH1-P when a voltage of 50 is applied, which increases to 346 K on increasing the voltage to 60.A steady state temperature of 332 K is reached for TH1-P when a voltage of 60 is applied, however after the post treatment process the steady state temperature was reached to 424 K with the same applied voltage.

Example 12

Heating performance of the heaters

Figure 9 shows the heating performance of fabricated heaters. The acid-alcohol treated heaters show increased performance than the as coated heaters. TH3-P show the highest heating performance among all the six fabricated heaters because of lowest Rs. Maximum temperature can be achieved by this SWCNT film is 763 K. SWCNT will degrade if the temperature is raised above 763 K in the oxygen environment. The best heater TH3-P consumed 0.34 W/cm2 to reach the temperature of 424 K. All the heaters are showing the linear increase in the temperature with the heating power shows the desired temperature can be easily controlled by controlling the supplied power. All the fabricated heaters are low power consuming heaters when compared to conventional heaters.

Example 13

IR thermal studies

Figure 10 depicts the Infrared thermal images of (a) TH1, (b) TH1-P, (c) TH2, (d) TH2-P, (e) TH3, (f) TH3-P at 60 V. The temperature distribution of the SWCNT films at the applied voltages is homogeneous. This even distribution of heat is attributed to the excellent thermal and electrical conductivity of SWCNT as well as uniform surface of the films.

Example 14

Defogging experiments

Owing to the excellent optical and electrothermal properties of these SWCNT films, transparent defoggers are fabricated (Figure 11a). TH3-P is exposed to a temperature of 263 K for 3 h to allow the frost to be formed over the heater. After forming the frost, the background is not clearly visible (Figure 11b). Frost on the surface of the heater is completely removed after 30 V is applied for only 52 sec. After defrosting the background is clearly visible (Figure 11c).