THERMAL MATERIAL AND A METHOD OF MAKING THE SAME

Abstract

There is provided a thermal material comprising an electrode, a film of reduced graphene oxide, a porous membrane that is sandwiched between the electrode and the film of reduced graphene oxide, and an ionic liquid that is disposed within pores of the porous membrane. There is also provided a method of preparing a thermal material. There is further provided a method of changing an article's apparent temperature. There is further provided a device comprising the thermal material as described herein.

Claims

1. A thermal material comprising: (a) an electrode; (b) a film of reduced graphene oxide; (c) a porous membrane that is sandwiched between the electrode and the film of reduced graphene oxide; and (d) an ionic liquid that is disposed within pores of the porous membrane.

2. The thermal material of claim 1, wherein the electrode comprises gold, copper, silver, titanium, platinum, tungsten, or combinations thereof.

3. The thermal material of claim 1, wherein the electrode has a thickness in a range of 10 nm to 2000 nm.

4. The thermal material of claim 1, wherein the film of reduced graphene oxide comprises a plurality of single-layered reduced graphene oxide.

5. The thermal material of claim 1, wherein the film of reduced graphene oxide has a thickness in a range of 100 nm to 2000 nm.

6. The thermal material of claim 1, wherein the porous membrane comprises polyethersulfone.

7. The thermal material of claim 1, wherein the porous membrane has a pore size in a range of 10 nm to 1000 nm.

8. The thermal material of claim 1, wherein the porous membrane has a thickness of at least 10 μm.

9. The thermal material of claim 1, wherein the ionic liquid is 1-butyl-3-methylimidazolium hexafluorophosphate.

10. The thermal material of claim 1, wherein the electrode and the porous membrane are flexible.

11. A method of preparing a thermal material, the method comprising: (a) disposing a film of reduced graphene oxide on a first side of a porous membrane; (b) adding an electrode on a second side of the porous membrane, the second side being opposite to the first side of the porous membrane; and (c) filling pores of the porous membrane with an ionic liquid.

12. The method of claim 11, wherein the disposing comprises: filtering a dispersion of graphene oxide through the porous membrane to form a film of graphene oxide on the porous membrane; and reducing the film of graphene oxide to form a film of reduced graphene oxide.

13. The method of claim 11, wherein the film of reduced graphene oxide comprises a plurality of single-layered graphene oxide.

14. The method of claim 11, wherein the filling is undertaken by exposing the porous membrane to the ionic liquid.

15. A method of changing an apparent temperature of an article, the method comprising: (a) coating a surface of the article with a thermal material, the thermal material comprising: (i) an electrode; (ii) a film of reduced graphene oxide; (iii) a porous membrane that is sandwiched between the electrode and the film of reduced graphene oxide; and (iv) an ionic liquid that is disposed within pores of the porous membrane; and (b) applying a bias voltage between the electrode of the thermal material and the film of reduced graphene oxide of the thermal material to drive anions of the ionic liquid to the film of reduced graphene oxide.

16. The method of claim 15, wherein the bias voltage is 3 V.

17. The method of claim 15, further comprising reversing the bias voltage to drive anions of the ionic liquid to the electrode.

18. A device comprising: (a) an article; (b) a thermal material coated on a surface of the article, the thermal material comprising: (i) an electrode; (ii) a film of reduced graphene oxide; (iii) a porous membrane that is sandwiched between the electrode and the film of reduced graphene oxide; and (iv) an ionic liquid that is disposed within pores of the porous membrane; and (c) a power supply connected to the thermal material.

19. The device of claim 18, wherein the article, the thermal material and the power supply are integral parts of the device.

20. The device of claim 18, wherein the power supply applies a bias voltage between the electrode of the thermal material and the film of reduced graphene oxide of the thermal material to drive anions of the ionic liquid to the film of reduced graphene oxide.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0093] The accompanying drawings illustrate a disclosed embodiment and serves to explain the principles of the disclosed embodiment. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

[0094] FIG. 1A shows an embodiment of the thermal material comprising a film of reduced graphene oxide, a porous asymmetric polyethersulfone membrane and a back gold electrode.

[0095] FIG. 1B is a schematic illustration of working principles of the thermal material.

[0096] FIG. 1C is a schematic illustration of band structure of the film of reduced graphene oxide as described herein when doped with anions.

[0097] FIG. 1D is a photograph of an embodiment of the thermal material where flexible materials are used.

[0098] FIG. 1E and FIG. 1F are thermal camera images of a beaker filled with boiling water, wherein the beaker is wrapped by the thermal material. In FIG. 1E, no bias voltage is applied on the thermal material, while in FIG. 1F, a bias voltage of 3 V is applied between the electrode and the film of reduced graphene oxide of the thermal material.

[0099] FIG. 2A is an atomic force microscope (AFM) height sensor image of graphene oxide flakes (taken on a SiO.sub.2/Si substrate) used in the preparation of the film of reduced graphene oxide.

[0100] FIG. 2B shows a corresponding AFM line scan plot of the graphene oxide flakes as shown in FIG. 2A. The graphene oxide flakes had a low height, thus demonstrating that they are single-layered.

[0101] FIG. 2C shows X-ray diffraction (XRD) spectra of a film of graphene oxide (that was prepared from the graphene oxide flakes as described above) and a film of reduced graphene oxide (that was prepared from the film of graphene oxide as described above).

[0102] FIG. 2D shows Raman spectra of the film of graphene oxide and the film of reduced graphene oxide as described above.

[0103] FIG. 2E is a scanning electron microscope (SEM) image of the film of reduced graphene oxide, showing the film's morphology.

[0104] FIG. 2F is a cross-sectional SEM image of a combination of the film of reduced graphene oxide and a porous polyethersulfone membrane. The film of reduced graphene oxide has a thickness of about 300 nm as shown in FIG. 2F.

[0105] FIG. 3A shows reflectance spectra of the thermal material under varying bias voltages. The reflectance of the thermal material increased with increasing bias voltages.

[0106] FIG. 3B shows a variation of emissivity of the thermal material under varying bias voltages.

[0107] FIG. 3C shows calculated apparent temperatures of the thermal material with varying emissivities ranged from 0.2 to 1, at a background temperature of 20° C.

[0108] FIG. 4A shows thermal camera images (taken at 8 to 13 μm) of a beaker filled with boiling water, wherein the beaker is wrapped by the thermal material. The beaker has a decreasing apparent temperature when a bias voltage of 3 V is applied on the thermal material. The beaker's apparent temperature increases back when the bias voltage is reversed.

[0109] FIG. 4B shows the thermal material's response time when the bias voltage is applied.

[0110] FIG. 4C shows a cycling test of the thermal material where a periodic voltage is applied (−3 to 3 V), highlighting the thermal material's robustness.

DETAILED DESCRIPTION

[0111] Referring to FIG. 1B, there is provided a schematic illustration of working principles of the thermal material.

[0112] The thermal material comprises a film of reduced graphene oxide (102), a porous membrane (104), an electrode (106) and an ionic liquid (108), wherein the porous membrane is sandwiched between the film of reduced graphene oxide and the electrode. The ionic liquid (108) comprises cations and anions and is initially disposed within pores of the porous membrane (104).

[0113] Where a power supply (110) is connected to the thermal material to apply a bias voltage between the electrode (106) and the film of reduced graphene oxide (102), the anions of the ionic liquid (108) may be driven towards the film of reduced graphene oxide (102), while the cations of the ionic liquid (108) may be driven towards the electrode (106). The reduced graphene oxide may be intercalated by the anions, thus leading to a lower infrared radiation (112) of the thermal material.

EXAMPLES

[0114] Non-limiting examples of the invention and a comparative example will be further described in greater detail by reference to specific Examples, which should not be construed as in any way limiting the scope of the invention.

Example 1—Synthesis of Thermal Material

[0115] 50 mL of diluted graphene oxide (GO, flakes with a size of 1000 to 10000 nm, purchased from Sigma Aldrich, Singapore) dispersion (10 mg/L) was self-assembled onto porous polyethersulfone (PES) having a pore size of 30 nm (purchased from Sterlitech of Auburn, Wash., the United States) under vacuum. A graphene oxide layered material with an interlayer distance from 6.4 to 8.8 Å depending on the graphene oxide's degree of oxidation.

[0116] The graphene oxide layered material was then reduced by an aqueous solution of ascorbic acid (30 mg/mL, purchased from Sigma Aldrich, Singapore) by immersion of the material in the solution for 24 hours. It was observed that the material's colour changed from light yellow to dark. In addition, the interlayer distance was reduced to 3.9 Å. Thus, it was shown that a layered composite material of reduced graphene oxide (rGO) was formed on the porous PES material. The layered composite rGO/PES material was then dried overnight at room temperature in a dry cabinet.

[0117] A back gold mesh was subsequently deposited on the bottom of the rGO/PES material to form a bottom electrode by atomic layer deposition using a mask. The rGO and the back gold then formed top and bottom electrodes, respectively, while the backside gold mesh also served as an obstruct layer to block transmission of background infrared (IR) radiation.

[0118] Eventually, an ionic liquid 1-Butyl-3-methylimidazolium hexafluorophosphate (BMIMPF.sub.6, 97%, purchased from Merck, Singapore) was injected into the porous PES through capillary action by soaking the material in the ionic liquid for 2 hours.

[0119] Referring to FIG. 1A, the thermal material may comprise a film of reduced graphene oxide, a porous polyethersulfone membrane and a back gold mesh. The film of reduced graphene oxide may serve as a top electrode while the back gold mesh may serve as a bottom electrode. Therefore, the thermal material may be adapted to be connected to a power supply.

Example 2—Characterization of Thermal Material

[0120] Referring to FIG. 1D, a photograph was taken for the fabricated active thermal material, which was flexible and light, and could be used as a cladding for target objects.

[0121] Atomic Force Microscope (AFM, Bruker Dimension Icon) images manifested that the GO flakes used could be homogenously suspended onto SiO.sub.2/Si substrate with a uniform contrast. The lateral size of GO ranged from 1 to 2 μm (FIG. 2A). Moreover, the GO's corresponding height profile demonstrated that it was single-layered with a thickness of about 0.8 nm (FIG. 2B). The rGO film was obtained by directly filtering GO dispersion onto asymmetric polyethersulfone filtration membrane with pore size of 30 nm, followed by ascorbic acid reduction. The XRD, Raman (WITEC ALPHA300R; 532 nm laser excitation, 100× object lens) and X-ray photoelectron spectroscopy (XPS, Kratos Analytical Axis-Ultra spectrometer using a monochromatic Al Kα X-ray source) results confirmed a successful reduction of GO by ascorbic acid. As shown in FIG. 2C, the typical 2θ value of GO was about 10.25° (d-spacing was about 8.60 Å) and the characteristic peak of rGO was dramatically shifted to higher 2θ angles (22.8°, corresponding interlayer spacing was approximately 3.9 Å) due to elimination of epoxy and hydroxyl groups from the GO during the reduction process, reducing the interlayer distance.

[0122] FIG. 2D showed the Raman spectra of rGO before and after reduction. It was observed that the G-band (around 1588 cm.sup.−1) became obvious because of recovery of the hexagonal structure of C atoms. The intensity of the 2D (2686 cm.sup.−1) band also increased, indicating a regraphitization of rGO during the reduction process. However, the D-band's intensity was still higher than that of the G-band, because the gentle reduction process could only partially remove defects and disorders in the rGO that had been formed by oxidation of graphite.

[0123] In addition, the C.sub.1S XPS spectra also confirmed the successful reduction of GO as indicated by decreases of C—O and O—C═O groups. As shown in the SEM image (Zeiss Sigma 300) of FIG. 2E, the surface morphology of synthesized rGO film was pretty smooth, indicating that transverse and interlayer contraction occurred during the reduction process. The cross-section SEM images manifested that the rGO film was firmly adhered to the PES filter membrane with a thickness of about 300 nm (see FIG. 2F). Further, no obvious gap was observed between the rGO film and the PES membrane.

Example 3—Doping Effect of Thermal Material

[0124] To evidence doping effect of the rGO by intercalation of ions, variation of the thermal material's optical response was measured by a Fourier transform infrared spectrometer (FTIR) under different bias voltages. As shown in FIG. 3A, the thermal material's optical reflectivity increased significantly with an increase in bias voltage over the visible and the full mid-infrared range, and the reflectance was increased by a factor of 1.6 at a bias voltage of 5 V. The suppressed infrared emissivity due to Pauli blocking and the enhanced Drude optical conductance led to increases of the infrared reflectance. However, the electrochemical window of the ionic liquid used limited the maximum value of the applied bias. Moreover, to avoid destruction of the rGO's structure, the applied maximum bias voltage for thermal stealth measurement was set to be 3 V.

[0125] Apparent temperatures as detected by a thermal camera followed the Stefan-Boltzmann law, i.e., P=εσT.sup.4, where P is power of received thermal radiation, ε is emissivity of an object's surface, σ is the Stefan-Boltzmann constant and T is the surface's actual temperature. Therefore, the apparent temperature of the thermal material could be modulated by controlling the emissivity during a reversible intercalation process. The IR emissivity is calculated according to the following equation: Total IR emissivity (%)=100%−Total IR transmittance (%)−Total IR reflectance (%), where the transmittance is negligible in our cases due to the thickness of rGO film and back gold layer. All the measurements were performed at ˜20° C. with the relative humidity of ˜60%. Thus, the emissivity of the thermal material could be modulated from 0.64 (under a bias voltage of 3 V) to 0.77 (under a bias voltage of 0 V, or under no bias voltage) at a spectral sensitivity range of 8 to 13 μm of the thermal camera (see FIG. 3B, FLUKE TiX580 thermal camera). As shown in FIG. 3C, the apparent temperature of the thermal material was calculated from the actual temperature in a range from 0 to 100° C. with the emissivity being in a range between 0 and 1, at a background temperature of 20° C. Apparent temperatures of the shaded areas in FIG. 3C were tuned by the thermal material and measured as shown in the figure.

Example 4—Performance of Thermal Material

[0126] To demonstrate the performance of the thermal material, the thermal material was wrapped around a beaker which was filled with boiling water. At a bias voltage of 0 V, the thermal material had a relatively high thermal emissivity, showing the actual temperature of its surface (see FIG. 1E). A suitable bias voltage of about 3 V was then applied on the thermal material while keeping the water in the beaker boiling. It was observed that the thermal material's emissivity was greatly suppressed, and the detected apparent temperature decreased. Referring to FIG. 1C, under a forward bias voltage, the reduced graphene oxide may be doped via intercalating of the anions of the ionic liquid. This may shift up the E.sub.F and lead to a higher carrier density of the reduced graphene oxide. Therefore, the thermal material may have a greatly suppressed optical absorption and emissivity due to Pauli exclusion principle. This effect was not observed in other materials, such as a device where the ionic liquid was sandwiched between two pieces of graphene glass (where graphene was inside).

[0127] Modulation of the thermal material's apparent temperature was subsequently systematically studied by placing the thermal material on a hot plate at 90° C. Bias voltages of 0 V and 3 V were applied on the thermal material and thermal images were taken. As shown in the left panel of FIG. 4A, the thermal material had a relatively higher apparent temperature revealing its actual temperature, due to its high emissivity at a bias voltage of 0 V. When the bias voltage was increased to 3 V, anions of the ionic liquid would intercalate into rGO interlayers and dope them, suppressing the emissivity and showing a lower apparent temperature. The apparent temperature could reach to 77° C. (see FIG. 4A, middle panel), which was consistent with the calculated results. Moreover, the thermal material could recover to its initial state when a reverse bias voltage was applied (see FIG. 4A, right panel).

[0128] Further, the thermal material's temperature response was monitored by plotting apparent temperature vs time (see FIG. 4B). The thermal material had a response time of 3.5 minutes at a bias voltage of 3 V, with a much faster decay time at a reverse voltage of −3 V. The thickness of the original GO flakes was considered as a key factor for the response time. A similar thermal material was fabricated with thicker GO flakes having a thickness of about 1.6 nm (compared to about 0.8 nm as described in Example 2). It was observed that a longer time was needed for anions of the ionic liquid to intercalate, and the performance of the comparative thermal material was also worse.

[0129] To demonstrate the thermal material's robustness, it was rigorously tested by cycling many times. It was observed that the values of response time and on/off ratio were almost the same after several cycles (see FIG. 4C). Moreover, no remarkable change was observed in the thermal material's Raman spectra after cycling, manifesting its high stability. Therefore, the thermal material could be effectively used for infrared stealth, owing to its remarkable on/off state, fast response, high durability and low threshold bias voltage.

Summary of Examples

[0130] In conclusion, a thermal material was developed based on rGO that has a low cost. The thermal material's E.sub.F may be tuned via intercalation of anions and the corresponding doping effect. The thermal material's emissivity may be modulated from 0.77 down to 0.6 at a bias voltage of 3 V with a fast response and a high durability. A hot object coated with the thermal material may be disguised as a cold one in a thermal camera or a thermal imager, thus achieving an active infrared stealth. Moreover, the thermal material had a simple geometry, which allowed for industrial-scale production for thermal management.

INDUSTRIAL APPLICABILITY

[0131] The thermal material may be used in a variety of applications such as thermal camouflage devices, adaptive IR optics and adaptive heat shields for satellites.

[0132] It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.