FIELD OF INVENTION

[0001 ] The present invention relates to carbon structure production, and more particularly, methods of forming a carbon structure by compression of carbon nanopowder.

BACKGROUND

[0002] Carbon, with its abundancy and low cost, demonstrates a wide range of industrial applications. Activated carbon films on conductive metal substrates (e.g., aluminum sheets) find applications as electrodes and current collectors in ultracapacitors, batteries, and capacitive deionization devices. Carbon deposited on insulating substrates (e.g., ceramics) is commonly used for electrical carbon film resistors. Free-standing carbon structures, which do not need any supporting substrate, can be desirable for applications such as air or water filtration.

[0003] The production of solid, continuous, and homogenous carbon structures with thickness in the range of 1 μιτι to 2000 μιτι thus becomes an important industrial process. However, producing uniform and high purity carbon structures has proven to be challenging. The current methods of carbon film production encompass gas-phase deposition and solvent-based coating. These methods generally have limitations in the total thickness that can be deposited, in energy efficiency and in scalability due to: the need to heat the substrate and/or carbon material, the need to perform the deposition in a low-pressure evacuated chamber and/or the need to form a carbon suspension. The production of free-standing carbon structures, such as carbon pellets, requires the addition of binder materials and/or high- temperature treatment.

[0004] Thus, what is needed is a method for forming a carbon structure that seeks to address one or more of the above-mentioned problems. Other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.

SUMMARY

[0005] According to one aspect, there is provided a method for forming a carbon structure. The method comprises steps of: providing carbon powder in a compression apparatus, wherein the carbon powder comprises carbon nanoparticles with particle size below 100 nanometers; and compressing the carbon powder using the compression apparatus at a compression pressure of 0.2 MPa to 2000 MPa, such that the carbon nanoparticles in the carbon powder bond to each other via attractive forces to form the carbon structure. [0006] In one embodiment, the step of providing the carbon powder in the compression apparatus may comprise providing the carbon powder with a substrate in the compression apparatus, such that the carbon powder is compressed against the substrate and the carbon structure adheres to the substrate during the compressing.

[0007] In one embodiment, the method may further comprise the step of pre-treating the substrate, prior to the step of providing the carbon powder with the substrate in the compression apparatus. In one embodiment, the substrate may be pre-treated in radiofrequency oxygen plasma at an oxygen pressure of 1 mbar to 100 mbar.

[0008] In one embodiment, the step of providing the carbon powder in the compression apparatus may comprise providing the carbon powder in a pressing die that is placed in the compression apparatus such that the carbon powder is compressed in the pressing die. In one embodiment, the carbon powder may be provided with a substrate in the pressing die such that the carbon powder is compressed against the substrate in the pressing die and the carbon structure adheres to the substrate during the compressing.

[0009] In one embodiment, the step of compressing the carbon powder may be performed uniaxially and the applied pressure may gradually increase to the compression pressure.

[0010] In one embodiment, the method may further comprise the step of dwelling at a dwelling pressure for a dwelling period of up to 60 minutes, after the step of compressing the carbon powder. The dwelling pressure may be constant through the dwelling period and the dwelling pressure is the same as the compression pressure. The dwelling pressure may also have repeated pulsing. [0011 ] In one embodiment, the method may further comprise the step of mechanically levelling the carbon powder, prior to the step of compressing the carbon powder.

[0012] In the embodiments, the carbon powder may comprise carbon black, activated carbon, carbon nanotube powder, graphene nanopowder, other carbon nanopowders, or any combination thereof, or any other carbon nanopowder that can be obtained from powder grading with particle size below 100 nanometers. The carbon powder may further comprise a dry binder.

[0013] In the embodiments, the resultant carbon structure may have a density of 0.2 g cc ¹ to 2.2 g cc ¹ .

[0014] In the embodiments, the attractive forces may comprise one or more of van der Waals forces, interfacial forces, chemical bonding, mechanical interlocking, and other adhesive and cohesive forces. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views and which together with the detailed description below are incorporated in and form part of the specification, serve to illustrate various embodiments and to explain various principles and advantages in accordance with a present embodiment.

[0016] FIG. 1 depicts a flowchart of a method for forming a carbon structure in accordance with a present embodiment. [0017] FIG. 2 depicts a schematic of a hydraulic pellet press for forming the carbon structure in accordance with the present embodiment.

[0018] FIG. 3 depicts a schematic of a pressing die for loading carbon powder in accordance with the present embodiment.

[0019] And FIG. 4 depicts an illustration of the carbon structure formation in accordance with the present embodiment.

[0020] Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been depicted to scale. For example, the dimensions of some of the elements in the illustrations or diagrams may be exaggerated in respect to other elements to help to improve understanding of the present embodiments. DETAILED DESCRIPTION

[0021 ] Embodiments of the present invention will be described, by way of example only, with reference to the drawings. Like reference numerals and characters in the drawings refer to like elements or equivalents. [0022] Referring to FIG. 1 , a flowchart of a method 1 00 of forming a carbon structure in accordance with a present embodiment is depicted. The carbon structure can be formed on a substrate or as a free-standing structure. The resultant carbon product can be a planar (i.e. two-dimensional) film, or can be a three-dimensional structure that is of any geometry and thickness. The method 100 can be performed using a setup as depicted in FIG. 2 with a hydraulic pellet press 200 including a top plate 206a and a bottom plate 206b. Instead of loading the substrate with carbon powder directly into the hydraulic pellet press 200, the method 1 00 can also be performed using a pressing die 300 as depicted in FIG. 3 for loading the carbon powder and substrate into the hydraulic pellet press 200.

[0023] At step 1 02, carbon powder 202, 302 is provided in a compression apparatus. The carbon powder 202, 302 comprises carbon nanoparticles with particle size below 100 nm, which can include carbon black nanopowder, activated carbon nanopowder, carbon nanotube powder, graphene nanopowder, other carbon nanopowders, or any mixture thereof. The nanopowder can be obtained from conventional carbon powder by sieving, grading, or other separation methods. Preferably, to form a carbon structure of high purity, separation of the carbon powder 202, 302 is performed if the carbon powder is in a mixture form which may contain impurities. It is worth noting that binding materials or polymer additives are not required in the carbon powder 202, 302 in order to form a continuous carbon structure, which advantageously allows the resultant carbon structure to have high carbon purity. Nonetheless, the method 100 is also suitable for carbon powder 202, 302 with added binding materials or polymer additives. In some embodiments, the carbon powder 202, 302 can be mixed with a dry binder.

[0024] At step 1 04, the carbon powder 202, 302 is compressed using the compression apparatus at a compression pressure of 0.2 MPa to 2000 MPa. Due to high powder flowability of the nanopowder and the high surface-to- volume ratio of nanoparticles, the carbon nanoparticles in the carbon powder bond to each other via attractive forces to form the continuous carbon product. The compressing can be performed using the compression apparatus, for example, a hydraulic pellet press 200, a roll press, or any other apparatus which is suitable. The pressure applied to compress the carbon powder 202, 302 can be uniaxial or multiaxial. Preferably, the applied pressure gradually increases to the compression pressure. The compression pressure is within a range of 0.2 MPa to 2000 MPa. Preferably, the compression pressure is 200 MPa. [0025] In some embodiments, the step of compressing the carbon powder is performed at ambient temperature, without heating the substrate, the starting carbon powder before or during the compressing, or the resultant carbon product formed after the compressing. Advantageously, less energy is required in comparison with methods which involve heating of the substrate or the carbon material or require the use of binding materials. The method 100 therefore can reduce production cost, and require fewer processing steps. [0026] In one arrangement, the carbon powder 202, 302 may be provided with a substrate 204, 304 in the compression apparatus. The resultant continuous carbon structure (e.g., a carbon thin film) therefore adheres to the substrate 204, 304 during the compressing. Preferably, the substrate 204, 304 may be pre-treated prior to providing the carbon powder 202, 302 on the substrate at step 1 02. Preferably, pre-treating the substrate 204, 304 may involve pre-treating the substrate 204, 304 in radiofrequency oxygen plasma. Preferably, the radiofrequency oxygen plasma is set at 1 mbar to 100 mbar oxygen pressure. Preferably, the duration of pre-treating the substrate in radiofrequency oxygen plasma treatment is between 0 minute and 10 minutes. Pre-treating the substrate can improve the adhesion of the resultant carbon product to the substrate, therefore may have the advantages of enhancing the carbon uniformity and reducing electrical contact resistance between the carbon and an electrically conductive substrate.

[0027] Preferably, the carbon powder 202, 302 is mechanically levelled in the compression apparatus before being compressed at step 104. In an embodiment depicted in FIG. 3, the carbon powder 302 can be mechanically levelled by vibration shaking of the pressing die 300. The levelling of the carbon powder 202, 302 advantageously ensures that the carbon powder 202, 302 is distributed evenly, thus generating a uniform carbon structure on the substrate 204, 304 after compression.

[0028] Preferably, the method 100 can further include a dwelling step after the step 1 04. The dwelling step can be performed at a dwelling pressure for a dwelling period of up to 60 minutes. In some embodiments, after compressing the carbon powder 202, 302 at a certain compression pressure (between 0.2 MPa and 2000 MPa), the compression pressure is maintained such that the dwelling pressure is the same as the compression pressure and the dwelling pressure is constant through the dwelling period. In other embodiments, the dwelling pressure can have repeated pulsing of the compression with pressure values between 0.2 MPa and 2000 MPa. Preferably, the dwelling period is between 0 min and 60 minutes. Properties of the resultant carbon structure can be controlled by the dwelling pressure and dwelling period for a given carbon powder. These properties include, but not limited to, density, electrical conductivity, adhesion to a substrate, and the electrical contact resistance between the carbon film and a substrate.

[0029] After step 104, the compression apparatus releases pressure and a resultant carbon structure can be unloaded from the compression apparatus. In some embodiments, the substrate 204, 304 with a resultant carbon structure can be unloaded from the compression apparatus after the pressure is lowered to ambient pressure. In some embodiments, the compression apparatus releases pressure by a release valve. In some embodiments, the resultant carbon structure has a density of 0.2 g cc ¹ to 2.2 g cc ^"1 . [0030] Referring to FIG. 2, a schematic of a hydraulic pellet press 200 for forming the carbon structure in accordance with the present embodiment is depicted. The hydraulic pellet press 200 includes a top plate 206a and a bottom plate 206b for performing step 104 of method 1 00, compressing the carbon powder 202. The hydraulic pellet press 200 is capable of providing a vertical uniaxial pressure as indicated by the arrows in FIG. 2. The hydraulic pellet press 200 can have a load of 24-ton, or any other load configurations which are suitable to perform method 100.

[0031 ] In an alternative embodiment, in addition to hydraulic pellet press, the method 1 00 may be performed with other compression apparatuses such as a roll press.

[0032] Referring to FIG. 3, a schematic of a pressing die 300 for loading carbon powder 302 in accordance with the present embodiment is depicted. In one embodiment, dry active carbon powder carefully graded for particle size below 1 00nm is used as the carbon powder 302. Preferably, the pressing die 300 is a cylindrical pressing die fabricated from W1 8Cr4V hardened carbon tool steel. Preferably, the cylindrical pressing die has a diameter of 15 mm and a substrate 304 can be prepared in circular shape with an equal diameter of 15 mm. The pressing die 300 includes a top core die 306a and an inner core die 306b for compressing the carbon powder 302. In one embodiment, the inner core die 306b together with the substrate 304 is placed on a steel support plate 308 and inserted into a pressing die sleeve 310. The substrate 304 is then covered with a predetermined amount of carbon powder 302, which can be a dry carbon nanopowder or a carbon powder mixture. Preferably, the amount of the carbon powder 302 is between 10 mg and 100 mg. Preferably, the carbon powder 302 can be distributed evenly and mechanically levelled by vibration shaking of the pressing die 300. The top core die 306a and a pushing rod 312 are subsequently placed onto the carbon powder 302. The pressing die 300, as a whole, is then loaded into the compression apparatus, such as a hydraulic pellet press 200 as depicted in FIG. 2, and compressed uniaxially at the compression pressure of 0.2 MPa to 2000 MPa. In an alternative embodiment, the substrate 304 is not required and the carbon powder 302 may be compressed directly in the pressing die 300.

[0033] In an alternative embodiment, in addition to the pressing die presented above, the carbon powder 302 may be loaded into pressing dies with various shapes and dimensions, which can result in different shapes and dimensions of the carbon product. This advantageously allows the formation of customized carbon structure of any geometry and thickness.

[0034] And referring to FIG. 4, an illustration 400 of the carbon structure formation in accordance with the present embodiment is depicted. At step 104 of method 100, under a pressure applied by a compression plate 406, the carbon structure 412 may be formed due to the high powder flowability, the high reactivity of nanoparticles, and bonding of the carbon nanoparticles in the carbon powder 402 to each other via attractive forces, like van der Waals forces, interfacial forces, chemical bonding, mechanical interlocking, or other adhesive and cohesive forces. The adhesion of the film to the substrate 404 may be due to similar physical and chemical forces, and may be controlled by the pressure applied, the time of the compressing and dwelling. [0035] The density of the carbon structure can be controlled by the applied pressure, the density of the carbon mixture, and the dwell time. When using an electrically conductive substrate and electrically conductive carbon powder, the described method 100 results in low contact resistance between the substrate and the carbon structure. For example, when formed from activated carbon powder on aluminum sheets, the carbon structure can be used as electrode and current collector in batteries and ultracapacitors. When formed on ceramics, such as alumina, the method 100 can be used for the fabrication of carbon-film electrical resistors.

[0036] The method 100 for forming a carbon structure on a substrate described herein can provide many advantages when compared to other methods for forming a carbon structure on a substrate. The method can be performed at ambient temperature and without heating. The described method can result in a carbon structure that exhibits carbon material purity as high as the purity of the carbon powder mixture in the starting material, because the method does not require the mixing of a liquid to form a suspension or slurry. Furthermore, the method is scalable and does not require deposition in an evacuated low-pressure chamber. The described method can be applied to substrates that are able to sustain the required pressure. As no heating step is required, the method can be more time efficient and energy efficient than gas-phase deposition and solvent-based deposition per kilogram of produced carbon film.

[0037] Different carbon nanopowders with particle size below 100 nm can be used as the starting material, so that the produced carbon structure exhibits different set of characteristics that might be useful for other applications than the ones listed herein. In the described embodiment, the carbon powder and the substrate are compressed uniaxially using a mechanical pressing die. Compression may be achieved using different pressurizing and compression techniques.

[0038] It should further be appreciated that the exemplary embodiments are only examples, and are not intended to limit the scope, applicability, operation, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements and method of operation described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.