Carbon nanotube film structure and method for fabricating the same

[0001] This application is related to commonly-assigned applications: U.S. patent application Ser. No. 12/002,169, entitled, “OPTICAL POLARIZER AND METHOD FOR FABRICATING THE SAME”, filed Dec. 14, 2007, which is now patented as U.S. Pat. No. 7,710,649; U.S. patent application Ser. No. 12/002,143, “ANODE OF LITHIUM BATTERY AND METHOD FOR FABRICATING THE SAME”, filed Dec. 14, 2007, which is now published as U.S. patent publication No. 20080299460, and U.S. patent application Ser. No. 12/002,144, “FIELD EMISSION CATHODE AND METHOD FOR FABRICATING THE SAME”, filed Dec. 14, 2007, which is now patented as U.S. Pat. No. 7,872,407. Disclosures of the above-identified applications are incorporated herein by reference.

[0002] 1. Field of the Invention

[0003] The present invention relates to a carbon nanotube film structure and a method for fabricating the same.

[0004] 2. Discussion of Related Art

[0005] Carbon nanotubes (CNTs) produced by means of arc discharge between graphite rods were first discovered and reported in an article by Sumio Iijima, entitled “Helical Microtubules of Graphitic Carbon” (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). CNTs are electrically conductive along their length, chemically stable, and each can have a very small diameter (much less than 100 nanometers) and large aspect ratios (length/diameter). Due to these and other properties, it has been suggested that CNTs can play an important role in many fields, such as microscopic electronics, field emission devices, thermal interface materials, etc.

[0006] However, the main obstacle to an actual application of carbon nanotube use is their difficulty to process, due to the powder form of the carbon nanotube products. Therefore, carbon nanotubes are formed into films to facilitate handling and processing thereof.

[0007] At present, different methods are widely used for fabricating carbon nanotube films including: a growing method, a spraying method, and a Langmuir-Blodgett (LB) method.

[0008] Through the growing method, a carbon nanotube film is grown on a chemically functionalized substrate directly by adopting multiple catalyst layers. However, the method typically requires high temperature during growing process and cannot be applied to such a substrate made of plastic.

[0009] Through the spraying method, carbon nanotubes, dispersed in a solvent, are sprayed on a substrate. In this method, however, carbon nanotubes cannot be well-dispersed, due to an aggregation thereof, and thus, a uniform film cannot be formed.

[0010] Through the Langmuir-Blodgett (LB) method, carbon nanotubes are soluble through an introduction of amide groups therein, and then through a step of “molecular self-assembly”, the film accumulates on a substrate. However, the method has such a problem that carbon nanotubes are disordered in the film.

[0011] What is needed, therefore, is to provide a carbon nanotube film structure and a method for fabricating the same, in which the above problems are eliminated or at least alleviated.

[0012] In one embodiment, a carbon nanotube film structure includes at least two overlapped carbon nanotube films aligned along different directions. Each carbon nanotube film includes a plurality of successive carbon nanotube bundles aligned along the same direction. The carbon nanotube film structure further includes a plurality of micropores formed by the carbon nanotube bundles.

[0013] In another embodiment, a method for fabricating the above-described carbon nanotube film structure includes the steps of: (a) providing an array of carbon nanotubes; (b) pulling out, by a tool, one carbon nanotube film from the array of carbon nanotubes; (c) providing a frame, adhering the carbon nanotube film to the frame; (d) repeating the step (b) and the step (c) to thereby achieve a multi-layer carbon nanotube film structure; and (e) peeling the carbon nanotube film structure off the frame to achieve a self-sustained carbon nanotube film structure.

[0014] Other advantages and novel features of the present carbon nanotube film structure and method for fabricating the same will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings.

[0019] Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one preferred embodiment of the present carbon nanotube film structure and related method for fabricating the same, in at least one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner.

[0020] Reference will now be made to the drawings to describe, in detail, embodiments of the present carbon nanotube film structure and method for fabricating the same.

[0021] Referring to FIG. 1, a method for fabricating the carbon nanotube film structure includes the steps of: (a) providing an array of carbon nanotubes, quite suitably, providing a super-aligned array of carbon nanotubes; (b) pulling out a first carbon nanotube film from the array of carbon nanotubes, by using a tool (e.g., adhesive tape or another tool allowing multiple carbon nanotubes to be gripped and pulled simultaneously); (c) providing a frame and adhering the first carbon nanotube film to the frame along a first direction and removing the excess film outside the frame; (d) pulling out a second carbon nanotube film from the array of carbon nanotubes and adhering the second carbon nanotube film to the frame along a second direction to overlap the first carbon nanotube film to thereby form a two-layer carbon nanotube film structure; and (e) peeling the carbon nanotube film structure off the frame to achieve the carbon nanotube film structure.

[0022] In step (a), a given super-aligned array of carbon nanotubes can be formed by the substeps of: (a1) providing a substantially flat and smooth substrate; (a2) forming a catalyst layer on the substrate; (a3) annealing the substrate with the catalyst at a temperature in the approximate range from 700° C. to 900° C. in air for about 30 to 90 minutes; (a4) heating the substrate with the catalyst at a temperature in the approximate range from 500° C. to 740° C. in a furnace with a protective gas therein; and (a5) supplying a carbon source gas into the furnace for about 5 to 30 minutes and growing a super-aligned array of the carbon nanotubes from the substrate.

[0023] In step (a1), the substrate can, beneficially, be a P-type silicon wafer, an N-type silicon wafer, or a silicon wafer with a film of silicon dioxide thereon. Quite usefully, a 4-inch P-type silicon wafer is used as the substrate.

[0024] In step (a2), the catalyst can, advantageously, be made of iron (Fe), cobalt (Co), nickel (Ni), or any alloy thereof.

[0025] In step (a4), the protective gas can, beneficially, be made up of at least one of nitrogen (N₂), ammonia (NH₃), and a noble gas. In step (a5), the carbon source gas can, advantageously, be a hydrocarbon gas, such as ethylene (C₂H₄), methane (CH₄), acetylene (C₂H₂), ethane (C₂H₆), or any combination thereof.

[0026] The super-aligned array of carbon nanotubes can, opportunely, be in a height of about 200 to 400 microns and includes a plurality of carbon nanotubes paralleled to each other and substantially perpendicular to the substrate. The super-aligned array of carbon nanotubes formed under the above conditions is essentially free of impurities, such as carbonaceous or residual catalyst particles. The carbon nanotubes in the super-aligned array are packed together closely by van der Waals attractive force.

[0027] In step (b), the first carbon nanotube film can, beneficially, be pulled out from the super-aligned array of carbon nanotubes by the substeps of: (b1) selecting a plurality of carbon nanotube segments having a predetermined width; and (b2) pulling the carbon nanotube segments at an even/uniform speed to form the first carbon nanotube film.

[0028] In step (b1), quite usefully, the carbon nanotube segments having a predetermined width can be selected by using a wide adhesive tape as the tool to contact the super-aligned array. In step (b2), the pulling direction is, usefully, substantially perpendicular to the growing direction of the super-aligned array of carbon nanotubes.

[0029] More specifically, during the pulling process, as the initial carbon nanotube segments are drawn out, other carbon nanotube segments are also drawn out end to end, due to the van der Waals attractive force between ends of the adjacent segments. This process of drawing ensures a successive carbon nanotube film can be formed. The carbon nanotubes of the carbon nanotube film are all substantially parallel to the pulling direction, and the carbon nanotube film produced in such manner is able to formed to have a selectable, predetermined width.

[0030] The width of the first carbon nanotube film depends on the size of the carbon nanotube array. The length of the first carbon nanotube film is arbitrary. In one useful embodiment, when the size of the substrate is 4 inches, the width of the first carbon nanotube film is in the approximate range from 1 centimeter to 10 centimeters, and the thickness of the first carbon nanotube film is in the approximate range from 0.01 to 100 microns.

[0031] In step (c), the frame is advantageously a square or rectangular metal frame using arbitrary metallic material (most suitably, a chemically and mechanically durable metal/alloy). The size of the frame is depends on the actual needs of utilization. When the width of the frame is larger than the width of the first carbon nanotube film, a plurality of the first carbon nanotube films can be adhered on the frame side by side and parallel to each other.

[0032] It is noted that because the carbon nanotubes in the super-aligned array in step (a) has a high purity and a high specific surface area, the first carbon nanotube film is adhesive. As such, in step (c), the first carbon nanotube film can be adhered to the frame directly. The first carbon nanotube film is, beneficially, fixed on/to the frame at an edge thereof.

[0033] In step (d), the first carbon nanotube film and the second carbon nanotube film are combined (i.e., attached to one another) by van de Waals attractive force to form a stable two-layer film structure. Additionally, a discernable angle between the first direction and the second direction is in the approximate range from greater than about 0° to about 90°. In one useful embodiment, the angle is about 90°.

[0034] Quite beneficially, a third carbon nanotube film or a plurality of carbon nanotube films can be provided and adhered to the frame one after another to form a multi-layer carbon nanotube film structure. The number of the layers is arbitrary and depends on the actual needs/use.

[0035] In one useful embodiment, an additional step (f) of treating the carbon nanotube film structure with an organic solvent can, advantageously, be further provided before the step (e).

[0036] In step (f), the organic solvent is volatilizable and can be selected from the group consisting of ethanol, methanol, acetone, dichloroethane, chloroform, and combinations thereof. The carbon nanotube film structure can, beneficially, be treated by either of two methods: dropping the organic solvent from a dropper to soak the entire surface of side carbon nanotube film structure or immerging the frame with the carbon nanotube film structure thereon into a container having an organic solvent therein. After being soaked by the organic solvent, the carbon nanotube segments in the carbon nanotube film can at least partially shrink into carbon nanotube bundles due to the surface tension created by the organic solvent. Due to the decrease of the specific surface via bundling, the coefficient of friction of the carbon nanotube film is reduced, but the carbon nanotube film maintains high mechanical strength and toughness. Further, due to the shrinking/compacting of the carbon nanotube segments into the carbon nanotube bundles, the parallel carbon nanotube bundles are, relatively, distant (especially compared to the initial layout of the carbon nanotube segments) to each other in one layer and cross with the parallel carbon nanotube bundles in each adjacent layer. As such, a carbon nanotube film having a microporous structure can thus be formed (i.e., the micropores are defined by the spacing/gaps between adjacent bundles). The resulting spacing can, beneficially, be about in the range of 100-500 mesh. Accordingly, the carbon nanotube film structure can be used, e.g., as a filtration membrane or as an electrode in a lithium battery. Besides, due to the excellent conductivity of the carbon nanotubes, the crossed carbon nanotube bundles can form a conductive network in the carbon nanotube film structure. This conductive network structure can be used in electromagnetic shielding.

[0037] It is to be understood that the microporous structure is related to the layer number of the carbon nanotube film structure. The greater the number of layers that are formed in the carbon nanotube film structure, the greater the number of bundles in the carbon nanotube film structure will be. Accordingly, the spacing/gaps between adjacent bundles and the diameter of the micropores will decrease. Further, a carbon nanotube film structure having arbitrarily width and length can be formed by piling a plurality of carbon nanotube films and partially overlapping such with each other. The width and length of the carbon nanotube film structure are not confined by the width and the length of the carbon nanotube film pulled from the array of carbon nanotubes.

[0038] Referring to FIG. 2, a two-layer carbon nanotube film structure, formed by the method described above, is shown. The carbon nanotubes in each layer are aligned. The angle between the aligned directions of the two layers is 90°. The two layers are combined by van de Warrls attractive force to form a stable film structure. After being treated with ethanol, the carbon nanotubes compact/shrink to bundles, and a space/distance is formed between every two adjacent bundles in each layer. Bundles in two layers cross with each other to form a microporous structure. The diameter of the respective micropores is in an approximate range of 10 nanometers to 10 microns.

[0039] Referring to FIG. 3, a four-layer carbon nanotube film structure, formed by the method described above, is shown. The carbon nanotubes in each layer are aligned. The angle between the aligned directions of any two adjacent layers is 90°. Four layers are combined by van de Warrls attractive force, with each subsequent carbon nanotube film being deposited directly on a preceding carbon nanotube film, to form a stable film structure. After being treated with ethanol, the carbon nanotubes compact/shrink to bundles and a space/distance is formed between every two adjacent bundles in each layer. Bundles in the four layers cross with each adjoining layer to form a microporous structure. The diameter of the respective micropores is in an approximate range from 1 nanometer to 1 micron.

[0040] Finally, it is to be understood that the above-described embodiments are intended to illustrate rather than limit the invention. Variations may be made to the embodiments without departing from the spirit of the invention as claimed. The above-described embodiments illustrate the scope of the invention but do not restrict the scope of the invention.