Abstract:
A CNT composite ( 10 ) includes a matrix ( 14 ) and a number of CNTs ( 12 ) embedded in the matrix. The matrix has a surface ( 102 ) and an opposite surface ( 104 ). Head portions of the respective CNTs are consistently oriented, parallel to the surfaces of the matrix. A method for manufacturing the composite includes (a) providing a substrate and depositing a catalyst film on the substrate; (b) forming the array of CNTs via the catalyst film on the substrate; (c) immersing the CNTs in a liquid matrix material, infusing the liquid matrix material into the array of CNTs; (d) taking the carbon nanotubes with the infused matrix out of the liquid matrix; (e) pressing the still-soft matrix and the CNTs therein, in order to arrange the CNTs consistently and parallel to the surfaces of the matrix; and (f) solidifying and peeling away the matrix to produce the CNT composite.

Description:
BACKGROUND 
     1. Field of the Invention 
     The invention generally relates to carbon nanotube composites and, more particularly, to a carbon nanotube composite having matrix-parallel nanotube structures and a method for manufacturing the carbon nanotube composite. 
     2. Discussion of Related Art 
     Carbon nanotubes (also herein referred to as CNTs) were first observed and reported in an article by Iijima in 1991 (Nature, Vol. 354, Nov. 7, 1991, pp. 56-58). Typically, CNTs are very small tube-shaped structures and are essentially consist of graphite. CNTs have interesting and potentially useful properties, such as electrical and mechanical properties, and offer potential for various application fields. 
     In polymers, CNTs have substantial potential for enhancing the carbon nanotube (CNT) composite&#39;s strength, toughness, electrical conductivity and thermal conductivity. Referring to FIG. 11, U.S. Pat. No. 6,924,335, the contents of which are hereby incorporated by reference, discloses a kind of CNT composite  40 . This CNT composite  40  has a number of CNTs  42  embedded in a polymer matrix  44 . In the CNT composite  40 , the CNTs  42  are parallel to one another and perpendicular to surfaces  46 ,  48  of the CNT composite  40 . However, the CNTs  42  do not contact one another. The configuration limits a thickness of the CNT composite  40  to be equal to a length of the CNTs  12 , i.e., a several hundreds microns, and limits a direction for thermal and/or electrical conduction. Furthermore, a range of thermal and/or electrical conduction is restricted to the length of the CNTs  12 . 
     Therefore, a CNT composite with good thermal/electrical conductivity in a direction parallel to a surface of the CNT composite and perpendicular to a growing direction of the CNTs and, more particularly, a method for manufacturing such a composite are desired. 
     SUMMARY OF THE INVENTION 
     A CNT composite includes a matrix and a number of CNTs embedded in the matrix. The matrix has a main surface and an opposite surface. The CNTs are arranged in a consistent orientation, and at least one portion of the CNTs is parallel to the main surface of the matrix. 
     A method for manufacturing the CNT composite includes: 
     providing a number of carbon nanotubes distributed in a number of parallel strip-shaped areas of a substrate; 
     immersing the carbon nanotubes into a liquid matrix in order to introduce the liquid matrix into clearances among the carbon nanotubes; 
     taking the carbon nanotubes with the matrix bound thereto out of the liquid matrix; 
     pressing the carbon nanotubes down along a consistent direction; 
     solidifying the matrix bound to the carbon nanotubes; and 
     peeling off the matrix bound with the carbon nanotubes from the substrate, thereby obtaining a CNT composite. 
     Other advantages and novel features of the CNT composite and the present method thereof will become more apparent from the following detailed description of preferred embodiments when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the present composite and method can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, the emphasis instead being placed upon clearly illustrating the principles of the present composite and method. 
         FIG. 1  is a schematic, cross-section view of a CNT composite, according to a preferred embodiment; 
         FIG. 2  is a schematic, top view of the CNT composite of  FIG. 1 ; 
         FIG. 3  is a schematic, cross-section view of the CNT composite, according to another preferred embodiment; 
         FIG. 4  is a schematic, top view of the CNT composite of  FIG. 3 ; 
         FIG. 5  is a top view of a substrate with strip-shaped catalyst films deposited thereon, according to a preferred embodiment; 
         FIG. 6  is a schematic, cross-section view showing a number of aligned CNTs deposited on the substrate of  FIG. 5 ; 
         FIG. 7  is similar to  FIG. 6 , but showing the substrate with the CNTs deposited thereon immersed in a liquid matrix material; 
         FIG. 8  is similar to  FIG. 7 , but showing the substrate with the CNTs deposited thereon embedded in a semi-solidified matrix material; 
         FIG. 9  is similar to  FIG. 8 , but showing the substrate with the CNTs deposited thereon embedded in a solidified matrix material after the CNTs are pressed down; 
         FIG. 10  is similar to  FIG. 1 , but showing the CNT composite utilized as a smart switch; and 
         FIG. 11  is a schematic, cross-section view of a conventional CNT composite, according to the prior art. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplifications set out herein illustrate at least one preferred embodiment of the present composite and method, in one form, and such exemplifications are not to be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 and 2 , a CNT composite  10 , according to a preferred embodiment, is shown. The CNT composite  10  includes a matrix  14  and a number of CNTs  12  uniformly embedded in the matrix  14 . The matrix  14  is in a thin-film form. The matrix  14  has a length, a width and a thickness. The matrix has a first planar surface  102  and a second planar surface  104  opposite to the first planar surface  102 . The first planar surface  102  and the second planar surface  104  are both defined by the width and the length of the matrix  14 . Head/upper portions of the CNTs  12  are aligned substantially parallel to the second planar surface  104 , and growth end portions of the CNTs  12  are substantially perpendicular and attached to the first planar surface  102 . Furthermore, the CNTs  12  are distributed in a number of parallel rows aligned along a length direction of the CNT composite  10  and in a number of columns aligned along a width direction of the CNT composite  10 . The CNTs  12  in the same row but in two adjacent columns approach but do not contact one another. The matrix  14  may, usefully, be a macromolecular material such as epoxy resin, acrylic acid resin, silicone, and thermal conductive grease, or a mixture thereof. A length of the CNTs  12  can be selected according to application need and/or other fabricating conditions. The length of the CNTs  12  is, advantageously, in a range of about 100-200 microns, in order to maximize their potential thermal/electrical conductivity. 
     Referring to  FIGS. 3 and 4 , a CNT composite  20 , according to the second preferred embodiment, is shown. The CNT composite  20  includes a matrix  24  and a number of CNTs  22  uniformly embedded in the matrix  24 . The matrix  24  is, most suitably, in a thin-film form. The matrix  24  has a length, a width and a thickness. The matrix has a first planar surface  202  and a second planar surface  204  opposite to the first planar surface  202 . The first planar surface  202  and the second planar surface  204  are both defined by the width and the length of the matrix  24 . The CNT composite  20  is similar to the CNT composite  10 , except that each of the CNTs  22  contacts other CNTs  22  in the same row and two adjacent columns. Each of the CNTs  22  can provide a thermal and/or electrical conduction path. The contacting CNTs  22  can provide a number of paths for thermal and/or electrical conduction, and the paths are parallel to the two opposite planar surfaces  202 ,  204 . Because of these paths, the CNT composite  20  has a good thermal and/or electrical conductivity in a direction parallel to the first and second planar surfaces  202 ,  204  thereof. 
     Referring to  FIGS. 3 and 4 , a CNT composite  20 , according to the second preferred embodiment, is shown. The CNT composite  20  includes a matrix  24  and a number of CNTs  22  uniformly embedded in the matrix  24 . The matrix  24  is, most suitably, in a thin-film form. The matrix has a first surface  202  and a second surface  204  opposite to the first surface  202 . The CNT composite  20  is similar to the CNT composite  10 , except that each of the CNTs  22  contacts other CNTs  22  in the same row and two adjacent columns. Each of the CNTs  22  can provide a thermal and/or electrical conduction path. The contacting CNTs  22  can provide a number of paths for thermal and/or electrical conduction, and the paths are parallel to the two opposite surfaces  202 ,  204 . Because of these paths, the CNT composite  20  has a good thermal and/or electrical conductivity in a direction parallel to the surfaces  202 ,  204  thereof. 
     Referring to  FIGS. 5 through 9 , a method for manufacturing the CNT composite  10  is described in detail, as follows. 
     In step  1 , as shown in  FIG. 5 , a substrate  16  is provided and a number of catalyst strips  18  are deposited thereon. The substrate  16  can be made of, for example, glass, quartz, silicon, alumina, etc. The catalyst film  18  can be made, e.g., of iron (Fe), cobalt (Co), nickel (Ni), or an alloy thereof. In this preferred embodiment, a silicon wafer is selected as the substrate  16 , an iron film of about 5 nanometers (nm) thick functioning as the catalyst film is deposited on an entire surface of the silicon wafer (i.e., substrate  16 ) by, for example, thermal deposition, electron-beam deposition, or sputtering deposition. The catalyst film is then divided into a number of separated strips by photolithography or masking, and, as such, the catalyst strips  18  are formed on the substrate  16 . 
     The catalyst strips  18  are in a number of parallel rows along a length direction of the substrate  16  and in a number of columns along a width direction of the substrate  16 . The widths and lengths of each the catalyst strips  18  are, usefully, substantially equal. The distances separating adjacent catalyst strips  18  are selected according to an application need. In the preferred embodiment, the distances separating adjacent catalyst strips  18  are a little longer than a predetermined length of the CNTs grown in a subsequent step. 
     The distribution density of the CNTs together with the width of the catalyst strips  18  determines the quantity of the CNTs. To obtain a sufficient quantity of CNTs, the width of each of the catalyst strips  18  is determined by the distribution density of the CNTs. The greater the distribution density of the CNTs, the less the width of the catalyst films  18 , vice verse. Accordingly, the widths of the catalyst films  18  are, advantageously, in a range from ten to several tens of microns (μm), approximately. 
     In step  2 , as shown in  FIG. 6 , an array of CNTs  12  is grown from the catalyst strips  18  on the substrate  16  by a chemical vapor deposition (CVD) process. Preferably, before the CVD process, the substrate  16  with the catalyst strips  18  deposited thereon is annealed in ambient air at 300-400° C. for approximate 10 hours, in order to transform the catalyst into nano-sized catalyst oxide particles. The catalyst oxide particles are then reduced to form the pure catalyst particles, by introducing a reducing agent such as ammonia or hydrogen. The annealing step is beneficial for transforming the catalyst of the catalyst strips  18  into uniform nano-sized catalyst particles, which will affect the uniformity of the CNTs grown in a subsequent step, since the CNTs directly grow from the catalyst particles. After that, the substrate  16  with the catalyst strips  18  deposited thereon is placed into a CVD reaction chamber, a carbon source gas is introduced into the chamber, and then the CNTs are formed on the substrate. The carbon source gas is, e.g., ethylene (C 2 H 4 ), methane (CH 4 ), acetylene (C 2 H 2 ), ethane (C 2 H 6 ), or another suitable hydrocarbon. In the preferred embodiment, the chamber is heated up to 700° C., an ethylene gas as a carbon source gas is introduced thereinto, and then the CNTs  12  are grown upon the catalyst strips  18  on the substrate  16 . 
     The length of the CNTs  12  determines the spacings between adjacent catalyst strips  18 . In other words, the longer the CNTs  12 , the wider the distances separating adjacent catalyst strips  18 . This configuration ensures that the distances between CNTs  12  in the same rows and adjacent columns are a little longer than the length of the CNTs  12 . According to the length of CNTs  12  in the preferred embodiment, the spacing between adjacent catalyst strips  18  is 100-200 μm, which is a little larger than the length of the CNTs  12 . 
     In step  3 , referring to  FIG. 7 , the CNTs  12  with the substrate  16  are immersed into a liquid matrix  14 ′, such as molten or solution of the matrix  14 , and then the CNTs  12  are surrounded with the matrix  14 . The matrix  14  is, advantageously, a resin, such as epoxy resin, acrylic resin, and silicone, thermal conductive grease, or a mixture thereof. In the preferred embodiment, a silicone and the steps include the following: a silicone, functioning as the matrix  14 , is dissolved into another liquid, e.g., ether, then a silicone solution is obtained. A small amount of a curing agent is added into the silicone solution to adjust a time for solidifying the solution in more than two hours. The CNTs are immersed into the silicon solution, surrounding the CNTs with the silicone. The CNTs are now physically combined with silicone, and then are taken out of the silicone solution, with the curing agent beginning to set the silicone matrix. The curing agent could be, e.g., an epoxy resin curing agent, alkaline type curing agent, and/or acid type curing agent. The alkaline type curing agent is a material, for example, selected from a group consisting of aliphatic diamine, aromatic polyamines, modified aliphatic amine, and other nitrogen compounds, and the acid type curing agent is a material, for example, selected from a group consisting of organic acid, anhydride, boron trifluoride complex, and other complex compound. 
     In step  4 , as shown in  FIGS. 8 and 9 , the CNTs  12  are pressed down when the matrix  14  is still soft and reflexible. Specifically, the CNTs  12  can be pressed down by a pressing means, such as a cylindrical tool or a polished plate. After the CNTs  12  are pressed down, the head/upper portions of the bent CNTs  12  are in a consistent direction perpendicular to the CNT  12  growing direction and parallel to the surface of the matrix  14 , and the attached/base end portions of the bent CNTs  12  are perpendicular or substantially so to the substrate  36 . The key actually is that the bent head portions of the CNTs  12  are essentially made parallel to the surface of the matrix  14  even while the base end portions thereof remain attached. It is not so much, in many instances, whether the base ends remain perpendicular to the substrate  36 . Then, the matrix  14  is cooled and solidified (i.e., curing is completed). The total time in step  3  and  4  should be controlled in a certain range to avoid being unable to press the CNTs down. The total time is determined by a curing rate of the matrix  14 , which, preferably, is about 15 minutes. 
     In step  5 , the solidified matrix  14  with the bent CNTs  12  embedded therein is peeled away from the substrate  16 , and then the CNT composite  10  is obtained. The CNT composite  10  includes the matrix  14  and a number of CNTs  12  uniformly embedded in the matrix  14 . The CNTs  12  are distributed in a number of parallel rows aligned along a length direction of the CNT composite  10  and in a number of columns aligned along a width direction of the CNT composite  10 . The CNTs  12  that are in the same row but in two adjacent columns approach but do not contact one another, in this particular embodiment. 
     It is noted that the method for fabricating the CNT composite can further include, after the peeling off step, a step of removing the remainder catalyst from the surface of the composite with a conventional approach such as cutting, grinding, etc. Depending on the application, the substrate may, however, be retained as part of the composite structure. 
     In a second embodiment, a method for manufacturing the CNT composite  20  is similar to that of the first preferred embodiment; expect that the distance separating adjacent catalyst strips are equal to or smaller than the length of the CNTs  22  in step  2 , in order to ensure that the CNTs  22  in the same row and in adjacent columns can contact one another after being pressed down. 
     The CNT composite can be applied in numerous fields. For example, according to whether or not the CNTs in the same row and in adjacent columns contact one another, they can be used as a thermal conductive material, electrical conductive material, smart switch, etc. 
     Referring to  FIG. 3 , in the second preferred embodiment, the length of CNTs  22  is larger than or equal to the spacing between adjacent CNTs columns. After being pressed down, the CNTs  22  in the same row and in adjacent columns can contact one another. Each of the contacting CNTs  22  can provide a thermal and/or electrical conduction path. The bent and contacted CNTs  22  embedded in the matrix  24  provide a number of thermal and/or electrical conduction paths parallel to the surface of the CNT composite  20 . Accordingly, the CNT composite  20  can function as a electrically/thermally conductive material with thermal/electrical conduction direction parallel to the surface thereof. 
     Referring to  FIG. 10 , the CNT composite  30  functions as a smart switch. The CNT composite  30  includes a matrix  34  and a number of CNTs  32 . The length of CNTs  32  is a little smaller than the spacing between the CNTs  32  in the same row and in adjacent columns. After being pressed down, the bent CNTs  32  in the same row and in adjacent columns cannot contact one another and are isolated with a layer/amount of matrix  34 . A voltage can be applied perpendicular to the CNTs  32  growing direction and parallel to the surface of matrix  34 . When the voltage is low, there is not a current passing through the CNT composite  30 ; and when the voltage is high enough, an electronic tunnel (i.e., essentially, arcing) occurs in the layer of matrix  34  between the CNTs in the same row and in adjacent columns. In such a high voltage state, the CNT composite  30  is electrically conductive parallel to the surface thereof. Because of these behavior differences based on applied voltage, the CNT composite  30  can, usefully, be applied as a smart switch by the control of the voltage applied thereto. 
     Finally, it is to be understood that the embodiments mentioned above 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.