Abstract:
Aligned multi-walled carbon nanotubes were grown on both sides of a metallic or metal-coated substrate by water vapor-assisted chemical vapor deposition. Aligned carbon nanotube films of thickness ranging from 1 μm to over 100 μm were obtained. By manipulating various operating factors—position of substrate in the reactor, amount of water vapor, amount of catalyst, reactor temperature, and growth time, the morphology and thickness of these carbon nanotube films could be adjusted.

Description:
[0001]     This application claims priority to provisional application Ser. No. 60/847,597, filed on Sep. 28, 2006 and which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present inventive subject matter relates to a method of synthesis of an aligned carbon nanotube (CNT) multilayer composite on both sides of a metallic substrate through catalytic pyrolysis of hydrocarbon in the presence of water. More particularly, the inventive subject matter relates to controlling the thickness of the CNTs on both sides of the metallic substrate by varying the operating parameters.  
       BACKGROUND OF THE INVENTION  
       [0003]     With the continually diminishing size of electronic devices, the heat generated by such devices can cause structural damage due to over-heating. It is highly desirable to design and fabricate thermal interface materials (TIMs) with exceptionally high thermal conductivity for transporting heat efficiently from electronic components to a heat sink.  
         [0004]     One such thermal interface material is a carbon nanotube (CNT), which has shown unique and attractive mechanical, electrical, and thermal properties. Several studies have revealed that CNTs have unusually high thermal conductivity in their axial direction. For example, molecular dynamic simulations of a single-walled nanotube (SWCNT) indicated that the thermal conductivity of the SWCNT can be as high as 6600 Wm −1 K −1  at room temperature. See “Unusually High Thermal Conductivity Of Carbon Nanotubes” by Berber et al. “Thermal Transport Measurements of Individual Multiwalled Nanotubes”, Kim et al.,  Phys Rev. Lett.,  87, 215502-1, 2001 disclosed measuring the thermal conductivity of a single multi-walled carbon nanotube (MWCNT) using a micro-fabricated suspended device, and their measurement showed that the thermal conductivity was larger than 3000 Wm −1 K −1  at room temperature. Realizing that CNTs are good TIM candidates, efforts have been made on the use of dispersed CNTs as thermal conducting fillers in polymer composites. For example, “Thermal Conductivity Improvement of Silicone Elastomer with Carbon Nanotube Loading,” Liu et al.,  Appl Phys Lett.,  84, 4248, 2004 reported a thermal conductivity ranging from 1.1 to 1.9 Wm −1 K −1  as the CNT loading in a polymer composite increased from nil to 3.8 wt %. The below-expectation enhancement could be attributed to the random orientation of CNTs in the polymer matrix and the existence of interface thermal resistances between the actual heat source/sink and the TIM device. To avoid these problems, Huang et al. disclosed in “Aligned Carbon Nanotube Composite Films for Thermal Management”  Adv. Mater.,  17, 1652, 2005 growing aligned CNTs on a silicon substrate and then fabricated a polymer composite film with such protruded, aligned CNTs running from one side of the TIM device to the other. An enhancement in thermal conductivity from 0.56 Wm −1 K −1  for the pure elastomer matrix to 1.21 Wm −1 K −1  for the same polymer embedded with a 0.4 vol % aligned CNT array was obtained. Despite the fact that aligned CNTs should have formed ideal thermal conducting paths through the composite structure, this enhancement was still far below expectation.  
         [0005]     Various techniques have been developed for growth of aligned CNTs in well-aligned configurations, including plasma or laser enhanced chemical vapor deposition (hereinafter referred to as “CVD”) process by Vander et al.,  J. Phys. Chem. B.,  106, 13122, 2002, hydrocarbon-ferrocene/ferric carbonyl mixture by Srivastava et al.,  Carbon,  39, 201, 2001 and also by Lee et al.,  Appl Phys Lett.,  82, 448, 2003 and CVD of hydrocarbon on metal thin films by Ant et al.,  Appl Phys Lett.,  81, 3464, 2002 and also by Merchan et al.,  Carbon,  42, 599; 2004 or pattern printing catalysts by Lee et al.,  Letters to the Editor/Carbon,  42, 667, 2004 on substrates.  
         [0006]     There remains a need for a method of synthesizing an aligned carbon nanotube multilayer composite on both sides of a metallic substrate.  
       SUMMARY OF THE INVENTION  
       [0007]     In the present inventive subject matter, the use of aligned CNTs as TIM was further explored in order to take advantage of the exceptional thermal conductivity of CNTs. Instead of growing the CNTs on silicon, they were grown on a metal surface by water vapor-assisted CVD.  
         [0008]     The present invention provides a powerful method for producing aligned carbon nanotubes on double-sided metallic substrate through catalytic pyrolysis of hydrocarbon in the presence of water. The present inventive subject matter provides a design with two layers of aligned CNTs, which grow in two opposite directions (up and down) from a suspended metal layer with bottom and top surfaces exposed. The nanotubes with any desired diameter in the range of about 10˜100 nm and lengths up to about 200 μm, are generated by the pyrolysis of a suitable hydrocarbon on double-sided metallic substrate.  
         [0009]     The method comprises sputtering a thin layer of Cr and Au on both sides of metal foil as substrate. The method includes synthesizing aligned carbon nanotubes on both sides of substrate through catalytic pyrolysis of a suitable hydrocarbon gas at an appropriate temperature in the presence of water.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]     The process of growth of aligned carbon nanotubes on double-sided metallic substrate will now be described with reference made to the accompanying drawings, in which:  
         [0011]      FIG. 1  is a schematic diagram showing the steps of synthesizing aligned carbon nanotubes with the present inventive subject matter;  
         [0012]      FIG. 2  represents the relationship between the thickness of CNTs film and volume of water;  
         [0013]      FIG. 3  represents the relationship between the thickness of CNTs film and growth temperature;  
         [0014]      FIG. 4  represents the relationship between the thickness of CNTs film and amounts of ferrocene;  
         [0015]      FIG. 5  represents the relationship between the thickness of CNT film and growth time;  
         [0016]      FIGS. 6   a  and  6   b  show scanning electron micrograph (SEM) images of aligned carbon nanotubes grown on top and bottom of metallic substrate by pyrolysis, respectively;  
         [0017]      FIG. 7   a  shows a transmission electron micrograph (TEM) of a bundle of aligned carbon nanotubes;  
         [0018]      FIG. 7   b  shows a TEM image of a CNT with trapped Fe catalyst particles;  
         [0019]      FIG. 7   c  shows a TEM image of a CNT with a metal particle at its tip;  
         [0020]      FIG. 7   d  shows a TEM image of a typical CNT with bamboo structure;  
         [0021]      FIG. 8   a  shows photo of aligned carbon nanotubes grown on both sides of copper sheet (thickness: 0.3 mm);  
         [0022]      FIG. 8   b  shows photo of aligned carbon nanotubes grown on both sides of copper foil (thickness: 0.007 mm);  
         [0023]      FIG. 8   c  shows photo of aligned carbon nanotubes grown on both sides of copper foil (thickness: 0.0125 mm); and 
     
    
       [0024]     TABLE 1 shows thermal performance of different TIM devices in the literature.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0025]     The process of synthesizing aligned carbon nanotubes on double-sided metallic substrate  2  is shown schematically in  FIG. 1 . The proposed multilayer composite structure  4  has two films of aligned CNTs  6   a  and  6   b  grown on the opposite faces of a suspended substrate  2 . Provided that the substrate is sufficiently thin and highly conductive, this double-sided design for a TIM device has the distinct advantage that it can fit between the heat source and heat sink to minimize the resistance to heat flow due to the presence of an air gap. Two kinds of substrates were chosen in the experiments. One was an electrically conductive material, for example metal plate or metal foil; the other was an electrically insulating material, for example silicon wafer. Whether the material selected for the substrate was electrically conducting or electrically insulating depended on the desired application for the thermal interface device. Different metals including silver, copper, and gold were used as substrates. Gold was found to be the best substrate for growth of aligned CNTs. However, copper is much cheaper and has a higher thermal conductivity (388 Wm −1 K −1 ) than that of gold (315 Wm −1 K −1 ). For these reasons, a thin film of gold was sputtered on the surface of the copper substrate. It is known that the adhesion of gold on copper is not sufficiently strong. By trial and error, it was found that good adhesion could be achieved with chromium  8  between gold  10  and copper. Thus, chromium films of thickness 12 nm and gold films of thickness 20 nm were deposited on both sides of three substrates—silicon wafer, copper plate and copper foil using an ARC-12M sputtering machine.  
         [0026]     CVD synthesis of aligned CNTs was performed in a horizontal quartz tube (2.5 cm in diameter and 70 cm in length) housed in a single-zone furnace (Carbolite, UK). A quartz boat (2.2 cm wide and 10 cm long) was placed in the middle of the quartz tube, where the temperature was the highest along the tube length. The substrates were placed in the middle of the boat except otherwise specified. Ferrocene, with a decomposition temperature of about 190° C., was stationed at the inlet of the quartz tube. Each reaction run started with the CVD reactor at room temperature. A specified amount of double deionized (DDI) water was injected using a micro-syringe under the quartz boat holding the ferrocene. Then, flow of argon was initiated while heating up the CVD reactor. Typically 20 minutes was required to reach the desirable reactor temperature and at that point, ethylene and hydrogen were introduced. CVD growth of aligned CNTs was carried out at about 725˜800° C. with the flow rates of ethylene, hydrogen and argon set at 40, 16 and 200 sccm, respectively. The temperature at the inlet of the quartz tube was about 200° C. in a typical reaction run. The flow of ethylene/hydrogen/argon mixture was maintained for the entire growth period ranging from about 10 to 60 min. After the reaction, the furnace was allowed to cool down to room temperature while maintaining the flow of argon.  
         [0027]     In addition, operating factors that can significantly influence the growth of CNTs on such a double-sided metal substrate were investigated in detail. The following examples are illustrative of an embodiment of the present inventive subject matter and, thus, are not intended to limit the scope of the present inventive subject matter.  
       EXAMPLE 1  
       [0028]     Aligned CNTs were produced by using a chemical vapor deposition system, which included a horizontal quartz tube (2.5 cm diameter; 70 cm long) housed in a sing-zone furnace (Carbolite, UK). The substrate was placed in the middle of the heating zone, which had the maximum temperature of 750° C. The following conditions were used in experiments where a different volume of water was added into the reaction system:  
         [0029]     Substrate: Thin Cr film (12 nm) and Au film (20 nm) were deposited on both sides of the silicon wafer.  
         [0030]     Volume of water added: 0 mL, 0.2 mL, 0.3 mL, 0.4 mL, 0.6 mL  
         [0031]     Amount of catalyst: 150 mg ferrocene  
         [0032]     Growth temperature: 750° C.  
         [0033]     Growth time: 20 min  
         [0034]     Flow rate of argon: 200 sccm  
         [0035]     Flow rate of hydrogen: 16 sccm  
         [0036]     Flow rate of ethylene: 40 sccm  
         [0037]     Under the above conditions, different thicknesses of CNT films were synthesized on double-sided metallic substrate. The relationship between the thickness of CNT film and the volume of water is shown in  FIG. 2 . It is seen that much longer and denser aligned CNT&#39;s are obtained with the injection of water.  
       EXAMPLE 2  
       [0038]     The following conditions were used in experiments where different growth temperatures were chosen:  
         [0039]     Substrate: Thin Cr film (12 nm) and Au film (20 nm) were deposited on both sides of the silicon wafer.  
         [0040]     Volume of water added: 0.4 mL  
         [0041]     Amount of catalyst: 150 mg ferrocene  
         [0042]     Growth temperature: 725° C., 750° C., 775° C., 800° C.  
         [0043]     Growth time: 20 min  
         [0044]     Flow rate of argon: 200 sccm  
         [0045]     Flow rate of hydrogen: 16 sccm  
         [0046]     Flow rate of ethylene: 40 sccm  
         [0047]     Under the above conditions, different thicknesses of CNT films were synthesized on double-sided metallic substrate. The relationship between the thickness of CNT film and growth temperature is shown in  FIG. 3 . It is seen that a thicker aligned CNT film can be synthesized by higher temperature. Temperature is a factor for aligned CNT synthesis, the thickness of CNT film can be controlled by varying CVD growth temperature.  
       EXAMPLE 3  
       [0048]     The following conditions were used in experiments where different amounts of ferrocene were chosen as catalyst:  
         [0049]     Substrate: Thin Cr film (12 nm) and Au film (20 nm) were deposited on both sides of the silicon wafer.  
         [0050]     Volume of water added: 0.4 mL  
         [0051]     Amount of catalyst: 30 mg, 100 mg, 150 mg, 200 mg ferrocene  
         [0052]     Growth temperature: 750° C.  
         [0053]     Growth time: 20 min  
         [0054]     Flow rate of argon: 200 sccm  
         [0055]     Flow rate of hydrogen: 16 sccm  
         [0056]     Flow rate of ethylene: 40 sccm  
         [0057]     Under the above conditions, different thicknesses of CNT films were synthesized on double-sided metallic substrate. The relationship between the thickness of CNT film and amounts of ferrocene is shown in  FIG. 4 . Thicker CNT film can be synthesized by larger amounts of ferrocene. The amounts of ferrocene are another important factor for aligned CNTs synthesis, the thickness of CNTs film can be controlled by varying the amount of the catalyst.  
       EXAMPLE 4  
       [0058]     The following conditions were used in experiments where different growth time was chosen for synthesis:  
         [0059]     Substrate: Thin Cr film (12 nm) and Au film (20 nm) were deposited on both sides of the silicon wafer.  
         [0060]     Volume of water added: 0.4 mL  
         [0061]     Amount of catalyst: 150 mg ferrocene  
         [0062]     Growth temperature: 750° C.  
         [0063]     Growth time: 10 min, 20 min, 30 min, 60 min  
         [0064]     Flow rate of argon: 200 sccm  
         [0065]     Flow rate of hydrogen: 16 sccm  
         [0066]     Flow rate of ethylene: 40 sccm  
         [0067]     Under the above conditions, different thicknesses of CNT films were synthesized on double-sided metallic substrate. The relationship between the thickness of CNT film and the growth time is shown in  FIG. 5 .  FIG. 5  shows that the rate of growth is gradually reduced as the growth time is increased. No remarkable change of thickness of CNT film on both sides of the substrate when the growth time is increased from 30 min to 60 min. This clearly indicates that CNT films grow rapidly up to about 30 min and growth almost terminates after 30 min. The decreased growth rate could be attributed to the fixed amount of ferrocene in the reaction system.  
       EXAMPLE 5  
       [0068]     On the basis of the optimization experiments, the thickness of CNT films on double-sided substrate can be controlled by varying the operating parameters. The following conditions were used in experiments for synthesis:  
         [0069]     Substrate: Thin Cr film (12 nm) and Au film (20 nm) were deposited on both sides of the silicon wafer (400 μm thickness), copper plate (0.5 inch diameter; 0.3 mm thickness) and copper foil (2 cm wide; 4 cm long; 0.007 mm thickness) respectively  
         [0070]     Volume of water added: 0.4 mL  
         [0071]     Amount of catalyst: 150 mg ferrocene  
         [0072]     Growth temperature: 750° C.  
         [0073]     Growth time: 20 min  
         [0074]     Flow rate of argon: 200 sccm  
         [0075]     Flow rate of hydrogen: 16 sccm  
         [0076]     Flow rate of ethylene: 40 sccm  
         [0077]     Under the above conditions, aligned CNTs multilayer composites were produced. It was very difficult to characterize samples with a ductile substrate such as a thin metal plate or foil by scanning electron microscopy (SEM). Therefore, optimization of the operating factors was accomplished by using a silicon wafer as substrate.  FIGS. 6   a  and  6   b  show SEM images of aligned carbon nanotubes grown on the top and bottom surfaces of a metallic silicon wafer substrate, respectively. The nanotubes are perpendicular to the substrate.  
         [0078]     The detailed structure of the CNTs was studied by High Resolution Transmission Electron Microscopy (“HRTEM”) ( FIG. 3 ). As can be seen, the CNTs were of different sizes but had a similar structure.  FIG. 7   a  shows a bundle of synthesized aligned CNTs, comprising mainly small carbon nanotubes.  FIG. 7   b  shows a CNT with trapped Fe catalyst particles. Most of the CNTs had an end closed with a metal particle at the tip ( FIG. 7   c ).  FIG. 7   d  shows a particular nanotube with an inner and outer diameter of approximately 8 and 20 nm, respectively, and contained about 25 graphitic layers in each sidewall.  
         [0079]     Based on experiments on silicon substrate, the following conditions were used to synthesize 50 μm thick CNT films on both sides of a copper substrate sputtered with Cr and Au: volume of water injected: 0.3 mL, catalyst: 100 mg ferrocene, growth temperature: 750° C., growth time: 20 min.  FIGS. 8   a ,  8   b  and  8   c  show the photographs of a copper sheet (0.5 inch diameter; 0.3 mm thickness); a copper foil (0.5 inch diameter, 0.007 mm thick); and a copper film (1 inch by 1 inch, 0.0125 mm thick) with aligned CNTs grown on both sides, respectively.  
         [0080]     The present method is highly advantageous over prior art methods of growing aligned CNTs since the thickness of CNT films on both sides of a metallic substrate can be controlled by varying different operating parameters. With the presence of water, the thickness of CNT film can be controlled from 2 μm to 200 μm. It is very important to control the density, length, and alignment of CNTs for thermal interface devices.  
         [0081]     In the present design of an aligned CNT multilayer composite, it is not necessary to strip off the substrate from the CNT film. Thus, a self-supporting thin film, which can provide great practical and potential commercial value, can be finally obtained.  
         [0082]     The thermal resistance of the TIM device with the metal substrate was evaluated using a steady state measurement system which was designed in accordance with ASTM D5470. The thermal resistance of the 12.5 μm copper foil (or a total TIM device thickness of 112.5 μm) was determined to be 12 mm 2  K/W under an applied contact pressure of 0.3 MPa. This is lower than those reported in the literature (Table 1). For example, the thermal resistance of the aligned CNT composite film synthesized by Huang et al.  Adv. Mater.,  17, 1652, 2005 ranged from 100 to 450 mm K/W as the sample thickness increased from 100 to 500 μm. The thermal resistance of the CNT film synthesized by Xu et al. “Proc 9 th  Intersociety Conference On Thermal And Thermomechanical Phenomena In Electronic Systems, Las Vegas, Nev. New York,” IEEE, 549, 2004 using plasma enhanced CVD was 100 mm 2  K/W under a contact pressure of 0.16 MPa and 23 mm 2  K/W under 0.445 MPa. Results are comparable to those of Zhang et al., who reported a thermal resistance of 15 mm 2  K/W with a film thickness ranging from 30 to 70 μm.  
         [0083]     The foregoing description of the preferred embodiments of the process for producing aligned CNTs has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiments disclosed. It is intended that the scope of the invention be defined by all the embodiments encompassed within the following claims and their equivalents.