US20060177369A1

US20060177369A1 - Method of using copper alloy substrate for growing carbon nanotubes

Info

Publication number: US20060177369A1
Application number: US11/052,927
Authority: US
Inventors: Hsin-Yuan Miao
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-02-09
Filing date: 2005-02-09
Publication date: 2006-08-10

Abstract

A method of using copper alloy substrate for growing carbon nanotubes, wherein copper is used as the base for copper matrix, and catalysts (ferrum, cobalt, and nickel) are dissolved in the copper matrix to form an alloy substrate. During the course of production for the carbon nanotube, the alloy substrate is positioned on the sample holder of the chamber, and the mechanical pump is used to extract the air pressure of the chamber under 10-3 torrs, and then the reaction gas is introduced into the chamber, and the total pressure is maintained at about 3 torrs. The reaction temperature of the alloy substrate at 850˜950° C. is provided by a heat-resistant tungsten fuse, and then preliminary dissociation of the reaction gas before introduced into the chamber is provided by a microwave generator. Furthermore, a radio frequency generator functions at a rate of 100˜600 W to completely dissociate the reaction gas and produces a self bias at −150V˜−450V to direct the straight growth and maintain the steady growth for 20 minutes for the carbon nanotube.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a method for growing carbon nanotube, which uses copper as the base for alloy substrate and methane as carbon source, introduces hydrogen and ammonia as dilute gas, and the time frame for hydrogen etching is utilized to control the effect of alloy catalyst being etched into nano-sized particles on the surface of alloy substrate, and then uses the nano-sized catalyst to control the growing diameter of the carbon nanotube during the growth period of the carbon nanotube.
2. Description of the Prior Art
“Carbon nanotube” is a material of the generation that was being predicted as full of potential values in applications. Currently, all of the researchers are focusing on the issue achieving carbon nanotube's full potential on applications, as well as efficiently control the growth of high quality carbon nanotubes. On the topic of production techniques, for example, (1) high yield and high quality during mass production, (2) demand for the application to be uniform and perpendicular to the array of matrix, control in density, control in average diameter, and control in average length during massive area production, (3) adjusting of the geometric shape (length, straightness, degree of chrility, etc.), size of the tube diameter, length, layers of tube walls, and whether or not the structure of graphite is perfect, etc; however, the topic mentioned previously is undoubtedly the most critical and difficult subject in the current research of carbon nanotubes.
Examples on using alloy as substrate to grow carbon nanotubes directly are rarely found in recent references, but each one has its distinct end use and growth method. Dissertations that focus on the application of electrical field emission and high capacitance, or on its low priced substrate and products that grow carbon are all trying to take the advantage of alloy substrate's electrical conductivity to increase the electrical use of carbon nanotube, such as its high capacitance property, strengthening the electrochemical property, etc; however, such growth method needs to be compensated with higher cost, for example, different growth methods are required to grow carbon nanotubes, or more expensive rare metals are used during one growth procedure.
The inner diameters of carbon nanotubes can go from 0.4 nm to several-tens nm, and the outer diameters of carbon tubes can also go from about 1 nm to several-hundreds nm, and the length could be within several μm to several-tens μm; however, the arrays of the stacking graphite layers on the tube walls can go from mono-layered nanotube to up to more than 50 layers of multi-layered nanotubes, and the gap interval between each graphite layer is about 0.34 nm.
Some data were shown to have effects on the physical and chemical properties of carbon nanotubes; all of the researchers use computer data simulations and the results of experiments to infer the actual relationship between the observed data and the physical and chemical properties of carbon nanotubes according to the basic data. The result shows that once the property of nanotube's “diameter” can be controlled effectively, all of the answers for other properties can be found. The end geometry (diameter) of the nanotube is a collaborated result, because the size of nanotube's diameter is affected by factors including number of layers, chrility, structure of layers, and the gap interval between layers; therefore, controlling the production parameters to manipulate the diameter of carbon nanotube in the production techniques becomes every researcher's mutual goal.
Due to the above mentioned disadvantages on growing carbon nanotubes, the inventor of the present application devoted his effort on improving the research and development; after several years of dedicated study, the method of using copper alloy substrate to grow carbon nanotube is finally successfully developed.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a method for growing carbon nanotube, which uses copper as matrix, where the catalysts component (ferrum, cobalt, and nickel) are dissolved in the copper matrix to form an alloy substrate, in order to provide the growth of nanotubes. Controlling the time frame of hydrogen etching during the earlier stage of growth aids in the manipulation of the size of nano-sized micro-particles being etched on the surface of alloy substrate; taking advantage of these nano-sized catalyst particles during the growth period, the growth diameter of carbon nanotubes is subsequently controlled. The secondary objective of the present invention is to provide a method for growing carbon nanotube, which controls the concentration parameter of the catalyst in the alloy substrate to change the growth profundity of carbon nanotube.
Another objective of the present is to provide a novel negative bias frequency, which at a rate of −150V˜−450V, produced by a radio frequency generator; not only could it dissociate the reaction gas, it could also increase the strength of conductivity within the electrical field in the sheath layer, which leads to a higher level of straightened growth for carbon nanotube.
Still another objective of the present invention is to provide a way to control the time frame of hydrogen etching on the alloy substrate to adjust the size of alloy catalysts being nano-sized. During hydrogen etching process, the surface of alloy substrate is activated causing the surface atoms to re-array into nanoscale particles; these nano-sized catalyst particles during the carbon nanotube growth period are taken into effect to further aid in the control of the condition for the size of the growth diameter of the carbon nanotube.
A method to accomplish the above invention objectives to create a method of using copper alloy substrate to grow carbon nanotubes, which comprises the following steps:

- (a) dissolving catalyst metals on top of a copper metal matrix to form an alloy substrate;
- (b) forming a reaction region on the surface of alloy substrate by grinding the alloy substrate with sand papers;
- (c) setting the alloy substrate onto a sample holder of the frequency adapting heated-tungsten-fuse chemical vapor deposition reaction chamber;
- (d) using a heat-resistant tungsten fuse to maintain the reaction temperature of the copper alloy substrate;
- (e) using a microwave generator to carry out preliminary dissociation of reaction gas introduced into the reaction chamber;
- (f) using a 13.56 MHz radio frequency generator to produce a self bias at a rate of −150V˜−450V;
- (g) using hydrogen etching in the reaction region to form nanoscale alloy catalyst particles;
- (h) carbon atoms start to grow into carbon nanotubes on top of the nanoscale alloy catalyst particles in the reaction region;
  wherein the reaction gas comprises diluted hydrogen gas and carbon source, methane. First of all, 80 sccm of hydrogen gas is introduced into the reaction chamber, and then in order to raise the reaction temperature of the alloy substrate to 850˜950° C. and maintain it for a steady time to go through hydrogen etching within the reaction chamber, the tungsten-fuse is heated along with the activation of the radio frequency generator to produce a self bias at a rate of −150V to −450V.

Subsequently, carbon source gas is introduced, and the procedure for introducing the carbon source gas is to emit 30 sccm first, and then following with an amount of 10 sccm/2 min consecutively until it reaches 60 sccm and then maintaining the steady growth for 20 minutes.
By controlling the time frame of hydrogen etching to manipulate the size of nano-refined catalyst particles would aid in the control of the dissolved and precipitated carbon atoms, of which would grow into the diameter size of the carbon nanotube.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings disclose an illustrative embodiment of the present invention which serves to exemplify the various advantages and objects hereof, and are as follows:
FIGS. 2 (a)˜(c) show the comparison of eutectic phase diagram between copper metal and transition metals, cobalt, ferrum, nickel;
FIG. 3 (a) shows the back appearance of metallography observation analysis using scanning electron microscope on the 80% copper—20% ferrum alloy substrate of the present invention;
FIGS. 3 (b)˜(c) show the spectrophotometer energy distribution analysis diagrams between dendrites crystal and dendrites of the metallography observation analysis on the 80% copper—0% ferrum alloy substrate of the present invention;
FIG. 3 (d) shows the mapping analysis diagram between dendrites crystal and dendrites of the metallography observation analysis on the 80% copper—20% ferrum alloy substrate of the present invention;
FIGS. 3 (e)˜(f) show line scanning analysis diagrams between dendrites crystal and dendrites of the metallography observation analysis on the 80% copper—20% ferrum alloy substrate of the present invention;
FIG. 4 (a) show the back appearance diagram of metallography observation analysis using scanning electron microscope, on the 70% copper˜10% ferrum—10% cobalt—10% nickel alloy substrate of the present invention;
FIGS. 4 (b)˜(c) show the spectrophotometer energy distribution analysis diagrams between dendrites crystal and dendrites of the metallography observation analysis on the 70% copper—10% ferrum—10% cobalt—10% nickel alloy substrate of the present invention;
FIG. 4 (d) shows the mapping analysis diagram between dendrites crystal and dendrites of the metallography observation analysis on the 70% copper—10% ferrum—10% cobalt—10% nickel alloy substrate of the present invention;
FIGS. 4 (e)˜(f) show line scanning analysis diagrams between dendrites crystal and dendrites of the metallography observation analysis on the 70% copper—10% ferrum—10% cobalt—10% nickel alloy substrate of the present invention;
FIG. 5 shows the reaction equation of the methane gas;
FIG. 6 shows the phase relationship diagram of the diamond and graphite;
FIGS. 7 (a)˜(b) show the scanning electron microscope diagrams of nanotubes growing from the mass area of the 80% copper—20% ferrum alloy substrate of the present invention;
FIGS. 7 (c)˜(d) show the scanning electron microscope diagrams of nanotubes growing from the mass area of the 80% copper—20% cobalt alloy substrate of the present invention;
FIGS. 7 (e)˜(f) show the scanning electron microscope diagrams of nanotubes growing from the mass area of the 80% copper—20% nickel alloy substrate of the present invention;
FIGS. 7 (g)˜(h) show the scanning electron microscope diagrams of nanotubes growing from the mass area of the 70% copper—10% ferrum—10% cobalt—20% nickel alloy substrate of the present invention;
FIG. 8 (a) shows the scanning electron microscope diagram of nanotube growing from the mass area of the copper—ferrum, copper—cobalt respective alloy substrates of the present invention;
FIG. 8 (b) show the scanning electron microscope diagram of nanotube growing from the mass area of the copper—nickel, copper—ferrum—cobalt—nickel respective alloy substrates of the present invention;
FIG. 9 (a) shows the scanning electron microscope diagram of the 80% copper—20% nickel alloy substrate of the present invention;
FIG. 9 (b) shows the scanning electron microscope diagram of the 80% copper—5% nickel alloy substrate of the present invention;
FIG. 10 shows the diagram of the reacting heat-resistant tungsten fuse of the present invention;
Table 1 shows the parameter of the growing carbon nanotube in the early stage in the experiment;
Table 2 shows the parameter of growing carbon nanotube after adjusting carbon supplying method, reaction temperature, and radio frequency self bias;
Table 3 shows the comparison of the carbon nanotube growth characteristics on different alloy substrates;
Table 4 shows the comparison of nanotube growth density on the copper—nickel alloy substrate with different concentrations of catalysts; and
Table 5 is the comparison table of hydrogen etching time vs. the size of nano-sized catalyst particles.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a method of using copper alloy substrate for growing carbon nanotubes, which uses “copper” as the substrate to grow carbon nanotube, whereas the alloy substrate is made up of copper (Cu) as matrix; in addition, catalysts, such as ferrum (Fe), cobalt (Co), nickel (Ni), etc. are dissolved to synthesize the alloy substrate. The high electrical conductivity of copper metal is the advantage of utilizing copper, as well as its strength, which is sturdy enough to support the alloy substrate. Since the copper metal has a low carbon melting rate (compared to catalysts, ferrum, cobalt, and nickel), and it is hard to be covered by carbon; moreover, the melting point for copper metal is not high as well (m.p. 1083° C.), hence, copper is suitable for the use of common manufacture procedure, and the steps are as follows:
1. Choosing the Combining Material of the Alloy Substrate
As shown in FIGS. 1 (a)˜(d), under the condition of melting carbon or not, appropriate catalysts and metal materials for the matrix are selected in the present invention, because “under the same temperature, ones that have higher carbon melting rate can be considered as catalysts, and ones that have low carbon melting rate can be considered as matrix”; hence, within the tolerable temperature limit (1000˜1200° C.), the carbon melting rates are as follows: nickel (0.53 wt %)>cobalt (0.47 wt %)>ferrum (0.38 wt %)>copper (0.00019 wt %). Since all of these metals are easy to obtain and stable, and copper metal to have the lowest carbon melting rate through the comparison, of which has the best electrical conductivity property, it is chosen as the best material for the matrix. Although ferrum, cobalt, and nickel have their own respective carbon melting rates, but they can still be selected as the best catalyst materials (compared to copper).
Accordingly, under the condition of the tolerable temperature range, copper metal reaches melting point as the material for the matrix; however, after catalyst metals are dissolved, the behavior of copper will be changed consequently with respect to the amount of catalyst metals added. Therefore, from FIGS. 2(a)˜(c), we can get the best amount to dissolve. Take FIG. 2 (b) as an example, from the right side of the diagram, liquid phase line in 90 wt % copper appears to be a “Knee,” which means the amount of solute changes too drastically around this point causing the liquid phase line to fluctuate dramatically. Because it is hoped that the whole alloy substrate would appear to be in liquid phase or liquefied on the surface, therefore, at the “Knee”, the ratios of solutes and solvents should take more considerations. After the completion of the alloy substrate of the present invention, “metallographic” analysis for the alloy substrate must be carried out, and the procedures are as followed: (1) cut to sample, (2) snag, (3) fine grind, (4) polish, (5) metallographic observation, etc. The procedures for metallographic observation comprise observation of the micro-appearance using scanning electron microscope and the qualitative element analysis of energy distribution spectrophotometer (EDX), which further includes line scanning and mapping, etc.
1-1. Metallographic Analysis on Copper—Ferrum Alloy Substrate
Due to the fact that the micro-structure of alloy substrate between the associations of copper with transition metals, ferrum, cobalt, nickel, etc. are quite similar, copper—ferrum alloy substrate is therefore used as an example here.
Referred to FIG. 3 (a), the appearance of the micro-structure using the scanning electron microscope can be clearly observed that the whole structure is divided into two phases: one is the dendrite, and the size is about 2˜3 μm which appears to be oval-shaped distributed on the surface of the alloy substrate; the other is the inter dendrite, which completely comprises the dendrites to build up the embodiment of alloy substrate. By analyzing the whole surface with the energy distribution spectrophotometer (EDX), a distribution ratio of weight percent of ferrum K 25.87 at % and copper K 74.13 at % is obtained.
Also referred to FIGS. 3 (b)˜(c), the energy distribution spectrophotometer analysis is focused on the independent dendrite and inter dendrite, where the dendrite from FIG. 3 (b) has ferrum K 88.11 at % and copper K 11.89 zt %, and the inter dendrite from FIG. 3 (c) has ferrum K 6.83 at % and copper K 93.17 at %. The numbers above suggest that the dendrite on the alloy substrate is composed of the catalysts which contains more than 88%, and the inter dendrite is taken up by the copper matrix which contains more than 93%; hence, catalysts and copper matrix are distributed amongst the whole embodiment of the alloy substrate, but they will change their respective concentrations according to the areas between dendrite and the inter dendrite.
FIG. 3 (d) shows the result of element image mapping on the copper element (left) and ferrum element (right). Apparently, the distribution of copper element occupies the entire alloy substrate region, but only a few appear on the dendrite. The distribution of ferrum element appears to be a strong contrast, where ferrum element occupies the whole dendrite region.
Referred to FIGS. 3 (e)˜(f), the line scanning element distribution is done on an independent straight line that trespasses the dendrite and inter dendrite. It is easy to see that the region where copper element disappears is where the ferrum element appears, and the small fluctuation on top of the rectangular wave is the reason of the compensation in between the two elements; therefore, catalysts will collectively become dendrite, which includes some copper components inside, in the binary alloy substrate formed by the combination of single catalyst and copper element, and copper element as the matrix would contain all of the catalysts and play the role of the solvent.
1-2. Metallographic Analysis of Copper—Ferrum, Cobalt, Nickel Alloy Substrate
The present invention has a multi-element alloy substrate of copper—ferrum—cobalt—nickel. This particular alloy substrate is designed to differentiate from the above mentioned binary alloy substrate. Referred to FIG. 4 (a) on the appearance of the micro-structure using scanning electron microscope can be clearly seen that the whole embodiment is divided into two phases; one is the “dendrite,” and the size is about 3˜8 um appeared to be rectangular-shaped dispersing throughout the surface of alloy substrate; the other is the “inter dendrite,” which completely holds the dendrites to build up the embodiment of alloy substrate. From the analysis of energy distribution spectrophotometer on the whole surface, the result of ferrum K 14.38 at %, cobalt K 13.67 at %, nickel K 12.18 at %, copper K 59.77 at % is obtained, which would suggest that even more catalyst metals (ferrum, cobalt, nickel) are distributed throughout the surface of the alloy substrate.
Also referred to FIGS. 4 (b)˜(c), the energy distribution spectrophotometer analysis is focused on the independent dendrite and inter dendrite, where the dendrite from FIG. 4 (b) has ferrum K 27.29 at %, cobalt K 28.75 at %, nickel K 20.05 at %, and copper K 23.92 at %, and the inter dendrite from FIG. 4 (c) has ferrum K 5.59 at %, cobalt K 5.84 at %, nickel K 8.58 at %, and copper K 79.99 at %. The numbers above suggest that the dendrite on the alloy substrate is composed of catalysts and copper matrix equally, and the inter dendrite is taken up by the copper matrix which comprises about 80%. This also shows that the catalysts and copper matrix are distributed throughout the whole embodiment of the alloy substrate, but by comparison to the above mentioned binary alloy substrate, the catalyst distribution area is even broader now.
FIG. 4 (d) shows the results of element image mapping on the copper element (left) and ferrum, cobalt, nickel elements (right). Apparently, the distribution of copper element occupies the entire alloy substrate region but only specifically appears in the region of dendrite region quantitatively, whereas others just appear slightly.
FIGS. 4 (e)˜(f), the line scanning element distribution is done on an independent straight line that trespasses the dendrite and inter dendrite. It is obvious to see that the region where copper element disappears is where the ferrum, cobalt, nickel elements appear; therefore, catalysts will distribute even broader throughout the whole alloy substrate quantitatively and with higher concentration on the dendrite in the multi-element alloy substrate comprising of multiple catalysts (ferrum, cobalt, nickel) and copper element (matrix).
2. Selecting Growth Parameter for Carbon Nanotubes
The present invention uses methane (CH₄) as the primary carbon source gas for the carbon atom; and uses hydrogen (H₂) and ammonia (NH₃) as the diluted gas. The gas flow, pressure, and reaction temperature required during reaction are decided by the arguments below:
During gas reaction, the source of carbon atom comes from the introduced gas containing carbon; when the gas was dissociated by the high power source (heat or radio frequency), free radicals are being stimulated and produced thereof and react with the introduced hydrogen. Hydrogen atom plays a very important role here; other than being inhibited by the growth of graphite, but it also aids in stimulating the production of carbon hydrogen free radicals. More importantly, hydrogen in the free radical form will take away the hydrogen in the carboxyl bond, leaving behind the carbon atom and the catalyst reaction on the surface of alloy substrate. It would also form a bond in sp2 or sp3 structure between the carbon and carbon atoms. Partial C—C sp2 non-diamond bond might be etched by hydrogen atoms leaving only pure sp3 bond. Concluding the role that hydrogen plays as shown in FIG. 5:

- (a) Forms sp3 C—H bond on the surface and saturates sp2 or sp3 carbon—carbon bonds on the surface;
- (b) Forms an active growing surface;
- (c) Eliminates the hydrogen on the surface and prevents the hydrogen entering the solid phase, consequently forming membrane similar to diamond;
- (d) Coated process leaning towards etching sp or sp3 carbon bonds;
- (e) Helps in producing gaseous phase C₂H₂

The present invention uses tungsten hot filament to provide heat; it could be used to maintain the reaction temperature required in the alloy substrate and also dissociates and ionizes the reaction gas. Apart from that, the structure of graphite is connected by sp2 C—C bond; to prevent the C—C bonds to be connected by sp3 forming membrane similar to diamond, the pressure of reaction gas and reaction temperature must be controlled within appropriate region. According to FIG. 5, the reaction parameters are decided as Table 1, where the selected reaction temperature is 1000˜1100° C., total reaction pressure is 3 torrs, and the flow rate of carbon source gas methane and diluted gas hydrogen are 60 sccm and 80 sccm, respectively. The radio frequency self bias (RF self bias) selected is −100V with 20 minute reaction time. There are four prototypes of alloy substrate for the growth of carbon nanotubes as shown in Table 1: (1)80 wt % copper—20 wt % ferrum
(2)80 wt % copper—20 wt % cobalt
(3)80 wt % copper—20 wt % nickel
(4)70 wt % copper—10 wt % ferrum—10 wt % cobalt—10 wt % nickel, etc.
3. Result and Parameter Adjustments in the Early Stage Growth
The present invention creates a radio frequency (RF) assisted hot filament chemical vapor deposition (HFCVD) machine according to the parameters in Table 1 to provide for the growth of carbon nanotube. The procedures are as follows:
Prepared the smelted copper alloy substrate into 10×5×0.5 mm size use waterjet or line cutting methods and position it onto the sample holder of radio frequency assisted hot filament chemical vapor deposition chamber.
Thereafter, extracting the air pressure inside the reaction chamber under 10-3 torrs with the mechanical pump and introducing the reaction gas (methane, hydrogen gas) according to the requirements of the experiment to maintain the total pressure at about 3 torrs. Before the reaction gas entering into the reaction chamber, preliminary dissociation of the reaction gas is provided by the microwave cavity to increase the degree of dissociation. Tungsten hot filament is used in the present invention primarily to heat to dissociate the reaction gas and maintain the required temperature for the alloy substrate in between 1000˜1100° C.
Furthermore, a 13.56 MHz radio frequency generator is added into the present device to operate in between 100˜600 W of afferent power, which would aid in the complete dissociation of reaction gas and produce plasma in the reaction region; the holder contrasting to ground could produce a −50˜−450V of negative self bias because of the addition, whereas the original radio frequency negative self bias is stabilized at −50V and the reaction time is maintained steady for 20 minutes.
In order to prevent the sudden production of the carbon atoms in the reaction region and without reacting with the catalysts, large amount of carbon atoms are gathered to form carbon black precipitating on the surface of the alloy substrate; therefore, diluted gas, hydrogen, and carbon source gas, methane, cannot enter into the reaction chamber at the same time, where the carbon source gas cannot be provided in full amount for reaction at once. 80 sccm of diluted gas, hydrogen, is introduced into the reaction chamber first and then increases the temperature inside the reaction chamber. When the chamber has reached the required reaction temperature, full amount of carbon source gas is introduced to proceed with the reaction. However, the reaction temperature is a problem. Previous decision on the reaction temperature is based on the range of the liquid phase after two metals had melted and taken the ratio of the two metals' melted proportion by weight into account; hence, the reaction temperature is lowered from the originally selected temperature 1000˜1100° C. to 850˜900° C. The procedure on introducing the carbon source gas into the chamber is being modified as well; 30 sccm of carbon source gas is introduced first and then added consecutively at a rate of 10 sccm/2 min until reaching an amount of 60 sccm.
The above procedures found that there are still many carbon precipitations stacked on the surface of the alloy substrate, because the surface of the alloy substrate is too smooth causing a decrease on the reaction surface for the catalyst and the carbon atoms on the alloy substrate. Not only the growth number of the carbon nanotubes would decrease but also cause the precipitation of the carbon black to stack up even more; therefore, #1000 sandpaper is used for grinding instead of using the metallographic grinding operation, and the result of the surface condition differs after the grinding. Slight scratches and bumps would remain on the surface, which not only would it increase the reaction regions for the carbon atoms and catalysts but also eliminate the inactive and oxygenated catalyst layer on the surface of alloy substrate; in addition, the radio frequency negative self bias is being raised to −150 W to increase the afferent power, which would then aid in the complete dissociation of the reaction gas producing more carbon atoms to participate in the reaction and compensate for the lack of heat from the tungsten heat filament.
Proceed on the continuous experiments according to the parameters from Table 2, the growth length of carbon nanotubes closely reach 3 μm within 20 minutes of reaction time, regardless of the components of the alloy substrate. The results are shown in FIGS. 7 (a)˜(h). On the alloy substrate with the four components, the mass areas of growth for the carbon nanotubes occur, but the degree of density of growth, appearance, and direction are differed according to different kinds of catalysts. The results are summarized in Table 3. Wherein, different properties of carbon nanotubes growing from the surface and corners occurred on copper—ferrum alloy substrate; the sizes of tube diameters are similar, but the length, appearance, and density are totally different as shown in FIGS. 7 (a)˜(b). The carbon nanotubes appear to be flat, disarrayed, and curvy, yet with higher growth density on the copper—cobalt alloy substrate; however, comparing to the three other alloy substrates, they have the characteristics of bigger diameters and longer length as shown in FIGS. 7 (c)˜(d).
The most important discovery is shown in FIGS. 7 (e)˜(f). Whether it's an single nickel—copper alloy substrate or a multi-catalysts nickel—ferrum—cobalt—copper alloy substrate, the growth of carbon nanotubes appear to be the best in the growth area, density, degrees of straightness on the alloy substrate with nickel catalyst; from FIGS. 7 (g)˜(h), the carbon nanotubes from the multi-catalysts alloy substrate not only have the properties mentioned above which are similar to the ones from the alloy substrate with nickel as the catalyst, but the diameter of the carbon nanotubes is smaller than the alloy substrate with nickel as the catalyst.
However, the smaller diameters of the carbon nanotubes is caused by the surface activation energy of the surface particles on the multi-alloy substrate, which larger than those of the other alloy substrates with less components, and the large surface activation energy would cause the phenomenon of smaller diameters in the carbon nanotubes to occur due to the effect of Laplace Pressure Effect during the precipitation of the carbon atoms in the formation of graphite.
From the result of previous metallography analysis, carbon nanotubes also grow under the phenomenon of mass area growth. In the copper—ferrum and copper—cobalt alloy substrate, dendrite often appear to be in the size of 3˜10 μm and distribute throughout the surface of the alloy substrate in oval-shaped. Above 88% of the components in dendrite are catalysts (ferrum or cobalt), and the primary component which takes up above 93% of the inter dendrite is the copper metal of the alloy substrate. As shown in FIG. 8 (a), within the two regions with different component ratios, they appear to be applicable methods for growing carbon nanotubes although there's an difference in the degree of density. Apparently, the carbon nanotubes would grow more intensively on the dendrite rather than the ones on the inter dendrite, and the whole appearance would resemble the dendrites. The carbon nanotubes grow on the inter dendrites are more sparse, dispersed, and without obvious shape.
However, the appearance of carbon nanotubes grow on the copper—nickel and copper—ferrum—cobalt—nickel alloy substrates are very different from the ones on copper—ferrum and copper—cobalt alloy substrates. There are still two phases occurred on alloy substrates, dendrites and inter dendrites; dendrites are placed quite intensively on the surface of the alloy substrate, and the components are distributed quite equally throughout, due to the fact that they appear in rectangular-shaped with width and length at several tlm and in grid-like distribution, therefore as shown in FIG. 8 (b), the growth of the carbon nanotubes has mass area and high degree of density, and it is hard to differentiate whether the carbon nanotubes are grown on dendrites or inter dendrites.
The phenomenon occurred in FIG. 7 could prove that the idea of using alloy substrate for growing carbon nanotubes is accurate and applicable; however, appropriate growth factors or parameters must be coordinated. For example, the method of providing carbon source must be combined with the reaction time from less to more, which should agree with the formation theory of cultivation and growth of the carbon structure; other than that, the reaction temperature of the alloy substrate must be maintained in between 850˜950° C., though the reaction gas would not be completely dissociated under this temperature, hence the power produced by the radio frequency generator would aid in the inefficiency of tungsten hot filament to dissociate reaction gas. The total pressure of the present invention is maintained at about 3 torrs, which is contrast to the total pressure of the growth membrane of semi-diamond.
The present invention proves that using big block of alloy substrate can grow carbon nanotubes that have mass area, good directional aptitude, and high degree of density; different types and densities of carbon nanotubes can be produced by manipulating the types of catalyst in the alloy substrate and different positions on the alloy substrate, thought the best result would come from the growth of copper—nickel or copper—ferrum—cobalt—nickel alloy substrates. As for the polishing step for the surface of alloy substrate, the growth of carbon nanotubes is enhanced by doing so. The property of the relationship between the value of half-height and width of Raman Spectroscopy and the thickness of the catalysts in the present invention implies there is a constant that moves toward the tendency of flattening.
4. Method for Controlling the Growth of Carbon Nanotubes
The most concentrated topic at the moment is to efficiently control the growth of high quality carbon nanotubes; on the production technique wise, the common goal is such as to high-throughput manufacture, increase productivity and control quality; on the mass area production wise, the most prominent and crucial problem at the moment is the desire for overall uniformity and perpendicular to the array of alloy substrate, control of the density, average diameter, and average length, etc.
Due to the problems described above, the present invention resorts to the self-made radio frequency assisted hot filament chemical vapor deposition machine to proceed with the production of carbon nanotubes; after ensuring the suitable reaction temperature and total reaction pressure, the radio frequency negative self-bias was adjusted complementing with the parameters like concentration of the dissolved catalyst metals, hydrogen etching time, change the elements of the reactants, etc, to achieve the characteristics of controlling the properties during the growth of carbon nanotubes.
4-1. Using the Concentration of the Catalysts to Control the Growth Density
The present invention comprises alloy substrate with different percentage of catalyst nickel metal, where (a) 80 wt % copper—20 wt % nickel and (b) 95 wt % copper—5 wt % nickel. In order to prevent the high heat caused by the line cutting method, of which might contribute to the sources of error due to the heaving surface, on the alloy substrate, the surface has to go under metallography grinding adjustment to obtain a smooth surface, along with the parameters for the growth of carbon nanotubes method in Table 2. The result of the experiment is shown in FIGS. 9 (a)˜(b).
FIGS. 9 (a)˜(b) can clearly be observed that the more leveling of the surface on the alloy substrate, the more flat and orderly of the growth on the carbon nanotubes after the metallography grinding on the appearance, which means the level of flatness on the surface would affect the growth of carbon nanotubes. More importantly, FIG. 9 (a) is of 80 wt % copper—20 wt % nickel alloy substrate, and FIG. 9 (b) is of 95 wt % copper—5 wt % nickel alloy substrate; the figures suggest that the density on the growth of carbon nanotubes is higher when the concentration of nickel in the alloy substrate is higher. Take the two pictures in the FIGS. 9 (a)˜(b) for a spot size analysis, the results are shown in Table 4. Within a constant unit area, the conclusion is that the density on the growth of carbon nanotubes is higher when the concentration of catalyst nickel in the alloy substrate is higher as well.
4-2. Using the Value of Radio Frequency Negative Self-Bias to Control the Degree of Straightness
The present invention raise the level of dissociation on the reaction gas in the reaction regions by increasing the work power through the radio frequency generator; hence, it would aid in the lack of heat generated by the tungsten hot filaments to dissociate reaction gas. The degree of dissociation can be evaluated by the negative self-bias though the outer potentiometer, and the electrical field produced by the self-bias has another important purpose.
The basic theory of the negative self bias is as follows: since the frequency (13.5 MHz) produced by the radio frequency generator is fixed, when the power is increased, the bonds between atoms are being destroyed producing large amount of cations, ions, and elections, which would in turn increase the Irf (t) between the two polar plates on the holder because of the sudden transformation in the electrical field of the reaction gas in the reaction region.
Within the components of Irf (t), the mass of electron is very small; hence, they are easy to move about during the sudden transformation in the electrical field, which would cause a built-up between the two polar plates producing a self-bias contrast to the ground. If the amount of dissociation in the reaction gas increases, the amount of built-up in the electrons would increase as well, and subsequently, the value of self bias would enlarge; therefore, the negative self bias can be used as a standard to evaluate the level of dissociation in the reaction gas.
The positive atoms or neutral atoms are heavier in mass; hence, the movement during the sudden transformation in the electrical field is not as easy, which would cause a built-up between the two polar plates forming plasma. As shown in FIG. 10, the transparent film that appears to be in purplish-blue color on the graphite holder is the plasma. The gap between plasma and the graphite holder is the so called “sheath.” Within sheath, there's a strong electrical field, and the properties of the field is created by the transformation and interaction of the electrons and plasma; hence, the line of electrical force will continuously affect the region between plasma and the alloy substrate. When the carbon nanotubes are slowly produced, the line of electrical force will keep working on the tip of the carbon nanotubes and guiding the growth of carbon nanotubes upward, so the effort is not only wasted on the portion on the alloy substrate. This is a property that only DC self bias produced by the transformation of the radio frequency would only have (rapid compilation of electrons). Furthermore, changing the value of self bias measured by ways of modifying the afferent work rate in the radio frequency generator could promote the change in the dynamics of the electric field in the sheath, which in turn would give the carbon nanotubes different levels of upward guiding force enforcing the growth of each carbon nanotube to develop into its distinctive degree of straightness.
4-3. Using Hydrogen Etching Time to Control the Growth of Diameters
Adding 80 sccm of hydrogen gas at 1 torr into the reaction chamber in the present invention and raising the temperature to the reaction temperature, hydrion will keep on “etching” the surface of the alloy substrate. Using the reduction property of the hydrogen gas, procedure on the addition of hydrogen is to clean off the oxidation portion on the surface of the alloy substrate; however, prolonging the time under the stable temperature without providing the source of carbon (methane) for the growth of carbon nanotubes, the atoms on the surface of the alloy substrate are rearranged due to the increase of the surface tension because of the activation affected by the intrusion of the hydrion on the atoms on the surface and the strong reduction effect. The process of turning the catalysts into nanoscale particles on the surface of the alloy substrate is called “hydrogen etching.”
Through the observation of electron microscope, the production of nano-sized particles are found on the surface of the alloy substrate after proceeding with the hydrogen etching on the four alloy substrates for 30 minutes. As shown in Table 5, the size of nano-sized particles are found to be on the average of 20˜70 nm, 100˜120 nm, and 200˜400 nm at the hydrogen etching times of 5 minutes, 10 minutes, 30 minutes, respectively. This phenomenon implies that the surface of the alloy substrate will be activated by the reduction of hydrogen, which further re-arrange the atoms on the surface into arrays of nanoscale particles, where the alloy substrate is under the high temperature and the bombardment of hydrogen plasma. As a result, controlling the hydrogen etching time of the alloy substrate can adjust the size of catalysts being nano-sized and further affect the size of diameters of the carbon nanotubes during growth.
The present invention provides a method of using copper alloy substrate for growing carbon nanotubes, comparing with the prior art described and other techniques known in the art described above, the advantages are shown below:
(1) The present invention uses copper as the matrix, and the components of catalysts (ferrum, cobalt, nickel) are dissolved into copper matrix to form alloy substrate for the growth of carbon nanotubes.
(2) During the early stage of the production procedures of the present invention, the length of hydrogen etching time is used to control the size of nano-sized micro-particles on the surface of the alloy substrate (containing catalysts), and then the size of particles are subsequently used to control the size of diameters on the carbon nanotubes during growth.
(3) During the growth of carbon nanotubes in the present invention, the manufacturing process of the thermal CVD produces carbon nanotubes that are of the structural quality producing from the Arc discharge and Laser ablation methods.
Many changes and modifications in the above described embodiment of the invention can, of course, be carried out without departing from the scope thereof. Accordingly, to promote the progress in science and the useful arts, the invention is disclosed and is intended to be limited only by the scope of the appended claims.

Claims

1. A method of using copper alloy substrate for growing carbon nanotubes, which comprises the following steps:

(a) dissolving the catalyst metals to form an alloy substrate in a copper metal matrix;

(b) grinding the alloy substrate with a sand paper to from a reaction region on the surface of the alloy substrate;

(c) setting the alloy substrate on the top of the sample holder in the radio frequency assisted hot filament chemical vapor deposition reaction chamber;

(d) maintaining the reaction temperature of the alloy substrate with a tungsten hot filament;

(e) proceeding with a preliminary dissociation action on the reaction gas entering into the reaction chamber with a microwave generator;

(f) producing a self bias at a rate of −150V˜−450V with a radio frequency generator;

(g) forming the nanoscale catalyst particles in the reaction region with hydrogen etching;

(h) a method of growing carbon nanotubes in the reaction region with carbon atoms;

wherein the reaction gas comprises diluted gas (hydrogen) and carbon source gas (methane); first of all, 80 sccm of hydrogen gas is introduced into the reaction chamber and raise the reaction temperature for the alloy substrate inside the reaction chamber to a temperature of 850˜950° C. using heat generated from a tungsten hot filament; proceeding with the hydrogen etching after maintaining time for a while, carbon source of gas is then introduced, and the procedure of introducing the carbon source gas is as follows: 30 sccm of carbon source gas (methane) is introduced first, and then additional amount is added consecutively at a rate of 10 sccm/2 min until reaching an amount of 60 sccm and sustaining the growth time for 20 minutes.

2. The method of using copper alloy substrate for growing carbon nanotubes according to claim 1, wherein the components of the alloy substrate is 80 wt % copper—20 wt % ferrum.

3. The method of using copper alloy substrate for growing carbon nanotubes according to claim 1, wherein the components of the alloy substrate is 80 wt % copper—20 wt % cobalt.

4. The method of using copper alloy substrate for growing carbon nanotubes according to claim 1, wherein the components of the alloy substrate is 80 wt % copper—20 wt % nickel.

5. The method of using copper alloy substrate for growing carbon nanotubes according to claim 1, wherein the components of the alloy substrate is 70 wt % copper—10 wt % ferrum—10 wt % cobalt—10 wt % nickel.

6. The method of using copper alloy substrate for growing carbon nanotubes according to claim 1, wherein the frequency of the radio frequency generator is at 13.56 MHz and the work rate is 100˜600 W.

7. The method of using copper alloy substrate for growing carbon nanotubes according to claim 6, wherein the radio frequency generator can completely dissociate the reaction gas and produce plasma to generate a self bias at −50˜−550V contrasting to the ground from the holder in the reaction region.

8. The method of using copper alloy substrate for growing carbon nanotubes according to claim 1, wherein after addition of 80 sccm hydrogen gas alone into the reaction chamber and raise the reaction temperature to 850˜950° C., hydrion will etch the surface of the alloy substrate.

9. The method of using copper alloy substrate for growing carbon nanotubes according to claim 8, wherein when addition of hydrogen gas alone into the reaction chamber the atoms on the surface of the alloy substrate are rearranged due to the increase of the surface tension and because of the activation affected by the bombardment of the hydrion on the atoms on the surface and the strong reduction effect; hence, the catalysts on the surface of the alloy substrate will exist in nanoscale particles.

10. The method of using copper alloy substrate for growing carbon nanotubes according to claim 1, wherein adjust the size of catalysts being nano-refined by controlling the length of the hydrogen etching time of the alloy substrate, to further control the size of diameters of the carbon nanotubes grown from the carbon atoms dissolved and precipitated thereafter.