Patent Application
20110091646
1. Field of the Invention
The present invention relates to chemical reaction chambers, and particularly to a chemical vapor deposition reactor with regulated and controlled gas flow for production of carbon nanotubes.
2. Description of the Related Art
Chemical vapor deposition (CVD) is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer or substrate is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Frequently, volatile by-products are also produced, which are typically removed by gas flow through the reaction chamber.
Microfabrication processes widely use CVD to deposit materials in various forms, including monocrystalline, polycrystalline, amorphous, and epitaxial. These materials include silicon, carbon fiber, carbon nanofibers, filaments, carbon nanotubes, SiO2, silicon-germanium, tungsten, silicon carbide, silicon nitride, silicon oxynitride, titanium nitride, and various high-k dielectrics. The CVD process is also used to produce synthetic diamonds.
In recent years, a great deal of interest has been directed towards carbon nanostructure materials. Carbon nanostructure materials have considerable commercial importance, with interest constantly growing since the discovery of buckminsterfullerenes, carbon nanotubes, and carbon nanofibers. Carbon nanotubes (CNTs) are of particular interest in the development of nanotechnologies.
Carbon nanotubes have a wide variety of properties and features that are of interest in electronic, mechanical, optical and chemical applications. Carbon nanotubes have been formed using an arc discharge method, laser vaporization and through catalytic chemical vapor deposition of hydrocarbons. Since carbon-carbon covalent bonds are one of the strongest known bonds, a structure based on a perfect arrangement of these bonds oriented along the axis of nanotubes would produce an exceedingly strong material.
For commercial applications, large quantities of purified carbon nanotubes are needed. Different types of carbon nanotubes, carbon nanofibers, vapor-grown carbon fiber and other types of carbon nanostructure materials can be produced in a variety of ways. As noted above, carbon nanotubes may be formed through arc discharge, laser vaporization or through catalytic chemical vapor deposition of hydrocarbons. None of these options, however, is economically viable for large-scale production and purification. In the arc discharge method, a vapor is created by an arc discharge between two carbon electrodes, with or without a catalyst. In the laser ablation technique, a high-power laser beam is projected on a small volume of the carbon in the presence of a feedstock gas, such as methane or carbon monoxide. Laser ablation produces a very small amount of pure nanotubes, while the arc-discharge method produces, typically, large quantities of nanotubes, but with very low purity. CVD presents a promising method for possible industrial production of nanotubes, due to the relatively low growth temperature, high yields and high purities of the nanotubes that can be achieved using CVD.
A typical prior art chemical vapor deposition reactor is illustrated in
After loading the boat 210 into the reaction furnace 200, the pressure of the reaction furnace 200 is maintained at atmospheric pressure (in the case of using an atmospheric pressure CVD apparatus) or on the order of a few hundred mTorr to a few Torr (in the case of using a low-pressure CVD apparatus). Then, the temperature of the reaction furnace 200 is raised to approximately 700 to 1000° C. by a resistance coil 250 mounted around the outer wall of the reaction furnace 200. When the temperature of the reaction furnace 200 reaches a predetermined processing temperature, a first valve 180 is opened to allow an etching gas from an etching gas supply source 110 to flow into the reaction furnace 200 through a gas inlet 220.
The etching gas may be ammonia gas, hydrogen gas, hydride gas or the like. The growing of carbon nanotubes is performed in situ with the formation of nano-sized catalytic metal particles. The first valve 180 is closed to cut off the supply of ammonia gas (or the like), and a second valve 120 is opened to supply a carbon source gas from the gas supply source 130 into the reaction furnace 200 through the gas inlet 220. The temperature of the reaction furnace 200 is kept at the same temperature as when the nano-sized isolated catalytic metal particles are formed; i.e., in the range of approximately 700 to 1000° C. Hydrocarbon gas having 1 to 3 carbon atoms may be used as the carbon source gas. Acetylene, ethylene, ethane, propylene, propane or methane gas is typically used as the carbon source gas. The lower limit of the processing temperature (i.e., 700° C.) refers to the minimum temperature that enables full pyrolysis of the carbon source gas.
To control the growth rate and time of carbon nanotubes, a carrier gas (i.e., an inert gas, such as hydrogen or argon) and/or a diluent gas (hydride gas) can be supplied along with the carbon source gas into the reaction furnace 200 from a carrier and/or diluent gas supply source 150 by opening a third valve 140. The density and growth pattern of carbon nanotubes synthesized over the substrate can also be controlled by supplying an etching gas (ammonia gas, hydrogen gas or hydride gas) in a predetermined ratio along with the carbon source gas. Preferably, the carbon source gas and the etching gas are supplied in a ratio of 2:1 to 3:1 by volume.
After the synthesis of carbon nanotubes is completed, the carbon nanotubes can be optionally subjected to in-situ purification. Carbon lumps or carbon particles, which are present on the surface of the grown carbon nanotubes, are removed in-situ with the growing of the carbon nanotubes. The second valve 120 is closed to cut off the supply of the carbon source gas 130 and a fourth valve 160 is opened to supply a purification gas from a purification gas supply source 170 to the reaction furnace 200 through the gas inlet 220. Ammonia gas, hydrogen gas, oxygen gas, or a mixture of these gases is typically used as the purification gas. When ammonia gas or hydrogen gas is selected as the purification gas, the purification gas can be supplied from the etching gas supply source 110 or the carrier gas and/or diluent gas supply source 150, without a need for the purification gas supply source 170. The remaining carbon source gas is exhausted from the reaction furnace 200 through a gas outlet 260. One such prior art reactor is shown in U.S. Pat. No. 6,350,488, which is herein incorporated by reference in its entirety.
Thus, an orifice chemical vapor deposition reactor solving the aforementioned problems is desired.
The orifice chemical vapor deposition reactor provides controlled and regulated reaction gas and vapor flow in order to produce high yields of carbon nanotubes with relatively high purity. The orifice chemical vapor deposition reactor includes a first reaction chamber having an inlet and an outlet, with an input gas including a carrier gas and/or diluents gas and a carbon source gas being injected into the first reaction chamber through the inlet. A feed pipe is preferably mounted on an exterior surface of the first reaction chamber, with the feed pipe covering, and being in fluid communication with, the inlet thereof for delivering the carrier gas and/or diluents gas and the carbon source gas into the first reaction chamber.
A catalyst boat is received within the first reaction chamber, with the catalyst boat being adapted for receiving a volume of reaction catalyst and being positioned within the gas stream of the gas being delivered into the first reaction chamber. A first reaction pipe is preferably positioned within the first reaction chamber. The first reaction pipe extends between, and joins, the inlet and the outlet thereof. The catalyst boat is preferably positioned substantially centrally within the first reaction pipe.
A second reaction chamber is also provided, with the second reaction chamber also having an inlet and an outlet formed therethrough. The inlet thereof is in fluid communication with the outlet of the first reaction chamber. Preferably, a connection pipe joins and extends between the outlet of the first reaction chamber and the inlet of the second reaction chamber.
A second reaction pipe is positioned within the second reaction chamber, with the second reaction pipe extending between, and joining, the inlet and the outlet thereof. An exhaust pipe is preferably mounted on an exterior surface of the second reaction chamber, with the exhaust pipe covering, and being in fluid communication with, the outlet thereof for removing exhaust gas from within the second reaction chamber.
A flow regulating member is positioned within the second reaction chamber adjacent the inlet thereof, with the flow regulating member having an orifice formed therethrough for regulating gas flow into the second reaction chamber. Preferably, the first and the second reaction pipes have substantially cylindrical contours having equal diameters. The flow-regulating member also preferably has a substantially cylindrical contour. An outer diameter of the flow-regulating member equals an inner diameter of the second reaction pipe in order to form a fluid-tight seal. The orifice is preferably formed axially through the cylindrical, flow regulating member and may have a cylindrical contour or may be formed as a venturi passage.
At least one product boat is received within the second reaction chamber, with the at least one product boat being adapted for receiving at least one substrate upon which a carbon material product, such as carbon nanotubes or the like, is formed. The at least one product boat is preferably positioned within the second reaction pipe.
These and other features of the present invention will become readily apparent upon further review of the following specification and drawings.
Similar reference characters denote corresponding features consistently throughout the attached drawings.
Referring to
A feed pipe 16 is preferably mounted on an exterior surface of the first reaction chamber 14, with the feed pipe 16 covering, and being in fluid communication with, the inlet 13 thereof for delivering the carrier gas and/or diluents gas and the carbon source gas into the first reaction chamber 14.
A catalyst boat 20 is received within the first reaction chamber 14, with the catalyst boat 20 being adapted for receiving a volume of reaction catalyst and being positioned within the gas stream of the gas being delivered into the first reaction chamber 14. A first reaction pipe 32 is preferably positioned within the first reaction chamber 14, as shown. The first reaction pipe 32 extends between, and joins, the inlet 13 and the outlet 15. The catalyst boat 20 is preferably positioned substantially centrally within the first reaction pipe 32, as shown.
A second reaction chamber 12 is also provided, with the second reaction chamber 12 also having an inlet 17 and an outlet 19 formed therethrough. The inlet 17 is in fluid communication with the outlet 15 of the first reaction chamber 14. Preferably, a connection pipe 22 joins, and extends between, the outlet 15 of the first reaction chamber 14 and the inlet 17 of the second reaction chamber 12.
A second reaction pipe 30 is positioned within the second reaction chamber 12, with the second reaction pipe 30 extending between, and joining, the inlet 17 and the outlet 19. An exhaust or outlet pipe 18 is preferably mounted on an exterior surface of the second reaction chamber 12, with the exhaust pipe 18 covering, and being in fluid communication with, the outlet 19 for removing exhaust gas from within the second reaction chamber 12 (illustrated by output arrow O).
First and second reaction chambers 14, 12, respectively, may have any desired contouring or relative dimensions. Similarly, first and second reaction pipes 32, 30, respectively, may have any desired contouring or dimensions. Exemplary dimensions for the reaction pipes include equal lengths of approximately 1.8 meters, with each pipe being formed as a cylindrical shell having a diameter of approximately 50 mm. Each reaction chamber may be heated by any suitable type of heating element (such as heating coil 250 in the prior art system 100 of
A flow regulating member 24 is positioned within the second reaction chamber 12 adjacent the inlet 17, with the flow regulating member 24 having an orifice 21 formed therethrough for regulating gas flow into the second reaction chamber 12. Preferably, the first and the second reaction pipes 32, 30, respectively, have substantially cylindrical contours having equal diameters. The connection pipe 22 is also preferably cylindrical having an equal diameter. The flow-regulating member 24 also preferably has a substantially cylindrical contour. An outer diameter of the flow-regulating member 24 equals an inner diameter of the first and second reaction pipes 32, 30, respectively, in order to form a fluid-tight seal. Using the exemplary dimensions given above, flow-regulating member 24 may have an outer diameter of approximately five cm and a length of approximately five cm.
The orifice 21 is preferably formed axially through the cylindrical, flow regulating member 24 and has a diameter less than the inner diameter of second reaction pipe 30, thus forming a venturi flow path to regulate the gas flow. This venturi flow path, and the regulation of the gas flow, allows for the controlled growth of the carbon nanotubes within the second reaction pipe. The orifice 21 may be formed as a cylindrical passage through member 24 or may, alternatively, be formed as a venturi passage through member 24. Using the exemplary dimensions given above for second reaction pipe 30, the orifice 21 may have a diameter of approximately three mm. Further, multiple members 24, having a variety of orifice contours and dimensions may be provided, allowing for the alteration of flow conditions and the production of differing types of carbon products. For example, removable members 24 may be provided having orifice diameters of three mm, six mm and twelve mm, respectively.
At least one product boat is received within the second reaction chamber 12. In
Any suitable types of gas may be used. For example, hydrogen gas may be used as a reactant gas for activation of the catalyst, and combined with argon gas for flushing air from the system. Acetylene gas may be used as the hydrocarbon source. Any suitable type of gas input system may be used, such as that shown in the prior art of
In operation, argon gas flows into the first reaction pipe 32 via the feed pipe 16 to flush air from the system and prevent oxidation of the catalytic metal during the heating phase of the reaction. Once the air is flushed, the first and second reaction chambers 14, 12, respectively, are heated to the reaction temperature. The ferrocene powder catalyst within boat 20 is heated in the first reaction chamber 14 at a temperature of approximately 120° C. in order to produce ferrocene vapor.
The hydrogen and the acetylene gases are mixed and passed into the vapor of the heated ferrocene. the ferrocene vapor then first decomposes to form atomic iron, which agglomerates into iron clusters or iron particles for the growth of the carbon nanotubes. The mixture of the hydrogen and acetylene gases and the vapor of the catalyst then pass through the orifice 21. The venturi action caused by flow through orifice 21 of flow regulating member 24 causes a high flow rate with a smooth laminar gas mixture for production of high-purity carbon nanotubes.
Reaction temperatures may be varied in the range from approximately 500° C. to 1200° C., with the hydrogen and hydrocarbon flow rates being in the range from approximately 50 ml/min to 500 ml/min, with a reaction time from approximately one minute to 1 hour. After the growth of the carbon nanotubes on the substrates within boats 26, 28, the reactor chambers are allowed to cool down to room temperature. The carbon products formed in the reactor are then collected from the ceramic boats 26, 28, from the catalyst boat 20, and from the walls of reaction tubes. System produces a variety of carbon nanostructure materials, including amorphous carbon layers, nanoparticles, and multi-walled carbon nanotubes.
It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims.
