BACKGROUND OF THE INVENTION
This invention relates to a continuous-flow process of making continuous-length nanofibers by laser vaporization.
The fantastic properties and applications of nanotubes have been described in a presentation of a new field called “Fractal Tube Reinforcement Microengineering” by Russell in 32nd International SAMPE Technical Conference, p. 224 (Nov. 5-9, 2000). There has been great interest in using nanotubes as building blocks to construct all kinds of products with superior performance, but nanotubes have been far too costly and of insufficient length and quantity to be useful for most industrial applications. Smalley, et al. in U.S. Pat. No. 6,183,714 disclose pulse laser vaporization of carbon mixed with one or more Group VIII transition metals to make a carbon nanotube. They then use a second laser pulse to maintain this nanotube end in an annealing zone, which allows growth of ropes of nanotubes. These ropes, however, are not continuous-length ropes and not attached to anything that can be pulled or wound. So, this process is not continuous and must be stopped to recover nanotube ropes from condensed vapor.
Kalaugher reported in “Nanotubes go to great lengths”, Nanotechweb.org (Mar. 11, 2004) that Windle and colleagues at Cambridge University have used chemical vapor deposition in a furnace of ethanol with ferrocene and thiophene and catalyzed with iron to make ribbons of carbon nanotubes. However, they do not disclose any way to apply tension to the nanofibers to constrain them to grow parallel to each other with controlled geometry to form useful structures.
- SUMMARY OF THE INVENTION
So, there remains a need to make continuous-length nanofiber in parallel, controlled arrangements in order to make commercially useful structures economically in large quantities.
A main object of the instant invention is to provide a continuous laser fabrication process of making all kinds of nanofibers of unlimited length. This is accomplished herein by laser vaporization of a feed-material made of all of the elements required to grow a chosen kind of nanofiber in a reactor, such that new nanofibers of the chosen kind grow onto seed-nanostructures attached to a filament. More feed-material is supplied to the reactor as needed to support the continuous growth of new nanofibers, which are pulled out and wound up at a rate controlled by their growth rate in a continuous-flow process.
Another object is to provide a process of controlling the growth of new nanofibers in order to make useful structures like cables and cylinders in one continuous operation. This is accomplished herein by applying tension to the growing nanofibers by focusing at least one laser light beam into the shape of a doughnut at a vapor of feed-material in a reaction zone. This provides a photon density gradient, which gathers atoms of the vapor together and constrains the growth of new nanofibers to be parallel to each other and to assume a cylindrical form. In another aspect an electrical field is applied to align the new nanofibers and constrain them into chosen shapes and useful structures.
BRIEF DESCRIPTION OF DRAWINGS
These and other objects and advantages of the invention will be better understood from the following detailed description of preferred embodiments of the inventive process when taken in conjunction with the accompanying drawings.
FIG. 1 a is a schematic cross-sectional view of a laser-assisted continuous nanofiber reactor.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
FIG. 1 b is an enlarged view of section X in FIG. 1 a showing nanofibers attached to filament.
In a first embodiment seed-nanostructures are placed in a reactor 1 shown in FIG. 1 a, which is capable of sustaining pressure. The pressure in the reactor 1 depends on the type of nanofibers to be built and controls their rate of growth to a manageable rate. The seed-nanostructures comprise nanotubes of any type and chemical composition including fullerenes and nanotube ropes together with catalyst nanoparticles, such as a mixture of Co and Ni. The catalyst nanoparticles are, alternatively, any of the transition elements or combinations thereof. In another example, seed-nanostructures comprise elements from which nanofibers are built. Nanostructures are defined to be structures less than 1 micron in thickness. An electric charge is imparted to an assembly of these seed-nanostructures relative to a filament 2 in the reactor 1 by means of a static charge generator until they become aligned relative to each other. Oxygen and water vapor are evacuated and inert gas is injected in the reactor 1. The preferred inert gas is a gas selected from the group consisting of argon, helium, and any combination of these. Each lower tip of at least one filament 2 is heated by reflecting at least one input laser light beam 3 from mirror 8 to focusing mirror 7 and focusing the laser beam 3 on the lower tip by the focusing mirror 7 until the lower tip becomes molten hot. A preferred filament is one made of sapphire having a tapered lower tip. Another preferred filament is a nanofiber. The filament 2 may be made of any other material. The lower tip of the at least one filament 2 is then moved out of a path of the focused laser beam 3 toward the seed-nanostructures such that they become attached to the filament 2. In one example the seed-nanostructures become attached to the filament 2 by embedding themselves into the molten tip. In another example they are adhered to the filament 2 by means of an applied adhesive. In another example they become attached by means of the electrostatic charge placed on them. A preferred laser emits a light beam 3 with a power of at least 10 mW in the visible frequency range. Visible light lasers are available from New Lamda Corp., Clearwater, Fla. Feed-material 4 is provided in the reactor 1, which comprises all elements needed to build new nanofibers 5 including a total of at most 5 atom % of at least one transition element. More preferably the feed-material 4 comprises all elements needed to build new nanofibers 5 including a total of at most 3 atom % of at least 3 transition elements. Two examples of highly effective transition element combinations in catalyzing new nanofiber growth are 1 atom % each of Ni, Fe, & Co, and Y, Ir, & Pt. In this paper atom % is defined as the percentage of atoms relative to all other atoms present. Most preferably only a negligible amount of transition elements are included and only in the beginning of the inventive process, because this costly part of the process is minimized. In one example the feed-material 4 is in a solid state. In a second example the feed-material is in a liquid state. In a third example the feed-material is in a gas state. Examples of feed-material 4 are carbon, hydrocarbons, fullerenes, SiC, BN, AlN, and any combination of these. The feed-material 4 is then heated to form vapor. A preferred way to heat the feed-material 4 is by means of at least one infrared laser beam 6. Infrared lasers are available from New Lambda Corp., Clearwater, Fla. In other examples electric arc, plasma arc, and radio frequency induction are used as means to heat the feed-material 4. The vapor is heated by means of the focused laser beam 3 sufficiently so that new nanofibers grow in a local region of the focus of the beam 3. A preferred reaction temperature of the vapor is 500° C. to 1400° C. More preferably the reaction temperature of the vapor is 600° C. to 1250° C. The at least one laser beam 3 creates a photon density gradient, which gathers atoms and molecules from the vapor together and constrains them to assemble into new nanofibers 5. In this paper, nanofiber is defined as any fiber that has a thickness of less than 1 micron. The new nanofibers 5 include single-wall nanotubes. In another example the new nanofibers 5 include multi-wall nanotubes. In another example the new nanofibers 5 include a mixture of single-wall and multi-wall nanotubes. The well-known technology of using a photon density gradient from highly focused light to manipulate matter is documented by Plewa et al. in “Processing Carbon Nanotubes with Holographic Optical Tweezers”, Physics.nyu.edu/grierlab/nanotube3b (Feb. 14, 2004), and by Dholakia et al. in “Optical tweezers: the next generation”, Nanotechweb.org/articles/feature/1/10/2/1 (October 2002). The lower tip of the filament 2 is then raised to a position just above the focus of the beam 3 such that new nanofibers form from the vapor on the seed-nanostructures. The new nanofibers 5 are structurally continuous with the seed-nanostructures. In one example the new nanofibers 5 are chemically identical with the seed-nanostructures. In another example the new nanofibers 5 are made of different elements than the seed-nanostructures. The filament 2 is then pulled out with the new nanofibers 5 attached as shown in FIG. 1 b at a rate controlled to match a rate of new nanofiber growth. Tension is provided on the new nanofibers 5 as they grow by at least one tensioning means. This constrains the new nanofibers 5 to grow parallel to each other. Feed-material 4 is continuously provided, as required, to allow continuous growth of new nanofibers 5 in a continuous-flow process. The means of providing tension is by focusing the at least one laser beam 3 into a toroid or doughnut shape. Preferably 3 laser beams 3 are used together to manipulate the nanofiber growth. This further constrains the new nanofibers 5 to assemble in a substantially cylindrical shape, so that cables, cylinders and hollow cylinders and other structures are manufactured in one efficient operation. Bonding agents may be used in this process to bond the new nanofibers 5 comprising these structures. Another means of providing tension and manipulating the new nanofibers 5 to control a final product geometry is by applying an electric field to the new nanofibers by an electric field generator. As the filament 2 is pulled out with the new nanofibers 5 attached it is wound up onto a spool, for example. The length of the new nanofibers 5 is not limited. The chemical make-up of the new nanofibers 5 depends on the type of feed-material used. Examples of new nanofibers 5 are C, SiC, BN, AlN, and any combination of these.
In a second embodiment the seed-nanostructures are replaced with seed-nanotubes, which are comprised of nanotubes. In another example the seed-nanotubes are comprised of short sections of nanotube ropes. There are no transition elements used in this embodiment to catalyze formation of new nanofibers 5. An electric charge is placed on the at least one filament 2 relative to the seed-nanotubes. In all other respects the process steps are the same in this embodiment as in the first embodiment.
While there is described herein certain specific process steps embodying the invention, it will be manifest to those skilled in the art that modifications may be made without departing from the spirit and the scope of the underlying inventive concept. The present invention shall not be limited to the particular processes herein shown and described except by the scope of the appended claims.