Abstract
A reactor and method for seeded growth of nano-products such as carbon nanotubes, wires and filaments in which selected precursors are introduced into the reactor which is heated to a temperature sufficient to induce nano-product formation from interaction between the precursor gases and a nanopore templated catalyst. The selected precursors are segregated in the reactor through a plate defining two chambers which are sealed off from each other except for a void space provided in the plate. The void space is closed off by a membrane having nanopores and a catalyst formed as a layer. Atomic transfer of material from the selected precursors to form the nano-products on the catalyst layer in the other of the chambers occurs by diffusion through the catalyst layer to form the nano-product on the other of the chambers absent a pressure drop between the two chambers.
Claims
1. A reactor heatable to high temperatures for seeded growth of nano-products by flowing selected precursors therewithin, comprising a sealed-interior housing having a gas inlet at one end for introducing the selected precursors into the housing and a gas outlet at another end for exhausting the selected precursors, a plate arranged within the housing to define two chambers in the housing which are sealed off from each other except for a void space provided in the plate with the selected precursors from the gas inlet entering one of the chambers, and a membrane having nanopores and a catalyst layer selected and being arranged to close off the void space, with the nanopores being sized and the gas inlet and gas outlet being arranged so that the selected precursors are constrained to flow along only one surface of the membrane from the gas inlet to the gas outlet, wherein the membrane is positioned such that the gas flow is substantially parallel to the one surface of the membrane such that atomic transfer of material from the selected precursors to form the nano-products on the catalyst layer occurs only by diffusion through the catalyst layer on the one surface.
2. The reactor of claim 1, wherein the plate is removably arranged within the housing.
3. The reactor of claim 1, wherein the housing is configured as one of a hollow circular tube and a hollow polygon.
4. The reactor of claim 1, wherein the reactor is comprised of one of quartz and ceramic material.
5. The reactor of claim 1, wherein the gas inlet is configured to supply the selected precursors as a mixture of hydrocarbon gas, water vapor, hydrogen and an inert carrier gas to the one of the chambers.
6. The reactor of claim 1, wherein the nano-products comprise one of carbon nanotubes, nanowires and nanofilaments.
7. The reactor of claim 1, wherein the housing and chambers are configured to employ chemical vapor deposition for the seeded growth.
8. The reactor of claim 1, wherein the catalyst layer is selected and tuned to form a meniscus shape at the nanopores such that the meniscus shape influences nanostructure growth.
9. The reactor of claim 1, wherein the catalyst layer is selected and tuned to form a meniscus shape at the nanopores by selection of at least one of reactor temperature, size of the nanopores, surface composition of the membrane and catalyst material.
10. The reactor of claim 1, wherein the catalyst layer is within, and at least partially along walls of, the nanopores.
11. The reactor of claim 1, wherein the membrane is a ceramic material.
12. The reactor of claim 1, wherein the catalyst layer is selected to induce carbon nanotube growth.
13. A reactor heatable to high temperatures for seeded growth of nano-products by flowing selected precursors therewithin, comprising a sealed-interior housing having a gas inlet at one end and a gas outlet at another end, a plate arranged within the housing to define two chambers in the housing which are sealed off from each other except for a void space provided in the plate with the selected precursors from the gas inlet entering one of the chambers, and a membrane having nanopores and a catalyst layer selected and being arranged to close off the void space, with the nanopores being sized so that the selected precursors flow along only one surface of the membrane from the direction of the gas inlet to the gas outlet such that atomic transfer of material from the selected precursors to form the nano-products on the catalyst layer occurs only by diffusion through the catalyst layer on the one surface, wherein a selected dopant gas is introduced into the other of the two chambers.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
(1) FIG. 1 is a schematic assembly view of a cylindrical embodiment of the multi-chambered apparatus of the present invention shown in partial cross-section;
(2) FIG. 2 is an enlarged cross-sectional view of the porous membrane affixed to the void in the segregating plate shown in FIG. 1 with catalyst applied to one end of the open porous membrane;
(3) FIGS. 3A-D are further enlarged cross-sectional views of the porous membrane with affixed catalyst shown in FIG. 2 but with various catalyst meniscus alternatives due to altered membrane/catalyst interfacial energies; and
(4) FIG. 4 is a Raman spectra indicating CNT growth on both sides of a template subject to CNT growth in the SFCVD reactor.
DETAILED DESCRIPTION OF THE DRAWINGS
(5) Now referring to FIG. 1, the disclosed apparatus is composed of a reactor tube 101 which is composed of quartz but could also be composed of alumina, titania or another high temperature ceramic as known to the art for use in high temperature furnaces. In one embodiment as shown, the tube 101 is round but can also be configured as a polygon of any number of sides such that a segregating plate 102 may be affixed within tube 101 either permanently or removably such that the reactor is divided into two chambers A and B. The segregating plate 102 has a void space 103 between the chambers A, B. The void space 103 is closed off by a removable nanoporous membrane 104 with a catalyst film 203 (FIG. 2) across one surface such that material can only transition between the reactor chambers by diffusion through the catalyst layer 203. Gases enter the reactor chambers A,B through gas inlet tubes 105 which pass through a compression seal designated generally by the numeral 106 to prevent oxygen from entering the system though the system may be sealed in any manner known to the art such that the chambers A,B remain segregated. One chamber A or B of the reactor is fed a mixture of a hydrocarbon gas, water vapor, hydrogen, a carrier gas, and potentially other gases that benefit the growth process. The hydrocarbon may consist of ethylene, acetylene or another hydrocarbon; the carrier gas may be Ar, N.sub.2 or another inert gas such that it is non-reactive with the other gases. Dopant gases may be fed to either chamber along with gases such as water vapor to restrict the formation of amorphous carbon or provide a benefit to continuous growth processes. The reactor is heated using a conventional resistive wire heating element (not shown for sake of clarity) although it can be heated by any other known method. The effluent end 107 of the tube 101 is connectable in a known way to a reducing coupling such that the gas exits through a inch stainless steel tube to induce an increase or decrease in pressure relative to the outside environment. The two chambers A, B may be separately vented or combined into a single vent where it may flow out through a treatment system or a vacuum.
(6) FIG. 2 is a cross-sectional isolated view of the segregation plate 102, void space 103, and porous membrane 104 from FIG. 1. The porous membrane consists of annular region of non-anodized aluminum or other nanoporous template (such as ceramic material) 201 with a center anodized region 202 although the anodized region may make up the entirety or just a portion of the membrane 104. A thin film catalyst 203 is applied to one surface of the membrane 104, but it may also be placed in either orientation such that atomic transfer between the reactor chambers A, B must occur due to diffusion through the catalyst 203 which consists of, for example, an iron-group element (Fe, Ni, Co), an alloy thereof, or one of the many other catalyst materials known in the art. The catalyst film 203 is applied to the surface either as a bulk film, through a physical deposition process such as sputtering or evaporation, a vapor deposition or another known type of method which forms a cohesive film across the nanoporous surface. The porous membrane 104 rests on a lip 204 such that gas flow cannot transfer around the membrane although the lip is unnecessary if the porous membrane 104 forms a cohesive seal without. The catalyst film 203 may also be applied by layer-by-layer, sol-gel, colloidal self-assembly or any other related deposition technique to form a film or coating on the surface or within the nanoporous template.
(7) FIG. 3A, FIG. 3B, and FIG. 3C represent three contemplated catalyst-substrate interactions where the catalyst meniscus is dependent on the interfacial energies between the catalyst 203, porous membrane 201 and gas and the capillary pressure within each nanopore. The catalyst curvature is required to form CNTs but the ultimate interfacial energy will determine whether the CNT grows in a root or tip growth methodology. Root growth occurs when the catalyst material remains in place while the CNT grows from this point. Tip growth dislodges a particle of catalyst from the well and ceases growth upon ejection of all carbon. CNTs grown via tip growth from a catalyst well will be short and potentially be joined to other CNTs by catalyst particles continuously dislodging from the well at the base of the initial nanotube. This is due to the fact that the CNT precursor material must traverse the catalyst layer and if a catalyst particle dislodges from this layer it loses communication with the carbon source and growth ceases. If the CNT is formed via root growth, it continues growing as long as carbon is diffusing through the catalyst layer to the root of the CNT.
(8) FIG. 3A represents a low catalyst-substrate interfacial energy with a positive or convex) (out from the catalyst film 203) capillary pressure driving the molten catalyst 301 into the pore. The low catalyst-substrate interfacial energy causes the molten catalyst to minimally wet the pore and upon formation of a CNT should demonstrate a tip-growth mechanism leading to a series of short interconnected CNTs.
(9) FIG. 3B. is similar to FIG. 3A in that itrepresents the same low catalyst-substrate interfacial energy leading to a high catalyst contact angle, but in this case, a negative or concave (into the catalyst film 203) capillary pressure drives a negative meniscus within the catalyst film. CNT formation in this catalyst-substrate system will drive root growth due to the catalyst's preference to remain attached to the bulk film.
(10) FIG. 3C shows a phillic substrate-catalyst interaction where the catalyst film 203 will wick into the pore due to massive positive capillary pressure forces similar to how water wicks in to a glass capillary. This wicking will induce the concave curvature 303 necessary for CNT formation and cause root growth due to the catalysts propensity to remain attached the pore wall and catalyst bulk.
(11) FIG. 3D shows an alternative catalyst application method whereby the catalyst 304 is coated along with walls of the nanoporous template 202. In this embodiment, the chambers A, B are not completely segregated; however, the immense aspect ratio provides enough catalyst surface area to catalyze the majority of the gas flowing through the nanocapillary. The catalyst material may applied through vapor deposition, atomic layer deposition, liquid phase self-assembly or other known material deposition processes without departing from the scope of our invention.
(12) FIG. 4 is the Raman spectroscopy signals of the top (catalyst coated) 401 and bottom sides 402 of an AAO template coated with an iron catalyst and subjected to the growth conditions described herein with the template situated such that the catalyst bulk was in the reactant gas side FIG. 1, chamber A of the reactor. Two distinct peaks are represented in the Raman spectra at 1350 cm.sup.1 and 1580 cm.sup.1 which represent the disorder induced and graphitic peaks respectively in a carbon network. The presence of these distinct peaks indicates CNT growth on both the sides of the template.
(13) While we have shown and described several embodiments in accordance with our invention, it should be understood that the same is susceptible to further changes and modifications without departing from the scope of our invention. Therefore, we do not want to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.
