Fibrils

Details Fibrils Fibrils

2001-04-26

2002-07-23

Kunemund, Robert (Department: 1774)

Chemistry of inorganic compounds

Carbon or compound thereof

Elemental carbon

C423S450000, C423S453000, C423S458000

Reexamination Certificate

active

06423288

ABSTRACT:

This invention relates to fibrils. It more particlarly refers to carbon/graphite fibrils and to an improved process for producing such. Carbon fibrils as used herein means graphitic fibrils having high surface area, high Young's modulus of elasticity and high tensile strength which are grown catalytically from available sources of carbon.
BACKGROUND OF THE INVENTION
It has been known for some time that one could make fibrils by decomposing various carbon contributing molecules, such as light hydrocarbons, in contact with a suitable metal catalyst, such as for example iron alone or in combination with other metals. In the past, the fibrils which have been made have been somewhat thicker than desirable and/or have been burdened with an overcoat of thermally deposited generally amorphous carbon which tended to reduce the desirable physical properties thereof or have been made in poor yields. Prior workers have sought to ameliorate the disadvantages of the amorphous carbon overcoating by subjecting the finished fibril to a very high temperature graphitizing treatment whereby generally rendering the fibrils of substantially greater cross sectional consistency from both a composition and a crystallinity point of view.
It is obvious that the improved fibril properties engendered by this high temperature graphitization process are expensive, because high temperature treatments are expensive. Additionally, such graphitized fibrils may still be too thick for many purposes because, graphitizing does not significantly reduce the fibril diameter. Thus, it is desired to produce high yields of high quality fibrils, preferably thin fibrils, which, in a preferred aspect of this invention, do not need post production graphitization.
SUMMARY OF THE INVENTION
More particlarly refers to carbon/graphite fibrils and to an improved process for producing such. Fibrils are made according to this invention in a high temperature, catalytic process. The Fibril can be made of a variety of materials, e.g. carbon, silicon nitride, silicon carbide, etc. In one important embodiment, such fibrils have the atoms in their composition relatively ordered at their outer surfaces as they are made by this process. Thus, it can be said that this process preferably directly produces a product having a relatively crystalline outer region for substantial portions of its length and may have inner regions where its atoms are less ordered. It may, and often does, even have a hollow region axially positioned along substantial portions of its length.
Fibrils according to this invention are characterized by small diameters, e.g. about 35 to 70 nanometers and high L/D up to about 100 and even more. Where the preferred structure described above is produced, it is suitably produced directly in the fibril forming process without further processing being required.
Where the fibrils of this invention are to be made of carbon, such can be produced in quite high yields. In this embodiment, a suitable source of carbon may be a hydrocarbonaceous material illustrated by: methane, ethane, propane, butane, benzene, cyclohexane, butene, isobutene, ethylene, propylene, acetylene, toluene, xylene, cumene, ethyl benzene, naphthalene, phenanthrene, anthracene, formaldehyde, acetaldehyde, acetone, methanol, ethanol, carbon monoxide, other similar materials, and mixtures of two (2) or more thereof. Such feed is contacted with a suitable, catalyst at elevated, fibril forming temperatures for a time sufficient to cause graphitic carbon fibrils to grow.
It is within the scope of this invention to provide a non-hydrocarbonaceous gas along with the carbon contributing reactant. Such gas might for example be hydrogen or carbon monoxide. Inert diluents are also suitable.
The temperature of the process of this invention can vary widely depending upon the nature of the carbon source being used, however, for best results, it should be kept below the thermal decomposition temperature thereof. In the case of using a mixture of such carbon sources, the operating temperature should be maintained below the thermal decomposition temperature of the most temperature-sensitive carbon source in the system. Temperatures in the range of 500 to 1500° C. may be found to be generally usable, depending on the carbon source used, preferably between about 600 and 900° C.
Subatmospheric, atmospheric and/or super atmospheric pressures may be used as dictated by other processing considerations. It has been found that it is desirable to provide the carbon source in the vapor state, and thus, the pressure should not be so high as to cause the carbon source to be in the liquid state under fibril forming temperature conditions. Further, it is desirable although not essential to provide a suitable gaseous diluent, such as hydrogen or inert gases, for example, nitrogen.
It is preferred that the system as a whole be non-oxidizing wherefor preferably avoiding the presence of oxygen if practical. Small amounts of these materials can be tolerated. It should be understood that the existence of oxidizing conditions, at the elevated temperatures operative for this process, will cause oxidation of the carbon source and therefor reduce the amount of carbon from such source which is available for conversion into fibrils as desired.
It may be desirable to provide suitable heat to this reaction system where and when needed. Temperature of different parts of the reactor zone may be suitably controlled to different temperatures and this is easily accomplished by using electrical resistance heating. However in larger scale industrial practice, electric resistance heating may sometimes be economically replaced by direct heating, such as for example by burning some of the carbon contributing feed to raise the temperature of the remainder of the feed, or by feeding the catalyst or the carbon contributing feed, or the diluent into the system at a sufficiently elevated temperature such that direct heat exchange of the component with each other will cause the fibril forming reaction to proceed as desired.
The nature of the catalyst seems to have a significant effect upon the yield of fibrils produced according to this invention. It is known to use iron group metals such as iron, cobalt or nickel to catalyze the conversion of carbon contributing compounds to fibrils, and such metals are within the scope of this invention. In addition, many other multivalant transition metals, including lanthanides, appear to be operative. Particularly useful catalytic metals include inter alia: iron, molybdenum cobalt, nickel, platinum, palladium, vanadium, and chromium. Of specific interest in this process are certain combinations of transition metals. Particularly useful combinations include iron and molybdenum, iron and chromium, copper and nickel, iron and platinum, iron and tin, iron and nickel, iron and manganese, and iron and cerium.
The yield of fibrils produced according to the practice of this invention appears to be related to the physical state of the catalyst used to produce such. According to this invention, it is important that the multivalent transition metal fibril forming catalyst be present on a suitable substrate as relatively discrete catalytic sites, each about 35 to 700A preferably 60 to 300A in size during fibril formation. These relatively discrete catalytic sites are produced by suitably applying the transition metal (in an appropriate state) to a substrate, suitably an inorganic substrate material which can include carbon/graphite.
The size of the substrate particle is a matter of some importance dependent upon the engineering of the process itself. For example, if the fibril formation is to take place in a fluid bed type of reaction zone, the substrate particle size will suitably be less than about 400 microns. If the fluid bed is an evullient bed of catalyst particles, particle sizes of about 50 to 300 microns have been found to be preferable. If the fluid bed is an evullient bed of fibrils containing small amounts of catalyst particles, i.e. up to about ten percent, these should prefera

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