PatentDe  


Dokumentenidentifikation EP1406836 08.11.2007
EP-Veröffentlichungsnummer 0001406836
Titel WHISKERFREIE SILICIUMCARBIDFASERN
Anmelder Industrial Ceramic Solutions LLC, Oak Ridge, Tenn., US
Erfinder NIXDORF, Richard D., Knoxville, TN 37922, US
Vertreter derzeit kein Vertreter bestellt
DE-Aktenzeichen 60222665
Vertragsstaaten DE, FR, GB, IT
Sprache des Dokument EN
EP-Anmeldetag 16.07.2002
EP-Aktenzeichen 027523752
WO-Anmeldetag 16.07.2002
PCT-Aktenzeichen PCT/US02/22590
WO-Veröffentlichungsnummer 2003009338
WO-Veröffentlichungsdatum 30.01.2003
EP-Offenlegungsdatum 14.04.2004
EP date of grant 26.09.2007
Veröffentlichungstag im Patentblatt 08.11.2007
IPC-Hauptklasse C01B 31/36(2006.01)A, F, I, 20051017, B, H, EP
IPC-Nebenklasse C04B 35/565(2006.01)A, L, I, 20051017, B, H, EP   D01F 9/08(2006.01)A, L, I, 20051017, B, H, EP   

Beschreibung[en]
FIELD OF INVENTION

This invention relates to silicon carbide discontinuous fibers, which are substantially free of whiskers, and methods for their manufacture.

BACKGROUND OF INVENTION

The International patent application WO 92/06769 discloses a filter and means for regeneration thereof. A method for the preparation of silicone carbide whiskers is disclosed in the United States patent US 4,873,069 . The United States patent US 5,458,181 discloses shaped bodies containing short inorganic fibers or whiskers and methods forming such bodies. A cast piston with fiber reinforcements is disclosed in United States patent US 4,662,326 .

Silicon carbide whiskers are known in the art. Articles manufactured employing such whiskers are also known to be good to excellent absorbers of microwave energy, particularly low energy microwave energy, e. g., about 2.45 GHz and about 1000- 3000 watts. These whiskers are commonly 0,001 - 0,003 mm (1-3 microns) in diameter and 0,01 - 0,2 mm (10 - 200 microns) long.

Silicon carbide whiskers, however, suffer problems relating to their potential carcinogenicity. More specifically, these whiskers, which are very small, readily inhaled, difficult to contain against dispersion into the ambient environment, among other undesirable characteristics or properties, are both difficult and expensive to manufacture and/or to be formed into a product which is useful. One proposed specific use for silicon carbide whiskers is in the fabrication of a filter for carbonaceous or organic components of a gaseous discharge stream, such as, for example, a filter for the exhaust system of a diesel engine.

Due to their small size, e. g. , many times smaller than cellulose paper-making fibers which commonly are of about 0,007 - 0,02 mm (7 - 20 microns) diameter and about 0,05 - 1,0 mm (50 - 1000 microns) length, these whiskers also are unsuitable for use in the well-known and relatively inexpensive paper-making processes for forming the whiskers into a sheet, which can subsequently be formed into a pleated filter paper product, for example.

Heretofore, it has been proposed to produce silicon carbide fibers, as opposed to whiskers, to serve as heating elements in a microwave field. However, certain of these methods produce spun continuous filaments which must be cut to produce discontinuous fibers. This method is expensive and time-consuming. Moreover, cutting of the filaments tends to expose their cut ends to oxidation or other deleterious degradation. Also, it has been proposed to produce silicon carbide fibers which are somewhat free of whiskers, but the method employed requires an initial step of producing carbon fibers which are thereafter converted to silicon carbide. However, these fibers do not heat rapidly in a low energy (e.g., 2.45 MHz) microwave field. None of these known methods provides a cost-efficient and environmentally friendly means for the manufacture of substantially whisker-free silicon carbide fibers which are individually on the order of the size not materially less that the size of cellulose paper-making fibers, hence suitable for use in the manufacture of desired geometrical shapes.

SUMMARY OF INVENTION

The present invention provides a method of producing discontinuous silicon carbide fibers according to appended claim 1, discontinuous fibers produced by said method, as well as a sheet comprising said fibres.

Embodiments of the present invention are defined in the dependent claims.The present invention comprises silicon carbide fibers of an individual size equal to or not substantially less than the size of cellulose paper-making fibers and which are substantially free of whiskers. On one embodiment, the fibers are formed employing cotton fibers, preferably chopped cotton fibers. The chopped cotton fibers are carbonized in an inert atmosphere at a temperature of between about 700°C and about 1200°C. These fibers, in water, are blended with calcium oxalate monohydrate mixed in hot methanol, ferrous sulfate, and fumed silica, and thereafter dried with heating. This mix is loaded into graphite tubes and heated at an elevated temperature for a time sufficient to effect the principal reaction of:



        SiO2+ 3C = SiC + 2CO     Eq.1



and resultant conversion of the fibers of the mix to silicon carbide. The process yields about 25% by weight of the original weight of carbonized cotton fibers. The vast majority of the discontinuous silicon carbide fibers are of a size approximating the size of cellulose paper-making fibers. Other components of the process product include smaller silicon fibers and/or particulates of non-fibrous geometry. This product is readily suspended in a slurry which is suitable as one of the feed materials for a slurry employed in a substantially conventional paper-making process and equipment comprises a self supporting sheet of discontinuous silicon carbide fibers which are intertangled in the manner of cellulosic fibers and additives found in a conventional paper product. The sheet product so produced has been found to be foldable, such as pleated on a pleating machine, to form a filter medium comprising principally silicon carbide fibers and, in certain circumstances, lesser quantities of entrapped silicon carbide particulates of non-fibrous geometry.

Importantly, the silicon carbide fibers of the present paper-like product obtained is strongly susceptible to relatively low-energy microwave energy and thus may be heated to a temperature of about 800 degrees C in less than about 15 seconds. At such temperature, common organic materials entrapped in a silicon carbide fiber filter, for example, are combusted and converted to environmentally friendly products.

BRIEF DESCRIPTION OF DRAWINGS

The single Figure is a photographic representation of silicon carbide fibers admixed with silicon carbide non-fibrous particulates as produced by the process of the present invention.

DETAILED DESCRIPTION OF INVENTION

In accordance with a preferred embodiment of the present invention, a quantity of cleaned, bleached and fully carbonized, cotton fibers, of about 10 microns in diameter, chopped to lengths approximating the length of, or longer than, cellulosic papermaking fibers, e. g., to between about 0.1 and about 4 millimeters in length are admixed, preferably in typical liquid-solids V-blender, equipped with an intensifier and a liquidus bar, with ferrous sulfate suspended in water, calcium oxalate monohydrate suspended in hot methanol or hot water, and low density (e. g. , fumed) silica, until homogenized. The homogeneous mixture is dried, preferably at about 19°C (300 degrees F). This dry mix is loaded into suitable closed containers, such as semi-porous graphite tubes, which, in turn are loaded into a furnace which preferably is preheated to at least about 1450 degrees C., i. e. below about 1750 degrees C where the formation of whiskers and particulate silicon carbide forms, for about one hour. The silicon carbide fibers formed within the tubes is recovered for further processing. Such further processing, in accordance with one embodiment, comprises formation of silicon carbide fibrous sheet material, employing convention cellulosic papermaking processes and equipment. The silicon paper-like product may be formed into any of various geometrical shapes, including pleating and incorporation into a regeneratable filter for carbonaceous products contained in a gas stream.

Carbonized cotton fibers, as opposed to carbonized PAN fibers or other organic carbonized fibers, is an important aspect of the present invention. For reasons not known with certainty, all non-cotton carbonized fibers known and available to the present inventor fail to yield the desired silicon carbide fibers, as opposed to whiskers. As noted, the useful cotton fibers should be cleaned and bleached cotton fibers which have been carbonized. Examples of suitable carbonized cotton fibers are those available from E & L Enterprises, Inc. of Oakdale, Tennessee, or Aerospace Enterprise, Inc. of Gardner, Maine and identified as AEI 1000 degree C carbonized cotton fibers. Raw, non-carbonized cotton fibers have been found to exhibit unacceptable mixing characteristics in the present invention even when added to the mix in relatively smaller proportions of a mixture of carbonized and raw cotton fibers.

For use in the present invention, the cotton fibers are chopped following their carbonization to individual fiber lengths of between about 3,175 mm to about 12,7 mm (about one-eighth to about one-half inch). The silicon carbide fibers produced from these cotton fibers retain the morphology of the carbonized cotton fibers. As noted, however, a small percentage of the carbonized cotton fibers end up as short silicon carbide fibers or particulates of silicon carbide. These particulates, however, are of insubstantial significance in the present invention in that, in a papermaking process, such particulates pass through the screen and/or are captured within the formed sheet where they can serve the beneficial function of enhancing the formation of the sheet material.

In the present process, a quantity of the carbonized chopped cotton fibers and water are loaded into a conventional rotary blender or V-blender which preferably is provided with an intensifier and liquidus bar. One suitable blender is a Littleford Model FM-130 rotary blender having a 0,085 m3 (3 cubic foot) capacity. Using this blender, preferably only one-half this capacity is employed. Blending commonly takes place within one to five minutes, using the intensifier.V-blender of the common laboratory type are also acceptable.

To the mix of cotton fibers and water, there is added, via the liquidus bar of the blender, ferrous sulfate and calcium oxalate monohydrate, followed by fumed silicon dioxide powder. The order of addition of the ingredients of the desired mix is not critical, but preferably, the carbonized cotton fibers are initially introduced into the blender, followed by the addition of the calcium oxalate, followed by the addition of the ferrous sulfate, and finally, addition of the low density silica. Preferably, the calcium oxalate monohydrate is suspended in hot methanol and added to the blender via the liquidus bar. Similarly, the ferrous sulfate is suspended in water and also added to the blender via the liquidus bar. The silicon dioxide powder is added in the dry powder form to the blender.

In a preferred mixture, there is employed 31. 6 parts of carbonized cotton fibers, 0.4 parts of calcium oxalate monohydrate, 12. 5ml of a solution of 25 mgFe++/ml water (in the form of ferrous sulfate ; equivalent of 0.0036 gm Fe) and 68.2 parts of fumed silicon carbide.

The quantity of cotton fibers in the mix may vary between about 25 and about 35 parts; the calcium oxalate may vary between about 0.3 and about 0.6 parts; the ferrous sulfate may vary between about 10 and about 25 mi of the noted suspension; and the silicon oxide may vary between about 66 and about 70 parts. Preferred results are obtained when the mix is mixed to homogeneity. In particular, homogeneity of the dispersion of the ferrous sulfate within the mix is important in ensuring conversion of the fibers to silicon carbide. Further, it is of importance in the present invention that the mix be free of any significant amount of a whisker growth component, such as boric oxide.

Drying of the mixture may be carried out by dispensing the mix from the blender into flat pans, for example, and heating the mixture in the pans within an oven at 149°C (300 degrees F).

The dried mix is thereafter loaded into semi-porous graphite closed containers for conversion of the fibers of the mix to silicon carbide. In a preferred embodiment, the mix within a tube is heated as rapidly as possible to a temperature of about 1700 degrees C. In a preferred embodiment, this activity is carried out by preheating an oven to a temperature of 1700 degrees C and, after expulsion of air from the mix in the tube by means of a brief (e. g. 45 minutes) argon purge, the tube with its mix contents is moved into the preheated oven, having an inert atmosphere, and held therein for between about one and about 5 hours. Preferably, the residence time within the oven is about two hours.

Employing the process of the present invention, there is achieved substantially 100% conversion of the fibrous material to silicon carbide, with a yield of between about 20% and 30% of the original weight of the carbonized cotton fibers. As may be seen from the single Figure, the vast majority of the silicon carbide fibers produced are of a size approximating the size of cellulosic paper-making fibers. The remainder of the mix comprises relatively small amounts of silicon carbide particulates and/or shorter silicon carbide fibers. The product so produced contained an insignificant quantity (e. g., less than 1%) of silicon carbide whiskers. Microscopic examination of the product showed silicon carbide fibers having individual diameters of about 0,005 mm - 0,025 mm (about 5-25 microns) in diameter and lengths of between about 0,1 mm and about 3 mm (about 100 and about 3,000 microns).

The product produced by the present invention was formed into a sheet employing conventional paper-making techniques. This paper was thereafter pleated employing a conventional pleating machine, preferably which the sheet was captured between first and second cellulosic paper sheets. The pleated sheet was formed into a filter geometry and tested for susceptibility to microwave radiation, employing a conventional household microwave oven of 2.45 GHz (about 600 watts). It was found that the product produced by the present invention consistently was heated in this oven to greater than 700 degrees C within about 30 seconds.


Anspruch[de]
Verfahren zum Herstellen diskontinuierlicher Siliziumkarbidfasern, das die folgenden Schritte umfasst: Vermischen einer Menge Metallsalz, einer Menge Kalziumoxalat und einer Menge Siliziumdioxid über eine Zeit, die ausreicht, um ein im Wesentliches homogenes Gemisch auszubilden, in einer Menge karbonisierter Baumwollfasern, wobei die Menge Metallsalz als Aktivator für die Umwandlung der karbonisierten Fasern in Siliziumkarbidfasern dient, Trocknen des Gemischs, Erhitzen des Gemischs in einem semiporösen Karbidbehälter über eine Zeit und bei einer Temperatur, durch die die karbonisierten Fasern in dem Gemisch in Siliziumkarbidfasern umgewandelt werden. Verfahren nach Anspruch 1, wobei die Fasern karbonisierte Baumwollfasern sind. Verfahren nach Anspruch 2, wobei das Metallsalz Eisensulfat ist. Verfahren nach Anspruch 1, wobei das Siliziumdioxid Siliziumdioxid niedriger Dichte umfasst. Verfahren nach Anspruch 4, wobei das Siliziumdioxid pyrogene Kieselsäure umfasst. Verfahren nach Anspruch 1, wobei die Komponenten des Gemischs zwischen ungefähr 25 und ungefähr 35 Teile karbonisierte Baumwollfasern, zwischen ungefähr 0,3 und ungefähr 0,6 Teile Kalziumoxalatmonohydrat, zwischen ungefähr 66 und ungefähr 70 Teile pyrogene Kieselsäure und zwischen ungefähr 10 und ungefähr 25 ml einer Lösung umfassen, die eine Konzentration von ungefähr 25 mg Fe++/ml Wasser in der Form von Eisensulfat hat. Verfahren nach Anspruch 1, wobei das Gemisch durch Wärme bei ungefähr 300 °C getrocknet wird. Verfahren nach Anspruch 1, wobei das getrocknete Gemisch in einen semiporösen Graphitbehälter gegeben wird, einer inerten Gasspülung über eine Zeit ausgesetzt wird, die ausreicht, um im Wesentlichen sämtliche Luft aus dem Behälter und dem Gemisch auszutreiben, und der gespülte Behälter sowie das Gemisch in eine vorgeheizte inerte Umgebung bei einer Temperatur zwischen ungefähr 1450 und ungefähr 1750 °C eingeleitet werden. Verfahren nach Anspruch 8, wobei der gespülte Behälter und das Gemisch in der vorgeheizten inerten Umgebung über eine Verweilzeit gehalten werden, die ausreicht, um Umwandlung der karbonisierten Fasern in Siliziumkarbidfasern zu bewirken. Verfahren nach Anspruch 9, wobei die Umwandlung der karbonisierten Fasern mehr als 90 % beträgt und die umgewandelten Fasern zwischen ungefähr 20 % und ungefähr 30 % des Gewichtes der nicht umgewandelten karbonisierten Fasern umfassen. Verfahren nach Anspruch 10, wobei die Verweilzeit des Behälters und des Gemischs in der vorgeheizten Umgebung zwischen ungefähr ein und ungefähr fünf Stunden beträgt. Diskontinuierliche Siliziumkarbidfasern, die gemäß dem Verfahren nach Anspruch 1 hergestellt wurden. Diskontinuierliche Siliziumkarbidfasern nach Anspruch 12, wobei die Fasern stark anfällig für Erhitzung durch Mikrowellenenergie sind. Diskontinuierliche Siliziumkarbidfasern nach Anspruch 13, wobei die Fasern innerhalb weniger als einer Minute in einem herkömmlichen Mikrowellenherd mit einer Leistung von ungefähr 800 Watt auf mehr als 800 °C erhitzt werden. Siliziumkarbidfasern nach Anspruch 12, wobei einzelne der Fasern eine Länge zwischen ungefähr 0,01 mm und ungefähr 3,0 mm (ungefähr 10 und ungefähr 3000 Mikron) haben. Blatt, das Siliziumkarbidfasern umfasst, die gemäß dem Verfahren nach Anspruch 1 und unter Verwendung eines herkömmlichen Papierherstellungsverfahrens hergestellt wurden. Blatt aus Siliziumkarbidfasern nach Anspruch 16, wobei das Blatt zum Falten desselben geeignet ist.
Anspruch[en]
A method of producing discontinuous silicon carbide fibers comprising the steps of

blending together in a quantity of carbonized cotton fibers, a quantity of metal salt which serves as a promoter for the conversion of said carbonized fibers to silicon carbide fibers, a quantity of calcium oxalate, and a quantity of silicon dioxide for a time sufficient to develop a substantially homogenous admixture,

drying said admixture,

in a semiporous graphite container, heating said admixture for a time and at a temperature at which said carbonized fibers in said admixture are converted to silicon carbide fibers.
The method of Claim 1 wherein said fibers are carbonized cotton fibers. The method of Claim 2 wherein said metal salt is ferrous sulfate. The method of Claim 1 wherein said silicon dioxide comprises low density silicon dioxide. The method of Claim 4 wherein said silicon dioxide comprises fumed silica. The method of Claim 1 wherein said components of said admixture comprise between about 25 and about 35 parts carbonized cotton fibers, between about 0.3 and about 0.6 parts calcium oxalate monohydrate, between about 66 and about 70 parts fumed silicon dioxide, and between about 10 and about 25 ml of a solution having a concentration of about 25 mg Fe++/ml of water in the form of ferrous sulfate. The method of Claim 1 wherein said admixture is dried with heat at about 300 degrees C. The method of Claim 1 wherein said dried admixture is disposed within a semi-porous graphite container, exposed to an inert gas purge for a time sufficient to expel substantially all air from the container and admixture, and said purged container and admixture are introduced into a preheated inert environment at a temperature of between about 1450 and about 1750 degrees C. The method of Claim 8 wherein said purged container and admixture are held within said preheated inert environment for a residence time sufficient to effect conversion of said carbonized fibers to silicon carbide fibers. The method of Claim 9 wherein said conversion of carbonized fibers is greater than 90 percent, said converted fibers comprising between about 20% and about 30% of the weight of the weight of the non-converted carbonized fibers. The method of Claim 10 wherein said residence time of said container and admixture within said preheated environment is between about one and about five hours. Discontinuous silicon carbide fibers produced in accordance with the method of Claim 1. The discontinuous silicon carbide fibers of Claim 12 wherein said fibers are strongly susceptable to heating by microwave energy. The discontinuous silicon carbide fibers of Claim 13 wherein said fibers are heated greater than about 800 degrees C within less than one minute in a conventional microwave oven of about 800 watts power. The silicon carbide fibers of Claim 12 wherein individual ones of said fibers are of a length between about 0,01 mm and about 3,0 mm (about 10 and about 3000 microns). A sheet comprising silicon carbide fibers as produced in accordance with the method of Claim 1 and employing a conventional paper-making process. The sheet of silicon carbide fibers of Claim 16 wherein said sheet is amenable to pleating thereof.
Anspruch[fr]
Procédé de préparation de fibres discontinues de carbure de silicium comprenant les étapes de

mélange dans une quantité de fibres de coton carbonisées, d'une quantité de sel métallique qui sert de promoteur pour la conversion desdites fibres carbonisées en fibres de carbure de silicium, d'une quantité d'oxalate de calcium, et d'une quantité de dioxyde de silicium pendant une durée suffisante pour développer un mélange substantiellement homogène,

séchage dudit mélange,

dans un récipient en graphite semi-poreux, chauffage dudit mélange pendant une durée et à une température auxquelles lesdites fibres carbonisées dans ledit mélange sont converties en fibres de carbure de silicium.
Procédé selon la revendication 1, dans lequel lesdites fibres sont des fibres de coton carbonisées. Procédé selon la revendication 2, dans lequel ledit sel métallique est le sulfate ferreux. Procédé selon la revendication 1, dans lequel ledit dioxyde de silicium comprend du dioxyde de silicium à basse densité. Procédé selon la revendication 4, dans lequel ledit dioxyde de silicium comprend de la silice sublimée. Procédé selon la revendication 1, dans lequel lesdits composants dudit mélange comprennent entre environ 25 et environ 35 parties de fibres de coton carbonisées, entre environ 0,3 et environ 0,6 partie d'oxalate de calcium monohydraté, entre environ 66 et environ 70 parties de dioxyde de silicium sublimé, et entre environ 10 et environ 25 ml d'une solution ayant une concentration d'environ 25 mg de Fe++/ml d'eau sous la forme de sulfate ferreux. Procédé selon la revendication 1, dans lequel ledit mélange est séché avec de la chaleur à environ 300 degrés C. Procédé selon la revendication 1, dans lequel ledit mélange séché est disposé dans un récipient en graphite semi-poreux, exposé à une purge au gaz inerte pendant une durée suffisante pour évacuer substantiellement tout l'air du récipient et du mélange, et lesdits récipient et mélange purgés sont introduits dans un environnement inerte préchauffé à une température comprise entre environ 1450 et environ 1750 degrés C. Procédé selon la revendication 8, dans lequel lesdits récipient et mélange purgés sont maintenus dans ledit environnement inerte préchauffé pendant un temps de séjour suffisant pour effectuer la conversion desdites fibres carbonisées en fibres de carbure de silicium. Procédé selon la revendication 9, dans lequel ladite conversion des fibres carbonisées est supérieure à 90 pour cent, lesdites fibres converties comprenant entre environ 20% et environ 30% du poids des fibres carbonisées non converties. Procédé selon la revendication 10, dans lequel le temps de séjour desdits récipient et mélange dans ledit environnement préchauffé est compris entre environ une et environ cinq heures. Fibres de carbure de silicium discontinues préparées conformément au procédé selon la revendication 1. Fibres de carbure de silicium discontinues selon la revendication 12, dans lesquelles lesdites fibres sont fortement sensibles au chauffage par une énergie à micro-ondes. Fibres de carbure de silicium discontinues selon la revendication 13, dans lesquelles lesdites fibres sont chauffées à plus d'environ 800 degrés C en moins d'une minute dans un four à micro-ondes classique d'une puissance d'environ 800 watts. Fibres de carbure de silicium selon la revendication 12, dans lesquelles certaines desdites fibres ont une longueur comprise entre environ 0,01 mm et 3,0 mm (environ 10 à environ 3000 microns). Feuille comprenant des fibres de carbure de silicium telles que produites conformément au procédé selon la revendication 1 et en utilisant un procédé de fabrication de papier classique. Feuille de fibres de carbure de silicium selon la revendication 16, dans laquelle ladite feuille est apte à être plissée.






IPC
A Täglicher Lebensbedarf
B Arbeitsverfahren; Transportieren
C Chemie; Hüttenwesen
D Textilien; Papier
E Bauwesen; Erdbohren; Bergbau
F Maschinenbau; Beleuchtung; Heizung; Waffen; Sprengen
G Physik
H Elektrotechnik

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