Composite Carbonaceous Articles and Process for Making Same

This invention relates to shaped carbon and graphite articles and to an improved process for making same. These articles find utility in applications requiring the combination of carbon's inertness with a porous side and an imprevious side. Thus, typical applications include electrodes for zinc/halogen cells, sodium/sulfur batteries and as fuel cell separators.

Traditionally, shaped carbon articles have been made by forming a "green" mix of finely divided carbonaceous particles and pitch, shaping the mix and then baking or graphitizing the resulting shape. The art also has learned to form graphite fibers and to make fabric therefrom, such as screens, separators and the like.

The present invention provides a composite article consisting of vitreous carbon intimately linked to a carbon felt fabric. The present invention also provides an improved process for making such an article by linking to a felted carbon material or its precursor a polymerisable carbonizable high carbon resin and then baking the resulting composite resin felt material to a carbonizing or graphitizing temperature.

A sandwich structure consisting of alternate vitreous carbon and carbon or graphite felt layers can also be obtained by simultaneous or successive operations.

In the accompanying drawing forming part hereof similar elements have been given the same reference numerals, in which drawings;

FIG. 1 is a cross sectional fragementary view of a composite carbonaceous article made in accordance with the present invention.

FIG. 2 is a view similar to FIG. 1 showing a second embodiment of the present invention.

FIG. 3 is a view similar to FIGS. 1 and 2 showing still a further embodiment of the present invention.

The resin used in the present invention is a carbonizable, polymerizable resin having a high carbon content in the range of 40 to 90 percent such as a furfural or phenolic resin or polyimides which after suitable heat treatment produces "vitreous" carbon. This material is called vitreous carbon because it has the appearance and characteristics of a black glass: it has a closed microporosity and is impervious to gas.

The material is obtained by carbonization and subsequent thermal treatment of carbonaceous materials with strong transversal molecular bonds. Carbon and graphite shapes made from this material are availble from Carbone U.S.A. Inc., 400 Myrtle Ave., Boonton, N.J. 07005.

Two grades are supplied which are obtained after two different heat treatments:

V10 is heat treated at 1,000.degree. C.

V25 is heat treated at 2,000.degree. C.

This last grade is more stable in dimension during thermal cycles and is to be preferred when sharp variations of temperature can occur or when maximum operating temperature is above 1,000.degree. C.

The above described material forms one half of the composite.

The felted carbon fabric which forms the other half of the composite preferably has the following characteristics:

The fibers constituting the felted fabic preferably have a diameter of four to eight microns. The materials above designated as RVC and RVG grades also are marketed by Carbone U.S.A. Corp.

The Catalyst used varies on the particular resin used in making the vitreous carbon and is not critical, as any of the well known polymerization catalysts can be employed. The preferred catalyst is aniline perchlorate which is formed in situ in the resin by separately adding aniline and perchloric acid. The amount of catalyst used normally ranges from one to ten weight percent basis resin, preferably about five percent.

The resin can be applied by either casting onto the carbon felt or by centrifuging a roll of felt with the resin. Generally from 0.1 to 2 grams of resin should be applied per cm.sub.2 of fibers.

Referring to the drawing, FIG. 1 illustrates a composite carbonaceous article 9 formed of a carbon felt 10 made of carbon fibers 11. The transverse carbon fibers 12 of the felt 10 penetrate into the vitreous carbon 14 on each side of the felt.

In FIG. 2 there is shown a composite carbonaceous article 15 having a single layer of vitreous carbon 14 intimately linked to a single face of a carbon felt fabric in accordance with the present invention.

FIG. 3 combines the structure of FIGS. 1 and 2 in providing for an article 16 in which alternate layers of felt fibers and vitreous carbon are linked together.

The invention is further illustrated in non-limiting fashion by the following examples.

This example illustrates the casting aspect of the process. A furfural resin (Durez 16470) is mixed with five percent of aniline and perchloric acid and poured on a flat, chemically inert surface. The resin is allowed to thicken for four to five hours at ambient temperature (18.degree. C.) or until its viscosity increases above 20,000 cps. Next a piece of felted fabric made of carbon fibers (RVC 1000) is cut and applied onto the mix of catalyzed resin. The assembly is then allowed to harden for twenty hours at room temperature after which it is cut to the desired dimension. In a modification, the fibers are precarbonized prior to being applied onto the resin. In another modification the untreated felt precursor is used.

This example illustrates the centrifugal version of the present process. This version provides a better product because the resin can be applied more homogeneously to the fibers.

The resin is made up as in the preceding example and poured into a centrifuge rotated at a speed of about 100 rpm and allowed to thicken until its viscosity increases above about 20,000 cps. A roll of the carbon felt is then introduced into the centrifuge and the same is restarted to a speed of 150 rpm. Heat is applied to the centrifuge to bring the resin to its polymerization temperature of 100.degree. to 200.degree. C. while the machine is operated for three to four more hours. The centrifuge is stopped before the article hardens, the composite roll is unrolled and placed on a flat surface for cutting to the desired dimensions.

This example illustrates the baking step of the invention. "Green" shapes obtained by either of the above examples are stacked in an iron crucible and separated from one another by flat separator plates such as graphite sheets of the same initial dimensions as the shapes. The "green" shapes then are heated to 700.degree. to 1200.degree. C. depending on the size and characteristics of shapes to obtain carbon/carbon composites. If a graphitized or a highly graphitized article is desired and if there is a need for increased purity or reduced specific surface area, the article is baked to 1500.degree. to 3000.degree. C.

Where a resin thickness of between about 0.2 mm and 2.5 mm is used, it is preferred to raise the baking temperature at a rate of about 3 to 5 degrees C./per hour. If the resin thickness is over 2.5 mm, it is preferred to raise the baking temperature by about 1 degree C. per hour.

The shrinkage due to baking is about 21%. The thickness of the vitreous carbon can be varied by incrmenets of 2/10 mm, up to 5 mm maximum. The thickness of the felt can be varied in the same fashion with no maximum limit.

From the foregoing description, one skilled in the art can easily ascertain the essential characteristics of this invention and, without departing from the spirit and scope thereof, can make various changes and modifications of the invention to adapt it to various usages and conditions.