ARF Makes Strong, Fast-Molding Graphite Furfuryl alcohol prepolymer acts as binder to form graphite with potential for a nuclear fuel matrix A fine-bodied graphite that is strong and uniform has been developed by Armour Research Foundation. Preliminary work has shown that the graphite can be loaded heavily with uranium dicarbide powder, and that this actually doubles the strength of the graphite matrix, say Carl W. Boquist and John M. Neff of ARF's ceramic and metals research division. ARF believes that the development opens the way for improved graphite matrices for high temperature gascooled nuclear reactors. The binder is the essential feature of the graphite. It's made by a patented process in which furfuryl alcohol is highly condensed to form a prepolymer. The graphite made using this binder can be loaded with 3Qr/c UCo, and exhibits strengths in flexure of more than 12,000 p.s.i. both at room temperature and at 2000° C , ARF says. Furfuryl alcohol resins or prepolymers are available and are responsible for some of the strongest graphites made to date. They're thermosetting, in contrast to thermoplastic bitumens or pitch binders, and consist mainly of polyfurfuryl chains and unreacted furfuryl alcohol. ARF's resin, also a polymer, is highly resinified and contains almost no unreacted alcohols or volatiles. It's made by a process patented by ARF's Erik Nielsen (U.S. 2,681,896), in which furfuryl alcohol is condensed in contact with activated alumina. Getting rid of the unreacted alcohols and volatiles, ARF finds, is a key point both for producing a dense graphite and for being able to dope it with UCo. Another advantage of the resin is its viscosity—ARF can make the liquid with viscosities as low as 40 to 50 centipoises, or as high as 100,000 centipoises. ARF's ceramic research department decided early to concentrate on finebodied molded graphites—those made from the finest commercial coke flour, of which 90% passes a 200-mesh screen. Most ceramic materials and graphitized and carbonized bodies show higher strength-to-density ratios when made from fine materials. 62
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Preparation. In making conventional graphite a green mix is first formed from pitch and suitable petroleum cokes. The binder fraction during carbonization is polymerized, condensed, and partially volatilized. This leaves an amorphous carbon residue binding the coke particles together. Subsequent high temperature treatment converts the carbon body to graphite. This process in itself is full of variables that can affect the finished graphite. Major variables in finebodied graphites are variation of the green mix, binder migration, and dimensional change. The green mix is often not uniform, even after considerable mixing. If the binder releases large amounts of gases, these can escape from the green body leaving a porous structure. If they don't escape, they can build up pressures that cause localized voids or binder migration. The resulting small differences in density will be magnified by carbonization and graphitization.
BINDER CHECK. ARF's H. Nakamura checks a specimen of prepolymerized furfuryl alcohol binder in a differential thermal analyzer and balance
Dimensional change is also a result of gas pressure. Thermoplastic binders soften during the early stages of carbonization. The gases released can bloat the carbonizing body. The binder itself shrinks as it carbonizes. At some point in the process, the outside will be shrinking while the inside is still evolving gases. This can result in weakness. Binder. With these conditions in mind, ARF started looking for a binder that mixes easily and uniformly, one that is also thermosetting and releases minimal amounts of gas during carbonization. The commercial furfuryl alcohol came closest to meeting these requirements. But even it gives off more volatiles than ARF likes to see. The unreacted alcohol can volatilize, and large quantities of water and other alcohols can condense on thermosetting. Hence this led to development of Nielsen's patented polymer. Warm-Molding. ARF forms the prepolymer, Thermax, and fine coke flour into mixes that can be graphitized by warm molding techniques in from six to 14 hr., gets graphite with densities above 1.7 grams per cc. The green mix is easy to keep uniform with low viscosity liquid binder. Using another fabrication technique it recently developed, ARF has taken lab sized samples from green mix to graphite in two hours. The mix was taken to 1000° C. in about an hour for carbonization, up to 2500° C. for graphitization in another hour without removal from the mold. Pressures reached 6000 to 8000 p.s.i. These graphite samples showed flexural strengths around 6000 p.s.i. at room temperature and densities above 2 grams per c c , ARF says. Variation in properties runs a factor of two less than previous molded graphite. Nuclear Fuel. A big interest in graphite is to make it a suitable matrix for the addition of nuclear fuel. Up to now, fueling has been difficult. Coated fuel particles—to minimize oxidation reactions, retain fission products, and prevent fuel migration—result in decreasing strengths with increasing fuel levels. Recently, though, ARF has put UCo into the high density matrix and obtained the increased strength. This high strength plus the low porosity of the fine-bodied graphite leads ARF to believe that it could be used in uncoated fuel elements for high-temperature reactors.
