SBIR-STTR Award

Carbon-carbon composites have not been considered for use in high-energy neutron fields primarily because of the high shrinkage rate of the polyacrylonitrile (PAN) fibers. The high shrinkage rate is primarily due to the nongraphitic structure of the fibers. During the past several years new fibers prepared from mesophase pitch and from decomposed hydrocarbon gases or liquids have become available. These new fibers are made from precursors that are known to graphitize well when heated to high temperatures. It is proposed to select new carbon fibers that can be easily graphitized and process them into fiber-reinforced carbon-carbon composites that will be stable to highenergy neutrons and as such can be considered as first wall tiles and limiters in a magnetically confined power fusion reactor. Composite fabrication will be tailored to permit flexibility of design in the removal of surface heat resulting from interaction of the plasma with the wall. In Phase I, the feasibility of fabrication will be demonstrated. The approach will be to maximize the fiber content of the composites, to increase the amount of highly graphitizable carbon, and to control orientation to provide for the most advantageous surface heat removal. Raman spectroscopy, x-ray diffraction, electrical resistivity, and scanning electron microscopy will be used to characterize the composites and provide data for estimating irradiation stability.Anticipated Results/Potential Commercial Applications as described by the awardee:The contractor anticipates the results from Phase I will demonstrate the feasibility of fabricating highly graphitic onedimensional and two-dimensional carbon-carbon composites. These composites are expected to be stable under high-energy neutrons and provide flexibility in design for removing heat from the first wall in a magnetically confined power fusion reactor. The potential commercial application is a feasible first wall material for power fusion devices.

Carbon fibers and matrices that can be readily graphitized have been selected for processing into fiberreinforced carbon-carbon composites that will have properties that permit their use in fusion test reactors and also have a high degree of resistance to fast-neutron damage at high temperatures. Composite fabrication will include pitch fibers (PF), vapor-deposited fibers (VDF), and petroleum pitch matrix carbons. Composites will be tailored to provide optimum thermal conductivity and strength in the through-the-thickness direction, to ease the removal of heat due to plasma interactions with the wall and limiters and ease their design in test reactors such as the Tokamak Fusion Test Reactor (TFTR), the Compact Ignition Tokamak (CIT), and TIBER II/ETR. Continuous-fiber 2D lay-ups and chopped-fiber, quasi-3D, and 3D composites will be fabricated from the PFs and VDFS. Staple-fiber mat and needle-punched continuous-fiber cloth precursors and 3D woven fabric will be prepared to provide fiber lengths parallel to the through-the-thickness direction. Petroleum pitch and vapor-deposited pyrolytic carbon will be utilized as densifying agents to maximize mechanical properties and reduce anisotropy. The composites will be heat-treated in the range 2600' to 3000'C to fully crystallize their structures. The composites win be characterized for crystallinity by measuring electrical resistivity and Raman microscopy. Thermal and mechanical properties will be measured at room temperature and at elevated temperatures. Structural, property, and dixnensional changes will be measured after neutron irradiation to confirm the neutron stability of selected composites and provide a data base for design.Anticapated Results Potential Commercial Applications as described by the awardee:It is anticipated that the results will demonstrate the feasibility of fabricating highly graphitic carbon-carbon composites that will have excellent stability to fast neutron damage alonl with properties that will ease the design of first-war tiles and limiters for fusion test reactors. The composites should have commercial potential as fusion reactor tiles and limiters that will improve maintenance schedules and extend the life of the reactor wall. They may also be useful as space radiator materials for space power reactors or satellites.