TECHNOLOGY
New Process Found To Grow Carbon Fibers When physicist Gary G. Tibbetts and coworkers at General Motors' research laboratories in Warren, Mich., began their experiment, they expected to find out how quickly carbon produced by pyrolysis of natural gas would diffuse through the walls of a stainless steel tube. What they found, instead, "is a method for greatly increasing the value of the carbon in natural gas by converting it to carbon as fibers/' Tibbetts says. "After 10 hours, visible fibers suddenly began growing from the surface of the . . . tube," Tibbetts says. Further experiments showed that by varying the reaction conditions the researchers could produce carbon fibers up to 12 cm long (which is the length of the uniform temperature zone in the furnace they are using) and from 5 to 1000 jtim in diameter. Most graphite fibers used to strengthen composite materials have diameters in the 10-jum range. Growing carbon filaments by pyrolyzing hydrocarbons in the presence of iron or other group VIII metals is not a new discovery. In fact, published reports of such work go back some 30 years, Tibbetts points out, although this early work produced sootlike material and not a high-strength carbon fiber like that produced by the GM procedure. Work that parallels that at GM currently is going on in Japan using benzene, rather than methane, as the hydrocarbon source, Tibbetts says. In the General Motors procedure, the fibers are generated within a thin-walled tube made of stainless steel containing 18% chromium and 8% nickel. The outer wall of this tube is in contact with circulating hydrogen which has been bubbled through water. Natural gas (about 97% methane) flows through the tube at a rate of 20 cc per minute, and the whole apparatus is inserted into a furnace that heats it to 970 ° C At this temperature the surface
Simple apparatus grows carbon fibers
Furnace
Hydrogen flow
Steel
-Steel tube
uhp
Natural gas flow
Browing-^* îarbon fibers
of the steel tube picks up carbon from the methane stream in a process common in steelmaking, called carburization. After 10 hours, when the steel has been carburized, carbon fibers suddenly begin to grow inward from the inner surface of the tube. This growth will continue for several hours. Quartz tubes will not grow carbon fibers, the researchers find, nor will steel tubes with less than 0.1% chromium. John R. Bradley, a GM metallurgist, has joined Tibbetts in investigating the process that produces carbon fibers in this system. Earlier work had established that w h e n stainless steels are carburized at high temperatures for long periods, as is the case in the GM experiment, a fine dust composed of metal, metal carbide, and graphite particles is ejected from the steel surface. The researchers believe that some of these fine particles, probably those containing Fe7C3 or Cr7C3 clusters, are serving as catalysts for the carbon fiber formation. Electron microscopy clearly shows that each
growing carbon filament has a small crystalline particle at its tip. Electron diffraction studies have not yet positively identified the composition of these particles, Bradley says, but energy-dispersive x-ray analysis indicates that the particles contain iron and chromium. The hydrogen flowing outside the steel tube diffuses through the steel, thereby preventing too much carbon, which could bury or deactivate the fiber growth, from building up on the growth surface. "Once we learn more about the metallurgical factors and processing conditions that cause metal dusting corrosion, we should be able to increase the yield an'd possibly simplify processing," Bradley suggests. Greater yields and more reproducible fiber properties will be needed before the process will produce carbon fibers at sufficiently low cost to make them competitive with glass fibers as reinforcers in automobile parts and other high-strength plastics markets. Rebecca Rawls, Washington August 8, 1983 C&EN
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