Ames Laboratory, U.S. Department of Energy, Ames, Iowa
Ames Laboratory, U.S. Department of Energy, Ames, Iowa
Ames, Iowa -- Scientists at the U.S. Department of Energy's (DOE) Ames Laboratory have applied for a patent for a "glue" suitable for joining parts made of a promising new class of ceramic materials. The difficulty of joining parts made of these silicon carbide ceramics had been an obstacle to their commercialization.
The glue was invented by four scientists at the Ames Lab: Iver Anderson, a metallurgist, Sina Ijadi-Maghsoodi, a chemist, and Özer Ünal and Mohammed Nosrati, both ceramists.
The ceramics in question are composite materials called CFCCs. The ceramic equivalent of fiberglass, they consist of silicon carbide fibers embedded in a silicon carbide matrix. (Most people know silicon carbide as the hard, brittle grit on sandpaper.) The fibers give the composite toughness the single-phase ceramic lacks.
There is strong commercial interest in these materials because they can survive conditions too hot or corrosive for even stainless steel or aerospace "superalloys." Potential applications for silicon carbide CFCCs include massive heat exchangers for the oil refining industry and large convection fans for heat-treating ovens.
The Ames Lab glue neatly avoids the need for high-temperature curing to develop useful strength, a crucial requirement of other proposed joining methods for these materials. If a joint must be cured at high temperatures, residual products of the fiber-making process participate in reactions that degrade the fibers in the ceramic composite and weaken the joined parts.
"Everybody and their brother could join monolithic silicon carbide at 1600 or 1800 C," says Anderson. "That's duck soup. But everybody was scratching their heads trying to figure out how to do it [join silicon carbide CFCCs] at temperatures below 1200 C."
The Ames Lab glue forms a strong joint at temperatures well below the onset temperature of the fiber degradation reaction. Indeed joints can be made by local heating with a propane torch. According to Anderson, this gives it a "tremendous real world advantage" over other joining methods.
The main ingredients in the glue are a silicon-bearing polymer (a molecule that has a carbon backbone) and a low-melting-point alloy of aluminum and silicon in powder form. It is the alloy that gives the mixture the ability to form a strong joint at comparatively low temperatures.
The polymer breaks down into silicon carbide and excess carbon when it is heated, and the aluminum alloy powder melts, reacts with the carbon to form more silicon carbide and forms small islands of aluminum oxide and aluminum boride within the silicon carbide. These islands act much like the flbers in the composite material, toughening the joint.
"It's a very symbiotic bond," Anderson says. "Everything likes everything else in there, and as a result we form some nice hard phases that provide for composite strengthening of the silicon carbide "
The glue has other advantages as well. Unlike other glues, it can be prepared outside a chemical fume hood because it includes no solvent. The viscosity of the polymer depends on the size of its molecules; bccause the glue is made with intermediate-length molecules called oligomers, it naturally has a pastelike consistency and doesn't have to be thinned with solvent.
Without solvent, far less gas is released during curing and the resulting joint has fewer trapped bubbles and much higher strength. In fact, the joint strength is at least double that of joints made by the best competitive techniques.
Now that the patent application has been filed, the Ames group is conducting more extensive tests of the joint compound in collaboration with three major CFCC manufacturing companies and discussing a licensing agreement with another firm.
Ames Laboratory is operated by Iowa State University for the DOE. The Lab conducts research into various areas of national concern, including energy resources, high- speed computer design, environmental cleanup and restoration, and the synthesis and study of new materials.
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