For release: Dec. 9, 1998
Contacts:
Sina Maghsoodi, Materials
Chemistry, (515) 294-1110
Iver Anderson, Metallurgy and Ceramics, (515)
294-4446
Susan Dieterle, Public Affairs, (515) 294-1405
AMES, Iowa -- A unique glue invented by scientists at the U.S. Department of Energy's Ames Laboratory should for the first time enable manufacturers to join parts made of a rugged class of ceramic materials known as silicon-carbide composites.
Silicon-carbide composites are considered possible replacements for steel and the superalloys used in the aerospace industry because they can withstand higher temperatures, do not melt and are less susceptible to corrosion. The composites are made of silicon-carbide fibers woven together like a mat and then encased in a silicon-carbide matrix. Just as steel rebar strengthens concrete, the fibers strengthen the brittle matrix material.
Until now manufacturers have been unable to make complex objects -- such as fans, heat exchangers, engine parts and fuel cells -- out of the composites because there was no way to form reliable joints between parts made of the material.
The glue developed and tested at Ames Lab appears to solve that problem. Tests indicate that ceramic joints made with the glue exhibit strength of up to 14,500 pounds per square inch (the equivalent of 100 megapascals) at temperatures of 1200 C (2200 F). That's a vast improvement over steel, which has little strength at temperatures above 700 C (1300 F).
The glue has the added advantage of being relatively easy to use compared to traditional ceramic-joining methods, which involve an elaborate, high-temperature curing process. Instead, the glue can be heated with a propane torch and cured in a regular air atmosphere without clamping pressure.
"This is probably the only glue that can join ceramic composites in an air atmosphere and produce the strength that is needed," said Sina Maghsoodi, an Ames Lab chemist who helped invent the glue.
Scientists have spent the past dozen years searching for a silicon-carbide joining process. The problem has been difficult to solve because, although silicon carbide itself can withstand temperatures of up to 2000 C (3600 F), the fibers in the composite material begin to degrade at about 1200 C (2200 F). Traditional ceramic-joining methods don't work with the composites because they involve curing the joints at temperatures of at least 1600 C (2900 F).
Maghsoodi and three Ames Lab colleagues -- Iver Anderson, director of the Lab's Metallurgy and Ceramics Program; Ozer Unal, an associate ceramist; and Mohammad Nosrati, a graduate student -- set out three years ago to solve that problem. Their work resulted in development of a pastelike glue made of silicon-bearing polymers and an aluminum-silicon alloy powder. When heated to about 500 C (900 F), the polymers in the glue begin to break down into silicon carbide and excess carbon. As the heat reaches 600 C (1100 F), the alloy powder starts to melt. Silicon from the alloy reacts with the excess carbon to form more silicon carbide, while the aluminum reacts with available oxygen to form aluminum oxide. These additional ceramic particles, or whiskers, diffuse to strengthen the joint in the same way that the fibers toughen the composite.
"The glue forms an in-situ composite that gives strength to the joint area," Maghsoodi said.
Maghsoodi said several companies that produce ceramic materials have requested samples of the patented glue compound and are exploring its possible uses. He said they are attracted to the strength of the joints as well as the relatively simple joining process. Because the joint can be made by propane-torch heating, manufacturers could use the glue in a variety of field applications rather than having to cure the joints in high-temperature furnaces.
Ames Laboratory is operated for the Department of Energy by Iowa State University. 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.
Last revision: 12/10/98 sd
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