By Susan Dieterle
Think back to the lore of Hercules' battle with the Hydra and you'll understand science's frustrating search for a material that can withstand high temperatures.
The Hydra, a nine-headed serpent that Hercules was bidden to slay, was a formidable foe: every time Hercules cut off one of its heads, two more would grow to replace it. Even for a man possessing the incredible strength of Hercules, it was an exhausting task.
The mythical battle has modern-day parallels. Scientists are searching for a new material that can withstand temperatures of up to 1600 C (2900 F). It would replace steel and other alloys in jet engines, car motors, furnaces and turbines, and would allow fuel to burn hotter and more efficiently. However, they haven't found a material that possesses all of the necessary attributes.
Every time they try a new material, new "heads" pop up:
But the battle is far from over. Scientists across the country have united behind a material whose promising properties were discovered at Ames Laboratory. By surrounding the problem with a multi-lab effort, they hope to lop off the remaining heads and slay the dragon once and for all.
"That's how research is," says Mufit Akinc, an Ames Lab senior ceramic engineer and chair of the Materials Science and Engineering Department at Iowa State University. "You go one step at a time."
Akinc is working with molybdenum-silicon-boron (Mo5Si3B), the material at the center of a five-year project funded by the DOE. Participating scientists from nine labs, including Ames Laboratory, are trying to work out the kinks in the intermetallic compound and make it viable as the high-temperature material of the future.
Akinc took up his sword in the battle six years ago when Pat Thiel, director of the Lab's Materials Chemistry Program, suggested that he take a fresh look at silicides as a high-temperature material. Silicides are compounds made of silicon and other elements. They have high melting points, but some silicides are prone to oxidation, a corrosion process that causes them to degrade in the presence of air. Other silicides have poor creep strength, meaning that they deform or sag at high temperatures.
Several decades ago researchers discovered molybdenum disilicide (MoSi2), which withstands temperatures of more than 1600 C without oxidizing, but sags in the intense heat. "It has no strength at that temperature," Akinc says. The material is used as a heating element in furnaces, since strength isn't needed in that kind of an application.
To work in the next generation of jet engines and other load-bearing applications, researchers want a material with oxidative stability and good creep strength at temperatures of at least 1400 C (2000 F). "It may sound very simple, but it's so drastic," Akinc says. "You're getting into a temperature regime where the material becomes almost white-hot. So far, no one has been able to get a material that, at this kind of temperature, won't melt, sag or deform, or oxidize in the presence of air."
Akinc began working with a Mo5Si3 alloy, which had strength but poor oxidative stability, and employed a "doping" technique used by Ames Lab senior chemist John Corbett. Doping involves introducing small amounts of a new element in order to dramatically change the properties of the original material. Akinc discovered that when Mo5Si3 is doped with boron, the oxidation problem is virtually eliminated. "This material essentially meets all of the molybdenum disilicide (MoSi2) performance with respect to being able to stay at 1600 C and being able to resist oxidation, but it's at least 10 to 100 times more resistant to deformation," he says.
As he continued investigating Mo5Si3B with the help of former graduate students Mitch Meyer and Andy Thom, word about his work began to spread. One of the students approached Matt Kramer, assistant program director for Metallurgy and Ceramics, for help in understanding the deformation processes affecting the alloy. Kramer was intrigued by the material's properties and began doing some microstructural characterization work.
The material's properties also caught the attention of associate metallurgist Bruce Cook and associate ceramist Ozer Unal. Cook is measuring the electrical properties of the material, while Unal is concentrating on its mechanical properties.
Scientists throughout the nation also were interested. Last year the Ames Lab group received a three-year, $900,000 grant from DOE's Advanced Energy Projects Division to continue work on the molybdenum-silicon-boron material.
In 1996, DOE wanted to select a high-temperature intermetallic substance for a five-year, multi-lab study. "They wanted something that would leapfrog the current technology, not something that just moved it ahead in small increments," says Kramer, who presented the proposal from Ames Lab suggesting Mo5Si3B for the project.
Other materials that were discussed included iron-aluminides and nickel-aluminides. "Most of the iron- and nickel-aluminides had a maximum operating temperature of 800 C to 1000 C," Kramer says. "We had already pushed the molybdenum material to almost 1400 C, so this fit very well with what the DOE was looking for."
The molybdenum-silicon-boron material was selected for the Design and Synthesis of Ultrahigh Temperature Intermetallics Project. DOE is dividing $300,000 a year among the eight participating national labs -- Ames Lab, Argonne, INEEL, Lawrence Berkeley, Lawrence Livermore, Los Alamos, Oak Ridge and Sandia -- and the University of Illinois. The funding is helping to redirect research efforts toward Akinc's material and to stimulate collaborative efforts among the 25 scientists involved in the project.
The two project coordinators are R. Bruce Thompson, director of Ames Lab's Nondestructive Evaluation Program, and Roddie Judkins, manager of the Fossil Energy Program at Oak Ridge. Thompson, who also directs Iowa State's Center for Nondestructive Evaluation, says that even though the project is in its early stages, it is already generating a lot of excitement in several scientific disciplines. "There's a tremendous amount of interest in this material and the questions it opens up, both scientifically and technically. Given the creative talents of the scientists involved, that should lead to something quite useful," he says.
Judkins notes that use of the molybdenum material could help the environment. If the operating efficiency of turbines and engines improves, less fuel would be needed to generate the same amount of power. Using less fuel would also cut the amount of emissions and "greenhouse gases" in the atmosphere, thereby reducing the threat of global warming. "I think this project clearly falls under the category of public-good R&D, and the public good derived from this is energy security and an improved environment," Judkins says.
Even though the molybdenum-silicon compound represents a large step forward in the search for a new high-temperature material, several problems remain.
Mo5Si3B, like many other high-temperature intermetallic compounds, is extremely brittle at room temperature. "It's not a very tough material," Akinc says. "You can hammer steel and it won't do any damage, but if you put this material at an angle and whack at it, you can break it. You don't want that to happen to an airplane engine."
It's also difficult to process. "Processing means you have to synthesize the materials and turn them into certain shapes. This material is extremely difficult to deform at high temperatures and has great strength," Akinc says. "That's good, but not if you're trying to shape it."
That's not all. "In order to cast the material from the molten state, you need to heat it to 2200 C," he says. "How do you melt it and where do you melt it? And how do you pour it into a mold that will withstand that kind of temperature? These are the problems we need to work out."
The DOE labs are tackling different aspects of those problems, and the researchers hope their combined efforts will make the material viable. "We're making significant progress in a relatively short amount of time," Judkins says. "We have some of the best people at some of the best labs doing the work on this. We've got a lot of good science in this investigation. That's one reason I think it will be successful."
Aside from the work on the material itself, Kramer says the multi-lab effort represents an enormous achievement. "One of the objectives of the project is to get the labs to work together, and regardless of whether the project works or doesn't work in terms of the technology, it really has helped the collaborative research," he says.
Akinc believes this kind of unified effort is the best hope for slaying the high-temperature beast. "It takes perhaps one person to see an opportunity, but it takes a large team of scientists with different backgrounds to turn it into a reality," he says. "That's what we're doing."
With that upbeat attitude, "heads" are sure to roll.
For more information:
Mufit Akinc, (515) 294-0744, makinc@ameslab.gov
Matt Kramer, (515) 294-0981, mjkramer@ameslab.gov
Current research funded by:
DOE Basic Energy Sciences Office
Last revision: 12/17/99 sd
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