Ames Laboratory, U.S. Department of Energy, Ames, Iowa
Ames Laboratory, U.S. Department of Energy, Ames, Iowa
Lab Recognized for Work in Solid State Physics and Metallurgy and Ceramics
Ames, Iowa -- The U.S. Department of Energy's (DOE) Ames Laboratory at Iowa State University (ISU) has won two DOE awards for research in the materials sciences.
A 1996 Materials Sciences Award went to Cai-Zhuang Wang and Kai-Ming Ho for "Sustained Outstanding Research" in solid state physics. This is the second year in a row that Ames Lab has won the award. The 1995 award was for work on quasicrystals.
R. William McCallum, Matthew Kramer and Kevin Dennis of Ames Laboratory collaborated with Daniel Branagan, Timothy Hyde and Charles Sellers of Idaho National Engineering Laboratory (INEL). The partnership resulted in a 1996 Materials Sciences Award for "Significant Implication for Department of Energy Related Technologies" in metallurgy and ceramics.
"In these times of tighter budgets, it was a great pleasure to learn the Lab had won two of the national Materials Sciences Awards this year," says Bruce Harmon, deputy director for the Ames Laboratory.
"In competition with much larger national labs, such as Los Alamos, the Ames Laboratory has garnered a number of these awards in the last several years," Harmon continues. "The recognition by scientific peers and the Department of Energy is a great tribute to the high-quality science performed in Ames, and a good morale booster."
For the past eight years, Wang and Ho have been developing a new technique for simulating the dynamics of atoms as accurately and far more quickly than can be done with first principle, or ab initio, methods.
The technique, called tight-binding molecular dynamics (TBMD), now represents a well recognized method for accurate computer simulations of large systems, that is systems on the order of 10,000 covalently bonded atoms (two atoms that share electrons, forming a chemical bond between the atoms).
"Until recently, molecular dynamics simulations have been restricted to using simple classical potentials between atoms," says Wang. "A breakthrough came with the introduction of ab initio molecular dynamics (MD), in which the quantum electronic structure is updated as part of the dynamics," he adds. "But the problem with ab initio MD is that it's very demanding computationally and so limits the number of atoms that can be studied at any one time." This lack of efficiency also makes the method expensive to use.
Ho continues, "A major challenge has been to find a scheme that can describe the interatomic interactions accurately, yet is fast enough to treat systems with a large number of atoms. There is a large class of problems that requires more atoms than ab initio techniques can handle and that demands more accuracy than classical potential simulations can provide. The goal of our work on TBMD is to bridge the gap between the two," he explains.
Wang and Ho have performed extensive TBMD simulations to study the structures, dynamics and electronic properties of complex silicon and carbon systems. "We've investigated the structures of fullerenes (cages of carbon atoms) from C20 to C100,'' says Ho. "Usually people can do a couple structures with ab initio methods, but it would be quite expensive to do the whole range. Furthermore, for a given cluster size (for example, C84), there are lots of ways you can put the atoms together to form a cage. We've actually predicted the ground state structure for C84 using TBMD, and the results have subsequently been confirmed by experiment."
TBMD has also proven a valuable method for studying complicated systems involving large numbers of atoms, such as amorphous and liquid phases, as well as crystalline defects.
The results of their simulations show the speed and accuracy of the TBMD method, which Wang and Ho will use to tackle other puzzles, such as the movement of defects in materials under stress and the pathways atoms take as they skim along the surface of a material.
In a unique collaboration that led to a Materials Sciences Award for both organizations, McCallum, Kramer and Dennis of Ames teamed up with INEL researchers Branagan, Hyde and Sellers to develop a means of controlling the solidification process of rare earth permanent magnets during rapid solidification and then used it to tailor an alloy for gas atomization. The partnership was part of the program for Tailored Microstructures in Permanent Magnets sponsored by the DOE Center of Excellence for Synthesis and Processing of Advanced Materials.
In an unusual but fortunate twist, an exchange of personnel further advanced the award-winning research when Branagan completed his Ph.D. with McCallum at Ames Laboratory and then took a position at INEL. Branagan's work with McCallum and the basis of his doctoral thesis had been to investigate what might be called "model" rare earth permanent magnet systems, in particular pure neodymium-iron-boron (Nd2Fe14B) doped with titanium carbide (TiC), from a fundamental standpoint. By looking at the systematics over a wide range, Branagan and McCallum were able to develop a process that would allow them to achieve a particular end control of the grain size within a material.
"Typically, when people are looking at permanent magnets, they start from the best material and try to improve it," says McCallum. "The problem with that is the best material is quite complex, so that when you try to make improvements, it's difficult to tell just exactly what each change you make is doing."
To avoid that dilemma, the model alloying process was intentionally designed to allow the change of only one particular factor -- the means of controlling the grain size or microstructure, which, in turn, leads to significant improvements in hard magnetic properties and a reduction in processing costs for permanent magnets.
At INEL, the concepts resulting from Branagan's thesis research with McCallum have been applied to technically useful compositions with potential impact for better magnets that will allow more energy-efficient motors for industrial, automotive and consumer applications.
"The results of INEL's work have been fed back to us, which has had a profound impact on our understanding of how the microstructure develops," says McCallum. "Their work has allowed us to expand our fundamental understanding substantially. What Ames Lab and INEL have done," he continues, "is not something that says we're going to make a better permanent magnet -- what it says is that we can make a good magnet much more easily."
Last revision: 4/17/98 mab
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