Metals for tomorrow

A Sandia research team is forging a new approach to making alloys with atomic arrangements that give the metals extraordinary properties.

By modeling on computers the complex chemical reactions that take place during the formation of nanoparticle clusters, and by using radiation to create the precise chemical conditions needed for clusters to form, the researchers believe they can construct alloys with such molecular exactness that the resulting bulk materials are stronger, lighter, and more corrosion-resistant than alloys made by conventional means.

Although the three-year project, funded by the program, is in its earliest stages, the exploratory research, if successful, could advance the science of creating alloys. That might revolutionize any number of products — aircraft engines, satellite structures, and consumer products such as bicycle frames and fuel-efficient automobiles, for example.

Cluster formation

An alloy is a material of two or more chemical elements, at least one being a metal, having properties different from its parent materials. Steel, for example, is stronger than iron, its primary component. Traditional alloy-making techniques typically involve heating and cooling, melting, annealing, tempering, and other metallurgical processes.

In the Sandia effort, the researchers start with a water-based solution into which they suspend, or dissolve, salt-like metallic particles, which are the alloy’s seed ingredients. The unique twist in the work, says chemist Tina Nenoff, is the use of radiation to break up the water molecules and interact with the metal salts to produce metallic particles in a highly reactive soup.

“We’re trying to form the perfect reaction conditions so the metal ions in the solution can start joining together to form clusters of nanoparticles,” she says.

In these conditions, the ions begin to form metallic clumps, which then combine to form larger and highly regular alloy structures.

Transient states

The team is focusing its research on the science that happens in the “novel metastable phase spaces” — transient states of matter — that are not accessible with traditional alloy production methods such as melting, says Nenoff. Understanding these stages is important for determining what alloys are created and how they form.

“The method of synthesis we’re studying — known as radiolysis — breaks down the water structure allowing it to react with metal salts to form nanoparticles, a synthetic approach that is flexible and versatile for making large quantities of nanoparticle compositions that can’t be easily created otherwise,” she says.

Two unique Sandia experimental facilities, the Gamma Irradiation Facility and the Ion Beam Materials Research Lab, provide the radiation environments needed for the research. In them, target solutions are subjected safely to a variety of controlled radiation exposures.

Following irradiation the samples are studied using spectroscopy and microscopy to understand what effects time and experimental variables have on particle formation, size, shape, and composition.

Depending on the combination of reactants, dose, and dose rate of radiation, researchers have been able to create nanometer-sized particles of various metals, including gold, in a variety of shapes including spheres, rods, and pyramids. The synthesis principles they are testing may become building blocks for essentially defect-free alloys.

The results of the experiments are being translated into computer simulations. Kevin Leung is leading an effort to use atomistic models, along with other methods, to interpret and understand the controlling factors in cluster formation — to determine if and why certain compositions form, and to suggest promising molecular compositions.

Synthesis by design

Because the simulations involve modeling the electronic structures and various charge states of complex particles, as well as the reorganization of attached particles and water molecules around formative clusters, they are large problems requiring the use of Sandia’s highperformance computers.*

“We’ll simulate the structure of the nanocrystal initiation right after radiation has been applied,” Leung says. “By examining the interface between metal clusters and nearby molecules, we will be able to understand what factors govern the size of these nanocrystals at the initial stages of formation and how the radiolysis affects clustering.”

“The calculations are telling us which nanoparticle compositions will be energetically more favorable than others,” adds Nenoff. “We attempt to synthesize those nanoparticles and compare notes with the modeling predictions. By developing our understanding of the basic materials science behind these nanoparticle formations, we’ll then be able to expand our research into other classes of materials.”

Nenoff adds that because of the novel techniques being used by the team, success of the project is not assured. “Improving on centuries of metallurgy won’t be easy,” she says. “But using computers to model these processes may mitigate some of the risk.”