When you think about the hundreds of new products that come onto the market each year, it quickly becomes apparent how important new materials are. From the latest touch-screen phone to high-tech sports fabrics that keep you warm — or cool and dry — nothing would be possible without new materials. Even existing products benefit when the materials that go into them become stronger, smaller, cleaner, smarter, lighter or cheaper to produce.
But just how do researchers go about developing new materials?
At the Ames Laboratory, the search for new materials takes a specific path referred to as materials discovery, design and synthesis. The process leverages the vast existing knowledge of materials and their various properties to create new materials that are a blend of the desired characteristics of the known. It also requires a mix of experimentation, characterization and theory.
“To design materials with specific behaviors and functions, we need to accurately describe the physics and chemistry that’s at work,” says Tom Lograsso, Ames Laboratory’s Materials Sciences and Engineering Division director. “The tools – high-speed computing and characterization equipment – to help understand the physics behind a material are becoming more robust and faster, which is helping narrow the design space.”
This knowledge base gives researchers the understanding of what attributes a material has to have to give it certain properties. Typically, the experimentalist uses this information to develop an educated “recipe” for the proposed material. The resulting material is then poked and prodded to measure its various characteristics, such as magnetism, crystal structure, etc. The theorist then uses these measurements to explain what’s actually taking place in the material. The theory then helps guide the next round of experimentation.
“It’s a very coordinated and integrated effort,” Lograsso says. “We do have an unusually high level of collaboration that you won’t find many places and a proven record of new discoveries.”
While Lograsso, who specializes in single-crystal growth, collaborates with researchers both here and elsewhere around the country and the world, those collaborations occur at very different levels of interactions.
“For example, at the Lab we’re working with (Ames Lab Chief Research Officer and physicist) Duane Johnson on topological insulators,” Lograsso explains. “He’s able to make precise calculations and we’re able to go out in the lab and make materials based directly on his work and then pass it along for detailed characterization.”
Materials known as topological insulators have been found to have unique electronic properties for electron transport that can be described theoretically by a warped (Dirac) cone. Binary tetradymite insulators bismuth-selenium (Bi2Se3) and bismuth-tellurium (Bi2Se3) have distinctly different electronic properties (Dirac cone) features.
This X-ray diffraction image of the Bi2SeTe2 crystal supports the theoretical predictions for the ternary tetradymite.
Duane Johnson’s group calculated the stability and electronic properties of bismuth-selenium-tellurium (Bi2SeTe2) ternary tetradymites to find an optimal composition and configuration where the two desirable Dirac cone features can coexist, as a stable “best in class” system. Using this proposed composition, the MPC was able to grow this Bi2SeTe2 crystal.
According to Lograsso, the ability to quickly make and test materials, predictably based on valid theoretical concepts, is a major step toward more efficient discovery of materials. And even the synthesis itself provides insights.
“From an experimentalist point of view, we have a wonderful store of information on how materials behave during synthesis, which can help shorten the design process,” he says.
Researchers here have the advantage of that technical expertise in large part because the Ames Laboratory is home to the Materials Preparation Center. In addition to expertise in alloying and single-crystal growth, the MPC also gives Ames Lab researchers ready access to the highest purity raw ingredients, particularly the rare-earth elements, that are essential to understanding the intrinsic behavior of materials with characteristics far beyond the limits of the individual components themselves.
“It’s hard to overstate the importance of using pure materials,” Lograsso says, “because impurities interfere with and inhibit crystal growth. Further inclusion of low level of impurities can have dramatic effects on the properties of the material. Without pure materials, you don’t really know if a material truly exhibits a particular characteristic or if the impurities are causing or affecting the characteristic.”
Lograsso likens it to a problem with a car. Is an intermittent rough idle caused by a cracked distributor cap, or is there a bad sensor that’s feeding false information to the computer module, which in turn is feeding the engine too much fuel? Replacing the distributor cap (eliminating the impurities) lets you then determine if it’s actually the sensor that’s acting up.
On the other hand, sometimes impurities are responsible for the functionalities we depend on every day.
“Semiconductors rely on impurities to change the electron transport properties of silicon,” Lograsso says. “Silicon is specifically and precisely doped with impurities on the level of parts per million to achieve desirable and useful characteristics.”
Likewise using single crystals is important so that the theorist gets a true picture of the actual crystal structure, the position of various atoms and their corresponding interactions. In a single, or monocrystal, the crystal structure runs continuously throughout the sample with no grain boundaries. With a polycrystalline sample, multiple crystal structures overlap or are scattered randomly so at best you get an average of how atoms are interacting within the material.
“We have a number of options when it comes to single-crystal growth methods,” Lograsso says. “We’re one of the few labs that can handle materials with high vapor pressures or that melt over a range of temperatures. We can also deal with reactive materials so that they don’t pick up impurities during processing, such as reacting with the crucible material.”
The Ames Laboratory’s range of single-crystal growth options includes arc-zone melting, horizontal-levitation-zone melting, cyclic-temperature technique, float-zone melting, recrystallization grain growth, Bridgman method, Czochralski method, and solution-growth techniques to produce single crystals. Depending on the material, these methods can be used in combination to address a particular challenge.
Although the combination of good theory, excellent materials, and top-notch expertise in both synthesis and characterization is yielding good results, Lograsso would like to see more done to capture the knowledge base that exists in the expertise at the Laboratory and to utilize that base to accelerate materials discovery. In fact, building such a resource is one of the long-term goals in the Lab’s scientific strategic plan.
“Over the years the Laboratory has developed an enormous amount of information that is essentially an empirical database,” he says. “We need to develop a way of capturing that information, along with the knowledge we continue to gain through the process of creating new materials, and utilize it to enhance and guide future materials discovery efforts.”
Knowing what makes one material perform in a particular way might simplify creating a new material with similar properties. And using that knowledge to shorten the path to discovery is an important step on the road to materials by design.
~ by Kerry Gibson