There’s a critical need for new materials that can function under extreme temperature, pressure, or radiation environments. Scientists are hoping MaRIE will fill that need by forever changing the way materials are developed.
It’s a material world, all right. Materials are the backbone of modern civilization, which wouldn’t exist without the metals, plastics, ceramics, semiconductors, and other marvelous solids we’ve fashioned from the elements. We’ve made everything from aluminum alloys to Zytel thermoplastics, but truth be told, we don’t exactly know what we’re doing.
Materials-savvy scientists cannot modify the composition of, say, a steel alloy and be certain of the results, nor can they predict which modifications will produce a desired product. Rather, their approach to material development is largely Edisonian—tinkering with an existing material’s recipe until, through trial-and-error and perseverance, something novel is produced. It’s a slow process that’s always been stunningly successful.
However, at a 2007 Department of Energy workshop,† some of the country’s best scientists made it clear that a different approach was needed. They stressed that solving many of our most challenging technical problems will require revolutionary new materials, and the nation can’t afford to wait for serendipity to supply them.
Enter MaRIE, a proposed national user complex for materials research that will be Los Alamos National Laboratory’s signaturescience center in the 21st century. Intended to hasten the development of materials that must function within extreme temperature, pressure, radiation, or other environments, MaRIE (for Matter-Radiation Interactions in Extremes) will also be part of a general effort to transform materials development from a “try it and see what happens” discipline into a “predict it and control what happens” science.
A Complex Overview
The MaRIE approach to materials research and development will be a science-based methodology. Specific steps will include fabricating a sample, exposing it to an extreme environment, and making measurements in situ that reveal how the sample changes over time. Experimentalists will coordinate their efforts with theorists and modelers to identify parameters or indicators that lend predictability to materials development. Their collective insights will then guide new experiments and new discoveries. The ultimate goal is to create materials that are perfectly suited to any given extreme environment. “We’re looking at a very different approach to materials research,” says John Sarrao, project lead of the MaRIE effort, “a much more complete integration of experiment, theory, simulation, and fabrication than has ever been done before.”
Plans for MaRIE call for a complex of three facilities and two particle accelerators, with the accelerators—the beating hearts of the complex—providing high-energy beams of protons, electrons, or x-rays for experiments. The beams will serve as common links between the facilities, create some of the extreme environments, and most important, provide scientists with state-of-the-art tools, such as proton microscopy and an x-ray free-electron laser (XFEL), for interrogating samples. Initially, MaRIE will focus on materials and environments relevant to national security, with an emphasis on energy security. Each facility will play a role in achieving that focus.
The Making, Measuring, and Modeling Materials Facility (M4) is where new samples will be fabricated as well as characterized before and after they’re tested in extreme environments. The facility will also house a cadre of theory, modeling, and computation (TMC) experts who will use MaRIE data to validate computer simulations and inform the next generation of materials models.
The bulk properties of a material, say, plutonium (Pu), are determined by a combination of its crystal structure (atom-scale), microstructure (micron-scale), and defects (atom-to-bulk scale). (A) Plutonium has six distinct solid-state crystal structures, or phases. The complex, room-temperature alpha phase makes the metal brittle. (B) The high-temperature delta phase makes the metal ductile. (C) This micrograph shows several Pu grains. The different colors indicate different orientations. (D) The threadlike lines are dislocation defects that run through the Pu grains. (E) Plutonium decays naturally by emitting helium nuclei (alpha particles), and helium gas slowly accumulates within the metal. The dark bubbles are voids filled with helium gas. Such voids can affect the thermal conductivity of the metal.
The Fission and Fusion Materials Facility (F3) (pronounced “eff cubed”) will specialize in creating neutron environments to mimic what’s expected in advanced reactors. The Laboratory has already been a pioneer in nuclear-reactor design and safety and has a strong interest in using MaRIE to develop materials for nuclear power. Researchers will be able to interrogate a sample in situ, using the XFEL or other photon sources.
The third facility, the Multi-Probe Diagnostic Hall (MPDH), will be well suited to the study of materials for national security. There, samples will be hit by shock waves or subjected to other dynamic extremes and probed by the XFEL for analysis of the sample’s interior with atom-scale resolution (better than one-billionth of a meter). Simultaneously, researchers will be able to use proton microscopy to make images (similar to an x-ray image) of larger features inside the sample and/or do spectroscopy on the sample’s surface.
Why is MaRIE focused on extreme environments? Because operating at the extremes opens the door to new solutions, especially in energy security.
It is estimated that the already-enormous global need for electric power—approximately 15 billion-billion watts (15 terawatts)—may double within the next 40 years because 2.5 billion people will be added to the planet and because China, India, and other countries will continue their steady industrialization. The immense deficit between the energy we’ll need versus the energy we currently produce requires full development of every known energy source, from fossil fuels to renewables to nuclear fission and fusion (although commercial fusion power is still several decades away).
One way to ease the situation is to increase the efficiency with which electricity is generated. The standard coal-fired power plant, for example, uses the heat from urning coal to convert water into high-pressure steam, which spins a steam turbine that turns an electric generator. The plant’s efficiency would nearly double if the temperature of the steam were increased to around 750 degrees Celsius and its pressure doubled to about 380 atmospheres, butpressure vessels inside the boiler (where the water-to-steam conversion takes place) would fatigue and burst. Developing stronger high-temperature steel would remove one barrier to making more than 50,000 of the world’s coal power plants burn less coal.
Similarly, next-generation nuclear reactors will be designed to achieve higher fuel efficiency and produce far less long-term nuclear waste because they’ll run nearly 3 times hotter than today’s reactors and produce 10 times the neutron flux. Existing structural materials or nuclear fuel claddings could not survive such extreme conditions but instead would become brittle, swollen, and structurally unsound. Next-generation reactors will require next-generation materials.
Renewable energy resources such as solar and wind power also present some formidable material challenges, with needs for lower-cost, higher-efficiency photovoltaic materials and stronger wind turbine blades. And what’s generally not appreciated is that using renewable energy sources is often tied to extreme chemical and electric-field environments.
For example, energy storage is crucial if solar or wind power is to be a reliable source of electricity. For small-scale systems, batteries are often the storage medium of choice. A battery stores energy by changing the oxidation state of a metal electrode through processes that are intrinsically extreme—chemical bonds get ripped apart. Indeed, the electric field near the electrode’s surface is more than 10,000 times greater than the field in a lightening bolt. Unfortunately, charging a battery also changes the metal’s surface structure, and a battery loses the ability to be recharged because of repeated structural changes.
“Maybe there’s a way to build a better battery,” says Mark McCleskey, one of MaRIE’s technical leads. “With the ability to image the surface structure, scientists at MaRIE could examine the electrochemical interfaces to see microscopically why breakdown occurs, information that can help us build more-durable high-capacity energy-storage devices.”
Maintaining the Nuclear Arsenal
MaRIE will also help scientists fulfill the Laboratory’s mission of maintaining the nation’s nuclear deterrent. Furthermore, on April 6, 2010, the Obama administration released its Nuclear Posture Review, which establishes U.S. nuclear policy for the next 5 to 10 years. In a follow-up commentary, Vice President Joe Biden wrote that “although we will not develop new warheads or add military capabilities as we manage our arsenal for the future, we will pursue needed life-extension programs so the weapons we retain can be sustained.”
Los Alamos plays a large role in the life-extension programs, including evaluating the safety and reliability of the weapons. Radiation emitted by plutonium and uranium components in the weapons causes many parts to age at an accelerated pace, so one aspect of life extension involves replacing various weapon parts. Some of the original parts were manufactured more than 30 years ago, so new ones will likely be manufactured differently and be made from different materials. In the extreme environment of an exploding weapon, what guarantee is there that those new parts will function as intended?
Los Alamos’ Tim Germann and co-workers recently conducted a simulation of phase transitions in a polycrystalline iron sample. Initially, in a body-centered-cubic crystal phase (gray with yellow boundary), the sample was shocked, resulting in primarily a low-symmetry, hexagonal-close-packed phase (red) with some twinning (green). The challenge is to understand what controls the evolution of these phases.
“In the absence of nuclear testing, such guarantees can come about only through advanced computer simulations that, closely coupled with experiment, can predict the outcome of detonating a weapon,” says Deputy Associate Director of Weapons Mary Hockaday. “Those simulations must account for any changes that might happen to a part when it sits for years in a radiation environment. Data from MaRIE could tell us how the microstructure evolves in that environment; if we know that, we know a lot.”
A material’s microstructure refers to structural features—crystal grains, grain boundaries, defects—that are usually on the order of a millionth of a meter in size micron scale). Explicitly, a metal forms crystals and, under the right conditions, can be a single crystal, its atoms all sitting at precise positions in an imaginary lattice. Normally, however, it will be polycrystalline, its interior composed of millions of micron-scale crystals (or grains) of different sizes and shapes, each randomly oriented. The grains are typically riddled with defects, ranging from point defects—an atom sitting where it shouldn’t be (an interstitial) or not sitting where it should be (a vacancy)—to larger defects.
The microstructure is important because, in combination with the crystal grains and defects, it determines the material’s macroscopic engineering properties, such as its strength, its stability under heat and pressure, or its elastic properties.
“Much of today’s materials research is focused on nanoscience because new phenomena arise when quantum mechanical effects dominate observed behavior,” says Sarrao. “So people are trying to understand things on the scale of nanometers. But that understanding isn’t enough to translate into actual utility. What matters for doing the materials revolution is a material’s microstructure and the micron frontier.”
The micron frontier is the evocative name given to the gap that exists between understanding how changes to the microstructure occur and knowing how they result in changes to the bulk material. Conquering the frontier will take more than just making micronscale measurements—materials scientists have been doing that for a long time. Rather, it means knowing where atoms are and are not inside a three-dimensional (3-D), macroscopic solid (as a function of time) and how the atoms move from place to place.
The tool that can perform that minor miracle is the XFEL, a technology that is just now becoming available. The two current XFELs, the Linac Coherent Light Source in California and the European XFEL in Germany, will begin to revolutionize materials sciences because they allow users to watch chemical bonds break and form, but only in highly prepared samples that are often so thin as to be essentially two dimensional (2-D). Such samples are once-only affairs; they vaporize from the heat that’s generated when they absorb or scatter too many high-energy x-rays.
Los Alamos scientists are working hard to develop a next-generation XFEL that will produce x-rays with 5 to 20 times higher energy than either the Californian or European machine. Higher-energy x-rays (knownas very-hard x-rays) can penetrate dense, multigranular metal samples that are thick enough to exhibit real-world properties.
Through a technique known as coherent x-ray diffractive imaging, already demonstrated on thin samples, researchers may be able to make 3-D images of the atoms making up the thick sample’s microstructure.
“Fewer very-hard x-rays will be absorbed by the sample, and any heat that does get deposited from x-ray interactions will be distributed among more atoms,” says Cris Barnes, the MPDH technical lead. “The result is that even thick samples will survive long enough to be probed several times.”
Furthermore, because XFELs can take a series of rapid “snapshots” (a couple every billionth of a second), scientists will be in a position to follow transient yet critically important processes such as the introduction and early development of environment-induced damage.
As an example of MaRIE science, consider a metal support structure inside a nuclear reactor and the general sequence of events that occurs after neutron bombardment causes atomic-scale defects in the material.
A neutron penetrates the metal, strikes an atom, and like a cue ball hitting a rack of billiard balls, initiates a cascade of atom displacements. The displacements take place within a volume much smaller than a crystal grain. In nearly no time (about 10 picoseconds) the cascade ends, and most of the displaced atoms find new homes within the grain’s crystal structure. But some don’t, and the grain harbors a nanoscale cluster of interstitial atoms and vacancies.
The high temperature inside the reactor allows the interstitials to migrate and coalesce into larger defects, while the vacancies merge to become voids. The defect structures continue to merge with others until the damage has grown into micron-scale cracks and voids.
Surprisingly, recent theoretical research at Los Alamos suggests a way to design a material that can “heal” itself before the damage reaches the micron scale. Simulations show that if the initial impact occurs inside a crystal grain close to a grain boundary, the boundary will trap and hold onto some of the homeless atoms. Later it will “unload” these excess atoms into the body of the grain, where they will annihilate vacancies. Thus, nanocrystalline materials with very small crystal grains (and a high grain-boundary to grain-body ratio) may be highly resistant to neutron damage.
The leaders of the MaRIE project (left to right): Mark Bourke (F3), Mark McCleskey (M4), Turab Lookman (TMC), Cris Barnes (MPDH), and John Sarrao (project lead).
Without MaRIE, it would be very difficult to confirm that result experimentally. But once the F3 facility is built, a nanocrystalline alloy can be placed in it and bombarded with fast neutrons. Training the XFEL on the sample while it’s being irradiated, researchers could image the atom displacements and then record the exact distribution of defects that remain after localized melting and recrystallization have occurred. A series of images would show how the defects diffuse and coalesce into nanoscale and micron-scale clusters and to what extent the atoms are “loaded” onto the grain boundary. More images would reveal the extent of unloading and self-healing.
The integrated MaRIE facility would then provide the tools to purposely manipulate the sample’s microstructure and accelerate materials development. Before long, high-performance computing simulations of the data could be compared in real time with the real thing, and the cycle would repeat until the birth of a new, neutron-resistant material.
Sometime in the rapidly approaching future, a switch will close and coherent x-rays and a highpower proton beam will simultaneously strike a thick sample in the MPDH: MaRIE will be fully operational. Thanks to breakthroughs in experimental characterization, theoretical modeling, and multiscale simulation on ultrafast supercomputers, scientists will then have an unprecedented opportunity to address what were once considered to be impossible materials problems. And although large facilities like MaRIE can take 10 or more years to build, plans for it can be driven by ideas scientists are having now about the future. Currently, the MaRIE design and program remain flexible and will continue to incorporate new research ideas.
—Jay Schecker (contributions from Anthony Mancino)
In this issue...