Advanced Structural Materials
JIAM research affiliates rely on a range of advanced tools—including supercomputers and a suite of state-of-the-art microscopes—to visualize, manipulate, model, and characterize new materials at the scale of the nanometer. Their research is advancing fields as wide-ranging as energy production, transportation, and computing.
Small-scale Mechanical Behavior
In the not-too-distant past, structural engineers might have tugged on a steel cable a centimeter in diameter to determine its strength. Today JIAM’s materials researchers are creating and testing steel threads that are millions of times thinner. What they learn about the material at the minute scale of the nanometer (1 billionth of a meter) will help them determine how the material will perform in larger-scale applications.
Likewise, earlier materials researchers might have pursued a macro-scale process of trial and error in evaluating candidate materials for use in an airplane wing or a bridge span. If one material didn’t work, they tried others, until they arrived at a material that possessed the desired properties. Often, these earlier engineers and scientists were limited by what they could see, or, or more accurately, what they could not see.
Modern Visualization Tools
Tremendous advancements in microscopy allow JIAM research affiliates to visualize and characterize materials at the scale of the nanometer and, in some cases, even at the scale of the atom. Among their primary tools are atomic force microscopes, transmission electron microscopes, scanning electron microscopes, and focused ion beam microscopes. JIAM research affiliates are exploring ways to adapt these microscopes to allow manipulation, measurement, and testing of ever-smaller material samples.
JIAM possesses a full suite of these leading-edge visualizing technologies, but the process of creating novel materials can begin at a much more basic level. Armed with pencil and paper, JIAM’s theorists often begin by working with equations and exploiting the laws of physics and what is known about a material’s atomic-scale characteristics and properties.
When the theorists and their brains have taken the equations as far as they will go, they turn the work over to the simulators and modelers. While physicists and chemists understand the basic forces of interaction between two atoms, UT’s Kraken and ORNL’s Jaguar supercomputers allow JIAM research affiliates to simulate the interaction among billions of atoms and to determine a material’s properties at larger scales.
Despite their power and speed, today’s supercomputers still face limitations. For one thing, they top out at simulating interactions among billions of atoms—still a relatively small scale. Consider, for instance, that a sugar-cube-sized sample can contain more than a trillion-trillion atoms. A material’s unique properties might become fully apparent only at the scale of 1 trillion atoms or more.
The second limitation involves time scales. In testing and characterizing a material, scientists measure the vibration among atoms. These vibrations can occur in a trillionth of a second. Modern supercomputers are capable of simulating vibrations to the nanosecond (one billionth of a second), but being able to simulate vibrations over much longer time scales—seconds, minutes, and even years—would provide them with much more detailed information on the material’s properties.
Continuing advances in computing will allow JIAM research affiliates to simulate material behavior at increasing time scales and with larger aggregates of atoms.
Through a technique termed nanoindentation, JIAM research affiliates press the tip of a tiny pyramid-shaped diamond into test materials. By measuring the force necessary to push the probe in to a certain depth and the amount of material displaced, scientists are able to determine a material’s hardness. This technique is particularly useful in assessing the hardness of thin-film materials, which may be only a few nanometers thick. Among their many other applications, thin-films provide the protective coating on the surfaces of our computers’ hard disk drives.
The functional layer of a hard drive is magnetic, allowing for storage of binary (zero or one) bits of information. But magnetic materials tend to be soft and thus are vulnerable to damage when the read-write heads, which typically hover a few nanometers above the rapidly spinning disk, actually make contact with the platter. The effect is like an F15 jet flying low over a forest and occasionally brushing the treetops with the tips of its wings. While the F15 might snap off a few limbs, the read-write head can actually dig into the magnetic layer and damage the disk.
The thicker the protective covering, the greater the distance between the read-write head and magnetic storage layer and the bigger the magnetic information has to be. In the early 1990s, a relatively thick layer (100 nanometers) of amorphous carbon provided the protective coating. Its thickness required larger disks, with storage capacities measured only in kilobytes (KB) or megabytes (MB).
Thanks in part to the research of JIAM research affiliates, in today’s personal computers, a protective film of diamond like carbon a mere 2- to 3-nanometers thick, coats the functional magnetic layer, allowing for smaller hard disks capable of data storage measured not in KB, but in gigabytes (1 million KB) or terabytes (1 billion KB). The diamond-tipped nanoindeter played a fundamental role in testing candidate coating materials for these modern hard disk drives.
The future for nuclear power in the United States began to dim with the accidents at Three Mile Island (1979) and Chernobyl (1986) and with continuing issues surrounding disposal or storage of spent nuclear fuel. But the current focus on reducing emissions of carbon dioxide and other greenhouse gases—which are linked to climate change—has triggered renewed interest in and support for development of nuclear power, which approaches being carbon neutral.
While fossil-fuel power plants subject containment materials to high heat, nuclear plants add to the strain on these materials by emitting heat plus high levels of radiation. Both forces can degrade the strength of first-wall materials intended to prevent the escape of heat and radiation.
Stainless steel, which is inexpensive to produce and whose properties are well known, has been the material of choice in containment structures, both for fossil-fuel and nuclear plants. But while stainless steel can withstand high heat, it can begin to degrade when bombarded by both heat and radiation. This degradation can trigger the forced shutdown of a nuclear reactor.
Faced with this challenge, researchers at UT and ORNL are exploring the creation of specialty steels that can withstand heat and radiation and thus provide superior first-wall containment for nuclear plants. One promising line of research involves embedding ceramic nano particles into steel. This composite material demonstrates tremendous strength and resilience against heat and radiation, but processing the material is prohibitively expensive.
JIAM research affiliates currently are investigating less-expensive ways to process the specialty steel, even as they search for analogous materials that may provide the same function but at a greatly reduced cost.
Composites: A Growth Area
Development of specialty steels and other advanced structural composites could represent a significant growth area for JIAM. Composites—whose combined properties surpass those of its individual components—are ubiquitous in today’s world, forming the structural elements of air and spacecraft, sports equipment, electronic devices, automobiles, bridges, and homes and commercial buildings.
Currently, JIAM research affiliates are testing a rubber-cement composite that can be used to create energy-efficient buildings capable of withstanding the tremors of an earthquake. Other researchers are testing carbon fiber reinforced polymer composites for possible use in shipbuilding. The composite material is one-fifth the weight of aluminum but equally as strong. The material is also more buoyant than aluminum. These features would significantly reduce a ship’s energy consumption. This composite material also possesses other desirable properties: it resists corrosion and can deflect enemy radar.
The challenge with many composites lies in forcing unlike—or nonequilibrium—component materials to combine. The specialty steel for use in nuclear containment structures illustrates the point. Embedding the ceramic particles into the steel requires enormous effort, which is why the composite material is so expensive to process.
A new joint ORNL-UT facility on the UT campus may offer a solution.
The new joint UT-ORNL ion accelerator and implantation system, which is housed in Senter Hall on the UT campus, will provide materials scientists with a powerful new tool. The facility will allow scientists to strip away electrons from any atomic species—for instance, helium, nitrogen, or carbon—and create positively charged ions. The ions are then accelerated to high velocity and shot at a target material.
In the case of the specialized steel for use in nuclear containment, this process can accomplish two key goals. First, the accelerator’s end stations will allow scientists to visualize and characterize the effects of bombardment of ionizing radiation on a test material. Second, it will allow researchers to ion-implant particles into the surface of the steel, making it more resistant to heat and radiation. Ion implantation could also contribute to improvements in many other products and devices, including artificial joints. Embedding carbon particles into the surface of the titanium ball will make it more resistant to wear against the ceramic socket.
The rubber-cement composite for use in buildings also illustrates the challenges inherent in combining nonequilibrium materials. The cement particles readily combine with water, but the rubber particles do not. To blend these materials, JIAM researchers add a molecular coating to the rubber, which inclines it to combine with water. The result is a strong chemical bond between unlike materials.
Energy in some form cuts across most—if not all—of JIAM’s advanced structural materials research. Durable containment walls will make nuclear power production safer and more reliable. Composite materials will create lighter-weight and more energy-efficient aircraft, automobiles, and ships. Smaller computer components and other electronic devices draw less power.