CALL them nanolaminates, or call them multilayers. In either case, they are atomic-scale sandwiches, composites made from dozens of alternating layers of materials, with each layer just 0.2 to 200 nanometers thick. The thickest layer may be only a few thousand atoms across, 1 one-hundredth the width of a human hair.
By carefully combining various elements and layer thicknesses, researchers can fabricate multilayers to almost any specification. These “designer” materials may be extremely strong; highly reflective; unusually ductile; or exceptionally resistant to heat, wear, and corrosion. Or they may incorporate several of those properties at the same time.
Many space-based solar astrophysics mirrors incorporate highly reflective multilayer coatings, allowing researchers to view the Sun’s corona in the x-ray and extreme ultraviolet wavelengths. Another early use of multilayer synthesis technology was to manufacture magnetic hard-disk drives for computers. In this application, various metallic alloys are layered together with a magnetic layer on top for storing data. A more recent nanolaminate development—a layered foil that releases heat energy in a controlled manner—can be used to bond dissimilar materials without damaging them.
Most of these innovations are the brainchild of materials scientist Troy Barbee, who has been leading the Laboratory’s multilayer effort since he arrived in 1985. Many of his successes received funding in the incubation stage from Livermore’s Laboratory Directed Research and Development Program. Now 71, Barbee shows no sign of slowing and remains as creative as ever.
After receiving a Ph.D. in materials science engineering from Stanford University, Barbee was named Laboratory Director of the Stanford Center for Materials Research, where he performed some of the earliest work with atomically engineered multilayers. Barbee notes that multilayer technology proved so successful in magnetic memory hard drives, technologists put it to use before anyone thought to patent it.
Atomic engineering does not rely on thermodynamics and kinetics to create new or advanced materials. Fabricating nanolaminates using atom-by-atom sputter deposition transcends the limits of standard manufacturing methods, resulting in materials with surprising properties, always different from those of the component substances in bulk form. These properties are due to the nanometer-scale environment of the atoms in each layer. Layers range from several atoms to a few thousand atoms thick, and the atoms within a layer are strongly influenced by the interfaces between the component layers.
Finding applications for these unusual materials is one part of the fun for Barbee, who works in Livermore’s Physical and Life Sciences Directorate. “I get great enjoyment from seeing these creations put to work,” he says. “So I have always emphasized working with collaborators and finding uses for our new multilayers.”
Collaborations with the National Aeronautics and Space Administration (NASA) and various branches of the U.S. government put multilayered telescope mirrors and other optical devices into space. Working with Pratt & Whitney, researchers developed refractory oxide nanolaminates to enhance the performance of aircraft turbine blades. That cooperative effort produced the technology now used to fabricate high-energy-density electrical capacitors. Livermore nanolaminates also make possible extreme ultraviolet lithography (EUVL), a new lithographic method for cramming more devices onto a silicon chip.
Companies have worked with the Laboratory to determine whether the reactive nanolaminates can deploy automobile air bags or deliver inhaled medications. Government agencies have discussed using a reactive nanolaminate as an anti-tamper device on a weapon or other high-value item. And the list goes on.
The Shortest Wavelengths
“A combination of tungsten and carbon layers turned out to demonstrate the optical quality needed for x-ray optics,” says Barbee. That finding led to the NASA collaboration to design multilayer coatings for telescope mirrors. Eventually, the coatings enabled the mirrors to focus light from space not only in the x-ray wavelengths but also in the extreme ultraviolet wavelengths. Livermore multilayer coatings are on the mirrors of NASA’s Transition Region and Coronal Explorer (TRACE) satellite, which orbits Earth, pointing constantly at the Sun. Launched in 1998, TRACE records images and other data about the Sun’s fluctuating magnetic fields, which cause sunspots, the corona, and other plasma structures. TRACE has vastly increased knowledge of the physics of sunspots and the Sun’s corona.
A seemingly unrelated payoff to the successful use of multilayers in solar astrophysics was for computer chip manufacturing. Multilayer mirror coatings of molybdenum and silicon are a key technology for EUVL, a manufacturing technique that uses extremely short wavelengths of light to “write” on computer chips. EUVL will increase computer performance by shrinking the features printed on chips, thus allowing manufacturers to develop even smaller chips. Livermore is collaborating with other national laboratories and industrial partners to develop EUVL for commercial applications. (See S&TR, September/October 2008, Smoothing Out Defects for Extreme Ultraviolet Lithography.)
Bulk metallic glasses such as window glass are brittle at room temperature. Likewise, most nanoscale microstructural materials easily crack under stress. Since the discovery of the highly ductile nanolaminate, various team members have explored the mechanical properties of this unusual material.
Using transmission electron microscopy (TEM) and subjecting a sample to various tensile strains, scientists can “watch” the microstructures change and see where dislocations and voids between grains of materials appear in the Cu–CuZr. Today, materials scientist Morris Wang, also of the Physical and Life Sciences Directorate, is collaborating with Barbee to determine why Cu–CuZr is so ductile. This past summer, Nanotech Briefs honored Wang with a 2008 Nano 50 Award for his work, part of which is with nanolaminates. The annual competition, held for the first time in 2005, recognizes the top 50 technologies, products, and innovators that have significantly advanced nanotechnology.
In the Cu–CuZr research, Wang tested amorphous layers ranging from 3 to 15 nanometers thick, with crystalline layers from 5 to 100 nanometers thick. Wang and Barbee’s results, combined with molecular dynamics simulations by Ju Li of Ohio State University, indicate that the interfaces between the many layers are the key to the material’s unusual ductility. Dislocations start at one interface, travel across a crystalline copper layer, and are absorbed at the next interface. TEM experiments also revealed that the tensile elongation occurs earlier in the amorphous CuZr at lower stress levels than in crystalline–crystalline multilayers. The interfaces are again critical for absorbing the dislocations, with all amorphous layers remaining intact during stress and strain tests.
A New Way to Join
Barbee and his postdoctoral researcher Timothy Weihs addressed the question of how or why energetic nanolaminates have such unique properties. As a result of their broad-ranging, systematic experiments, they developed a substantial advance in understanding the relationship between nanolayering and energetic response. Their research showed that materials in nanometer form, layered or not, would have this energetic property. Inherent in this understanding is the ability to relate the amount of stored potential energy, the scale of the material, and the rate of reaction and energy release in a quantitative, though parametric manner.
Weihs has since joined the faculty of Johns Hopkins University. In 2001, he and another Johns Hopkins faculty member founded Reactive NanoTechnologies, Inc., (RNT) in Hunts Valley, Maryland. Barbee and RNT won an R&D 100 Award in 2005 for NanoFoil®, the patented reactive nanolaminate developed in the Livermore–RNT collaboration. The same year, RNT received a Nano 50 Award for the material.
NanoFoil can be used to bond metals, ceramics, semiconductors, and polymers. It can replace lead-based soldering, which causes collateral damage to parts and is potentially toxic, and epoxies, which tend to degrade over time. With NanoFoil, a heat sink can easily be attached directly to a computer chip to conduct heat away from the chip.
Today, RNT is bonding sputtering targets for companies that manufacture integrated circuitry, data storage devices, photovoltaic cells, and flat-panel displays. Says Weihs, “Since we make the bonding foil by sputtering, it’s an interesting twist.”
In a detonator project, a Livermore team proposed using nanolaminate materials for the U.S. Strategic Environmental Research and Development Program, which is managed jointly by the Departments of Energy and Defense and the Environmental Protection Agency. “The project was part of the government’s goal to ‘green’ the arsenal,” says Laboratory chemist Alex Gash. In addition to Barbee and Gash, the Livermore team included solgel expert Joe Satcher from the Physical and Life Sciences Directorate and Randy Simpson, who along with Gash works in the Laboratory’s Energetic Materials Center. The center conducts research on the performance of high explosives not only for the nuclear weapons program but also for advanced conventional weapons, rocket and gun propellants, homeland security, demilitarization, and industrial uses of energetic materials.
The team’s objective was to develop an environmentally safe stab detonator for medium-caliber (20- to 60-millimeter) munitions. Stab detonators, which are activated by a mechanical stimulus, ignite to detonate the main charge. The primer mix in conventional stab detonators contains two forms of lead and other highly toxic substances that pose a danger during manufacture and after detonation. In addition, some primer constituents are no longer made, so substitutes are needed.
The Livermore team developed an energetic nickel–aluminum nanolaminate that serves as the mechanically sensitive igniter for the energetic explosives in conventional munitions. Sensitivity can be enhanced with an energetic nanolaminate, which is environmentally safe yet fully functional as an igniter.
“By varying the layer thickness, we can modify the ignition sensitivity and the reaction speed,” says Gash. A slower reaction could be used in a delay mechanism, similar to the device that keeps a parachute closed until a pilot is ejected from a plane. Specialized reactive nanolaminates with a delay mechanism may someday be used in commercial technologies, but at this time, the materials are too expensive for widespread application.
“Most energetic nanocomposites are powders whose combustion is difficult to control,” says Gash, who also received a 2008 Nano 50 Award. In addition, most energetic nanocomposites do not age well because they are made of materials that readily oxidize in air. “With nanolaminates, the layers are self-contained, so they aren’t as exposed to oxygen in the air,” says Gash. “These materials also allow us to control the total energy output as well as the ignition conditions.” Controlling the reaction is easily achieved by manufacturing the multilayer with specific layer thicknesses.
Barbee and others have explored many combinations of common elements for various weapons-related projects. For example, hafnium or zirconium layered with carbon may produce reaction temperatures well above 3,000 kelvins, perhaps even reaching 4,100 kelvins. “For all we know about nanolaminates, there is still a lot we don’t understand,” says Barbee. “Our experience in nanolaminate research has taught us to expect the unexpected.”
The prototype capacitor is 4 centimeters square and 1 millimeter thick and packs 50 joules per cubic centimeter of dielectric energy density. The goal is to develop a device no more than 1 centimeter on a side and half as thick as the prototype. These miniature capacitors could be used in power electronics control circuitry, automotive control systems, telecommunications, computers, radar systems, and other pulsed radio-frequency applications.
“Thirty years ago, the first nanolaminate pair I fabricated, niobium–copper, was designed for studying physical properties,” says Barbee. “Today, that same material is being tested at Los Alamos [National Laboratory] as a radiation-damage-resistant material for use in future nuclear and fusion energy systems.”
Who knows where a Livermore nanolaminate might turn up next. “Troy is amazing,” says Weihs. “He just keeps going and going with lots of great ideas. In fact, I’d say he has more ideas than he can handle.”
Key Words: energetic materials, extreme ultraviolet lithography (EUVL), magnetron sputter deposition, multilayers, nanolaminates, reactive materials.
For further information contact Troy Barbee (925) 423-7796.
Lawrence Livermore National Laboratory
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