Materials scientist Xavier Lepró can’t grow his “spider-webs” fast enough to swing from skyscrapers like Spider-man, but he can best another web-maker, Mother Nature, when it comes to consistency.
Lepró has helped pioneer the spinning of spider-silk-like yarns for use in suspending target capsules inside NIF hohlraums. These yarns are even stronger than the silks spun by the technology’s inspiration, real-life spiders. The ultrathin fibers are being developed by a team led by Michael Stadermann, group leader for the LLNL Target Fabrication group’s science and technology arm.
The team spins carbon nanotube yarns with diameters on the scale of a few microns, or millionths of a meter. Like spider silk, the NIF yarns are stronger than steel and so gossamer thin as to be almost invisible. Their goal is to field the fibers as a capsule-support alternative to the “tents” traditionally used to hold the peppercorn-size capsules inside NIF hohlraums (see “How NIF Targets Work”).
“Spider silk properties change with the mood of the spider.”
Stadermann’s group sought a material that would provide both low contact and staunch support for the capsules. The current supports are nanometer-thick membranes that partially wrap around the capsule; experiments revealed that these “tents” induce perturbations at the point of contact that can interfere with NIF implosions (see “Studying Effects of Target ‘Tents’ on NIF Implosions”).
The team wanted an alternative that would minimize the perturbations by reducing the contact surface between the support and capsule but still hold the capsule firmly in place at four points, a technical challenge. They required a material that was strong and thin like wire and capable of supporting 100,000 times its weight without breaking or yielding during assembly and handling. When seeking a material as strong as steel yet light as a feather, the connection to spider silk was obvious.
NIF was one of a number of scientific organizations that tried running a spider “farm” to harvest actual spider silk for experiments. In NIF’s case, the territorial arachnids proved hard to sustain, and there were also quality-control issues.
As Lepró explained, “We need a material we can rely on. That is not spider silk, because spider silk properties change with the mood of the spider.”
A Carbon Nanotube ‘Forest’
When Lepró came to the Lab in 2016 as a post-doctoral fellow from the University of Texas at Dallas with experience in nanotechnology, Stadermann decided to harness his ability working with nanometric and micrometric threads to try synthesizing an alternative fiber for use in NIF targets.
Lepró, along with Chantel Aracne-Ruddle and Alicia Calonico-Soto, a NIF & Photon Science Directorate Summer Scholar, begins with what they dubbed a “carbon nanotube forest.” These are arrays of tubes approximately 12 nanometers (0.00000047 inches) in diameter, Lepró said. “They grow in such a way that the carbon nanotubes arrange themselves perpendicular to the substrate, like a bamboo ‘forest’ on a field, where each bamboo will be equivalent to a nanotube.”
By pulling from the edge of the nanotube “forest,” the team assembles bundles of hundreds of individual nanotubes into threads. They then weave millions of the nanotube threads into a yarn, in much the same way that weavers have spun wool to make yarn throughout the centuries. “This is ultra-high tech meeting ultra-low tech,” Stadermann said.
The product is a round “knit” tube composed of millions of carbon nanotubes twisted together. Each nanotube is made solely of carbon atoms and is stronger than steel when normalized by weight. The yarns are flexible, like spider silk, with the added advantage of being thermally and electrically conductive. Stadermann says the carbon nanotube yarn is among the thinnest fibers his team has ever made. The diameter of carbon nanotube yarn is similar to the diameter of a typical spider-silk thread, Lepró confirmed.
The first test of the impact of the nanotube yarn was performed this month in a hydrogrowth radiography experiment on NIF. The test had encouraging results that will be fully analyzed in the coming months.
Two NIF & Photon Science technologies—the PEEL process for fabricating ultrathin polymer films and the neodymium-doped fiber amplifier and laser—have received regional technology transfer awards for outstanding technology development from the Federal Laboratory Consortium (FLC).
Started in 1974, the FLC assists the U.S. public and private sectors in utilizing technologies developed by federal government research laboratories. It comprises more than 300 federal labs and research centers.
Polyelectrolyte Enabled Liftoff
The Polyelectrolyte Enabled Liftoff (PEEL) technology is a robust, scalable method of fabricating freestanding polymer films that are larger, stronger and thinner than conventional methods can produce. PEEL, which won a 2016 R&D 100 award, is used at NIF for the fabrication of membranes as thin as 30 nanometers (billionths of a meter) that serve as supports for laser targets.
With the new PEEL process, very thin films can be directly delaminated from their deposition substrates over very large areas. PEEL eliminates the need for often-used sacrificial interlayers which can negatively impact the properties of the final freestanding film and are difficult to scale in quantity and size.
PEEL allows, for the first time, the remarkable properties of polymers such as polyvinyl fluoride (PVF) to be exploited in vanishingly thin films. Because the process is easily scalable in size and manufacturing quantity, it eventually could be applied to sensing, catalysis, filtration and wound-healing applications. The work was done in collaboration with San Diego-based General Atomics.
LLNL team members who worked on PEEL are chemical engineer Salmaan Baxamusa; materials scientist Tayyab Suratwala; physicist Art Nelson; chemists Michael Stadermann, Philip Miller and Chantel Aracne-Ruddle; and three other researchers who have left the Lab: summer interns Maverick Chea and Shuaili Li and postdoc William Floyd III. Two General Atomics employees, chemical technician Anatolios Tambazidis and chemist Kelly Youngblood, also aided in the development of PEEL.
Annemarie Meike, a business development executive in the Lab’s Innovation and Partnerships Office (IPO), has overseen the tech transfer efforts for the PEEL technology. “The PEEL technology is a great example of an innovation that was developed for a narrow application, the fabrication of NIF targets, that may enable a broad range of technologies of the future,” Meike said.
Neodymium-doped Fiber Amplifier and Laser
The neodymium-doped fiber amplifier and laser is a new type of optical fiber amplifier that could potentially double the information-carrying capacity of fiber-optic cables—an important step forward in addressing the telecommunications industry’s need for a bigger and faster bandwidth for Internet users.
Most of the data for the Internet travel on fiber-optic cables, which are made up of bundles of threads that transmit laser light. As the fiber gets longer, however, power is lost due to attenuation. A Livermore team developed a new fiber that generates laser power and optical gain with relatively good efficiency. This discovery opens up the potential for installed optical fibers to operate in a transmission region known as E-band, in addition to the C and L bands where they currently operate—effectively doubling a single optical fiber’s information-carrying potential.
LLNL team members who developed the new fiber amplifier are physicists Paul Pax, Mike Messerly, Victor Khitrov, Leily Kiani, Reggie Drachenberg, Graham Allen, Diana Chen, Jay Dawson and Chris Ebbers (now retired), optical technician Parker Crist, and laser technicians Nick Schenkel and Matt Cook.
David Dawes, a business development executive in the Lab’s Innovation and Partnerships Office, directs the tech transfer work for the new fiber amplifier.
“The neodymium-doped fiber amplifier and laser technology is one of those true breakthrough developments that have the potential for global impact by opening up new Internet telecommunications bandwidth options that don’t require laying more cable,” Dawes said. “It’s another example of the out-of-the-box innovative solutions generated by Jay Dawson (NIF&PS deputy program director for Department of Defense Technologies) and his prolific fiber development team. They are all a true pleasure and an honor to work with.”
The awards were presented on Aug. 30 during the FLC’s three-day Far West/Mid-Continent regional meeting at the Sheraton Hotel in Pasadena, California.