Silk nanosensor could speed development of new infrastructure, aerospace and consumer materials (Middle) Mechanophore-labeled silk fiber fluoresces in response to damage or stress. (Right) Control sample without the mechanophore. (Image: Chelsea Davis and Jeremiah Woodcock/NIST)
Posted: Mar 17, 2017
Consumers want fuel-efficient vehicles and high-performance sporting goods, municipalities want weather-resistant bridges, and manufacturers want more efficient ways to make reliable cars and aircraft.
What’s needed are new lightweight, energy-saving composites that won’t crack or break even after prolonged exposure to environmental or structural stress.
To help make that possible, researchers working at the National Institute of Standards and Technology (NIST) have developed a way to embed a nanoscale damage-sensing probe into a lightweight composite made of epoxy and silk.
The probe, known as a mechanophore, could speed up product testing and potentially reduce the amount of time and materials needed for the development of many kinds of new composites.
The NIST team created their probe from a dye known as rhodamine spirolactam (RS), which changes from a dark state to a light state in reaction to an applied force. In this experiment, the molecule was attached to silk fibers contained inside an epoxy-based composite.
As more and more force was applied to the composite, the stress and strain activated the RS, causing it to fluoresce when excited with a laser. Although the change was not visible to the naked eye, a red laser and a microscope built and designed by NIST were used to take photos inside the composite, showing even the most minute breaks and fissures to its interior, and revealing points where the fiber had fractured.
The results were published today in the journal Advanced Materials Interfaces (“Observation of Interfacial Damage in a Silk-Epoxy Composite, Using a Simple Mechanoresponsive Fluorescent Probe”).
The materials used in the design of composites are diverse. In nature, composites such as crab shell or elephant tusk (bone) are made of proteins and polysaccharides. In this study, epoxy was combined with silk filaments prepared by Professor Fritz Vollrath’s group at Oxford University using Bombyx mori silk worms. Fiber-reinforcedpolymer composites such as the one used in this study combine the most beneficial aspects of the main components–the strength of the fiber and the toughness of the polymer.
What all composites have in common, though, is the presence of an interface where the components meet. The resilience of that interface is critical to a composite’s ability to withstand damage. Interfaces that are thin but flexible are often favored by designers and manufacturers, but it is very challenging to measure the interfacial properties in a composite.
“There have long been ways to measure the macroscopic properties of composites,” said researcher Jeffrey Gilman, who led the team doing the work at NIST. “But for decades the challenge has been to determine what was happening inside, at the interface.”
One option is optical imaging. However, conventional methods for optical imaging are only able to record images at scales as small as 200-400 nanometers. Some interfaces are only 10 to 100 nanometers in thickness, making such techniques somewhat ineffective for imaging the interphase in composites.
By installing the RS probe at the interface, the researchers were able to “see” damage exclusively at the interface using optical microscopy.
The NIST research team is planning to expand their research to explore how such probes could be used in other kinds of composites as well. They also would like to use such sensors to enhance the capability of these composites to withstand extreme cold and heat.
There’s a tremendous demand for composites that can withstand prolonged exposure to water, too, especially for use in building more resilient infrastructure components such as bridges and giant blades for wind turbines.
The research team plans to continue searching for more ways that damage sensors such as the one in this study could be used to improve standards for existing composites and create new standards for the composites of the future, ensuring that those materials are safe, strong and reliable.
“We now have a damage sensor to help optimize the composite for different applications,” Gilman said. “If you attempt a design change, you can figure out if the change you made improved the interface of a composite, or weakened it.”
Source: National Institute of Standards and Technology (NIST)
Rice University scientists replace metal with carbon nanotubes for aerospace use
HOUSTON – (Jan. 27, 2016) – Common coaxial cables could be made 50 percent lighter with a new nanotube-based outer conductor developed by Rice University scientists.
The Rice lab of Professor Matteo Pasquali has developed a coating that could replace the tin-coated copper braid that transmits the signal and shields the cable from electromagnetic interference. The metal braid is the heaviest component in modern coaxial data cables.
The research appears this month in the American Chemical Society journal ACS Applied Materials and Interfaces.
Replacing the outer conductor with Rice’s flexible, high-performance coating would benefit airplanes and spacecraft, in which the weight and strength of data-carrying cables are significant factors in performance.
Rice research scientist Francesca Mirri, lead author of the paper, made three versions of the new cable by varying the carbon-nanotube thickness of the coating. She found that the thickest, about 90 microns – approximately the width of the average human hair – met military-grade standards for shielding and was also the most robust; it handled 10,000 bending cycles with no detrimental effect on the cable performance.
“Current coaxial cables have to use a thick metal braid to meet the mechanical requirements and appropriate conductance,” Mirri said. “Our cable meets military standards, but we’re able to supply the strength and flexibility without the bulk.”
Coaxial cables consist of four elements: a conductive copper core, an electrically insulating polymer sheath, an outer conductor and a polymer jacket. The Rice lab replaced only the outer conductor by coating sheathed cores with a solution of carbon nanotubes in chlorosulfonic acid. Compared with earlier attempts to use carbon nanotubes in cables, this method yields a more uniform conductor and has higher throughput, Pasquali said. “This is one of the few cases where you can have your cake and eat it, too,” he said. “We obtained better processing and improved performance.”
Replacing the braided metal conductor with the nanotube coating eliminated 97 percent of the component’s mass, Mirri said.
She said the lab is working on a method to scale up production. The lab is drawing on its experience in producing high-performance nanotube-based fibers.
“It’s a very similar process,” Mirri said. “We just need to substitute the exit of the fiber extrusion setup with a wire-coating die. These are high-throughput processes currently used in the polymer industry to make a lot of commercial products. The Air Force seems very interested in this technology, and we are currently working on a Small Business Innovation Research project with the Air Force Research Laboratory to see how far we can take it.”
Co-authors are graduate students Robert Headrick and Amram Bengio and alumni April Choi and Yimin Luo, all of Rice; Nathan Orloff, Aaron Forster, Angela Hight Walker, Paul Butler and Kalman Migler of the National Institute of Standards and Technology (NIST); Rana Ashkar of NIST, the University of Maryland and Oak Ridge National Laboratory; and Christian Long of NIST and the University of Maryland.
Pasquali is the A.J. Hartsook Professor of Chemical and Biomolecular Engineering, chair of the Department of Chemistry and a professor of materials science and nanoengineering and of chemistry.
The research was supported by the Air Force Office of Scientific Research, the Air Force Research Laboratories, the Robert A. Welch Foundation, NIST, the National Science Foundation and a NASA Space Technology Research Fellowship.
Read the abstract at http://pubs.acs.org/doi/abs/10.1021/acsami.5b11600
Video: Spinning nanotube fibers at Rice University: https://youtu.be/4XDJC64tDR0
|Source: Investigación y Desarrollo|