Tough Little Fibers

Project: Continuous nanofibers
School: University of Nebraska
Location: Lincoln, Neb.
Director: Yuris Dzenis

University-of-Nebraska-nanofibers-web

This high-resolution scanning electron microscopy image shows ultra-tough and strong continuous nanofibers developed by University of Nebraska – Lincoln engineers that can be easily aligned and bundled for handling and processing into various applications.

Yuris Dzenis, professor of mechanical and materials engineering at the University of Nebraska – Lincoln (UNL), and his group in the Advanced Nanomaterials and Nanomanufacturing Laboratory may have found the key that will soon enable nanofibers to be used in many more applications. Their work is part of a large research effort in advanced nanostructured fibers supported by the United States Army, Air Force Office of Scientific Research, National Science Foundation and research groups at several other universities. Dzenis’ group is investigating continuous nanofibers formed by the electrospinning process – a method of obtaining composite fibers from solutions as a liquid jet accelerates through an electric field.

“In testing hundreds of individual nanofibers with different diameters, we found a very unusual size effect,” says Dzenis. “Not only do strength and modulus increase dramatically when the diameter decreases (as expected), but also the toughness increases.” says Dzenis. Typically when a material’s strength increases, it becomes brittle.

UNL is studying several nanofibers with diameters ranging from 1/10 to 1/1,000 of the diameters of conventional fibers such as carbon and Kevlar. While electrospinning can produce continuous nanofilaments in the single nanometer to micron diameter range, the group found the largest increases in strength, ductility and toughness in ultrafine fibers with diameters below 250 nanometers.

These ultrafine fibers are also different from other nanomaterials because their continuous fibers are endless. “Most nanomaterials, including popular carbon nanotubes and graphene, are discontinuous particles,” says Dzenis. “While researchers projected a big future for nanotubes in structural materials 15 to 20 years ago, much of that promise is yet to be realized because they are difficult to align and process into composites or fibers. In contrast, it should be possible to process continuous nanofibers into composites using techniques similar to the ones used in industry, such as textile processing.”

The group at UNL demonstrated and published results producing fibers from polyacrylonitrile (PAN). Dzenis says that even this medium grade polymer – which wasn’t necessarily expected to be high performing – can yield high performance mechanical properties in fine continuous fiber format. His group is currently working on several other systems. Dzenis says all three major classes of reinforcing fibers – polymers, carbons and ceramics – can be processed by electrospinning into nanofibers with diameters much smaller than those of conventional microfibers.

Electrospinning has been around for decades, but there was little interest in using it to create nanofibers for structural applications. “They were considered too small for practical application and industry concentrated on traditional fibers that they knew how to produce more economically,” says Dzenis. In the last 10 to 15 years, companies have begun working on applications ranging from sensors to biomedical. There wasn’t as much interest in the composites industry, says Dzenis, because high mechanical properties hadn’t been demonstrated.

Now Dzenis’ group is evaluating all fundamental and applied aspects of the electrospinning process. Through systematic experimentation and modeling, they’re aiming to improve the process itself and the alignment and mechanical properties of the fibers. “With these further improvements, we may be able to generate a new generation of reinforcement fiber components that are both strong and tough,” says Dzenis. That would be useful in safety critical applications in aircraft or aerospace structures, bridges and other civil infrastructures.

“Normally, all structures have a trade-off,” says Dzenis. “Most high-performance fibers have high strength and modulus, but not high toughness. If we can get both, we will enter a new structural generation.”