Engineered Polymers with Tailored Mechanical and Thermal Properties

Polymers combine with designed nanocomposites for energy storage and harvesting

A material with the ability to store energy like a ceramic capacitor, with the flexibility, low cost, and low weight of plastic would make be an appealing combination for future electric vehicles. Zoubeida Ounaies, associate professor in Penn State’s Department of Mechanical and Nuclear Engineering, is creating such a material by working at the intersection of smart materials - materials that couple between different domains – and nanocomposites – polymeric materials modified by nanoscale fillers that in some way change the properties of the bulk material.

Her research focuses on four applications: sensing, actuation, energy harvesting, and energy storage. Ounaies doctoral studies at Penn State in the nineties gave her a strong background in ceramic ferroelectrics. Her postdoctoral work at NASA Langley Research Center involved making the polymers that NASA was using in space piezoelectric – a property where materials respond to an electric field by changing their shape or generate an electric charge under mechanical stress.

“We understood what it takes to make a material piezoelectric, and we designed it in the polymer,” Ounaies recalls. “That effort was successful in our niche application, but it didn’t really rival the performance of ferroelectric ceramics. With the advent of nanotechnology, we have the opportunity to create and enhance dipoles in the polymers, and essentially design a material which acts like a piezoelectric.”

For energy harvesting, her group is working to develop active nanocomposite fibers that could be woven into clothing to collect the mechanical energy of bending or walking and transform that mechanical strain into enough electricity to power mobile devices. Other uses for her active polymer nanocomposites are in the fields of advanced electronics, autonomous systems, aerospace, automotive, medical and consumer products industries.

When it comes to energy storage, Ounaies and her group are interested in developing flexible light weight capacitors. As previously mentioned, the standard electronic polymers for polymer-based capacitors are polypropylene or polyethylene. Although these polymers have a relatively high breakdown strength, they also exhibit a low dielectric permittivity, so their ability to store charge, as measured by the volumetric energy density, is low. For the past decade, the composites community hoped that nanocomposites would fill the gap between ceramic capacitors and polymeric ones. With few exceptions the results have been disappointing, Ounaies says.

“The issue with most nanocomposites is that it is very difficult to disperse the nanofillers in the polymer host, so the presence of aggregates and nanovoids deteriorates the performance of the material. When you mix the ceramic nanofillers in the polymer, the interface becomes chemically different from either the ceramic or the polymer. And that is the weakest link where you get electric stress and then catastrophic breakdown,” she says.

Advances in nanotechnology in recent years made it possible for her group to design the internal interface so that there is more of a gradual transition from one material to the other. Her group has developed methods to manipulate and pattern nanotubes and nanofibers in polymers using electrokinetic interactions with dramatic changes in electrical and stress coupling. For example, by orienting the direction of the nanofillers in such a way as to minimize the local field at the interface, they were able to enhance dielectric breakdown strength, gaining an order of magnitude improvement in energy density. One of their main focus is to control the spatial distribution and alignment of nanofillers, so that they are able to design nanocomposite material systems for prescribed performances, with important implications in energy storage and energy harvesting.

Like many of the Penn State researchers mentioned in this article, Ounaies is a few steps removed from working on the devices that are meant to store the energy of wind and sun or power the electric motor of a hybrid electric vehicle. Instead her group’s focus is on the nano-enhanced materials that will one day enable those energy storage devices to be made at a lower cost and at increased efficiency.