In the search for ways to harvest small amounts of energy to run mobile electronic devices or sensors, researchers typically add hard ceramic nanoparticles or nanowires to a soft, flexible polymer support. The polymer provides the flexibility, while the piezo nanoparticles convert the mechanical energy into electrical voltage.
However, these materials are relatively inefficient, because upon mechanical loading, the mechanical energy is largely absorbed by the bulk of the polymer, with a very small fraction transferred to the piezo nanoparticles. While adding more ceramic would increase the energy efficiency, it comes with the trade-off of less flexibility.
"The hard ceramics in the soft polymer is like stones in water," said Professor Qing Wang of Penn State. "You can slap the surface of the water, but little force is transferred to the stones. We call that strain-transfer capability."
Almost three decades ago, Penn State materials scientist Bob Newnham came up with the concept that the connectivity of the piezo filler determined the efficiency of the piezoelectric effect. A 3D material would be more efficient than what he classified as ‘zero-dimensional nanoparticles’, ‘one-dimensional nanowires’ or ‘2D films’, because the mechanical energy would be transported directly through the 3D material instead of dissipating into the polymer matrix. However, achieving a 3D structure with a well-defined microstructure remained a mystery.
Now, the Penn State team believe they have solved that with a polyurethane foam dusting sheet.
According to the team, the small uniform protrusions on the sheet act as a template for forming the microstructure of the piezoelectric ceramic. The researchers applied the ceramic to the polyurethane sheet in the form of suspended nanoparticles in solution. When the template and solution are heated to a high enough temperature, the sheet burns out and the solution crystalises into a solid 3D microform foam with uniform holes. The researchers then fill the holes in the ceramic foam with polymer.
"We see that this 3D composite has a much higher energy output under different modes," said Prof. Wang. "We can stretch it, bend it, press it. And at the same time, it can be used as a pyroelectric energy harvester if there is a temperature gradient of at least a few degrees."
"We were able to show theoretically that the piezoelectric performance of nanoparticle/nanowire composites is critically limited by the large disparity in stiffness of the polymer matrix and piezoceramics, but the 3D composite foam is not limited by stiffness," added Professor Sulin Zhang of Penn State. "This is the fundamental difference between these composite materials.”
Currently, Prof. Wang and his collaborators are working with lead-free and more environmentally friendly alternatives to the current lead-zirconium-titanate ceramic.