Piezoelectric materials offer the most direct way of converting mechanical energy into an electrical potential or vice versa. Applications that utilize these effects are far reaching, ranging from loud speakers and acoustic imaging to energy harvesting and electrical actuators. However, piezoelectrics are intrinsically low power density materials and it has been difficult to boost the performance of these materials. It has been shown that reducing the dimensionality of the material can slightly enhance the piezoelectric coefficient, but major breakthroughs in the efficiency of these materials are required if they are to compete with higher power output and more sensitive electronic/photonic devices. We are therefore interested in understanding if the efficiency of piezoelectric materials is fundamentally limited or engineering limited.
To help address this we have been investigating various ways to make new composite piezoelectric materials and interface the piezoelectric to different materials in order to enhance the energy transfer process or capture energy from non-mechanical energy sources (e.g., light, chemical, heat, etc.). For example, we have developed organic/inorganic hybrid energy-harvesting platforms by embedding piezoelectric nanowire arrays in an environment-responsive polymer matrix. Energy sources such as heat can then be used to swell the polymer which places a tensile load on the nanowires and produces a dc electric output from the nanowire array. Using this configuration we can achieve power densities of 20 nw/cm2 at a temperature of only 65°C. These figures can be boosted by over 2-fold by simply altering the nanowire/polymer interface via non-slip adhesion promoters.
(top left) Schematic of a hybrid piezoelectric nanowire-polymer energy nanoconverter. (bottom left) Image of ZnO nanowires embedded in a polymer matrix. (right) Electric current and power output of a nanowire-polymer energy nanoconverter under thermal (~ 65°C) stimulation.
In addition to leveraging nanowire structures, we have been investigating piezoelectric polymer composites that interface piezoelectric nanoparticles with a polymer matrix. Due to the processability and biocompatibility of the polymer systems, there is a tremendous interest in fabricating high-efficiency piezoelectric polymers. Furthermore, these materials are much more amenable to 3D structuring which is extremely difficult to do with conventional electroceramics such as lead zirconate titanate (PbZrxTi1-xO3, PZT) or barium titanate (BaTiO3, BTO) and should have a significant impact on the development of biodiagnostics, imaging technologies, and nano/microelectromechanical systems. To boost the performance of piezoelectric polymers and provide a means of creating 2D and 3D shapes, we have been developing rapid printing strategies which allow the user to define the size, shape, and composition of the piezoelectric material. For example, we have utilized digital projection printing (DPP) to fabricate piezoelectric structures in mere seconds by combining BTO nanoparticles with a photoliable polymer solution. By grafting photosensitive chemical groups on the BTO nanoparticles, the piezoelectric crystals crosslink with the polymer chains under light exposure and enhance the mechanical-to-electrical energy conversion process by efficiently funneling the stress in the polymer chains to the piezoelectric nanoparticles. Ongoing work includes optimizing the printing process, understanding the chemical interface between the piezoelectric nanoparticles and the polymer matrix, and incorporating these materials into novel sensor and imaging platforms.
Related references
1. X.Y. Wang, K. Kim, Y.M. Wang, M. Stadermann, A. Noy, A.V. Hamza, J.H. Yang, and D.J. Sirbuly "Matrix-assisted energy conversion in nanostructured piezoelectric arrays" Nano Lett.10, 4901-4907 (2010).
2. K. Kim and D.J. Sirbuly, "Enhanced output of nanostructured piezoelectric arrays via controlled matrix/transducer interfacial interactions" Appl. Phys. Lett. 101, 213114 (2012).
3. K. Kim, W. Zhu, X. Qu, C. Aaronson, W.R. McCall, S. Chen, and D.J. Sirbuly, "3D Optical Printing of Piezoelectric Nanoparticle-Polymer Composite Materials" ACS Nano, 8, 9799-9806 (2014).
4. W.R. McCall, K. Kim, C. Heath, G. La Pierre, and D.J. Sirbuly, "Piezoelectric nanoparticle-polymer composite foams" ACS Appl. Mater. Interfaces, DOI: 10.1021/am506415y (2014).