smart materials for artificial muscle and energy harvesting

Summary

Elastomers, in everyday language rubber, are soft materials useful in actuators and devices for the conversion of renewable mechanical energy. In these applications we use capacitors formed by the elastomer and conformable electrodes. An electric voltage is applied to the elastomer capacitor, which is then deformed by electrostatic forces. Currently one major obstacle in employing these actuators is the high voltage needed for driving the circuit, whereas withstanding high voltages is advantageous for dielectric elastomer generators. Material synthesis must account for these different requirements. Acrylic elastomers, natural and synthetic rubber and polydimethylsiloxane (PDMS) are potential candidates for the next generation of dielectric elastomer actuators and generators. In addition, dielectric elastomers may be promising materials for highly conformable piezoelectrics. In this project we first investigated the potential of charge controlled actuators, which should avoid the pull-in instability of voltage driven actuators. The pull-in instability causes an uncontrolled thickness reduction of the actuator, finally leading to catastrophic failure by electrical breakdown. Our investigations have shown that charge controlled actuation leads to a new instability, similar to the necking instability in mechanical testing. We also investigated temporal effects in minimum energy elastomer actuators, co-developed at the Johannes Kepler University, in order to derive guidelines for long term stable dielectric elastomer actuators. In the field of materials development, we showed the potential of natural rubber for dielectric elastomer generators. In the last year of the project and in concurrent work this year, we revealed, in close co-operation with the EMPA group, first piezoelectric signals in PDMS elastomers with strongly polar molecules. These results may pave a way towards highly conformable piezoelectric materials, that can adjust to arbitrary three-d-forms

Report

Within this project, 3 publications appeared in peer-reviewed journals. The total number of citations for these three papers is 32 on Google Scholar (checked on May 11, 2016). The results of this project also contributed to a review for the 25 years anniversary of Advanced Materials, listed with 102 citations on Google Scholar.

Research Project

There are several obstacles currently limiting the use of dielectric elastomers in actuators and energy generators, ranging from too large mechanical losses to high voltages needed for actuation. Acrylic elastomers, natural and synthetic rubbers and polydimethylsiloxane PDMS are interesting materials for the next generation of dielectric elastomer transducer materials.

Most important results

In order to be independent of novel materials in the beginning phase of the project, the two groups agreed that work at the Johannes Kepler University will also focus on fundamental problems of dielectric elastomer transducers, avoiding electromechanical instabilities during operation and seeking strategies to make the materials long term stable. In addition the materials base was widened to include natural rubber.

Peer-reviewed publications

  1. T. Lu, C. Keplinger, N. Arnold, S. Bauer and Z. Suo, Charge localization instability in a highly deformable dielectric elastomer, Applied Physics Letters Vol. 104, 022905 (2014).
    Google Scholar citations: 6
    Green OA: Harvard University http://www.seas.harvard.edu/suo/publications.html
  2. G. Buchberger, B. Hauser, J. Schoeftner, S. Bauer, B. Jakoby, and W. Hilber, Temporal change in the electromechanical properties of dielectric elastomer minimum energy structures, Journal of Applied Physics Vol. 115, 214105 (2014).
    Google Scholar citations: 1
  3. R. Kaltseis, C. Keplinger, S. J. A. Koh, R. Baumgartner, Y. F. Goh, W. H. Ng, A. Kogler, A. Tröls, C. C. Foo, Z. Suo and S. Bauer, Natural rubber for sustainable high-power electrical energy generation, RSC Advances Vol. 4, 27905-27913 (2014).
    Google Scholar citations: 25
  4. S. Bauer, S. Bauer-Gogonea, I. Graz, M. Kaltenbrunner, C. Keplinger, and R. Schwödiauer,  A soft future: From robots and sensor skin to energy harvesters, Adv. Mater. Vol. 26, 149-162 (2014).
    Google Scholar citations: 103
    Hybrid OA : http://onlinelibrary.wiley.com/doi/10.1002/adma.201303349/full