- Half of the energy is always lost when charging a capacitor. Even in the limit of vanishing resistance, half of the charging energy is still lost--to radiation instead of heat. While this fraction can technically be reduced by charging adiabatically, it otherwise places a fundamental limit on the charging efficiency of a capacitor. Here we show that this 1/2 limit can be broken by coupling a ferroelectric to the capacitor dielectric. Maxwell's equations are solved for the coupled system to analyze energy flow from the perspective of Poynting's theorem and show that (1) total energy dissipation is reduced below the fundamental limit during charging and discharging; (2) energy is saved by "recycling" the energy already stored in the ferroelectric phase transition; and (3) this phase transition energy is directly transferred between the ferroelectric and dielectric during charging and discharging. These results demystify recent works on low energy negative capacitance devices as well as lay the foundation for improving fundamental energy efficiency in all devices that rely on energy storage in electric fields.
- Dielectric elastomer actuators (DEAs) are soft, electrically powered actuators that have no discrete moving parts, yet can exhibit large strains (10%–50%) and moderate stress (∼100 kPa). This Tutorial describes the physical basis underlying the operation of DEA's, starting with a simple linear analysis, followed by nonlinear Newtonian and energy approaches necessary to describe large strain characteristics of actuat
- New evidence for molecular alignment under an electric field is also presented. In the discussion of compliant electrodes, the rationale for carbon nanotube (CNT) electrodes is presented based on their compliance and ability to maintain their percolative conductivity even when stretched. A procedure for making complaint CNT electrodes is included for those who wish to fabricate their own. Percolative electrodes inevitably give rise to only partial surface coverage and the consequences on actuator performance are introduced. Developments in actuator geometry, including recent 3D printing, are described. The physical basis of versatile and reconfigurable shape-changing actuators, together with their analysis, is presented and illustrated with examples. Finally, prospects for achieving even higher performance DEAs will be discussed.
- I. INTRODUCTION
- The goal of creating artificial muscles with performances comparable to mammalian muscles has proven to be a rallying call to scientists and engineers, particularly in the soft robotics community. Several different active polymer approaches have been proposed1 for reaching this goal, but in this Tutorial, we focus on dielectric elastomer actuators (DEAs). DEAs are solid electrostatic actuators, with no discrete moving parts, that can be controlled electrically to produce large actuation strains and high energy densities on a par with mammalian muscles and over a moderate range of frequencies (up to <1 kHz). Fundamental to the actuator performance is the use of thin elastomers as the dielectric, sandwiched between compliant electrodes, because elastomers exhibit the unusual combination of being soft, highly extensible, and nearly incompressible.
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