KEYWORDS: Wind energy, Capacitance, Capacitors, Power supplies, Dielectrics, Energy harvesting, Picosecond phenomena, Servomechanisms, Energy efficiency, Energy conversion efficiency
Dielectric elastomer generators (DEGs) are attractive candidates for harvesting electrical energy from mechanical work since they comprise relatively few moving parts and large elastomer sheets can be mass produced. Successfully demonstrations of the DEG prototypes have been reported from a diverse of energy sources, including ocean waves, wind, flowing water and human movement. The energy densities achieved, however, are still small compared with theoretical predictions. We show that significant improvements in energy density (550 J/kg with an efficiency of 22.1%), can be achieved using an equi-biaxial mechanical loading configuration, one that produces uniform deformation and maximizes the capacitance changes. Analysis of the energy dissipations indicates that mechanical losses, which are caused by the viscous losses both within the acrylic elastomer and within the thread materials used for the load transfer assembly, limits the energy conversion efficiency of the DEG. Addressing these losses is suggested to increase the energy conversion efficiency of the DEG.
Mechanical energy and electrical energy can be converted to each other by using a dielectric elastomer transducer. Large
voltage-induced deformation has been a major challenge in the practical applications. The voltage-induced deformation
of dielectric elastomer is restricted by electromechanical instability (EMI) and electric breakdown. We study the loading
path effect of dielectric elastomer and introduce various methods to achieve giant deformation in dielectric elastomer and
demonstrate the principles of operation in experiments. We use a computational model to analyze the operation of DE
generators and actuators to guide the experiment. In actuator mode, we get three designing parameters to vary the
actuation response of the device, and realize giant deformation with appropriate parameter group. In the generator mode,
energy flows in a device with inhomogeneous deformation is demonstrated.
Soft active materials have many important applications. We develop a theory of large deformation in a family
of soft active materials known as dielectric elastomers. We show that the Maxwell stress is not applicable to deformable
dielectrics in general, and that the effect of electric field on deformation is material specific. Based on available
experimental data, we construct for a class of model materials, which we call ideal dielectric elastomers, a free-energy
function comprising contributions from stretching and polarizing. We show that the free-energy function is typically
non-convex, causing the pull-in instability of dielectric elastomers.
The emergence of wearable electronics is leading away form glass substrates for the display backplane, to plastic and metal. At the same time the substrate thickness is reduced to make displays lighter. These two trends cooperate toward the development of compliant substrates, which are designed to off load mechanical stress from the active circuit onto the substrate. Compliant substrates made the circuit particularly rugged against rolling and bending. Design principles for compliant substrates include: (a) Moving the circuit p;lane as close as possible to the neutral plane of the structure, and (b) Using substrate and encapsulation materials with low stiffness. Design principle (a) is demonstrated on thin-film transistors made on thin steel foil. Such transistors function well after the foils are rolled to small radii of curvature. Principle (b) of compliant substrates is demonstrated with bending experiments of a-Si:H TFTs made on thin substrates of polyimide foil. TFTs on 25-micrometers thick polyimide foil may be bent to radii of curvature as low as 0.5 mm without failing. The reduction in bending radius, from R-2 mm on same- thickness steel foil, agrees with the theoretical prediction that changing from a stiff to a compliant substrate reduces the bending strain in the device plane by a factor of up to 5.
Several degradation mechanisms in ferroelectric ceramics are analyzed in this article. A ferroelectric crystal under cyclic electric field fatigues by forming a-domain bands. An energy based model is proposed, indicating that these bands are retarded by the electric field, but driven by the shear stress resolved onto the bands. The stress has been attributed to the misfit strain near the 180 degree(s) domain wall and the edge of the electrodes. A second problem is related to fracture of piezoelectric ceramics. A double-cantilever beam subjected to combined electrical and mechanical loadings is analyzed using finite elements. Also analyzed is electrode debonding, which is shown to decrease capacitance.
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