Properties of particulate-filled polymer matrix composites are highly dependent on the spatial position, orientation and assembly of the particles throughout the matrix. External fields such as electric and magnetic have been individually used to orient, position and assemble micro and nanoparticles in polymer solutions and their resulting material properties were investigated, but the combined effect of using more than one external field on the material properties has not been studied in detail. Applying different configurations of electric and magnetic fields on geometrically and magnetically anisotropic particulates can produce varying microarchitectures with a range of material properties. Experimentally and with simulations, we systematically probe the effect of combined electric and magnetic fields on the microstructure formation of geometrically and magnetically anisotropic barium hexaferrite (BHF) in polydimethylsiloxane (PDMS). The magnetic and dielectric properties resulting from different microstructures are characterized and microstructure-property relationships are analyzed. Our results demonstrate that a variety of microarchitectures can be produced using multi-field processing depending on the nature of the applied external field. For example, the application of an electric field creates macro-chains where the orientation of the BHF stacks inside the macro-chains is random. On the other hand, application of a magnetic field rotates the BHF stacks within the macro-chain in the direction dictated by the magnetic field. In simulations, the dielectrophoretic, magnetic, and viscous forces and torques acting on the particles show that particle anisotropies are central to the ability to control orientation along the orthogonal magnetic and geometric axes, mirroring experimental results. The authors refer to the ability to manipulate particle orientation along orthogonal axes as “orthogonal control”. Using this technique, not only are a variety of microstructures possible, but also a range of dielectric and magnetic properties can result. For example, for 1 vol% BHF-PDMS composites, the experimental dielectric permittivity is found to vary from 2.84 to 5.12 and the squareness ratio (remnant magnetization over saturation magnetization) is found to vary from 0.55 to 0.92 (from 0.52 to 0.99 in simulations) depending on the applied external stimuli. The ability to predict and produce a variety of microstructures with a range of properties from a single material set will be particularly beneficial for resin pool based additive manufacturing and 3-D printing.
Dielectric elastomer actuators (DEAs), unlike traditional actuators, are lightweight, soft, smart materials that are proving to be attractive in a broad range of potential applications such as robotic arms, artificial muscles, medical devices, stretchable sensors, grippers, loudspeakers, and automotive lightweighting. Though they have been known and studied for decades, their use is not widespead in part due to a number of practical implementation difficulties including the need for and handling of thin dielectric layers to reduce operating voltage, strechable and stackable electrodes that don’t overly degrade performance, and poor longevity due to electrical breakdown contributed by potential flaws in the thin film and material inclusions/irregularities. This paper investigates the construction of DEA multi-layer and single-layer articles, exploring the effect of non-negligible elastomer-based electrodes with various dielectric/electrode layer thickness ratios as well as the influence of stacked dielectric layers on breakdown voltage. A balance between actuator performance and the level of required voltage for operation is shown given a set of assumptions. Furthermore, the influence of stacked dielectric layers on breakdown voltage is demonstrated, confirming previous results performed with polyvinylidene fluoride (PVDF).
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