Confocal microscopy observations of electrical pre-breakdown of bi-layer elastomer dielectrics
Introduction
Most dielectrics (including semiconductors) in which electrical breakdown has been investigated are materials having a high elastic modulus (several GPa to hundreds of GPa) and consequently the mechanical deformation associated with high electric fields (1 V/micron) are likely to be small. This is because the electrical energy per unit volume scales with electric field, , as whereas elastic strain energy density scales with elastic strain, , as . and are the dielectric constant and elastic modulus, respectively. Comparison of these energies suggest that they are comparable and result in significant strains at normalized electric fields, , given by [1], [2], [3], [4]: where and h are the applied voltage and elastomer thickness, respectively. With the increasing use of soft elastomers ( 0.1–10 MPa) under large electric fields (10 V/micron) in the development of a variety of dielectric elastomer actuators and energy harvesting devices, these dielectrics are being used under conditions where the elastic and electrostatic energies are comparable. One manifestation of this is that an initially flat surface of a soft dielectric is unstable to morphological perturbations above a critical electric field and a pseudo-periodic array of pits forms. The pits are visible to the eye, as well as by optical microscopy, and have average spacing that depends on the thickness of the soft elastomer [5], [6]. (A video recording of the pitting, observed in reflected light optical microscopy, with increasing and then decreasing electric field is included in the Supplementary Materials). The instability has been extensively analyzed [1], [3], [7], [8]. As the electric field is increased beyond this instability condition, the pits extend deeper into the elastomer and above an even higher electric field, catastrophic electrical breakdown occurs. If electrical breakdown does not occur and the field is reduced, the pitting phenomenon is reversible. This forms the basis for a reversible, privacy window [5], [9] as the pits scatter light and the optical transmittance of the elastomer can be controlled over a wide range of values by controlling the applied voltage [6]. In contrast to the electro-mechanical analyzes which predict a bifurcation instability [3], the pits form by a nucleation and growth process. Although studies of the optical scattering by the pits have been reported, they are neither precise enough nor sufficiently interpretable to provide insight into the mechanisms by which the pits extend post-instability into the elastomer. Furthermore, microscopy studies have been unable to reveal the shape evolution once the surface becomes steep, as illustrated by the confocal image in Fig. 1(a), possibly because of large optical refraction effects and low contrast. Consequently, the defects well below the macroscopic surface of the elastomer cannot be resolved. As will be described in this work, using fluorescence confocal microscopy to enhance contrast, the pit evolution consists of a series of large strain and complex morphological changes until, ideally, an array of hyperbolic funnels form penetrating through the elastomer. At each step, the morphological instabilities are reversible when the electric field is subsequently decreased unless permanent breakdown occurs. The observed behavior is quite distinct from the usual filamentary pre-breakdown that occurs in stiff materials and has implications for dielectric breakdown in elastomers.
The observations reported in this work are made in a configuration consisting of two dielectric elastomer layers, one significantly softer than the other, deposited on an ITO coated glass substrate with a compliant CNT mat network on top of the softer dielectric. To enhance contrast in the otherwise transparent elastomers, they were each dyed with a fluorescent dye. Fig. 1(b). This is the same structure used in previous experiments relating optical transmittance to the electric-field induced surface morphological instability of a soft elastomer [5], [9]. Following the work of Wang et al. [1], the bi-layer elastomer configuration was selected because the electrical field induced surface instabilities in the soft elastomer, once formed, growing stably with increasing electric field and do not lead to irreversible, catastrophic breakdown, until the electric field approaches the breakdown field of the stiffer elastomer layer. The rationale for using a stiffer elastomer layer is to raise the voltage at which catastrophic breakdown of the bilayer occurs since the dielectric breakdown field of an elastomer generally increases with its elastic modulus [1]. In turn, this enables the electric field in the softer elastomer to be varied at fields below which complete failure occurs. In the experiments reported here, a voltage between the ITO electrode and the CNT network was applied in discrete steps and observations were recorded by confocal optical imaging at successively higher voltages.
Section snippets
Materials and methods
All the devices studied comprised a layer of 10:1 mixing ratio of base to cross-linker of silicone elastomer (Sylgard 184, Dow Corning) on an ITO coated glass substrate with a layer of 50:1 Sylgard 184 on top, unless otherwise stated. (The ITO coated glass had a thickness 150 um, and a 30–60 Ohm resistivity, and was purchased from SPI Supplies). According to the literature, the elastic shear modulus of the 10:1 and 50:1 composition, are 700 kPa and 3 kPa [10], [11], respectively. The dielectric
Confocal microscopy observations
Before any voltage is applied to the bilayer elastomer structure, the surface of the soft elastomer, as well as its interface with the stiffer elastomer, are flat and smooth. Fig. 1(c). Sequences of confocal images indicate that the two interfaces remained planar with increasing voltage until circular depressions (“pits”) nucleate randomly at the CNT coated surface over a narrow range of voltage. The initial, electric field induced surface morphological instability shape is consistent with
Discussion
No theoretical or analytical model currently exists to describe the electric field induced surface morphological behavior beyond the initial surface perturbation instability to compare with our observations. So, in the absence of any model, the different stages in the evolution of the pre-breakdown defects with increasing electric field, in what we term the post-instability regime, are considered sequentially. We assume that the elastomer surface is conducting so that potential everywhere along
Concluding remarks
Once the electric field across the thickness of a soft, flat elastomer exceeds the initial bifurcation condition at which surface undulations grow, they undergo a series of complex morphological changes that are reversible when the voltage is removed. The evolution of each, described in this work as pre-breakdown defects, is slightly different in detail but they all transition from an approximately periodic array of axi-symmetric pits to closed “crack-like” features that propagate in the field
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
We thank Professor Joanna Aizenberg for the use of the confocal microscope and to both Matthias Kollosche and Ehsan Hajiesmaili for invaluable discussions and assistance. This work was partially supported by the National Science Foundation, United States through the MRSEC Program (DMR 20-11754).
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