Heat Transfer in Dielectric Elastomers - Simulating thermal breakdown in multilayered dielectric elastomers

Research output: Book/ReportPh.D. thesis

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Abstract

Dielectric elastomers are an interesting type of electro-mechanical transducers that can operate both as actuators, generators and sensors. They are composed of an elastomer layer sandwiched between two compliant electrodes and may be arranged in various configurations, including multilayered stacks. During operation dielectric elastomers may undergo several types of electrical breakdown, including thermal breakdown. Thermal breakdown occurs when the thermal energy generated within the dielectric elastomer, due to Joule heating, cannot be balanced by the dissipated heat. Thus a thermal runaway occurs. Thermal breakdown is more prone to occur in a multilayered stack of dielectric elastomers, due to the high volume-to-surface-ratio. This thesis is dedicated to modelling the performance of a multilayered cylindrical stack of dielectric elastomers, with the aim of gaining a better understanding of how various geometrical and operational parameters affects thermal breakdown. Initially a simplified analytical electro-thermal model is set up, which accounts for Joule heating within the stack as well as heat transfer. In the analytical model it is assumed that the top and bottom surfaces of the stack are equal to the ambient temperature and that the cylindrical surface is thermally insulated. Subsequently, two increasingly complex 2D axisymmetric models are set up in the commercial finite-element-method software COMSOL Multiphysics®, where natural convection is assumed to occur at all surfaces of the stack. The first model is a pure electro-thermal model which combines the effects of Joule heating and heat transfer, while the second model is an electrothermal and -mechanical model, thus it also includes the electro-mechanical deformation of the stack. The output of all the models is the estimated point of thermal breakdown, determined as the possible amount of layers in the dielectric elastomer stack before thermal breakdown occurs. It is investigated how various geometrical and operational parameters affects the breakdown point, with the overall desire to increase the amount of layers in the stack as well as the applied voltage. For modelling the elasticity of the elastomer material in the electro-thermal and -mechanical model three types of hyperelastic material models are used; Gent, Mooney-Rivlin and Ogden. It is found that using the Gent model yields the most conservative prediction of breakdown point, whereas using the Ogden model resulted in the least restrictive breakdown point. However, generally the difference in breakdown point, when utilising the three different material models, is small. Furthermore, the electrical conductivity of the elastomer material follows an Arrhenius expression with respect to temperature, but it is concluded that utilising the mathematically simpler Frank-Kamenetskii expression as a function of temperature is acceptable, albeit it yields a more conservative prediction of breakdown point. It is observed that having a single entrapped particle with a higher electrical conductivity than the elastomer material, although still in the range of semiconductors, in the dielectric elastomer stack drastically reduces the breakdown point of the entire stack. Furthermore, it is found that a high temperature of the surroundings also influence the breakdown point significantly, due to the limited heat transfer at high ambient temperatures. On the contrary, including an inactive area in the dielectric elastomer stack has little influence on the breakdown point, although it imposes some mechanical restrictions on the active area, yielding a non uniform electric field as well as stretch ratio. This thesis introduces a model for simulating the performance of a multilayered stack of dielectric elastomers, which accounts for both the electro-thermal and electro-mechanical effects. It is able to determine when a thermal breakdown will occur, and what geometrical and operational parameters that affects the point of breakdown. Furthermore, it is set up in the commercial software COMSOL Multiphysics®, thus accessible to all. Consequently, the electro-thermal and -mechanical model put forth in this work is a great tool when designing and optimizing multilayered stacks of dielectric elastomers.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages104
Publication statusPublished - 2019

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