Alexander Kogler

Stretch dependence of the electrical breakdown strength and dielectric constant of dielectric elastomers

Dielectric elastomers are used for electromechanical energy conversion in actuators and in harvesting mechanical energy from renewable sources. The electrical breakdown strength determines the limit of a dielectric elastomer for its use in actuators and energy harvesters. We report two experimental configurations for the measurement of the stretch dependence of the electrical breakdown strength of dielectric elastomers, and compare the electrical breakdown fields for compliant and rigid electrodes on the elastomer. We show that the electrode configuration strongly influences the electrical breakdown field strength. Further, we compare the stretch dependent dielectric function and breakdown of the acrylic elastomer VHB 4910 TM from 3M TM , and of the natural rubber ZruElast TM A1040 TM from Zrunek rubber technology. While the dielectric permittivity of VHB decreases with increasing stretch ratio, the dielectric constant of rubber is insensitive to stretch. Our results suggest natural rubber as a versatile material for dielectric elastomer energy harvesting. (Some figures may appear in colour only in the online journal)

Natural rubber for sustainable high-power electrical energy generation

Clean, renewable and abundant sources of energy, such as the vast energy of ocean waves, are untapped today, because no technology exists to convert such mechanical motions to electricity economically. Other sources of mechanical energy, such as motions of people and vibrations of buildings and bridges, can potentially power portable electronics and distributed sensors. Here we show that natural rubber can be used to construct generators of high performance and low cost. Natural rubber has higher elastic modulus, fracture energy and dielectric strength than a commonly studied acrylic elastomer. We demonstrate high energy densities (369 mJ g−1) and high power densities (200 mW g−1), and estimate low levelized cost of electricity (5–11 ct kW−1 h−1). Soft generators based on natural rubber enable clean, low-cost, large-scale generation of electricity.

Directional, passive liquid transport: the Texas horned lizard as a model for a biomimetic ‘liquid diode’

Moisture-harvesting lizards such as the Texas horned lizard (Iguanidae: Phrynosoma cornutum) live in arid regions. Special skin adaptations enable them to access water sources such as moist sand and dew: their skin is capable of collecting and transporting water directionally by means of a capillary system between the scales. This fluid transport is passive, i.e. requires no external energy, and directs water preferentially towards the lizards snout. We show that this phenomenon is based on geometric principles, namely on a periodic pattern of interconnected half-open capillary channels that narrow and widen. Following a biomimetic approach, we used these principles to develop a technical prototype design. Building upon the Young–Laplace equation, we derived a theoretical model for the local behaviour of the liquid in such capillaries. We present a global model for the penetration velocity validated by experimental data. Artificial surfaces designed in accordance with this model prevent liquid flow in one direction while sustaining it in the other. Such passive directional liquid transport could lead to process improvements and reduction of resources in many technical applications.

Ferroelectret based flexible keyboards and tactile sensors

Ferroelectrets easily allow for the preparation of multilayer keyboards, where keys are digitally encoded with a ternary code. Contact electrode charging turned out to be a reliable way for the fast preparation of charge patterns on cellular polymers. A three layer keyboard, allowing for encoding 26 keys was fabricated to demonstrate the feasibility of the proposed input device. Further work is necessary to better understand the underlying mechanisms during the preparation of charge patterns on cellular ferroelectrets.

Heteropolar Charging of Ferroelectrets for Flexible Keyboards and Tactile Sensors

Ferroelectrets are flexible and conformable polymer transducer foils with a dominant longitudinal piezoelectric effect. Ferroelectrets also have a negligible transverse piezoelectric effect, which makes them ideal for the fabrication of roll- and bendable keyboards. Cellular polypropylene ferroelectrets with heteropolar charge patterns are useful for a simple and effective key encoding. The preparation of charge patterns is, however, a technical challenge. We have investigated and produced such structures in cellular polypropylene ferroelectrets by corona poling, in combination with shadow masks and unstructured and structured counter electrodes. Furthermore, we have also produced lateral charge distributions by contact poling with specially designed high voltage electrodes. We discuss the two different poling concepts and the related problems, their strengths, weaknesses and experimental pitfalls. We found in particular contact poling to be a reliable and well reproducible poling method for an effective preparation of heteropolar charge patterns of ferroelectrets.