Lifetime of dielectric elastomer actuators under DC electric fields

Dielectric elastomer actuators (DEAs) are interesting soft actuators compared to other actuation technologies owing to large achievable actuation strains, high energy density and fast actuation. However, because DEAs need to be operated under high electric fields to achieve the required strain and force ranges for industrial applications, a trade-off between performance and lifetime exists, owing to the limited dielectric breakdown strength of the dielectric elastomers used in DEAs. The development of reliable and durable DEAs would not only increase its industrial and academic interest, but also catalyze future investments in the field with the objective of commercializing DEA-based devices.The lifetime of DEAs under DC actuation has not been studied in the literature. The aim of the thesis is to present the first extensive studies on the lifetime of silicone-based DEAs under constant (DC) electric fields, and proposing effective approaches to enhance their lifetime under ambient and harsh environmental conditions, without compromising on actuation strain.The comprehensive investigations include the selection of the dielectric elastomers, the selection of the electrode materials, the type of prestretch, the selection of surface area, the encapsulation of the device layers, the influence of electric field, the effect of polarity reversal, the influence of humidity and temperature, and multilayering. The lifetime experiments are conducted with an automated lifetime test bench designed at LMTS. This setup allows a continuous monitoring of the deformation and the electrical resistance of the electrodes under different temperature and humidity conditions.The lifetime of silicone-based DEAs is strongly limited by increasing humidity, temperature, and electrode surface area. There is a significant trade-off between DC lifetime and actuation strain for thin (12 um) prestretched silicone-based DEAs. All tested DEAs fail exclusively by dielectric breakdown, which cannot be anticipated with sudden strain or electrode resistance changes. The selection of the dielectric elastomer and of the electrode materials also plays a key role in lifetime at constant strain. At 20°C 90% RH, DC lifetime decreases in the following order: Electro 242-1 > Sylgard 186 > Elastosil 2030/20 > Sylgard 184.The combination of high humidity and high electric field significantly decreases the DC lifetime of equibiaxially prestretched silicone-based DEAs. For Elastosil 2030/20 DEAs at 85°C 85% RH, the mean DC lifetime decreases by a factor of 62x when increasing the electric field from 80 V/um (2% actuation strain) to 100 V/um (5% actuation strain). At constant temperature, increasing the humidity significantly reduces the DC lifetime. For Elastosil 2030/20 DEAs at 100 V/um (4-5% actuation strain), the DC lifetime decreases by a factor of >125x when the relative humidity is increased from 20% RH to 85% RH at 85°C. The addition of encapsulation layers is a facile solution to greatly increase the DC lifetime in dry and humid conditions without compromising on the actuation performance and without complexifying the DEA fabrication processes. Adding a soft and thin (4 um) silicone (LSR 4305) encapsulation layer on both electrodes improves the DC lifetime of DEAs by a factor of over 6x. Adding additional DEA layers increases the output force of DEAs, but at the expense of lower lifetimes.

Conductivity, dielectric and piezoelectric properties of SrBi4Ti4O15

Strontium Bismuth Titanate is a very promising material for high temperature piezoelectric applications, its elevated ferroelectric phase transition (530°C), linear piezoelectric properties under low field and relatively low room temperature conductivity (compared to others Bismuth Titanates) make it very attractive for precision sensors. However, under severe conditions (low frequency, high field, high temperature or low oxygen partial pressure) some of those advantages disappear. Piezoelectric response is dominated by charge drift in general becomes unstable. Above all, at high temperature and low oxygen partial pressure, a large conductivity increase reduces the piezoelectric efficiency of the material, in this work, electrical conductivity, piezoelectric properties and dielectric permittivity of SrBIT ceramic have been investigated in conditions of high temperature, low oxygen partial pressure, low to moderate driving field and frequency. As a result of this research, a better understanding of SrBIT properties was achieved. Thanks to a careful study of SrBIT processing as a bulk ceramic, a reproducible route was established. Many basic mechanisms leading to both SrBIT crystallization and sintering have been identified. It was demonstrated that a detailed knowledge of the exact processing conditions is required in order to achieve high quality material. DC conductivity measurements were carried out as a function of temperature, oxygen partial pressure and dopant concentration. It was found that the apparent activation energy for conduction for undoped SrBIT was 1 eV between 140 and 220°C and 1.5 eV between 450 and 700°C. Decrease of the activation energy in the lower temperature range has been discussed considering grain boundary conductivity, as a transition from electronic to ionic conduction or as consequence of small polarons conduction. It was shown that lower activation energy resulted from Manganese doping (0.5 eV), this was interpreted as either growing influence of grain boundary conductivity as dopant concentration increases or as shallow hole trap formation. DC conductivity measurements and acceptor/donor doping experiments demonstrated p-type conductivity in the low temperature range (up to 220°C) as donor doping decreases conductivity, while oxygen partial pressure controlled measurements indicated n-type conductivity at higher temperature (above 700°C). An electrical impedance analysis was performed with several equivalent circuits. The aim of these models was to simulate the impedance of SrBIT. The best approximation was found with a distributed element of Havriliak-Negami type. However, as the physical justification for this circuit was not clear, the investigation of the grain, grain boundary and electrode impedances was performed with multiple discrete parallel RC elements. With temperature, grain size and oxygen activity variations, the identification of three separate contributions as grain, grain boundary and electrode was realized. The anisotropy of conductivity and permittivity was demonstrated with textured material and both DC and AC analysis. With the master curves built for the electrical modulus, it was found that the impedance probably related to Bismuth oxide layers produces an additional high frequency are. From this finding and by comparing characteristic relaxation frequencies of undoped, 2 mol.% Mn and 4 mol.% Nb SrBIT, it was determined that conductivity is higher in the ab plane direction than in the c direction within both perovskite units and Bismuth oxide layers. With conductivity measurements under controlled oxygen partial pressure, it was found that an acceptor-based (intrinsic or extrinsic) model could be used to describe the electrical conductivity under controlled oxygen partial pressure of both undoped and 2 mol.% Mn doped SrBIT. However, as neither a pO2-1/6 region (intrinsic oxygen vacancies compensated by electrons) for undoped SrBIT nor a pO2-1/4 region (oxygen vacancies compensated by singly ionized acceptors) for Mn doped SrBIT were seen, it was concluded that the acceptorcontrolled model is not sufficient for a complete description of SrBIT. For this reason and in order to include the low oxygen partial pressure behavior of undoped SrBIT, a donor-based (intrinsic) model was also considered. The source of intrinsic donors would be in that case Bi3+ cations sitting on Sr2+ sites in the perovskite sublattice. Considering Bismuth vacancies as the negative compensating species, both pO2-1/4 and pO2-independent regions could be predicted with the model. However, even if the donor-controlled model seems to better match conductivity measurements in the full PO2) range, rejecting the acceptor-based model would be an error. It is actually not demonstrated that in SrBIT the concentration of exchanged Bismuth cations is always (all temperature, pressure) larger than the natural acceptor impurity concentration. It is very likely that the cation exchange is dependent of the oxygen partial pressure. It is also not proved as suggested in the literature that direct compensation between exchanged Strontium and Bismuth exists, reducing the net donor-excess. With the acceptor-controlled model, the mass-action constants for reduction and for ionization of intrinsic carriers across the band gap were determined. The band gap of SrBIT was estimated to be 3.5 eV. The ionic conductivity of SrBIT was determined at high temperature with measurements under controlled oxygen partial pressure. It was found that the electrical conductivity of SrBIT is probably mixed (electronic and ionic) as the estimation of the transference number provided quite large values (t=0.8 at 800°C). From electronic and ionic conductivity data, mixed conduction can actually be predicted in a large temperature range (above room temperature). The piezoelectric measurements using direct effect demonstrated that it is actually possible to unlock piezoelectrically active ferroelectric domain walls and create non-linear piezoelectric properties in undoped SrBIT. This occurs above a threshold elastic field, which is thermally activated. With a piezoelectric composite, it was demonstrated that the electromechanical coupling between two different phases creates a piezoelectric relaxation. This one could be positive or negative depending on the respective properties of each composite's component. It was shown experimentally that a small temperature change is sufficient to transform a positive relaxation into a negative one. While these experiments did not provide a detailed microstructural explanation for the piezoelectric relaxation observed in 2 mol.% Mn doped SrBIT, they gave a first insight into an original phenomenological approach. Microstructure and piezoelectric properties were related with the calculation of a piezoelectric relaxation composite made of two textured samples. This demonstrated that a piezoelectric relaxation may occur just because of anisotropy. Microstructural reproduction of this coupling is actually an important feature of Aurivillius phases.

EPFL2000

Production d'hydrogène dans un réacteur microstructuré

Pierre Reuse

The aim of this work was to operate the steam reforming of methanol and the total oxidation of methanol in a microstructured two-passage reactor. The steam reforming enables the production of hydrogen at relatively low temperatures (250 °C) with only a small amount of carbon monoxide (~ 1%). After a cleaning step (CO preferential oxidation, for example), this hydrogen would be a good feed for a fuel cell to produce electricity for mobile applications. Steam reforming is an endothermic reaction. The energy needed to run this reaction is produced by the total oxidation, a strongly exothermic reaction. To allow an efficient heat transfer between these two reactions, a microstructured reactor was used. These structures, with dimensions in the micron range, are widely known to have far better heat transfer capacity than traditional heat exchangers. Furthermore, these structures have some unique features with respect to classical reactors. A narrow residence time distribution, close to that of a plug flow reactor, can be obtained, keeping selectivity high for complex reactions. Microstructured reactors also have small residence time, allowing very quick variations in the feed and experimental conditions. In chapter 4, the concepts relating to the channel coatings will be presented. A Cu/Zn/Al catalyst provided by Süd-Chemie was transformed into a suspension of nanoparticles in isopropanol by using an attrition grinding apparatus. A thin layer (~5 µm) of catalyst was formed on the microchannel walls. Several tests were performed to characterize the adhesion of this layer. The influence of the layer on the residence time distribution was also investigated; it was possible to maintain Bodenstein numbers as high as 50. Chapter 5 is focused on the steam reforming of methanol, the reaction that generates hydrogen. A kinetic analysis in differential and integral modes established that the rate of the reaction is inhibited by hydrogen, the main product of the reaction; the partial order is - 0.2. The influence of water on the rate of reaction is negligible (partial order of 0.1), and methanol has a positive partial order of 0.7. A kinetic constant of 5.57 · 10-6 mol0.4 · (m3)0.6 · gcat-1 · s-1 at 200 °C and an activation energy of 73.9 kJ · mol-1 complete the description of the kinetics of the steam reforming. Throughout the comparisons, we showed that the catalyst coated in the microchannels has an initial rate for the steam reforming 30% higher than the original catalyst in fixed bed. We can therefore conclude that the catalyst modification procedure does not diminish the catalyst activity. This was also shown by comparing our results with results available in the scientific literature for similar catalysts. CO2 selectivity remains high and constant then decreases as 90% conversion is approached. Working with an excess of water with respect to methanol contributes to keeping the selectivity high. A molar ratio of 1.2 is a good compromise between selectivity and increased energy costs due to the vaporization of the additional water. It's also beneficial to work at conversions below 90% to be in the high selectivity domain and to diminish the inhibiting influence of hydrogen. Finally, we showed that this catalyst deactivates over 100 hours at a temperature of 300 °C. A kinetic model for the deactivation at temperatures between 300 °C and 330 °C was established. In chapter 6 the total oxidation of methanol was studied. A cobalt based catalyst was developed to allow the synchronization of the two reactions temperatures. The minimum temperature for complete conversion for the total oxidation reaction must clearly not exceed an acceptable working temperature for the steam reforming catalyst (i.e. a temperature that results in an acceptable catalyst lifetime). A kinetic analysis determined partial orders for methanol and oxygen as 0.67, the kinetic constant at 230 °C as 5.45 · 10-3 mmol-0.33 · m4 · (gcat · s)-1 and the activation energy as 130 kJ · mol-1. The copper based catalyst used for the steam reforming of methanol was also active for the total oxidation of methanol. Unfortunately the minimal temperature to ensure a total conversion was not compatible with the working temperature of the steam reforming. In chapter 7, the results concerning the coupling of the two previously studied reactions are presented. The numerous fittings and cables necessary for the operation of the reactor didn't allow the reactor to be run in an autothermal mode. Therefore constant, not regulated, heating was used to compensate the thermal losses. By coupling the two reactions and by studying the dynamic response time of the reactor (temperature and conversion of the steam reforming reaction) at the time of stopping and starting the oxidation reaction, we've seen that the temperature increase was around +10 °C within 60 seconds and that the conversion for the steam reforming of the methanol fitted well with the temperature increase. This reactor also has good dynamics and responds quickly to feed variations. When the coupling mode was changed from co-current to counter-current we noticed a significant increase in conversion. Furthermore, selectivity in counter-current mode remained high, even for conversion approaching 100%.

EPFL2003

High Permittivity Silicones for Dielectric Elastomer Actuators

Simon Johannes Dünki

Many technical advances of modern society sprout from the creation of specially customized polymer materials exhibiting unique combinations of properties uncharted so far. Amid this plurality of highly versatile materials, the class of electroactive polymers (EAPs) bears the special ability to undergo large shape changes when exposed to electrical stimulation. Such materials offer new perspectives in automation, robotics and medicine. Especially the dielectric elastomer actuators (DEA) technology is considered as highly promising among EAPs due to the simple working principle and actuation characteristics similar to mammalian muscles. Despite great research efforts and remarkable progress in regard to materials and devices, the commercial breakthrough of the DEA technology is still outstanding. A major obstacle for their application is the high driving voltage needed to obtain a reasonable actuation. The development of tailored elastomers that overcome this deficiency is therefore a main research focus in academia and industry. The exceptional elastic and dielectric characteristics of silicones nominate them as an ideal vantage point to start this endeavor. Their low permittivity inherently decreases the actuation performance and should therefore be optimized with all other properties remaining unaffected. In the present work, dipoles were covalently attached to a siloxane backbone in order to increase the permittivity thereof and achieve a material with an enhanced actuation performance at low driving voltages for DEAs. The introduction of the polar groups to the polymer side chains was carried out via thiol-ene chemistry, which enabled a quantitative functionalization of the polymer backbone. Three different polar groups, nitrile, thioether and sulfones, were investigated and the corresponding functionalized polysiloxanes were prepared and characterized. Thorough dielectric, mechanical and electromechanical investigations were conducted on polysiloxane based elastomers with different content of nitrile groups. In addition, first results were obtained with thioethers and sulfones which are also promising candidates to enhance the permittivity of polysiloxanes. Most of the synthesized polysiloxanes show glass transition temperatures well below room temperature, an important prerequisite to ensure elastic properties. Polysiloxanes that carry a butylthioether moiety at every repeat unit doubled the permittivity of the elastomer. With the same amount of the nitrile or sulfone dipoles, a six respectively eight times higher permittivity as compared to regular siloxane is achieved. However, with enhanced dipole content also an increase of the electrical conductivity of the polymer was observed. A simple one-step process to prepare cross-linked polar silicone films was developed using UV light triggered thiol-ene addition. A strong correlation between the dipole content in the material and the resulting actuation strain was found. Finally, a polar silicone elastomer was prepared that showed up to 20.5% lateral strain under a field as low as 10.8 V/um. Compared to commercial silicone this corresponds to an effective voltage reduction by about 70%. Processing such materials in films the thickness of which is below 10 um would result in an actuator that can be driven below 100 V.

EPFL2017