Mechanical properties of electroactive polymer microactuators with ion-implanted electrodes

Abstract

We report on the use of the bulge test method to characterize the mechanical properties of miniaturized bucklingmode dielectric elastomer actuators (DEA). Our actuator consists of a Polydimethylsiloxane (PDMS) membrane bonded to a silicon chip with through holes. Compliant electrodes are fabricated on both sides of the membrane by metal ion implantation. The membrane buckles when a critical voltage is applied to the electrodes. The maximum displacements as well as the efficiency of such actuators strongly depend on the mechanical parameters of the combined electrode-elastomer-electrode layer, mainly effective Young's modulus E and residual stress σ. We report measured E and σ obtained from bulge tests on PDMS membranes for two PDMS brands and for several different curing methods, which allows tuning the residual stress by controlling the rate of solvent evaporation. Bulge test measurements were then used to study the change in membranes' mechanical properties due to titanium ion implantation, compared to the properties obtained from depositing an 8 nm thick gold electrode. At the doses required to create a conductive layer, we find that the Ti ion implantation has a low impact on the membrane's overall rigidity (doubling of the Young's modulus and reducing the tensile stress) compared to the Au film (400% increase in E). The ion implantation method is an excellent candidate for DEAs' electrodes, which need to be compliant in order to achieve large displacements.

Flexible and stretchable electrodes for dielectric elastomer actuators

“…First, the Young's modulus of metal is several order of magnitude higher than that of dielectric elastomers (50 − 100 GPa compared to 0.2 − 1 MPa), which means that even 50 nm-thick electrodes on a 50 µm-thick elastomer will have a significant stiffening impact on the elastomer, leading to a negligible actuation strain. For example, we have shown that sputtering a 8 nm gold layer on a 30.6 µm PDMS membrane with an initial Young's modulus of 0.77 MPa caused a relative increase of the Young's modulus of the membrane of 440% , up to 4.2 MPa [49]. Secondly, the limit of elasticity for metals is very low, typically 2-3% and if a metal electrode is strained above this limit, it will crack and form islands separated by non-conductive polymer.…”

Metal Ion Implanted Compliant Electrodes in Dielectric Electroactive Polymer (EAP) Membranes

“…With an acceleration potential of a few kV on the sample, the electric field accelerates the ions that then penetrate a few nm into the sample. Due to the low heat conductivity of polymers (for instance 0.22WK −1 m −1 for the Polydimethylsiloxane model Sylgard 186), thin Polydimethylsiloxane (PDMS) membranes can burn if the implantation process generates too much heat as can occur at high ion fluence [6]. The plasma can be operated in a pulsed manner to reduce charging and heating of the sample.…”

“…We use a bulge test setup where we can apply a well defined gas differential pressure on the membrane and record its displacement with an optical profilometer [6]. With this setup we can characterize the mechanical properties of the membrane, i.e.…”

“…3), 600 µs pulses were created at a frequency of 2 Hz. Implantation rate is relatively high and doses of 10 16 at/cm 2 were obtained after 170 pulses [6]. We built our custom experimental FCVA setup, with a 1 cm 2 beam, which is coupled to a x-y stage for larger area scanning.…”

“…We use this technology to fabricate DEAPs micro-actuators whose relative displacement is the same as for macro-scale DEAPs [5], [6]. The implantation process creates a nano-composite a few nm thick, located in the top tens of nm of the surface of the elastomer [7], [8].…”