Infoscience

Thesis

Metal-Ion Implanted Elastomers: Analysis of Microstructures and Characterization and Modeling of Electrical and Mechanical Properties

This thesis reports on the microstructural analysis of metal ion implanted Polydimethylsiloxane (PDMS), and on the characterization and modeling of its electrical and mechanical properties. Low energy (below 35 keV) metal ion implantation into PDMS forms metal nanoparticles in the top 10 nm to 120 nm of the polymer, creating a metal–insulator composite. Above a certain ion dose, the percolation threshold, the particles form a conductive path. By suitable choice of the volume-ratio between the two constituents (metal atoms and PDMS), one is able to create stretchable electrodes capable of sustaining uniaxial strains of up to 175% while remaining conductive, and remaining operational after 105 cycles at 30% strain. These outstanding properties are especially required for flexible electronic and for polymer actuators and sensors. Low energy metal ion implantation into 30 µm thick PDMS was performed at 10 keV and 35 keV with Low Energy Broad Beam Implanter (LEI), and at 2.5 keV, 5 keV and 10 keV with Filtered Cathode Vacuum Arc (FCVA). The metals used for the implantation were Titanium and Gold. Doses ranged from 0.1×1016 at/cm2 to 7×1016 at/cm2, leading to surface resistivities between 100 Ω/square and 100 MΩ/square. Generally lower implantation energy and higher ion doses lead to better conductivities. However doses above the percolation threshold lead to an important increase of stiffness. The effective Young's modulus measurements for FCVA implanted samples were in the range of 5 MPa. The samples implanted with LEI showed much important increase of the stiffness reaching 80 MPa for the gold and 170 MPa for the titanium implantations. Together the electrical and the mechanical measurements showed the best conductivity-to-compliance-ratio is obtained with FCVA implantation with Gold at 2.5 keV and doses around 1.5×1016 at/cm2. A TEM sample preparation method based on cryo-ultramicrotomy, was developed, adapted for extremely low modulus (1 MPa) elastomers with hard inclusions, allowing high-resolution TEM cross-section micrographs for microstructural analysis of the implanted layers. Gold ions penetrate PDMS by up to 30 nm (for FCVA, 60 nm for LEI) and form crystalline nanoparticles whose size increases with the dose and the energy. Titanium forms a nearly homogeneous amorphous composite with the PDMS up to 18 nm thick (for FCVA) and 120 nm thick for LEI). The penetration depths were confirmed with computer simulations. Using TEM micrographs the metal volume fraction of the composite was accurately determined, allowing conductivity and the Young's modulus to be plotted vs. the volume fraction. The graphs showed different scalings dependant on the microstructure and on the ion species, allowing for the first time quantitative use of the percolation theory for ion implanted thin films. This allowed linking the composite's Young's modulus and conductivity directly to the implantation parameters and volume fraction. Both electrical and mechanical properties were measured on the same samples, and different percolation thresholds and exponents were found, showing that while percolation explains very well both conduction and stiffness of the composite, the interaction between metal nanoparticles occurs differently for determining mechanical and electrical properties. Flexible electrodes fabricated by this ion implantation technique were used to fabricate small arrays of 1 to 3 mm diameter tunable lenses, consisting of electroactive polymer actuators bonded to a socket that provides fluidic coupling between devices. The focal length was electrically tuned from 4 mm to 8 mm by applying a voltage from 0 kV to 1.7 kV.

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