Fabrication Of Grin-Lenses By Contactless Electrophoretic-Gradient Formation In Transparent Nanocomposites
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Mat. Res. Soc. Symp. Proc. Vol. 576 © 1999 Materials Research Society
However, the direct electric contact between the electrodes and the sol causes difficulties regarding possible electrochemical reactions and poor surface quality after demoulding. Furthermore, the electrical potential function showed strong local maxima in the vicinity of the electrodes [15]. This hinders a simple computer simulation of the GRIN formation process, which would be very interesting for future index profile turning. Therefore, the aim of this paper was to investigate the GRIN formation in this nanocomposite system with a contactless approach, in order to avoid mechanical post-treatment of the GRIN lenses and to allow a simulation of the GRIN formation process. The general approach was to use thin polymer foils for the electrical insulation of the sol from the electrodes, which would allow a certain discharging of the nanoparticles (as the basis for electrophoresis) but can avoid direct electrical contact to the matrix sol. 3 EXPERIMENTAL 3.1 Sol synthesis The materials used to produce GRIN structures were synthesized from methacryloxypropyl trimethoxy silane (MPTS), methacrylic acid (MA), zirconium-n-propoxide (ZR) and dodecandioldimethacrylat (DDDMA). The ZrO 2 nanoparticles were generated in-situ by hydrolysis and condensation of ZR with MA and water as it has described elsewhere [17-20]. In order to enhance the chain flexibility and hence the mobility of diffusive nanoparticles, the organic monomer of DDDMA was added, which acts simultanously as a network former during polymerisation of the C=C double bonds. Irgacure 184 was added as a photoinitiator in concentration of 0.2 mol%/mol C=C and the material was photocured by illumination with a Hg-lamp with intensity of 0.6 J/cm 2. 3.2 Assembling of the electrophoretic cell In order to realize a contactless electrophoretic process, the electrodes (a centre electrode dot of 1 mm diameter and two ring electrodes (width 0.5 mm) with a diameter of 4 mm and 2 mm respectively were deposited on float glass plates by silk screen printing using a commercial silver paste. The so prepared electrodes were covered with a 50 ý.im thick polyester (PE) foil and mounted on the top and bottom side of a metallic cylinder (1 cm in diameter and 1 cm in height). The sol was filled into the mounted cell by a syringe through an opening. 3.3 Characterization The variation of the electric potential in the electrophoretic cell was measured with a 100 ý.tm metallic point probe [15]. The electrical current during electrophorensis, performed at constant voltage was measured by DM 25XT Wavetek amperemeter. The refractive index profile was determined in-situ using a Mach-Zehnder interferometer setup [15] with a He-Ne laser (632.8 nm) as light source. 4 RESULTS 4.1 Theoretical aspects If insulating foils are used in an electrophoretic cell, the formation of an electric counter potential due to increasing Coulomb interaction between even charged particles approaching each other by the driving force of an electri
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