Tantalum oxide nanomers for optical applications

Abstract

The synthesis of transparent nanomers by the incorporation of nanoscaled tantalum oxide into an organic-inorganic composite matrix and their subsequent characterization are presented. The matrix materials used consist of a mixture of organically functionalized silanes and polymerizable monomers. The mixture does not exhibit phase separation, even down to the lowest nanometer scale, as revealed by SAXS measurements. The addition of nanoscaled Ta2O5 particles (mean particle diameter: 4 nm as determined by photon correlation spectroscopy) aims to increase the refractive index of the nanomers. The preparation of the oxide sol and the optimization of the synthesis with respect to compatbility with the matrix material, thereby avoiding agglomeration effects, is described. After incorporation of the particles in the monomer mixture, a photopolymerization step, followed by curing with a temperature program up to 90°C, led to colourless and transparent monoliths. The volume shrinkage, caused by polymerization, decreases from 8.2% for the unfilled matrix material to 5.8% for a nanomer containing 30 wt. % tantalum oxide. The shrinkage decreases linearly with increasing filler content of tantalum oxide. The increase in refractive index is about 7.3 x 10-4 per wt. % oxide (measured at a wavelength of 546.1 nm). The colouration of the monoliths is expressed as yellowness index G according to DIN 6167. Colour values attained for nanomers with up to 15wt. % tantalum oxide are comparable to values for commercial optical polymer materials. Nanomers containing 15 wt. % tantalum oxide show transparency losses at a wavelength of 850 nm below 0.1 dB/cm.