Ceramic coatings for silica and sapphire optical waveguides for high temperature embedding and sensing

Abstract

Glass, sapphire and polymer fibers transparent to visible and infrared electromagnetic frequencies are extensively used in communication and sensing. The lifetimes of these waveguides are extended considerably by suitably coating them. Plastic coated silica waveguides are gradually replacing metal coaxial cables used in communications and they have been used successfully in various types of sensing. Unfortunately plastic coatings cannot withstand very high temperatures. In order to perform contact or invasive sensing in the medium to high temperature range and in harsh environments, other appropriate coating materials have to be used. This thesis examines various refractory materials as candidate coating materials.

Coating materials should not react chemically with the waveguide material but should have matching thermal expansion coefficients. Refractory materials are examined in detail for thermodynamic suitability for both sapphire and silica waveguide cores and claddings. The candidate coating materials selected are alumina, silicon carbide, zirconia and metal niobium. Experimental verification of the chemical inertness of these materials with silica and sapphire in very low pressure and at 857°C temperature is studied. The materials found suitable for coating can be coated using the various methods discussed. Fibers suitably coated with these materials would be suitable for high temperature sensing in harsh environments and in situ within advanced high temperature composites.

Metal niobium does not react with sodium and is thermodynamically compatible with alumina which is also a very stable refractory material. Multilayer coatings of niobium and alumina on sapphire exposed to harsh environmental conditions can prolong the life of the sapphire waveguide.

X-ray diffraction and electron microprobe analyses of the single oxides and carbides, namely, alumina, silicon carbide and zirconia and the metal niobium, were conducted. It was found that sapphire did not react with any of the selected ceramics; the silica fiber underwent structural change in the silicon carbide matrix and the change was macroscopic. Within the restricted environment, the silica fiber appeared not to react with the alumina, zirconia and niobium matrices.

This thesis specifically considers the possibility of using the various ceramics as coating materials without analyzing the nature of the phases present. Hence detailed analyses of phases were not made when macroscopic change in fiber structure was observed or as observed during the x-ray analyses and microprobe analyses.