Three silicon oxycarbide glasses (SiCO) with increasing C content were obtained through pyrolysis in inert atmosphere at 1000 °C of sol-gel derived siloxane networks containing Si-CH 3 and Si-H bonds. The glasses were further annealed at 1200, 1400, and 1500 °C to follow their evolution at high temperature. Quantitative information concerning the structure of glasses before and after annealing at high temperature was collected with a wide range of techniques (some of them used for the first time in this field) with the aim of probing the following: (i) the short-range order and chemical composition ( 29 Si and 1 H MAS NMR, RDF derived from X-ray and neutron scattering, inelastic neutron scattering, FT-IR, and elemental analysis), and (ii) the long-range order (X-ray and neutron diffraction) and microstructural features (HR-TEM combined with electron diffraction, Raman, porosity, and surface area measurements). This extensive collection of data, carried out on the same set of specimens, provided detailed and sound structural information on nearly-stoichiometric SiCO glasses and their high-temperature behavior.

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Silica sol-gel anti-reflective (AR) coatings have been investigated with particular reference to their laser induced damage threshold (LIDT) when subjected to irradiation from a Nd pulse laser at 1064nm. Coatings (whose thickness was optimised for minimum reflection at 1064nm) were deposited by spinning silica sols (average particle size 1 5nm) produced formed by the base (ammonium hydroxide) catalysed hydrolysis/condensation of TEOS in ethanol. Addition of polyethyleneglycols (PEGs) increased the size ofthe colloidal silica particles and also induced some particle aggregation in the sol, unlike a similar chain length diol. Increases in the LIDT ofthe coatings possibly depend upon the impurity levels, the wettability of the substrate and the presence of PEG. LIDT improvements may be obtained by control of substrate and coating surface wettability, hydrophilicity and surface chemistry. INTRODUCTIONThe advantageous properties of silica sol-gel anti-reflection (AR) coatings, in routine use in terrawatt Nd glass laser systems, include their ease of synthesis and deposition, their good optical properties, low cost and their reasonably high thresholds at which laser-induced damage occurs. Sol-gel chemistry can be used to prepare these coatings from colloidal silica particles produced via base catalyzed hydrolysis and condensation oftetra-ethyl-ortho-silicate (TEOS) in ethanol'. Recently the addition oforganic binder (e.g. polyethylene glycol; PEG) to the silica sol-gel has also been investigated4. Incorporation oforganic binders (e.g. soluble siloxane, PEG, PVP, PVA, etc) into oxide (e.g. 5i022' , Zr02, Hf02, etc) sol-gel coatings has led to increased resistance to laser-induced damage36. 5i02-PEG hybrid gels have a narrow pore size distribution and larger pores than pure 5i02 gel7 but the reason for this and the role of the PEG in these inorganic-organic systems is not fully understood. Here we have studied the effect of the PEG concentration and molecular chain length and the time of its addition during preparation on the properties of the silica sol-gel and those of the coatings produced from these. This is a step in planned progress towards Si02/Zr02 multilayer stacks. Elsewhere PEG-doped Zr02 is shown to have beneficial properties3 and here the authors wanted to be sure of the effect of the PEG diffusing from the Zr02 layers on the properties ofthe adjacent silica layers. As high damage thresholds have been achieved for at least one high index component (i.e. Zr02)8 the work presented here was also aimed at reducing the amount of laserinduced damage resistance in the low index component as well as increasing the mechanical strength. EXPERIMENTALColloidal (3%v/) silica sol was prepared from the hydrolysis and condensation of tetra-ethyl-orthosilicate (TEOS, Aldrich) in water (hplc grade) and ethanol (Aldrich) catalyzed by ammonium hydroxide in the molar ratio of 1:4:37:0.001 at ambient conditions9. The pH of the system was 10.5-11.0. The 464 ISPIE Vol. 2633 0819419974/951$6.OO Downloaded From: http://proceedings.sp...