Stabilization, Characterization and Optical Applications of Niobium and Tantalum Oxide Sols Prepared Via Alkoxide Routes
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STABILIZATION, CHARACTERIZATION AND OPTICAL APPLICATIONS OF NIOBIUM AND TANTALUM OXIDE SOLS PREPARED VIA ALKOXIDE ROUTES
STEPHEN PARRAUD*, LILIANE G. HUBERT-PFALZGRAF* AND HERVE FLOCH** *Laboratoire de Chimie Mol6culaire, Associ6 au CNRS, Universit6 de Nice-Sophia Antipolis, Parc Valrose, 06034 Nice, France "*Commissariat A l'Energie Atomique, Centre de Limeil-Valenton, 94195 Villeneuve St Georges Cedex, France
ABSTRACT Hydrolysis of niobium and tantalum pentaethoxides in ethanol and in the presence of basic (ammonia, tetramethylammonium hydroxide, di and triethylamine) or acidic (nitric or hydrochloric acids) additives was investigated. Triethylamine gives monodispersed, small particles of amorphous M2 0 5 ,nH 2 0 (M = Nb, Ta). These colloidal suspensions were used to obtain thin films by spin-coating techniques. The coatings display a thickness of 100-300 nm and a refractive index around 1.7 . Laser damage tests at 1064 nm wavelength with a pulse length of 3 ns were carried out on the single layer systems. The threshold values (one-onone) are in favor of the Ta2 0 5 coatings with an average of 14.5 ± 2.1 J/cm 2 , by comparison with 8.3 ± 1.6 J/cm 2 for the Nb 2 0 5 films. INTRODUCTION Two methods are commonly used to prepare coatings by the sol-gel process [1]. The first consists in an application of a precursor solution to a substrate with subsequent conversion of the precursor to an oxide on the surface; this usually requires water and heat. The second consists in an application of a colloidal suspension (sol) of a chemically converted oxide to a substrate with subsequent evaporation of the suspending medium at room temperature. The first method is in fact the most convenient one in view of the number of readily available suitable precursors, in contrast to the difficulty in preparing adequate colloidal oxide suspensions. However, it is now well established that this second method offers a route to films of much less internal stress, which can be easily stacked, and thus to antireflection coatings having a higher laser damage resistance than those conventionally deposited (PVD) [2]. This paper deals with our efforts directed towards the preparation of suitable colloidal suspensions of niobium and tantalum oxides. The availability of monodispersed M2 0 5 oxides should also facilitate their various other applications, for instance as high frequency oscillators, ceramic filters, composite fillers or catalysts [3]. Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
398
EXPERIMENTAL PROCEDURE The alkoxides [M(OEt) 5] 2
M = Nb, Ta were synthesized according to the literature
and purified by distillation [4]. Hydrolysis of M(OEt) 5 (M = Nb, Ta) in ethanol (0.2 M) was achieved by adding a similar volume of an ethanol solution of water (8- 40M) and triethylamine (0.018-0.05M. Vigorous stirring was maintained during the hydrolysis and a fluid, translucent, slightly blue, stable sol was obtained. Particle sizes were estimated by sedimentation measurement (L 100 nm) or
by
transmission electron m
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