Microscopy Studies of Sol-Gel Derived Lithium Niobate
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MICROSCOPY STUDIES OF SOL-GEL DERIVED LITHIUM NIOBATE
VIKRAM JOSHI*, GRACE K. GOO**, AND MARTHA L. MECARTNEY** * University of Minnesota, Dept. of Chemical Engineering and Materials Science, Minneapolis, MN 55455 ** Univeristy of California, Dept. of Mechanical and Aerospace Engineering, Irvine, CA 92717 ABSTRACT Lithium niobate prepared by sol-gel techniques can crystallize at temperatures as low as 200°C, and achieve a high homogeneity. In this sutdy, nucleation, crystallization, and morphological development are examined by TEM for alkoxide derived LiNbO 3. Preliminary studies show that the as-dried gel is completely amorphous and homogeneous. Between 150 200'C, crystallization begins with crystallites on the order of 10-20 nm appearing in the amorphous matrix. By 450'C, single crystal spheroidal particles (0.05 - 0.2 gm) form. By 6000 C, facetted large crystallites on the order of 1 gtm develop. Grain size and porosity are strongly dependent on the amount of water of hydrolysis in thin films. INTRODUCTION Lithium niobate is a very attractive material due to its good ferroelectric and electrooptic properties. It is a promising material for integrated optic devices due to its fast respond time and low absorption. LiNbO 3 has a very high Curie temperature, high electro-optic coefficients, and good acousto-optic properties. Its applications include usage in dielectric waveguides, Q-switches, integrated optics and many others. The interest in preparing the lithium niobate by sol-gel is due to the homogeneity and high purity it can achieve as a final product as well as the precise chemical stoichiometry which is extremely important for optimum electro-optic properties. In addition, thin films can be easily prepared by the sol-gel processing, at a relatively low temperature as compared to traditional film preparations. Several researchers have reported synthesis of LiNbO 3 powders and thin films using the solgel method [1-5]. The goal of this investigation is to study the nucleation and crystallization in sol-gel processed LiNbO 3 , and the effects of variations in temperature and water of hydrolysis on microstructural development. EXPERRIvENTAL PROCEDURE Figure 1 illustrates the flow chart of the preparation of LiNbO 3 bulk powders and thin films. For the bulk material, we have used a 1:2 alkoxide to water ratio for hydrolysis. The obtained gels were allowed to set at room temperature for a week. Heat treatment was done in air. The heating rate was 10°C/min. The temperature was held at 200°C, 400°C, and 600°C for 30 min each to allow for the removal of water and organics and finally complete crystallization. Samples were taken out for examination at different steps in the temperature profile. For the thin film 0studies, we have used 1:2 and 1:1 alkoxide to water ratio, and holding temperature of 400 C were used after dip coating onto glass substrate. Holding time of both 10 min. and 60 min. were used on both batches to study the amount of water of hydrolysis on nucleation and growth of the thin films. Routine TEM sa
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