Sol-gel route to high T c ceramic precursors
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Sol-gel route to high Tc ceramic precursors T. E. Karis and J. Economy a) IBM Research Division, Almaden Research Center, 650 Harry Road, San Jose, California 95120 (Received 30 October 1989; accepted 17 April 1991)
An aqueous sol-gel procedure to obtain fine Y - B a - C u particles with an approximately 1-2-3 stoichiometric ratio is described. This involves precipitating an aqueous solution of the oxides in HC1 by neutralization with Ba(OH)2, and precipitation of BaCO 3 from solution with CO 2 . A novel treatment with H 2 O 2 decreased the weight loss while sintering. The average particle size and shape was characterized by rheology, light scattering, and scanning electron microscope. Particle composition was studied with x-ray diffraction, electron microprobe, and infrared spectroscopy. The suspension was formed into a fiber and sintered, and measurements were made indicating a superconducting transition temperature near 60 K.
I. INTRODUCTION The potential of liquid nitrogen temperature superconducting materials from Y, Ba, and Cu has given rise to numerous synthetic approaches to obtain materials with various microstructures. These approaches include repeated heating and grinding, sputter deposition, chemical vapor deposition, and a variety of precursor routes involving nitrates, oxalates, carbonates, citrates, cyanates, or hydroxides.1"21 All of these methods have met with some degree of success; however, most are limited in their ability to produce a mechanically viable product.8-20"23 Here we report an alternative route to the ceramic, using an aqueous sol-gel precipitation method.6'24"26 In the past this route has met with little success due to the partial solubility of Ba2+ in water.19 We circumvent this problem by precipitating the Ba2+ with dry CO 2 . We then demonstrate that the process closely maintains the required 1-2-3 stoichiometry, and that the suspension can be formed into high Tc superconducting monoliths. II. EXPERIMENTAL A. Preparation Ten grams of the dry powdered oxides (Y 2 O 3 , BaO, and CuO) were combined in a mortar and pestle to obtain the stoichiometric ratio Y 1 Ba 2 Cu 3 . Four grams of this mixture was then dissolved in 160 ml of 1 M HC1 held at 50 °C for several hours while stirring. This formed a clear blue solution. The solution was filtered while warm, and then rapidly neutralized with 160 ml of warm 0.5 M Ba(OH)2 in a 400 ml beaker while stirring. During neutralization, a bluish-green colloidal suspension of and YxOH 3a; formed. With continued stirring, a)
Current address: Department of Materials Science and Engineering, 1304 West Green Street, Urbana, Illinois 61801. J. Mater. Res., Vol. 6, No. 8, Aug 1991 http://journals.cambridge.org
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dry CO2 was bubbled through the suspension as it cooled to room temperature to precipitate the stoichiometric amount of BaCO 3 onto the particles. The suspension was washed four times with de-ionized water in a centrifuge to remove the BaCl2 formed during the neutralization step. The final precipitate was blue-green i
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