Electrophoretic Deposition of High-Temperature Superconductor Thick Films
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ELECTROPHORETIC DEPOSITION OF HIGH-TEMPERATURE SUPERCONDUCTOR THICK FILMS J. B. MOONEY, A. SHER, M.L. RIGGS, K.A. SABO, A. ROSENGREEN, B. KINGSLEY, AND J.C. TERRY SRI International, 333 Ravenswood Ave., Menlo Park, California 94025. ABSTRACT Thick films of highi-temperature superconductors have been deposited electrophoretically using techniques developed for office copier toning. The resultant films are dense because of the high charge density per particle and the high fields during deposition. Orientation by electrostatic and magnetic fields has been demonstrated. INTRODUCTION High-temperature superconductors are being widely studied, and various techniques for the preparation of thick films have been reported including screen printing or painting, spraying, spin coating, and settling. Most of these processes have taken advantage of preprocessed superconducting powders to prepare the paint or suspension. The advantage of the preprocessed superconductors is that the two processes-superconductor preparation and film formation--can be separated using conditions that are specific for the material or the application. An additional advantage is that a single deposition process can be used with various prepared superconductors and can follow the evolving technology. TONER PREPARATION AND DEPOSITION Koura1 , Hein et al. 2 , and Chu et al.3 have reported on an electrophoretic deposition. We have been working on a similar method using techniques developed for liquid-toner-based office copiers. The toners are prepared by coating the particles with a binder, dispersing the coated particles in a dielectric liquid (a kerosene-type hydrocarbon), and adding a charge director. The charge director is ionized upon mixing with the binder-coated particles: the anion attaches to chemical sites on the binder surface, and the cation remains in the solution. The result is a colloidal suspension of negatively charged particles, with the counter ions maintaining charge neutrality. Except for particles that chemically react with the binder, the sole limitation appears to be that the particle size must be smaller than 10 gtm. This method of charge direction is not only universal, but it is more efficient than the usual method 1 in that the functional groups on the binder provide a large number of chemical sites for reaction with the charge director providing high charge-to-mass ratios and efficient deposition. The toner is deposited on an electrically conducting substrate by applying a positive potential to the substrate, with a counter electrode spaced 1 or 2 mm away. Typically, 4000 V is applied for 0.2 second. Following deposition and removal of the excess fluid, the deposit is cured by heating to about 1200C, a temperature at which the binder melts to form a coherent film. The binder provides green strength to the deposit until sintering. The electric field during deposition is on the order of 5 x 104 V/cm, but the effective fields are actually greater because of the difference in the mobility of the large toner particles and the small c
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