Freestanding thick films of YBa 2 Cu 3 O 6.9 by screenprinting techniques
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I. INTRODUCTION The recent discovery1'2 of superconductivity above 90 K has led to considerable attention on the production of single phase YBa 2 Cu 3 0 6 9 (123 material) with optimum electrical and mechanical properties. The practical application of these high temperature superconductors can be in the form of large and small-scale devices. For a number of these applications, especially in the sensor and detector areas, for example IR detectors, freestanding films with very low thermal masses are desirable. In this paper we describe a screen printing technique capable of producing freestanding films with properties comparable to those of bulk ceramic 123 material. A preliminary version of this work has been published.3
II. SYNTHESIS AND MICROSTRUCTURE Pellets of 123 material were synthesized by taking stoichiometric mixtures of dry CuO, BaO, and Y2O3 powders, pressing them into pellets at 5.6 kbar, sintering in air for 4 h at 950 °C, and then annealing under 640 Torr of oxygen for 12 h at 900 °C followed by a 5-h slow cool. X-ray powder patterns showed single phase 123 material with all of the observed diffraction lines accounted for by an orthorhombic unit cell with a = 3.825, b = 3.883, and c = 11.680 A. The fully processed 123 ceramic was then ground in an agate mortar and sieved through a 20 jam standard screen. Scanning electron micrographs showed the powder to consist primarily of grains with diameters between 5 and 10 jam. Powders were also ground under n-butanol or n-heptanol in an agate mortar and pestle. The resultant powders had average particle diameter between 1 and 2 ptm with the maximum particle diameter near 4-5 jam. These solutions were directly screen printed onto precleaned fused silica substrates through 20 /am or smaller metal or polyester mesh. Dry ground powders were similarly printed after an ink was prepared by suspension of the YBa 2 Cu 3 0 6 9 powders in spectral grade n-butanol. The
alcohols were removed from the printed powders by air drying (150 °C), and the film and substrate were placed in a box furnace in air at 950 °C for 30 min to resinter the grains. An interfacial reaction between the 123 material and the silica substrate occurs at this point that efficiently debonds the resintered film upon cooling. The films adhered strongly to other substrates such as alumina, sapphire, and strontium titanate. To assure optimum superconducting properties, the freestanding films were then annealed in 640 Torr of oxygen for 12 h at 900 °C followed by a 5-h slow cool to room temperature. Films have been grown in this fashion with thicknesses from 20 to 300 jam and areas up to 10 cm2. To obtain films thinner than 20 jam, finer particle size will be required and the resultant mechanical strength of the film will become a problem. The powders wet ground under n-heptanol or n-butanol suspend much better and under identical processing protocol produce denser and apparently better sintered films. The dry ground films were 65-75% of theoretical density while the wet ground films were 75-85% of theoret
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