The Preparation of Bi-Pb-Sr-Ca-Cu-O Superconducting Powder for Thick Film Pastes
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THE PREPARATION OF Bi-Pb-Sr-Ca-Cu-O SUPERCONDUCTING POWDER FOR THICK FILM PASTES J. HAGBERG, A. UUSIMAKI, J. LEVOSKA, S LEPPAVUORI and R RAUTIOAHO* Microelectronics and Material Physics Laboratories and *Materials Engineering Laboratory,University of Oulu,OULU, FINLAND
ABSTRACT The oxalate route was used to prepare (Bi-Pb) -Sr-Ca-Cu-O powder with the nominal composition Bi1.7 5 Pb0.4 Sr 1 . 9 Ca 2 .1Cu3. 2Oy . This powder was decomposed/annealed at different temperatures between 434 and 795 0 C. They were investigated by thermogravimetric analysis, X-ray diffraction and transmission electron microscopy. Particle size distribution and surface area were also determined. The specific surface area decreased from a value of 16.5 m`/g for powder decomposed at 434 IC to 1.0 m2 /g for powder decomposed at 795 OC. The measured mean particle size reached a minimum of 0.52 pm for powder decomposed at 589 *C.
INTRODUCTION Thick film technology is used to produce 5 to 50 pm thickness films. Films are printed from a paste through a screen. The viscous paste for printing is made by mixing a powdery material with an organic vehicle. Printing accuracy and smoothness of the printed film is, in addition to the vehicle properties, a function of the morphology of the powder. If the particle size of the powder is large or if it includes large agglomerates, the smoothness of the film will be poor. In the worst case it will cause holes in the films. In a previous study [II, an oxalate powder decomposed at 600°Cwas used for paste preparation. Thick films printed from this paste were melt annealed above 885 *C and then at 852 °C over a long period. In order to improve printability, morphology and structure of the films, an investigation of the structure and morphology of the oxalate powders when decomposed at different temperatures has been conducted.
MATERIALS AND METHODS Oxalate powders were prepared by the precipitation method [2]. Bismuth nitrate was added to an aqueous solution of oxalic acid. An aqueous solution of nitrates of lead, strontium, calcium and copper was added to this solution. The magnetically stirred mixture was allowed to react over two days. The pH was regulated to 2.9 with
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ammonium hydroxide. The continuously magnetically stirred mixture was allowed to react for a further five days. The solution was then filtered, the residue washed and dried. The precursor powder was decomposed at temperatures from 434 *C to 795 *C for 12 h. Decomposition was carried out in aluminium oxide crucibles in normal atmosphere. All the annealed powders were studied by X-ray diffraction (XRD) using Cuc-radiation. Specific surface area was determined by the BET method. Pastes were made for particle size distribution analysis by adding 45 to 60 wt.%of solvent to the powder. The solvent was a mixture of 90 wt.% terpineol, 6 wt.% ethylcellulose and 4 wt.% antharox. To ensure paste homogeneity, they were milled about 20 times on a three roll paste mill. Ultrasonic st
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