Preparation of thick films of (Pb,Cd)Sr2(Y,Ca)Cu2O7 using a mist pyrolysis technique

Abstract

Mist pyrolysis is a relatively new innovation in the technology of ceramic processing, but the use of related spray processes is well established [1]. The mist pyrolysis technique relies on the ultrasonic atomization of a feed solution to form solute droplets of uniformly small size, usually , 1 im, which are reacted in situ to form a fully-reacted ultra-®ne powder with a narrow size distribution. The formation of a fully-reacted product at the end of a single process is the major advantage of mist pyrolysis over other spray based techniques, and was ®rst developed for the preparation of thin ®lms of metallic compounds [2]. Since its invention, the technique has been widely researched in the ®eld of electro-ceramics for both the production of powders and the deposition of ®lms [3±6]. Mist pyrolysis has been applied to the fabrication of high Tc superconductors. Single step preparation of YBa2Cu3O7-a (YBCO) and (Bi2yxPbx)Sr2Ca2Cu3O10 a (BSCCO) has been reported by either using direct mist spraying onto a hot substrate or by particulate deposition [7±14]. Direct mist spraying involves the deposition of a mist onto a heated substrate maintained at relatively low temperatures (typically 673 K). The low temperature of the substrate invariably leads to the formation of amorphous or poorly crystallized ®lms but, when annealed, fully superconducting coatings are obtained. The fabrication of ®lms by particulate deposition involves the deposition of fully-reacted sub-micrometer-sized particulates onto a substrate which, after annealing, consolidates to form a thick ®lm coating. We have designed and built a mist pyrolysis system to fabricate thick ®lms of a new high temperature superconducting material (Pb1y yCd y)Sr2(Y1yxCax)Cu2O7 a ((Pb,Cd)-1212) by both direct mist spraying and particulate deposition. The system con®gurations used to achieve ®lm deposition by both these mechanisms are shown in Fig. 1. A three zone furnace was used, allowing a controlled temperature gradient to be set up in a 500 mm hot zone. A high volume nebuliser generated mist at a controlled rate which was fed into a 50 mm SiO2 reactor tube. The design of the nebuliser allowed a known volume of reactant solution to be placed in a plastic diaphragm which was immersed in a water bath, thus preventing any solution coming into contact with the ultrasonic transducer. The sample holder was constructed from Al2O3 and was positioned in the centre of the furnace hot zone for direct mist spraying and at the far end of the furnace for particulate deposition. A powder extraction=exhaust gas ®ltering system, consisting of a ®lter, dessicant jar, ow meter and diaphragm pump, was attached to the end of the reactor tube. The ®lter arrangement consisted of a disposable glass ®bre ®lter with nominal pore size 1 im held in position on a large pore sized sintered glass ®lter by the suction of the diaphragm pump. Direct powder synthesis was accomplished by passing 0.1 M nitrate or EDTA solution directly down the furnace and trapping the reacted particles in the ®lter at the collection end. We were able to retain 99.98% of < 0:3 im-sized particles under the optimized operating conditions of T ˆ 823 K and