Preparation of high-Tc superconducting ceramic/polymer composites using gamma radiation

Abstract

High temperature superconducting materials, like many other ceramics, are inherently brittle in nature and therefore pose problems in converting them into simple usable forms such as flexible wires, tapes, sheets, tubes etc. This difficulty has been overcome to some extent by an ingenious use of high temperature superconductor (HTSC)/polymer composites with proper choice of polymeric materials to achieve the desired physical properties such as good flexibility and mechanical strength [1–4]. The polymers used were mostly thermoplastics such as polymethyl methacrylate (PMMA), silicone rubber, polyvinyl chloride, polyethylenes etc. [5–7]. In the synthesis process the solid polymer granules were blended with the superconducting ceramic and other additives such as antioxidants and plasticizers, and processed into sheets on a roll mixing machine. The resultant polymers showed good flexibility and mechanical strength. However, a blend of ceramic particles and polymer granules has an inherently heterogeneous character. In order to improve the homogeneity, we therefore blended liquid polymers with superconducting materials, whereby the coating can be made more homogeneous. Subsequently the polymer can be converted to a hard yet flexible system by radiation curing. Our new technique can produce materials with a wide range of properties. We synthesized Y123-polymer composites with copolymer of styrene, butyl acrylate and ethyl hexyl acrylate using thermal initiation followed by gamma radiation curing. These composites could have applications in the areas of shielding and levitation since the bulk conductivity values are very low at liquid nitrogen temperatures. The YBa2Cu3O7yx powder was prepared by a wet process developed in our laboratory (unpublished work). Oxalates of yttrium and copper were coprecipitated from acetate solutions and the mixed oxalates were treated with barium acetate solution to give a uniform slurry. The slurry was further evaporated to dryness to give a free-flowing blue powder. After initial firing at 200 8C for 4 h, the material was further calcined at 850 8C for 18 h. Finally, the product was heated to 950 8C under oxygen for 4 h followed by slow cooling to 400 8C. The product was further annealed in oxygen for 4 h and finally cooled to room temperature. Formation of single-phase product (orthorhombic) was confirmed by X-ray diffraction (XRD) using Ni filtered CuK radiation on a Philips pw 1710 diffractometer. Scanning electron microscope (SEM) microstructural analyses of selected samples were prepared at 25 kV (Jeol T330A). A.C. magnetic susceptibility measurements were carried out in the temperature range 75– 100 K using an EG & G PAR 5208, two-phase lockin amplifier. The room temperarure resistance to the samples was of the order of kilo-ohms. A number of copolymer compositions were tried for the purpose of blending with the superconducting ceramic particles: (i) styrene ‡ butyl acrylate ‡ methyl methacrylate in the ratios of 1:7:2 (vol %) and 1:5:4 (vol %) (ii) butyl acrylate ‡ methyl acrylate in the ratio of 1:1 (vol %) (iii) styrene ‡ butyl acrylate ‡ ethyl hexyl acrylate ‡ methyl methacrylate in the ratio of 1:7:1:1 (vol %) and (iv) styrene ‡ butyl acrylate ‡ ethyl hexyl acrylate in the ratio of 5:4:1 (vol %). Preparation of the ceramic/polymer composites was carried out in two steps. In the first step, partial copolymerization of the monomers was carried out to ,75% conversion by heating the mixture in the given proportions at 80 8C with 3% benzoyl peroxide in nitrogen atmosphere until a viscous liquid solution was obtained. The copolymers of compositions (i) and (ii) gave sticky and soft polymers and were not tried for the composites. Compositions (iii) and (iv) gave good copolymers. However, composition (iv) was chosen to keep to the minimum the number of compositional variations. In the second step, the viscous copolymer was mixed with superconducting material (grain size ,10 im) in various proportions by weight (25, 50 and 75 wt %) to form a slurry. It is advisable to use a paint mill to get a thorough and homogeneous mixture of the composite. A crosslinking agent (0.5%) was included to achieve the required mechanical strength of the final composite. This slurry was then poured into moulds of 1 in. (2.54 cm) diameter and 8 in. (0.32 cm) height, which were then irradiated with gamma rays (total dose of 8 Mrads) under nitrogen atmosphere. The composite was found to have good flexibility and mechanical strength and did not break on bending. To study the effect of grain size on the superconducting properties, composites were synthesized with Y-123 of three different mesh sizes (10, 18 and 85). The various ceramic/polymer composites prepared were characterized by XRD, SEM, and a.c. magnetic susceptibility. Fig. 1a shows the XRD pattern for Y-123, which indicates that the parent material used in this study was single phase and orthorhombic. Fig. 1b–e shows