JOURNAL OF MATERIALS SCIENCE: MATERIALS IN ELECTRONICS 3(1992) 227-231 Electrical impedance of superconductor/ polymer composites J. DU, J. UNSWORTH, B. J. CROSBY Centre for Materials Technology, Faculty of Science, University of Technology, Sydney, NSW 2007, Australia Electrical impedance measurements were carried out on high-Tc superconducting ceramic/ polymer composite materials, in which superconducting particles are embedded in a polymer matrix. The results for the impedance magnitude and phase angle versus frequency (1 kHz-1 MHz), temperature (50-290 K) and volume percentage of superconductor (0-70%) are presented. The results reveal that the a.c. electrical properties of the new composite materials go through a large change at volume percentage of powder filler around 40%, which indicates that particle-particle contacts are partially formed. Far below this composition the composite is an insulator, and above this composition the composite behaves as a semiconductor. No marked transition in impedance and phase angle was observed when the material went through the superconducting transition temperature. 1. Introduction Hard, brittle high-T~ superconducting ceramics can be incorporated into polymer matrices [1-5] to produce composite materials with superior mechanical proper- ties, greater machinability, processability and flexi- bility. In particular, 0-3-type composites, in which superconducting ceramic powders embedded in poly- mer matrices~ are extremely flexible. They can be easily formed into various useful shapes with versatile poly- mer processing techniques, such as compression, ex- trusion and injection moulding. Studies [4, 5] have shown that even though there is no d.c. zero-resistance transition in 0-3 superconducting composites, the diamagnetic properties are preserved and large lev- itation forces are observed. This suggests that 0-3 superconducting composites could be very promising materials for superconducting applications. We have not yet seen any publication on electrical impedance measurements on these new supercon- ducting composites. It is important to study the a.c. properties of these new materials, which would give information about their microstructures, composite dielectric behaviour, and frequency and temperature dependence. In this way we can evaluate their possible applications in electronics. We fabricated YBa 2Cu30 7- x/thermop lastic com- posites with a wide range of compositions. We report in this paper the impedance measurement results for these composites. Measurements were carried out at frequencies between 1 kHz and 1 MHz and temper- atures between 50 and 290K for various volume percentages (0-70% YBCO) composites. 2. Sample preparation Sintered bulk YBa2Cu3Ov_= (123) was milled into fine powder. The 123 powder was mixed with poly(vinyl chloride) (PVC) powder containing 5% antioxidant and 10% plasticizer, and was processed into composite sheets on a two-roll mixing machine. The composite sheets were then cut into pieces and hot pressed into discs with a diameter of 25 mm in a cylindrical mould. The volume percentage of super- conductor powder ranged from 0 to 70%. Basic evaluation of the microstructure, d.c. electrical and magnetic properties for these composites can be found elsewhere [4]. Scanning electron microscope (SEM) micrographs showed their structure to be homogen- eous and X-ray diffraction showed that only super- conducting phase was detectable in the composites [-4]. D.c. electrical measurement [4] showed that there was no d.c. zero-resistance transition but a typical percolation curve for the room-temperature resistivity versus the volume fraction with pronounced decrease of resistivity between 30 and 50% (see Fig. 1). All disc samples were lapped down to 2.5 mm thick- ness and coated with silver on both sides with a d.c. E v t,w 10~2 10~0 lOS 10 ~ 10 ~ 10~ 10 ~ 100 \ \ \ 0 20 40 60 80 Volume fraction of 123 (%) Figure 1 Room-temperature d,c. resistivity versus volume fraction of I23 powder for the 123/PVC composites. The percolation starts at around 30 vol %. 0957-4522 ,~) 1992 Chapman & Hall 227