The Effect of Cadmium Doping on Bscco Superconductors Prepared Via Coprecipitation Method

The coprecipitation technique was used in the preparation of cadmium doped Bi1.6Pb0.4Sr2Ca2Cu3O10 and Bi1.7Pb0.3Sr2Ca2Cu3O10 polycrystalline ceramic superconductors. In this study, the calcium site was subjected to Cd doping with nominal composition ranging from x=0 to x=0.1 at different sintering times. The superconducting properties of the samples have been investigated. The undoped samples, which exhibits TC(R=0) around 102 –104 K and TC-onset around 112-116 K, showed large flaky grains around 9-11 m in size which are randomly distributed. However, at longer sintering time (100 h) the undoped sample showed a better orientation as compared to that of the shorter sintering time (24 hr) sample. The calculated value of Josephson current, Io, obtained from the AC susceptibility data showed a much higher value (Io =100.04 A) as compared to the sample prepared by conventional method (Io =55.9 A). This indicates better grain connectivity and higher 2223 phase content, which was confirmed by SEM photographs. In addition, the nature of the ultra fine particles of the oxalate powders produced by coprecipitation method have increased the diffusion reaction and shortened the heat treatment procedure for the sample preparation, this leads to better superconducting properties as compared to the samples prepared by conventional solid state technique where its diffusion reaction requires high sintering temperatures for long duration and sometimes several grindings. The resistivity measurements showed the normal metallic behaviour followed by decreased in TC(R=0) as the cadmium concentration increased due to the decrease in the 2223 phase and an increase in the formation of 2212 phase. The temperature dependence of ac susceptibility data, shows the shifting of the onset diamagnetism towards lower temperature as the Cd concentration increased due to the presence of low-TC phase. The imaginary component, shows a shift in the intergranular coupling peak, Tp, towards lower temperature as the Cd concentration increased. Hence it can be deduced that the dynamic magnetic response of the samples are not only phase dependent but also dependant on the intergranular coupling. The calculated I0 which revealed the quality of the coupling of the grains, showed a decrease in its value as the cadmium concentration increased. The results of x-ray diffraction (XRD) patterns show that all samples doped with Cd contain 2212 peaks which correspond to the low-superconducting phase. The intensity of these peaks increases towards higher value, as the Cd concentration increases. The volume of 2223 phase decreases gradually as the Cd concentration increases. In addition, there is a possibility that either Cd2+ might have occupied other sites in the sample. When long sintering time was applied, the improvement in superconducting properties was obvious at low doping concentrations x=0.02 where the sample was still dominated by 2223 phase. Above that concentration, the grain size decreased and became shorter and thicker, randomly distributed as compared to the undoped samples. It is also observed that the superconducting properties and the microstructure improved when the sample was sintered for 48 hours and 100 hours, the high-TC phase dominates, indicating that the optimum time must be above 48 hours. The study shows that the substitution of cadmium in calcium site does not improve the TC of the BSCCO system. This is due to the formation of low-TC phase which weakened the coupling of the grains. However, all samples showed an obvious improvement in TC as the sintering time increases.

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