Synthesis of Er1-xAxBa2Cu3O7-δ (A=Ca, Zn, Pb and Nd) superconductor ceramics via coprecipitation and their electrochemical characteristics

ErBa2Cu3O7-δ is a type II high temperature superconductor, which is known as the 123-system. This type of superconductor is classified as the most stable high temperature superconductor. This dissertation presents the doping effect of diamagnetic elements (Zn and Pb) and paramagnetic elements (Ca and Nd) in the Er-123 System using the coprecipitation method as a procedure of synthesis. Electrochemical studies of free and doped samples have been done to emphasize the doping effect on the copper electrochemical behaviour in the superconductor. A series of Zn, Ca, Pb, and Nd were successfully doped in the Er-123 system using the co-precipitation method. Metal acetates were used as starting materials, which were dissolved in acetic acid. The precipitating agent, oxalic acid in an alcoholic solution of (water: iso-propanol) was used to form metal oxalates. Samples were dried and calcined for 24 hours at 900 °C before they were pressed into pellets and sintered for 24 hours at 920 °C. A second calcination and sintering period at 600 °C was applied for 2 and 3 hours, respectively. All heat treatments were carried out in the oxygen environment. The transport property of sintered samples was measured using the four-point probe electrical resistance measurement and AC Susceptibility. A scanning electron microscope and energy dispersive X-ray were used to identify the surface morphology and the chemical composition while the crystalline structure of the samples were determined using the X-ray diffraction (XRD) technique. Cyclic voltammetry was used to study the electrochemical behaviour in 0.1 M NH4Cl as a supporting electrolyte. The XRD pattern of the samples showed the presence of the secondary phase of 211 in the Zn-doped samples at x≥0.15 with 10-12 % peak intensity, while in the case of the Ca-doped samples a single-phase material was produced. Two phases of PbO2 and Pb3O4 were produced for the Pb-doped sample, while for the Nd-doped, the Nd-123 phase was recorded at x=0.15. The orthorhombic phases of all doped samples are similar to the pure sample. Lattice parameters slightly change due to the replacement of the substituted element in the structure. In the case of the Zn and Ca-doped samples, the substitution occurred at the Cu site and the substitution in the Pb-doped and Nd samples occurred at the Ba and Er sites, respectively. The TC values recorded were 85, 87, 85, and 91 K for 0.05 mole of doped Zn, Ca, Pb and Nd, respectively. These values decreased by 0.2 mole of dopant to 48, 68, 85 and 87 K. The resistance versus temperature curve shows that the higher content of Zn in Zn-doped samples (x≥0.15) lowers the critical resistance temperature, TC, below the liquid nitrogen temperature. A semiconducting-like response appeared for the Zn-doped and Ca sample at a higher content of dopant (x≥0.10). Where in the Pb and Nd-doped samples, a typical superconducting curve was produced. The microstructure study of the doped samples showed that the presence of the pores and boundaries between the grains reduced the TC value due to the air pockets and the weak-links, which resist the current to flow. This effect increased by the content of the increase of dopant concentration in the sample. The electrochemical study of the doped samples shows that the significant sensitivity depends on the pH and scan rate. The change of current has a semi-linear relationship with the acidity. The graph of current (I) versus scan rate (v) and current versus the square root of scan rate (v ½) were a straight line, which indicates that the electron transfer process is diffusion controlled and corresponds to an adsorption controlled process. The diffusion coefficient values increased with the content of dopant where the values recorded at 0.05 and 0.2 mole of dopant were (7.45×10-5, 8.67×10-5), (8.67×10-5, 1.07×10-4) and (7.42×10-5, 9.95×10-5) cm2/s for Ca, Pb and Nd, respectively, where the value of free doped sample ErBCO was 7.45×10-5 cm2/s. The value of charge increased by the content of doped element in the ErBCO system, where the recorded values were 808.01, 1230.91, 1042.92 and 990.61 μC/cm2 with 0.2 mole of doped Zn, Ca, Pb and Nd respectively, where the undoped sample value was 497.25 μC/cm2.

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