Synthesis, Characterization and Ageing of MgB2

ABSTRACTWe studied the influence of sample preparation and defects in the superconducting properties samples using atomic ratios of Mg:B=1:1 and Mg:B=1:2. Samples were characterized by SEM, and XRD, and the magnetization properties were examined in a SQUID magnetometer. The presence of Mg vacancies was determined by Rietveld analysis. Most of the samples exhibited sharp superconducting transitions with Tcs between 37–39 K.We found a strong correlation between the crystal strain and the Tc. This strain was related to the presence of Mg vacancies. In addition, results showed that some samples degraded with time when exposed to ambient conditions. In these samples the Tc did not change with time, but the superconducting transition became broader and the Meissner fraction decreased. This effect was only present in samples with poor connectivity between grains and smaller grain sizes. The degradation was related to a surface decomposition as observed by X-ray Photoelectron Spectroscopy. No correlation was found between this effect and the presence of Mg vacancies.

Two-zone annealing of Tl0.5Pb0.5(Sr0.8Ba0.2)2Ca2Cu3Oy

Stable conditions have been identified for two-zone processing of the superconducting thallium cuprate Tl0.5Pb0.5(Sr0.8Ba0.2)2Ca2Cu3Oy. With P(O2) of 0.8 atm, P(Tl2O) of 4.4 × 10−3 atm, and a sample temperature of 920 °C, single-phase Tl0.5Pb0.5(Sr0.8Ba0.2)2Ca2Cu3Oy, is produced with a Tc of 115 K, complete diamagnetic shielding, and Meissner fraction greater than 70%. Although a small amount of melting occurs under these conditions, a comparison of the low-field diamagnetic shielding for these samples with samples of Pb- and Sr-free TlBa2Ca2Cu3Oy and Tl2Ba2Ca2Cu3Oy, suggests that such melting is not necessary to produce the triple-CuO2-layer superconductors.