Synthesis, Characterization and Ageing of MgB2
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Synthesis, Characterization and Ageing of MgB2 A. Serquis1, R. Schulze2, Y. T. Zhu1, J.Y. Coulter1, D. E. Peterson1, N.O. Moreno3, P. G. Pagliuso3, S.S. Indrakanti4, V. F. Nesterenko4 and F. M. Mueller1 1 Superconductivity Technology Center, MS K763 2 Materials Technology Metallurgy Group, MS G755 3 Condensed Matter and Thermal Physics, MS K764 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM 87544, USA 4 Department of Mechanical and Aerospace Engineering, University of California, San Diego, La Jolla, CA 92093 ABSTRACT We 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. INTRODUCTION Since the discovery of superconductivity in MgB2 at 39 K [1], the highest Tc observed for a non-copper-oxide bulk superconductor, considerable progress has been made in the understanding of the fundamental properties of this material. MgB2 appears to be a suitable candidate for superconducting technologies currently based on Nb-alloys wires and films. However, to make practical devices using MgB2, it is essential to have a stable material. Taking into account that most high-temperature superconductors are highly sensitive to water and moist air it is important to explore the influence of exposure MgB2 to air and water. Zhai et al.[2] have studied the degradation of superconducting properties in MgB2 films by exposure to water and have observed that the Tc(onset) of the films remains unchanged throughout the degradation process. Nevertheless, there are no detailed studies on the chemical stability of MgB2, specially regarding its sensitivity to O2, H2O and CO2 in ambient conditions. Besides, different sample preparation methods lead to materials with different superconducting properties [3-5]. It is interesting to note that the origin of lower Tc of the in-situ prepared films, the possible non-stoichiometry and the effect of possible O contamination remain topics of debate. Cooper et al [6] proposed that the compounds with the AlB2-type structure could be non-stoichiometric. While Rogado et al [7] f
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