Phase formation and superconductivity of fe-tube encapsulated and vacuum-annealed MgB2
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Date
2006
Journal Title
Journal ISSN
Volume Title
Publisher
World Scienti fic Publishing Company
Abstract
We report optimization of the synthesis parameters viz. heating temperature (TH), and
hold time (thold) for vacuum-annealed (105 Torr) and LN2 (liquid nitrogen) quenched
MgB2 compound. These are single-phase compounds crystallizing in the hexagonal struc-
ture (space group P6=mmm) at room temperature. Our XRD results indicated that for
phase-pure MgB2, the TH for 105 Torr annealed and LN2-quenched samples is 750 C.
The right stoichiometry i.e., MgB2 of the compound corresponding to 105 Torr and TH
of 750 C is found for the hold time (thold) of 2.30 hours. With varying thold from 1{4
hours at xed TH (750 C) and vacuum (105 Torr), the c-lattice parameter decreases
rst and later increases with thold (hours) before a near saturation, while the a-lattice
parameter rst increases and later decreases beyond a thold of 2.30 hours. The c=a ratio
versus thold plot showed an inverted bell-shaped curve, touching the lowest value of 1.141,
which is the reported value for perfect stoichiometry of MgB2. The optimized stoichi-
metric MgB2 compound exhibited superconductivity at 39.2 K with a transition width
of 0.6 K. In conclusion, the synthesis parameters for phase pure stoichimetric vacuum-
annealed MgB2 compound are optimized and are compared with widely-reported Ta
tube encapsulated samples.
Description
Keywords
MgB2, Fe-encapsulations, superconductivity, vacuum synthesis