Browsing by Author "Nigam, Rashmi"

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    Phase formation and superconductivity of fe-tube encapsulated and vacuum-annealed MgB2
    (World Scienti fic Publishing Company, 2006) Singh, K. P.; Awana, V. P. S.; Shahabuddin, M. D.; Husain, M.; Saxena, R. B.; Nigam, Rashmi; Ansari, M. A.; Gupta, Anurag; Narayan, Himanshu; Halder, S. K.; Kishan, H.
    We report optimization of the synthesis parameters viz. heating temperature (TH), and hold time (thold) for vacuum-annealed (10􀀀5 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 10􀀀5 Torr annealed and LN2-quenched samples is 750 C. The right stoichiometry i.e., MgB2 of the compound corresponding to 10􀀀5 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 (10􀀀5 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.