PHASE FORMATION AND SUPERCONDUCTIVITY OF Fe-TUBE ENCAPSULATED AND VACUUM-ANNEALED MgB₂

Abstract

Lesotho Southern AfricaWe report optimization of the synthesis parameters viz. heating temperature (T H ), and hold time (t hold ) for vacuum (10 -5 torr) annealed and LN 2 (liquid nitrogen) quenched MgB 2 compound.These are single-phase compounds crystallizing in the hexagonal structure (space group P 6 /mmm) at room temperature. Our XRD results indicated that for phase-pure MgB 2 , the T H for 10 -5 torr annealed and LN 2 quenched samples is 750 0 C. The right stoichiometry i.e., MgB 2 of the compound corresponding to 10 -5 Torr and T H of 750 0 C is found for the hold time (t hold ) of 2.30 hours. With varying t hold from 1-4 hours at fixed T H (750 0 C) and vacuum (10 -5 torr), the c-lattice parameter decreases first and later increases with t hold (hours) before a near saturation, while the alattice parameter first increase and later decreases beyond t hold of 2.30 hours. c/a ratio versus t hold plot showed an inverted bell shape curve, touching the lowest value of 1.141 which is reported value for perfect stoichiometry of MgB 2 . The optimized stoichimetric MgB 2 compound exhibited superconductivity at 39.2 K with transition width of 0.6 K. In conclusion, the synthesis parameters for phase pure stoichimetric vacuum annealed MgB 2 compound are optimized and are compared with widely reported Ta tube encapsulated samples.

Physical property characterization of bulk MgB2 superconductor

“…The lattice parameters are though close to our vacuum annealed MgB 2 [27], the presence of MgO is comparatively less in Argon annealed sample, as seen by XRD pattern (Fig.2). As far as the presence of minute amount of MgO is concerned, the same is seen in earlier reports as well [2,15,[25][26][27][28][32][33][34][35][36][37], but not always marked on the respective XRD pattern.…”

“…Magnetization measurements are carried out with a Quantum-Design 14 Tesla Physical Property measurement System (PPMS) having Vibrating Sample Magnetometer (VSM) attachment. , corresponding to pure MgO [2,17,[25][26][27][28], the rest of the Bragg reflections are characteristic of the hexagonal MgB 2 structure. MgO peak is marked in the pattern in Fig.…”

“…In this case the Fe tube, which contains the raw MgB2 pellet inside, was sealed inside a quartz tube at high vacuum of 10 -5 Torr. The encapsulated raw MgB 2 pellet is than heated at desired heating temperatures (750 0 C) with a hold time (3 hours) and is finally quenched in liquid nitrogen (LN 2 ) [26,27]. However we realized that the vacuum annealing method results in highly porous samples [28], which in turn adversely affect the transport properties like electrical and thermal conduction, in particular.…”

“…2. Except for a weak reflection at 2θ = 63 0 , corresponding to pure MgO [2,17,[25][26][27][28], the rest of the Bragg reflections are characteristic of the hexagonal MgB 2 structure. MgO peak is marked in the pattern in Fig.…”

Physical property characterization of Fe-tube encapsulated and vacuum annealed bulk MgB2

“…Our MgB 2 samples were synthesized by encapsulation of well mixed and palletized high quality (above 3 N purity) Mg and B powders with some added Mg turnings in a soft Fe-tube and its subsequent 3 heating to 750 0 C for two and half hours in an evacuated (10 -5 Torr) quartz tube and quenching to liquidnitrogen temperature. Details of synthesis procedure are given elsewhere 9,10 . In our case the resultant sample was a bulk polycrystalline compound.…”