Critical currents in magnetically pressed MgB₂rods

Abstract

Using a magnetic compression method a 2.3 mm diameter superconducting MgB 2 rod was synthesized in a stainless steel tube. After the compression the MgB 2 density inside the tube reached d = 2.45 g cm −3 , i.e. very near to the theoretical value 2.62 g cm −3 . At T = 4.2 K and B = 0.5 T the quench current reached 870 A. The critical current density versus temperature was determined from electric and magnetic measurements.

Superconductivity in porous MgB2

“…Initial tablets ( 0.28 extruded in Fig. 2) have a small susceptibilities which goes to zero at a temperature equal to the critical temperature T co 38K corresponding to a bulk sample [5,6,7]. The weak diamagnetism in extruded sample is explained by the fact that MgB 2 particles in a magnesium matrix have no superconducting interface.…”

“…As seen in Fig.2 the dense samples (with > 0.7) exhibit a single superconducting transition. The susceptibility of the dense sample with = 0.94 [7] is labeled by asterisks. With increasing porosity (decreasing ), two superconducting transitions are observed.…”

“…It was noted that machining causes the formation numerous micro cracks and defects that disappear after annealing owing to recrystallization processes promoted by the Mg excess. The method of magnetic compression and the subsequent annealing allows one to obtain a highdensity MgB 2 rod with the 860 A current in a core of 2.3 mm diameter at T= 4.2K [7]. Scanning tunnel microscopy [8] of polycrystalline bulk samples has revealed the presence of a normal inter-grain layer in the form of an amorphous region extending over 5 to 20 nm.…”

“…Average susceptibility was calculated as <>= (B(Т)-H)/4Н. A standard dense pallet (=0.94) [7] was used to calibrate the <>. In such samples local induction is zero (B=0) at low temperature and fields, but observed value 4<χ(T)>0.75 was less than 1.…”

Equal channel angular extrusion (ECAE) of intermetallics

“…Even though the critical temperature of MgB 2 is three times lower than the mercury based high T C superconductors (HTSC) cuprates (134 K [24,112,[126][127][128], hot pressing [129], magnetic compression [25], shock wave consolidation [26]. The mechanical properties of MgB 2 bulks prepared by all the above techniques are detailed in Table 2.1.…”