Improvement of the critical current density of spark plasma sintered MgB₂by C₆₀addition

Abstract

We investigated the influence of fullerene (C 60 ) addition on the superconducting parameters of MgB 2 bulk samples obtained by spark plasma sintering (SPS). It was found that 1.5 wt% C 60 addition leads to the reduction of the critical temperature by a few K, whereas the magnetically determined critical current density increases by approximately one order of magnitude in an applied magnetic field H = 70 kOe at temperature T = 10 K. This indicates substitution of B by C in C 60 added MgB 2 processed by SPS, contrary to some data reported in the literature obtained using conventional methods. For our relatively large specimens, the occurrence of macroscale flux jumps was observed over a wide (H, T ) domain. The relaxation of the irreversible magnetization suggests that the macroscale flux jumps transform into microscale flux jumps with increasing H and T .

Microwave Synthesis of Fullerene-Doped MgB2

“…Figure c shows the relaxation curves at H = 4 × 10 6 A/m obtained for the B6 (C 60 -doped) MW sample at 6 and 10 K. The applied magnetic field is above the low- H range where the macroscopic flux jumps appear on the m ( H ) curves in the T domain. The downward curvature of the relaxation m ( t ) curves (Figure c) for the MW sample can be explained by associating effects of the thermally activated flux creep with a cumulative effect of the microscopic flux jumps in the MW sample . The transformation of macro scale flux jumps into micro scale flux jumps with increasing T and H is in agreement with the stability diagram .…”

“…Table presents some data of the pellets obtained by MW. We used a sample of C 60 -doped MgB 2 produced by the spark plasma sintering method (sample B7) as a reference . It should be noted that a detailed comparative analysis between C 60 -doped samples B6 and B7 is not possible because processing routes and techniques are very different, and also starting compositions are not identical.…”

“…It is worth mentioning that previous work on conventional in situ powder-in-tube processing of Mg, B, and C 60 mixtures was not successful, so that the critical current density, J c , and irreversibility field, H irr , remained constant. On the other hand, unconventional ex situ spark plasma sintering (SPS) applied to mixtures of MgB 2 and C 60 powders produced excellent B-substituted samples with enhanced J c and H irr . In the first case, it was expected that fullerene (C 60 ) would provide a more efficient B substitution with carbon leading to the enhancement of properties because of its low decomposition temperature.…”

Enhancement of the critical current density in FeO-coated MgB2 thin films at high magnetic fields

“…The obtained value of J c ~ 10 5 A/cm 2 at T = 20 K and B = 7 T (see Fig. 6) is higher than the J c value of 10 4 A/cm 2 at T = 5 K and B = 7 T, and the J c value of 10 2 A/cm 2 at T = 10 K and B = 7 T reported recently in [13], which was obtained by doping of MgB 2 with C 60 .…”

Vortex pinning properties and microstructure of MgB2heavily doped with titanium group elements

“…Chemical doping or substitution is one of the solutions to improve the vortex pinning properties of MgB 2 . Substituting carbon (C), derived from various carbon sources including SiC [11], B 4 C [12], the fullerene (C 60 ) [13], carbohydrates [14] and so on, for the B-site enhances the upper critical field, m H 0 c2 , by shortening the coherence length, resulting in the highly enhanced J c in higher magneticfields. In addition to the carbon, the doping of titanium group elements (TGE; Ti, Zr, Hf) also significantly improved the J c -properties of MgB 2 under high magnetic fields [8,[15][16][17][18][19], in which the optimal TGE-doping level of 1%-20% depended on the species of TGE.…”