Strongly enhanced current-carrying performance in MgB2 tape conductors by C60 doping

Abstract

Abstract:MgB 2 is a promising superconductor for important large-scale applications for both high field magnets and cryocooler-cooled magnet operated at temperatures around 20 K. In this work, by utilizing C 60 as a viable alternative dopant, we demonstrate a simple and industrially scaleable rout that yields a 10~15-fold improvement in the in-high-field current densities of MgB 2 tape conductors. For example, a J c value higher than 4×10 4 A/cm 2 (4.2 K, 10 T), which exceeds that for NbTi superconductor, can be realized on the C 60 doped MgB 2 tapes. It is worth noting that this value is even higher than that fabricated using strict high energy ball milling technique under Ar atmosphere. At 20 K, H irr was ~10 T for C 60 doped MgB 2 tapes. A large amount of nanometer-sized precipitates and grain boundaries were found in MgB 2 matrix. The special physical and chemical characteristic of C 60 , in addition to its C containing intrinsic essence, is a key point in enhancing the superconducting performance of MgB 2 tapes.Keywords: MgB 2 , doping, current-carrying performance, C 60 * Author to whom correspondence should be addressed, E-mail: ywma@mail.iee.ac.cn 1The MgB 2 superconductor has aroused great research interest in the area of fundamental and applied research [1][2][3][4] In some case the C 60 molecule geometrical configuration can be broken and solo C atoms will be produced. The small molecule of C 60 gives a highly uniform mixture, while the high reactivity of freshly C substitutes B at low temperature. Therefore, C 60is an ideal dopant for MgB 2 composites to improve its J c -B performance.In this work, it is demonstrated that C 60 doping resulted in a significant enhancement of the J c -B performance over the entire applied high-field range. From the analysis of MgB 2 lattice parameter, the C substitution is actually happened in C 60doped MgB 2 tapes. Compared to nano-C doped samples, the sintering temperature is much lower to obtain the same C substitution level. The J c value is much higher in C 60 doped tapes than that of nano-C doped MgB 2 samples with similar irreversibility field (H irr ), showing different intrinsic property in these two type samples.C 60 -doped MgB 2 tapes were fabricated by the commonly reported powder-in-tube (PIT) method. Commercial powders of magnesium (-325 mesh, 99.8%), boron (amorphous, 2-5 μm, 99.99%) and C 60 (99%) were weighed out according to the nominal atomic ratio of (Mg 1.05 BB 2 ) 1-x (C) x with x = 0, 0.05, 0.08, 0.10, 0.15, respectively. When they were mixed in air for an hour, the powders were put into pure iron tubes with an outer diameter of 8 mm, and an inner diameter of 5 mm.After packing, the tubes were rotary-swaged and drawn to wires of 1.75 mm in diameter. The wires were subsequently rolled into flat tapes. It should be noted that all the tape processing procedures were carried out in the air condition. The final tapes were cut and raped in Ta foil, then sintered at 700-900°C for 1 h in flowing high purity Ar, followed by a furnace cooling to room temp...

The effect of reduced graphene oxide addition on the superconductivity of MgB2

“…As reported by many groups, carbon doping is a relatively simple and effective way of improving superconducting properties in MgB 2 . [2][3][4][5][6][7][8][9][10] Various types of carbon sources have been used as dopants for MgB 2 , and J c and H c2 have been significantly enhanced due to charge carrier scattering, thanks to the two-band nature of MgB 2 . Graphene is becoming recognized as a novel dopant for MgB 2 due to its specific way of improving J c , as it improves the intergrain connectivity, and at the same time, leaves micro-strains in the MgB 2 matrix which are beneficial for improving the flux pinning.…”

Significant improvement in critical current densities of C-doped MgB2tapes made by high-energy ball milling

“…Moreover, its J c is still up to 2.2 × 10 4 A cm −2 even at 4.2 K and 12 T, six times larger than that of the 10 h samples. The J c values are comparable not only to the best performance of C, SiC, C 60 or carbohydrate doped MgB 2 tapes made from amorphous boron [7,22,[33][34][35][36], but also to those of C 4 H 6 O 5 doped MgB 2 after cold high pressure densification tapes and mechanically alloyed C-doped MgB 2 /Fe samples with amorphous boron [8,11,16]. We should note that our J c values are lower than those of mechanically alloyed C-doped tapes using a harder Monel sheath [11].…”

“…Moreover, a large number of fine precipitates less than 10 nm are scattered throughout the MgB 2 grains of the 80 h sample (see figure 7(c)). These defects on the order of the coherence length can serve as effective pinning centers, which are also responsible for the enhanced flux pinning and J c in the 80 h MgB 2 [36,37]. Figure 8 shows the normalized pinning force as a function of magnetic field at 20 K of the different tapes sintered at 800 • C. The pinning force of MgB 2 tapes shows a shift to higher field with increasing milling time.…”

“…However, it is difficult to obtain commercially high purity amorphous B. In this work, by using the crystalline boron as one of the ball milled precursor powders, we got the MgB 2 tapes with J c being comparable to the best performance of C-containing doped MgB 2 tapes made from amorphous boron [7,22,[33][34][35][36]. More importantly, it is easy to get commercially crystalline boron powders.…”

Application Prospects of ${\rm MgB}_{2}$ in View of Its Basic Properties

“…Figure 2 Experimental data are shown for comparison (solid circles). They were extracted from literature in [5] and a view more recent data were added in this plot [27,28,29,30,31,32,33,34,35,36,37,38].…”