J. Horvat

Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping

Doping of MgB2 by nano-SiC and its potential for the improvement of flux pinning were studied for MgB2−x(SiC)x/2 with x=0, 0.2, and 0.3 and for 10 wt % nano-SiC-doped MgB2 samples. Cosubstitution of B by Si and C counterbalanced the effects of single-element doping, decreasing Tc by only 1.5 K, introducing intragrain pinning centers effective at high fields and temperatures, and significantly enhancing Jc and Hirr. Compared to the undoped sample, Jc for the 10 wt % doped sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At 20 K and 2 T, the Jc for the doped sample was 2.4×105 A/cm2, which is comparable to Jc values for the best Ag/Bi-2223 tapes. At 20 K and 4 T, Jc was twice as high as for the best MgB2 thin films and an order of magnitude higher than for the best Fe/MgB2 tapes. The magnetic Jc is consistent with the transport Jc which remains at 20 000 A/cm2 even at 10 T and 5 K for the doped sample, an order of magnitude higher than the undoped one. Because o...

Flutelike Porous Hematite Nanorods and Branched Nanostructures: Synthesis, Characterisation and Application for Gas-Sensing

Flute-like porous alpha-Fe2O3 nanorods and branched nanostructures such as pentapods and hexapods were prepared through dehydration and recrystallisation of hydrothermally synthesised beta-FeOOH precursor. Transmission electron microscopy (TEM), high-resolution TEM and selected area electron diffraction analyses reveal that the nanorods, which grow along the [110] direction, have nearly hollow cavities and porous walls with a pore size of 20-50 nm. The hexapods have six symmetric arms with a diameter of 60-80 nm and length of 400-900 nm. The growth direction of the arms in the hexapod-like nanostructure is also along the [110] direction, and there is a dihedral angle of 69.5 degrees between adjacent arms. These unique iron oxide nanostructures offer the first opportunity to investigate their magnetic and gas sensing properties. The nanostructures exhibited unusual magnetic behaviour, with two different Morin temperatures under field-cooled and zero-field-cooled conditions, owing to their shape anisotropy and magnetocrystalline anisotropy. Furthermore, the alpha-Fe2O3 nanostructures show much better sensing performance towards ethanol than that of the previously reported polycrystalline nanotubes. In addition, the alpha-Fe2O3 nanostructure based sensor can selectively detect formaldehyde and acetic acid among other toxic, corrosive and irritant vapours at a low working temperature with rapid response, high sensitivity and good stability.

Effect of carbon nanotube doping on critical current density of MgB2 superconductor

The effect of doping MgB2 with carbon nanotubes on transition temperature, lattice parameters, critical current density and flux pinning was studied for MgB2−xCx with x=0, 0.05, 0.1, 0.2, and 0.3. The carbon substitution for B was found to enhance Jc in magnetic fields but depress Tc. The depression of Tc, which is caused by the carbon substitution for B, increases with an increasing doping level, sintering temperature, and duration. By controlling the extent of the substitution and addition of carbon nanotubes we can achieve the optimal improvement on critical current density and flux pinning in magnetic fields while maintaining the minimum reduction in Tc. Under these conditions, Jc was enhanced by two orders of magnitude at 8 T and 5 K and 7 T and 10 K. Jc was more than 10 000 A/cm2 at 20 K and 4 T and 5 K and 8.5 T, respectively.

High-transport critical current density above 30 K in pure Fe-clad MgB2 tape

Abstract Fe-clad MgB 2 long tapes have been fabricated using a powder-in-tube technique. An Mg+2B mixture was used as the central conductor core and reacted in situ to form MgB 2 . The tapes were sintered in pure Ar at 800°C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 μm. The Fe-clad tape shows a sharp transition with transition width of ΔT c of 0.2 K and a T c0 of 37.5 K. We have achieved the highest transport critical current reported so far at 1.7×10 4 A/cm 2 for both 29.5 K in 1 T and 33 K in zero applied field. Resistivity temperature dependence and transport critical current were also measured in magnetic fields applied perpendicular and parallel to the tape plane. Not only is the use of an Fe sheath necessary for the successful processing of in situ reacted powder-in-tube MgB 2 , it confers on the finished wire the additional benefit of magnetic screening.

Very fast formation of superconducting MgB2/Fe wires with high Jc

Abstract In this paper, we have investigated the effects of sintering time and temperature on the formation and critical current densities of Fe-clad MgB 2 wires. MgB 2 wires were fabricated using the powder-in-tube process and sintered for different periods of time at predetermined temperatures. All the samples were examined using XRD, SEM, and magnetisation measurements. In contrast to the common practice of sintering for several hours, the present results show that there is no need for prolonged heat treatment in the fabrication of Fe-clad MgB 2 wires. A total time in the furnace of several minutes is more than enough to form nearly pure MgB 2 with high performance characteristics. The results from T c , J c and H irr show convincingly that the samples which were sintered for 3 min above 800°C are as good as those sintered for longer times. In fact, the J c field performance for the most rapidly sintered sample is slightly better than for all other samples. J c of 4.5×10 5 A/cm 2 in zero field and above 10 5 A/cm 2 in 2 T at 15 K has been achieved for the best Fe-clad MgB 2 wires. As a result of such a short sintering there is no need for using high purity argon protection and it is possible to carry out the heat treatment in a much less protective atmosphere or in air. These findings substantially simplify the fabrication process, making it possible to have a continuous process for fabrication and reducing the costs for large-scale production of MgB 2 wires.

Superconductivity, critical current density, and flux pinning in MgB2–x(SiC)x/2 superconductor after SiC nanoparticle doping

We investigated the effect of SiC nanoparticle doping on the crystal lattice structure, critical temperature Tc, critical current density Jc, and flux pinning in MgB2 superconductor. A series of MgB2−x(SiC)x/2 samples with x=0–1.0 were fabricated using an in situ reaction process. The contraction of the lattice and depression of Tc with increasing SiC doping level remained rather small most likely due to the counterbalancing effect of Si and C co-doping. The high level Si and C co-doping allowed the creation of intragrain defects and highly dispersed nanoinclusions within the grains which can act as effective pinning centers for vortices, improving Jc behavior as a function of the applied magnetic field. The enhanced pinning is mainly attributable to the substitution-induced defects and local structure fluctuations within grains. A pinning mechanism is proposed to account for different contributions of different defects in MgB2−x(SiC)x/2 superconductors.

Flux jumping and a bulk-to-granular transition in the magnetization of a compacted and sintered MgB2 superconductor

In this paper, we report the results of field (H) and temperature (T) dependent magnetization (M) measurements of a pellet of uniform, large-grain sintered MgB2. It was found that iron is compatible with MgB2 and can be used as sheath material for fabrication of Fe-clad MgB2 wires. We show that at low temperatures the size of the pellet and its critical current density, Jc(H) – i.e. its M(H) – ensure low-field flux jumping, which of course ceases when M(H) drops below a critical value. With further increase of H and T the individual grains decouple and the M(H) loops drop to lower lying branches, unresolved in the usual full M(H) representation. After taking into account the sample size and grain size, respectively, the bulk sample and the grains were deduced to exhibit the same magnetically determined Jcs (e.g. 105 A/cm2, 20 K, 0 T) and hence that for each temperature of measurement Jc(H) decreased monotonically with H over the entire field range, except for a gap within the grain-decoupling zone.

Improvement of critical current in MgB2/Fe superconducting wires by a ferromagnetic sheath

Transport critical current (Ic) was measured for Fe-sheathed MgB2 round wires. A critical current density of 5.3×104 A/cm2 was obtained at 32 K. Strong magnetic shielding by the iron sheath was observed, resulting in a decrease in Ic by only 15% in a field of 0.6 T at 32 K. In addition to shielding, interaction between the iron sheath and the superconductor resulted in a constant Ic between 0.2 and 0.6 T. This was well beyond the maximum field for effective shielding of 0.2 T. This effect can be used to substantially improve the field performance of MgB2/Fe wires at fields at least three times higher than the range allowed by mere magnetic shielding by the iron sheath. The dependence of Ic on the angle between field and current showed that the transport current does not flow straight across the wire, but meanders between the grains.

Effect of (Pb,Bi)3Sr2Ca2CuOy phase on critical current density of Ag/(Bi,Pb)2Sr2Ca2Cu3O10 tapes

Abstract The phase evolution, critical current density ( J c ) and irreversibility field ( H irr ) for (Bi,Pb) 2 Sr 2 Ca 2 Cu 3 O 10 (2223) tapes have been studied through a two-step sintering process. The tapes sintered at 840°C without second-step annealing at 820°C contain a small fraction of (Bi,Pb) 2 Sr 2 CuO 6 (2201). 2201 phase vanishes when the tapes are annealed at 820°C for 30 h after sintering at 840°C. (Bi,Pb) 2 Sr 2 CuO 6 (2201) phase was found to be the predominant factor for the depression of J c of the tapes. The J c vs. magnetic field shows that the depression of J c occurs only in the low field region, indicating that 2201 phase act as weak links in the tapes.

Transport critical current density in Fe-sheathed nano-SiC doped MgB/sub 2/ wires

The nano-SiC doped MgB/sub 2//Fe wires were fabricated using a powder-in-tube method and an in-situ reaction process. The depression of T/sub c/ with increasing SiC doping level remained rather small due to the counterbalanced effect of Si and C co-doping. The high level SiC co-doping allowed creation of the intra-grain defects and nano-inclusions, which act as effective pinning centers, resulting in a substantial enhancement in the J/sub c/(H) performance. The transport J/sub c/ for all the wires is comparable to the magnetic J/sub c/ at higher fields despite the low density of the samples and percolative nature of current. The transport I/sub c/ for the 10wt% SiC doped MgB/sub 2//Fe reached 660A at 5K and 4.5T (J/sub c/=133000A/cm/sup 2/) and 540A at 20K and 2T (J/sub c/=108000A/cm/sup 2/). The transport J/sub c/ for the 10wt% SiC doped MgB/sub 2/ wire is more than an order of magnitude higher than for the state-the-art Fe-sheathed MgB/sub 2/ wire reported to date at 5K and 10T and 20K and 5T respectively. There is a plenty of room for further improvement in J/sub c/ as the density of the current samples is only 50%.