The authors have fabricated a novel MgB2 superconducting wire using the in-situ powder-in-tube method with fine boron powder (0.25 μm) and coarse magnesium powder (150 μm). The core of the wire has a fibrous structure consisting of numerous discontinuous MgB2 filaments. The critical current density Jc increases as wire size decreases, and the Jc of a 1.00 mm wire is 1.4×104 A/cm2 at a temperature of 20 K in a magnetic field of 4 T.
Dense MgB2 bulk superconductors were prepared by a diffusion method in sealed stainless tubes, into which magnesium and boron powders were separately packed. After diffusion reaction at a temperature above 780°C for a long time, dense MgB2 bulks with densities of 2.35˜2.60 g cm-3 formed at the initially boron packed part. The critical current properties of the dense bulks were high, and the highest Jc was more than 0.8 MA cm-2 at 20 K in low magnetic fields. This value is almost threefold higher than that of conventionally processed MgB2 bulks with low densities of ˜1.3 g cm-3. A largely increased effective current path is essential for enhanced Jc characteristics. Doping of boron, carbon and silicon-carbide further improved the critical current properties of the dense MgB2 bulks, particularly under high magnetic fields. This is mainly due to the substitution effect of carbon for the boron site. In addition, starting from premixed MgB2 and boron powders was found to be effective for suppressing the generation of cracks, thereby resulting in the successful fabrication of long and dense MgB2 plates. The newly developed diffusion method can provide homogeneous and dense MgB2 bulks that are suitable for basic research on flux pinning properties including doping effects, as well as for application to large current carrying devices such as current leads.
The relationship between microstructures and critical current properties of magnesium diboride (MgB2) bulks was studied. MgB2 bulks synthesized by the solid-solid reaction of magnesium and boron below the melting point of magnesium (650°C) exhibited excellent critical current properties up to high magnetic fields. Low-temperature long-time heat treatment was found to be effective for forming a strongly connected MgB2 matrix with small grain size and poor crystallinity, and a very high Jc of 4.0×105 A/cm2 was achieved at 20 K. Furthermore, the strong relationship between Hirr and the crystallinity of MgB2 bulks was discovered. The Hirr was found to increase with an increase of full-width at half-maximum, FWHM, of MgB2 (110) peak, which corresponds to distortion in the honeycomb boron sheet. Excellent Jc characteristics under high magnetic fields were observed in samples with large FWHM of (110) due to the enhanced intraband scattering and strengthened grain boundary flux pinning. The relationship between crystallinity and Hirr explains the large variation of Hirr for MgB2 bulks, tapes, single crystals and films.
As-grown MgB2 thin films were prepared by an electron-beam evaporation method. In order to enhance the critical current density Jc in magnetic fields, the MgB2 thin film was deposited in an O2 atmosphere. E-J characteristics were measured in magnetic fields that were applied in various directions to investigate O2 doping effect and the angular dependence of pinning properties. The values of Jc and the glass-liquid transition temperature Tg of the O2-doped sample were larger than those of the non-doped sample and had peaks in the magnetic field applied parallel to the c-axis. It was found that c-axis correlated pinning centers were introduced by the deposition of the MgB2 thin film in an O2 atmosphere. The distributions of the local critical current density Jcl were derived by fitting obtained E-J characteristics with the theoretical expression based on the percolation transition model. As a result, it was found that the distribution of Jcl of the O2-doped sample became broad and shifted to the high current density region as compared to that of the non-doped sample. This is because pinning centers with strong pinning force were introduced by the deposition in the O2 atmosphere.
As-grown MgB2 films were grown by MBE (Molecular Beam Epitaxy) apparatus on MgO(100) substrates under low-temperature and low-growth rate conditions using a molecular beam epitaxy. We confirmed the temperature range for MgB2 fabrication predicted by Liu et al. Under the conditions of substrate temperature 200 °C, B 0.03 nm/s and Mg/B=8, the films were highly c-axis oriented with a superconducting transition of 35 K. We measured the upper critical fields (Hc2) using a 30 T pulsed magnet and critical current density (Jc). Hc2 and Jc at 4.2 K were about 30 T and 10 MA/cm2, respectively.
The preparation of MgB2 thin films, which can grow with fairly good superconducting properties at temperatures below 300°C, was examined on heat-resistant plastic substrates such as polyimide film, making use of this low-temperature growth characteristic. To obtain high-quality MgB2 films even at low temperatures, the idea of epitaxial growth is essential, and a metallic buffer with a hexagonal crystal structure of the same lattice parameter as that of MgB2 was previously proved to be effective for this purpose. Here, we have examined a double-layered metallic buffer Ti/Ag for growing MgB2 with good crystallinity. We then clarified that this buffer, each layer being 50 nm-thick and fabricated at an ambient temperature, is useful for polyimide film substrates. Preliminary results on the deformation properties of MgB2/polyimide films are also described.
The authors prepared powder-in-tube PIT MgB2 tapes through an in-situ process using a steel sheath. Magnesium hydride MgH2 and amorphous boron powders were mixed to synthesize MgB2 superconductors. The mixed powder was encased in a steel tube and rolled into a monocore tape of about 5 mm in width and 0.5 mm in thickness. Carbide powders (i.e., SiC and TiC) were added to the core. The addition of SiC or TiC nano-sized powder has been found to enhance appreciably the Jc in MgB2 tapes. The Ic for the 5 wt% SiC- or 10 wt% TiC-added tapes reaches 100 A at 4.2 K and 7 T, which corresponds to a core Jc higher than 10 kA/cm2. The improvement in Jc in a higher magnetic field may be the result of the increase in upper critical field Hc2 due to the incorporation of nano-sized powders such as SiC and TiC. Furthermore, the core Jc for the hot-pressed MgB2 tapes with 5 wt% SiC added reached 40 kA/cm2 at 7 T, which is an enhancement by a factor of 4 compared to that of tapes without pressing during heat treatment. The higher Jc of hot pressed tapes results from improving the density of the MgB2 structures, which have less voids and good linkage between the MgB2 grains.
We investigated the applicability of Cu and Cu-alloy sheaths to MgB2 wires prepared by the in-situ synthesis powder-in-tube (PIT) method. Raw materials pellets of Mg and B mixed with ZrH2 and/or C powder were inserted into Cu, Cu-Zr, Cu-Cr, Cu-Sn and Cu-Be sheath tubes. We fabricated round MgB2 wires 1.0mm in diameter utilizing rotary swaging, drawing and two-axial rolling at room temperature. The wires were annealed at 500 - 800°C for 2 h in an Ar gas atmosphere. Compared with Cu sheath wires, the critical current characteristics of some of the Cu-alloy sheath wires were improved. For example, the four-probe transport measurements at 4.2 K in a self-field showed that a MgB2/Cu short sample with a 1.0mm diameter had an IC of 726 A (JC = 545 kA/cm2), while that of the MgB2/Cu-Cr sample was 846 A (JC = 635 kA/cm2). This improvement may be due to the higher core density of the MgB2 wires fabricated using Cu-Cr sheaths with higher hardness. Futhermore, decreasing the Cu:Core ratio in the cross-section of the MgB2/Cu wire from 4.9 to 2.0, is was possible to increase the IC from 726 A to 1219 A.
The mechanical properties of copper and copper alloy (Cu-Zr, Cu-Be and Cu-Cr) sheath in situ PIT-processed MgB2 superconducting wires were studied at room temperature (RT) and 4.2 K. The effects of stress/strain on the critical current (Ic) of the wires have also been studied at 4.2 K and in magnetic fields up to 5 T. It has been clarified that alloying the Cu sheath significantly increases the yield and flow stresses of the wires at both RT and 4.2 K. The 0.5% flow stresses of the Cu alloy sheath wire were 147-237 MPa, whereas that of Cu was 55 MPa. At RT, serration corresponding to multiple cracking was observed around a strain of 0.4% and the stress-strain curves saturated beyond that point. The strain dependence of Ic prior to the critical strain (εirr) was different depending on the magnetic field; being almost constant at 2 T and increasing with strain at 5 T. The Ic decreased beyond εirr, which is much larger for Cu alloy sheath wires as compared to Cu sheath wire. This is due to the difference in the residual compressive strain in the MgB2 core during cooling from the heat-treatment temperature to 4.2 K, which is determined through relaxation by yielding in the sheath materials. The transverse compression tests revealed that the Ic of the Cu alloy sheath wire did not degrade up to 314 MPa, which is also higher than that of Cu sheath wire.
We investigate the influence of lengthwise mechanical strain on the critical current (Ic), the strain effects, for three kinds of “powder-in-tube (PIT)" route MgB2 conductors. We tested in situ monocore MgB2/iron, ex situ monocore MgB2/stainless steel, and ex situ 14-filament MgB2/nickel tapes. Ic was measured for each sample as a function of lengthwise external strain in magnetic fields (B) at 4.2, 15, 20, and 25K using a U-shaped rig made of stainless steel. Even though the fabrication process and conductor configuration (number of filaments, width, and thickness) are different, the Ic-strain relations were essentially the same in all MgB2 samples. By increasing the strain, large and irreversible degradation occurred after linear and reversible Ic-strain relation up to the maximum of Ic (Icm). The slope of linear Ic-strain relation in the reversible regime, d(Ic/Icm)/d(strain), depends on both temperature and magnetic field. We discuss the relations between d(Ic/Icm)/d(strain) and magnetic fields normalized by the irreversibility field (Birr.) We also examined the strain effect of ex situ PIT monocore in Nb3Sn/stainless steel tape. The Ic-strain relation obtained is a parabolic one that has been reported so far for many kinds of multifilamentary Nb3Sn conductors. This indicates that the linear strain dependence of Ic in MgB2 conductors is an essential feature of MgB2.
We prepared MgB2 thin films on polished sapphire C-plane single-crystal substrates using an electron beam evaporation technique and various deposition conditions in order to investigate the flux pinning centers in MgB2. We confirmed that the grain boundaries of MgB2 crystals act as effective pinning centers by comparing the magnetic properties of the thin films prepared at different substrate temperatures while maintaining the stoichiometric composition. We also found that the Mg deficiencies or the B substitutions on Mg sites in MgB2 crystals could be effective pinning centers by comparing the magnetic properties of the thin films prepared at the same substrate temperatures while changing the chemical composition. Evaluating the MgB2 films deposited with O2 gas in the chamber, we found that MgO in the MgB2 films may also act as effective pinning centers.