Recent studies on MgB2 superconductors are reviewed. MgB2 single crystals show Bc2 anisotropy factors of 2.6—4.5. The Bc2 of a single crystal at 4.2 K is ∼15 T when the field is parallel to the ab plane. MgB2 bulks, wires and tapes show a higher Bc2 than single crystals. This Bc2 enhancement can be explained by the introduction of defects that decrease the coherence length. MgB2 thin films show a much higher Bc2 than bulk materials. The highest Bc2, 49 T at 0 K, was reported for highly resistive MgB2 thin film. One of the effective pinning centers in MgB2 seems to be grain boundary. Effective pinning centers are also introduced by nanometer-size impurity additions such as SiC. SiC addition significantly increased Jc values in the high field regions, and hence Birr. Most tapes and wires are now fabricated by a powder-in-tube (PIT) method. The ex-situ PIT method employs MgB2-reacted powder and hard sheath materials such as stainless steel. The Jc of ex-situ tapes sensitively depends on the packing density of the MgB2 core. A maximum Jc of ∼4.5×105 A/cm2 is obtained at 4.2 K and zero fields. Ex-situ processed tapes show Jc anisotropy with respect to the field orientation due to the grain alignment. The in-situ PIT method employs a powder mixture of Mg and B. High-energy ball milling and mechanical alloying of the starting powder mixture enhanced the Jc values. The replacement of MgH2 with Mg is effective for improving the reactivity and enhancing Jc values. The Jc at 4.2 K and zero magnetic field reached 2—3×106 A/cm2. The highest Jc, 2.5×104 A/cm2, was obtained at 10 T and 4.2 K when SiC was added to in-situ processed tape prepared with MgH2+B powder. This Jc is comparable to that of a commercial Nb-Ti conductor.
Suspension spinning of commercially available MgB2 powder was examined to fabricate a long superconducting MgB2 wire. The MgB2 powders were suspended in a nonaqueous poly(vinyl alcohol) solution. The viscous spinning dope was extruded as a filament into a precipitating medium of methyl alcohol and coiled on a winding drum. The as-drawn filaments were pressed and heated in order to remove volatile compounds. The filamentary samples were then cold-pressed, enveloped by an iron sheet with a pellet of mixed powder of Mg and B or with Mg powders, vacuum-sealed in a fused quartz tube and sintered. The Jc of the sample was strongly dependent on the starting materials and sintering condition. Although the transport Jc value was low, such as 4500 A/cm2 at 4.2 K and self-fields, a Jc value of more than 200 A/cm2 was maintained by applying a field of 10 T. Doping of 5 at% nanoscale SiC resulted in an improvement in the Jc and superconductivity at 4.2 K was maintained by applying a field of 14 T.
The effect of low-melting-point metal powder addition on the transport current and stress-strain performance of ex-situ processed MgB2 tapes with a Ni sheath has been studied. The addition of low-melting-point metal powders, such as In and Sn, produces an appreciable increase in the Jc of the MgB2 core. The addition of 10vol% In enhances Jc by a factor of 6-7 after the combination of rolling and annealing at 200°C. The Jc of the core with In addition is on the order of 105 A/cm2 at 0.5 T and 4.2 K. The addition of 10vol% Sn enhances Jc by a factor of ∼3. The In and Sn additions also appreciably increase the n value of the Ic transition. Both In and Sn metals impregnate into gaps among MgB2 grains, improving the linkage of the grains. The current can then transfer through the impregnated metal by the proximity effect. Furthermore, the addition of In and Sn offers an appreciable improvement in the strain tolerance of the MgB2 tapes. The addition of low-melting-point metal powders is a relatively simple and easy approach to yield better transport and stress/strain performance in ex-situ MgB2 tapes.
We investigated the applicability of a Cu sheath to MgB2 wires that were prepared utilizing the in-situ synthesis PIT (powder-in-tube) method and compared it with that of stainless steel. Since the critical current density of MgB2 increases with TiH2 doping, we prepared TiH2-doped Cu sheath MgB2 wires 40 m in length and φ1.0 mm or 0.5x1.0 mm2 in cross-sectional dimension utilizing rotary swaging, drawing, and two-axial rolling at room temperature. We then annealed the samples at 600∼850°C for 1∼5hr in an Ar gas atmosphere. The critical current of the TiH2 (6%)-doped MgB2/Cu short sample annealed at 650°C reached 139A(JC = 132kA/cm2) at 4.2 K and ambient field. We also investigated the effect of repeated rolling and annealing on the critical current density of MgB2/SUS wires and confirmed that the enhancement of JC is caused by the densification of MgB2 cores. Finally, we concluded that Cu-sheathed MgB2 wires produced utilizing the in-situ PIT method are promising for practical use.
The effects of various impurity additions have been investigated for MgB2 tapes fabricated by the in-situ process of the powder-in-tube method. As impurities, we tried SiO2, SiC and ZrB2. These powders were added to powder mixtures of Mg and B and MgH2 and B. The powder mixtures were then stuffed into Fe tubes and cold worked into tapes using a combination of grove rolling and flat rolling. Heat treatment was performed at the relatively lower temperature of 600 °C for 1 h with argon flowing and in an Ar gas atmosphere in a closed chamber after evacuating up to 10−5 Torr. From the XRD observation of each tape, Mg2Si was formed for tapes to which SiO2 and SiC were added. ZrB2 did not react with Mg and B. We obtained JC enhancements for all of the additives in the Mg + B in-situ processed tapes. Much higher JC values were obtained using the MgH2+B powder. A high JC value of 1.6 x 104 A/cm2 at 4.2 K and 10 T was obtained for the MgH2+B powder mixture to which 10 at%SiC was added. JC values in the high magnetic fields were increased, while the JC values in the low magnetic fields were decreased by the addition of SiC. The irreversibility field of MgB2 tape was enhanced from 17 T to 23 T by the addition of SiC. However, at low fields of ∼4T, the JC enhancement resulting from the addition of SiC seemed to be very small. The highest JC value, 2.1 x 104 A/cm2 at 10 T, was achieved with the 5at% SiC-doped tape heated in an atmosphere with very small oxygen impurity.
The measurement and analysis of AC transport current loss (self-field loss) in MgB2 superconducting composites were studied. In experimental results, the normalized Ic dependency of AC loss in the MgB2 deviated from Norris' theoretical curve. Then a numerical analysis was performed by taking into consideration the model of a round superconducting wire that consisted of three concentric layers. The analysis clearly showed the experimentally obtained results, such as that the phenomenon originated in the inhomogeneity of the critical current density in the wire rod section.
This paper reports on the successful fabrication and testing of a MgB2 coil. We have fabricated a 15 m-long MgB2/stainless steel mono-core tape using a mixture of commercial MgB2 powder and tin powder (10wt% of MgB2 powder) using the PIT method. The tape is made applying an ex-situ process without any heat treatment during the processing procedure. The MgB2 superconducting tapes show good uniformity, with a high Jc value (180-290 A/mm2 at 4.2 K and 3 T) along the tape's length, as well as good bending tolerance. Ic degradation of the tape occurred at a bending strain of as high as 1.3%. Using wire over 12 m-long, we made a small solenoid coil, 48.5 mm in outer diameter and 40 mm height, and tested it at 4.2 K and self-field. The highest Ic value we obtained was 255 A, which generated a central field of 0.5 T, and a maximum magnetic field of 0.575 T.
Unlike high-Tc oxide superconductors, the boundaries of MgB2 grain do not have weak links. Hence, polycrystalline MgB2 thin films have a strong potential for use in practical high-field applications. We have therefore focused on polycrystalline MgB2 thin films fabricated on various substrates such as TiN-buffered Si and a YSZ-buffered Hastelloy tape. A low-temperature two-step process was employed for depositing of the MgB2 thin films. The highest zero-resistance transition temperature achieved was 34 K for the MgB2 films deposited on Si substrates. Critical current densities exceeding 105 A/cm2 at 4.2 K and 10T have been attained for MgB2 films grown on Hastelloy tape. The microstructures of the films consisting of nanometer-sized grains appeared to act as strong pinning centers. Furthermore, ZrB2 doping into MgB2 thin films has been attempted, resulting in a slightly enhanced upper critical field.
We have investigated the effects of in-situ annealing on the superconducting properties of MgB2 thin films for the purpose of obtaining a high critical temperature close to bulk value. MgB2 thin films were fabricated by rf magnetron sputtering on a C-plane sapphire (Al2O3) substrate. Thin films were produced by simultaneously sputtering pure B and a Mg metal target. Sputtering deposition was followed by in-situ annealing in a high vacuum. To prevent the evaporation of Mg from the film surface, a two-step annealing process was adopted: The first step is the crystallization stage during low-temperature annealing and the next is an improvement of the film's superconducting properties by annealing at a high temperature. Utilizing the optimal annealing process, in which a thin film is first heated at 600°C for two hours and then 670°C for two hours, we have consistently obtained thin films with a zero resistivity temperature of 32 K.