Fe Tapes Research Articles

MgB2 thin films fabricated on Fe tape and effects of annealing on their properties

Magnesium diboride (MgB2) thin films were fabricated on Fe tapes by an electron-beam evaporation method and post-annealed at 650 °C for 1–5 h. Appropriate post- annealing (1 h) resulted in a critical temperature (T c) of 34.4 K and infield critical current density (J c) of 0.20 MA cm−2 at 20 K under 6 T. Characterization suggests that annealing improves the crystallinity of the MgB2 thin film; however, Fe diffuses into the MgB2 layer when annealing for longer durations, which deteriorates the superconductivity. MgB2 thin films on Fe tape can be utilized in diverse superconducting applications under external magnetic fields.

Effect of Various C Sources Co-Doping on Transport Critical Current Density and Grains-Connectivity of MgB2/Fe Tapes

In this paper, we have prepared a series of Fe sheathed MgB2 tapes with SiC, Malic acid and C9H11NO mono-and co-doping by in-situ powder-in-tube (PIT) method, and studied the effects of various C sources co-doping on transport critical current density and flux pinning of MgB2/Fe tapes. The results suggest by various C sources co-doping, the amount of C substitution for boron largely increase, comparing to SiC, Malic acid or C9H11NO mono-doped sample. For MgB2/Fe tapes with 5 mol% SiC+10 wt% Malic acid and 4 wt% C9H11NO+10 wt% Malic acid co-doping, the transport Jc at 4.2 K and 10 T are 14.8 and 13.5 kA/cm2, respectively, which are clearly higher than the Jc of these dopants mono-doped samples. Furthermore, it is found that at the 4 wt.% C9H11NO doped MgB2, the poor in-field Jc should be attributing to the cracks at grains boundary, which results in the bad grains-connectivity.

Structural–microstructural characteristics and its correlationswith the superconducting properties of in situ PIT-processedMgB2 tapes with ethyltoluene and SiC powder added

We performed structural–microstructural investigations of pureMgB2, ethyltoluene and both ethyltoluene- and SiC-addedMgB2/Fe tapes. The analysis of the microstructure shows that the grain size for the pure and ethyltoluene-addedMgB2 tape sample is in the range of 10–100 nm. However, with the addition ofboth ethyltoluene and SiC, the grain size decreases to about 5–80 nm. Thea-axis length of the ethyltoluene-added tape samples is slightlydecreased, whereas for both ethyltoluene- and SiC-added samples, thea-axis length is decreased by 0.4% as compared to the pureMgB2 tape sample, showing the amount of carbon substitution is lessin ethyltoluene-added tape samples. The reason for the higherJc values of theethyltoluene-added MgB2 tape sample as compared to the pureMgB2 tapes is the presence of a lesser amount of the impurity phase, MgO. The large improvement inJc–B properties for the ethyltoluene- and SiC-addedMgB2 tape sample is attributed to (1) the enhancement of upper critical field,Bc2, by the substitution of carbon for boron, (2) pinning by nanosized (5–20 nm) particles ofMg2Si and other silicides, (3) enhanced grain boundary pinning due to the smaller grain size and(4) the presence of a lesser amount of impurity phase, MgO, as compared to the puresample.

Influence of sintering temperature on the superconducting properties of Zn-stearate-dopedMgB2 tapes

Correlation of phase purities, microstructure,Jc and Hc2 with sintering temperature has been studied in Zn-stearate-dopedMgB2/Fe tapes. All doped tapes show significant enhancements inJc and Hc2. TEM analyses revealed that strong flux pinning centers induced by grain boundaries andcrystal defects as well as carbon substitution effect are responsible for the improvement ofJc and Hc2 values. Although a high sintering temperature of850 °C increases the grain size, improved grain connection can compensate for this disadvantage, giving thebest Jc of 2.0 × 104 A cm−2 at 4.2 K and 10 T. On the other hand, tapes sintered at900 °C have alower Jc due to the larger grain size and more MgO impurities.

Enhancement of the critical current density and the irreversibility field in maleic anhydride doped MgB2 based tapes

A significant enhancement of Jc, Hirr, and Hc2 in MgB2 tapes has been achieved by using maleic anhydride (C4H2O3) as dopants. MgB2 tape samples with 0 up to 30wt% C4H2O3 added were prepared by an in situ processed powder-in-tube method. Compared to the pure tapes, Jc for the 10wt% C4H2O3 doped samples was improved by more than an order of magnitude in high fields up to 18T. At 4.2K, the transport Jc for the 10wt% doped samples reached 1.08×104, 5.42×103, and 2.18×103A∕cm2 at 12, 14, and 16T, respectively, and were better than those of the best MgB2∕Fe tapes reported by far. Furthermore, the Hirr for the 10wt% doped sample reached 9.6T at 20K, which was comparable to that of NbTi at 4.2K. The improvement of the superconducting properties in doped tapes can be attributed to the increase of Hc2 and the grain size refinement by C4H2O3 doping.

Determination of the texturing gradient in ex situ MgB2∕Fe tapes examined by x-ray diffraction and its effects on the pinning force

We found that the reduced pinning forces of ex situ MgB2∕Fe monofilamentary tapes do not scale with temperature if the external magnetic field is applied parallel to the tape surface. A model is proposed to explain the behavior of the volume pinning forces in parallel and perpendicular orientations of the magnetic field, accounting for the observed texturing gradient between the center of the filament and the MgB2∕Fe interface. The degree of texturing of the sample, as determined by x-ray diffraction, is confirmed by modeling the pinning force in parallel field as the superposition of two components, the first one due to a highly textured region on the external part of the filament and the latter due to a low texturing region in the core.

Superconducting properties of in situ powder-in-tube-processedMgB2 tapes fabricated with sub-micrometre Mg powder prepared by an arc-plasmamethod

We fabricated in situ powder-in-tube-processedMgB2/Fe tapes using sub-micrometre Mg powder prepared by applying an arc-plasma method. Wefound that the use of this sub-micrometre Mg powder was very effective in increasing theJc values. Thetransport Jc value of 10 mol% SiC-added tapes fabricated with this sub-micrometre Mg powder reached275 A mm−2 at 4.2 K and 10 T. This value was about six times that of 5 mol% SiC-added tapesfabricated with commercial Mg powder. Microstructure analyses suggest that thisJc enhancement is primarily due to the smallerMgB2 grain size.

Improved critical current densities in MgB2 tapes with ZrB2 doping

Mg B 2 ∕ Fe tapes with 2.5–15at.% ZrB2 additions were prepared through the in situ powder-in-tube method. The phases, microstructures, critical current density, and flux pinning were characterized by means of x-ray diffraction, scanning electron microscope, and magnetic and transport property measurements. Compared to the pure tape, a significant improvement in the in-field JC was observed for all the ZrB2 doped samples, while the critical temperature decreased slightly. The highest JC value was achieved for the 10at.% doped sample. At 4.2K, the transport JC increased by more than an order of magnitude than the undoped one in magnetic fields above 9T. Nanoscale segregates or defects caused by the ZrB2 additions which act as effective flux pinning centers are proposed to be the main reasons for the improved JC field performance.

Upper critical field, irreversibility field, and critical current density of powder-in-tube-processedMgB2/Fe tapes

We fabricated pure and SiC-addedMgB2/Fe compositetapes using a MgH2 starting powder and applying heat treatments at600–900 °C and systematically investigated their superconducting properties.For both the pure and SiC-added tapes, the critical temperature(Tc) increased with increasing heat-treatment temperature due to the improved crystallinity ofMgB2.The SiC additiondecreased the Tc butincreased the slope of the Bc2–T and Birr–T curves, d Bc2/d T andd Birr/d T, for all heat-treatmenttemperatures. The d Bc2/d T and d Birr/d T of the pure tape decreased with increasing heat-treatment temperature from 600 to700 °C because of the longer coherence length associated with the improved crystallinity. However,the SiC addition significantly decreased the heat-treatment temperature dependences ofd Bc2/d T andd Birr/d T. At atemperature of ∼20 K, which is easily obtained using a cryocooler, theBirr is governedby both the Tc and d Birr/d T. The Birr of a pure tape at 20 K decreased with increasing heat-treatment temperature from 600 to700 °C, butthe Birr of the 10 mol% SiC-added tape increased with the temperature. These behaviourscan be explained by the heat-treatment temperature dependence of theTc andd Birr/d T. At 20 Kthe highest Birr of 10 T was obtained under the conditions of a 10 mol% SiC addition and heat-treatment temperatureof 900 °C.This Birr at 20 K is comparable to that of commercial Nb–Ti at 4.2 K. The 10 mol% SiC-added tape heat treatedat 900 °C and the 5 at.% SiC-added tape heat treated at800 °C showedJc (MgB2 core) valueshigher than 104 A cm−2 at 20 K in 5 T.

The superconducting properties of MgB/sub 2//(stainless steel) tapes fabricated by the pit process

We have investigated the superconducting properties of monofilament MgB/sub 2/ tapes and wires with a stainless steel (SUS316) sheath. MgB/sub 2/ powder was put into a SUS316 tube with a 6.5 mm diameter; the tube was cold-rolled into a wire with a 1.4 mm-1.0 mm diameter using groove-rolling and cassette roller dies and then cold-rolled into tapes. The stainless steel sheath is effective to increase the density of the MgB/sub 2/ core. The transport critical current density depends on the wire diameter and tape thickness. The J/sub C/ values increase with increasing the total cross-sectional area reduction of tapes. However, the J/sub C/ value of the tape with a thickness of less than 0.3 mm deteriorated because of cracks introduced by rolling. Post-annealing increased the J/sub C/ properties of these tapes. The low annealing temperature of /spl sim/600/spl deg/C was effective to increase the J/sub C/ values for MgB/sub 2//(stainless steel) tapes. This temperature was much lower than the optimum annealing temperature of MgB/sub 2/ tapes with other sheath materials, such as MgB/sub 2//Fe tapes. The post-annealing improved the linkage of grains and enhanced the J/sub C/ values.

Transport critical current densities and n factors in mono- and multifilamentary MgB/sub 2/=Fe tapes and wires using fine powders

Mono- and multifilamentary MgB2/Fe tapes and wires with high transport critical current densities have been prepared using the powder-in-tube (PIT) process. The fabrication details are described. The effect of powder grain sizes and recrystallization temperature on jc has been investigated. At 25K and 1 T, jc values close to 105 A/cm2 were measured, while jc of 106 A/cm2 were extrapolated for 4.2K/0T in our monofilamentary tape. MgB2/Fe tapes exhibit high exponential n factors for the resistive transition: n ~ 80 and 40 were found at 5 T and 7 T, respectively. The highest transport jc values obtained so far in MgB2/Fe wires with 7 filaments were 1.1 * 105 A/cm2 at 4.2 K and in a field of 2 T, which is still lower than for monofilamentary tapes. Improved deformation and recovering processing is expected to lead to higher jc values.

Effect of the processing parameters of MgB 1.8(SiC) 0.1/Fe tapes on the critical current density

Nano-SiC doping into MgB 2 has induced extraordinary enhancement in critical current density ( J c) and flux pinning as a result of Si and C co-substitution into the lattice and the formation of nano-inclusions within the grains. In this work, a systematic study of the effect of the preparation parameters for processing MgB 1.8(SiC) 0.1/Fe tapes on critical temperature ( T c), critical current density ( J c) and microstructures was carried out. A mixture of Mg and B was doped with nano-SiC particle powder, and the SiC-doped samples were fabricated with the reaction in situ method. The samples were characterised using X-ray diffraction, scanning electronic microscopy and magnetisation measurements. It was found that the sintering temperature had a strong effect on both T c and J c. T c was increased with sintering temperature, while J c of the tapes sintered at 750 and 850 °C is higher than that of the tapes sintered at 680 and 950 °C. A longer sintering time was found to be beneficial to T c but has negligible effects on J c. Slow cooling improves the J c field performance at 20 and 30 K. Uniaxial pressure (250 and 680 MPa) applied to tapes before sintering has almost no effect on J c.

High transport critical currents in dense monofilamentary Fe- and Ni-clad MgB/sub 2/ tapes and MgB/sub 2//Fe wires with 7 filaments

Highly dense monofilamentary MgB/sub 2//Ni and MgB/sub 2//Fe tapes and multifilamentary MgB/sub 2//Fe wire with high transport critical current densities have been fabricated using a straightforward powder-in-tube (PIT) process. High density of the MgB/sub 2/ core and a fine starting powder appear to be decisive factors for reaching high critical current densities. After annealing, we measured transport j/sub c/ values up to 2.3/spl times/10/sup 5/ A/cm/sup 2/ at 4.2 K in a field of 1.5 T (corresponding to an I/sub c/ of 300 A) in a MgB/sub 2//Ni tape and up to 10/sup 4/ A/cm/sup 2/ at 4.2 K in 6.5 T in a MgB/sub 2//Fe tape. For higher currents these monofilamentary tapes quenched due to insufficient thermal stability. Thus, j/sub c/ at 4.2 K, 0 T can for instance only be extrapolated, yielding a value close to 1 MA/cm/sup 2/. The recrystallization during annealing leads to a densification and to j/sub c/ values which are more than a factor of /spl sim/10 higher than those measured in the as-deformed tapes. Ni shows a limited reaction with MgB/sub 2/, leading to the formation of Mg/sub 2/Ni reaction layers between the filament and the matrix. On the other hand, Fe stays chemically inert and turns out to be an excellent candidate material for preparation of multifilamentary MgB/sub 2/ tapes. A transport j/sub c/ value of 9/spl times/10/sup 4/ A/cm/sup 2/ at 4.2 K and 0.75 T in MgB/sub 2//Fe wire with 7 filaments produced by groove rolling was obtained.