M. Bhatia

Prospects for improving the intrinsic and extrinsic properties of magnesium diboride superconducting strands

The magnetic and transport properties of magnesium diboride films represent performance goals yet to be attained by powder-processed bulk samples and conductors. Such performance limits are still out of the reach of even the best magnesium diboride magnet wire. In discussing the present status and prospects for improving the performance of powder-based wire we focus attention on (1) the intrinsic (intragrain) superconducting properties of magnesium diboride, Hc2 and flux pinning, (2) factors that control the efficiency with which current is transported from grain-to-grain in the conductor, an extrinsic (intergrain) property. With regard to Item-(1), the role of dopants in Hc2 enhancement is discussed and examples presented. On the other hand their roles in increasing Jc, both via Hc2 enhancement as well as direct fluxoid/pining-center interaction, are discussed and a comprehensive survey of Hc2 dopants and flux-pinning additives is presented. Current transport through the powder-processed wire (an extrinsic property) is partially blocked by the inherent granularity of the material itself and the chemical or other properties of the intergrain surfaces. These and other such results indicate that in many cases less than 15% of the conductors cross sectional area is able to carry transport current. It is pointed out that densification in association with the elimination of grain-boundary blocking phases would yield five-to ten-fold increases in Jc in relevant regimes, enabling the performance of magnesium diboride in selected applications to compete with that of Nb-Sn.

Large upper critical field and irreversibility field in MgB2 wires with SiC additions

Resistive transition measurements are reported for MgB2 strands with SiC dopants. The starting Mg powders were 325 mesh 99.9% pure, and the B powders were amorphous, 99.9% pure, and at a typical size of 1–2 μm. The SiC was added as 10 mol % of SiC to 90 mol % of binary MgB2 [(MgB2)0.9(SiC)0.1]. Three different SiC powders were used; the average particle sizes were 200 nm, 30 nm, and 15 nm. The strands were heat treated for times ranging from 5 to 30 min at temperatures from 675 °C to 900 °C. Strands with 200 nm size SiC additions had μ0Hirr and Bc2 which maximized at 25.4 T and 29.7 T after heating at 800 °C for 30 min. The highest values were seen for a strand with 15 nm SiC heated at 725 °C for 30 min which had a μ0Hirr of 29 T and a Bc2 higher than 33 T.Resistive transition measurements are reported for MgB2 strands with SiC dopants. The starting Mg powders were 325 mesh 99.9% pure, and the B powders were amorphous, 99.9% pure, and at a typical size of 1–2 μm. The SiC was added as 10 mol % of SiC to 90 mol % of binary MgB2 [(MgB2)0.9(SiC)0.1]. Three different SiC powders were used; the average particle sizes were 200 nm, 30 nm, and 15 nm. The strands were heat treated for times ranging from 5 to 30 min at temperatures from 675 °C to 900 °C. Strands with 200 nm size SiC additions had μ0Hirr and Bc2 which maximized at 25.4 T and 29.7 T after heating at 800 °C for 30 min. The highest values were seen for a strand with 15 nm SiC heated at 725 °C for 30 min which had a μ0Hirr of 29 T and a Bc2 higher than 33 T.

High transport critical current density and large Hc2 and Hirr in nanoscale SiC doped MgB2 wires sintered at low temperature

We report a systematic study on the effect of sintering temperature on the phase formation, critical current density, upper critical field and irreversibility field of nanoscale SiC doped MgB2. Bulk and Fe sheathed wires doped with different nano-SiC particle sizes have been made and heat treated at temperatures ranging from 580 to 1000 °C. A systematic correlation between the sintering temperature, normal state resistivity, RRR, Jc, Hc2, and Hirr has been found in all samples of each batch. Samples sintered at a lower temperature have a very fine and well consolidated grain structure while samples sintered at a high temperature contain large grains with easily distinguishable grain boundaries. Low temperature sintering resulted in a higher concentration of impurity precipitates, larger resistivity, higher Jc up to 15 T and lower Tc values. These samples show higher Hc2 and Hirr at T near Tc but lower Hc2 near T = 0 than samples sintered at high temperature. It is proposed that huge local strains produced by nano-precipitates and grain boundary structure are the dominant mechanism responsible for higher Hc2 at T near Tc. However, higher impurity scattering due to C substitution is responsible for higher Hc2 in the low temperature regime for samples sintered at a higher temperature. In addition to high Hc2, it is also proposed that the large number of nano-impurities serve as pinning centres and improve the flux pinning, resulting in higher Jc values at high magnetic fields up to 15 T.

Irreversibility field and flux pinning in MgB2 with and without SiC additions

The critical current density (Jc) was measured at 4.2?K for MgB2 strands with and without SiC additions. In some cases measurements were performed on long (1?m) samples wound on barrels, the transport results being compared to the results of magnetic measurements. Most measurements were performed on short samples in fields of up to 18?T. It was found that in?situ processed strands with 10% SiC additions heat treated at 700?800??C showed improved irreversibility fields (Hr) and bulk pinning strengths (Fp) as compared to control samples; an increase in Hr of 1.5?T was noted. Heat treatment to 900??C gave even larger improvements, with Hr reaching 18?T and Fp values maximizing at 20?GN?m?3.

Transport properties of multifilamentary, in situ route, Cu-stabilized MgB2 strands: one metre segments and the Jc(B,T) dependence of short samples

The transport critical current density (Jc) was measured at 4.2 K for MgB2 monofilamentary and 7-, 19-, and 37-stack multifilamentary strands. Simple, one-step heat treatments (HT) were used, with temperatures of 675 and 700 °C, and times from 10–40 min. Most measurements were performed on 1 m segments of strands wound onto barrel holders. Transport properties of monofilament, 7-, 19-, and 37-stack strands were compared, and the influence of CuNi and monel outer sheaths was investigated. HT optimization studies were performed on various strands. Transport Jcs of 0.8 mm OD strands reached 2 × 105 A cm−2 at 4 T and 4.2 K (1 µV cm−1), and 1.15 × 106 A cm−2 at 4.2 K and zero field. Smaller 10-filament wires with ODs as small as 0.25 mm (40 µm filaments) exhibited good performance in some cases. The temperature and field dependences of the transport Jc were also measured; a typical example was 2 × 104 A cm−2 at 4 K, 20 T.

Multifilamentary, in situ route, Cu-stabilized MgB2 strands

Transport critical current densities and n-values were measured at 4.2 K in fields up to 15 T on 7-, 19-, and 37-stack multifilamentary MgB2 strands made using an in situ route. Some strands included SiC additions (particle size ~30 nm), while in others Mg-rich compositions were used. Two basic multifilamentary variants were measured; the first had Nb filamentary barriers, the second had Fe filamentary barriers. All samples incorporated stabilizer in the form of Cu 101. Simple, one-step heat treatments were used, with temperatures ranging from 700 to 800 °C, and times from 10 to 30 min. Transport critical current densities of 1.75 × 105 A cm−2 were seen at 4.2 K and 5 T in 37-stack strands. One metre segments of strand were also measured at 4.2 K; these showed critical current densities of 6 × 105 A cm−2 at zero field. Magnetic Jcs of 37-stack strands were measured; these measurements showed that the intrinsic Jc of the material reached 106 A cm−2 at 20 K.

Thermally assisted flux flow and individual vortex pinning in Bi2Sr2Ca2Cu3O10 single crystals grown by the traveling solvent floating zone technique

Magnetoresisitivity and critical current density Jc as a function of temperature and field are studied for Bi2Sr2Ca2Cu3O10 single crystals grown using the traveling solvent floating zone technique. Below a characteristic field B∗, Jc as a function of field exhibits a field-independent plateau associated with thermally activated pinning of individual vortices. Analysis of resistive transition broadening revealed that thermally activated flux flow is found to be responsible for the resistivity contribution in the vicinity of Tc. The activation energy U0 is 800K in low field, scales as B−1∕6 for B 2T.

Effect of various additions on upper critical field and irreversibility field of in-situ MgB/sub 2/ superconducting bulk material

The effect of silicon carbide, carbon and metal-diboride (NbB/sub 2/, ZrB/sub 2/, and TiB/sub 2/) additions on the irreversibility field, H/sub irr/ and the upper critical field H/sub c2/ of bulk superconducting MgB/sub 2/ have been studied. Samples with 10 mole % of SiC and C and 7.5% of above named metal diboride additions were made separately by a powder milling and compaction technique along with the control sample. These samples were heat-treated at various schedules and H/sub c2/ and H/sub irr/ values were measured. An increase in /spl mu//sub o/H/sub c2/ from 20.5 T for pure sample to more than 33 T and enhancement of /spl mu//sub o/H/sub irr/ from 16 T to a maximum of 28 T for SiC doped sample was observed at 4.2 K. For a ZrB/sub 2/ doped sample 24 T and 28.6 T of /spl mu//sub o/H/sub irr/ and /spl mu//sub o/H/sub c2/ respectively we obtained with only 2 K drop in the T/sub c/.

Increases in the irreversibility field and the upper critical field of bulk MgB2 by ZrB2 addition

In a study of the influence of ZrB2 additions on the irreversibility field, μ0Hirr and the upper critical field Bc2, bulk samples with 7.5at% ZrB2 additions were made by a powder milling and compaction technique. These samples were then heated to 700–900°C for 0.5h. Resistive transitions were measured at 4.2K and μ0Hirr and Bc2 values were determined. An increase in Bc2 from 20.5Tto28.6T and enhancement of μ0Hirr from 16Tto24T were observed in the ZrB2 doped sample as compared to the binary sample at 4.2K. Critical field increases similar to those found with SiC doping were seen at 4.2K. At higher temperatures, increases in μ0Hirr were also determined by M-H loop extrapolation and closure. Values of μ0Hirr which were enhanced with ZrB2 doping (as compared to the binary) were seen at temperatures up to 34K, with μ0Hirr values larger than those for SiC doped samples at higher temperatures. The transition temperature, Tc, was then measured using dc susceptibility and a 2.5K drop of the midpoint of Tc was obs...

Solenoidal coils made from monofilamentary and multifilamentary MgB2 strands

Three solenoids have been wound with MgB2 strands and tested for transport properties. One of the coils was wound with a Cu-sheathed monofilamentary strand and the other two with a seven-filament strand with Nb-reaction barriers, Cu stabilization, and an outer monel sheath. The wires were first S-glass insulated, then wound onto an OFHC Cu former. The coils were then heat treated at 675??C/30?min (monofilamentary strand) and 700??C/20?min (multifilamentary strand). Smaller (1?m) segments of representative strand were also wound into barrel-form samples and HT along with the coils. After HT the coils were epoxy impregnated. Transport Jc measurements were performed at various taps along the coil lengths. Measurements were made initially in liquid helium, and then as a function of temperature up to 30?K. Homogeneity of response along the coils was investigated and a comparison to the short sample results was made. Each coil contained more than 100?m of 0.84?1.01?mm OD strand. One of the seven-strand coils reached 222?A at 4.2?K, self-field, with a Jc of 300?kA?cm?2 in the SC and a winding pack Je of 23?kA?cm?2. At 20?K these values were 175 and 13.4?kA?cm?2. Magnet bore fields of 1.5 and 0.87?T were achieved at 4.2 and 20?K, respectively. The other multifilamentary coil gave similar results.