F. Buta

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.

Optimization studies for processing Nb/sub 3/Al by a rapid ohmic-heating and quenching method

Rapid heating to different temperatures in the vicinity of 2000/spl deg/C followed by quenching to room temperature was used to prepare short samples of jelly-roll-type Nb/sub 3/Al superconductor. The as quenched material has either a bcc or A15 structure depending on the presence or absence of a specific endothermic reaction during the rapid heating, but after a secondary heat treatment at 800/spl deg/C an A15 structure is obtained for both cases. Significantly higher critical current densities at magnetic fields up to 18 T (4.2 K) are obtained for samples quenched as bcc, while the A15-quenched material is superior at higher fields due to a larger upper critical field. For the bcc-quenched samples the critical current density is optimized (16 T, 4.2 K) when they are quenched from temperatures just above or 400/spl deg/C higher than the temperature of the endothermic reaction, with a noticeable drop in the middle range. A 10 hour secondary heat treatment gives rise to better superconducting properties than a 3 hour heat treatment (both at 800/spl deg/C). Some of the observed dependencies are explained using SEM microstructure analysis.

Short-sample quenching of Nb/sub 3/Al precursor strand in support of reel-to-reel process development

Niobium-aluminum precursor strands were fabricated using the conventional jelly roll method with two Nb:Al volume ratios, 3:1, and 5:1. These mono-elements were restacked in 7-, 19- and 36-core arrangements and drawn down to wires approximately 0.8 mm and 0.4 mm in diameter. Short lengths of wire were resistively heated under high vacuum to temperatures as high as 2200/spl deg/C for various times, and either allowed to cool by radiation plus lead conduction or quenched into liquid gallium. Vacuum cooling led to a disordered A15 phase, which was then ordered by annealing at 800/spl deg/C; gallium quenching tended to produce the bcc phase, which could be transformed into A15 also by annealing for various times at 800/spl deg/C. The superconducting properties of the final wires were examined by vibrating-sample magnetometry (/spl chi//sub dc/ and T/sub c/) as well as transport J/sub c/ measurement. Best results so far are for a 5:1 Nb:AI volume ratio sample Ga quenched from 2000/spl deg/C with a secondary reaction of 800/spl deg/C/3 h, giving J/sub c/=4.5/spl times/10/sup 8/ A/m/sup 2/ at 12 T, with a T/sub c/=18-18.5 K.

Influence of transformation heat treatment on microstructure and defects in RHQT-processed Nb/sub 3/Al

Nb-Al jelly-roll composite wires of stoichiometric composition (25 at% Al) have been processed by Rapid Heating-Quenching (RHQ) to temperatures just above the bcc formation reaction. Transformation heat treatments going to 800/spl deg/C and 1000/spl deg/C with various initial heating ramp-up times (from 30 sec to 30 min) and followed by 800/spl deg/C/10 hrs were applied to samples so obtained. The microstructure and planar defect distribution were investigated by transmission electron microscopy; the concentration of planar defects was found to decrease as the initial heating rate is increased. In situ high temperature X-ray diffraction studies were used to investigate the transformation heat treatment. It is proven that less ordering of the bcc solid solution prior to conversion to A15 occurs at high heating rates.

A new generation of in situ MgB2 wires with improved Jc and Birr values obtained by cold densification (CHPD)

By means of Cold High Pressure Densification (CHPD), the critical current density, <i>J</i>_c, of binary and alloyed MgB₂ wires has been enhanced by more than a factor 2 at 4.2 K and at fields up to 19 T. The relative MgB₂ mass density of binary MgB₂ wires was enhanced to ~ 54% after applying 2.5 GPa at 300 K before reaction. In C₄H₆O₅ (malic acid) alloyed wires, densification also caused the enhancement of <i>B</i>_irr, as a consequence of a slightly enhanced C content, determined by X ray diffraction. Almost isotropic <i>J</i>_c values were obtained for C₄H₆O₅ added wires of 1 × 0.6 mm² cross section, the values of <i>J</i>_c(4.2 K)=1 × 10⁴ A/cm² for parallel and perpendicular fields being obtained at 13.8 and 13.4 T, respectively (1 μV/cm criterion). The corresponding values for 20 K were both close to 6.2 T. The value of <i>B</i>_irr^// at 20 K was 11 T. The positive effects of cold densification on J_c and <i>B</i>_irr on MgB₂ was also observed on 150 mm long wires alloyed with C₄H₆O₅ (malic acid) or with SiC, by the succession of 6 overlapping pressure steps. This process can be extended to long wire lengths: by means of a newly developed prototype machine with an automatic press/release/advance sequence, a first wire length of 1 m was densified at 1.5 GPa, yielding J_c(4.2 K) = 1 x 10⁴ A/cm² at 13.1 T. Further improvements are expected after optimization.

Wind and React and React and Wind

Multifllamentary MgB2 wire was used to construct three coils that were tested for transport Jc and magnetic field in the bore between 4.2 K and 30 K. The solenoid coil achieved 85 A at 4.2 K with 3.9 T in a 3.8 cm bore. A racetrack coil was also fabricated with an attached compression plate to test the effects of compressive stress on an MgB2 racetrack coil. At compressive stresses up to 52 MPa, no degradation was observed in transport Jc at 20 K. A react and wind coil with a 56 cm bore and 828 m of MgB2 wire was also tested for transport Jc and magnetic field in the bore to demonstrate the feasibility of the react and wind process. At 5 K, the react and wind coil achieved 168 A and 0.17 T.

{\rm MgB}_{2}

The phases present in the Niobium-Aluminum system were investigated by X-ray diffraction and scanning electron microscopy on jelly-roll composites of 23.5, 27.5 and 35.0 at% Al, quenched after being heated in various time intervals (0.3 sec-5 min) to temperatures in the vicinity of the bcc formation temperature. In the present configuration, at concentrations around 27.5 at% Al the bcc phase is the product of an eutectoidal reaction between A15-structure Nb/sub 3/Al and sigma-phase Nb/sub 2/Al. This eutectoid has not been found in any of the published equilibrium phase diagrams, a revised equilibrium phase diagram is proposed.

Solenoid, Racetrack and Pancake Coils

Jelly-roll type Niobium-Aluminum multifilamentary composites of two Nb/Al ratios (3.30:1 and 3.75:1 by volume) with laminate sizes around 165 nm were rapidly heated slightly above the bcc formation reaction temperature and then quenched. Samples so obtained were subjected to A15 transformation heat treatments having different initial heating rates to induce different degrees of Al segregation as planar defects in the transformed A15 phase. It was found that as the heating rate is increased better superconducting properties are obtained. Of the tested methods the most promising is an ohmic heating pulse to temperatures around 1000 °C followed by an 800 °C/10 h ordering heat treatment. Critical current densities as high as 240 A/mm2 at 23.5 T and 4.2 K were obtained. Direct insertion in a furnace pre-heated at 1000 °C followed by 800 °C/10 h also leads to good properties but is sensitive to the thermal mass of the sample, making it less useful for the heat treatment of coils.

Phase stability at high temperatures in the Nb-Al system

Jelly-roll Nb/sub 3/Al wires with a Nb matrix were ohmically-heated to maximum temperatures ranging from 1800 to 2300/spl deg/C in vacuum in order to optimize the ohmic-heating conditions and to investigate the superconducting properties for this transformation method. The diameters of these wires were 0.80-1.34 mm. Surface temperature at the central point of the sample was measured by a photodiode during rapid ohmic-heating. After the surface temperature reached a maximum, the sample was quenched in liquid gallium. All the samples were annealed at 800/spl deg/C for 3-25 hours after the rapid heating process to transform the bcc-phase to the A15 phase. Critical currents were measured up to 23 T. The samples heated to 2000/spl deg/C showed a maximum critical current density of 64 A/mm/sup 2/ at 20 T. The critical current density decreased with increasing maximum temperature during rapid heating. This paper describes the superconducting properties, the rapid heating conditions and the achievement of high critical current density at high magnetic fields.

Processing-superconducting property correlation studies in RHQT-processed Niobium-Aluminum superconductors

By means of Cold High Pressure Densification (CHPD), the critical current density, <i>J</i>_c, of binary and alloyed MgB₂ wires has been enhanced by more than a factor 2 at 4.2 K and at fields up to 19 T. The relative MgB₂ mass density of binary MgB₂ wires was enhanced to ~ 54% after applying 2.5 GPa at 300 K before reaction. In C₄H₆O₅ (malic acid) alloyed wires, densification also caused the enhancement of <i>B</i>_irr, as a consequence of a slightly enhanced C content, determined by X ray diffraction. Almost isotropic <i>J</i>_c values were obtained for C₄H₆O₅ added wires of 1 × 0.6 mm² cross section, the values of <i>J</i>_c(4.2 K)=1 × 10⁴ A/cm² for parallel and perpendicular fields being obtained at 13.8 and 13.4 T, respectively (1 μV/cm criterion). The corresponding values for 20 K were both close to 6.2 T. The value of <i>B</i>_irr^// at 20 K was 11 T. The positive effects of cold densification on J_c and <i>B</i>_irr on MgB₂ was also observed on 150 mm long wires alloyed with C₄H₆O₅ (malic acid) or with SiC, by the succession of 6 overlapping pressure steps. This process can be extended to long wire lengths: by means of a newly developed prototype machine with an automatic press/release/advance sequence, a first wire length of 1 m was densified at 1.5 GPa, yielding J_c(4.2 K) = 1 x 10⁴ A/cm² at 13.1 T. Further improvements are expected after optimization.