CRITICAL CURRENT DENSITIES AND N-VALUES OF MGB2 CONDUCTORS FOR SMES, MRI, AND LOW AC LOSS APPLICATIONS.

Multifilamentary MgB2 strands (filament numbers 36 to 114) prepared by the in-situ power-in-tube (PIT) route with carbon doping contents of 0, 2, and 3.2% were wound on barrels for transport Jc and n-value measurement at 4.2 K in fields of up to 12 T. The strand and gauge lengths were 1 m and 0.5 m. Heat treatments at 675 °C and 650 °C centered around the melting point of Mg (650 °C) and both utilized the liquid-solid reaction. A pair of strands, with and without 2% C doping exhibited the Jc (B) crossover effect. Studied were the dependencies of Jc on field strength, dopant concentration, and cabling and the dependence of n-value on field strength.

Mechanism of enhanced critical fields and critical current densities of MgB2 wires with C/Dy2O3 co-additions.

A series of monofilamentary powder-in-tube MgB2 wires were fabricated with 2 mol. % C doping and co-additions of 0-3 wt. % Dy2O3. Irreversibility fields (µ 0 Hirr ), upper critical fields (µ 0 Hc 2), and transport critical currents were measured, and from these quantities, anisotropies ( γ ) and electronic diffusivities ( D π , σ ) were estimated. The addition of 1 wt. % Dy2O3 to already optimally C-doped MgB2 wires produced higher Hc 2//ab , Hc 2//c , and Hirr values at 4.2 K. In addition, the critical current density, Jc , increased with Dy2O3 concentration up to 1 wt. % where non-barrier Jc reached 4.35 × 104 A/cm2 at 4.2 K, 10 T. At higher temperatures, for example, 20 K and 5 T, co-additions of 2 mol. % C and 2 wt. % Dy2O3 improved non-barrier Jc by 40% and 93% compared to 2 and 3 mol. % C doping, respectively. On the other hand, measurements of Tc showed that C/Dy2O3 co-additions increase interband scattering rates at a lower rate than C doping does (assuming C doping levels giving similar levels of low-T µ 0 Hc 2 increase as co-addition). Comparisons to a two-band model for µ 0 Hc 2 in MgB2 allowed us to conclude that the increases in Hc 2//ab , Hc 2//c , and Hirr (as well as concomitant increases in high-field Jc ) with Dy2O3 addition are consistent with increases primarily in intraband scattering. This suggests C/Dy2O3 co-addition to be a more promising candidate for improving non-barrier Jc of MgB2 at temperatures above 20 K.

Magnetic, Mechanical and Thermal Modeling of Superconducting, Whole-body, Actively Shielded, 3 T MRI Magnets Wound Using MgB2 Strands for Liquid Cryogen Free Operation.

we present magnetic, mechanical and thermal modeling results for a 3 Tesla actively shielded whole body MRI (Magnetic Resonance Imaging) magnet consisting of coils with a square cross section of their windings. The magnet design was a segmented coil type optimized to minimize conductor length while hitting the standard field quality and DSV (Diameter of Spherical Volume) specifications as well as a standard, compact size 3 T system. It had an overall magnet length and conductor length which can lead to conduction cooled designs comparable to NbTi helium bath cooled 3 T MRI magnets. The design had a magnetic field homogeneity better than 10 ppm (part-per-million) within a DSV (Diameter of Spherical Volume) of 48 cm and the total magnet winding length of 1.37 m. A new class of MgB2 strand especially designed for MRI applications was considered as a possible candidate for winding such magnets. This work represents the first magnetic, mechanical and thermal design for a whole-body 3 T MgB2 short (1.37 m length) MRI magnet based on the performance parameters of existing MgB2 wire. 3 Tesla MRI magnet can operate at 20 K at 67 % of its critical current.

Enhanced higher temperature irreversibility field and critical current density in MgB2 wires with Dy2O3 additions.

Bulk samples of magnesium diboride (MgB2) doped with 0.5 wt% of the rare earth oxides (REOs) Nd2O3 and Dy2O3 (named B-ND and B-DY) prepared by standard powder processing, and wires of MgB2 doped with 0.5 wt% Dy2O3 (named W-DY) prepared by a commercial powder-in-tube processing were studied. Investigations included x-ray diffractometry, scanning- and transmission electron microscopy, magnetic measurement of superconducting transition temperature (T c), magnetic and resistive measurements of upper critical field (B c2) and irreversibility field (B irr), as well as magnetic and transport measurements of critical current densities versus applied field (J cm(B) and J c(B), respectively). It was found that although the products of REO doping did not substitute into the MgB2 lattice, REO-based inclusions resided within grains and at grain boundaries. Curves of bulk pinning force density (F p) versus reduced field (b = B/B irr) showed that flux pinning was by predominantly by grain boundaries, not point defects. At all temperatures the F p(b) of W-DY experienced enhancement by inclusion-induced grain boundary refinement but at higher temperatures F p(b) was still further increased by a Dy2O3 additive-induced increase in B irr of about 1 T at all temperatures up to 20 K (and beyond). It is noted that Dy2O3 increases B irr and that it does so, not just at 4 K, but in the higher temperature regime. This important property, shared by a number of REOs and other oxides promises to extend the applications range of MgB2 conductors.

The Role of CHPD and AIMI processing on enhancing J C and transverse connectivity of in-situ MgB2 strand.

Research into in-situ MgB2 strand has been focused on improvements in J C through reduction of porosity. Both of cold-high-pressure-densification (CHPD) and advanced-internal-magnesium-infiltration (AIMI) techniques can effectively remove the voids in in-situ MgB2 strands. This study shows the nature of the reduced porosity for in-situ MgB2 strands lies on increases in transverse grain connectivity as well as longitudinal connectivity. The CHPD method bi-axially applying 1.0 GPa and 1.5 GPa yielded 4.2 K J CM∥s of 9.6 × 104 A/cm2 and 8.5 × 104 A/cm2 at 5 T, respectively, with compared with 6.0 × 104 A/cm2 for typical powder-in-tube (PIT) in-situ strand. Moreover, AIMI-processed monofilamentary MgB2 strand obtained even higher J Cs and transverse grain connectivity than the CHPD strands.

Quench, Normal Zone Propagation Velocity, and the Development of an Active Protection Scheme for a Conduction Cooled, R&W, MgB2 MRI Coil Segment.

The development of coils that can survive a quench is crucial for demonstrating the viability of MgB2-based main magnet coils used in MRI systems. Here we have studied the performance and quench properties of a large (outer diameter: 901 mm; winding pack: 44 mm thick × 50.6 mm high) conduction-cooled, react-and-wind (R&W), MgB2 superconducting coil. Minimum quench energy (MQE) values were measured at several coil operating currents (I op ), and distinguished from the minimum energy needed to generate a normal zone (MGE). During these measurements, normal zone propagation velocities (NZPV) were also determined using multiple voltage taps placed around the heater zone. The conduction cooled coil obtained a critical current (I c ) of 186 A at 15 K. As the operating currents (I op ) varied from 80 A to 175 A, MQE ranged from 152 J to 10 J, and NZPV increased from 1.3 to 5.5 cm/s. Two kinds of heater were involved in this study: (1) a localized heater ("test heater") used to initiate the quench, and (2) a larger "protection heater" used to protect the coil by distributing the normal zone after a quench was detected. The protection heater was placed on the outside surface of the coil winding. The test heater was also placed on the outside surface of the coil at a small opening made in the protection heater. As part of this work, we also developed and tested an active protection scheme for the coil. Such active protection schemes are of great interest for MgB2-based MRIs because they permit exploitation of the relatively large MQE values of MgB2 to enable the use of higher J e values which in turn lead to competitive MgB2 MRI designs. Finally, the ability to use a quench detection voltage to fire a protection heater as part of an active protection scheme was also demonstrated.

Architecture and Transport Properties of Multifilamentary MgB2 Strands for MRI and Low AC Loss Applications.

Standard in-situ type MgB2 strands manufactured by Hyper Tech Inc have 19 - 36 subelements, a monel outer sheath, and a Cu interfilamentary matrix. Typical transport Jc s of the strands are 2×105 A/cm2 with n-values of 20 - 30 at 4.2 K and 5 T. This work introduces two new MgB2 conductor designs. First, a new class of MgB2 strand is designed for magnetic resonance imaging applications. This type has a higher Cu content designed to enhance protection of a magnet wound with it, and a larger diameter to increase the critical current. Second, a new class of low AC loss MgB2 strand with high filament count and a high resistance matrix is discussed. Transport properties at 4.2 K and fields up to 10 T are reported. Optical techniques are used to study the macro- and micro-structures of these MgB2 strands.

Demonstration of a Conduction Cooled React and Wind MgB2 Coil Segment for MRI Applications.

This study is a contribution to the development of technology for an MgB2-based, cryogen-free, superconducting magnet for an MRI system. Specifically, we aim to demonstrate that a react and wind coil can be made using high performance in-situ route MgB2 conductor, and that the conductor could be operated in conduction mode with low levels of temperature gradient. In this work, an MgB2 conductor was used for the winding of a sub-size, MRI-like coil segment. The MgB2 coil was wound on a 457 mm ID 101 OFE copper former using a react-and-wind approach. The total length of conductor used was 330 m. The coil was epoxy impregnated and then instrumented for low temperature testing. After the initial cool down (conduction cooling) the coil Ic was measured as a function of temperature (15-30 K), and an Ic of 200 A at 15 K was measured.

Influence of Metal Diboride and Dy2O3 Additions on Microstructure and Properties of MgB2 Fabricated at High Temperatures and under Pressure.

High temperatures and under pressure (HTP) processing has been used to study the effects of chemical doping in MgB2. ZrB2, TiB2 and NbB2 were selected as additives since, like MgB2, they have an AlB2-type structure and similar lattice parameters. Dy2O3 was selected as it has been reported to generate nanoscale, secondary intragrain phases in MgB2. While C is known to enter the B-sublattice readily, attempts to dope Zr and other elements onto the Mg site have been less successful due to slow bulk diffusion, low solubility in MgB2, or both. We have used high-temperature, solid-state sintering (1500 °C), as well as excursions through the peritectic temperature (up to 1700 °C), to investigate both of these limitations. Bulk MgB2 samples doped with MB2 (M = Zr, Ti and Nb) and Dy2O3 additions were synthesized and then characterized. Lattice distortion and high densities of crystal defects were observed in the MgB2 grains around nano-sized MB2 inclusions, this highly defected band contributed to a large increase in Bc2 but was not large enough to increase the irreversibility field. In contrast, distributed intragrain precipitates were formed by Dy2O3 additions which did not change the lattice parameters, Tc, Tc distribution or Bc2 of MgB2, but modified the flux pinning.

A model for the compositions of non-stoichiometric intermediate phases formed by diffusion reactions, and its application to Nb3Sn superconductors.

In this work we explore the compositions of non-stoichiometric intermediate phases formed by diffusion reactions: a mathematical framework is developed and tested against the specific case of Nb3Sn superconductors. In the first part, the governing equations for the bulk diffusion and inter-phase interface reactions during the growth of a compound are derived, numerical solutions to which give both the composition profile and growth rate of the compound layer. The analytic solutions are obtained with certain approximations made. In the second part, we explain an effect that the composition characteristics of compounds can be quite different depending on whether it is the bulk diffusion or grain boundary diffusion that dominates in the compounds, and that "frozen" bulk diffusion leads to unique composition characteristics that the bulk composition of a compound layer remains unchanged after its initial formation instead of varying with the diffusion reaction system; here the model is modified for the case of grain boundary diffusion. Finally, we apply this model to the Nb3Sn superconductors and propose approaches to control their compositions.

Microstructures and superconducting properties of high performance MgB2 thin films deposited from a high-purity, dense Mg-B target.

High quality, c-axis oriented, MgB2 thin films were successfully grown on 6H-SiC substrates using pulsed laser deposition (PLD) with subsequent in situ annealing. To obtain high purity films free from oxygen contamination, a dense Mg-B target was specially made from a high temperature, high pressure reaction of Mg and B to form large-grained (10~50 µm) MgB2. Microstructural analysis via electron microscopy found that the resulting grains of the film were composed of ultrafine columnar grains of 19-30 nm. XRD analysis showed the MgB2 films to be c-axis oriented; the a-axis and c-axis lattice parameters were determined to be 3.073 ± 0.005 Å and 3.528 ± 0.010 Å, respectively. The superconducting critical temperature, Tc,onset , increased monotonically as the annealing temperature was increased, varying from 25.2 K to 33.7 K. The superconducting critical current density as determined from magnetic measurements, Jcm , at 5 K, was 105 A/cm2 at 7.8 T; at 20 K, 105 A/cm2 was reached at 3.1 T. The transport and pinning properties of these films were compared to "powder-in-tube" (PIT) and "internal-infiltration" (AIMI) processed wires. Additionally, examination of the pinning mechanism showed that when scaled to the peak in the pinning curve, the films follow the grain boundary, or surface, pinning mechanism quite well, and are similar to the response seen for C doped PIT and AIMI strands, in contrast to the behavior seen in undoped PIT wires, in which deviations are seen at high b (b = B/Bc2 ). On the other hand, the magnitude of the pinning force was similar for the thin films and AIMI conductors, unlike the values from connectivity-suppressed PIT strands.

Influence of Twisting and Bending on the Jc and n-value of Multifilamentary MgB2 Strands.

The influences of strand twisting and bending (applied at room temperature) on the critical current densities, Jc , and n-values of MgB2 multifilamentary strands were evaluated at 4.2 K as function of applied field strength, B. Three types of MgB2 strand were evaluated: (i) advanced internal magnesium infiltration (AIMI)-processed strands with 18 filaments (AIMI-18), (ii) powder-in-tube (PIT) strands processed using a continuous tube forming and filling (CTFF) technique with 36 filaments (PIT-36) and (iii) CTFF processed PIT strands with 54 filaments (PIT-54). Transport measurements of Jc(B) and n-value at 4.2 K in fields of up to 10 T were made on: (i) PIT-54 after it was twisted (at room temperature) to twist pitch values, Lp , of 10-100 mm. Transport measurements of Jc(B) and n-value were performed at 4.2 K; (ii) PIT-36 and AIMI-18 after applying bending strains up to 0.6% at room temperature. PIT-54 twisted to pitches of 100 mm down to 10 mm exhibited no degradation in Jc(B) and only small changes in n-value. Both the Jc(B) and n-value of PIT-36 were seen to be tolerant to bending strain of up to 0.4%. On the other hand, AIMI-18 showed ±10% changes in Jc(B) and significant scatter in n-value over the bending strain range of 0-0.6%.

Kinetic analysis of MgB2 layer formation in advanced internal magnesium infiltration (AIMI) processed MgB2 wires.

Significantly enhanced critical current density (Jc) for MgB2 superconducting wires can be obtained following the advanced internal Mg infiltration (AIMI) route. But unless suitable precautions are taken, the AIMI-processed MgB2 wires will exhibit incomplete MgB2 layer formation, i.e. reduced superconductor core size and hence suppressed current-carrying capability. Microstructural characterization of AIMI MgB2 wires before and after the heat treatment reveals that the reaction mechanism changes from a "Mg infiltration-reaction" at the beginning of the heat treatment to a "Mg diffusion-reaction" once a dense MgB2 layer is formed. A drastic drop in the Mg transport rate from infiltration to diffusion causes the termination of the MgB2 core growth. To quantify this process, a two-stage kinetic model is built to describe the MgB2 layer formation and growth. The derived kinetic model and the associated experimental observations indicate that fully reacted AIMI-processed MgB2 wires can be achieved following the optimization of B particle size, B powder packing density, MgB2 reaction activation energy and its response to the additions of dopants.