Recent advances in MgB 2 conductors are leading to a new level of performance.Based on the use of proper powders, proper chemistry, and an architecture which incorporates internal Mg diffusion (IMD), a dense MgB 2 structure with not only a high critical current density J c , but also a high engineering critical current density, J e , can be obtained. In this paper, a series of these advanced (or second-generation, "2G") conductors has been prepared. Scanning electron microscopy and associated energy dispersive X-ray spectroscopy were applied to characterize the microstructures and compositions of the wires, and a dense MgB 2 layer structure was observed. The best layer J c for our sample is 1.07x10 5 A/cm 2 at 10 T, 4.2 K, and our best J e is seen to be 1.67x10 4 A/cm 2 at 10 T, 4.2 K.Optimization of the transport properties of these advanced wires is discussed in terms of Bpowder choice, area fraction, and the MgB 2 layer growth mechanism. PACS: 74.70.Ad; 74.25.Sv; 74.25.Qt; 74.62.Dh Keywords: MgB 2 , layer critical current density J c , engineering critical current density J e , internal magnesium diffusion (IMD)

An advanced internal Mg infiltration method (AIMI) in this paper has been shown to be effective in producing superconducting wires containing dense MgB 2 layers with high critical current densities. In this study, the in-field critical current densities of a series of AIMI-fabricated MgB 2 strands were investigated in terms of C doping levels, heat treatment (HT) time and filament numbers. The highest layer J c for our monofilamentary AIMI strands is 1.5 × 10 5 A/cm 2 at 10 T, 4.2 K, when the C concentration was 3 mol% and the strand was heat-treated at 675 °C for 4 hours. Transport critical currents were also measured at 4.2 K on short samples and one-meter segments of eighteen-filament Cdoped AIMI strands. The layer J c s reached 4.3 × 10 5 A/cm 2 at 5 T and 7.1 × 10 4 A/cm 2 at 10 T, twice as high as those of the best PIT strands. The analysis of these results indicates that the AIMI strands, possessing both high layer J c s and engineering J e s after further optimization, have strong potential for commercial applications.

Monocore powder-in-tube MgB 2 precursor strands were cold-drawn and heat- This effect is attributed to the fibrous macrostructure and its accompanying anisotropic connectivity. Magnetic measurements with the field directed along the strand axis yielded a critical density, J cm ┴ (B), for current flowing transversely to the strand axis that was less than and dropped off more rapidly than J ct (B). In the conventional magnetic measurement, the loop currents that support the magnetization are restricted by the lower of J ct (B) and J cm  (B). In the present case the latter, leading to the premature dropoff of the measured J cm (B) compared to J ct (B) with increasing field. This result is supported byKramer plots of the J cm  (B) and J ct (B) data which lead to an irreversibility field for transverse current that is very much less than the usual transport-measured longitudinal one, B irr,t .

A series of MgB 2 superconducting composite strands co-doped with Dy 2 O 3 and C were prepared via an advanced internal Mg infiltration (AIMI) route. The transport properties and MgB 2 layer growth were studied in terms of the Dy 2 O 3 doping level, reaction temperature, and reaction time. Transport studies showed that both critical current densities, J c s, and irreversibility fields, B irr s, were increased with Dy 2 O 3 doping. The highest layer J c was 1.35 × 10 5 A/cm 2 at 4.2 K, 10 T, 30 % higher than that of the best AIMI wires without Dy 2 O 3 doping. The highest "nonbarrier" J c reached 3.6 × 10 4 A/cm 2 at 4.2 K, 10 T, which was among the best results reported so far. The improvements were even more pronounced at higher temperatures where the field at which the layer J c reached 10 4 A/cm 2 was pushed out by 0.9 T at 20 K, 1.2 T at 25 K, and 1.4 T at 30 K. While little or no enhancement in B irr was seen at 10 K and 15 K, the increases in J c at higher temperatures were consistent with observed increases in B irr of 17% at 20 K, 44% at 25 K, and 400% at 30 K. Also, there were some indications that the reaction and layer growth of MgB 2 was enhanced by Dy 2 O 3 doping.

In this study we show that dominant point pinning mechanisms in SiC doped MgB2 wires can be obtained by annealing in high isostatic pressure. The results indicate that the point pinning centers increase the critical current density in medium and high magnetic fields, but not at low magnetic fields. In addition, our study shows that dominant pinning mechanism changes from point to surface type with increase of magnetic fields. An MgB2 wire heat treated in a high pressure of 1.4 GPa shows a high critical current density of 100 A mm−2 in 13 T at 4.2 K. Scanning electron microscope studies show that high isostatic pressure increases the density of the MgB2 material, eliminates voids, allows for small Si precipitates and homogeneous distribution of Si precipitates. Transport measurements E - B and E - I show that the MgB2 wires manufactured by Hyper Tech Research did not heat up after transition into a normal state. This is important for applications in coils.