The magnetic and transport properties of MgB 2 films represent performance goals yet to be attained by powder-processed bulk samples and conductors. Carbon-doped films have exhibited upper critical fields, μ 0 H c2 , as high as 60 T and a possible upper limit of more than twice this value has been predicted. Very high critical current densities, J c , have also been measured in films, e.g. 25 MA/cm 2 in self field and 7 kA/cm 2 in 15 T. Such performance limits are still out of the reach of even the best MgB 2 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 MgB 2 --H c2 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 H c2 enhancement is discussed and examples presented. On the other hand their roles in increasing J c , both via H c2 enhancement as well as direct fluxoid/pining-center interaction, are discussed and a comprehensive survey of H c2 dopants and flux-pinning additives is presented. Dopant selection, chemistry, methods of introduction (inclusion), and homogeneity of distribution (via the rounding of the superconducting electronic specific heat transition) are considered. 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. Overall porosity, including reduced density and intergranular blocking, is quantified in terms of the measured temperature dependence of the normal-state resistivity compared to that of a clean single crystal. Several experimental results are presented in terms of percent effective cross-sectional area for current transport. These and other such results indicate that in many cases less than 15% of the conductor's 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 J c in relevant regimes, enabling the performance of MgB 2 in selected applications to compete with that of Nb 3 Sn. imaging. But to take the next step, whether we are interested in high field 4 K operation, or in the more energy efficient 20 K temperatures regimes well out of the range of the present NbTi and Nb 3 Sn wires, further improvement of the in-field J c is needed. In addressing this issue we must recognize two classes of properties (what might be termed "intrinsic" and "extrinsic", respectively) that are in urgent need of fundamental research. Below, we summarize some of the key issues that need to be addressed. Keywords Intrinsic PropertiesThe essential intrinsic (i.e.intragranular) properties of polycrystalline MgB 2 are H c2 and flux pinning (or, alternatively, intra-grain J c Necessary ...

All rights reservedNo part of this book may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher To BETTY JANE AND CHRISTOPHER also to an international community of low-temperature physicists and cryogenic engineers whose work is represented on these pagesType-2 Superconductor in the Critical Mixed State.-Carbon-replicated electron micrograph of a ferromagnetic-particle-decorated representation of the arrangement of axially directed flux lines in an annealed-and-deformed Pb-In(6.3 at.OJo) 4-mm!6 rod in the trapping critical state (see, e.g., Figure 16-7). The arrangement of the flux lines was found to change continuously from long-range-ordered in the relatively high field (-70 G) interior region of the sample (;;, 400 ,an deep) [left-hand plate], to amorphous in the low-residual-field region just below the surface (,,; 100,an deep) [right-hand plate]. In this material, for which K = 2 at 1.2 K, the flux-lattice parameter at 70 G is about 0.6 I'm. Original micrograph courtesy of U. Essmann, Max-Planck-Institute fUr Metallforschung, Institut fUr Physik, Stuttgart, West Germany. (Reference: H. Trauble and U. Essmann, J. Appl. Phys. 39,4052-4059 (1968))1.General Preface xi property optimization. (iii) Superconductor and superconducting magnet design engineers for whom the chapters on Stability, AC Loss, and Conductor Design are intended. (iv) Manufacturers of "NbTi" alloy superconductors who wish to become reapprised of the Ti-alloy metallurgy and metal physics that underlies the behavior and properties of their product. Administrative Credits and Genesis

Resistive transition measurements are reported for MgB 2 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 MgB 2 [(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 minutes at temperatures from 675°C to 1000°C. Strands with 200 nm size SiC additions had µ 0 H irr and B c2 which maximized at 25.4 T and 29.7 T after heating at 800°C for 30 minutes. The highest values were seen for a strand with 15 nm SiC heated at 725°C for 30 minutes which had a µ 0 H irr of 29 T and a B c2 higher than 33 T.

Fe-clad MgB 2 long tapes have been fabricated using a powder-in-tube technique. An Mg + 2B mixture was used as the central conductor core and reacted in-situ to form MgB 2 . The tapes were sintered in pure Ar at 800 o C for 1 h at ambient pressure. SEM shows a highly dense core with a large grain size of 100 µm. The Fe clad tape shows a sharp transition with transition width of ∆T c of 0.2 K and T c0 at 37.5 K. We have achieved the highest transport critical current reported so far at 1.6 × 10 4 A/cm 2 for both 29.5 K in 1 Tesla and 33 K in null field. R-T and critical current were also measured for fields perpendicular and parallel to the tape plane. The iron cladding shielded on the core from the applied external field, with the shielding being less effective for the field in the tape plane. Fe cladding may be advantageous for some applications as it could reduce the effects of both the self-field and external fields.

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)