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

Abstract

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 ...

Drawing induced texture and the evolution of superconductive properties with heat treatment time in powder-in-tubein situprocessed MgB2strands

“…The Mg vapour that evolves during HT permeates the B and begins to convert it to veins of MgB 2 . As a result of the density difference between B and MgB 2 [67] these veins will swell during this conversion (resulting in a 1.2-fold increase in volume assuming an initial B density of 65%) and will stop growing when either all the B is converted to MgB 2 or until neighboring veins of …”

Superconducting Properties of MgB<SUB>2</SUB> Particle Impregnated with Mg-Based Alloys

“…The observed J c values with the present samples are, however, about one order of magnitude smaller than those of the in-situ PIT samples. 7) As claimed in the literature, a possible reason is that the used MgB 2 powder is rather clean; in-situ MgB 2 powders are usually dirty. In fact, the J c values measured with only the MgB 2 pressed-powder are about half of those in the M0 samples as seen in Fig.…”

“…Especially J c improvement in MgB 2 has been one of major issues of research; a well known problem is that J c drops rapidly with increasing magnetic field due to its poor flux pinning. Improvements in J c (enhancements of flux pinning) were achieved by various approaches; doping by chemical compounds, 7) substitution of Mg and/or B by other elements, 8,9) high-pressure application during process, 10) various heat-treatment, 11) etc. Theoretical studies for variations of the band structure by element substitution haven been also carried out.…”

Technical aspects of cold high pressure densification (CHPD) on long lengths of In Situ MgB2 wires with enhanced Jc values

“…However, a main factor limiting the transport properties in both the ex situ and in situ wires is the low mass density in the filaments, a common property of wires produced using powder metallurgical techniques. With respect to the theoretical mass density of 2.63 g/cm 3 , the relative mass densities in the MgB 2 filaments of filaments produced by the standard techniques vary between ~45 and 50% for in situ wires [1,16] and up to 80% for ex situ wires [16,17]. The reason for the different mass densities is linked to the fact that ex situ filaments are formed by sintering already formed MgB 2 grains, while the in situ technique involves a phase reaction process with a volume shrinkage of ~ 20% between the original Mg+B mixture and the final MgB 2 phase [1,16].…”