Correlated vortex pinning in Si-nanoparticle doped MgB2

168

134

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 ...

Section: Critical Current Density and Its Enhancement By Doping And F...mentioning

confidence: 99%

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

Collings

Sumption

Bhatia

et al. 2008

168

134

“…If intragranular pinning were dominant, then GB could simply provide transparency to the current, although in reality to some degree grain boundaries will always act to reduce J c . In fact, several studies [10,[13][14][15][16] have shown that small grain samples yield high J c and so the importance of GB pinning in MgB 2 appears to be unquestionable.…”

Section: Introduction: Pinning Mechanisms In Mgbmentioning

confidence: 99%

Grain boundaries and pinning in bulk MgB₂

Mikheenko¹,

Martínez²,

Bevan³

et al. 2007

We report the grain size dependence of critical current and grain boundary pinning in bulk MgB2. By combining polarized optical microscopy and electron backscatter diffraction, we obtain evidence of special grain boundaries with a high density of dislocations that are able to provide high critical current in MgB2 polycrystals. We argue that reduction of grain size to the nanoscale level is sufficient to provide the critical current densities required for large-scale applications at the boiling temperature of liquid hydrogen.

“…The disadvantage of pure MgB 2 is its quite low upper critical field B c2 (4.2 K) ∼ 18 T and weak flux pinning (which results in a low irreversibility field B irr (4.2 K) ∼ 12 T, and thus low critical current density in high fields). Flux pinning in pure MgB 2 is weak in spite of the same main pinning mechanism as in Nb 3 Sn [3] (grain boundary pinning [4,5]) due to rather clean boundaries and fairly large grains in normally prepared samples [4]. Further, in situ prepared MgB 2 samples [6] are very porous, which limits the magnitude of J c [7] and causes inhomogeneity within the sample [8].…”

Section: Introductionmentioning

confidence: 99%

Enhancement of critical fields and current of MgB₂by co-doping

Novosel¹,

Galić²,

Pajić³

et al. 2013

The electromagnetic properties of well-characterized iron-sheathed MgB 2 wires, undoped and doped with dextrin-coated magnetite nanospheres and nanorods, have been studied in the temperature range 5-300 K and a magnetic field up to 16 T. Doping hardly affected the superconducting transition temperature and the active cross-sectional area of the wires, and increased the low temperature upper critical field, B c2 . Wire doped with nanospheres also showed enhanced irreversibility field, B irr , and low temperature (T ≤ 15 K) critical current density. Magnetization measurements generally confirm the transport results and indicate an enhancement of flux pinning in nanosphere doped wire for T ≤ 20 K. Annealing of wire doped with nanorods at higher temperature (750 • C) enhanced its critical fields, as expected for co-doping.