John Hay Durrell

A trapped field of 17.6 T in melt-processed, bulk Gd-Ba-Cu-O reinforced with shrink-fit steel

The ability of large-grain (RE)Ba2Cu3O7−δ ((RE)BCO; RE = rare earth) bulk superconductors to trap magnetic fields is determined by their critical current. With high trapped fields, however, bulk samples are subject to a relatively large Lorentz force, and their performance is limited primarily by their tensile strength. Consequently, sample reinforcement is the key to performance improvement in these technologically important materials. In this work, we report a trapped field of 17.6 T, the largest reported to date, in a stack of two silver-doped GdBCO superconducting bulk samples, each 25 mm in diameter, fabricated by top-seeded melt growth and reinforced with shrink-fit stainless steel. This sample preparation technique has the advantage of being relatively straightforward and inexpensive to implement, and offers the prospect of easy access to portable, high magnetic fields without any requirement for a sustaining current source.

Self-assembled, rare earth tantalate pyrochlore nanoparticles for superior flux pinning in YBa2Cu3O7-δ films

Addition of pyrochlore rare earth tantalate phases, RE3TaO7 (RTO, where RE = rare earth, Er, Gd and Yb) to YBa2Cu3O7?? (YBCO) is shown to vastly improve pinning, without being detrimental to the superconducting transition temperature. The closely lattice matched to RTO phase provides a lower interfacial energy with YBCO than BaZrO3 (BZO) and produces very fine (~5?nm) particles with high linearity in their self-assembly along c. Critical current densities of 0.86, 0.38?MA?cm?2 at 1 and 3?T (for fields) parallel to the c axis were recorded at 77?K in 0.5?1.0??m thick films and a transition temperature of 92?K was observed even in the highest level doped sample (8?mol%).

The behavior of grain boundaries in the Fe-based superconductors

The Fe-based superconductors (FBS) are an important new class of superconducting materials. As with any new superconductor with a high transition temperature and upper critical field, there is a need to establish what their applications potential might be. Applications require high critical current densities, so the usefulness of any new superconductor is determined both by the capability to develop strong vortex pinning and by the absence or ability to overcome any strong current-limiting mechanisms of which grain boundaries in the cuprates are a cautionary example. In this review we first consider the positive role that grain boundary properties play in the metallic, low temperature superconductors and then review the theoretical background and current experimental data relating to the properties of grain boundaries in FBS polycrystals, bi-crystal thin films, and wires. Based on this evidence, we conclude that grain boundaries in FBS are weak linked in a qualitatively similar way to grain boundaries in the cuprate superconductors, but also that the effects are a little less marked. Initial experiments with the textured substrates used for cuprate coated conductors show similar benefit for the critical current density of FBS thin films too. We also note that the particular richness of the pairing symmetry and the multiband parent state in FBS may provide opportunities for grain boundary modification as a better understanding of their pairing state and grain boundary properties are developed.

A trapped field of >3 T in bulk MgB2 fabricated by uniaxial hot pressing

A trapped field of over 3 T has been measured at 17.5 K in a magnetized stack of two disc-shaped bulk MgB2 superconductors of diameter 25 mm and thickness 5.4 mm. The bulk MgB2 samples were fabricated by uniaxial hot pressing, which is a readily scalable, industrial technique, to 91% of their maximum theoretical density. The macroscopic critical current density derived from the trapped field data using the Biot–Savart law is consistent with the measured local critical current density. From this we conclude that critical current density, and therefore trapped field performance, is limited by the flux pinning available in MgB2, rather than by lack of connectivity. This suggests strongly that both increasing sample size and enhancing pinning through doping will allow further increases in trapped field performance of bulk MgB2.

Negative magnetocaloric effect from highly sensitive metamagnetism in CoMnSi1-xGex

We report a novel negative magnetocaloric effect in CoMnSi_{1-x}Ge_{x} arising from a metamagnetic magnetoelastic transition. The effect is of relevance to magnetic refrigeration over a wide range of temperature, including room temperature. In addition we report a very high shift in the metamagnetic transition temperature with applied magnetic field. This is driven by competition between antiferromagnetic and ferromagnetic order which can be readily tuned by applied pressure and compositional changes.

Importance of low-angle grain boundaries in YBa2Cu3O7-delta coated conductors

Over the past ten years the perception of grain boundaries in YBa2Cu3O7−δ conductors has changed greatly. They are now not a problem to be eliminated, but an inevitable and potentially favourable part of the material. This change has arisen as a consequence of new manufacturing techniques which result in excellent grain alignment, reducing the spread of grain boundary misorientation angles. At the same time there is considerable recent evidence which indicates that the variation of properties of grain boundaries with mismatch angle is more complex than a simple exponential decrease in critical current. This is due to the fact that low-angle grain boundaries represent a qualitatively different system to high-angle boundaries. The time is therefore right for a targeted review of research into low-angle YBa2Cu3O7−δ grain boundaries. This article does not purport to be a comprehensive review of the physics of grain boundaries as found in YBa2Cu3O7−δ in general; for a broader overview we would recommend that the reader consult the comprehensive review of Hilgenkamp and Mannhart (2002 Rev. Mod. Phys. 74 485). The purpose of this article is to review the origin and properties of the low-angle grain boundaries found in YBa2Cu3O7−δ coated conductors both individually and as a collective system.

Theory and experiment testing flux-line cutting physics

We discuss predictions of five proposed theories for the critical state of type-II superconductors accounting for both flux cutting and flux transport (depinning). The theories predict different behaviours for the ratio Ey/Ez of the transverse and parallel components of the in-plane electric field produced just above the critical current of a type-II superconducting slab as a function of the angle of an in-plane applied magnetic field. We present experimental results measured using an epitaxially grown YBCO thin film favouring one of the five theories, i.e.?the extended elliptic critical-state model. We conclude that when the current density J is neither parallel nor perpendicular to the local magnetic flux density B, both flux cutting and flux transport occur simultaneously when J exceeds the critical current density Jc, indicating an intimate relationship between flux cutting and depinning. We also conclude that the dynamical properties of the superconductor when J exceeds Jc depend in detail upon two nonlinear effective resistivities for flux cutting (?c) and flux flow (?f) and their ratio r = ?c/?f.

Thin-film metal catalyst for the production of multi-wall and single-wall carbon nanotubes

We present a detailed study of the growth of multiwall and single-wall carbon nanotubes (SWCNTs) by chemical-vapor deposition using a thin-film triple metal (Al∕Fe∕Mo) catalyst. Using Nanoauger spectroscopy, a full map of the metals in the sample surface is constructed and their evolution followed at different deposition temperatures. During the formation of SWCNTs at high temperatures (∼1000°C), the initial iron layer (∼1nm) is transformed into nanosized particles at the surface. In addition, the Al layer also plays a critical role during the annealing process by being altered into AlxOy particles. These particles act as a suitable underlayer to stabilize the nanosized Fe catalyst for nanotube growth. We also show that it is possible to resolve SWCNTs by mapping the areal intensity of carbon KVV Auger electrons.

Mechanisms for enhanced supercurrent across meandered grain boundaries in high-temperature superconductors

It has been well established that the critical current density Jc across grain boundaries (GBs) in high-temperature superconductors decreases exponentially with misorientation angle θ beyond ∼2°–3°. This rapid decrease is due to a suppression of the superconducting order parameter at the grain boundary, giving rise to weakly pinned Abrikosov-Josephson (AJ) vortices. Here we show that if the GB plane meanders, this exponential dependence no longer holds, permitting greatly enhanced Jc values: up to six times at 0T and four times at 1T at θ∼4°–6°. This enhancement is due to an increase in the current-carrying cross section of the GBs and the appearance of short AJ vortex segments in the GB plane, confined by the interaction with strongly pinned Abrikosov (A) vortices in the grains.

Enhanced critical current in YBa2Cu3O7?? thin films through pinning by ferromagnetic YFeO3 nanoparticles

Nanoscale ferromagnetic inclusions of YFeO3 have been incorporated into pulsed laser deposited YBa2Cu3O7 ? ? (YBCO) thin films. The poisoning of the YBCO through the addition of the magnetic material is minor, with 1?mol% doping resulting in an unsuppressed superconducting transition temperature of 90?K. The critical current density of the magnetically doped films is enhanced both in field and at self-field, and values of 3.0?MA?cm ? 2 have been achieved at 77?K, self-field in films 1??m thick, compared to 1.5?MA?cm ? 2 in an undoped film prepared by the same process. Such an enhancement in critical current at such low dopant levels is suggestive of an additional contribution to the flux pinning from the magnetic constituent.