[en] Inward unidirectional copper diffusion into a pellet and the resulting alloying of Cu-Mg during in situ reaction of MgB₂ were studied. The reaction between the copper and magnesium occurred at as low a temperature as 660 deg. C in a flowing argon atmosphere giving rise to the formation of a MgCu₂ layer at the interface between the Cu and the MgB₂. The kinetics of copper alloying in the core sample was much enhanced at 800 deg. C and above, as shown by energy dispersive x-ray analysis. Consequently, J_c was degraded drastically due to the decrease in the superconducting volume fraction. However, the alloying effect did not lower the T_c onset, but broadened the transition. A sharp T_c transition and J_c(6 K,4 T)∼2 x 10⁴ A cm^-2 was achieved when annealing was conducted at the lower temperature of 750 deg. C. Direct comparison between the pure and copper alloyed samples shows that optimization of the heat treatment and hence J_c rests on the competing effects of phase formation and grain connectivity versus MgCu₂ formation

[en] The powder-in-tube technique has been used to fabricate Cu-sheathed magnesium diboride (MgB₂) wires using an in situ reaction method. The effects of high heating rate, short duration heat treatments of MgB₂/Cu wires were studied by means of optical and scanning electron microscopy, x-ray diffraction, resistivity, ρ(T), and susceptibility, χ'(T), measurements. The transport critical currents, I_c(B), of the wires were measured using both direct current in a constant field and pulse field-pulse current methods. Usage of high heating rates allows the processes of interest to occur in conditions closer to isothermal, which permits the influences of heat treatment time and temperature to be more readily distinguished. The results show that the microstructure and properties of the MgB₂/Cu wires are strongly dependent on the heat treatment temperature but quite insensitive to the reaction time: a short heat treatment for 5 min at 700 ⁰C was sufficient for obtaining the highest critical current achieved in this work.

[en] Rapid heat generation is one of the biggest problems faced in carrying out pulsed field magnetization of (RE)BCO superconducting bulks compared to other methods of magnetization. The effects of various thermal conductivities in the ab-plane (k_ab) and along the c-axis (k_c) of a bulk on its trapped field performance following pulsed field magnetization were modelled. The E–J power law was used, coupled with the heat generation, to simulate the effect of anisotropic thermal conductivity combinations on the peak trapped field and total trapped flux. A thermally isolated bulk is not affected so strongly by thermal conductivity, but on cooling the bulk conductively from its base using a cold head, increasing k_c significantly enhances both the trapped field and the flux. Embedding highly thermally conducting copper structures in the bulk material was investigated as a practical way to locally increase k_ab and k_c. The structures investigated increased the trapped field and flux by a maximum of around 30% without increasing the size of the bulk. Different structures can be used depending on whether an application requires the highest trapped field or highest total flux. (paper)

[en] Coated conductors, although designed to carry transport current, are ideally suited to carrying persistent current and can therefore be cut and stacked to form a type of composite bulk which has superior thermal properties compared to existing bulks despite having less than 2 % superconductor by volume. The magneto-thermal modelling reported follows on from previous experimental work on pulsed magnetization of a 12 mm square tape stack. The magnitude of the applied field has a strong effect on the trapped field and flux. The optimum applied field depends on sample height and the maximum trapped field and flux saturates as the height reaches the diameter of the stack. The nature of a composite bulk made from a stack of tapes gives complete control over the height of the stack which needs to be optimised for pulsed magnetization.

[en] Copper sheathed MgB2 wires, prepared by an in-situ process, were exposed to neutron radiation in order to introduce defects into the superconductor. The high level of disorder (4.6 x 10^-2 dpa) leads to a decreases of the transition temperature by more than 4 K, but to an increase of the slope of the irreversibility line, thus resulting in higher irreversibility fields at low temperatures. The transport currents are significantly enhanced at 4.2 K for fields above 2 T

[en] Electromagnetic compaction of in situ MgB_2/Cu wire has been achieved using a custom-built 200 J device. The monofilament core packing density was increased by 8% and up to 31% for unreacted and reacted wires respectively. The higher density of the MgB_2 core resulted in a critical current density increase of up to 75% in comparison to that for cold-drawn-only wire. Applying this treatment to a wire with Cu powder additions to the core and with an optimized heat treatment resulted in one of the highest ever reported values of J_c for MgB_2/Cu wires of 6.83 × 10"3 A cm"−"2 at 4.2 K and 6 T. (paper)

[en] The ability of superconductors to sustain persistent currents has been well exploited with (RE)BCO superconducting bulks, which can be magnetized to form a compact source of high magnetic field. However, thin films can also sustain persistent currents, which can be utilized by stacking them in layers to create a type of composite bulk. Such a stack is capable of trapping higher fields than a bulk, as reported in this paper. 12 mm wide, 55 μm thick commercial (RE)BCO tape from Superpower Inc was cut into 12 mm by 12 mm squares, stacked together and magnetized at temperatures between 10 and 77.4 K using a sequence of pulsed magnetic fields. The results are compared to a commercial 14 mm diameter YBCO bulk, showing that the stack of tapes outperformed the bulk at temperatures below approximately 60 K. Particularly high trapped fields were achieved below 50 K, with a maximum of 2.0 T at 10 K measured 0.8 mm from the stack surface. The maximum trapped field possible for a stack of tapes increases significantly with decreasing temperature down to 10 K, rather than saturating at a higher temperature as in the case of a bulk, due to superior thermal stability. The J_c, thermal and mechanical properties of commercial (RE)BCO tapes give them great potential for use as trapped field magnets activated by pulsed magnetic fields. (fast track communication)

[en] Stacks of superconducting tape, made of second-generation coated conductors, present an interesting alternative to bulk superconductors. The layered geometry allows incorporation of other functional, namely ferromagnetic, materials. The ferromagnetic material changes the field configuration within the superconductor and can be used to shape or divert the flux density from its interior. This can be used to optimise the remanent flux density after magnetisation. FEM simulations are employed to test the performance of a variety of superconductor/ferromagnet structures and explain the role of the ferromagnetic material in the composite structure. Cases where trapped field is much higher and lower than remanent magnetisation of the ferromagnetic material are considered, showing that performance can be enhanced or maintained by using ferromagnetic material while reducing the usage of costly superconducting tape.

[en] Design requirements of the next generation of electric aircraft place stringent requirements on the power density required from electric motors. A future prototype planned in the scope of the European project ‘Advanced Superconducting Motor Experimental Demonstrator’ (ASuMED) considers a permanent magnet synchronous motor, where the conventional ferromagnets are replaced with superconducting trapped field magnets, which promise higher flux densities and thus higher output power without adding weight. Previous work has indicated that stacks of tape show lower cross-field demagnetisation rates to bulk (RE)BCO whilst retaining similar performance for their size, however the crossed-field demagnetisation rate has not been studied in the temperature, the magnetic field and frequency range that are relevant for the operational prototype motor. This work investigates crossed-field demagnetisation in 2G high temperature superconducting stacks at temperatures below 77 K and a frequency range above 10 Hz. This information is crucial in developing designs and determining operational time before re-magnetisation could be required. (paper)

[en] Stacks of superconducting tapes nowadays have multiple applications and many new modifications are currently considered to enhance their beneficial properties. We have studied the field trapping in stacks of commercial superconducting tapes with different configurations. Experimental and numerical analyses were performed. The superconducting stacks were magnetized to act as powerful permanent magnets using pulsed field magnetization and field cooling at 77 K. The configurations include a basic stack made of layered tapes, a stack interlayered with ferromagnetic material, a sectioned stack made of thin tapes and a shielded basic stack.

The present study shows that, in terms of total trapped flux, the basic stack performs best, closely followed by the shielded stack. No significant positive effect due to the presence of ferromagnetic layers was found in the studied configuration. The sectioned stack is the worst according to every analysed criterion. The possibility of application of the analysed modification is discussed. (paper)