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[en] A study is presented on structure and crystallographic orientation of Nb3Sn layers formed in the process of interaction of polycrystalline Nb substrate with Cu/sub 0.2/Sn/sub 0.8/ solution at 980 0C. Grain structure characterization of Nb3Sn layers has been carried out with scanning electron microscopy and TV automatic image analysis. The superconducting layer is divided into a fine-grained FG and coarse-grained CG zone. The X-ray patterns of FG and CG zones did not differ from one another and were the same as in randomly oriented Nb3Sn polycrystalline powder
[en] The 119Sn Moessbauer spectroscopy was used to examine the influence of temperature (1153 to 1253 K) and heating time (11 and 22 min) on the formation of the β-Nb3Sn phase in simultaneous processes: peritectic reaction of NbSn2 phase decomposition and reactive diffusion of Sn atoms into the Nb1.5%Zr base
[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, Jc was degraded drastically due to the decrease in the superconducting volume fraction. However, the alloying effect did not lower the Tc onset, but broadened the transition. A sharp Tc transition and Jc(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 Jc 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, Ic(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] 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] 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 (kab) and along the c-axis (kc) 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 kc 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 kab and kc. 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] 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] Results are given of the effect of the temperature of growth of the pinning force. Investigations were performed for the perpendicular H and parallel H orientation of the tape conductor of Nb3Sn face in relation to the magnetic field vector
[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)