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Nano- and micro-scale engineering of higher-performance MgB2 composite superconductors for macro-scale applications

Final Report Summary - HIPERMAG (Nano- and micro-scale engineering of higher-performance MgB2 composite superconductors for macro-scale applications)

The project aimed to develop the superconducting material MgB2 into a technical superconductor. This new material has the potential to become the conductor of choice in various existing applications, as well as to play a pivotal role in the breakthrough of superconducting technology in the energy domain.

The objectives for Work Package WP1 were to provide the know-how needed for optimized powder processing, i.e. to find methods for the modification of the microstructure of precursor powders and bulk samples to achieve enhanced superconducting properties. This precursor powder was also tested in monofilamentary tapes.

At the beginning of the project work, the influence of the Mg/B ratio in the precursor powder on the properties of MgB2 bulk samples was investigated.

In further work it turned out that the Boron quality is an essential parameter for the preparation of high-quality precursor powders. Oxygen impurities and the grain size of the boron precursor powder were found to affect the reactivity of the powder, the microstructure and the superconducting parameters of bulk samples.

It was demonstrated that the critical current densities Jc of bulk samples and tapes based on mechanically alloyed nanocrystalline precursor powder are also quite high without doping.

The results achieved in this work package were important for the potential manufacturing of high quality precursor powders. The excellent properties of these carbon doped nanosized precursor powders were demonstrated for bulk samples and monofilamentary tapes.

The work in WP2 led to a strong progress in the understanding of the micro-structure control in MgB2 wires. All milestones were achieved, and the conditions for obtaining higher Jc values in in-situ MgB2 wires can now be formulated quite precisely:
- a very high powder quality is required (> 99.99%), as well for amorphous Boron as for Mg and the additives;
- the amount of oxygen impurities has to be minimized, in order to prevent the formation of oxides at the grain boundaries;
- the size of all initial powder particles should be as small as possible;
- the mixing procedures used in WP2 also included mechanical alloying in dry environment under argon. Wet grinding allows better reduction, but introduces oxide contamination;
- the formation kinetics depends primarily on the powder size, but also on the chemical nature of the additive. Submicron silicon carbide (SiC) additives allow the lowest reaction temperatures;
- the metallic sheath has to be chosen in order to avoid too strong a reaction with the powder mixture;
- a thermal stabiliser is necessary for safe operation, and will usually be Cu.

The main task of the common investigations was to exploit the know-how generated in WP1 in order to establish a scalable, cost-effective and reproducible preparation route of MgB2 precursor powder.

One method of the powder processing was the high energy milling of the Mg and B powder e.g. Mechanical alloying (MA).

All efforts were concentrated on the augmenting of the high quality precursor material productions preserving their best parameters obtained in common WP1 and WP2 efforts.

It was found that, as in any material, the superconducting properties of MgB2 are strongly determined by the technology processing involved. The microstructure and composition are the main parameters of the improving of the Jc and Hc2 parameters.

Three main material properties are strongly important for high Jc parameters upon high magnetic fields: the grain size, the content of the carbon substituted for boron place and the connectivity of the grains.

The objectives for WP4 were to bring together the know-how developed in WP2 (Microstructural control) and WP3 (Powder processing) to ensure the compatibility between high-performance powders with carefully optimised nano-structure and the mechanical deformation and heat treatment needed for optimal stability.

The design of thin monofilament conductors with Nb barrier to prevent reaction of filament and sheath, with highly conductive Cu component to ensure good thermal stability and with reinforcing stainless steel sheath component seemed to be very promising also for a multifilament conductor design. Therefore, as continuation of this work multifilament conductors with Nb/Cu/stainless steel composite sheath were developed.

Good results for multifilamentary tapes with mechanically alloyed precursor powders where achieved for multifilament wires and tapes with Fe sheath. Cu-stabilised multifilament conductors with mechanically alloyed precursor powders could successfully be fabricated when harder barrier materials were used. A good barrier material - titanium - was found which not only prevents reaction of filament and sheath but is also hard enough to preserve the filament geometry even when the improved mechanically alloyed powders are used.

Another conductor design which proved to be suitable for introduction of mechanically alloyed precursor powders was a 14-filament tape with Ni sheath and a Cu component with Fe barrier to prevent alloying with Ni.

Even though the precursor powders were constantly improved, it became more and more clear that further enhancement of current carrying capabilities to values observed in thin films, can only be achieved by improving the connectivity of grains in the filament. Therefore, in a change of the work program basic research tasks with the aim to find ways to optimise the grain connectivity were established in WP4.

As a result of the investigations performed in the HIPERMAG project a multifilament conductor optimised with regard to thermal and mechanical stability and ac-losses should have the following design:
- a component with high electrical and thermal conductivity, e.g. Cu;
- barriers to prevent interdiffusion of elements between filament and sheath or between different sheath components. Nb is suitable for precursor powders with good deformation properties (powders that do not compact too much). Harder barrier materials like Ti or Fe are required in direct contact to fine-grained precursor powders with bad flow properties.
- a strong sheath component like Fe, Monel or stainless steel to ensure good compaction of the precursor powders during deformation.
- a strong sheath component with high thermal expansion coefficient to ensure precompression of the filament during cool-down from the temperature of the heat treatment to the temperature of the application.

Electron microscopy and spectroscopy techniques as well as high-energy X-ray diffraction were applied in WP5 for the analysis of superconducting MgB2 ceramics wires and tapes.

MgB2 wires, tapes and bulk samples were studied by a combination of X-ray diffraction and electron microscopy. The reaction layers forming at the interface between the ceramic core and Fe or Ni sheaths can be studied with both methods. Grain sizes can be determined either by direct observation or by analysis of the shape of X-ray diffraction peaks. Electron microscopy can detect B-rich secondary phases and phases present in small fractions that are not accessible by X-ray diffraction. On the other hand, synchrotron diffraction provides a fast and non-destructive method for the study of the main phases and their development during in-situ, high-temperature investigations. The combination of the two techniques is a very valuable tool for the optimisation of MgB2-based superconducting materials.

Performing diffraction studies with various angles between the incident beam and the plane of tapes, it was possible to determine the degree of preferential orientation of the MgB2 crystallites.

It was also observed that the degree of preferential orientation is broadly independent of the amount of carbon doped into the MgB2 phase. The differences in critical current anisotropy resulting from this type of doping are therefore due to other factors.

ICL's activities were aimed at characterising the changes in the key fundamental parameters as the MgB2 was modified in various ways. The techniques utilised were:
- measurement of the heat capacity in magnetic field, which provides a reliable evaluation of the Hc2, and
- Point contact spectroscopy (PCS) in magnetic field, which gives a direct measure of the two superconducting energy gaps and how they shrink with applied field. Both techniques were able to give information on another key aspect of the processed material - its homogeneity or lack of it.

The quench development of metal sheathed MgB2 conductors has been analysed. Cu-stabilised and non-stabilised conductors have been studied. Experimentally, energy pulses were deposited to the conductor by passing rectangular current pulses through a graphite-based-epoxy heater. The temperature and the electric field profiles around the point heat disturbance that gives rise to a quench, as well as their time evolution, were measured from multiple voltage taps and thermocouples along the conductor. The experimental results were in qualitative agreement with the simulated ones, obtained by solving the one-dimensional heat balance equation of the system. The results demonstrated that the Cu-stabilisation of MgB2 wires leads to enhanced stability with a higher MQE and a larger MPZ, hence significantly reducing the possibility of local burn-out as seen in standard Fe-sheathed wires. It was also found that the non-linear power-law current sharing in the normal zone has significant influence on the onset of the quench process and results in a marked deviation from the classical quench theory based on the critical state model. The unexpected increase of MPZ size with transport current for finite n-values, contradictory to the prediction of the standard current-share model for CSM, was explained on the basis of a non-linear current-share model based on power-law superconducting E(J) with a finite power exponent.

ICMA analysed the critical currents of bulk materials processed by Resistive sintering (RS) and Hot isostatic pressing (HIP) (Ubir, IFW). Various starting powders were used, including in situ and ex situ, mechanically alloyed and nanoparticles added. The effects of grain connectivity, flux-creep phenomena, grain size and the added nanoparticles on the critical current density were studied.

In terms of mechanical and AC loss properties, all benchmark conductors demonstrated a reversible and linear variation of the critical current with axial strain up to a critical limit, followed with an irreversible degradation. The critical strain limit is independent of temperature and magnetic field. Its value can be engineered by proper sheath design. The amplitude of the beneficial reversible strain variation is strongly temperature- and field dependent, but obeys a simple scaling relation. Preliminary AC loss measurements demonstrated hysteretic losses in ferromagnetic sheath and in filaments to dominate. Detailed measurement and analysis were carried out by IOC leading to the identification of different loss mechanisms including ferromagnetic hysteresis modulated by eddy / super-currents, coupling current, flux pinning hysteresis.