FEL & Utilization Section

Research on magnetic and superconducting materials

1. Magnetic materials:

The research on magnetic materials is currently focused on magnetocaloric effect and its possible usage in :

(i) Refrigeration in and around room temperature and
(ii) Gas liquefaction.

(i) Magnetic refrigeration is an energy-efficient and environmentally sound technology alternative to vapour-cycle refrigerators and air conditioners. It offers considerable saving of operating cost by eliminating the most inefficient part of the existing refrigerators– the compressor. It uses a solid refrigerant and a common heat transfer fluid (e.g. water, air or helium gas) with no ozone-depleting and global-warming effects. The technology right now is at a nascent stage, and its development will largely depend on discovering materials with a large magnetocaloric effect at or close to room temperature. The ongoing research activity is presently focused on two classes of materials namely NiMnX (X=In, Sn, Al etc.) based ternary Heusler alloys and FeRh based binary alloys.

Large magnetocaloric effect with significant refrigerant capacity has been observed around 240K in a Ni50Mn34In16 alloy, and the working temperature has been pushed further to 275 K by partial chemical substitution of Cr and Cu in this Ni50Mn34In16 alloy. Further research has revealed that the same first order magneto-structural phase transition, which is responsible for the magnetocaloric effect in these NiMnIn based alloys, also gives rise to a large magnetoresistance and large magnetic field induced strain. These results highlight the multifunctional nature of this alloy system. The same multifunctional properties associated with the first order magneto-structural phase transition have been found in the equi-atomic FeRh alloy. A very large and reproducible magnetocaloric effect with giant magnetic refrigeration capacity has been observed FeRh alloys in and around the room temperature.

At the root of such multifunctional properties is a disorder influenced first order phase transition, which in turn is a manifestation of an interesting interplay between electronic and lattice degree of freedom observed in various classes of magnetic materials. This generality has been highlighted with the help of an interesting model intermetallic compound CeFe2. Further it has been shown that under certain circumstances in the presence of an applied magnetic field, this first order magneto-structural phase transition often gets kinetically arrested, and thus gives rise to a highly non-equilibrium glass-like magnetic state. This magnetic-glass state is distinctly different from ‘spin-glass’ state, and has also been observed in NiMnIn, FeRh and Gd5Ge4, apart from the CeFe2 based alloys.

(ii) Liquid hydrogen, with its high volumetric density, is a useful medium for storing and transporting hydrogen efficiently and economically. In conventional liquefiers, the figure of merit currently is approximately 35%. In order to obtain higher efficiency (>50%), magnetic refrigeration based on magnetocaloric effect is a promising cooling method.

The ongoing research in this direction is focused on finding materials with large magnetocaloric effect in the temperature regime 20 to 70K. Several new magnetocaloric materials with significant potential in this direction- DyCu2, DyPt2, MnSi, NdRu2 and GdCu6 have been identified.

Further details on these activities can be found here.

2. Superconducting materials:

The research in superconducting materials is currently focused on:

(i) Materials for superconducting high-current applications (i.e. high-field superconducting magnets)
(ii) Materials for superconducting radio-frequency (SCRF) cavity applications.

(i) Materials for superconducting high-field applications:
The commercially available superconducting magnets are currently based on NbTi alloys (for fields < 7 T) and Nb3Sn (for fields 7T < H < 15 – 20T). The increasing demand for higher magnetic fields motivates research on newer superconducting materials with superior current carrying capacity. The A15-superconductor Nb3Al is an example of such a material, which has been identified for R&D on high-field superconducting magnets to be used in International Thermonuclear Experimental Reactor (ITER). The FEL Utilization Laboratory (FELUL) has worked on this material, and has discovered a new composite superconducting material consisting of Nb3Al nano-particles embedded in a Nb-Al matrix, with a large critical current carrying capacity. This research gives a new direction to the ongoing activity, and studies are presently underway on the possibility of tuning their properties for technological applications. The group also has interest in the study of refractory metal alloys superconductors involving Nb, Zr, Ti, V, Mo and Re, particularly in the correlation between their excellent mechanical properties and relatively less explored superconducting properties. Ti-V alloys in particular are promising alternatives to Nb-based superconductors because of their suitability for long term neutron irradiation environment, and high degree of machinability.

(ii) Materials for superconducting radiofrequency (SCRF) cavity applications:
SCRF cavities are used extensively in high energy particle accelerators operating in the continuous wave (CW) or long-pulse mode with high accelerating electric field gradients. Two fundamental limits for a SCRF cavity are: (i) critical RF magnetic field above which the perfect superconducting state is destroyed, which limits the ‘accelerating field’ or ‘gradient’, (ii) surface resistance as predicted by the microscopic BCS theory, which limits the quality factor Q. The current material of choice for such SCRF cavities is the type-II superconductor niobium (Nb) in its high purity form. An open question in SCRF cavity technology is why the RF surface resistance of Nb increases sharply in magnetic fields well below the expected limit of critical field of Nb. Recent research at RRCAT has provided some clues in this direction suggesting that “buffer chemical polishing” introduces plenty of hydrogen and oxygen in Nb, which impairs its microscopic superconducting properties. The ongoing research in the FELUL aims at a better understanding of the superconducting and thermal properties of Nb and its alloys (both in the bulk and in the thin-film form), and of other materials like the Ti-V and Mo-Re alloys. This research will help to identify materials leading to better and reproducible performance of a SCRF cavity. It is believed that there is still scope for the optimization of the qualifying criteria in choosing the appropriate superconducting materials for SCRF cavity fabrication in terms of the superconducting, thermal and mechanical properties of the starting materials, which can help insignificantly reducing the cost of production of an SCRF cavity.

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