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 the 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 240 K 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.
The present research on magnetocaloric materials, which could be useful for room temperature refrigeration, is being carried out on earth abundant materials, such as Mn-Co-Ge alloys. A large magnetocaloric effect has been observed near room temperature in an off-stoichiometric composition of this system. The work is aimed at finding a suitable alternative to costly rare-earth materials which are difficult to purify. The alloys being pursued do not consist of elements such as arsenic, which are toxic in nature
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.
2. Superconducting materials:
The research in superconducting materials is currently focused on:
(i) Superconductors for high-current high magnetic field applications (i.e. high-field superconducting magnets)
(ii)Materials for superconducting radio-frequency (SCRF) cavity applications.
(i) Superconductors for high-current high magnetic field applications:
The commercially available superconducting magnets are currently based on NbTi alloys (for fields < 7 T) and Nb3Sn (for fields 7 T < H < 15– 20 T). 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 Section 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 Nb-based superconductors are not very suitable for long term neutron irradiation environment as the latter gives rise to long term radioactivity in these materials. Moreover, with the increasing usage of superconductivity in different fields in the modern world, the use Nb everywhere may lead to the scarcity of this element in future. Hence there is a need for looking into alternate superconducting alloys. The FEL & Utilization Section is strongly interested in the study of refractory metal alloys involving Zr, Ti, V, Mo and Re as alternate to the Nb based superconductors. Of particular current interest are the Ti-V alloys because of their excellent mechanical properties and other interesting physical properties. Magnetism and superconductivity has been found to co-exist in this series of alloys and substitution of rare earth elements has been found to enhance the current carrying ability of these alloys up to 20 times in magnetic fields up to 6 T.
(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’, and (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. 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. New techniques of chemo-mechanical polishing, which do not use harsh acidic environments, are being explored to achieve very low surface roughness on pure Nb coupons. A rms surface roughness of nearly 10 nm could be achieved with a lesser degradation of lower critical field when compared to buffer chemical polishing. The ongoing research 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 in significantly reducing the cost of production of an SCRF cavity in future.