FEL & Utilization Section
Research on magnetic and superconducting materials
The research in this field mainly focusses on superconductors alternate to the Nb based materials for high-current high magnetic field applications (i.e. high-field superconducting magnets) and superconductors for cryogenic radiation detector applications
(i) Superconductors alternate to Nb based materials 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 high magnetic fields motivates research on newer superconducting materials with superior current carrying capability. Moreover, the Nb-based superconductors are not compatible with the neutron irradiation environment (as in a fusion based reactor) as the latter gives rise to long term radioactivity in these materials. Additionally, with the increasing usage of superconductivity in different fields, the ever-increasing use of Nb everywhere may also have a socioeconomic impact in future. Hence there is a need for looking into alternate superconducting alloys. The FEL & Utilization Section (FUS) 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 V-Ti alloys because of their excellent mechanical properties and other interesting physical properties. Experimental results indicate that magnetic spin fluctuations and superconductivity co-exist in the V-Ti alloys, where the spin fluctuations suppress the superconducting transition temperature by nearly 50% as compared to the theoretically predicted values. Addition of rare earth elements in the V-Ti alloys reduces the grain size in this system and also introduces additional flux line pinning centres. As a result, the alloys become more capable of carrying large currents in higher magnetic fields. Addition of Gd is found to introduce ferromagnetism in the V-Ti alloys near room temperature. The superconductivity in the system is not only sustained in the presence of this ferromagnetism; in fact, the superconducting transition temperature (Tc) is found to increase marginally along with the enhancement of critical current density Jc (the measure of current carrying capacity of a superconductor) in the Gd-containing V-Ti alloys.
While both the V-Ti and Mo-Re alloy systems are low temperature superconductors, they are found to exhibit high field paramagnetic Meissner effect and in fact, this effect can be observed up to 7 T fields and beyond in the T-V alloys after Y-addition. The Mo-Re alloys are found to exhibit higher Jc in lower fields as compared to the Ti-V alloys, and experimental evidence indicates the presence of a surface mixed-state state or ‘Kulik vortex-state’ in the bulk Mo-Re alloys and also the occurrence of a vortex–liquid to vortex–glass transition.
The Jc of the V-Zr alloys is very high and comparable to the commercial Nb based superconductors. This is found to be due to the presence of very large number of grain boundaries generated during eutectic reaction. The V-Zr alloys form with five different metallurgical phases. The β-V phase becomes superconducting below 5.2 K, whereas the α and β-Zr phases remain normal down to 2 K and lead to the formation of a normal channel of heat conduction in parallel to the superconducting channel even when bulk of the sample remains superconducting. This configuration makes these alloys more efficient in removing the heat than what is expected theoretically for a bulk superconductor, which is an important information for technological applications.
With consistent work on the V, Ti, Zr and RE (rare earth elements) containing alloy superconductors for more than a decade, the FUS has achieved significant success in the enhancement of Jc in these superconductors. The journey of this research is summarized in figure 9(a). With V0.6Ti0.4 as the starting material, the first success was achieved by adding atomic proportions of RE elements. By successive cold working and annealing stages (C3, A3, Ann3, etc. represents different cold work-anneal schedules) it was possible to further enhance the Jc and the upper critical field very significantly. Subsequently, after the inclusion of atomic proportions of Zr in the RE added Ti-V alloy system, after successive cold working and annealing stages, the Jc was enhanced to a level higher than the commercial Nb-Ti superconductors up to fields more than 7 T. The upper criticl field was also improved significantly. While the addition of Gd reduced the ultimate tensile strength (Us) by about 5 times [figure 9(b)], it was again improved by 3.4 times by successive cold work and annealing [figure 9(c)]. Thus, it was shown that RE and Zr containing Ti-V alloys could replace Nb in commercial and research applications, and the processes evolved towards the enhancement of the above superconducting properties may be useful in the further enhancement of Jc in the commercial superconductors as well.
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Figure 9: Enhancement of current carrying capacity and mechanical strength in the rare earth and Zr containing V-Ti alloy superconductors and comparison with the commercial materials (Nb-Ti and Nb3Sn)
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(ii) Superconductors for cryogenic radiation detector applications:
A superconducting film, electro-thermally biased at the superconducting-to-normal phase transition can be utilized as an extremely sensitive thermometer. These devices are known as Transition Edge Sensors (TES). Such sensors detect radiations ranging from γ and X-rays to the sub-millimeter and millimeter waves, which makes TES an indispensable tool for studies in basic sciences, enforcement of nuclear non-proliferation, nuclear forensics, non-destructive analysis of spent nuclear fuel and astronomy. The use of superconductors at sub-Kelvin temperatures helps in attaining the highest possible sensitivity as the superconducting gap is substantially lower than the band gap of the conventional detector materials. Moreover, the operation at very low temperatures reduces the noise thereby boosting the signal to noise ratio of the detectors.
As a part of the activity on the TES detectors, studies on the use of disordered Mo films for microwave and THz radiation has been taken up. The superconducting transition temperature of the disordered Mo films (nano-grain morphology, residual resistivity ratio ~1) is much higher than bulk Mo. The Tc (3-4 K) and microstructure of the films are being tuned with the Ar pressure maintained during deposition. Temperature dependent terahertz spectroscopy of these superconducting Mo films enable the experimental determination of the temperature dependence of penetration depth, superfluid density and superfluid stiffness (figure 10). The disordered films show promise for use as active materials in microwave kinetic inductance bolometers for THz and IR radiation detection.
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Figure 10: Temperature dependence of (a-b) penetration depth (λ) of Mo thin films of two different thicknesses, fitted with the dirty-limit BCS model, (c) Superfluid density (n) along with films and the two-fluid model fits and (d) Superfluid stiffness (J).
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