S.N. Ekbote

The CESR and spin wave studies of A-15 films in normal and superconducting states

Abstract The electron spin resonance studies have been reported for A-15 superconductors, namely Nb 3 Ge, Nb 3 Si and V 3 Si possessing different T c values and CESR, Platzmann-Wolff type spin waves, and spin waves of antiferromagnetic type are observed in all the samples. It is found that T c of Nb 3 Ge depends upon the presence and separation of spin wave absorptions from the CESR, and T c is found to increase when the separation is reduced. It is concluded that the exchange interactions in the conduction band, as manifested by the behaviour of spin waves, are of antiferromagnetic type and they are responsible for superconductivity in A-15 materials studied.

Preparation and characterization of Ag-added Bi1.84Pb0.4Sr2Ca2.2Cu3O10+x bulk tube conductors for cryogen free superconducting magnet

Bulk tube conductors of Bi1.84Pb0.4Sr2Ca2.2Cu3O10+xwith addition of silver varying from 0 to 25 wt% (not reported earlier) were systematically studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electrical transport and a.c. susceptibility techniques. The tube conductors formed by cold isostatic pressing (CIP) of the powders obtained from spray drying method have been made successfully. It was found that Ag addition has not only affected the formation of the desired Bi-2223 phase and the microstructure of these large bulk tube samples thereby influencing on the critical current (Ic),it also reduces the contact resistance to minimize the cryogen losses. These variations have been found to be Ag content dependent. An optimized value of 10 wt% Ag has been found to produce the best quality tubes showing reproducibleIcvalue > 120 Amp at 77 K which is in general a requirement to energies of the cryogen free conventional/HTSC superconducting magnets below 20 K.

The effect of high temperature sintering on the Tc of Tl1Ba2Can−1CunOx system

Abstract The effect of high temperature sintering ( T ≅ 970° C ) has been studied on the Tl 1 Ba 2 Ca n −1 Cu n O x systems where n varies from 1 to 5. It has been found that under optimised conditions of preparation the final phase composition obtained can vary considerably from the starting or nominal composition. XRD studies have revealed that in the case of n = 1 and 2, the Tl 1 nominal composition 1201 and 1212 have converted into 2201 and 2212 with a = 3.85 A and c = 23.23 A for 2201 and a = 3.86 A and c = 29.04 A for 2212, with an enhanced T c for 2201 ( T c ∼ 97.2K) over previously reported T c s. The other nominal compositions of n = 3, 4 and 5 (1223, 1234 and 1245) were showing only the Tl 1 series behaviour with slightly enhanced T zero c and T onset c . The XRD studies showed that in all the above phases a = 3.86 A and c = 15.5 A. The low angle peak appears at 2 π = 5.25° indicating the formation of n = 3 or Tl 1 Ba 2 Ca 2 Cu 3 O x compound.

The preparation of an enhanced-Tc superconducting Tl2Ba2CuOx phase by using low Tl concentrations

Calcium-free high-Tc superconducting compounds of the Tl2Ba2Can-1CunOx series have been fabricated with an enhanced onset critical temperature, Tcon approximately=97.2 K. The optimisation of parameters has revealed that the choice of nominal composition plays an important role in the formation of the final phase. While a nominal 2201 composition did not yield a high-Tc phase, compounds with low thallium concentration (1201) gave a 2201 high-Tc phase. XRD studies confirm the formation of a 2201 phase from 1201 and 1/2201 phases with a approximately=3.86 AA and c approximately=23.23 AA. It has also been found that a high sintering temperature of 97 degrees C maintained for a short duration of three to five minutes is favourable for the enhancement of Tc.

The Enhancement of Tc in Calcium Free Tl-Bilayer Superconducting System (Tl2Ba2Cu1Ox)

The observation of enhanced Tc in calcium free Tl compounds Tl2Ba2Cu1Ox (2201) with varying concentration of Tl and Cu has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under the optimised conditions: sintering temperature 970°C and duration 3–10 minutes the highest Tc (onset) ~115 K and Tc (zero) ~95 K was found. The XRD studies showed the transformation of all the three nominal compositions into 2201 phase with different Tcs.

Lead-doped electron-beam-deposited BiSrCaCuO superconducting thin films☆

Abstract Superconducting thin films of the lead-doped BiSrCaCuO system have been prepared on (100) single-crystal SrTiO 3 substrates by an electron beam deposition technique using a single sintered pellet as the evaporation source. As-deposited films are amorphous and non-superconducting; post-deposition annealing at an optimized temperature in air has been found to result in crystalline and superconducting films. The superconducting characteristics of the films have been observed to be sensitive not only to the duration and temperature of post-deposition annealing but also to the lead content and the sintering parameters for the pellet to be used as the evaporation source. A pellet with nominal composition Bi 3 Pb 1 Sr 3 Ca 3 Cu 4 O y that had been sintered for 200 h zero resistivity at T c 0 = 112 K . However, films deposited using such a pellet as the evaporation source had T c 0 ≈ 73–78 K, as had the films deposited from a pellet without any lead. We investigated systemativally films deposited from pellets with more lead and sintered for different durations. It is evident from these investigations that pellets with nominal composition Bi 3 Pb 2 Sr 3 Ca 3 Cu 4 O y , i.e. with an excess of lead, and sintered for about 75 h when used as the evaporation source yield films with T c 0 ≈ 100 K when annealed between 835 and 840 °C for an optimized long duration. The films are characterized by X-ray diffraction and energy-dispersive spectroscopy techniques and have been found to be highly c axis oriented. The effect of lead in promoting a high T c 0 = 110 K phase seems to be similar to that in bulk ceramics.

Enhancement ofT c (∼ 103 K) in calcium-free Tl2Ba2CuO x (2201) compound

EnhancedTc in calcium-free Tl compounds of the series TlmBa2Can−1CunOx (2201) has been reported. Three different starting compositions (2201, 1201 and 2202) were studied extensively with varying conditions of preparation. Under optimized conditions (sintering temperature 970°C and duration 3–10 min) the highestTc(onset) ranges from 103 K to ∼ 115 K andTc (zero) ∼ 95 K was found. XRD studies showed the transformation of all the three nominal compositions into 2201 phase with differentTcs.

CESR studies of exchange interactions in superconductors

Abstract A study of CESR and spin waves in V3Si, Nb3Si and InMo6S8 in the normal and superconducting states is reported. Our results on A-15s and ternary molybdenum sulphides, along with those of Al by other workers give a strong credence to the presence of exchange interactions in the conduction band to be of overriding importance in causing electron pairing, in conformity with our earlier contentions.