Engineering topics

Vitta, Satish

Publications and source records attributed to Vitta, Satish.

6 records

NASA NTRS1993

The effect of fluctuations on the electrical transport behavior in YBa2Cu3O(7-x)

The excess conductivity behavior of highly oriented YBa2Cu3O(7-x) thin films prepared by both coevaporation and laser ablation was studied in detail in the reduced-temperature range 9 x 10(exp -4) is less than t is less than 1. The excess conductivity in all the films studied was found to diverge sharply near T(sub c), in agreement with the conventional mean-field theory. However, the detailed temperature dependence could not be fitted to either the power-law or the logarithmic functional forms as predicted by the theory. The excess conductivity of all the films was found to be exponentially dependent on the temperature over nearly three decades for 9 x 10(exp -4) is less than t is less than 10(exp -1), in contradiction to the mean-field theory.

Vitta, Satish↗

NASA NTRS1992

Magnetic flux relaxation in YBa2Cu3O(7-x) thin film: Thermal or athermal

The magnetic flux relaxation behavior of YBa2Cu3O(7-x) thin film on LaAlO3 for H parallel c was studied in the range of 4.2-40 k and 0.2-1.0 T. Both the normalized flux relaxation rate (S) and the net flux pinning energy (U) increase continuously from 1.3 x 10 exp -2 to 3.0 x 10 exp -2 and from 70-240 meV respectively, as the temperature (T) increases from 10 to 40 K. This behavior is consistent with the thermally activated flux motion model. At low temperatures, however, S is found to decrease much more slowly as compared with kT, in contradiction to the thermal activation model. This behavior is discussed in terms of the athermal quantum tunneling of flux lines. The magnetic field dependence of U, however, is not completely understood.

Vitta, Satish↗

NASA NTRS1991

Rapid solidification of polymorphic transition metals induced by nanosecond laser pulses

An Nd-YAG laser giving 5 ns, 266 nm pulses was used to melt and subsequently quench thin, pure metallic films on a liquid Al/Al2O3 substrate at 10 to the 10th - 10 to the 12th K/s. Transmission electron microscopy together with electron diffraction was used to study the competitive nucleation and growth behavior of the crystals from the undercooled melt. In the case of Fe and Co, the high-temperature bcc and fcc structures were retained after laser quenching. Ti and Zr in spite of the structural similarities exhibit different preferences for nucleation from the undercooled melt. In all the metals the solid-state transformations were completely suppressed and the crystal growth was found to be limited by the collisional frequency of the atoms onto the growing interface.

Vitta, Satish↗

NASA NTRS1991

Temperature dependence of the anisotropy in magnetic relaxation in YBa2Cu3O(7-x) thin films

The relaxation of diamagnetic magnetization in the c-axis-aligned YBa2Cu3O(7-x) thin film is studied as a function of orientation and temperature in the range of 5-50 K at H = 0.2 T. The magnetization M(T,H) in the orientations H parallel to c and H perpendicular to c and at all the temperatures is found to decrease logarithmically with time t. The activation energy for the movement of flux lines U is found to be 30-110 meV in the range 5-50 K. For H parallel c, U increases continuously with T, whereas for H perpendicular c, U has two apparent maxima: at T = 10 K and T greater than 50 K. These results are discussed in terms of the thermally activated flux motion model.

Vitta, Satish↗

NASA NTRS1991

Electromigration failure in YBa2Cu3O(7-x) thin films

Electromigration failure in highly oriented YBa2Cu3O(7-x) thin films below the superconducting transition temperature is reported here for the first time. The film on SrTiO3 failed at 86 K, 230,000 A/sq cm, while that on LaAlO3 failed at 84 K, 930,000 A/sq cm. Scanning electron microscopy and energy dispersive X-ray analysis of the films after failure shows that Cu migrates preferentially away from the failure region toward the electrode.

Vitta, Satish↗

NASA NTRS1991

Magnetic flux relaxation in YBa2Cu3)(7-x) thin film: Thermal or athermal

The magnetic flux relaxation behavior of YBa2Cu3O(7-x) thin film on LaAlO3 for H is parallel to c was studied in the range 4.2 - 40 K and 0.2 - 1.0 T. Both the normalized flux relaxation rate S and the net flux pinning energy U increase continuously from 1.3 x 10(exp -2) to 3.0 x 10(exp -2) and from 70 to 240 meV respectively, as the temperature T increases from 10 to 40 K. This behavior is consistent with the thermally activated flux motion model. At low temperatures, however, S is found to decrease much more slowly as compared with kT, in contradiction to the thermal activation model. This behavior is discussed in terms of the athermal quantum tunneling of flux lines. The magnetic field dependence of U, however, is not completely understood.

Vitta, Satish↗