Polikristal MgB2 üstüniletkeninde akı dinamiği ve zaman etkileri

In this thesis, we have investigated the time dependent flux motion in a polycrystalline superconducting MgB2 sample by means of current-voltage (I ? V curves), transport relaxation (V ? t curves), and magnetovoltage (V ? H curves) measurements. The transport measurements were carried out as functions of temperature (T), transport current (I), external magnetic field (H) and orientation of H with respect to I. In addition to the V ? t curves, the time effects in the I ? V and V ? H curves were investigated with the help of the current sweep rate of transport current (dI/dt) and also the field sweep rate of external magnetic field (dH/dt), respectively. Angular dependence of magnetovoltage (V ? ?) curves was also measured to study the anisotropic behavior of the MgB2 sample. In order to understand better the flux dynamics, the experimental results for polycrystalline MgB2 sample were compared to previous similar studies on superconducting Y1Ba2Cu3O7-? (YBCO) and Bi1.7Pb0.3Sr2Ca2Cu3Ox (BSCCO) polycrystalline samples.It was observed that the I ? V curves exhibit nearly reversible behavior for both slow and high current sweep rates (dI/dt) upon cycling the transport current. The measured voltage dissipation in the I ? V curves is nearly stable with time for a given current value, and, the evolution of the I ? V curves is nearly independent of dI/dt. The I ? V curves obtained by standard and reverse procedures reveal that there is no difference between them. These results suggest that, in polycrystalline MgB2, the weak links between grains (standard procedure) and surface weak links (reverse procedure) do not have a considerable effect on the evolution of the I ? V curves and also on the other transport measurements as compared to the polycrystalline YBCO and BSCCO.At high dissipation levels, we observed that the sample voltage in the V ? t curves increases rapidly and reaches a steady state in a very short time; whereas, it shows a non-linear behavior at relatively low dissipation levels. The results were interpreted mainly in terms of flux pinning/depinning and current -and field- induced organization of the flux lines along the sample in time. Observation of a quenched state at high dissipation levels was correlated with the supercooling effect, whereas, at relatively low dissipation levels, the smooth voltage decay was correlated with the glassy state relaxation. In addition, the influence of bidirectional square wave (BSW) current on the evolution of the V ? t curves was studied and regular sinusoidal-like voltage oscillations were observed at well defined values of the amplitude and period of the BSW current, temperature and external magnetic field. These voltage oscillations were interpreted in terms of the plastic flow of flux lines (or current-induced self-magnetic field (SMF) lines). The experimental results suggest that the interaction between the flux lines (or SMF lines) and flux pinning centers could be elastic coupling in nature because of the rigidity of flux lines in MgB2.The magnetovoltage measurements (V ? H curves) showed that there is a strong counterclockwise (ccw) hysteresis effects in the forward region of V ? H curves, whereas, clockwise (cw) effects appear in the reverse region. The ccw and cw effects in the V ? H curves were attributed to the difference in the local fields between the increasing and decreasing branches. The hysteresis in the V ? H curves was interpreted in terms of flux pinning and depinning, together with different local magnetic fields. Furthermore, the measured dissipation in the V ? H curves was found to be practically independent of dH/dt, which could be related to weak pinning properties of MgB2. The V ? ? curves showed that measured voltage dissipation depends on the magnetic field orientation. This implies the presence of macroscopic anisotropic effects on average in polycrystalline MgB2.