Z.X. Gao

Phase formation of polycrystalline MgB2 at low temperature using nanometer Mg powder

Abstract The MgB2 superconductor synthesized in a flowing argon atmosphere using nanometer magnesium powder as the raw materials, denoted as Nano-MgB2, has been studied by the technique of in situ high temperature resistance measurement (HT-RT measurement). The MgB2 phase is identified to form within the temperature range of 430–490 °C, which is much lower than that with the MgB2 sample fabricated in the same gas environment using the micron size magnesium powder, denoted as Micro-MgB2 and reported previously. The sample density of the Nano-MgB2 reaches 1.7 g/cm3 with a crystal porosity structure less than a μm, as determined by the scanning electron microscope (SEM) images, while the Micro-MgB2 has a much more porous structure with corresponding density of 1.0 g/cm3. This indicates that the Mg raw particle size, beside the sintering temperature, is a crucial factor for the formation of high density MgB2 sample, even at the temperature much lower than that of the Mg melting, ∼650 °C. The X-ray diffraction (XRD) pattern shows a good MgB2 phase with small amount of MgO and Mg and the transition temperature, TC, of the Nano-MgB2 was determined to be 39 K by temperature dependent magnetization measurements (M–T), indicating good superconducting properties.

In situ resistance measurement of superconducting MgB2 in flowing argon atmosphere

Abstract In situ resistance measurement of MgB2 sample was performed in flowing argon atmosphere. This set of measuring curve was compared with the in situ resistance measurement result of MgB2 sample performed in vacuum. This measurement result presents a resistance decreasing taken as a whole as increasing temperature from room temperature to 800 °C, and it indicated that the Mg powder began to melt at about 530 °C. At this temperature the resistance values of MgB2 sample presented a sharp down, and it is lower than MgB2 sample prepared in vacuum conditions that appeared at 638 °C. This indicated that the phase formation of MgB2 in argon atmosphere was at lower temperature than in vacuum.

Effect of magnetization process on levitation force between a superconducting disk and a permanent magnet

The levitation forces between a permanent magnet and a coaxial superconducting disk after different magnetization processes are measured. Significant effect of the magnetization process on the levitation force is observed. Theoretical calculations of levitation force based on the equations for current motion with temperature and field-dependent critical current density, and the heat dissipation due to the flux motion are in perfect agreement with the experimental data.

Electric transport study of superconducting MgB2 wire

Abstract Superconducting MgB 2 wires were prepared by exposing boron fiber to magnesium powder in high temperature. Pulse current method has been used to study the V – I characteristics systematically in a large current range. Using a scaling function, all the R – T curves in different field converge into a single universal curve. From the V – I curves, we find a much sharp N–S transition and in high field a reasonable large J c is found.

Determination of U(x,t) and j in MTG YBCO bulk by magneto-optic imaging

Abstract We use the magneto-optic imaging to determine the spatial distribution and the time dependence of the magnetic induction B z ( x,t ) on the broad surface of the melt texture growth YBCO bulk. The model-independent effective activation energy U and the current density j could be calculated from B z ( x,t ), ∂ B z ( x,t )/∂ t and ∂ B z ( x,t )/∂ x . The good fit shows that the thermally activated flux creep in the melt texture growth bulk is consistent with the logarithmic model in the temperature range of liquid nitrogen.

Direct observation of remnant fields in Y2BaCuO5 and platinum doped YBa2Cu3O7 bulk superconductors

Abstract The dynamic decay process of the remnant field and the thermally activated flux creep in the melt texture growth Y2BaCuO5 and platinum doped YBa2Cu3O7 bulk samples were directly observed. The doping effect of Y2BaCuO5 and platinum on the trapped flux was clearly watched.

A new method to determine jc in bulk superconductor from saturated diamagnetic field by magneto-optic imaging

The diamagnetic field created by supercurrent will be saturated after superconducting bulk sample is fully penetrated by the external magnetic field. The critical current density of a sample could be determined from the saturated diamagnetic field, which is easily and precisely observed by magneto-optic imaging.

The interaction of phase slip centers in mesoscopic aluminum line

Abstract The I – V characteristics of mesoscopic aluminum wire have been studied. We found step structures in the I–V characteristics, which we believe are due to the formation of phase slip centers. With decreasing temperature, the step structure is replaced by peak structure. This anomalous behavior was analyzed in terms of the interaction between the phase slip centers.

Study on current-limiting mechanism of Bi-2223 tape by magneto-optic imaging

Abstract Crystalline holes, macroscopic inhomogeneity of superconductivity and weak remnant magnetic fields, directly observed in the metallograph and the magneto-optic images, may indicate the mean reason to limit the critical current density in our silver sheathed Bi 1.8 Pb 0.34 Sr 2 Ca 2.2 Cu 3 O 8+ x 37-filament long tape. The current paths show that thigh supercurrent regions preferentially located at the Bi-2223/Ag narrow interfae of filaments.

The anomaly of current–voltage characteristics of a mesoscopic aluminum line

The I - V characteristics of mesoscopic aluminum wire have been studied. Step structures are found near superconducting transition temperature and the number of the steps is different at different temperature. At lower temperatures, the steps gradually turn into strong peaks. These anomalous features can be suppressed by a magnetic field. We propose an interpretation based on the nonequilibrium superconductivity due to the formation of phase slip centers (PSCs). The PSCs are excited one by one with increasing of current and the interaction between them will affect the charge imbalance length, leading to the anomalous peaks in the I - V characteristics near the critical current