C. S. Pande

Microstructural and superconductive studies of Ag-YBa2Cu3O7−x composites

Composites of Ag-YBa2Cu3O7−x were synthesized and carefully characterized for crystal structure and microstructure by X-ray diffraction, optical microscopy, SEM, TEM, and EDAX techniques in order to investigate the effect of Ag additions on the superconducting properties of 123 compounds. The a.c. susceptibility data show thatTc (onset) of 123+Ag composites vary between 90.2 and 91.8 K. TheJc values we measured for 123 material without silver were in the range of earlier reported values for the pure 123 material. Whereas there is a relatively small increase in the critical current for the YBCO/Ag2O ratio of 3, the variation of the grain size of the composites shows that theseJc changes are due to slight variations in the grain size rather than any dramatic effect of Ag inclusions as speculated earlier.

Correlation of ‘‘twins’’ observed by optical microscopy and transmission electron microscopy in YBa2Cu3O7−x superconductors

By using transmission electron microscopy and optical microscopy on the same specimens, the patterns of light‐ and dark‐contrast lines seen in reflected polarized light were shown to be an interference pattern due to the variable spacing of suboptical microtwins. These microtwins are mostly [110] reflection twins. The [110] twinning was observed to be cyclic and occasionally pseudotetragonal because of the progressive cycling of contact twin domains. Within a domain, and occasionally in a whole grain, the [110] reflection twins occurred as polysynthetic lamellae. The morphology of the domain structure can be explained from the theory of martensitic transformation.

Color indicator of cuprate superconductivity observed by polarized light microscopy

A common mineralogical technique for identifying individual crystals in a heterogeneous, polycrystalline sample involves the observation at room temperature of their colors in reflected polarized light (colors of polarization). Here we present the observation that a brownish yellow (golden) color of polarization is not only characteristic of the YBa2Cu3O7 superconductor, but of the cuprate superconductors in general and is correlated with the occurrence of superconductivity.

Optical differentiation of orthorhombic superconductors such as YBa2Cu3O7 from their tetragonal precursor material

The orthorhombic superconducting phase of YBa2Cu3O7 is shown to be differentiable from its tetragonal precursor by optical microscopy. Individual grains 1 μm in diameter or larger are found to either exhibit the color of the compensator in reflected polarized light (tetragonal material) or to change colors from the compensator (orthorhombic material). By using color differences, retained tetragonal material and the orthorhombic nature of untwinned areas of grains are identified. The advantage to producing better superconducting single crystals and sintered polycrystalline materials is discussed.

Thermally activated flux motion under a distribution of activation energies in high Tc superconductors

Abstract Within a thermally activated flux motion model and assuming a distribution of barriers, we derive exact closed form expressions for various parameters for magnetic relaxation in high T c superconducting material. In particular, we introduce a new parameter S * proportional to S , the normalized logarithmic decay of magnetization, and show that S * is a monotonically increasing function of temperature.

Orthorhombic to tetragonal phase transition in Y1Ba2Cu3O7−x

Quenching experiments were performed to study the orthorhombic to tetragonal phase transition in Y1Ba2Cu3O7−x. The quenched and unquenched specimens were characterized by x‐ray diffraction and optical microscopy. The phase transition temperature was found to be dependent on the environment of the sealed specimens during quenching, and could be as high as 1173 K (900 °C). The phase transition is suggested to be of second order.

Columnar growth in combustion deposited diamond thin films

The morphology of the nucleation and growth surfaces of oxygen‐acetylene combustion deposited diamond thin films has been studied using scanning electron microscopy. The diameters of both nucleation cells and growth surface grains have been measured and analyzed statistically. The general shape of the distributions is found to change from nucleation to growth surface for each film. The frequency distribution of cells is generally normal, whereas, the distribution for the growth surface is found to be always lognormal. The change in shape of the distributions from normal for the nucleation surface to lognormal for the growth surface can be explained from a stochastic theory of growth.

The color of polarization in high temperature superconductors

Abstract We have studied the color of polarization of many members of the class of hole carrier cuprate superconductors and found a characteristic color and a strong correlation between color and the existence of superconductivity. We have extended this study to include one member of the electron carrier cuprate superconductors and have observed the same characteristic color indicating that the presence of this color is independent of the nature of charge carriers. We have also developed a method to quantify the color through photometric techniques.

Effects of alloying on the superconducting properties of ErBa/sub 2/Cu/sub 3/O/sub 7/

In high-T/sub c/ superconductors, the low critical current densities in polycrystalline materials have been attributed to a combination of critical current anisotropy and poor superconducting coupling across grain boundaries. Theoretical calculations indicate that although the flux pinning should vary roughly inversely as grain size, the polycrystalline critical current behavior could possibly be understood in terms of stresses due to the grain boundary. Experiments have been conducted to increase the coupling between adjacent grains by modifying grain boundary chemistry. These include adding either a conducting layer or a superconducting layer at the interfaces. The effect of additions such as Ag, B, Bi, Ga, and In to produce a conducting layer and the alloying of RBa/sub 2/Cu/sub 3/O/sub 7/ with another superconductor to produce a superconducting layer were analyzed by measuring T/sub c/ and J/sub c/ and observing changes to the microstructure. Early results indicate some J/sub c/ enhancement with silver addition. However, the addition of a different superconductor appears more promising. >

Novel technique for crystal growth of superconducting ErBa2Cu3O7

Millimeter-sized crystals of ErBa2Cu3O7 were fortuitously grown while sintering a flat disk. It was possible to separate individual crystals from the sintered mass and measure their properties. The resistive transition and onset of magnetic transition of a single crystal was 93 K and the resistive transition width was < 1 K. Flux expulsion was observed and individual crystals could be levitated by a magnetic field. Crystals showed unusual growth features. They were heavily corrugated along the part of the crystal that grew parallel to the top surface of the disk and that coincided with thec axis of the unit cell. Crystals were characterized by X-rays, electron microscopy, and optical microscopy.