P. Murugaraj

Preparation of highly oriented polycrystalline YBa2−y Cu3Ox superconductors

Abstract A simple processing technique for preparing highly oriented superconducting (BaO-deficient) YBaCu-ceramic is described. The maximum preparation temperature is 950°C. For that purpose small amounts of BaO (with respect to the 1:2:3 composition) are replaced by K 2 CO 3 in equimolar amounts or in excess of that. In this way a BaO-deficient material is obtained. The amount of potassium initially introduced is negligible after the process. The T c — value obtained is 94 K. The material consists of extremely oriented grains (only (001) — reflexes in the X-ray diffractogram). The so-prepared material is much more stable against humid atmosphere and need not be annealed again in air or oxygen. This procedure can also be extended to analogous superconductors (e.g. in the BiCaSrCuO system).

Far-infrared study of optical phonons in an YBa2Cu3O6 crystal

Abstract We have performed polarized reflectance measurements on a highly oriented multicrystalline block of YBa 2 Cu 3 O 6 for temperatures between 300 K and 10 K in the frequency range between 50 cm -1 and 10000 cm -1 . With the electric field E parallel to the c-axis of the crystal, the reflectance exhibits all of the five theoretically expected infrared active A 2 u modes. Polarization perpendicular to the c-axis reveals five of the six predicted E u modes. A Kramers-Kronig analysis of the spectra allows a comparison of the experimentally obtained eigenfrequencies and oscillator strengths with lattice dynamical calculations. The agreement between experiment and theory is good. The strength of all observed infrared modes appears to be normal, in contrast to the superconducting phase YBa 2 Cu 3 O 7 , where some of the infrared active phonons exhibit unusually high oscillator strengths.

Defect chemistry and anisotropic transport in YBa2Cu3O6…7

Special preparation procedures and analytical techniques allow us to prepare chemically and crystallographically well defined YBa 2 Cu 3 O 6−7 . The analyses reveal the possibility of disorder in the Y- and Ba-sublattice in addition to defects in the O-sublattice in addition to defects in the O-sublattice. Attempts are made to explain the O 2 partial pressure dependence of the electronic conductivity using simple defect models. In particular, a considerable hole trapping is suggested. The anisotropy of the electronic conductivity is found to be significantly large. Careful electrochemical polarisation experiments (ac,dc) with glass-sealed specimen allow the determination of the ionic conductivity and of the chemical diffusion in the same experiment. The chemical diffusion coefficient is also obtained using conductivity relaxation results. The diffusion analysis, which also takes into account the electron-ion interaction according to the theory of «conservative ensembles» strongly suggests a large ionic anisotroy. Preliminary measurements (chemical relaxation) confirm the anisotropy of D

Preparation of large crystals of Y-Ba-Cu-O superconductor

Abstract We report on the preparation of large crystals of Y-Ba-Cu-O superconductor extending our ‘K 2 CO 3 -method’ adopted earlier for the preparation of oriented Y-Ba-Cu-O ceramics. By this method, we obtain cm-size blocks consisting of a few uniformly oriented subregions. These blocks can be split into crystals of mm-size which themselves are made up of thin layers (20–100μm) in a highly correlated mosaic-like structure. The misorientation between the layers is less than 5 degrees. Due to the high temperature synthesis, some amount of second phases (Y 2 BaCuO 5 , CuO, BaCuO 2 ) is found to exist in the crystals as inclusions. The crystals annealed in oxygen atmosphere exhibit superconductivity with T c ≈90K with a transition width of 2K (resistively measured). The chemical analysis of these crystals suggests some mutual disorder on Y- and Ba-sites. The chemical formulae of these crystals can be described as ‘(Y x Ba 1−x ) 3 Cu 3 O y ’ (0.45≧x≧0.33).

Y-Ba-Cu-O superconductor: Preparation of highly oriented ceramics, improvement of durability and precise determination of redox state

Abstract A simple technique to prepare highly oriented superconducting YBaCuO ceramics is presented. The same technique has also been found to work in Bi-Sr-Ca-Cu-O system. The slightly BaO-deficient material obtained is more durable in humid environment as compared to the conventional ceramics. An additional improvement of durability is achieved by surface modification. Finally, two independent analytical methods are described for determining the redox state (O nonstoichiometry) precisely.

The effect of heterogenous doping on heterogeneous catalysis: Dehydrohalogenation of tertiary butyl chloride

Abstract The presence of AgCl heterogeneously doped with γ-Al2O3 increases the rate of dehydrohalogenation of tertiary butyl chloride significantly. The dependences of the catalytic activity on temperature, grain size and composition of the catalyst are quantitatively analyzed in terms of the concept of “heterogeneous doping”. The mechanism is interpreted in terms of Ag+-vacancy accumulation in the space charge region of silver chloride.

Use of potassium salts in the crystal growth of RBa2Cu3Ox superconductors (R=Y or lanthanides)

Abstract The advantage of using K 2 CO 3 or KCl as flux materials for material processing and crystal growth of RBa 2 Cu 3 O x ( R = Y , Gd , Dy , Pr , etc.) is reviewed. As far as their role in crystal growth is concerned, different possibilities are discussed. Large mosaic crystals with low misorientation can be obtained by using K 2 CO 3 or KCl as additives during synthesis. Preparation temperatures in the range of 1270–1330 K are important as well as the slow cooling process. Under defined conditions, it is possible to obtain potassium-free products. With the exception of the non-superconducting Pr compound all compounds exhibit a distinct platelet morphology. In contrast to Gd, Dy and Pr derivatives, Y derivative shows a considerable nonstoichiometry and antisite disorder in the R and Ba sublattices. While oxygenation of the Y-123 crystals is achieved with ease, it is extremely difficult for rare earth derivatives. After oxygenation, the Y derivatives show a fairly sharp transition to the superconducting phase whereas the Gd and Dy derivatives show a broad transition with a width of ∼50 K. The difficulties in the oxygenation of rare earth derivatives are attributed to the absence of significant antisite disorder.

Growth and characterization of large crystals of GdBa2Cu3O7 superconductor

We report here on the growth of large crystals of Gd-Ba-Cu-O superconductor by using the ‘K2CO3-method’ adopted earlier for preparing oriented ceramics and large crystals of Y-Ba-Cu-O superconductors. The temperature favourable for the growth of GdBaCuO crystals is found to be higher than that required for YBaCuO crystals. Also, the crystal perfection is better for these crystals and as a result, the oxygenation of the sample becomes more difficult. Resistivity measurements on these samples show a sharp drop in resistivity at 90 K which then tails off at lower temperatures and attains zero resistance around 35 K. Such behaviour is normally due to an inhomogeneous oxygen content in the sample. Chemical analysis shows that the deviation from ‘123’ composition in GdBaCuO-samples is much less as compared to YBaCuO-samples obtained by the same technique.