Document Type

Thesis

Date of Award

1-31-1991

Degree Name

Master of Science in Applied Chemistry - (M.S.)

Department

Chemical Engineering, Chemistry and Environmental Science

First Advisor

Lawrence Suchow

Second Advisor

R. P. T. Tomkins

Third Advisor

Barbara B. Kebbekus

Abstract

Substitution of some other elements for rare earth or yttrium in high-Tc superconducting RBa2Cu3Oy compounds (R = Y, Nd, etc.) may improve chemical and physical properties of the materials, shed light on the superconducting mechanism of the new materials, and help to find new superconducting materials with even higher transition temperatures.

Bi3+ and Tl3+ have been reported earlier to result in higher Tc's in the related layer-structure compounds. The effect of electronegativity on superconductivity could be studied if substitution of Bi or Tl for Y or other rare earth elements in RBa2Cu3Oy could be realized.

Earlier researchers have indicated that, under oxidizing conditions at the reaction temperature (about 900°C), Bi3+ does not enter YBa2Cu3Oy but that a non-superconducting perovskite phase Ba2BiYO6 (with Bi5+) is formed instead. In the present work, attempts have been made to prepare superconducting (R,Bi)Ba2Cu3Oy by first forming the non-superconducting, oxygen-deficient, tetragonal form in an inert atmosphere and then introducing additional oxygen and converting to the orthorhombic form by reheating in air or oxygen at lower temperature (about 250 - 500°C). x-ray diffraction, optical microscopy, thermogravimetric analysis, and electrical measurements were used to study the reaction products.

It was found that, in an inert atmosphere, YBa2Cu3Oy is very difficult to form while Ba2BiYO6, a non-superconducting phase, still forms as it does under oxidizing conditions, with oxidation of Bi3+ to Bi5+ probably due to reduction of Cu2+ to Cu+; (Y,Bi)Ba2Cu3Oy is not formed.

In an oxidizing atmosphere, both NdBa2Cu3Oy and Ba2BiNdO6 with a structure similar to that of Ba2BiYO6 result; there is no evidence for the formation of (Nd,Bi)Ba2Cu3Oy to any significant degree. The presence of Ba2BiNdO6, a minority and insulating phase, along with NdBa2Cu3Oy results in a broader transition temperature and an increase in normal state resistivity.

In an inert atmosphere, the formation of both NdBa2Cu3Oy and Ba2BiNdO6 is hindered and a tetragonal NdBa2Cu3Oy and poorly crystallized and probably oxygen-deficient Ba2BiNdOx result. Superconductivity of the samples with Bi addition is severely deteriorated since the tetragonal NdBa2Cu3Oy is not easily converted to an optimum orthorhombic structure while the insulating Ba2BiNdO6 appears in the samples during the after-annealing.

Tl was found not to enter the Y sites of YBa2Cu3Oy under oxidizing conditions.

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