Faculty Bibliography

The relative chemical reactivity toward water within the YBa2Cu3O7-x series (0 < x < 1) is found to be YBa2Cu3O6.59 < YBa2Cu3O7.00 << YBa2Cu3O6.05. Thus, factors other than copper valence, such as internal strain and lattice vacancies, are likely to be responsible for the high reactivity of the oxygen-deficient phase. For the two orthorhombic samples, YBA2Cu3O7.00 and YBa2Cu3O6.59, the reactivity follows the expected trend based on the copper valence. Additional useful information related to the mechanism of corrosion is acquired from an examination of the surface microstructure of water-degraded YBa2Cu3O7-x samples. Accordingly, inter- and intragrain cracking phenomena occur during water degradation of YBa2Cu3O7-x specimens and serve to enhance the rate of decomposition of the high-T(c) lattice. Interestingly, the surface microstructure of corroded samples reveals features which appear to be related to the twinning structure of the host lattice.

The oxygen deficient high-T(c) material, YBa2Cu3O6, decomposes rapidly when in the presence of water solution or water vapor. Accordingly, upon reaction with water, it decomposes into Ba(OH)2, Cu2O, CuO, a metastable ''Y2BaCuO5'' phase and a series of unidentified amorphous compounds. Although the majority of the decomposition products are insoluble in water, Ba(OH)2 leaches away into solution where it subsequently reacts with atmospheric CO2 to form sparingly soluble BaCO3 Crystals. Interestingly, the ''Y2BaCuO5'' phase forms only in the initial stages of corrosion and disappears after long water exposure times. On the other hand, authentic Y2BaCuO5 Samples are very stable in the presence of water. Thus, the chemical reactivity of the ''Y2BaCuO5'' phase generated via decomposition of the high-T(c) material is different from that of the authentic phase. In addition, comparisons of the relative reactivity of YBa2Cu3O7 and YBa2Cu3O6 samples reveal that the oxygen deficient material is more reactive than the fully oxygenated compound. This unusual reactivity trend suggests that factors other than copper formal valence dominate the chemical reactivity of the high-T(c) phase.

Although it is known that some of the high-T(c) phases react rapidly with water, CO2, CO, and acids, no systematic comparison of the relative reactivities of the various cuprate superconductors is available. In this letter, x-ray powder diffraction, scanning electron microscopy, x-ray photoemission spectroscopy, and electrochemical measurements are utilized to establish a comprehensive comparison of the intrinsic reactivity characteristics of the common copper-oxide superconductors. Consequently, the following reactivity scale has been determined: YBa2Cu3O7 > Tl2Ba2Ca2Cu3O10 > Bi2Sr2Cu2O8 greater-than-or-equal-to La1.85Sr0.15CuO4 > Nd1.85Ce0.15CuO4 > Nd1.85Th0.15CuO4.

Methods of making electrical contact to high-T(c) superconductors with conductive polymers are described. In addition, three- and four-point resistance measurements are acquired on samples of YBa2Cu3O7-delta, GdBa2Cu3O7-delta, and Pb0.3Bi1.7Sr1.6Ca2.4Cu3O10 using poly(3-hexylthiophene) contacts in order to determine the temperature dependence of the conductive polymer/superconductor contact resistance. Although the polymer/superconductor contact resistance displays activated behavior above T(c) and increases as the temperature is lowered, below T(c) there is a precipitous decrease in the resistance at that interface. Similar measurements completed on systems where the superconducting components are substituted by normal metals do not show any signs of such contact resistance decreases. Possible reasons for the decrease in the polymer/superconductor interface resistance at T(c) are discussed.

The electrochemical responses recorded at nine copper oxide ceramics, including six high-temperature superconductor phases, are described in this paper. The cyclic voltammetric behavior exhibited by these materials is found to depend greatly on the method of surface treatment, the same preparation technique, the amount of impurities in the high-T(c) phase, and the properties of the electrolytic fluid. The voltammetry acquired at these electrodes is used as a diagnostic tool to assess the surface quality of the electrodes and to measure the degree of reversibility of electron flow at the electrode/solution interface. It is shown that the preferred method of surface preparation for the copper oxide materials is a diamond-scribed technique in that well-resolved voltammetry is obtained using this treatment for the majority of copper oxide superconductors.

Measurements of interfacial (double layer) capacitance, C(DL), and charge transfer resistance, R(CT), have been made as a function of temperature at fluid electrolyte interfaces with electrodes made from two different Tl-based high-temperature superconductor materials. Measurements spanning the 112-119 K superconducting transition temperatures, T(c), of the HTSC electrodes reveal a smooth decrease in C(DL) with decreasing temperatures, except that an abrupt, ca. 1 deg wide change in the shape of the C(DL) Vs temperature curve occurs at the T(c) of the electrode. These are the first data which show that the onset of superconductivity can be reflected in a chemical phenomenon at a molecular fluid/HTSC interface. Of the several contributors to interfacial capacitance, it is hypothesized that alteration in charge carrier distribution or polarizability over the outermost electrode lattice layer as electron pairs start to appear at T(c) is the most likely origin of the T(c)-correlated capacitance feature. Alterations in the charge transfer resistance for electrochemical solvent reduction appear over the same temperature interval as the capacitance feature, but not as consistently.