Faculty Bibliography

By varying the oxygen content in the high temperature superconductor phase YBa2Cu3O7-x (0 < X < 1), the electrical properties of this material can be manipulated from that of a superconductor to that of an insulating semiconductor. In this paper, a method for preparing YBa2Cu3O7-x samples with different oxygen contents is described and a corresponding procedure for fabricating electrodes from these samples is outlined. The described procedure provides a simple and systematic means to tailor the conductive properties of electrode specimens. The electrochemical responses of the variable oxygen content electrodes are examined by cyclic voltammetry under conditions where the host electrodes are stable. Furthermore, the voltammetry recorded at these high temperature superconductors is used to assess the surf ace quality and thereby determine the degree of reversibility of electron transfer at superconductor/solution interfaces. The ability to control the density of states within these electrode specimens and thereby examine the effects this quantity has on the heterogeneous charge transfer rate is also addressed.

The chemical reactions leading to the decomposition of superconducting YBa2Cu3O7-x (0 > x > 0.05) in water solution and water vapor at different temperatures were investigated and the corrosion products were characterized using X-ray diffraction and scanning electron microscopy. Although the YBa2Cu3O7-x phase is stable in some nonaqueous solvents, it reacts readily with water, acids, carbon dioxide, and carbon monoxide. Samples of YBa2Cu3O7-x soaked in water solution at 25, 50, and 75-degrees-C all produce BaCO3 and CuO corrosion products as determined by X-ray powder diffraction studies. Although there is some evidence for the formation of small amounts of Y2BaCuO5 and Y2O3, the majority of the Y-containing corrosion products are produced as amorphous phases. Our studies suggest that "Y2BaCuO5" is formed with a metastable or distorted structure. Moreover, when YBa2Cu3O7-x was exposed to water vapor equilibrated at a temperature of 75-degrees-C, BaCO3 forms and increases in amount with exposure time. On the other hand, CuO and "Y2BaCuO5" form only in the initial stages of corrosion. For comparison, superconducting Tl2Ba2Ca2Cu3O10+x was also investigated, and similar results were obtained. Details related to the corrosion of these two high-T(c) phases are discussed herein.

Fluid electrolyte solutions based on mixtures of butyronitrile (PrCN) and ethyl chloride (EtCl) with Bu4NPF6 or Bu4NClO4 as electrolyte freeze below -180-degrees-C and provide excellent media for cryogenic electrochemical experiments. A 1:2 mixture of PrCN and EtCl exhibits the best combination of freezing point and ionic conductivity for ultralow temperature electrochemistry. Diffusion coefficients for bis(pentamethylcyclopentadienyl) iron (Cp2*Fe) are measurable by protential step chronoamperometry down to -160-degrees-C using a conventionally sized electrode, but the resistivity of the solvent mixture is such that potential sweep voltammetry benefits from the use of microdisk (10 and 25-mu-m diam Pt) or microband (0.2-mu-m wide Au) electrodes. Voltammetry at a chemically modified electrode down to -170-degrees-C is presented for the case of thin films of poly-[Os(bpy)2(vpy)2][PF6]2.

Fabrication of a well-behaved superconductor microband electrode fashioned from the edges of thin films of Bi2Sr2CaCu2O8 sandwiched between two insulating layers is demonstrated. In room temperature solutions of redox species, these microbands exhibit well-defined voltammograms and currents interpretable by standard microband electrode theory. The behavior of freshly cleaved electrodes indicates preparation of reasonably pristine surfaces. The new superconductor electrode geometry along with the recently reported fluid electrolyte cryosolvent will be useful in the context of future experiments with superconductor electrodes at temperatures below their T(c) values. The first results for double-layer capacitance of a superconductor/electrolyte solution interface are also provided.