Brewton-Parker College: A Private Christian/Baptist College located in Southeast Georgia Mathematics and Natural Sciences: Biology, Chemistry, Mathematics, PhysicsMathematics and Natural Sciences: Biology, Chemistry, Mathematics, PhysicsMathematics and Natural Sciences: Biology, Chemistry, Mathematics, PhysicsMathematics and Natural Sciences: Biology, Chemistry, Mathematics, Physics

2.6 Electrochemistry of Thin Film Superconductor Electrodes

While barriers to acquiring good electrochemistry on ceramic superconductors have been overcome,18 the electrochemical response of redox active solution species using bare YBa2Cu3O7-x thin film electrodes has been problematic.21, 22† Redox active assemblies anchored on the surface, 37-40 on the other hand, have successfully been used to obtain faradaic electron transfer reactions.† One advantage that bulk YBa2Cu3O7-x electrodes have over thin films is that they can be mechanically resurfaced just prior to electrochemical measurements in an inert atmosphere.† This provides a pristine surface, free of insulating corrosion products.† Unfortunately, there are no methods currently available for mechanically resurfacing YBa2Cu3O7-x thin films to a pristine state, though polishing and cleaving of microband edge electrodes have proved successful.13, 14† Chemical etching of YBa2Cu3O7-x thin films with bromine / ethanol solutions has been employed,41 but limitations of controlling the etching rate and non-uniform etching prevent the formation of a clean uniform surface.

The electrolyte tetraethylammonium tetrafluoroborate, Et4NBF4, (Aldrich) was recrystallized from ethyl acetate / ethanol and vacuum dried overnight prior to use.† The compound 7,7’,8,8’-tetracyanoquinodimethane, TCNQ, (Aldrich) was used as the redox couple.† Acetonitrile was distilled with P2O5 under N2 and passed over activated alumina in an inert atmosphere before use.† Here, 0.1 M Et4NBF4 in CH3CN was used as electrolyte solution.†† Solutions containing 1mM TCNQ were also made from the electrolyte solution.† Cyclic voltammetry was accomplished using an EG&G PAR 273 potentiostat.† All electrochemical measurements were carried out in an inert atmosphere glovebox.† Thin films were transferred from the laser ablation chamber to an inert atmosphere with less than 2-3 minutes atmospheric exposure to prevent surface corrosion.†

Investigations targeted YBa2Cu3O7-x thin films since these are the most commonly researched high-temperature superconducting thin film materials.† The orientation of the thin films can be manipulated by careful control of the deposition temperature.34† At a temperature of 750oC, deposition of the c-axis orientation is preferred.† The a-axis orientation of YBa2Cu3O7-x is favored when the deposition temperature is 630oC.† At temperatures between 630oC and 750oC, a mixture of both orientations is obtained.† The choice of substrates also plays a factor in the determination of orientation.34† For c-axis orientation, the lattice mismatches for MgO and LaAlO3 are 2.00% and 2.62%, respectively.† While the lattice mismatches for a-axis oriented thin films on MgO and LaAlO3 are 8.41% and 0.81%, respectively.† There are two primary conduction pathways in the YBa2Cu3O7-x structure.† The first conduction pathway is aligned within the a-b plane through the copper-oxide sheets.† The second pathway is along the copper

 

Structural schematic for YBa2Cu3O7-z orientations a) a-axis b) c-axis (tilted view)

Illustration 2.5:†††† Structural schematic for YBa2Cu3O7-z orientations a) a-axis b) c-axis (tilted view).

Structural schematic for YBa2Cu3O7-z orientations a) a-axis b) c-axis (side view). Arrows indicate the locations where electronic conductivity is thought to be highest within the lattice. oxide chains. 

Illustration 2.6:†Structural schematic for YBa2Cu3O7-z orientations a) a-axis b) c-axis (side view).† Arrows indicate the locations where electronic conductivity is thought to be highest within the lattice.††††††††† oxide chains.†

Illustrations 2.5(a) and 2.6(b) show that the a-axis oriented thin film positions its copper-oxide sheets perpendicular to the electrode interface, whereas the c-axis oriented thin film, shown in Illustrations 2.5(b) and 2.6(b), has its copper-oxide sheets parallel to the interface.†† Perpendicular alignment of the copper-oxide sheets at the surface of the electrode may provide active sites near the electrode’s surface which promotes electron transfer to the solution-dissolved redox species.

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The mission of Brewton-Parker College, a Georgia Baptist college, is to develop the whole student through the application of Biblically-centered truth to a liberal arts curriculum in a community of shared Christian values.
 
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Updated on: April 15, 2010 8:26 PM