Assessment of a High-Throughput Methodology for the Study of Alloy Oxidation using AlxFeyNi1-x-y Composition Gradient Thin Films.

The high-temperature oxidation of multicomponent metal alloys exhibits complex dependencies on composition, which are not fully understood for many systems. Combinatorial screening of the oxidation of many different compositions of a given alloy offers an ideal means for gaining fundamental insights into such systems. We have previously developed a high-throughput methodology for studying AlxFeyNi1-x-y alloy oxidation using ∼100 nm thick composition spread alloy films (CSAFs). In this work, we critically assess two aspects of this methodology: the sensitivity of CSAF oxidation behavior to variations in AlxFeyNi1-x-y composition and the differences between the oxidation behavior of ∼100 nm thick CSAFs and that of bulk AlxFeyNi1-x-y alloys. This was done by focusing specifically on AlxFe1-x and AlxNi1-x oxidation in dry air at 427 °C. Transitions between phenomenologically distinguishable types of oxidation behavior are found to occur over CSAF compositional ranges of <2 at. %. The oxidation of AlxFe1-x CSAFs is found to be very similar to that of bulk AlxFe1-x alloys, but some minor differences between CSAF and bulk behavior are observed for AlxNi1-x oxidation. On the basis of our assessment, high-throughput studies of CSAF oxidation appear to be an effective method for gaining fundamental insights into the composition dependence of the oxidation of bulk alloys.

On the Structural and Chemical Characteristics of Co/Al2O3/graphene Interfaces for Graphene Spintronic Devices.

We report a detailed investigation of the structural and chemical characteristics of thin evaporated Al2O3 tunnel barriers of variable thickness grown onto single-layer graphene sheets. Advanced electron microscopy and spectrum-imaging techniques were used to investigate the Co/Al2O3/graphene/SiO2 interfaces. Direct observation of pinhole contacts was achieved using FIB cross-sectional lamellas. Spatially resolved EDX spectrum profiles confirmed the presence of direct point contacts between the Co layer and the graphene. The high surface diffusion properties of graphene led to cluster-like Al2O3 film growth, limiting the minimal possible thickness for complete barrier coverage onto graphene surfaces using standard Al evaporation methods. The results indicate a minimum thickness of nominally 3 nm Al2O3, resulting in a 0.6 nm rms rough film with a maximum thickness reaching 5 nm.

All-Diamond Microelectrodes as Solid State Probes for Localized Electrochemical Sensing.

The fabrication of an all-diamond microprobe is demonstrated for the first time. This ME (microelectrode) assembly consists of an inner boron doped diamond (BDD) layer and an outer undoped diamond layer. Both layers were grown on a sharp tungsten tip by chemical vapor deposition (CVD) in a stepwise manner within a single deposition run. BDD is a material with proven potential as an electrochemical sensor. Undoped CVD diamond is an insulating material with superior chemical stability in comparison to conventional insulators. Focused ion beam (FIB) cutting of the apex of the ME was used to expose an electroactive BDD disk. By cyclic voltammetry, the redox reaction of ferrocenemethanol was shown to take place at the BDD microdisk surface. In order to ensure that the outer layer was nonelectrically conductive, a diffusion barrier for boron atoms was established seeking the formation of boron-hydrogen complexes at the interface between the doped and the undoped diamond layers. The applicability of the microelectrodes in localized corrosion was demonstrated by scanning amperometric measurements of oxygen distribution above an Al-Cu-CFRP (Carbon Fiber Reinforced Polymer) galvanic corrosion cell.