Charles F. Windisch

Direct Assembly of Large Arrays of Oriented Conducting Polymer Nanowires

Although oriented carbon nanotubes, oriented nanowires of metals, semiconductors and oxides have attracted wide attention, there have been few reports on oriented polymer nanostructures such as nanowires. In this paper we report the assembly of large arrays of oriented nanowires through controlled nucleation and growth during a stepwise electrochemical deposition process in which a large number of nuclei were first deposited on the substrate using a large current density. After the initial nucleation, the current density was reduced step by step to grow the oriented nanowires from the nucleation sites created in the first step. A very different morphology was also demonstrated by first depositing a monolayer of close-packed colloidal spheres using a similar step-wise deposition process. As a result, the polymer nanofibers grew from the spheres in a radial fashion and formed the continuous three-dimensional network of nanofibers in the film. The principles of control nucleation and growth in electrochemical deposition investigated in this paper should be applicable to other electrical conducting and electrochemical active materials, including metals and conducting oxides. We also hope the oriented electroactive polymer nanostructure will open the door for new applications, such as miniaturized biosensors.

Epitaxial growth and properties of MBE-grown ferromagnetic Co-doped TiO2 anatase films on SrTiO3(001) and LaAlO3(001)

Abstract We have investigated the heteroepitaxial growth and materials properties of pure and Co-doped TiO 2 anatase on SrTiO 3 (001) and LaAlO 3 (001), grown by oxygen plasma assisted molecular beam epitaxy. This material is a promising new diluted magnetic semiconductor that shows large magnetization and a Curie temperature well above room temperature. We have found that epitaxial films with the highest crystalline quality and most uniform distribution of Co result when a rather slow growth rate (∼0.01 nm/s) is used over a substrate temperature range of 550–600 °C on LaAlO 3 (001). These conditions result in layer-by-layer growth of single-crystal films and a very low density of extremely small nanocrystalline inclusions. In contrast, growth at a higher rate (∼0.04 nm/s) leads to extensive formation of secondary-phase rutile nanocrystals to which Co diffuses and segregates. The rutile nanocrystals nucleate on the evolving anatase film surface in such a way that lattice strain between the two phases is minimized. Cobalt appears to substitute for Ti in the lattice and exhibits a +2 formal oxidation state. Both pure and Co-doped films can be grown as n-type semiconductors by controlled incorporation of oxygen atom vacancies. Free electrons are required to couple the Co(II) spin to a ferromagnetic state.

Infrared transparent spinel films with p-type conductivity

Spinel oxide films containing at least two transition metal cations were found to exhibit p-type conductivity with high optical transparency from the visible to wavelengths near 15 μm. Resistivities as low as 0.003 Ω cm were measured on 100-nm thick rf sputter deposited films that contained nickel and cobalt. Optical spectra, Raman scattering and XPS measurements indicated the valency of nickel localized on octahedral sites within the spinel lattice determines these properties. A resistivity minimum was found at the composition NiCo 2 O 4 deposited from aqueous or alcoholic solutions followed by subsequent annealing at 400°C in air. Solution deposited films richer in nickel than this stoichiometry, were always found to phase separate into nickel oxide and a spinel phase with concomitant loss in conductivity. However, the phase stability region could be extended to higher nickel contents when rf-sputter deposition techniques were used. Sputter deposited spinel films having a cobalt to nickel ratio <2 were found to exhibit the highest conductivity. Results suggest that the phase stability region for these materials can be extended through appropriate choice of deposition conditions. A possible mechanism that promotes high conductivity in this system is thought to be charge transfer between the resident di- and trivalent cations that may be assisted by the magnetic nature of the oxide film.

Stress corrosion cracking of Al-Mg and Mg-Al alloys

Abstract Al- and Mg-based alloys are being used for reducing the weight of automobiles. For structural applications, they must have adequate stress corrosion resistance, and yet, under some circumstances, stress corrosion cracking can occur in both alloy systems. Precipitation of the Mg-rich β-phase (Al3Mg2) at grain boundaries of Al-Mg alloys and the γ-phase (Mg17Al12) at grain boundaries of the Mg-Al alloys is a critical factor in their stress corrosion performance. In Mg-Al, the γ-phase is cathodic (noble) to the matrix, while in the Al-Mg case, the β-phase is anodic (active) to the matrix. These phases produce localized galvanic-induced corrosion that leads to intergranular stress corrosion cracking and crack growth rates of 350 and 1,800 times faster than the solution-treated condition, for Al-Mg and Mg-Al, respectively.

Influence of Mg on the corrosion of Al

This article summarizes a series of experiments to determine the influence of Mg on the corrosion and electrochemical behavior of Al. Magnesium is commonly added to increase the strength of lightweight nonheat treatable Al alloys. However, these alloys are susceptible to grain boundary dissolution, stress corrosion cracking, or hydrogen induced embrittlement due to changes in the alloy structure and elemental distribution during processing, welding, or in-service exposure to elevated temperatures. Auger electron spectroscopy and transmission electron microscopy measurements show that alloys having a distribution of Al3Mg2 (β phase) precipitates and segregated Mg on grain boundaries are more susceptible to cracking. To understand the roles of Mg on the cracking process we compared the corrosion potential and film formation of pure Al, Al implanted with Mg, a 7 wt % Mg–Al alloy and pure Al3Mg2 phase. The surfaces of the specimens were cleaned and prepared in a surface analysis system and transferred in a va...

In situ observations of the early stages of localized corrosion in Type 304 SS using the electrochemical atomic force microscope

In situ, time-lapse images of the early stages of pitting in dilute NaCl solutions, and intergranular corrosion in dilute oxalic acid solutions, have been obtained in Type 304 SS (UNS S30400) using an electrochemical atomic force microscope (ECAFM). A pit was observed to grow from an irregular shape with ledges along the pit walls, to a round pit with an island formation, over a 6-min time span. Grain boundary chromium carbides were exposed, and then partially dissolved, over a 26-min time span. The intergranular corrosion was most prominent along the grain boundary between the carbides, rather than between the carbides and the matrix, leaving the carbides intact via ligaments of matrix material. This study demonstrated the usefulness and complexities in using the ECAFM for studying the early stages in pitting and intergranular corrosion.

Spectroscopic characterization of processing-induced property changes in doped ZnO films

Abstract The optical response and electronic conductivity of neat and trivalent cation doped ZnO films are dependent upon film deposition parameters and subsequent post-deposition processing. As-deposited films which show high resistivity can be made conducting through gas phase reduction with hydrogen at elevated temperatures or by electrochemical reduction at room temperature using both aqueous and non-aqueous solvents. Resistive sputter and solution deposited films which exhibit c-axis or random crystallite orientation respectively, become conducting after these treatments and show increased infrared reflectivity owing to free carrier absorption. Raman measurements are used to confirm the wurtzite crystalline phase and dopant incorporation into the lattice. Optical properties of selected films are determined from transmission and spectroscopic ellipsometry measurements using models parameterized with previously determined microstructural information. Based upon the electrochemical results, a mechanism is proposed to describe the reduction process which requires the generation of atomic hydrogen as a reducing species.

Structural model of homogeneous As–S glasses derived from Raman spectroscopy and high-resolution XPS

The structure of homogeneous bulk As x S100− x (25 ≤ x ≤ 42) glasses, prepared by the conventional rocking–melting–quenching method, was investigated using high-resolution X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. It is shown that the main building blocks of their glass networks are regular AsS3/2 pyramids and sulfur chains. In the S-rich domain, the existence of quasi-tetrahedral (QT) S = As(S1/2)3 units is deduced from XPS data, but with a concentration not exceeding ∼3–5% of total atomic sites. Therefore, QT units do not appear as primary building blocks of the glass backbone in these materials, and an optimally-constrained network may not be an appropriate description for glasses when x < 40. It is shown that, in contrast to Se-based glasses, the ‘chain-crossing’ model is only partially applicable to sulfide glasses.

Electrochemistry of TiN in 6 M KOH Solution

Cyclic polarization of TiN electrodes in 6 M KOH at room temperature revealed relatively large anodic and cathodic overpotentials for water decomposition that depend on the surface structure of the material. Polished TiN electrodes also underwent a slow reaction during anodic polarization above the reversible potential for O{sub 2} evolution that involved the formation of a reaction product film. The film was uniform over the surface of the electrodes and contained hydrous potassium titanate. A corrosion product, similar in composition but less uniform in appearance, formed on TiN after 4 months under open-circuit conditions in the same solution. Results indicate that the film forms slowly by a dissolution/precipitation mechanism and that the dissolution step gives rise to an oxidation peak observed in the cyclic voltammogram during anodic polarization. Since the film forms above the reversible potential for O{sub 2} evolution and since the overvoltage for O{sub 2} evolution depends on surface structure, it is not yet clear whether the formation of the corrosion film will compete with electrolysis reactions during the charging of structurally tailored, high surface area, TiN electrodes that are under development for ultracapacitor applications.

High temperature corrosion and crack growth of SiCSiC at variable oxygen partial pressures

Abstract Thermal gravimetric analysis (TGA) and subcritical crack growth measurements of chemical-vapor-infiltrated SiC matrix reinforced with Nicalon fibres and with a 1 μm thick C fiber-matrix interface have been conducted at 1100 °C over O 2 Ar mixtures ranging from 0.25% to 20.0% O 2 . The TGA and interface recession measurements both gave linear reaction kinetics for O 2 concentrations of 2.0% or less and a reaction order of unity. Subcritical crack growth measurements demonstrated that the crack velocity, in the stress-intensity-independent stage II regime, increases with increasing O 2 /Ar ratio. Also, the transition from stage II to the stress-intensity-dependent stage III regime is shifted to lower stress intensities with increasing O 2 /Ar ratio. A time-dependent crack growth model that incorporates creep of the bridging SiC fibers and the removal of the C interfacial layer by oxidation successfully explains the subcritical crack growth characteristics.