James Anthony Ohlhausen

Corrosion of Lithium‐Ion Battery Current Collectors

The primary current‐collector materials being used in lithium‐ion cells are susceptible to environmental degradation: aluminum to pitting corrosion and copper to environmentally assisted cracking. Localized corrosion occurred on bare aluminum electrodes during simulated ambient‐temperature cycling in an excess of electrolyte. The highly oxidizing potential associated with the positive‐electrode charge condition was the primary factor. The corrosion mechanism differed from the pitting typically observed in aqueous electrolytes because each site was filled with a mixed metal/metal‐oxide product, forming surface mounds or nodules. Electrochemical impedance spectroscopy was shown to be an effective analytical tool for characterizing the corrosion behavior of aluminum under these conditions. Based on X‐ray photoelectron spectroscopy analyses, little difference existed in the composition of the surface film on aluminum and copper after immersion or cycling in electrolytes made with two different solvent formulations. Although Li and P were the predominant adsorbed surface species, the corrosion resistance of aluminum may simply be due to its native oxide. Finally, copper was shown to be susceptible to environmental cracking at or near the lithium potential when specific metallurgical conditions existed (work hardening and large grain size).

Mechanisms of friction in diamondlike nanocomposite coatings

Diamondlike nanocomposite (DLN) coatings (C:H:Si:O) processed from siloxane precursors by plasma enhanced chemical vapor deposition are well known for their low friction and wear behaviors. In the current study, we have investigated the fundamental mechanisms of friction and interfacial shear strength in DLN coatings and the roles of contact stress and environment on their tribological behavior. Friction and wear measurements were performed from 0.25to0.6GPa contact pressures in three environments: dry (<1% RH) nitrogen, dry (<1% RH) air, and humid (50% RH) air, with precise control of dew point and oxygen content. At 0.3GPa contact stress, the coefficient of friction (COF) in dry nitrogen was extremely low, ∼0.02, whereas in humid air it increased to ∼0.2, with minimal amount of wear in both environments. The coatings also exhibited non-Amontonian friction behavior, with COF decreasing with an increase in Hertzian contact stress. The main mechanism responsible for low friction and wear under varying cont...

Oxygen transport and isotopic exchange in iron oxide/YSZ thermochemically-active materials via splitting of C(18O)2 at high temperature studied by thermogravimetric analysis and secondary ion mass spectrometry

Ferrites are promising materials for enabling solar-thermochemical cycles for the production of synthetic fuels. Such cycles utilize solar-thermal energy for the production of hydrogen from water, or carbon monoxide from carbon dioxide. Recent work studying the thermochemical behaviour of iron oxides co-sintered with yttria-stabilised zirconia (YSZ) using thermogravimetric analysis revealed a striking difference in behaviour of iron that is in solid solution with the YSZ and that which exists as a second iron oxide phase. Materials in which the majority of iron was dissolved in the YSZ exhibited enhanced utilization of iron over those which possessed larger fractions of un-dissolved, bulk iron oxides. To illuminate this phenomena further, several samples of thermally-reduced iron oxide/8YSZ were re-oxidised using isotopically labelled C(18O)2. Post mortem characterization by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), with the application of multivariate analysis tools, enables the differentiation between 18O and 16O signals emanating from iron oxide particles. The distribution of 18O is uniform throughout the iron-doped 8YSZ, but concentrated at the surface of iron oxide particles embedded in this matrix. After identical thermal reduction and re-oxidation treatments, the gradient of 18O/16O across the iron oxide particles is found to depend on the size of the iron oxide particles, as well as the method of synthesis of the iron oxide/YSZ material. Comparative thermogravimetric analyses of the 18O-labelled materials and analogous un-labelled materials revealed that exposure to CO2 at 1100 °C results in rapid oxygen isotopic exchange.

Tribochemical polymerization of adsorbed n-pentanol on SiO2 during rubbing: when does it occur and is it responsible for effective vapor phase lubrication?

The origin and role of tribochemical reaction products formed while sliding silicon oxide surfaces in the presence of adsorbed alcohol molecules in equilibrium with the vapor phase were studied. Wear and friction coefficient studies with varying contact loads and n-pentanol vapor environments were used to determine under what operating conditions the tribochemical reaction species was produced. Imaging time-of-flight secondary ion mass spectrometry and microinfrared spectroscopy found that hydrocarbon species with a molecular weight higher than the starting vapor molecules are produced when there is wear of the SiO(2) surface. When the n-pentanol vapor lubrication is effective and the silicon oxide surface does not wear, then the tribochemical polymerization products are negligible. These results imply that the tribochemical polymerization is associated with the substrate wear process occurring due to insufficient adsorbate supply or high mechanical load. The tribochemical reactions do not seem to be the primary lubrication mechanism for vapor phase lubrication of SiO(2) surfaces with alcohol, although they may lubricate the substrate momentarily upon failure of the alcohol vapor delivery to the sliding contact.

Forensic analysis of bioagents by X-ray and TOF-SIMS hyperspectral imaging

Hyperspectral imaging combined with multivariate statistics is an approach to microanalysis that makes the maximum use of the large amount of data potentially collected in forensics analysis. This study examines the efficacy of using hyperspectral imaging-enabled microscopies to identify chemical signatures in simulated bioagent materials. This approach allowed for the ready discrimination between all samples in the test. In particular, the hyperspectral imaging approach allowed for the identification of particles with trace elements that would have been missed with a more traditional approach to forensic microanalysis. The importance of combining signals from multiple length scales and analytical sensitivities is discussed.

Nanoscale Void Nucleation and Growth in the Passive Oxide on Aluminum as a Prepitting Process

Nanometer scale morphological changes in the passive oxide on aluminum have been tracked as a function of polarization in aqueous chloride and borate electrolytes. Nanoscale void formation has been detected and characterized in the passive oxide on single and polycrystalline Al as well as nanocrystalline Al films. Void nucleation occurs at the metal/oxide interface and growth proceeds into the oxide. Nucleation and growth are continuous processes that occur well below the pitting potential. Void growth is related to the rate and extent of the passive oxide growth. Chloride is shown not to be necessary for the nucleation and growth of voids. The extent of void growth correlates with the faradaic charge density produced due to Al oxidation. Void densities on the order of 5 X 10 10 cm -2 form with as little as 2 to 4 monolayers of Al oxidation and at volumetric efficiencies of 10 to 50%. The mechanistic origin of the voids is consistent with point defect saturation at the Al/oxide interface. The shape factors for the voids are inconsistent with two leading pit initiation models where stable pitting is argued to result from disruption of the remnant oxide over a void or voidlike structures.

Multivariate statistical analysis for x-ray photoelectron spectroscopy spectral imaging: Effect of image acquisition time

The acquisition of spectral images for x-ray photoelectron spectroscopy (XPS) is a relatively new approach, although it has been used with other analytical spectroscopy tools for some time. This technique provides full spectral information at every pixel of an image, in order to provide a complete chemical mapping of the imaged surface area. Multivariate statistical analysis techniques applied to the spectral image data allow the determination of chemical component species, and their distribution and concentrations, with minimal data acquisition and processing times. Some of these statistical techniques have proven to be very robust and efficient methods for deriving physically realistic chemical components without input by the user other than the spectral matrix itself. The benefits of multivariate analysis of the spectral image data include significantly improved signal to noise, improved image contrast and intensity uniformity, and improved spatial resolution—which are achieved due to the effective sta...

Effect of Humidity on Scintillation Performance in Na and Tl Activated CsI Crystals

Time dependent photoluminescence and radioluminescence for sodium (Na) and thallium (Tl) activated cesium iodide (CsI) single crystals exposed to 50% and 75% relative humidity (RH) has been investigated. These results indicate that Tl activated crystals are more robust than the Na activated crystals against humidity induced scintillation degradation. The development of “etching pits” and “inactive” domains are the characteristics of deteriorated Na activated CsI crystals. These “inactive” domains, bearing a resemblance to a polycrystalline appearance beneath the crystal surface, can be readily detected by a 250 nm light emitting diode. These features are commonly observed at the corners and deep scratched areas where moisture condensation is more likely to occur. Mechanisms contributing to the scintillation degradation in Na activated CsI crystals were investigated by Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). ToF-SIMS depth profiles indicate that Na has been preferentially diffused out of CsI crystal, leaving the Na concentration in these “inactive” domains below its scintillation threshold.

Activation of erbium films for hydrogen storage

Hydriding of metals can be routinely performed at high temperature in a rich hydrogen atmosphere. Prior to the hydrogen loading process, a thermal activation procedure is required to promote facile hydrogen sorption into the metal. Despite the wide spread utilization of this activation procedure, little is known about the chemical and electronic changes that occur during activation and how this thermal pretreatment leads to increased rates of hydrogen uptake. This study utilized variable kinetic energy X-ray photoelectron spectroscopy to interrogate the changes during in situ thermal annealing of erbium films, with results confirmed by time-of-flight secondary ion mass spectrometry and low energy ion scattering. Activation can be identified by a large increase in photoemission between the valence band edge and the Fermi level and appears to occur over a two stage process. The first stage involves desorption of contaminants and recrystallization of the oxide, initially impeding hydrogen loading. Further he...

Accelerating aging failures in MEMS devices

The feasibility of using temperature and humidity to age vapor-deposited SAM-coated electrostatic-actuated MEMS devices with contacting surfaces was determined. Failures were dependent on both temperature and humidity. The trend indicated longer life at both lower temperatures and lower humidity levels. Using cantilever beams, measurements reveal degradation of the VSAM (vapor-deposited self assembled monolayer) surface coating when stressed at 300/spl deg/C with controlled humidity environments of 500 and 2000 ppmv. In particular, we have seen the surface adhesion change for these beams stressed at 300/spl deg/C for time intervals of 10, 24, 50, 100, and 200 hours. However, there is no measurable change after 2 hours. The higher humidity case promotes the same surface adhesion change in a factor of ten less time. The complex MEMS devices tested followed the same trends as the beam test structures. We definitely observe a failure of the MEMS devices due to the environment with most failures occurring at 300/spl deg/C and some failures at 200/spl deg/C. These failures are due to an adhesion site in the hub of the load gear where the typical gap is 0.3 /spl mu/m.