Willem G. Sloof

Determination of thickness and composition of aluminium-oxide overlayers on aluminium substrates

Abstract A novel method is presented to determine the thickness and the composition of oxide films on metal substrates using the measured XPS spectra of the clean and the oxidised metal. The method is applied to aluminium-oxide films on aluminium substrates. The oxide-film thickness is derived from the primary zero-loss intensities of only the metallic and oxidic Al 2p main peaks, utilising the known value of the intrinsic bulk plasmon excitation probability for the Al 2p core-level photoelectron emission process. It is shown that this new method for layer-thickness determination gives results that can differ 20% from results obtained by the less correct methods used in common practice. The actual composition, expressed as the O/Al atomic ratio, of the oxide film is determined from the total primary zero-loss intensities of the O 1s peak and the Al 2p peak of Al 2p oxidic rest spectrum. This spectrum of the oxide is obtained after subtraction of a reconstructed metallic Al 2p peak from the measured XPS spectrum.

Repeated crack healing in MAX-phase ceramics revealed by 4D in situ synchrotron X-ray tomographic microscopy.

MAX phase materials are emerging as attractive engineering materials in applications where the material is exposed to severe thermal and mechanical conditions in an oxidative environment. The Ti2AlC MAX phase possesses attractive thermomechanical properties even beyond a temperature of 1000 K. An attractive feature of this material is its capacity for the autonomous healing of cracks when operating at high temperatures. Coupling a specialized thermomechanical setup to a synchrotron X-ray tomographic microscopy endstation at the TOMCAT beamline, we captured the temporal evolution of local crack opening and healing during multiple cracking and autonomous repair cycles at a temperature of 1500 K. For the first time, the rate and position dependence of crack repair in pristine Ti2AlC material and in previously healed cracks has been quantified. Our results demonstrate that healed cracks can have sufficient mechanical integrity to make subsequent cracks form elsewhere upon reloading after healing.

Interface Microstructure and Adhesion of Zinc Coatings on TRIP Steels

Hot-dip galvanized transformation induced plasticity (TRIP) steel sheets were recently developed for automotive applications. The microstructure and the adhesion of zinc coated CMnSi TRIP steel alloyed with P were studied. The α-Zn coating adjacent to the steel substrate consists of a continuous η-Fe2Al5-xZnx inhibition layer with columnar ζ-FeZn13 intermetallic particles on top. Along the interface between the inhibition layer and the steel substrate Mn/Mn-P oxides were frequently observed. Although these oxides at the steel surface reduce the adhesion between the zinc coating and the TRIP steel, they do not cause any bare spots during galvanizing. Upon tensile deformation of the galvanized steel sheet, cracking along the α-zinc grain boundaries preceded fracture of the interface between the α-Zn layer and the inhibition layer. After 4 % deformation the average interface crack length increased linearly with the applied strain. This interface fracture was strongly influenced by the crystalline orientation of the α-Zn grains.

Pyrococcus furiosus 4Fe-ferredoxin, chemisorbed on gold, exhibits gated reduction and ionic strength dependent dimerization.

Pyrococcus furiosus ferredoxin is a small metalloprotein that shuttles electrons between redox enzymes. In its native 4Fe–4S form the protein is highly thermostable. In addition to three cluster-ligating cysteines, two surface cysteine residues (C21 and C48) are present. We used the reactivity of these surface thiols to directly immobilize ferredoxin on a bare gold electrode, with an orientation in which the cluster is exposed to solution. Voltammetry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM) studies established the immobilization of the 4Fe form. Native and recombinant wild-type ferredoxins were compared with the C48S, C21S, and C21S/C48S mutants. The variants with one and two surface cysteines can be directly chemisorbed on bare gold. Cyclic voltammetry demonstrated that the reduction potentials are similar to those in solution. The interfacial electron transfer kinetics revealed that the reduction is gated by the interconversion between two oxidized species. AFM images showed that dimers are chemisorbed at low ionic strength, while monomers are present at high ionic strength. XPS spectra revealed the presence of S, Fe, C, N, and O at the surface, which are assigned to the corresponding atoms in the peptide and the cofactor. Analysis of the sulfur spectrum corroborates that both C21 and C48 form gold–thiolate bonds. Moreover, two inorganic sulfide and two iron species were identified, suggesting an inhomogeneous charge distribution in the 4Fe–4S cluster. In conclusion, P. furiosus ferredoxin can be directly and vectorially chemisorbed on gold with retention of its properties. This may provide a biocompatible electrode surface with docking sites for redox enzymes.

Modeling Internal Oxidation of Binary Ni Alloys

A numerical model is presented to simulate the diffusional transport of oxygen and that of an alloying element, within a 1-D binary Ni alloy, leading to the selective oxidation of the alloying element and the formation of an internal oxide precipitate. This specific model is written in MATLAB and, with the aid of the Matlab Toolbox, is coupled to the ThermoCalc extensive database. A reaction time is introduced to overcome problems related to the difficulty of formation of the internal oxide. Two cases are considered: Al as the alloying element for which the solubility product of the oxide forming elements is small, and Mn for which it is large.