SIMS Investigations on the Distribution of Trace Elements in Modified Aluminium–Silicon–Magnesium Alloys

 Strontium modified aluminium–silicon cast alloys are well known for their outstanding mechanical properties as they combine excellent strength with good ductility that is due to a modification of brittle Si in the eutectic with traces of Sr (0.3–0.5 wt%). Although the level of Sr addition is very low, formation of ternary AlSiSr phases with deleterious effects on the ductility can take place. Consequently, there is a certain need of alternative modifying elements. Following the theory of Lu and Hellawell which predicts an optimal atomic size for good modification, Eu, which has an atomic radius close to that optimal size, was additionally used as a modifying element. This study presents SIMS (secondary ion mass spectrometer) investigations of the 3D-distributions of the main alloying and trace elements in Al–Si feedstock alloys for thixocasting modified with Sr and Eu, respectively. Received October 1, 2001; accepted May 21, 2002     

AES and SIMS analysis of non-metallic inclusions in a low-carbon Chromium-steel

In the final step of secondary metallurgical steel processing, calcium is added. Besides Mg, Ca is the most powerful deoxidiser and desulfurisation agent. It reacts with dissolved oxygen and sulfur and reduces oxides and sulfides thereby forming non-metallic inclusions. Within this paper we present the analysis of such inclusions in a low-carbon chromium-steel. Depending on the time of quenching of the steel sample, different structures were revealed by REM, Auger and SIMS: If the steel was quenched immediately after Ca-addition, non-metallic inclusions that appeared to have "cavities" could be detected with SEM. SIMS investigations of these particles showed ring-shaped structures and revealed that the ring is made up of Al, Ca, Mg, O and S. No secondary ions however could be retrieved from the core inside the ring, thus leaving the nature of the "cavities" unclear. If the steel sample was quenched 3 min after Ca addition, inclusions did not have a ring-shaped structure but a compact one.     

Imaging surface spectroscopy for two- and three-dimensional characterization of materials

Secondary ion mass spectrometry (SIMS) exhibits a unique potential for the measurement of two-and three-dimensional distributions of trace elements in advanced materials, which is demonstrated on relevant technological problems. One example is the characterization of high purity iron. With this material segregation experiments have been performed and the initial and final distribution of the trace elements have been measured. Another example is the investigation of the corrosion behaviour of high purity chromium. Samples oxidized with (16)O and (18)O have been measured to explain the growing and adhesion of the oxide layer. All imaging techniques generate a vast quantitiy of data. In order to extract the important information the assistance of chemometric tools is essential. Detection of chemical phases by classification using neural networks or de-noising of scanning-SIMS images by wavelet-filtering demonstrates the increase of the performance of analytical imaging techniques.     

Investigation of the formation and properties of protective oxide layers on high purity chromium with SIMS imaging techniques

Spallation of the protective oxide layer formed during hot gas oxidation is the main reason for the corrosion of high purity powder metallurgically produced chromium[1]. To explain the formation and occasional spallation of the oxide layer a chromium sample subjected to two consecutive oxidation processes in¹⁶O and¹⁸O atmosphere at high temperature was characterised by 2D and 3D SIMS.The formation of the protective oxide can be described by the diffusion of chromium from the bulk to the surface and the reaction of the chromium atoms with the oxygen from the gaseous phase. The very low solubility of nitrogen in chromium oxide indicates its inability to diffuse through the growing oxide layer and explains the enrichment of nitrogen (same mechanism applies for chlorine) in the interface metal/oxide. The accumulation of trace elements within the interface during the oxidation process explains the reduced adhesion power of the passivation layer and its spallation.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Separating drugs from nondrugs: a statistical approach using atom pair distributions

A computational approach to quantify the druglike character of chemical compounds is presented. For this purpose, the distribution of atom types and their pair-wise combinations in known drugs and nondrugs was examined. Statistical analysis of the occurrence probabilities was used to derive a drug-likeliness score on a logarithmic scale. "Typical" pharmaceutical agents exhibit scores greater than 0.3, while for ordinary substances, values below 0 are expected. Although any kind of fitting or error minimization scheme is absent in this method, confirmed drugs are predicted with an accuracy of at least 71%. Many falsely predicted nondrugs were found to closely resemble actual drugs or to contain unsuitable substitution patterns that can easily be ruled out by applying medicinal knowledge. As the outlined method is computationally inexpensive, this drug-likeliness score can therefore be used as a filter for the in silico screening of large substance databases.     

3D-SIMS analysis of ultra high purity molybdenum and tungsten: a characterisation of different manufacturing techniques and products

Applying a recently developed three dimensional SIMS imaging technique major differences in the distribution of trace elements in ultra high purity Mo and W have been found. In the electron beam melted material severe grain boundary segregation of trace elements have occurred whereas in the hot pressed material trace elements have been present as precipitates with a size of 5–15 m. Guided by the results of the 3D-SIMS images and the advantages of the sintering process a material with homogeneous distribution of trace elements has been developed and characterised. To test the applicability for the microelectronics industry, sputtering targets have been manufactured out of this new material and layers with a thickness of 350 nm have been sputterdeposited on silicon. The quality of these layers, with respect to particle emission and the distribution of trace elements, was evaluated by EPMA and 3D-SIMS imaging. Further improvement of the sintering process led to a material with a completely homogeneous distribution of C, H, N, O and S to minimise the outgassing and diffusion of impurities.Abbreviations BSE Back scattered electron - EPMA Electron probe micro analysis - GAAS Graphite furnace atomic absorption spectrometry - GDMS Glow discharge mass spectrometry - ICP-AES Inductively coupled plasma atomic emission spectrometry - ICP-MS Inductively coupled plasma mass spectrometry - SIMS Secondary ion mass spectrometry - ULSI Ultra large scale integration     

Simultaneous quantification of total medium- and long-chain fatty acids in human milk by capillary gas chromatography with split injection

Four different quantification methods for the capillary gas chromatographic determination of medium-chain fatty acids (6:0-12:0) and myristic acid in human milk samples, using a split injector, were compared. Odd-carbon-numbered fatty acids (5:0-17:0) were added as internal standards. Each medium-chain fatty acid and myristic acid was calculated on the basis of: the peak area of the internal standard with one methylene group less; the peak area of the internal standard with one methylene group more; half the sum of the peak areas of the internal standards with one methylene group less and more (bracketting method); the peak area of 17:0. The peak-area ratio of each analyte and 17:0 in a standard was found to be subject to an unacceptably high coefficient of variation. From the methods using internal standards with one methylene group more and less, the bracketting method was found to be the best, resulting in recoveries close to 100%, with the lowest coefficients of variation. The method was applied for the determination of the fatty acid composition of mature milk samples of 47 Cura?aoan women.     

Car-parrinello molecular dynamics simulation of liquid formic acid

First-principles molecular dynamics has been used to investigate the structural, vibrational, and energetic properties of formic acid, formic acid-formate anion dimers, and liquid formic acid in a periodically repeated box with 32 formic acid molecules. We found that in liquid formic acid the hydrogen-bonded clusters mainly consist of linear branching chains. From our simulation, we got good agreement with the available structural and dynamical data. We also studied the proton transfer in the cis-formic acid-formate anion dimer, and we showed that this proton transfer does not have any potential barrier. The hydrogen bonding statistics as well as the mean lifetime of the hydrogen bonds are analyzed.     

A photochemical activation scheme of inert dinitrogen by dinuclear Ru(II) and Fe(II) complexes

A general photochemical activation process of inert dinitrogen coordinated to two metal centers is presented on the basis of high-level DFT and ab initio calculations. The central feature of this activation process is the occupation of an antibonding pi* orbital upon electronic excitation from the singlet ground state S0 to the first excited singlet state S1. Populating the antibonding LUMO weakens the triple bond of dinitrogen. After a vertical excitation, the excited complex may structurally relax in the S1 state and approaches its minimum structure in the S1 state. This excited-state minimum structure features the dinitrogen bound in a diazenoid form, which exhibits a double bond and two lone pairs localized at the two nitrogen atoms, ready to be protonated. Reduction and de-excitation then yield the corresponding diazene complex; its generation represents the essential step in a nitrogen fixation and reduction protocol. The consecutive process of excitation, protonation, and reduction may be rearranged in any experimentally appropriate order. The protons needed for the reaction from dinitrogen to diazene can be provided by the ligand sphere of the complexes, which contains sulfur atoms acting as proton acceptors. These protonated thiolate functionalities bring protons close to the dinitrogen moiety. Because protonation does not change the pi*-antibonding character of the LUMO, the universal and well-directed character of the photochemical activation process makes it possible to protonate the dinitrogen complex before it is irradiated. The pi*-antibonding LUMO plays the central role in the activation process, since the diazenoid structure was obtained by excitation from various occupied orbitals as well as by a direct two-electron reduction (without photochemical activation) of the complex; that is, the important bending of N2 towards a diazenoid conformation can be achieved by populating the pi*-antibonding LUMO.