Non-destructive testing of tubes using a time reverse numerical simulation (TRNS) method

A method for the detection of defects in cylindrical structures and the determination of their positions and orientations is presented in this paper. The scattered field, which is generated by the interaction of excited guided waves with a defect, is evaluated with an approach named time reverse numerical simulation method (TRNS). Since the excited waves and the scattered field propagate along the sample, the time-consuming scanning of the whole tube can be eliminated. The scattered displacement field is measured in three dimensions over time with a laser vibrometer at different locations distributed equally around the circumference at a fixed axial coordinate far away from the defect. Instead of analyzing the complicated time signals directly, they are played back in time. If the recorded displacement histories of the scattered field are reversed in time and played back in an identical structure, the waves travel back the same path and interfere to a maximum at their origin. The result is an amplitude increase at the position of the defect where the scattered field was generated. Instead of playing back the recorded time signals in an experiment, this step is replaced by a numerical simulation. Only this enables the visualization and detection of the amplitude increase. As long as the simulation is of high accuracy, the position of the maximum interference corresponds exactly to the location of the defect in the experiment, although no defect is implemented in the simulation.     

New pyrazolium-carboxylates as structural analogues of the pseudo-cross-conjugated betainic alkaloid nigellicine

Pyrazolium-3-carboxylates were examined as relatives of the betainic alkaloid Nigellicine and as new examples of the sparsely populated class 16 of heterocyclic pseudo-cross-conjugated mesomeric betaines (PCCMB). The title compounds were prepared in a 4-step procedure starting from beta-diketo compounds 8 which were cyclized with substituted hydrazines. The resulting isomeric pyrazole esters 9 and 10 were separated and subsequently quaternized with dimethyl sulfate in the presence of nitrobenzene to pyrazolium esters 11 and 12. Saponification was best accomplished in diluted sulfuric acid, which resulted in the formation of the pseudo-cross-conjugated mesomeric betaines 13 and 14 in one step. Protonation to the corresponding carboxylic acids required the treatment of the betaines with tetrafluoroboric acid in dichloromethane. The effect of negative solvatochromism proves the charge separation in the ground state of the molecules. X-ray crystallographic analyses, semiempirical calculations, and ESI mass spectrometric measurements were performed to gain knowledge about the phenomenon of pseudo-cross-conjugation.     

Elastic stability of DNA configurations. I. General theory

Results are presented in the theory of the elastic rod model for DNA, among which are criteria enabling one to determine whether a calculated equilibrium configuration of a DNA segment is stable in the sense that it gives a local minimum to the sum of the segment's elastic energy and the potential of forces acting on it. The derived stability criteria are applicable to plasmids and to linear segments subject to strong anchoring end conditions. Their utility is illustrated with an example from the theory of configurations of the extranucleosomal loop of a DNA miniplasmid in a mononucleosome, with emphasis placed on the influence that nicking and ligation on one hand, and changes in the ratio of elastic coefficients on the other, have on the stability of equilibrium configurations. In that example, the configurations studied are calculated using an extension of the method of explicit solutions to cases in which the elastic rod modeling a DNA segment is considered impenetrable, and hence excluded volume effects and forces arising from self-contact are taken into account.     

Strongly coupled optical phonons in the ultrafast dynamics of the electronic energy and current relaxation in graphite

Ultrafast charge carrier dynamics in graphite has been investigated by time-resolved terahertz spectroscopy. Analysis of the transient dielectric function and model calculations show that more than 90% of the initially deposited excitation energy is transferred to a few strongly coupled lattice vibrations within 500 fs. These hot optical phonons also substantially contribute to the striking increase of the Drude relaxation rate observed during the first picosecond after photoexcitation. The subsequent cooling of the hot phonons yields a lifetime estimate of 7 ps for these modes.     

Precipitates originating from tungsten crucible parts in AlN bulk crystals grown by the PVT method

In order to evaluate the possible involvement of crucible materials in the growth of AlN bulk crystals grown by physical vapor transport, we applied growth conditions with a high vertical thermal gradient and hence high supersaturation of aluminum vapor. Under these conditions, precipitates formed causing diffuse grayish substructures at the initial growth interface and in the crystal body, decorating dislocations. Electron microscopy studies revealed that the precipitates are elongated, single‐phase particles with sizes of 50–500 nm of commensurate structure, oriented along the <110> direction. Chemical analysis of the precipitates showed tungsten as well as carbon and oxygen. The lattice parameters of the precipitates are in close agreement to hexagonal tungsten hemicarbide (W₂C). The possible transport from the tungsten parts and its conversion into tungsten hemicarbide precipitates is discussed. We thus conclude that the W₂C precipitates may contribute to the decoration of dislocations, even in growth with moderate thermal gradients.     

P9]+ [Al(OR(F))4)]-, the salt of a homopolyatomic phosphorus cation

Positive at last: The first condensed-phase homopolyatomic phosphorus cation [P(9)](+) was prepared using a combination of the oxidant [NO](+) and weakly coordinating anion, [Al{OC(CF(3))(3)}(4)](-). [P(9)](+) consists of two P(5) cages linked by a phosphonium atom to give a D(2d)-symmetric Zintl cluster. NMR (see picture), Raman, and IR spectroscopy, mass spectrometry, and quantum-chemical calculations confirmed the structure.     

How to confirm identified toxicants in effect-directed analysis

Due to the production and use of a multitude of chemicals in modern society, waters, sediments, soils and biota may be contaminated with numerous known and unknown chemicals that may cause adverse effects on ecosystems and human health. Effect-directed analysis (EDA), combining biotesting, fractionation and chemical analysis, helps to identify hazardous compounds in complex environmental mixtures. Confirmation of tentatively identified toxicants will help to avoid artefacts and to establish reliable cause–effect relationships. A tiered approach to confirmation is suggested in the present paper. The first tier focuses on the analytical confirmation of tentatively identified structures. If straightforward confirmation with neat standards for GC–MS or LC–MS is not available, it is suggested that a lines-of-evidence approach is used that combines spectral library information with computer-based structure generation and prediction of retention behaviour in different chromatographic systems using quantitative structure–retention relationships (QSRR). In the second tier, the identified toxicants need to be confirmed as being the cause of the measured effects. Candidate components of toxic fractions may be selected based, for example, on structural alerts. Quantitative effect confirmation is based on joint effect models. Joint effect prediction on the basis of full concentration–response plots and careful selection of the appropriate model are suggested as a means to improve confirmation quality. Confirmation according to the Toxicity Identification Evaluation (TIE) concept of the US EPA and novel tools of hazard identification help to confirm the relevance of identified compounds to populations and communities under realistic exposure conditions. Promising tools include bioavailability-directed extraction and dosing techniques, biomarker approaches and the concept of pollution-induced community tolerance (PICT). Figure Toxicity confirmation in EDA as a tiered approach