On the formation and technical importance of the microsegregation in austenitic chromium-nickel alloyed steels

The austenitic chromium nickel alloyed steels are showing different types of concentration-distributions of the elements chromium and nickel in the austenite in dependance on the content of alloys. As a consequence of the crystallization of the heat to austenite the “equal directed” type of segregation forms. The ferritic solidification of a heat and the following ferrite to austenite transformation leads to “unequal directed” segregation in the austenite. Heats having both the types of segregation solidify by way a 3-phase reaction. For heats ferritically solidified better hot plasticity, lower susceptibility to hot cracking in the weld and better polishability is observed compared with heats solidified austenitically. The advantages are accounted to the lower separation during the ferritic solidification as consequence of the higher diffusivity of the alloying and accompanying elements in the ferrite compared with the austenite.     

Routes to control of H2 Coulomb explosion in few-cycle laser pulses

We have measured coincident ion pairs produced in the Coulomb explosion of H2 by 8-30 fs laser pulses at different laser intensities. We show how the Coulomb explosion of H2 can be experimentally controlled by tuning the appropriate pulse duration and laser intensity. For laser pulses less than 15 fs, we found that the rescattering-induced Coulomb explosion is dominated by first-return recollisions, while for longer pulses and at the proper laser intensity, the third return can be made to be the major one. Additionally, by choosing suitable pulse duration and laser intensity, we show H2 Coulomb explosion proceeding through three distinct processes that are simultaneously observable, each exhibiting different characteristics and revealing distinctive time information about the H2 evolution in the laser pulse.     

Phase control in a coherent superposition of degenerate quantum states through frequency control

We demonstrate a controlled phase change of π in a degenerate superposition by altering a laser frequency by only 10 MHz. The method relies on the preparation of an adiabatic state involving the M = ±2 and M = 0 states of the ³P₂ (J = 2) level of metastable neon. Dependent on the frequency, the preparation proceeds either by stimulated Raman adiabatic passage (STIRAP) or by coherent population trapping (CPT). In the former case the superposition is prepared by adiabatic transfer induced in an extended tripod linkage scheme. In the latter case population is optically pumped into the Zeeman manifold of the level ³P₂. The population which does not reach a dark state decays to the ground state of neon. The amplitudes and relative phases of the dark states differ for the two cases. The phase change is monitored using the method of phase-to-population mapping.     

Electrochemically decorated carbon nanotubes for hydrogen sensing

Low-density networks of single-wall carbon nanotubes have been modified by palladium nanoparticles using an electrochemical method. A major advantage of this approach is that it allows for selective metal deposition on the electrically contacted nanotubes, whereas the remaining substrate surface and the non-contacted tubes remain essentially unaffected. The Pd-decorated networks function as effective hydrogen sensors enabling the detection of hydrogen concentrations as low as 10 ppm at room temperature. The electrochemical metal deposition scheme is promising for the development of sensor arrays suitable for analysing gas mixtures.     

Interaction dynamics of elastic waves with a complex nonlinear scatterer through the use of a time reversal mirror

This Letter reports on work performed to locate and interrogate a nonlinear scatterer in a linearly elastic medium through the use of a time reversal mirror in combination with nonlinear dynamics. Time reversal provides the means to spatially and temporally localize elastic energy on a scattering feature while the nonlinear dynamics spectrum allows one to determine whether the scatterer is nonlinear (e.g., mechanical damage). Here elastic waves are measured in a solid and processed to extract the nonlinear elastic response. The processed elastic signals are then time reversed, rebroadcast, and found to focus on the nonlinear scatterer, thus defining a time-reversed nonlinear elastic wave spectroscopy process. Additionally, the focusing process illuminates the complexity of the nonlinear scatterer in both space and time, providing a means to image and investigate the origins and physical mechanisms of the nonlinear elastic response.     

The effects of plastic deformation on band gap, electronic defect states and lattice vibrations of rutile

The extensive polygonization of 200 nm rutile crystals in high-energy dry milling allowed to study the spectral properties of grain boundaries and adjacent microstrained crystalline matter. Changes in UV, VIS, NIR, IR and FIR spectra during milling were followed. For the UV absorption edge the value of unstrained rutile was retained while residual traces of anatase, intergrown with the rutile phase, continued to act as traps for photoinduced charges. The evolving broad absorption in VIS and NIR could be attributed to electrons weakly bound to defects in the packing of oxygen anions at the grain boundaries, which may relax to face-sharing Ti³⁺-O octahedra. Among the IR-active lattice vibrations, the narrow E_u⁽²⁾ band showed a shift to higher frequencies by 15 cm⁻¹ which is definitively not due to phonon confinement or Fröhlich surface modes but probably to coupling of the bulk phonon to a plasmon at the grain boundary. At the external surface of the polygonized primary particle, the regular atomic order is destroyed by milling so that hydroxylation is replaced by physisorption of H₂O, as shown by IR and TG.     

Variable angle NMR spectroscopy and its application to the measurement of residual chemical shift anisotropy

The successful measurement of anisotropic NMR parameters like residual dipolar couplings (RDCs), residual quadrupolar couplings (RQCs), or residual chemical shift anisotropy (RCSA) involves the partial alignment of solute molecules in an alignment medium. To avoid any influence of the change of environment from the isotropic to the anisotropic sample, the measurement of both datasets with a single sample is highly desirable. Here, we introduce the scaling of alignment for mechanically stretched polymer gels by varying the angle of the director of alignment relative to the static magnetic field, which we call variable angle NMR spectroscopy (VA-NMR). The technique is closely related to variable angle sample spinning NMR spectroscopy (VASS-NMR) of liquid crystalline samples, but due to the mechanical fixation of the director of alignment no sample spinning is necessary. Also, in contrast to VASS-NMR, VA-NMR works for the full range of sample inclinations between 0° and 90°. Isotropic spectra are obtained at the magic angle. As a demonstration of the approach we measure 13C-RCSA values for strychnine in a stretched PDMS/CDCl? gel and show their usefulness for assignment purposes. In this context special care has been taken with respect to the exact calibration of chemical shift data, for which three approaches have been derived and tested.     

Force localization in contracting cell layers

Epithelial cell layers on soft elastic substrates or pillar arrays are commonly used as model systems for investigating the role of force in tissue growth, maintenance, and repair. Here we show analytically that the experimentally observed localization of traction forces to the periphery of the cell layers does not necessarily imply increased local cell activity, but follows naturally from the elastic problem of a finite-sized contractile layer coupled to an elastic foundation. For homogeneous contractility, the force localization is determined by one dimensionless parameter interpolating between linear and exponential force profiles for the extreme cases of very soft and very stiff substrates, respectively. If contractility is sufficiently increased at the periphery, outward directed displacements can occur at intermediate positions. We also show that anisotropic extracellular stiffness can lead to force localization in the stiffer direction, as observed experimentally.