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 ¹⁶O and ¹⁸O 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.     

Peristaltic flow of a Johnson-Segalman fluid through a deformable tube

To understand theoretically the flow properties of physiological fluids we have considered as a model the peristaltic motion of a Johnson–Segalman fluid in a tube with a sinusoidal wave traveling down its wall. The perturbation solution for the stream function is obtained for large wavelength and small Weissenberg number. The expressions for the axial velocity, pressure gradient, and pressure rise per wavelength are also constructed. The general solution of the governing nonlinear partial differential equation is given using a transformation method. The numerical solution is also obtained and is compared with the perturbation solution. Numerical results are demonstrated for various values of the physical parameters of interest.      

Rotating flow of a second-order fluid on a porous plate

A study is made of the unsteady flow engendered in a second-order incompressible, rotating fluid by an infinite porous plate exhibiting non-torsional oscillation of a given frequency. The porous character of the plate and the non-Newtonian effect of the fluid increase the order of the partial differential equation (it increases up to third order). The solution of the initial value problem is obtained by the method of Laplace transform. The effect of material parameters on the flow is given explicitly and several limiting cases are deduced. It is found that a non-Newtonian effect is present in the velocity field for both the unsteady and steady-state cases. Once again for a second-order fluid, it is also found that except for the resonant case the asymptotic steady solution exists for blowing. Furthermore, the structure of the associated boundary layers is determined.     

Large-scale vortical structure,supported by small-scale turbulent motions. Helicity as a cause for inverse energy cascade

In this article we demonstrate that turbulent stress contributions which depend on the rotation of the frame of reference (and therefore are system dependent) give rise to the inverse energy cascade, and thus introduce an ordering in the structure of turbulence.We first demonstrate that a non-rotating Boussinesq fluid subject to an artificial force that is not invariant under parity changes of the orthogonal group has a destabilizing effect in the B'enard problem. This destabilization is due to helicity and the stability regimes are divided into two regions: (1) If the helicitys is below a threshold values ^*, then long and very short wavelength disturbances at Rayleigh numbers Ra > Ra^crit are stable whereas those with intermediate wavelengths are unstable. (2) If the helicitys >s _* then all disturbances are unstable.For a rotating turbulent Boussinesq fluid we derive the most simple rotation dependent expression for the stress divergence and demonstrate that it leads quantitatively to a similar helicity dependent force. In the B6nard problem it gives rise to an analogous division of the stability/instability regime as obtained for non-rotating fluids subject to the artificial helicity dependent force.     

Comparisons of numerical methods with respect to convectively dominated problems

A series of numerical schemes: first‐order upstream, Lax–Friedrichs; second‐order upstream, central difference, Lax–Wendroff, Beam–Warming, Fromm; third‐order QUICK, QUICKEST and high resolution flux‐corrected transport and total variation diminishing (TVD) methods are compared for one‐dimensional convection–diffusion problems. Numerical results show that the modified TVD Lax–Friedrichs method is the most competent method for convectively dominated problems with a steep spatial gradient of the variables. Copyright © 2001 John Wiley & Sons, Ltd.     

TOF-SIMS depth profiling and element mapping on oxidized AlCrVN hard coatings

V-alloyed AlCrN hard coatings were deposited on silicon wafers (Si (100)) by reactive arc evaporation in a commercial coating system at 500 °C for 10 min, resulting in a coating thickness of ∼500 nm. The chemical composition of the stoichiometric coatings is constant at approximately Al_0.70Cr_0.05V_0.25N regardless of the applied bias voltage during deposition. Coatings synthesized at a low bias of −40 V show a dual-phase structure (hexagonal close-packed and face-centered cubic (fcc)), whereas coatings deposited at a high bias of −150 V have a metastable single-phase structure (face-centered cubic). All samples were oxidized for 15 min under 20 mbar O₂ atmosphere and at four different temperatures (550, 600, 650, and 700 °C). The oxidized coatings were subject to depth profiling and element mapping by a time of flight secondary ion mass spectrometry instrument, equipped with a Bi-cluster analysis gun and Cs⁺-sputter gun. The evaluation of the in-depth distribution of several elements and species points out distinctive differences in the oxidation behavior of the two different coatings, whereas element mapping shows the formation of islands made of oxidized vanadium and aluminum species as the top-most layer of the single-phase (fcc) coating at temperatures above 650 °C.     

Spherical lead zirconate titanate emulsion powder from water-soluble inorganic precursors via plasma burning

Coprecipitation of metal hydroxides inside water-in-oil (W/O)-emulsion droplets leads to spherical gel-particles. Stoichiometric Nd-doped lead zirconate titanate (PZT) ceramic powders are prepared from the water-soluble inorganic precursors Pb(NO₃)₂, ZrO(NO₃)₂, TiO(NO₃)₂, and the dopant Nd(NO₃)₃. The injection of gaseous NH₃ into the emulsion provides the coprecipitation of the corresponding hydroxides. The spray-dried powder consists of surfactant-covered spheres containing portions on HN₄NO₃. Thermal decomposition of NH₄NO₃ during calcination may destruct the gel spheres. Therefore, NH₄NO₃ was removed by sublimation during plasma burning and simultaneously the oxidation of the organic compounds was accelerated. Surprisingly, plasma burning (< 180°C) results in a PZT-phase with small amounts of unreacted PbO. SEM-micrographs show discrete narrow-distributed spheres without destructions caused by NH₄NO₃ decomposition or by organic burnout.