Nonstoichiometry in steps on AB crystals (I). General aspects,including bulk imperfections

Starting with a quasichemical treatment, simple model calculations are given for the equilibrium composition in surface steps containing A and B kinks, the amount and ratio of which is varied by changing the adjacent gas composition. In connection with deviations from step stoichiometry, the total amount of kinks increases strongly. A nearest neighbour approximation has been applied to the attachment of A or B atoms at kinks from the gas phase, and also from the bulk by formation of vacancies V_A or V_B. These processes can be formulated separately for the two elements, taking into account the effective formation of bulk atoms of the other kind (B_B or B_A).     

Undercompressive shocks for nonstrictly hyperbolic conservation laws

We study 2×2 systems of hyperbolic conservation laws near an umbilic point. These systems have Undercompressive shock wave solutions, i.e., solutions whose viscous profiles are represented by saddle connections in an associated family of planar vector fields. Previous studies near umbilic points have assumed that the flux function is a quadratic polynomial, in which case saddle connections lie on invariant lines. We drop this assumption and study saddle connections using Golubitsky-Schaeffer equilibrium bifurcation theory and the Melnikov integral, which detects the breaking of heteroclinic orbits. The resulting information is used to construct solutions of Riemann problems.     

Synthesis of stereoregular poly(alkyl malolactonates)

Optically pure methyl, ethyl, isopropyl, and benzyl (R)-malolactonate were prepared from (S)-(-)-malic acid and were polymerized in the bulk with tetraethylammonium benzoate as the initiator to yield high-molecular-weight, crystalline polymers. The optical purity of methyl and benzyl malolactonate was determined by ¹H NMR spectroscopy of the β-lactone complexed with a chiral europium shift reagent. Enantiomeric excesses of 100% were found (the experimental error was 3%). Optically active poly(β-malic acid) was obtained from optically active poly[benzyl (S)-malate] by catalytic hydrogenolysis of the pendent benzyl esters. Ethyl and benzyl (R)-malolactonate were also copolymerized, and the benzyl esters of the resulting copolymer were converted into carboxylic acid units by hydrogenolysis.     

The microscopic visualisation of the sonocrystallisation of ice using a novel ultrasonic cold stage

This work reports dynamic video images of the influence of ultrasonic cavitation on the sonocrystallisation of ice at a microscopic level. This has been achieved through the construction of a unique ultrasonic system for an optical microscope. The system consists of (1). an ultrasonic cold stage, (2). a temperature control system, and (3). a microscope and imaging setup. This allows the temperature of a sample to be systematically controlled while it is subjected to simultaneous excitation with alternating pressures in the ultrasonic frequency range. Both the amplitude of excitation and the frequency can be varied. Experiments on ice crystals in pure water and sucrose solutions were conducted. Three distinct phenomena were observed. Firstly, there is a tendency for cavitation bubbles to form at the grain boundaries between ice crystals. Secondly, there is a progressive melting of ice by cavitation bubbles which appear to eat their way into the ice phase. Thirdly, the dendritic ice structures may fragment under the influence of ultrasound, thus increasing the number of nuclei which may subsequently grow (secondary nucleation). These observations form the basis of a significantly enhanced understanding and exploitation of the sonocrystallisation of ice.     

Flow field analysis by large-area interrogation in particle image velocimetry

The evaluation of flow records obtained by particle image velocimetry (PIV) employs the interrogation of certain regions in the flow image. Basic relations are derived for the case that the velocity is not constant within the region of interrogation. The influence of a distribution of velocities on the Young's fringe system and its Fourier transform, the autocorrelation of the flow record, are discussed. The considerations show that there is an easy access to statistical parameters of the flow by carrying out large-field interrogation. The autocorrelation function will reproduce the probability density function of the velocity, and there is no need for a large number of time-consuming small-area interrogations. Furthermore, the effects of velocity variations in ordinary fringe analysis are discussed. Examples are given from synthetic PIV records and measurements in a wind tunnel.     

Lithium nickel oxide Li(Ni0.75Al0.17Co0.08)O2 as cathode material for lithium ion batteries

Al- and Co-substituted lithium nickel oxide of the nominal composition Li(Ni_0.75Al_0.17Co_0.08)O₂ was synthesised by a coprecipitation technique and by several solid state routes. Rietveld analysis of XRD profiles and galvanostatic cycling in glass cells were performed for structural and electrochemical characterisation. Depending on the reactivity of the respective precursor, there is in each case a minimal synthesis temperature, at which a single phase of the R layered structure could be obtained. The coprecipitation technique and solid state routes using pre-substituted nickel hydroxide are suitable for the synthesis of single phase Al- and Co-substituted lithium nickel oxide, even at rather low synthesis temperatures. The electrochemical performance of lithium nickel oxide Li(Ni_0.75Al_0.17Co_0.08)O₂ synthesised in air is poor due to an enhanced lithium nickel disorder. Synthesis in oxygen atmosphere seems to be required. Paper presented at the 6th Euroconference on Solid State Ionics, Cetraro, Calabria, Italy, Sept. 12–19, 1999.     

Global estimates of fundamental solutions for higher-order Schr?dinger equations

In this paper we first establish global pointwise time-space estimates of the fundamental solution for Schr?dinger equations, where the symbol of the spatial operator is a real non-degenerate elliptic polynomial. Then we use such estimates to establish related L ^p ?CL ^q estimates on the Schr?dinger solution. These estimates extend known results from the literature and are sharp. This result was lately already generalized to a degenerate case (cf. [4]).     

A Hydrocarbon-sensitized Argon Ionization Detector for the Detection of Inorganic Compunds

Noble gas ionization detectors make it possible to determine inorganie trace components in gases. Among the several modes of operation of these detectors the hydrocarbon-sensitized argon detector is a farourable compromise with regard to the relation of the expense to the attainable detection limit. The suitability of this detector for the determination of H₂, O₂, N₂, CH₄ and CO₂, in pure gases is shown.     

An economic¯X-chart approach to the joint control of the means of independent quality characteristics

Summary In controlling the mean of a multivariate normally distributed quality characteristic the main disadvantage of the globalT ²-control charts is that they don't indicate the component of the quality characteristic which gives rise to an alarm. To avoid this disadvantage the joint use of¯x-charts is proposed in this paper, where — for the sake of simplicity — the investigations are restricted to the case of independent components and to one-sided¯x-charts. On the basis of an economic objective function an approximation to the optimal design of the¯x-charts procedure is derived and discussed. It turns out that at least in the bivariate case the nearly optimal economic design is very close to the exact solution.Supported by the Deutsche Forschungsgemeinschaft (DFG).