Showing papers by "University of Stuttgart published in 1980"

Mechanism and kinetics of the diffusion of gold in silicon

Abstract: The diffusion of Au in Si is known to take place via the interchange of Au atoms between substitutional (Au s ) and interstitial (Au i ) sites. So far it has generally been believed that this interchange involves lattice vacancies (V) and that it occurs via the Frank-Turnbull mechanism V+Au i ⇆Au s . It is stated in the literature that this model explains the observation that the Au s concentrationC s m in the centre of Au-diffused Si wafers increases with timet according to $$C_s^m \propto \sqrt t $$ . We show that this statement is incorrect, i.e., the Frank-Turnbull model cannot account for the $$C_s^m \propto \sqrt t $$ law. Such a dependence is expected in the case of Si wafers with a sufficiently low density of internal sinks for self-interstitials if the Au i −Au s interchange is controlled by the so-called kick-out mechanism Au i ⇆Au s +1. Since this mechanism involves self-interstitials (I) the present result is in accordance with the fact that under high-temperature equilibrium conditions the dominating intrinsic point defects in Si are self-interstitials and not vacancies as in Ge or metals.

A computer program for the design and analysis of low-speed airfoils

Abstract: A conformal mapping method for the design of airfoils with prescribed velocity distribution characteristics, a panel method for the analysis of the potential flow about given airfoils, and a boundary layer method have been combined With this combined method, airfoils with prescribed boundary layer characteristics can be designed and airfoils with prescribed shapes can be analyzed All three methods are described briefly The program and its input options are described A complete listing is given as an appendix

On the use of local basis sets for localized molecular orbitals

Abstract: Two procedures are discussed for the direct variational optimization of localized molecular orbitals which are expanded in local subsets of the molecular basis set. It is shown that a Newton-Raphson approach is more efficient than an iterative diagonalization scheme. The effect of the basis-set truncation on the quality ofab-initio SCF results is investigated for Be, Li2, HF, H2O, NH3, CH4 and C2H6.

Correlation energies in the spin-density functional formalism

Abstract: It has been shown recently that dynamical correlation effects can be adequately described by using an electron-gas expression for correlation between electrons of different spins. In this paper the method is applied to the calculation of excitation energies for the first- and second-row atoms and to the determination of ground-state properties for small polyatomic molecules, such as CH2, CH4, CH 4 + , CH 5 + . Additionally, deficiencies of the method for cases with few electrons and strongly varying electron density are investigated and an empirical correction to the electron-gas approximation is proposed. This correction is based on atomic data and gives an overall improvement for test molecules with two to four electrons.

Correction of local surface irregularities using reflection lines

Abstract: A family of straight lines can be reflected onto a surface as seen from a fixed point of vision. Curves generated in this way on the surface are called reflection lines. Deviations of the surface will give irregularities in these reflection lines. To correct these deviations, a procedure can be developed to find the necessary changes of the surface for a given variation of the reflection lines, the point of vision and the family of straight lines being kept constant. An example is given of correcting the surface of a car body using reflection lines.

Localized exciton bound to an isoelectronic trap in silicon

Abstract: A photo- and electroluminescence study is reported on the recombination radiation from excitons bound to an isoelectronic trap in silicon. This type of luminescence is novel in silicon. The luminescence which was obtained from only few samples consists of three long-lived emission lines $A$, $B$, and $C$, and a number of associated weak phonon wings. The decay characteristics and the temperature dependence of the fluorescence along with the "forbidden" $B$ line exhibited under perturbations of a magnetic or elastic strain field give rise to our interpretation in terms of an isoelectronic trap binding the electron-hole pair. These features are highly reminiscent of familiar isoelectronic systems which can bind an exciton such as GaP:N, GaP:Bi, or ZnTe:O. Orientational Zeeman measurements show that the trap has axial symmetry in the 111g direction. The $g$ factors are ${g}_{e}=1.6\ifmmode\pm\else\textpm\fi{}0.1$ and ${g}_{h}=1.1\ifmmode\pm\else\textpm\fi{}0.1$. The ${g}_{e}$ value is largely reduced in comparison with known bound-electron values and suggests that an electron is tightly bound to the trap capturing the hole by its Coulomb field. This conception also explains the observed cw-excitation dependence of the fluorescence. The samples showing the $A$, $B$, and $C$ fluorescence are $n$ and $p$ type ($\ensuremath{\rho}\ensuremath{\ge}2$ \ensuremath{\Omega} cm) and are grown by different techniques. The strongest fluorescence is observed from an aluminumdoped sample (2.5 \ensuremath{\Omega} cm) for which the trap density is estimated to be no more than some ${10}^{13}$ ${\mathrm{cm}}^{\ensuremath{-}3}$. No correlation of luminescence with shallow-donor or -acceptor doping could be established, and the nature of the trap remains unidentified. A correlation of luminescence with carbon concentration in a number of samples and the annealing behavior of the exciton luminescence may hint that carbon is one of the trap constituents.

Magnetic after-effects in Fe3O4and vacancy-doped magnetite

Abstract: The magnetic relaxation spectrum of stoichiometric and vacancy-doped magnetite has been investigated in the temperature range between 4·2 and 130 K. The observed relaxation processes are classified as short- and long-range rearrangements of Fe2+ and Fe3+ ionic states within domain walls. It is proposed that below 35 K a coherent tunnelling process governs the mobility of electrons, but that above 50 K is replaced by a thermally activated hopping process leading to the Verwey transition at 123 K.

Flow Induced Spring Coefficients of Labyrinth Seals for Application in Rotor Dynamics

Abstract: Flow induced aerodynamic spring coefficients of labyrinth seals are discussed and the restoring force in the deflection plane of the rotor and the lateral force acting perpendicularly to it are also considered. The effects of operational conditions on the spring characteristics of these components are examined, such as differential pressure, speed, inlet flow conditions, and the geometry of the labyrinth seals. Estimation formulas for the lateral forces due to shaft rotation and inlet swirl, which are developed through experiments, are presented. The utilization of the investigations is explained and results of stability calculations, especially for high pressure centrifugal compressors, are added. Suggestions are made concerning the avoidance of exciting forces in labyrinths.

Microdynamics and nonlinear stochastic processes of gross variables

Abstract: Starting from classical Hamiltonian mechanics, we derive for the dynamics of gross variables in nonequilibrium systems exact nonlinear generalized Fokker-Planck and Langevin equations in which the effect of the initial preparation is taken into account explicitly. This latter concept allows for the construction of a uniquely determined projection operator. The memory functions occurring in the Langevin equations are related to the random forces by a fluctuation-dissipation theorem of the second kind. We discuss the connection with the generalized Fokker-Planck equation. The known results for equilibrium fluctuations are recovered as a special case.

Die Spiegeloptik des Flußkrebsauges

Abstract: In the superposition eye of the crayfish (Astacus leptodactylus), images are formed by radial plane mirrors arranged in an orthogonal pattern. The optical structure of the crayfish eye can be described as a family of virtual reflecting cone envelopes concentric around each direction in space. There exist two reflection mechanisms: total internal reflection at the sides of the crystalline cones and reflection by multilayer mirrors attached to the distal parts of the cones. Image-forming rays have to be reflected twice in the general case, and once in the case of perpendicular position of the plane of incidence and the mirror plane. For rays incident at small angles to the ommatidial axis, this condition is almost satisfied due to a particular axial variation of the refractive index of the crystalline cone, and for rays incident at large angles, due to the spectral reflecting properties of the multilayer reflector.

Electrophilic Reagents—Recent Developments and Their Preparative Application

Abstract: The most widely known electrophilic agents are protic acids and compounds with an electronsextet partial structure. Recent research has aimed at the development of new electrophilic reagents, with greater reactivity on the one hand and higher selectivity on the other, which would largely obviate the addition of Lewis acids (Friedel-Crafts catalysts), and also alIow control of the isomer ratio in reactions with ambivalent substrates. Compounds with “super-leaving groups”, such as trifluoromethanesulfonate and fluorosulfate, have been demonstrated as most advantageous in this respect since they are sufficiently polarized or dissociated for reactions to take place even in the absence of Friedel-Crafts catalysts. Heterocycles such as pyridones or imidazole are likewise suitable leaving groups; they are employed for their high selectivity, and also because they allow working under non-aqueous conditions.

Nonlinear stability of parallel flows by high-order amplitude expansions

Abstract: * * The Landau-Stuart theory and its subsequent modifications suffer from some restrictions and from the nonuniqueness in determining higher-order terms of the amplitude expansions, which limit the range of applicability as well as the validity of the results. In the present paper, a well-defined amplitude is introduced a priori. In this way, uniqueness for terms of any order is achieved. Moreover, Watson's method is no longer restricted to almost neutral disturbances. This offers not only more accurate approximations and numerical studies on convergence but, as a consequence, a whole series of new applications. As a first example, the nonlinear equilibrium states of the plane Poiseuille flow are investigated. The numerical results are discussed in context with the author's solutions of the nonlinear equations and with special emphasis on the convergence of Landau's series. f

The Role of Anthraquinonoid Dyes in the Catalytic Fading'of Dye Mixtures– Substituent–dependent Triplet Yield of Diaminoanthraquinones

Abstract: Taking various diaminoanthraquinones as model compounds for commercial anthraquinonoid dyes it was found that the ability of initiating catalytic fading via singlet oxygen mechanism is closely related to the quantum yield of triplet formation in these dyes. The quantum yields of triplet formation show a marked difference between the various diaminoanthraquinones. Whereas 1, 4– and 1, 2–diaminoanthraquinones have very small quantum yields, the 1, 5– and 1, 8–derivatives pass into the triplet state with a fairly high efficiency. From this it was concluded that catalytic fading occuring via singlet oxygen mechanism must be expected in dye mixtures consisting of 1, 5– or 1, 8–diamino substituted anthraquinonoid dyes and easily oxidizable azo–compounds.

Short term thermal energy storage

Abstract: The present paper reviews the problem of short term thermal energy storage for low temperature solar heating applications. The techniques of sensible and latent heat storage are discussed, with particular emphasis on the latter. Requirements for hot water storage subsystems are provided and the importance of stratification in hot water storage tanks is described. Concerning latent heat storage, both material and heat exchanger aspects are considered in detail. The example of a passively operating latent heat store employing a finned heat pipe heat exchanger is used to elaborate upon the heat transfer problems in the generally poorly conducting phase change heat storage materials. Finally, some data pertaining to the current economics of heat storage are presented.

On the theory of the diffusion of gold into silicon

Abstract: Gold diffuses into silicon by the so-called kick-out mechanism, in which self-interstitials present in thermal equilibrium displace substitutionally dissolved Au atoms into interstices, so that these may undergo rapid interstitial diffusion. Except for an initial period this process may be described in terms of a strongly concentration-dependent effective diffusion coefficient for substitutionally dissolved gold atoms. An exact solution is given for the diffusion of Au from a planar surface into a crystal with a negligible density of sinks for self-interstitial atoms. Comparison with existing data supports the main conclusions of the theory. Die Diffusion von Gold in Silizium erfolgt uber einen “Verdrangungsmechanismus”, bei dem Eigenzwischengitteratome im thermischen Gleichgewicht Goldatome aus Gitterplatzen verdrangen und auf Zwischengitterplatze befordern, zwischen denen diese rasch diffundieren konnen. Nach einer kurzen Anlaufphase last sich dieser Mechanismus durch einen stark konzentrationsabhangigen effektiven Diffusionskoeffizienten fur die substitutionell gelosten Goldatome beschreiben. In der vorliegenden Arbeit wird eine exakte Losung fur die Diffusion von Gold von einer ebenen Oberflache in das Innere eines senkenarmen Siliziumkristalls gegeben. Die wichtigsten Ergebnisse der Theorie sind im Einklang mit experimentellen Daten.

Elektrophile Agentien : neue Entwicklungen und präparative Anwendungen

Abstract: Zu den bekanntesten elektrophilen Agentien gehoren Protonensauren und Verbindungen, die Atome mit Elektronensextett enthalten Ein Ziel der Forschung ist die Entwicklung neuer elektrophiler Agentien mit groserer Reaktivitat einerseits und hoherer Selektivitat andererseits, um auf den Zusatz von Elektronenacceptoren (Friedel-Crafts-Katalysatoren) moglichst weitgehend verzichten und das Isomerenverhaltnis bei Reaktionen ambivalenter Verbindungen beeinflussen zu konnen Verbindungen mit Super-Austrittsgruppen wie Trifluormethansulfonat und Fluorosulfat erwiesen sich als sehr vorteilhaft, denn ihre Polarisierung oder Dissoziation reicht auch in Abwesenheit von Friedel-Crafts-Katalysatoren fur viele Reaktionen aus Auch Heterocyclen, z B Pyridone und Imidazole, sind als Austrittsgruppen geeignet; Agentien mit diesen Austrittsgruppen zeichnen sich besonders durch ihre hohe Selektivitat aus

Variability in brain gangliosides of fishes.

Abstract: Gangliosides are membrane-bound glycosphingolipids containing different residues of sialic acid. In the nervous tissue gangliosides seem t o be involved in the process of synaptic transmission (Rahmann et al., 1976) and adaptive neuronal functions (Rahmann 1978: Rahmann et al., 1976). In the CNS of adult mammals less polar monoand d is ia logangl ios ide f r a c t i o n s d o m i n a t e (68-84%). trisialogangliosides occur at 8-2257, and more polar fractions make up only about 412% (Ledeen and Yu. 1976). In contrast to these findings the brain ganglioside patterns of the few fish species investigated up to now (Tettamanti et al.. 1965; Gielen. 1968; Ishizuka and Wiegandt, 1972: Avrova, 197 I ; McCluer and Agranoff, 1972; Rosner e t al.. 1973: Breer, 1975) show a great variability. In some elasmobranchs partly contradictory results are reported concerning the occurrence of polar ganglioside fractions. With the exception of the elasmobranchs, the fish brain ganglioside pattern in comparison to that of mammals seems to be characterized by the preponderance of more polar fractions. In order to elucidate the question of whether the brain ganglioside pattern of vertebrates might generally be correlated with the phylogenetic position of the different systematic groups-or with any other, probably environmental, factors in the present study-the brain ganglioside composition of 16 different fish species was compared.

Distinction of 2e− and le− Reduction Modes of the Flavin Chromophore as Studied by Flash Photolysis

Abstract: Evidence is given for the fact that the excited flavin triplet (3F1*ox) exhibits competitive le− and 2e− transfer chemistry, depending on the nature of the photosubstrate. As an ‘external’ photoreductant, the 2e− donor borohydride has been investigated. Borohydride is found to compete effectively with the ‘internal’ 1 e− donors, namely excess starting flavin in the ground state (Flox) and, as primary product, (alky1)dihydroflavin (RFlredH). It will be shown that some of the flavin radicals, observed by earlier authors in the reaction of 3Fl*ox with CH or C−COO− substrates or in the autophotolytic side-chain cleavage of riboflavin, are due to the dye-dye reaction (I): In contrast flavin photoreduction by borohydride or hydrocarbon substrates need not involve radicals, but may in fact be a hydride or ‘carbanion-plus-proton’ addition towards the highly polar and considerably basic (pK= 4.4) acceptor triplet (cf. reaction 11) The products are much more photoreactive than the starting substrates, which leads to the secondary photocomproportionation (111): This latter reaction is the second source of radicals in the system. This (cf. 11) ‘photohydrogenation’ of flavin is mechanistically related to the biological reduction of flavin by CH substrates.

Study of atomic jump processes in sodium crystals by quasi-elastic neutron scattering

Abstract: The atomic jump processes involved in self-diffusion, in high-purity sodium single crystals were studied by means of quasi-elastic neutron scattering using a back-scattering spectrometer. In the temperature range between 323 K and the melting temperature (370.9 K) the dependence of the quasi-elastic line-width on the momentum transfer was measured in the major crystallographic and in various low-symmetry directions. The results are compared with calculations for various monovacancy and divacancy mechanisms of self-diffusion, incorporating spatial as well as temporal correlation effects of the atomic jumps on the basis of the encounter model and taking into account both nearest-neighbour and non-nearest-neighbour jumps. It is concluded that even close to the melting temperature self-diffusion in sodium occurs by nearest-neighbour jumps only. Satisfactory agreement between the diffusion coefficients obtained from our neutron-scattering data and the radiotracer experiments of Mundy (1971) can be ach...

High Purity Organic Molecular Crystals

Abstract: The article reviews the preparation of highest purity organic crystals which are of rapidly growing importance for the understanding of basic processes and for the innovation of practical applications. High purity can be obtained by zone refining in combination with other methods. For the characterization of the crystals, grown from the melt or by sublimation, sensitive methods have been developed, being able to detect impurities down to well below 1 ppm. The author’s extensive practical experience is condensed into reliable technical prescriptions and valuable advices and hints.