The role of gravity on macrosegregation in alloys

During dendritic solidification liquid flow is induced both by buoyancy forces and solidification shrinkage. There is strong evidence that the major reason for the liquid flow is the former, i.e., thermosolutal convection. In the microgravity environment, it is thought that the thermosolutal convection will be greatly diminished so that convection will be confined mainly to the flow of interdendritic liquid required to satisfy the solidification shrinkage. An attempt is made to provide improved models of dendritic solidification with emphasis on convection and macrosegregation. Macrosegregation is an extremely important subject to the commercial casting community. The simulation of thermosolutal convection in directionally solidified (DS) alloys is described. A linear stability analysis was used to predict marginal stability curves for a system that comprises a mushy zone underlying an all-liquid zone. The supercritical thermosolutal convection in directionally solidified dendritic alloys was also modeled. The model assumes a nonconvective initial state with planar and horizontal isotherms and isoconcentration that move upward at a constant solidification velocity. Results are presented for systems involving lead-tin alloys and show significant differences with results of plane-front solidification

Cross sections for the production of fragments with Z greater than or equal to 8 by fragmentation of Z greater than or equal to 9 and less than or equal to 26 nuclei

Charge changing nuclear collisions in plastic nuclear track detectors were studied using a new experimental technique of automatic track measurement for etched tracks in plastic detectors. Partial cross sections for the production of fragments of charge Z approximately 8 were measured for projectile nuclei of charge 9 approximately Z approximately 26 in the detector material CR39 and in silver. for this purpose three independent experiments were performed using Bevalac beams. The first one was an exposure of a stack of CR39 plastic plates to 1.8 GeV/nucl. Ar-40 nuclei. The second one was an exposure of another CR39 stack of 1.7 GeV/nucl. Fe-56 projectiles. In the third experiment a mixed stack of CR39 plates and silver foils was irradiated with 1.7 GeV/nucl. Fe-56 nuclei. Thus the measurement of nuclear cross sections in a light target (CR39 = C12H18O7) and as well in a heavy target (silver) was possible

Neue Synthesestrategien für Halbleitende Polymere: Verwendung von Zinn, Bor und Gold als Metallfunktionelle Gruppen und Synthese von Dinukleophilen

In this thesis a general method for the synthesis of dinucleophilic compounds, containing both a stannyl and a boron moiety, was developed. Using a thiophene derivative as dinucleophile, it was possible to show that a both nucleophile and electrophile selective cross-coupling reaction could be performed successfully. This type of reaction allows the combination of different aromatic systems, where the products still contain both a halogen and a boron functionality. It was shown for electron poor, rich and neutral dielectrophiles that this reaction can be used as a general method. The resulting products can be used, besides other potential applications, as monomers in polymerization reactions and thus for the synthesis of semiconducting polymers. Furthermore the first compound of another class of dinculeophiles could be successfully synthesized: A thiophene derivative which contains both a boron and a gold moiety as metal functional groups. Organogold reagents have been rarely used in stoichiometric amounts so far. It was shown that these compounds perform cross-coupling reactions under mild reaction conditions and in high yields. Therefore this type of both gold and boron functionalized dinucleophiles are suitable for a potential use in dually-selective cross-coupling reactions...In dieser Arbeit wurde eine generell anwendbare Methode zur Synthese von dinukleophilen Verbindungen entwickelt, die sowohl mit Zinn als auch mit Bor funktionalisiert sind. Am Beispiel eines Thiophenderivates konnte die Methode einer sowohl Nukleophil- als auch Elektrophil-selektiven Kreuzkupplung erfolgreich gezeigt werden. Diese Art der Reaktion erlaubt es, unterschiedliche Aromaten miteinander zu verbinden, wobei die Produkte jeweils noch über eine Bor- und eine Halogenfunktion verfügen. Am Beispiel elektronen-armer, reicher und neutraler Dielektrophile konnte die allgemeine Anwendbarkeit der Reaktion gezeigt werden. Die Produkte eignen sich unter anderem für die Verwendung als Monomere in Polymerisationsreaktionen und damit für die Synthese halbleitender Polymere. Darüber hinaus konnte die erste Verbindung einer weiteren Klasse von Dinukleophilen synthetisiert werden: Es gelang, ein Thiophenderivat herzustellen, welches sowohl über eine Bor- als auch eine Goldfunktion verfügt. Goldorganyle wurden bisher nur selten stöchiometrisch verwendet. Es zeigt sich allerdings, dass diese unter milden Reaktionsbedingungen und mit guten Ausbeuten Kreuzkupplungsreaktionen eingehen. Damit eignet sich diese neue sowohl Gold- als auch Bor-funktionalisierte Verbindung ebenfalls für eine potentielle dual-selektive Kreuzkupplung..

Cantilever-based Resonant Gas Sensors with Integrated Recesses for Localized Sensing Layer Deposition

This work presents mass-sensitive hammerhead resonators with integrated recesses as a gas-phase chemical microsensor platform. Recesses are etched into the head region of the resonator to locally deposit chemically sensitive polymers by ink-jet printing. This permits the sensing films to be confined to areas that (a) are most effective in detecting mass loading and (b) are not strained during the in-plane vibrations of the resonator. As a result of the second point, even 5-μm thick polymer coatings on resonators with a 9-12 μm silicon thickness barely affect the Q-factor in air. This translates into higher frequency stability and ultimately higher sensor resolution compared to uniformly coated devices

Exchange coupling between magnetic layers across non-magnetic superlattices

The oscillation periods of the interlayer exchange coupling are investigated when two magnetic layers are separated by a metallic superlattice of two distinct non-magnetic materials. In spite of the conventional behaviour of the coupling as a function of the spacer thickness, new periods arise when the coupling is looked upon as a function of the number of cells of the superlattice. The new periodicity results from the deformation of the corresponding Fermi surface, which is explicitly related to a few controllable parameters, allowing the oscillation periods to be tuned.Comment: 13 pages; 5 figures; To appear in J. Phys.: Cond. Matte

With Joy And Love

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Damping by slow relaxing rare earth impurities in Ni80Fe20

Doping NiFe by heavy rare earth atoms alters the magnetic relaxation properties of this material drastically. We show that this effect can be well explained by the slow relaxing impurity mechanism. This process is a consequence of the anisotropy of the on site exchange interaction between the 4f magnetic moments and the conduction band. As expected from this model the magnitude of the damping effect scales with the anisotropy of the exchange interaction and increases by an order of magnitude at low temperatures. In addition our measurements allow us to determine the relaxation time of the 4f electrons as a function of temperature

Exponential behavior of the interlayer exchange coupling across non-magnetic metallic superlattices

It is shown that the coupling between magnetic layers separated by non-magnetic metallic superlattices can decay exponentially as a function of the spacer thickness $N$, as opposed to the usual $N^{-2}$ decay. This effect is due to the lack of constructive contributions to the coupling from extended states across the spacer. The exponential behavior is obtained by properly choosing the distinct metals and the superlattice unit cell composition.Comment: To appear in Phys. Rev.

Gilbert Damping in Magnetic Multilayers

We study the enhancement of the ferromagnetic relaxation rate in thin films due to the adjacent normal metal layers. Using linear response theory, we derive the dissipative torque produced by the s-d exchange interaction at the ferromagnet-normal metal interface. For a slow precession, the enhancement of Gilbert damping constant is proportional to the square of the s-d exchange constant times the zero-frequency limit of the frequency derivative of the local dynamic spin susceptibility of the normal metal at the interface. Electron-electron interactions increase the relaxation rate by the Stoner factor squared. We attribute the large anisotropic enhancements of the relaxation rate observed recently in multilayers containing palladium to this mechanism. For free electrons, the present theory compares favorably with recent spin-pumping result of Tserkovnyak et al. [Phys. Rev. Lett. \textbf{88},117601 (2002)].Comment: 1 figure, 5page