Inhibition of CO oxidation on RuO2(110) by adsorbed H2O molecules

Catalytic CO oxidation on the RuO(2)(110) surface was studied at 300 K by scanning tunneling microscopy (STM), high-resolution electron-energy-loss spectroscopy (HREELS), and thermal desorption spectroscopy (TDS). Upon repeatedly exposing the surface to several 10 L of CO and O(2) at 300 K, STM shows that unreactive features accumulate with each CO and O(2) titration run. HREELS and TDS show formation of increasing amounts of H(2)O, retarded formation of O-cus atoms and incomplete removal of CO-bridge molecules during O(2) dosing, and a changing ratio of single- and double-bonded CO-bridge molecules. It is concluded that H(2)O (presumably from the residual gas) is accumulating at the Ru-cus sites thus blocking them, so that the dissociative adsorption of oxygen is prevented and the CO oxidation reaction is suppressed. Some 10% CO- bridge remains on the surface even during oxygen exposure. Consistent with this interpretation, deactivation of the surface is suppressed at 350 K, at the onset of H(2)O desorption.     

Investigation of Al diffusion in different oxide thin-film layers by SNMS/HFM method

Depth profiles of Ga₂O₃/a-SiO₂/Al₂O₃- substrate, Ga₂O₃/a-Si₃N₄/Al₂O₃- substrate, and Ga₂O₃/Al₂O₃ substrate thin layers were determined by the SNMS/HFM method. Al diffusion from the Al₂O₃ substrate was investigated after 50, and in some cases after 600 hours of heat treatment time at different temperatures (600 °C,850 °C,950 °C,1050 °C and 1150 °C). The diffusion coefficient of Al at 850 °C was found to be D _Al=8.7 * 10^–18 cm²/s in amorphous SiO₂; D _Al=1.5*10^–17 cm²/s in amorphous Si₃N₄ and D _Al=5.5* 10^–16 cm²/s in Ga₂O₃ at 600 °C, respectively. The possible diffusion mechanism is explained in terms of the metal-oxygen bond-strengths. Although the studied materials have high resistivity at room temperature, the applied SNMS/HFM method has proven to be an efficient surface analytical tool even in these cases.Dedicated to Professor Dr. rer. nat. Dr. h.c. Hubertus Nickel on the occasion of his 65th birthday     

Directional coupling of optical signals by odd dark beams with mixed phase dislocations

Numerical simulations on the evolution of step-screw and edge-screw optical phase dislocations in bulk saturable self-defocusing nonlinear media are presented, with emphasis on their ability to induce steering waveguides for signal beams (pulses). Two schemes for directional coupling of such signals, both ensuring a reasonable coupling-to-cross-talk efficiency ratio, are investigated. The parameters useful for performance optimization of the couplers are discussed. Received: 19 July 2000 / Revised version: 5 February 2001 / Published online: 27 April 2001     

A Microgel Construction Kit for Bioorthogonal Encapsulation and pH‐Controlled Release of Living Cells

pH‐Cleavable cell‐laden microgels with excellent long‐term viabilities were fabricated by combining bioorthogonal strain‐promoted azide–alkyne cycloaddition (SPAAC) and droplet‐based microfluidics. Poly(ethylene glycol)dicyclooctyne and dendritic poly(glycerol azide) served as bioinert hydrogel precursors. Azide conjugation was performed using different substituted acid‐labile benzacetal linkers that allowed precise control of the microgel degradation kinetics in the interesting pH range between 4.5 and 7.4. By this means, a pH‐controlled release of the encapsulated cells was achieved upon demand with no effect on cell viability and spreading. As a result, the microgel particles can be used for temporary cell encapsulation, allowing the cells to be studied and manipulated during the encapsulation and then be isolated and harvested by decomposition of the microgel scaffolds.     

Dilatometric study of CaFeO2.5 single crystal

CaFeO_2.5±δ single crystal was obtained after different steps of mechanical and thermal treatment and finally a crystallization in an image furnace. We performed dilatometric tests of this material according to the three crystallographic directions of the network, from ambient to 1,473 K, accompanied by DSC and TG tests. The tests performed on a single crystal CaFeO_2.5 confirmed the transitions already observed by other authors. For the first time, a dilatometric anomaly has been observed at 1,308 K. It has the highest intensity of all the anomalies observed and appears only along the b crystallographic direction reflecting a change of state of CaFeO_2.5 at high temperature. This anomaly was confirmed using DSC and TG analysis. Moreover, this study confirmed the important dilatometric anisotropy of the material.     

Unidimensional SPH simulations of reactive shock tubes in an astrophysical perspective

Smoothed Particle Hydrodynamics (SPH) is a Lagrangian method widely used for the modelling of a large variety of astrophysical fluid flows in more than one dimension. Simulations of thermonuclear explosions in stars require, besides the hydrodynamic equations, a realistic equation of state, an energy source term, and a set of nuclear kinetic equations to follow the composition changes of the gas during the explosion. The implementation of a realistic stellar equation of state, and the coupling of hydrodynamics and nuclear burning are investigated in the framework of the simple shock tube geometry. We present and discuss the results of a series of SPH simulations of a detonation in the presence of (1) a single exothermic nuclear reaction, and (2) a restricted network of nuclear reactions. Our results are compared to those of identical simulations performed by other authors using a different hydrodynamic method.