Publikationen

The paper presents a quantitative model to elucidate the role of impinging photons on the final response towards oxidizing gases of light-activated metal oxide gas sensors. The model is based on the competition between oxygen molecules in air and oxidizing target gases (such as NO2) for the same adsorption sites: the surface oxygen vacancies (OV). The model fairly reproduces the experimental measurements of both the steady-state and the dynamic response of individual SnO2 nanowires towards oxidizing gases. Quantitative results indicate that: (1) at room temperature NO2 adsorbs onto OV more avidly than oxygen; (2) the flux of photons and the NO2 concentration determine the partition of the two gas populations at the surface; and (3) the band-to-band generation of electron-hole pairs plays a significant role in the photodesorption process of gas molecules. The model also offers a methodology to estimate some fundamental parameters, such as the adsorption rates and the photodesorption cross sections of oxidizing molecules interacting with the nanowires' surface. All these results, enabled by the use of individual nanowires, provide deep insight about how to control the response of metal oxide nanowires towards oxidizing gases, paving the way to the development and consolidation of this family of low consumption conductometric sensors operable at room temperature.

The oligoalumosiloxanes {[Ph2SiO]8[Al(O)OH]4∙2,5Et2O∙HOtBu} (6) and {[Ph2SiO]8[Al(O)OH]4∙2Et2O∙2HOiPr} (7) have been obtained from the reaction of diphenylsilanediol with aluminium-tri-tert-butoxide and aluminium-tri-iso-propoxide in ethyl ether with reasonable yields. In a 1:1 molar mixture of toluene and the respective alcohol (iso-propanol or tert-butanol), the ethyl ether molecules in {[Ph2SiO]8[Al(O)OH]4∙4Et2O}, in 6 or 7 can be completely displaced forming the compounds [Ph2SiO]8[Al(O)OH]4∙4HOiPr (8) and [Ph2SiO]8[Al(O)OH]4∙nHOtBu (9). Whereas 6, 7 and 8 are crystalline, 9 is obtained as a viscous liquid. An X-ray structure determination on {[Ph2SiO]8[Al(O)OH]4∙3Et2O∙HOtBu} reveals different bonding modes of the diethyl ether molecules to the oligoalumosiloxane compared to the tert-butanol, which forms two hydrogen bonds (one to the OH-group of the inner Al4(OH)4 cycle and one through the alcohol OH-group to a Si-O-Al moiety The alcohol adducts have been characterized in solution through 1H-, 13C- and 29Si-NMR and show dynamic equilibria between the oligoalumosiloxane [Ph2SiO]8[Al(O)OH]4 and the alcohol molecules.

Nanometric powders in the TiO2-CeO2 system were synthesized using sol-gel methods aiming at corrosion inhibitor preparation. The precursors employed were Ti(OiPr)4 as TiO2 source and Ce(NO3)3·6H2O or Ce(CH3CO2)3·xH2O as CeO2 source. Several parameters were varied, like the cerium precursor (cerium nitrate or cerium acetate) and the TiO2:CeO2 molar ratios. For both cerium sources binary powders with TiO2:CeO2 = 4:1 or 1:1 molar ratios were prepared. The obtained materials were dried 12 h in an oven at 100 °C and then thermally treated for 1 h at 400 °C with a heating rate of 1 °C·min-1 in order to eliminate the water and organic residues. In all cases, nanometric – predominantly amorphous – powders have been obtained which crystallized after the 1 h thermal treatment at 400 °C. The only exception was recorded for the TiO2:CeO2 = 4:1 composition prepared from the Ce(NO3)3·6H2O precursor which remained amorphous after the thermal treatment. Even though the primary particle sizes were ~ 4-5 nm, the material is constituted from aggregates larger than 50 nm. The desired presence of cerium in two valence states, necessary in active corrosion protection, was achieved and it was put into evidence by XPS and DTA/TGA for the TiO2:CeO2 = 4:1 binary compositions. Future studies have to be performed in order to reduce the agglomeration of the material.

Tinnitus related distress corresponds to different degrees of attention paid to the tinnitus. Shifting attention to a signal other than the tinnitus is therefore particularly difficult for patients with high tinnitus related distress. As attention effects on Event Related Potentials (ERP) have been shown this should be reflected in ERP measurements (N100, phase locking). In order to prove this hypothesis single sweep ERP recordings were obtained in 41 tinnitus patients as well as 10 control subjects during a period of time when attention was shifted to a tone (attended) and during a second phase (unattended) when they did not focus attention to the tone. Whereas tinnitus patients with low distress showed a significant reduction in both N100 amplitude and phase locking when comparing the attended and unattended measurement condition a group of patients with high tinnitus related distress did not show such ERP alterations. Using single sweep ERP measurements the results of our study show, that attention in high tinnitus related distress patients is captured by their tinnitus significantly more than in low distress patients. Furthermore our results provide the basis for future neurofeedback based tinnitus therapies aiming at maximizing the ability to shift attention away from the tinnitus.

We report on the observation of real-time-resolved room temperature grain growth in nanocrystalline metals. We find that neither the time evolution of size can be modeled by standard growth theories nor are there any other systems aware to us that manifest a similar growth behaviour. We detect a transition from an initially self-similar slow growth to abnormal grain growth. Its onset seems to be associated with the simultaneous decrease of microstrain with increasing grain size. Abnormal grain growth is considered as a generic feature of nanocrystallinity but is a transient state since we observed in the late stage of coarsening, using orientational imaging microscopy, a monomodal grain size distribution. We empirically find a nonlinear-response-type of growth law which is in agreement with the observed coarsening kinetics. © 2008 Acta Materialia Inc.

Capillary forces can significantly contribute to the adhesion of biological and artificial micro- and nanoscale objects. In this paper, we study numerically the effect of meniscus size on the force between two homogeneous flat plates for different contact angles. The force distance curves show excellent quantitative agreement with previous investigations. The results for n menisci of equal total liquid volume reveal interesting scaling properties and an unexpected maximum force for moderately hydrophilic surfaces (i.e., contact angles around 70°). Further, we calculate the Minimum solid-liquid area for multiple bridges, the cohesive stress (i.e., force per area) between the plates, and the work required to separate them. The results are presented in two-dimensional maps, which may be useful in the understanding of biological attachment structures and in the design of artificial contact systems.

Motivated by experimental results, we present numerical and analytical calculations of the capillary force exerted by a capillary bridge spanning the gap between two parallel flat plates of asymmetric wettabiltiy. Depending on whether the sum of the two contact angles is smaller or larger than 180°, the capillary force is either attractive or repulsive at small separations D between the plates. In either cases the magnitude of the force diverges as D approaches zero. The leading order of this divergence is captured by an analytical expression deduced from the geometry of the meniscus of a flat capillary bridge. The results for substrates with different wettability reveal an interesting behavior: with the sum of the contact angles fixed, the magnitude of the capillary force and the rupture separation decreases as the asymmetry in contact angles is increased. In addition, we present the rupture separation, i.e., the maximal extension of a capillary bridge, as a function of the contact angles. Our results provide an extensive picture of surface wettability effects on capillary adhesion.

We conduct experimental investigations of macroscopic capillary forces between two flat rigid substrates characterized by their advancing and receding contact angles with water. Our results exhibit excellent agreement with theoretical predictions obtained by the numerical solution of the capillary equation. On the basis of this comparison, we use the measurements of the capillary force to investigate the phenomenon of contact angle hysteresis. We present examples of force measurements for surfaces that display low, moderate, and high contact angle hysteresis and compare results for a larger variety of substrates. Finally, we show that for the case of water, the role of viscosity is insignificant within the range of force and velocity measured in the present work.

The development of the contact is crucial in indentation testing, yet only limited knowledge exists on the true contact size for compliant materials. In this investigation the contact evolution and the deformation behavior of polydimethylsiloxane was studied during indentation in situ inside a scanning electron microscope and by observation in a light microscope. Since detailed information on the true contact area and the amount of sink-in can be acquired from finite element analysis, simulations on the indentation process have been performed in order to complement the in situ testing. Comparison of results revealed that the contact areas calculated according to the standard Oliver-Pharr procedure deviated from the real contact size by approximately 10% for the elastomeric PDMS material. © 2008 Acta Materialia Inc.