Surface phenomena on the Tin-dioxide polycrystalline layers

Polycrystalline tin-dioxide is widely used in the detection of reducing gases (such as H2, CO, CH4, C2H5OH,...) in air by measuring its conductivity changes. The advantage of gas sensors based on such sensing devices is low cost and high sensitivity. In contrast to their widespread applications and to their successful empirical research and development work, the present understanding of the chemical sensing mechanisms is still immature. In this thesis, for gas sensors based on thick and porous tin-dioxide layers, a study of the response function upon variation of the partial pressure of ethanol vapors in 100% humidified air has been carried out. The influence of the working temperature and of the water vapors on the conductivity of the sensor was particularly emphasized. Based on our experimental data, a theoretical model of the sensing mechanism in thick and porous tin-dioxide layers is presented. The model accepts the conduction mechanism as being governed by the Schottky potential barriers at the junction between grains. For describing the adsorption of gas molecules on the solid surface a method of conditioned adsorption was developed. The central idea was to assume that the reducing gas molecules are adsorbed (i.e. react) only on pre-adsorbed oxygen. The predictions made in the frame of our theoretical model are in good agreement with the experimental data

The SrTiO3_3 displacive transition revisited by Coherent X-ray Diffraction

We present a Coherent X-ray Diffraction study of the antiferrodistortive displacive transition of SrTiO$_3$, a prototypical example of a phase transition for which the critical fluctuations exhibit two length scales and two time scales. From the microbeam x-ray coherent diffraction patterns, we show that the broad (short-length scale) and the narrow (long-length scale) components can be spatially disentangled, due to 100 $\mu$m-scale spatial variations of the latter. Moreover, both components exhibit a speckle pattern, which is static on a $\sim$10 mn time-scale. This gives evidence that the narrow component corresponds to static ordered domains. We interpret the speckles in the broad component as due to a very slow dynamical process, corresponding to the well-known \emph{central} peak seen in inelastic neutron scattering.Comment: 4 pages, 3 figures, accepted in PR

Dynamics in shear flow studied by X-ray Photon Correlation Spectroscopy

X-ray photon correlation spectroscopy was used to measure the diffusive dynamics of colloidal particles in a shear flow. The results presented here show how the intensity autocorrelation functions measure both the diffusive dynamics of the particles and their flow-induced, convective motion. However, in the limit of low flow/shear rates, it is possible to obtain the diffusive component of the dynamics, which makes the method suitable for the study of the dynamical properties of a large class of complex soft-matter and biological fluids. An important benefit of this experimental strategy over more traditional X-ray methods is the minimization of X-ray induced beam damage. While the method can be applied also for photon correlation spectroscopy in the visible domain, our analysis shows that the experimental conditions under which it is possible to measure the diffusive dynamics are easier to achieve at higher q values (with X-rays).Comment: 9 pages, 7 figures, to appear in Eur. Phys. J.

First πK\pi K atom lifetime and πK\pi K scattering length measurements

The results of a search for hydrogen-like atoms consisting of $\pi^{\mp}K^{\pm}$ mesons are presented. Evidence for $\pi K$ atom production by 24 GeV/c protons from CERN PS interacting with a nickel target has been seen in terms of characteristic $\pi K$ pairs from their breakup in the same target ($178 \pm 49$) and from Coulomb final state interaction ($653 \pm 42$). Using these results the analysis yields a first value for the $\pi K$ atom lifetime of $\tau=(2.5_{-1.8}^{+3.0})$ fs and a first model-independent measurement of the S-wave isospin-odd $\pi K$ scattering length $\left|a_0^-\right|=\frac{1}{3}\left|a_{1/2}-a_{3/2}\right|= \left(0.11_{-0.04}^{+0.09} \right)M_{\pi}^{-1}$ ($a_I$ for isospin $I$).Comment: 14 pages, 8 figure

X-ray fluorescence from the element with atomic number Z = 120

Accepted for publication in Physical Review LettersAn atomic clock based on X-ray fluorescence yields has been used to estimate the mean characteristic time for fusion followed by fission in reactions 238U + 64Ni at 6.6 MeV/A. Inner shell vacancies are created during the collisions in the electronic structure of the possibly formed Z=120 compound nuclei. The filling of these vacancies accompanied by X-ray emission with energies characteristic of Z=120 can take place only if the atomic transitions occur before nuclear fission. Therefore, the X-ray yield characteristic of the united atom with 120 protons is strongly related to the fission time and to the vacancy lifetimes. K X-rays from the element with Z = 120 have been unambiguously identified from a coupled analysis of the involved nuclear reaction mechanisms and of the measured photon spectra. A minimum mean fission time $\tau$_f$ = 2.5×10−18s has been deduced for Z=120 from the measured X-ray multiplicity

Many-Body Hydrodynamic Interactions in Charge-Stabilized Suspensions

In this joint experimental-theoretical work we study hydrodynamic interaction effects in dense suspensions of charged colloidal spheres. Using x-ray photon correlation spectroscopy we have determined the hydrodynamic function H(q), for a varying range of electrosteric repulsion. We show that H(q) can be quantitatively described by means of a novel Stokesian dynamics simulation method for charged Brownian spheres, and by a modification of a many-body theory developed originally by Beenakker and Mazur. Very importantly, we can explain the behavior of H(q) for strongly correlated particles without resorting to the controversial concept of hydrodynamic screening, as was attempted in earlier work by Riese et al

Aging behavior of the localization length in a colloidal glass

The localization length $r_{loc}$ associated with a fast secondary relaxation in glassy Laponite is determined by X-ray photon correlation spectroscopy (XPCS) through a Debye–Waller fit of the non-ergodicity parameter. Quantitative differences are observed between the time dependence (aging) of $r_{loc}$ in spontaneously aged and rejuvenated samples. This behavior is also reflected in the calculated shear modulus which matches well with data obtained by rheological measurements