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

Dynamics at the crystal-melt interface in a supercooled chalcogenide liquid near the glass transition.

Direct quantitative measurements of nanoscale dynamical processes associated with structural relaxation and crystallization near the glass transition are a major experimental challenge. These type of processes have been primarily treated as macroscopic phenomena within the framework of phenomenological models and bulk experiments. Here, we report x-ray photon correlation spectroscopy measurements of dynamics at the crystal-melt interface during the radiation induced formation of Se nano-crystallites in pure Se and in binary AsSe4 glass-forming liquids near their glass transition temperature. We observe a heterogeneous dynamical behaviour where the intensity correlation functions g2(q, t) exhibits either a compressed or a stretched exponential decay, depending on the size of the Se nano-crystallites. The corresponding relaxation timescale for the AsSe4 liquid increases as the temperature is raised, which can be attributed to changes in the chemical composition of the melt at the crystal-melt interface with the growth of the Se nano-crystallites

Large-area alginate/PEO-PPO-PEO hydrogels with thermoreversible rheology at physiological temperatures

Alginate hydrogels have shown great promise for applications in wound dressings, drug delivery, and tissue engineering. Here, we report the fabrication, rheological properties, and dynamics of a multicomponent hydrogel consisting of alginate and poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PEO-PPO-PEO) triblock copolymers, and the achievement of thick, castable gels with tunable, thermoreversible behavior at physiological temperatures (Figure 1). PEO-PPO-PEO triblock copolymers can form temperature-sensitive hydrogels that exist as liquids at low temperatures and soft solids at high temperatures. In this work, we have employed PEO-PPO-PEO triblock copolymers to impart thermoresponsive properties to alginate hydrogels in the form of a multicomponent hydrogel. These systems can transition between a weak gel and a stiff gel, with a corresponding increase in the viscoelastic moduli of approximately two orders of magnitude as temperature is increased. The temperatures corresponding to the upper and lower boundaries of the stiff gel region, as well as the storage modulus at physiological temperatures (e.g., 36 – 40 C), can be controlled through the PEO-PPO- PEO concentration. Additionally, we explore the properties of these materials under compression and large deformations, and describe how alginate and F127 concentration can be used to control the fracture stress and strain. Finally, we compare the results from bulk rheology to the structure and dynamics of the gels measured via small-angle X-ray scattering (SAXS) and X-ray photon correlation spectroscopy (XPCS) experiments. Please click Additional Files below to see the full abstract

Local step-flow dynamics in thin film growth with desorption

Desorption of deposited species plays a role in determining the evolution of surface morphology during crystal growth when the desorption time constant is short compared to the time to diffuse to a defect site, step edge or kink. However, experiments to directly test the predictions of these effects are lacking. Novel techniques such as \emph{in-situ} coherent X-ray scattering can provide significant new information. Herein we present X-ray Photon Correlation Spectroscopy (XPCS) measurements during diindenoperylene (DIP) vapor deposition on thermally oxidized silicon surfaces. DIP forms a nearly complete two-dimensional first layer over the range of temperatures studied (40 - 120 $^{\circ}$C), followed by mounded growth during subsequent deposition. Local step flow within mounds was observed, and we find that there was a terrace-length-dependent behavior of the step edge dynamics. This led to unstable growth with rapid roughening ($\beta>0.5$) and deviation from a symmetric error-function-like height profile. At high temperatures, the grooves between the mounds tend to close up leading to nearly flat polycrystalline films. Numerical analysis based on a 1 + 1 dimensional model suggests that terrace-length dependent desorption of deposited ad-molecules is an essential cause of the step dynamics, and it influences the morphology evolution.Comment: 21 pages, 9 figures, and one tabl

Domain fluctuations in a ferroelectric low-strain BaTiO3 thin film

A ferroelectric BaTiO3 thin film grown on a NdScO3 substrate was studied using x-ray photon correlation spectroscopy (XPCS) to characterize thermal fluctuations near the a/b to a/c domain structure transformation present in this low-strain material, which is absent in the bulk. XPCS studies provide a direct comparison of the role of domain fluctuations in first- and second-order phase transformations. The a/b to a/c domain transformation is accompanied by a decrease in fluctuation timescales, and an increase in intensity and correlation length. Surprisingly, domain fluctuations are observed up to 25 degrees C above the transformation, concomitant with the growth of a/c domains and coexistence of both domain types. After a small window of stability, as the Curie temperature is approached, a/c domain fluctuations are observed, albeit slower, potentially due to the structural transformation associated with the ferroelectric to paraelectric transformation. The observed time evolution and reconfiguration of domain patterns highlight the role played by phase coexistence and elastic boundary conditions in altering fluctuation timescales in ferroelectric thin films

X Ray Photon Correlation Spectroscopy for the study of polymer dynamics

11 pags., 6 figs., 1 tab.X Ray Photon Correlation Spectroscopy, XPCS, is a novel technique developed for the study of slow dynamics in condensed matter. The principle of this technique is based on the time variations of the speckle pattern originated by the scattering of coherent light from a material with spatial inhomogeneities. Although laser photon correlation spectroscopy was long established, XPCS has only been possible with the advent of new synchrotron radiation X-ray sources that can provide sufficient coherent flux. The results from these techniques and future perspectives in the field of polymer science are discussed here. © 2016 Elsevier Ltd. All rights reserved.The authors gratefully acknowledge financial support from the MINECO, Spain (MAT2014-59187-R) and from NSLS-II under contract No. DE-SC0012704 with the US Department of Energy.Peer Reviewe

Coherent x-ray studies of non-equilibrium processes

X-ray Intensity Fluctuation Spectroscopy (XIFS) is an ideal technique to perform measurements on the dynamics of fluctuations in condensed matter systems. Over the past few years, XIFS has been used in several studies of dynamics in both hard condensed matter and soft condensed matter equilibrium systems. Its extension to study the dynamics of non-equilibrium systems is currently under way. In this thesis, we present the first applications of XIFS to study dynamics during a first order phase transition with nonconserved order parameter (model A). The order-disorder phase transition in the binary alloy Cu3Au has been chosen as a case study for such a non-equilibrium process and has been studied using XIFS. Our experiments have confirmed the theoretical predictions for the scaling laws describing the evolution of the density-density correlation functions, which are measured by the autocorrelation function of the scattered intensity. The covariance of the scattered intensity was found to be proportional with scaling functions with natural variables delta t = |t1 - t 2| and t¯ = t1+t22 , as predicted by theory. However, some significant early-time deviations from this scaling picture have been observed and are currently under investigations.In order to study non-equilibrium processes in condensed matter systems, a very precise and fast temperature control system is often required. In order to trigger the particular process under study and to obtain meaningful experimental data, the temperature has to change by hundreds of degrees in very short times, while completely avoiding any over/under shooting. To achieve this, we designed and implemented a computer-controlled temperature tracking system which combines standard Proportional-Integral-Derivative (PID) feedback, thermal modeling and finite difference thermal calculations (feedforward) and Kalman filtering of the temperature readings in order to reduce the noise. The resulting Kalman-Predictive-Proportional-Integral-Derivative (KPPID) algorithm allows us to obtain accurate control, to minimize the response time and to avoid over/under shooting, even in systems with inherent noisy temperature readings and time delays. The KPPID temperature controller was successfully implemented at the Advanced Photon Source at Argonne National Laboratories and was used to perform the coherent X-ray diffraction experiments described in this thesis