Search results for "phase transition"
showing 10 items of 1281 documents
A new approach to the growth of ZnO by vapour transport
2005
The crystal growth of ZnO by vapour transport is classically made in presence of additional species which enhance the growth process. Usually, additional species have been considered as chemical transport agents that promote a typical CVT (Chemical Vapour Transport) process. Recently, we have proposed a new interpretation of the chemical role of some of these species. This new interpretation considers that, in some cases, the additional species promote a partial consumption of the O2 provided by the ZnO decomposition and, consequently, a Zn excess is generated. This excess of Zn pressure activates the ZnO decomposition and the growth rate is enhanced. Among those species, carbon shows an ad…
Dipolar and orientational glass formation in two-dimensional films of CO mixtures
2002
Heat-capacity and dielectric measurements have been performed to study orientational and dipolar order and disorder phenomena in submonolayer films of CO and binary mixtures of CO physisorbed on graphite and boron nitride (BN). With both methods transitions to quadrupolar and dipolar ordered low-temperature phases have been found for pure CO. Calorimetric measurements revealed that the dipolar transition of CO on graphite belongs to the universality class of the two-dimensional (2D) Ising model. It will be shown that in dilute mixtures of CO with CH 4 molecules a dramatic suppression of this phase transition occurs. This effect turned out to be much stronger than that previously observed in…
Memory effects and coverage dependence of surface diffusion in a model adsorption system
1999
We study the coverage dependence of surface diffusion coefficients for a strongly interacting adsorption system O/W(110) via Monte Carlo simulations of a lattice-gas model. In particular, we consider the nature and emergence of memory effects as contained in the corresponding correlation factors in tracer and collective diffusion. We show that memory effects can be very pronounced deep inside the ordered phases and in regions close to first and second order phase transition boundaries. Particular attention is paid to the details of the time dependence of memory effects. The memory effect in tracer diffusion is found to decay following a power law after an initial transient period. This beha…
Environmental scanning electron microscopy (ESEM) as a new technique to determine the ice nucleation capability of individual atmospheric aerosol par…
2007
Abstract Heterogeneous ice nucleation on synthetic silver iodide, natural kaolinite and montmorillonite particles via condensation, freezing and deposition modes was studied by environmental scanning electron microscopy (ESEM) in the temperature range of 250–270 K. By increasing the H2O pressure in the sample chamber at constant temperature, ice formation can be studied in situ and can be related to the chemical composition of the particles that can be determined simultaneously. For silver iodide and kaolinite, supersaturation values of first ice formation are in good agreement (1–2% absolute) with diffusion chamber experiments. For both substances, threshold temperatures for the condensati…
New particle-dependent parameterizations of heterogeneous freezing processes: sensitivity studies of convective clouds with an air parcel model
2015
Abstract. Based on the outcome of laboratory results, new particle-dependent parameterizations of heterogeneous freezing were derived and used to improve and extend a two-dimensional spectral microphysics scheme. They include (1) a particle-type-dependent parameterization of immersion freezing using the numbers of active sites per mass, (2) a particle-type and size-resolved parameterization of contact freezing, and (3) a particle-type-dependent description of deposition freezing. The modified microphysical scheme was embedded in an adiabatic air parcel model with entrainment. Sensitivity studies were performed to simulate convective situations and to investigate the impact of ice nuclei con…
Nucleation in physical and nonphysical systems
2003
Abstract The aggregation of particles out of an initially homogeneous situation is well known in physics. Depending on the system under consideration and its control parameters, the cluster formation in a supersaturated (metastable or unstable) situation has been observed in nucleation physics as well as in other branches. We investigate the well-known example of condensation (formation of liquid droplets) in an undercooled vapour to conclude that the formation of bound states as a phase transition is related to transportation science. We present a comparison of nucleation in an isothermal–isochoric container with traffic congestion on a circular one-lane freeway. The analysis is based, in …
Atomic layer deposition of polyimide thin films
2007
The atomic layer deposition (ALD) of different polyimide thin films has been studied. We have demonstrated self-limiting ALD deposition of PMDA–DAH, PMDA–EDA, PMDA–ODA and PMDA–PDA thin films at 160 °C. The maximum deposition rate of 5.8 A cycle−1 was obtained for the PMDA–DAH process. Although the deposition rate was high at 160 °C, a sudden decrease was observed when the temperature was increased. Regardless of the process studied, no film growth was obtained at 200 °C or above. Deposited polyimide films were analysed by FTIR, AFM and TOF-ERDA. According to the FTIR measurements, imide bonds were formed already in as-deposited films indicating polyimide formation without any additional th…
Oscillatory Changes of the Heterogeneous Reactive Layer Detected with the Motional Resistance during the Galvanostatic Deposition of Copper in Sulfur…
2015
Metallic copper was galvanostatically deposited on quartz|gold resonant electrodes by applying a constant current in a 0.5 M CuSO4/0.1 M H2SO4 aqueous solution. Galvanostatic copper deposition is one of the best methodologies to calibrate the electrochemical quartz crystal microbalances (EQCM), a gravimetric sensor to evaluate changes in mass during the electrochemical reactions through the Sauerbrey equation. The simultaneous measurement of mass, current density, and motional resistance by an EQCM with motional resistance monitoring allows us to characterize the processes occurring on the electrode surface and at the interfacial regions with unprecedented detail. During the galvanostatic c…
Bader’s topological analysis of the electron density in the pressure-induced phase transitions/amorphization in α-quartz from the catastrophe theory …
2013
In this work, the Bader's topological analysis of the electron density, coupled with Thom's catastrophe theory, was used to characterize the pressure-induced transformations in α-quartz. In particular, ab initio calculations of the α-quartz structures in the range 0-105 Gpa have been performed at the HF/DFT exchange-correlation terms level, using Hamiltonians based on a WC1LYP hybrid scheme. The electron densities calculated throughout the ab initio wave functions have been analysed by means of the Bader's theory, seeking for some catastrophic mechanism in the sense of Thom's theory. The analysis mainly showed that there is a typical fold catastrophe feature involving an O-O interaction at …