FTIR and XPS studies on corrosion resistant SiO2 coatings as a function of the humidity during deposition

The degradation of SiO2 coatings deposited on alloys by metal organic chemical vapour deposition (MOCVD) in sulphidizing high-temperature environments is determined by delamination and crack formation. With increasing water concentration during deposition, the crack density in silica decreases and the critical thickness for delamination of SiO2 coatings increases. This improvement is supposed to be caused by compositional changes in the SiO2 coating. In this study presence of water and silanol groups as measured by Fourier transform infrared spectroscopy(FTIR) and the Si:O ratio as measured by XPS are discussed in relation to the protective properties. The FTIRmeasurements show that the coatings deposited in more humid environments contain more silanol groups and have lower stress levels. The coatings obtained under all deposition conditions consisted of stoichiometric SiO2.0 as determined by XPS. The presence of silanol groups reduces the viscosity of the coating, and stress relaxation by viscous flow becomes enhanced, thereby improving the coating performance

The protection of alloys against high temperature sulphidation by SiO2-coatings deposited by MOCVD

Silica coatings have been deposited on various alloys by MOCVD (Metal Organic Chemical Vapor Deposition) to protect them against high temperature corrosion in coal gasification environments. DiAcetoxyDitertiaryButoxySilane (DADBS) has been used as a metal organic precursor at deposition temperatures between 773 - 873 °K and amorphous layers were produced with a growth rate of about 1 μm. h-1. These coatings have been tested at 823°K in a sulphiclizing atmosphere with a low oxygen (9.3 10 -29 bar) and a high sulphur partial pressure (1.2 10 bar). In this environment the sulphidation resistance of various alloys has improved by a factor of at least 100 by the coating. The observed corrosion reaction is local and is explained by a model in which in the first stage cracks are formed due to mechanical stresses in the coating. In the second stage metal sulphides are formed by outward diffusion of metal and inward diffusion of sulphur through the cracks. When stainless steels are used as the alloy the outer layer consists of FeS and the lower one of CrS

Thin alumina and silica films by chemical vapor deposition (CVD)

Alumina and silica coatings have been deposited by MOCVD (Metal Organic Chemical Vapor Deposition) on alloys to protect them against high temperature corrosion. Aluminium Tri-lsopropoxide (ATI) and DiAcetoxyDitertiaryButoxySilane (DAOBS) have been used as metal organic precursors to prepare these ceramic coatings. The influence of several process steps on the deposition rate and surface morphology is discussed. The deposition of SiO2 at atmospheric pressure is kinetically limited below 833 K and is a mixed first and second order reaction with an activation energy of 155 kJ.mole-1. The deposition of Al2O3 is kinetically limited below 673 K and is a first order reaction with an activation energy of 30 kJ.mole-1 at atmospheric pressure. The deposition of Al2O3 is kinetically limited below 623 K and is a second order reaction at low pressure (3 torr) with an activation energy of 30 kJ.mole-1. The decomposition of both precursors involves a B-hydroge n elimination reaction by which DADBS decomposes to acetic acid anhydride, 2-methyl propane, SiO2 and H2O, while ATI decomposes to 2-propanol, propane, Al2O3 and H2O

The pyrolytic decomposition of metal alkoxides (di-acetoxy-di-t-butoxy-silane, DADBS) during chemical vapour deposition of thin oxide films

In this study the effects of the nature of metal alkoxides on their vapour pressures and thermal decomposition chemistry are reported. The vapour pressure and the volatility of a metal alkoxide strongly depends on the steric effect of its alkoxy group.\ud \ud The thermal decomposition chemistry of one metal alkoxide (di-acetoxy-di-t-butoxy-silane, DADBS) has been studied by mass spectrometry at temperatures between 423 and 923 K. The pyrolytic products were acetic acid anhydride and 2-methyl propene. The acetic acid anhydride is formed at temperatures above 473 K and 2-methyl propene is formed above 673 K by a ß -hydride elimination mechanism. In these steps, a 6-ring intermediate is supposed to be formed. The silicon acid finally remaining is proposed to react by poly-condensation to SiO2 coatings or powder

A Dual-loop Model Predictive Voltage Control/Sliding-mode Current Control for Voltage Source Inverter Operation in Smart Microgrids

The design of a robust controller for the voltage source inverter is essential for reliable operation of distributed energy resources in future smart microgrids. The design problem is challenging in the case of autonomous operation subsequent to an islanding situation. In this article, a dual-loop controller is proposed for voltage source inverter control. The outer loop is designed for microgrid voltage and frequency regulation based on the model predictive control strategy. This outer loop generates reference inverter currents for the inner loop. The inner loop is designed using a sliding-mode control strategy, and it generates the pulse-width modulation voltage commands to regulate the inverter currents. A standard space vector algorithm is used to realize the pulse-width modulation voltage commands. Performance evaluation of the proposed controller is carried out for different loading scenarios. It is shown that the proposed dual-loop controller provides the specified performance characteristics of an islanded microgrid with different loading conditions.http://www.tandfonline.com/loi/uemp20hb201

Ising model for distribution networks

An elementary Ising spin model is proposed for demonstrating cascading failures (break-downs, blackouts, collapses, avalanches, ...) that can occur in realistic networks for distribution and delivery by suppliers to consumers. A ferromagnetic Hamiltonian with quenched random fields results from policies that maximize the gap between demand and delivery. Such policies can arise in a competitive market where firms artificially create new demand, or in a solidary environment where too high a demand cannot reasonably be met. Network failure in the context of a policy of solidarity is possible when an initially active state becomes metastable and decays to a stable inactive state. We explore the characteristics of the demand and delivery, as well as the topological properties, which make the distribution network susceptible of failure. An effective temperature is defined, which governs the strength of the activity fluctuations which can induce a collapse. Numerical results, obtained by Monte Carlo simulations of the model on (mainly) scale-free networks, are supplemented with analytic mean-field approximations to the geometrical random field fluctuations and the thermal spin fluctuations. The role of hubs versus poorly connected nodes in initiating the breakdown of network activity is illustrated and related to model parameters

Dynamical robustness in complex networks: the crucial role of low-degree nodes

Many social, biological, and technological networks consist of a small number of highly connected components (hubs) and a very large number of loosely connected components (low-degree nodes). It has been commonly recognized that such heterogeneously connected networks are extremely vulnerable to the failure of hubs in terms of structural robustness of complex networks. However, little is known about dynamical robustness, which refers to the ability of a network to maintain its dynamical activity against local perturbations. Here we demonstrate that, in contrast to the structural fragility, the nonlinear dynamics of heterogeneously connected networks can be highly vulnerable to the failure of low-degree nodes. The crucial role of low-degree nodes results from dynamical processes where normal (active) units compensate for the failure of neighboring (inactive) units at the expense of a reduction in their own activity. Our finding highlights the significant difference between structural and dynamical robustness in complex networks