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

Optical characteristics of Nd:YAG optics and distortions at high power

The intensity profile and beam caustics of a fiber coupled high power Nd:YAG laser beam through a lens system are studied. The thermal lensing effect and its influence on the beam profile and focal position are discussed. Asymmetry of the intensity profile in planes above and below the focal plane is demonstrated. Also the influence of small pollutions on the protective window is explained. Three different methods are used to measure the occurrence\ud of thermal lensing and quantify these effects

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

Teachers' sense of their professional identity

Abstract The overall aim of this dissertation was to contribute to the conceptualization of teachers’ professional identity. Based on the literature and previous research, teachers’ job satisfaction, self-efficacy, occupational commitment, and change in level of motivation were perceived as indicators of teachers’ sense of their professional identity. The relationships between these indicators were explored using data of 1214 teachers working in secondary education in the Netherlands. Teachers’ relationship satisfaction (referring, for instance, to teachers’ satisfaction with the support they receive or their satisfaction with their co-workers) and teachers’ classroom self-efficacy play a pivotal role in the relationships between the indicators. By further analysing the same dataset, three distinct professional identity profiles were identified: an unsatisfied and demotivated identity profile consisting of teachers who scored relatively low on the indicators, a motivated and affectively committed identity profile consisting of teachers who scored relatively high on the indicators, and a competence doubting identity profile consisting of teachers with a more diverse score pattern. Differences between the profiles were observed regarding teachers’ beliefs about the objectives of education: ‘stimulating personal and moral development’ and ‘importance of qualification and schooling’. No differences between the profiles were observed regarding the teachers’ amount of experience. An additional exploratory study among eighteen teachers showed that the students of teachers with an unsatisfied and demotivated identity profile observed their teachers’ behaviours ‘providing clear instruction’ and ‘efficient classroom management’ more often than students’ ratings of teachers with a motivated and affectively committed or a competence doubting identity profile. Samenvatting De professionele identiteit van docenten moet gezien worden als het resultaat van de voortdurende interactie tussen de persoonlijke kenmerken van een docent en de kenmerken van diens werkomgeving. Deze voortdurende interactie wordt weerspiegeld in de motivatie, het vertrouwen in het eigen kunnen, de arbeidssatisfactie en de professionele betrokkenheid van de docent. De relaties tussen deze indicatoren zijn onderzocht met behulp van gegevens van 1214 leerkrachten in het voortgezet onderwijs in Nederland. De tevredenheid met de contacten op het werk (verwijzend naar, bijvoorbeeld, de tevredenheid met de ontvangen ondersteuning) en het vertrouwen in het eigen kunnen in de klas spelen een centrale rol in de relaties tussen de indicatoren. Op basis van dezelfde dataset werden drie verschillende professionele identiteitsprofielen onderscheidden: een unsatisfied and demotivated profiel, (docenten met dit profiel scoren relatief laag op de indicatoren), een motivated and affectively committed profiel (docenten met dit profiel scoren relatief hoog op de indicatoren) en een competence doubting profiel (bestaande uit docenten met een divers score patroon). Verschillen tussen de profielen werden waargenomen ten aanzien van opvattingen over de doelstellingen van het onderwijs: ‘bevordering van de persoonlijke en morele ontwikkeling’ en ‘belang van kwalificatie en scholing’. De docenten behorende tot de drie profielen verschilden niet in hun hoeveelheid ervaring in het onderwijs. Uit een extra verkennende studie onder achttien leraren bleek dat de leerlingen van de leerkrachten met een unsatisfied and demotivated profiel vaker het gedrag ‘duidelijke instructie’ en ‘efficiënt klasmanagement’ waarnamen bij hun docent dan leerlingen van leerkrachten met een ander identiteitsprofiel.

In Quest of the Best Theoretical Description of Excess Molar Functions of Binary Mixtures of Alcohols

An attempt to describe excess molar functions of alkane + alcohol and alcohol + alcohol binary mixtures was made using the theoretical approach constructed on the basis of S-ERAS model and the chemical theory proposed by Campbell. The results obtained seem to be promising, however in order to complete the work, the procedure of determination of fitting parameters as well as relative magnitude of physical and chemical contributions to regarded excess molar quantities must be established

Winding Strings in AdS_3

Correlation functions of one unit spectral flowed states in string theory on AdS_3 are considered. We present the modified Knizhnik-Zamolodchikov and null vector equations to be satisfied by amplitudes containing states in winding sector one and study their solution corresponding to the four point function including one w=1 field. We compute the three point function involving two one unit spectral flowed operators and find expressions for amplitudes of three w=1 states satisfying certain particular relations among the spins of the fields. Several consistency checks are performed.Comment: 35 pages. v2. Important additions: one more author, complete results for the 3-point function with two w=1 states and new section with computation of 4-point function with one w=1 state. Acknowledgments and references modifie

Modeling for simulation of hybrid drivetrain components

Designing a hybrid drivetrain is a complex task, due to the unknown sensitivity of vehicle performance to system components specifications, the interaction between systems components, and the ability to operate the system components at different set points at any time. Therefore, many researchers have made efforts formulating, and developing holistic hybrid drivetrain analysis, design, and optimization models including the top-level vehicle system control. However, an integral design approach is usually characterized by large computation times, complex design problem formulations, multiple subsystem simulations, analyses, and non-smooth, or non-continuous models. In this paper, the influence of the component efficiencies, whereby the engine operation strategy (engine-, or system optimal operation) on the fuel economy, and the energy management strategy (EMS) is investigated. Thereby, a relative simple rule-based (RB) EMS is used, and is compared with the strategy based on dynamic programming (DP). The series-parallel transmission of the Toyota Prius has been used as a case study. The component modeling, and simulation results from the RB EMS, and DP are compared with results from the simulation platform ADVISOR. Finally, it is shown, that modeling the component efficiencies by only a few characteristic parameters, and using the RB EMS, the fuel consumption can be calculated very quickly, and with sufficient accuracy. In future work, the influence of topology choice on the fuel economy, and the EMS will also be investigate