Chemically controlled vapour-phase

Many thin-film processing techniques have been developed and further improved in the search for the most suitable approach for a specific application. Typically thin films can be prepared from either the hquid or gaseous phase. Vapour-phase deposition processes, which are more popular, fall into the two main categories of physical and chemical vapour deposition. Atomic layer deposition holds a special position among the chemical vapour deposition techniques because it offers the possibility to produce thin films in a controlled, self-hmiting manner. 

This paper presents an on-line model based level control of a batch reactor with reaction rate uncertainties. The analyzed chemical batch process is catalyzed by a catalyst which decomposes in the reactor therefore it is fed several times during the batch. The chemical reaction produces a vapour phase by-product which causes level change in the system. The on-line control method is based on the shrinking horizon optimal control methodology based on the detailed model of the process. The results demonstrate that the on-line optimization based control strategy provides good control performance despite the disturbances. 

Chemical modification with different amounts of tween-80, a nonionic surfactant, was foimd to enhance the mesoporous area in pillared montmorillonite samples. Deactivation of the modified clays in the vapour phase catalysis of alkylation of toluene by methanol showed that the pillar density and the rate of deactivation could be controlled by the amount of surfactant used during the preparation of pillared samples. Presence of surfactant within the gallery affects the distribution of pillars perhaps during washing and dehydration. This offers a method to suppress deactivation in pillared samples to catalyze organic reactions. 

Doping can be achieved from the vapour phase (if the vapour pressure is sufficiently high), or in solution, chemically as well as electrochemically. In the latter case the polymer is one of the electrodes in an electrochemical cell and the process can be carefully controlled. The incorporation of solvent molecules is not considered in any of the diffraction studies, although it is sometimes stated that it may occur for smaller, polar molecules (e.g. for NH3, but not, say, in the case of cyclohexane). 

From an examination of literature reports one can classify sensor applications into several categories. The earliest applications of piezoelectric sensors were as simple mass detectors where material was deposited to the device surface in a non-selective manner. In later work, mass sensors were transformed into chemical sensors by developing surface coatings which selectively bound gases from a vapour-phase chemical system. This increase in sophistication represented a major advance in the analytical capabilities of piezoelectric sensors. The manner and selectivity with which surface binding is controlled is the critical element in producing a practical chemical sensor and... 

Abstract A growing tendency in chemical vapour deposition is to produce ultra-thin films or nano-objects as particles, tubes or wires. Such an objective addresses the question of a better control of the main parameters which govern the nucieation and growth steps of the deposit. This chapter focuses on the interfacial phenomena that occur at both the solid surface and the gaseous phase levels. The role of surface defects, surface reactive groups, and autocatalytic phenomena on the nucieation step are discussed by means of representative examples from the literature. In an attempt to clarify gas-phase properties, the influence of the supersaturation parameter on the nucieation step is also described. 

Interest in the crucial processes of nucleation and the growth of solids from fluid phases has a long and multidisciplinary history [50-53]. This research topic involves chemistry, chemical physics, material science, chemical engineering and physics, and, as a consequence, both theoretical and experimental studies were carried out by specialists in these fields. Thus, the following discussion does not pretend to be an exhaustive literature coverage of what is known about nucleation and growth, but rather, through recent articles, tries to review contributions especially relevant to controlled chemical vapour deposition of nanoparticles, always from a multidisciplinary point of view. 

The exchange of chemical compounds from the gas phase to a surface, e.g. atmospheric particles, soil, water, vegetation or other surfaces, is controlled by the affinity of the compound to this surface. The ratio of vapour pressure to water solubility can be used as indicator between levels in the atmosphere and water surface (Henry s law H constant). In many model calculations, the ratio between POP levels in octanol and water, the octanol-water partitioning coefficient (Kow), is used as reference for the distribution of POP in organic material [14]. Consequently, the expression ///RT (Cair/Cwalcr) and Kow (Coctanoi/Cwater) provide the octanol-air partitioning coefficient (Koa) ... 

Plasma-enhanced chemical vapour deposition has gained importance rapidly in recent years, because this technique provides some unique advantages over conventional CVD. The important advantages include lower deposition temperatures, deposition of non-epuilibrium phases and a better control of stoichiometry and purity of deposits. In this technique, the activation energy for the breakdown of reactive species, and their subsequent interaction with other species to form a deposit, is provided by the high kinetic energy of electrons in the plasma (figure 13.2). 

Deposition occurs by adsorption or reaction from a gas phase. This method may ensure excellent dispersion and very well controlled distribution of the active species. Chemical vapour deposition is an example of gas-phase deposition. 

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