Vapour-solid process

The prediction of the thermal conductivity is not as simple for the composite reinforced with SiC whiskers. In fact, as mentioned earlier, the thermal conductivity of the reinforcement phase plays an important role. However, for SiC whiskers, their chemical composition can vary drastically depending on the manufacturing process, which is not the case for Si3N4 whiskers. In fact, the influence of the manufacturing process is drastic the thermal conductivity of SiC(w) produced by the vapour-solid process is around 20 W/m K as opposed to 100-250 W/m K for whiskers produced by the vapour-liquid-solid process. Using a Si3N4 matrix with a nominal thermal conductivity of... 

Solids processes clearly differ from vapour liquid systems in two main ways. Firstly and most obviously, solid systems have a particle size or size distribution added to their specification. Secondly, solids processing systems use different... 

The term sublimation strictly refers to the phase change solid -> vapour, with no intervention of a liquid phase. In industrial applications, however, the term usually includes the reverse process of condensation or desublimation solid -> vapour -> solid. In practice, it is sometimes desirable to vaporise a substance from the liquid state and hence the... 

The process is characterised by the electrofluorination of volatile organic substrates within the matrix of pores of a carbon anode immersed in molten KF 2HF as electrolyte (as in a mid-temperature fluorine generator cell), and depends on the phenomenon that the anodically charged porous carbon is not wetted by the electrolyte. The fluorination probably takes place at the three phase interface of organic vapour, solid carbon, and liquid electrolyte in close proximity to, or at the sites where fluorine is being evolved. 

The catalytic hydration of olefins can also be performed in a three-phase system solid catalyst, liquid water (with the alcohol formed dissolved in it) and gaseous olefin [258,279,280]. The olefin conversion is raised, in comparison with the vapour phase processes, by the increase in solubility of the product alcohol in the excess of water [258]. For these systems with liquid and vapour phases simultaneously present, the equilibrium composition of both phases can be estimated together with vapour-liquid equilibrium data [281]. For the three-phase systems, ion exchangers, especially, have proved to be very efficient catalysts [260,280]. With higher olefins (2-methylpropene), the reaction was also performed in a two-phase liquid system with an ion exchanger as catalyst [282]. It is evident that the kinetic characteristics differ according to the arrangement (phase conditions), i.e. whether the vapour system, liquid vapour system or two-phase liquid system is used. However, most kinetic and mechanistic studies of olefin hydration were carried out in vapour phase systems. 

Alkene hydration to alcohols is a reaction of some industrial importance, although there have been few fundamental investigations in recent years. Beranek and Kraus have pointed out that the reaction equilibrium for the vapour phase process, though more favoured by low temperatures, still favours dehydration even at room temperature. Consequently, when high temperatures are employed to give more rapid reaction, high pressures must also be employed and even then the maximum attainable conversion may be low. Matters are improved somewhat by use of a three phase system (solid catalyst, liquid water, and gaseous alkene), for which conversion is improved by virtue of the alcohol solubility in water. 

In addition, the vapour-solid (VS) mechanism was proposed to explain whisker growth for some materials. It should be pointed out that the morphology of the crystal growth has a very complex relationship with the catalyst and the processing conditions. 

The use of solid acids has been traditionally biased towards large-scale continuous vapour phase processes such as catalytic cracking and paraffin isomerisations. However, it is increasingly recognised that there is also a great need for solid acid catalysts which are effective in liquid-phase organic reactions such as those employed in many batch-type reactors by fine, speciality and pharmaceutical intermediate chemical manufacturers. This has contributed towards a substantial recent research effort into the development of new solid acid catalysts.86-91... 

Modem alkylation processes make use of solid catalysts based on zeolites. According to different technologies, the reaction can be performed in vapour or liquid phase. The selection of a suitable chemical reactor for ethylbenzene is discussed in the Example 8.3. A conceptual flowsheet is depicted in Fig. 7.31 for a vapour-phase process (Mobil-Badger), one of the most widely used. The reactor works at 390-440 C and 0.6-3 MPa. Besides the main product ethylbenzene (EB), polyethylbenzenes (PEB) are formed, their amount depending on the reaction conditions. Large excess of benzene, over 6 1, is needed to shift the equilibrium to the desired product. The reaction mixture is sent to the separation section. Final yield can increase over 99% by converting PEB s to EB in a separate transalkylation reactor. 

In the vapour-liquid-solid process, as described in Figures 4.38a and b, a liquid alloy droplet composed of metal cntalyst component (An, Fe, etc.) and nanowire component (Si, M-V component, E-VI compound, oxide, etc.) is first formed under... 

Figure 5 Demonstration of the activity of printed particles. In printed arrays, 60-nm Au particles retain both their catalytic and optical activities, a. Silicon nanowires grown through a vapour-liquid-solid process from a printed array of Au particles (inset is tilted), b, AFM topography and the corresponding dark-field image of particles on the printing plate (top row) and an SEM image and the corresponding dark-field micrograph on a silicon substrate (bottom row the images were mirrored for convenience).

The electrodeposition of conducting polymer from a solution phase is a transformation reaction. The usual cooperative processes (solid -o- liquid, liquid <-> vapour, solid vapour) which possess a latent heat of transition and present a discontinuous volumic modification are called first-order transitions. The absence of any latent heat and density variations and the presence of a discontinuity in the heat capacity-temperature curve are of second-order transitions. 

The development of the chemistry of pol)mrers and their application in nanotechnology many researchers have been seen alternative forms for obtaining nanosized films. Our studies have focused primarily on vapour deposition of precursors and solid state imidization reaction [7,10,11,12]. Vapour deposition processes of organic layers play an important role in polytronics. They allow the construction of systems without solvents, based on principle of bottom-up and have significant role in the formation of intermediate and protective layers. Important advantages of vapour processes are follows (i) vacuum deposition is basically a cleaning process from impurities and resulting deposited layers... 

A crystal growth process which is receiving more attention for certain compounds is mineralisation which, although involving a complex series of mechanisms, certainly includes a vapour-solid stage. It is not intended to discuss this route here but small crystals of many rare earth pnictides and chalcogenides have been grown in this manner. 

Miblimation The volatilization of a solid substance into the vapour phase without passing through the liquid phase. Also used to describe the process of purification in which the vapour is condensed directly from the vapour phase to a solid (on a cold-finger often cooled by refrigerant). In the latter case this substance may melt during the initial vaporization. Used for purification. 

The reason for the constancy and sharpness of the melting j)oint of a pure crystalline solid can be appreciated upon reference to Fig. 7,10, 1, in which (a) is the vapour pressure curve of the solid and (6) that of the liquid form of the substance. Let us imagine a vessel, maintained at constant temperature, completely filled with a mixture of the above liquid and solid. The molecules of the solid can only pass into the liquid and the molecules of the liquid only into the solid. We may visualise two competitive processes taking place (i) the solid attempting to evaporate but it can only pass into the liquid, and (ii) the liquid attempting to distil but it can only pass into the solid. If process (i) is faster, the solid will melt, whereas if process (ii) proceeds with greater speed the... 

The term distillation is applied to vaporisation and subsequent condensation according to (i) it should also be applied to (ii) since it is really the liquid which is converted into vapour and is first formed by condensation. Strictly speaking, the term sublimation should be applied to changes according to (iii). However, in practice, a substance when heated may first melt and then boil, but on cooling it may pass directly from the vapour to the solid the process is then also called sublimation. Indeed the mode of vaporisation, whether directly from solid to vapour or through the intermediate formation of a liquid, is of secondary importance it is the direct conversion of vapour to solid which is really the outstanding feature of sublimation in the laboratory. 

The formation of a liquid phase from the vapour at any pressure below saturation cannot occur in the absence of a solid surface which serves to nucleate the process. Within a pore, the adsorbed film acts as a nucleus upon which condensation can take place when the relative pressure reaches the figure given by the Kelvin equation. In the converse process of evaporation, the problem of nucleation does not arise the liquid phase is already present and evaporation can occur spontaneously from the meniscus as soon as the pressure is low enough. It is because the processes of condensation and evaporation do not necessarily take place as exact reverses of each other that hysteresis can arise. 

The process of sintering by vapour phase transport is probably important to the growth of single crystals of solids from polycrystalline samples in a closed system, where the vapour species are the transporting gas, which may have the same or different chemical composition from die solid. An example where... 

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