Vapour-liquid-solid deposition

Gas-phase methods-. Nanorods and wires have been produced by vapour-liquid-solid deposition. The substrate is first coated with a thin layer of a catalyst such as gold. A ZnO/graphite mix is placed close to the substrates in a furnace tube and heated at about 900 °C under an argon flow. Zn and CO vapour are formed, and the Zn forms an alloy with the Au particles. ZnO starts to grow when the alloy becomes saturated with Zn. Epitaxial growth of oriented nanowires on sihcon substrates has been reported.f ... 

Low temperature plants contain large amounts of fluids kept in the liquid state only by pressure and temperature. If for any reason it is not possible to keep the plant cold then the liquids begin to vapourize. Another hazard in low temperature plant is possible impurities in the fluids, which are liable to come out of solution as solids. Deposited solids may be the cause not only of a blockage but also, in some cases, of an explosion. 

Bulk Fluids. - Here we focus on liquid-vapour transitions and liquid-solid transitions, and briefly mention some recent deposition studies. 

Ethanol purchased from Verbiese (France) (purity > 99.77 %) is mixed with double distilled and deionized water (resistivity 18 MO/cm) for the preparation of aqueous solutions of concentration 0.6, 1, 2, 3, 4.5, 9 and 17 mol%. The solutions are placed in an insulated glass bulb connected to a vacuum line. The vapour-liquid equilibrium conditions are first obtained at 295 K before a contact between the gas phase in equilibrium above the liquid and the pre-cooled sample holder of a cryostage is established. The deposition takes place at 10 Torr and 88 K in 3-5 min. Using thermodynamic modelling and a condensation kinetic model (relation below), one derives the concentration XEton) of EtOH in the deposited solids as ... 

Co-condensed EtOH-water mixtures reveal the formation of distinct EtOH hydrate phases in different temperature domains. A hydrate 1 appears in the 130 K - 163 K range depending on the EtOH content. It is proposed to have a cubic lattice similar to that of the clathrate type I. Hydrate 2 is found to crystallize at 158 K or 188 K-193 K in correlation with the absence or the presence of ice Ic and EtOH content. Its composition seems to correspond to the monohydrate. The deposited solids undergo crystallization 10 K lower in comparison to frozen aqueous solutions. This reflects the remarkable ease with which water molecules initiate molecular rearrangement at low temperature. This seems most likely due to EtOH generating defects that facilitate the water reorientation . This may also reflect the generation of clusters (in the vapour phase before deposition) having a different nature relative to those encountered in the liquid solutions. These unusual structures may have implications in atmospheric chemistry or astrophysics. 

Glen et al [1988, 1992] have described a model developed essentially for particulate deposition in gas streams, in particular for high temperature systems. Under these conditions material carried by the gas phase may exist as vapour, liquid or solid. The transport and deposition will depend on the nature of the contaminating material. The mechanisms already identified as being responsible for particle deposition include inertial impaction diffusion (Brownian and eddy) gravitation thermophoresis... 

Alloys are usually made by melting the components and mixing them together while liquid, though you can make them by depositing the components from the vapour, or by diffusing solids into each other. No matter how you make it, a binary alloy can take one of four forms ... 

The vap. press, curve of solid iodine is indicated by PO, Fig. 16 that of liquid iodine by 00 and the effect of press, on the m.p. of iodine by ON. At the triple point 0 these curves meet. Fig. 18 shows a similar curve for water. The curve PO thus represents the sublimation curve or hoar-frost line OC. the boiling or vaporization curve, i.e. the effect of press, on the b.p. of the liquid. The same phenomenon occurs with water, iodine, etc., and the principle involved is the same as indicated in the law represented by Clapeyron-Clausius equations with respect to the lowering of the m.p. by an increase of press. Consequently, if the vap. press, of iodine be less than that of th,e triple point, the solid does not melt, but rather sublimes directly without melting at the triple point at 114-15° (89 8 mm.) and A. von Richter at 116 1° (90 mm.). According to R. W. Wood, if the condensation of iodine vapour occurs above —60°, a black granular deposit is formed, but below that temp, a deep red film is produced. 

Solid samples that have a sufficient vapour pressure at 300 °C are deposited on the tip of a heated metal probe which is then inserted into the instrument through a vacuum lock. With some ionisation methods, the solid sample is mixed with a liquid matrix (e.g. glycerol or benzoic acid). 

Microwaves and Radar yes yes liquids, powders, granular solids. Action very similar to ultrasonic signals. Has been employed for discrete sensing. Pulsed energy used in transmitted or reflected inodes. Transmitter and receiver can be mounted outside the vessel. Very useful for determining mean levels io closed vessels under difficult conditions, e.g. in cases of high turbulence, in the presence of vapour or condensate, or where there is encrustation of surfaces with deposits. Typical accuracies are of the order of 0.2 per cent FSD 1. 

The composition has been established by the vapour density and by analysis (e.g. with silicon, see p. 87). The fluoride is a colourless gas which does not fume in the air, and is highly poisonous. It condenses to a colourless liquid at - 95° C. and freezes to a white solid at -160° C. The heat of formation is 106-2 to 109-7 Cals.8 per mol, therefore much greater than that of the trichloride, and the trifluoride also proves to be the more stable of the two compounds. It can be decomposed by electric sparks with deposition of phosphorus and formation of the pentafluoride, thus —... 

A hollow drum of stainless steel, which is filled with liquid nitrogen, spins about a vertical axis at high speed within an evacuated outer vessel. A beam of halohydrocarbon vapour, RX, elfuses from a slit in the end of jet A, which is placed close to the outer surface of the drum, and immediately freezes to form a solid layer. The rate of deposition is adjusted so that the thickness of the halohydrocarbon layer laid down each revolution is about ten monolayers. A much less intense beam of sodium vapour effuses through a second jet B and is deposited on top... 

The Au sample is cooled with liquid nitrogen to 100 K and exposed to water or methanol by backfilling the chamber. Exposures are stated in Langmuirs (L) (1 L=10 Torr s). It is expected that amorphous solid water and methanol are formed during the slow deposition of the respective vapours at temperatures below 120 K (see also ref [11]). 

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