Vapour phase coatings

Apart from the reactions described above for the formation of thin films of metals and compounds by the use of a solid source of the material, a very important industrial application of vapour phase transport involves the preparation of gas mixtures at room temperature which are then submitted to thermal decomposition in a high temperature furnace to produce a thin film at this temperature. Many of the molecular species and reactions which were considered earlier are used in this procedure, and so the conclusions which were drawn regarding choice and optimal performance apply again. For example, instead of using a solid source to prepare refractory compounds, as in the case of silicon carbide discussed above, a similar reaction has been used to prepare titanium boride coatings on silicon carbide and hafnium diboride coatings on carbon by means of a gaseous input to the deposition furnace (Choy and Derby, 1993) (Shinavski and Diefendorf, 1993). 

Pretreatment primers. In this method of use the silane may be applied from a solvent solution, by vapour phase deposition or by plasma deposition although solvent application is the more usual. The solution usually contains water and silane at a concentration of 1-2 wt%. The applied film may be water washed before subsequent coating/bonding and/or heat cured. The solvent(s) used may be important in both the stability of the solution and the performance, particularly in the wet adhesion. It has been shown that the presence of water either in the solution or as a final rinse is important, particularly in the case of AAMS and presumably other silanes [1]. Other factors which are important include the concentration of silane the pH of the solution the thickness of the silane film deposited. 

There are two ways in which coatings can be applied thermomechanical processes (e.g. detonation gun, flame spraying and plasma spraying) and vapour phase deposition processes. The latter category can be subdivided into CVD (chemical vapour deposition) and PVD (physical vapour deposition). In the case of a CVD process, a chemical reaction takes place in an oven and as a result the coating material is formed and deposited on the object. Figures 11.7.9 and 11.7.10 are representations of two methods to apply coatings. 

For the modification of silica with aminosilanes, the liquid phase procedure is usually applied. Only few studies have described the vapour phase APTS modification.6,7 The modification proceeds in three steps, (i) A thermal pretreatment of the silica determines the degree of hydration and hydroxylation of the surface, (ii) In the loading step, the pretreated substrate is stirred with the silane in the appropriate solvent, (iii) Curing of the coating is accomplished in a thermal treatment. On industrial scale ethanol/water is used as a solvent, on lab-scale an organic solvent is used. The reasons for this discrepancy is the increased control on the reaction processes, possible in an organic solvent. This will be clarified by the discussion of the modification mechanism in aqueous solvent and the effect of water in the different modification steps. 

Milanko, O.A, Milinkovic, S.A. and Rajakovic, L.V. Evaluation of coating materials used on piezoelectric sensors for the detection of organophosphorus compounds in the vapour phase, Anal. Chim. Acta, 269, 289 (1992). 

Obviously, it is unfeasible to process polymers directly in the vapour phase. Most gas phase operations feature a fluidized bed, either constituted of inert sand particles, or of a cracking or reforming catalyst. The plastics fed into the bed are almost immediately melted, coating the individual bed particles and pyrolyzing as a multitude of thin layers. 

ITO/PEDOT PSS/CuPc C6o/Mg/Ag organic solar cells were fabricated on ITO (5 Q/squarc sheet resistance)-coated glass substrates. After solvent cleaning, the ITO/glass substrates were spin-coated by a PEDOT PSS layer and immediately transferred into the deposition chamber. A 70 nm-thick CuPc C60 blend layer was prepared by organic vapour phase deposition (OVPD ) [2, 3], The Mg/Ag back contacts were deposited by thermal evaporation in high vacuum (p 10 7 mbar) on non-air-exposed absorber surfaces. The device preparation details can be found elsewhere [4], The compositional and substrate temperature (Tsubstrate) investigations are carried out on type A and B devices with nonoptimised and optimised contacts, respectively. 

In physical vapour deposition, PVD, coatings are produced on solid surfaces by condensation of elements and compounds from the vapour phase. The principles are based generally on purely physical effects, but PVD may also be associated occasionally by chemical reactions. Some of these chemical reactions are used intentionally in a special physicochemical film deposition technology, reactive deposition. Reduced to its essence, physical vapour deposition involves three steps ... 

Titanium dioxide exists in nature in three crystalline forms rutile, anatase and brookite. Brookite is an alkali stabilized modification and has never been observed in evaporated films but was found in dip-coated ones. In the case of reactive evaporation of TiO (vapour phase Ti + TiO) and condensation of the vapour on hot substrates (glass, or SiO and C films) depending on the temperature of the substrate Ts, various 2 phases are obtained ... 

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