Film Formation Basics

Lead usually has excellent resistance to seawater owing to the formation of a passive film of basic carbonate and carbonate-chloride double salts , which should be compared with its behaviour in solutions of alkali chlorides (see salts p. 4 87). 

A. Basic Features of Langmuir-Blodgett Film Formation and Study... 

In the sol-gel process, ceramic polymer precursors are formed in solution at ambient temperature shaped by casting, film formation, or fiber drawing and then consolidated to furnish dense glasses or polycrystalline ceramics. The most common sol-gel procedures involve alkoxides of silicon, boron, titanium, and aluminum. In alcohol water solution, the alkoxide groups are removed stepwise by hydrolysis under acidic or basic catalysis and... 

The basic features of the ion-by-ion and hydroxide cluster film-forming mechanisms are shown schematically in Figures 2.1 and 2.2, respectively. Film formation involving complex decomposition will proceed in a similar manner (Fig. 2.3 shows this for a molecule-by-molecule deposition). 

Homogeneous precipitation using urea (which hydrolyzes to give an alkaline solution) has been used extensively, and in a few cases films of basic salts (sulphates of A1 and Sn(lV) and formate of Fe) have been obtained. These are not considered semiconductors in the conventional sense, but do provide examples for extension of the CD method beyond the conventional sulphide-selenide-oxide compounds. 

The basic science behind nucleation and forces between materials have been treated in Chapter 1. For those interested in this section, it is assumed that this basic science is (more or less, at least) understood. However, the basics treated in Chapter 1, while important to an understanding of film (as opposed to isolated crystal) formation, are not enongh by themselves to provide a phenomenological explanation of film formation. We would ideally like to be able to predict in advance, from fundamental principles, whether a particular bath formulation will result in adherent films or not. We cannot However, if we cannot reliably predict adhesion, we can at least choose conditions so that the probability of adhesion is good. 

A variation of the CD process for PbSe involved deposition of a basic lead carbonate followed by selenization of this film with selenosulphate [64]. White films of what was identified by XRD as 6PbC03-3Pb(0H)2-Pb0 (denoted here as Pb—OH—C) were slowly formed over a few days from selenosulphate-free solutions that contained a colloidal phase and that were open to air (they did not form in closed, degassed solutions). CO2 was necessary for film formation—other than sparse deposits, no film formation occurred of hydrated lead oxide under any conditions attempted in this study. Treatment of these films with selenosulphate solution resulted in complete conversion to PbSe at room temperature after 6 min. The selenization process of this film was followed by XRD, and it was seen to proceed by a breakdown of the large Pb—OH—C crystals to an essentially amorphous phase of PbSe with crystallization of this phase to give finally large (ca. 200 nm) PbSe crystals covered with smaller (15-20 nm) ones as well as some amorphous material. 

There are purely electrochemical methods for finding the amount of simple radicals such as H or O on noble metal electrodes. Basically, they rely upon die assumption that when some electrical variation in die state of die electrode is brought about, die only effect it has is to reduce or augment die H or the O on die electrode surface. Now of course this is not so if die substrate is, say, iron, or indeed all but the noble metals, for there may be a co-dissolution of die substrate, or competing oxide film formation, etc. Spectroscopic methods (e.g., FUR in a millisecond response version) or ellipsometiy are not affected by such difficulties. 

The reductive decomposition of thiocyanato complexes should be applicable to the electrodeposition of other metal sulfides. We have tried this with Pd2, Co2+, Ni2+, Zn2+ and In3+.I8 While thin films of PdS, CoS and NiS could be successfully electrodeposited, other metal sulfides such as ZnS and In2S3 could not be obtained. This is an interesting series of results when we think of the softness (hardness) of these metals as acid. TC coordinates with its sof basic S atom to soft acidic Cd2+ and Pd2+, while hard acidic In3+ only permits coordination with hard basic N atom to form an isothiocyanato-complex. Other metals are at the borderline accepting coordination of both S and N. Because reduction of TC is catalyzed by a central metal,75,76) such ligand reduction may result in the formation of metal sulfides only for thiocyanato-complexes. The difference in bahavior among Co2+, Ni2+ and Zn2+ could be reasoned as the consequence of efficient catalysis of the electron transfer reaction by the transition metals. Such trends fit nicely with the previous findings by electrochemical analyses. 7) It is therefore understood that the chemical structure of the active species is decisive to the film formation. Thus, designing such molecular precursors which are chemically stable but can be electrochemically decomposed to metal sulfides should broaden the possibilities of electrochemical thin film synthesis. 

Surface film formation on noble metal electrodes at reduction potentials was studied extensively with solutions of DME, THF, 2Me-THF, and DN. Basically, these solvents are much less reactive at low potentials than are alkyl carbonates and esters. However, in contrast to ethereal solutions of TBA+ whose electrochemical window is limited cathodically by the TBA+ reduction at around OV (Li/Li+), in Li+ solutions, ether reduction processes that form Li alkoxides occur at potentials below 0.5 V (Li/Li+) [4], It should be emphasized that the onset potential for surface film formation on noble metals in ethereal solutions is as high as in... 

Basic issues such as surface reactions, surface film formation, passivation, ionic and electronic transport phenomena through surface films, problems in uniformity of deposition and dissolution processes, correlation between surface chemistry, morphology, and electrochemical properties are common to all active metal electrodes in nonaqueous solutions and are dealt with thoroughly in this chapter. It is believed that many conclusions related to Li, Mg, Ca, and A1 electrodes can be extended to other active metal electrodes as well. 

There is another phenomenon, regarded as a deteriorative change in the protein of soy milk, caused also by the evaporation of water. This is a film formation on the surface of soy milk, which occurs when heated soy milk is kept open to the air. This phenomenon is observed not only in heated soy milk but also in heated cow s milk. Film formation of soy milk occurs only when the soy milk is heated above 60°C and there is evaporation of water from the surface of the soy milk. The mechanism of protein insolubilization is basically the same as that of soy milk powder produced from heated soy milk (10. When water is removed from the surface of heated soy milk by evaporation, the molecular concentration of protein near the surface increases locally and the exposed reactive groups of the denatured molecules come close enough to interact intermolecularly both by hydrophobic interactions and through the sulfhydryl/disulfide interchange reaction to form a polymerization (film) on the surface. The upper side of the film contains more hydrophobic amino acids because of orientation of the hydrophobic portions of the unfolded molecules to the atmosphere rather than into the aqueous solution. 

Their primary property of extremely tough film formation give them extreme resistance to degradation while maintaining basic substrate tactile character. 

Several reviews addressing the polarization behavior, d ion adsorption, competition between Cr adsorption and OH codeposition, oxide film formation, and cr ion discharge, as well as the kinetic aspects of the reaction on various oxide-covered and oxide-free surfaces that have been investigated during the past 15 years, have been published (55/, 333-338). Of these, particular mention should be made of Refs. 555, 335, 336, and 439-441, where the basic aspects of the properties of oxide electrodes and the kinetic aspects of oxide film formation in relation to Cl adsorption and the kinetics of Cr ion discharge were addressed. Mechanistic aspects of chlorine evolution were critically analyzed recently in an excellent article by Trasatti (338). In this article, the focus is primarily on the nature and characterization of the adsorbed intermediates partipatingin the course of CI2 evolution and their role in the electrocatalysis of the chlorine evolution reaction. As with the OER, in aqueous solutions CI2 evolution takes place on an oxidized surface of metals or on bulk oxide films, so that their surface states often have to be considered in treating the electrocatalysis of the reaction. 

The film formation process from the aqueous dispersion is shown schematically in Fig. 3. The mechanisms of film formation from the aqueous polymeric dispersions have been discussed for a long time, and many theories have been proposed. The detail was reviewed by Muroi from a basic point of view. Film formation in pharmaceutical applications was discussed by Lehmann, Steuernagel, and Fukumori. ... 

At the initial stage of film deposition, a sufficient number of gaseous species are condensed to establish a permanent residence on the surface of the substrate. The model of film formation can be categorised into three basic types, which are illustrated schematically in Figure 6.2. This work was first realised by Bauer [7] and the details of these categorisations are described as follows. 

Kasrai, M., Fuller, M. S., Bancroft, G. M., Yamaguchi, E. S. and Ryason, P. R. X-Ray Absorption Study of the Effect of Calcium Sulphonate on Antiwear Film Formation Generated from Neutral and Basic ZDDPs Part 1 - Phosphoras Species , Tribol. T. 2003,46, 534-542. 

Chemical.- lt ia unaltered in diy air at the ordinaxy temperature but when heated to redness is oxidized to CuO. In damp air it becomes coated with a brownish film of oxide a green film of basic carbonate or, in salt air, a green film of basic chloride. Hot H SO, dissolves it with formation df CuSO, and SO, it is dissolved by HNO, with formation of CuiNO,), and NO and by rlCl with liberation of H Weak acids form witn it soluble salts in presence of air and moisture. It is dissolved by NH,HO, in presence of air, with formation of a blue solution. It combines directly with Cl, frequently with light... 

Interestingly the formation of excitons in nanoparticles stabilized on thin films -a basic cluster formation process in thin film physics - leads to the creation of quantum dots, i.e. quasi-zero dimensional structures that confine carriers in all the three spatial dimensions, thereby enhancing applicable phenomena such as light emission and gas sensing. 

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