Metal oxides multilayers

Metal effect pigments are mainly based on small metallic flakes of aluminium or bronze. In another class are mica-metal oxide multilayer interference pigments for perlescent effects. 

Horio N, Hiramatsu M, Nawata M, hnaeda K, Torri T. Preparation of zinc oxide/ metal oxide multilayered thin films for low-voltage varistors. Vacuum. 1998 51 719-722. DOI 10.1016/S0042-207X(98)00293-0. 

The in situ wet chemical approach requires less nanocarbon modification, especially for electrodeposition, and can produce thin, uniform, multilayer films. This is the method of choice for nanocarbon-polymer hybrids as the increased interfacial area reduces problems of nanocarbon insolubility and subsequent aggregation. Gas phase deposition offers the greatest control of thin film thickness but is suitable almost exclusively to the deposition of metals and metal oxides. 

Inorganics can also be synthesized and used as templates. Thus, controlled siloxane networks were formed when dispersions of alkoxysilanes (such as (MeO)3SiMe) are mixed with the suitable template matrixes. Ultrafine particles of metal oxides can be used as starting materials for the formation of metal oxide films. For instance, a mixture of a double-chained ammonium amphiphile and an aqueous solution of aluminum oxide particles (diameter about 10 100 nm) gives a multilayered aluminum oxide film when calcinated at over 300°C. 

Fig. 4 shows a simple phase diagram for a metal (1) covered with a passivating oxide layer (2) contacting the electrolyte (3) with the reactions at the interfaces and the transfer processes across the film. This model is oversimplified. Most passive layers have a multilayer structure, but usually at least one of these partial layers has barrier character for the transfer of cations and anions. Three main reactions have to be distinguished. The corrosion in the passive state involves the transfer of cations from the metal to the oxide, across the oxide and to the electrolyte (reaction 1). It is a matter of a detailed kinetic investigation as to which part of this sequence of reactions is the rate-determining step. The transfer of O2 or OH- from the electrolyte to the film corresponds to film growth or film dissolution if it occurs in the opposite direction (reaction 2). These anions will combine with cations to new oxide at the metal/oxide and the oxide/electrolyte interface. Finally, one has to discuss electron transfer across the layer which is involved especially when cathodic redox processes have to occur to compensate the anodic metal dissolution and film formation (reaction 3). In addition, one has to discuss the formation of complexes of cations at the surface of the passive layer, which may increase their transfer into the electrolyte and thus the corrosion current density (reaction 4). The scheme of Fig. 4 explains the interaction of the partial electrode processes that are linked to each other by the elec-... 

Figure 6.12. Photocurrent generation from metal oxide/TCPP alternate films, (a) Schematic illustration of a metal oxide/TCPP multilayer, (b) current/bias plots (irradiation 0.3mW/cm2 at 420 nm), (c) photocurrent from (Ti02-gel/TCPP)5 assembly prepared on an ITO electrode (0.3 mW/cm2 at 420 nm, -0.2 V vs Ag/AgCl).

A.F. Hofmann, in 1868, prepd formaldehyde by passing a mixture of methanol vapors and air over a heated platinum spiral, serving as a catalyst. The following reaction takes place CH3OH+ / 02 (air) - CH20+H20. The present method of preparation is essentially the same as used by Hofmann, except that cheaper catalysts than platinum are used, such as Ag and Cu in multilayer gauzes or screens, vanadium and molybdenum oxides, iron oxides or other metal oxides. Limited amounts of formaldehyde are also prepd by the oxidation of natural gas and the lower petroleum hydrocarbons... 

The aluminum flakes can be coated with aluminum oxide and/or silicon dioxide in the liquid phase before the CVD process to reduce the reactivity of the metal and to create multilayer color effects. These pigments show strong angle-dependent coloristic effects (color travel) [5.158]. Metal halogenides are difficult to use as gas-phase precursors for metal oxide coating because of the reactivity of the metal flakes and the corrosion of steel in chloride atmospheres. The use of TiCU and water vapor was described about ten years ago. However, no commercial products have, as yet, been introduced. 

From the perspective of chemical analysis, the ability to observe spectra from surface monolayers without requiring field enhancement demonstrates the high sensitivity of modem Raman instramentation. In addition, it indicates that multilayer films such as polymer coatings, lubricants, or metal oxides are readily observable on a variety of substrates, provided the multilayer is reasonably transparent. However, the sensitivity constraints in the absence of field enhancement are generally severe when dealing with monolayers,... 

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