Thin deposit

Figure 4.4 Corrosion product mounds covering localized areas of metal loss on an aluminum heat exchanger tube. Attack initiated beneath a thin deposit layer.

Properties of deposits Deposits can be produced that are adherent, coherent and finely crystalline. Addition agents, e.g. organic sulphonamides can improve the deposit structure so that thick coatings can be produced free of nodules and blisters. The production of very smooth thick deposits of copper has been reported Thin deposits tend to reproduce the substrate topography, but some cases of levelling have been reported. The brightness tends to fall with increasing thickness. 

Hydrogen sulfide also is formed, and this instantly reacts with iron and steel to form thin deposits of black ferrous sulfide in the superheater tubes ... 

Ultrathin films of CdS ranging in coverage from 25 to 200 ML were grown also by the previous method on Au substrates (of non-specified nature) and were characterized by quantitative Raman resonance [41], It was found that the electronic structure of the films in this coverage regime corresponds to that of bulk CdS. It was concluded also that ECALE does not involve growth by random precipitation of CdS onto the Au surface the thin deposited layers of the material were contiguous. 

Although not essential, one often uses a previously coated AgCl-Ag electrode or a silver-plated Pt wire coated electrolytically in KC1 solution with a thin deposit of AgCl. Such dry AgCl-Ag electrodes are much in favour as reference electrodes (although in the absence of oxidants), in addition to calomel electrodes (Pt wire in contact with Hg, covered with calomel paste in contact with KC1 solution), which also belong to the second kind, viz.,... 

This is dipping without the assistance of ancillary aids such as coacervants. Thin deposits are formed by viscous latex wetting the former. Single dips are prone to defects such as pinholes, whereas multi-dipping can build up thicknesses of 0.005 to 0.010 inches (127 to 254 ycm) and prevent the occurrence of pinholes the deposit is allowed to partially, or totally dry between dips. Condoms are an example of a product produced by simple dipping. 

Insufficient rinsing can also result in some codeposition if the previous reactant is not fully removed. The main drawback is the possibility of 3-D growth, which can be hard to identify with very thin deposits. Alternatively, the rinse solution may not be important. Some high quality CdTe films were formed in this group without using a separate rinse solution. That is, the reactant solutions were exchanged by each other, under potential control, suggesting some small amount of codeposition probably did occur. 

Weaver et al., formed superlattices with CdSe and CdS, using EC-ALE, without an automated system [163]. They studied their relatively thin deposits by surface enhanced Raman (SERS), examining stress build-up in the deposits. 

An automated flow system has also been used by Foresti et al. to form CdS layers, with up to 150 cycles, using pH 9.2 solutions for both elements on Ag(lll) electrodes [116], In their case, the deposits appeared stoichiometric, without the excess S previously observed by this group [111]. Their cycle produced relatively thin deposits, similar to this author s, or about 1/3 ML/cycle. 

An alternative method for producing thin deposits consists in avoiding single-step processes by chemisorbing the precursor and then having it decomposed thermally [1]. Such a method, often named atomic layer deposition or gas phase impregnation-deposition, provides very thin deposits due to the presence of a... 

If reaction is allowed to proceed until the termination stage is reached, the terminal thickness of many CD films is typically several hundred nanometers, although it may reach a micron or more in some cases. This terminal thickness depends to a large extent on the deposition parameters. To take an extreme case, addition of sulphide to a solution of Cd ions will give an immediate precipitate of CdS, but no (or at most an extremely thin) deposit on the walls of the deposition vessel, which may thicken somewhat with time, but will not be visible (which means a terminal thickness less than ca. 20 nm). For a normal CD reaction, if precipitation occurs homogeneously in solution, then that precipitate is lost for film... 

The reduction of 2- and 4-cyanopyridine to the aminomethylpyridine was mentioned in Part I. 3-Aminomethylpyridine has been prepared from 3-cyanopyridine in an electrocatalytic reduction in aqueous hydrochloric acid solution, using an electrode consisting of a thinly deposited layer of palladium black on graphite. The reduction proceeds with electrolytically generated hydrogen not via an electron transfer to the substrate.418... 

Other workers, who have had occasion to repeat the work of Kalmus report considerable difficulty in obtaining thick deposits of cobalt consistent with a high quality of adhesion. The solutions recommended by Kalmus yield quite satisfactory results for very thin deposits, but the case is otherwise where deposits are required ranging in thickness from 3 to 5 thousandths of an inch and upwards. In these eases the deposited metal is found to separate very readily from its base.3 Furthermore, in view of more recent research on the rapid deposition... 

With this solution a current density of 5 amperes per square foot may be employed, equivalent to 0-54 amperes per square decimetre. The initial voltage may be 5, falling to 3 volts or somewhat lower when a thin deposit has formed on the cathode. 

A spot analysis, using a Kevex-ray unit on a scanning electron microscope was performed on a piece of the fabric remnant and a piece of the washing test specimen. These specimens were mounted on a copper stud and had a thin deposition layer of carbon deposited over them. The following results were obtained from the technician who devised and performed the tests. 

This works well with a current density of 0-15 to 0 17 ampere per sq. decimetre, the initial density being 0 2 ampere until a thin deposit has been obtained on the cathode. Voltage 0 7. The anodes should consist of pure Swedish charcoal iron. These become covered with a black carbonaceous slime after a time, and require cleaning. It is desirable that the anodic area should somewhat exceed that of the cathode. 

The deposition time could, indeed, be neglected, if it is small compared to the age of the formation - it is practically so in thin (hundereds of meters) deposits - although it would not be the same with the pressure effect. In these thin deposits the variation in quality of the organic matter is not very well defined. 

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