Deposition of metal

Most metals commence to be deposited from solutions of their own salt in the course of cathodic polarization near their reversible potentials so that at sufficiently low current densities these processes can be considered to be practically reversible (see Fig. 27.). At higher current densities the cathode 

Contrary to most metals, iron, cobalt, and nickel are deposited at the cathode even at very low current densities at a potential that is by 0.2 to 0.3 V more negative than the reversible potential. The least comparative irreversibility on deposition is manifested by iron, the greatest by nickel. Polarization decreases as temperature is raised so that at temperatures exceeding 70 °C iron will be deposited in a reversible way whilst deposition of nickel and cobalt are associated with an overvoltage of about 0.05 V even at 100 °C. It seems that a higher temperature accelerates the conversion of the unstable form in which metals are initially deposited to a stable one. 

A rather high degree of polarization is sometimes encountered on deposition of metals from solutions of their complex salts (for example silver from a cyanide solution). 

The deposition of metals may be accompanied by the discharge of hydrogen ions which are present in every aqueous solution. Taking into account the respective equilibrium potentials and possible polarization phenomena as well it is possible to determine beforehand whether at a given temperature and concentration the deposition of metal or the discharge of hydrogen ions will have priority. In the same manner it can be determined in what sequence the individual metallic ions will be deposited from a solution containing two different salts or more. 

Should there be no interaction of the deposited metals (i. e. should no alloy be formed), a simultaneous deposition of different metals is possible only when their deposition potentials are equal. As these deposition potentials are determined by the sum of reversible (reduction) potentials and of the corresponding overvoltage value o, the condition of a simultaneous discharge of cations A+ and B+ (one of which may be the hydrogen ion) can be written in the following form  

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Madey and co-workers followed the reduction of titanium with XPS during the deposition of metal overlayers on TiOi [87]. This shows the reduction of surface TiOj molecules on adsorption of reactive metals. Film growth is readily monitored by the disappearance of the XPS signal from the underlying surface [88, 89]. This approach can be applied to polymer surfaces [90] and to determine the thickness of polymer layers on metals [91]. Because it is often used for chemical analysis, the method is sometimes referred to as electron spectroscopy for chemical analysis (ESCA). Since x-rays are very penetrating, a grazing incidence angle is often used to emphasize the contribution from the surface atoms. 

Huckaby D A and Blum L 1991 A model for sequential first-order phase transitions occuring in the underpotential deposition of metals J. Eiectroanai. Chem. 315 255-61... 

This chapter is an attempt to present the important results of studies of the synthesis, reactivity, and physicochemical properties of this series of compounds. The subject was surveyed by Bulka (3) in 1963 and by Klayman and Gunther (4) in 1973. Unlike the oxazoles and thiazoles. there are few convenient preparative routes to the selenazoles. Furthermore, the selenium intermediates are difficult to synthesize and are often extremely toxic selenoamides tend to decompose rapidly depositing metallic selenium. This inconvenience can be alleviated by choice of suitable reaction conditions. Finally, the use of selenium compounds in preparative reactions is often complicated by the fragility of the cycle and the deposition of metallic selenium. 

Material Protection. The graft copolymers of ethylene sulfide on polyethyleneimine can be used as an antifouHng anticorrosion substrate for iron (439). PEIs or their derivatives are also used in electrolysis baths as brighteners in the electrochemical deposition of metals (440,441). 

In contrast to the older techniques, a newer method is to use a scanning tuimeling electron microscope to deposit metal coatings in microscopic images as small as 0.001 pm. The ultimate surface metallization techniques allow deposition of metals atom by atom in controlled three-dimensional arrays. 

Rhenium exhibits a greater resistance than tungsten to the water cycle effect, in which lamps and electron tubes become blackened by deposition of metal. This phenomenon involves catalysis by small quantities of water that react with the metal in a hot filament to produce a volatile metal oxide and hydrogen. The oxide condenses on the surface of the bulb and is reduced back to the metal by hydrogen. 

Acoustic Wave Sensors. Another emerging physical transduction technique involves the use of acoustic waves to detect the accumulation of species in or on a chemically sensitive film. This technique originated with the use of quartz resonators excited into thickness-shear resonance to monitor vacuum deposition of metals (11). The device is operated in an oscillator configuration. Changes in resonant frequency are simply related to the areal mass density accumulated on the crystal face. These sensors, often referred to as quartz crystal microbalances (QCMs), have been coated with chemically sensitive films to produce gas and vapor detectors (12), and have been operated in solution as Hquid-phase microbalances (13). A dual QCM that has one smooth surface and one textured surface can be used to measure both the density and viscosity of many Hquids in real time (14). 

Porosity and Pore Size. The support porosity is the volume of the support occupied by void space and usually is described in units of cm /g. This value represents the maximum amount of Hquid that may be absorbed into the pore stmcture, which is an especially important consideration for deposition of metal salts or other active materials on the support surface by Hquid impregnation techniques. The concentration of active material to be used in the impregnating solution is deterrnined by the support porosity and the desired level of active material loading on the catalyst. If the porosity is too low, inefficient use of the support material and reactor volume may result. If the porosity is too high, the support body may not contain sufficient soHd material to provide the strength necessary to survive catalyst manufacture and handling. 

In many of the other processes that use base metal catalysts, irreversible poisoning of the catalyst occurs as a result of deposition of metal contaminants from the process feedstock onto the catalyst surface. These catalysts are not considered to be regenerable by ordinary techniques. 

The deposition of metals directly from these halides would require high temperatures to be efficient, but because of the thermodynamic stabilities of the hydrogen halides, direct reduction can readily be carried out with hydrogen at lower temperamres. The general reaction... 

The important beneficial effects that substrate roughness can bring were firmly established in the late sixties and early seventies, principally as a result of work in two areas. The first was associated with the electroless deposition of metals onto plastics such as ABS and polypropylene. In the process the plastics must be etched in a way which produces pits on a micrometre scale. Such a topography had been shown to be a necessary, but not sufficient condition for adequate adhesion [40]. 

Vacuum Deposition-also vapor deposition or gas plating the deposition of metal coatings by means of precipitation (sometimes in vacuum) of metal vapor onto a treated surface. The vapor may be produced by thermal decomposition, cathode sputtering or evaporation of the molten metal in air or an inert gas. 

Remond, G. B. and Johnson, A. R., Vacuum Deposition of Metals , Metal Finishing Journal, 4, 393 (1958)... 

The corrosion rates of the materials of construction are always of importance, but it has been found that, whereas the uniform removal of metal from the hot leg may not impair the load-carrying ability of the container, the deposition of metal in the cold leg can cause the cessation of flow, and the measure of the suitability of an alloy is often the time, under given conditions, that it takes for plugging to occur. Again, the flow velocity and the cross-sectional area are of primary importance in relating test results to operating conditions. 

Oxygen overpotential is about 0.4-0.5 volt at a polished platinum anode in acid solution, and is of the order of 1 volt in alkaline solution with current densities of 0.02-0.03 A cm-2. As a rule the overpotential associated with the deposition of metals on the cathode is quite small (about 0.1-0.3 volt) because the depositions proceed nearly reversibly. 

Table 14.1 Deposition of metals at controlled potential of the mercury cathode...

The unhindered ionic charge transfer requires many open pores of the smallest possible diameter to prevent electronic bridging by deposition of metallic particles floating in the electrolyte. Thus the large number of microscopic pores form immense internal surfaces, which inevitably are increasingly subject to chemical attack. 

The pores of tire separating membrane are to be most uniformly distributed and of minimum size to avoid deposition of metallic particles and thus electronic bridging. One distinguishes between macroporous and microporous separators, the latter having to show pore diameters below I micron (/urn ), i.e., below one-thousandth of a millimeter. Thus the risk of metal particle deposition and subsequent shorting is quite low, since active materials in storage batteries usually have particle diameters of several microns. 

Hydrogen adsorption from solution Oxygen adsorption from solution Underpotential deposition of metals Adsorption of probe molecules from solution ... 

The acetylacetonates are stable in air and readily soluble in organic solvents. From this standpoint, they have the advantage over the alkyls and other alkoxides, which, with the exception of the iron alkoxides, are not as easily soluble. They can be readily synthesized in the laboratory. Many are used extensively as catalysts and are readily available. They are also used in CVD in the deposition of metals such as iridium, scandium and rhenium and of compounds, such as the yttrium-barium-copper oxide complexes, used as superconductors. 1 1 PI Commercially available acetyl-acetonates are shown in Table 4.2. 

Green, M. L., and Levy, R. A., Chemical Vapor Deposition of Metals for Integrated Circuit Applications, /. ofMetals, pp. 62-71 (June 1985)... 

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