Protection layers disadvantages

There is no intermetallic compound formation and the electrodeposit behaves as a simple mixture of the two metals. It can be considered as basically a stable wick of tin through which zinc is fed to be consumed at a rate lower than its consumption from a wholly zinc surface. If the conditions are such that zinc is rapidly consumed, and no protective layer of corrosion products is formed, the coating may break down, but in mildly corrosive conditions some of the benefits of a zinc coating, without some of its disadvantages, are obtained. 

Field emission displays are VFDs that use field emission cathodes as the electron source. The cathodes can be molybdenum microtips,33-35 carbon films,36,37 carbon nanotubes,38" 16 diamond tips,47 or other nanoscale-emitting materials.48 Niobium silicide applied as a protective layer on silicon tip field emission arrays has been claimed to improve the emission efficiency and stability.49 ZnO Zn is used in monochrome field emission device (FED) displays but its disadvantage is that it saturates at over 200 V.29... 

The main advantage of this group of materials is resistance to pressure. However, there are also disadvantages, such as a high rigidity in comparison to bone, local pitting corrosion due to the fact that there is no protective layer, metal fatigue and friction corrosion. The latter results in rejection which is caused by released ions. 

The disadvantage of physically protective barriers is the rapid and localized corrosion that occurs when the protective layer is scratched or removed locally (Figs. 16.10 and 16.11). Thus, in many cases, the utilization of methods involving continuous electrochemical protection is necessary. 

The increased interest in energy sensitive polymers probably evolves from the shortcomings of the conventional image recording media-silver halide emulsion. It is both difficult and expensive to apply emulsion films as protective layers or use as printing plates or etch resists. Synthetic polymers are one to two orders of magnitude less radiation sensitive than halide emulsion, but their unique properties of chemical and physical resistance easily overcome this disadvantage, ( ., Table IV). 

The disadvantage of this diffusion barrier is that it slightly changes the gas composition at the electrode because of the different diffusion coefficients of the constituents of the raw exhaust gas. On average the rich gas components, especially the small H2, are faster than the bigger 02 or NOx molecules causing a lean shift that is typical for all sensors. The closer the exhaust gas is to equilibrium, the less lean shift is observed. Downstream of the catalytic converter the lean shift completely disappears. Furthermore, precatalytic layers in front of the electrode, or catalytic materials inside the protection layer, reduce the effect... 

Electroless plating is usually limited to very thin layers, typically less than 1 im. This could be an advantage, when the purpose is to create a very thin barrier layer. When a protective layer is needed (to prevent corrosion or abrasion) this is an obvious disadvantage. This, however, is not an inherent limitation of the method, and it would seem that technologies for electroless plating of thicker layers could be developed. 

Although insulators other than aluminum oxide have been tried, aluminum is still used almost universally because it is easy to evaporate and forms a limiting oxide layer of high uniformity. To be restricted, therefore, to adsorption of molecules on aluminum oxide might seem like a disadvantage of the technique, but aluminum oxide is very important in many technical fields. Many catalysts are supported on alumina in various forms, as are sensors, and in addition the properties of the oxide film on aluminum metal are of the greatest interest in adhesion and protection. 

Consequently, mirror optics are more common, in particular in the mid-IR. The mirrors used are usually aluminium- or gold-coated flat or curved substrates. While near-IR mirrors are usually protected by thin SiO-layers, in the mid-IR unprotected mirrors have to be used. Disadvantages of mirror optics are the elevated space consumption and the higher prices in comparison to refractive optics, especially comparing non-standard mirrors against non-standard lens. In total, mirror optics are so preferable to fibres and refractive optics, at least in the mid-IR, that in some technical applications they are used to replace waveguides to transport IR radiation between source, sensor head and spectrometer. 

There are also disadvantages. First, litliium batteries are very expensive. Compared on tiie basis of equal energy content, they may cost three to five times more than Leclanch cells. More serious is the matter of safety. For low power applications, these batteries are quite safe, but high power lithium batteries have been known to explode. For example, accidental heating may melt the lithium (m.p. 180.5°C). This can rupture the protective SET layer, leading to a violent reaction between the metal and the solvent, and eventually to explosion. 

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