Alternative Passivation Processes

A number of alternative passivation processes to anodic oxidation have been investigated for (Hg,Cd)Te. The primary motivations for this search have been the relatively high fixed charge associated with most anodic oxides, and low thermal stability. The fixed charge is especially a problem with p-type devices, since it produces surface inversion which requires additional device structures to compensate. The thermal stability has been pursued because of the likely association of thermal stability with thermodynamic stability, with the ultimate result being increased longevity and reliability of infrared imaging systems 

Perhaps the most significant development in alternative passivation has been another anodic electrochemical process, sulfidization. The 

Little fundamental electrochemical information is presently available about anodic suifidization. One work (86) using oxide marker and compositional profiling found the suifide growth to be into the substrate, as the same report concluded for anodic oxidation. The original reports from Nemirovsky, et ai. (83), suggested that the reaction was a six electron process  

Furthermore, this later work draws a possible connection between the anodic suifidization process and photocorrosion of ll-VI solar cells in polysulfide electrolytes. If this connection could be established, a well of knowledge could be applied to gain a better understanding of this passivation process. (For example Ref 88-90). 

Anodic fluoride films have also been produced on (Hg,Cd)Te as an 

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In view of its susceptibility to sulphide tarnishing, silver may itself require some measure of protection in many decorative and industrial applications. Chromate passivation processes are commonly employed, but as an alternative, thin coatings of gold, rhodium or palladium may be used. 

Study of the action of AR on . coli precA luxCDABE-Anvp has confirmed it photoprotective effects and has shown features of such activity in live systems. Surprising was the interrelation between preservation of viability of AR-processed bacterial cells in the conditions of a long and intensive UV-irradiation and depression of activity their reparing SOS-systems. It has assumed AR action and the SOS-answer as alternative "passive" and "active" mechanisms for protection of bacterial cells DNA at various intensivity of UV-irradiation. 

Compounds can cross biological membranes by two passive processes, transcellu-lar and paracellular mechanisms. For transcellular diffusion two potential mechanisms exist. The compound can distribute into the lipid core of the membrane and diffuse within the membrane to the basolateral side. Alternatively, the solute may diffuse across the apical cell membrane and enter the cytoplasm before exiting across the basolateral membrane. Because both processes involve diffusion through the lipid core of the membrane the physicochemistry of the compound is important. Paracellular absorption involves the passage of the compound through the aqueous-filled pores. Clearly in principle many compounds can be absorbed by this route but the process is invariably slower than the transcellular route (surface area of pores versus surface area of the membrane) and is very dependent on molecular size due to the finite dimensions of the aqueous pores. 

Bioavailability can be either an active or a passive process. Consider for a moment a pond filled with organisms (fish, algae, microbes). If a lipid-soluble molecule is added to the pond, there will be a sttong propensity for it to enter the lipid components of the environment—in other words, the living organisms. The partitioning of the lipid-soluble compounds into the animals will be passive, as the compounds will be able to absorb fairly easily into the aquatic biota. Alternatively, if there are metal ions in the water, they will... 

The basic mechanism of passivation is easy to understand. When the metal atoms of a fresh metal surface are oxidised (under a suitable driving force) two alternative processes occur. They may enter the solution phase as solvated metal ions, passing across the electrical double layer, or they may remain on the surface to form a new solid phase, the passivating film. The former case is active corrosion, with metal ions passing freely into solution via adsorbed intermediates. In many real corrosion cases, the metal ions, despite dissolving, are in fact not very soluble, or are not transported away from the vicinity of the surface very quickly, and may consequently still... 

The small particles are reported to be very harmful for human health [98]. To remove particulate emissions from diesel engines, diesel particulate filters (DPF) are used. Filter systems can be metallic and ceramic with a large number of parallel channels. In applications to passenger cars, only ceramic filters are used. The channels in the filter are alternatively open and closed. Consequently, the exhaust gas is forced to flow through the porous walls of the honeycomb structure. The solid particles are deposited in the pores. Depending on the porosity of the filter material, these filters can attain filtration efficiencies up to 97%. The soot deposits in the particulate filter induce a steady rise in flow resistance. For this reason, the particulate filter must be regenerated at certain intervals, which can be achieved in the passive or active process [46]. 

Some laboratories have found an alternative to the short-term cultures by using cell lines other than Caco-2 cells. The most popular of these is Madin-Darby canine kidney (MDCK) cells, an epithelial cell line from the dog kidney. MDCK cells have been suggested to perform as well as Caco-2 cells in studies of passive drug permeability [56]. These cells have also been used to optimise the conditions for studies of low-solubility drugs [53]. However, as noted previously, the active transport processes of this cell line can be quite different to those of Caco-2 cells [28-30], Another cell line that only requires short-term culture is 2/4/A1, which is a conditionally immortalised rat intestinal epithelial cell line [86]. The 2/4/A1 cell line is discussed in Section 4.3.2.2 below. 

Hydride Compressors using reversible metal hydride alloys offer an economical alternative to traditional mechanical compressors for GH2. The simplicity and passive operation of the hydride compression process offers many advantages over mechanical compressors. Hydride compressors are compact, silent, do not have dynamic seals, require very little maintenance, and can operate unattended for long periods. However, they are a very new and may be difficult to be built at the scale required for GH2 transmission pipeline service. 

Industrial fire protection and safety engineers attempt to eliminate hazards at their source or to reduce their intensity with protective systems. Hazard elimination may typically require the use of alternative and less toxic materials, changes in the process, spacing or guarding, improved ventilation or, spill control or inventory reduction measures, fire and explosion protective measures - both active and passive mechanisms, protective clothing, etc. The level or protection is dependent on the risk prevalent at the facility versus the cost to implement safety measures. 

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