Partially Blocked Contacts

The current density-voltage relationship of a contact in which a thin layer of insulating oxide covers the metal and the selective layer is electronically conducting follows the Fowler-Nordheim equation, which describes the tunneling current at metal-insulator-metal junctions. 

Here V is the applied voltage, 0b is the average barrier height, d is the insulator thickness, h is Planck s constant, m is the mass, e is the charge of electron, and B is a constant dependent on barrier height and thickness. Both barrier height and the thickness of the oxide can be chemically modulated, giving rise to chemiresistive behavior. Such barrier junctions apparently exist in most chemiresistors, which do not use noble metals for the contact. 

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Partial blocking effect was first identified for pure iron in contact with aerated sulphuric acid medium [55]. Corrosion of carbon steel in sodium chloride media clearly showed the porous layer effect (see Section 5.2) [74]. The same effect was found for zinc corrosion in sodium sulphate [75] and the properties of the layer which was demonstrated to be formed of an oxide/hydroxide mixture were further used for building a general kinetic model of anodic dissolution [76], usable for measurement of the corrosion rate from impedance data. 

Studies from our laboratoiy (Challa et al., 1998) have recently shown that GTP treatment amehorated UVB-induced oxidative burst as measured by H2O2 and myeloperoxidase production. GTP treatment to mice also partially blocked UVB-induced infiltration of leukocytes and appeared to inhibit lL-10 production in skin, as shown by immunohistochemistiy. In this study, GTP apphcation to mice prior to UVB irradiation was found to result in complete reversal of UVB-induced inhibition of contact hypersensitivity, but showed only partial reversal of induction of tolerance to 2,4-dinitrofluorobenzene. These data suggested that green tea, and the polyphenols present therein, might be useM against inflammatory dermatoses and immunosuppression caused by solar radiation in humans. The validation of these studies to human population exposed to low levels of UV radiation chronically through solar radiation is an area for further study. 

It is of interest that glucose suppression of alkaloid metabolism in P. cyclopium (D 8.4.2) can be at least partially blocked by the morphological organization of the mycelium. In mycelial mats growing at the surface of solid or liquid media, i.e., under natural growth conditions, cell specialization, as indicated by alkaloid production and conidiospore formation, is suppressed by administration of high glucose concentrations. This is because only one side of the mat has contact with the medium which slows the permeation of nutrients to the other cells. 

Hurst (19) discusses the similarity in action of the pyrethrins and of DDT as indicated by a dispersant action on the lipids of insect cuticle and internal tissue. He has developed an elaborate theory of contact insecticidal action but provides no experimental data. Hurst believes that the susceptibility to insecticides depends partially on the cuticular permeability, but more fundamentally on the effects on internal tissue receptors which control oxidative metabolism or oxidative enzyme systems. The access of pyrethrins to insects, for example, is facilitated by adsorption and storage in the lipophilic layers of the epicuticle. The epicuticle is to be regarded as a lipoprotein mosaic consisting of alternating patches of lipid and protein receptors which are sites of oxidase activity. Such a condition exists in both the hydrophilic type of cuticle found in larvae of Calliphora and Phormia and in the waxy cuticle of Tenebrio larvae. Hurst explains pyrethrinization as a preliminary narcosis or knockdown phase in which oxidase action is blocked by adsorption of the insecticide on the lipoprotein tissue components, followed by death when further dispersant action of the insecticide results in an irreversible increase in the phenoloxidase activity as a result of the displacement of protective lipids. This increase in phenoloxidase activity is accompanied by the accumulation of toxic quinoid metabolites in the blood and tissues—for example, O-quinones which would block substrate access to normal enzyme systems. The varying degrees of susceptibility shown by different insect species to an insecticide may be explainable not only in terms of differences in cuticle make-up but also as internal factors associated with the stability of oxidase systems. 

The adsorption of block copolymers from a selective solvent was considered by Ligoure (1991). He predicted the existence of surface micelles (see Fig. 3.22) in the case when the block interacting unfavourably with the solvent only partially wets the surface. The model predicts a critical surface micellar concentration (csmc) that differs from the bulk cmc. When the contact angle, which characterizes the interfacial interactions between the copolymer, adsorbing surface, and solvent is lower than some universal value, surface micelles were predicted to appear at a lower copolymer concentration than bulk ones. Experimental results on surfaces are discussed in Section 3.8.4. 

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