Porosity/pores oxide films

The most important considerations with respect to sensor characteristics are surface properties (hydrophilic-hydrophobic), pore-size distribution, and electrical resistance. To ensure adequate sensitivity and response a sensor of this type should consist of a very thin film of porous ceramic with a porosity >30%. The contacting electrodes may be interdigitated or porous sandwich-type structures made from noble metals (e.g., Pd, Pt, Au) and so constructed that they do not obstruct the pores of the oxide film. Humidity affects not only the resistance of a porous ceramic but also its capacitance by extending the surface area in contact with the electrodes. The high dielectric constant of adsorbed water molecules also plays an important role. 

Traditional powder metal bearings (Table 7) consist of bronze of 90% copper—10% tin. The common pore volume of 20—30% is usually impregnated with an oxidation-resistant oil of SAE 30 viscosity (22). High porosity with high oil content is favored for higher speed, light load applications. Lower porosity with up to 3.5% added graphite is desirable for low speeds and oscillation where oil-film formation is difficult. 

It can be seen from Fig. lOthat above lO gion/l the dissolution rate is no longer proportional to the concentration of the oxidizing species. Surface films formed at this range apparently inter -fere with the diffusion mechanism. It is believed that here dissolution proceeds through the pores of the films, as diffusion through the films is unlikely under the present experimental conditions. Since the porosity and other physical characteristics of the film depend in a complex manner on the rate of the film formation, it is likely that at... 

The PS layer formed on a Si substrate is the most frequently used as a PS host. Sometimes, partial or complete modification of the PS layer (by annealing, oxidation, carbonization, etc.) is performed prior to pore filling or coating process [2]. Multi-layered PS structures (with different porosity) [8], self-supported PS films [9], and PS grains (PS layer separated from Si substrate and divided onto grains) [10] are used as well. 

Another unique approach toward low 6 is to disperse fine foams in PI films, since the e of air is unity. This technique developed by Hedrick et al. [208] typically involves the preparation of PS-PAA-PS (PS polystyrene) triblock copolymer, imidization, and finally higher temperature annealing where thermally labile PS block undergoes thermolysis (depolymerization) to form submicron pores. They utilized a variety of other thermally unstable block such as poly(a-methylstyrene), poly(propylene oxide), PMMA, poly(e-caprolactone), and aliphatic polyesters and examined the effects of chemical structure, fraction, and molecular weight of the block on the resultant morphology (pore size, shape, porosity) and dielectric and thermal, and mechanical properties. In this case, the resulting porous structure depends on the initial microphase separation domain structure of the thermally labile triblock. For example, nano-foamed PI (19% porosity) prepared from triblock consisting of PMDA-3F [3F = l,l-bis(4-amino-phenyl)-l-phenyl-2,2,2-trifluoroethane] (see Fig. 62 for its structure) and poly(propylene oxide) showed a considerably lower e = 2.3) than that of the non-porous homo PMDA-3F e = 2.9) [209]. 

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