Porosity effects Volume resistivity

The objective of this smdy is to be able to estimate the effective transport resistance of a porous medium by characterizing its void morphology by mercury porosimetry. A series of porous catalyst solids were obtained differing only in void morphology, overall porosity and pore sizes. We cahnilated the tortuosity by a dynamic experiment employing solid-gas chromato phy, SGC. Tortuosities of aU solids were very si ar, in the range of 5-25. Transport resistance is more easily related to overall volume porosity rather than specific network architectu features observable by porosimetry. 

Gel batteries require an additional separator to fix the plate distance and to prevent electronic shorts. The most effective protection against shorts is achieved by means of separators with low pore size ideally, microporous materials should be used (pore size less than 1 pm). Additionally, the separator should have a low acid-displacement since the fumed silica and the cracks in the gel already reduce the volume available for electrolyte. To minimize the internal resistance of the battery, the electrical resistance of the separator should be as low as possible. These two requirements, viz., low acid-displacement and low electrical resistance, translate into a need for separators with good wettability, high porosity, and low geometrical volume, i.e., rib configuration and backweb thickness should both be optimized. 

Finally, it is important to mention the effect of porosity. Since the thermal conductivity of air is negligible compared to the solid phases, the addition of large (>25 percent) volume fractions of pores can significantly reduce Ath. This approach is used in the fabrication of firebrick. As noted above, the addition of large-volume fractions of porosity has the added advantage of rendering the firebricks thermal-shock-tolerant. Note that heat transfer by radiation across the pores, which scales as has to be minimized. Hence for optimal thermal resistance, the pores should be small and the pore phase should be continuous. 

For process modeling proposes the effective chemical reaction rate has to be expressed as a function of the liquid bulk composition x, the local temperature T, and the catalyst properties such as its number of active sites per catalyst volume c, its porosity e, and its tortuosity t. As discussed in Section 5.4.2, the chemical reaction in the catalyst particles can be influenced by internal and external mass transport processes. To separate the influence of these transport resistances from the intrinsic reaction kinetics, a catalyst effectiveness factor p is introduced by... 

It might be expected that inclusion of small particles in a powder could decrease porosity if they fit into voids between larger particles. In practice, the opposite effect is usually observed. The explanation for this follows from consideration of the main factors that determine closeness of packing and flow properties of powders the size, shape, and surface properties of the particles. It is often found that the effects of surface properties outweigh the others because they govern the friction and adhesion between particles. As the size of particles decreases, the ratio of surface to volume increases, thus magnifying frichonal resistance. Other factors that may contribute to increased friction or stickiness are the presence of liquid films and electrical charge effects. 

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