Pore structure porous glass

Systems used in practice have a spongy structure (porous glass or carbon) or have the structure common in ceramic membranes. The latter have an interconnected, tortuous and randomly oriented pore network with constrictions and dead ends (Fig. 9.1) and are formed by packing of particles. 

Recently, many experiments have been performed on the structure and dynamics of liquids in porous glasses [175-190]. These studies are difficult to interpret because of the inhomogeneity of the sample. Simulations of water in a cylindrical cavity inside a block of hydrophilic Vycor glass have recently been performed [24,191,192] to facilitate the analysis of experimental results. Water molecules interact with Vycor atoms, using an empirical potential model which consists of (12-6) Lennard-Jones and Coulomb interactions. All atoms in the Vycor block are immobile. For details see Ref. 191. We have simulated samples at room temperature, which are filled with water to between 19 and 96 percent of the maximum possible amount. Because of the hydrophilicity of the glass, water molecules cover the surface already in nearly empty pores no molecules are found in the pore center in this case, although the density distribution is rather wide. When the amount of water increases, the center of the pore fills. Only in the case of 96 percent filling, a continuous aqueous phase without a cavity in the center of the pore is observed. 

Finally, we described the permeation characteristics of a thermosensitive gel supported on porous glass. The switch functional ability of the membrane was demonstrated in permeation experiments. It was pointed out that the change in the permeation characteristics resulted from that in the pore structure in the gel. 

Fukasawa, T., Ando, M., Ohji, T. Fabrication of porous ceramics with complex pore structure by freeze drying process. Ceram. Trans. 112, Innovative Process-ing/Synthesis Ceramics, Glasses and Composites IV, 217-226, 2001... 

In contrast to many other nanomaterials, silica nanoparticles do not acquire any peculiar property from their submicrometric size, except for the corresponding increase of the surface area. As a matter of fact, they can simply be regarded as extremely small and highly porous glass spheres. What makes silica nanoparticles very interesting from the supramolecular point of view is the presence of a well-defined structure with compartments (bulk, surface, pores, shells, etc.) that can be rather... 

The third relaxation process is located in the low-frequency region and the temperature interval 50°C to 100°C. The amplitude of this process essentially decreases when the frequency increases, and the maximum of the dielectric permittivity versus temperature has almost no temperature dependence (Fig 15). Finally, the low-frequency ac-conductivity ct demonstrates an S-shape dependency with increasing temperature (Fig. 16), which is typical of percolation [2,143,154]. Note in this regard that at the lowest-frequency limit of the covered frequency band the ac-conductivity can be associated with dc-conductivity cio usually measured at a fixed frequency by traditional conductometry. The dielectric relaxation process here is due to percolation of the apparent dipole moment excitation within the developed fractal structure of the connected pores [153,154,156]. This excitation is associated with the selfdiffusion of the charge carriers in the porous net. Note that as distinct from dynamic percolation in ionic microemulsions, the percolation in porous glasses appears via the transport of the excitation through the geometrical static fractal structure of the porous medium. 

Patel ct at. (P5I) moist-cured blocks of cement pastes of w/c ratio 0.59 for 7 days and then sealed the prism surfaces of each and exposed the ends to air at 20 C and 65% RH. Methanol sorption data obtained using porous glass as a reference standard (Section 8.3.5) showed that, near to the exposed surfaces, the pore structure was markedly coarser and the diffusion time lower. TG evidence showed that less hydration had occurred and that carbonation was increased. These effects were detectable even at a distance of 50 mm from the exposed surface. Further work conlirmed the marked effect of RH on hydration rates (P28) (Section 7.7.1). 

Figure 15.10. (a) Pore structure in a Shirazu Porous Glass (SPG) membrane, featuring highly tortuous pores, (b) Top-layer structure of a ceramic alumina membrane, made by sintering ceramic particles together. 

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