Disordered pores

Prior to the 1990s, materials with pore size in the meso range such as silica gels and activated carbons show disordered pore structme with a broad distribution of pore size. In 1992, by using cationic smfactants as template, Mobil scientists... 

On the other hand, the peak shape may reflect the distribution of pore volume over pore size of the samples For MCM-41 samples, sharp peaks are observed This means that filling of all mesopores occurs at almost the same relative pressure In contrast, for the commercial mesoporous silica, of which a disordered pore stmcture is known, a broad peak on the differential curve is observed, reflecting a wide pore size distribution. 

Pang12831 successfully synthesized millimeter-sized mesoporous Co-SiC>2 spheres through the introduction of Conl-(3-aminopropyl)trimethoxysilane (APTMOS) complex into the reaction mixture with CTAB as template in basic media. The mesoporous Co-Si02 spheres have a high surface area and disordered pores. 

Macroporous titania films have been prepared on titanium surfaces [32] using PS spheres (0.5,16, and 50 pm in diameter) with TiOj particles (39 nm). A slurry of the mixed PS and Ti02 particles was spread on the titanium substrate and dried slowly (1 day). After heating to 450-950°C, titania with a disordered pore structure was obtained with a thickness of 1 to 0.1 mm on the titanium. 

Figure 2.6 Scheme of the mechanism for the formation of mesoporous silica. Silica polymers formed initially from silica monomers, and associated with surfactant monomers, which form composite self-organised primary particles which can either continue to grow via monomer addition (path 1) or themselves aggregate in a directional fashion (path 2) to form the final mesophase composite. Nondirectional aggregation would cause formation of disordered pore structures. Reprinted with permission from Nooney, R.I. Thirunavukkarasu, D. Chen, Y. Josephs, R. Ostafin, A.E., Synthesis of Nanoscale Mesoporous Silica Spheres with Controlled Particle Size, Chem. Mater., 14, 4721—4728. Copyright (2002) American Chemical Society... 

Other notions that are very important in consideration of pore systems are percolation and percolation threshold. The permeability of the system composed of strongly disordered pores is non-linearly related to pore number and total volume, and that relation may be characterized by a threshold - as long as a certain pore density is not reached, the system remains impermeable. When the density becomes higher than that critical value, then permeability appears and increases rapidly (Stauffer 1986). As this concerns the connections between pores, these may be distinguished interconnected pores, closed pores and pores closed at one end. The roles of these kinds of pores in the flow of fluids and gases across material are different. 

The first synthesis of mesoporous mbced oxides was reported by Schwickardi et al. [64]. These authors reported the synthesis of a series of phase-pure spinels (ZnCr204, CoCr204, and NLA.I2O4) and LaFeOs perovskites with disordered pore structure using activated carbon as the hard templates and metal nitrates as... 

It is not clear how two phases coexist in disordered pores as alternating domains or as two infinite networks. Disordered porous materials with low porosity are more reminiscent of interconnected cylindrical pores and therefore a domain structure seems to be more probable [299, 311-315]. In highly porous materials, such as highly porous aerogels, infinite networks of two coexisting phases may be assumed. The critical point of fluids in disordered pores is expected to belong to the universality class of the random-field Ising model [316-318]. 

---