Formation of macropores

Column sorbents for aqueous media show )ust average properties. This is due to the different copolymerization process, which does not allow easy formation of macroporous beads with proper pore topology. This fact also reflects many experiences of SEC users, who have to input much more effort to get good aqueous SEC work accomplished. 

Photogenerated carriers are needed for the formation of macropores in n-type electrodes, as discussed in Chapter 9. 

For p-type electrodes with doping densities below 1018 cm-3 diffusion and thermionic emission of charge carriers across the SCR is dominant. For p-type doping densities below 1016 cm4 this charge transfer is associated with the formation of macropores, as discussed in Chapter 9. 

This section is devoted to the formation mechanisms that have been proposed as being responsible for formation of macropores on p-type and n-type silicon electrodes. 

Macropore formation on p-type silicon electrodes was first observed for anodization in water-free mixtures of anhydrous HF and an organic solvent [Pr7, Ril]. Later it was observed that organic HF electrolytes with a certain fraction of water [Pol, We5], or even non-organic, aqueous HF electrolytes [We2, Le21], are also sufficient for the formation of macropores on p-type Si electrodes. This indicates that macropore formation on such electrodes cannot be ascribed to the chemical iden-... 

A general three-step procedure for the formation of macroporous materials by colloidal crystal templating is illustrated in Figure 6. In the first step, monodispersed colloidal spheres assemble into ordered 3D or sometimes 2D arrays to serve as templates. Secondly, the voids of colloidal crystals are filled by precursors that subsequently solidify to form composites. Finally, the original spheres are removed, creating a solid framework with interconnected voids, which faithfully replicate the template arrays. 

FIGURE 8.25. The relationship between pore diameter and n-type doping density of the silicon substrate for stable formation of macropore arrays. The upper and lower limits of stable pore formation are shown as a function of substrate resistivity (dashed lines). (Reprinted from Lehmann and Griining. 1997, with permission from Elsevier Science.)... 

Thus, in general, the dissolution of silicon at an anodic current results in three regions of the silicon substrate which is exposed to the electrolyte as shown in Fig. 8.41. Such an etched layer prior to the initiation of pores is involved in all types of PS because the etching canses roughening of the surface, which is required for pore initiation. The etching phase is associated with the formation of macropores on both n and p types of silicon and with formation of micropores on For example, for n-... 

It is important to note that although surface defect sites are associated with the initiation of pores, they do not determine the density and dimension of the pores in the bulk PS. The bulk morphology of PS is determined by the property of semiconductors and anodization conditions. However, under certain conditions such as those for the formation of macropores on lowly doped materials, control of the initiation sites by surface patterning can to some extent change the PS morphology. 

Two-Layer PS. Two-layer PS with a micro PS on top of a macro PS layer is formed on lowly doped p-Si or illuminated n-Si. For lowly doped p-Si, two-layer PS can form when the conditions are such that the space charge layer and the resistive layer differ in dimension by several orders of magnitude and both are significantly involved in the rate-limiting process due to the effect of surface curvature on the current flow near the surface and in the substrate. For n-Si, two-layer PS can form on a front-illuminated substrate as long as the conditions exist for the formation of macropores. It may also form in the dark under conditions similar to those for the formation of two-layer PS on lowly doped p-Si. 

In all but extreme climates, the upper portion of the soil profile is extensively occupied by plant roots, which remove both water and mineral nutrients. Plants and other biota (such as insects and small mammals) create extensive networks of voids often referred to as macropores, which result in a heterogeneous, biporous (i.e., there are two porosity values, for micro- and macropores), and structurally very complex material. Macropores (and pipes, which are larger, continuous macropores) can play a significant role in water transport, although the exact role of flow through macropores versus flow through the rest of the soil matrix is not completely understood. For an overview of the types and mechanisms of formation of macropores, the reader is referred to Beven and Germann (1982). 

Takadama H, Kim HM, Koknbo T, Nakamma T (2001a) An X-ray photoelectron spectroscopy stndy of the process of apatite formation on bioactive titanium metal. J Biomed Mater Res 55 185-193 Takadama H, Kim HM, Kokubo T, Nakamttra T (2001b) TEM-EDX study of mechanism of bonelike apatite formation on bioactive titanirrm metal in simulated body flttid. J Biomed Mater Res 57 441-448 Takagi S, Chow LC (2001) Formation of macropores in calcium phosphate cement implants. J Biomed Mater Res 12 135-139... 

Formation of macroporous copolymers in the presence of precipitating diiuents... 

The third approach to the formation of macroporous structure of styrene— DVB copolymers consists of adding a linear polymer, largely polystyrene, to the monomer mixture. 

This book contains 17 chapters and is divided into 3 parts. Chapters 1-5 deal with the more fundamental aspects of known types of polystyrene networks, like type of networks, the formation of macroporous polymers, the preparation of continuous polymeric beds and the properties of ion-exchange resins, among other examples. 

The other paste components mentioned in the subtitle of this section have analogous effects. Moreover, they promote formation of macropores in NAM, thus facilitating the movement of ions into the plate interior. All these functions of the additives stabilizing mechanically the structure of NAM make them an indispensable component of the paste formulations for negative battery plates. Usually, polypropylene fibres with a length of 3 mm are introduced in amount within the range 0.075—0.08 wt% versus the leady oxide used for paste preparation. 

Takagi S, Chow LC. Formation of macropores in calcium phosphate cement implants. J Mater Sci-Mater M. 2001 Feb 12(2) 135-9. 

FIGURE 3.55 Formation of macroporous starPEG-heparin cryogels by combined cryotreatment of the aqueous gel-forming reaction mixture and lyophiUzation of the incompletely frozen gel. Yellow rods, heparin grey crosses, starPEG. Source Welzel et al. [78], Reproduced with permission from American Chemical Society. 

Another model proposed by Kohl and coworkers [77, 78] is based upon the strain - induced preferential etching described earlier. The model accounts for the formation of macropores and highly branched micropores when the silicon is rendered porous in either nonaque-ous or aqueous HF solutions, respectively. 

In the case of a-SiH and Sii xCx, the thickness of the nanoporous layer is limited by the formation of macropores, an instability of the growth front attributed to the high resistivity of the starting material [160]. 

Porous photoetching of -type GaP in H2SO4/H2O/H2O2 leads to the formation of macroporous GaP (random networks of single crystalline GaP with pore sizes of 150 nm and porosity of 35 to 50% for porous etched and porous photoetched, respectively), which appears to be the most strongly scattering material in visible light reported to date and localization effects are anticipated [224, 225]. 

If formation of macroporous hcaieycomb structures using polymers, nanocomposites, and hybrids have been described, only few examples of honeycomb films based on new building blocks introduced via the different methods described above have been recently achieved as functional materials. Indeed, functional honeycomb film with self-assembled Horseradish peroxidase (HRP) enzyme nanogels at the pore walls for biocatalysis can be mentioned for clinical diagnostic kits and for immunoassays [193]. 

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