Macroporous walls

Figure 3. Schematic representation of the micro- and nanoscale morphology of gel-type (a) and macroreticular (b) resins [13], Level 1 is the representation of the dry materials. Level 2 is the representation of the microporous swollen materials at the same linear scale swelling involves the whole polymeric mass in the gel-type resin (2a) and the macropore walls in the macroreticular resin (2b). The morphology of the swollen polymer mass is similar in both gel-type and macroreticular resins (3a,b). Nanopores are actually formed by the void space surrounding the polymeric chains, as shown in level 4, and are a few nanometer wide. (Reprinted from Ref [12], 2003, with permission from Elsevier.)...

Because the SCR width depends not only on doping density but also on bias, the average pore density is expected to decrease with the square root of bias. This, however, is not observed. An increase in bias often leads to the formation of breakdown-type mesopores at the macropore walls. Because these spiking pores show diameters on the order of a few tens of nanometers they are hard to identify even in an SEM. Spiking pores can be identified if they are enlarged by subsequent chemical etching, as shown in Fig. 8.10. Details of their branched morphology become visible by formation of an oxide replica and after etchback of the sub-... 

The macropore walls are covered by micro PS, indicated by a broken line at the interface to bulk Si. (i, h) Sectional views of the pore shown in (g) at positions h and i. (i) Note that the pore tip is free of micro PS. 

A bare monolithic structure can be coated with a catalyst support layer in several ways. Figure 21 shows a SEM image of a typical commercial cordierite monolith structure. Washcoating can be done by (partly) filling the pores of the macroporous walls with the washcoat material or by depositing a washcoat as a layer on top of the walls. These methods are shown schematically in Figure 22. 

Those monoliths can be produced from a piece of structured foam polymer with macropores. The piece of polymer is soaked in a sol that will form a ceramic of the desired material after heat treatment. The sol-soaked structure is dried, and it is burned at a suitable temperature to remove the polymer. The remaining structure will be a ceramic one with macropores, permitting the wall-flow of gases. This technique is also used to produce heat plates and pipes with macroporous walls for gas separation purposes. The polymers used are often derived from polyurethanes [45-46]. 

PS in nonoxidizing electrolyte such as acetonitrile (MeCN), whereas they are not filled in oxidizing electrolytes such as DMF. However, it has been reported that the PS on p-Si (13 Qcm) formed in anhydrous HF-MeCN solution has only the macropore layer with no nanometer PS covering the surface of the macropore walls and the sample surface. ... 

Dual template synthesis combined with a modified bulk sol-gel process can be used to prepare the 3-D bimodal ordered porous silica, in which the macropore wall is mesoporous and both the pores are interconnected. The macropores were replicated... 

In order to produce a three-dimensionally ordered macroporous structure, the metal precursor solution should fulfil the following criteria (1) Sufficient metal should be present in the voids to form a macroporous wall, and the metal concentration should therefore be high. (2) Reactivity of the metal precursor should be mild so that it can infiltrate the voids. If the metal precursor reacts with a functional group on the surface of the template or moisture in the air before it infiltrates the voids, an ordered porous structure cannot be obtained. (3) The metal precursor should be solidified in the voids before the template is removed and the produced... 

A recent systematic study of macropore formation performed on various doped n-type Si substrates with rear illumination, by Foil and coworkers [106] showed that a strong influence of the SCR on the average macropore density is indeed observed in accordance with the Lehmann model [72] (i.e. an increased anodic bias decreases the density of pores), except for highly doped Si. It was observed that an increasing anodic bias increases the pore density, in contrast to the prediction. The pore growth seems to be dominated by the chemical-transfer rate and most likely calls for a chemical passivation mechanism of the macropore walls. 

The SEM image sample surface analysis and porosity measurements lead to the conclusion that smaller micro- and mesopores are present on the macropore walls (see Figure 7 and Figure 8). The ASAP analysis made on the unetched samples did not reveal micro- and mesopores with diameter <100 nm - so the surface is relatively continuous and flat (in the ASAP analysis the large pores observed on Figure 6 were not taken into account). 

Sponge-Uke cryogels with a [30, 91] rather brittle macropore walls were obtained... 

Sponge-like cryogels were [30, 91] prepared the fragility of macropore walls increased with the growth of initial polymer concentration... 

Thus, in the native hydrated state, polymer- and protein-based hydrogels produced by cryogelation are characterized by a large macropore volume but a small specific surface area of macropore walls, high pore interconnectivity, and high hydrophilicity. The main portion of water in macroporous hydrogels can be attributed to bulk water located in macropores. The amount of bound water located... 

Figuer 14.10. Typical structural evolution of a smeltable RF-MOx system. Scale bars white, 5 pm black, 2 pm. Arrow points to the xerogel-like structure inside the macroporous walls [53]. 

In the procedures considered above, the structure is subjected to a global treatment. A local transformation of the macroporous structure requires that a pattern should be created in it. The simplest operation is local pore opening. For this purpose, a mask is created on the backside of an oxidized (or Si3N4-coated) structure by photolithography, the silicon substrate is anisotropically etched to a depth required for reaching the pore tips, and the oxide protecting the macropore walls is... 

The zirconia meso-macroporous particles used as catalyst support in this work have a size of around 10pm [7,8]. The synthesized zirconia particles contain regular arrays of macropores having a diameter range from 300 to 500 nm. The hollow macrochannels are always orthogonal to the face of the monolithic particle. Moreover, macroporous walls are mesostructured with a disordered wormhole like assembly of meso/micropores into macroporous framework. 

Subsequently, it was found that the surfactant in the synthesis system can modify the mesoporous textural properties but does not affect the self-formation of the meso-macroporous hierarchy. Zirconium oxides with the similar meso-macropo-rous structure could also be synthesized via the self-formation phenomenon in the absence of a surfactant [131]. The resulting hierarchically meso-macroporous zir-coniiun oxides are mainly tens of micrometers in size with a regular array of parallel or funnel-like macrochannek of 300-800 nm in diameter. The macropore walls are constmcted from mesochannels (around 2.0 nm) with a wormhole-like array. These materials obtained via this self-formation phenomenon present high surface area and pure crystalline phases in meso-macroporous structures. 

Recently, the self-formation phenomenon used for the formation of hierarchically porous metal oxides was ako applied to the formation of hierarchically micro-macroporous niobium oxides [12,141,144]. These synthesized niobium oxide particles with amorphous nature are mainly 100 in size with a regular array of parallel macropores (Figure 32.13a). The macropores with the pore size in the range of 0.3-10 pm extend through almost the whole particle. The macropore walls are constructed from accessible micropores. The hierarchically porous niobium oxides can be obtained via a self-formation phenomenon under different pH values (2, 7, and 12) [141,144]. 

Meso-Macroporous Yttrium Oxides The self-formation phenomenon was also used for the preparation of hierarchically porous yttriiun oxides by a controlled polymerization of yttrium butoxide in aqueous media [12,141,144], The synthesized yttrium oxides are 0.5-2 pm in size and are covered by a smooth surface. The fissure particles with funnel-hke and parallel macrochannels below the smooth surfeice were observed by higher resolution SEM observations (Figure 32.13b). The yield of the synthesis as well as the amount of macropores per particle continuously decreases with increasing initial synthesis pH values. The macropore diameters are 1-5, 2-8, and 5-10 pm for syntheses carried out in acidic, neutral, and basic media, respectively. The macropore walls are formed by a regation of mes-ostructured nanoparticles giving a supplementary interparticle porosity centered at 30 nm. A third level of porosity is demonstrated by the inhomogeneous pores centered at 3-7 nm for syntheses in acidic and neutral media and 5-15 nm in an alkaline medium. As for all the previously described compositions, the meso-macroporous yttria structures are amorphous at the atomic scale. 

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