Pores columnar porous

Figure 1.9 SEM images [34] of (a) original porous SiC surface after PECE, (b) early stage formation of columnar pore in cross-section, (c) porous surface structure 20 pm below the original surface after 90 min of RIE (the inset shows the Fourier transform of a larger area of this picture), and (d) the self-ordered columnar porous structure below the cap layer in cross-section. Reproduced from Y. Ke, R.P. Devaty and W.J. Choyke, Self-ordered nanocolumnar pore formation in the photoelectro-chemical etching of 6H SiC, Electrochem. Solid-State Lett., 10(7), K24-K27 (2007). Copyright 2007, with permission from The Electrochemical Society...

Based on the weight loss during the PEC process measured using the microbalance, the porosity is estimated to be 0.1. Because the columnar pores are generally uniform throughout the porous layer, it is reasonable to assume that the columnar porous structure we obtain is comparable with a perfect hexagonal packed cylindrical pore lattice. For such a pore lattice, the porosity P can be calculated using ... 

Figure 8.8 Plan-view transmission electron micrograph of columnar porous SiC with vertical pores of diameter 20 nm. Most of the SiC, except the top porous layer, was removed by polishing for this transmission image...

The other type of porous glass that has cylindrical pores is mesoporous silicate (MPS) (14,15). The advantage of MPS is in its feasibility to make a small pore diameter, typically below 10 nm. A columnar-phase liquid crystal, formed from surfactant molecules with a long alkyl chain tail and silicate molecules, is calcined to remove hydrocarbons. At the end, a hexagonal array of straight and uniform cylindrical holes is created in a crystalline order. MPS is not available commercially either. 

Porous anodic aluminum oxide (AAO) which characterized by a closely packed regular array of columnar cells is well-established and widely-used material for formation of nanostructures for SERS [2,3]. Particularly, promising SERS-active substrates were prepared by vacuum deposition of silver onto commercially available alumina filters with open pores of 200-300 nm diameters [4], Nanowires and nanorods have been fabricated by filling the AAO pores with transition- or noble-metals. However, due to multistage procedure these nanoarrays being sensitive are rather complicated in fabrication. 

FIGURE 6.13 Schematic representation of a section parallel to the pore axis of an elongated, columnar cell of a porous anodic alumina film. 

Porous anodic alumina attracts an attention of a scientific community due to an ordering nanostmcture resulting from self-regulated electrochemical anodic process. Aluminum is a unique material, which forms a regular porous oxide, composed of a packed array of columnar hexagonal cells, each having a cylindrical pore in the center [1], The cell size is known to be determined by the electrolyte and anodization regimes [1,2]. We have found that the cell size can be also tuned by Ti in Al-Ti alloys. The results obtained are presented in this paper. 

The main disadvantages of the mechanieal proeesses in some applications could be the isotropic character of the pore distribution and morphology, nonuniform grain size, and the impossibility to achieve regular columnar pores, perpendieular to the surface, as in the ease of electrochemically etched wafers (see handbook ehapter Porous Silieon Formation by Anodisation ). 

M.M. Yusuf et al. have reported mesoporous Ti02 films with controlled pore sizes prepared by smfactant templating sol-gel method and its photocatalytic activity. The porous films of anatase with rectangular and columnar structure were obtained by annealing after immersion of the wet gel films in benzyltrimethylammonium chloride- and cetyltrimethyl-ammonium chloride-solutions, respectively. They concluded that the microstructure of the sol-gel derived films can be controlled by changing the type of surfactant species and the concentration of surfactant solutions. The photocatalytic activity of their films for the oxidation of NO , was improved by the surfactant modification (Yusuf, 2002). 

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