Pores macro

Pore size optimization is one area where developmental efforts have been focused. Unimodal pore (NiMo) catalysts were found highly active for asphaltene conversion from resids but a large formation of coke-like sediments. Meanwhile, a macroporous catalyst showed lower activity but almost no sediments. The decrease of pore size increases the molecular weight of the asphaltenes in the hydrocracked product. An effective catalyst for VR is that for which average pores size and pore size distribution, and active phase distribution have been optimized. Therefore, the pore size distribution must be wide and contain predominantly meso-pores, but along with some micro- and macro-pores. However, the asphaltene conversion phase has to be localized in the larger pores to avoid sediment formation [134],... 

Macro pores covered with micro PS on p-Si reported33... 

Conditions for macro PS formation on p-Si55 Micro PS on macro pore walls ... 

Deep straight array of macro pores on n-Si with back illumination20... 

Morphology, which is determined by the distribution of materials in space, is the least quantifiable aspect of a material. It is thus very difficult to characterize morphology of PS, which has extremely rich details with respect to the range of variations in pore size, shape, orientation, branch, interconnection, and distribution. Qualitatively, the diverse morphological features of PS reported in the literature can be summarized by Figure 9 with respect to four major different aspects pore orientation, fill of macro pores, branching, and depth variation of a PS layer. 

The pores at the surface are smaller than those in the bulk of PS as, for example, shown in Figure 11, 8,16,24 Such an increase in pore diameter from the surface to bulk is due to the transition from pore initiation to steady growth. Also, two-layer PS, a micro PS layer on top of a macro PS can form for on illuminated n-Si or on lowly doped p-Si. For the micro PS layer formed on front illuminated n-Si, pore diameter is less than 2 nm and thickness of PS changes with illumination intensity and the amount of charge passed. Also, the diameter of macro pores on front illuminated n-Si changes with the amount of charge Passed.20 Pore size and depth variation of PS on n-Si are very different for front and back illuminated n-Si samples. 

The orientation of primary pores is in general in the <100> direction for all the PS formed on all types of (100) substrates.7,14,84 For the PS with dendritic structure, as shown in Figure 12, pores propagate along (100) direction even on the (110) and (111) substrates.20,84,94 The branched pores of non-dendritic types formed on (100) substrate may not be strictly perpendicular to the primary pores but deviate to various extents from the <100> direction toward to the source of holes.12,14,95 Ronnebeck et al.95 found that the macro pores formed on... 

For pores of extremely small diameters, in the order of a few nm, the direction of individual pores is totally random. On the other hand, large pores tend to have less anisotropic effect and grow more dominantly in the direction of carrier supply, that is, perpendicular to the surface. The macro pores formed on p-Si generally have smooth walls and an orientation toward the source of holes that is perpendicular to the surface, even on (110) and (111) samples.34,39... 

There is a correlation between the occurrence of two-layer PS and the saturation photo current value.78 Only a single micro PS layer forms at a photo current density below the photo saturation value while two-layer PS forms at current densities above the saturation current as shown in Figure 17. Also, macro PS layer forms only after a certain amount of charge determined by the amount of etch required for the initiation of macro pores.77... 

Figure 18. Three layers are resulted after formation of PS on illuminated n-Si sample an etch layer, a micro PS layer and a macro PS layer. The walls of the macro pores may also be fully or partially covered with micro PS.

The PS formed on p-Si generally consists of micro pores, but when the resistivity is above a certain value macro pores can form underneath a layer of micro pores. The resistivity, at which this occurs, depends on type of solvent, HF concentration and... 

Well aligned macro pores, > 50 nm are perpendicular to the surface for (100)... 

In PS of two distributions, macro pores may be partially or fully filled by micro... 

PS with macro pores has the same composition and crystalline structure as the... 

Figure 28. Schematic illustration of formation of a macro pore partially filled by a micro PS on n-Si under illumination.

Figure 29. Schematic illustration for different types of PS and the current distributions (a) void macro pore (b) micro PS (c) macro pore filled with micro PS (d) macro pore partially filled with micro PS. 

Macro pores on p-Si controlled by resistivity of Si relative to electrolyte35 I... 

Macro pores on p -Sidue to hole diffusion in space charge layer39... 

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