Micropore Formation Mechanism

This section is devoted to the mechanism responsible for the formation of micropores on silicon electrodes. Micropore formation on silicon electrodes illustrates the importance of the pore position with respect to the neighboring pores, i.e. the 

An extension of this QC model, including tunneling probabilities between the confined crystallites and the bulk, has been developed [Fr6]. The QC model for microporous silicon formation, however, is still qualitative in character, and a quantitative correlation between anodization parameters and the morphology and properties of the porous structure is at yet beyond the capability of the model. 

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Polyethylene (PE) and polypropylene (PP) have high porosity, low resistance, high tensile strength, good acid and alkali resistance, good elasticity, etc. Thus, the commercial membranes of lithium or lithium-ion battery are mainly made of PE or PP. The membranes of lithium batteries can be produced by dry method and wet method according to the different preparation technology. Their membrane micropore formation mechanisms are different [53,54]. 

The average pore size of PS structures covers four orders of magnitude, from nanometers to tens of micrometers. The pore size, or more precisely the pore width d, is defined as the distance between two opposite walls of the pore. It so happens that the different size regimes of PS characterized by different pore morphologies and different formation mechanisms closely match the classification of porous media, as laid down in the IUPAC convention [Iu2]. Therefore the PS structures discussed in the next three chapters will be ordered according to the pore diameters as mostly microporous (d<2 nm), mostly mesoporous (2 nm50 rim). Note that the term nanoporous is sometimes used in the literature for the microporous size regime. 

If the pore density is plotted versus the doping density of the silicon electrode, it can be seen that the micropore density is independent of doping, while the macropore and mesopore densities increase linearly with doping density, as shown in Fig. 6.10. This is a consequence of the QC formation mechanism being independent of doping, while the SCR-related mechanisms are not, as discussed in Section 6.2. 

Frcilich, D. and G. B. Tanny. 1978. The formation mechanism of dynamic hydrous Zr (IV) oxide membranes on microporous supports. J. Coll. Interface Sci. 64(2) 362-70. 

Numerous techniques have been applied for the characterization of StOber silica particles. The primary characterization is with respect to particle size, and mostly transmission electron microscopy has been used to determine the size distribution as well as shape and any kind of aggregation behavior. Figure 2.1.7 shows a typical example. As is obvious from the micrograph, the StOber silica particles attract a great deal of attention due to their extreme uniformity. The spread (standard distribution) of the particle size distribution (number) can be as small as 1%. For particle sizes below SO nm the particle size distribution becomes wider and the particle shape is not as perfectly spherical as for all larger particles. Recently, high-resolution transmission electron microscopy (TEM) has also revealed the microporous substructure within the particles (see Fig. 2.1.8) (51), which is further discussed in the section about particle formation mechanisms. 

Tsai, F.-J. Torkelson, J. M., "Microporous Poly(methyl methacrylate) Membranes Effect of a Low-Viscosity Solvent on the Formation Mechanism," Macromolecules, 23, 4983 (1990). 

Yamazaki S and Tsutsumi K. Synthesis of A-type zeolite membrane using a plate heater and its formation mechanism. Micropor Mesopor Mater 2000 37 67-80. 

In conclusion, the employment of templates provides a feasible strategy to prepare microporous compounds with special morphologies. However, the formation mechanism of these compounds is still not very clear, and more work on this subject should be carried out. 

The use of the latest experimental analysis and detection techniques including liquid/ solid NMR, XRD, electron diffraction, and in situ analysis is very necessary for the studies on the roles of the SDAs in the crystallization process of microporous materials, which could help us gain a better understanding of the formation mechanism of the pore systems and reveal the real correlation between guest molecules or ions and the resultant frameworks. 

Zhang, J. and Goldfarb, D., In situ investigations of the formation mechanism of mesoporous materials via the dynamics and ordering of spin-probes — pH and Si/surfactant effect, Microporous Mesoporous Mater., 48, 143, 2001. 

The Formation Mechanism of Microporous Symmetric or Asymmetric Membranes. The original recipes and subsequent modifications for preparing asymmetric membranes are deeply rooted in empiricism. Detailed descriptions of membrane preparation techniques are given in the literature.2 30 Only after extensive use of the scanning electron microscope, which provided the necessary structural information, was it possible to rationalize the various parameters for membrane preparation processes. At first, the asymmetric structure was obtained mainly in membranes made from cellulose acetate. But later it became ap-... 

The Phase Separation Process and Its Relation to the Formation Mechanism of Microporous Membranes... 

General Observations Concerning Structures and Properties of Phase Inversion Membranes. Before going into any detailed discussion of the formation mechanism of microporous membranes, several general observations concerning the membrane structure, preparation procedures, and mass transport properties are described. 

Series of Special Issues cover various aspects of microporous molecular sieves, metal-organic frameworks and ordered mesoporous materials. Synthesis principles, templating, formation mechanisms, characterization methods, functionalization strategies, and applications are discussed in excellent and comprehensive review articles and more specific research reports. 

Both microporous manganese oxides " and the Mo-V were prepared by heating aqueous solutions of the metal precursors. In order to produce pure samples, conditions (pH, temperature, reaction time, and concentration of metal precursor) must be carefully controlled. Further investigation is needed to demonstrate the importance of these bifunctional (redox and microporosity) materials for understanding the formation mechanism of these microporous materials and for the development of a new strategy to form other microporous transition metal oxides. 

A. Laybourn, R. Dawson, R. Clowes, T. Hasell, A. I. Cooper, Y. Z. Khimyak and D. J. Adams, Network formation mechanisms in conjugated microporous polymers, Polym. Chem., 2014, 5(21), 6325-6333. 

In some applications, porous silicon (PSi) is, however, desired to increase the specific surface area (up to 1000 mVcm ). Several books and reviews have been already published about PSi formation mechanisms, morphologies and optical properties (cf [130-133]). Briefly, PSi is usually prepared from (100) Si wafers by constant-current anodization in an ethanolic solution (mixture of HF with ethanol). The characteristics of the pores are determined by the doping of the substrate, the HF concentration and the cturent density used during the anodization process [134-139]. PSi surfaces present =Si-H, =SiH2> and trihydride (-SiHj) sites. Potential steric hindrances could appear most hkely when micropores formation is obtained. It should be mentioned that PSi received increasing interest in the 1990s because of its luminescence properties in the visible range. 

Asahi Chemical Industry carried out an exploratory investigation to determine the requirements for cellulose based separators for lithium-ion batteries. In an attempt to obtain an acceptable balance of lithium-ion conductivity, mechanical strength, and resistance to pinhole formation, they fabricated a composite separator (39—85 /cellulosic fibers (diameter 0.5—5.0 /pore diameter 10—200 nm) film. The fibers can reduce the possibility of separator meltdown under exposure to heat generated by overcharging or internal short-circuiting. The resistance of these films was equal to or lower than the conventional polyolefin-based microporous separators. The long-term cycling performance was also very comparable. 

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