Porous structure formation

In this case, the mechanism of the porous structure formation can be subdivided into several stages that differ from one another by the gel particle size, composition of gel, and particle arrangement. Undoubtedly, these parameters will change with composition of the solution. In particular, at a high concentration of the first component (with a lower pH of precipitation) and insignificant content of the second, completely adsorbed on gel particles, precipitated by the first component, the structure of the adsorbent obtained will be represented by gel partially encapsulated by the other component. As the concentration of the second component increases, the number of encapsulated particles will... 

Such systems and the mechanism of the adsorbent porous structure formation can be used for purposeful modification of the nature of their surfaces and the value of their sorption capacity. In this case, during calcinating of a sample, the modifying additive can interact with the prevailing component, forming a new crystalline compound with properties differing essentially from those of the initial compounds. This fact opens unrestricted possibility of changing the chemical nature of the solid surfaces and, consequently, their adsorption and catalytic properties. 

The reason for these changes and causative factors will be considered in more detail later here. Therefore, we will not dwell on this matter and only note that apart from the factors considered, the character of the porous structure formation is affected essentially by the nature of hydroxide, the shape of its particles, tendency for crystallization, etc. In other words, the action of the controlling factor (the pH of hydroxide precipitation), responsible for structure formation and shape of the Vg-composition curve, is also affected... 

Apart from examination the porous structure formation of binary co-precipitated adsorbents as a function of pH of initial and final precipitation of hydroxide components [4,15,20] it was interesting to investigate more complex compositions and the mechanism of their structure formation. Komarov, Repina and Skyrko [21] studied the three-component systems Fe(OH)3 - Zn(OH)2 - Cu(OH)2, Fe(0H)3 - Al(OH)3 - Zn(OH)2 and Fe(OH)3 -Cr(OH)3 - Ni(OH)2. The fact that in the first of the systems the pH of initial and final co-precipitation of zinc and copper coincide is a characteristic feature of these systems. In this case the -composition curve is represented by a straight line connecting the sorption capacities of individual components [15]. In the first and third systems pH values of initial and final precipitations of hydroxide of all the components of the mixture do not coincide (Tables 1 and 3). This selection of compositions allows the mechajiism of co-precipitation of hydroxide components and relations of porous structure formation of adsorbents obtained to be followed. 

In order to understand the porous structure formation of adsorbents co-precipitated from both three and four-component systems, succession of formation of hydrogels and... 

Apart from the investigations of the relations governing the porous structure formation of coprecipitated adsorbents in terms of the pH of initial and complete precipitation of hydrogels, it was of interest to find out how the temperature of co-precipitation of hydrogels and freezing of salt solutions and hydrogels affects the structure of adsorbents produced. 

Note that here the formation of nanoporous structure in crystalline matrix of the anodized semiconductor is considered as a primary event, which results from the specific physical processes caused by current flow localization. However, the contribution of chemical processes cannot be excluded either. In particular, coexistence of two competing mechanisms (porous structure formation and SiC dissolution) resulting in formation of non-SPSC, was proposed above. 

Rabelo D., and Coutinho Fernanda Margarida Barbosa. Porous structure formation and swelling properties of styrene-divinylbenzene copolymers. Eur. Polym. J. 30 no. 6 (1994) ... 

G. Elyashevich, A. Kozlov, L Meneva, Study of polyethylene orientation in the course of porous structure formation, Vysokomol Soyed B, 40,483-486,1998. 

The major design concept of polymer monoliths for separation media is the realization of the hierarchical porous structure of mesopores (2-50 nm in diameter) and macropores (larger than 50 nm in diameter). The mesopores provide retentive sites and macropores flow-through channels for effective mobile-phase transport and solute transfer between the mobile phase and the stationary phase. Preparation methods of such monolithic polymers with bimodal pore sizes were disclosed in a US patent (Frechet and Svec, 1994). The two modes of pore-size distribution were characterized with the smaller sized pores ranging less than 200 nm and the larger sized pores greater than 600 nm. In the case of silica monoliths, the concept of hierarchy of pore structures is more clearly realized in the preparation by sol-gel processes followed by mesopore formation (Minakuchi et al., 1996). 

On ferrierite, ZSM-22 and EU-1 zeolite catalysts, 10MR monodimensional zeolite structures (ID), the main reaction is the isomerization of ethylbenzene (figure la). ZSM-5, 10MR three-dimensional structure (3D) zeolite is very selective in dealkylation (90%) (figure lb) and no deactivation was observed within 8 hours of reaction. This particular selectivity of the zeolite ZSM-5 can be partly explained by the presence of strong acid sites and its porous structure that on one hand promotes the containment of molecules in the pores (presence of 8-9A cages at the intersection of channels) and on the other hand prevents the formation of coke and therefore pore blockage. 

Depending on the conditions of synthesis, copolymerization of divinyl/vinyl-monomers in the presence of an inert solvent leads to the formation of expanded (preswollen) or heterogeneous (porous) structures [54,99,100]. If the solvent remains in the network (gel) phase throughout the copolymerization, expanded networks are formed. If the solvent separates from the network phase the network becomes heterogeneous. According to Dusek et al., heterogeneities may appear in poor solvents due to the polymer-solvent incompatibility (x-induced syneresis), while in good solvents due to an increase in crosslink density (v-induced syneresis) [99]. 

The fundamental reason for the uneven distribution of reactions is that the rate of electrochemical reactions on a semiconductor is sensitive to the radius of curvature of the surface. This sensitivity can either be associated with the thickness of the space charge layer or the resistance of the substrate. Thus, when the rate of the dissolution reactions depends on the thickness of the space charge layer, formation of pores can in principle occur on a semiconductor electrode. The specific porous structures are determined by the spatial and temporal distributions of reactions and their rates which are affected by the geometric elements in the system. Because of the intricate relations among the kinetic factors and geometric elements, the detail features of PS morphology and the mechanisms for their formation are complex and greatly vary with experimental conditions. 

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. 

The electrochemical formation of porous structures is also observed for III—V semiconductors like GaP [Anl, Erl], GaN [Pe7, Myl], InP [Ki2, Kol6, Tal3, LalO] or GaAs [Be5, Fa4, Scl5]. Structural dimensions in the macroporous regime are observed for n-type GaAs of moderate doping (1017 cm4) anodized in KOH in the... 

Two types of structures were seen a needle type bundled structure pointing inward towards the center of the sphere and surface bumps. The needle type structures overall were less than 10 pm in length and less than 1 pm in thickness. To confirm that these structures corresponded to sodium alanate. elemental mapping of the alanate s sodium and aluminum constitutes was made. As can be seen in Fig.5, the presence of intense sodium distribution was noted on the needle structures while the both the surface bumps and needle structures showed the presence of aluminum distribution. These results confirmed spheres filling with the alanate. While these needle structures are not usual for sodium alanate. it is speculated that the porous silica could have provided a nucleation surface for these unique structures formation. Nevertheless, since the samples were exposed to ambient air prior to the... 

The porous membrane templates described above do exhibit three-dimensionality, but with limited interconnectedness between the discrete tubelike structures. Porous structures with more integrated pore—solid architectures can be designed using templates assembled from discrete solid objects or su-pramolecular structures. One class of such structures are three-dimensionally ordered macroporous (or 3-DOM) solids, which are a class of inverse opal structures. The design of 3-DOM structures is based on the initial formation of a colloidal crystal composed of monodisperse polymer or silica spheres assembled in a close-packed arrangement. The interconnected void spaces of the template, 26 vol % for a face-centered-cubic array, are subsequently infiltrated with the desired material. 

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