Closed pores

The presence of closed pores was demonstrated by Kozawa [22] by measuring the BET surface area of EMD samples of various particle sizes. Kozawa s new method for the determination of the closed pore is based on the relationship of the BET surface area and the particle size, by extrapolating the surface area value to zero particle size (Fig. 17). Table 8 shows the percentage of closed pores of various EMD samples. 

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Trapped gas in closed pores often limits densification when sintering witlr a liquid or viscous (glass) phase because rapid material transport tlirough tlie liquid often results in pore closure early in tlie sintering process. 

Closed pore Cavity not communicating with the surface. 

True density Mass of the solid divided by the volume of the solid excluding open and closed pores. 

Fused-cast refractory is very dense but may contain a system of closed pores and large, highly oriented grains may exist in a particular casting. The size and distribution of the pore and grain phases must be controlled. 

Materials to be subjected to HIP must be impermeable to the pressurizing gas. Glasses and refractory metals have been used to "can" ceramics, or ceramics can be presintered to the closed pore stage, ie, >92% theoretical density (I JJ), to produce an impermeable surface for HIP. 

Thus he modified Poiseuille s equation (Eq. 23) to describe the penetration of such a liquid into a closed pore of length L ... 

A or less diam. C, closed pores with no outside opening ----------- superficial surface ---------- true surface... 

Gas-filled plastics are polymer materials — disperse systems of the solid-gas type. They are usually divided into foam plastics (which contain mostly closed pores and cells) and porous plastics (which contain mostly open communicating pores). Depending on elasticity, gas-filled plastics are conventionally classified into rigid, semi-rigid, and elastic, categories. In principle, they can be synthesized on the basis of any polymer the most widely used materials are polystyrene, polyvinyl chloride, polyurethanes, polyethylene, polyepoxides, phenol- and carbamideformaldehyde resins, and, of course, certain organosilicon polymers. 

The results collected in Table 2 show that the average activity is also dependent on the architectural system of channels. Thus, samples with close pore size, Ni and acid sites density, but with different topology showed very different catalytic behavior Ni-MCM-48 (with a 3D pore system) was more active than Ni-MCM-41 (with a ID pore system). In fact, the three-dimensional interconnecting mesopore system is very beneficial with respect to the molecular diffusion of heavy products in the pore channels. 

Pores are classified into two types open pores, which connect to the outside of the material, and the closed pores, which are totally within the material. Penetrating pores are kind of open pores these have at least two openings located on two sides of a porous material. Penetrating pores are permeable for fluid, and therefore are important in applications such as filters. Many porous materials have been used in many applications. They are classified by many different criteria such as pore size, pore shape, materials and production methods. Classification by pore size and by pore shape is useful while considering the applications of porous materials. The classification of porous materials by pore size (according to Schaefer30) differentiates between microporous pores (pore diameter < 2 nm), mesoporous pores (2 nm < pore diameter <50 nm) and macroporous pores (pore diameter > 50 nm). 

Let us assume that a sphere with radius a is immersed in a liquid of finite volume, e.g., a mineral in a hydrothermal fluid. Diffusion in liquids is normally fast compared to diffusion in solids, so that the liquid can be thought of as homogeneous. Similar conditions would apply to a sphere degassing into a finite enclosure, e.g., for radiogenic argon loss in a closed pore space. Given the diffusion equation with radial flux and constant diffusion coefficient... 

In contrast to the highly interconnected pores mentioned previously, closed pores can also be obtained by microemulsion polymerization if the initial volume fraction of the dispersed phase is kept lower than 30%. Recently two systems have been reported where the polymerization of the continuous phase and the subsequent removal of the Hquid dispersed phase resulted in the formation... 

This also provides useful strategies to achieve porous thermosets having tailored morphologies such as interconnected or closed pores. These guidelines are summarized in Fig. 7 and discussed below. 

Porous materials can be either open-pored such as a common sponge, or closed-pored such as the bubble-wrap packaging. Aerogels are open-pored materials such that unbonded material can move from one pore to another. 

Porosity is one of the most important properties of a stationary phase, since it severely inflnences the chromatographic colnmn performance, the speed of separation, as well as the specific surface area and consequently loading capacity. Porosity refers to the degree and distribution of the pore space present in a material [114], Open pores indicate cavities or channels, located on the surface of a particle, whereas closed pores are situated inside the material. The sum of those pores is defined as intraparticular porosity. Interparticular porosity, in contrast, is the sum of all void volume between the particles. According to their diameter, pores have been internationally (lUPAC) classified as follows [114] ... 

The atmosphere is also important in sintering. Gas trapped in closed pores will limit pore shrinkage unless the gas is soluble in the grain boundary and can diffuse from the pore. Alumina doped with MgO can be sintered to essentially zero porosity in hydrogen or oxygen atmospheres, which are soluble, but not in air, which contains insoluble nitrogen. The density of oxides sintered in air is commonly less than 98% and often only 92-96%. The sintering atmosphere is also important in that it may influence the sublimation or the stoichiometry of the principal particles or dopants. 

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