Closed-pore materials

Fig. 3B (right) Temperature dependence of the lower closure point pressure (p/po)L of the hysteresis loop of CHF3 and C2F6 in the closed-pore material Cl. 

The chemical reactivity is also appreciably affected by the degree of porosity, since high porosity leads to large increase in surface area with resulting increase in reactivity. Differences in reactivity between one form of graphite or another can be considerable. Obviously, high surface area materietls such as activated carbon are far more reactive than dense, pore-free or closed-pore materials such as glassy carbon. 

Pores contained in solid samples can be classified into closed and open pores. The meaning of closed and open depends on the size of pores into which a probe fluid (gas or liquid) can diffuse. In any case, when the probe fluid cannot penetrate into certain spaces, they are denoted as closed pores. Materials such as foamed metals or polymers have a considerable amount of isolated voids, individually surrounded by a dense matrix. Most of their porosities are denoted as closed pores. Gels prepared from a relatively soft network tend to retain molecular scale pores. They are open pores in nature, but virtually... 

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. 

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. 

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. 

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). 

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] ... 

A porous particle contains many interior voids known as open or closed pores. A pore is characterized as open when it is connected to the exterior surface of the particle, whereas a pore is closed (or blind) when it is inaccessible from the surface. So, a fluid flowing around a particle can see an open pore, but not a closed one. There are several densities used in the literature and therefore one has to know which density is being referred to (Table 3.15). True density may be defined as the mass of a powder or particle divided by its volume excluding all pores and voids. True density is also referred to as absolute density or crystalline density in the case of pure compounds. However, this density is very difficult to be determined and can be calculated only through X-ray or neutron diffraction analysis of single-crystal samples. Particle density is defined as the mass of a particle divided by its hydrodynamic volume. The hydrodynamic volume includes the volume of all the open and closed pores. Practically, the hydrodynamic volume is identified with the volume included by the outer surface of the particle. The particle density is also called apparent or envelope density. The term skeletal density is also used. The skeletal density of a porous particle is higher than the particle one, since it is the mass of the particle divided by the volume of solid material making up the particle. In this volume, the closed pores volume is included. The interrelationship between these two types of density is as follows (ASTM, 1994 BSI, 1991) ... 

Symbol Solid material volume Open pore volume Closed pore volume Interparticle void volume... 

In contrast to gas-phase carbonization, most thermosetting resins, such as phenol-formaldehyde and furfuryl alcohol, and also cellulose can be converted to carbon materials by solid-phase carbonization. When the carbonization of most of these precursors proceeds rapidly, the resultant carbon materials become porous. If the carbonization is performed so slowly that the resultant carbonaceous solids can shrink completely, the so-called glass-like carbons are produced, which contain a large number of closed pores. 

The apparent solid density is the ratio of the mass of the dry material to its apparent solid volume. Apparent solid volume is the sum of the volume of closed pores and true volume where the true volume is that of a body occupied by solid material excluding all forms of porosity (BS... 

Most solids of high surface area are to some extent porous. The texture of such materials is defined by the detailed geometry of the void and pore space. Porosity, , is a concept related to texture and refers to the pore space in a material. An open pore is a cavity or channel communicating with the surface of a particle, as opposed to a closed pore. Void is the space or interstice between particles. In the context of adsorption and fluid penetration powder porosity is the ratio of the volume of voids plus the volume of open pores to the total volume occupied by the powder. Similarly, particle porosity is the ratio of the volume of open pores to the total volume of the particle. It should be noted that these definitions place the emphasis on the accessibility of pore space to the adsorptive. 

Porosity is usually defined as the ratio of the volume of pores and voids to the volume occupied by the solid. However, it should be kept in mind that the recorded value of porosity is not always a simple characteristic property of the material, since it is likely to depend also on the methods used to assess both the pore volume and the volume of the solid. The pore volume is usually regarded as the volume of open pores, but it may include the volume of closed pores. Moreover, the recorded value may depend on the nature of the probe molecule or the experimental conditions. 

Skeletal density Ratio of the mass of discrete pieces of solid material to the sum of the volumes of the solid material in the pieces and closed pores within the pieces (ASTM) ... 

Figure 7.20 Lifetime results versus porogen load shown on three separate time scales. The shortest lifetimes on the bottom frame are due to annihilations of positrons and positronium in the MSSQ material. The middle frame shows the positronium annihilations from closed pores and from open pores in the top frame. Statistical errors are shown or smaller than the symbols. See text.

We prepared two carbons by pyrolysis of saccharose followed by heat treatment at two different temperatures 400 C (CS400) and 1000"C (CSIOOO). We performed X-ray diffraction and SAXS on each of these porous materials and obtained the structure factors, S q), following the procedure described by Franklin [14]. The resulting S(q) s are shown in Figure 1 (bold line). We performed Hg porosimetry to obtain the density of both carbons, accounting for the volume occupied by C atoms, closed pores and smaller open pores. We also measured the H/C and the O/C ratios by combustion experiments. The results are summarized in Table 1. 

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