Porous porosity

Most catalysts of high surface area are to some extent porous. Porosity is a concept related to texture and refers to the pore space in a material. It can be defined as the fraction of the bulk volume that is occupied by pore or void space. 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 interstices between particles. 

Another major distinction between ceramics and hydraulic cements is the porosity. Ceramics are made dense unless their application requires some degree of porosity. Hydraulic cements, however, are inherently porous. Porosity is <1 vol% for the best ceramics, but typically 15-20 vol% for cements. Ceramics tolerate very high temperatures, and are corrosion resistant in a wide range of pH, while cements are made for use at ambient temperatures and are affected by high temperature as well as acidic environment. Compared to cements, ceramics are more expensive thus, cement is produced in high volume while ceramics, except few products such as bricks, are specialty products. 

The first two methods yield a highly porous (porosity above 0.9) and transparent aerogel the other methods produce less perfect and more dense porous solids called xerogels. The principles and practice of these methods are summarized in the following sections. 

No convincing conclusion has been reached as to the biodegradation mechanisms of bioactive ceramics. Many researchers have reported different results, as described above. These discrepancies are considered to be caused by the fact that materials used for the experiments were different, and that experimental methods and analytical methods were also different. Therefore, when these reported results are compared, it is important to consider the characteristics of the material used (chemical compositions, impurity, crystallinity, dense or porous, micro- or macro-porous, porosity), experimental methods used in vivo or in vitro, animal species, implanted duration, implanted sites, load bearing or not), and analytical methods used (radiographic, optical microscopic, electron microscopic). Futhermore, a good understanding of the characteristics of the materials to be used becomes important when bioactive ceramics are used clinically. 

Gas diffusion media are by definition porous. Porosity is typically between 70% and 80% as shown in Table 4-3. Porosity of a gas diffusion layer may be easily calculated from its areal weight, thickness, and the density of the solid phase (for carbon-based materials, preai varies between 1.6 and 1.95gcm ). The porosity, e, also depends on the compressed thickness ... 

In applying Eq. XVI-13 to an actual porous bed, r is taken to be proportional to the volume of void space Ale, where e is die porosity, divided by the amount of surface alternatively, then,... 

Isotherms of Type 111 and Type V, which are the subject of Chapter 5, seem to be characteristic of systems where the adsorbent-adsorbate interaction is unusually weak, and are much less common than those of the other three types. Type III isotherms are indicative of a non-porous solid, and some halting steps have been taken towards their use for the estimation of specific surface but Type V isotherms, which betoken the presence of porosity, offer little if any scope at present for the evaluation of either surface area or pore size distribution. 

In Unger and Fischer s study of the effect of mercury intrusion on structure, three samples of porous silica were specially prepared from spherical particles 100-200 pm in diameter so as to provide a wide range of porosity (Table 3.16). The initial pore volume n (EtOH) was determined by ethanol titration (see next paragraph). The pore volume u (Hg, i) obtained from the first penetration of mercury agreed moderately well with u fEtOH),... 

Solid Density. SoHds can be characterized by three densities bulk, skeletal, and particle. Bulk density is a measure of the weight of an assemblage of particles divided by the volume the particles occupy. This measurement includes the voids between the particles and the voids within porous particles. The skeletal, or tme soHd density, is the density of the soHd material if it had zero porosity. Fluid-bed calculations generally use the particle... 

Fig. 3. Microporous membranes are characterized by tortuosity, T, porosity, S, and their average pore diameter, d. (a) Cross-sections of porous membranes containing cylindrical pores, (b) Surface views of porous membranes of equal S, but differing pore size.

Porous metal stmctures can also be created by spraying molten metal onto a base. Porosity is controlled by spraying conditions or by an additive that may be removed later. 

Space Applications. The growth of powder metahurgy in space technology has arisen from the difficulty of handling many materials in conventional fusion-metahurgy techniques, the need for controhed porosity, and the requirement of many special and unique properties (60,61). Powder metahurgy is applied in low density components with emphasis on porous tungsten for W—Ag stmctures, beryhium compounds, titanium and... 

Filtered-Particle Inspection. Solids containing extensive inteiconnected porosity, eg, sintered metallic or fired ceramic bodies formed of particles that ate typically of 0.15-mm (100-mesh) screen size, are not inspectable by normal Hquid penetrant methods. The preferred test medium consists of a suspension of dyed soHd particles, which may be contained in a Hquid vehicle dyed with a different color. Test indications can form wherever suspensions can enter cracks and other discontinuities open to the surface and be absorbed in porous material along interior crack walls. The soHd particles that form test indications ate removed by filtration along the line of the crack at the surface where they form color or fluorescent indications visible under near-ultraviolet light (1,3). 

Dehydration or Chemical Stabilization. The removal of surface silanol (Si—OH) bonds from the pore network results in a chemically stable ultraporous soHd (step F, Fig. 1). Porous gel—siHca made in this manner by method 3 is optically transparent, having both interconnected porosity and sufficient strength to be used as unique optical components when impregnated with optically active polymers, such as fiuors, wavelength shifters, dyes, or nonlinear polymers (3,23). 

---