Open porosity

There are various methods devised for characterizing open pores for their size by regarding them as capillaries and determining their equivalent diameter from the rate of fluid flow through them or the extent to which liquid mercury can be forced into them. 

In the mercury method, for example, the sample is placed in a container and evacuated. Mercury is then admitted by applying pressure. The pressure necessary to force mercury into the capillary depends on the contact angle and surface tension, and is given by  

P is in kg/cm, d is the pore diameter (p), y is the surface tension of mercury, and 9 is the contact angle (140° for most oxides). As the pressure is increased, smaller-sized pores are permeated at the increased pressure. Their amount is measured by the decrease in the apparent volume of mercury plus sample. Thus, the results obtained can be used to determine the distribution of open pore sizes. 

The following sections deal with the evolution of microstructure in different ceramic compositions. 

Ceramic compositions forming the basis for the whiteware industry are the mixtures of clay, feldspar, and flint. These compositions include hard porcelain for tableware, sanitary ware, electrical porcelain, and so on. 

It is well known that, depending on the field of application, preference is given either to closed-cell or open-cell foams. The latter are widely used for sound and vibration insulation, in chromatography, as absorbents and filters of liquids and gases, for the preparation of metallized foamed materials, etc. 

Considerable information has been accumulated regarding the relationdiip between the portion of open cells and the technical properties of oligomeric foams 83-85) xhere are considerably less data available on the relationship between the amount of open cells and other morphological parameters of RO foams, for example apparent density. Reticular foams based on RO are not considered in this survey because the open-cell structure is created by secondary processing of finished products and not during foaming. 

In many works it has been established that the relative number of open cells increases with decreasing apparent density of RO foams. This phenomenon has been discussed in detail in The incr se in the size of cells during foaming is accompanied by a decrease in wall thickness, that finally results in a rupture of the polymeric films forming the cell membranes and in the occurrence of open, i.e. communicating cells. 

According to the data obtained by Shutov and Chaikin for several RO (polyurethane and phenolic) foams, the curve = f (7) exhibits two maxima where is 

Dependence of the volume fraction of open cells on apparent density 7 and volume fraction of polyn ric phase dp for PUR foams 1 composition based on polyester foamed by carbon dioxide 2 precipitation plastic foam, (according to data of Salyer and co-work-ers83) 

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Cold-isostatic-pressing foUowed by vacuum sintering or HIP is also used to manufacture smaller intricate shapes. In this instance beryUium powder is loaded into shaped mbber bags and pressed isostaticaUy in a pressure chamber up to 410 MPa (60,000 psi). After the pressing operation the mbber bag is stripped from the part which is then vacuum sintered to about 99% of theoretical density at about 1200°C. If full theoretical density is required, the sintered part may be simply given a HIP cycle because there is no open porosity after vacuum sintering. In a similar manner, conventional axial cold-pressing... 

The single-point BET surface area measurement was used to check for open pores. The results for some soft and hard carbon samples heated at 700°C and 1000°C are presented in Table 2 for comparison. The hard carbon samples studied here have about ten times more open porosity than the soft carbons. 

As already mentioned, the first pyrolysis cycle leads to an open porosity of about 20-30 %. This high porosity limits the interlaminar shear strength (ILS). A reduction in the porosity is possible by reimpregnation with polymers and subsequent pyrolysis or CVI (Chemical Vapour Infiltration). 

Overall the spheres were of good quality judging from images obtained and could be used for filling with sodium alanate. However, to confirm through wall open porosity formation across the spheres wall, approximately 900 A cross section was made across a sphere wall and SEM and elemental mapping was conducted and Fig. 3 shows the results obtained. 

CA 70, 69760k(l969) (Blasting expls of high.deton rate can be prepd from AN of open porosity of ca 10% by wt or greater... 

The quality of a silica gel depends on many factors such as its internal structure, the size of its particles, its open porosity (dimension and distribution of pores), its specific area, its resistance to crushing and its polarity. 

Area of the pay zone Thickness of the pay zone Open porosity Residual water saturation Initial reservoir pressure Reservoir temperature range Reservoir water salinity Water-free gas composition... 

With the help of figure 9.27 you can derive yet another formula for the percentage of open porosity, an important ceramic parameter. When a ceramic mass has been baked, it sometimes still possesses so-called open and closed pores. The open pores are in contact with the outside air and so can be filled with water. From figure 9.27 it appears that ... 

Fig. 32. Percent of open porosity and percent of theoretical density of air sintered samples versus alumina content in the system La0.g2Sr0jjCrj Al.. From ref. [105], This paper was originally presented at the Fall 1989 Meeting of The Electrochemical Society held in Hollywood, Florida.

Chiche et a/.[56] have studied the oligomerization of butene over a series of zeolite (HBeta and HZSM-5), amorphous silica alumina and mesoporous MTS-type aluminosilicates with different pores. The authors found that MTS catalyst converts selectively butenes into a mixture of branched dimers at 423 K and 1.5-2 MPa. Under the same reaction conditions, acid zeolites and amorphous silica alumina are practically inactive due to rapid deactivation caused by the accumulation of hydrocarbon residue on the catalyst surface blocking pores and active sites. The catalytic behaviour observed for the MTS catalyst was attributed to the low density of sites on their surface along with the absence of diffusional limitations due to an open porosity. This would result in a low concentration of reactive species on the surface with short residence times, and favour deprotonation and desorption of the octyl cations, thus preventing secondary reaction of the olefinic products. 

In general terms the microstructure (Fig. 5.20(c)) consists of the filler particles embedded in a glass and crystalline matrix (mullite Al6Si2013) derived from the clay and flux. There must be no open porosity and total porosity must be kept to a minimum, typically 4vol.%, to avoid adverse effects due to ionization of the gas in the pores (see Section 5.2.2). 

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