Pores, porosity ceramics

Y.H. Koh, J.H. Song, E.J. Lee, and H.E. Kim, Freezing Dilute Ceramic/Camphene Slurry for Ultra-High Porosity Ceramics with Completely Interconnected Pore Networks, J. Am. Ceram. Soc., 89, 3089-93 (2006). 

AI2O3 ceramics with lamellar pores and ceramic walls can be fabricated by a freeze-drying process using aqueous ceramic slurries. The relative density, porosity and strength of the porous AI2O3 ceramic is about 61,59%, 37.04 % and 31.0 MPa, respectively. 

Pore-free ceramic materials and low porosity surface coating to avoid undercut corrosion and to eliminate substrate attack. 

This is very important as several other properties are dependent upon it. If the porosity is too high, the article will be weak and will not retain liquid. The pore structure should also be taken into account. When a ceramic material is hred, although the internal surface area decreases as the material approaches zero porosity, the mean radius of the pores increases. Thus, when the internal surface area is 3 mVg the mean pore radius may be of the order of 10 m, while when the internal surface has dropped to 0-5 mVg the mean pore radius may be about 4-5 x 10 m. The mean pore radius may reach a value as high as 9 x 10 m as the ware approaches zero porosity during firing. It is thus obvious that at some point the pores must start to close up. This closing of the pores with the approach of vitrification is borne out by results of permeability measurements. 

The small particles are reported to be very harmful for human health [98]. To remove particulate emissions from diesel engines, diesel particulate filters (DPF) are used. Filter systems can be metallic and ceramic with a large number of parallel channels. In applications to passenger cars, only ceramic filters are used. The channels in the filter are alternatively open and closed. Consequently, the exhaust gas is forced to flow through the porous walls of the honeycomb structure. The solid particles are deposited in the pores. Depending on the porosity of the filter material, these filters can attain filtration efficiencies up to 97%. The soot deposits in the particulate filter induce a steady rise in flow resistance. For this reason, the particulate filter must be regenerated at certain intervals, which can be achieved in the passive or active process [46]. 

The choice of solid carriers spans a wide spectrum (Table 1) from materials most suitable for research purposes (sintered glass beads, laterite stone deposited on a gramophone disk) to industrial materials (pumice, activated carbon, etc.). Key properties that affect the performance of the carrier are porosity (from impervious to controlled-size pores), composition (from ceramics to activated carbon), and hydrophilic behavior. It is difficult to perform a direct comparison of different carriers. Colonization and biofilm growth depend strongly on the nature of bacteria and on their intrinsic propensity to adhere on hydrophilic vs. hydrophobic surfaces. 

One or more 12.7-centimeter (5-inch) OD porous layers can be installed in the rig at any axial location in the three sections. Each layer could be positioned using one custom-made retaining ring behind. In all of the tests reported here the porous material was a SiC ceramic foam supplied by Hi-Tech Ceramics of Alfred, New York. All the ceramic foams were 12.7 cm (5 in.) in diameter with varying thickness from 1.3 cm (0.5 in.) to 2.5 cm (1 in.). Two different pore sizes were tested, including 8 ppcm (20 ppi) and 18 ppcm (45 ppi). According to the manufacturer the porosity of the ceramic foams was about 80%. 

When multicomponent alkoxide solutions, or a single alkoxide and a soluble inorganic salt, are mixed, a multicomponent alkoxide may result. In this way, such complex oxides such as the YBCO superconductor (cf. Section 6.1.2.4) can be formed. Sol-gel processing can also be used to coat fibers for composites and to form ceramics with very fine pore sizes called xerogels. A xerogel commonly contains 50-70% porosity, a pore size of 1-50 nm, and a specific surface area exceeding 100 m /g. 

Laboratory-scale bubble columns for ozonation preferably have a reactor liquid phase volume of VL = 2-10 L, with a height-to-diameter-ratio of hid = 5-10. The ozone/oxygen (ozone/air) gas mixture is supplied through a ceramic or stainless steel porous plate fine pore diffuser (porosity 3,10-40 pm hole diameter). PTFE-membranes are a comparatively new alternative for the ozone gas-to-water transfer (Gottschalk et al., 1998). 

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 ... 

Consequently ceramics break easily, mainly because they are so brittle. Defects in the material result in cracks when loads are applied. The main defects are porosity, foreign particles which have been incorporated and surface cracks resulting from the surface treatment of the baked product. For example volume percentage and pore dimensions strongly affect the strength of the object (figure 9.30). 

A ceramic is rarely homogeneous, a common inhomogeneity being porosity. There is strong evidence to show that breakdown can be initiated at pores and that the occurrence of gas discharges within pores is an important factor. There... 

The effects of moisture must be limited by ensuring that the residual porosity in ceramics consists of isolated pores so that water does not penetrate into the bulk. Units must be protected from humidity either by encapsulation in polymers with a low permeability to water vapour or by enclosure in sealed metal cans. 

Undoubtedly, changes in porosity (such as bulk and surface porosity and diameter of individual pores) of nanophase ceramic formulated by Webster et al. (1999) provide an explanation for the observed differences in mechanical properties of respective nanophase and conventional ceramic formulations for example, individual surface pores four times smaller were achieved in nanophase (67-nm grain size) compared to conventional (179-nm grain size) HA formulations (Webster et al., 2000a). Compared to conventional formulations, therefore, the bending modulus of nanophase ceramics... 

The selection and chemical modification of the current generation of chemically and physically robust stationary phases with narrower particle and pore size distributions has been based on the developmental effort that has occurred over the past 20 years. Initially chemically modified, deformable polymeric gels were used, such as the crosslinked agaroses, dextrans, or acrylate-based copolymers, but more recently various classes of highly refined type I and type II silicas and other ceramic materials, or new classes of controlled porosity polymeric organic materials have found increasing application. 

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