**2. What are porous ceramics?**

Porous ceramics are categorized as those ceramics having high percentage porosity between 20 and 95%. These materials composed of at least two phases like solid ceramic phase, and the gas-filled porous phase [7]. The gas content of these pores usually regulates itself to the environment, as an exchange of gas with the environment is possible through pore channels.

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Honeycombs also with well-defined unidirectional channels can be paste extruded from a variety of ceramic powders and more complex three-dimensional porous ceramics can be made by rapid prototyping techniques, such as 3D printing. Apart from these methods, it is possible to distinguish many different types of fabrication routes (**Figure 2**) for producing

Introductory Chapter: A Brief Introduction to Porous Ceramic

http://dx.doi.org/10.5772/intechopen.74747

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**Figure 2.** Schematic of porous ceramic processing methods: (a) partial sintering, (b) sacrificial fugitives, (c) replica

templates, and (d) direct foaming [1].

**Figure 1.** Magnification of a single open pore in the alumina ceramic body.

Closed pores can contain a composition of gases that is independent of the environment [8]. When porosity is determined for any ceramic body, porosity can be distinguished for several types such as open (accessible from the outside) porosity (**Figure 1**) and closed porosity. Where open porosity can be further categorized into open dead-end pores and open pore channels. The presence of porosity depends on the specific application, so a more open porosity may be needed to be permeable such as a closed porosity or filters/membrane such as thermal insulator may be wanted. The sum of the open and closed porosity is mentioned to as the total porosity [9]. If the fractional porosity of a material is relatively low, then the closed porosity will dominate; at the fractional porosity increases, the open porosity level increases.

The porous ceramics have been classified on the basis of nature of porosity, volume fraction and size of these pores [10]. The nature of porosity in natural ceramics depend on their genesis while in synthetic ceramics, it depends on their manufacturing and generally, it can be controlled. The pore size of these materials can be classified into three grades depending on the pore diameter: microporous (less than 2 nm), mesoporous (between 2 and 50 nm) and macroporous (more than 50 nm). The pore size distributions are usually measured by mercury intrusion porosimetry technique. The pore size distribution of the closed porosity is not determinable using this technique, but may occur, for example, by optical and electronic examination of a polished cross section. The pore size distribution represents the pore volume in function of pore size and commonly is given as percentage or a derivative [11].
