Pores in ceramic materials

Closed and open pores (Fig. 56) are an important feature of ceramic materials. They exert a strong influence on chemical resistance, strength, thermal conductivity, modulus of elasticity, and thermal shock characteristics. In characterizing a ceramic, it is important to determine not only its total porosity, but also the pore types, pore shapes, pore sizes, and pore size distribution that are present. 

Accurate porosity data can be obtained by means of quantitative image analysis and microscopy, including electronic methods. However, this requires optimum conditions in the preparation of the polished section, in order to prevent or at least minimize common artifacts, such as  

Avoidance of these artifacts will make it easier to use microscopic methods to distinguish between pores, pull-outs, phases, and smearing. This requires preparation techniques which are well-matched to material in question, especially with respect to its microstructure and manufacture. It is always necessary to perform microscopic examinations of the section after the individual process steps, so that the preparation 

Tubular pores and occluded pores Gas bubbles caused by thermal decomposition phase boundary reaction 

Lenticular cavities caused by inhomogeneities or combustion of impurities 

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Figure 57. Schematic representation of characteristic pore formations in ceramic materials.

The presence of porosity in ceramic materials can also lead to an increase in the dielectric loss. The surfaces of pores contain large numbers of crystalline defects. These surface defects can provide sites for the hopping of ions. Further absorption of moisture in the pore and subsequent leaching of the ions in the ceramic by adsorbed moisture can lead to increases in dielectric loss. 

Once defined that 6 h of milling is enough for a good phase formation, samples with different strontium content (Lai.xSrxMnOs, x= 0.2, 0.3 and 0.4) were synthesized. Table 3 shows the values of the surface area, pore volume and average particle size of the LSM powders synthesized by solid-state method with different strontimn content. One can observe that samples presented veiy low surface areas and pore volumes, in accordance with results described in the literature [14,34]. The low pore volume in ceramic materials obtained by this methodology is related to the fact it is a physical process, which hinders the formation of pores. 

Porosity in ceramic materials may have a dramatic influence on thermal conductivity under most circumstances, increasing the pore volume results in a reduction of the thermal conductivity. In fact, many ceramics used for thermal insulation are porous. Heat transfer across pores is typically slow and inefficient. Internal pores normally contain stiU air, which has an extremely low thermal conductivity—approximately 0.02 W/m K. Furthermore, gaseous convection within the pores is also comparatively ineffective. 

Inorganic membranes commercially available today are dominated by porous membranes, particularly porous ceramic membranes which are essentially the side-products of the earlier technical developments in gaseous diffusion for separating uranium isotopes in the U.S. and France. Summarized in Table 3.1 are the porous inorganic membranes presently available in the market (Hsieh 1988). They vary greatly in pore size, support material and module geometry. 

When a ceramic material (green or baked) is porous, we speak of true density and apparent density. True density is the density of a ceramic material without pores. In order to determine this, the mass of the object must be known as well as the volume less the pore volume. In figure 9.27 all of this is drawn and some calculations are given. 

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). 

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