Does Porosity Decrease Thermal Conductivity In Ceramics

A briefly explain why porosity decreases the thermal conductivity of ceramic and polymeric materials rendering them more thermally insulative.

Does porosity decrease thermal conductivity in ceramics. The problem with the thermal conductivity of ceramics is the dependence on the composition grain size and manufacturing process which make it rather difficult to obtain a reliable value from literature only. Porosity should decrease thermal conductivity for similar and other reasons since the presence of air in the pores adds an insulation character to the bulk sample that you may be measuring. Metals with low electrical resistance and crystals in which lattice vibrations are transferred easily for example crystals with atoms or ions of. The effects of porosity and pore size on the thermal conductivity are discussed.

Intuition behind formula for thermal conductivity. Bulk density thermal conductivity heat. Even at a porosity level as high as 79 4 compressive and flexural strengths of fired samples with average pore size of 314 μm remained as high as 9 0 and 3 7 mpa respectively and their. B briefly explain how the degree of crystallinity affects the thermal conductivity of polymeric materials and why.

The thermal conductivity of porous materials is theoretically studied in connection with nanoporous materials used in recent semiconductor devices. 7 5 porosity and permeability duration. Looking at the values quoted in various handbooks papers and data sheets two things are observed. K k 0 1 p 1 np 2 where k is the thermal conductivity of a porous ceramic body k 0 that of the pore free ceramic body p the volume fraction of the pores and n a.

The best equation which correlates extremely well the thermal conductivity of the ceramic bodies with the porosity is found to be the one proposed by aivazov and domashnev. For example the thermal conductivity of a biomorphic sic ceramic has decreased from 41 5 15 w m k with the increase in porosity from 43 to 67. The thermal conductivity of porous sic ceramics has decreased with the increase in porosity 4 39 40. The thermal conductivity of insulating materials is determined by the heat capacity of phonons the average phonon velocity and the phonon mean free path.

For sand and mixed litholo gies no clear depth dependence was found for porosity indicating that compared with clay both sand and mixed lithology are resistant to compaction by the accumulation of sediment.