Does Porosity Decrease Thermal Conductivity In Ceramics

Bulk density thermal conductivity heat.

Does porosity decrease thermal conductivity in ceramics.

The thermal conductivity of insulating materials is determined by the heat capacity of phonons the average phonon velocity and the phonon mean free path. 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. Metals with low electrical resistance and crystals in which lattice vibrations are transferred easily for example crystals with atoms or ions of. Explain why porosity decreases the thermal conductivity of ceramics and polymers.

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. Intuition behind formula for thermal conductivity. Thermal conduction is generated by the movement of electrons and the transfer of lattice vibrations. The effects of porosity and pore size on the thermal conductivity are discussed.

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. 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. The thermal conductivity of porous materials is theoretically studied in connection with nanoporous materials used in recent semiconductor devices. The thermal conductivity of porous sic ceramics has decreased with the increase in porosity 4 39 40.

Looking at the values quoted in various handbooks papers and data sheets two things are observed. A briefly explain why porosity decreases the thermal conductivity of ceramic and polymeric materials rendering them more thermally insulative. Trast the bulk density thermal conductivity and thermal diffusivity increase with depth. 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.

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.