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.

Does porosity decrease thermal conductivity in ceramics. 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. Explain why porosity decreases the thermal conductivity of ceramics and polymers. The effects of porosity and pore size on the thermal conductivity are discussed. Thermal conduction is generated by the movement of electrons and the transfer of lattice vibrations.

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. 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. Trast the bulk density thermal conductivity and thermal diffusivity increase with depth.

B briefly explain how the degree of crystallinity affects the thermal conductivity of polymeric materials and why. The thermal conductivity of porous sic ceramics has decreased with the increase in porosity 4 39 40. 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. 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.

Looking at the values quoted in various handbooks papers and data sheets two things are observed. Metals with low electrical resistance and crystals in which lattice vibrations are transferred easily for example crystals with atoms or ions of. Bulk density thermal conductivity heat. Intuition behind formula for thermal conductivity.

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