A number of conductivity models are available with approximations regarding the dispersion and λ p. Using the single-mode relaxation time approximation (∂ f p /∂ t | s = − f p / τ p) and the gas kinetic theory, Callaway phonon (lattice) conductivity model as k p , s = 1 8 π π 3 ∑ ∑ α α ∫ ∫ c v , p τ τ p ( u p , g ⋅ ⋅ s ) 2 d κ κ for component along s , k p = 1 6 π π 3 ∑ ∑ α α ∫ ∫ c v , p τ τ p u p , g 2 κ κ 2 d κ κ for isotropic conductivity .