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Role of boundary scattering in presence of the correction term on thermal conductivity of isotopically pure LiF crystal
B. K. Singh
This paper is to show the significance of Callaway’s correction term ?c for theoretical analysis of lattice thermal conductivity ? of pure LiF, over the temperature range 2–50 K, by using authors adapted Callaway-Holland model. To study the effect of scattering of phonons by crystal boundary alone, different perspectives have been looked into. That includes samples of various sized crystals cleaved from the same isotopically and chemically pure boule. Different empirical expressions for three phonon relaxation rates τ -1 3 ph for the entire temperature range, as have been proposed by the author earlier, have been used. Anisotropic and dispersive point defect as well as boundary scattering relaxation rates are used. The main argument of the paper is that ?c cannot be neglected as its contribution is even more or nearly equal to the Callaway’s Debye term contribution near the conductivity peak. Hence, its inclusion in the calculation of total lattice thermal conductivity ? is the only way to provide the best fitting of the experimental data for crystals of different sizes with simultaneously taking the same three phonon and point defect parameters. It has also been shown that the effect of boundary scattering is sizable even beyond the conductivity maximum, in the case of isotopically pure LiF. This is in contradiction to other researchers who suggest that boundary scattering is only a low temperature phenomenon. Theoretically justified parameters have been used to obtain excellent agreement with the result reported by Thacher.
Keywords: Phonons, Thermal conductivity, Boundary scattering