Heat transfer in the orientationally disordered phase of SF6

Abstract

The isobaric thermal conductivity of solid SF6 is investigated in the high-temperature phase. The experimental results are rescaled to a constant density. The isochoric thermal conductivity initially decreases with increasing temperature, then passes through a smooth minimum and begins to grow. A modified version of the reduced coordinate method is used to calculate the phonon–phonon and phonon–rotational contributions to the total thermal resistance. The growth of the isochoric thermal conductivity is explained by a weakening of the scattering of phonons on collective rotational excitations of the molecules as the correlations of the rotation weaken.

Phonon scattering and heat transfer by "diffusive" modes in solid C_{6}H_{6}

“…The discussion of the lower limit of thermal conductivity of molecular crystals brings up inevitable question: should the siteto-site transport of the rotational energy of the molecules be taken into account? The minimum values of the experimental thermal conductivity Λ v is 1.2 times higher than Λ * min calculated by equation (19), and 2.5 times higher than Λ min calculated by equation (18). The above correlation between the Λ min and Λ * min suggests a positive answer.…”

“…The calculated values of Λ min were as a rule considerably smaller than the experimental ones [2,15,18]. The most obvious reason for this difference is that the site to site transfer of the rotational energy was not taken into account.…”

“…Nevertheless, isochoric studies of thermal conductivity of molecular crystals [2,15,18] show a considerable deviation from this law. These discrepancies may be due to the thermal conductivity approaching its lower limit.…”

Molecular rotation and volume dependence of the thermal conductivity in liquid N2

“…Despite evidence for that molecular translation oscillations in liquids can be collective, it is uncertain whether Debye approximation is generally applicable to liquids. Therefore, the translational component C tr was determined using a modified method of reduced coordinates [18]. It is important that with this method there is no need to involve any approximate model.…”

Experimental setup for measuring the isobaric thermal conductivity of molecular crystals

“…With the temperature increasing, the rotational motion can in principle pass through the following stages: a growth of the libration amplitudes, the appearance of jump-like reorientations of the molecules, an increase in the frequency of the reorientations, a hindered rotation of the molecules and, finally, a nearly free rotation of the molecules [1,8]. In this case, the transition of the molecules from low-amplitude librations to a weakly hindered rotation is accompanied by a decrease in the translational-orientational component of the thermal resistance, and the general thermal conductivity can increase with an increase in the temperature [9,10]. The translation-rotation coupling also leads to a softening of elastic constants and affects phonon spectra.…”