Isochoric thermal conductivity of solid n-alkanes: propane C3H8

Abstract

The isochoric thermal conductivity of solid propane C 3 H 8 has been investigated on three samples of different densities in the temperature interval from 50 K to the onset of melting. In all the cases the isochoric thermal conductivity exhibits dependences that are weaker than Lµ1/T. The results obtained are compared with those for other representatives of n-alkanes. The experimental data are discussed within a model in which the heat is transported by low-frequency phonons and by «diffusive» modes above the phonon mobility edge.PACS: 66.70.-f Nonelectronic thermal conduction and heat-pulse propagation in solids; thermal waves; 63.20. K-Phonon interactions.Keywords: isochoric thermal conductivity, phonons, diffusive modes, solid propane.Normal alkanes C n H 2n+2 form a class of substances taking an intermediate, transitional position between simple molecular solids and long-chain polymers which attracts considerable interest. The n-alkanes have a deceptively simple composition and a molecular packing in the solid state: the lamellae of molecules are aligned as chains parallel to one another and nearly perpendicular to the lamella plane. Yet, despite this seeming simplicity, they show an extraordinary variety of dynamical behavior both in the solid and liquid states. There is an intriguing «odd-even» effect alternating the orthorhombic versus triclinic unit cells between the adjacent members of the series as well as the presence or absence of premelting rotator phases. Glassy crystalline «rotator phases» with a hexagonal symmetry are found below the melting points of the odd-numbered n-alkanes starting with n = 9. The difference between the phase transition and melting points increases with the chain length. The hexagonal modifications are also observed for the even-numbered n-alkanes starting with n = 22 [1,2]. The nature of the molecular disorder in the «rotator phases» of n-alkanes is not fully clear. Besides the more or less hindered rotation of the molecules about the long axes other additional forms of disorder have been discussed [3]. The melting point alternation is another well-known phenomenon in n-alkanes; that is, the even-numbered members of series melt at a relatively higher temperature than the odd-numbered members [1].The short-chain (n < 6) n-alkanes are the least known members of the series. Previously, we studied the isochoric thermal conductivity of methane [4] and ethane [5]. Here we explore studies of the heat transfer in saturated hydrocarbons further. The isochoric thermal conductivity of solid propane C 3 H 8 was studied on samples of different densities in the temperature interval from 50 K to the onset of melting. Constant-volume investigations are possible for molecular solids having comparatively high compressibility coefficients. Using a high pressure cell, it is possible to grow samples of sufficient density which can be cooled then with practically invariable volume whereas the pressure in the cell decreases. As samples of moderate densities are cooled the pressure in the sell dro...

Isochoric thermal conductivity of solid n-alkanes: Hexane C6H14

“…The Bridgman coefficient ( ln / ln ) T g V = − ∂ Λ ∂ was calculated from experimental data to be 7.6 ± 0.6 at T = 178 K. The isochoric thermal conductivity of all solid n-hexane samples decreases with increasing temperature following a dependences weaker than Λ ∝ 1/T. A similar behavior was observed previously in the low-temperature phase of ethane [7] and in propane [8]. It was noted previously that the thermal conductivity of long-chain odd n-alkanes (C 9 H 20 -C 19 H 40 ) has some features in common [5].…”

“…2 (data of this study and [5,7,8] along with the thermal conductivities of the liquid phases of these compounds measured immediately after melting [5,14]. Data for ethane, propane and hexane correspond to zero pressure; thermal conductivity of "odd" n-alkanes with n = 9-19 was studied at constant pressure 30 MPa.…”

“…Constant-volume investigations are possible for molecular solids having comparatively high compressibility coefficients. Using a high pressure cell, it is possible to grow samples of sufficient density which can be cooled then with practically invaria- Isobaric thermal conductivity of some n-alkanes (this study and [5,7,8]). Data for ethane, propane and hexane correspond to zero pressure; thermal conductivity of "odd" n-alkanes with n = 9-19 correspond to pressure 30 MPa.…”

“…Table 2. The structure of n-alkanes [2,5,10], the temperature T α-β and the entropy ΔS α-β /R of the transition to the "rotational" phase, the temperature T m and the melting entropy ΔS m /R [NIST Standard Reference Data: http://webbook.nist.gov/chemistry/form-ser.html], a complete change of the entropy and variations of thermal conductivity Λ α /Λ L during the ordered phase -liquid transition [5,7,8,14].…”

“…Previously, thermal conductivity was investigated only in "odd" n-alkanes with n = 9-19 at constant pressure 30 MPa [5]. We have investigated the isochoric thermal conductivity of methane [6], ethane [7] and propane [8]. Here we report the isochoric thermal conductivity of solid n-hexane (C 6 H 14 ) measured on samples of various densities in the temperature interval from 100 K to the onset of melting.…”