Unusual PT dependence of thermal conductivity for a clathrate hydrate

Abstract

Molecular clathrate solids¹ are crystals with peculiar and interesting properties. Their characteristic feature² is that one chemical species, the ‘host’, forms a strongly bonded structure, which encages individual molecules of a second ‘guest’ species. There is virtually no chemical bonding between host and guest molecules. The guests are, nevertheless, firmly constrained by high energy barriers. X-ray and neutron diffraction experiments have shown that clathrates have a periodic structure typical of crystalline solids. A clathrate hydrate² is a host substance consisting of H₂O molecules which form an ice-like, hydrogen-bonded structure. Huge geological deposits of clathrate hydrates containing natural gas are known to exist^2,3—these deposits may be exploitable, and in searching for clathrate hydrates knowledge of the thermal conductivity, λ, will be important. We present here a preliminary account of the first measurements of λ versus pressure (P) and temperature (T) for a clathrate hydrate. We studied a clathrate hydrate encaging molecules of tetrahydrofuran. Thermal conductivity was measured using the transient hot-wire method⁴. The accuracy of the measurements was 3% and the precision 1%. The temperature dependence was found to be unusual in that λ(T) has a positive slope (Fig. 1), a feature never before observed with crystalline organic materials. Furthermore, when λ was measured versus P, it was found unexpectedly that λ (P) was constant—behaviour never before reported for a non-metallic material.