Russell G. Ross

Thermal Conductivity of Solar System Ices, with Special Reference to Martian Polar Caps

Methods of measurement of thermal conductivity are briefly reviewed and representative data are presented for Solar System ices, including polymorphic modifications of H2O, clathrate hydrates, and the solidified gases CO2, CH4, and N2. Some general planetological implications are considered, with a special examination of the thermal state of the Martian polar caps. The total range of thermal conductivity of ices relevant to objects in the Solar System is about 2 orders of magnitude, not including the effects of pressure, porosity, or crystallinity. For the range of pressures encountered in icy satellites, pressure controls the thermal conductivity of crystalline water ice phases with a factor of six variation. The range of porosities expected in the regoliths of icy satellites ought to produce up to 3 orders of magnitude variation in thermal conductivity. The effect of these factors (composition, crystallinity, pressure, and porosity) on thermal conductivity is among the primary causes of differences in the geologic evolution of icy bodies.

Effect of guest molecule size on the thermal conductivity and heat capacity of clathrate hydrates

Using the transient hot-wire method, the authors have measured both the thermal conductivity, lambda , and the heat capacity per unit volume, rho cp, for structure II clathrate hydrats of 1,3-dioxolane (DO) and cyclobutanone (CB), and compared the results with similar previous measurements for tetrahydrofuran (THF). All three clathrate hydrates showed a glass-like temperature dependence of lambda . The magnitude of lambda was small, about the same as liquid water, and decreased in the sequence DO-THF-CB which corresponds to an increase in the size of the guest molecule. There was some evidence to support the suggestion that the dominant process in the thermal resistivity was associated with excitation of rotation vibrations of the guest molecules. In all cases, the pressure dependence of lambda was unusually small. The authors found evidence that there was a phase transition on the CB clathrate hydrate (and almost certainly also in the DO clathrate) at 135K and 1 GPa, which are essentially the same conditions under which they previously found an analogous transition for THF. In each case, the resultant presumably metastable phase may be a new type of clathrate hydrate arising from densification of the host structure. For the three clathrates, the authors results for rho cp did not depend on the nature of the guest species, and at 0.1 GPa, were the same as previously found for ice Ih.

EFFECTIVE THERMAL CONDUCTIVITY OF BINARY DISPERSED COMPOSITES OVER WIDE RANGES OF VOLUME FRACTION, TEMPERATURE, AND PRESSURE

The effective thermal conductivity of binary dispersed composites prepared from mixed and compacted powders of AgCl and low‐density polyethylene was measured over the entire range of volume fraction at temperatures in the range 100–330 K together with pressures up to 1.75 GPa. Measurements were made using the transient hot‐wire method. The ratio of the thermal conductivities of AgCl and LDPE was an adjustable parameter in the range 2–8 at the temperatures and pressures employed. The rigorous effective medium approximation (EMA) provided a good description for most of our results although some limits to its applicability were also indicated. An empirical theory due to Nielsen provided a significantly less successful description. Suggestions are made for an improved theoretical approach going beyond the limits of the EMA.

Effects of H and D order on the thermal conductivity of ice phases

Thermal conductivities (λ) of nine ice phases were determined by the transient hot wire method in a temperature range of 100–300 K and under pressures up to 2.2 GPa. Pure D2O as well as D2O–H2O mixtures were found to have values of λ close to those of pure H2O, except in phase VIII. For this phase λ was found to be about 9% lower in the case of pure D2O as well as in 1:1 and 1:3 mixtures of D2O:H2O. The hydrogen ordered phases systematically exhibit higher λ than the disordered ones, and in particular the presence of ice VI′ could be detected by a change of slope at low T in the curve of ln λ versus ln T for phase VI. If the results are fitted to the function λ=CT−n, the values of n fall into two groups. One group with n<1 contains only paraelectric phases, whereas the other group, having higher values of n, contains antiferroelectric phases and phase Ih. The latter phase is exceptional in that λ has a negative pressure coefficient. The results are interpreted in terms of dynamic decoupling of the hydroge...

Thermal conductivity, heat capacity and phase diagram of cyclooctanol in liquid, solid and glassy crystal states under high pressure

Using the transient hot-wire method, thermal conductivity and heat capacity per unit volume are measured for solid and liquid phases and glassy crystal states of cyclooctanol, and information is provided on the phase diagram under high pressure. A new solid phase (III) is detected and characterized as a normal crystal phase, whereas all other solid phases (I, II, IV, V) are characterized as plastic crystal phases. We find evidence that the plastic crystal phases I, II and IV could each be the source for a distinct glassy crystal state. It is argued for phase II that its possession of both a low thermal conductivity and a low dielectric permittivity could be accounted for by assuming restricted reorientational motion of the molecules. The unusual (although small) decrease of thermal conductivity observed through the glassy to plastic crystal transitions may indicate that phonons can couple to reorientational motion in the plastic crystal phases I, II and IV.

Thermal conductivity of crystalline and glassy crystal cyclohexanol under pressure

The thermal conductivity λ of phases liquid, I, II, III and the glassy crystal state of cyclohexanol has been measured using the transient hot-wire method at temperatures in the range 100–370K and at pressures up to 0·7 GPa. It was inferred from the observed temperature dependence of the thermal resistivity W (= λ-1) that molecular orientational disorder made a dominant contribution to the resistivity for the glassy crystal state under isobaric conditions. The same was found for plastic crystal phase I under isochoric conditions and also under isobaric conditions especially at the highest pressure. It was suggested that conformational disorder may make an important contribution to W for molecular crystal phases II and III.

The phase diagram of ammonium iodide (NH4I) under pressure, and a comparison with NH4Cl and NH4Br

During recent measurements of thermal conductivity using the transient hot-wire method, data were obtained for pressure-temperature coordinates corresponding to transitions amongst phases I, II, III and IV and NH4I which extended reliable knowledge of the phase diagram to pressures of 2 GPa. Comparison of the phase diagrams of NH4Cl, NH4Br and NH4I showed that a generalised phase diagram covering these substances was a poor approximation if the higher-order critical points or triple points (as appropriate) were made coincident. However, the ferro-ordered-to-disordered phase transition boundaries of these substances could be made to follow a common curve with an alternative empirical adjustment of pressure and temperature. When this was done, the adjustments of temperature required were correlated with differences in barrier height opposing re-orientational motion of the ammonium species.

Thermal conductivity and phase diagram of CCl4 under pressure

The thermal conductivity has been measured for the phases; liquid, Ib, II, III and IV of CCl4 using the transient hot-wire method. Measurements were made at pressures up to 2·2 GPa, and in the temperature range 166–413 K. The thermal conductivity of the plastic crystal phase Ib was found to have features which are closely similar to those found for the plastic crystal phases of cyclopentane and cyclohexane. Phase IV of CCl4 was found to form at pressures in the region of 1 GPa—substantially lower than reported in previous work.

Experimental test of theories for the effective thermal conductivity of a dispersed composite

The effective thermal conductivity of binary dispersed composites prepared from mixed and compacted powders of NaCl and low‐density polyethylene (LDPE) was measured over the entire range of volume fraction at temperatures in the range 120–320 K together with pressures up to 1.73 GPa. Measurements were made using the transient hot‐wire method. The ratio of thermal conductivities of NaCl and LDPE varied over the range 10–50 at the temperatures and pressures employed. Using our results (together with previous data for AgCl‐LDPE composites), we tested theoretical predictions from both the effective‐medium approximation and the real‐space renormalization group approximation. The two approximations exhibited a comparable overall level of agreement with experiment and neither was successful in describing our data within experimental inaccuracy over the entire range of volume fraction. For both approximations, the extent to which agreement with experiment was achieved depended significantly on the volume fraction...

Thermal conductivity and heat capacity of ammonium halides under pressure: NH4Br, NH4I and a comparison with NH4Cl

The thermal conductivity lambda and heat capacity per unit volume rho cp have been measured for solid phases of NH4Br and NH4I using the transient hot-wire method in the temperature T range 100-400 K and at pressures P up to 2 GPa. Comparison of present results with those of previous work for NH4Cl showed similar heat conduction behaviour amongst these ammonium halides. Continuous or near-continuous phase transitions were indicated by an abrupt change in the derivative of lambda with respect to either T or P as well as by a maximum in rho cp. lambda and rho cp changed discontinuously at the first-order I-II phase transition in NH4I. A decrease in NH4 orientational order was concluded to be mainly responsible for the observed increase of thermal resistivity W(= lambda -1) in both ferro- and antiferro-ordered phases during the isobaric temperature increase leading up to an order-disorder transition temperature. Negative (or near-negative) values of ( delta lambda / delta P)T observed for the antiferro-ordered phases were consistent with corresponding negative values of thermal expansivity reported by other workers.