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Clathrate hydrates are crystalline compounds in which guest molecules are encaged within an ice-like lattice. They occur naturally and possess properties of significant interest for energy and storage applications. Here, we report the thermal conductivity κ of structure I CO₂ clathrate hydrate across a broad temperature range (90–265 K) and at pressures up to 1.2 GPa. Similar to structure II clathrate hydrates, κ decreases with decreasing temperature, displaying almost identical temperature dependence. However, the absolute values are 10–30% lower. Notably, κ of CO₂ clathrate hydrate is among the lowest observed for structure I clathrate hydrates, with κ = (426 ± 8) mW m^–1 K^–1 under stable conditions at 270 K and 1 MPa. Furthermore, the isothermal dependencies of κ on density ρ and pressure p─parameters crucial for thermal modeling at elevated pressures─are relatively weak, with (d ln κ/d ln ρ) = 1.2 ± 0.2 and (d ln κ/dp) = (12 ± 1) % GPa^–1 . The measurements show significantly lower κ values and a different temperature dependence compared with previously reported simulation results. Nevertheless, the experimental data confirm the simulation prediction that κ for CO₂ clathrate hydrate is significantly lower than for other structure I clathrates. Our findings further indicate that κ in both structures I and II clathrate hydrates tends to decrease with increasing van der Waals radius of the guest molecules, as reviewed here. This trend may arise from enhanced distortion and anharmonicity within the ice framework. We tentatively propose that the pronounced anharmonicity of the clathrate hydrate lattice leads to frequent phonon–phonon scattering, effectively suppressing phonon-mediated heat transport and resulting in predominantly diffusive thermal conduction.

The thermal conductivity κ of solid CO2 was studied in the temperature T range of 100–220 K and at pressures up to 200 MPa using the transient hot-wire method. The results are consistent with those expected for a polycrystal composed of small molecules, with κ increasing significantly as the temperature decreases and as pressure and density increase. The variation in κ with temperature is primarily attributed to changes in phonon–phonon scattering and density. The thermal conductivity behaviour is described using a two-basis model, where heat is transported by both phonons and diffuse modes. The density ρ dependence of the thermal conductivity, represented by the Bridgman parameter g = (d ln κ/d ln ρ)T, was found to be g = 6.7 at 190 K, increasing to 9.4 at 110 K as the temperature decreases. This increase is attributed to an enhanced phonon contribution to the total κ.

The thermal conductivity κ of cyclopentane clathrate hydrate (CP CH) of type II was measured at temperatures down to 100 K and at pressures up to 1.3 GPa. The results show that CP CH displays amorphous-like κ characteristic of many crystalline clathrate hydrates, e.g., tetrahydrofuran (THF) CH. The magnitude of κ is 0.47 W m⁻¹ K⁻¹ near the melting point of 280 K at atmospheric pressure, and it is almost independent of pressure and temperature T: ln κ = −0.621−40.1/T at atmospheric pressure (in SI-units). This is slightly less than κ of type II CHs of water-miscible solvents such as THF. Intriguingly, unlike other water-rich type II clathrate hydrates of water-miscible molecules M (M·17 H₂O), CP CH does not amorphize at pressures up to 1.3 GPa at 130 K and also remains stable up to 0.5 GPa at 240 K. This shows that CP CH is mechanically more stable than the previously studied water-rich type II CHs, and suggests that repulsive forces between CP and the H2O cages increase the mechanical stability of crystalline CP CH. Moreover, we show that κ of an ice-CH mixture, which often arises for CHs that form naturally, is described by the average of the parallel and series heat conduction models to within 5% for ice contents up to 22 wt%. The findings provide a better understanding of the thermal and stability properties of clathrate hydrates for their applications such as gas storage compounds.