Heat transfer in crystalline clathrate hydrates at low temperatures

The experimental results on the thermal conductivity k(T) of crystalline Xe, CH₄, and THF clathrate hydrates have been analyzed. In a wide region of temperatures above 2 K, k(T) exhibits a behavior typical of disordered solids, which depends weakly on their chemical composition, crystalline structure and microstructure. The results are discussed in the context of phenomenological models of phonon scattering by local modes. It has been found that the Xe clathrate has a feature unusual for glasses, namely, k(T) decreases almost two-fold as the temperature increases from 50–100 K. The behavior of k(T) is presumably determined mainly by the strong phonon scattering on water molecules

The effect of proton ordering in thermal conductivity of clathrate tetrahydrofuran hydrate

The effect of proton ordering in tetrahydrofuran hydrate has been detected using a technique based on measurement of thermal conductivity. The thermal conductivity of tetrahydrofuran hydrate was measured by the steady-state potentiometric method in an interval of 2–150 K. Two regimes were selected to observe the effect: (i) slow cooling of the hydrate sample and (ii) doping the hydrate with a small quantity of KOH alkali to a concentration of 10⁻⁴. Proton ordering affects the temperature dependence of the thermal conductivity changing its glass-like behavior to crystal-like below 150 K. The phonon maximum that appears in the temperature dependence of the thermal conductivity at T = 17 K is attributed to the cooperative process of local proton ordering stimulated by orientational and ionic defects

Influence of thermal treatment on thermal properties of adamantane derivatives

Heat transport mechanisms present in 2-adamantanone and 1-cyanoadamantane crystals were investigated in a broad temperature range. To characterize scattering processes, thermal conductivity and heat capacity measurements were carried out. A particular care was paid to the cooling rate of specimen which influenced the thermal history of the samples. The experimental results led to a conclusion that under slow cooling the thermal conductivity reaches the highest values and resembles the behavior of ordered molecular crystals. As for fast cooling, the “quenching” resulted in changes in both the structure and the temperature dependence of the thermal conductivity, the latter resembling that of amorphous solids. In heat capacity measurements the thermal history made on samples did not reflect the preliminary findings known from thermal conductivity results, which could imply that the observed mechanisms are more complex

Thermal conductivity of argon–SiO₂ cryocrystal nanocomposite

The effective thermal conductivity of samples of cryocrystal nanocomposite obtained from argon and SiO2 nanopowder was determined in the temperature interval 2–35 K using the steady-state method. The thermal conductivity of crystalline argon with nanoparticles of amorphous silica oxide embedded in its structure shows a weak dependence on particle linear dimension in the interval 5–42 nm. The temperature dependence of the thermal conductivity of the nanocomposites can be well approximated by taking into account only the two mechanisms of heat carrier scattering: phonon-phonon interaction in U-processes and scattering of phonons by dislocations

Orientational isotopic effects in the thermal conductivity of CH4/CD4 solid solutions

The thermal conductivity of (CH4)1–c(CD4)c solid solutions with c = 0, 0.03, 0.065, 0.13, 0.22, 0.4, 0.78, and 1.0 has been measured in the region of existence of three orientational phases: disordered (phase I), partially ordered (phase II) and completely ordered (phase III). The temperature range is 1.3–30 K. It is shown that the thermal conductivity has different temperature dependences k(T) in these phases. Its value increases with the degree of the orientational order in the phase. In phase I the thermal conductivity is independent of c and weakly dependent on T. The impurity effect in k(T) is much stronger in the low-temperature part of phase II than in phase III. As the concentration c grows, the k(T) curve of phase II approaches the dependence k(T) typical of phase I. There is a hysteresis in the vicinity of the II↔III phase transition. In phase III the impurity effect in k(T) can be considered as phonon scattering at rotational defects developing due to the difference between the moments of inertia of the CH4 and CD4 molecules. The obtained dependences of thermal conductivity on temperature and concentration can be explained qualitatively assuming that the dominant mechanism of phonon scattering is connected with the interaction of phonons with the rotational motion of the molecules in all of the three orientational phases of the CH4–CD4 system

Quantum effects in the thermal conductivity of solid krypton—methane solutions

The dynamic interaction of a quantum rotor with its crystalline environment has been studied by measurement of the thermal conductivity of the Kr₁–c(CH₄)c solid solutions at c = 0.05–0.75 in the temperature region 2–40 K. The thermal resistance of the solutions was mainly determined by the resonance scattering of phonons on CH₄ molecules with the nuclear spin I = 1 (the nuclear spin of the T species). The influence of the nuclear spin conversion on the temperature dependence of the thermal conductivity к(T) leads to a well-defined minimum on к(T). The temperature of the minimum depends on the CH₄ concentration. It was shown that the nonmonotonic increase of the anisotropic molecular field with the CH₄ concentration is caused by a compensation effect due to corrections in the mutual orientations of the neighboring rotors at c > 0.5. The temperature dependence of Kr₁–c(CH₄)c is described within the Debye model of thermal conductivity taking into account the lower limit of the phonon mean free path. It is shown that phonon–rotation coupling is responsible for the anomalous temperature dependence of the thermal resistance at varying temperature. It increases strongly when the quantum character of the CH₄ rotation at low temperatures changes to a classical one at high temperatures. A thermal conductivity jump (a sharp increase in к(T) within a narrow temperature range) was also observed. The temperature position of the jump varies from 9.7 to 8.4 K when the CH₄ concentration changes from 0.25 to 0.45

Anomaly in temperature dependence of thermal transport of two hydrogen-bonded glass-forming liquids

6 págs.; 3 figs.; PACS number s : 66.70. f, 63.50. x, 65.20. w, 65.60. aThe thermal conductivity of two molecular glasses (ethanol and 1-propanol) decrease with increasing temperature up to their glass transitions at Tg 97 and 98 K, respectively. Within their supercooled liquid phases, the conductivity increases with rising temperature up to a maximum which roughly coincides with the liquidus (or melting temperatures Tm 159 K and Tm 149 K, respectively). From there on, the conductivity decreases with increasing temperature, a behavior common to most liquids examined so far, exception made of liquid water. The origin of the rather different dependencies with temperature of thermal transport is understood as a competition between phonon-assisted and diffusive transport effects which are amenable to experiments using high resolution quasielastic neutron scattering and visible and ultraviolet Brillouin light-scattering spectroscopies. © 2007 The American Physical Society.Peer Reviewe

Heat transfer in solid methyl alcohol

Thermal conductivity coefficient к(T) of two crystalline (orientationally-ordered and orientationally-disordered) phases of pure methanol (at temperatures from 2 K to Tm , Tm is the melting temperature), CH₃OH + 6.6 % H₂O glass from 2 K to Tg , Tg is the glass transition temperature and a supercooled liquid from Tg to 120 K has been measured under equilibrium vapor pressure. The dependence к(T) is described approximately as a sum of two contributions: кI(T) describing heat transport by acoustic phonons and кII(T) —by localized high-frequency excitations. The temperature dependences of the thermal conductivity of primary monoatomic alcohols CH₃OH, C₂H₅OH, and C₃H₇OH in the glass state have been compared. Different mechanisms of phonon scattering in the crystalline phases and glass have been analyzed. The кII(T) has been calculated within the Cahill–Pohl model. There is an anomaly of the thermal conductivity of the glass state near Tg (a smeared minimum in the к(T) — curve)

Experimental evidence of the role of quasilocalized phonons in the thermal conductivity of simple alcohols in orientationally ordered crystalline phases

The thermal conductivity к(T) of crystalline alcohols (methyl, ethyl and 1-propyl) within their thermodynamic equilibrium phases for T ≥ 2 K and under the equilibrium vapor pressures has been measured and analyzed. While such compounds usually exhibit a rich polymorphism including amorphous and partially ordered crystals, the phases here explored correspond to crystals showing complete orientational order. The results show that the temperature dependence of к(T) above its maximum deviates from the expected decrease following a 1/T law with increasing temperature arising from anharmonic interactions involving acoustic excitations. Such a deviation is here attributed to the presence of a component кII(T) corresponding to the shortest-lifetime phonons (Cahill–Pohl model) additional to that кI(T) related to propagating phonons and thus: к(T) = кI(T) + кII(T). Above T = 40 K кI(T) does follow the law 1/T and кII(T) is basically temperature independent. The component кI(T) is well described by the Debye–Peierls model taking into account the phonon–phonon Umklapp processes and phonon scattering by dislocations. In turn, the contribution кII(T) is attributed to the effects of higher lying excitations which get thermally populated above some 40 K. Finally, a systematic trend is found concerning the strength of phonon–phonon scattering which is seen to diminish as the number of carbon atoms in the alcohol molecule increases

Deuteration effects in the thermal conductivity of molecular glasses

The thermal conductivity κ(T) of pure deuterated ethanol has been measured under its equilibrium vapor pressure in its orientationally-ordered crystal (T = 2 K – Tm), orientational glass and the glass state (T = 2 K – Tg, Tg is the glass transition temperature) solid phases. The temperature dependence of the conductivity is well described by a sum of two contributions: κ(T) = κI(T) + κII(T), where κI(T) account for the heat transport by acoustic phonons and κII(T) for the heat transfer by localized high-frequency excitations respectively. The thermal conductivities of deuterated and hydrogenated ethanols are compared in different phases. The phonon scattering mechanisms in the glasses have been analyzed. In the investigated glasses the effect of complete deuteration shows up as a contribution κII(T)