Thermal Conductivity Of Glasses Research Articles

Thermal Conductivity in Glasses Below 1K: New Technique and Results

Using the capacitance of a glass sample as its own thermometer, we have measured the thermal conductivity of two glasses below 700mK. This technique avoids any thermal boundary resistance between the thermometer and the sample. In addition, since the loss tangent goes to zero as the temperature is lowered, the amount of heat dissipated in a capacitive measurement is very small at low temperatures. We were able to measure the thermal conductivity at temperatures which were previously experimentally inaccessible and found that the thermal conductivity, κ, scales as T 1.86 down to 10mK, in agreement with the two-level system model.

On the increase of thermal conductivity in glasses above the plateau region

Hopping of localized vibrational states in glassy systems via vibrational anharmonicity, C eff, provides an additional heat-carrying channel. The associated thermal conductivity, κ hop( T), increases linearly with increasing temperature T in the regime where vibrational lifetimes are reasonably long. Recent work of Yamaguchi et al. has shown that, for a- GeS 2, C eff for the strongly localized states associated with the Bose peak is at least 20 times larger than the acoustic anharmonicity C 111. Insertion of this value into the theoretical expression for κ hop( T), along with accepted values for the other parameters, yields a magnitude for κ( T) quite comparable to that found for network-forming glasses above the plateau temperature.

Thermal conductivity of an organic clathrate: Possible generality of glasslike thermal conductivity in crystalline molecular solids

The present investigation of an organic clathrate, hexakis(phenylthio)benzene with carbon tetrabromide guests, shows its thermal conductivity (determined from 39 to 146 K) to be similar to the “universal” thermal conductivity of glasses, despite its crystalline structure. We attribute the low thermal conductivity and its positive temperature derivative to efficient scattering of the heat-carrying acoustic phonons by localized optic modes. This behavior may well be common for molecular solids.

Do phonons exist in non-crystalline matter?

The low-frequency properties of glasses are usually explained by phonons. It will be shown here that the motions at these frequencies are vibrational and are not phonons, but localized vibrations of the multifractal cluster structures of the glasses that should be called clustrons. Because of the distinct elements of these structures (clusters) far fewer atoms hold a vibrational degree of freedom than in crystals, thereby explaining the greater density of states in glasses than in the crystal. The extremely strong localization of these clustrons (because the neighbour clusters vibrate at different frequencies) explains the decreased thermal conductivity in glasses apart from the specific heat increase (which would cause a parallel increase in the thermal conductivity in crystals).

Low-temperature specific heat and thermal conductivity of glasses.

The soft potential model (an extension of the tunneling model to include soft localized vibrations) is shown to describe the anomalous features of the specific heat ${\mathit{C}}_{\mathit{P}}$ and the thermal conductivity of glasses over the entire low-temperature range, up to an including the peak in ${\mathit{C}}_{\mathit{P}}$/${\mathit{T}}^{3}$ and the second rise of the thermal conductivity above the plateau.

Interaction of soft modes and sound waves in glasses.

The soft potential model (an extension of the tunneling model to include soft localized vibrations) is shown to describe the anomalous features of the specific heat, the sound absorption and the thermal conductivity of glasses over the entire low temperature range.

Thermal Conductivity of Glasses: Theory and Application to Amorphous Si

Thermal Conductivity of Glasses: Theory and Application to Amorphous Si

Thermal conductivity of glasses at low temperatures

It is shown that at low temperatures the heat conductivity of glasses is determined by a narrow region of the phase space of phonons, the characteristic size of which corresponds to √2 α . It has been found possible to determine the functional dependence of thermal conductivity in wide range of low temperatures and, in particular, explain the plateau portion of the curve.

Thermal conductivity of glasses: Theory and application to amorphous Si.

Vibrations of amorphous Si are represented by the network model of Wooten, Winer, and Weaire and the forces of Stillinger and Weber. It is shown how the Kubo formula for thermal conductivity can be evaluated "exactly" assuming harmonic vibrations, making only finite-system-size errors. The results support a "shunt-resistor model" which fits data for glasses very well.

Origin of the plateau in the low-temperature thermal conductivity of silica.

Thermal conductivities of glasses at low temperatures show strikingly similar behavior irrespective of their chemical composition. While for T T>2 K is totally unexplained. While Rayleigh scattering of phonons by structural disorder should be the natural cause for limiting the mean free path of phonons in this temperature range, it has been concluded before that in glasses a strong enough source of such scattering does not exist. In this study we show by a proper structural analysis in at least one material (namely, silica) that a strong enough source of Rayleigh scattering of phonons in glasses does exist so that the ‘‘universal plateau’’ can be explained without invoking any new mechanism. This may be for the first time that the low-temperature property of a structural glass has been correlated to its structure.

Atomic tunneling states with dissipation in glasses: temperature-dependent two-level systems

By taking account of the influence of dissipation on atomic tunneling states in glasses, we introduce a density of states of two-level systems which is non-linearly temperature dependent. Based on this model, we can explain the “excess T 3” anomaly in the specific heat and the “plateau” in the thermal conductivity of glasses.

Thermal conductivity of gamma-irradiated ternary borate glasses

The thermal conductivity of a variety of ternary borate glasses before and after gamma irradiation has been measured by the steady-state method at 25°C. The results obtained show that the thermal conductivity decreases with the increase of alkali oxide content or when the glass structure becomes more disrupted. The effect of gamma radiation is believed to result from the creation of disorder and shortening of the phonon mean free path leading to a decrease in the thermal conductivity of the glass.

Thermal conductivity of sodium disilicate melt at high pressures

The thermal conductivity of sodium disilicate (Na 2O · 2SiO 2) has been measured over the temperature range 300 to 1600 K at high pressures up to 5 GPa. Data have been obtained for four states of sodium disilicate: crystal, melt and two types of glass (normal and densified in the non-rigid state). The conductivity of the crystalline state obeys an inverse temperature dependence. Whereas the thermal conductivity of glass at atmospheric pressure increases with temperature, the results at high pressures exhibit an initial decrease of conductivity up to ∼ 900 K, followed by a premonitory increase before melting. The conductivity decreases by ∼ 70% in the melt range and the conductivity of the melt becomes almost independent of temperature. The pressure dependence of the conductivity for the melt has a minimum at ∼ 2 GPa. This is close to the pressure where a change of melt structure is inferred from viscosity and density measurements for the same melt at high pressures.

Low-temperature behavior of potassium and sodium silicate glasses.

In a study of the low-temperature anomalies which appear in disordered solids we have measured the specific heat, thermal conductivity, thermal expansion, and dielectric dispersion for a carefully prepared series of (${\mathrm{SiO}}_{2}$${)}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$(${\mathrm{K}}_{2}$O${)}_{\mathrm{x}}$ and (${\mathrm{SiO}}_{2}$${)}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$(${\mathrm{Na}}_{2}$O${)}_{\mathrm{x}}$ glasses. Each sample had a known glass transition temperature ${T}_{g}$, mass density, phonon velocities, covalency, and density and concentration fluctuations. The only significant correlation occurred between the phonon mean free path and the glass-transition temperature. No explicit support was found for any model developed to explain the specific heat and/or thermal conductivity of glasses.

Effect of transition metal oxides on the thermal conductivity of glass

The dependence of the thermal conductivity of a barium borosilicate glass on the type and concentration of transition metal oxides has been studied. The thermal conductivity was measured by the steady-state method at 25°C. The results obtained showed that the introduction of TiO 2 decreased the thermal conductivity whereas MnO 2, Fe 2O 3, CoO, NiO or CuO increased it. The thermal conductivity increased with the atomic weight of the transition element. Increasing the concentration of the transition metal oxide magnified both effects. The factors representing the specific contribution of the various transition metal oxides to thermal conductivity were calculated from thermal conductivity values measured by the least-square method. Only TiO 2 gave negative values whereas other oxides gave positive values which increased with the atomic weight of the transition metal. The effects brought about by transition metal oxides on the parameters affecting the thermal conductivity are considered.

Linear dispersion of transverse high-frequency phonons in vitreous silica

Using phonon interference in thin films, we have measured the dispersion curve of transverse phonons in vitreous silica. A linear dispersion was found up to 450 GHz. This result has important implications for theories on the specific heat and thermal conductivity of glasses and is consistent with recent model calculations of phonons in a glass.

ChemInform Abstract: THERMAL CONDUCTIVITY OF LEAD BORATE GLASSES

AbstractDie Wärmeleitfähigkeit von Gläsern im System PbO‐B2O3 bei 30°C (stationäre Bestimmungsmethode) nimmt mit zunehmendem PbO‐Gehalt immer stärker ab.

Thermal Conductivity of Lead Borate Glasses

Thermal conductivity of glasses in the system PbO‐B2O3 at 30°C were determined by steady‐state methods. The values decrease progressively with increasing PbO content. The experimental thermal conductivities and those calculated from factors representing the contributions of individual oxides (on a weight percent basis) agree fairly well.

Theoretical method of determining the temperature dependence of the thermal conductivity of glasses

Theoretical method of determining the temperature dependence of the thermal conductivity of glasses

Long-range refractive index fluctuations in optical glass as revealed by light scattering

Light-scattering measurements have been made on optical-quality glass before and after annealing. Small-angle scattering in both specimens indicates the existence of long-range correlations in refractive index of order several hundred nanometres. In addition, wide-angle scatter demonstrates the existence of shorter-range (a few nanometres) correlation distances. All of our observations have been consistent with Goodman's strained mixed cluster model of glasses (1975, 1977). It is shown that the amplitude of the fluctuations is insufficient to support the explanation of the low-temperature (<1K) thermal conductivity of glasses in terms of structural scattering of phonons.