Low-temperature thermal conductivity of an amorphous palladium silicon alloy

The thermal conductivity of a glassy PdSi alloy has been measured in the temperature range 0.1–30 K. The thermal conductivity contributed by the phonons has the same anomalous behavior as found for all non-metallic amorphous materials. Heat treatment increases the phonon thermal conductance by ～50%, a change which does not appear to be related to partial crystallization.     

Low-temperature specific heats of porous silica xerogels of low densities

Low-temperature specific heat measurements have been performed in porous silica xerogels with densities varying from 670 to 1730 kg m⁻³ to study the low-energy vibrational dynamics. The specific heat, C_p, shows a bump in the temperature range above 4 K, when reported in a plot of C_p/T³ against the temperature, T. The bump is almost independent of the sample density and is close to the boson peak observed in melt-quenched amorphous silica (a-SiO₂). At temperatures <4 K, an additional contribution to that predicted by the Debye theory is observed. It follows an approximately linear temperature dependence (C_exc=aT^1+v, v being equal to about 0.25). In the xerogel with the largest density, specific heat of about a factor 5 larger than that of a-SiO₂ is measured, which increases with decreasing sample density. By comparison with the corresponding properties of a-SiO₂, we conclude that the disorder introduced by the presence of pores does not measurably affect the excess density of vibrational states in a frequency range of the boson peak (BP), but increases the density of the two-level systems (TLS).     

Fractal slice model analysis for effective thermal conductivity and temperature distribution of porous crystal layer via layer crystallization

A fractal slice model was established based on the reported model to predict the effective thermal conductivity of a porous crystal layer via layer crystallization. The temperature distribution of the crystal layer was obtained by the fractal slice method. The simulation results agreed with the experimental data better than the other theoretical models. The results were helpful to enhance the thermal transport by mitigating the thermal resistance effectively.     

The thermal conductivity of some chalcogenide glasses

The temperature-dependent thermal conductivities of several chalcogenide glasses were determined above 300 K. At 300 K, the thermal conductivity for most of these glasses is about 3 mW/cm · deg.     

Synthesis and thermal diffusion of nanostructured porous GaAs

Nanostructured porous GaAs samples were prepared by electrochemical anodic dissolution of GaAs for various current densities and etching periods. The samples are characterized by SEM and photoluminescence (PL) where a blue shift is observed in PL. Thermal properties of this nanostructured GaAs are then studied by photoacoustic (PA) spectroscopy, where one order decrease in thermal conductivity of porous GaAs compared to bulk is observed. Further it is shown that the thermal conductivity of porous GaAs decreases with decrease in size of the particles. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Low temperature thermal conductivity and specific heat of elastomers

The specific heat C and thermal conductivity κ of polybutadiene are characteristic of all non-crystalline materials at temperatures below ≈ K, reflecting the presence of localized excitations. The changes in C and κ with variation in crosslinking suggest that the relaxation times of the localized excitations may vary with crosslink density. Extension of an elastomer does not reveal a change in density of localized excitations as monitored by measuring κ, possibly because other phonon scattering mechanisms mask the effect. For T ? 10 K the phonon mean-free-path is independent of the microscopic anisotropy of the elastomer.     

Low-temperature X-ray studies of crystal-lattice parameters and thermal expansion of KTiOPO4 crystals

The unit-cell parameters a, b, and c of KTiOPO₄ crystals have been measured by the X-ray diffraction method in the temperature range 80–320 K. The parameters obtained were used to determine the thermal expansion coefficients α_[100], α_[010], and α_[001] along the principal crystallographic axes. It was established that thermal expansion in the crystals is essentially anisotropic and that α_[010] > α _[100], whereas α_[001] is close to zero.     

Explanation of the anomalous thermal conductivity of crosslinked cis-1,4 polybutadiene

An anomaly in the low-temperature thermal conductivity of crosslinked polybutadiene is found to be due to partial crystallization. The behaviour is qualitatively explained, and compared with other semi-crystalline polymers.     

On the change of low-temperature thermal conductivity of gap after bending

The thermal conduction of a sulphur-doped single crystal plastically deformed by bending at 963 K was measured between 2 and 50 K. The thermal resistivity W at temperatures > 14 K is practically independent on deformation. At lower temperatures W is found to be proportional to AT⁻³ before and to AT⁻³ + BT⁻² after deformation. A is due to boundary scattering, B due to dislocations.     

The generalized Jonscher's relationship for conductivity and its confirmation for porous structures

The theoretical generalization of the Jonscher's relationship for the complex conductivity of carriers moving in self-similar medium is derived. It is shown that the correction derived enters to more general expression, which, in turn, we define as the generalized Jonscher's relationship. The basic idea which was used for the derivation of the relationship is based on the supposition that disordered medium has self-similar property. The derived relationship is confirmed on dielectric spectroscopy data related to sodium nitrite embedded to porous glasses. Based on new relationship there is a possibility to extract additional information about relaxation processes of a system of dipoles from the processes related to conductivity. It is important in the cases when the contribution to relaxation peaks is small and unnoticeable on the background of essential domination of processes related to conductivity.     

Effect of thermal history on conductivity and electrical relaxation in alkali silicate glasses

Electrical conductivity σ₀ and electric field relaxation measurements have been carried out as a function of thermal history for two alkali silicate glasses, Na₂O3SiO₂ and K₂O3SiO₂. Specimens of each glass with three different thermal histories, two of the anneal-and-quench type and one of the rate-cool type, were studied. The average structural or fictive temperature T_f of each of the specimens was characterized by measuring their indices of refraction. Effects of thermal history on σ₀ and its activation enthalpy $\text{H}{}_{\mathrm{\sigma }}{}^1$ were in accord with results of previous investigators. That is, for a given type of thermal history σ₀ was lower and $\text{H}{}_{\mathrm{\sigma }}{}^1$ higher the lower T_f. In addition it was found that for two specimens with the same T_f or index of refraction but different thermal histories the rate-cooled specimen exhibited a lower conductivity than the annealed-and-quenched specimen, in accord with the results of Ritland. The distribution of relaxation times τ_σ for decay of the electric field due to ionic migration was found to be due primarily to a distribution in the pre-exponential term ln $\text{τ}{}_{\mathrm{\sigma }}{}^1$ in the equation $\text{ln}\text{τ}{}_{\mathrm{\sigma }}\text{=}\text{ln}\text{τ}{}_{\mathrm{\sigma }}{}^1\text{+ H}{}^1\text{/RT}$; the distribution in $\text{H}{}^1$ was extremely narrow. Differences in thermal history caused small differences in the distribution of τ_σ, but no difference in the average activation enthalpy $\text{〈H}{}^1\text{〉}$ for τ_σ. From this result it appeared that the dependence of the conductivity activation enthalpy $\text{H}{}_{\mathrm{\sigma }}{}^1$ on thermal history was due to the effect of thermal history on the temperature dependence of the distribution in τ_σ.     

Influence of cellular substructure on the thermal conductivity of heterovalent solid solutions of fluorides

The thermal conductivity of single-crystal samples of Sr_0.88La_0.09Nd_0.03F_2.03 and Sr_0.80La_0.15Nd_0.05F_2.20 solid solutions, both with a cellular substructure and without it, have been investigated by the method of stationary longitudinal heat flow in the temperature range of 50–300 K. An anisotropy of thermal conductivity (related to the substructure) is established for a Sr_0.995Nd_0.005F_2.005 single crystal.     

Structural,morphological, ionic conductivity,and thermal properties of pectin-based polymer electrolytes

New polymer electrolytes (PEs), potentially interesting for solid-state electrochemical devices applications, were synthesized by a solvent casting method using pectin and ionic liquid (IL) N,N,N-trimethyl-N-(2-hydroxyethyl)ammonium bis(trifluoromethylsulfonyl)imide ([N_{1 1 1 2(OH)}] [NTf₂]. The resulting electrolytes besides being moderately homogenous and thermally stable below 155°C, they also exhibited good mechanical properties. The SPE membranes were analyzed by differential scanning calorimetry (DSC), X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and complex impedance spectroscopy.     

Electrical conductivity and thermal dehydration studies of hydrated barium oxalate and barium cadmium oxalate crystals

The measurement of electrical conductivity for the investigation of the number of water molecules present in the hydrated barium oxalate, and barium cadmium oxalate lattice have been carried out in the temperature range 30 to 415 °C. The dehydration temperature and the number of water molecules jettisoned out of the structure at a particular temperature is estimated from the sharp increase in conductivity at these points. The almost abrupt increase of conductivity is attributed to the increase in the number of mobile charge carriers viz., H⁺ and OH⁻ ions generated from the escaping water molecules. Samples in the form of pellets with and without silver paste applied, showed different behaviour with respect to conductivity of the material upto the completion of dehydration. The study of electrical conductivity in association with the thermal behaviour has been used to understand the mechanism of conduction.     

Influence of phase separation on the thermal conductivity of potassium borosilicate glasses between 1 and 10 K

The thermal conductivities of five potassium borosilicate samples which had undergone thermal treatment for different periods of time, were measured between 1 and 10 K. The results showed that, initially, as the particles grow bigger, thermal conductivity decreases until it reaches a minimum corresponding to an average dimension of 900 Å, thereafter increasing. An attempt was made to analyse the results in terms of a resonant scattering theory, which proved to be insufficient. As an alternative, a phenomenological model taking into account Kapitza's resistance was developed. This effect being important mainly during the first stage of the thermal treatment when the boundaries between phases are not yet well defined, it could explain the existence of a minimum of the thermal conductivity.     

A molecular dynamics study of the thermal conductivity of nanoporous silica aerogel,obtained through negative pressure rupturing

In this study, classical molecular dynamics with the well-known van Beest, Kramer and van Santen potential are used for the first time to investigate the solid thermal conductivity of silica aerogel. Aerogel samples at various densities are obtained through negative pressure rupturing of dense silica samples, and reverse non-equilibrium molecular dynamics is employed to determine the thermal conductivity at each density. Results indicate that a power-law fit of the thermal conductivity obtained varies almost linearly with density, where decreasing density and increasing porosity led to an almost linear decrease in thermal conductivity. This is reflective of the trend observed in experimental bulk sintered silica aerogel. The results also showed that the thermal conductivity is of the same order of magnitude as bulk sintered aerogel. The power-law fit of the results also accurately reflected the variation found in bulk sintered aerogel.     

Low-temperature dehydration of gypsum single crystals

On the basis of the analysis of the electron spin resonance spectrum of the SO ₃ ^? radical ion, it is established that the transformation of gypsum (CaSO₄ · 2H₂O) into bassanite (CaSO₄ · 0.5H₂O) with partial removal of water molecules occurs through the formation of an anhydrous metastable phase. This phenomenon, reflecting the processes of recrystallization of materials of this class, serves as the scientific basis for the production of binders from sulfate mineral associations.