Thermal conductivity of solid argon from molecular dynamics simulations

The thermal conductivity of solid argon in the classical limit has been calculated by equilibrium molecular dynamic simulations using the Green-Kubo formalism and a Lennard-Jones interatomic potential. Contrary to previous theoretical reports, we find that the computed thermal conductivities are in good agreement with experimental data. The computed values are also in agreement with the high-temperature limit of the three-phonon scattering contribution to the thermal conductivity. We find that finite-size effects are negligible and that phonon lifetimes have two characteristic time scales, so that agreement with kinetic theory is obtained only after appropriate averaging of the calculated phonon lifetimes.     

Role of bond strength on the lattice thermal expansion and oxide ion conductivity in quaternary pyrochlore solid solutions

Quaternary pyrochlore-type solid solutions, CaGdZrNb(1-x)Ta(x)O(7) (x = 0, 0.2, 0.4, 0.6, 0.8, 1), were prepared by a high-temperature ceramic route. The pyrochlore phases of the compounds were confirmed by powder X-ray diffraction (XRD), Raman spectroscopy, and transmission electron microscopy. The crystallographic parameters of the pyrochlore compounds were accurately determined by Rietveld analysis of the powder XRD data. The isovalent substitution of Ta in place of Nb at the B site can reveal the effect of chemical bonding on lattice thermal expansion and oxide ion conductivity because both Nb and Ta have the same ionic radius (0.64 ?). Lattice thermal expansion coefficients of the samples were calculated from high-temperature XRD measurements, and it was found that the thermal expansion coefficient decreases with substitution of Ta. Oxide ion conductivity measured by a two-probe method also shows the same trend with substitution of Ta, and this can be attributed to the high bond strength of the Ta-O bond compared to that of the Nb-O bond. Microstructural characterization using scanning electron microscopy proves that the size of the grains has a small effect on the oxide ion conductivity. Our studies established the role of chemical bonding in deciding the conductivity of pyrochlore oxides and confirmed that the 48f-48f mechanism of oxide ion conduction is dominant in pyrochlore oxides.     

Thermal conductivity of solid argon at high pressure and high temperature: a molecular dynamics study

The thermal conductivity of solid argon at high-pressure (up to 50 GPa) and high-temperature (up to 2000 K) has been calculated by equilibrium molecular dynamics simulations using the Green-Kubo formalism and an exponential-6 interatomic potential. A simple empirical expression is given for its pressure and temperature dependence. The results are compared with predictions based on kinetic theory. The relative change of the thermal conductivity lambda with density rho is found to be consistent with a partial differential ln lambda/ partial differential ln rho slope of approximately 6 in a wide range of pressures and temperatures, in good agreement with predictions based on kinetic theory.     

Low temperatures lattice thermal conductivity on non-crystalline polymers

The density fluctuation model is used to analyze the lattice thermal conductivity data of two samples of polycarbonate between 0.04 and 1K. The study is carried out by calculating the latice thermal conductivity of a noncrystalline polymer as the sum of two contributions asK=K _BM+K _Em, whereK _BE is attributed to phonons which interact with the crystal boundaries,K _EM is due to phonons which interact with the empty spaces. The relative importance of each contribution has also been examined by estimating their percentage contributions to the lattice thermal conductivity. An excellent fit to the experimental data was obtained over the whole temperature range.     

Thermal analysis of chemically synthesized polyaniline and effects of thermal aging on conductivity

Thermal characteristics of chemically synthesized polyaniline with various dopants have been studied by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared spectroscopy, gel-permeation chromatography (GPC), and chemical titration. The HCl-doped polyaniline shows three major weight losses at around 100, 200, and 500°C which are assigned to removal of H₂O and HCl, and decomposition of the polymer, respectively. Thermal aging of the HCl-doped polyaniline performed at 100, 150, and 200°C for various periods of time results in a decrease in conductivity. After the thermal treatments, the polymer can be re-doped with HCl to partially recover the conductivity. However, both the conductivity and the doping level cannot be restored to the level of the original materials owing probably to changes in morphology, crosslinking, or other chemical reactions.     

Studies on lattice thermal expansion and XPS of ThO2NdO1.5 solid solutions

The lattice parameter changes with respect to temperature (T) have been measured by high temperature X-ray diffraction (HTXRD) technique for ThO₂NdO_1.5 solid solutions containing 23.8 and 42.5 mol% NdO_1.5 in the temperature range from 298 to 2000 K. The temperature versus lattice parameter data have been made use of in calculating the lattice thermal expansivity. The values of thermal expansion of the solid solutions were found to be increased with increase in neodymium oxide content and temperature. The mean linear thermal expansion coefficients in this temperature range for ThO₂NdO_1.5 solid solutions are 12.28 × 10⁻⁶ and 12.90 × 10⁻⁶ K⁻¹, respectively. The binding energies of Th 4f_7/2 and Nd 3d_5/2 energy levels of the solid solutions containing 13.1, 23.8, 31.9, 37.2 and 42.5 mol% NdO_1.5 and two-phase mixtures containing 47.6 and 51.8 mol% NdO_1.5 were experimentally determined by X-ray photoelectron spectroscopy (XPS).     

Thermal conductivity of nanotubes revisited: effects of chirality, isotope impurity, tube length, and temperature

We study the dependence of the thermal conductivity of single-walled nanotubes on chirality, isotope impurity, tube length, and temperature by nonequilibrium molecular-dynamics method with accurate potentials. It is found that, contrary to electronic conductivity, the thermal conductivity is insensitive to the chirality. The isotope impurity, however, can reduce the thermal conductivity up to 60% and change the temperature dependence behavior. We also found that the tube length dependence of thermal conductivity is different for nanotubes of different radii at different temperatures.     

Analysis of lattice thermal conductivity of GaAs at high temperatures

The lattice thermal conductivity of GaAs has been analysed in the entire temperature range 100–800 K in the frame of the Sharma-Dubey-Verma (SDV) model of phonon conductivity, and very good agreement has been found between the calculated and experimental values of the lattice thermal conductivity in the entire temperature range of study. The temperature exponentm(T) for the three-phonon scattering relaxation rate for GaAs has also been calculated in the above temperature range. The separate percentage contributions due to transverse and longitudinal phonons have also been studied.

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Zusammenfassung Die Gitter-Warmeleitfähigkeit von GaAs wurde im Temperaturbereich zwischen 100 und 800 K im Rahmen des Modells der Phononen-Leitfähigkeit von Sharma-Dubey-Verma (SDV) untersucht und eine sehr gute übereinstimmung zwischen den berechneten und Versuchswerten der Gitter-Wärmeleitfähigkeit im ganzen untersuchten Temperaturbereich gefunden. Der Temperaturexponentm=m(T) für die Drei-Phonenen-Streuungs-Relaxationsgeschwindigkeit bei GaAs wurde im obengenannten Temperaturbereich ebenfalls berechnet. Die durch transversale und longitudinale Phonone verursachten einzelnen prozentualen Beiträge wurden ebenfalls untersucht.

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Résumé La conductibilité thermique du réseau de GaAs a été analysée dans tout l'intervalle de température allant de 100 à 800 K, dans le cadre du modÊle de conductibilité de phonons de Sharma-Dubey-Verma (SDV). Une trÊs bonne concordance a été observée entre les valeurs calculées et expérimentales de la conductibilité thermique du réseau dans l'intervalle de température étudié. L'exposant de températurem(T) a également été calculé dans ce domaine de température pour la vitesse de relaxation de la diffusion de trois phonons. Les pourcentages individuels des contributions des phonons transversaux et longitudinaux ont également été étudiés.

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100–800 CaAs -- . . m() CaAs. , .

     

Thermal expansion of the crystal lattice of novel thermoplastic polyimides

Measurements of the coefficient of thermal expansion (CTE) of the crystalline lattice of two semicrystalline thermoplastic polyimides are reported. NEW-TPI and LARC-CPI polyimides were studied using elevated temperature wide-angle x-ray scattering (WAXS) from 25°C. To 325°C. To examine possible shifts in the c-axis, a novel approach developed in our lab was used to create highly oriented samples. Films were treated in 1-methyl-2-pyrrolidinone (NMP) at the reflux temperature, washed and then dried under constraint. The films treated in this manner were highly oriented, with c-axes preferentially aligned along the film normal. The advantage of this orientation is that it allows numerous reflections of type (00l) to be examined for temperature shifts of the c-axis using reflection mode WAXS. No systematic shift of the c-axis lattice parameter as a function of temperature was observed in either NEW-TPI or LARC-CPI. The c-axis thermal expansion is concluded to be smaller than 8 x 10^?6/°C for LARC-CPI, and for NEW-TPI may be weakly negative. From WAXS of unoriented films, systematic shifts in the a and b lattice parameters were deduced as a function of temperature. The linear CTEs relating these unit cell parameters at temperature T to their values at 0°C are: ©1995 John Wiley & Sons, Inc.     

Degree of crystallinity and lattice thermal conductivity of polyethylene at low temperatures

The lattice thermal conductivity of a semicrystalline polymer was studied at low temperatures by calculating the total lattice thermal conductivities of four samples of polyethylene with different degrees of crystallinity between 0.43 and 0.81 and temperatures between 0.4 and 20 K. The contributions of the crystalline and noncrystalline natures and their percentage contributions were taken into account. The predicted lattice thermal conductivity of polyethylene was in fairly good quantitative agreement with the experimental value, and showed a strong crystallinity dependence, with a distinctive cross-over point at about 2 K.     

Band-state interpretation of lattice thermal conductivity and microhardness of ternary chalcopyrite semiconductors

A new approach utilising the concept of band states and periodicity has been used to explain the lattice thermal conductivity and microhardness of ternary chalcopyrite crystals. The experimental values agree quite well with the calculated values using our model. A single fitting parameter used in each case explains the uniqueness of the model.     

Thermal conductivity and thermal expansivity of thermotropic liquid crystalline polymers

The thermal conductivity and thermal expansivity of a thermotropic liquid crystalline copolyesteramide with draw ratio λ from 1.3 to 15 have been measured parallel and perpendicular to the draw direction from 120 to 430 K. The sharp rise in the axial thermal conductivity K_par; and the drastic drop in the axial expansivity α_∥ at low λ, and the saturation of these two quantities at λ > 4 arise from the corresponding increase in the degree of chain orientation revealed by wide-angle x-ray diffraction. In the transverse direction, the thermal conductivity and expansivity exhibit the opposite trends but the changes are relatively small. The draw ratio dependences of the thermal conductivity and expansivity agree reasonably with the predictions of the aggregate model. At high orientation, K_par; of the copolyesteramide is slightly higher than that of polypropylene but one order of magnitude lower than that of polyethylene. In common with other highly oriented polymers such as the lyotropic liquid crystalline polymer, Kevlar 49, and flexible chain polymer, polyethylene, α_par; of the copolyesteramide is negative, with a room temperature value differing from those of Kevlar 49 and polyethylene by less than 50%. Both the axial and transverse expansivity show transitions at about 390 and 270 K, which are associated with large-scale segmental motions of the chains and local motions of the naphthalene units, respectively. ©1995 John Wiley & Sons, Inc.     

Thermal expansion effects in a silo

We consider a vertical container filled with granular material and exposed to temperature cycles which induce changes in the pressure distribution. Our starting point is an elastic model for the grains leading to classical Janssen behaviour in isothermal conditions. We assume complete mobilisation of the wall frictions. The dynamics superposes static friction plus a viscous friction. After a T jump favouring an expansion, we predict that the resulting overpressures are relaxed only in a region (of size comparable to the Janssen screening length λ) near the top. When T returns to the initial value, we expect decohesion in a certain layer near the top (for rapid cooling). For slow cooling, we expect a smooth return to the original Janssen state. We also discuss a different, but related, effect: instead of changing the temperature, we switch off the gravity effects (on a small column) by suddenly rotating the column from vertical to horizontal. Here, a rarefaction wave should invade the whole column.     

A new approach to lattice thermal conductivity: Application to Ge

The new expression τ _3ph ^?1 =g(ω) (B_N + B_UE^?Θ/αT)T^m is proposed for the three-phonon scattering relaxation rate, considering contributions due to three-phonon normal and umklapp processes, which give a new approach to the lattice thermal conductivity. With use of the above expression, the lattice thermal conductivity of Ge has been calculated in the entire temperature range 2–1000 K: good agreement is found between the experimental and calculated values of the phonon conductivity in the entire temperature range of investigation. Analytical expressions are also obtained to calculate an approximate value of the lattice thermal conductivity. The role of four-phonon processes is also included in the present study.     

Role of the Debye temperature in the lattice thermal conductivity of silicon

This paper is a theoretical study of the effect of the variation in the Debye temperature _D with temperature on the lattice thermal conductivity of Si in the temperature range 2–300 K. Expressions for the three-phonon scattering relaxation rates previously proposed by Shermaet al. are used here. The percentage changes in the lattice thermal conductivity due to the Debye temperature for the transverse and the longitudinal phonons are studied separately.

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Zusammenfassung Dieses Manuskript ist eine theoretische Untersuchung des Einflusses der Veränderung der Debey-Temperatur D mit der Temperatur auf die Gitterwärmeleitfähigkeit von Si im Temperaturbereich 2–300 K. Unlängst von Shermaet al. vorgeschlagene Ausdrücke für die drei Phononen-Streuungsrelaxations-Geschwindigkeiten fanden dabei Anwendung. Die prozentuelle Änderung der Gitterwärmeleitfähigkeit in Abhängigkeit von der Debey-Temperatur wurde für transversale als auch für longitudinale Phononen separat untersucht.

     

Sol-gel synthesis,thermal characterization and conductivity of new glass-polymer solid electrolytes

Poly(ethylene oxide) (PEO) based-polymeric solid electrolytes are of growing interest for their applications in electrochemical devices. Their major limitations are structural and electrochemical stability, and low cationic transport number. A possible response to these problems is given by composite or nanostructured materials. We present sol-gel synthesis, thermal and electrical characterization of new electrolytes made of a composite glass-polymer matrix doped with LiClO₄ and LiBF₄. Emphasis to the critical aspects of preparation is given. We obtain a conductivity at room temperature better than 10^?5 ohm^?1 cm^?1, which is high enough to envisage technological applications.     

Temperature dependency of thermal conductivity of solid phases for fatty acids

Thermal conductivity variations with temperature of solid phases for lauric acid (LA), myristic acid (MA), pivalic acid (PA), and stearic acid (SA) have been measured with radial heat-flow method. Temperature dependencies of the thermal conductivity for same organic materials have been obtained by linear regression analysis. From graphs of thermal conductivity versus temperature, the thermal conductivity of solid phase at their melting temperature and temperature coefficients of thermal conductivity for LA, MA, PA, and SA have been found to be 0.37, 0.39, 0.23, and 0.35 W K^?1 m^?1 and 0.00935, 0.00446, 0.01095, and 0.00295 K^?1, respectively. The ratios of thermal conductivity of liquid phase to thermal conductivity of solid phase for LA, MA, PA, and SA have also been measured to be 0.52, 0.48, 0.25, and 0.59, respectively, with a Bridgman-type directional solidification apparatus.