High proton conductivity of water channels in a highly ordered nanowire

A proton-conductive material based on a crystalline assembly of trimesic acid and melamine (TMA?M, see picture) is reported. Because of the ordered structure of the assembly, the water-saturated proton conductivity for the TMA?M assembly is 5.5?S?cm(-1) , which is the highest proton conductivity measured to date. This exceptionally high conductivity and low-cost fabrication of the material make applications feasible for fuel-cell devices.     

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.     

Thermal conductivity of methane hydrate from experiment and molecular simulation

A single-sided transient plane source technique has been used to determine the thermal conductivity and thermal diffusivity of a compacted methane hydrate sample over the temperature range of 261.5-277.4 K and at gas-phase pressures ranging from 3.8 to 14.2 MPa. The average thermal conductivity, 0.68 +/- 0.01 W/(m K), and thermal diffusivity, 2.04 x 10(-7) +/- 0.04 x 10(-7) m2/s, values are, respectively, higher and lower than previously reported values. Equilibrium molecular dynamics (MD) simulations of methane hydrate have also been performed in the NPT ensemble to estimate the thermal conductivity for methane compositions ranging from 80 to 100% of the maximum theoretical occupation, at 276 K and at pressures ranging from 0.1 to 100 MPa. Calculations were performed with three rigid potential models for water, namely, SPC/E, TIP4P-Ew, and TIP4P-FQ, the last of which includes the effects of polarizability. The thermal conductivities predicted from MD simulations were in reasonable agreement with experimental results, ranging from about 0.52 to 0.77 W/(m K) for the different potential models with the polarizable water model giving the best agreement with experiments. The MD simulation method was validated by comparing calculated and experimental thermal conductivity values for ice and liquid water. The simulations were in reasonable agreement with experimental data. The simulations predict a slight increase in the thermal conductivity with decreasing methane occupation of the hydrate cages. The thermal conductivity was found to be essentially independent of pressure in both simulations and experiments. Our experimental and simulation thermal conductivity results provide data to help predict gas hydrate stability in sediments for the purposes of production or estimating methane release into the environment due to gradual warming.     

Thermal conductivity of Al2O3/water nanofluids

A considerable number of studies can be found on the thermal conductivity of nanofluids in which Al₂O₃ nanoparticles are used as additives. In the present study, the aim is to measure the thermal conductivity of very narrow Al₂O₃ nanoparticles with the size of 5 nm suspended in water. The thermal conductivity of nanofluids with concentrations up to 5 % is measured in a temperature range between 26 and 55 °C. Using the experimental data, a correlation is presented as a function of the temperature and volume fraction of nanoparticles. Finally, a sensitivity analysis is performed to assess the sensitivity of thermal conductivity of nanofluids to increase the particle loading at different temperatures. The sensitivity analysis reveals that at a given concentration, the sensitivity of thermal conductivity to particle loading increases when the temperature increases.     

Thermal conductivity of molten alkali halides from equilibrium molecular dynamics simulations

The thermal conductivity of molten sodium chloride and potassium chloride has been computed through equilibrium molecular dynamics Green-Kubo simulations in the microcanonical ensemble (N,V,E). In order to access the temperature dependence of the thermal conductivity coefficient of these materials, the simulations were performed at five different state points. The form of the microscopic energy flux for ionic systems whose Coulombic interactions are calculated through the Ewald method is discussed in detail and an efficient formula is used by analogy with the methods used to evaluate the stress tensor in Coulombic systems. The results show that the Born-Mayer-Huggins-Tosi-Fumi potential predicts a weak negative temperature dependence for the thermal conductivity of NaCl and KCl. The simulation results are in agreement with part of the experimental data available in the literature with simulation values generally overpredicting the thermal conductivity by 10%-20%.     

Thermal reduction of Pd molecular cluster precursors at highly ordered pyrolytic graphite surfaces

Highly ordered pyrolytic graphite (HOPG) surfaces were modified by the adsorption of Pd molecular precursors from solution. Two palladium-containing molecular precursors were studied, a mononuclear one and a trinuclear one, to compare their affinities and distributions at substrate surfaces. To obtain Pd nanoparticles, these neutral molecular precursors were reduced under a hydrogen atmosphere. Thermogravimetric analysis was carried out to establish the behavior of these precursors at various temperatures. Understanding the thermal stability of these compounds is very important to establish the appropriate conditions to form metallic Pd. The modified surface has been characterized by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy; also, the reductive process was monitored by XPS. Remarkable differences were observed between the mononuclear and trinuclear compounds in terms of dispersion, particle size, and homogeneity. The preference of the trinuclear compound was to deposit at HOPG defects, in contrast to that of the mononuclear one, which was agglomeration on all surfaces. After the application of this technique, not only Pd nanoparticles but also Pd nanowires were obtained.     

Thermal conductivity of ethanol

We present a factorization of the Ewald sum permitting efficient computation of the reciprocal space part of the molecular representation for the heat flux vector. We use the derived expression to evaluate thermal conductivity of a model of ethanol at several near-ambient state points.     

Thermal conductivity of carbon nanotube-polyamide-6,6 nanocomposites: reverse non-equilibrium molecular dynamics simulations

The thermal conductivity of composites of carbon nanotubes and polyamide-6,6 has been investigated using reverse non-equilibrium molecular dynamics simulations in a full atomistic resolution. It is found, in line with experiments, that the composites have thermal conductivities, which are only moderately larger than that of pure polyamide. The composite conductivities are orders of magnitude less than what would be expected from nai?ve additivity arguments. This means that the intrinsic thermal conductivities of isolated nanotubes, which exceed the best-conducting metals, cannot be harnessed for heat transport, when the nanotubes are embedded in a polymer matrix. The main reason is the high interfacial thermal resistance between the nanotubes and the polymer, which was calculated in addition to the total composite thermal conductivity as well as that of the subsystem. It hinders heat to be transferred from the slow-conducting polymer into the fast-conducting nanotubes and back into the polymer. This interpretation is in line with the majority of recent simulation works. An alternative explanation, namely, the damping of the long-wavelength phonons in nanotubes by the polymer matrix is not supported by the present calculations. These modes provide most of the polymers heat conduction. An additional minor effect is caused by the anisotropic structure of the polymer phase induced by the nearby nanotube surfaces. The thermal conductivity of the polymer matrix increases slightly in the direction parallel to the nanotubes, whereas it decreases perpendicular to it.     

Thermal conductivity of cemented carbides

The hypothetical salts Li₄SiN₂O and Li₇SiN₃O were sought in the course of studies on the reactions of Li₂SiN₂ and Li₅SiN₃ with lithium oxide, and of LiSiNO with lithium nitride.

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Zusammenfassung In einer Reihe von Studien der Reaktionen von Li₂SiN₂ und Li₅SiN₃ mit Lithiumoxid bzw. LiSiNO mit Lithiumnitrid wurde nach den hypothetischen Salzen Li₄SiN₂O und Li₇SiN₃O gesucht.

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Li₂SiN₂ Li₅SiN₃ LiSiNO Li₄SiN₂O Li₇SiN₃O.

     

Thermal conductivity measurements on ferrofluids

The thermal conductivity of a number of ferrofluids consisting of colloidally dispersed Fe₃O₄ particles in diester, hydrocarbon, water and fluorcarbon carriers have been measured at 38°C. The variation in thermal conductivity with particle concentration is well described by Tareef's equation (1940). This has enabled the ratio of the physical to magnetic size to be determined and compared with estimates of the ratio obtained from electron micrographs and magnetic measurements.The fit between theory and experiment is particularly good for hydrocarbon carrier fluids giving the ratio of solid to magnetic radiusR _i/R _m=1.24±0.03 compared with the value obtained from magnetic data and electron micrographs of 1.19±0.07. The corresponding value from the fluids with a diester carrier ranges between 1.1<R _d/R _m<1.3 which is again consistent with microscopy and magnetic data.The application of a magnetic field of 0.1 T had no noticeable effect on the thermal conductivities of ferrofluids.     

Thermal conductivity of exfoliated graphite nanocomposites

Since the late 1990’s, research has been reported where intercalated, expanded, and/or exfoliated graphite nanoflakes could also be used as reinforcements in polymer systems. The key point to utilizing graphite as a platelet nanoreinforcement is in the ability to exfoliate graphite using Graphite Intercalated Compounds (GICs). Natural graphite is still abundant and its cost is quite low compared to the other nano–size carbon materials, the cost of producing graphite nanoplatelets is expected to be ~$5/lb. This is significantly less expensive than single wall nanotubes (SWNT) (>$45000/lb) or vapor grown carbon fiber (VGCF) ($40–50/lb), yet the mechanical, electrical, and thermal properties of crystalline graphite flakes are comparable to those of SWNT and VGCF. The use of exfoliated graphite flakes (xGnP) opens up many new applications where electromagnetic shielding, high thermal conductivity, gas barrier resistance or low flammability are required. A special thermal treatment was developed to exfoliate graphite flakes for the production of nylon and high density polypropylene nanocomposites. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to assess the degree of exfoliation of the graphite platelets and the morphology of the nanocomposites. The thermal conductivity of these composites was investigated by three different methods, namely, by DSC, modified hot wire, and halogen flash lamp methods. The addition of small amounts of exfoliated graphite flakes showed a marked improvement in thermal and electrical conductivity of the composites.     

Thermal conductivity of ternary gas mixtures

Thermal conductivities of ternary mixtures of N₂-Ne-H₂, O₂-N₂-Ne, and Kr-Ar-H₂ gas systems are measured at six temperature levels in the range 40–175°C. These data are used to evaluate the predictive procedures of the thermal conductivity of multicomponent gas mixtures as developed by several authors. This and a parallel study on quaternary mixtures provide a guidance to predicting conductivity values of multicomponent gas mixtures.     

Thermal conductivity of gel-spun polyethylene fibers

The thermal conductivities of unidirectional gel-spun polyethylene fiber-reinforced composites have been measured parallel (K∥?) and perpendicular (K⊥) to the fiber axis from 15 to 300K. The axial thermal conductivity K∥? varies linearly with volume fraction v_f of fiber, while the transverse thermal conductivity K⊥ follows the Halpin-Tsai equation. Extrapolation to v_f = 1 gives the thermal conductivity of gel-spun polyethylene fiber which, at 300K, has values of 380 and 3.3 mW cm^?1K^?1 along and perpendicular to the fiber axis, respectively. The axial thermal conductivity is exceptionally high for polymers, and is more than twice the thermal conductivity of stainless steel. This high value arises from the presence of a large fraction of long (> 50 nm) extended chain crystals in the fiber. Further improvement of up to a factor of 10 is possible if the length and volume fraction of the extended chain crystals can be increased. © 1993 John Wiley & Sons, Inc.     

Thermal conductivity of a polymerizing liquid

Thermal conductivity kappa of seven polymerizing liquids has been measured in real time at different temperatures, and calorimetry and dielectric spectroscopy of one liquid are performed to help interpret the results. As a covalently bonded linear chain or a network structure in the liquid grows, kappa of the Debye equation initially increases with the polymerization time t(polym) as the molecular weight, density, and sound velocity increase, as on cooling a liquid. The measured kappa reaches a maximum and then decreases, thus showing a peak at a certain t(polym) and finally becomes constant, which is not the true behavior of steady state kappa. The dielectric relaxation time of the covalently bonded structure at the t(polym) for the kappa peak is less than 5 s and the extent of polymerization is below the vitrification plateau value. The peak height increases when the pulse time for kappa measurement is increased. An increase in the liquid's temperature shifts the kappa peak to a shorter t(polym). Liquid compositions polymerizing rapidly show a similar shift, and those polymerizing slowly or whose viscosity does not reach a high enough value show a small kappa peak or none. The kappa peak may be an artifact of the time dependence of heat capacity during the pulse time used for the kappa measurement, as proposed for glasses and supercooled liquids, similar to the changes in other properties observed as an artifact of kinetic freezing/unfreezing. For a polymerizing liquid, the peak may additionally arise when the rate of increase in the elastic modulus becomes equal to the rate of decrease in equilibrium Cp. In either case, its appearance does not distinguish the Brownian motions' slowing on polymerization from that on cooling or compressing a liquid.     

Thermal conductivity of binary liquid mixtures

A semi-empirical method is developed for the prediction of the thermal conductivity of binary liquid mixtures. The proposed method is tested by calculating the thermal conductivity of twelve binary liquid mixtures and an excellent agreement between the observed and calculated values is obtained.     

Thermal conductivity of vitreous sodium phosphates

Summary The thermal conductivity was measured for vitreous sodium phosphates of a variety of compositions in the temperature range from 50° to 160°. The temperature coefficient of the thermal conductivity decreases with the increase ofR (Na₂O/P₂O₅ mole ratio), and changes its sign from positive to negative at the value ofR near unity. In the composition range fromR=0.7 toR=1.4, where the thermal conductivity increases with the increase ofR, the composition dependence of thermal conductivity can be explained by the composition dependences of density and of compressibility. The residual water in the phosphates on the ultraphosphate side of composition seems to have some large effect on the thermal conductivity.

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Zusammenfassung Die thermische Leitfähigkeit für glasige Natriumphosphate verschiedener Zusammensetzung wurde im Temperaturbereich von 50 bis 160° gemessen. Der Temperaturkoeffizient thermischer Leitfähigkeit nimmt mit dem Anteil anR (Na₂O/P₂O₅ molares Verhältnis) zu und ändert sein Vorzeichen von positiven zum negativen Werten vonR in der Nähe von eins. Im Bereich der Zusammensetzung vonR=0,7 bisR=1,4, indem die thermische Leitfähigkeit mit Zunahme vonR zunimmt, läßt sich die Abhängigkeit von der Zusammensetzung für die thermische Leitfähigkeit durch die Abhängigkeit von Dichte und Kompressibilität von der Zusammensetzung erklären. Das restliche Wasser in den Phosphaten auf der Ultraphosphat-Seite der Zusammensetzung scheint einen großen Einfluß auf die thermische Leitfähigkeit zu haben.

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With 7 figures and 1 table     

Thermal conductivity of polytetrafluoroethylene and polytrifluorochloroethylene

Summary Measurements of thermal conductivity were made on polytetrafluoroethylene and polytrifluorochloroethylene which were crystallized by various heat treatments. The variation of thermal conductivity with the degree of crystallinity can be explained qualitatively in terms of the difference of the mean free path. An approximate calculation of the mean free path for the amorphous polytrifluorochloroethylene was made. The calculation gives the value of the mean free path which is very close to the atomic distances in the polymer molecule.

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Zusammenfassung Die thermische Leitf?higkeit von Polytetrafluor?thylen und Polytrifluorchlor?thylen wurde an Proben gemessen, die durch verschiedene W?rmebehandlungen kristallisiert wurden. Die ?nderung der thermischen Leitf?higkeit mit dem Kristallisationsgrad kann qualitativ durch die Unterschiede der mittleren freien Wegl?nge erkl?rt werden. Eine absch?tzende Berechnung der mittleren freien Wegl?nge des Polytrifluorchlor?thylens wurde durchgeführt. Die berechneten Werte liegen nahe an den Atomabst?nden der Polymermoleküle.

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With 4 figures and 1 table

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This article is Part VI of the series “Studies on Thermal Conductivity of High Polymers”. Part I–V of this series were presented in High Polymer Chem., Japan (Kobunshi Kagaku).