Motoo Fujii

Thermal and electrical conductivity of a suspended platinum nanofilm

This letter reports on the measurements of the in-plane thermal conductivity and the electrical conductivity of a microfabricated, suspended, nanosized platinum thin film with the width of 260nm, the thickness of 28nm, and the length of 5.3μm. The experimental results show that the electrical conductivity, the resistance-temperature coefficient and the in-plane thermal conductivity of the nanofilm are greatly lower than the corresponding bulk values from 77to330K. The comparison results indicate that the relation between the thermal conductivity and the electrical conductivity of this nanofilm might not follow the Wiedemann–Franz law that describes the relation between the thermal conductivity and the electrical conductivity of a bulk metallic material.

Short hot wire technique for measuring thermal conductivity and thermal diffusivity of various materials

A transient short hot wire technique (SHWT) is developed for simultaneous determination of the thermal conductivity and thermal diffusivity of various materials such as liquids, gases or powders. A metal wire with (or without) insulation coating serves both as a heating unit and as an electrical resistance thermometer and the wire is calibrated using water and toluene with known thermophysical properties. This SHWT includes correlation of the experimental data with numerically simulated values based on a two-dimensional heat-conduction model. For the measurements with proportional relation between temperature rise and logarithmic heating time interval, the thermal conductivity and thermal diffusivity are obtained from the slope and the intercept of the measured temperature rise and those of calculated non-dimensional temperature rise by including the heat flux and the properties of the wire. For the measurements with nonlinear relation between temperature rise and logarithmic heating time interval, the thermal conductivity and thermal diffusivity are extracted from a curve fitting method by using the downhill simplex method to match the experimental data and the numerical values. This technique is applied here using air as a testing sample. The effect of natural convection is investigated and the accuracy of this measurement is estimated to be 2% for thermal conductivity and 7% for thermal diffusivity.

Measurements of Thermal Conductivity and Electrical Conductivity of a Single Carbon Fiber

In this paper, the thermal conductivity of a single carbon fiber under different manufacturing conditions is measured using the steady-state short-hot-wire method. This method is based on the heat transfer phenomena of a pin fin attached to a short hot wire. The short hot wire is supplied with a constant direct current to generate a uniform heat flux, and both its ends are connected to lead wires and maintained at the initial temperature. The test fiber is attached as a pin fin to the center position of the hot wire at one end and the other end is connected to a heat sink. One-dimensional steady-state heat conduction along the hot wire and test fiber is assumed, and the basic equations are analytically solved. From the solutions, the relations among the average temperature rise of the hot wire, the heat generation rate, the temperature at the attached end of the fiber, and the heat flux from the hot wire to the fiber are accurately obtained. Based on the relations, the thermal conductivity of the single carbon fiber can be easily estimated when the average temperature rise and the heat generation rate of the hot wire are measured for the same system. Further, the electrical conductivity of the single carbon fiber is measured under the same conditions as for the thermal conductivity using a four-point contact method. The relation between the thermal conductivity and electrical conductivity is further discussed, based on the crystal microstructure.

Stochastic thermal transport of nanoparticle suspensions

Both the Langevin equation of the Brownian motion and the concept of the stochastic thermal process are adopted to describe the temperature fluctuation of the nanoparticles suspended in carrier liquids. The heat transfer chain between the nanoparticles and the ambient liquid is analyzed. Based on the superposition principle and the Green-Kubo theorem, a thermal conductivity model that is able to account for the effects of the volume fraction and sizes of nanoparticles has been developed. Comparisons show that the results predicted by the present model are well coincident with the experimental data.

A NUMERICAL ANALYSIS OF LAMINAR FLOW AND HEAT TRANSFER OF AIR IN AN IN-LINE TUBE BANK

A numerical analysis of laminar flow and heat transfer in a tube bank is described. The two-dimensional Navier-Stokes and energy equations are solved numerically for successive subregions from the inlet to the exit of the bank by a procedure catted the “one step forward and half step backward iteration method” and by using a hybrid grid system. Calculations are carried out for an in-line square tube bank up to five rows deep, with pitch-to-diameter ratios 1.5 × 1.5, under the conditions of uniform tube wall temperature, Re = 60, 120, and 300, and Pr = 0.7. Stream, isovorticity, and isothermal lines through the whole tube bank, local and average Nusselt numbers, pressure distributions along the tube walls and in the flow directions, and friction factor are presented. The friction factor and average Nusselt number agree well with published experimental results. An analogous relation between an in-line tube bank and parallel plates with respect to developing heat transfer characteristics is presented.

Influence of grain boundary scattering on the electrical properties of platinum nanofilms

The electrical conductivity and temperature coefficient of resistance of polycrystalline platinum nanofilms have been investigated experimentally and theoretically. The results show that these electrical properties have been greatly reduced mainly by grain boundary scattering. By applying the theory of Mayadas and co-workers [Appl. Phys. Lett. 14, 345 (1969); Phys. Rev. B 1, 1382 (1970)] to predict the electrical conductivity and temperature coefficient of resistance with the same reflection coefficient, however, obvious discrepancies have been found. These discrepancies indicate that Drude’s relation for bulk metals cannot be applied directly in the nanosized grain interior of polycrystalline metallic films.

Simultaneous measurements of thermal conductivity and thermal diffusivity of liquids under microgravity conditions

A transient short-hot-wire technique is proposed and used to measure the thermal conductivity and thermal diffusivity of liquids simultaneously. The method is based on the numerical evaluation of unsteady heat conduction from a wire with the same length diameter ratio and boundary conditions as those in the experiments. To confirm the applicability and accuracy of this method. Measurements were made for five sample liquids with known thermophysical properties and were performed under both normal gravity and microgravity conditions. The results reveal that the present method determines both the thermal conductivity and the diffusivity within 2 and 5%. respectively. The microgravity experiments clearly indicate that even under normal gravity conditions, natural-convection effects are negligible for at least l s after the start of heating. This method would be particularly suitable for a valuable and expensive liquid, and has a potential for application to electrically conducting and or corrosive liquids when the probe is effectively coated with an insulating and anticorrosive material.

Simultaneous Measurements of the Thermal Conductivity and Thermal Diffusivity of Molten Salts with a Transient Short-Hot-Wire Method

A transient short-hot-wire technique has been successfully used to measure the thermal conductivity and thermal diffusivity of molten salts (NaNO3, Li2CO3/K2CO3, and Li2CO3/Na2CO3) which are highly corrosive. This method was developed from the hot-wire technique and is based on two-dimensional numerical solutions of unsteady heat conduction from a short wire with the same length-to-diameter ratio and boundary conditions as those used in the actual experiments. In the present study, the wires are coated with a pure Al2O3 thin film by using a sputtering apparatus. The length and radius of the hot wire and the resistance ratio of the lead terminals and the entire probe are calibrated using water and toluene with known thermophysical properties. Using such a calibrated probe, the thermal conductivity and thermal diffusivity of molten nitrate are measured within errors of 3 and 20%, respectively. Also, the thermal conductivity of the molten carbonates can be measured within an error of 5%, although the thermal diffusivity can be measured within an error of 50%.

Size effects on the thermal conductivity of polycrystalline platinum nanofilms

The surface and grain-boundary effects on the in-plane thermal conductivity of polycrystalline platinum nanofilms have been investigated. The thicknesses of the nanofilms range from 15.0 to 63.0 nm and the mean grain sizes measured by x-ray diffraction vary from 9.5 to 26.4 nm. The thermal conductivities of the nanofilms measured by a direct electrical heating method are greatly reduced from the bulk values. The measured results are compared with the values predicted by the Qiu and Tien model and the Kumar and Vradis theory. It is found that the reduction in the thermal conductivity is mainly caused by grain-boundary scattering and the reflection coefficient of electrons striking the grain boundaries is around 0.35. The relaxation time model is also applied to study the size effects to check whether the Matthiessen rule is still valid in predicting the in-plane thermal conductivity of polycrystalline metallic nanofilms. The results indicate that by considering only grain-boundary scattering and background scattering the Matthiessen rule is still valid. If surface scattering, however, is included, deviations of the Matthiessen rule from other theories mentioned above have been found.

A NUMERICAL ANALYSIS OF LAMINAR FREE CONVECTION AROUND AN ISOTHERMAL SPHERE: FINITE-DIFFERENCE SOLUTION OF THE FULL NAVIER-STOKES AND ENERGY EQUATIONS BETWEEN CONCENTRIC SPHERES

A numerical solution of transient laminar free convection between two concentric spheres is presented for Pr= 0.7 and Ra =100. The steady average heat transfer coefficient for the inner sphere becomes independent of the radius of the outer sphere when it is more than 40 times that of the inner sphere, and independent of the number of grid points when they are at least 61 and 46 for the radial and azi-muthal directions, respectively. Steady state is reached at a dimensionless time τ = 6 for the average heat transfer coefficient and at τ = 12 for the convection flow. The steady local Nusselt number near the top and bottom stagnation points is different from that of the previous solution of the boundary-layer equations containing curvature terms, while the average Nusselt numbers of both solutions agree within 2%.