Xing Zhang

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.

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.

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.

Temperature dependence of the thermal conductivity of individual pitch-derived carbon fibers

Abstract The thermal conductivity of individual pitch-derived carbon fibers was measured in the temperature range 100–400 K by a T type method, in which a hot wire served both as a heating source and thermometer, and the electrical and thermal properties of the hot wire were measured by direct current heating. When a tested carbon fiber was attached to the center position of the hot wire, the thermal conductivity of the fiber was determined by a comparison of the average temperature rise of the hot wire with and without the fiber. Results show that the thermal conductivity of the fiber was limited by boundary scattering below 300 K, and saturated around 350 K at a value of about 800 W/(m·K). An unexpectedly high thermal conductivity of about 920 W/(m·K) was observed at around 400 K. The effect of the thermal contact resistance on the measurement was estimated by changing the length of the fiber in the same contact conditions and the radiation effect was also discussed. The uncertainty of the thermal conductivity was estimated to be ± 13%.

Simultaneous measurements of the specific heat and thermal conductivity of suspended thin samples by transient electrothermal method.

The electrothermal technique is developed to simultaneously measure the specific heat and thermal conductivity of individual thin samples suspended across two heat sinks, resorting to pulsed direct currents with or without a dc offset. The temperature evolution due to Joule self-heating is recorded and compared with the numerical solutions of transient heat conduction equations using the finite volume method. The thermal conductivity is determined by the steady temperature level and the specific heat by the transient temperature rise or relaxation. This technique is applied to a 10 microm thick platinum wire and the thermal conductivity and specific heat are in good agreement with the literature values. In addition, the influences of thermal radiation and thermal boundary resistance between the sample and heat sinks on the experimental results are discussed.

Measurements of thermal effusivity of a fine wire and contact resistance of a junction using a T type probe.

The thermal effusivity of a fine wire and the thermal contact resistance of a junction have been measured by a modified T type probe using a periodic heating method. The modified T type probe is made of a short periodic heated platinum wire and a test wire with one end contacting to the midpoint of the hot wire. Dimensionless expressions for the temperature responses of the hot wire with respect to the thermal effusivity of the test wire and the thermal contact resistance of the junction between the test wire and the hot wire were presented. A measurement system based on a flexible resolution A/D board and a LABVIEW-based virtual lock-in was setup. The probe was further verified by measuring four kinds of commercially available metallic wires at room temperature. The obtained thermal contact resistances were repeatable, with the calculated thicknesses of about 1 to 2 microm. The present method can further be applied to measure the thermal effusivity of nonconductive wires, and to analyze the thermal contact resistance of nano/microscale junction.

Particle swarm optimization based on Gaussian mutation and its application to wind farm micro-siting

In this paper, a particle swarm optimization algorithm with Gaussian mutations, denoted by GPSO, is proposed to solve constrained optimization problems. Two Gaussian mutation operators are employed to search the promising regions for better solutions. One operator is for the region between the personal best position and the global best one. The other operator is for the region around the global best position. The Gaussian mutations help the population jump out of local optima and find better solutions with more probability. The feasibility-based method compares the performance of different particles. Evaluated by three typical optimization problems, GPSO is more accurate, robust and efficient for locating global optima. The GPSO method is applied to a wind-farm micro-siting problem. Simulation results demonstrate that the power generation of the wind farm is further improved while the execution time is substantially reduced.

Noncontact measurement of internal temperature distribution in a solid material using ultrasonic computed tomography

Abstract Numerical simulations and experiments have been carried out for a noncontact measurement of the internal temperature distribution in a solid material using ultrasonic computed tomography (CT). The method is based on the fact that the sound propagation velocity in a material depends on its temperature as well as its density and structure. From the numerical simulations, the convolution method is found to be an effective algorithm for the reconstruction of the sound velocity distribution. To obtain an accurate temperature distribution, it is found to be necessary to measure the sound propagation time with a resolution of 1 ns. In the experiments, the temperature distributions are measured in an agar-gel cylinder of 40 mm in diameter, along the center axis of which a platinum wire with 0.1 mm in diameter is located. By comparing the experimental results with the theoretical ones, the temperature distribution inside the agar-gel can be reconstructed with an error of 0.1 K, except for the region close to the platinum heater wire where temperature gradient is high. Further, the effects of an obstacle to the sound propagation, such as an acrylic resin cylinder inside the agar-gel, are investigated. Although the obstacles causes a part of projection to be missed, by using a linear-interpolation method to compensate for the incomplete projection, the temperature distribution can be reconstructed well but with a little larger error of 0.2 K, except for the regions close to the platinum heater wire and obstacle.

Gaussian particle swarm optimization with differential evolution mutation

During the past decade, the particle swarm optimization (PSO) with various versions showed competitiveness on the constrained optimization problems. In this paper, an improved Gaussian particle swarm optimization algorithm (GPSO) is proposed to improve the diversity and local search ability of the population. A mutation operator based on differential evolution (DE) is designed and employed to update the personal best position of the particle and the global best position of the population. The purpose is to improve the local search ability of GPSO and the probability to find the global optima. The regeneration strategy is employed to update the stagnated particle so as further to improve the diversity of GPSO. A simple feasibility-based method is employed to compare the performances of different particles. Simulation results of three constrained engineering optimization problems demonstrate the effectiveness of the proposed algorithm.