Viscosity and Thermal Conductivity of Stable Graphite Suspensions Near Percolation

Nanofluids have received much attention in part due to the range of properties possible with different combinations of nanoparticles and base fluids. In this work, we measure the viscosity of suspensions of graphite particles in ethylene glycol as a function of the volume fraction, shear rate, and temperature below and above the percolation threshold. We also measure and contrast the trends observed in the viscosity with increasing volume fraction to the thermal conductivity behavior of the same suspensions: above the percolation threshold, the slope that describes the rate of thermal conductivity enhancement with concentration reduces compared to below the percolation threshold, whereas that of the viscosity enhancement increases. While the thermal conductivity enhancement is independent of temperature, the viscosity changes show a strong dependence on temperature and exhibit different trends with respect to the temperature at different shear rates above the percolation threshold. Interpretation of the experimental observations is provided within the framework of Stokesian dynamics simulations of the suspension microstructure and suggests that although diffusive contributions are not important for the observed thermal conductivity enhancement, they are important for understanding the variations in the viscosity with changes of temperature and shear rate above the percolation threshold. The experimental results can be collapsed to a single master curve through calculation of a single dimensionless parameter (a Péclet number based on the rotary diffusivity of the graphite particles).United States. Air Force Office of Scientific Research (FA9550-11-1-0174)National Natural Science Foundation (China) (51036003

Simulation and Analysis of an Integrated Device to Simultaneously Characterize Thermal and Thermoelectric Properties

For many applications, multiple material properties impact device performance and characterization of multiple properties using a single sample is desirable. In this article, the authors focus on thermoelectric materials characterization, which requires the thermal conductivity, electrical conductivity, and Seebeck coefficient to be quantified. Specifically, the authors present a design analysis using numerical COMSOL simulations of the 3w technique to optimize a measurement structure for thermoelectric films while also including the capability for electrical measurements to be performed on the same sample without detachment or repositioning. Thermal optimization of the structure is achieved through investigation of temperature spatial uniformity, the impact of heater line width on the fitted thermal conductivity, and the impact of uncertainty in material properties and geometric parameters on the fitted thermal conductivities for the material of interest

Simulation and Optimization of an In-plane Thermal Conductivity Measurement Structure for Silicon Nanostructures

Silicon-on-insulator based measurement structures have recently been developed to measure the thermal conductivity of nanostructured materials. For example, suspended steady-state measurement structures are often used for measuring the in-plane thermal conductivity of thin silicon films as the heat transfer is confined to the lateral direction. However, few researchers have focused on optimizing the important structural and measurement parameters, such as geometry and applied heater power levels, to ensure accurate measurements. In this article, numerical simulations are first compared with existing experimental data for suspended steady-state joule heating measurement structures with a large suspended region (~10 mm2). Then, a smaller scale (suspended surface area ~500 μm2) structure is developed and optimized for measurement of porous nanostructured silicon materials to maximize the measurement accuracy for the range of expected sample thermal properties

Solar steam generation by heat localization

Currently, steam generation using solar energy is based on heating bulk liquid to high temperatures. This approach requires either costly high optical concentrations leading to heat loss by the hot bulk liquid and heated surfaces or vacuum. New solar receiver concepts such as porous volumetric receivers or nanofluids have been proposed to decrease these losses. Here we report development of an approach and corresponding material structure for solar steam generation while maintaining low optical concentration and keeping the bulk liquid at low temperature with no vacuum. We achieve solar thermal efficiency up to 85% at only 10 kW m[superscript −2]. This high performance results from four structure characteristics: absorbing in the solar spectrum, thermally insulating, hydrophilic and interconnected pores. The structure concentrates thermal energy and fluid flow where needed for phase change and minimizes dissipated energy. This new structure provides a novel approach to harvesting solar energy for a broad range of phase-change applications.United States. Dept. of Energy. Office of Basic Energy Sciences (Energy Frontiers Research Center. Award DE-SC0001299)United States. Dept. of Energy. Office of Basic Energy Sciences (Energy Frontiers Research Center. Award DE-FG02-09ER46577))United States. Air Force Office of Scientific Research (FA9550-11-1-0174)Masdar Institute of Science & Technology - MIT Technology & Development ProgramNatural Sciences and Engineering Research Council of Canad

3D Printing Nanostructured Thermoelectric Device

Thermoelectric materials convert thermal energy to electrical energy and vice versa. Thermoelectrics have attracted much attention and research efforts due to the possibility solving electronic cooling problems and reducing energy consumption through waste heat recovery. The efficiency of a thermoelectric material is determined by the dimensionless figure of merit ZT, which depends on both thermal and electrical properties. Researchers have worked for several decades to improve the ZT, but there had been little progress until nanomaterials and nanofabrication became widely available. Nanotechnology makes the ZT enhancement attainable by disconnecting the linkage between thermal and electrical transport. Printing customized, flexible thermoelectric devices opens the door to new applications and energy saving solutions, while probing the impact of different structure on properties and performance. This study combines nanostructured materials with 3D printing technology to enable development of customized thermoelectric devices with mechanical flexibility, which is not possible in commercially-available devices. A 3D printer is fabricated to allow printing of nanostructured thermoelectric inks, and can print customized devices by controlling the movement of the substrates and the mechanisms of ink dispensing. The properties and performance of the devices are measured with the modified Harman method. Although the selected nanoink (zinc oxide in ethanol) yields low figures of merit, this work demonstrates the feasibility of using 3D printing to fabricate flexible thermoelectric devices. This technology will contribute to ongoing research of energy recycling and waste heat recovery