|
Research:
Nanoscale Energy Transport in Unique Materials
Compressed Bismuth Nanoparticles: The nanoEngineering lab is investigating how thermal and electrical transport is affected by nanoscale dimensions in materials (e.g. nanowires, nanoparticles, nanocomposites, nanostructured bulk materials, at interfaces, etc.). One such project in the lab involves the exploration of zero-dimensional structures for high-efficiency thermoelectric materials. In an effort to realize theoretical predictions of increased thermoelectric efficiency for small, low-dimensional structures a pellet composed of bismuth nanoparticles was created through uniaxial compression. Even with knowledge that a surface layer of oxide was present surrounding each particle, a focus of the investigation was to determine whether high compression levels could sufficiently break through the oxide layer to form a conductive network. The thermoelectric properties (thermal conductivity, electrical conductivity, Seebeck coefficient) of this pellet were measured and compared to a series of pellets produced from commercially-available microparticles over a range of compression levels. Interestingly, the nanoparticle pellet demonstrated a porosity much higher than the microparticle pellet, which may suggest that yielding was not significant in the nanoparticle sample, and implying that the nanoparticles have a much higher yield strength than the microparticles. Both types of pellets showed a large reduction in electrical conductivity relative to bulk bismuth due to porosity and the presence of the oxide layer on the particles though the reduction was not as dramatic as expected, providing evidence that the oxide sheet surrounding the particles was breached to some extent.
Polyaniline and Bismuth Nanoparticle Nanocomposites: Another specific material system under consideration is a composite consisting of bismuth nanoparticles embedded in a matrix of the conducting polymer, polyaniline (PANI). The network of bismuth particles constitutes the zero-dimensional structure. PANI serves as a binder and potential conduction bridge for the particles. The spherical bismuth particles have a mean diameter of 10 nm, which is likely small enough for quantum confinement to prevail. Bismuth was selected because it is the best thermoelectric material (ZT = 1.6 at 300 K) of the pure elements. Moreover, bismuth is likely sufficient to illustrate any increase in efficiency due to dimensional confinement. PANI was chosen because it is one of the few electrically-conductive polymers that is environmentally stable at room temperature. The goal is to have the thermoelectric properties of bismuth dominate the composite with the PANI serving merely to hold everything together mechanically and, in some sense, electrically. A series of polymer-nanoparticle composites have been created and tested. To date, the figure of merit measured is quite low (largely due to a low electrical conductivity), and only a small enhancement in the ZT of PANI is demonstrated.
Nanostructured Aeroclays: Aerogels provide superior thermal insulation due to their extrememly porous structure. Common aerogels are composed of materials that are both expensive and caustic. A new type of aerogel or aeroclay, as it may be more appropriately called, has been synthesized by our collaborators in David Schiraldi's laboratory in the Macromolecular Science and Engineering Department at Case, that is made up of harmless and inexpensive clay. The nanoEngineering laboratory has measured aeroclays of various configurations to determine their thermal properties and has also developed a theoretical model to describe thermal transport in these micro/nanostructured materials. The thermal conductivity of one orientation of the aeroclay was found to be 0.026 ± 0.006 W/mK, which is comparable to air. A custom designed heat flow meter apparatus is being used for thermal characterization.
|
