Ph.D. Graduates (2014)
Reductions in the size of materials down to the nanoscale have led to the discovery of nanoparticles with exceptionally unique optical, mechanical, electrical and thermal properties. As a result of their extraordinarily high thermal conductivities, carbon-based nanoparticles are projected to become integral components in a wide variety of thermal management systems and devices in the future, including their incorporation into thermal interface materials, microchannel heat sinks and waste heat recovery devices. Despite nearly two decades of intense research in this area, however, carbon-based nanoparticles have not yet been widely adopted in these systems. One major impediment to their integration in these applications is the high degree of phonon boundary scattering that occurs across individual nanoparticle interfaces, particularly when they come into contact with an amorphous material. The physical phenomena responsible for this include: 1) mismatches in the vibrational spectra of the nanoparticle and the surrounding amorphous material, 2) a low adhesion energy (or bonding strength) at the interface and 3) differently sized constrictions that are formed at contacting junctions. Of interest in this work is the last of these, whose magnitude effect on thermal transport in bulk materials is not well known. To this end, the effect of nanoparticle contact area on thermal transport within a bulk paraffin phase change material is quantified and reported for a variety of nanoparticle types. Paraffins are amorphous in nature and are common materials used for the storage of thermal energy. The effect of nanoparticle inclusions on both the micro-scale and macro-scale heat conduction phenomena within paraffin are quantified as a function of nanofillers geometry and type. The results are expected to aid in the analysis and design of next-generation nanocomposites and thermal energy storage materials.
Ph.D. Graduates (2012)
Current Position: Mechanical Engineer, Naval Surface Warfare Center Carderock Division
This work investigates the role that dopant clustering plays in the aging phenomena observed in scandia-stabilized zirconia electrolytes. Molecular dynamics simulations have been conducted on supercells containing compositions of xSc2O3 + (11x)Y2O3 + 89 ZrO2 for x = f0; 1; 2; 11g and also on the composition 8Y2O3 + 92 ZrO2. It was discovered that individual dopant-dopant barriers have a relatively small effect on bulk ionic conductivity when the dopants are arranged in small clusters, as the vacancies rarely migration through the small clusters. However, larger clustering of ions can have a significant impact on ionic conductivity, mostly due to localized destabilization of the high-conductivity cubic phase, which forces oxygen ions to migrate through the remaining dopant-rich cubic matrix. Part of the destabilization occurs due to vacancies becoming trapped within the dopant clusters, which forces the Zr ions in the zirconia-rich regions to assume a higher coordinated state with oxygen. However, the zirconium ions are known to abhor eight-fold coordinated states, and these regions form precipitates of low-conductivity phases. These phases have been identifed as tetragonal, and their formation coincides with the simultaneous reductionin ionic conductivity. The tetragonal precipitates are found to be energetically favorable at temperatures of 1073 K, indicating that colloidal stratification of the mixture is inevitable due to cationic migration during long term annealing.
However, clustering of dopants appears to reach a limiting point as the dopants exhibit a repulsive interaction that limits the thermodynamic stability of the zirconia precipitates. Therefore, the dopant clusters will reach a certain maximum size at which point clustering is expected to terminate at an equilibrium state. Scandia stabilized zirconia was observed to provide better conductivity, lower zirconium coordination and superior stabilization of the cubic phase for identical conditions compared to yttria stabilized zirconia, but the energy barriers for dopant clustering was higher for samples containing yttrium, indicating that yttria stabilized zirconia was more resistant to clustering due to aging of the electrolyte.