57th Annual Report on Research 2012 Under Sponsorship of the ACS Petroleum Research Fund

Introduction

The New Directions grant from the Petroleum Research Fund to continues to enhance my transition from a leadership position in a semiconductor industry research and development environment to that of a professor and research scientist in an academic setting.   With the finds provided by PRF over the last year, two graduate students and one undergraduate student have been supported.  These students have focused on developing phonon scattering based approaches to realizing improved efficiency thermoelectric materials.

The PRF funds have provided seed funding that has led to one $250000 grant from the New York State Energy Research and Development Authority, a $250000 grant focused on Anderson Localization of Phonons in Thermoelectric Nanostructures (Office of Naval Research, Part of a larger program focused on thin film energy generating materials), and 775000 service units (valued by NSF at $1/service unit) for molecular dynamic simulations of heat flow in these nanostructured materials.  The funds have also partially supported a graduate student and undergraduate student to present their research results at national conferences.

From a research perspective, the focus of this award is to explore opportunities for using controlled phonon scattering in nanocomposite materials to create sustainable and efficient thermoelectric materials.  Two classes of materials are being explored.  One a nanocomposite material formed from a nanowire embedded a strong phonon scattering medium.  The nanowire is formed from a material with large Seebeck coefficient and excellent electrical conductivity while the phonon scattering medium is formed from a material that has both a high phonon density of states as well as a low lattice thermal conductivity.  The second material being explored is that of a material in which 20% of the lattice planes have their mass artificially increased.  These "heavy" lattice planes are being explored to understand the conditions under which Anderson localization of the phonons occurs.  Anderson localized phonons are interesting for thermoelectric materials in that these phonons, ideally, have zero lattice thermal conductivity.

ZnO Nanowire/Aerogel NanoComposites

ZnO is a material that in bulk form has large a Seebeck coefficient (500µV/K) but with a lattice thermal conductivity too large for efficient energy generation.  In this experimental work, we seek to utilize controlled phonon scattering at the surface of nanowires to redirect the energy carrying lattice vibrations into a strong phonon scattering medium, that of an aerogel.  Thin ZnO nanowires are formed using solution-based processes.  A solution of TEOS and water is spin-coated onto the ZnO nanowire containing substrate, followed by ethanol- water exchanges in this thin sol-gel film.  This material is then supercritically dried in CO₂ to produce the desired nanocomposite.  SEM cross-sections of both the ZnO nanowires and the ZnO nanowires embedded in aerogel are shown in figure 1. 

The thermal conductivity of the ZnO nanowire/Aerogel nancomposite was measured using the well-known three-omega technique.  In figure 2 (left), the thermal conductivity as a function of temperature of the ZnO nanowires, the thin film silica aerogel, and the ZnO nanowire/aerogel nanocomposite is shown along with the data for bulk ZnO.  The data show that when the ZnO nanowires are embedded in an aerogel matrix, the thermal conductivity of the composite is reduced by over an order of  magnitude at room temperature.  We note that based on simple geometrical consideration, the thermal conductivity of the nanocomposite would be expected to increase slightly once embedded in the aerogel. We attribute the thermal conductivity reduction to the diffuse scattering of phonons at the surface of the ZnO nanowire with the subsequent emission of the scattered phonon into the aerogel.  Given the large disparity between the phonon density of states found between the aerogel and ZnO nanowire, virtually all scattered phonons should be emitted into the aerogel.  Given the measured value of the thermal conductivity for the nanocomposite, the measured charge carrier mobility in ZnO nanowires, and a bulk ZnO Seebeck coefficient, a thermoelectric figure of merit of 2 is predicted for this unique material, potentially enabling a significant improvement in the efficiency of energy generation from heat sources.

Anderson Localization of Phonons in Random Multilayer Thermoelectrics

Reverse nonequilibrium molecular dynamics simulations have been used to quantify the impact of randomly placed mass-altered atomic planes, such as those produced in pseudomorphically grown heterostructures, on the thermal conductivity of silicon.  The results indicate that the room temperature thermal conductivity of these silicon-based structures can be reduced to values below 0.050 W/m-K (see figure 2 right).  These values are significantly less than those found in random alloy or superlattice structures containing the same percentage of mass-altered atoms and are attributed to Anderson localization of phonons.  Based on these simulations, we have experimentally constructing random multilayer thin films formed in the Si/Sn system.  In these materials, Si and Sn thin films are deposited using conventional sputtering techniques.  Using this process Sn monolayers are inserted at random locations in the silicon thin film.  A TEM cross-section of the Si/Sn structure is shown in Figure 4 where we see well-defined layers of silicon (light material) and Sn (dark material).  In figure 5, the thermal conductivity of this random multilayer structure is shown as a function of temperature.  The data for a-Si are included for reference.  The results shown that the thermal conductivity of this random multilayer structure is reduced to values of approximately 100mW/m-K at room temperature.  Hall measurements on the structure indicate a electron mobility of 10 cm²/V-s (as expected if the electrons are being scattered predominantly by the Sn planes).  If the Seebeck coefficient in this material can be maintained at levels similar to silicon (~800 µV/K), thermoelectric figures of merit greater than three should be possible.  This would enable energy generation at greater than 15% efficiency from energy flow between hot and cold reservoirs held at 500K and 300K, respectively.  Opportunities for incorporating this material into large area energy generating panels using roll to roll processing are being explored.