Quantum Simulation of Nanoscale Transport in Direct Energy Conversion Materials: From Thermal-Field Emitters to Thermoelectrics

In the ongoing struggle to resolve our current energy crisis, many agencies and researchers have spearheaded the application of direct energy conversion materials, such as thermoelectric and thermionic devices for waste heat recovery and power generation. However, the current state-of-the-art direct energy conversion materials are plagued by extremely low efficiencies that prevent a widespread solution. Recent effort to improve the efficiencies of these direct energy conversion materials has demonstrated a drastic increase through the inclusion of nanoscale features. With new advances in nanoscale materials comes the need for new models that can capture the underlying physics. Thus, this research has developed a necessary tool and a unique modeling approach (based on NEGF quantum simulations) that couples both the electrical and thermal response of nanoscale transport accounting for both the dissipative interactions of electron-phonon and phonon-phonon scattering. Through the aid of high performance computing techniques, the models developed in this research are able to explore the large design space of nano-structured thermoelectrics and thermionic materials. The models allow computational predictions to drive innovation for new, optimized, direct energy conversion materials.;A specific device innovation that has come from this research is the development of variably spaced superlattice (VSSL) devices, which are the next progression in band engineering thermoelectric materials. Computational findings of VSSL materials predict a seven times increase in ZT at room temperature when compared to traditional superlattice devices. Other thermoelectric materials studied include nanocrystalline composites (NCC) which were predicted to outperform equivalent superlattice structures as a results of decreases electron filtering. In addition to thermoelectric materials, this research has developed a quantum modeling technique to investigate and optimize nano-tipped thermionic and thermal-field devices. Results have provided incite into the applicability of Richardson's theory in characterizing the emission from wide-band gap thermionic materials. Ultimately, the quantum models developed in this research are a necessary tool for understanding nanoscale transport and innovating new nanostructured materials.