Bulk Nanostructured Thermoelectric Materials for Solid-State Cooling and Power Generation
The decoupling of electrical and thermal transport has been a long-time fundamental scientific challenge for both bulk crystals, nanomaterials, and nanocomposites. For the cases of nanomaterials and nanocomposites, it is mainly due to the fact that most solution-synthesize nanomaterials, such as conductive polymers and inorganic nanowires/nanoparticles can not be effectively doped to achieve the optimum doping level like the bulk inorganic crystals due to the well-known “self-purification” effect. Associated with the lower doping level are the three intrinsic problems: (1) low electrical conductivity (σ) due to low carrier concentration; (2) low Seebeck coefficient (S) due to bipolar transport (both electrons and holes are contributing to electrical transport when the material is nearly at intrinsic doping level); and (3) relatively high thermal conductivity (κ) due to bipolar transport. In this area, we will explore an innovative materials chemistry approach, based on the success in our proof-of-concept investigation, to solve this challenge in nanocomposite systems by either design nanoscale heterostructures or grafting well-designed functional conductive organic molecules such as small molecules to the surface of thermoelectric chalcogenide nanowires through a theoretical guided exploration. The significant breakthroughs, which have been suggested in our research, include advantages like: (1) improved electrical conductivity due to the surface doping from the heterostructure or functional groups on the organic molecules; (2) enhanced Seebeck coefficient due to the blocking of minority carrier or energy filtering; and (3) significantly lowered thermal conductivity due to minority carrier blocking and phonon scattering from compositional interface, surface-bound organic molecules, and phase transition.
As a complimentary approach, our group is also exploring novel complex metal oxide materials and new processing method for electricaloric cooling. The entropy change, as function of applied electrical field, can be significantly improved with optimized crystallinity and domain orientation. We are using in-situ TEM to understand the break down mechanism in the thin film electrocaloric materials.
The main efforts include:
Rational Design of Nanoscale Heterogenous System - Through the band structure calculation, we utilize the theories of (1) Energy Filtering; (2) Minority Carrier Blocking; and (3) Phonon Scattering at Surface/Interface and during phase transition to select matrix and nano-inclusion materials to increase power factor and decrease thermal conductivity
Scalable Synthesis of Nanomaterials - We use solution-phase method to produce highly uniform nanocrystals, nanowires, and their heterostructures in kilogram range to minimize sample variation and perform systematic investigation
Controlled Consolidation of Nanomaterials into Bulk Nanostructured Composite - We use spark plasma sintering process to rapidly consolidate nanocrystals, nanowires, and their heterostructures into bulk nanostructured composite to minimize elemental diffusion and grain growth. The in-plane and cross-plane electrical and thermal properties will be investigated.