Recently, nano-enabled thermoelectric (TE) devices have attracted much attention for cost-effective embedded and portable cooling applications such as in high-performance integrated circuits, lasers, and medical and food chillers. However, efficiency of this novel technology, largely, relies on: (i) Fine-tuning the basic material parameters that are strong function of coupled structural-electronic processes at the nanoscale; and (ii) System-level optimization considering the geometry, substrates, and contacts. These issues have not been fully assessed experimentally and, therefore, demand a careful numerical investigation. As a response to this need, in this project, we have mainly worked on the following Thrusts:

i) Develop a multiscale simulator for modeling non-classical nanostructured Bi₂Te₃, nitride and ZnO thermoelectric devices with the capability of handling millions of atoms.

ii) Investigate the competing effects of atomicity, various long-range built-in structural and electrostatic fields, size-quantization, and operating temperature on the performance of TE devices.

iii) Explore new design space for improved TE efficiency and performance.

iv) Involving the undergraduate (REU) students: (a) Design an accelerated GUI with interactive features for the multiscale TE simulator and made it freely accessible on NSF’s nanoHUB.org for the broader community for use in research and teaching activities. (b) Explore and use advanced synthesis and printing technologies for the creation and characterization of a prototypical TE module.

Listed below are some of the key Outcomes from this project.

1) An educational version of the Multiscale Modeling of Thermoelectric Cooler (multiscaleTEC) software was published on NSF’s nanoHUB (URL: https://nanohub.org/tools/multiscaletec).

2) Detailed numerical investigation of the performance of Bi₂Te₃ nanowire based thermoelectric devices suggests that active hotspot cooling of as much as 23 °C with a high heat flux is achievable using such low-dimensionality structures. However, it has been observed that thermal and electrical contact resistances, which are quite large in nanostructures, play a critical role in determining the cooling range and lead to significant performance degradation that must be addressed before these devices can be deployed in such applications.

3) Another simulator called Monte Carlo Phonon Transport Simulator (MCPT) was created and uploaded on nanoHUB.org for free-access and use by the community (URL: https://nanohub.org/tools/mcpt). MCPT calculates lattice thermal conductivity by solving Boltzmann Transport Equation (BTE) numerically using a particle-based Monte Carlo simulation. The modified valence force-field model used here accounts for effects of strain and phonon-phonon interactions. In addition, diffusive boundary scattering of phonons due to rough surfaces is modeled via the use of Gaussian distribution and the Bechmann-Kirchhoff surface scattering formalism for electromagnetic waves.

4) As compared to the empirically fitted model, the thermal con­ductivity calculated using the MCPT simulator deviates by about 8% (~18 W/m-K). Also, it was observed that with the increase of lattice temperature the diffusive boundary scattering increases as the percentage of phonons with larger wavevector increases. The observation can be useful in designing materials with low thermal conductivity for TE cooling purposes.

5) First-principles DFT based method was used to investigate the piezoelectric polarization coefficient of ZnO nanowires. The study indicates that piezoelectric coefficient for bare ZnO nanowires is a highly diameter sensitive parameter, where free boundary leads the surface atoms to move towards the center of the nanowire. The results indicate small size nanowires have a promising potential in the application of energy harvesting.

6) Tensile strain in MoS₂ based electron conducting channels for TE use reduces phonon energy for E+ and E- Raman modes, energy bandgap, and electron effective mass, which modulate the electron density, velocity, and conductivity. For both modes, the current peaks at ~4% of applied strain. 

7) The synthesis of n-Bi₂Te₃, n-Bi₂Se₃, and p-Sb₂Te₃ thermoelectric nanoplates was carried out using a microwave-stimulated reaction between chalcogen and pnictogen complexes formed in an organic solvent. These two complexes reacted to precipitate black chalcogen-pnictogen nanoplates. It was observed, in one case, that a black precipitate already began to form even before the solution was microwaved. These plates were then examined under SEM to determine their size and shape. The nanoplates were then dissolved in another solvent producing printer ink.

8) A characterization system was constructed that allows a straightforward measurement of ZT figure-of-merit of TE modules with minimal heat loss. The various parts employed in this system include wood blocks as chassis, copper plate and blocks using a 3-D printer, thin-film heater, Arduino Uno, two linear actuators, an H-Bridge motor controller, a 12-volt battery, two voltage divider circuits, a potentiometer, and a liquid CPU cooler.

The award has fully/partially supported the publication of 2 freely-accessible software on NSF’s nanoHUB cyberinfrastructure, 1 book chapter, 4 journal articles, and 6 refereed conference papers. Results from this project have been presented in about 10 technical seminars/workshops. In total, 4 PhD (2 of whom already graduated), 2 MS (1 female), and 6 REU students have partially/fully contributed and been trained in this project.

Last Modified: 09/28/2017
Modified by: Shaikh S Ahmed