Energy, Environment and Water Research Center

Energy, Environment and Water Research Center

Abstract

Waste heat accounts for almost half the energy wasted during conventional power generation systems. Thermoelectric (TE) and Thermophotovoltaics (TPV) systems can be used as waste heat recovery systems in a wide range of applications such as cogeneration in power plants, substituting alternators in automobiles, solid state cooling of microprocessors etc. This talk will focus on how nanostructures can help improve the overall performance of these systems and also, how new characterization techniques can help search for the right nanoscale materials.

One dimensional (1D) nanostructures such as quantum wires and quantum wells have received great attention this past decade because of their unique thermo-physical properties due to size confinement effects. These effects are said to have a significant impact in their TE properties, potentially enhancing the TE figure of merit ZT. ZT is defined as ZT=S2σ/κ, where S is the Seebeck coefficient, σ is the electrical conductivity and κ is the thermal conductivity.

TE characterization of nanowires still remains a challenge because of their small size as well as the lack of suitable characterization techniques able to measure all the relevant thermo-physical properties on the same sample and along the same direction. This work presents the development of a Micro-Electro-Mechanical Systems (MEMS) device able to measure the TE properties κ, σ, and S on the same nanostructure. Additionally, the device has an etched through hole that facilitates the structural characterization of the sample using transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The results obtained for individual BixTe1-x nanowires using this method will be presented.

Furthermore, the MEMS device was modified to extend its use to characterization of the in-plane TE properties of thin films. Recent developments in thin film growth techniques made it possible to produce films with unique and very anisotropic thermo-physical properties. Existing practice for TE characterization of thin films is obtaining κ in the cross plane direction using techniques such as the 3-ω method or time domain laser reflectance technique, whereas the σ and S are usually obtained in the in-plane direction. However, transport properties of thin films can be anisotropic making this combination of measurements along different directions unsuitable for obtaining the actual ZT value for these types of materials. We were able to use the MEMS device to measure all three TE properties in the in-plane direction obtaining the in-plane ZT. The TE properties of two classes of thin films will be presented. The first one is InGaAlAs thin films grown by Molecular Beam Epitaxy embedded with ErAs nanoparticles. The second class is superlattice thin films grown by the Modulated Elemental Reactant Technique (MERT). Three different superlattice thin films were characterized, namely WSe2, Wx(WSe2)y and  (PbSe0.99)x(WSe2)x films. These measurements reveal intriguing size effects on the TE properties.

Additionally a new approach for enhancing the efficiency and power density of TPV cells will be introduced. TPV cells operate in a similar manner as conventional solar cells, but use a terrestrial thermal radiation emitter rather than the sun. They have the additional advantage that they can operate continuously in all weather conditions and allow for greater power density outputs. A nanostructured selective surface emitter will be proposed that has high short wavelength absorptivity and low long wavelength absorptivity. Furthermore the power density of the TPV could be increased by taking advantage of the near field enhancements of radiative energy transfer. A new method for measuring the near field radiation properties of these emitters will be introduced and coupled that with far-field characterization techniques a better understanding will be gained in designing more efficient selective surfaces and TPV solar cells.

For further information, contact Eroulla Cadd on 22208600 or email e [dot] cadd [at] cyi [dot] ac [dot] cy