Abstract
Thermoelectric materials directly convert heat into electricity by generating a voltage from a temperature differential, without any moving parts or noise. Improving the efficiency of thermoelectric materials is important to advancing clean energy generation, especially given current US policy and societal views on reducing oil consumption and generation of greenhouse gases. Select automotive manufacturers have demonstrated improved miles per gallon (MPG) when using thermoelectric generators in test vehicles, however performance needs to be enhanced to facilitate market penetration.
Thermoelectric material performance is described by the dimensionless figure of merit “ZT”, where Z is the figure of merit and T is the average temperature. The material performance is proportional to 1) thermopower squared and inversely proportional to 2) electrical resistivity and 3) thermal conductivity. The goal is to produce a phonon-glass electron-crystal (PGEC), in other words to maintain high electrical conductivity yet keep heat transfer low. Recently, much research to increase ZT has focused on reducing thermal conductivity via nanostructuring.
In this work, we investigate laser sintering of thin films of doped silicon-germanium nanoparticles using a Continuous-Wave (CW) diode laser for higher performance based on nanostructuring and lower cost. Si-Ge is non-toxic, stable at temperatures up to 1000 °C, and can be doped both n- and p- type. Laser processing allows extremely fast heating and cooling rates, which in turn minimizes the amount of nanoparticle size growth and hence generates a large density of interfaces to reduce thermal transport by phonons. By using nanoclusters with a starting grain size of 5-9 nm prior to laser sintering, a bottom-up approach is taken for achieving thermoelectric films. Thermopower and electrical resistivity are characterized using an Ulvac ZEM-3, and Time-Domain ThermoReflectance (TDTR) is used to measure the thermal conductivity of the sintered layer. Morphology and composition are analyzed using a Scanning Electron Microscope (SEM) and Energy Dispersive x-ray Spectroscopy (EDS). High-Temperature X-Ray Diffraction (HT-XRD) is used for structural characterization of the raw and laser sintered films.
Our laser sintering method achieved one of the lowest reported thermal conductivity values for Si-Ge of 1.36 W/m/K, near the Si-Ge amorphous limit of ~1 W/m/K at room temperature, when sintering on fused silica substrate. Using silicon substrate, a high thermopower and good electrical conductivity were measured, where the absolute Seebeck coefficient exceeded 300 µV/K and electrical conductivity measured as 2.4*104 S/m (0.004 Ω∙cm) at room temperature. The peak power factor of 0.0029 W/m/K2 occurred at 600 K. Due to sample inhomogeneity over the measurement volumes between thermal and electrical measurements, exact ZT values could not be calculated. The peak ZT value is expected to be near 0.1 due to high thermal conductivity of the Si substrate, which reduces efficiency to ~ 2% given a 300 to 1000 K operating range. High-temperature XRD confirmed the sintered materials are stable to temperatures > 1000 °C.
Laser processing has the potential for cost reduction and facilitates processing simplicity via flexibility with both materials and geometries. This work serves as the foundation for CW laser sintering of Si-Ge thermoelectrics, a stepping stone to other thermoelectric materials, and the possibility of enhanced thermoelectric performance for recovery of waste heat worldwide.
