| Summary: | Mg2Si-based thermoelectrics are currently the most promising, environmentally benign and inexpensive materials for power generation. The objective of this thesis is to characterize Mg2Si-based thermoelectric materials using state-of-the-art synchrotron radiation techniques including infrared reflectivity/absorption spectroscopy and high resolution X-ray powder diffraction. This is complemented by density functional theory (DFT) calculations. Also reported here is the main research: the analysis of the electronic structure and transport properties of doped Mg2Si using experimental and theoretical methods.
To enhance the thermoelectric performance, Mg2Si doped with a single component of Bi or Sb were studied. The investigation showed by doping the electron carrier concentrations were increased. In particular, dc conductivities of the doped samples were extracted from the analysis of infrared reflectivity spectra employing the Drude free electron model. We found the conductivity was lower when determined by infrared measurements rather than in-situ four point probe measurements of the bulk sample because of the limited penetration depth of infrared (IR) radiation and the very small spot size. In particular, we were able to extract the electrical conductivity, relaxation times and electron effective masses of the samples. DFT calculations reproduced the experimental observations and show a substantial increase in the Seebeck coefficients.
The next step was to study the effect after doping with two different dopants. For this purpose, we investigated the effect of Ge substitute Si in Bi doped Mg2Si. In particular, the dc conductivities of the doped samples were extracted from the analysis of infrared reflectivity spectra. From the IR data, we extracted the relevant parameters for electrical transport. The experimental data were explained with theoretical DFT calculations in which the calculated densities of states (DOS) of the Ge- and Bi-doped Mg2Si samples were found to be very similar, and therefore to have comparable Seebeck coefficients. The steep curvatures of the DOS at the Fermi level indicate a light electron band. We found the thermal conductivity of Mg2Si is substantially lower from 7 Wm-1K-1 to 2.7 Wm-1K-1 in Mg2Si0.677Ge0.3Bi0.023 at 300 K. A performance figure of merit of 0.7 was achieved at 773 K for this sample.
We further investigated the effect of multi-doping with Sb, Al and Zn on the enhancement of the thermoelectric and electrical transport properties of Mg2Si. A maximum ZT of 0.964 was found for Sb0.5%Zn0.5% doped Mg2Si (Mg1.995Zn0.005Si0.995Sb0.005) at 880 K. This value is comparable to those of PbTe based thermoelectrics which are the currently the materials used in commercial products.
We also studied the effect of pressure on the thermoelectric performance of a Al-doped Mg2Si sample. From in-situ X-ray diffraction, we observed a structural transform in which the electrical conductivity was increased after the phase transition. The experimental observed maximum thermoelectric power at 1.9 GPa was reproduced by DFT calculations and explained by the increase of electronic density of states at the Fermi level.
The effect of multi-wall carbon nanotubes (MWCNTs) to increase the electrical conductivity of Mg2Si0.877Ge0.1Bi0.023 was examined. At 323 K the conductivity was found to increase from 450 Ω-1cm-1 to 500 Ω-1cm-1. However, this effect diminished at higher temperature and the conductivity drop to 470 Ω-1cm-1 at 773 K. Raman study showed the persistent of disorder (D) and tangential (G) mode characteristics of a carbon nanotube in the doped sample indicating that there was no decomposition or substantial chemical reaction of the MWCNTs with Mg2Si0.877Ge0.1Bi0.023.
Finally, we present the results on the analysis of valence electron topologies of Mg2Si multi-doped with Al, Zn and Sb thermoelectric materials by the Maximum Entropy Method (MEM) using data obtained from synchrotron X-ray powder diffraction measurements. The results showed the qualitative feature of valence electron distributions were correctly located. However, due to the limited number of Bragg diffraction peaks in the experimental patterns, the effect of the dopants to the core charge density cannot be reliably obtained. An error analysis was performed from the analysis of diffraction pattern of Al-doped Mg2Si which included high angle Bragg reflections. We concluded that the density maps extracted from MEM analysis of the doped samples were qualitatively correct.
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