Magnesium Silicide Based Thermoelectric Nanocomposites

The major objective of this thesis was to investigate how far the thermoelectric (TE) properties of magnesium silicide based materials can be enhanced via doping, alloying, and nanostructuring. The investigation of Sb and Bi doped Mg2Si showed experimentally that the dopants can indeed substitute Si in the crystal lattice. The excess Sb and Bi atoms were found in the grain boundaries, most likely in the form of Mg3Sb2 and Mg3Bi2. As a consequence, the sample showed lower carrier concentration than the formal Sb/Bi concentration suggests, and the thermal conductivity was significantly reduced. The investigation of the effect of germanium substitution for silicon in bismuth doped Mg2Si, showed that the alloying drastically reduced the room temperature thermal conductivity partially due to the added mass contrast and the existence of Ge-rich domains within the sample. Due to the increased in the amount of scattering centers caused by Ge alloying, the electrical conductivity was also decreased while the Seebeck coefficient was increased only very slightly. In summary, the positive effect of Ge substitution on the TE properties of Bi doped Mg2Si resulted in a figure of merit of 0.7 at 773 K for Mg2Si0.677Ge0.3Bi0.023 sample. The addition of multi wall carbon nanotubes (MWCNT) to the Mg2Si0.877Ge0.1Bi0.023 resulted in an improved electrical conductivity, in particular around room temperature. The Seebeck coefficient of all nanocomposites is enhanced at 773 K due to energy filtering that stems from the introduction of CNTs - Mg2Si0.877Ge0.1Bi0.023 interfaces. The lattice thermal conductivity of the nanocomposites is reduced due to the phonon scattering by nanodomains and grain, particularly at medium temperatures, resulting in a slight reduction in total thermal conductivity. All in all, the thermoelectric figure of merit of the sample containing 0.5 weight-% MWCNT was enhanced by about 22% as compared to the pristine sample. Finally, the investigation of the effect of silicon carbide (SiC) nanoparticles on the TE properties of Mg2Si0.676Ge0.3Bi0.024 revealed that increasing the concentration of SiC nanoparticles systematically reduced the electrical conductivity, while enhancing the Seebeck coefficient. In spite of its high thermal conductivity, SiC could successfully decrease the lattice thermal conductivity through adding more interfaces. The HRTEM study showed the existence of both Ge and Bi in the Si position, and some Bi segregation at the boundary. In summary, the figure of merit reached its maximum value of 0.75 at 773 K for the sample containing 0.5 wt.-% SiC, which is among the highest achieved in the Mg2Si1-xGex system

Synthesis of p-type Mg2Si1-xSnx with x = 0-1 and optimization of the synthesis parameters

Mg2Si is a promising thermoelectric material in the mid-temperature region 500 – 800 K. Development of Mg2Si based thermoelectric generators requires both good n- and p-type materials. While the thermoelectric properties n-type Mg2(Si,Sn) materials are good, those of the corresponding p-type are not as much. Therefore, optimizing p-type solid solution of magnesium silicide and magnesium stannide is highly desired. We employ high energy ball milling for efficient synthesis of p-type Mg2(Si,Sn) and investigate the effect of milling time, sintering temperature, and holding time on the thermoelectric properties of p-type Mg2Si1-xSnx with x = 0-1. We can show the synthesis of p-type Mg2(Si,Sn) for the whole compositions using Li as a dopant. We have also studied the effect of the synthesis parameters (milling time, sintering temperature, and holding time) on the phase purity, functional homogeneity and thermoelectric properties. The phase purity increases with longer milling time. The functional homogeneity decreases with higher sintering temperature and longer holding time. The optimum synthesis condition for x = 0.6 leads to zTmax0.6 at 700 K, which is one of the highest value reported for p-type Mg2(Si,Sn)

Developing Contacting Solutions for Mg2Si1–xSnx-Based Thermoelectric Generators: Cu and Ni45Cu55 as Potential Contacting Electrodes

Magnesium silicides can be used for thermoelectric energy conversion as high values of figure of merit zT were obtained for n-type (1.4 at 500 °C) and p-type (0.55 at 350 °C) materials. This, however, needs to be complemented by low resistive and stable contacting to ensure long-term thermogenerator operation and minimize losses. In this study, we selected Cu and Ni45Cu55 as contacting electrodes for their high electrical conductivity, similar coefficient of thermal expansion (CTE), and good adhesion to Mg2(Si,Sn). Both electrodes were joined to Mg2Si0.3Sn0.7 pellets by hot pressing in a current-assisted press. Microstructural changes near the interface were analyzed using SEM/EDX analysis, and the specific electrical contact resistance rc was estimated using a traveling potential probe combined with local Seebeck scanning. Good contacting was observed with both electrode materials. Results show low rc with Cu, suitable for applications, for both n- and p-type silicides (<10 μΩ·cm2), with the occurrence of wide, highly conductive diffusion regions. Ni45Cu55 joining also showed relatively low rc values (∼30 μΩ·cm2) for n- and p-type but had a less inhomogeneous reaction layer. We also performed annealing experiments with Cu-joined samples at 450 °C for 1 week to investigate the evolution of the contact regions under working temperatures. rc values increased (up to ∼100 μΩ·cm2) for annealed n-type samples but remained low (<10 μΩ·cm2) for p-type. Therefore, Cu is a good contacting solution for p-type Mg2(Si,Sn) and a potential one for n-type if the diffusion causing contact property degradation can be prevented