Final Report Summary - NANOSICON (High-temperature stable nano-structured silicides for highly efficient thermogenerators and their contacting technology)
TE devices have been known as a viable and flexible principle of direct energy conversion for cooling and power generation for manifold practical applications. Silicides of Mg and Mn are known to exhibit promising TE properties for power generation at temperatures up to about 600 degrees Celsius. The advantage of TE devices over conventional conversion techniques is their extremely high reliability (over 30 years of operation free of failure under space conditions have been reported), silent and vibration-less operation, low volume, no need of maintenance, light weight, free scalability from the sub-microwatt up to the MW region, no involvement of compressed gases or cycling fluids and environmentally friendly operation. The development and investigation of new TE materials has become recently an attractive area for research and application. The main disadvantage of TE devices is their still moderate efficiency.
The main objective of the proposal was the development of improved Mg2Si and its solid solutions with Mg2Sn. These are promising n-type materials for thermogenerators operated at intermediate temperatures. A detailed study of the influence of doping and formation of a solid solution on the electronic and thermal transport properties has been carried out here. Firstly, the synthesis method was optimised. This is crucial due to the high vapour pressure of Mg. Under optimised synthesis conditions, doped Mg2Si and Mg2(Si,Sn) were prepared and studied.
(a) Sb and Bi doping in Mg2Si
- Mg2Si1-xRx (R - Sb, Bi) was synthesised in several compositions. Phase identification by X-ray diffraction (XRD) indicates biphasic nature at higher Bi concentrations while solid solubility for all Sb values.
- Electrical conductivity and Seebeck coefficient were studied. Bi substitution results in an increase in the absolute value of the conductivity. For Sb substitution, an increase is observed up to x = 0.025; further addition results in a weak decrease.
- The thermal conductivity and the lattice component were determined. A decrease with temperature is observed, with a pronounced decrease in the presence of Sb. This might be due to the mass disorder from Sb at Si site and Mg vacancies.
- Maximum ZT values are similar for both Sb and Bi substitution with a maximum of 0.56 for Bi and 0.52 for Sb.
(b) Sb doping in Mg2(Si0.4Sn0.6)
- Measurements of carrier concentration indicated systematic increase with increased dopant addition.
- The thermal conductivity increases with doping. The lattice values show a significant reduction compared to the doped Mg2Si compositions. A maximum ZT value of 1.15 at 673 K was achieved.
The two-year MCF-IIF period was completed successfully. Remarkable improvement of the TE figure of merit was achieved. Six papers related to the project have been published. These contributed significantly to TE materials research. International collaborations were enriching the success of the activity. Along with the laboratory activities, the fellow participated in research conferences and other scientific meetings. The activity has considerably opened the research scope and enhanced the academic communication between the fellow and international colleagues.
The effect of doping on the properties in Mg2Si and Mg2(Si0.4Sn0.6) was studied with Sb resulting in pinning of carrier concentration and enhanced reduction of thermal conduction. Bi doping leads to a prominent increase in the power factor, explained by vacancy formation and secondary phases in Sb/Bi, respectively. Sb doping in Mg2(Si0.4Sn0.6) resulted in large enhancement of ZT with a maximum of 1.15 at 673 K caused by increased band degeneracy and reduced thermal conductivity in the solid solutions. Overall, this makes these materials excellent candidates for thermogenerator applications, for example in the automobile.