Researchers at the Materials Function Control Laboratory at the Toyohashi University of Technology and the Nagoya Institute of Technology have successfully synthesized a new thermoelectric material, CaMgSi, an intermetallic compound. The key to this development was the synthesis procedure; bulk CaMgSi intermetallic compound was synthesized by combining mechanical ball-milling (MM) and pulse current sintering (PCS) processes.
The new thermoelectric material is lightweight, with a low density of 2.2 g/cm3. One of the possible applications of the material is in automobiles to utilize waste heat emitted from engines, the research team suggested. A paper on the work is published in the Journal of Alloys and Compounds.
Thermoelectric energy conversion through the Seebeck effect is a clean and environmentally compatible power-generation technology. However, thermoelectric devices are less common in practical use because most thermoelectric materials include toxic and/or expensive elements.
Previous work by the research team had investigated Mg2Si-based systems as suitable thermoelectric materials for the effective utilization of automotive waste heat. The Mg2Si intermetallic compound is non-toxic, inexpensive and low-density. Its thermoelectric property can be improved by substituting Ge or Sn for a part of the Si.
The researchers had earlier synthesized Mg-Ca-Si thermoelectric materials with a molar ratio of 3:1:2, comprising Mg2Si, CaMgSi and Ca7Mg7.25Si14 phases. The Seebeck coefficient (α) of the Ca7Mg7.25Si14 intermetallic compound was that of a metallic behavior (α ≈ 0). Further, Mg2Si is widely known as an n-type compound semiconductor (α < 0). Although the primary phase of the synthesized material was Mg2Si, nevertheless the thermoelectric properties of the materials showed a positive α value. This result suggested to the team that the CaMgSi phase itself has high thermoelectric performance.
Appearance of thermoelectric property in the intermetallic compound, CaMgSi, has been predicted by both theoretical and experimental studies. However, the biggest issue in front of us was the synthesis of thermoelectric CaMgSi of optimal size.—Nobufumi Miyazaki and Nozomu Adachi
In general, alloys are produced by mixing the constituent elements in their molten forms. However, the vapor pressures of Ca and Mg are higher than that of Si, and the boiling point of Mg (1363 K) is lower than the melting point of CaMgSi (1507 K) and Si (1687 K). This makes conventional production of CaMgSi problematic.
To overcome that problem, the team used a mechanical ball milling process to mix the elements homogeneously, without melting, and then induced a chemical reaction between Ca, Mg, and Si using the pulse current sintering process, said Associate Professor Yoshikazu Todaka.
The thermoelectric property of the thus-synthesized CaMgSi exhibited a performance comparable to that of previously developed Mg-based thermoelectric materials.
The researchers expect that adding a a fourth element to CaMgSi will further enhance its thermoelectric properties. Interestingly, they found that the novel thermoelectric can exhibit both n- and p-type conductivity with a slight change in the composition of CaMgSi. Such a property for the material is of interest for its application in power-generation modules.
Nobufumi Miyazaki, Nozomu Adachi, Yoshikazu Todaka, Hidetoshi Miyazaki, Yoichi Nishino (2016) “Thermoelectric property of bulk CaMgSi intermetallic compound,” Journal of Alloys and Compounds, Volume 691, Pages 914-918 doi: 10.1016/j.jallcom.2016.08.227