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A3: High temperature materials
Oxide thermoelectric materials, which are highly durable at high temperature in air, non-toxic, low cost with minimal environmental impact, are apparently promising for recuperation of decentralized waste heat energy at the temperature range of > 400 °C, where all the non-oxide candidate materials will eventually be oxidized under aerobic conditions. Although strongly ionic characters of oxide materials has been regarded as an inherent disadvantage leading to low carrier mobility and high lattice thermal conductivity, it has been revealed that such disadvantages are not always the case with all oxides. Recent reports on reduced ferroelectric oxides [1] and cage-like structure oxides [2] are convincing that the simple picture of ionic compounds no longer holds for these oxides.
In this paper, some new aspects in metal oxides that show unconventionally enhanced phonon scattering will be highlighted in terms of their transport properties and crystal structures. In particular, β-pyrochlore oxides ABB'O6 (A = K, Rb, Cs) show lower lattice thermal conductivity with decreasing the mass and size of the A-site alkali cations, clearly evidencing that the larger size mismatch between the A-site cations and the surrounding oversized cage framework enhances the “rattling” motion of the A-site cations and thereby efficiently shortens the phonon mean free path more for the smaller A-site cations. As a consequence, the thermal conductivity of the oxide with the smallest A cation, KBB'O6, was revealed to be virtually the same with its theoretical minimum, κmin [3].
[1] S. Lee, J. A. Bock, S. Trolier-McKinstry, C. A. Randall, J. Eur. Ceram. Soc., 32, 3971 (2012).
[2] M. Ohtaki, S. Miyaishi, J. Electron. Mater., 42, 1299 (2013).
[3] C. Dames, G. Chen, J. Appl. Phys., 95, 682 (2004).