Thermoelectric and thermomechanical properties of invar 36: Comparison with common thermoelectric materials

Most thermoelectric (TE) materials are brittle and exhibit high coefficient of thermal expansion (CTE), both of which are undesirable considering their service conditions. In this context, invar 36 is known to be a ductile material with a very low CTE feature. However, its TE performance is yet to be reported. Accordingly, the present work involves preparation of invar 36 and investigation of its TE performance. The CTE and mechanical properties of invar 36 were also measured to calculate its thermal shock resistance parameter (R') and to comment on its thermal shock resistance. Further, we calculated R' of state-of-the-art TE ma-terials using literature data for a detailed comparison. The results show that Seebeck coefficient of invar 36 is -14.4 mu V/K at RT and its carrier type is changing from n-to p-type at similar to 550 K. This is an unusual behavior, observed in some well-known TE materials like PbTe and CoSb3, and might suggest a mixed conduction in plain invar 36. Also, we report that invar 36 exhibits a PF of similar to 2.6 mu W/cmK2 at RT, which is comparable to some TE materials. On the other hand, the ZT of invar 36 (similar to 0.0 07) was found to be 10-100 times lower than common TE materials at RT. However, considering a possible minority carrier compensation and as -mea-sured thermal conductivity (similar to 11.9 W/mK), ZT might be improved through careful doping and/or grain size reduction strategies in the future. Besides, unlike most TE materials, invar 36 is ductile with excellent mechanical properties (Elongation: 39%, YS: 436 MPa, UTS: 583 MPa, E: 146 GPa, upsilon: 0.28, Toughness: 200 MJ/m3). Importantly, invar 36 exhibits 10 times lower CTE (1.7 x10-6 K-1 between RT - 473 K) and 75-1000 times higher R' (20125 W/m) than state-of-the-art TE materials, thus, it might attract interest in applications where durability is equally important.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

The study investigates the thermoelectric performance of invar 36, a ductile material with low coefficient of thermal expansion. The results show that invar 36 exhibits a negative Seebeck coefficient and comparable power factor to some thermoelectric materials, but significantly lower figure of merit. However, it has excellent mechanical properties and significantly lower CTE and higher thermal shock resistance, which make it potentially interesting for applications where durability is important.