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

Abstract

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 materials using literature data for a detailed comparison. The results show that Seebeck coefficient of invar 36 is −14.4 μV/K at RT and its carrier type is changing from n- to p-type at ∼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 ∼2.6 μW/cmK2 at RT, which is comparable to some TE materials. On the other hand, the ZT of invar 36 (∼0.007) was found to be 10–100 times lower than common TE materials at RT. However, considering a possible minority carrier compensation and as-measured thermal conductivity (∼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, υ: 0.28, Toughness: 200 MJ/m3). Importantly, invar 36 exhibits 10 times lower CTE (1.7 ×10-6 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.