Synthesis, sintering, transport and thermal properties of Na2Fe2Ti6O16 freudenbergite

Abstract

Na2Fe2Ti6O16 freudenbergite has been synthesized using a simple solid-state route and conventionally sintered to reach 94% density ceramics. High temperature electrical and thermal transport properties have been investigated for the first time. In the temperature range of 468 K–1000 K, the electrical conductivity of Na2Fe2Ti6O16 can be well fitted with small polaron hopping model. For a further verification of the model, we measured the room-temperature optical reflectivity and derived the optical conductivity (σ(ω)) with Kramers–Kronig analysis. A mid-infrared peak in σ(ω) confirms the presence of small polarons. From 468 K to 1000 K, Na2Fe2Ti6O16 is an n-type oxide and exhibits a temperature-independent Seebeck coefficient (∼−590 μV K−1), in agreement with the small polaron hopping mechanism. The thermal conductivity decreases with increasing temperature from 16.5 W m−1.K−1 (373 K) to 3.4 W m−1.K−1 (1000 K). Na2Fe2Ti6O16 shows a maximum figure of merit (ZT) ∼ 3 × 10−4 at 1000 K.