Protocols for the Fabrication, Characterization, and Optimization of n-Type Thermoelectric Ceramic Oxides

Abstract

The development of oxides with high figure of merit, ZT, at modest temperatures (∼300−500 °C) is desirable for ceramic-based thermoelectric generator technology. Although ZT is a compound metric with contributions from thermal conductivity (κ), Seebeck coefficient (S), and electrical conductivity (σ), it has been empirically demonstrated that the key to developing thermoelectric n-type oxides is to optimize σ of the ceramic to ∼1000 S/cm at the operating temperature. Titanate-based perovskites are a popular choice for the development of n-type oxide ceramics; however, the levels of σ required cannot be achieved without control of the ceramic quality, significant reduction of the ceramic in low P(O 2 ) atmosphere (e.g., N 2 /5%H 2 ), and the use of specific dopants and dopant mechanisms, which allow the egress of oxygen homogeneously from the lattice. Here, we discuss the processing protocols to fabricate reliable, reproducible ceramic oxides and schemes for inducing high levels of σ, thereby optimizing the power factor (PF = σS 2 ) and ZT. The problems associated with measuring κ, σ, and S to achieve reproducible and accurate values of ZT are discussed, as are future directions which should enable further optimization. Finally, we comment on how these protocols may be applied to other systems and structures.

A and B site doping of a phonon-glass perovskite oxide thermoelectric

“…8 Metal oxides offer increased stability at high operating temperatures (automotive exhaust waste heat recovery typical operation temperature is 350-700 K, and industrial furnace waste heat recovery at 700-1100 K) 4,9 as well as higher natural abundance and lower toxicity of the constituent elements, leading to much lower costs and smaller environmental impacts when compared with the intermetallics used currently. 10,11 Titanate-based thermoelectrics, such as donor doped SrTiO 3 , are the current state-of-the-art n-type thermoelectric oxides with a recent report of ZT z 0.6 at 1000 K which was achieved through nanoscale modulation of SrTiO 3 and TiO 2 inclusions in Sr 0.9 La 0.1 Ti 0.9 Nb 0.1 O 3 . 12 The unique band structure of SrTiO 3 with contributions from both heavy and light carriers at the Fermi level, 13 gives rise to high power factors of 28-36 mW K À2 cm À1 at 300 K for x ¼ 0.015-0.1 in Sr 1Àx La x -TiO 3Àd single crystals but ZT is limited to 0.09 due to the large k of 9-12 W m À1 K À1 in these materials.…”

Low‐temperature crystallization of La0.15Sr0.775TiO3 using ionic liquids

“…SEM (Figure ) demonstrates the typical particle sizes obtained, of the order 26 ± 10 nm at 300°C, increasing to 36 ± 12 after heating to 600°C (Figure C) at which point the sample is phase pure. This represents a significant decrease in particle size as compared to powders produced using standard solid‐state processes, which are more usually of the order of 0.5‐2 μm …”

“…Thermoelectric materials enable the capture and recycling of waste heat energy; however further optimization of materials is required to enable commercialization of oxide‐based technology . One of the primary reasons for this is the lack of n‐type thermoelectric oxides with high figure of merit . One of the highest values for ZT achieved thus far in an n‐type oxide has been found in La 0.15 Sr 0.775 TiO 3 (LST, 0.41 at 973 K), however this material has only been synthesized using solid‐state synthesis.…”

Performance of a Thermoelectric Module Based on n-Type (La0.12Sr0.88)0.95TiO3−δ and p-Type Ca3Co4−xO9+δ

“…In addition, donor doping with La on the A-site increases the electrical conductivity corresponding to a significant enhancement of zT. 8,11 SrTiO 3 -based materials have rarely been used for fabrication of TE modules; however, 50 pairs of n-type La 0.035 Sr 0.965 TiO 3 and a non-oxide p-type Ni 0.9 Mo 0.1 yielded 450 mW/cm 2 at thermal gradient (DT) 360 K under 5% H 2 /Ar. 12 Ca 3 Co 4Àx O 9+d (p-type electrical conductivity) is stable up to 926°C in air, 13 and since the valence state of Co is strongly dependent on the partial pressure of oxygen, the stability is dependent on working conditions such as atmosphere and temperature.…”