Redox-Promoted Tailoring of the High-Temperature Electrical Performance in Ca3Co4O9 Thermoelectric Materials by Metallic Cobalt Addition

Abstract

This paper reports a novel composite-based processing route for improving the electrical performance of Ca3Co4O9 thermoelectric (TE) ceramics. The approach involves the addition of metallic Co, acting as a pore filler on oxidation, and considers two simple sintering schemes. The (1-x)Ca3Co4O9/xCo composites (x = 0%, 3%, 6% and 9% vol.) have been prepared through a modified Pechini method, followed by one- and two-stage sintering, to produce low-density (one-stage, 1ST) and high-density (two-stage, 2ST) ceramic samples. Their high-temperature TE properties, namely the electrical conductivity (σ), Seebeck coefficient (α) and power factor (PF), were investigated between 475 and 975 K, in air flow, and related to their respective phase composition, morphology and microstructure. For the 1ST case, the porous samples (56%–61% of ρth) reached maximum PF values of around 210 and 140 μWm−1·K−2 for the 3% and 6% vol. Co-added samples, respectively, being around two and 1.3 times higher than those of the pure Ca3Co4O9 matrix. Although 2ST sintering resulted in rather dense samples (80% of ρth), the efficiency of the proposed approach, in this case, was limited by the complex phase composition of the corresponding ceramics, impeding the electronic transport and resulting in an electrical performance below that measured for the Ca3Co4O9 matrix (224 μWm−1·K−2 at 975K).

Reduction in Processing Time in Ca3Co4O9+δ Ceramics through Nanoprecursors Produced by an Easily Scalable and Environmentally Friendly Process

“…Moreover, when increasing sintering time up to 12 h in these samples, electrical resistivity is further decreased, while it is maintained constant for longer sintering times. The minimum values determined at 800 °C (16 mΩ cm) in samples sintered for 12 h is about 25% lower than those measured in samples produced through the ceramic route in this work (20.6 mΩ cm) or previously published results (20–41.7 mΩ cm) [ 20 , 22 , 23 ]. Furthermore, they are similar to those reported in samples produced using solution methods (about 16 mΩ cm) [ 24 ] or by directional solidification (around 15 mΩ.cm) [ 25 ].…”

“…Moreover, when increasing sintering duration, their performances are raised up to 12 h and maintained constant for larger sintering times. The highest PF values at 800 °C (~0.23 mW/K 2 m), obtained in samples sintered for 12 h, are about 10% higher than the obtained through the classical ceramic route in this work, or in the literature (0.10–0.20 mW/K 2 m) [ 22 , 23 ]. On the other hand, they are much lower than the reported for textured materials produced by spark plasma texturing [ 26 ], or laser floating zone technique (~0.40 mW/K 2 m) [ 25 ].…”

“…The highest value at 800 °C in these samples (~195 μV/K) is higher than the reported in spark plasma sintered samples (170–175 μV/K) [ 26 ], but slightly lower than the determined in laser floating zone grown materials (~205 μV/K) [ 25 ], which induces higher oxygen vacancies in the thermoelectric phase. Furthermore, they are in the order of those published for classically sintered materials (170–210 μV/K) [ 22 , 23 ].…”

Prospects for Electrical Performance Tuning in Ca3Co4O9 Materials by Metallic Fe and Ni Particles Additions

“…Single-phase Ca 3 Co 4 O 9 ceramic materials used as matrices for the (1 − x) Ca 3 Co 4 O 9 /xFe or Ni (x = 0%, 3%, 6% and 9% vol.) compositions were prepared through a ‘wet-chemistry’ modified Pechini method [ 49 ]. Appropriate amounts of micrometric Fe and Ni particles and Ca 3 Co 4 O 9 powders were mixed in an agate mortar, in ethanol, to produce the desired compositions.…”

“…The present research work takes advantages of the previously published results for metallic Co additions [ 49 ] and reports on a similar approach for improving the high-temperature electrical performance of bulk Ca 3 Co 4 O 9 materials, by similar metallic Fe and Ni additions and employment of two different sintering schemes, for producing porous and dense Ca 3 Co 4 O 9 -based composites. The degree of success of this approach is measured by the electrical counterpart of the TE performance (electrical conductivity, Seebeck coefficient, and resulting power factor) of the resulting composites, which are compared to the best ones obtained for the Co additions and to some values reported in literature, being related to the prepared samples’ phase compositions, morphologies, and microstructural features.…”