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

Abstract

This work further explores the possibilities for designing the high-temperature electrical performance of the thermoelectric Ca₃Co₄O₉ phase, by a composite approach involving separate metallic iron and nickel particles additions, and by employing two different sintering schemes, capable to promote the controlled interactions between the components, encouraged by our recent promising results obtained for similar cobalt additions. Iron and nickel were chosen because of their similarities with cobalt. The maximum power factor value of around 200 μWm^-1K^-2 at 925 K was achieved for the composite with the nominal nickel content of 3% vol., processed via the two-step sintering cycle, which provides the highest densification from this work. The effectiveness of the proposed approach was shown to be strongly dependent on the processing conditions and added amounts of metallic particles. Although the conventional one-step approach results in Fe- and Ni-containing composites with the major content of the thermoelectric Ca₃Co₄O₉ phase, their electrical performance was found to be significantly lower than for the Co-containing analogue, due to the presence of less-conducting phases and excessive porosity. In contrast, the relatively high performance of the composite with a nominal nickel content of 3% vol. processed via a two-step approach is related to the specific microstructural features from this sample, including minimal porosity and the presence of the Ca₂Co₂O₅ phase, which partially compensate the complete decomposition of the Ca₃Co₄O₉ matrix. The obtained results demonstrate different pathways to tailor the phase composition of Ca₃Co₄O₉-based materials, with a corresponding impact on the thermoelectric performance, and highlight the necessity of more controllable approaches for the phase composition tuning, including lower amounts and different morphologies of the dispersed metallic phases.

Keywords: composite approach; controlled interactions; electrical performance; thermoelectric cobaltites; transition metals additions.

Figure 1

Normalized XRD patterns of the 1ST sintered reference matrix and Fe added samples. The vertical lines of different colors mark the main identified phases.

Figure 2

Normalized XRD patterns of the 1ST sintered reference matrix and Ni added samples. The vertical lines of different colors mark the main identified phases. The arrows mark the peak displacements of the phases suggesting substitution with Ni, namely Ca₃Co₄O₉ (black arrows) and CoO (blue arrow).

Figure 3

Representative SEM micrographs (I,III,V,VII) and EDS maps (II,IV,VI,VIII) of selected 1ST sintered samples (fractures).

Figure 4

Electrical conductivity (I), Seebeck coefficient (II), and power factor (III) for 1ST sintered samples. The results for selected Co additions are presented for comparison.

Figure 5

ln(σ·T) vs. 1000/T (Arrhenius) plots for all 1ST sintered samples. The red lines represent linear fittings at high temperatures for activation energy (E_a) calculations.

Figure 6

Normalized XRD patterns of the 2ST sintered reference matrix and Fe added samples. The vertical lines of different colors mark the main identified phases.

Figure 7

Normalized XRD patterns of the 2ST sintered reference matrix and Ni added samples. The vertical lines of different colors mark the main identified phases.

Figure 8

Representative SEM micrographs (I,III,V,VII) and EDS maps (II,IV,VI,VIII) of selected 2ST sintered samples (fractures).

Figure 9

Electrical conductivity (I), Seebeck coefficient (II) and power factor (III) for 2ST sintered samples. The results for selected Co additions are presented for comparison.

Figure 10

Compositional dependence of the power factor at 925 K, for the 1ST and 2ST sintered samples. The results for selected Co additions are shown for comparison.