Microstructural Influence on Thermoelectric Properties of CaMnO3 Ceramics

Abstract

Thermoelectric properties of pure polycrystalline CaMnO 3 ceramics were significantly enhanced by increasing sintering holding time from 1 to 24 h. The Seebeck coefficient values were reduced while the sintering holding time was increased, and the DC electrical conductivity was enhanced from 255 S/m to 1.748 S/m. This same effect was observed in electronic thermal conductivity, increasing from 5x10-3 to 3.5x10-2 W/m K, whereas the lattice thermal conductivity decreased from 5.0 to 4.0 W/m K at 873K. Overall, CaMnO 3 ceramic sintered for 24 h demonstrated the best performance, which presented a Figure-of-Merit value of about 0.03. The grain size of the samples ranged from 2.79 µm to 6.45 µm, due to the sintering holding time, directly influencing the high-temperature thermoelectric properties of CaMnO 3 ceramics.

Thermoelectric properties of CaMnO3 ceramics produced by using powder synthesized by modified Pechini method

“…where σ 0 , E a and k B are a pre-exponential factor, activation energy and the Boltzmann constant (1.38 × 10 23 m 2 kg/s 2 K), respectively. As observed in previous work [17], a direct proportionality between σ values and grain size can be noticed. The exception is the CMO-3h sample, which showed the highest values of electrical conductivity, reaching 1830 S/m, at 873 K, which could be related to the presence of ∼10% of the secondary phase.…”

“…Figure 4 shows the SEM images of the ceramics. It is possible to observe that there is a trend of grain size growth with the longer sintering time [17]. Grains exhibited a polyhedral morphology, the same as those presented for the CMO obtained by solid-state reaction [28].…”

“…According to the previous study, increasing the sintering time further improves the electrical conductivity values due to the decrease in grain boundary area, which acts as a scattering centre to the charge carrier [17]. Thereby, this study reports the effects of sintering time on the grain size, structure, microstructure and thermoelectric properties of produced CMO-based ceramics considering the presence of secondary phases generated by chemical synthesis, i.e.…”

“…The smallest average grain size of 1.04 µm was obtained for the ceramics sintered for 1 h and the largest of 4.85 µm was achieved for the CMO-24h sample. According to the previously published work, the chemical synthesis pro- duces smaller grain sizes compared to solid-state reaction method [17].…”

Tuning the Thermoelectric Performance of CaMnO3-Based Ceramics by Controlled Exsolution and Microstructuring

“…The zT values reported for pristine CaMnO 3−δ are typically below 0.10, such as 0.03 15 and 0.04 16 at 600 °C and 0.06 17 and 0.08 10 at 800 °C, whereas 0.12 at 800 °C has been reported 18 but not confirmed in other investigations. A giant power factor was reported for a CaMnO 3−δ sample fabricated by the laser floating-zone technique, reaching 0.39 mW•K −2 • m −1 at 800 °C.…”