Thermoelectric response of porous Ca3Co4O9 prepared by an eco-friendly technique

In this paper, we have studied the thermoelectric response of porous Ca3Co4O9 (CCO) samples. The porosity in the CCO samples was increased by introducing wood particles in the material followed by a high-temperature sintering to completely burn off the wood. The density of the resulting samples was found to decrease gradually with increase in porosity. Structural and chemical characterization of the samples was performed by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. All these characterization experiments showed that, except for an increase in the ratio of the relative concentration of Co3+ and Co4+, increasing porosity did not coincide with any change in phase or elemental composition of the material. The increase in the ratio of Co3+/Co4+, did serve to enhance the value of Seebeck coefficient of the more porous samples. Porosity also caused a reduction in the thermal conductivity due to increased phonon scattering, thereby enhancing the figure of merit, ZT, of the sample. A ZT value of 0.19 at 800 K was obtained for the CCO sample with optimum porosity of 47%. These promising results coming from materials prepared by an eco-friendly process give a path to a new era of porous materials for practical applications in thermoelectric modules.