Rational interface-enriched defects induce excellent thermoelectric performance of sandwich-type Ca₃Co₄O₉ textured composites

Ca₃Co₄O₉ composed of distorted rock-salt-type slab layers of Ca₂CoO₃ and tilted octahedral CoO₂ is a potential high-temperature thermoelectric compound. However, its low charge carrier mobility and high electrical resistivity lead to poor service performance. In this study, a multilayer co-firing strategy was adopted to build sandwich-type Ca₃Co₄O₉/Ag/Ca₃Co₄O₉ textured composites by introducing a metallic silver layer into the Ca_2.87Ag_0.1La_0.03Co₄O₉ matrix, thereby tuning the complex and enriching point defects at the interface. The metal-semiconductor interface between silver and the lamella-shaped matrix was smooth along the parallel orientation and greatly reduced resistance grain boundaries. In addition, atomic-scale distorted lattices were observed, suggesting that a large number of charge carrier transport channels existed, which was of considerable importance for electron-phonon transporting properties. Ultimately, a large ZT value of 0.493 was obtained at 1073 K for three-layer silver composites. Additionally, a minimum thermal conductivity of 0.98 W (m⁻¹ K⁻¹) was achieved. The results increased the fundamental understanding of the crucial effect of point defects at the interface, which synergistically modulated the electrical and thermal transport properties to enhance the thermoelectric performance of oxide-based materials.