Microstructure Of Melted And Annealed Ca-Co-O Series Materials

In Ca-Co-O series materials, Ca₃Co₄O₉, Ca₂Co₂O₅ and Ca₃Co₂O₆ all have thermoelectric properties and thus been investigated widely, most of these studies focus on theoretical calculations, doping and controlling of microstructure, a little on the microstructure transformation in heating and cooling process, considering the decomposition of Ca₃Co₄O₉ and Ca₃Co₂O₆, the sintering temperature were mostly controlled below the decomposition temperature, that may be one of the reason for valuable thermoelectric materials prepared unsuccessfully. This paper attempts to explore a new preparing method of Ca-Co-O materials above the decomposition temperature.With CaCO₃ and Co₂O₃ as starting materials, samples with nominal composition of Ca₃Co₄O₉, Ca₂Co₂O₅ and Ca₃Co₂O₆ have been prepared by pre-sintering, arc-melting and annealing at different temperature in this paper, and the microstructure of samples investigated respectively. All of these are the foundation for exploring new preparing method of Ca-Co-O materials.It is shown that the melted samples are compact except needle holes and shrinkage caves in centre, consist of two solid solutions, calcium oxide containing divalent cobalt oxide (a phase) and divalent cobalt oxide containing calcium oxide (βphase), with large dendrite in microstructure.In the samples with nominal composition of Ca₃Co₄O₉ annealed at 880℃, Ca₃Co₄O₉ and Ca₃Co₂O₆ were observed when annealed for 36h, but Ca₃Co₂O₆ disappeared when annealed for 96h, so the samples with unique Ca₃Co₄O₉ can be obtained. In the samples with nominal composition of Ca₃Co₂O₆ annealed at 880℃for 100h, Ca₃Co₂O₆ are the main phase with a few of Ca₃Co₄O₉ and a phase. In the samples with nominal composition of Ca₂Co₂O₅ annealed at 880℃for 100h, Ca₃Co₂O₆ and Ca₃Co₄O₉ are the main phase, with no expected Ca₂Co₂O₅.In the samples annealed at 1000℃and 1050℃for 9h, homogenized microstructure can be observed.Samples with nominal composition of Ca₂Co₂O₅ and Ca₃Co₂O₆ annealed at 1200℃for 9h consist of a andβ. Samples with nominal composition of Ca₃Co₄O₉ annealed at 1200℃for 9h consist of a andβ, too, butβphase is spherical, a with signs of decomposition locates at the gap between sphericalβ. After being further annealed at 880℃for 5h, the decomposition of a is even more obvious, andβarrays obtained between which there may be certain orientation relationship.Based onβarrays, which perhaps appear only in a certain range of temperature and composition, there is the possibility of preparing Ca₃Co₄O₉ thermoelectric materials with a certain orientation.