Considering the ZT expression a high performance thermoelectric material

Ca3−xNaxCo4O9 polycrystalline ceramic materials, with x=0.00, 0.01, 0.03, 0.05, 0.07, and 0.10, were prepared by the conventional solid state route using CaCO3 (Panreac, 98+%), Na2CO3 (Panreac, 98+%), and Co2O3 (Aldrich, 98+%) commercial powders as starting materials. They were weighed in the appropriate proportions, well mixed and ball milled for 30 min at 300 rpm, in acetone media, in an agate ball mill. The obtained suspension has then been dried under infrared JWH 133 until all the acetone has been evaporated. The resulting powder mixture was manually milled, to break the agglomerates, and subsequently thermally treated at 750 and 800 °C for 12 h under air, with an intermediate manual milling to decompose the carbonates [15]. After the thermal treatments, the powders were uniaxially pressed at 400 MPa for 1 min in order to obtain green ceramic parallelepipeds (3 mm×2.5 mm×14 mm), with an adequate size for their thermoelectric characterization. The green ceramics were then sintered in the optimal conditions found in previous works, consisting in one step heating at 900 °C during 24 h with a sinoatrial (SA) node final furnace cooling [38].