ICT2014 Abstracts

Nanostructuration has been proven to be a successful route to improve the properties of thermoelectric materials. Among the different approaches, the addition of secondary phases within a thermoelectric material matrix has allowed increasing the properties due to a lower thermal conductivity attributed to phonon scattering at the interfaces matrix-particles. Other authors have demonstrated that the improvement can also be correlated to a better power factor due to low energy carrier filtering. First reports on particles additions in thermoelectric doped SrTiO₃ oxides seems also to show lower thermal conductivities and improved ZT values. However, samples low density, grain growth and reactions between the thermoelectric oxide and the added oxides make the results difficult to interpret. Concerning the Ca₃Co₄O₉ compound, only silver/Ca₃Co₄O₉ composites have been studied, showing reaction between silver and the oxide, and improved ZT value of 0.5 at 1000 K for the (Ca_2.7Ag_0.3Co₄O₉ /Ag 10 wt%) composite.

The Ca₃Co₄O₉-Co₃O₄ composite samples have been prepared by solid state reaction and sintered by spark plasma sintering (SPS, FCT Systeme GmbH HP D 25). Co₃O₄ has been added to Ca₃Co₄O₉ to produce (1-x)Ca₃Co₄O₉/xCo₃O₄ composites samples with x = 0, 1, 10, 20 and 50 vol % by thoroughly mixing the powders 5 min at 400 rpm in an agate ball mill (Retsch PM 100). The SPS cycle consists in applying a pressure of 70 MPa whereas the temperature was raised at 100°C/min up to 850°C for 5 minutes and then cooled at 100°C/min to room temperature. Phase purity and absence of reaction between the two phases have been checked by powder X-ray diffraction and DTA/TG analyzes respectively. The transport properties have been measured in a ZEM III equipment (ULVAC Technologies) whereas thermal diffusivity was measured using the laser flash diffusivity technique (Netzsch LFA 457) from 323 to 1000 K.