Nowadays, fossil fuels and energy costs are dramatically increasing. When considering the relatively low energy conversion efficiency of classical systems, an impressive amount of these natural resources are wasted, usually in form of heat. If only a small fraction of the waste heat could be recovered and transformed in useful energy, it would lead to enormous natural resources savings. This would not only decrease energy costs, but reduce environmental impacts, as CO₂ release to the atmosphere, and decrease global warming impact. In this situation, thermoelectric (TE) technology can be regarded as one of the most promising methods to produce useful energy from waste and/or renewable heat sources [1]. When used to harvest waste heat, this technology can play an important role in fighting against global warming by increasing energy conversion efficiency, reducing the fossil fuel consumption, and leading to the decrease CO² emissions. For these applications, it is necessary to have TE materials with high performances, determined through the dimensionless figure of merit, ZT (= TS²/rk; T absolute temperature, S Seebeck coefficient, r electrical resistivity, and k thermal conductivity, where S²/r is the power factor, PF) [2]. On the other hand, if considering massive applications, these thermoelectric materials should have as low costs as possible. In this regard the use of thick films allows material saving, decreasing the need of raw materials, which will decrease the impact of extraction industry on the environment.
In this work, the objective is producing p-type Bi₂Ca₂Co₂O_x thick films from optimized suspensions using the dip-coating technique, to obtain adequate thermoelectric properties for their use in thermoelectric power generators. In a first step, Bi₂O₃, CaCO₃, and CoO were ground in a planetary mill for 10h to obtain a mean particle size of 2.98mm. The powder was then used to prepare a stable suspension with Ethanol, MEK, PVB, PAG, and DBP with 35vol.% solid content. These suspensions were used to prepare thick films on alumina substrates. Some thick films were sintered at 805ºC for 24h, while others were partially melted at 935ºC for 12min and annealed at 805ºC for 24h. SEM observations indicate that partially melted films have lower thickness and better grain orientation than the sintered ones. Consequently, the former films display lower electrical resistivity and Seebeck coefficient. However, power factor of partially melted films at 800ºC (~0.21mW/K²m) is around 33% higher than the obtained in sintered ones.
 
[1] M. H. Elsheikh, et al, Renew. Sust. Energy Rev. 30, 337-355 (2014)
[2] D. M. Rowe (2006) In: D. M. Rowe (ed.), Thermoelectrics Handbook: Macro to Nano, 1st edn. CRC Press, Boca Raton, FL.