Improved Thermoelectric Properties of Ca3-xBaxCo4O9 (x=0~0.4) Bulks by Sol-Gel and SPS Method

Ca_3-xBa_xCo₄O₉ (x = 0~0.4) bulks were prepared by sol-gel and spark plasma sintering (SPS) method, and the effects of preparation process on texture and thermoelectric properties were discussed in detail. The results show that SPS process is helpful to form texture. The bulks are composed of plate-like grains, which are aligned to (001) plane. To some extent, the orientation and density of Ca₃Co₄O₉ bulks are enhanced with the increasing SPS temperature, pressure and sintering time, which decrease electrical resistivity as well as keep the lower thermal conductivity and higher Seebeck coefficient. SPS sintering time has the most immense effect on thermoelectric properties. The sample sintered at 800degC and 30MPa for 5min exhibits figure of merit (ZT) value of 0.2 at 700degC, while the ZT value is improved to 0.4 for the bulk sintered for 2h. A certain amount of Ba doping can reduce the electrical resistivity and the thermal conductivity, and so improve the ZT value. When the doping content of Ba is 0.2, the maximum ZT value of 0.28 at 700degC for Ca_2.8Ba_0.2Co₄O₉ samples sintered by SPS at 800degC, 30MPa for 5min can be achieved. When the sintering time is prolonged to 2h, the ZT value is improved to 0.43. But the ZT value decreases when the doping content exceeds some level because of two main reasons: on the one hand, too much doping content causes defect scattering, which reduces carrier mobility and so decreases the electrical conductivity; on the other hand, the radius of Ba ions is very large, and when the content is higher, it´s difficult for them to completely enter the lattice to substitute Ca sites and so the carrier concentration can´t be enhanced