Effect of carbon nanotubes addition on thermoelectric properties of Ca₃Co₄O₉ ceramics

Increasing the phonon scattering center by adding nanoparticles to thermoelectric materials is an effective method of regulating the thermal conductivity. In this study, a series of Ca_3Co_4O_9/x wt.% CNTs (x=0, 3, 5, 7, 10) polycrystalline ceramic thermoelectric materials by adding carbon nanotubes (CNTs) were prepared with sol-gel method and cold-pressing sintering technology. The results of x-ray diffraction and field emission scanning electron microscopy show that the materials have a single-phase structure with high orientation and sheet like microstructure. The effect of adding carbon nanotubes to the thermoelectric properties of Ca_3Co_4O_9 was systematically measured. The test results of thermoelectric properties show that the addition of carbon nanotubes reduces the electrical conductivity and Seebeck coefficient of the material. Nevertheless, the thermal conductivity of the samples with carbon nanotubes addition is lower than that of the samples without carbon nanotubes. At 625 K, the thermal conductivity of Ca_3Co_4O_9/10 wt.% CNTs sample is reduced to 0.408 W\cdotm^-1\cdotK^-1, which is about 73% lower than that of the original sample. When the three parameters are coupled, the figure of merit of Ca_3Co_4O_9/3 wt.% CNTs sample reaches 0.052, which is 29% higher than that of the original sample. This shows that an appropriate amount of carbon nanotubes addition can reduce the thermal conductivity of Ca_3Co_4O_9 ceramic samples and improve their thermoelectric properties.