Thermoelectrics in misfit-layered oxides [(Ca,Ln) _2CoO _3] _(0.62)[CoO _2]: From bulk to nano

摘要

Thermoelectric (TE) properties of [(Ca,Ln) _2CoO _3] _(0.62)[CoO _2] family, including ceramic bulks synthesized by a solid-state reaction and nano-sized compacts fabricated by a simple cold-pressure method, are systematically investigated, in terms of nanocrystallization technology, effects of doped rare-earth Ln ions, and doping level. The doped Ln ions have noticeable influences on the TE characteristics of [Ca 2CoO _3] _(0.62)[CoO _2] system. The substitutions of Ln ~(3+) for Ca ~(2+) change hole concentration and meanwhile introduce a strong point-defect phonon scattering. From Ln=La to Yb, the increased structural distortion, enhanced electron correlation, and increasing ionic mass fluctuation are responsible for the different TE response. Furthermore, compared with the ceramic bulks, in the highly textured nano-sized compacts the thermal conduction is efficiently suppressed. A doubling of ZT has been achieved by the nanoengineering (ZT exceeds 0.45 at 1000K). On the basis of these results, some strategies for further improvement in ZT are proposed.