A structural change in Ca₃Co₄O₉associated with enhanced thermoelectric properties

Abstract

Temperature dependent electrical resistivity, crystal structure and heat capacity measurements reveal a resistivity drop and electrical transport behavior change corresponding to a structural change near 400 K in Ca(3)Co(4)O(9). The lattice parameter c varies smoothly with increasing temperature while anomalies in a, b(1) and b(2) lattice parameters occur near 400 K. The Ca site in the Ca(2)CoO(3) block becomes distorted and a change in electrical transport behavior is found above 400 K. Resistivity and heat capacity measurements as a function of temperature under magnetic field combined with Co L-edge x-ray absorption spectra reveal only a weak spin contribution to this change. Reduced resistivity associated with the structural change enhances the thermoelectric properties at moderately high temperatures and points to the electrical transport behavior change as a mechanism for improved ZT in this thermoelectric oxide.

Effect of grain size and porosity on phonon scattering enhancement of Ca3Co4O9

“…The resistivities for samples S3 and S5, 16 and 16.7 mX cm at 1000 K, respectively, fall in the middle range. The decreasing resistivity and transformation from metallic to semiconducting behavior associated with the first-order phase transition, which lead to the enhancement of the TE properties at high temperatures [70,71], are due to the decrease in the density and the random orientation distribution of the grains in the ceramics. Moreover, the charge carriers are scattered more severely in the nano-sized porous materials than in the macroscale porous materials due to the higher density of scattering sites.…”

High-temperature thermoelectric properties of Na- and W-Doped Ca3Co4O9 system

“…This kind of behavior in resistivity of C-349 system has been previously attributed to the spin-state transition, 52 removal of oxygen atoms from porous layered cobaltites 38 and structural distortion in Ca 2 CoO 3 sheets. 53 On the other hand, all doped samples show metallic behavior at low temperatures before showing a transition to semiconducting behavior above 400 K. The absolute values of resistivity at 1000 K for x ¼ 0.025 and 0.05 samples are smaller than that of undoped sample, but higher for x ¼ 0.075. This shows that dual doping of small amounts of Na and W has benecial effect on resistivity of our samples.…”

Suppressed spin-density-wave transition and enhanced electrical conductivity in chlorine doped Ca3Co4O9−xClx ceramics

“…Driven by the global need to utilize waste heat and the application of clean energy source, environmental friendly thermoelectric (TE) materials that can directly convert heat energy into electrical energy without utilizing moving parts or generating carbon dioxide gases, radioactive substances, or other emissions have attracted much interest [1][2][3]. The performance of TE materials is determined by the dimensionless figure of merit ZT = S 2 T/qj, where T, S, q and j are the absolute temperature, the thermopower, the thermal conductivity and the electrical resistivity, respectively.…”