1. Themoelectric properties of Pb and Sr doped Ca3Co4O9
Hiroshi Nakatsugawa1, Hyeon Mook Jeong1, Rak Hee Kim2, Natsuko Gomi1 and Hiroshi Fukutomi1
1 Yokohama National University, Yokohama Japan,
2 Changwon National University, Changwon Korea
INTRODUCTION
EXPERIMENT
Since the discovery of large thermoelectric power in the layered compounds NaCo2O4 and Ca3Co4O9, misfit-layered cobalt oxides particularly have attracted much interest as candidates for thermoelectric materials.
The Crystal structure of Ca3Co4O9 consists of an alternate stack of a distorted three-layered rock salt (RS) - type Ca2CoO3 block layer (BL) and a CdI2 - type CoO2 conducting sheet parallel to the c-axis. Owing to the size difference between the RS - type BL and the CoO2 sheet, however, the compound has an incommensurate periodicity parallel to the b-axis. Thus the resulting structural formula becomes [Ca2CoO3]pCoO2, where p equals bCoO2 / bCa2CoO3 ～ 0.62.
・conventional solid-state reaction
・electrical resistivity ρ
・Seebeck coefficient S
・thermal conductivity κ
・Hall coefficient RH
・carrier concentration n = 1/eRH
・Hall mobility μ = RH / ρ
・Scanning Electron Microscopy
・neutron powder diffraction (HERMES)
・Rietveld refinement program (PREMOS 91)
・magnetic susceptibility χ
Pb2+
Sr2+
Thermoelectric properties
x = 0.01
x = 0
x = 0.02
x = 0.03
RWp = 7.4%
a = 4.84Å, c = 10.86Å, p = 0.618
RWp = 7.1%
a = 4.84Å, c = 10.87Å, p = 0.619
RWp = 8.0%
a = 4.84Å, c = 10.89Å, p = 0.617
RWp = 7.2%
a = 4.84Å, c = 10.89Å, p = 0.616
Co3.08+
Co3.13+
Co3.14+
Co3.11+
Co3.61+
Co3.61+
Co3.65+
Co3.66+
CONCLUSION
・Seebeck coefficient showed similar large S in all the samples because the substitution of Pb2+ for Ca2+ would keep the carrier (holes) concentration. The Co2 – O3 modulation reveal that Pb ions take Pb2+ in the RS – type BL.
・The magnetic susceptibility reveal that Co1 ions take Co3.1+ and Co2 ions take Co3.6+ in all the samples, which also suggests that Pb ions mainly take Pb2+ in the RS – type BL.
・The maximum ZT is in x = 0.02 at RT which resulted from minimum ρ at RT in x = 0.02.
J. Sugiyama et al., Phys. Rev. B 68, (2003)