Layered Co-based ceramics with a nominal composition Bi2-xLaxSr2Co2O8-δ (x=0.0, 0.4, 0.8, short by BLC-222) were prepared using conventional solid state reaction method. X-ray photoemission spectroscopy (XPS) was used to investigate their electronic structures. The cobalt ions are highly mixed valences of Co3+ and Co4+. The fraction of Co4+ almost keeps unchanged with the increase of x. The O-1s photoemission spectra show that there are lattice oxygen and chemical absorbed oxygen in all the samples. The substitution of Bi3+ by La3+ results in a change from metallic-like behavior to semiconductor behavior. This abnormal phenomenon means that La3+ plays a key role in effecting the electrical transport property of BLC-222. The O-Co covalence bond is strengthened by the increase of La3+, which results in the decrease of conductivity.
Polycrystalline particles of Ca3-xErxCo2O6 (x=0.0, 0.15, 0.3, 0.45 and 0.6) were synthesized using sol-gel method combined with Low Temperature Sintering procedure (LTS) to evaluate the effect of Er substitution on the thermoelectric properties of Ca3Co2O6. The crys- tal structure and microstructure were investigated using X-ray diffraction, infrared spectroscopy and scanning electron microscope. The elec- trical conductivity and Seebeck coefficient of the complex oxides were measured from 300 to 1073 K. The results showed that all the sam- ples were p-type semiconductors. The electrical conductivity increased with the increase in temperature. Er substitutions at Ca site affected carrier concentrations and carrier mobility, resulting an increase in Seebeck coefficient and decrease in electrical conductivity. The power factor of Ca2.85Er0.15Co2O6 reached 10.66 μw/mK2 at 1073 K.
Polycrystalline Ca2.7-xEr0.3NaxCo4O9+δ (x=0, 0.05, 0.1, 0.15) samples were prepared using a sol-gel process followed by hot-pressing technique sintering and their high-temperature thermoelectric properties were measured from 350 up to 1073 K. The results showed that with the increase of the dopant amount of Na, the electrical conductivity increased, while the Seebeck coefficient and thermal conductivity decreased. The electrical conductivity was dominated by polaron hopping conductivity mechanism above about 550 K and the Na+ substitution for Ca2+ could not change the transport mechanism. The dimensionless figure of merit ZT value reaches 0.31 for Ca2.55Er0.3Na0.15Co4O9+δ at 1073 K, which suggested that Ca2.7-xEr0.3NaxCo4O9+δ (x=0, 0.05, 0.1, 0.15) were good potential oxides for thermoelectric application.
Tetragonally layered perovskite manganites of Nd2-2xSr1+2xMn2O7 (x=0.25, 0.3, 0.4) were fabricated by using solid-state reaction technique. Structural characterization of the compounds was investigated by X-ray diffraction (XRD) and FT-IR absorption spectra. The XRD patterns revealed that all the samples were single phase. X-ray photoemission spectroscopy (XPS) was used to investigate their electronic structures. It was found that manganese was in mixed states of Mn3+ and Mn4+ whereas lattice oxygen and chemical absorbed oxygen were existed in all the samples. The high temperature electrical properties of Nd2-2xSr1+2xMn2O7 (x=0.3, 0.4) were measured by standard four-probe technique. The results showed that both compounds had semi-conductivity behavior in the temperature range of 300～1073 K, and the electrical conduction was dominated by thermally activated behavior above 500 K.
Layered perovskite manganites with a nominal chemsitry of Sm2-2xSr1+2x-2yCa2yMn2O7 (x=0.2, 0.3, 0.4, 0.5; y=0, 0.2, 0.3) were prepared using sol-gel method. The crystal structures of these compounds were studied by X-ray diffraction (XRD) and FTIR absorption spectra. The absorption peaks become weaker and move a little to higher frequency with increasing of Sm/Sr concentration. As the Sm doping increases to x=0.2 and x=0.3, the absorption peaks show a cubic structure character, reflecting that the samples suffer from a transition from tetragonal structure to cubic structure. This coincides with the X-ray diffraction results. The high temperature electrical properties were studied by conventional four-probe method. Although all samples exhibit the semiconductive behavior, lnρ-1 000/T curves are not linear and they obey the small polaron hopping mechanism. Moreover, the resistivity decreases with x reducing. This is due to that Sm doping increases the Jahn-Teller ion Mn3+ / Mn4+ ratio, and decreases the eg bandwidth.
