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Polycrystalline samples of Ca3−xBixCo4O9+δ (x = 0.00, 0.05, 0.10, 0.15, 0.20 and 0.30) have been prepared by conventional solid-state synthesis. Thermopower of all the samples is positive, indicating that the predominant carriers are holes over the entire temperature range. The resistivity of all the samples, except the one with x = 0.30, exhibits nonmetal to metal transition (TMI) in the low temperature regime. The resistivity results indicate that all the doped samples obey the variable range hopping in the low temperature regime. The TMI and T* (transition temperature from Fermi liquid metal to incoherent metal) increase, and the slope of A value (Fermi-liquid transport coefficient) decreases with the increasing Bi content due to an increase in chemical pressure in the lattice. Among the samples, Ca2.7Bi0.3Co4O9+δ has the highest dimensionless figure of merit of 0.091 at 300 K. This value represents an improvement of about 135 % compared to the undoped Ca3Co4O9+δ. Magnetic measurements indicate that all the samples exhibit a low-spin state of cobalt ion. The ferrimagnetic transition temperature is suppressed by the Bi dopant. These results suggest that Bi is an effective doping element for improving the thermoelectric properties of Ca3Co4O9+δ.

Polycrystalline samples of Ca1−xGdxMnO3−δ (x=0.00, 0.02, and 0.05) have been studied by X-ray diffraction (XRD), electrical resistivity (ρ), thermoelectric power (S), and thermal conductivity (κ). All the samples were single phase with an orthorhombic structure. The Seebeck coefficient of all the samples was negative, indicating that the predominant carriers are electrons over the entire temperature range. The iodometric titration measurements indicate that the electrical resistivity of Ca1−xGdxMnO3−δ correlated well with the average valence of Mnv+ and oxygen deficiency. Among the doped samples, Ca0.98Gd0.02MnO3−δ had the highest dimensionless figure of merit 0.018 at 300 K, representing an improvement of about 125% with respect to the undoped GaMnO3−δ sample at the same temperature.

Polycrystalline samples of Ca 1 − x Gd x MnO 3 − δ ( x = 0.00 , 0.02, and 0.05) have been studied by X-ray diffraction (XRD), electrical resistivity (ρ), thermoelectric power (S), and thermal conductivity (κ). All the samples were single phase with an orthorhombic structure. The Seebeck coefficient of all the samples was negative, indicating that the predominant carriers are electrons over the entire temperature range. The iodometric titration measurements indicate that the electrical resistivity of Ca 1 − x Gd x MnO 3 − δ correlated well with the average valence of Mn v + and oxygen deficiency. Among the doped samples, Ca 0.98 Gd 0.02 MnO 3 − δ had the highest dimensionless figure of merit 0.018 at 300 K, representing an improvement of about 125% with respect to the undoped GaMnO 3 − δ sample at the same temperature.

Polycrystalline samples of Ca 1-x Bi x Mn y O 3-δ ( x  = 0.00, 0.02, and 0.05) are prepared by the conventional solid-state reactions. Thermoelectric properties are measured at 300 K–700 K. All the samples are single phase with an orthorhombic structure. Thermopower of all the samples is negative, indicating that predominant carriers are electrons over the entire temperature range. Among the samples, Ca 2.95 Bi 0.05 MnO 3-δ has the highest dimensionless figure of merit of 0.047 at 300 K, representing an improvement of about 487 % with respect to the parent compound of CaMnO 3-δ .

We report the synthesis of the superconductive P3 cobalt oxyhydrates by immersing [alpha]-NaCoO2 in aqueous NaMnO4 solution for the first time. It is a novel route to form the superconductive phase of cobalt oxyhydrates without resort to the commonly used Br2/CH3CN solution. Topotactic transformation of the parent compound [alpha]-NaCoO2 results in water intercalation and hence a significant expansion of the c-axis in the unit cell. The P3 cobalt oxyhydrates exhibits an onset superconducting transition at about 4.2 K.

We report the synthesis of the superconductive P3 cobalt oxyhydrates by immersing alpha -NaCoO sub(2) in aqueous NaMnO sub(4) solution for the first time. It is a novel route to form the superconductive phase of cobalt oxyhydrates without resort to the commonly used Br sub(2)/CH sub(3)CN solution. Topotactic transformation of the parent compound alpha -NaCoO sub(2) results in water intercalation and hence a significant expansion of the c-axis in the unit cell. The P3 cobalt oxyhydrates exhibits an onset superconducting transition at about 4.2 K.

Polycrystalline samples of Ca 0.98 RE 0.02 MnO 3− δ (RE = Sm, Gd, and Dy) have been prepared by conventional solid-state reactions and their properties measured at 300 K to 700 K. All samples were single phase with orthorhombic structure. The average valence and oxygen content of Ca 0.98 RE 0.02 MnO 3− δ were determined by iodometric titration. Doping at the Ca site by rare-earth metals causes a strong decrease of electrical resistivity due to the creation of charge carrier content by Mn 3+ in the Mn 4+ matrix, as evidenced by iodometric titration results. The Seebeck coefficient of all the samples was negative, indicating that the predominant carriers are electrons over the entire temperature range. Among the doped samples, Ca 0.98 Dy 0.02 MnO 3− δ had the highest dimensionless figure of merit of 0.073 at 612 K, representing an improvement of about 115% with respect to the undoped CaMnO 3− δ sample at the same temperature. All the samples exhibited an antiferromagnetic transition with Néel temperature of around 120 K. Magnetization measurements indicated that Ca 0.98 RE 0.02 MnO 3− δ samples exhibited a high-spin state of Mn 3+ .

Polycrystalline samples of Ca^sub 3-x^Ag^sub x^Co^sub 3.95^Fe^sub 0.05^O^sub 9+δ^ (x = 0.0, 0.1, 0.2, and 0.3) have been prepared by conventional solid-state synthesis. The x-ray diffraction patterns reveal that all the samples are single phase. The thermopower of all the samples is positive, indicating that the predominant carriers are holes over the entire temperature range. The electrical resistivity and thermopower simultaneously decrease with increasing Ag content. The highest power factor (2.66 [mu]W cm^sup -1^ K^sup -2^ at 150 K) is reached for the Ca^sub 2.8^Ag^sub 0.2^Co^sub 3.95^Fe^sub 0.05^O^sub 9+δ^, representing an improvement of about 16 % compared to the undoped sample. These results suggest that the Ag is an effective doping element for enhancing the thermoelectric properties of Ca^sub 3^Co^sub 3.95^Fe^sub 0.05^O^sub 9+δ.^ [PUBLICATION ABSTRACT]

Polycrystalline samples of Ca 3− x Bi x Co 4 O 9+ δ ( x  = 0.00, 0.05, 0.10, 0.15, 0.20 and 0.30) have been prepared by conventional solid-state synthesis. Thermopower of all the samples is positive, indicating that the predominant carriers are holes over the entire temperature range. The resistivity of all the samples, except the one with x  = 0.30, exhibits nonmetal to metal transition ( T MI ) in the low temperature regime. The resistivity results indicate that all the doped samples obey the variable range hopping in the low temperature regime. The T MI and T * (transition temperature from Fermi liquid metal to incoherent metal) increase, and the slope of A value (Fermi-liquid transport coefficient) decreases with the increasing Bi content due to an increase in chemical pressure in the lattice. Among the samples, Ca 2.7 Bi 0.3 Co 4 O 9+ δ has the highest dimensionless figure of merit of 0.091 at 300 K. This value represents an improvement of about 135 % compared to the undoped Ca 3 Co 4 O 9+ δ . Magnetic measurements indicate that all the samples exhibit a low-spin state of cobalt ion. The ferrimagnetic transition temperature is suppressed by the Bi dopant. These results suggest that Bi is an effective doping element for improving the thermoelectric properties of Ca 3 Co 4 O 9+ δ .

Polycrystalline samples of Ca₃₋ₓ Bi ₓ Co₄O₉₊δ (x = 0.00, 0.05, 0.10, 0.15, 0.20 and 0.30) have been prepared by conventional solid-state synthesis. Thermopower of all the samples is positive, indicating that the predominant carriers are holes over the entire temperature range. The resistivity of all the samples, except the one with x = 0.30, exhibits nonmetal to metal transition (TMI) in the low temperature regime. The resistivity results indicate that all the doped samples obey the variable range hopping in the low temperature regime. The TMIand T*(transition temperature from Fermi liquid metal to incoherent metal) increase, and the slope of A value (Fermi-liquid transport coefficient) decreases with the increasing Bi content due to an increase in chemical pressure in the lattice. Among the samples, Ca₂.₇Bi₀.₃Co₄O₉₊δ has the highest dimensionless figure of merit of 0.091 at 300 K. This value represents an improvement of about 135 % compared to the undoped Ca₃Co₄O₉₊δ . Magnetic measurements indicate that all the samples exhibit a low-spin state of cobalt ion. The ferrimagnetic transition temperature is suppressed by the Bi dopant. These results suggest that Bi is an effective doping element for improving the thermoelectric properties of Ca₃Co₄O₉₊δ .