High-temperature thermoelectric properties of

Physica. B, Condensed matter, 2004

Ca 1Àx Pr x MnO 3Àd (x=0, 0.05, 0.15, 0.1, 0.2, 0.4, 0.67; d=0.02) samples were prepared by a solid-state reaction method. X-ray diffraction analysis showed that all samples prepared were of single phase with orthorhombic structure. Electrical resistivity measurements from room temperature to 1300 K showed that a metallic conducting tendency dominated at high temperatures. The hopping nature of the charge carriers was well interpreted in the framework of polaron theory. The Seebeck coefficient was measured in the same temperature interval, and its concentration dependence was analyzed using the high-temperature (HT) thermopower theory proposed by Marsh-Parris. The thermal conductivity and the figure of merit of the prepared samples were also compared with those of other similar perovskite compounds. The observed figure of merit of the sample with x=0.15 was Z=1.5 Â 10 À4 K À1 at T=1100 K, indicating a good potential for application as a HT thermoelectric material.

Materials, 2019

High-temperature instability of the Ca 3 Co 4−y O 9+δ and CaMnO 3−δ direct p-n junction causing the formation of Ca 3 Co 2−x Mn x O 6 has motivated the investigation of the thermoelectric performance of this intermediate phase. Here, the thermoelectric properties comprising Seebeck coefficient, electrical conductivity, and thermal conductivity of Ca 3 Co 2−x Mn x O 6 with x = 0.05, 0.2, 0.5, 0.75, and 1 are reported. Powders of the materials were synthesized by the solid-state method, followed by conventional sintering. The material Ca 3 CoMnO 6 (x = 1) demonstrated a large positive Seebeck coefficient of 668 µV/K at 900 • C, but very low electrical conductivity. Materials with compositions with x < 1 had lower Seebeck coefficients and higher electrical conductivity, consistent with small polaron hopping with an activation energy for mobility of 44 ± 6 kJ/mol and where both the concentration and mobility of hole charge carriers were proportional to 1−x. The conductivity reached about 11 S•cm −1 at 900 • C for x = 0.05. The material Ca 3 Co 1.8 Mn 0.2 O 6 (x = 0.2) yielded a maximum zT of 0.021 at 900 • C. While this value in itself is not high, the thermodynamic stability and self-assembly of Ca 3 Co 2−x Mn x O 6 layers between Ca 3 Co 4−y O 9+δ and CaMnO 3−δ open for new geometries and designs of oxide-based thermoelectric generators.

Nippon Seramikkusu Kyōkai gakujutsu rombunshi, 2003

Ca 1|x Dy x MnO 2.98 (0[x[0.2) m•MdÁ« Pham Xuan ThaoEÒ GE´c¹¹ERº×v k¤ae[ÈwZpåw@åwÞ¿Èw¤ÈEim}eAeNmWZ^[C9231292 Îì §\üSCû¬®ä 11 High temperature thermoelectric properties of Ca 1|x Dy x MnO 2.98 ( x0, 0.05, 0.10, 0.15, 0.20) have been measured in the temperature range from 300 to 1273 K. Ca 1|x Dy x MnO 2.98 ( x0.10, 0.15, 0.20) exhibited the metalinsulator transition in the relation between electrical resistivity and temperature, and the transition temperature increased with increasing x. Electrical resistivity in the metallic region decreased with increas ing x content, reecting the increase of Mn 3{ ions, and the absolute value of Seebeck coecient (a) also decreased. Thermal conductivity ( l) did not considerably change with increasing x content. Among substit uted CaMnO 3 , the highest gure of merit of Z1.05~10 |4 K |1 was obtained for x0.05 and 0.10 at 773 K, while a Z1.63~10 |4 K |1 was obtained for x0.20 at 1273 K.

TheScientificWorldJournal, 2012

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, Ca₀.₉₈Gd₀.₀₂MnO(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.

Progress in Solid State Chemistry, 2007

The possibility to change the Seebeck coefficient sign has been evidenced in the LaCoO 3 perovskites. A small hole doping (Co 3þ /Co 4þ ) will result in a large positive Seebeck coefficient, while a small electron doping (Co 2þ /Co 3þ ) will give a large negative Seebeck coefficient at room temperature. This mechanism is shown to be efficient as well in 1D Ca 3 Co 2 O 6 deriving from hexagonal perovskites. By doping Ca 3 Co 2 O 6 with Ti 4þ , a mixed valency Co 2þ /Co 3þ is introduced and the thermopower turns negative.

Journal of the Korean Ceramic Society, 2010

Ceramics with perovskite-type structure are interesting functional materials for several energy conversion processes due to their flexible structure and a variety of properties. Prominent examples are electrode materials in fuel cells and batteries, thermoelectric converters, piezoelectrics, and photocatalysts. The very attractive physical-chemical properties of perovskite-type phases can be modified in a controlled way by changing the composition and crystallographic structure in tailor-made soft chemistry synthesis processes. Improved thermoelectric materials such as cobaltates with p-type conductivity and n-type manganates are developed by following theoretical predictions and tested to be applied in oxidic thermoelectric converters.

Microelectronics Journal, 2008

Oxide ceramics with nominal composition of La 0:8 Sr 0:2 Co 1Àx Mn x O 3 ð0pxp0:05Þ were grown by using the citrate sol-gel method followed by high temperature sintering. The thermoelectric properties were studied in the temperature range between 100 and 290 K. The magnitude of Seebeck coefficient SðTÞ and electrical resistivity rðTÞ increases with the manganese doping level, reaching maximum values close to 180 mV=K and 4 mO cm, respectively. On the contrary, the total thermal conductivity kðTÞ decreases with the manganese content. The behavior of SðTÞ and rðTÞ was interpreted in terms of small-pollaron hopping mechanism. From SðTÞ, rðTÞ and kðTÞ data it was possible to calculate the dimensionless thermoelectric figure of merit ZT, which reaches maximum values close to 0.12; the structural and morphological properties of the samples were studied by powder X-ray diffraction analysis and scanning microscopy (SEM), respectively. r

Crystal Engineering, 2002

Metal-transition oxides are potential materials for thermoelectric devices operating at high temperatures (T 300 K). In particular, compounds exhibiting small resistivity and large Seebeck coefficient are required. Two kinds of oxides have been retained: the 'misfit' cobaltites which are p-doped metallic phases and the electron doped perovskite manganites. For the former, it is found that the Ca for Sr substitution enhances the thermopower values in the Pb/Sr/Co/O misfit cobaltite whereas it has only a moderate effect in the case of the Tl/Sr/Co/O phase. From this study, it appears that the oxidation state of Co in the conducting layers plays a crucial role on the physical properties. Moreover, for the manganites, we show that electrondoped SrMnO 3 can be prepared by a two-steps method. The doping, created via substitution at either the A-site (Pr 3+ for Sr 2+ ) or B-site (Mo 6+ for Mn 4+ ), leads to metallic compounds with large thermopower values. 

Journal of Electronic Materials, 2009

We have investigated the effects of Bi doping on the crystal structure and high-temperature thermoelectric properties of the n-type layered oxide Ca 2 MnO 4Àc . The electrical conductivity r and the absolute value of the Seebeck coefficient S were, respectively, found to increase and decrease with Bi doping. The thermal conductivity j of doped Ca 2 MnO 4Àc is relatively low, 0.5 W/m K to 1.8 W/m K (27°C to 827°C). Consequently, the ZT value, ZT = rS 2 T/j, increases with Bi doping. The maximum ZT is 0.023 for Ca 1.6 Bi 0.18 MnO 4Àc at 877°C, which is ten times higher than that of the end member, Ca 2 MnO 4Àc . The increase of ZT mainly results from the considerable increase of r, which can be explained in terms of structural change. The Mn-O(1) and the Mn-O(2) distances in the c-direction and ab-plane, respectively, increase with increasing Bi concentration, indicating that the valence state of Mn ions decreases with the increase of electron carriers in the CaMnO 3 layers. In addition, the Mn-O(2)-Mn bond angle increases linearly with Bi doping, leading to an improvement of the electron carrier mobility.

Inorganic Chemistry, 2008

Perovskite-type CaMn 1-x Nb x O 3(δ (x) 0.02, 0.05, and 0.08) compounds were synthesized by applying both a "chimie douce" (SC) synthesis and a classical solid state reaction (SSR) method. The crystallographic parameters of the resulting phases were determined from X-ray, electron, and neutron diffraction data. The manganese oxidations states (Mn 4+ /Mn 3+) were investigated by X-ray photoemission spectroscopy. The orthorhombic CaMn 1-x Nb x O 3(δ (x) 0.02, 0.05, and 0.08) phases were studied in terms of their high-temperature thermoelectric properties (Seebeck coefficient, electrical resistivity, and thermal conductivity). Differences in electrical transport and thermal properties can be correlated with different microstructures obtained by the two synthesis methods. In the high-temperature range, the electron-doped manganate phases exhibit large absolute Seebeck coefficient and low electrical resistivity values, resulting in a high power factor, PF (e.g., for x) 0.05, S 1000K)-180 µV K-1 , F 1000K) 16.8 mΩ cm, and PF > 1.90 × 10-4 W m-1 K-2 for 450 K < T < 1070 K). Furthermore, lower thermal conductivity values are achieved for the SC-derived phases (κ < 1 W m-1 K-1) compared to the SSR compounds. High power factors combined with low thermal conductivity (leading to ZT values > 0.3) make these phases the best perovskitic candidates as n-type polycrystalline thermoelectric materials operating in air at high temperatures.