Formation mechanism and thermoelectric properties of CaMnO3 thin films synthesized by annealing of Ca0.5Mn0.5O films (original) (raw)
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Thermoelectric properties of CaMnO3 films obtained by soft chemistry synthesis
Journal of Materials Research, 2012
Polycrystalline randomly oriented CaMnO 3 films were successfully deposited on sapphire substrates by soft chemistry methods. The precursor solutions were obtained from a mixture of metal acetates dissolved in acids. The Seebeck coefficient and the electrical resistivity were measured in the temperature range of 300 K , T , 1000 K. Modifications of thermal annealing procedures during the deposition of precursor layers resulted in different power factor values. Thermal annealing of CaMnO 3 films at 900°C for 48 h after four-layer depositions (route A) resulted in a pure perovskite phase with higher power factor and electrical resistivity than four-layer depositions of films annealed layer by layer at 900°C for 48 h (route B). The studied films have negative Seebeck coefficients indicative of n-type conduction and electrical resistivities showing semiconducting behavior.
RSC Advances, 2020
CaMn 1Àx Nb x O 3 (x ¼ 0, 0.5, 0.6, 0.7 and 0.10) thin films have been grown by a two-step sputtering/annealing method. First, rock-salt-structured (Ca,Mn 1Àx ,Nb x)O thin films were deposited on 1 100 sapphire using reactive RF magnetron co-sputtering from elemental targets of Ca, Mn and Nb. The CaMn 1Àx Nb x O 3 films were then obtained by thermally induced phase transformation from rock-salt-structured (Ca,Mn 1Àx Nb x) O to orthorhombic during post-deposition annealing at 700 C for 3 h in oxygen flow. The X-ray diffraction patterns of pure CaMnO 3 showed mixed orientation, while Nb-containing films were epitaxially grown in [101] out of-plane-direction. Scanning transmission electron microscopy showed a Ruddlesden-Popper (R-P) secondary phase in the films, which results in reduction of the electrical and thermal conductivity of CaMn 1Àx Nb x O 3. The electrical resistivity and Seebeck coefficient of the pure CaMnO 3 film were measured to 2.7 U cm and À270 mV K À1 at room temperature, respectively. The electrical resistivity and Seebeck coefficient were reduced by alloying with Nb and was measured to 0.09 U cm and À145 mV K À1 for x ¼ 0.05. Yielding a power factor of 21.5 mW K À2 m À1 near room temperature, nearly eight times higher than for pure CaMnO 3 (2.8 mW K À2 m À1). The power factors for alloyed samples are low compared to other studies on phase-pure material. This is due to high electrical resistivity originating from the secondary R-P phase. The thermal conductivity of the CaMn 1Àx Nb x O 3 films is low for all samples and is the lowest for x ¼ 0.07 and 0.10, determined to 1.6 W m À1 K À1. The low thermal conductivity is attributed to grain boundary scattering and the secondary R-P phase.
Ceramics International, 2018
CaMnO 3-based materials are very attractive among n-type thermoelectric oxides for high-temperature applications when they are appropriately doped. The main drawback of these materials is the cost associated to the necessary rare earth cations. This work aims decreasing the amount of these materials through a partial substitution of Ca 2+ by an equimolar mixture of K + and Yb 3+ , Ca 1-x (K 0.5 Yb 0.5) x MnO 3 , with x = 0.05, 0.10, 0.15, and 0.20. XRD studies have confirmed that the thermoelectric phase is the major one in all samples. Microstructure has shown the formation of large crystals, and an increasing porosity when the substitution is raised. This evolution has been confirmed through density measurements. Electrical resistivity has been drastically decreased for the 0.10 substituted samples, compared with the 0.05 ones, slightly increasing for higher substitution. On the other hand, absolute Seebeck coefficient and thermal conductivity are lower when the substitution is raised. The best ZT values have been achieved for the 0.10 substituted samples, which are around the typical reported in the literature for higher doping level. These results clearly point out to a decrease of the necessary rare earth dopant content to achieve similar performances in CaMnO 3 ceramics, which is of the main economic significance for their industrial production.
High-temperature thermoelectric properties of Ca1−xPrxMnO3−δ (0⩽x<1)
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.
Microstructural Influence on Thermoelectric Properties of CaMnO3 Ceramics
Materials Research, 2020
Thermoelectric properties of pure polycrystalline CaMnO 3 ceramics were significantly enhanced by increasing sintering holding time from 1 to 24 h. The Seebeck coefficient values were reduced while the sintering holding time was increased, and the DC electrical conductivity was enhanced from 255 S/m to 1.748 S/m. This same effect was observed in electronic thermal conductivity, increasing from 5x10-3 to 3.5x10-2 W/m K, whereas the lattice thermal conductivity decreased from 5.0 to 4.0 W/m K at 873K. Overall, CaMnO 3 ceramic sintered for 24 h demonstrated the best performance, which presented a Figure-of-Merit value of about 0.03. The grain size of the samples ranged from 2.79 µm to 6.45 µm, due to the sintering holding time, directly influencing the high-temperature thermoelectric properties of CaMnO 3 ceramics.
Flash combustion synthesis of electron doped-CaMnO3 thermoelectric oxides
Powder Technology, 2014
Ceramics In situ high-temperature X-ray diffraction Electrical properties Nano-crystalline particles of calcium manganite Ca 1 − x R x MnO 3 (R = Yb, Dy, Sm and Bi, with x = 0.0 and 0.1), having a complex orthorhombic-perovskite structure, have been successfully synthesized by flash combustion method. The powders obtained were thoroughly characterized by scanning electron microscopy and thermal analyses. The average crystallite size is found in the range between 100 and 200 nm. The crystallization was followed by in situ high-temperature X-ray diffraction in air. The XRD analyses show that the cell volume of the orthorhombic-perovskite structure for Ca 0.9 R 0.1 MnO 3 increased with a rise in the ionic radius of the R 3+ cations. The stoichiometry of the samples was confirmed by energy dispersive spectroscopy analysis. The temperature dependence of thermoelectric properties was measured from 75 to 400°C and the R-dopant effects were investigated. The electrical resistivity is strongly dependent on the nature of the substituent ions. All the samples exhibit negative values of the thermopower, which indicates that the electrons are the predominant charge carriers (n-type conduction). Finally, the highest power factor of 165 μW•m −1 K −2 has been obtained at 400°C for Ca 0.9 Yb 0.1 MnO 3. In this paper, we suggest to use the fine grains of Ca 0.9 R 0.1 MnO 3 as promising candidates for thermoelectric applications.
Characterization and Studies on AC Conductivity of CaMnO3 Material
Journal of physics, 2018
Synthesis of CaMnO 3 has been performed by solid state reaction method using CaCO 3 and MnCO 3 powder as raw materials. The raw materials were weighed, milled, compacted into a pellet and then sintered at 1250°C. Phase of material, microstructure, and conductivity of the samples were observed. The refinement results of X-ray diffraction pattern shows that CaMnO 3 formed as a single phase, which has a structure orthorhombic (P n m a) with lattice parameters, a = 5.277 Å, b = 7.452 Å, and c = 5,261 Å. The atomic density of the refinement result is 4.591 gr.cm-3. The morphology of CaMnO 3 sample has a good particle homogeneity with the particle size 1-2 μm. The value of AC conductivity on the CaMnO 3 is directly proportional with the increasing ofthe temperature. The highest value of the AC conductivity of the CaMnO 3 sample is 2.8 x 10-3 S/cm at a temperature of 400°C. whereS, σ, and κ are respectively the Seebeck coefficient, the electrical conductivity and the thermal conductivity. CaMnO 3 material can be synthesized through a solid-state reaction. J.W. Park, et al carry out synthesis of CaMnO 3 using solid-state reaction method[8]. The experiment was begun by mixing raw materials, followed by calcination, compaction and sintering at 1300°C for 12 hours. Recently, several studies involving successful synthesis ofCaMnO 3 have been reported, especially those performed at high temperatures (≥ 1000°C). The interesting featuresin theCa-Mn-O system are the electrical and magnetic properties that are found in the Ca 2 MnO 4 , Ca 3 Mn 2 O 7 , Ca 4 Mn 3 O 10 , CaMnO 3 , and Ca 2 MnO 4 compounds[9]. The stable activity of the composition of CaO-MnO occurs in the temperature range of 1100-1300 °C. Based on the description that CaMnO 3 material can be applied as a thermoelectric device, so the aim of this study is to synthesize and characterize CaMnO 3 material.
Journal of Applied Physics, 2013
Polycrystalline tungsten-substituted CaMn 1Àx W x O 3Àd (0.00 x 0.05) powders were synthesized from a polymeric precursor, pressed and sintered to high density. The impact of tungsten substitution on the crystal structure, thermal stability, phase transition, electronic and thermal transport properties is assessed. Tungsten acts as an electron donator and strongly affects high-temperature oxygen stoichiometry. Oxygen vacancies form in the high figure-of-merit (ZT)-region starting from about T ¼ 1000 K and dominate the carrier concentration and electronic transport far more than the tungsten substitution. The analysis of the transport properties yields that in the investigated regime the band filling is sufficiently high to overcome barriers of polaron transport. Therefore, the Cutler-Mott approach describes the electrical transport more accurately than the Mott approach for small polaron transport. The lattice thermal conductivity near room temperature is strongly suppressed with increasing tungsten concentration due to mass-difference impurity scattering. A ZT of 0.25 was found for x ¼ 0.04 at 1225 K. V C 2013 AIP Publishing LLC.