La 1−xCa x MnO3 semiconducting nanostructures: morphology and thermoelectric properties
© Culebras et al.; licensee Springer. 2014
Received: 14 May 2014
Accepted: 7 July 2014
Published: 21 August 2014
Semiconducting metallic oxides, especially perosvkite materials, are great candidates for thermoelectric applications due to several advantages over traditionally metallic alloys such as low production costs and high chemical stability at high temperatures. Nanostructuration can be the key to develop highly efficient thermoelectric materials. In this work, La 1−xCa x MnO3 perosvkite nanostructures with Ca as a dopant have been synthesized by the hydrothermal method to be used in thermoelectric applications at room temperature. Several heat treatments have been made in all samples, leading to a change in their morphology and thermoelectric properties. The best thermoelectric efficiency has been obtained for a Ca content of x=0.5. The electrical conductivity and Seebeck coefficient are strongly related to the calcium content.
KeywordsNanostructures Seebeck Thermoelectricity Perovskites
S being the Seebeck coefficient, σ the electrical conductivity, κ the thermal conductivity, and T the absolute temperature. The power factor (PF) defined as PF≡σ S2 can be used to compare the relative efficiency when the thermal conductivity is similar in different samples.
Over the past 30 years, semiconductor alloys based on Bi 2Te3, PbTe, and SiGe [7–9] have been extensively studied and optimized for their use in thermoelectric applications. However, most of these compounds present disadvantages related to the shortage of raw materials, toxicity, or high costs of production.
For these reasons, research on the new materials to build up efficient thermoelectric devices is a scientific subject of current interest [10, 11]. Recently, several oxides such as NaCoO 2, Ca 3Co4O9, Sr 1−xLa x TiO3, La 1−xSr x CoO3, Nd 1−xCa x CoO3, or Ca 0.8Dy0.2MnO3 have shown excellent thermoelectric properties. More precisely, perosvkite-type transition metal oxide single crystals have depicted large thermoelectric responses . The electrical properties of La 1−xA x MnO3 (A = Ca, Sr, Ba, and Pb) perosvkite-type oxides are related to their stoichiometry . Significant variations appear when the degree of substitution of the alkali-earth element for La varies from 0% to 50% . The novelty of perovskite-type oxides is due to their low cost, non-toxicity, and possibility of being used for high-temperature applications. The origin of the thermoelectric properties in these oxides is not yet fully understood, but it could be related to the high spin-orbit interaction as well as the large electron effective mass .
In 1993, the work of Hicks and Dresselhaus  suggested that the morphology of a thermoelectric system can be used to improve both the electronic transport and the phonon scattering. Nanostructuration can increase ZT over unity by changing σ and S independently. The density of electronic states in a nanostructured system, when the Fermi energy is close to a maximum in the density of electronic states, depicts usually sharp peaks and theoretically larger Seebeck coefficients than the same material in bulk . Furthermore, the phonon dynamics and heat transport in a nanostructured system can be suppressed by means of size effects. Nanostructures with one or more dimensions smaller than the phonon mean free path (a phonon glass) but larger than that of electrons (electron crystal) will noticeably reduce the thermal conductivity κ without affecting much the electrical transport. In other words, phonon transport will be strongly disturbed, while the electronic transport can remain bulk-like in nanostructured systems.
In this report, La 1−xCa x MnO3 nanocrystals have been obtained by the hydrothermal method as a function of the Ca content. Several heat treatments have been made to determine the temperature when the perosvkite phase is obtained. Scanning electron microscopy and X-ray diffraction studies have been used to determine the perosvkite phase. The electrical conductivity and Seebeck coefficient have been determined as a function of temperature in order to analyze their thermoelectric performance.
The reactants MnCl 2·4H 2O, Ca(NO 3) 2, La(NO 3) 3, KMnO 4 and KOH were purchased from Sigma Aldrich Co., Madrid, Spain.
Synthesis of La 1−xCa x MnO3nanostructures
La 1−xCa x MnO3 samples with x=0.005,0.05,0.1 and 0.5 have been prepared by a conventional hydrothermal treatment [20–22]. Stoichiometric amounts of reactants were used to have an aqueous solution of 0.55 M in cations (Mn 7+, Mn 2+, Ca 2+, and La 3+) by keeping a molar ratio between KMnO 4 and MnCl 2·4H 2O according to the average valence of Mn ions in La 1−xCa x MnO3. The pH of the solution was adjusted to 13 by adding KOH. After ultrasonic stirring, the solution was transferred into a Teflon autoclave and heated for 30 h at 230°C. Then, the reactor was cooled down to room temperature, and the obtained solid was washed with water and ethanol and dried at 230°C for 12 h. The powder was subjected to different temperatures, 650°C and 900°C for 12 h. The powder obtained after 900°C was pressed to form compact pellets (0.5-in. diameter) by using a pellet die at 490 MPa. Further, the pellet was sintered at 900°C for 24 h.
The scanning electron microscopy (SEM) analysis was carried on a Hitachi 4800S microscope (Hitachi, Ltd., Tokyo, Japan) at an acceleration voltage of 20 kV and at a working distance of 14 mm for gold-coated surfaces. The wide-angle X-ray diffraction (WAXRD) patterns were acquired on a Bruker AXS D5005 diffractometer (Bruker AXS GmbH, Karlsruhe, Germany). The samples were scanned at 4°/min using Cu K α radiation (λ=0.15418 nm) at a filament voltage of 40 kV and a current of 20 mA. The diffraction scans were collected within the 2θ= 20° to 80° range with a 2θ step of 0.01°.
where d is the sample thickness. A Keithley 2400 current source (Keithley Instruments Inc., Cleveland, OH, USA) was used as driving source.
Results and discussion
The X-ray diffraction patterns for the La 1−xCa x MnO3 (x=0.05) powder, resulting from the thermal treatment at 230°C, 650°C, and 900°C are depicted in Figure 1D. Similar diffraction patterns are obtained for all the samples regardless the Ca content. X-ray diffraction analysis has been made in order to know when the orthorhombic perovskite phase appears because only this phase presents thermoelectric activity [26–28]. At 230°C, the perovskite phase was not obtained, resulting in an insulating material. The diffraction peaks observed at 230°C are related to segregated metallic oxides of Ca, La, and Mn (CaO, Mn 3O4, CaMn 2O4, etc.). At 650°C, the WAXDR spectrum indicates that the orthorhombic perovskite-type structure was present. The material obtained after this treatment was a semiconductor material. The WAXDR spectrum of the sample heated at 900°C is similar to that obtained at 650°C, indicating that most of the material has the perovskite phase. The perosvkite phase is attained at 650°C; however, the electrical conductivity of the compacted powder (without sintering) obtained at 650°C and 900°C is very low (around 10 −3 S/cm). In addition, the sample size and shape are more homogeneous after treatment at 900°C. Thus, in order to use these materials for thermoelectric applications, we have realized a sintering process by keeping the compact pellet at 900°C for 24 h.
Generally, a p-type conductivity is observed in LaMnO 3[31, 32]. It has been attributed to the excess of oxygen (O 3+δ) and La vacancies and probably also to Mn vacancies , although it is not completely clear. Doing a literature search, it is clear that LaMnO 3 is a p-type semiconductor, while CaMnO 3 is an n-type semiconductor and contains an oxygen defect (O 3−δ). In the work of Zeng et al. , electrical conductivity is analyzed as a function of the oxygen defect and they obtain a decrease of the activation energy as soon as the defect of oxygen is higher. From these observations, we can argue that the type of conduction in La 1−xCa x O3 goes from p to n as soon as the Ca content increases. We have found in our measurements that only the sample with x=0.005 is a p-type semiconductor, while all the samples with a higher Ca concentration are n-type semiconductors. There are several empirical models in the literature [27, 33] to explain the conductivity based on different vacancies, but the location of the Mn(d) and O(p) levels is not clear. There are also several ab initio calculations, but we have found contradictions in the location of the Mn(d) and O(p) levels, probably due to the Jan-Teller distortion.
Thermoelectric parameters of La 1−x Ca x MnO 3 nanostructures at 330 K
S ( μ V/K)
Power factor ( μ W/mK2)
La 1−xCa x MnO3 perovskite nanostructures have been synthesized by the hydrothermal method. The perovskite-type structure has been obtained at 650°C and 900°C. The nanostructure morphology changes from fibrillar to nanoparticle type when increasing the temperature treatment. The electrical conductivity increases 3 orders of magnitude after the sintering process. The electrical conductivity depends on the calcium content. The sign of Seebeck coefficient changes from positive to negative. The best power factor of 0.16 μ V/mK 2 has been obtained for the sample La 0.5Ca0.5MnO3. The magnitude of PF indicates that these materials have a modest efficiency at room temperature. More research is needed in order to increases the thermoelectric efficiency.
We acknowledge the financial support of the Ministry of Finances and Competitiveness through the Grant CDS2010-0044 belonging to the ‘Consolider-Ingenio Programme’, Grant MAT2012-33483, and the FPU Programme for young researchers.
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