Enhancement on thermoelectric properties of BaTiTaO by Ca doping

Thermoelectric (TE) materials are characterised by their ability to directly transform heat into electric energy, thus making them suitable for energy harvesting/recovery applications. The performances of these materials is evaluated by the dimensionless figure-of-merit, ZT (ZT= 2T/rk; T: Absolute temperature; : Seebeck coefficient; r: electrical resistivity; k: thermal conductivity). Among promising ceramic n-type TE materials, ones based on TiOx family are most studied due to their performances while they contain nontoxic and environment friendly substances as compared with other thermoelectric systems. They are characterised by a perovskite crystal structure with cubic, tetragonal or orthorhombic symmetry. These materials show very attractive thermoelectric properties when they sintered in reductive atmosphere, producing oxygen vacancies in their structure. In this work the effects of different degree of Ca doping in Ba site on their microstructure and thermoelectric properties will be explored. The purpose of dopant implantation is mainly to modify the unit cell parameter through substituting cation with lower ionic radii ones. These adjustments will lead to increase the free charge carrier concentration, enhancing overall thermoelectric properties. (Ba1-xCax)yTi0.85Ta0.15Oδ ceramics (x = 0, 0.10, 015, and 0.20, y = 1-0.925) were prepared by solid-state reaction route and sintered in forming gas N2/H2 at 1350°C. Presented phases were identified from XRD recorded data while materials characterization have been done through microstructural studies (SEM/EDS), thermal analysis, measurements of electrical conductivity and Seebeck coefficient as function of temperature.publishe

Thermoelectric ceramic materials, and the quest to improve their performance

El pdf del artículo es la versión post-print.Peer Reviewe

Improvement of thermoelectric performances of Bi2Sr2Co1.8Ox textured materials by Pb addition using a polymer solution method

El pdf del artículo es la versión pre-print.Bi1.6Pb0.4Sr2Co1.8Ox ceramics have been prepared through a polymer solution method using polyethyleneimine. From these powders, bulk textured materials have been prepared using a melt growth technique. Microstructure has been observed by scanning electronic microscopy (SEM) which has shown that samples are mainly composed by the thermoelectric phase, with small amounts of secondary phases. Electrical resistivity measurements showed very small values nearly constant with temperature, whilst thermopower increases from room temperature to 650 °C. Power factor at 50 °C is about 0.15 mW/K2·m and 0.30 at 650 °C, which makes these ceramics good candidates for power generation applications. © 2012 Elsevier B.V. All rights reserved.This research has been supported by the Spanish Ministry of Science and Innovation-FEDER (Project MAT2008-00429) and by the Gobierno de Aragón (Consolidated Research Groups T12 and T87). Sh. Rasekh also acknowledges a JAE-PreDoc 2010 grant from CSIC.Peer Reviewe

Influence of Ca substitution by Mg on the Ca3Co4O9 performances

Ca3-xMgxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Mg have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Mg has been incorporated into the Ca3Co4O9 structure and no Mg-based secondary phases have been identified. Apparent density measurements have shown that samples do not modify their density until 0.05 Mg content, decreasing for higher contents. Electrical resistivity decreases and Seebeck coefficient slightly raises when Mg content increases until 0.05 Mg addition. The improvement in both parameters leads to higher power factor values than the usually obtained in samples prepared by the conventional solid state routes and close to those obtained in textured materials.Se han preparado cerámicas termoeléctricas policristalinas de Ca3-xMgxCo4O9, con pequeñas cantidades de Mg, utilizando el método clásico de estado sólido. La caracterización microestructural ha mostrado que el Mg se ha incorporado en la estructura del Ca3Co4O9 y que no se han formado fases secundarias con el Mg. Las medidas de densidad aparente han mostrado que las muestras prácticamente poseen la misma densidad hasta un contenido de 0.05 Mg, disminuyendo para contenidos mayores. La resistividad eléctrica disminuye y el coeficiente Seebeck aumenta cuando el contenido de Mg se incrementa hasta valores de 0.05. La mejora de ambos parámetros conduce a valores del factor de potencia mayores que los obtenidos en muestras preparadas por la vía convencional de estado sólido, alcanzando valores próximos a los que se encuentran en materiales texturados

Thermoelectric properties in Ca3Co4-xMnxOy ceramics

Ca3Co4-xMnxOy polycrystalline thermoelectric ceramics with small amounts of Mn have been prepared by the classical solid state method. X-ray diffraction data have shown that Ca3Co4O9 is the major phase, with small amounts of the Ca3Co2O6 one. Moreover, they show that the Mn has been incorporated into these two phases. Electrical resistivity decreases, compared with the values for undoped samples, with Mn content until a minimum for the 0·03 doped ones, increasing for higher Mn substitution. Seebeck coefficient does not change in all the measured temperature range, independently of Mn content. The improvement in electrical resistivity leads ∼30% higher power factor values for the 0·03 Mn doped samples than that obtained in the undoped ones. The maximum power factor at 800°C, ∼0·28 mW K-2 m-1, is close to that obtained in much higher density samples, clearly indicating the good thermoelectric properties of these samples.The authors wish to thank the Gobierno de Aragón (Research Groups T12 and T87) and the MINECO-FEDER (MAT2013-46505-C3-1-R) for financial support. Authors would also like to acknowledge the use of Servicio General de Apoyo a la Investigación-SAI, Universidad de Zaragoza. The technical contributions of C. Estepa and C. Gallego are also acknowledged. Sh. Rasekh acknowledges a JAE-PreDoc 2010 grant from CSIC.Peer Reviewe

Decrease of Ca3Co4O9+δ thermal conductivity by Yb-doping

In this study, the effect of Yb-substitution on the structural, electrical and thermal transport properties of the CaCoO system has been investigated in the low temperature region (between 10 and 300 K). The resistivity of samples increases with raising the Yb-concentration in the system. All the samples show a metal-semiconductor transition below 85 K. In the analysis based on Strongly Correlated Fermi Liquid Model, an increased bandwidth and a reduced electronic correlation are found. It has also been found that the energy gap, E , value also decreases with Yb-substitution. The samples show positive thermoelectric power, indicating that dominant charge carriers are holes in all the samples. The thermoelectric power value decreases with Yb-substitution. From Mott equation, it is determined that the Fermi energy and hole concentration decrease in the Yb-susbtituted samples, compared to the undoped ones. Thermal conductivity, κ, is decreased in about 50% of the measured in undoped samples for the 0.01 and 0.03 Yb-doped samples at 300 K. Highest figure of merit, ZT, value is found to be 5.4 10 at 300 K for the unsubstituted sample and the ZT value decreases by the substitution.The authors wish to thank the MINECO-FEDER (Project MAT2013-46505-C3-1-R) and the Gobierno de Aragón and Fondo Social Europeo (Research Groups T12 and T87) for financial support.Peer Reviewe

Influence of Ca substitution by Mg on the Ca3Co4O9 performances

Ca3-xMgxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Mg have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Mg has been incorporated into the Ca3Co4O9 structure and no Mg-based secondary phases have been identified. Apparent density measurements have shown that samples do not modify their density until 0.05 Mg content, decreasing for higher contents. Electrical resistivity decreases and Seebeck coefficient slightly raises when Mg content increases until 0.05 Mg addition. The improvement in both parameters leads to higher power factor values than the usually obtained in samples prepared by the conventional solid state routes and close to those obtained in textured materials.Se han preparado cerámicas termoeléctricas policristalinas de Ca3-xMgxCo4O9, con pequeñas cantidades de Mg, utilizando el método clásico de estado sólido. La caracterización microestructural ha mostrado que el Mg se ha incorporado en la estructura del Ca3Co4O9 y que no se han formado fases secundarias con el Mg. Las medidas de densidad aparente han mostrado que las muestras prácticamente poseen la misma densidad hasta un contenido de 0.05 Mg, disminuyendo para contenidos mayores. La resistividad eléctrica disminuye y el coeficiente Seebeck aumenta cuando el contenido de Mg se incrementa hasta valores de 0.05. La mejora de ambos parámetros conduce a valores del factor de potencia mayores que los obtenidos en muestras preparadas por la vía convencional de estado sólido, alcanzando valores próximos a los que se encuentran en materiales texturados

Role of Ag in textured-annealed Bi2Ca2Co1.7Ox thermoelectric ceramic

Bi2Ca2Co1.7Ox thermoelectric ceramics with small Ag additions (0, 1, 2, 3, 4, and 5 wt.%) have been successfully grown from the melt, using the laser floating zone method and subsequently annealed at 800 °C for 24 h. The microstructure has shown a reduction of the amount of secondary phases for Ag contents up to 4 wt.%. This microstructural evolution leads to a decrease of the electrical resistivity values until an Ag content of 4 wt.%, whereas Seebeck coefficient has been maintained unchanged. This is in agreement with the presence of metallic Ag in all samples, confirmed not only by Energy Dispersive Spectrometry but also by X-ray photoelectron and Auger spectroscopy. These electrical properties lead to maximum power factor values of about 0.30 mW/K2.m at 650 °C for the 4 wt.% Ag containing samples, which is among the best results obtained for this type of materials

Thermoelectric properties of directionally grown Bi2Ba2Co2Oδ/Ag composites: effect of annealing

Bi2Ba2Co2Oδ thermoelectric ceramics with Ag additions (0, 1, 3, and 5 wt%) have been successfully textured using the laser floating zone method. Microstructure has shown that samples are composed by thermoelectric grains, together with a high amount of secondary phases, and that 3 wt% Ag addition leads to the best grain orientation. These microstructural benefits have been reflected in the highest thermoelectric performances determined in these samples. It has also been found that annealing procedure leads to a drastic decrease of secondary phases amount and raises the oxygen content in the thermoelectric phase. These modifications are reflected in an important decrease of electrical resistivity, ρ (compared to the as-grown samples) without drastic modification of Seebeck coefficient, S. As a consequence, the highest thermoelectric performances, determined through the power factor, have been reached in the 3 wt% Ag textured and annealed samples in the whole measured temperature range. The maximum PF (=S/ρ) values at 650 °C determined in these samples are very close to the highest reported in single crystals or very low rate LFZ grown Bi2Ba2Co2Oδ compounds, making them very attractive for practical applications.This research has been supported by the Spanish MINECO-FEDER (MAT2013-46505-C3-1-R). The authors wish to thank the Gobierno de Aragón and Fondo Social Europeo (Consolidated Research Groups T87 and T12) for financial support.Peer Reviewe