Electrical performance tuning in thermoelectric Ca3Co4O9 materials by transition metals additions

This work reports on the effects on high-temperature thermoelectric (TE) properties in bulk, polycrystalline p-type Ca3Co4O9 ceramics, after employing a composite approach consisting of metallic particles additions and two simple sintering schemes. The added Fe, Co and Ni particles are expected to act as porosity fillers upon oxidation in air and provide improved grain connectivity, changing the microstructural features and electrical properties of the resulted materials. The composites have been prepared through a modified Pechini method, followed by one- and two-stage sintering, to produce low-density (one-stage, 1ST) and high-density (two-stage, 2ST) ceramic samples. The electrical conductivity (σ), Seebeck coeffcient (α) and power factor (PF) values have been investigated between 475 and 975 K, in air flow, and related to the sample’s respective phase compositions, morphologies and microstructures. For the Co additions in the 1ST sintering case, the porous samples reached maximum PF values of around 210 μWm-1K2 , being around two times higher than those of the pure Ca3Co4O9 matrix. For the 1STsintered Fe and Ni added samples, the highest PF values of 80 and 90 μWm-1K-2 have been measured for the 3% vol. Ni and 3 and 6% vol. Fe additions, respectively, very close to some of the best reported values from literature. In contrast, 2ST sintering resulted in much denser samples and more complex phase compositions and microstructures, leading to lower electrical performance. The improvements of electrical properties achieved in the present work are promoted by a simultaneous increase in electrical conductivity and Seebeck coefficient values, stemming from pore filling effects and subsequent microstructural modifications.publishe

Thermal transport measurement techniques for the low dimension bulk thermoelectric materials

Thermoelectric (TE) phenomena were extremely gain attention due to their directly conversion of heat energy into electrical one base on well-known Seebeck effect. Thus reason made these materials a promising way in order to harvest wasted energy and as a consequence helping the global warming The conversion efficiency of such materials is quantified by the dimensionless figure of merit ZT, TS2/ρ where S is the Seebeck coefficient (or thermopower), ρ the electrical resistivity, κ the thermal conductivity, and T is the absolute temperature. Therefore intense studies were carried out by various groups in order to obtain high performance thermoelectric modules. It is evident that the one pair of thermoelectric will not produce considerable electric energy for daily applications. The high volumes of work were done in order to minimize the dimensions of the TE pairs so the more pairs can be placed in the single modules. However, in order to obtaining the performance of these materials we should capable of measuring the ZT parameters, among them thermal conductivity. As for today there is not any commercial system capable of measuring the thermal conductivity of so low dimensions. In this work after the exploring the different methods and techniques, a simple but practical thermal conductivity measurement system and analysis for the low dimension bulk thermoelectric materials were successfully develope

Estudio de nuevos materiales termoeléctricos basados en óxidos de hierro

El proyecto consiste en la preparación y el estudio de dos tipos de materiales que se espera que posean propiedades termoeléctricas. Así como el estudio del efecto termoeléctrico y sus aplicaciones. Los dos tipos de materiales preparados, se constituyen de hierro además de otros componentes. Se espera de los materiales termoeléctricos en un futuro, gran aceptación y utilización en sistemas energéticos

Unravelling the effects of calcium substitution in BaGd2CoO5 Haldane gap 1D material and its thermoelectric performance

Ecobenign and high-temperature-stable oxides are considered a promising alternative to traditional Bi2Te3-, Bi2Se3-, and PbTe-based thermoelectric materials. The quest for high-performing thermoelectric oxides is still open and, among other challenges, includes the screening of various materials systems for potentially promising electrical and thermal transport properties. In this work, a new family of acceptor-substituted Haldane gap 1D BaGd2CoO5 dense ceramic materials was characterized in this respect. The substitution of this material with calcium results in a general improvement of the electrical performance, contributed by an interplay between the charge carrier concentration and their mobility. Nevertheless, a relatively low electrical conductivity was measured, reaching ∼5 S/cm at 1175 K, resulting in a maximum power factor of ∼25 μW/(K × m2) at 1173 K for BaGd1.80Ca0.20CoO5. On the other hand, the unique anisotropic 1D structure of the prepared materials promotes efficient phonon scattering, leading to low thermal conductivities, rarely observed in oxide electroceramics. While the BaGd2–xCaxCoO5 materials show attractive Seebeck coefficient values in the range 210–440 μV/K, the resulting dimensionless figure of merit is still relatively low, reaching ∼0.02 at 1173 K. The substituted BaGd2–xCaxCoO5 ceramics show comparable thermoelectric performance in both inert and air atmospheres. These features highlight the potential relevance of this structure type for thermoelectric applications, with future emphasis placed on methods to improve conductivity.publishe

Redox-promoted tailoring of the high-temperature electrical performance in Ca3Co4O9 thermoelectric materials by metallic cobalt addition

This paper reports a novel composite-based processing route for improving the electrical performance of Ca3Co4O9 thermoelectric (TE) ceramics. The approach involves the addition of metallic Co, acting as a pore filler on oxidation, and considers two simple sintering schemes. The (1-x)Ca3Co4O9/xCo composites (x = 0%, 3%, 6% and 9% vol.) have been prepared through a modified Pechini method, followed by one- and two-stage sintering, to produce low-density (one-stage, 1ST) and high-density (two-stage, 2ST) ceramic samples. Their high-temperature TE properties, namely the electrical conductivity (σ), Seebeck coefficient (α) and power factor (PF), were investigated between 475 and 975 K, in air flow, and related to their respective phase composition, morphology and microstructure. For the 1ST case, the porous samples (56%-61% of ρth) reached maximum PF values of around 210 and 140 μWm-1·K-2 for the 3% and 6% vol. Co-added samples, respectively, being around two and 1.3 times higher than those of the pure Ca3Co4O9 matrix. Although 2ST sintering resulted in rather dense samples (80% of ρth), the efficiency of the proposed approach, in this case, was limited by the complex phase composition of the corresponding ceramics, impeding the electronic transport and resulting in an electrical performance below that measured for the Ca3Co4O9 matrix (224 μWm-1·K-2 at 975K).publishe

Environmentally friendly synthesis methods to obtain the misfit [Ca2CoO3-δ]0.62[CoO2] thermoelectric material

This work reports the microstructural and thermoelectric characterization of the misfit [Ca2CoO3-δ]0.62[CoO2] compound obtained by a solid-state synthesis using mollusk shells and a proteic sol-gel method, which uses gelatin as a polymerizing agent. The results clearly demonstrate the capability of these routes to produce pure Ca3Co4O9 with plate-like morphology. Sintered ceramic samples show randomly oriented grains and relative densities in the range of 63–67%. The obtained microstructures provide reasonable electrical properties and result in competitive thermoelectric performance for the material prepared by the proteic sol-gel synthesis (P.F. of 205 μW/K2 m at 700 °C).publishe

Exploring the high-temperature electrical performance of Ca3-xLaxCo4O9 thermoelectric ceramics for moderate and low substitution levels

Aliovalent substitutions in Ca3Co4O9 often result in complex effects on the electrical prop-erties and the solubility, and impact of the substituting cation also depends largely on the prepara-tion and processing method. It is also well-known that the monoclinic symmetry of this material’s composite crystal structure allows for a significant hole transfer from the rock salt-type Ca2CoO3 buffer layers to the hexagonal CoO2 ones, increasing the concentration of holes and breaking the electron–hole symmetry from the latter layers. This work explored the relevant effects of relatively low La-for-Ca substitutions, for samples prepared and processed through a conventional ceramic route, chosen for its simplicity. The obtained results show that the actual substitution level does not exceed 0.03 (x < 0.03) in Ca3-xLaxCo4O9 samples with x = 0.01, 0.03, 0.05 and 0.07 and that further introduction of lanthanum results in simultaneous Ca3Co4O9 phase decomposition and secondary Ca3Co2O6 and (La,Ca)CoO3 phase formation. The microstructural effects promoted by this phase evolution have a moderate influence on the electronic transport. The electrical measurements and determined average oxidation state of cobalt at room temperature suggest that the present La sub-stitutions might only have a minor effect on the concentration of charge carriers and/or their mobil-ity. The electrical resistivity values of the Ca3-xLaxCo4O9 samples with x = 0.01, 0.03 and 0.05 were found to be ~1.3 times (or 24%) lower (considering mean values) than those measured for the pris-tine Ca3Co4O9 samples, while the changes in Seebeck coefficient values were only moderate. The highest power factor value calculated for Ca2.99La0.01Co4O9 (~0.28 mW/K2m at 800 °C) is among the best found in the literature for similar materials. The obtained results suggest that low rare-earth substitutions in the rock salt-type layers can be a promising pathway in designing and improving these p-type thermoelectric oxides, provided by the strong interplay between the mobility of charge carriers and their concentration, capable of breaking the electron–hole symmetry from the conduc-tive layers.publishe

Exploring tantalum as a potential dopant to promote the thermoelectric performance of zinc oxide

Zinc oxide (ZnO) has being recognised as a potentially interesting thermoelectric material, allowing flexible tuning of the electrical properties by donor doping. This work focuses on the assessment of tantalum doping effects on the relevant structural, microstructural, optical and thermoelectric properties of ZnO. Processing of the samples with a nominal composition Zn1-xTaxO by conventional solid-state route results in limited solubility of Ta in the wurtzite structure. Electronic doping is accompanied by the formation of other defects and dislocations as a compensation mechanism and simultaneous segregation of ZnTa2O6 at the grain boundaries. Highly defective structure and partial blocking of the grain boundaries suppress the electrical transport, while the evolution of Seebeck coefficient and band gap suggest that the charge carrier concentration continuously increases from x = 0 to 0.008. Thermal conductivity is almost not affected by the tantalum content. The highest ZT~0.07 at 1175 K observed for Zn0.998Ta0.002O is mainly provided by high Seebeck coefficient (-464 µV/K) along with a moderate electrical conductivity of ~13 S/cm. The results suggest that tantalum may represent a suitable dopant for thermoelectric zinc oxide, but this requires the application of specific processing methods and compositional design to enhance the solubility of Ta in wurtzite lattice

Variation of mechanical and electrical performances of Bi2Ca2Co1.7Ox ceramics above working conditions

This work explores the effect of exposing the Bi2Ca2Co1.7Ox-textured ceramics at temperatures above working conditions, on mechanical and electrical properties. Microstructural studies have shown a first improvement of microstructure with an important grain growth, followed by the formation of porosity and the appearance of cracks for larger times. These features were reflected on their mechanical and electrical properties. Three point bending tests have revealed an increase of bending strength with the thermal treatment, reaching the maximum at 24 h and decreasing for further treatment time, which is very slight for times larger than 196 h. On the other hand, electrical resistivity is drastically reduced with the thermal treatment when compared to the as-grown samples. Moreover, the samples behavior is modified from semiconducting-like for as-grown samples to metallic-like for the thermally treated ones. Accordingly, Seebeck coefficient is decreased with the thermal treatment, being very similar for all thermally treated samples. As a consequence of the drastic decrease of electrical resistivity, although with lower reduction on the Seebeck coefficient, all thermally treated samples display higher power factor values than the as-grown ones. The highest values at 650 °C (0.29 mW/K2 m) have been obtained in textured samples thermally treated for 48 h, which are comparable to the best values reported in the literature without the use of expensive materials.M. A. Torres, M. A. Madre, and A. Sotelo acknowledge the Spanish MINECO-FEDER (MAT2017-82183-C3-1-R) and Gobierno de Aragón-FEDER (Research group T54-20R), for financial support. Sh. Rasekh acknowledges the support of the Research Employment Contract FCT–CEECIND/02608/2017.Peer reviewe