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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.

[EN]: Ca3Co4-xNixO9 (x=0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been prepared by the classical solid state method. As a result of the Ni addition an increase in porosity has been detected. Moreover, the presence of Ni has been related with the increase of Ca2Co3O6 secondary phase and the appearance of a new NiO-CoO solid solution. However, for the 0.01-Ni doped samples an improvement in the thermoelectric performances has been measured. This effect has been related with a decrease in the resistivity values and an increase in the Seebeck coefficient. The raise in the power factor for the 0.01-Ni doped samples, compared with the undoped ones, is between 10 and 25% at 50 and 800 ºC respectively. Moreover, the maximum power at 800 ºC, around 0.25 mW/K2.m, is significantly higher than the best results obtained in Ni doped samples reported previously in the literature., [ES]: Se han preparado muestras policristalinas de Ca3Co4-xNixO9 (x = 0,01, 0,03 y 0,05) mediante reacción en estado sólido. Se ha detectado un incremento de la porosidad en las muestras dopadas con Ni. La adición de Ni también ha provocado el aumento de la fase secundaria Ca2Co3O6, así como la aparición de una nueva fase consistente en una solución sólida de NiO-CoO. A pesar de ello se ha detectado una mejora de las prestaciones termoeléctricas en las muestras con un 0,01 de Ni. Esta mejora ha sido provocada por una disminución de la resistividad eléctrica y un aumento del coeficiente Seebeck.

Ca3−xBaxCo4O9 bulk polycrystalline thermoelectric ceramics with x = 0.00, 0.01, 0.03 and 0.05 have been prepared through a classical solid state method. Microstructural characterizations of sintered samples have shown that Ba has been incorporated into the Ca3Co4O9 and/or Ca3Co2O6 structures and that no Ba-based secondary phases have been produced. The apparent density values increase when the amount of barium is raised with respect to the undoped samples, reaching a maximum value of around 77 % of the Ca3Co4O9 theoretical density, for samples with the highest amount of barium. Electrical resistivity of doped samples decreases in the whole measured temperature range with respect to the undoped ones, while Seebeck coefficient remains practically unchanged. In conclusion, the improvement in resistivity leads to power factor values which are higher than the ones measured in undoped samples, fact which makes this bulk polycrystalline compound a promising candidate for high-temperature power generation applications in oxidative environments.

Bi-2212 samples prepared by a solid-state reaction technique have been grown from the melt using the laser floating zone method. After annealing the as-grown bars, the samples showed a good grain alignment and a high transport critical current density. Secondary annealing processes were performed on the annealed samples with the aim of producing Bi-2212 phase controlled decomposition. Hence, the Bi-2201 phase and the secondary phases, which act as effective pinning centers, were obtained with the secondary annealing process. After these thermal treatments, the transport critical current densities of samples significantly increased, when compared to the annealed ones. The maximum critical current density was achieved when the samples were subjected to secondary annealing at 680°C for 168 h with an improvement of ~80%, compared to the annealed ones. Moreover, it was found that magnetization of the secondarily annealed samples was also increased. The magnetic critical current densities in these secondary annealed samples were about 3 times higher than the values obtained for the annealed ones. These results clearly indicate that the secondary annealing processes lead to the formation of effective pinning centers in the bulk material.

Ca3-xNaxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Na (x=0, 0.01, 0.03, 0.05, 0.07, and 0.10) have been prepared using the classical solid state method. Microstructural characterization has shown that Na has been incorporated into the Ca3Co4O9 phase and that no Na-based secondary phases have been produced. It has also been found that Na addition promotes grain growth and favours sintering due to the formation of a small amount of liquid phase. Electrical resistivity decreases when Na content increases until 0.07Na addition while Seebeck coefficient is maintained practically unchanged. The improvement in electrical resistivity leads to higher power factor values than the usually obtained in samples prepared by more complex and expensive techniques.

Cobaltite ceramics are very promising materials for new thermoelectric devices, as they exhibit high thermopower and relatively low electrical resistivity, as well as high chemical stability at high temperatures. They show anisotropic thermoelectric properties linked to their layered structures. As a consequence, different texturing methods developing oriented grains, such as sinterforging, and template grain growth processes have been successfully used to improve the performances of these materials. There are some other less used methods, like the directional growth from the melt, which have already shown their applicability to this kind of compounds. One of these methods is the laser floating zone (LFZ) melting technique. This process has demonstrated its suitability in the Co-oxide based thermoelectric materials processing in the last years in our laboratories. In this work, two examples of the versatility and usefulness of the LFZ technique will be shown. The first one will be based on the processing of the well known Ca3Co4O9 ceramic material. In this case, a very dense and homogeneous material is obtained in two steps (growth and annealing), reaching PF values as high as 0.42 mW/K2m at 800 ºC. In the second case, a new and well textured thermoelectric composite (Bi2Ca2Co1.7Ox + Ca3Co4O9) is obtained, reaching PF values of 0.31 mW/K2m at 650 ºC.

The laser radiation has been found to be a very useful tool in the materials processing during the last years. Between all these processes, some of them, probably less known, have been used to produce texture in the processed materials. This grain orientation can be easily produced in the whole material volume by the laser floating zone (LFZ) technique. With the help of this process, based on the directional growth from the melt, it is possible to enhance the performances of highly anisotropic materials. In the last years this process has been successfully applied on different materials characterized by their high crystalline and electrical anisotropy, as high-Tcsuperconductors and oxide thermoelectric materials. This is the case for the pure and doped Bi-AE-Co-O (AE: Alkaline earth) misfit layered thermoelectric (TE) compounds. In spite of the formation of multiphase materials after the directional solidification, the high-temperature measured properties are higher than the usually obtained on classically textured materials (sinter-forging). Moreover, when these materials are subjected to an annealing process, their TE performances are further improved. This technique has also been successfully applied on the Ca3Co4O9 (349). In this system, in accordance with the phase diagram, a dense CaO-ss and CoO-ss eutectic microstructure is produced. After an annealing process, performed under air, the solid solutions react to produce the 349 TE phase. It has been found that the optimal annealing process lead to the highest reported TE performances measured on bulk 349 materials.

[EN]: Thermoelectric (TE) materials can transform a temperature gradient into electricity directly, due to the well-known Seebeck effect. Nowadays, due to the global warming effect, these materials are gaining increased interest for practical power generation applications using natural and waste heat sources. Their performance is usually quantified by the dimensionless figure-of-merit (ZT=S^2T/(rho*kappa) S: Seebeck coefficient, T: absolute temperature, kappa: total thermal conductivity, rho: electrical resistivity). Since 1997, the misfit-layered cobalt oxides (cobaltites) have been found to possess attractive TE properties at high temperatures in oxidative environments. Among the different TE cobaltites, the so-called Ca3Co4O9 compound presents some of the best TE properties and has been extensively studied in the past years, having the largest amount of available information. Ca3Co4O9 has a monoclinic crystal structure, composed of two different alternately-stacked layers (CdI2-type [CoO2] and NaCl-type [Ca2CoO3]) which present a misfit in the b-direction. It has been shown that the TE performances of this system (compound) can be further improved, especially in bulk form, making it a very-promising candidate for practical power generation applications in a not-so-distant future. The main objective of this work has been to study the improvement of the electrical performances (quantified by the power factor, PF=S^2/rho) of bulk polycrystalline Ca3Co4O9 samples, via doping, synthesis and densification., [ES]: Los materiales termoeléctricos (TE) pueden transformar un gradiente de temperatura en un potencial eléctrico, de forma directa sin ningún tipo de parte móvil, debido al efecto Seebeck. En la actualidad, más de la mitad de la energía usada en los diferentes procesos primarios de conversión de la misma se desperdicia en forma de calor arrojado al medio ambiente. Por lo tanto, la conversión termoeléctrica constituye una solución alternativa para mejorar la eficiencia de los actuales procesos y equipos, tanto de tipo industrial como personal. Por otra parte, y dado que los dispositivos termoeléctricos no poseen partes móviles, son dispositivos dotados de una gran fiabilidad, estimándose su vida útil en más de 100000 horas. Así mismo, esta ausencia de movimiento los hace también especialmente silenciosos y ligeros. Por todo ello, hay un gran interés en el campo del desarrollo de nuevos, y más eficientes materiales TE. Los materiales termoeléctricos clásicos están basados en intermetálicos de metales pesados que presentan inconvenientes medioambientales pues se degradan fácilmente a temperaturas modestas en aire. Una posible solución a estos problemas llegó con el relativamente reciente descubrimiento de propiedades TE interesantes en compuestos basados en óxidos de cobalto. En la actualidad, el estudio de estos óxidos se centra en las familias Ca3Co4O9, LaCoOx, y Bi2AE2Co2Ox (AE: alcalinotérreo), entre otras. La estructura cristalina de estos óxidos de cobalto está compuesta por dos capas diferentes que se disponen de forma alternada. Una de ellas, la correspondiente al CoO2 (tipo CdI2) con características conductoras, y otra con carácter aislante (tipo NaCl), donde se disponen el resto de los iones. Estas dos subredes presentan los mismos parámetros de red, excepto en la dirección b, lo que le da su caracter misfit. Las prestaciones TE se suelen evaluar mediante la figura de mérito adimensional ZT=S2T/ (S: Coeficiente Seebeck, T: Temperatura absoluta, : Resisti

Ca3-xRExCo4O9 thermoelectric ceramics with different rare earth (RE = Dy, Er, Gd, and Tb) substitution for Ca have been successfully produced by the classical solid state method. All these rare earths in small proportions produce the same effects on the thermoelectric phase, reflected on a similar and significant decrease of the electrical resistivity, compared with the undoped samples. The Seebeck coefficient slightly decreases with all the studied substitutions while the power factor values appreciably increase at high temperatures for small amounts (0.01) of doping elements. Further doping produces the decrease of power factor which indicates that the optimal doping level has been determined to be 0.01 for all the tested rare earths which produces, approximately, the same improvement, around 20 % of the power factor obtained in the undoped samples at 800 °C.

Ca3-xNdxCo4Oy polycrystalline thermoelectric ceramics with small amounts of Nd have been synthesized by the classical solid state method. X-ray diffraction data have shown that all the Nd has been incorporated into the Ca3Co4O9 and Ca3Co2O6 phases and no Nd-containing secondary phases have been produced. Apparent density measurements have revealed that all samples are very similar, with densities around 70 % of the theoretical one. Electrical resistivity decreases in all Nd-doped samples, compared with the pure one, while Seebeck coefficient remains constant for all samples. The improvement in resistivity leads to higher power factor values than the obtained for the undoped samples. © 2013 Springer Science+Business Media New York.

Ca3Co4-xZnxOy (x = 0.01, 0.03, and 0.05) polycrystalline thermoelectric ceramics have been prepared by the classical solid state method. XRD data have shown that Ca3Co 4O9 is the major phase, with small amounts of the Ca 3Co2O6 one. Moreover, it has been found that the Zn has been incorporated into these two phases. Slight Zn doping decreases electrical resistivity compared with the values obtained in undoped samples. The minimum values have been obtained for the 0.01-Ni doped samples, increasing for further Zn substitution. Seebeck coefficient does not appreciably change in all the measured temperature range, independently of Zn content. The improvement in electrical resistivity leads to higher power factor values for the 0.01 Zn-doped samples (about 30 %) than for the undoped ones. The maximum power factor at 800 °C, around 0.27 mW/K2 m is significantly higher than the best results obtained in Zn doped samples reported in the literature. © 2014 Springer Science+Business Media New York.

Ca3Co4−xTixO9 polycrystalline thermoelectric ceramics with small amounts of Ti 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 Ti has been incorporated into these two phases. Electrical resistivity decreases, compared with the values for undoped samples, until 0.03-Ti doped ones. Further Ti addition produces an increase of resistivity with respect to the 0.03 Ti doped samples. Seebeck coefficient does not appreciably change in all the measured temperature range, independently of Ti content. The improvement in electrical resistivity leads to about 55 % higher power factor values for the 0.03 Ti-doped samples than that obtained in the undoped ones. The maximum power factor at 800 °C, around 0.33 mW/K2 m, is slightly higher than the obtained in higher density samples, clearly indicating the good thermoelectric performances of these doped samples.

Bi2Ca2Co1.7Ox nominal composition ceramic rods were textured by the electrically assisted laser floating zone technique. A new composite formed by three phases, two of them with well-known thermoelectric properties, Bi2Ca2Co 1.7Ox and Ca3Co4O9, was obtained for the first time when the growth process was performed with a 300 mA applied current. Under these conditions the thermoelectric performances of this new composite reach values above the highest ones reported in the literature for polycrystalline Bi2Ca2Co1.7Ox materials. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Ca3-xGaxCo4Oy polycrystalline thermoelectric ceramics with small amounts of Ga have been synthesized by the classical solid state method. XRD data have shown that all the Ga has been incorporated into the Ca3Co4O9 and Ca 3Co2O6 thermoelectric phases and no Ga-containing secondary phases have been produced. Moreover, for Ga contents ≥0.07, the Ca3Co2O6 phase disappears. Apparent density measurements have revealed that all samples are very similar, with densities around 70% of the theoretical one. Electrical resistivity decreases in all Ga-doped samples, compared with the pure ones, while the Seebeck coefficient is practically the same for all samples, except for the 0.12 and 0.15 Ga substituted ones, at high temperatures which present slightly higher values. Both factors in the 0.15 Ga-substituted samples lead to power factor values around 0.35 mW/K2 m at 800 C which is about 70% higher than the values obtained for the undoped samples. © 2013 Elsevier Ltd and Techna Group S.r.l.

Bi2-xPbxSr2CaCu2Oy textured materials (x=0.0, 0.2, 0.4, and 0.6) have been successfully prepared by the laser floating zone technique. Microstructure and electrical properties (JC and TC) have been clearly affected by Pb addition. From the E-I curves, slope of the transition between the superconducting and the normal state (n) at 77K reaches a maximum of about 16 for the 0.4 Pb doped samples. This value is much higher than the typical ones for the Bi-2212 materials. Moreover, when the electrical properties of the 0.4 Pb doped samples are measured at lower temperatures (between 65 and 77K), n values increase when the temperature is decreased. A maximum n value of 32 has been reached at 65K which makes this material very attractive for its use as resistive fault current limiters. © 2014 Elsevier Ltd.

OBJECTIVE: The relative impact of early intervention approach on speech perception and language skills was examined in these 3 well-matched groups of children using cochlear implants. STUDY DESIGN: Eight children from an auditory verbal intervention program were identified. From a pediatric database, researchers blind to the outcome data, identified 23 children from auditory oral programs and 8 children from bilingual-bicultural programs with the same inclusion criteria and equivalent demographic factors. PATIENTS: All child participants were male, had congenital profound hearing loss (pure tone average >80 dBHL), no additional disabilities, were within the normal IQ range, were monolingual English speakers, had no unusual findings on computed tomography/magnetic resonance imaging, and received hearing aids and cochlear implants at a similar age and before 4 years of age. MAIN OUTCOME MEASURES: Open-set speech perception (consonant-nucleus-consonant [CNC] words and Bamford-Kowal-Bench [BKB] sentences) and the Peabody Picture Vocabulary Test (PPVT) were administered. RESULTS: The mean age at cochlear implant was 1.7 years (range, 0.8-3.9; SD, 0.7), mean test age was 5.4 years (range, 2.5-10.1; SD, 1.7), and mean device experience was 3.7 years (range, 0.7-7.9; SD, 1.8). Results indicate mean CNC scores of 60%, 43%, and 24% and BKB scores of 77%, 77%, and 56% for the auditory-verbal (AV), aural-oral (AO), and bilingual-bicultural (BB) groups, respectively. The mean PPVT delay was 13, 19, and 26 months for AV, AO, and BB groups, respectively. CONCLUSION: Despite equivalent child demographic characteristics at the outset of this study, by 3 years postimplant, there were significant differences in AV, AO, and BB groups. Results support consistent emphasis on oral/aural input to achieve optimum spoken communication outcomes for children using cochlear implants.

[EN]: Bi1.6Pb0.4Sr2Co1.8Ox thermoelectric ceramics with small Ag additions (0, 1, and 3 wt.%) have been successfully produced by a sol-gel method via nitrates. Microstructure has shown a reduction on the amount of secondary phases and an increase on the bulk density with increasing Ag contents. The microstructural evolution, as a function of Ag content, is confirmed with the electrical resistivity values which show an important decrease for the 3 wt.% Ag samples, leading to maximum power factor values of about 0.025 mW/K2.m at room temperature, which is about two times higher than the obtained for the Ag-free sintered samples., [ES]: Se han preparado cerámicas termoeléctricas de composición Bi1.6Pb0.4Sr2Co1.8Ox con pequeñas adiciones de Ag (0, 1, y 3 % en peso) por medio de un método sol-gel a partir de los nitratos metálicos. La microestructura ha mostrado una reducción de la cantidad de fases secundarias y un aumento de la densidad del material al aumentar la cantidad de Ag. La evolución microestructural, en función del contenido de Ag, se ha confirmado con los valores de la resistividad eléctrica que muestra una reducción importante para las muestras con 3 % en peso de Ag, lo que lleva a alcanzar valores máximos del factor de potencia de alrededor de 0.025 mW/K2.m a temperatura ambiente, que es dos veces mayor que el obtenido para muestras sinterizadas sin Ag.

High-performance Ca3Co4O9 thermoelectric ceramic has been prepared from a Ca1-xCoxO/CayCo1-yO divorced eutectic structure produced by a directional melt-grown using the laser floating zone technique. This material has been grown at very high solidification rate in order to produce a very fine microstructure to reduce the necessary annealing time to recover the Ca3Co4O9 thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases and estimate the extent of Ca3Co4O9 formation with time and related with their thermoelectric performances. The optimum annealing time, 72h, has been determined by the maximum power factor value (about 0.42mWK-2m-1), which is around the best values reported in textured materials (∼0.40mWK-2m-1). This high power factor outcome from the high Ca3Co4O9 phase content, apparent density and Co3+/Co4+ relationship determinations performed in the present work. © 2013 Elsevier Ltd.

It is well know that H2O and CO2 (both present in the atmosphere) can react chemically with BSCCO superconductors, especially with the alkaline¿earth cations, producing chemical degradation and reducing their performances. In this work, Bi-2212 highly textured rods were grown from the melt through a laser floating zone melting technique. In order to determine the influence of H2O and CO2 on these bulk textured samples, they were immersed in distilled water and in CO2 saturated water. The evolution of the electrical and mechanical properties of these rods was determined as a function of the immersion time in both media and compared with their initial properties. It has been found that performance degradation was higher in distilled water than in CO2 saturated one. Nevertheless, in any case, the degradation has been found to be not very important.

Ca3Co4-xCuxO9 polycrystalline thermoelectric ceramics with small amounts of Cu have been synthesized by the classical solid state method. X-ray diffraction data have shown that all the Cu has been incorporated into the Ca3Co4O9 structure and no Cu-containing secondary phases have been produced. Apparent density measurements have shown that all samples are very similar, with densities around 75 % of the theoretical one. Electrical resistivity decreases when Cu content increases until x = 0.03 while Seebeck coefficient remains practically constant for all samples. The improvement in resistivity leads to higher power factor values than the obtained for the undoped samples. © 2013 Springer Science+Business Media New York.

Thermoelectric ceramics are based in a limited number of transition metal oxides (Co, Mn, Ni,.) which produce materials with high thermoelectric performances. Based on previously existing thermoelectric phases, the phase diagram equilibrium can help to design new thermoelectric ceramics based on other transition metals (for example, Fe). In this work, BaFeOx ceramics have been prepared by the classical solid state method using different sintering temperatures. The produced materials have shown promising thermoelectric properties when treatment temperatures are in the perovskite zone domain of the phase equilibrium diagram. In spite of the good values for the Seebeck coefficients, power factor is low due to the high resistivities measured in all cases. © 2012 Springer Science+Business Media New York.

Ca3-xSrxCo4O9 polycrystalline thermoelectric ceramics with small amounts of Sr have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Sr has been incorporated into the Ca3Co4O9 and Ca3Co2O 6 structures and no Sr-based secondary phases have been produced. Apparent density measurements have shown that samples slightly increase their density until 0.05 Sr content, decreasing for higher contents. Electrical resistivity decreases and Seebeck coefficient slightly raises when Sr content increases until 0.07 Sr 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. © 2013 Elsevier B.V. All rights reserved.

Ca3Co4-xCrxO9 polycristalline thermoelectric ceramics with small amounts of Cr have been synthesized by the classical solid state method. Microstructural characterizations have shown that all the Cr has been incorporated into the Ca3Co4O 9 structure and no Cr-containing secondary phases have been produced for Cr contents≤0.05. Apparent density measurements have shown that all samples are very similar, with densities around 75% of the theoretical one. Electrical resistivity decreases and Seebeck coefficient slightly raises when Cr content increases until 0.05 Cr addition. The improvement in both parameters leads to higher power factor values than the usually obtained by conventional solid state routes. © 2013 Elsevier Ltd and Techna Group S.r.l.

Bi2Ba2Co2Ox thermoelectric ceramics were textured from the melt using the laser floating zone method, at a growth rate of 5 mm h−1. The microstructure showed good grain alignment with the growth axis. These microstructural features are reflected in the thermoelectric performance, with a significant increase in the power factor values, reaching ∼0.4 mW K−2 m−1 at 650 °C, much higher than the values typically obtained in these materials. © 2012 Published by Elsevier Ltd.

Bi1.6Pb0.4Sr2Co1.8Ox thermoelectric ceramics with small Ag additions (0, 1, and 3 wt%) have been successfully grown from the melt, using the laser floating zone method. Microstructure has shown a reduction in the amount of secondary phases and a better grain alignment with respect to the growth direction for an Ag content of 3 wt%. The microstructural evolution, as a function of Ag content, is confirmed with the electrical resistivity values, which show an important decrease for the 3 wt% Ag samples, leading to maximum power factor values of about 0.42 mW/K2 m at 650 °C, which are among the best results obtained in this type of material. © 2012 Elsevier Ltd and Techna Group S.r.l.

Bi1.6Pb0.4Sr2Co1.8Oy/Ag composites with small amounts of Ag (0, 1 and 3wt.%) have been synthesized by a sol‐gel via nitrates reaction and directionally grown from the melt. As‐grown samples were microstructurally characterized to determine the phases distribution and alignment. Moreover, thermoelectric characteristics were determined by the four‐probe technique. Scanning electron microscopy revealed that Ag particles appear dispersed among well oriented ceramic grains with large size. The composites electrical resistivity is lower than that of pure Bi1.6Pb0.4Sr2Co1.8Oy. The resistivity reduction leads to power factor improvements higher than 50%, compared with pure samples, for Ag additions of 3wt.%Ag.

Bi2Sr2.1Ca0.9Cu2O8 superconducting ceramics have been prepared by the classical solid state route and textured using the Laser Floating Zone (LFZ) melting technique at 30 mm/h growth speed. The obtained long (more than 12 cm) textured rods were cut into 3 cm long pieces. Silver paint has been applied on each piece to produce the electrical contacts after annealing. All the samples were then annealeded at 860 ºC during 60 h and 800 ºC for 12 h with a final quench to room temperature. Samples characteristics have been determined through electrical resistivity measurements, from 77 K to room temperature, and transport critical current density at 77 K. Based on the phase diagram equilibria, half of the samples were postannealed at 750 ºC for times between 12 and 300 h and the other half at 900 ºC between 5 and 120 minutes. After postannealing, the transport electrical properties have been determined and related with the changes on microstructure.

Bi2Sr2Co1.8Ox/Ag composites with small amounts of Ag have been synthesized by a sol-gel via nitrates reaction and directionally grown from the melt. Some of the obtained samples were annealed in order to obtain the thermoelectric phase as the major one. As-grown and annealed samples were microstructurally characterized to determine the phases distribution and alignment. Moreover, thermoelectric and mechanical characteristics of annealed samples were determined by the four-probe technique and by three point flexural strength tests, respectively. Scanning electron microscopy revealed that Ag particles appear dispersed among well oriented ceramic grains with large size, providing a plastic flow region which increases the flexural strength for the optimally Ag added samples (1wt.%). The composites electrical resistivity is lower than that of pure Bi 2Sr 2Co 1.8O x while Ag addition does not significantly affect thermopower values. The resistivity reduction leads to power factor improvements of ∼50%, compared with pure samples, for Ag additions of 1wt.% Ag. © 2012 Elsevier Ltd.

The effect of annealing on directionally solidified Bi 2Sr 2Co 1.8O x ceramic rods has been studied for different times up to 1008 h. Microstructure has shown five different phases in the as-grown materials which have been reduced to two major ones after 1008 h thermal treatment, accompanied by an important grain growth. These microstructural changes are reflected on the mechanical properties which are higher than those measured in the as-grown materials in all cases. Moreover, they also produce an important decrease on the resistivity and increase of thermopower, leading to a raise on the power factor on thermally treated samples, about two times, compared to the as-grown samples. © 2012 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Two solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obtain Ca 3Co 4O 9 misfit compounds with improved thermoelectric properties, compared to the classical solid state reaction. A comparison among the final products obtained by these different methods has been performed using DTA, TGA, FTIR, X-ray diffraction, scanning electron microscopy, and thermoelectric characterizations. All the samples obtained by solution synthesis show a very significative reduction on the secondary phases content. As a consequence, an important decrease on the electrical resistivity values is produced, compared to the solid state prepared samples, leading to a relatively important power factor raise. © 2012 Elsevier Ltd.

[EN]: Several solution synthetic methods, sol-gel and a polymeric route, have been studied in order to obtain Bi 2Sr 2Co 1.8O x ceramics with improved thermoelectric properties, compared to the classical solid state reaction. The products obtained by these different methods have been compared using DTA-TGA, powder X-ray diffraction, scanning electron microscopy, and thermoelectric characterizations. All the samples obtained by solution synthesis show higher homogeneity and lower content of secondary phases. The main differences in thermoelectrical properties are due to the decrease of electrical resistivity in samples obtained by solution methods, compared with the solid state obtained samples. Between them, the decrease is especially high for those samples prepared by the polymer solution method. Therefore, the polymeric solution synthesis route is shown to yield a power factor four times higher than the obtained for the solid state and sol-gel methods at room temperature., [ES]: Se han obtenido cerámicas termoeléctricas Bi2Sr2Co1.8Ox utilizando varios métodos de síntesis, el clásico por estado sólido, sol-gel y de matriz polimérica. Se han comparado los productos obtenidos por las diferentes vías a través de técnicas de DTA-TGA, difracción de rayos X en polvo, microscopía electrónica de barrido y de caracterización termoeléctrica. Todas las muestras obtenidas por los métodos en disolución poseen una mayor homogeneidad y menor contenido de fases secundarias. La principal diferencia en las propiedades termoeléctricas es debida a la disminución de la resistividad eléctrica en las muestras obtenidas por medio de métodos en disolución, comparado con las que se obtienen por estado sólido. Entre ellas, la reducción de la resistividad es mucho mayor para las que se obtienen por el método de matriz polimérica. Además, los materiales obtenidos por este último método poseen un factor de potencia, a temperatura ambiente, cuatro veces mayor que el obtenido en las sintetizadas por los otros dos métodos estudiados.