Utilizando el método de reacción sólida fue posible preparar compósitos de Ca2.95Sr0.05Co4O9+δ/Ag (Ca349-Sr/Ag). Sus propiedades de transporte fueron estudiadas en el rango de temperatura entre 80K y 300K mediante mediciones de coeficiente Seebeck (S) temperatura absoluta (T), conductividad térmica (κ?(T) y resistividad eléctrica (ρ?(T). El rendimiento termoeléctrico se determinó a través del cálculo del factor de potencia (PF) y la figura de mérito adimensional (ZT), estos parámetros alcanzaron valores máximos cercanos a 13μW/K2-cm y 0,18, respectivamente. El incremento observado en el rendimiento termoeléctrico es el resultado de los efectos combinados de la sustitución de estroncio (Sr) por calcio (Ca) y la adición de plata sobre las propiedades de transporte de compósito. Los resultados obtenidos hacen de estas estructuras materiales promisorios para ser usados en el diseño de dispositivos termoeléctricos útiles en aplicaciones a bajas temperaturas.

Actualmente existe un creciente interés en la investigación de materiales termoeléctricos, generado por las nuevas posibilidades de diseño, preparación y caracterización que ofrece hoy la ciencia de materiales y por la necesidad de encontrar materiales que permitan la fabricación de refrigeradores y generadores eléctricos de estado sólido eficientes y ambientalmente amigables. La eficiencia de un material termoeléctrico es una función de su figura de mérito (ZT= S2T/pK, donde S es el coeficiente Seebeck, p la resistividad eléctrica y Kla conductividad térmica). El objetivo central de las investigacio­nes en materiales termoeléctricos es desarrollar compuestos con altos valores de ZT. En este sentido, algunas investigaciones están enfocadas hacia la reducción de la conductividad térmica, mientras que otras lo están hacia el incremento del factor de potencia (PF = S2/ p). Los óxidos cerámicos se encuen­tran entre los materiales promisorios como termoelementos dada su estabilidad química y sus interesan­tes propiedades eléctricas y térmicas. Aquí se muestran los resultados de varios estudios los cuales están enfocados hacia la determinación de las propiedades termoeléctricas de los compuestos de YBa2Cu307_0 (YBCO ), La 2-x_xSrCuO 4+ 0(LSCO) y La_1-xSr ,.CoO 3 ( LSCoO ). Los mejores valores obtenidos para el factor de potencia (PF=l8µWIK2cm) y la figura de mérito (ZT=0,5) pueden compararse con los exhibidos por los materiales semiconductores, convencionalmente utilizados hoy en la elaboración de dispositivos termoeléctricos. Esto permite considerar estos óxidos como materiales termoeléctricos promisorios, los cuales podrían funcionar a temperatura ambiente y bajo ella, rango de temperatura en el cual los semiconductores convencionales presentan serias dificultades en su desempeño.

Oxide ceramics with the nominal composition Ca2-xSrxCo2O5 were prepared using the sol-gel method. The transport properties were evaluated using Seebeck coefficient S(T) and electrical resistivity ρ(T) measurements. ST exhibited positive values, suggesting a p-type material, which increased with the Sr content. The electrical resistivity revealed a metallic behavior. ρ(T) decreased with the Sr content, reaching minimum values close to 18mΩcm. From S(T) and ρ(T) data, the thermoelectric (TE) power factor was calculated, which reached maximum values close to 4μWK-2cm-1. Thus, these ceramics can be considered to be promising materials for use in low-temperature thermoelectric applications

Se describe un sistema de calentamiento escalonado para la medición de la conductividad térmica, el cual mezcla las ventajas de los métodos dinámicos con las propias de los métodos estacionarios, permitiendo la realización de mediciones rápidas y confiables en el rango de temperatura entre 85K y la temperatura ambiente. Este sistema es especialmente apropiado para la medición de materiales sólidos con una relación longitud-espesor mayor que la unidad.A fin de ilustrar las capacidades y la fiabilidad del sistema se presentan mediciones de conductividad térmica en muestras de plomo (99.9 %) y material cerámico del tipo La0,92Sr0,08CoO3. Se concluye que este arreglo experimental permite realizar mediciones rápidas y seguras, permitiendo además controlar de manera eficiente las pérdidas por radiación y convección.

We describe a step heating method, which is used for thermal conductivity measurements. This system takes the advantages of both dynamic and steady-state methods. It is especially suitable for solid materials having large length-to-thickness ratio. In order to illustrate the capabilities and reliability of the system we show some measurements of thermal conductivity on pure lead (99.9 %) and La0.92Sr0.08CoO3 ceramic samples. This technique provides very good precision and fast measurements in the temperature range between 85K and room temperature, along with an effective control of radiative and convective heat losses.

Utilizando el método de reacción de estado sólido se prepararon muestras policristalinas del compuesto Ca1-xLaxMnO3 (0 ≤ x ≥ 0.15). Sus propiedades de transporte se estudiaron en función de la temperatura y el contenido de lantano, a partir de mediciones de resistividad eléctrica ro(T) y coeficiente Seebeck S(T) en el rango de temperatura entre 100 y 290K. Las propiedades estructurales y morfológicas se estudiaron mediante estudios de difracción de rayos-X (XRD) y microscopia electrónica de barrido (SEM), respectivamente. El coeficiente Seebeck es negativo en todo el rango de temperatura estudiado, indicando una conducción dada por portadores de carga negativos, su magnitud disminuye con el contenido de lantano desde |-261| mV/K hasta |-120| mV/K. La resistividad eléctrica muestra un comportamiento de carácter semiconductor, descrito mediante un mecanismo tipo hopping de pequeños polarones. Las propiedades termoeléctricas de los compuestos obtenidos se evaluaron a través de su factor de potencia termoeléctrico PF, el cual alcanza valores máximos cercanos a 2mW/K2cm, estos valores permiten considerar esta familia de compuestos como posibles materiales útiles en aplicaciones termoeléctricas.

By using the solid state reaction method samples of Ca1-xLaxMnO3 (0 <= x <= 0.15) were prepared. Their transport properties were studied by electrical resistivity rho(T) and Seebeck coefficient S(T) measurements as a function of temperature and lanthanum content, in the temperature range between 100 and 290K. The structural and morphological properties were studied by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM), respectively. The Seebeck coefficient is negative throughout the studied temperature range indicating a conduction given by negative charge carriers, its magnitude decreases with the lanthanum content from vertical bar -261 vertical bar mu V/K to vertical bar-120 vertical bar mu V/K. The electrical resistivity shows a semiconducting behavior, it was interpreted in terms of small polaron hopping model. Thermoelectric properties of the obtained compounds were studied by the thermoelectric power factor PF, which reaches maximum values around 2 mu W/K-2 cm, these values become this kind of ceramics promising thermoelectric compound, to be used in technological applications.

By using the solid state reaction method composites of Ca-2.94 Y0.06C04O9+delta/Ag were growth, the silver weight ratios take values between 0 % and 10 %. Their transport properties were studied from electrical resistivity p(T) and Seebeck coefficient S (T) measurements as function of temperature. The magnitude of the electrical resistivity decreases with the Ag content present in the composites. The Seebeck coefficient is positive throughout the studied temperature range suggesting a conduction given by positive charge carriers. Thermoelectric performance was assessed through the thermoelectric power factor, P F, which increases with the added Ag level. This parameter reaches maximum values around 9.5 mu W/K(2)cm. The observed behaviour in the transport properties become these ceramic composites promising thermoelectric materials.

Utilizando el método de reacción de estado sólido se prepararon compósitos de Ca2:94Y0:06Co4O9+d/Ag, donde el contenido de plata tomó valores en proporciones de masa entre 0% y 10 %. Sus propiedades de transporte se estudiaron en función de la temperatura a partir de mediciones de resistividad eléctrica r(T) y coeficiente Seebeck S(T). La magnitud de la resistividad eléctrica decrece con el contenido de plata presente, hasta valores próximos a 3 mW -cm. El coficiente Seebeck es positivo en todo el rango de temperatura estudiado sugiriendo una conducción eléctrica dada por portadores de carga positivos. El rendimiento termoeléctrico se evaluó a partir del cálculo del factor de potencia PF, el cual se incrementa con el nivel de plata adicionada alcanzando valores máximos cercanos a 9:5 mW=K2cm. El comportamiento mostrado por las propiedades de transporte permiten proponer estos compósitos como potenciales materiales con aplicaciones termoeléctricas.

By using the solid state reaction method composites of Ca2:94Y0:06Co4O9+d/Ag were growth, the silver weight ratios take values between 0% and 10 %. Their transport properties were studied from electrical resistivity r(T) and Seebeck coefficient S(T) measurements as function of temperature. The magnitude of the electrical resistivity decreases with the Ag content present in the composites. The Seebeck coefficient is positive throughout the studied temperature range suggesting a conduction given by positive charge carriers. Thermoelectric performance was assessed through the thermoelectric power factor, PF, which increases with the added Ag level. This parameter reaches maximum values around 9:5 mW=K2cm: The observed behaviour in the transport properties become these ceramic composites promising thermoelectric materials.

By using the solid-state reaction method samples of Ca1-xYxMnO3 CaMnO-Y were prepared. Transport properties of the samples were studied from electrical resistivity rho(T) and Seebeck coefficient ST measurements. The structural and morphological properties were researched by X-ray diffraction analysis and scanning electron microscopy SEM, respectively. The Seebeck coefficient is negative throughout the studied temperature range indicating a conduction given by negative charge carriers. The magnitude of ST decreases with the yttrium level, which suggest an increase in the charge carrier density. The temperature behavior of electrical resistivity exhibit a semiconducting character; the magnitude of rho(T) decreases with yttrium level reaching minimum values close to 0.5 mu cm. From the electrical resistivity and Seebeck coefficient experimental data it was possible to calculate the thermoelectric power factor, which shows maximum values around 0.12W-K(2)cm., Utilizando el metodo de reaccion de estado solido se prepararon muestras del compuesto Ca1-xYxMnO3 CaMnO-Y. Sus propiedades de transporte se estudiaron a partir de mediciones de resistividad electrica rho T y coeficiente Seebeck ST, en funcion de la temperatura y el contenido de itrio. Por medio de analisis de difraccion de rayos-X y microscopia electronica de barrido SEM se estudiaron las propiedades estructurales y morfologicas, respectivamente. El coeficiente Seebeck es negativo en todo el rango de temperatura estudiado sugiriendo una conduccion dada por portadores de carga negativos. La magnitud de ST decrece con el nivel de itrio presente, evidenciando un incremento en la densidad de portadores de carga. El comportamiento de la resistividad electrica es de caracter semiconductor y su valor minimo a temperatura ambiente es cercano a 0.5 Omega cm. Utilizando los datos experimentales de resistividad electrica y coeficiente Seebeck se calculo el factor de potencia termoelectrico cuyo maximo valor a temperatura ambiente fue cercano a 0.12 mu W/K(2)cm.

Utilizando el metodo de reaccion de estado solido se prepararon muestras del compuesto Ca 1-x Y x MnO 3 (CaMnO-Y). Sus propiedades de transporte se estudiaron a partir de mediciones de resistividad electrica ρ(T) y coeficiente Seebeck S (T), en funcion de la temperatura y el contenido de itrio. Por medio de analisis de difraccion de rayos-X y microscopia electronica de barrido (SEM) se estudiaron las propiedades estructurales y morfologicas, respectivamente. El coeficiente Seebeck es negativo en todo el rango de temperatura estudiado sugiriendo una conduccion dada por portadores de carga negativos. La magnitud de S(T) decrece con el nivel de itrio presente, evidenciando un incremento en la densidad de portadores de carga. El comportamiento de la resistividad electrica es de caracter semiconductor y su valor minimo a temperatura ambiente es cercano a 0.5 Ω·cm. Utilizando los datos experimentales de resistividad electrica y coeficiente Seebeck se calculo el factor de potencia termoelectrico cuyo maximo valor a temperatura ambiente fue cercano a 0.12μW/K 2 cm.

Thermoelectric properties of La1-xSrxCuO3-delta (LSCuO) samples prepared by sal-gel method were studied in the temperature range between 100 and 290 K From Seebeck coefficient S(T) and electrical resistivity p(T) measurements the thermoelectric power factor PF was calculated S(T) is positive over the studied temperature range suggesting a p type conduction its magnitude decreases with the Sr level from 400 mu V/K to 40 mu V/K The temperature dependence of p(T) changes from semiconducting-like to metallic as the strontium increases at room temperature p(T) exhibit values less than 17 m Omega cm The thermoelectric performance was evaluated through the PF parameter which shows maximum values close to 60 mu W/K-2 cm These results make these oxygen deficient ceramics promising thermoelectric material for low temperature applications (C) 2010 Elsevier B V All rights reserved

The price of fossil fuel accompanied by an increasing awareness of environmental problems associated with global warming, resulted in an upsurge of scientific activity to identify and develop environmentally friendly sources of electrical power. In this sense, direct energy conversion by thermoelectric devices using residual heat as heating source, could help to solve the energetic problems of the modern society. A thermoelectric converter is a solid state heat engine, in which the charge carriers are the work fluid, allowing the direct conversion of a thermal flow in electric power. This kind of devices are technologically attractive taking in to account that they do not have mobile parts, are silent, versatile and very reliable. Through last years the research of thermoelectric materials is focused on the development of thermoelectric generation systems, destined to optimize the low temperature waste heat recovery systems, which could generate useful electricity from different thermal sources as the human body, automobiles, and industrial plants. If the evolution of thermoelectric materials allows to improve the thermoelectric performance, in the next future these devices will contribute to reduce the use of fossil fuel and as a consequence to reduce the effects of global warming.

The price of fossil fuel accompanied by an increasing awareness of environmental problems associated with global warming, resulted in an upsurge of scientific activity to identify and develop environmentally friendly sources of electrical power.In this sense, direct energy conversion by thermoelectric devices using residual heat as heating source, could help to solve the energetic problems of the modern society.A thermoelectric converter is a solid state heat engine, in which the charge carriers are the work fluid, allowing the direct conversion of a thermal flow in electric power. This kind of devices are technologically attractive taking in to account that they do not have mobile parts, are silent, versatile and very reliable.Through last years the research of thermoelectric materials is focused on the development of thermoelectric generation systems, destined to optimize the low temperature waste heat recovery systems, which could generate useful electricity from different thermal sources as the human body, automobiles, and industrial plants.If the evolution of thermoelectric materials allows to improve the thermoelectric performance, in the next future these devices will contribute to reduce the use of fossil fuel and as a consequence to reduce the effects of global warming.

Oxide ceramics with a nominal composition La2-xSrxCuO4-delta (0<x <= 0.2) (LSCuO) were grown by using the solid-state reaction method. The transport properties of the obtained samples were modified by submitting the samples to a post-growth high temperature thermal process. The thermoelectric properties were studied from Seebeck coefficient S(T), electrical resistivity rho(T) and thermal conductivity kappa(T) measurements. These transport properties were studied in the temperature range between 100 and 290K. The magnitude of S(T), rho(T) and kappa(T) decreases with the Sr doping level reaching maximum values close to 300 mu V/K, 4 m Omega-cm and 4.7 W/K-m, respectively. From S(T), rho(T) and kappa(T) data it was possible to calculate the thermoelectric power factor (PF) and the dimensionless figure of merit (ZT), which reach maximum values close to 80 mu W/K-2-cm and 0.4, respectively. The observed values in these performance parameters become these oxide ceramics promissory thermoelectric compounds for low temperature thermoelectric applications.

Oxide ceramics of La1.85Sr0.15CuO4-delta (LSCuO) were grown by using the solid-state reaction method. The oxygen stoichiometry was modified by submitting the obtained samples to a post -growth thermal process at 900 degrees C under an inert atmosphere. Powder X-ray diffraction analysis shows all samples are single-phase with K2NiF4-type structure. The thermoelectric properties were studied from Seebeck coefficient S(T) and electrical resistivity p(T) measurements in the temperature range from 90 to 290K. The magnitude of S(T) and p(T) increases with the processing time reaching maximum values, at room temperature, close 150/mu V/K and 0.8m Omega cm, respectively. From S(T) and p(T) data it was possible to calculate the thermoelectric power factor PF, which reaches maximum values close to 38/mu W/K(2)cm, in the temperature range between 90 and 160K. The behavior of the transport properties and PF becomes these ceramics promissory thermoelectric materials, which could be used in low temperature thermoelectric applications.