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Thermoelectrical properties of cobaltite ceramics can be tuned up by choosing the adequate synthetic method, cation substitution and subsequent grain orientation. This can be performed preparing the ceramics by a polymer solution method, using Pb partially substituting Bi, and texturing by directionally growing from the melt. In this work, Bi2-xPbxCa2Co1.7Oy (x = 0.0, 0.2, 0.4 and 0.6) ceramics, prepared by a polymer solution method, have been directionally grown, with the Laser Floating Zone (LFZ) technique, at 30 mm/h. In all the cases, the microstructure shows alternated layers with small CoO inclusions. It has been found a very important decrease on the resistivity and, at the same time, on the thermopower. However, the power factor values are improved with Pb addition, reaching power factor values, for samples with 0.4 Pb substitution, as high as two times the values obtained for undoped ones.Las propiedades termoeléctricas de cerámicas basadas en óxidos de cobalto pueden ser ajustadas eligiendo adecuadamente el método de síntesis, la substitución catiónica y posterior orientación de grano. Esto puede realizarse preparando las cerámicas precursoras por un método de solución con adición de un polímero, substituyendo parcialmente el Bi por Pb y texturando el material por crecimiento direccional a partir del fundido. En este trabajo, cerámicas con composición Bi2-xPbxCa2Co1.7Oy (x = 0.0, 0.2, 0.4 y 0.6), han sido preparadas por un método en disolución y crecidas direccionalmente con la técnica de Zona Fundida Flotante Inducida por láser (LFZ), a 30 mm/h. En todos los casos, la microestructura está formada por capas alternas con pequeñas inclusiones de CoO. Se ha encontrado una apreciable disminución en los valores de resistividad y, al mismo tiempo, del poder termoeléctrico .Sin embargo, el factor de potencia aumenta con la adición de plomo, llegando a doblar, en muestras con una adición del 0.4 Pb, los valores obtenidos para las muestras sin dopar.

Thermoelectric performances on cobaltite ceramics can be changed by grain orientation. This can be performed by directional solidification on samples prepared from different synthetic methods. Three synthetic methods, conventional solid state reaction, sol-gel and polymer solution were used for preparation of Bi2Ca2Co1.7Ox powders. The powders have been directionally grown, at a rate of 30 mm/h, by the laser floating zone method. In all the cases, the microstructure shows alternated layers, of Bi2Ca2Co2Ox and a Bi-Ca-O solid solution, with small CoO inclusions. It has been found an improvement on the power factor at 50 ºC for the solution methods with respect to the conventional solid sate sintered samples.Las prestaciones de las cerámicas basadas en óxidos de cobalto pueden modificarse por medio de la orientación de los granos. Este proceso puede llevarse a cabo mediante solidificación direccional de las muestras, que previamente han podido ser preparadas por diferentes rutas. En este trabajo se han utilizado tres métodos de síntesis, estado sólido, sol-gel y por vía polimérica, para preparar cerámicas con composición Bi2Ca2Co1.7Ox. Estos polvos se han crecido direccionalmente a 30 mm/h, por el método de zona flotante inducida por láser. En todos los casos la microestructura consiste en capas alternas, de Bi2Ca2Co2Ox y una solución sólida Bi-Ca-O, con pequeñas inclusiones de CoO. Se ha encontrado que los métodos de disolución mejoran el factor de potencia a 50 ºC, con respecto a los valores que se obtienen a través de los métodos de sinterizado en estado sólido.

Thermoelectric performance on cobaltite ceramics can be increased raising the electrical conductivity. This can be performed by increasing grain size and aligning them along a preferential direction by the Laser Floating Zone (LFZ) method. In this work, Bi2Ca2Co1.7Ox ceramics have been directionally grown at three different speeds, 15, 30 and 50 mm/h. It has been found a small influence of the growth conditions on the thermoelectric properties. In all the cases, the microstructure shows alternated cobaltite layers with small CoO inclusions. In despite, the thermopower is higher than usual in misfit cobaltites obtained by conventional solid state routes. This preliminary result originates probably from oxygen vacancies.Las prestaciones termoeléctricas de las cerámicas basadas en óxidos de cobalto pueden ser aumentadas por la disminución de la resistividad eléctrica. Esto puede realizarse aumentando el tamaño de grano y alineándolo en una dirección preferencial, utilizando el método de zona flotante inducida por láser (LFZ). En este trabajo, se han crecido cerámicas termoeléctricas de composición Bi2Ca2Co1.7Ox a tres velocidades diferentes, 15, 30 y 50 mm/h. Se ha encontrado que las condiciones de crecimiento tienen poca influencia en las propiedades termoeléctricas. En todos los casos, la microestructura está formada por capas alternadas de cobaltitas, con pequeñas inclusiones de CoO. A pesar de ello, el poder termoeléctrico medido es mayor de lo habitual para este tipo de materiales preparados por métodos convencionales en estado sólido. Este resultado preliminar está originado, probablemente, por la formación de vacantes de oxígeno.

Understanding the mechanisms that connect heat and electron transport with crystal structures and defect chemistry is fundamental to develop materials with thermoelectric properties. In this work, we synthesized a series of self‐doped compounds Cu2+xMn1−xGeS4 through Cu for Mn substitution. Using a combination of powder X‐ray diffraction, high resolution transmission electron microscopy and precession‐assisted electron diffraction tomography, we evidence that the materials are composed of interconnected enargite‐ and stannite‐type structures, via the formation of nanodomains with a high density of coherent interfaces. By combining experiments with ab initio electron and phonon calculations, we discuss the structure–thermoelectric properties relationships and clarify the interesting crystal chemistry in this system. We demonstrate that excess Cu+ substituted for Mn2+ dopes holes into the top of the valence band, leading to a remarkable enhancement of the power factor and figure of merit ZT. [ABSTRACT FROM AUTHOR]

Polycrystalline samples in the series Ti1-xTaxS2 with x varying from 0 to 1 were prepared using solid-liquid-vapor reaction and spark plasma sintering. Rietveld refinements of X-ray diffraction data are consistent with the existence of a full solid solution for x ≤ 0.4. Transport measurements reveal that tantalum can act as electron donor when substituted in the Ti sites. As a consequence, the electrical resistivity and the absolute value of the Seebeck coefficient decrease with Ta content due to an increase in the carrier concentration. The lattice thermal conductivity being reduced due to mass fluctuation effect, the ZT values in Ti0.95Ta0.05S2 is slightly increased as compared to TiS2. [ABSTRACT FROM AUTHOR]

Dense ceramics of the itinerant ferromagnet CoS2 have been prepared by the spark plasma sintering technique. The structural study confirms a cubic unit-cell for CoS2 with a = 5.539(6)Å ([formula]). Scanning and transmission electron microscopy reveal the existence of sulfur deficient regions of composition 'Co2S3' with a structure compatible to that of CoS2 where 0.5 S atom are missing in one out of two successive layers along the b direction. The volume of these regions estimated to be less than 1% is explained by possible S losses from the surface during the sintering. The physical properties measurements show a high TC (122 K), large saturated magnetization (0.88 μB/f.u.), with a ∼100% magnetoresistance at 5 K in 9 T. Also, the value of the residual resistivity ratio, ρ300 K/ρ5 K = 45, between those of crystal and conventional ceramics, attest for both good compacity and grain connectivity. Interestingly, the Seebeck coefficient exhibits negative values reaching S = -47.5 μV K-1 at 675 K and below TC, S can be fitted by a diffusion model with a magnon term related to the ferromagnetism. A large power factor (S2/ρ) of 1 mW m-1 K-2 is obtained over a broad T range but the too high thermal conductivity (κ675 K ∼ 8.7 W K-1 m-1) is a limiting physical parameter to use CoS2 ceramics in thermoelectric devices. [ABSTRACT FROM AUTHOR]

Exploring the possibility to obtain n-type oxide thermoelectrics, the transparent conducting oxide Ga3-xIn5+xSn2O16 (0.32O3 and its much lower indium content, more promising for applications. The absolute value of the Seebeck coefficient decreases almost linearly with x, whereas improvement in the power factor is obtained by increasing the indium content. The figure of merit ZT value reaches about 0.28 at 1000 K for x=0.6. The observed results are explained taking into account the peculiarities of the oxygen-deficient fluorite structure. [ABSTRACT FROM AUTHOR]

Transport properties including electrical resistivity, thermopower, and thermal conductivity of polycrystalline Mo2.5Ru0.5Sb7-xTex compounds for 0⩽x⩽1 have been investigated in the 2-1000 K temperature range. Additional information on the concentrations and the scattering mechanisms of the charge carriers as well as on the magnetic properties has been obtained through Hall effect and magnetic susceptibility measurements performed in the 5-300 K temperature range. The enhancement in the Te content results in a decrease in the carrier concentration which is at the origin of the simultaneous increase in the electrical resistivity and thermopower. A single parabolic band model with acoustic phonon scattering enables to explain the compositional and temperature dependence of the thermopower while such a simple model fails to adequately describe the electronic thermal conductivity. This characteristic together with the unusual dependence of the thermal conductivity upon alloying might be a direct consequence of strong phonons-dimers interactions displayed by Mo3Sb7. Magnetic susceptibility data demonstrate that the antiferromagnetically-coupled dimers tend to disappear as x is enhanced, lending further support to a crucial role played by these interactions to understand the thermal transport in these materials. Low thermal conductivity coupled with high thermopower values result in a high dimensionless figure of merit ZT~0.70 at 1000 K in Mo2.5Ru0.5Sb6.5Te0.5 positioning this material as an interesting candidate for power generation applications. [ABSTRACT FROM AUTHOR]