(Microwave - assisted synthesis and characterization of inorganic materials)

Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Químicas, Departamento de Química Inorgánica I, leída el 23-05-2014El objetivo principal de esta tesis es mostrar que la utilización de radiación microondas como fuente de calentamiento es una alternativa prometedora en la síntesis de materiales inorgánicos no moleculares. El método presenta un gran ahorro energético y de tiempo en comparación con el método cerámico tradicional Se han estudiado los principios básicos de los distintos mecanismos de calentamiento por microondas, relacionándolo con la interacción entre los dipolos del material y la radiación aplicada. Se ha observado que tanto la parte magnética como la parte eléctrica de la onda son importantes en el calentamiento, no sólo la parte eléctrica como se ha descrito previamente en la literatura. Se ha preparado una amplia gama de materiales inorgánicos mediante el uso de diferentes técnicas de microondas, obteniendo una gran variedad de materiales desde un punto de vista composicional, estructural, de sus propiedades y aplicaciones. - Se han sintetizado materiales con estructura perovskita , pirocloro, holandita, fluorita y estructuras unidimensionales. - Los materiales preparados muestran una amplia gama de propiedades funcionales: ferromagnetismo, ferroelectricidad , multiferroicidad , termoelectricidad , conductividad de ion litio y oxígeno. - Los materiales sintetizados pueden tener aplicación en una amplia gama de diferentes tecnologías: tecnología de la información, telecomunicaciones, aplicaciones termoeléctricas, baterías de ión litio y pilas de combustible de óxido sólido . La amplia variedad de diferentes sistemas de microondas o de técnicas de síntesis asistida por microondas abre nuevas oportunidades para la preparación de nanopartículas inorgánicas y nanoestructuras . Esto permite la posibilidad de modificar la morfología, y las propiedades físicas y químicas de los materiales al nivel de la nanoescala. . Se han empleado diversos sistemas de síntesis asistida por microondas: microondas de estado sólido, microondas monomodal utilizando una cavidad TE10p y síntesis hidrotermal asistida por microondasDepto. de Química InorgánicaFac. de Ciencias QuímicasTRUEunpu

Experimental conditions required for accurate measurements of electrical resistivity, thermal conductivity, and figure of merit (ZT) using Harman and impedance spectroscopy methods

The Harman method is used extensively for the characterization of the dimensionless figure of merit ZT of thermoelectric (TE) materials and devices. However, its accuracy has often been questioned, since in many cases there are relatively high errors associated with the method. The impedance spectroscopy technique, which has recently been shown as a suitable tool to also characterize TE materials and devices, has some similarities with the Harman method, and can also directly provide ZT. In order to obtain reliable measurements in both methods, there are some common critical points that must be taken into account, such as for example the requirement of fully adiabatic conditions, and a negligible Joule effect. In this study, we have evaluated the effect of different experimental conditions in the accuracy of both methods using a sample with known TE properties. Our analysis has led to the identification of different sources of errors and other issues that have not been clearly identified to date that can lead to inaccurate results. Namely, the need of a homogeneous Peltier effect at the junctions, problems arising from the use of Ag paint, and the selection of the right value for the current perturbation applied to the system. These problems and sources of error need to be identified and carefully considered if accurate results are to be obtained

Skutterudite thermoelectric modules with high volume-power-density: scalability and reproducibility

The construction and evaluation of wholly-skutterudite thermoelectric modules with a high volume-power-density is described. Such modules afford the maximum power output for the minimum use of material. Synthesis of the component n-type unfilled skutterudite CoSb2.75Sn0.05Te0.20 and p-type filled skutterudite Ce0.5Yb0.5Fe3.25Co0.75Sb12, was achieved using a scalable ball-milling route that provides sufficient material for the construction and assessment of performance of 12 modules. Impedance spectroscopy at room temperature is shown to provide a rapid means of evaluating the quality of module fabrication. The results show a high degree of reproducibility in module performance across the 12 modules, with an average internal resistance of 102(4) m. Electrical measurements on the component n- and p-type materials reveal power factors (S2σ) of 1.92 and 1.33 mW m-1 K-2, respectively, at room temperature and maximum figures of merit of ZT = 1.13 (n-type) and ZT = 0.91 (p-type) at 673 K and 823 K, respectively. The figure of merit of the module at room temperature (ZT = 0.12) is reduced by ca. 39% from the average of the n- and p-type component materials at the same temperature, as a result of thermal- and electrical-contact resistance losses associated with the architecture of the module. I-V curves for the module determined for T in the range 50 – 450 K show an almost linear dependence of the open-circuit voltage on T and allow calculation of the power output, which reaches a maximum value of 1.8 W (0.9 W cm-2) at T = 448 K

LaNi0.6Co0.4−xFexO3−δ as Air-Side Contact Material for La0.3Ca0.7Fe0.7Cr0.3O3−δ Reversible Solid Oxide Fuel Cell Electrodes

The goal of the current work was to identify an air-side-optimized contact material for La0.3Ca0.7Fe0.7Cr0.3O3−δ (LCFCr) electrodes and a Crofer22APU interconnect for use in reversible solid oxide fuel cells (RSOFCs). LaNi0.6Co0.4−xFexO3 (x = 0–0.3) perovskite-type oxides were investigated in this work. The partial substitution of Co by Fe decreased the thermal expansion coefficient values (TEC) closer to the values of the LCFCr and Crofer 22 APU interconnects. The oxides were synthesized using the glycine–nitrate method and were characterized using X-ray thermodiffraction and 4-probe DC electrical conductivity measurements. Based on the materials characterization results from the Fe-doped oxides investigated here, the LaNi0.6Co0.2Fe0.2O3−δ composition was selected as a good candidate for the contact material, as it exhibited an acceptable electrical conductivity value of 395 S·cm−1 at 800 °C in air and a TEC value of 14.98 × 10−6 K−1 (RT-900 °C).Depto. de Química InorgánicaFac. de Ciencias QuímicasTRUEMitacsComunidad de MadridMinisterio de Ciencia, Innovaciónpu

The impact of charge transfer and structural disorder on the thermoelectric properties of cobalt intercalated TiS2

A family of phases, CoxTiS2 (0 ≤ x ≤ 0.75) has been prepared and characterised by powder X-ray and neutron diffraction, electrical and thermal transport property measurements, thermal analysis and SQUID magnetometry. With increasing cobalt content, the structure evolves from a disordered arrangement of cobalt ions in octahedral sites located in the van der Waals’ gap (x ≤ 0.2), through three different ordered vacancy phases, to a second disordered phase at x ≥ 0.67. Powder neutron diffraction reveals that both octahedral and tetrahedral inter-layer sites are occupied in Co0.67TiS2. Charge transfer from the cobalt guest to the TiS2 host affords a systematic tuning of the electrical and thermal transport properties. At low levels of cobalt intercalation (x < 0.1), the charge transfer increases the electrical conductivity sufficiently to offset the concomitant reduction in |S|. This, together with a reduction in the overall thermal conductivity leads to thermoelectric figures of merit that are 25 % higher than that of TiS2, ZT reaching 0.30 at 573 K for CoxTiS2 with 0.04 ≤ x ≤ 0.08. Whilst the electrical conductivity is further increased at higher cobalt contents, the reduction in |S| is more marked due to the higher charge carrier concentration. Furthermore both the charge carrier and lattice contributions to the thermal conductivity are increased in the electrically conductive ordered-vacancy phases, with the result that the thermoelectric performance is significantly degraded. These results illustrate the competition between the effects of charge transfer from guest to host and the disorder generated when cobalt cations are incorporated in the inter-layer space

Low thermal conductivity in La-filled cobalt antimonide skutterudites with an inhomogeneous filling factor prepared under high-pressure conditions

La-filled skutterudites LaxCo4Sb12 (x : 0.25 and 0.5) have been synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. The structural properties of the reaction products were characterized by synchrotron X-ray powder diffraction, clearly showing an uneven filling factor of the skutterudite phases, confirmed by transmission electron microscopy. The non-homogeneous distribution of La filling atoms is adequate to produce a significant decrease in lattice thermal conductivity, mainly due to strain field scattering of high-energy phonons. Furthermore, the lanthanum filler primarily acts as an Einstein-like vibrational mode having a strong impact on the phonon scattering. Extra-low thermal conductivity values of 2.39 W/mK and 1.30 W/mK are measured for La0.25Co4Sb12 and La0.5Co4Sb12 nominal compositions at 780 K, respectively. Besides this, lanthanum atoms have contributed to increase the charge carrier concentration in the samples. In the case of La0.25Co4Sb12, there is an enhancement of the power factor and an improvement of the thermoelectric properties

Structural and dielectric properties of ultra-fast microwave-processed La_0.3Ca_0.7Fe_0.7Cr_0.3O3-delta ceramics

Perovskite La_0.3Ca_0.7Fe_0.7Cr_0.3O_(3-delta) (LCFCr) is a mixed ionic and electronic conductor (MIEC) that can be employed as an electrode material in reversible solid oxide fuel cells (RSOFCs). In this work, an ultra-fast (15 min) one-step microwave (MW)-assisted combustion synthesis route has been developed to obtain phase pure and highly crystalline LCFCr powder. The synthesized powders exhibited a sponge-like microstructure with increased electrochemical reaction sites. Neutron thermodiffraction analysis revealed a structural transition above 500 degrees C from the room temperature (RT) orthorhombic Pnma to a rhombohedral R3c perovskite phase. The oxygen vacancy concentration was found to increase from delta = 0.272(7) at RT to delta = 0.333(5) at 900 degrees C. Furthermore, a 3-dimensional G-type antiferromagnetic structure was detected at RT. MW-sintering of pressed green ceramic pellets was carried out at 950 degrees C for 1 h, using a MW-transparent quartz fiber crucible or alternatively a SiC crucible acting as a MW-absorber. Impedance spectroscopy data on sintered ceramic pellets revealed electronic inhomogeneity as demonstrated by the occurrence of three dielectric relaxation processes associated with two grain boundary (GB)-like contributions and one bulk. The dielectric inhomogeneity encountered may be restricted to the extrinsic GB areas, which may be rather thin. More homogeneous dielectric properties of the GBs were found in the pellet that was sintered in the SiC crucible

Enhancing the thermoelectric properties of single and double filled p-type skutterudites synthesized by an up-scaled ball-milling process

The single and double filled p-type skutterudites Ce0.8Fe3CoSb12 and Ce0.5Yb0.5Fe3.25Co0.75Sb12 have been prepared by mechanical alloying. This offers a rapid method for the preparation of skutterudites that could be scaled up for adoption at industrial level. The large-scale samples prepared by ball-milling exhibit enhanced figures of merit ZT, compared with materials prepared by conventional solid-state reaction. At room temperature ZT is increased by ca. 19 % for both single and double filled skutterudites. Maximum figures of merit, ZT = 0.68 and ZT = 0.93 are attained for Ce0.8Fe3CoSb12 at 773 K and Ce0.5Yb0.5Fe3.25Co0.75Sb12 at 823 K respectively. The improvement in thermoelectric values at room temperature may be traced to a reduction in thermal conductivity in the ball-milled samples arising from the reduced grain size. The influence of the microstructure on the thermoelectric properties, together with the stability in air and the performance of the materials after several heating and cooling cycles has been studied and are detailed in this work. The densified samples prepared by ball-milling also show a higher resistance to oxidation, which starts at 694 K for Ce0.8Fe3CoSb12 and at 783 K for Ce0.5Yb0.5Fe3.25Co0.75Sb12