(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

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

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

Microwave-Assisted Routes for the Synthesis of Complex Functional Oxides

The synthesis of complex functional inorganic materials, such as oxides, can be successfully performed by using microwave irradiation as the source of heat. To achieve this, different routes and set-ups can be used: microwave-assisted synthesis may proceed in the solid state or in solution, aqueous or not, and the set ups may be as simple and accessible as domestic oven or quite sophisticated laboratory equipment. An obvious advantage of this innovative methodology is the considerable reduction in time—minutes rather than hours or days—and, as a consequence, energy saving. No less important is the fact that the particle growth is inhibited and the broad variety of different microwave or microwave-assisted synthesis techniques opens up opportunities for the preparation of inorganic nanoparticles and nanostructures. In this work, various microwave synthesis techniques have been employed: solid-state microwaves, single-mode microwaves using a TE10p cavity and microwave-assisted hydrothermal synthesis. Relevant examples are presented and discussed

Síntesis asistida por microondas de sólidos inorgánicos

The use of microwaves to produce inorganic solids is an attractive option because of potential benefits: energy saving, short processing times, increased product yields, economy and environment friendly, and so on. In a microwave process, there is an inverse heating profile as compared to conventional methods: the heating occurs by energy conversion rather than energy transfer. In this work different approaches to the synthesis of materials have been followed: direct solid state reaction, performed in a domestic set-up (NiO, LaMO3, RECrO3�), solvothermal synthesis by using a more sophisticated apparatus (ã-AlOOH, BiFeO3). Sol-gel and combustion methods combined with microwaves are also describedLa utilizacion de radiacion microondas para la sintesis de solidos inorganicos presenta grandes beneficios: ahorro energetico, tiempos cortos de procesado, mayor rendimiento, procedimiento economico y respetuoso con el medio ambiente, etcetera. En un proceso de sintesis por microondas existe un perfil inverso de temperatura comparado con los metodos convencionales: el calentamiento ocurre por conversion en lugar de por transferencia de energia. En este trabajo se muestran diferentes rutas de sintesis en las que se ha empleado radiacion microondas: estado solido con un microondas domestico (NiO, LaMO3, TRCrO3,�c), sintesis solvotermal empleando un aparato mas sofisticado (�Á-AlOOH, BiFeO3). Tambien se describen casos en los que se combina sol-gel o combustion con microondas

Microwave-Assisted Routes for the Synthesis of Complex Functional Oxides

The synthesis of complex functional inorganic materials, such as oxides, can be successfully performed by using microwave irradiation as the source of heat. To achieve this, different routes and set-ups can be used: microwave-assisted synthesis may proceed in the solid state or in solution, aqueous or not, and the set ups may be as simple and accessible as domestic oven or quite sophisticated laboratory equipment. An obvious advantage of this innovative methodology is the considerable reduction in time—minutes rather than hours or days—and, as a consequence, energy saving. No less important is the fact that the particle growth is inhibited and the broad variety of different microwave or microwave-assisted synthesis techniques opens up opportunities for the preparation of inorganic nanoparticles and nanostructures. In this work, various microwave synthesis techniques have been employed: solid-state microwaves, single-mode microwaves using a TE10p cavity and microwave-assisted hydrothermal synthesis. Relevant examples are presented and discussed