This investigation demonstrates the feasibility to fabricate high quality ceramic-carbonate membranes based on mixed-conducting ceramics. Specifically, it is reported the simultaneous CO 2 /O 2 permeation and stability properties of membranes constituted by a combination of ceramic and carbonate phases, wherein the microstructure of the ceramic part is composed, in turn, of a mixture of fluorite and perovskite phases. These ceramics showed ionic and electronic conduction, and at the operation temperature, the carbonate phase of the membranes is in liquid state, which allows the transport of CO 3 2and O 2species via different mechanisms. To fabricate the membranes, the ceramic powders were uniaxially pressed in a disk shape. Then, an incipient sintering treatment was carried out in such a way that a highly porous ceramic was obtained. Afterwards, the piece is densified by the infiltration of molten carbonate. Characterization of the membranes was accomplished by SEM, XRD, and gas permeation techniques among others. Thermal and chemical stability under an atmosphere rich in CO 2 was evaluated. CO 2 /O 2 permeation and long-term stability measurements were conducted between 850 and 940 ℃. The best permeation-separation performance of membranes of about 1 mm thickness, showed a maximum permeance flux of about 4.46×10-7 mol•m-2 •s-1 •Pa-1 for CO 2 and 2.18×10-7 mol•m-2 •s-1 •Pa-1 for O 2 at 940 ℃. Membranes exhibited separation factor values of 150-991 and 49-511 for CO 2 /N 2 and O 2 /N 2 respectively in the studied temperature range. Despite long-term stability test showed certain microstructural changes in the membranes, no significant detriment on the permeation properties was observed along 100 h of continuous operation.
Lithium aluminate (LiAlO2) polymorphs have been synthesized by solid-state reaction, but these ceramics usually show certain limitations attributed to the low control on the particle size, morphology and specific surface area. In this sense, different chemical synthesis pathways, citrate precursor among them, have been studied to obtain ultrafine powders exhibiting enhanced textural and morphological features. Synthesis by citrate precursor method would involve the use of alternative chelating agents for the formation of more stable metal-chelate species, such as ethylene-diamine-tetra-acetic acid (EDTA). Thus, the aim of this work was to study the g-LiAlO2 synthesis by EDTA-citrate complexing approach to establish the effect of the synthesis route on the structural and microstructural characteristics of the resultant powders. The synthesized ceramic powders were calcined (600-900°C) and characterized by simultaneous TG-DTA, XRD, SEM, TEM and N2 adsorption-desorption techniques. Crystallization transition process from the precursors to the g-LiAlO2 phase is reported. Results show that chemical synthesis by EDTA-citrate complexing method can produce pure and crystalline g-LiAlO2 nanoparticles at relative low temperatures (700 ºC). The possible formation mechanism is discussed.
Resumen. El aluminato de litio (LiAlO2) en sus diferentes fases polimórficas se ha sintetizado por la técnica convencional de reacción en estado sólido; sin embargo, este método presenta ciertas limitaciones desde el punto de vista del control que se tiene en el tamaño de partícula, morfología y área específica. En este sentido, se han estudiado diferentes rutas de síntesis química para la obtención de polvos ultrafinos que presenten propiedades texturales y características morfológicas mejoradas. Entre éstas, se encuentra la síntesis por citratos precursores; un método que además del ácido cítrico, puede involucrar el uso de agentes complejantes o quelantes alternativos para promover la formación de especies más estables. Un ejemplo de lo anterior es el ácido etilendiaminotetraacético o EDTA por sus siglas en inglés. El objetivo de este trabajo es estudiar la síntesis del g-LiAlO2 por el método del citrato precursor-EDTA y establecer el efecto de la ruta de síntesis sobre las características estructurales y microestructurales de los compuestos obtenidos. Los polvos cerámicos sintetizados fueron calcinados a diferentes temperaturas (600-900 ºC) y caracterizados por diferentes técnicas como ATG-ATD, DRX, MEB, MET y adsorción-desorción de N2 a baja presión. Se estudió el proceso de descomposición y cristalización de los precursores hasta la obtención del óxido metálico de aluminato de litio. Los resultados muestran que el método de síntesis propuesto es adecuado para la obtención, a baja temperatura (700 ºC), de nanopartículas de la fase cristalina y pura del g-LiAlO2. Se discute un posible mecanismo de formación del compuesto de estudio a partir de los geles precursores usados.
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