In the 25 years since the first report of mixed ionic and electronic conducting ceramics, perovskite membranes underwent both research and development phases, with the latest works entering pilot trials for oxygen separation from air. During this time a number of perovskite synthesis methods were investigated from the original solid state chemistry through the more advanced and well established sol-gel route via the Pechinni method. The advances in synthesis methods were possible due to the desirable full incorporation of cations into the A and B-sites of perovskites with a general ABO 3 structure. In terms of membrane manufacturing, perovskite hollow fibres attracted a major research effort due to small membrane thickness and high fluxes. This led to a number of investigations by doping with other cations or by surface modification, all aiming at increasing oxygen fluxes. Recently, advanced ceramic processing by tape casting has led to the preparation of very thin dense films either on porous supports or as monoliths containing internal porous regions. All these developments in perovskite synthesis and membrane preparation methods, together with other types of methods requiring special equipment are addressed in this review, including an 2 analysis of the state of the art. Finally, future challenges are discussed in terms of competing technologies and potential industrial design directions of perovskite membranes for oxygen separation from air.
Porous substrates play a major role in devices requiring the flow of fluids such as in filters and membranes. Porous substrates are also extensively used for the preparation of inorganic membranes for microfiltration, [1][2][3] desalination, [4][5][6][7] gas separation, [8][9][10][11] and percrystallization processes. [12,13] These inorganic membranes are known as asymmetric membranes as they are conventionally prepared by coating ceramic [14][15][16] or carbon [17][18][19] thin films as top layers on porous substrates. The importance of the porous substrate is to provide the mechanical strength otherwise not available in thin films. Many inorganic materials such as ceramics [20][21][22] and stainless steel [23][24][25] have been manufactured as porous substrates, though alumina is the most used material due to lower costs and processing versatility. Porous substrates come in several geometries such as tubes, [14,[26][27][28] hollow fibers, [29][30][31] and flat surfaces. [19,32] Traditionally, inorganic porous substrates are prepared from conventional ceramic processes such as slip-casting, [33,34] extrusion, [35,36] dry pressing, [37] tapecasting, [38][39][40] and phase inversion hollow fibers. [41,42] A relatively more recent method is the freeze-casting technique for processing porous ceramic substrates. [43][44][45][46] Due to the ability of forming an aligned pore structure, this technique can overcome problems associated with lower fluid fluxes conferred by conventional ceramic processing methods such as tortuous pore structure, constrictions, and dead-end and isolated pores. [47] Freeze-cast starts with the preparation of a stable ceramic suspension (or slurry) at normal conditions followed by controlled freezing of the solvent of the ceramic
Este é um artigo publicado em acesso aberto (Open Access) sob a licença Creative Commons Attribution, que permite uso, distribuição e reprodução em qualquer meio, sem restrições desde que o trabalho original seja corretamente citado. Determinação e verificação de como a gordura trans é notificada nos rótulos dealimentos, em especial naqueles expressos "0% gordura trans" Determination and verification of how "trans" fat is notified on food labels, particularly those expressed as being "0% trans fat" ResumoNeste trabalho, foram analisadas 251 amostras de alimentos quanto aos teores de gordura total, ácidos graxos TRANS, saturados, monoinsaturados e poli-insaturados de produtos amplamente consumidos pela população, principalmente aqueles com alegação "zero TRANS". De acordo com a Resolução RDC 360/2003 da ANVISA, é obrigatória a declaração dos níveis de ácidos graxos TRANS na rotulagem dos alimentos embalados quando os teores forem superiores a 0,2 g na porção do alimento. Produtos com valores inferiores a 0,2 g podem ser notificados na rotulagem como "zero TRANS". O objetivo deste estudo foi avaliar a quantidade de gorduras saturadas e TRANS em diversos grupos de alimentos industrializados, e a adequação, frente à legislação brasileira, da declaração dessas gorduras nos respectivos rótulos. Para as análises, foram utilizados os métodos oficiais de análise de gorduras AOAC 996.01 (modificado) e de ésteres metílicos de ácidos graxos AOAC 996.06. Conforme os resultados obtidos, alguns produtos apresentaram, em 100 g, teores de ácidos graxos TRANS superiores aos recomendados para ingestão total diária em diversos países (2 g/dia). Vários produtos estão conforme a legislação (0,2 g de gordura TRANS por porção), o que não implica necessariamente em zero absoluto, dando a falsa ideia ao consumidor de não estar ingerindo gordura TRANS, independentemente da quantidade consumida. Em apenas 12 amostras, não se detectou gordura TRANS em 100 g de produto analisado. Ressalte-se, ainda, que, muitas vezes, a quantidade consumida é maior do que a porção sugerida, acarretando uma ingestão significativa de ácidos graxos TRANS. Os resultados obtidos nesta avaliação oferecem subsídios para modificações na legislação em vigor, exigindo a declaração de ácidos graxos TRANS na rotulagem. Palavras-chave: Gorduras TRANS; Gorduras saturadas; Legislação brasileira; Rotulagem nutricional. SummaryIn this study 251 food samples widely consumed by the population, especially those claiming to be "zero TRANS", were analys5ed for their total fat contents and for their "TRANS", saturated, monounsaturated and polyunsaturated fatty acid contents. According to the RDC Resolution 360/2003 of ANVISA, the TRANS fatty acid content of a food must be printed on the label when the level is greater than 0.2 g per food portion. Products containing less than 0.2 g/ per portion can be reported on the label as "zero TRANS." The aim of this study was to evaluate the amounts of saturated and "TRANS" fats in many groups of processed foods, and the adequacy of the disc...
Development of new ceramic membranes has recently grown due to its superior thermal and mechanical stability. An interesting approach to manufacture asymmetric membranes is the production of aligned pore structure by the freeze-casting method. The lack of studies involving membrane production with tubular freeze-cast substrates warrants more research. In this study, a novel tubular freeze-cast alumina substrate was used for deposition of a silica top layer. The substrate showed radially aligned pores, indicating precise structure control. The obtained pore structure shows high potential for membrane manufacture. The silica layer was produced by the sol-gel method and dip-coated on the substrates with two different withdrawal speeds. The microporous silica showed pores smaller than 2 nm. The highest withdrawal speed resulted in broader substrate coverage. However, a uniform silica layer was only obtained after a second deposition. These results confirm the viability to use tubular freeze-cast substrates for production of nanofiltration membranes.
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