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.

This work deals with the deposition of a thin layer of porous silica antireflective coating onto glass substrates. The films were deposited with different withdrawal speeds and heat-treated at 425°C for 30 minutes. The effects of heat treatment and film deposition rate on the films reflectance were evaluated. The diffuse reflectance was measured using ultraviolet-visible (UV-Vis) spectroscopy. Scanning electron microscopy (SEM) was used for microstructural evaluation of the films. The water contact angle upon the films surface was evaluated using a tensiometer and was based on the sessile drop technique. The mechanical characteristics of the films were evaluated by tape test and pencil hardness. The obtained sol-gel silica coatings were homogeneous and free of cracks. UV-Vis analysis of the glass substrate revealed a reflectance value of 3.86%, whereas the lowest reflectance value obtained for antireflective coatings was 2.72%. The contact angle measurement showed that, for all films, there was wetting of the film by water, characterizing them as hydrophilic. The adhesion of the films were 4B and the pencil hardness were 3H.

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ResumoEste trabalho trata da deposição de um nanofilme de sílica através do processo solgel na superfície de um substrato vítreo para sua aplicação em usinas térmicas de energia solar. Os filmes foram confeccionados pelo processo de revestimento por imersão utilizando-se um equipamento dip-coating e passaram por um tratamento térmico à 425°C durante 30 minutos. Na etapa de caracterização foram feitos ensaios de espectroscopia de fluorescência de raios-X, refletância difusa no UV-Vis, microscopia eletrônica de varredura e medidas de molhabilidade. As propriedades mecânicas foram avaliadas por teste de adesão da fita e dureza a lápis. Resultados dos ensaios de espectroscopia de fluorescência de raios-X confirmaram haver a deposição de sílica sobre o substrato. Foi obtida uma refletância difusa de 2,72% para o filme antirreflexivo. Imagens no MEV revelaram uma superfície lisa, uniforme e isenta de trincas. Medidas de molhabilidade mostraram que houve o molhamento dos filmes pela água, caracterizando-os como hidrofílicos. Os filmes apresentaram uma adesão de 4B e dureza de 3H. Os nanofilmes de sílica confeccionados apresentaram propriedades ópticas e mecânicas satisfatórias para a aplicação solar. Palavras-chave: Nanofilme de sílica; Revestimento antirreflexivo; Processo sol-gel; Energia solar. ANTIREFLECTIVE SILICA NANOFILMS PRODUCED BY THE SOL-GEL PROCESS FOR SOLAR APPLICATION AbstractThis paper deals with the deposition of silica nanofilms by the sol-gel process in a glass substrate for its application in solar power plants. The films were prepared by dip-coating process and heat-treated at 425°C for 30 minutes. The characterization was performed by X-ray fluorescence spectroscopy, scanning electron microscopy and wettability measurements. Mechanical properties were evaluated by adhesion tape test and pencil hardness. X-ray fluorescence spectroscopy has confirmed the silica deposition on the substrate. A diffuse reflectance of 2.72% was obtained to the antireflective films. Images in SEM revealed a smooth surface, free from cracks. The wettability measurements showed that, for all films, there was wetting of the film by water, characterizing them as hydrophilic. The films' adhesion was 4B and its hardness was 3H. The silica nanofilms showed good optical and mechanical properties for its application in solar area.