|
Tubular alumina porous support systems for nanostructure ceramic-composite membranes were prepared by gel-casting method. The support systems made in this work, have an open porosity about 48% and pore size in the range of 100â€"600 nm. Due to important role of the supports surface modification in the nanocomposite-ceramic membranes performance, the outer surface of supports was modified via dip-coating of submicron and nano î±-alumina powder (synthesized via novel methods in our lab) slurries respectively as intermediate layers. Also, the effects of several parameters such as: solid content, dipping time, vacuum pressure, heating rate and number of coated layers on intermediate layers microstructure and morphology were investigated. Experimental results showed that using vacuum at the first coating step causes undesired penetration of submicron particles in pores of the support systems, so that the more vacuum pressure, the more powder penetration. Therefore, the optimum routine for modification was obtained as twice coating of 5 wt.% submicron slurry without applying vacuum and a dipping time of 30 s for each stage followed by vacuum dip-coating (0.2-0.3 bar) of 5 wt.% submicron slurry for 15 s. Finally, the supports surface was modified by the same vacuum dip-coating conditions of 1 wt.% alumina nanoparticles slurry. The heating rate for all stages was 1 °C/min up to 850oC for the slow elimination of polymeric binder and dispersant followed by temperature increasing at a rate of 5 °C/min up to 1350 °C. Then the articles were maintained at this temperature for 2 hours and cooled slowly to ambient temperature. The optical microscope micrographs showed that applying the above mentioned optimum routine results in uniform modified surfaces and a dramatically reduction in rough surface of supports. In addition, SEM images confirmed that the final modified surfaces were crack-free and intermediate layers with appropriate thickness (20-100 î¼m) were obtained. |