Production and Characterization of a Silica-Alumina Membrane Using Novel Tubular Freeze-Cast Substrates

Abstract

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.

Microstructure Evolution During the Sintering of Freeze-Cast Alumina

“…The alumina freeze-cast tubes produced in this study have already been reported in previous studies describing the structure of the macropores produced by freeze-casting and its influence on the mechanical behavior 21,47 , as well as for production of membranes 14,57 . Representative micrographs of the radially aligned porosity are reproduced for reader's ease in Figure 1a, with pore sizes ranging within 5-10 µm 57 .…”

“…The alumina freeze-cast tubes produced in this study have already been reported in previous studies describing the structure of the macropores produced by freeze-casting and its influence on the mechanical behavior 21,47 , as well as for production of membranes 14,57 . Representative micrographs of the radially aligned porosity are reproduced for reader's ease in Figure 1a, with pore sizes ranging within 5-10 µm 57 . The microstructure exhibiting the grains after sintering is also reproduced in Figure 1b, c for each sintering temperature used in this study, with average grain sizes of 237±44 nm, 250±34 nm and 500±105 nm for the samples sintered at 1300 °C, 1400 °C and 1500 °C, respectively 21 .…”

Mechanical Behavior of Tubular Freeze-Cast Substrates with Organized Pore Structure

“…The highly organized pore structure of the tubular freezecast substrates has already been discussed in other reports previously published and is only briefly reproduced in this study for reader's ease 17,26 . The substrates showed radially aligned pores with lamellar geometry (Figure 1a, b), typical of freeze-cast substrates produced from suspensions using water as solvent.…”

“…The production method of the tubular substrates is already reported elsewhere 17,26 . In summary, ceramic suspensions were prepared with alumina concentrations varying within 10-30 vol%.…”

“…The use of tubular substrates for separation applications usually involves pressure-driven processes, in which the substrates are immersed in a fluid and a pressure difference between the external and internal surfaces is applied. For instance, ceramic substrates used for membrane manufacture must be stable enough to endure pressures up to 10 MPa 17 . Therefore, assessment of the mechanical behavior of ceramic substrates is critical for safe and widespread commercial use.…”