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

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.