Manufacture of Highly Porous Tubular Alumina Substrates with Anisotropic Pore Structure by Freeze‐Casting

Abstract

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

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

“…The spatial distribution allowed an analysis of the regions where tensile stresses are prevalent, as well as regions with compressive stresses. The tensile stresses found were slightly higher than the ones obtained by Athayde et al 26 . In addition, the honeycomb model was applied to the freeze-cast alumina and proven adequate in predicting the tensile stresses for material failure with tubular geometry.…”

“…Experimental data used for the modeling in this study were already reported by Athayde et al 26 , specifically: porosity by Archimedes' method 27 and pore diameter by mercury intrusion porosimetry (these results were reproduced in the Appendix, Table A1 and Table A2, respectively). In addition, stereological analyses of the scanning electron microscopy (SEM) micrographs (Figure 1a, b) by the mean linear intercept method 28 were also used.…”

“…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 maximum tensile stresses were analyzed varying processing parameters (solid loading of the initial ceramic suspension and the sintering temperatures) in order to achieve the substrate with most adequate mechanical stability. The higher crushing strengths were obtained for higher values of both the solid loading and the sintering temperature, due to the effect that the processing parameters showed on structural properties (pore and grain sizes) 26 .…”

“…In previous work 26 , the freeze-cast substrates with radially aligned porosity were submitted to O-ring compression tests and the crushing strengths of such tubular substrates were analyzed by the Martinez's model 18 . The maximum tensile stresses were analyzed varying processing parameters (solid loading of the initial ceramic suspension and the sintering temperatures) in order to achieve the substrate with most adequate mechanical stability.…”