Controlling ceramic porosity using carboxylate-alumoxane nanoparticles
AbstractThe formation of carboxylate substituted alumina nanoparticles, [Al(O) x(OH)y(O2CR)z]n, is an environmentally benign and inexpensive process. The alumoxane nanoparticles are synthesized from the mineral boehmite with carboxylic acids. The physical characteristics of carboxylate-alumoxanes, such as ceramic yield, particle size, pore volume, and pore diameter are dependent upon the carboxylate ligand that is substituted on the surface of the boehmite mineral. Of the synthesized alumoxanes, acetic acid-alumoxane (A-alumoxane), has the smallest particle size (11 nm), the highest ceramic yield (76%), and produces the smallest pore diameter (5 nm). These physical characteristics led to the investigation of the A-alumoxane ceramic precursor as a strengthening agent for porous alumina ceramics, and for the fabrication of ultra and nanofiltration ceramic membranes.
Infiltration is one mechanism used to deposit strengthening agents, such as ceramic precursors, into porous alumina ceramics. Due to the particle size associated with the alumoxanes, they can be easily infiltrated into 94 nm pore sized alumina ceramics, and upon thermolysis forming a homophase, Al2O3, ceramic composite. When the alumoxanes are doped with metals forming MxAlyOz upon thermolysis, a heterophase ceramic composite forms. The additional ceramic material (either a homo or hetero-layer) into the interior of the porous ceramic decreases the material's porosity; a decrease in porosity presumably increases the ceramic's strength.
The alumina derived from the thermolysis of A-alumoxane produces pore diameters between 5--17 nm, thus making the ceramic desirable for the use in ultrafiltration membranes. The small pore sizes associated with ultrafiltration membranes, hinders the flux through the resulting membrane. To improve the flux, the membrane layer should be as thin as possible without sacrificing the membrane's performance and rejection characteristics. Once the thinnest possible membrane is achieved, the flux can be further improved by altering the surface chemistry and/or the surface features of the membrane. Adjusting the surface pH by metal doping, or adding functional groups, and forming a hierarchical membrane were investigated to improve the membrane's performance