Herstellung und Charakterisierung hoch poröser nanostrukturierter Filtermembranen

Highly porous, nanostructured filter membranes were investigated and their properties characterized. These nanomembranes promise to minimize adhesion forces of strongly adhesive dusts to the membrane surface. This is achieved by reducing the grain sizes of the nanomembranes and by making available a few contact points on the nanomembrane only. The nanomembranes were produced in the form of a coating by dispersion, drying, and filtration of nanoscaled ceramic Al2O3 or TiO2 membrane grains from suspensions on porous, ceramic substrates. The suspensions were characterized by means of dynamic and static light scattering. The size distributions of the dispersed membrane grains were measured in the gas phase by a DMPS (differential mobility particle sizer) or by a laser scattering light spectrometer. Following filtration, the nanomembranes were sintered for mechanical solidification. The structures of non-sintered and sintered nanomembranes were characterized by scanning electron microscopy. To produce the cross-sections, the structures of the nanomembranes were first fixed with cyanacrylate in the gas phase and then embedded in a cast resin. After hardening of the cast resin, the cross-sections were cut, polished, and smoothed or etched by ion thinning. Microscopies of the nanomembrane surfaces and cross-sections were made using an ESEM (environmental scanning electron microscope). From these microscopies, the grain size distributions, pore size distributions, porosities, and layer thicknesses of the nanomembranes were determined by image analyses. Build-up of nanomembranes was investigated using two different Al2O3 suspensions. The suspensions had a mean grain size of 98 nm and 155 nm, respectively. The layer thickness of the nanomembranes was observed to increase linearly with the filtration time. The filtration rate was proportional to the growth rate of the nanomembranes. Using the 155 nm membrane grains, this increase was much steeper compared to the membrane grains of 98 nm in size. It was also studied whether the diffusion deposition of the membrane grains could be influenced at filtration rates between 2.5 cm/s and 8.75 cm/s and whether it was possible to control the filtrated membrane structure in this way. For the 155 nm large membrane grains, permeabilities, pore sizes, and porosities were found to decrease slightly with an increasing filtration rate. At a mean grain size of 98 nm, no significant influence of the filtration rate on the structure of the filtrated nanomembranes could be measured. The nanomembrane with a mean grain diameter of 98 nm had porosities around 0.86 and mean pore sizes of 200 nm. The Al2O3 nanomembranes with a mean grain diameter of 155 nm had slightly higher porosities of 0.94 and pore sizes of 300 nm on the average. The non-sintered nanomembranes are extremely fragile. By sintering, the nanomembranes gain strength, but their structure could change significantly. The sintering conditions of the Al2O3 and TiO2 nanomembranes were derived from literature values and dilatometries of compacted Al2O3 and TiO2 nanoparticles with a mean grain size of 155 nm and 196 nm, respectively. The TiO 2 nanomembranes sintered at 1000 °C already with a dwell time of 5 hr. Al 2 O 3 exhibited an increased sintering activity at sintering temperatures of 1130 °C and 1250 °C over a sintering duration of 10 hr.