Investigations of the correlation between granule structure and deformation behavior

For the improved handling and processing of fine and nano-sized materials in ceramic industries a granulation step is required. The processability of granulated material is characterized by flowability and compactibility. Microstructure in combination with deformation behavior affects the residual defects within pressed products and with this the quality of sintered compounds. For applications like redispersion or dissolving of granulated material, the influence of microstructure is also known. This study focuses on evaluation of different methods for structure characterization and their applicability on defined generated ceramic granule structures. The determined structure parameters were used for the interpretation of measured resulting mechanical properties of the spray-dried granules. Alumina oxide granules with different internal structures (hollow and homogeneous) were produced by spray drying. The correlations between granule microstructure and deformation behavior were investigated using a single granule strength tester. To interpret correlations it is necessary to characterize the internal granule structures by an objective and quantitative method. For characterizing the internal total porosity several techniques (mercury intrusion porosimetry, calculation of granule density including pores and image analysis of cross-sectional images) were compared. Additionally the promising technique of image analysis of computer tomography scans is also investigated regarding applicability for granules smaller 160 µm. As the total porosity is not sufficient for a definite internal structure characterization a further parameter H defining the distribution of solid material over the cross-sectional area is introduced. The internal granule structures are analyzed following the described method. Single granule deformation tests were done with several size classes of granules to investigate correlations between structure, size and deformation behavior. Hollow granules seem to show a different deformation behavior compared to homogeneous ones as their force increases much steeper until reaching the higher value fracture point. Also the deformation behavior of hollow granules after the fracture point is different from homogeneous ones; the force drops impressive in some cases even to force zero (brittle) whereas the force decreases gentler for homogeneous samples (ductile)