Agglomeration naturally occurs in today's industrially important powders due to adhesion forces between fine particles. The characterization of these powder agglomerates, especially the determination of the strength of these agglomerates and the nature of effective bonding forces is critical for a series of industries. The successful dispersion of powders reproducibly often necessitates the elimination of agglomerates totally. This objective can only be accomplished by a careful characterization of agglomerates, the strength of bonds in between the primary units forming them in relation to the powder processing technique used for their generation.
Although it is known that the presence of strong agglomerates in powders is the main reason for a series of undesirable phenomena in post processing, there are no known techniques for the quantitative determination of the strength of agglomerates. Most prior studies employ models assuming monosize spherical particles forming the pellets or sizeable granules which are held together by relatively weaker Van-der Waals forces or liquid bridges. Generally the most common and adverse affects are due to solid bridging generated during sintering or dissolution/precipitation rather than these relatively weaker bonding mechanisms. Because it would be very hard to detect the presence of small fractions of strong agglomerates in such pellets, development of a characterization technique which focusses on single agglomerates in a powder would be very helpful. It has been shown that these types of bonding mechanisms effective in forming ceramic powder agglomerates are very critical in determining the powder's sintering behaviour (1,2). In those studies ultrasonic forces were utilized to break dispersed suspended agglomerates in solutions. These forces which are a result of the cavitation phenomena are able to break most agglomerates and have been utilized in powder dispersion for a long time.
It is of course natural that almost all of the powder characteristics can be traced back to the powder processing technique utilized for the preparation of the powder. The powder particles are mostly formed by precipitation of a solute from a liquid or by nucleation and growth in a vapor. A large number of pathways can be utilized for the precipitation of the precursors to the final material. Precipitation involves the phase change of a sparingly soluble solute at high levels of supersaturation where at least initially, high nucleation rates are favored. Techniques used for the transformation of the precipitated precursor to the final form through heat treatment along with various precipitation conditions ( reactant concentrations, temperature, pH, surface properties of the new phase, concentration levels of impurities and additives etc.) all may have significant effects on the state of agglomeration of the prepared powder, Agglomeration during the creation of the precursor phase depends on how well the nucleation and growth stages are separated. The way solvents are removed from these precursor precipitates and accompanying events like dissolution/reprecipitation have a determining role on the nature of bonds in the final agglomerates. Thus a study on the powder agglomerate characterization can not be complete without an effort to tie the ultimate properties back to the powder processing technique.
In our three year project, we plan to address the determination of agglomerate strength distributions for hard agglomerates via a, three step process. These are:
1) The demonstration of the validity of using hollow glass microspheres as a measurement of disruptive forces in an ultrasonic field.
2) The synthesis of model hard agglomerates with narrow and well- defined bond strength distribution.
3) The study of hard agglomerate formation in commercially significant powder processing schemes.
In the first year of this project, experimental work on the calibration of the effective ultrasonic forces on suspended particles in liquids have been completed. Strength distributions of hollow glass bubbles were determined by using a mercury porosimeter. Samples of these bubbles were suspended in water and ultrasonically treated at different energy output levels. Similar mercury porosimetry tests were done on the recovered treated glass bubbles. The strength distributions of the untreated and treated glass bubbles were compared with each other in order to reach an effective strength value for the specific energy output level. Model agglomerates formed from monosize submicron silica spheres were aged in basic solutions or heat treated at different temperatures to change the nature of bonding between the particles. Suspensions of these powder agglomerates were ultrasonically treated at different energy output settings and the agglomerate breakdown process was followed in situ by sedimentation type particle size analysis. The results of these changes in particle size distributions were combined with the results obtained during calibration studies to obtain strength values for the synthesized model agglomerates. These agglomerates were characterized by a range of methods.