INTRODUCTION
Agglomerate strength is an important property in a number of industries. In some processes agglomerates may limit the strength and unitormity of final or intermediate products, thus requiring nearly total breakdown of agglomerates. Conversely, agglomerates may be required to enhance flow ability and thus the agglomerated product must be capable of resisting breakage during the handling and prrxessing stages.
In processing of ceramic powders, there are some type of agglomerates which are more detrimental than others. It is generally accepted that weak or “soft” agglomerates i.e. which disintegrate during green forming of compacts, do not impede densification. On the other hand strong or “hard” agglomerates are not broken down during compaction and can lead to incomplete densification and/or strength limiting flaws.
Hard agglomeration often results during certain stages of powder processing (such as filtration and drying) when the dispersed particles are subject to large hydrostatic or capillary forces which overcome the repulsive inter-particle forces. Under these conditions, a number of reaction pathways can lead to the formation of metal-oxygen-metal bonds between these individual primary particles. For small particle size, this can lead to a dissolution-reprecipitation process removing material from the particle surface and depositing it in the toroidal region at the particle-particle contact. Once these oxide bridges form, it is difficult to disperse (or break the agglomerate bonds) the resulting hard agglomerate by either chemical or mechanical means.
The inter-particle forces that can contribute to the cohesive strength of agglomerates can be separated into those which act independently of the metal- oxide bridges and those that are a result of such bridges. The Van&w&s force, which is always present, an electrostatic force and a magnetic force are examples of the former, while the force due to metal-oxygen-metal bond and a meeha,nica.l force that arises from the interlocking of irregularly shaped particles are examples of the latter.
The problem with the rational design of powder processing/treatment to avoid oxide bridging and subsequent hard agglomerate formation is the lack of knowledge concerning the kinetics and equilibrium of the various reactions associated with powder processing. These reactions depend upon a number of processing variables, including the Metal (M), the particle size, the degree of condensation/coordination of M, saturation level (defined as volume of liquid per unit void volume), temperature (i.e. drying conditions), contact angle, surface tension, pH, and the alkyl group, R.
In our previous IFPRI project it was demonstrated that the addition of a alcohol (EtOH) wash step during titania processing was able to reduce the formation of hard agglomerates. Although various alcohols have been used previously to control agglomeration [1,2], little work has been reported on the mechanism(s) involved. It has been suggested [2] that the formation of surface ethoxy groups during alcohol washing has a direct influence on the strength of, resulting agglomerates.
The present study deals with the effect of various parameters including alcohol washing on the formation of oxide bridges during powder processing. Changes in both the chemical and physical structure of both titania and silica during processing will be assessed using low field NMR, high field MAS NMR, small angle x-ray scattering, TGA, FTIR, particle size analysis, and agglomerate strength distribution. These changes will be correlated with the dispersibility of the final dried powder and the processing conditions.