Rheological and surface chemical characterization of alkoxysilane treated, fine aluminum powders showing enhanced flowability and fluidization behavior for delivery applications

• Aluminum powders with 4 different particle size distributions were surface modified.
• Surface chemical characterization showed retention of a siloxane-like layer.
• Bulk property analysis demonstrated enhanced flow behavior in all cases.

The effect of surface treatment on the properties of fine aluminum powders was investigated using both chemical and rheological characterization techniques. Four different particle size distributions (PSD) were surface treated with methyltriethoxysilane and subsequently evaluated through classic density measurements and by using an FT4 powder rheometer (Freeman Technologies, UK). The chemical surface properties were analyzed using diffuse reflectance infrared spectroscopy (DRIFTs) and X-ray photoelectron spectroscopy (XPS) which confirmed retention of the silane and production of a siloxane-like layer on all PSD samples. The results of the bulk measurements, both direct and empirical, showed enhanced flowability and easier fluidization, indicating reduction of intrinsic cohesion for all samples. Fluidization testing showed decrease in total energy of the uniformly fluidized bed by ~ 80–90% in all cases. Shear stress values as a function of applied normal stress were collected to produce Mohr diagrams which generated extrapolated cohesion and unconfined yield strength values using the Mohr–Coulomb criteria. Cohesion and unconfined yield strength were reduced in all surface treated powders which indicated easier flowability and an increase in the flow function at 6 kPa. The compressibility of the treated powders at 8 kPa was decreased by > 60% in all cases, indicating significant reduction in entrained air and improvement in packing density. Classical density measurements showed small improvements in the Hausner Ratio and little to no improvement in the Carr index, whereas directly measured properties using powder rheology showed substantial changes in the flow and fluidization properties. Overall, fluidization behavior and flowability properties were enhanced in all PSDs and showed the increased potential for the use of fine aluminum powders after surface treatment in delivery applications.

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