Abstract: The evolution of powder particle size, crystal structure, and surface chemistry was evaluated
for micron scale NiO powders subjected to impact milling with commonly employed milling additives:
methanol, Vertrel XF, and amorphous carbon. The effect of the different comminution protocols on
sintered body microstructure was evaluated for high temperature sintering in inert atmosphere (N2).
X-ray photoelectron spectroscopy showed that NiO powder surface chemistry is surprisingly sensitive
to milling additive choice. In particular, the proportion of powder surface defect sites varied with
additive, and methanol left an alcohol or alkoxy residue even after drying. Upon sintering to
intermediate temperatures (1100 ℃), scanning electron microscopy (SEM) showed that slurry milled
NiO powders exhibit hindered sintering behaviors. This effect was amplified for NiO milled with
methanol, in which sub-500 nm grain sizes dominated even after sintering to 1100 ℃. Upon heating
to high temperatures (1500 ℃), simultaneous differential scanning calorimetry/thermogravimetric
analysis (DSC/TGA) showed that the powders containing carbon residues undergo carbothermal
reduction, resulting in a melting transition between 1425 and 1454 ℃. Taken together, the results
demonstrated that when processing metal oxide powders for advanced ceramics, the choice of milling
additive is crucial as it exerts significant control over sintered body microstructure.
Keywords: nickel oxide; impact milling; sintering; densification; grain growth