Effects of −OH Groups on Fe 3 O 4 Particles on the Heterogeneous Coating in a Hydrous Alumina Coating Process

The effects of the −OH groups on Fe 3 O 4 particles on heterogeneous coating in an aqueous process were studied. Three kinds of Fe 3 O 4 particles, nontreated, base-treated, and microwave hydrothermally treated, with different amounts and nature of −OH groups on their surfaces were used in a hydrous alumina coating process. The −OH groups on the Fe 3 O 4 particles were characterized by TGA, FTIR, and base titration. The relative amounts of −OH groups were in the order microwave hydrothermally treated > nontreated > base treated. The microwave hydrothermally treated Fe 3 O 4 particles were basic at pH <7.7 and acidic at pH >7.7, while the nontreated and base-treated particles did not show clear acidic or basic nature. HRTEM images showed that the microwave hydrothermally treated Fe 3 O 4 particles were more easily film coated by hydrous alumina, while the nontreated and base-treated Fe 3 O 4 particles gave rise to more homogeneous precipitation of hydrous alumina under the same condition. It is inferred that the precipitation and reaction occurred at surface hydroxyl sites in the coating process.