Effects of −OH Groups on Fe3O4 Particles on the Heterogeneous Coating in a Hydrous Alumina Coating Process

The effects of the −OH groups on Fe3O4 particles on heterogeneous coating in an aqueous process were studied. Three kinds of Fe3O4 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 Fe3O4 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 Fe3O4 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 Fe3O4 particles were more easily film coated by hydrous alumina, while the nontreated and base-treated Fe3O4 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.