Surface Modification of Magnetite by Chemical Vapor Deposition of 1,3,5,7-Tetramethylcyclotetrasiloxane and Its Effect for Calcination

Abstract

1,3,5,7-Tetramethylcyclotetrasiloxane (H₄) was deposited on magnetite at 80°C by utilizing a chemical vapor deposition method. As a result of the deposition, the magnetite was covered with a 1.4 nm thickness of poly (methyl siloxane) (PMS) and became hydrophobic. Magnetite treated with H₄ (PMS-magnetite) was calcinated at below 1000°C and its color was measured.
Although magnetite turned brown at about 200°C and to red-brown at about 500°C, the color changes of PMS-magnetite occurred at higher temperatures.
The reason for the color change was investigated by TG-DTA and X-ray diffraction. In the color change at about 200°C, an exothermic weight increase was observed, which suggests the change of magnetite to maghemite. On the other hand, the color change at about 500°C is attributable to. crystal dislocation from maghemite to hematite. Thus, the oxidation of magnetite and the crystal dislocation of maghemite were depressed by the coating of the PMS film. These results show that the surface definitely played an important role even in crystal dislocation which is regarded as the transformation of the entire crystal itself.