Hydrothermal Synthesis of Nanodispersed “Iron Mica” from Yellow Iron Oxide Pigment

Abstract

The temperatures within the 150–200 °С interval of preliminary heat treatment of yellow iron oxide pigment (the a-FeОOН phase) influence the disperse composition of “iron mica” (the a-Fe₂O₃ phase with platelet crystals), which is formed at hydrothermal treatment (230±5 °С) of the samples of yellow iron oxide pigment in aqueous solution of potassium hydroxide with 3.5 M concentration. It has been found that increasing temperature of preliminary heat treatment from 150 to 200 °С leads to decreasing sizes of coherent scattering regions, calculated from the X-ray phase analysis data, as well as the mean size of the d crystals of “iron mica” (in the plate plane), observed with the use of scanning electron microscope, by ~ 2 and ~ 20 times, respectively, compared to the samples without preliminary heat treatment. Significant change in the form of the crystals of initial (a-FeOОН has needle-like crystals) and resultant phases (a-Fe₂O₃ has platelet crystals) during phase transition testifies that the transformation of a-FeOОН
into a-Fe₂O₃ proceeds according to the “dissolution – precipitation” mechanism, by way of dissolution of the phase that is nonequilibrial at the given thermodynamic conditions (a-FeOОН), formation of nuclei and growth of the equilibrium phase crystals (a-Fe₂O₃). The conclusion has been made concerning formation of the a-Fe₂O₃ nuclei on the surface of the yellow iron oxide crystals in the process of preliminary heat treatment. The number of the formed nuclei is greater at higher temperature t and duration t of the preliminary heat treatment. These parameters (t and t) let intentionally vary the number of the α-Fe₂O₃ nuclei, and, consequently, the disperse composition of “iron mica”.