Hydrothermal synthesis of high refractive index thin films from chromium slag

Chromium slag is a hazardous waste, and its high-value-added comprehensive utilisation would be advantageous. High refractive index thin films were prepared on glass substrates by a hydrothermal method from chromium slag with different mineralisers. The samples were characterised by inductively coupled plasma, X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, an atomic force microscope, a transmission electron microscope and by film thickness and refractive index. The results show that a few Fe₂O₃, FeOOH, Al₂O₃, AlOOH or MgO crystals are formed, and most compounds of iron, aluminium, magnesium and chromium are still in amorphous state. The film materials are combined with a glass substrate by a chemical bond. The coordination ability of mineraliser anion and metal ion affects the sediment reaction rates. With NaClO₃ or NaHCO₃ as a mineraliser, the film surface is a perfect three-dimensional space orientation network sediment with fine twines and small pores. The thinner the film, bigger the refractive index will be. The films´ refractive index is mainly related to the film structure and non crystalline phase.