Effect of post heating on the phase transformation of FeCr2O4 to (Fe,Cr)2O3 solid solution pigment powders

Iron chromite was synthesized via solution combustion route using Iron (III) nitrate nonahydrate and Chromium (III) nitrate nonahydrate as starting materials, as well as Glycine-Urea, Glycine-Citric acid, and Glycine-Ethylene glycol mixtures as fuels. The effect of post heating at different temperatures on the structure, molecular, microstructure and chromatic properties of powders was studied. The X-ray diffraction (XRD) patterns showed that as synthesized powders were obtained in crystalline FeCr2O4 phases Moreover, post heating of the powders led to d-space shift and oxidation and formation of (Fe,Cr)2O3 solid solution phase regardless of fuel type. Phase transformation of FeCr2O4 to (Fe,Cr)2O3 solid solution was observed at 500/750°C depending of dominant phase of as synthesized particles. Fourier transform infrared (FTIR) analysis illustrated that band position of octahedral M-O and tetrahedral M-O bonds were shifted due to Fe cations movement from their position and lattice shrinkage by increasing of post heating temperature. Moreover, scanning electron micrographs showed that Fe0.7Cr1.3O3 semi spherical fine particles were formed from porous spongy FeCr2O4 particles due to oxidation and phase transformation during the post heating. Furthermore, chromatic properties of the samples were represented. The color properties of the pigments showed that the formation of brown pigments is provided with the phase transformation from FeCr2O4 to (Fe,Cr)2O3 in the temperature range of 750 °C. Also, increasing the color purity to this temperature is related to the removal of residual carbonaceous matters. The best sample prepared at this temperature has the following specifications: a*= 8.81, b*= 8.94 L* = 23.7, which can be considered as a suitable candidate for brown ceramic pigment.