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Thermal evolution of sol-gel derived cobalt aluminate spinel

Cobalt compounds range in color from pink to green and blue depending upon the crystal lattice, oxidation state and coordination of cobalt ion. One of the most common crystalline structure used as intense blue ceramic pigment is cobalt aluminate, CoAl2O4, known as Thénard blue. It belongs to ternary oxides of the general formula AB2O4 which possess a cubic spinel-type structure in the space group Fd-3m. The aim of this work was to characterize the influence of processing temperature on oxidation state and coordination of cobalt in the sol-gel derived cobalt aluminate spinel. Precursor for cobalt aluminate was prepared from aluminum-sec-butoxide and cobalt(II) nitrate hexahydrate by a sol-gel technique. It was grinded to fine powder and divided in 5 samples which were annealed in the furnace at 500, 600, 700, 800 and 900 °C for 2 h respectively, then slowly cooled to RT. Annealed samples were characterized by means of X-ray diffraction and UV–Vis diffuse reflectance spectroscopy. The samples annealed at T<700°C were green, while for samples annealed at higher temperatures the color changed to blue. All annealed samples contained the spinel-type crystal phase and amorphous phase, the quantity of latter decreased with the increase of annealing temperature. UV-Vis spectra revealed that in the samples annealed below 700 °C a part of cobalt existed as Co3+. The sample annealed at 600 °C contained 40.91 wt% of amorphous phase. In annealing above 600 °C, the CoAl2O4 phase started to form by solid state reaction between Co2AlO4 and amorphous aluminum oxide. The inversion parameter increased with the increase in annealing temperature. The increase in delta reflects in the decrease of oxygen positional parameter u, indicating on the expansion of the octahedral site volume and simultaneous decrease of the tetrahedral site volume. XRD line broadening analysis was also performed since the crystallite dimension is of fundamental importance for materials application as coloring agents. The crystallite size increased with the increase in annealing temperature, from 10.8 nm for annealing at 500 oC to 50.3 nm for 900 oC.

Cobalt aluminate; Spinel structure; X-ray powder diffraction; Rietveld method

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