Naslov
Sol-gel synthesis and characterization of cobalt-based nano-pigments

Autori
Kurajica, Stanislav

Vrsta, podvrsta i kategorija rada
Sažeci sa skupova, sažetak, znanstveni

Izvornik
XXII. Croatian meeting of chemists and chemical engineers, Book of Abstracts / Tomašić, Vesna ; Maduna Valkaj, Karolina - Zagreb : CSCE/Chemistry in industry, 2011, 16-16

ISBN
978-953-6894-42-0

Skup
XXII. Croatian meeting of chemists and chemical engineers

Mjesto i datum
Zagreb, Hrvatska, 13.02.2011. - 16.02.2011

Vrsta sudjelovanja
Pozvano predavanje

Vrsta recenzije
Nije recenziran

Ključne riječi
sol-gel; cobalt; nano-pigment

Sažetak
Majority of blue ceramic pigments are based on cobalt ion. Beside ceramics, cobalt-based pigments are widely used for the coloration of plastics, paint, fibers, paper, rubber, glass and cement. Cobalt and its salts are widely considered to be toxic and environmentally hazardous substances. Also, the scarcity of Co in the earth and its difficult metallurgy make the use of cobalt raw materials expensive. Additionally, some cobalt-based pigments are unstable in high-temperature oxidizing environment. The use of nano-particles can improve the pigment performance. Being smaller than the wavelengths of visible spectrum, nano-particles produce no scattering in the visible-light range, enabling transparency. Therefore, in order to minimize the use of cobalt in the preparation of ceramic pigments, enhance their thermal and chemical stability and obtain nano-crystalline pigments, mullite (Al4+2xSi2-2xO10-x) and gahnite (ZnAl2O4) were investigated as a host lattice for cobalt ions. In the course of preparation of nano-crystalline powders sol-gel method was used because of its advantage of producing pure, homogeneous and ultra fine powders at low temperatures and short calcinations times. The obtained as-received, as well as thermally-treated powders were investigated using differential scanning calorimetry (DSC), X-ray diffraction (XRD) followed by Rietveld structure refinement, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and UV–VIS diffuse reflectance spectroscopy (DRS). The course of thermal reactions in premullite powder containing small amount of cobalt is dominated by the intermediate formation of two faint crystallized phases. The former phase was attributed to cobalt-containing spinel structure and the latter to Al–Si spinel. With temperature increase Co-containing spinel transforms progressively in CoAl2O4, while Al–Si spinel forms mullite above 1100 ◦C. Mullite lattice parameters, Rietveld refinement data and the CoAl2O4/Co2+ ratio in annealed samples suggest that the majority of cobalt is incorporated in CoAl2O4 and only about 0.6 at.% enters mullite structure or the glassy phase, or both. On the other hand, in the course of thermal evolution of cobalt doped powders with gahnite stoichiometry, nano-crystalline spinel powders were produced by gel heat treatment at temperatures as low as 400 °C. The green color of samples thermally treated at lower temperatures has been explained as a consequence of partial oxidation of Co2+ ions and accommodation of Co3+ ions in octahedral sites. Thermal treatment at higher temperatures promote change of color from green to blue, characteristic of Co2+ ions in tetrahedral coordination. The spectra evolution could be interpreted as a progressive reduction of Co3+ to Co2+ ions at higher temperatures. Beside calcinations temperature, the appearance of Co3+ and Co2+ ions, as well the distribution of Co2+ ions in tetrahedral and octahedral sites of zinc cobalt aluminate crystal lattice depend on Co loading.

Izvorni jezik
Engleski

Znanstvena područja
Temeljne tehničke znanosti

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