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Significance statement
Ni-MgTi2O5 pseudobrookites (Mg1-xNixTi2O5; x=0-1.0) were investigated in detail for the first time aiming to develop new yellow-orange ceramic pigments or dyes, under rapid (industrial-like) firing conditions. Interestingly, Ni solubility increased with firing temperature due to enhanced configurational entropy (40 and 60-70 mol% at 1200 and 1400ºC, respectively). Fired powders exhibited intense yellowish-orange colors, increasing the color intensity and chroma (red and yellow hues) with Ni doping up to x=0.4-0.5 (L*/a*/b* = 61.8/25.2/49.3 for x=0.4 at 1400ºC). Considering Ni toxicity or environmental criteria, relatively intense orange hues may be also obtained with a much lower Ni doping (L*/a*/b* = 73.0/15.7/44.1 for x= 0.1). As for its technological application, Ni-karrooite pigments were found only partially stable within the tested low temperature (1000-1050ºC) ceramic glazes; however, glazed samples developed nice yellowish colorations comparable to those obtained with Ni-rutile ceramic pigments, and therefore Ni-karrooite powders could be used as alternative yellowish ceramic dyes. Also noteworthy, the employment of an alternative aqua-based citrate-gel route, instead of the classical ceramic route, yielded to the formation of Ni-karrooite at lower temperatures (1000ºC, 20 mol% Ni) and produced finer-grained powders, although the resulting yellow-orange colors were very similar to those of the ceramic route.
Journal Reference
Journal of the European Ceramic Society,Volume 35, Issue 1, 2015, Pages 357–376. M. Llusar, , E. García, M.T. García, C. Gargori, J.A. Badenes, G. Monrós.
Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, Avda. Sos Baynat s/n, 12071, Castellón, Spain.
Abstract
Solids solutions of Ni in MgTi2O5 pseudobrookite (karrooite) were investigated to develop new yellowish ceramic pigments or dyes. Ni-Karrooite samples (Mg1−xNixTi2O5) were prepared by ceramic method and through a citrate-gel metalorganic decomposition route, using rapid firing conditions (800–1500 °C). Fired pigments were characterized by XRD, SEM/EDX, UV–vis–NIR and color measurement (CIE-L*a*b*) techniques. The entropy-stabilization of MgTi2O5 with respect to ilmenite and rutile decreased with Ni doping, and Ni solid-solubility increased with temperature (40 mol% at 1200 °C, 60 mol% at 1400–1500 °C). Optical absorptions of Ni2+ in karrooite octahedral sites (mainly M1 sites) produced intense yellowish-orange colors, increasing saturation and red/yellow hues with Ni doping (up to x = 0.4–0.5). The pigments showed a limited stability within low-temperature (1000–1050 °C) ceramic glazes, although glazed samples developed intense yellowish colorations. Noteworthy, metalorganic decomposition route enabled the formation of Ni-karrooite at lower temperatures (1000 °C, 20 mol% Ni) and produced finer-grained powders, although the colors of powders and enameled samples were similar.
