Cobalt minimisation in willemite (CoxZn2−xSiO4) ceramic pigments

Abstract

Industrially available blue ceramic pigments, with the exception of vanadium-zircon blue, (Zr,V)SiO 4 , contain Co. To minimise the use of toxic and scarce cobalt, while also maintaining an intense blue colour, the performance as blue ceramic pigments of both Co 2+ -Zn 2 SiO 4 (willemite) and Zn 2+ -Co 2 SiO 4 (olivine) solid solutions was studied. The addition of cobalt in the willemite lattice was minimised. A wide range of compositions, Co x Zn 22x SiO 4 (x = 2, 1.95, 1.5, 1, 0.9, 0.5, 0.05 and 0), were prepared and characterised by X-ray diffraction, lattice parameter measurements, UV-VIS-NIR spectroscopy, CIE-L*a*b* colour measurements and electronic microscopy (SEM/EDAX). The results show that an intense and more blue ceramic pigment is obtained based on the willemite lattice, with an optimised, low Co content, ranging from 25 mol% Co (Co 0.5 Zn 1.5 SiO 4 ) to 2.5 mol% Co (Co 0.05 Zn 1.95 SiO 4 ). This last composition (with only 2.5 mol% Co) exhibits the best blue hue once enamelled (b* = 239), though it is also considerably lighter (L* = 57).

Raman microscopy study of synthetic cobalt blue spinels used in the field of art

“…In fact, to counter the high toxicity of cobalt compounds, intense blue pigments with less important amounts of cobalt (non-stoichiometric) were reported to have been recently manufactured under the form of Co-containing host minerals such as gahnite (ZnAl 2 O 4 ). [27] Gazulla et al [34] report that no less than a minimum of 40% Zn in weight is necessary for obtaining and detecting by XRD the mixed spinel form ZnCoAl 2 O 4 , whereas other studies reported that no difference is observed in the XRD pattern of ZnO in Co-doped ZnO minerals, even up to 12% Co addition. [18,35,36] As a matter of fact, while the XRD technique confirmed its limitation in distinguishing between Co/Al and Co/Al/Zn spinel pigments, the Raman spectra acquired on these pigments clearly showed additional features in the Zn-containing spinels and hence make the mineralogical identification and differentiation of these two kinds of pigments from now on possible.…”

“…In addition, the range of hues available for Co-spinels is much more diverse than for olivines. [27] The spinel-like mineral has a cubic structure (isometric), which may be expressed with the stoichiometry AB 2 O 4 consisting of a packed oxygen arrangement in which the cations reside in the octahedral and tetrahedral sites. A represents a divalent metal ion such as Co II , Mg II , Fe II , Ni II , Mn II or Zn II , whereas B represents trivalent metal ions such as Co III , Al III , Fe III , Cr III , Mn III or Ti III .…”

The vast colour spectrum of ternary metal oxynitride pigments

“…For example, Calbo et al minimised the Cr content and partially substituted Ni by Mg and Zn in the spinel type black pigment Ni(Fe, Cr) 2 O 4 [4]. Fores et al obtained the pigment Co 0.05 Zn 1.95 SiO 4 with an intense blue colour, which contains only 2.5% mol of cobalt [5]. Furthermore, Munoz et al minimised the amount of Cr in the green pigment formulation Cr 1.14 Al 0.86 O 3 [6].…”

Exploring the Colour of 3d Transition‐Metal Ions in Trigonal Bipy­ramidal Coordination: Identification of Purple‐Blue (CoO5) and Beige‐Red (NiO5) Chromophores in LiMgBO3Host

“…CIE chromaticity diagram ( Figure 7 and [20] Whereas a number of Co II oxides in tetrahedral coordination are known (for example spinel CoAl 2 O 4 , and Zn 1-x Co x Al 2 O 4 , [28] willemite Co x Zn 2-x SiO 4 , [29] Co 2 SnO 4 , [30] and Co-doped ZnO [31] ), which are blue in colour (except the latter, which is green), very few Ni II oxides in tetrahedral coordination are known: Ni-doped β-Al 2 O 3 , [20] Ni-doped hibonite (CaAl 12 O 19 ) [32] and BaZn 2-x Ni xSi 2 O 7 , [20] to our knowledge, displaying green, turquoise, and red colours.…”