In situ formation of coloured M(II)–doped Zn2SiO4–willemite in ceramic glazes (M=Mn, Co, Ni, Cu)

Abstract

The family of M(II) substituted ZnO–wurtzite was investigated as potential new ceramic pigments. When MxZn1−xO or (MM׳)xZn1−xO with (M/M′=Mn, Co, Ni, Cu) oxides are incorporated to industrial frits the development of new colour hues is observed. This is attributed to the in situ formation of coloured M(II)–doped Zn2SiO4–willemite in ceramic glazes. The pigments were prepared by solid state reaction through the traditional ceramic route. Prior to enamelling, the materials were characterized by XRD analysis and FTIR spectroscopy. The colour performance and technological properties of the enamels were evaluated by enamelling in batches containing 5 wt% with a transparent frit, representative of single firing industrial procedures, and submitting them to thermal treatment at 1000 °C. The colours obtained, once the glazes were formed are attributed mainly to the formation of crystalline Zn2SiO4 within the glassy matrix, when chromophore M(II) ions enter into the tetrahedral coordination Zn(II) site available in willemite. The colorimetric CIELab parameters measured are in good agreement with the structural and spectroscopic analyses. The colour of some glassy coatings was interpreted by analysing the reflectance UV–visible electronic spectra of the samples. This study demonstrates that this family of pigment is capable of forming its own Zn-based crystals in a conventional transparent ceramic frit giving rise to a wide palette of colours. When Mn, Co, Ni and Cu are present in the Zn(II) site light brown, deep blue, pale yellow and green, are achieved respectively. Furthermore the simultaneous doping with a combination of MM′ results in various nice new shades of colours, enlarging the palette of green, yellow, brown and blue hues. The homogeneity of the enamels was confirmed by SEM–EDX microscopy. These materials are promising for ceramic applications since a low amount of pigment would be necessary to achieve intense colouration.