Raising cost, limited reserves and toxicity make a pressing need to reduce the consumption of cobalt in the ceramic industry, trying to improvefficiency and sustainability of pigments [1,2]. A novel blue colorant, based on the melilite structure, has been developed by searching for a ceramic pigment stable in very aggressive media, like the calcium- and zinc-rich glazes used in porous tiles (stoneware and monoporosa). Melilite-like compound crystallizes in the tetragonal system (s.g. P-421m), with general formula X2T1T22A7, where X is a typically a large monovalent to trivalent cation (e.g. Na+, Ca2+, Y3+), T1 and T2 are small divalent to tetravalent cations (e.g. Zn2+, Mg2+, Al3+ in T1; Si4+, Al3+ in T2); and A is O2-, but also F-, S2- or N3-. Its structure is characterized by layers of linked tetrahedra (T22O7 dimers and four-fold coordinated T1 cations) connected by X cations in a distorted eight-coordinated antiprismatic site. Tetrahedral site T1 represents a quasi-regular environment with 4 identical metal-oxygen lengths, but one pair of O-M-O angles different from the other two [3]. Hardystonite, Ca2ZnSi2O7, was selected as a typical melilite-like compound which has just one four-fold crystallographic site where Co2+ ions can be accommodated, thus ensuring its unrivalled blue color. Five samples (Ca2Zn1-xCoxSi2O7 with x = 0.05, 0.1, 0.2, 0.3 and 0.4) were prepared by solid state synthesis in industrial-like conditions (95% yield) and characterized by XRD, DRS, SEM-EDS and technological testing. Increasing cobalt doping gives rise to a gradual expansion of the hardystonite unit cell, unexpected on the basis of Zn2+ and Co2+ ionic radii, attributed to a change of the covalent character of M-O bonding. Optical spectra are dominated by the strong absorption bands of Co2+ in tetrahedral coordination (crystal field strength Dq = 421 cm-1, Racah B parameter = 793 cm-1). The best compromise between cobalt concentration and optical response was found to be around x=0.3. The hardystonite pigment bestows a deep blue color on glazes and glassy coatings, withstanding to aggressive media rich in CaO and ZnO better than industrial blue pigments (cobalt aluminate, spinel) with similar color efficiency than industrial blue dyes (cobalt silicate, olivine) but with the advantage to avoid specking defects of highly staining colorants. Furthermore, exploring the wide range of isomorphic substitutions allowed in the melilite structure, a comparison through a set of compounds with a fixed cobalt fraction (Co2+ = 0.3 apfu) in the T1 site (Ca2MgSi2O7, Sr2MgSi2O7, Ba2MgSi2O7, and Sr2ZnSi2O7) was attempted. References. [1] R. Eppler, (1987) Am Ceram Soc Bull, 66, 1600. [2] M. Llusar, A. Fores, J. Badenés, J. Calbo, M. Tena, G. Monros, (2001) J Eur Ceram Soc, 21, 1121. [3] L. Bindi, M. Czank, F. Röthlisberger, P. Bonazzi, (2001) Am Mineral, 86, 747.
Co-doped hardystonite, Ca2(Zn,Co)Si2O7, a new blue ceramic pigment
ARDIT, Matteo;CRUCIANI, Giuseppe
2010
Abstract
Raising cost, limited reserves and toxicity make a pressing need to reduce the consumption of cobalt in the ceramic industry, trying to improvefficiency and sustainability of pigments [1,2]. A novel blue colorant, based on the melilite structure, has been developed by searching for a ceramic pigment stable in very aggressive media, like the calcium- and zinc-rich glazes used in porous tiles (stoneware and monoporosa). Melilite-like compound crystallizes in the tetragonal system (s.g. P-421m), with general formula X2T1T22A7, where X is a typically a large monovalent to trivalent cation (e.g. Na+, Ca2+, Y3+), T1 and T2 are small divalent to tetravalent cations (e.g. Zn2+, Mg2+, Al3+ in T1; Si4+, Al3+ in T2); and A is O2-, but also F-, S2- or N3-. Its structure is characterized by layers of linked tetrahedra (T22O7 dimers and four-fold coordinated T1 cations) connected by X cations in a distorted eight-coordinated antiprismatic site. Tetrahedral site T1 represents a quasi-regular environment with 4 identical metal-oxygen lengths, but one pair of O-M-O angles different from the other two [3]. Hardystonite, Ca2ZnSi2O7, was selected as a typical melilite-like compound which has just one four-fold crystallographic site where Co2+ ions can be accommodated, thus ensuring its unrivalled blue color. Five samples (Ca2Zn1-xCoxSi2O7 with x = 0.05, 0.1, 0.2, 0.3 and 0.4) were prepared by solid state synthesis in industrial-like conditions (95% yield) and characterized by XRD, DRS, SEM-EDS and technological testing. Increasing cobalt doping gives rise to a gradual expansion of the hardystonite unit cell, unexpected on the basis of Zn2+ and Co2+ ionic radii, attributed to a change of the covalent character of M-O bonding. Optical spectra are dominated by the strong absorption bands of Co2+ in tetrahedral coordination (crystal field strength Dq = 421 cm-1, Racah B parameter = 793 cm-1). The best compromise between cobalt concentration and optical response was found to be around x=0.3. The hardystonite pigment bestows a deep blue color on glazes and glassy coatings, withstanding to aggressive media rich in CaO and ZnO better than industrial blue pigments (cobalt aluminate, spinel) with similar color efficiency than industrial blue dyes (cobalt silicate, olivine) but with the advantage to avoid specking defects of highly staining colorants. Furthermore, exploring the wide range of isomorphic substitutions allowed in the melilite structure, a comparison through a set of compounds with a fixed cobalt fraction (Co2+ = 0.3 apfu) in the T1 site (Ca2MgSi2O7, Sr2MgSi2O7, Ba2MgSi2O7, and Sr2ZnSi2O7) was attempted. References. [1] R. Eppler, (1987) Am Ceram Soc Bull, 66, 1600. [2] M. Llusar, A. Fores, J. Badenés, J. Calbo, M. Tena, G. Monros, (2001) J Eur Ceram Soc, 21, 1121. [3] L. Bindi, M. Czank, F. Röthlisberger, P. Bonazzi, (2001) Am Mineral, 86, 747.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
