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Physics Procedia 00 (2010) 000–000

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VI Encuentro Franco-Español de Química y Física del Estado Sólido VIème Rencontre Franco-Espagnole sur la Chimie et la Physique de l’État Solide

Synthesis of a new CaxY2 xVxSn2 xO7 yellow pigment. C. Gargori, R. Galindo, S. Cerro, A. García, M. Llusar, G. Monrós

*

1. Departamento de Química Inorgánica y Orgánica, Universitat Jaume I, 12004, Castellón

Abstract

In this communication a new ceramic pigment based on codoping pyrochlore Y2Sn2O7 with V5+ and Ca2+ has been obtained. The limit of solid solution of CaxY2 xVxSn2 xO7 is around x=0.16. Pigment becomes stable in double firing glazes (CIEL*a*b*=78/5/35 5% w. enamelled) but unstable in single firing glazes such as based on CaO-ZnO-SiO2 chemical system.Using unconventional methods of synthesis the reactivity of the system and final pigmenting power of the powder is enhanced in the case of ammonia coprecipitation of a mixture of nitrates and tin chloride.

Keywords: pyrochlore, ceramic pigment, vanadium.

1. Introduction

Pyrochlore crystal structure (Fd-3m) describes materials of the type A2B2O6 and A2B2O7 where the A and B species are generally rare-earth or transition metal species; e.g. Y2Ti2O7.The pyrochlore structure is a super structure derivative of the simple fluorite structure (AO2 = A4O8, where the A and B cations are ordered along the <110> direction. The additional anion vacancy resides in the tetrahedral interstice between adjacent B-site cations. These systems are particularly susceptible to geometrical frustration and novel magnetic effects. The pyrochlore structure shows varied physical properties ranging from electronic insulators, La2Zr2O7, ionic conductors. Gd1.9Ca0.1Ti2O6.9, , metallic conductivity, Bi2Ru2O7-y, mixed ionic and electronic conductivity, spin ice systems, Dy2Ti2O7, spin glass systems, Y2Mo2O7 and superconducting materials, Cd2Re2O7 [1]. A yellow pigment with the pyrochlore structure CaxY2 xVxTi2 xO7 is known from 1993 as a substitute for the decreasing variety of available yellow ceramic pigments due to the severe regulation of toxic lead and cadmium. The solubility limit of vanadium in this pigment was found to be 1.5 wt% as V2O5 or 0.13 as x in the above formula expression. Characterization of vanadium in the vanadium pyrochlore yellow pigment by electron spectroscopy for chemical analysis and electron spin resonance showed that the oxidation state of vanadium was V5+ and its yellow colour mostly originated from V5+ substituted for Ti4+ [2,3]. In this communication CaxY2 xVxSn2 xO7 x= 0.04, 0.08, 0.16, 0.32 ceramic compositions have been synthesised and adequately mineralised in order to optimize yellow colour. The optimized composition has been

* Corresponding author. Tel.: +34-96-47-28250; fax: +34-96-47-28214. E-mail address: monros@qio.uji.es

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Physics Procedia 8 (2010) 84–87

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1875-3892 c© 2010 Published by Elsevier Ltd.doi:10.1016/j.phpro.2010.10.016

C. Gargori / Physics Procedia 00 (2010) 000–000

synthesised by ammonia coprecipitation and Pechini routes in order to obtain a yellow pigment stable in Na2O-CaO-PbO-SiO2 (1000ºC) double firing glaze conventional ceramic tile glaze [4].

2. Experimental and discussion

Ceramic samples CaxY2 xVxSn2 xO7 x= 0.04, 0.08, 0.16, 0.32 have been prepared from V2O5 and CaCO3,SnO2 (PANREAC), Y2O3 (ALDRICH) mixed in acetone media and fired at 1200ºC during 6 hours and adding 0,14 mol of Na2SiF6 as flux agent. Fired powders have been characterized by several techniques: a) XRD analysis carried out on a Siemens D5000 diffractometer using Cu K radiation, 20º-70º 2 range, scan rate 0.05 º2 /s, 10 s per step and 40 kV and 20 mA conditions (Figure 1). b) 5% w. enamelled in a single firing ceramic glaze based on CaO-ZnO-SiO2 chemical system that is fired at 1.080ºC and also in a double firing glaze from Na2O-CaO-PbO-SiO2 system, fired at only 1.000ºC for enamel production. Characterization of powders and glazed samples was carried out by optical microscopy and CIEL*a*b* measurements using standard lighting C. On this method, L* is a measure of brightness (100=white, 0=black) and a* and b* of chroma (-a*=green, +a*=red, -b*=blue, +b*=yellow) [5]. c) UV-Vis-NIR spectroscopy of enamelled samples collected using a Lambda 2000 spectrometer supplied by Perkin Elmer through diffuse reflectance technique (Figure 2). d) Electron microscopy carried out by Scanning Electron Microscopy (SEM), using a Leo-440i microscope supplied by LEYCA (Fig. 3). Likewise SEM-EDXA microstructural analysis were obtained in a LEO-440i electronic microscope equipped by a microanalysis system supplied by Oxford. XRD results (Fig 1) indicate the crystallization of Y2Sn2O7 as the only crystalline phase in all samples except in x=0.32 that shows very weak peaks associated to YVO4 along with perovskite peaks. UV-Vis-NIR reflectance diffuse spectroscopy (Fig. 2) show a sharp band centred at 250 nm associated to the Sn4+-O2- band transfer and an additional band centred at 500 nm responsible of yellow colour and associated to V5+-O2- band transfer. The maximum of absorbance is observed for x=0.16 sample. CIEL*a*b* results summarised on Table I. indicate that the yellow b* parameter increase with x until x=0.16 and decrease for x=0.32 indicating a solubility limit of vanadium in this pigment around x=0.16. The enamelled powders were 5 wt%. glazed in a CaO-ZnO-SiO2(1080ºC) single firing glaze but do not produce colour, in Na2O-CaO-PbO-SiO2 (1000ºC) double firing glaze the colour of all samples is similar (CIEL*a*b*=78/5/35).

In order to increase the reactivity of the system, the ceramic optimised composition x=0.16 was prepared by an ammonia coprecipitation route (CO) and a Pechini route (CI). In both routes Y(NO3)3.6H2O, Ca(NO3)2.4H2O, SnCl2.2H2O and VOSO4.8H2O (ALDRICH) were used as precursors. In CO route the precursor were solved in water (250 ml for 5 g of final product) and concentrated ammonia was dropped at 70ºC and vigorous stirring until pH=8, finally powder was obtained by drying in oven at 110ºC. In CI route, a molar ratio metallic cations:ethylenglycol:citric acid=1:1:1 was used. Firstly precursors were solved in water (250 ml for 5 g of final product) and citric acid was solved at 70ºC in vigorous stirring, then ethylenglycol was added and maintained 8 hours for esterification. The obtained ester was dried at 110ºC and submitted to charring treatment at 250ºC. Na2SiF6flux agent was added to CO or CI powders by manually mixture in an agatha mortar using acetone media. XRD results obtained at 1000ºC/6h and 1200ºC/6h are shown in Fig. 1.b and c. At 1000ºC both CI and CE show peaks associated to pyrochlore along with unreacted SnO2 and Y2O3 peaks, CO sample only show weak peaks associated to SnO2 along with pyrochlore. At 1200ºC all samples show pyrochlore as the only crystalline phase detected. CO at 1200ºC show the best pigmenting results in double firing glaze (L*a*b*=80,0/5,0/43,7) than CE(79,3/2,6/35,4) and CI (81,7/0,1/31,8).

Table I. CIEL a*b* colorimetric coordinates for the CaxY2 xVxSn2 xO7 yellow pigments

SAMPLE (x) L* a* b*

0.04 84.3 1.2 26.60.08 87.2 0.2 30.8

0.16 85.7 1.1 34.5

0.32 85.8 0.8 33.5

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SEM micrographs of powders fired at 1200ºC shown on Figure 3 indicate the presence of aggregates of submicrometric particles in all samples. Size of aggregates in CE and CO sample (2-6 μm) are similar but more compact in CO sample and higher than in citrate powder (1-4 μm). Also size of particles forming aggregates are higher for CE (500 nm) than for CO (300 nm) and higher than CI powder (150 nm). Likewise SEM-EDXA microstructural analysis of powders fired at 1200ºC (not shown) indicate that codopants, vanadium and calcium, present a homogeneous distribution on samples. However the global content of calcium is higher than vanadium, probably due to vanadium evaporation during firing, Likewise the content of vanadium is slightly higher in CO and CE sample than in CI sample in agreement with colour intensity observed on glazed samples.

3. Conclusions

A new ceramic pigment based on codoping pyrochlore Y2Sn2O7 with V5+ and Ca2+ has been obtained. The limit of solid solution of CaxY2 xVxSn2 xO7 is around x=0.16. Pigment becomes stable in double firing glazes (CIEL*a*b*=78/5/35 5% w. enamelled) but unstable in single firing glazes such as based on CaO-ZnO-SiO2chemical system. Using unconventional methods of synthesis the reactivity of the system and final pigmenting power of the powder is enhanced in the case of ammonia coprecipitation of a mixture of nitrates and tin chloride, however citrate-ethylenglycol esterification method show worse yellow shade than ceramic sample. All powders show aggregates of submicronic particles high sized in CO and CE samples than in citrate powders. Vanadium content is slightly higher in CO and CE sample than in CI sample in agreement with colour intensity observed on glazed samples.

10 15 20 25 30 35 40 45 50 55 60 65 70

Figure 1. (a) XRD Difractogrammes of CaxY2 xVxSn2 xO7.0,14 Na2SiF6 ceramic powders, (b) XRD Difractogrammes of x=0.16 CE, CO and CI simples fired at 1000ºC/6h, (c) XRD Difractogrammes of x=0.16 CE, CO and CI simples fired at 1000ºC/6h.CRYSTALLINE PHASES: P=Y2Sn2O7, S=SnO2, Y=Y2O3.

x=0.32

0.16

0.08 0.04

P P P P

S

Y

YVO4

CO

CI

CE

CO

CI

CES

S YP

(a)

(b)

(c)

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-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

200 400 600 800 1000 1200 1400 1600

wavelength (nm)

A (

a.u

.)

0

0,2

0,4

0,6

0,8

1

1,2

1,4

200 375 550 725 900 1075 1250 1425 1600

Figure 2. UV-Vis-NIR spectra (a) CaxY2 xVxSn2 xO7.0,14 Na2SiF6 ceramic powders, (b) CE,CO at 1000ºC.

Figure 3. SEM micrographs of powders fired at 1200ºC.

References

[1] M.A. Subramanian, G. Aravamudan and G. V. Subba Rao, Oxide Pyrochlores: A Review Progress in Solid State Chemistry, 15,55-143 (1983). [2] G. Monrós, J.A. Badenes, A. García, M.A. Tena, El Color de la Cerámica Pub. Universitat Jaume I, Spain, 2003. [3] S. Ishida, F. Ren, N. Takeuchi, New yellow ceramic pigment based on codoping pyrochlore-type Y2Ti2O7 with V5+ and Ca2+

Journal of the American Ceramic Society,76,10, 2644-2648 (1993). [4] A. García, M. Llusar, J. Badenes, M.A. Tena, G. Monrós, Encapsulation of hematite in zircon by microemulsion and Sol-Gel methods J. of Sol-Gel and Tech, 27,3, 267-276 (2003). [5] CIE Comission International de l'Eclairage, Recommendations on Uniform Color Spaces Color Difference Equations, Psychometrics Color Terms. Suplement nº 2 of CIE Pub. Nº 15 (E1-1.31) 1971, Bureau Central de la CIE, Paris (1978).

x=0.16

x=0,32 and 0.08

x=0.04

(a)

(b)

CO

CE

CE

2 m__

CO

2 m___

CI

2 m__

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