Pigments in the Y 2 O 3 Al 2 O 3 Cr 2 O 3 System

Transcription

1 MATERIA Y CERAMICZNE /CERAMIC MATERIALS/, 62, 4, (2010), Pigments in the Y 2 O 3 Al 2 O 3 Cr 2 O 3 System MIROS AW M. BU KO, EWA STOBIERSKA*, JERZY LIS, BEATA MOLASY AGH University of Science and Technology, Faculty of Materials Science, Krakow, Poland * estob@agh.edu.pl Abstract The paper presents the results of research on preparation and properties of ceramic pigments in the Y 2 O 3 -Al 2 O 3 -Cr 2 O 3 system which were obtained by the solid state reaction. The chromium ions were used as chromophores in these pigments. The pigments were nally synthesized for 6 h at 1500ºC. They were applied for colouring of transparent glaze used for glazing gres porcellanato tiles. Keywords: Pigments, Yttrium-aluminium system, Gres porcellanato PIGMENTY W UK ADZIE Y 2 O 3 Al 2 O 3 Cr 2 O 3 Artyku przedstawia wyniki bada nad otrzymywaniem i w a ciwo ciami pimentów ceramicznych w uk adzie Y 2 O 3 -Al 2 O 3 -Cr 2 O 3, wytworzonych za pomoc reakcji w stanie sta ym. Jony chromu wykorzystano jako chromofory w badanych pigmentach. Pigmenty ostatecznie syntezowano przez 6 h w 1500ºC. Wykorzystano je do barwienia prze roczystego szkliwa przeznaczonego do szkliwienia p ytek gres porcellanato. S owa kluczowe: pigment, uk ad Y 2 O 3 -Al 2 O 3, gres porcellanato 1. Introduction The appearance of new products with enhanced functional properties of new ceramic body masses, particularly gres porcellanato, sets high requirements to substances used in their preparation, especially pigments. The observed high ring temperatures of these masses, exceeding 1200ºC, the pigments used in their compositions have to show high temperature stability as well as low chemical reactivity. In the case of pigments covering broad spectrum of colours of high intensity, from pink through red to brown-red, the research for new solutions is still ongoing. One of such solutions may be the application of double yttria-alumina oxide of perovskite structure, in which part of aluminum ions is replaced by chromium(iii) ions, YCr x Al 1-x O 3 (YAP). Red ceramic pigments in the Y 2 O 3 -Al 2 O 3 -Cr 2 O 3 system have been investigated since 1999 [1]. Generally, it was stated that the synthesis of YAP from the appropriate mixture of the respective oxide powders with addition of some mineralisers led to almost phase pure pigments [2-4]. Our earlier studies showed that some amounts of other phases can be formed during the high-temperature solid state reaction [5]. It can be assumed that the presence of the second phase may in uence colour properties of the YAP pigment. Besides YAP phase two other phases with monoclinic (YAM) and garnet (YAG) structure exist in the Y 2 O 3 -Al 2 O 3 system [6]. A possibility of formation of solid solutions of such phases with Cr 2 O 3 and their colours are still unknown. So, the aim of the present work was to study colour properties of all yttrium-aluminium oxide phases doped with chromia. 2. Experimental Yttrium-aluminum pigments coloured with chromium were obtained via a direct solid state reaction route [7, 8]. The mixture of precursor powders was subjected to dry homogenization. Initially the chromium oxide powder was mixed with aluminum hydroxide (gibbsite), and later the yttrium oxide powder was added together with 3 wt% of calcium uoride as a mineraliser. All powders were Sigma-Aldrich chemical reagents. The initial substances were matched stochiometrically to give phases in which a part of aluminium or yttrium ions were substituted by chromium ones. Chromium oxide level was xed at 3 mol.% in each solid solution. The mixtures were calcined at 1300, 1400 and 1500ºC soaked in nal calcination temperature up to 6 h. In such a way six powders with nominal chemical compositions were prepared: (Y 2.91 Cr 0.09 )Al 5 O 12 Y Cr AG (Y 3.88 Cr 0.12 )Al 2 O 9 Y Cr AM (Y 0.97 Cr 0.03 )AlO 3 Y Cr AP Y 3 (Al 4.85 Cr 0.15 )O 12 YAl Cr G Y 4 (Al 1.94 Cr 0.06 )O 9 YAl Cr M Y(Al 0.97 Cr 0.03 )O 3 YA Cr P 577

2 M.M. BU KO, E. STOBIERSKA, J. LIS, B. MOLASY The preparation procedure is schematically presented in Fig. 1. To examine pigments properties the powders were applied for colouring transparent glaze used for gres porcellanato tiles. Each pigment was added to glaze in the amount of 10 %. The glaze as well as the gres porcellanato body mass were of industrial origin and the glazed specimens were fast- red in a laboratory electric furnace, according to industrial ring curves [9]. Diffraction analysis of X-ray radiation (CuK ) was used for phase identi cation of the prepared powders (X Pert Pro, Panalytical). Quantitative ratios of identi ed phase as well as their lattice parameters were determined using the Rietveld method. Morphological observations of powders were performed with the use of a scanning microscope (Nova NanoSEM 200, FEI Company) and color parameters with the use of an SP-62 spectrophotometer made by X-Rite. 3. Results and discussion Results of the XRD phase analysis are collected in Table 1. Fig. 1. Preparation route of pigments in the Y 2 O 3 -Al 2 O 3 -Cr 2 O 3 system. Table 1. Quantitative analysis [wt%] of the pigments calcined at different temperatures with 6 h soaking. Calcination temperature [ºC] YAG YAM YAP CaF 2 YOF CaAl 4 O 7 Y Cr AG Y Cr AM Y Cr AP YA Cr G YA Cr M YA Cr P a) b) c) Fig. 2. Changes of cell volume of the yttrium-aluminium oxide phases as a function of calcination temperature. 578 MATERIA Y CERAMICZNE /CERAMIC MATERIALS/, 62, 4, (2010)

3 PIGMENTS IN THE Y 2 O 3 Al 2 O 3 Cr 2 O 3 SYSTEM a) b) c) d) e) f) Fig. 3. Morphology of the yttrium-aluminium oxide pigments calcined at 1300ºC with 6 h soaking: a) Y Cr AG, b) Y Cr AM, c) Y Cr AP, d) YA Cr G, e) YA Cr M, f) YA Cr P. As it can be seen in each case, the increase of calcination temperature causes a progress in the synthesis reaction but only YAG powders show a lack of other yttrium-aluminium oxide phases. The YAM phase is the most dif cult to synthesize; even at the highest temperature, the presence of perovskite and yttrium oxide is observed. Such a phenomenon can be attributed to different structural possibilities of the formation of solid solutions with the substitution of chromium ions for yttrium or aluminium ones. Changes of the lattice cell volume of the all yttriumaluminium oxides reveal a formation of the respective solid solutions with temperature, (Figs. 2a-c). The increase of MATERIA Y CERAMICZNE /CERAMIC MATERIALS/, 62, 4, (2010) 579

4 M.M. BU KO, E. STOBIERSKA, J. LIS, B. MOLASY Table 2. Colour parameters of the pigments ring at different temperatures with 6 h soaking. Colour Firing conditions parameter 1300ºC 6 h 1400ºC 6 h 1500ºC 6 h Y Cr AG L * a * b * Y Cr AM L * a * b * Y Cr AP L * a * b * YA Cr G L * a * b * YA Cr M L * a * b * YA Cr P L * a * b * Table 3. Colour parameters of gres porcellanato tiles covered with glazes contained 10 % of the respective pigment. Colour parameter Pigment L * a * b * Y Cr AG Y Cr AM Y Cr AP YA Cr G YA Cr M YA Cr P such values and differences between respective ionic radii (Cr pm, Al nm, Y pm) suggest that the substitution of chromium ions for aluminium ones acts easier than for yttrium ions. The real chemical compositions of the respective solid solutions are dif cult to determine. Figs. 3a-f present SEM images of the powders synthesized for 6 h at 1300ºC. All powders are composed of angular crystallites with well shaped walls and sharp edges connected in the form of aggregates. The smaller crystallites, up to 2 m, can be observed in both garnet phases (Figs. 2a and 2d). The grain sizes in other powders range from 1 to 7 m. There are no distinct differences in sizes and shapes of the grains of the a) d) b) e) c) f) Fig. 4. Images of the pigments prepared at 1300ºC: a) Y Cr AG, b) Y Cr AM, c) Y Cr AP, d) YA Cr G, e) YA Cr M, f) YA Cr P. same yttrium-aluminium oxide phase caused by their chemical compositions. The colour parameters of the pigments synthesized for 6 h at different temperatures and measured by the CIE L * a * b * system are presented in Table 2. Fig. 4 shows typical images of the pigments prepared at 1300ºC. The data of Table 2 reveal that the powder of perovskite phase with chromium ions substituted for aluminium ions (YA Cr P) is the most red coloured. It is also seen that the optimum temperature for this pigment synthesis is 1300ºC; the increase of the temperature leads to a decrease of a * parameter and red colour becomes less intensive. The powder of YA Cr M phase has the similar red colour, calcined at the lowest temperature what is probably caused by a signi cant amount of the perovskite phase in this powder (see Table 1). The other YAM and YAG powders are gray yellow, yellowish and even light brown. A comparison of the phase composition and colour parameters allows us to nd that the colour of the pigment do not depend only on amount of the perovskite phase. The YA Cr P powder synthesized at 1300ºC is composed of ~83 % of perovskite and ~11 % of the garnet phase and is the most intensive red among the all pigments. The same powder calcined at 1500ºC is almost pure perovskite (over 580 MATERIA Y CERAMICZNE /CERAMIC MATERIALS/, 62, 4, (2010)

5 PIGMENTS IN THE Y 2 O 3 Al 2 O 3 Cr 2 O 3 SYSTEM a) b) c) d) e) f) Fig. 5. Pictures of the gres porcellanato tiles covered with transparent glazes contained 10 % of the respective pigments: a) Y Cr AG, b) Y Cr AM,c) Y Cr AP, d) YA Cr G, e) YA Cr M, f) YA Cr P. 97 %) but its a * parameter is lower than the previous one (see Table 2). It can be stated that colour of the pigments depends not only on their phase composition but it is a result of complex structural and microstructural relations. Fig. 5 shows colours of gres porcellanato tiles covered with transparent glaze containing 10 % of the pigments. The L * a * b * parameters of these tiles are collected in Table Conclusions In the Y 2 O 3 -Al 2 O 3 -Cr 2 O 3 system only perovskite powders (YAP) are red colour regardless on the type of substitution and the chemical composition of solid solution with chromium oxide. The garnet phase (YAG) seems to be the most stable; the amount of YAG in the calcined powders and their colour properties are independent of temperature. The pigments with nominal YAM stoichiometry are the most sensitive to temperature of calcination. Their colours are strictly connected with phase and chemical composition of the powders. Acknowledgements References [1] Baldi G., Dolen N.: Mater. Eng., 10, (1999), [2] Marinova Y., Hohemberger J.M., Cordoncillo E., Escribano P., Carda J.B.: J. Eur. Ceram. Soc., 23, (2003), [3] Matteucci F., Lepri Neto C., Dondi M., Cruciani G., Baldi G., Boschi A.O.: Adv. Appl. Ceram., 105, (2006), [4] Shirpour M., Faghihi Sani M.A., Mirhabibi A.: Ceram. Int., 33, (2007), [5] Stobierska E., Lis J., Bu ko M.M., Wali ska M.: Ceramika/ Ceramics, 91/2, (2005), [6] Phase Diagrams for Ceramists. The American Ceramic Society Inc., (1964). [7] Stobierska E., Lis J., Bu ko M.M. Gubernat A.: Adv. Sci. Tech., 45, (2006), [8] Stobierska E., Lis J., Bu ko M., Gubernat A.: Proceedings of the 10th International Conference of the European Ceramic Society, Heinrich J.G., Aneziris C.G. (Eds.), Göller Verlag, Baden-Baden, (2007), [9] Beata Molasy, Master Thesis, AGH, Received 1 March 2010; accepted 8 May 2010 This work was carried out within the scope of research project MNiSW, Nr N N MATERIA Y CERAMICZNE /CERAMIC MATERIALS/, 62, 4, (2010) 581