- Email: [email protected]
Synthesis of partially stabilized leucite
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 68 (2007) 1207–1210 www.elsevier.com/locate/jpcs
Synthesis of partially stabilized leucite Alexandra Klouzˇkova´a,, Martina Mra´zova´b, Martina Kohoutkova´b a
Department of Glass and Ceramics, Institute of Chemical Technology, Prague, Technicka´ 5, 166 28 Prague, Czech Republic Laboratory of Inorganic Materials, Institute of Inorganic Chemistry of AS CR and ICT, Prague, Technicka´ 5, 166 28 Prague, Czech Republic
b
Received 19 March 2007; received in revised form 27 March 2007; accepted 29 March 2007
Abstract Leucite is the dominating crystalline phase in most feldspathic dental porcelains used for ceramic-fused-to-metal restorations. The stable form of leucite at room temperature is a tetragonal one, which undergoes a phase transition to cubic form at 625 1C. This transformation is associated with 1.2% volume contraction that leads to the formation of microcracks in and around the crystals. The high amount of tetragonal leucite crystals may be the reason of developing large flaws within the glass matrix that would lead to a decrease of mechanical properties. Potentially, cesium-stabilized cubic leucite could be added to leucite porcelains to increase crystalline content without increasing flaw size and frequency. The aim of this work was to develop a suitable and reproducible technology for the preparation of cubic leucite. Previously, the hydrothermal synthesis of tetragonal leucite was described. This method was modified for the preparation of cubic leucite. It was found that the final products consist of both tetragonal and cubic leucite up to 9.52 mol% of Cs2O. Above this value fully stabilized leucite was observed. It was proved that the products of the hydrothermal syntheses are formed by spherolites having particle size of approximately 2 mm that consist of particles tens of nm in size. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; B. Chemical synthesis; C. X-ray diffraction; D. Phase transitions
1. Introduction Leucite has become a very important part of composite dental ceramics in recent few years. The presence of leucite in a porcelain matrix increases the thermal expansion of final composite material, making possible its fusion to a metal reinforcement. In addition, leucite is used as a major crystalline phase in a new generation of dental porcelains for all-ceramics restorations. Thermal and mechanical properties of these materials are affected by the amount, average crystal size and structure of the crystalline phase. Leucite undergoes a crystallographic transformation from tetragonal to cubic form at 625 1C. For dental porcelains this transformation occurs in the range of 400–600 1C [1,2]. The tetragonal (low-temperature) leucite possesses a high coefficient of thermal expansion (27.2 10 6 1C over range from 25 to 650 1C) whereas Corresponding author. Tel.: +420 22044 3777; fax: +420 22431 3200.
E-mail address: [email protected] (A. Klouzˇkova´). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.03.055
cubic form exhibits lower value (15.9 10 6 1C). The coefficient of thermal expansion of feldspathic matrix is about 8.4 10 6 1C [3]. Leucite was initially introduced into dental porcelains to raise the composite coefficient of thermal expansion. In addition in the last few years it was found that the presence of leucite, owing to its martensitic tetragonal–cubic transition behavior, also improves fracture toughness and persistence of a final dental product. A high amount of tetragonal leucite crystals may be the source of microcracking within the matrix that would lead to decrease of especially fracture toughness. Potentially, cesium stabilized cubic leucite could be added to leucite porcelains to increase the crystallic content without increasing flaw size and frequency [3,4]. The demand for leucite ceramics having high-fracture toughness requires a new suitable technology of the preparation of composite material in which leucite and matrix are synthesized separately seems to be promising. Both these phases are subsequently homogenized and sintered in a dentist laboratory after application on framework (metal or
ARTICLE IN PRESS 1208
A. Klouzˇkova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1207–1210
Fig. 1. Scheme of the preparation procedure of Cs–leucite by ion-exchange of analcime.
12 10 Cs2O [mol. %]
ceramic). This technology ensures reproducible control of leucite ceramics microstructure, which is required to improve its fracture toughness and persistence. The aim of this research was to develop a suitable technology leading to the preparation of partially stabilized cubic leucite powders with controlled particle size distribution.
3. Results and discussion 3.1. Preparation of cubic leucite by dual ion exchange The synthesis of precursor analcime is a very important part of preparation of Cs–leucite. It was proved that the
6 4 3 µm analcime 14.5 µm analcime
2
2. Experimental The method of preparation of Cs–leucite consisted of three steps. The first step comprised synthesis of analcime (NaAlSi2O6 H2O), that was prepared from a gel containing aluminum powder (Al, Lachema, Czech Republic), amorphous silica powder (SiO2, Polskie Odczynniki Chemiczne Gliwice, PL) and 4 M solution of sodium hydroxide (NaOH, Lachema, Czech Republic). Analcime was subsequently used for dual ion exchange under hydrothermal conditions. In the second step, pollucite (CsxNa(1 x)AlSi2O6 H2O) was prepared by partial ion exchange of Na+ ions from analcime for Cs+ ions from 4 M solution of CsCl. The final step involves ion exchange of remaining Na+ ions for K+ ions from 4 M solution of KCl. t-Leucite was prepared by single ion exchange of analcime (Na+ ions for K+ ions, 4 M solution of KCl) [5,6]. All syntheses were carried out in autoclaves at a temperature of 200 1C. After the hydrothermal treatment, the content of the autoclave was washed with boiling distilled water, vacuum filtered and dried in an oven at 100 1C (Fig. 1). A Philips X‘Pert PRO y–y powder diffractometer was used for X-ray powder diffraction analysis. The results for each specimen were analyzed using computer X‘Pert High Score program. Chemical composition of powders and the degree of ion exchanges were determined by XRF analysis using ARL 9400 XP sequential WD-XRF spectrometer. Particle size and their distribution were observed by optical (Jenapol, Zeiss, Germany) and scanning electron microscope (SEM, Philips XL 30 CP, The Netherlands).
8
0
0
20
40
60
80
time [hours]
Fig. 2. Mol% Cs2O versus time is plotted into the graph for 3 and 14.5 mm analcime.
Table 1 Reaction times of the products of single synthesis Sample
A B C D
Cs2O (mol%)
— 3.84 5.22 9.52
Reaction time (h) Analcime
Pollucite
c-Leucite
t-Leucite
2 2 2 2
— 5 8 72
— 4 4 4
4 — — —
particle size of analcime is the main control factor for subsequent ion exchanges of pollucite and cubic leucite, respectively. By the observance of optimal reaction conditions [5], i.e. reaction time of synthesis—2 h, reaction temperature—200 1C and 4 M solution of NaOH, homogenous powders with uniform particle size at intervals 2–3 mm were obtained. X-ray fluorescence analysis was used to estimate the degree of ion exchange in the pollucite powder. It was found that the degree of ion exchange of Na+ ions for Cs+ ions depends both on the reaction time and on the particle size of analcime. The amount of Cs2O increased with the duration of the ion-exchange treatment and decreased with increasing particle size of analcime. The results of XRF analyses are graphically displayed in Fig. 2. Reaction times of individual ion exchanges to Cs–leucite are listed in Table 1. X-ray diffraction patterns of tetragonal leucite, partially stabilized Cs–leucite and fully stabilized leucite are shown in Fig. 3. It is very important to
ARTICLE IN PRESS A. Klouzˇkova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1207–1210
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Fig. 3. XRD diffractograms of the products: A—t-leucite, B—leucite stabilized with 3.84 mol% Cs2O, C—leucite stabilized with 5.22 mol% Cs2O, D— leucite stabilized with 9.52 mol% Cs2O. Table 2 Chemical composition of powders by XRF [wt%] Sample
Na2O
Al2O3
SiO2
K2O
Cs2O
A B C D
1.00 1.07 1.10 1.09
27.72 25.10 24.22 22.06
50.69 45.64 44.17 40.16
20.68 14.62 12.74 7.36
0.00 13.25 17.49 29.03
Table 3 Quantitative phase analysis and crystallite-size determination of products by Rietveld method. Sample
t-Leucite (wt%)
c-Leucite (wt%)
dt (nm)
dc (nm)
A B C D
100 54 38 0
0 46 62 100
50 77 46 —
— 33 31 39
notice peaks from 25 to 28 12y. X-ray diffraction patterns for non-stabilized t-leucite A showed two peaks on position 25.86 and 27.31 1 2y and for fully stabilized leucite D one peak on position 26.22 1 2y in this range. It is evident from B and C X-ray patterns that leucite powders stabilized by the addition of 3.84 and 5.22 mol% Cs2O comprised both two tetragonal and one cubic peaks with different relative intensity. It is also evident that the amount of stabilized leucite increased with the amount of Cs2O contained in the pollucite lattice. Additionally, no detectable amount of pollucite was found in any of the preset powders. Chemical compositions of the final products (t-leucite and Cs–leucite) are shown in Table 2. Table 3 gives results of quantitative phase analysis and crystallite determination
of products by the Rietveld method. Fig. 4 shows SEM images of analcime powder (I), used for preparation of pollucite (II) and fully stabilized cubic leucite (III, D). Scanning electron microscopy of the grains shows homogenous powders with uniform particle size 2 mm and shape. SEM images revealed that these particles are spherolites that are compounded of much smaller particles. This result was confirmed by X-ray profile analysis, by which a particle size in the range 30–70 nm was identified.
4. Conclusion The research was focused on the partial and full stabilization of Cs–leucite powders. It was shown that hydrothermal synthesis of analcime followed by dual ion exchange to Cs–leucite is a suitable method for the controlled preparation of partially stabilized cubic leucite. Leucite powders have been synthesized from relatively inexpensive precursors at a relatively low temperature (200 1C). Optimal conditions for the preparation of pollucite (CsxNa(1 x)AlSi2O6 H2O) with controlled Cs2O fraction were examined. XRD analyses showed that the powdered products are crystalline pollucite or crystalline leucite with no traces of other phases. The degree of the stabilized leucite was evaluated by the Cs2O ratio in pollucite. It was determined that the addition of 3.8 mol% Cs2O is required for stabilization of 46% of cubic leucite. Fully stabilized cubic form of leucite was obtained by the addition of 9.5 mol% Cs2O. The main advantage of the hydrothermally prepared leucite powders is fine grain size. It was proved by evaluation from XRD data and microscopical analysis that the products of the hydrothermal synthesis—alcime, t-leucite, pollucite and Cs–leucite consist of spherolites
ARTICLE IN PRESS 1210
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Fig. 4. SEM images of analcime (I), pollucite (II) and c-leucite (III).
having particle size of 2 mm. These spherolites consist of much smaller particles (crystallites) in the range 30–70 nm. The prepared leucite precursors are very promising dental raw materials, because the use of submicron powders leading to better homogeneity of the final composite microstructure. One of the major advantages of the technology consists in the achievement of a high proportion of homogenous dispersion of stabilized leucite grains in the matrix leading to the significant increase of fracture toughness in the leucite dental porcelain. Acknowledgments The authors are very grateful to J. Maixner and S. Bakardjieva for their kind assistance with XRD and SEM, respectively.
This work was a part of the Project no. 2A-1TP1/063, ‘‘New glass and ceramic materials and advanced concepts of their preparation and manufacturing’’, realized under financial support of the Ministry of industry and trade. References [1] J.R. Mackert Jr, M.B. Butts, C.W. Fairhurst, Dent. Mater. 2 (1986) 32. [2] J.R. Mackert Jr, A.L. Evans, J. Am. Ceram. Soc. 74 (1991) 450. [3] S.T. Rasmussen, C.I. McLaren, W.J. O‘Brien, J. Biomed. Mater. Res. B 69 (2004) 195. [4] I.L. Denry, J.R. Mackert, J.A. Holloway, S.F. Rosenstiel, J. Dent. Res. 75 (12) (1996) 1928. [5] M. Novotna´, A. Klouzˇkova´, J. Maixner, V. Sˇatava, Ceram-Silik. 49 (4) (2005) 252. [6] M. Novotna´, V. Sˇatava, J. Maixner, A. Klouzˇkova´, P. Kostka, D. Lezˇal, Glass Technol. 45 (2004) 105.
Journal of Physics and Chemistry of Solids 68 (2007) 1207–1210 www.elsevier.com/locate/jpcs
Synthesis of partially stabilized leucite Alexandra Klouzˇkova´a,, Martina Mra´zova´b, Martina Kohoutkova´b a
Department of Glass and Ceramics, Institute of Chemical Technology, Prague, Technicka´ 5, 166 28 Prague, Czech Republic Laboratory of Inorganic Materials, Institute of Inorganic Chemistry of AS CR and ICT, Prague, Technicka´ 5, 166 28 Prague, Czech Republic
b
Received 19 March 2007; received in revised form 27 March 2007; accepted 29 March 2007
Abstract Leucite is the dominating crystalline phase in most feldspathic dental porcelains used for ceramic-fused-to-metal restorations. The stable form of leucite at room temperature is a tetragonal one, which undergoes a phase transition to cubic form at 625 1C. This transformation is associated with 1.2% volume contraction that leads to the formation of microcracks in and around the crystals. The high amount of tetragonal leucite crystals may be the reason of developing large flaws within the glass matrix that would lead to a decrease of mechanical properties. Potentially, cesium-stabilized cubic leucite could be added to leucite porcelains to increase crystalline content without increasing flaw size and frequency. The aim of this work was to develop a suitable and reproducible technology for the preparation of cubic leucite. Previously, the hydrothermal synthesis of tetragonal leucite was described. This method was modified for the preparation of cubic leucite. It was found that the final products consist of both tetragonal and cubic leucite up to 9.52 mol% of Cs2O. Above this value fully stabilized leucite was observed. It was proved that the products of the hydrothermal syntheses are formed by spherolites having particle size of approximately 2 mm that consist of particles tens of nm in size. r 2007 Elsevier Ltd. All rights reserved. Keywords: A. Ceramics; B. Chemical synthesis; C. X-ray diffraction; D. Phase transitions
1. Introduction Leucite has become a very important part of composite dental ceramics in recent few years. The presence of leucite in a porcelain matrix increases the thermal expansion of final composite material, making possible its fusion to a metal reinforcement. In addition, leucite is used as a major crystalline phase in a new generation of dental porcelains for all-ceramics restorations. Thermal and mechanical properties of these materials are affected by the amount, average crystal size and structure of the crystalline phase. Leucite undergoes a crystallographic transformation from tetragonal to cubic form at 625 1C. For dental porcelains this transformation occurs in the range of 400–600 1C [1,2]. The tetragonal (low-temperature) leucite possesses a high coefficient of thermal expansion (27.2 10 6 1C over range from 25 to 650 1C) whereas Corresponding author. Tel.: +420 22044 3777; fax: +420 22431 3200.
E-mail address: [email protected] (A. Klouzˇkova´). 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.03.055
cubic form exhibits lower value (15.9 10 6 1C). The coefficient of thermal expansion of feldspathic matrix is about 8.4 10 6 1C [3]. Leucite was initially introduced into dental porcelains to raise the composite coefficient of thermal expansion. In addition in the last few years it was found that the presence of leucite, owing to its martensitic tetragonal–cubic transition behavior, also improves fracture toughness and persistence of a final dental product. A high amount of tetragonal leucite crystals may be the source of microcracking within the matrix that would lead to decrease of especially fracture toughness. Potentially, cesium stabilized cubic leucite could be added to leucite porcelains to increase the crystallic content without increasing flaw size and frequency [3,4]. The demand for leucite ceramics having high-fracture toughness requires a new suitable technology of the preparation of composite material in which leucite and matrix are synthesized separately seems to be promising. Both these phases are subsequently homogenized and sintered in a dentist laboratory after application on framework (metal or
ARTICLE IN PRESS 1208
A. Klouzˇkova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1207–1210
Fig. 1. Scheme of the preparation procedure of Cs–leucite by ion-exchange of analcime.
12 10 Cs2O [mol. %]
ceramic). This technology ensures reproducible control of leucite ceramics microstructure, which is required to improve its fracture toughness and persistence. The aim of this research was to develop a suitable technology leading to the preparation of partially stabilized cubic leucite powders with controlled particle size distribution.
3. Results and discussion 3.1. Preparation of cubic leucite by dual ion exchange The synthesis of precursor analcime is a very important part of preparation of Cs–leucite. It was proved that the
6 4 3 µm analcime 14.5 µm analcime
2
2. Experimental The method of preparation of Cs–leucite consisted of three steps. The first step comprised synthesis of analcime (NaAlSi2O6 H2O), that was prepared from a gel containing aluminum powder (Al, Lachema, Czech Republic), amorphous silica powder (SiO2, Polskie Odczynniki Chemiczne Gliwice, PL) and 4 M solution of sodium hydroxide (NaOH, Lachema, Czech Republic). Analcime was subsequently used for dual ion exchange under hydrothermal conditions. In the second step, pollucite (CsxNa(1 x)AlSi2O6 H2O) was prepared by partial ion exchange of Na+ ions from analcime for Cs+ ions from 4 M solution of CsCl. The final step involves ion exchange of remaining Na+ ions for K+ ions from 4 M solution of KCl. t-Leucite was prepared by single ion exchange of analcime (Na+ ions for K+ ions, 4 M solution of KCl) [5,6]. All syntheses were carried out in autoclaves at a temperature of 200 1C. After the hydrothermal treatment, the content of the autoclave was washed with boiling distilled water, vacuum filtered and dried in an oven at 100 1C (Fig. 1). A Philips X‘Pert PRO y–y powder diffractometer was used for X-ray powder diffraction analysis. The results for each specimen were analyzed using computer X‘Pert High Score program. Chemical composition of powders and the degree of ion exchanges were determined by XRF analysis using ARL 9400 XP sequential WD-XRF spectrometer. Particle size and their distribution were observed by optical (Jenapol, Zeiss, Germany) and scanning electron microscope (SEM, Philips XL 30 CP, The Netherlands).
8
0
0
20
40
60
80
time [hours]
Fig. 2. Mol% Cs2O versus time is plotted into the graph for 3 and 14.5 mm analcime.
Table 1 Reaction times of the products of single synthesis Sample
A B C D
Cs2O (mol%)
— 3.84 5.22 9.52
Reaction time (h) Analcime
Pollucite
c-Leucite
t-Leucite
2 2 2 2
— 5 8 72
— 4 4 4
4 — — —
particle size of analcime is the main control factor for subsequent ion exchanges of pollucite and cubic leucite, respectively. By the observance of optimal reaction conditions [5], i.e. reaction time of synthesis—2 h, reaction temperature—200 1C and 4 M solution of NaOH, homogenous powders with uniform particle size at intervals 2–3 mm were obtained. X-ray fluorescence analysis was used to estimate the degree of ion exchange in the pollucite powder. It was found that the degree of ion exchange of Na+ ions for Cs+ ions depends both on the reaction time and on the particle size of analcime. The amount of Cs2O increased with the duration of the ion-exchange treatment and decreased with increasing particle size of analcime. The results of XRF analyses are graphically displayed in Fig. 2. Reaction times of individual ion exchanges to Cs–leucite are listed in Table 1. X-ray diffraction patterns of tetragonal leucite, partially stabilized Cs–leucite and fully stabilized leucite are shown in Fig. 3. It is very important to
ARTICLE IN PRESS A. Klouzˇkova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1207–1210
1209
Fig. 3. XRD diffractograms of the products: A—t-leucite, B—leucite stabilized with 3.84 mol% Cs2O, C—leucite stabilized with 5.22 mol% Cs2O, D— leucite stabilized with 9.52 mol% Cs2O. Table 2 Chemical composition of powders by XRF [wt%] Sample
Na2O
Al2O3
SiO2
K2O
Cs2O
A B C D
1.00 1.07 1.10 1.09
27.72 25.10 24.22 22.06
50.69 45.64 44.17 40.16
20.68 14.62 12.74 7.36
0.00 13.25 17.49 29.03
Table 3 Quantitative phase analysis and crystallite-size determination of products by Rietveld method. Sample
t-Leucite (wt%)
c-Leucite (wt%)
dt (nm)
dc (nm)
A B C D
100 54 38 0
0 46 62 100
50 77 46 —
— 33 31 39
notice peaks from 25 to 28 12y. X-ray diffraction patterns for non-stabilized t-leucite A showed two peaks on position 25.86 and 27.31 1 2y and for fully stabilized leucite D one peak on position 26.22 1 2y in this range. It is evident from B and C X-ray patterns that leucite powders stabilized by the addition of 3.84 and 5.22 mol% Cs2O comprised both two tetragonal and one cubic peaks with different relative intensity. It is also evident that the amount of stabilized leucite increased with the amount of Cs2O contained in the pollucite lattice. Additionally, no detectable amount of pollucite was found in any of the preset powders. Chemical compositions of the final products (t-leucite and Cs–leucite) are shown in Table 2. Table 3 gives results of quantitative phase analysis and crystallite determination
of products by the Rietveld method. Fig. 4 shows SEM images of analcime powder (I), used for preparation of pollucite (II) and fully stabilized cubic leucite (III, D). Scanning electron microscopy of the grains shows homogenous powders with uniform particle size 2 mm and shape. SEM images revealed that these particles are spherolites that are compounded of much smaller particles. This result was confirmed by X-ray profile analysis, by which a particle size in the range 30–70 nm was identified.
4. Conclusion The research was focused on the partial and full stabilization of Cs–leucite powders. It was shown that hydrothermal synthesis of analcime followed by dual ion exchange to Cs–leucite is a suitable method for the controlled preparation of partially stabilized cubic leucite. Leucite powders have been synthesized from relatively inexpensive precursors at a relatively low temperature (200 1C). Optimal conditions for the preparation of pollucite (CsxNa(1 x)AlSi2O6 H2O) with controlled Cs2O fraction were examined. XRD analyses showed that the powdered products are crystalline pollucite or crystalline leucite with no traces of other phases. The degree of the stabilized leucite was evaluated by the Cs2O ratio in pollucite. It was determined that the addition of 3.8 mol% Cs2O is required for stabilization of 46% of cubic leucite. Fully stabilized cubic form of leucite was obtained by the addition of 9.5 mol% Cs2O. The main advantage of the hydrothermally prepared leucite powders is fine grain size. It was proved by evaluation from XRD data and microscopical analysis that the products of the hydrothermal synthesis—alcime, t-leucite, pollucite and Cs–leucite consist of spherolites
ARTICLE IN PRESS 1210
A. Klouzˇkova´ et al. / Journal of Physics and Chemistry of Solids 68 (2007) 1207–1210
Fig. 4. SEM images of analcime (I), pollucite (II) and c-leucite (III).
having particle size of 2 mm. These spherolites consist of much smaller particles (crystallites) in the range 30–70 nm. The prepared leucite precursors are very promising dental raw materials, because the use of submicron powders leading to better homogeneity of the final composite microstructure. One of the major advantages of the technology consists in the achievement of a high proportion of homogenous dispersion of stabilized leucite grains in the matrix leading to the significant increase of fracture toughness in the leucite dental porcelain. Acknowledgments The authors are very grateful to J. Maixner and S. Bakardjieva for their kind assistance with XRD and SEM, respectively.
This work was a part of the Project no. 2A-1TP1/063, ‘‘New glass and ceramic materials and advanced concepts of their preparation and manufacturing’’, realized under financial support of the Ministry of industry and trade. References [1] J.R. Mackert Jr, M.B. Butts, C.W. Fairhurst, Dent. Mater. 2 (1986) 32. [2] J.R. Mackert Jr, A.L. Evans, J. Am. Ceram. Soc. 74 (1991) 450. [3] S.T. Rasmussen, C.I. McLaren, W.J. O‘Brien, J. Biomed. Mater. Res. B 69 (2004) 195. [4] I.L. Denry, J.R. Mackert, J.A. Holloway, S.F. Rosenstiel, J. Dent. Res. 75 (12) (1996) 1928. [5] M. Novotna´, A. Klouzˇkova´, J. Maixner, V. Sˇatava, Ceram-Silik. 49 (4) (2005) 252. [6] M. Novotna´, V. Sˇatava, J. Maixner, A. Klouzˇkova´, P. Kostka, D. Lezˇal, Glass Technol. 45 (2004) 105.