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Transparent phosphosilicate glasses containing crystals formed during cooling of melts
Journal of Non-Crystalline Solids 357 (2011) 3897–3900
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Transparent phosphosilicate glasses containing crystals formed during cooling of melts S.J. Liu a, Y.F. Zhang a, W. He a, Y.Z. Yue a, b,⁎ a b
Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, Shandong Polytechnic University, Jinan 250353, China Section of Chemistry, Aalborg University, DK-9000 Aalborg, Denmark
a r t i c l e
i n f o
Article history: Received 23 May 2011 Received in revised form 20 July 2011 Available online 3 September 2011 Keywords: Transparent glass ceramics; Phosphosilicate; Spontaneous crystallization
a b s t r a c t Effect of P2O5–SiO2 substitution on spontaneous crystallization of SiO2–Al2O3–P2O5–Na2O–MgO melts during cooling was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and rotation viscometry. Results show that addition of P2O5 leads to amorphous phase separation (APS), i.e., phosphate- and silicate-rich phases. It is due to the tendency of Mg 2+ to form [MgO4] linking with [SiO4]. Molar substitution of P2O5 for SiO2 enhances the network polymerization of silicate-rich phase in the melts, and thereby the spontaneous crystallization of cubic Na2MgSiO4 is also enhanced during cooling of the melts. In addition, the sizes of the local crystalline and separated glassy domains are smaller than the wavelength of the visible light, and this leads to the transparency of the obtained glasses containing crystals. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, much attention has been devoted to optically transparent glass-ceramic (TGC) materials with better optical performances, greater thermal stability and higher strength than their parent glasses [1,2]. These transparent crystalline materials have been applied in many fields such as substrates for arrayed waveguide grating [3,4], optical amplifiers for wavelength up-conversion [5], solar collectors, printed optical circuits, gradient refraction optical lenses, optical waveguides [6–8], and so on. Development of TGCs with different types of crystalline phases, e.g., β-quartz solid solution, mullite, spinel and oxyfluoride, has been reported [1,4,7]. Usually, one of the conditions for obtaining low-scattering TGCs is that the refractive index of the crystalline phase should be close to that of the residual glass phase, and the birefringence of the crystal is sufficiently small. The other condition is that the grain size of the crystal should be much smaller than the wavelength of visible light [9]. Regarding the production procedure of TGCs, parent glasses are first synthesized, and subsequently they are subjected to the controlled nucleation and crystallization processes via appropriate heat treatment, during which phase separation and growth of crystals may take place and consequently result in formation of crystals in the glass matrix. However, if controlled nucleation and/or crystallization processes could be conducted during cooling of a melt in a controlled way, the production of TGCs will be energy-saving and more efficient. ⁎ Corresponding author at: Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, Shandong Polytechnic University, Jinan 250353, China. Tel.: + 45 9635 8522; fax: + 45 9635 0558. E-mail address: [email protected] (Y.Z. Yue). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.07.029
Opal glass is a kind of glass ceramic obtained via spontaneous crystallization during quenching of the melts, usually forming fluoride as the primary crystalline phases [10,11]. Our previous work indicated that the type of alkaline earth oxide has important effect on the transparency of the phosphosilicate glasses containing crystals formed during cooling of melts [12,13]. To our best knowledge, few studies have been carried out so far concerning phosphosilicate transparent glass ceramics formed by spontaneous crystallization during cooling of melts. The emphasis of the present work is placed on the impact of P2O5–SiO2 substitution on spontaneous crystallization behavior in the phosphosilicate glass system. 2. Experimental procedure The glass composition studied in this work is SiO2–Al2O3–P2O5– Na2O–MgO system as shown in Table 1, for which P2O5 content increases from 3.25 to 4.0 mol% at the price of decrease of SiO2, so the glasses are named as P1, P2, P3 and P4, respectively. As a comparison, MgO in the sample P1 is replaced with CaO and results in the sample C. The mixtures of reagent-grade chemicals (SiO2, Al2O3, (NH4)2HPO4, Na2CO3, MgO, and CaCO3) were preheated at 673 K for 12 h and at 1273 K for 2 h to remove NH3 and CO2, and then completely melted in a lidded platinum crucible at 1773 K in an electric furnace for 2 h. Then the melts were poured into stainless steel molds and cooled naturally down to room temperature in air. The obtained samples were cut into the dimensions 4 × 4 × 1 mm, and were performed for the isobaric heat capacity (Cp) measurement by using differential scanning calorimetry (DSC) (Setaram STA Labsys evo). A platinum crucible containing the samples and an empty platinum crucible were placed on the sample carrier of the DSC at
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In this study, all the samples were obtained under the same conditions; dense opalescence can be observed in the appearance of sample C, for which CaO is the only type of alkaline earth oxide. This indicates that phase transformation must take place in sample C during cooling of the melt, and this is verified by the XRD patterns in Fig. 1. It is clear that sharp diffraction peaks are present in sample C, and the main crystalline phase is identified to be NaCaPO4 (ICDD 030751). However, when MgO is introduced into the glass composition, as shown in Fig. 1, samples P1–P4 show quite different diffraction peaks from that of sample C. Na2MgSiO4 (ICDD 19-1216) is found to be the main crystalline phase for these samples instead of NaCaPO4
crystals, and the intensity of diffraction peaks drastically increases with increasing the P2O5 content. It should be noted that the four samples P1–P4 are transparent and colorless even though crystals have precipitated during cooling of the melts, and particularly for sample P4 which shows the largest degree of crystallization, transparency still remains (see the photograph of sample P4 in the inset of Fig. 1). The mean crystalline sizes were calculated using the Scherrer equation for the samples P1–P4 [15], and they are 14, 16, 20 and 25 nm in the sequence of P1, P2, P3 and P4, respectively. The formation of these nano-crystals should be ascribed to a diffusion layer which acts as the barrier to hamper the further crystal growth [16–18]. In detail, the diffusion layer around Na2MgSiO4 is enriched in the glass-forming components since the present samples belong to non-isochemical glass system, and this increases the viscosity, and hence, hinders the diffusion of modifying ions (e.g. Na + and Mg 2+ ions) to Na2MgSiO4 crystals. To further verify this inference, the XRD measurement was also conducted on sample P4 that was subjected to the heat treatment at the onset temperature of crystallization (Tonset = 920 K) for 2 h. As shown in Fig. 1, the intensity of diffraction peaks of the heat-treated sample P4 is slightly higher than that of the as-cooled sample P4. However, the former still exhibits transparency. The mean crystalline size of heat-treated sample P4 was calculated to be 27 nm, indicating that crystalline size is almost the same, within the limits of error, as that of the as-cooled sample P4. In addition, the compositional comparison between sample C and sample P1 indicates that alkaline earth oxide type (MgO and CaO) should be the key factor for different crystalline phases and degree of crystallization, since the content of alkaline earth oxide is the same for both compositions. The temperature dependences of Cp for the samples with various P2O5 content are shown in Fig. 2. Samples P1–P3 show similar Cp curves on heating the temperature, i.e., the rises of Cp around 790 to 820 K are ascribed to glass transition followed by a broad exothermic peak around 920 to 1050 K due to crystallization. However, sample P4 has an endothermic peak at lower temperature of 745 K, besides the characteristics peaks corresponding to glass transition and crystallization like samples P1–P3. This low temperature endothermic peak may be related to polymorphic transformation of sample P4, since a sharp endothermic peak occurs at 710 K on the downscan DSC curve. For sample P4 heat-treated at Tonset for 2 h, a sharper endothermic peak below glass transition region can be seen compared to as-cooled sample P4 (Fig. 2), and this is attributed to an increasing extent of crystallization as shown in Fig. 1. Furthermore, the increase of degree
Fig. 1. XRD patterns for samples P1–P4 obtained by cooling of the melts in air, and for sample P4 subjected to a subsequent heat-treatment at 920 K for 2 h. Inset: photograph of sample P4 with a thickness of about 7 mm.
Fig. 2. Isobaric heat capacities (Cp) versus temperature (T) for the obtained samples P1– P4 (solid lines) and sample P4 heat-treated at 920 K for 2 h (dashed line). The Cp curves were obtained by a differential scanning calorimeter at the upscan rate of 10 K/min. Inset: the Cp curve of sample P4 during downscan at 10 K/min.
Table 1 The chemical composition of phosphosilicate glasses (mol%).
C P1 P2 P3 P4
SiO2
Al2O3
P2O5
Na2O
MgO
CaO
59.25 59.25 59 58.75 58.5
2.5 2.5 2.5 2.5 2.5
3.25 3.25 3.5 3.75 4.0
19 19 19 19 19
0 16 16 16 16
16 0 0 0 0
room temperature. Both crucibles were held 5 min at an initial temperature of 333 K, and then heated at a rate of 10 K/min to the maximum temperature. Before measuring each sample, a baseline was measured by using two empty platinum crucibles according to the above-stated heating procedure. To determine the specific heat capacity, a sapphire sample as reference was measured after the measurement of the baseline. All the measurements were performed under N2 atmosphere. In order to identify the crystalline phases in all samples, X-ray powder diffraction (XRD) measurements were performed using X-ray diffractometer (BRUKER AXS D8-Advance) with graphite monochromatized Cu Kα1 radiation. For scanning electron microscope (SEM) observation (FEI Quanta 200) fresh etched fracture surfaces of samples were prepared by etching them in 5 wt% HF solution for 20 s. High temperature viscosity data are measured using SRV-1600 viscometer. The standard glass NIST SRM 717A was used for calibration of the viscometers [14]. Good agreement was achieved between the measured and the standard values, with the measurement error of ±0.2 Pa s. 3. Results
S.J. Liu et al. / Journal of Non-Crystalline Solids 357 (2011) 3897–3900
of crystallization is also reflected in the decrease of intensity of exothermic peak between 920 and 1050 K, which is correlated with crystallization [19,20]. The SEM micrographs of the fracture surfaces for the obtained samples are shown in Fig. 3. For sample P1, although individual crystals are not clearly observable, a phase separation-like morphology is observed. This is consistent with the results of other phosphosilicate systems [21,22]. However, further substitution of P2O5 for SiO2 leads to different morphologies for samples P2–P4. The fine continuous and interconnected textures with an amount of isolated pores can be observed, and the isolated pores should be related to the erosion of glassy phase with weaker acid resistance. Moreover, the size of isolated pores in heat-treated sample P4 is larger than that of not heat treated one (see Figs. 3(d) and (e)). This indicates that the heat treatment at 920 K for 2 h enhances the tendency of phase separation. 4. Discussion According to the present work, P2O5 is the key factor to the spontaneous crystallization during cooling of the melts. In phosphosilicate glass melts, both P 5+ and Si 4+ have high ionic field strength (43.2 and 23.8, respectively) [23], and have strong attraction to oxygen ions in the glass network. When P2O5 concentration is high enough, the competition in attracting oxygen ions between two network-former ions causes amorphous phase separation (APS), i.e., phosphate- and silicate-rich phases [24,25], and hence induces the subsequent crystallization during cooling; this is reflected by the XRD results. As demonstrated by Ryerson and Mysen [26,27], the structural role of P2O5 can change in metal silicate melts, and it depends on the degree of polymerization. For all compositions in this study, we suggest that P2O5 tends to form discrete anion complexes via attracting cations around [PO4] units due to abundant modifier ions [26,27]. For the sample C, the phase separation gives rise to the transfer tendency of network modifier such as Na + and Ca 2+ ions to phosphate portion, and form discrete metal phosphate complex in the melt. Such cooling of the melt leads to the formation of NaCaPO4 crystals in sample C, this is consistent with the result of O'Donnell [28,29]. However, the appearance and crystalline phase of samples P1–P4 which contain only alkaline earth oxide MgO are quite different from
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those of sample C. This means that the role of MgO differs from that of CaO for spontaneous crystallization of the melts in this study. Generally, the coordination number of Mg 2+ in glass depends on the value of R + (alkali ion)/Mg 2+[30], and Mg 2+ exists all along in form of [MgO4] as network former when the value of R +/Mg 2+ is larger than 1.33 [31]. Since Na +/Mg 2+ values are much higher than 1.33 in the samples P1–P4, i.e., the presence of a large amount of oxygen available from Na2O can meet the coordination of [MgO4] units; Mg 2+ is mainly present in tetrahedral coordination. Tetrahedrally-coordinated Mg 2+ tends to link to [SiO4] rather than [PO4], since the structure of [MgO4] is similar to that of [SiO4]. In other words, a kind of medium-range ordered structure might exist in this glass system, i.e., the structural units in the melts are not fully randomly distributed to a certain extent in a certain range of the temperature above liquidus temperatures [32]. In this circumstance, P 5+ ions do not participate in the formation of crystals during cooling. In addition, two Na + ions have to be bonded to each [MgO4] unit to keep the charge-neutralization, which decreases the amount of nonbridging oxygen in the silicate-rich phase. This is reflected by the dependence of viscosity (η) of melts on temperature (T) as shown in Fig. 4, from which the viscosity of sample P1 is larger than that of sample C. Furthermore, equilibrium viscosity measurement cannot be obtained at T b 1600 K for sample C due to crystallization. Crystallization is not detected in samples P1–P4, and this is attributed to the formation of [MgO4] units [33], which suppresses the crystallization. It is obvious that compositions P1–P4 are more favorable for designing an efficient and energy-saving production process of glass ceramics than composition C, since the former ones are more suitable for a continuous forming process (without sudden boost of viscosity in a large temperature range) in comparison to the latter one. In other words, glass ceramics with a targeted degree of transparency can be obtained from compositions P1–P4 simply by controlling the cooling rate of the casting or forming processes of their corresponding melts. For the samples P1–P4, the substitution of P2O5 for SiO2has an effect on the spontaneous crystallization during cooling of the melts. On the one hand, the P2O5–SiO2 substitution hinders the spontaneous crystallization due to the increase of η as shown in Fig. 4, and this is because the substitution results in an increase of the amount of the network forming ions, since the number of P 5+ doubles that of Si 4+ in per mole network forming oxides. On the other hand, the P2O5–SiO2 substitution enhances the crystallization due to the fact that the
Fig. 3. SEM images of the fracture surfaces of the obtained samples P1–P4 and sample P4 heat-treated at 920 K for 2 h. (a) P1; (b) P2; (c) P3; (d) P4 and (e) P4 heat-treated at 920 K for 2 h.
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than to [PO4] units leads to spontaneous crystallization of cubic NaMgSiO4 during cooling of the melts. The transparency of the samples mainly results from the small sizes of the optical scattering centers, i.e., from the weak optical scattering. In addition, a partial substitution of P2O5 for SiO2 enhances the spontaneous crystallization of Na2MgSiO4, but transparency still remains when the P2O5 content is up to 4.0 mol%. Acknowledgements This work was supported by the Natural Science Fund of Elite Young Researchers of Shandong Province No. 2008BS04004, Science and Technology Star Plan of Young Researchers of Jinan City No. 20080118 and the Taishan Scholar Fund of Shandong Province. References Fig. 4. Dependence of viscosity on temperature of the glass melts with various P2O5 content.
linking trend of [MgO4] to [SiO4] is promoted by addition of P2O5. This is verified by an increase of diffraction peak intensity of Na2MgSiO4 as shown Fig. 1. Moreover, it can be seen that glass transition temperatures (Tg) increases with substituting P2O5 for SiO2 from the Cp curves in Fig. 2. This is also related to the increase of crystallization degree of Na2MgSiO4, i.e., the network polymerization degree of residual silicate-rich glassy phase increases with a decrease in the non-bridging oxygen concentration. In the present work, the extent of spontaneous crystallization increases with increasing P2O5 content, but relatively high transparency can be observed for the samples P1–P4. As shown in Fig. 3, typical phase separation is seen in the SEM images, and the samples contain at least three phases such as silicate- and phosphate-rich glassy phases, and crystalline phase. However, the sizes of the second phases as scattering center are far less than the minimum wavelength of visible light, indicating weak optical scattering, and hence high transparency. In addition, theoretically there is no optical birefringence for cubic crystal Na2MgSiO4, which also favors the transparency. From the evolution of the diffraction peak intensity of crystals and morphologies of the samples as a function of R = P2O5/(SiO2 + P2O5), it is possible to obtain a kind of transparent glass ceramics, in which the spontaneous crystallization takes place during cooling of the melts. 5. Conclusion A series of transparent glass-ceramics is prepared from the SiO2– Al2O3–P2O5–Na2O–MgO system by changing the P2O5/SiO2 ratio. The coexistence of SiO2 and P2O5 causes phase separation, and the linking preference of tetrahedrally-coordinated Mg 2+ to [SiO4] units rather
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H. Scheidler, E. Rodek, Am. Ceram. Soc. Bull. 68 (1989) 1926. G.H. Beall, D.A. Duke, J. Mater. Sci. 4 (1969) 340. A.S. Gouveia-Neto, E.B. da Costa, L.A. Bueno, S.J.L. Ribeiro, J. Alloys Comp. 375 (2004) 224. X.L. Duan, D.R. Yuan, C.N. Luan, Z.H. Sun, D. Xu, M.K. Lv, J. Non-Cryst. Solids 328 (2003) 245. A.S. Gouveia-Neto, E.B. da Costa, L.A. Bueno, S.J.L. Ribeiro, J. Lumin. 110 (2004) 79. M.C. Gonçalves, L.F. Santos, R.M. Almeida, C. R. Chim. 5 (2002) 845. L.L. Kukkonen, I.M. Reaney, D. Furniss, M.G. Pellatt, A.B. Seddon, J. Non-Cryst. Solids 290 (2001) 25. S. Todoroki, S. Inoue, Appl. Surf. Sci. 223 (2004) 39. G.H. Beall, L.R. Pinckney, J. Am. Ceram. Soc. 82 (1999) 5. J.L. Stempin, D.R. Wexell, U. S. Patent 5 (591) (1997) 683. N.F. Borrelli, J.E. Dickinson, J.E. Pierson, S.D. Stookey, U.S. Patent 4 (979) (1990) 975. S.J. Liu, G.R. Li, Y.Z. Yue, L.N. Hu, W. He, Glass Technol.: Eur. J. Glass Sci. Technol. A 52 (2011) 67. S.J. Liu, Y.F. Zhang, Y.Z. Yue, Phys. Chem. Glasses: Eur. J. Glass Sci. Technol. B 52 (2011) 85. NIST, Gaithersburg, MD, 20899, USA; September 18, 1996. H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures: For Polycrystalline and Amorphous Materials, Wiley, New York, 1974. C. Bocker, S. Bhattacharyya, T. Hoche, C. Rüssel, Acta Mater. 57 (2009) 5956. S. Bhattacharyya, T. Hoche, Cryst. Growth Des. 10 (2010) 379. A. de Pablos-Martín, G.C. Mather, F. Munoz, S. Bhattacharyya, T. Höche, J.R. Jinschek, T. Heil, A. Durán, M.J. Pascual, J. Non-Cryst. Solids 356 (2010) 3071. M. Moesgaard, H.D. Pedersen, Y.Z. Yue, E.R. Nielsen, J. Non-Cryst. Solids 353 (2007) 1101. M.M. Smedskjaer, Y.Z. Yue, S. Morup, J. Am. Ceram. Soc. 93 (2010) 2161. W. Holand, V. Rheinberger, S. Wegner, M. Frank, J. Mater. Sci. Mater. Med. 11 (2000) 11. M. Arbab, V.K. Marghussian, H. Sarpoolaky, Ceram. Int. 33 (2007) 943. M. Mirsaneh, I.M. Reaney, P.V. Hatton, J. Non-Cryst. Solids 354 (2008) 3362. J.M. Oliveira, R.N. Correia, M.H. Fernandes, J. Non-Cryst. Solids 273 (2000) 59. T. Höche, C. Moisescu, I. Avramov, C. Russel, W.D. Heerdegen, C. Jager, Chem. Mater. 13 (2001) 1320. F.J. Ryerson, P.C. Hess, Geochim. Cosmochim. Acta 44 (1978) 611. B.O. Mysen, F.J. Ryerson, D. Virgo. Am. Mineral. 66 (1981) 106. M.D. O'Donnell, S.J. Watts, R.V. Law, R.G. Hill, J. Non-Cryst. Solids 354 (2008) 3554. M.D. O'Donnell, S.J. Watts, R.V. Law, R.G. Hill, J. Non-Cryst. Solids 354 (2008) 3561. H. Scholze, Springer Verlag, Berlin Heidelberg, 1988, p. 60. V.V. Gorbachev, A.S. Bystrikov, S.K. Vasilev, L.V. Bogomolova, Soviet J. Glass Phys. Chem. 9 (1983) 447. Y.Z. Yue, J. Non-Cryst. Solids 345–346 (2004) 523. K.D. Kim, J. Non-Cryst. Solids 354 (2008) 1715.
Contents lists available at SciVerse ScienceDirect
Journal of Non-Crystalline Solids j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j n o n c r y s o l
Transparent phosphosilicate glasses containing crystals formed during cooling of melts S.J. Liu a, Y.F. Zhang a, W. He a, Y.Z. Yue a, b,⁎ a b
Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, Shandong Polytechnic University, Jinan 250353, China Section of Chemistry, Aalborg University, DK-9000 Aalborg, Denmark
a r t i c l e
i n f o
Article history: Received 23 May 2011 Received in revised form 20 July 2011 Available online 3 September 2011 Keywords: Transparent glass ceramics; Phosphosilicate; Spontaneous crystallization
a b s t r a c t Effect of P2O5–SiO2 substitution on spontaneous crystallization of SiO2–Al2O3–P2O5–Na2O–MgO melts during cooling was studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and rotation viscometry. Results show that addition of P2O5 leads to amorphous phase separation (APS), i.e., phosphate- and silicate-rich phases. It is due to the tendency of Mg 2+ to form [MgO4] linking with [SiO4]. Molar substitution of P2O5 for SiO2 enhances the network polymerization of silicate-rich phase in the melts, and thereby the spontaneous crystallization of cubic Na2MgSiO4 is also enhanced during cooling of the melts. In addition, the sizes of the local crystalline and separated glassy domains are smaller than the wavelength of the visible light, and this leads to the transparency of the obtained glasses containing crystals. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Recently, much attention has been devoted to optically transparent glass-ceramic (TGC) materials with better optical performances, greater thermal stability and higher strength than their parent glasses [1,2]. These transparent crystalline materials have been applied in many fields such as substrates for arrayed waveguide grating [3,4], optical amplifiers for wavelength up-conversion [5], solar collectors, printed optical circuits, gradient refraction optical lenses, optical waveguides [6–8], and so on. Development of TGCs with different types of crystalline phases, e.g., β-quartz solid solution, mullite, spinel and oxyfluoride, has been reported [1,4,7]. Usually, one of the conditions for obtaining low-scattering TGCs is that the refractive index of the crystalline phase should be close to that of the residual glass phase, and the birefringence of the crystal is sufficiently small. The other condition is that the grain size of the crystal should be much smaller than the wavelength of visible light [9]. Regarding the production procedure of TGCs, parent glasses are first synthesized, and subsequently they are subjected to the controlled nucleation and crystallization processes via appropriate heat treatment, during which phase separation and growth of crystals may take place and consequently result in formation of crystals in the glass matrix. However, if controlled nucleation and/or crystallization processes could be conducted during cooling of a melt in a controlled way, the production of TGCs will be energy-saving and more efficient. ⁎ Corresponding author at: Key Laboratory of Processing and Testing Technology of Glass & Functional Ceramics of Shandong Province, Shandong Polytechnic University, Jinan 250353, China. Tel.: + 45 9635 8522; fax: + 45 9635 0558. E-mail address: [email protected] (Y.Z. Yue). 0022-3093/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2011.07.029
Opal glass is a kind of glass ceramic obtained via spontaneous crystallization during quenching of the melts, usually forming fluoride as the primary crystalline phases [10,11]. Our previous work indicated that the type of alkaline earth oxide has important effect on the transparency of the phosphosilicate glasses containing crystals formed during cooling of melts [12,13]. To our best knowledge, few studies have been carried out so far concerning phosphosilicate transparent glass ceramics formed by spontaneous crystallization during cooling of melts. The emphasis of the present work is placed on the impact of P2O5–SiO2 substitution on spontaneous crystallization behavior in the phosphosilicate glass system. 2. Experimental procedure The glass composition studied in this work is SiO2–Al2O3–P2O5– Na2O–MgO system as shown in Table 1, for which P2O5 content increases from 3.25 to 4.0 mol% at the price of decrease of SiO2, so the glasses are named as P1, P2, P3 and P4, respectively. As a comparison, MgO in the sample P1 is replaced with CaO and results in the sample C. The mixtures of reagent-grade chemicals (SiO2, Al2O3, (NH4)2HPO4, Na2CO3, MgO, and CaCO3) were preheated at 673 K for 12 h and at 1273 K for 2 h to remove NH3 and CO2, and then completely melted in a lidded platinum crucible at 1773 K in an electric furnace for 2 h. Then the melts were poured into stainless steel molds and cooled naturally down to room temperature in air. The obtained samples were cut into the dimensions 4 × 4 × 1 mm, and were performed for the isobaric heat capacity (Cp) measurement by using differential scanning calorimetry (DSC) (Setaram STA Labsys evo). A platinum crucible containing the samples and an empty platinum crucible were placed on the sample carrier of the DSC at
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S.J. Liu et al. / Journal of Non-Crystalline Solids 357 (2011) 3897–3900
In this study, all the samples were obtained under the same conditions; dense opalescence can be observed in the appearance of sample C, for which CaO is the only type of alkaline earth oxide. This indicates that phase transformation must take place in sample C during cooling of the melt, and this is verified by the XRD patterns in Fig. 1. It is clear that sharp diffraction peaks are present in sample C, and the main crystalline phase is identified to be NaCaPO4 (ICDD 030751). However, when MgO is introduced into the glass composition, as shown in Fig. 1, samples P1–P4 show quite different diffraction peaks from that of sample C. Na2MgSiO4 (ICDD 19-1216) is found to be the main crystalline phase for these samples instead of NaCaPO4
crystals, and the intensity of diffraction peaks drastically increases with increasing the P2O5 content. It should be noted that the four samples P1–P4 are transparent and colorless even though crystals have precipitated during cooling of the melts, and particularly for sample P4 which shows the largest degree of crystallization, transparency still remains (see the photograph of sample P4 in the inset of Fig. 1). The mean crystalline sizes were calculated using the Scherrer equation for the samples P1–P4 [15], and they are 14, 16, 20 and 25 nm in the sequence of P1, P2, P3 and P4, respectively. The formation of these nano-crystals should be ascribed to a diffusion layer which acts as the barrier to hamper the further crystal growth [16–18]. In detail, the diffusion layer around Na2MgSiO4 is enriched in the glass-forming components since the present samples belong to non-isochemical glass system, and this increases the viscosity, and hence, hinders the diffusion of modifying ions (e.g. Na + and Mg 2+ ions) to Na2MgSiO4 crystals. To further verify this inference, the XRD measurement was also conducted on sample P4 that was subjected to the heat treatment at the onset temperature of crystallization (Tonset = 920 K) for 2 h. As shown in Fig. 1, the intensity of diffraction peaks of the heat-treated sample P4 is slightly higher than that of the as-cooled sample P4. However, the former still exhibits transparency. The mean crystalline size of heat-treated sample P4 was calculated to be 27 nm, indicating that crystalline size is almost the same, within the limits of error, as that of the as-cooled sample P4. In addition, the compositional comparison between sample C and sample P1 indicates that alkaline earth oxide type (MgO and CaO) should be the key factor for different crystalline phases and degree of crystallization, since the content of alkaline earth oxide is the same for both compositions. The temperature dependences of Cp for the samples with various P2O5 content are shown in Fig. 2. Samples P1–P3 show similar Cp curves on heating the temperature, i.e., the rises of Cp around 790 to 820 K are ascribed to glass transition followed by a broad exothermic peak around 920 to 1050 K due to crystallization. However, sample P4 has an endothermic peak at lower temperature of 745 K, besides the characteristics peaks corresponding to glass transition and crystallization like samples P1–P3. This low temperature endothermic peak may be related to polymorphic transformation of sample P4, since a sharp endothermic peak occurs at 710 K on the downscan DSC curve. For sample P4 heat-treated at Tonset for 2 h, a sharper endothermic peak below glass transition region can be seen compared to as-cooled sample P4 (Fig. 2), and this is attributed to an increasing extent of crystallization as shown in Fig. 1. Furthermore, the increase of degree
Fig. 1. XRD patterns for samples P1–P4 obtained by cooling of the melts in air, and for sample P4 subjected to a subsequent heat-treatment at 920 K for 2 h. Inset: photograph of sample P4 with a thickness of about 7 mm.
Fig. 2. Isobaric heat capacities (Cp) versus temperature (T) for the obtained samples P1– P4 (solid lines) and sample P4 heat-treated at 920 K for 2 h (dashed line). The Cp curves were obtained by a differential scanning calorimeter at the upscan rate of 10 K/min. Inset: the Cp curve of sample P4 during downscan at 10 K/min.
Table 1 The chemical composition of phosphosilicate glasses (mol%).
C P1 P2 P3 P4
SiO2
Al2O3
P2O5
Na2O
MgO
CaO
59.25 59.25 59 58.75 58.5
2.5 2.5 2.5 2.5 2.5
3.25 3.25 3.5 3.75 4.0
19 19 19 19 19
0 16 16 16 16
16 0 0 0 0
room temperature. Both crucibles were held 5 min at an initial temperature of 333 K, and then heated at a rate of 10 K/min to the maximum temperature. Before measuring each sample, a baseline was measured by using two empty platinum crucibles according to the above-stated heating procedure. To determine the specific heat capacity, a sapphire sample as reference was measured after the measurement of the baseline. All the measurements were performed under N2 atmosphere. In order to identify the crystalline phases in all samples, X-ray powder diffraction (XRD) measurements were performed using X-ray diffractometer (BRUKER AXS D8-Advance) with graphite monochromatized Cu Kα1 radiation. For scanning electron microscope (SEM) observation (FEI Quanta 200) fresh etched fracture surfaces of samples were prepared by etching them in 5 wt% HF solution for 20 s. High temperature viscosity data are measured using SRV-1600 viscometer. The standard glass NIST SRM 717A was used for calibration of the viscometers [14]. Good agreement was achieved between the measured and the standard values, with the measurement error of ±0.2 Pa s. 3. Results
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of crystallization is also reflected in the decrease of intensity of exothermic peak between 920 and 1050 K, which is correlated with crystallization [19,20]. The SEM micrographs of the fracture surfaces for the obtained samples are shown in Fig. 3. For sample P1, although individual crystals are not clearly observable, a phase separation-like morphology is observed. This is consistent with the results of other phosphosilicate systems [21,22]. However, further substitution of P2O5 for SiO2 leads to different morphologies for samples P2–P4. The fine continuous and interconnected textures with an amount of isolated pores can be observed, and the isolated pores should be related to the erosion of glassy phase with weaker acid resistance. Moreover, the size of isolated pores in heat-treated sample P4 is larger than that of not heat treated one (see Figs. 3(d) and (e)). This indicates that the heat treatment at 920 K for 2 h enhances the tendency of phase separation. 4. Discussion According to the present work, P2O5 is the key factor to the spontaneous crystallization during cooling of the melts. In phosphosilicate glass melts, both P 5+ and Si 4+ have high ionic field strength (43.2 and 23.8, respectively) [23], and have strong attraction to oxygen ions in the glass network. When P2O5 concentration is high enough, the competition in attracting oxygen ions between two network-former ions causes amorphous phase separation (APS), i.e., phosphate- and silicate-rich phases [24,25], and hence induces the subsequent crystallization during cooling; this is reflected by the XRD results. As demonstrated by Ryerson and Mysen [26,27], the structural role of P2O5 can change in metal silicate melts, and it depends on the degree of polymerization. For all compositions in this study, we suggest that P2O5 tends to form discrete anion complexes via attracting cations around [PO4] units due to abundant modifier ions [26,27]. For the sample C, the phase separation gives rise to the transfer tendency of network modifier such as Na + and Ca 2+ ions to phosphate portion, and form discrete metal phosphate complex in the melt. Such cooling of the melt leads to the formation of NaCaPO4 crystals in sample C, this is consistent with the result of O'Donnell [28,29]. However, the appearance and crystalline phase of samples P1–P4 which contain only alkaline earth oxide MgO are quite different from
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those of sample C. This means that the role of MgO differs from that of CaO for spontaneous crystallization of the melts in this study. Generally, the coordination number of Mg 2+ in glass depends on the value of R + (alkali ion)/Mg 2+[30], and Mg 2+ exists all along in form of [MgO4] as network former when the value of R +/Mg 2+ is larger than 1.33 [31]. Since Na +/Mg 2+ values are much higher than 1.33 in the samples P1–P4, i.e., the presence of a large amount of oxygen available from Na2O can meet the coordination of [MgO4] units; Mg 2+ is mainly present in tetrahedral coordination. Tetrahedrally-coordinated Mg 2+ tends to link to [SiO4] rather than [PO4], since the structure of [MgO4] is similar to that of [SiO4]. In other words, a kind of medium-range ordered structure might exist in this glass system, i.e., the structural units in the melts are not fully randomly distributed to a certain extent in a certain range of the temperature above liquidus temperatures [32]. In this circumstance, P 5+ ions do not participate in the formation of crystals during cooling. In addition, two Na + ions have to be bonded to each [MgO4] unit to keep the charge-neutralization, which decreases the amount of nonbridging oxygen in the silicate-rich phase. This is reflected by the dependence of viscosity (η) of melts on temperature (T) as shown in Fig. 4, from which the viscosity of sample P1 is larger than that of sample C. Furthermore, equilibrium viscosity measurement cannot be obtained at T b 1600 K for sample C due to crystallization. Crystallization is not detected in samples P1–P4, and this is attributed to the formation of [MgO4] units [33], which suppresses the crystallization. It is obvious that compositions P1–P4 are more favorable for designing an efficient and energy-saving production process of glass ceramics than composition C, since the former ones are more suitable for a continuous forming process (without sudden boost of viscosity in a large temperature range) in comparison to the latter one. In other words, glass ceramics with a targeted degree of transparency can be obtained from compositions P1–P4 simply by controlling the cooling rate of the casting or forming processes of their corresponding melts. For the samples P1–P4, the substitution of P2O5 for SiO2has an effect on the spontaneous crystallization during cooling of the melts. On the one hand, the P2O5–SiO2 substitution hinders the spontaneous crystallization due to the increase of η as shown in Fig. 4, and this is because the substitution results in an increase of the amount of the network forming ions, since the number of P 5+ doubles that of Si 4+ in per mole network forming oxides. On the other hand, the P2O5–SiO2 substitution enhances the crystallization due to the fact that the
Fig. 3. SEM images of the fracture surfaces of the obtained samples P1–P4 and sample P4 heat-treated at 920 K for 2 h. (a) P1; (b) P2; (c) P3; (d) P4 and (e) P4 heat-treated at 920 K for 2 h.
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than to [PO4] units leads to spontaneous crystallization of cubic NaMgSiO4 during cooling of the melts. The transparency of the samples mainly results from the small sizes of the optical scattering centers, i.e., from the weak optical scattering. In addition, a partial substitution of P2O5 for SiO2 enhances the spontaneous crystallization of Na2MgSiO4, but transparency still remains when the P2O5 content is up to 4.0 mol%. Acknowledgements This work was supported by the Natural Science Fund of Elite Young Researchers of Shandong Province No. 2008BS04004, Science and Technology Star Plan of Young Researchers of Jinan City No. 20080118 and the Taishan Scholar Fund of Shandong Province. References Fig. 4. Dependence of viscosity on temperature of the glass melts with various P2O5 content.
linking trend of [MgO4] to [SiO4] is promoted by addition of P2O5. This is verified by an increase of diffraction peak intensity of Na2MgSiO4 as shown Fig. 1. Moreover, it can be seen that glass transition temperatures (Tg) increases with substituting P2O5 for SiO2 from the Cp curves in Fig. 2. This is also related to the increase of crystallization degree of Na2MgSiO4, i.e., the network polymerization degree of residual silicate-rich glassy phase increases with a decrease in the non-bridging oxygen concentration. In the present work, the extent of spontaneous crystallization increases with increasing P2O5 content, but relatively high transparency can be observed for the samples P1–P4. As shown in Fig. 3, typical phase separation is seen in the SEM images, and the samples contain at least three phases such as silicate- and phosphate-rich glassy phases, and crystalline phase. However, the sizes of the second phases as scattering center are far less than the minimum wavelength of visible light, indicating weak optical scattering, and hence high transparency. In addition, theoretically there is no optical birefringence for cubic crystal Na2MgSiO4, which also favors the transparency. From the evolution of the diffraction peak intensity of crystals and morphologies of the samples as a function of R = P2O5/(SiO2 + P2O5), it is possible to obtain a kind of transparent glass ceramics, in which the spontaneous crystallization takes place during cooling of the melts. 5. Conclusion A series of transparent glass-ceramics is prepared from the SiO2– Al2O3–P2O5–Na2O–MgO system by changing the P2O5/SiO2 ratio. The coexistence of SiO2 and P2O5 causes phase separation, and the linking preference of tetrahedrally-coordinated Mg 2+ to [SiO4] units rather
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