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Crystallization behavior and microstructure of barium borosilicate glass–ceramics
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CERAMICS INTERNATIONAL
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Crystallization behavior and microstructure of barium borosilicate glass–ceramics Huidong Li, Lang Wun, Xin Wang, Dong Xu, Yuancheng Teng, Yuxiang Li State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, China Received 17 July 2015; received in revised form 14 August 2015; accepted 19 August 2015
Abstract The crystallization mechanism and activation energy of glass–ceramics belonging to SiO2–B2O3–BaO–Na2O–CaO–TiO2–ZrO2–Nd2O3 system were investigated by Kissinger and Ozawa methods using differential thermal analysis (DTA) with different heating rates and particle sizes. The effect of different thermal treatment temperatures on the crystalline phases, microstructure of barium borosilicate glass–ceramics was also studied. The results show that two exothermal effects shift towards higher temperatures with increasing heating rate. In addition, the exothermal effects in the temperature range of 900–1050 1C shift towards higher temperatures with increasing particle size. The activation energy E (Kissinger method) and Avrami constant n associated with zirconolite are 124.38 kJ/mol and 3.4, respectively, indicating three-dimensional crystallization mechanism. The activation energy E (Kissinger method) and Avrami constant n associated with titanite are 166.13 kJ/mol and 2.2, respectively, indicating two-dimensional crystallization mechanism. Only strip-shaped zirconolite crystals are observed in the bulk of the glass–ceramics when the thermal treatment temperatures are 750–850 1C. Brick-shaped titanite crystals appear in the bulk of the glass–ceramics when the thermal treatment temperatures are higher than 900 1C. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: D. Glass–ceramics; Crystallization; Zirconolite; Titanite
1. Introduction High-level liquid wastes (HLLW), generated during reprocessing of spent fuel, contain fission product and actinides which must be immobilized in highly durable solid matrices for long-term storage and disposal. Borosilicate glass is one of the most widely used matrices for immobilization of HLLW due to its good chemical durability and ease of fabrication [1]. However, the major undesired properties such as metastable nature of glass and the low solubility ( 2 wt%) of actinides (Np, Pu and Am) in glass matrix limit the development of glass immobilization [2]. Moreover, due to poor solubility of sulfate (o 1 wt% SO3) in the borosilicate glass, an immiscible layer (known as yellow phase) usually forms during the vitrification of HLLW [3]. In this consideration, highly stable matrices, n
Corresponding author. Tel.: þ86 13890105181. E-mail address: [email protected] (L. Wu).
such as ceramics and glass–ceramics, have been developed for long-term disposal of HLLW. Glass–ceramics containing both glass matrix and durable crystalline phase have been recognized as host materials for HLLW immobilization, because they are more easily manufactured when compared with ceramics and generally possess higher durability, thermal stability and superior mechanical properties than glass [4–9]. Zirconolite (CaZrTi2O7)-based glass–ceramics have attracted a great deal of interest, because it is well-known for its excellent capacity to incorporate actinides into Ca and Zr sites of its structure [6–8]. In general, a better understanding of crystallization mechanism could lead to improved knowledge in varying glass–ceramic properties. The activation energy for the crystallization and the Avrami exponent (n) are the most important parameters to evaluate the crystallization mechanism of glass–ceramic materials. Several studies have been carried out on different glass systems to evaluate these parameters by
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Please cite this article as: H. Li, et al., Crystallization behavior and microstructure of barium borosilicate glass–ceramics, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.08.095
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differential thermal analysis (DTA) with different heating rates [10,11]. For example, Guo et al. [10] investigated the effects of complex nucleation agents (CaF2, CaF2 þ TiO2, CaF2 þ ZrO2, CaF2 þ P2O5) on the crystallization kinetics, microstructure and properties of CaO–MgO–Al2O3–SiO2 glass–ceramics. Wu et al. [11] reported that the activation energy (Eα) and the Avrami constant (n) of leucite in K2O–CaO–Al2O3–SiO2 glass–ceramics were 463.81 kJ/mol and 3.74, respectively. Additionally, they found that column crystals were transformed into fine granular grains when the sintering temperature changed from 900 1C to 1100 1C. Park et al. [12] reported that activation energies of crystallization in SiO2–MgO-fly ash-TiO2 glass–ceramics were 468–293 kJ/mol and decreased with increasing powder particle size. Loiseau et al. [7] studied both the bulk and surface crystallization processes of SiO2–Al2O3–CaO–TiO2–ZrO2 glass– ceramic system. The activation energy of defect-fluorite crystal growth in the bulk was found to be 440 kJ/mol. Titanite and anorthite growth from glass surface with activation energies of 493 and 405 kJ/mol, respectively. Recently, Mishra et al. [13] have reported that the incorporation of BaO as modifier in the borosilicate glass belonging to SiO2– B2O3–BaO–Na2O system shows marked improvement in enhancing the solubility of sulfate ( 3 mol%), which is very helpful to prevent the formation of yellow phase in the glass. Based on the above consideration, barium borosilicate glass–ceramic containing zirconolite crystalline phase as the matrix is proposed to enhance the solubility of actinides and sulfate [14]. In a previous work, the effects of nucleating agents CaO, TiO2 and ZrSiO4 addition on the crystalline phase, microstructure and chemical durability of barium borosilicate glass–ceramics were mainly investigated [14]. The zirconolite-2 M and titanite phases were obtained when the content of nucleating agents was greater than or equal to 45 wt% [14]. In order to better understand the crystallize mechanism of the barium borosilicate glass–ceramics, the effects of glass particle sizes (o75, 75–150, 150–380, 380–830 and 830–1400 μm) and heating rates (5, 10, 20, 30, 40 and 50 1C/min) on exothermic peaks of the glass–ceramics with 45 wt% nucleating agents were investigated using differential thermal analysis (DTA) in this study. The activation energy for crystallization of the glass–ceramics was calculated by Kissinger and Ozawa methods. The effect of different thermal treatment temperatures on the crystalline phases, microstructure of barium borosilicate glass–ceramics was also studied. 2. Experimental procedures The composition of barium borosilicate glass studied in this work was the following levels (wt%): SiO2 (25.50), B2O3 (10.20), BaO (10.20), Na2O (5.10), CaO (12.77), TiO2 (18.20), ZrO2 (14.03), Nd2O3 (4). Neodymium (III) ion was used as a surrogate to simulate the trivalent actinide ions, e.g. Am3 þ and Cm3 þ in a waste form. The mixture of high purity oxide precursors was calcined at 850 1C for 2 h and then melted at 1250 1C for 3 h in an alumina crucible to form homogeneous melt. The melt was poured into cold water to form homogeneous glass. The other desired compositions were mixed and melted, and then controlled from melting temperature to thermal treatment temperatures in the muffle furnace to prepare glass–ceramics.
The differential thermal analysis (DTA) was used with heating rates of 5, 10, 20, 30, 40 and 50 1C/min (particle size o 150 μm) to investigate crystallization kinetics occurring during reheating of the glass sample. Since the particle sizes of glass powders also usually have significant influence on crystallization kinetics [15], the as quenched glass was crushed and sieved in this work to obtain five different particle sizes, e. g. o 75 μm, 75–150 μm, 150–380 μm, 380–830 μm, 830– 1400 μm. All the glass–ceramics were characterized by Xray diffraction (XRD) using a X'Pert PRO Roentgen diffractometer system with Cu Kα rays (λ¼ 1.5418 Å) in the range of 2θ ¼ 10–801. The microstructure, elemental compositions of these samples was analyzed using a scanning electron microscope (SEM, S400) fitted with an energy-dispersive X-ray analyzer (EDX) accessory and optical microscope (4XC-PC). The slice glass–ceramics firstly were etched in 10 wt% HF solution for 10–15 s and then rinsed with distilled water and sonicated to remove any debris, glass–ceramics were coated with a film of gold before the microstructure of glass–ceramics was studied. In order to determine the kinetic parameters of crystallization under non-isothermal conditions, activation energy (E) of crystallization from the relationship between heating rate (α) and temperature of the maximum (Tp) in the exothermic peak of DTA curves was obtained using Kissinger Eq. (1) and Ozawa Eq. (2): ln
T 2p E ¼ þC RT p α
ln α ¼
E þ C1 RT p
ð1Þ ð2Þ
where Tp is the crystallization exothermal peak temperature of DTA curves, α is the heating rate, R is the universal gas constant and C, C1 are constant. A linear relationship between ln(T2p/α) or ln(α) and 1/Tp for the crystallization temperatures was obtained in according with Eqs. (1) or (2). From the value of the activation energy, the Avrami parameter (n) was calculated by Augis-Bennett Eq. (3). 2
2:5 RT p ð3Þ T E where ΔT is the half-height temperature width of the maximum exothermic peak of DTA, the crystallization index n is related to crystallization manner, n¼ 1 indicates one-dimensional growth (surface crystallization), n¼ 2 means two-dimensional growth and n¼ 3 implies three-dimensional growth (volumetric crystallization) [16]. n¼
3. Results and discussion 3.1. Crystallization behavior of glass–ceramics 3.1.1. Effect of heating rate Fig. 1 shows DTA curves of the fine glass powder (o150 μm) at different heating rates of 5, 10, 20, 30, 40 and 50 1C/min. It can be observed from Fig. 1 that there is no significant difference on
Please cite this article as: H. Li, et al., Crystallization behavior and microstructure of barium borosilicate glass–ceramics, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.08.095
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the glass transition temperatures (Tg 650 1C). Moreover, the DTA curves exhibit two exothermal peaks (Tp1 and Tp2), indicating the crystallization phenomenon of crystalline phases from the parent glass [17,18]. A progressive displacement of two exothermal peaks (Tp) towards higher temperature is observed with increasing heating rate from 5 to 50 1C/min. The detailed values of glass transition temperatures (Tg) and exothermal peak temperatures (Tp1 and Tp2) of glass at different heating rates are summarized in Table 1. Plots used for the determination of activation energy (E) of crystallization using Kissinger equation and Ozawa equation are shown in Fig. 2. The activation energy E1 and E2 associated with two exothermal peaks Tp1 and Tp2, which are calculated from the slope of the plots using the Kissinger method, are 124.38 and 166.13 kJ/mol, respectively. While using the Ozawa method, the activation energy E1 and E2 are 137.40 and 181.15 kJ/mol, respectively. The values obtained using the two equations are very similar. It is not surprising because these two equations are equivalent for a small Tp range. Usually, the activation energy E is related to the certain energy barrier when glassy phase transforms to crystalline phase [19,20]. In this work, the value of E1 is lower than E2 suggesting that it is easier for the glass to crystallize at crystallization exothermic peak Tp1. Moreover, it is noted that the values of the activation energy (E1 and E2) are remarkably lower than those in the literatures [10,11,21,22]. Using activation energy values, the Avrami parameters (n) are also determined. The value of n associated with the first crystallization exothermic peak Tp1 is about 3.4, suggesting that the crystallization might occur by threedimensional crystallization mechanism [16]. While the value of n associated with the second crystallization exothermic peak Tp2 is
3
about 2.2. This value appears close to 2, indicating that the crystallization might occur by two-dimensional crystallization mechanism [16]. 3.1.2. Effect of particle size DTA curves for glass powders with different particle sizes are presented in Fig. 3 (the heating rate remained constant 20 1C/ min). As shown in Fig. 3, the exothermal peaks of Tp1 are too weak to observe. The different particle size of glasses has also no significant effect on the glass transition temperatures (Tg). Moreover, it can be observed that the exothermal peaks corresponding to the Tp2 shift towards higher temperatures and the maximum height (δTp) decreases with increasing particle size. It is known that maximum height (δTp) is generally proportional to the total number of nuclei (internal and surface nuclei) present in the glass sample and to the maximum crystallization rate [7,23]. The decrease of δTp with increasing particle size can be explained by the fact that the total number of surface nuclei decreases, which suggests that the Tp2 might be associated with surface crystallization mechanism phenomenon [7]. Additional experimental data will be provided below for further discussion. 3.2. Microstructure of glass–ceramics In order to study the effects of the exothermic effects observed in Figs. 1 and 3 on the structure of glass–ceramics, the glass–ceramics were heated first at Tn=700 1C for 2 h and then heated at different crystallization temperatures (Tc=750 1
Fig. 1. DTA curves of glass powders with different heating rates.
Table 1 The glass transition temperatures (Tg) and exothermal peak temperatures (Tp1 and Tp2) of glass powders (o 150 μm) with different heating rates. α (1C/min)
Tg (1C)
Tp1 (1C)
Tp2 (1C)
5 10 20 30 40 50
628 637 645 649 654 656
742 751 774 799 811 818
861 884 907 926 937 947
Fig. 2. Plots used for the determination of activation energy (E) of crystallization using the Kissinger method (a) and Ozawa method (b).
Please cite this article as: H. Li, et al., Crystallization behavior and microstructure of barium borosilicate glass–ceramics, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.08.095
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C, 800 1C, 850 1C, 900 1C, 950 1C, 1000 1C and 1050 1C, respectively) for 2 h. The XRD patterns of the glass–ceramics are shown in Fig. 4. Only zirconolite phase (PDF no.17-0495) is observed at temperatures between 750 1C and 850 1C. As crystallization temperature rises to 900 1C, a few amounts of weak peaks corresponding to titanite (PDF no.73-2066) are present. The existence of titanite crystalline phase in the glass– ceramics is not serious because the titanite crystalline phase is also known for its excellent stability and good capacity to incorporate actinides [24]. And zirconolite–sphene synroc has recently been proposed for HLLW immobilization in our group [25]. Moreover, the intensity of the diffraction peaks of zirconolite phase increases as the thermal treatment temperature increases from 750 to 950 1C, and then decreases with further increasing temperatures. While the intensity of the diffraction peaks of titanite phase increases as the thermal treatment temperature increases from 900 to 1050 1C. Based on the above analyses, it seems that the exothermal peaks Tp1 and Tp2 (Fig. 1) correspond to the crystallization of zirconolite and titanite, respectively. Fig. 5 shows SEM images and EDX spectrums of the bulk and surface for glass–ceramics at different thermal treatment temperatures. As shown in Fig. 5(a)–(g), strip-shaped zirconolite crystals exist in the bulk of all the glass–ceramics. Thermal treatment temperatures have no significantly influence on the shape of crystals in the bulk of glass–ceramics when the temperatures are lower than 1050 1C. It is interesting to notice
Fig. 3. DTA curves of glass powders with different particle sizes.
that for the glass–ceramics heated at 1050 1C (Fig. 5(g)), some small crystal particles around strip-shaped crystals are observed. According to EDX analysis (Fig. 5(i) and (k)), the strip-shaped and the small crystal particles correspond to zirconolite and titanite, respectively. This might be due to the fact that the zirconolite can react with excess silica of the glass matrix to form titanite when the crystal growth temperature is relatively high [4,26]. Fig. 5(h) shows the SEM image of the bulk and surface for glass–ceramics heated at 1000 1C. It can be observed in Fig. 5(h) that there exist some strip-shaped crystals in the bulk and platelet-like crystals on the surface. The platelet-like crystals are also zirconolite according to the EDX analysis (Fig. 5(l)). The different shapes of the zirconolite crystals might be due to the different angles of observation. In this work, titanite crystals are not detected on the surface of barium borosilicate glass– ceramics. This is different from that of SiO2–Al2O3–CaO–TiO2– ZrO2 system reported by Loiseau et al. [7]. The optical micrograph and EDX spectrum of the bulk for glass–ceramics heated at Tc ¼ 950 1C without etching in the HF solution are shown in Fig. 6. It can be seen that the strip-shaped zirconolite and brick-shaped titanite crystals are observed in the glass–ceramics. As shown in Fig. 5, the titanite crystals are not observed in the bulk of the glass–ceramics after etching in the HF solution. This might be due to the fact that the titanite can react with HF. While using optical microscope without etching in the HF solution, the titanite crystals are observed in the bulk of the glass–ceramics. It is generally accepted that excess energy may exist at the defects (e.g., grain boundary, impurity, pores, macro and micro surface) of materials. It is easier to nucleate and grow at the defects when compared to that of the glass and crystals in the bulk. In this study, it is thought that as heterogeneous nucleation, titanite may also nucleate and grow at the defects such as the interface between glass and zirconolite crystals, pores, and so on. Further investigation on the interface effect of the crystal growth is necessary in future works. Moreover, due to the low values of the activation energy (E1 and E2) in this work, the glass–ceramic was also prepared from melting temperature to room temperature in muffle furnace without any thermal treatment. The XRD patterns and SEM image of the sample are shown in Fig. 7. It can be observed in Fig. 7 that the primary crystalline phase of the glass–ceramics is zirconolite. A small amount of ZrO2 phase is also observed. Compared with Fig. 5, the size of zirconolite crystals in the glass– ceramics (Fig. 7) is smaller than that in the sample with thermal treatment. It is interesting to note the formation of zirconolite phase from melting temperature to room temperature in muffle furnace without any thermal treatment, which is very helpful to implement the glass–ceramics for immobilization of HLLW. 4. Conclusions
Fig. 4. XRD patterns of glass–ceramics at different thermal treatment temperatures (indicated in the figure).
Crystallization mechanism and activation energy of glass– ceramics belonging to SiO2–B2O3–BaO–Na2O–CaO–TiO2– ZrO2–Nd2O3 system were investigated by Kissinger and Ozawa methods using differential thermal analysis (DTA) with different heating rates and particle sizes. And the effect of different thermal treatment temperatures on the crystalline phases, microstructure of
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Fig. 5. SEM images of the bulk for glass–ceramics heated at (a) Tc ¼ 750 1C, (b) Tc ¼ 800 1C, (c) Tc ¼850 1C, (d) Tc ¼900 1C, (e) Tc ¼950 1C, (f) Tc ¼ 1000 1C, (g) Tc ¼1050 1C, of the bulk and surface for glass–ceramics heated at (h)Tc ¼1000 1C, and EDX spectrums of (i) T: zirconolite, (j) RG: residual glass, (k) S: titanite, (l) T1: zirconolite.
Fig. 6. Optical micrograph and EDX spectrum of glass–ceramics heated at Tc ¼ 950 1C.
barium borosilicate glass–ceramics was also studied. The results show that the different heating rates and particle sizes have no significant influence on the glass transition temperatures (Tg). With the heating rate increases, two exothermal effects shift towards higher temperatures. The obvious exothermal effects in the temperature range 900–1050 1C shift towards higher temperatures, and the maximum height (δTp) decreases with increasing particle size. Using the Kissinger method, the activation energies of zirconolite and titanite are E1 ¼ 124.38 kJ/mol and E2 ¼ 166.13 kJ/mol, respectively. While using the Ozawa method, the activation energies of zirconolite and titanite are E1 ¼ 137.40 kJ/mol and E2 ¼ 181.15 kJ/mol, respectively. The Avrami
parameters n of zirconolite and titanite are 3.4 and 2.2, respectively, indicating that zirconolite and titanite correspond to three-dimensional and two-dimensional crystallization mechanism, respectively. Only strip-shaped zirconolite crystals are observed in bulk of the glass–ceramics when the thermal treatment temperatures are 750–850 1C. Both strip-shaped zirconolite and brick-shaped titanite crystals exist in the bulk when the thermal treatment temperatures are higher than 900 1C. Moreover, the formation of zirconolite phase from melting temperature to room temperature in muffle furnace without any thermal treatment is very helpful to implement the glass–ceramics for immobilization of HLLW.
Please cite this article as: H. Li, et al., Crystallization behavior and microstructure of barium borosilicate glass–ceramics, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.08.095
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Fig. 7. XRD pattern (a) and SEM image (b) of glass–ceramics that from melting temperature to room temperature without any thermal treatment.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 11305135), the Scientific Research Innovation Team Project (No. 14tdfk02) and the Open Project (No. 11zxfk26) of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (Southwest University of Science and Technology). References [1] J. Ramkumar, S. Chandramouleeswaran, V. Sudarsan, R.K. Mishra, C.P. Kaushik, K. Raj, T. Mukherjee, A.K. Tyagi, Borosilicate glasses modified with organic ligands: a new selective approach for the removal of uranyl ion, J. Hazardous Mater. 154 (2008) 513–518. [2] P.G. Eller, G.D. Jarvinen, J.D. Purson, R.A. Penneman, R.R. Ryan, F.W. Lytle, R.B. Greegor, Actinide valences in borosilicate glass, Radiochim. Acta 39 (1985) 17–22. [3] D. Manara, A. Grandjean, O. Pinet, J.L. Dussossoy, D.R. Neuville, Sulfur behavior in silicate glasses and melts: implications for sulfate incorporation in nuclear waste glasses as a function of alkali cation and V2O5 content, J. Non-Cryst. Solids 353 (2007) 12–23. [4] D. Caurant, O. Majerus, P. Loiseau, I. Bardez, N. Baffier, J.L. Dussossoy, Crystallization of neodymium-rich phases in silicate glasses developed for nuclear waste immobilization, J. Nucl. Mater. 354 (2006) 143–162. [5] M. Malek, M.R. Khani, P. Alizadeh, H. Kazemian, Composite wasteform based on SiO2–PbO–CaO–ZrO2–TiO2–(B2O3–K2O) parent glass with zircon as the second component, Ceram. Int. 35 (2009) 1689–1692. [6] M. Mahmoudysepehr, V.K. Marghussian., SiO2–PbO–CaO–ZrO2–TiO2– (B2O3–K2O), A new zirconolite glass–ceramic system: crystallization behavior and microstructure evaluation, J. Am. Ceram. Soc. 92 (2009) 1540–1546. [7] P. Loiseau, D. Caurant, O. Majerus, N. Baffier, Crystallization study of (TiO2, ZrO2)-rich SiO2–Al2O3–CaO glasses part II surface and internal crystallization processes investigated by differential thermal analysis (DTA), J. Mater. Sci. 38 (2003) 853–864. [8] Y. Zhang, M.W.A. Stewart, H. Li, M.L. Carter, E.R. Vance, S. Moricca, Zirconolite-rich titanate ceramics for immobilisation of actinides – waste form/HIP can interactions and chemical durability, J. Nucl. Mater. 395 (2009) 69–74. [9] Y. Zhang, Z. Zhang, G. Thorogood, E.R. Vance, Pyrochlore based glass– ceramics for the immobilization of actinide-rich nuclear wastes: from concept to reality, J. Nucl. Mater. 432 (2013) 545–547. [10] X.Z. Guo, X.B. Cai, J. Song, G.Y. Yang, H. Yang, Crystallization and microstructure of CaO–MgO–Al2O3–SiO2 glass–ceramics containing complex nucleation agents, J. Non-Cryst. Solids 405 (2014) 63–67. [11] J.F. Wu, Z. Li, Y.Q. Huang, F. Li, Crystallization behavior and properties of K2O–CaO–Al2O3–SiO2 glass–ceramics, Ceram. Int. 39 (2013) 7743–7750.
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Please cite this article as: H. Li, et al., Crystallization behavior and microstructure of barium borosilicate glass–ceramics, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.08.095
CERAMICS INTERNATIONAL
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Crystallization behavior and microstructure of barium borosilicate glass–ceramics Huidong Li, Lang Wun, Xin Wang, Dong Xu, Yuancheng Teng, Yuxiang Li State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials, Southwest University of Science and Technology, Mianyang 621010, China Received 17 July 2015; received in revised form 14 August 2015; accepted 19 August 2015
Abstract The crystallization mechanism and activation energy of glass–ceramics belonging to SiO2–B2O3–BaO–Na2O–CaO–TiO2–ZrO2–Nd2O3 system were investigated by Kissinger and Ozawa methods using differential thermal analysis (DTA) with different heating rates and particle sizes. The effect of different thermal treatment temperatures on the crystalline phases, microstructure of barium borosilicate glass–ceramics was also studied. The results show that two exothermal effects shift towards higher temperatures with increasing heating rate. In addition, the exothermal effects in the temperature range of 900–1050 1C shift towards higher temperatures with increasing particle size. The activation energy E (Kissinger method) and Avrami constant n associated with zirconolite are 124.38 kJ/mol and 3.4, respectively, indicating three-dimensional crystallization mechanism. The activation energy E (Kissinger method) and Avrami constant n associated with titanite are 166.13 kJ/mol and 2.2, respectively, indicating two-dimensional crystallization mechanism. Only strip-shaped zirconolite crystals are observed in the bulk of the glass–ceramics when the thermal treatment temperatures are 750–850 1C. Brick-shaped titanite crystals appear in the bulk of the glass–ceramics when the thermal treatment temperatures are higher than 900 1C. & 2015 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Keywords: D. Glass–ceramics; Crystallization; Zirconolite; Titanite
1. Introduction High-level liquid wastes (HLLW), generated during reprocessing of spent fuel, contain fission product and actinides which must be immobilized in highly durable solid matrices for long-term storage and disposal. Borosilicate glass is one of the most widely used matrices for immobilization of HLLW due to its good chemical durability and ease of fabrication [1]. However, the major undesired properties such as metastable nature of glass and the low solubility ( 2 wt%) of actinides (Np, Pu and Am) in glass matrix limit the development of glass immobilization [2]. Moreover, due to poor solubility of sulfate (o 1 wt% SO3) in the borosilicate glass, an immiscible layer (known as yellow phase) usually forms during the vitrification of HLLW [3]. In this consideration, highly stable matrices, n
Corresponding author. Tel.: þ86 13890105181. E-mail address: [email protected] (L. Wu).
such as ceramics and glass–ceramics, have been developed for long-term disposal of HLLW. Glass–ceramics containing both glass matrix and durable crystalline phase have been recognized as host materials for HLLW immobilization, because they are more easily manufactured when compared with ceramics and generally possess higher durability, thermal stability and superior mechanical properties than glass [4–9]. Zirconolite (CaZrTi2O7)-based glass–ceramics have attracted a great deal of interest, because it is well-known for its excellent capacity to incorporate actinides into Ca and Zr sites of its structure [6–8]. In general, a better understanding of crystallization mechanism could lead to improved knowledge in varying glass–ceramic properties. The activation energy for the crystallization and the Avrami exponent (n) are the most important parameters to evaluate the crystallization mechanism of glass–ceramic materials. Several studies have been carried out on different glass systems to evaluate these parameters by
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Please cite this article as: H. Li, et al., Crystallization behavior and microstructure of barium borosilicate glass–ceramics, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.08.095
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differential thermal analysis (DTA) with different heating rates [10,11]. For example, Guo et al. [10] investigated the effects of complex nucleation agents (CaF2, CaF2 þ TiO2, CaF2 þ ZrO2, CaF2 þ P2O5) on the crystallization kinetics, microstructure and properties of CaO–MgO–Al2O3–SiO2 glass–ceramics. Wu et al. [11] reported that the activation energy (Eα) and the Avrami constant (n) of leucite in K2O–CaO–Al2O3–SiO2 glass–ceramics were 463.81 kJ/mol and 3.74, respectively. Additionally, they found that column crystals were transformed into fine granular grains when the sintering temperature changed from 900 1C to 1100 1C. Park et al. [12] reported that activation energies of crystallization in SiO2–MgO-fly ash-TiO2 glass–ceramics were 468–293 kJ/mol and decreased with increasing powder particle size. Loiseau et al. [7] studied both the bulk and surface crystallization processes of SiO2–Al2O3–CaO–TiO2–ZrO2 glass– ceramic system. The activation energy of defect-fluorite crystal growth in the bulk was found to be 440 kJ/mol. Titanite and anorthite growth from glass surface with activation energies of 493 and 405 kJ/mol, respectively. Recently, Mishra et al. [13] have reported that the incorporation of BaO as modifier in the borosilicate glass belonging to SiO2– B2O3–BaO–Na2O system shows marked improvement in enhancing the solubility of sulfate ( 3 mol%), which is very helpful to prevent the formation of yellow phase in the glass. Based on the above consideration, barium borosilicate glass–ceramic containing zirconolite crystalline phase as the matrix is proposed to enhance the solubility of actinides and sulfate [14]. In a previous work, the effects of nucleating agents CaO, TiO2 and ZrSiO4 addition on the crystalline phase, microstructure and chemical durability of barium borosilicate glass–ceramics were mainly investigated [14]. The zirconolite-2 M and titanite phases were obtained when the content of nucleating agents was greater than or equal to 45 wt% [14]. In order to better understand the crystallize mechanism of the barium borosilicate glass–ceramics, the effects of glass particle sizes (o75, 75–150, 150–380, 380–830 and 830–1400 μm) and heating rates (5, 10, 20, 30, 40 and 50 1C/min) on exothermic peaks of the glass–ceramics with 45 wt% nucleating agents were investigated using differential thermal analysis (DTA) in this study. The activation energy for crystallization of the glass–ceramics was calculated by Kissinger and Ozawa methods. The effect of different thermal treatment temperatures on the crystalline phases, microstructure of barium borosilicate glass–ceramics was also studied. 2. Experimental procedures The composition of barium borosilicate glass studied in this work was the following levels (wt%): SiO2 (25.50), B2O3 (10.20), BaO (10.20), Na2O (5.10), CaO (12.77), TiO2 (18.20), ZrO2 (14.03), Nd2O3 (4). Neodymium (III) ion was used as a surrogate to simulate the trivalent actinide ions, e.g. Am3 þ and Cm3 þ in a waste form. The mixture of high purity oxide precursors was calcined at 850 1C for 2 h and then melted at 1250 1C for 3 h in an alumina crucible to form homogeneous melt. The melt was poured into cold water to form homogeneous glass. The other desired compositions were mixed and melted, and then controlled from melting temperature to thermal treatment temperatures in the muffle furnace to prepare glass–ceramics.
The differential thermal analysis (DTA) was used with heating rates of 5, 10, 20, 30, 40 and 50 1C/min (particle size o 150 μm) to investigate crystallization kinetics occurring during reheating of the glass sample. Since the particle sizes of glass powders also usually have significant influence on crystallization kinetics [15], the as quenched glass was crushed and sieved in this work to obtain five different particle sizes, e. g. o 75 μm, 75–150 μm, 150–380 μm, 380–830 μm, 830– 1400 μm. All the glass–ceramics were characterized by Xray diffraction (XRD) using a X'Pert PRO Roentgen diffractometer system with Cu Kα rays (λ¼ 1.5418 Å) in the range of 2θ ¼ 10–801. The microstructure, elemental compositions of these samples was analyzed using a scanning electron microscope (SEM, S400) fitted with an energy-dispersive X-ray analyzer (EDX) accessory and optical microscope (4XC-PC). The slice glass–ceramics firstly were etched in 10 wt% HF solution for 10–15 s and then rinsed with distilled water and sonicated to remove any debris, glass–ceramics were coated with a film of gold before the microstructure of glass–ceramics was studied. In order to determine the kinetic parameters of crystallization under non-isothermal conditions, activation energy (E) of crystallization from the relationship between heating rate (α) and temperature of the maximum (Tp) in the exothermic peak of DTA curves was obtained using Kissinger Eq. (1) and Ozawa Eq. (2): ln
T 2p E ¼ þC RT p α
ln α ¼
E þ C1 RT p
ð1Þ ð2Þ
where Tp is the crystallization exothermal peak temperature of DTA curves, α is the heating rate, R is the universal gas constant and C, C1 are constant. A linear relationship between ln(T2p/α) or ln(α) and 1/Tp for the crystallization temperatures was obtained in according with Eqs. (1) or (2). From the value of the activation energy, the Avrami parameter (n) was calculated by Augis-Bennett Eq. (3). 2
2:5 RT p ð3Þ T E where ΔT is the half-height temperature width of the maximum exothermic peak of DTA, the crystallization index n is related to crystallization manner, n¼ 1 indicates one-dimensional growth (surface crystallization), n¼ 2 means two-dimensional growth and n¼ 3 implies three-dimensional growth (volumetric crystallization) [16]. n¼
3. Results and discussion 3.1. Crystallization behavior of glass–ceramics 3.1.1. Effect of heating rate Fig. 1 shows DTA curves of the fine glass powder (o150 μm) at different heating rates of 5, 10, 20, 30, 40 and 50 1C/min. It can be observed from Fig. 1 that there is no significant difference on
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the glass transition temperatures (Tg 650 1C). Moreover, the DTA curves exhibit two exothermal peaks (Tp1 and Tp2), indicating the crystallization phenomenon of crystalline phases from the parent glass [17,18]. A progressive displacement of two exothermal peaks (Tp) towards higher temperature is observed with increasing heating rate from 5 to 50 1C/min. The detailed values of glass transition temperatures (Tg) and exothermal peak temperatures (Tp1 and Tp2) of glass at different heating rates are summarized in Table 1. Plots used for the determination of activation energy (E) of crystallization using Kissinger equation and Ozawa equation are shown in Fig. 2. The activation energy E1 and E2 associated with two exothermal peaks Tp1 and Tp2, which are calculated from the slope of the plots using the Kissinger method, are 124.38 and 166.13 kJ/mol, respectively. While using the Ozawa method, the activation energy E1 and E2 are 137.40 and 181.15 kJ/mol, respectively. The values obtained using the two equations are very similar. It is not surprising because these two equations are equivalent for a small Tp range. Usually, the activation energy E is related to the certain energy barrier when glassy phase transforms to crystalline phase [19,20]. In this work, the value of E1 is lower than E2 suggesting that it is easier for the glass to crystallize at crystallization exothermic peak Tp1. Moreover, it is noted that the values of the activation energy (E1 and E2) are remarkably lower than those in the literatures [10,11,21,22]. Using activation energy values, the Avrami parameters (n) are also determined. The value of n associated with the first crystallization exothermic peak Tp1 is about 3.4, suggesting that the crystallization might occur by threedimensional crystallization mechanism [16]. While the value of n associated with the second crystallization exothermic peak Tp2 is
3
about 2.2. This value appears close to 2, indicating that the crystallization might occur by two-dimensional crystallization mechanism [16]. 3.1.2. Effect of particle size DTA curves for glass powders with different particle sizes are presented in Fig. 3 (the heating rate remained constant 20 1C/ min). As shown in Fig. 3, the exothermal peaks of Tp1 are too weak to observe. The different particle size of glasses has also no significant effect on the glass transition temperatures (Tg). Moreover, it can be observed that the exothermal peaks corresponding to the Tp2 shift towards higher temperatures and the maximum height (δTp) decreases with increasing particle size. It is known that maximum height (δTp) is generally proportional to the total number of nuclei (internal and surface nuclei) present in the glass sample and to the maximum crystallization rate [7,23]. The decrease of δTp with increasing particle size can be explained by the fact that the total number of surface nuclei decreases, which suggests that the Tp2 might be associated with surface crystallization mechanism phenomenon [7]. Additional experimental data will be provided below for further discussion. 3.2. Microstructure of glass–ceramics In order to study the effects of the exothermic effects observed in Figs. 1 and 3 on the structure of glass–ceramics, the glass–ceramics were heated first at Tn=700 1C for 2 h and then heated at different crystallization temperatures (Tc=750 1
Fig. 1. DTA curves of glass powders with different heating rates.
Table 1 The glass transition temperatures (Tg) and exothermal peak temperatures (Tp1 and Tp2) of glass powders (o 150 μm) with different heating rates. α (1C/min)
Tg (1C)
Tp1 (1C)
Tp2 (1C)
5 10 20 30 40 50
628 637 645 649 654 656
742 751 774 799 811 818
861 884 907 926 937 947
Fig. 2. Plots used for the determination of activation energy (E) of crystallization using the Kissinger method (a) and Ozawa method (b).
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C, 800 1C, 850 1C, 900 1C, 950 1C, 1000 1C and 1050 1C, respectively) for 2 h. The XRD patterns of the glass–ceramics are shown in Fig. 4. Only zirconolite phase (PDF no.17-0495) is observed at temperatures between 750 1C and 850 1C. As crystallization temperature rises to 900 1C, a few amounts of weak peaks corresponding to titanite (PDF no.73-2066) are present. The existence of titanite crystalline phase in the glass– ceramics is not serious because the titanite crystalline phase is also known for its excellent stability and good capacity to incorporate actinides [24]. And zirconolite–sphene synroc has recently been proposed for HLLW immobilization in our group [25]. Moreover, the intensity of the diffraction peaks of zirconolite phase increases as the thermal treatment temperature increases from 750 to 950 1C, and then decreases with further increasing temperatures. While the intensity of the diffraction peaks of titanite phase increases as the thermal treatment temperature increases from 900 to 1050 1C. Based on the above analyses, it seems that the exothermal peaks Tp1 and Tp2 (Fig. 1) correspond to the crystallization of zirconolite and titanite, respectively. Fig. 5 shows SEM images and EDX spectrums of the bulk and surface for glass–ceramics at different thermal treatment temperatures. As shown in Fig. 5(a)–(g), strip-shaped zirconolite crystals exist in the bulk of all the glass–ceramics. Thermal treatment temperatures have no significantly influence on the shape of crystals in the bulk of glass–ceramics when the temperatures are lower than 1050 1C. It is interesting to notice
Fig. 3. DTA curves of glass powders with different particle sizes.
that for the glass–ceramics heated at 1050 1C (Fig. 5(g)), some small crystal particles around strip-shaped crystals are observed. According to EDX analysis (Fig. 5(i) and (k)), the strip-shaped and the small crystal particles correspond to zirconolite and titanite, respectively. This might be due to the fact that the zirconolite can react with excess silica of the glass matrix to form titanite when the crystal growth temperature is relatively high [4,26]. Fig. 5(h) shows the SEM image of the bulk and surface for glass–ceramics heated at 1000 1C. It can be observed in Fig. 5(h) that there exist some strip-shaped crystals in the bulk and platelet-like crystals on the surface. The platelet-like crystals are also zirconolite according to the EDX analysis (Fig. 5(l)). The different shapes of the zirconolite crystals might be due to the different angles of observation. In this work, titanite crystals are not detected on the surface of barium borosilicate glass– ceramics. This is different from that of SiO2–Al2O3–CaO–TiO2– ZrO2 system reported by Loiseau et al. [7]. The optical micrograph and EDX spectrum of the bulk for glass–ceramics heated at Tc ¼ 950 1C without etching in the HF solution are shown in Fig. 6. It can be seen that the strip-shaped zirconolite and brick-shaped titanite crystals are observed in the glass–ceramics. As shown in Fig. 5, the titanite crystals are not observed in the bulk of the glass–ceramics after etching in the HF solution. This might be due to the fact that the titanite can react with HF. While using optical microscope without etching in the HF solution, the titanite crystals are observed in the bulk of the glass–ceramics. It is generally accepted that excess energy may exist at the defects (e.g., grain boundary, impurity, pores, macro and micro surface) of materials. It is easier to nucleate and grow at the defects when compared to that of the glass and crystals in the bulk. In this study, it is thought that as heterogeneous nucleation, titanite may also nucleate and grow at the defects such as the interface between glass and zirconolite crystals, pores, and so on. Further investigation on the interface effect of the crystal growth is necessary in future works. Moreover, due to the low values of the activation energy (E1 and E2) in this work, the glass–ceramic was also prepared from melting temperature to room temperature in muffle furnace without any thermal treatment. The XRD patterns and SEM image of the sample are shown in Fig. 7. It can be observed in Fig. 7 that the primary crystalline phase of the glass–ceramics is zirconolite. A small amount of ZrO2 phase is also observed. Compared with Fig. 5, the size of zirconolite crystals in the glass– ceramics (Fig. 7) is smaller than that in the sample with thermal treatment. It is interesting to note the formation of zirconolite phase from melting temperature to room temperature in muffle furnace without any thermal treatment, which is very helpful to implement the glass–ceramics for immobilization of HLLW. 4. Conclusions
Fig. 4. XRD patterns of glass–ceramics at different thermal treatment temperatures (indicated in the figure).
Crystallization mechanism and activation energy of glass– ceramics belonging to SiO2–B2O3–BaO–Na2O–CaO–TiO2– ZrO2–Nd2O3 system were investigated by Kissinger and Ozawa methods using differential thermal analysis (DTA) with different heating rates and particle sizes. And the effect of different thermal treatment temperatures on the crystalline phases, microstructure of
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Fig. 5. SEM images of the bulk for glass–ceramics heated at (a) Tc ¼ 750 1C, (b) Tc ¼ 800 1C, (c) Tc ¼850 1C, (d) Tc ¼900 1C, (e) Tc ¼950 1C, (f) Tc ¼ 1000 1C, (g) Tc ¼1050 1C, of the bulk and surface for glass–ceramics heated at (h)Tc ¼1000 1C, and EDX spectrums of (i) T: zirconolite, (j) RG: residual glass, (k) S: titanite, (l) T1: zirconolite.
Fig. 6. Optical micrograph and EDX spectrum of glass–ceramics heated at Tc ¼ 950 1C.
barium borosilicate glass–ceramics was also studied. The results show that the different heating rates and particle sizes have no significant influence on the glass transition temperatures (Tg). With the heating rate increases, two exothermal effects shift towards higher temperatures. The obvious exothermal effects in the temperature range 900–1050 1C shift towards higher temperatures, and the maximum height (δTp) decreases with increasing particle size. Using the Kissinger method, the activation energies of zirconolite and titanite are E1 ¼ 124.38 kJ/mol and E2 ¼ 166.13 kJ/mol, respectively. While using the Ozawa method, the activation energies of zirconolite and titanite are E1 ¼ 137.40 kJ/mol and E2 ¼ 181.15 kJ/mol, respectively. The Avrami
parameters n of zirconolite and titanite are 3.4 and 2.2, respectively, indicating that zirconolite and titanite correspond to three-dimensional and two-dimensional crystallization mechanism, respectively. Only strip-shaped zirconolite crystals are observed in bulk of the glass–ceramics when the thermal treatment temperatures are 750–850 1C. Both strip-shaped zirconolite and brick-shaped titanite crystals exist in the bulk when the thermal treatment temperatures are higher than 900 1C. Moreover, the formation of zirconolite phase from melting temperature to room temperature in muffle furnace without any thermal treatment is very helpful to implement the glass–ceramics for immobilization of HLLW.
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Fig. 7. XRD pattern (a) and SEM image (b) of glass–ceramics that from melting temperature to room temperature without any thermal treatment.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant no. 11305135), the Scientific Research Innovation Team Project (No. 14tdfk02) and the Open Project (No. 11zxfk26) of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (Southwest University of Science and Technology). References [1] J. Ramkumar, S. Chandramouleeswaran, V. Sudarsan, R.K. Mishra, C.P. Kaushik, K. Raj, T. Mukherjee, A.K. Tyagi, Borosilicate glasses modified with organic ligands: a new selective approach for the removal of uranyl ion, J. Hazardous Mater. 154 (2008) 513–518. [2] P.G. Eller, G.D. Jarvinen, J.D. Purson, R.A. Penneman, R.R. Ryan, F.W. Lytle, R.B. Greegor, Actinide valences in borosilicate glass, Radiochim. Acta 39 (1985) 17–22. [3] D. Manara, A. Grandjean, O. Pinet, J.L. Dussossoy, D.R. Neuville, Sulfur behavior in silicate glasses and melts: implications for sulfate incorporation in nuclear waste glasses as a function of alkali cation and V2O5 content, J. Non-Cryst. Solids 353 (2007) 12–23. [4] D. Caurant, O. Majerus, P. Loiseau, I. Bardez, N. Baffier, J.L. Dussossoy, Crystallization of neodymium-rich phases in silicate glasses developed for nuclear waste immobilization, J. Nucl. Mater. 354 (2006) 143–162. [5] M. Malek, M.R. Khani, P. Alizadeh, H. Kazemian, Composite wasteform based on SiO2–PbO–CaO–ZrO2–TiO2–(B2O3–K2O) parent glass with zircon as the second component, Ceram. Int. 35 (2009) 1689–1692. [6] M. Mahmoudysepehr, V.K. Marghussian., SiO2–PbO–CaO–ZrO2–TiO2– (B2O3–K2O), A new zirconolite glass–ceramic system: crystallization behavior and microstructure evaluation, J. Am. Ceram. Soc. 92 (2009) 1540–1546. [7] P. Loiseau, D. Caurant, O. Majerus, N. Baffier, Crystallization study of (TiO2, ZrO2)-rich SiO2–Al2O3–CaO glasses part II surface and internal crystallization processes investigated by differential thermal analysis (DTA), J. Mater. Sci. 38 (2003) 853–864. [8] Y. Zhang, M.W.A. Stewart, H. Li, M.L. Carter, E.R. Vance, S. Moricca, Zirconolite-rich titanate ceramics for immobilisation of actinides – waste form/HIP can interactions and chemical durability, J. Nucl. Mater. 395 (2009) 69–74. [9] Y. Zhang, Z. Zhang, G. Thorogood, E.R. Vance, Pyrochlore based glass– ceramics for the immobilization of actinide-rich nuclear wastes: from concept to reality, J. Nucl. Mater. 432 (2013) 545–547. [10] X.Z. Guo, X.B. Cai, J. Song, G.Y. Yang, H. Yang, Crystallization and microstructure of CaO–MgO–Al2O3–SiO2 glass–ceramics containing complex nucleation agents, J. Non-Cryst. Solids 405 (2014) 63–67. [11] J.F. Wu, Z. Li, Y.Q. Huang, F. Li, Crystallization behavior and properties of K2O–CaO–Al2O3–SiO2 glass–ceramics, Ceram. Int. 39 (2013) 7743–7750.
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Please cite this article as: H. Li, et al., Crystallization behavior and microstructure of barium borosilicate glass–ceramics, Ceramics International (2015), http: //dx.doi.org/10.1016/j.ceramint.2015.08.095