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Observation of nucleation effect on crystallization in lithium aluminosilicate glass by viscosity measurement
Journal of Non-Crystalline Solids 336 (2004) 195–201 www.elsevier.com/locate/jnoncrysol
Observation of nucleation effect on crystallization in lithium aluminosilicate glass by viscosity measurement Ki-Dong Kim a
a,*
, Seung-Heun Lee a, Hyo-Kwon Ahn
b
Department of Materials Science & Engineering, Kunsan National University, Kunsan, Chunbuk, South Korea b Technical Research Institute, Hankuk Glass Industry Co., Kunsan, Chunbuk, South Korea Received 12 December 2003; received in revised form 12 December 2003
Abstract The crystal nucleation effect in lithium aluminosilicate glasses was investigated by the viscosity measurement with aid of the fiber elongation method. The abrupt increase of viscosity due to the crystallization of glass was observed in viscosity–temperature curve but the minimum viscosity temperature (Tg ) related with crystallization showed a strong dependence on the nucleation state such as nucleation temperature, nucleation time and heating rate. The results by viscosity agreed well with those of DTA. The nucleation effect on the microstructure of glass-ceramics was also discussed. Finally, the nucleation effect on the crystallization kinetics was approached quantitatively by calculating the crystal volume from viscosity value. 2004 Elsevier B.V. All rights reserved. PACS: 64.70.P
1. Introduction Crystal nucleation is an important theme in glass science as well as in glass technology. In scientific viewpoint, crystal nucleation is the basis for the phase transition of glass. Therefore, knowledge of crystal nucleation is essential for understanding the meta-stable state of glasses and thus their transformation to thermodynamically stable phases, crystallization. The crystal nucleation in glass technology is undesirable for producing glass without devitrification but it is an important process for preparation of glass-ceramics via controlled crystallization. In studying the nucleation behavior of glasses such as determination of the maximum nucleation temperature/rate etc., two techniques are commonly used. One technique is to observe the crystals grown from nuclei by heat treatment because the dimension of nuclei is too small. This is time-consuming technique but it is possible to evaluate the nucleation effect quantitatively by using microscope [1–
*
Corresponding author. Tel.: +82-63 469 4737; fax.: +82-63 466 2086. E-mail address: [email protected] (K.-D. Kim). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.01.001
3]. The other one proposed recently is based on differential thermal analysis (DTA). Since the crystallization peak temperature in DTA curve depends on nucleation treatment of glass, the shift of peak temperature plays a role of indicator showing the influence of nucleation state indirectly [4–7]. As the phase separation has influence on the resulting viscosity of glasses [8], the viscosity of partially crystallized glass is also different from that of original glass [9]. Especially, according to some works [10–12] the occurrence of crystal precipitation could be recognized from the slope change of viscosity curve when the viscosity of glass was measured continuously under isothermal or non-isothermal condition in crystallization temperature range. Therefore, it is expected that the continuous determination of viscosity could be also a good method in studying the nucleation state of glasses. In fact, in the processing of glass-ceramics the knowledge of viscosity change during crystallization is important practically because it can give information about the body stiffness and the annealing [12]. In the present work, the influence of various nucleation states on the crystallization of a lithium aluminosilicate glass was investigated by viscosity–temperature and viscosity–time relation determined with aid of fiber elongation method. Based on the
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201
results of viscosity, the maximum nucleation temperature, the dependence of microstructure on the nucleation state and the crystallization kinetics were discussed.
2. Experimental The investigated glass composition belongs to the system of Li2 O–Al2 O3 –SiO2 glass-ceramics with low thermal expansion. Table 1 contains the detailed composition of the experimental glass. The raw materials used were reagent grade chemicals. As a fining agent, As2 O3 was used. The batches of 300 g were mixed thoroughly and melted in a Pt/20Rh crucible at 1650 C in an electric furnace. About 30 min was allowed for the melts to be homogenized by Pt/Rh-stirrer. Two types of glass specimen were prepared. One is fiber type for viscosity measurement. Glasses in fiber type were prepared by direct drawing from melts in crucible. They had no inclusion and their dimension was 0.65 ± 0.1 mm in diameter and 65 mm in length. The other was disk type formed on a graphite mold for another measurements. To determine the glass viscosity continuously, fiber elongation method using a vertical tube furnace whose temperature is controlled within ±0.5 C was employed. Glass fiber specimens were as quenched or subjected to isothermal treatment for nucleation prior to viscosity measurement. After inserting glass fiber into the tube furnace, its length change was checked with increase of temperature or time. The viscosity (g) was calculated by using a following equation suggested by Robbinson [13]: g¼
gM l1 l2 ðt2 t1 Þ; 3V ðl2 l1 Þ
where, g is the acceleration of gravity (m/s2 ), M is the load applied to the fiber (kg), l1 and l2 are length of fiber at the time of t1 and t2 (m), respectively and V is the volume of the glass fiber specimen before heating (m3 ). In the present work the load (M) of 50 g was applied. Hence, the maximum value of ðl2 l1 Þ and ðt2 t1 Þ in a measured viscosity range lead to 0.04 m and 1400 s approximately. Viscosities obtained were reproducible within less than 5%. From the viscosity–temperature curve the crystallization temperature, namely the viscosity minimum temperature (Tg ) was determined. Unlike the homogeneous glasses, the viscosity of the heterogeneous glasses like partially crystallized glass via bulk crystallization is called an apparent viscosity because their viscosity is regarded as the viscosity of the
composite consisting of glass phase and crystalline solids. Hence, from the beginning of crystallization, namely passing through Tg the viscosity in the above equation is described as an apparent viscosity that increases generally with proceeding of crystallization [14]. For the comparison with the results obtained from the viscosity measurement, DTA run (Setaram 92-18, France) for some specimens with the same nucleation state as viscosity measurement was performed. The glass powders of 60 mg with particle size between 425 and 500 lm were used for DTA. For nucleation the glass powders had been subjected to isothermal heat treatment and then they were subsequently heated at 10 C/min up to 1000 C to complete the crystallization. The crystallization peak temperature (Tp ) occurring in DTA curve and its shift were compared with the viscosity minimum temperature (Tg ) shown in viscosity–temperature curve. In order to confirm the nucleation effect on the crystallization by occurrence of new phase or its peak intensity, X-ray diffraction analysis (XRD: M18XCE, Bruker, Germany) was performed for some heat-treated specimens. Finally, the microstructure analysis was conducted on some specimens using scanning electron microscopy (SEM: JSM-5410, Jeol, Japan) to examine the dependence of the microstructure of resulting glassceramics on the nucleation state.
3. Results 3.1. Influence of nucleation temperature The viscosity of five glass specimens was plotted as a function of temperature in Fig. 1. Prior to viscosity measurement the glasses were nucleated at 650, 680, 700, 725 and 750 C for 1 h, respectively. The viscosity was measured at a heating rate of 3 C/min from 755 C. The viscosity curves in Fig. 1 have a minimum around 800
10.8
10.6
10.4
logη (dPas)
196
10.2
650 °C 680 °C 700 °C
10.0
725 °C 750 ° C
Table 1 Composition (wt%) of experimental glass
9.8 770
SiO2
Al2 O3
R2 O
RO
RO2
P2 O5
As2 O3
61.3
21.3
4.1
6.9
3.5
2.1
0.8
R2 O: Li2 O + K2 O, RO: MgO + BaO + ZnO, RO2 : ZrO2 + TiO2 .
780
790
800
810
820
830
o
Temperature ( C)
Fig. 1. Viscosity–temperature curves under heating rate of 3 C/min of glasses nucleated at denoted various temperature for 1 h.
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201
C irrespective of nucleation temperature. The viscosity decrease with temperature increase is a natural behavior of glass. However, passing through about 800 C, an increase in viscosity occurs due to crystallization of glass as confirmed in DTA. Hence, the resulting viscosity minimum temperature, Tg , may be regarded as the onset of crystallization detected by viscosity change in the present glass. Tg varies with the nucleation state of glass as shown in Fig. 1. The details are summarized in Table 2 and presented graphically in Fig. 2. The lowest Tg and the highest log g is positioned at 725 C, where the nucleation density is expected to be maximum. Increase of viscosity above Tg in Fig. 1 is related to the increase of solid content in glass, in other words the apparent viscosity above Tg depends on the degree of crystallization, namely crystal volume fraction due to the crystal growth. By comparing the apparent viscosity of five specimens one another above Tg , it is found that they have different value because of difference in volume fraction of crystal. If the crystal growth rate is same for all specimens, a specimen with the highest nucleation density (or nuclei concentration) shall indicate the largest crystal volume fraction above Tg . As expected, the specimen nucleated at 725 C shows the largest apparent viscosity value above Tg . SEM image of the specimen treated thermally at Tg showed no surface but bulk crystallization. XRD analysis indicated that h-quartz solid solution is formed at Tg and its transformation into b-Spodumene (Li2 O Æ Al2 O3 Æ
Table 2 Viscosity minimum temperature (Tg ) and log g at Tg of glasses nucleated at various temperatures for 1 h Nucleation temperature (C)
Tg (C)
log g (dPa s) at Tg
650 680 700 725 750
804 801 800 800 807
9.98 10.02 10.09 10.11 9.97
197
4SiO2 ) begins at about 900 C. The shoulder shown at viscosity curve in the range of 810–830 C in Fig. 1 may be related to this transition. The compositional change of the glass phase occurring during the phase transition would result in the viscosity change of the shoulder type [11]. However, according to DTA results no exothermic peak for this transition was observed. It is known that the absence of the exothermic peak in transition of h-quartz solid solution into b-Spodumene is typical because of extremely small transition enthalpy [12]. After phase transition to b-Spodumene, Gahnite (ZnO Æ Al2 O3 ) as a minor crystal phase was also found for some specimens. Fig. 3 illustrates the XRD patterns of the six specimens treated thermally at 1050 C for 30 min. Their nucleation state except as-quenched specimen was same as that of Fig. 1. The intensity of b-Spodumene peaks is almost same so that the comparison between specimens may be impossible. However, the Gahnite peaks are most clear for the specimen nucleated at 725 C among them. The XRD results demonstrate that the nucleation state of glass influences not only on the initial crystallization process but also on the further phase transition. 3.2. Influence of nucleation time and heating rate Fig. 4 shows viscosity–temperature curves for the three specimens nucleated at 725 C for 0.5, 1 and 3 h. The specimens with nucleation time of 0.5 h and 1 h have almost same Tg , 801 C. However, the specimen nucleated for 3 h shows lower Tg (785 C) and higher viscosity at Tg than those of other two specimens. It is recognized that the increase of nucleation density in glass with increase of nucleation time is related to lowering of Tg where crystallization is detected. The shoulder in viscosity curve due to the phase transition shifts also toward lower temperature with increase of nucleation time. In the case of specimen nucleated for 3 h the
β-spodumene
10.15
808
Gahnite 10.10
10.05
802 10.00
Intensity
804
740 °C
logη (dPas) at Tη
Tη ( °C)
806
725 °C 700 °C 680 °C 650 °C As-quenched
800
640
660
680
700
720
740
9.95 760
Nucleation tempeature (°C)
Fig. 2. Tg () and log g at Tg ( ) of glasses as a function of nucleation temperature.
10
15
20
25
30
35
40
45
50
55
60
65
70
75
Degree (2θ)
Fig. 3. XRD patterns of crystallized glasses treated thermally at 1050 C for 30 min after nucleation at denoted various temperatures for 1 h.
198
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201 12.5
12.0
logη (dPas)
11.5
logη (dPas)
11.0
10.5
12.0
3 °C/min
11.5
5 °C/min 7 °C/min
11.0
10.5 10.0
10.0
9.5 750
775
800
825
9.5 725
850
750
775
800
825
850
875
900
925
Temperature (°C)
Temperature (°C)
Fig. 4. Viscosity–temperature curves for three glasses nucleated at 725 C for 0.5 (), 1 ( ) and 3 h (M). Heating rate: 3 C/min.
Fig. 6. Viscosity–temperature curves of glasses with three different heating rates.
shoulder already appears at 795 C. However, there seems to be little difference in slope (Dg=DT ) between Tg and the temperature at shoulder beginning shown in three viscosity curves. The apparent viscosity value above Tg increases due to the increase of crystal volume fraction. On the other hand, the viscosity of the residual glass phase except crystalline phase may decrease due to the change of its composition. This opposite effect on the apparent viscosity would result in similar value in Dg=DT . The XRD results of Fig. 5 demonstrate also the nucleation time effect on crystallization for specimens treated thermally at 1050 C for 30 min. The specimen nucleated for 0.5 h exhibits the lower intensity in Gahnite peak than the other two specimens. Viscosity behavior under heating rate of 3, 5 and 7 C/min for the specimen nucleated at 725 C for 1 h is shown in Fig. 6. With increase of heating rate, the position of Tg and the shoulder in viscosity curve shift to higher temperature, and the viscosity value at Tg de-
creases. Although glasses have undergone the same nucleation state, their crystallization process depends on the heating rate.
4.1. Comparison with DTA results Recently, several studies performed by DTA [5,6] reported that the exothermic peak temperature and peak height due to crystallization were important to understand the nucleation and crystallization behavior of a glass. Based on the dependence of the crystallization peak temperature on the nucleation state of glass, they determined the state of maximum nucleation, and the results by DTA technique were confirmed by the microscopy method [3,7]. DTA results of the present glass showed a strong exothermic peak due to the crystallization of h-quartz solid solution. In Fig. 7 the peak temperature (Tp ) and normalized peak height (dTp )
3 Hr
1 Hr
0.5 Hr
7
855
6
840
5
825
4
810
3
795
2
780
1
765
0 650 10
15
20
25
30
35
40
45
50
55
60
65
70
75
Degree (2θ)
Fig. 5. XRD patterns of crystallized glasses treated thermally at 1050 C for 30 min after nucleation at 725 C for 0.5, 1 and 3 h.
675
700
725
750
DTA Peak Temperature (Tp : °C)
Gahnite
Intensity
4. Discussion
Normalized DTA Peak Heigt (δTP )
750 775
Nucleation temperature (°C)
Fig. 7. Peak temperature (Tp : ) and normalized peak height (dTp : ) of glasses determined by DTA as function of the nucleation temperature.
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201 Table 3 DTA peak temperature (Tp ) at various nucleation time of glasses nucleated at 725 C Nucleation time (h)
Peak temperature (Tp : C)
0.5 1 3
827 823 810
of the present glass are depicted as function of the nucleation temperature. Tp is 848 C for the as-quenched glass, but it decreases as increase of nucleation temperature then reaches to a minimum value, 823 C for the specimen nucleated at 725 C. In the case of dTp , it showed a maximum value for the specimen nucleated at 700–725 C. Both results mean that the nucleation rate (or nucleation density) of present glass is a maximum near 725 C. Comparison of the viscosity behavior in Fig. 2 with the peak behavior shown in Fig. 7 indicates that the dependence of Tg on the nucleation temperature is similar to that of Tp . Especially the temperature of 725 C where the maximum nucleation occurs is same in DTA and viscosity technique. Additionally, Table 3 contains results of the nucleation time effect performed by DTA technique. Tp corresponding to the nucleation time of 0.5, 1 and 3 h is 827, 823 and 810 C, respectively. Such a tendency in Tp with increase of nucleation time occurred also in Tg as shown in Fig. 4. The consistency in results of the two techniques demonstrates that the viscosity measurement during heating is also a good method in studying the influence of the initial nucleation state of glasses on the crystallization. 4.2. Microstructure As discussed above, temperature and time for nucleation, and heating rate after nucleation have an influence on nucleation density and crystallization rate. Therefore, the final microstructure would depend on the thermal history of the specimen. Fig. 8 shows the
199
microstructure of two specimens with different thermal history. One specimen (a) was nucleated under the condition regarded as the optimum based on the foregoing viscosity results, namely at 725 C for 1 h, and then heated to 900 C at 3 C/min. The other (b) is nucleated at 650 C for 1 h and then heated to 900 C at 7 C/min. The specimen (a) has a microstructure composed of crystals with relatively uniform size of 0.5–1.0 lm. On the other hand, the specimen (b) shows a nonhomogeneous microstructure in which small crystals less than 0.5 lm and large crystals with size of 1.5–2.0 lm are mixed. Being considered the influence of nucleation temperature and heating rate on crystallization expressed as viscosity behavior, it is clear the microstructure difference between specimens with different thermal history. The nucleation density of the specimen (b) before crystallization would be lower than that of the specimen (a) because its lower nucleation temperature than the optimal one. Tg of the specimen (b) appearing at higher temperature and its lower apparent viscosity at the same temperature as shown in Fig. 1 reflect such effect of the nucleation temperature. This relative low nucleation density of specimen (b) may have some relation with non-uniform crystal growth during heating and finally it shows the heterogeneous microstructure. 4.3. Kinetics of crystallization As observed in experimental results of present work, the volume of crystal contained in specimens seems to be almost reflected by their viscosity value. Fig. 9 shows viscosity behavior of glasses with increase of time under isothermal condition of 795 C where h-quartz solid solution is formed. Prior to the viscosity measurement the three glasses were nucleated at 725 C and the concrete nucleation time is denoted in hour. In Fig. 9, the effect of not only the nucleation time but also the
11.5
logη (dPas)
11.0
10.5
10.0 0
Fig. 8. SEM image of crystallized glasses with different thermal history (·10 000). (a) Heat treatment to 900 C at 3 C/min after nucleation at 725 C for 1 h, (b) heat treatment to 900 C at 7 C/min after nucleation at 650 C for 1 h.
20
40
60
80
100
Time (min)
Fig. 9. Viscosity–time curves under isothermal condition of 795 C for three glasses nucleated at 725 C for 0.5 (), 1 ( ) and 3 h (M).
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201
gapp n ¼ ð1 þ maÞ ; gg
ð2Þ
0.3
0.1
0.0 0
1
2
3
4
5
lnt (min)
Fig. 10. Crystal volume (a)–time (ln t) curves under isothermal condition of 795 C for three glasses nucleated at 725 C for 0.5 (), 1 ( ) and 3 h (M).
occurrence of the time-lag effect under overall crystallization. Based on Eq. (4), the relation between ln½ lnð1 aÞ and lnðtÞ at 795 C can be plotted and the case of specimen nucleated for 3 h is presented in Fig. 11. The determined values of n and k for three nucleation states are summarized in Table 4. The average value of n approaches to 3 and thus the crystal morphology is expected to be sphere as shown in the foregoing microstructure. However, k value consisting of nucleation rate (I) and crystal growth rate (U ) shows an increase with increase of nucleation time. It was already
0.0
-0.5
ð3Þ
where a is the crystal volume fraction at time t, n is the Avrami’s exponent depending on the nucleation mechanism and the crystal morphology, and k is called Avrami kinetic coefficient including the rate of nucleation (I) and crystal growth (U ), namely k IU n . Rearrangement of Eq. (3) allows following Eq. (4): ln½ lnð1 aÞ ¼ ln k þ n ln t:
0.4
ð1Þ
This equation is valid for the concentration range of 0 < a < 0:6 according to the comparison between experimental and theoretical results [14]. The knowledge of a value with increase of time allows the performance of crystallization kinetics under isothermal condition. The isothermal crystallization kinetics in glasses is usually approached by Eq. (3) known as JMA (Johnson, Mehl, Avrami) equation [15]: a ¼ 1 exp½ktn ;
0.5
0.2
where gapp is the apparent viscosity of crystallized glass with volume fraction of crystal phase, gg is the viscosity of homogeneous glass without crystal phase (a ¼ 0) and a is the volume fraction of crystal phase in crystallized glass. m and n are empirical constants depending on the shape of dispersed crystalline solids and their orientation in crystallized glass. As shown in the forgoing microstructure, the shape of crystalline solids in present specimens was similar to sphere. Accordingly, under assumption that spherical crystals are randomly oriented in specimen, Eq. (1) can be altered to gapp 3 ¼ ð1 aÞ : gg
0.6
α
heat treatment time on the crystallization degree in specimen is well depicted by viscosity change. According to several studies [10,14], under the assumption that the partial crystallized glasses are a kind of composite consisting of glass phase and crystalline solids, it is possible to perform the calculation of their viscosity by using following Eq. (1):
-1.0
ln[-ln(1-α)]
200
-1.5
-2.0
-2.5
ð4Þ
The linear relation between ln½ lnð1 aÞ and ln t makes it possible to determine slope (n) and intercept (ln k). By using Eq. (2), the y-axis (g) of Fig. 9 can be altered to a. Fig. 10 represents the crystal volume fraction a at 795 C as function of time (ln t) for h-quartz solid solution. The curves show that a value is constant, nearly zero for initial some minutes. That means that a delay time exists prior to crystallization although the nucleation is already undergone. Especially it is clear for the specimen with short nucleation time. The long delay time may be due to the low nucleation density determined in short nucleation time. The existence of the delay time for crystallization in Fig. 9 implies an
-3.0 2.0
2.2
2.4
2.6
2.8
3.0
3.2
ln(t) (min)
Fig. 11. Relation between ln½ lnð1 aÞ and lnðtÞ at 795 C for glass nucleated at 725 C for 3 h.
Table 4 Summary of Avrami’s exponent (n) and Avrami kinetic coefficient (k) for glasses treated isothermally at 795 C Item
n k (·106 )
Nucleation time (h) at 725 C 0.5
1
3
2.82 2.3
2.43 153
2.48 370
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201
mentioned from Dg=DT of Fig. 4 that the growth rate of initial crystal phase is almost constant irrespective of nucleation time. Therefore, k may be determined only by the nucleation density depending on the nucleation time.
Acknowledgement
5. Conclusion
References
In the present work, the viscosity of lithium aluminosilicate glasses was determined continuously with increase of temperature by fiber elongation method. The viscosity–temperature curve provided useful information, so called a viscosity minimum temperature (Tg ) with respect to the crystallization of glasses. Tg of glass showed a dependence on its nucleation temperature, nucleation time and heating rate. The same results in DTA confirmed that the viscosity measurement during heating is also a good method in studying the nucleation effect of glasses on the crystallization. Nucleation effect was also observed in the microstructure of crystallized glasses with different nucleation state. The crystallization kinetics discussed based on the time dependence of viscosity at Tg showed reasonable results relatively for nucleation effect although the equations for composite were applied to calculate the crystal volume.
201
This work was supported by Korea Research Foundation Grant (KRF-2002-002-D00100).
[1] P.F. James, Phys. Chem. Glasses 15 (1974) 95. [2] U. Schiffner, W. Pannhorst, Glastech. Ber. 60 (1987) 211. [3] L.L. Burgner, M.C. Weinberg, J. Non-Cryst. Solids 261 (2000) 161. [4] I.W. Donald, B.L. Metcalfe, D.J. Wood, J.R. Copley, J. Mater. Sci. 24 (1989) 3892. [5] X.J. Xu, C.S. Ray, D.E. Day, J. Am. Ceram. Soc. 74 (1991) 909. [6] C.S. Ray, D.E. Day, J. Am. Ceram. Soc. 80 (1997) 3100. [7] T. Wakasugi, L.L. Burgner, M.C. Weinberg, J. Non-Cryst. Solids 244 (1999) 63. [8] O.V. Mazurin, G.P. Roskova, in: O.V. Mazurin, E.A. PoraiKoshits (Eds.), Phase Separation in Glass, North-Holland, Amsterdam, 1984, p. 243. [9] P.L. Higby, J.E. Shelby, J. Am. Ceram. Soc. 67 (1984) 445. [10] M. Yamane, T. Sakaino, T. Moriya, J Ceram. Soc. Jpn. 81 (1973) 184. [11] G. Patridge, Glass Technol. 26 (1985) 129. [12] H. Bach (Ed.), Low Thermal Expansion Glass Ceramics, Springer, 1995, p. 39, 136. [13] H.A. Robinson, C.A. Peterson, J. Am. Ceram. Soc. 27 (1944) 129. [14] K.D. Kim, S.H. Lee, J. Mater. Sci. 32 (1997) 6561. [15] I. Gutzow, J. Schmelzer, The Vitreous State-Berlin, 1995, p. 389.
Observation of nucleation effect on crystallization in lithium aluminosilicate glass by viscosity measurement Ki-Dong Kim a
a,*
, Seung-Heun Lee a, Hyo-Kwon Ahn
b
Department of Materials Science & Engineering, Kunsan National University, Kunsan, Chunbuk, South Korea b Technical Research Institute, Hankuk Glass Industry Co., Kunsan, Chunbuk, South Korea Received 12 December 2003; received in revised form 12 December 2003
Abstract The crystal nucleation effect in lithium aluminosilicate glasses was investigated by the viscosity measurement with aid of the fiber elongation method. The abrupt increase of viscosity due to the crystallization of glass was observed in viscosity–temperature curve but the minimum viscosity temperature (Tg ) related with crystallization showed a strong dependence on the nucleation state such as nucleation temperature, nucleation time and heating rate. The results by viscosity agreed well with those of DTA. The nucleation effect on the microstructure of glass-ceramics was also discussed. Finally, the nucleation effect on the crystallization kinetics was approached quantitatively by calculating the crystal volume from viscosity value. 2004 Elsevier B.V. All rights reserved. PACS: 64.70.P
1. Introduction Crystal nucleation is an important theme in glass science as well as in glass technology. In scientific viewpoint, crystal nucleation is the basis for the phase transition of glass. Therefore, knowledge of crystal nucleation is essential for understanding the meta-stable state of glasses and thus their transformation to thermodynamically stable phases, crystallization. The crystal nucleation in glass technology is undesirable for producing glass without devitrification but it is an important process for preparation of glass-ceramics via controlled crystallization. In studying the nucleation behavior of glasses such as determination of the maximum nucleation temperature/rate etc., two techniques are commonly used. One technique is to observe the crystals grown from nuclei by heat treatment because the dimension of nuclei is too small. This is time-consuming technique but it is possible to evaluate the nucleation effect quantitatively by using microscope [1–
*
Corresponding author. Tel.: +82-63 469 4737; fax.: +82-63 466 2086. E-mail address: [email protected] (K.-D. Kim). 0022-3093/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2004.01.001
3]. The other one proposed recently is based on differential thermal analysis (DTA). Since the crystallization peak temperature in DTA curve depends on nucleation treatment of glass, the shift of peak temperature plays a role of indicator showing the influence of nucleation state indirectly [4–7]. As the phase separation has influence on the resulting viscosity of glasses [8], the viscosity of partially crystallized glass is also different from that of original glass [9]. Especially, according to some works [10–12] the occurrence of crystal precipitation could be recognized from the slope change of viscosity curve when the viscosity of glass was measured continuously under isothermal or non-isothermal condition in crystallization temperature range. Therefore, it is expected that the continuous determination of viscosity could be also a good method in studying the nucleation state of glasses. In fact, in the processing of glass-ceramics the knowledge of viscosity change during crystallization is important practically because it can give information about the body stiffness and the annealing [12]. In the present work, the influence of various nucleation states on the crystallization of a lithium aluminosilicate glass was investigated by viscosity–temperature and viscosity–time relation determined with aid of fiber elongation method. Based on the
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201
results of viscosity, the maximum nucleation temperature, the dependence of microstructure on the nucleation state and the crystallization kinetics were discussed.
2. Experimental The investigated glass composition belongs to the system of Li2 O–Al2 O3 –SiO2 glass-ceramics with low thermal expansion. Table 1 contains the detailed composition of the experimental glass. The raw materials used were reagent grade chemicals. As a fining agent, As2 O3 was used. The batches of 300 g were mixed thoroughly and melted in a Pt/20Rh crucible at 1650 C in an electric furnace. About 30 min was allowed for the melts to be homogenized by Pt/Rh-stirrer. Two types of glass specimen were prepared. One is fiber type for viscosity measurement. Glasses in fiber type were prepared by direct drawing from melts in crucible. They had no inclusion and their dimension was 0.65 ± 0.1 mm in diameter and 65 mm in length. The other was disk type formed on a graphite mold for another measurements. To determine the glass viscosity continuously, fiber elongation method using a vertical tube furnace whose temperature is controlled within ±0.5 C was employed. Glass fiber specimens were as quenched or subjected to isothermal treatment for nucleation prior to viscosity measurement. After inserting glass fiber into the tube furnace, its length change was checked with increase of temperature or time. The viscosity (g) was calculated by using a following equation suggested by Robbinson [13]: g¼
gM l1 l2 ðt2 t1 Þ; 3V ðl2 l1 Þ
where, g is the acceleration of gravity (m/s2 ), M is the load applied to the fiber (kg), l1 and l2 are length of fiber at the time of t1 and t2 (m), respectively and V is the volume of the glass fiber specimen before heating (m3 ). In the present work the load (M) of 50 g was applied. Hence, the maximum value of ðl2 l1 Þ and ðt2 t1 Þ in a measured viscosity range lead to 0.04 m and 1400 s approximately. Viscosities obtained were reproducible within less than 5%. From the viscosity–temperature curve the crystallization temperature, namely the viscosity minimum temperature (Tg ) was determined. Unlike the homogeneous glasses, the viscosity of the heterogeneous glasses like partially crystallized glass via bulk crystallization is called an apparent viscosity because their viscosity is regarded as the viscosity of the
composite consisting of glass phase and crystalline solids. Hence, from the beginning of crystallization, namely passing through Tg the viscosity in the above equation is described as an apparent viscosity that increases generally with proceeding of crystallization [14]. For the comparison with the results obtained from the viscosity measurement, DTA run (Setaram 92-18, France) for some specimens with the same nucleation state as viscosity measurement was performed. The glass powders of 60 mg with particle size between 425 and 500 lm were used for DTA. For nucleation the glass powders had been subjected to isothermal heat treatment and then they were subsequently heated at 10 C/min up to 1000 C to complete the crystallization. The crystallization peak temperature (Tp ) occurring in DTA curve and its shift were compared with the viscosity minimum temperature (Tg ) shown in viscosity–temperature curve. In order to confirm the nucleation effect on the crystallization by occurrence of new phase or its peak intensity, X-ray diffraction analysis (XRD: M18XCE, Bruker, Germany) was performed for some heat-treated specimens. Finally, the microstructure analysis was conducted on some specimens using scanning electron microscopy (SEM: JSM-5410, Jeol, Japan) to examine the dependence of the microstructure of resulting glassceramics on the nucleation state.
3. Results 3.1. Influence of nucleation temperature The viscosity of five glass specimens was plotted as a function of temperature in Fig. 1. Prior to viscosity measurement the glasses were nucleated at 650, 680, 700, 725 and 750 C for 1 h, respectively. The viscosity was measured at a heating rate of 3 C/min from 755 C. The viscosity curves in Fig. 1 have a minimum around 800
10.8
10.6
10.4
logη (dPas)
196
10.2
650 °C 680 °C 700 °C
10.0
725 °C 750 ° C
Table 1 Composition (wt%) of experimental glass
9.8 770
SiO2
Al2 O3
R2 O
RO
RO2
P2 O5
As2 O3
61.3
21.3
4.1
6.9
3.5
2.1
0.8
R2 O: Li2 O + K2 O, RO: MgO + BaO + ZnO, RO2 : ZrO2 + TiO2 .
780
790
800
810
820
830
o
Temperature ( C)
Fig. 1. Viscosity–temperature curves under heating rate of 3 C/min of glasses nucleated at denoted various temperature for 1 h.
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201
C irrespective of nucleation temperature. The viscosity decrease with temperature increase is a natural behavior of glass. However, passing through about 800 C, an increase in viscosity occurs due to crystallization of glass as confirmed in DTA. Hence, the resulting viscosity minimum temperature, Tg , may be regarded as the onset of crystallization detected by viscosity change in the present glass. Tg varies with the nucleation state of glass as shown in Fig. 1. The details are summarized in Table 2 and presented graphically in Fig. 2. The lowest Tg and the highest log g is positioned at 725 C, where the nucleation density is expected to be maximum. Increase of viscosity above Tg in Fig. 1 is related to the increase of solid content in glass, in other words the apparent viscosity above Tg depends on the degree of crystallization, namely crystal volume fraction due to the crystal growth. By comparing the apparent viscosity of five specimens one another above Tg , it is found that they have different value because of difference in volume fraction of crystal. If the crystal growth rate is same for all specimens, a specimen with the highest nucleation density (or nuclei concentration) shall indicate the largest crystal volume fraction above Tg . As expected, the specimen nucleated at 725 C shows the largest apparent viscosity value above Tg . SEM image of the specimen treated thermally at Tg showed no surface but bulk crystallization. XRD analysis indicated that h-quartz solid solution is formed at Tg and its transformation into b-Spodumene (Li2 O Æ Al2 O3 Æ
Table 2 Viscosity minimum temperature (Tg ) and log g at Tg of glasses nucleated at various temperatures for 1 h Nucleation temperature (C)
Tg (C)
log g (dPa s) at Tg
650 680 700 725 750
804 801 800 800 807
9.98 10.02 10.09 10.11 9.97
197
4SiO2 ) begins at about 900 C. The shoulder shown at viscosity curve in the range of 810–830 C in Fig. 1 may be related to this transition. The compositional change of the glass phase occurring during the phase transition would result in the viscosity change of the shoulder type [11]. However, according to DTA results no exothermic peak for this transition was observed. It is known that the absence of the exothermic peak in transition of h-quartz solid solution into b-Spodumene is typical because of extremely small transition enthalpy [12]. After phase transition to b-Spodumene, Gahnite (ZnO Æ Al2 O3 ) as a minor crystal phase was also found for some specimens. Fig. 3 illustrates the XRD patterns of the six specimens treated thermally at 1050 C for 30 min. Their nucleation state except as-quenched specimen was same as that of Fig. 1. The intensity of b-Spodumene peaks is almost same so that the comparison between specimens may be impossible. However, the Gahnite peaks are most clear for the specimen nucleated at 725 C among them. The XRD results demonstrate that the nucleation state of glass influences not only on the initial crystallization process but also on the further phase transition. 3.2. Influence of nucleation time and heating rate Fig. 4 shows viscosity–temperature curves for the three specimens nucleated at 725 C for 0.5, 1 and 3 h. The specimens with nucleation time of 0.5 h and 1 h have almost same Tg , 801 C. However, the specimen nucleated for 3 h shows lower Tg (785 C) and higher viscosity at Tg than those of other two specimens. It is recognized that the increase of nucleation density in glass with increase of nucleation time is related to lowering of Tg where crystallization is detected. The shoulder in viscosity curve due to the phase transition shifts also toward lower temperature with increase of nucleation time. In the case of specimen nucleated for 3 h the
β-spodumene
10.15
808
Gahnite 10.10
10.05
802 10.00
Intensity
804
740 °C
logη (dPas) at Tη
Tη ( °C)
806
725 °C 700 °C 680 °C 650 °C As-quenched
800
640
660
680
700
720
740
9.95 760
Nucleation tempeature (°C)
Fig. 2. Tg () and log g at Tg ( ) of glasses as a function of nucleation temperature.
10
15
20
25
30
35
40
45
50
55
60
65
70
75
Degree (2θ)
Fig. 3. XRD patterns of crystallized glasses treated thermally at 1050 C for 30 min after nucleation at denoted various temperatures for 1 h.
198
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201 12.5
12.0
logη (dPas)
11.5
logη (dPas)
11.0
10.5
12.0
3 °C/min
11.5
5 °C/min 7 °C/min
11.0
10.5 10.0
10.0
9.5 750
775
800
825
9.5 725
850
750
775
800
825
850
875
900
925
Temperature (°C)
Temperature (°C)
Fig. 4. Viscosity–temperature curves for three glasses nucleated at 725 C for 0.5 (), 1 ( ) and 3 h (M). Heating rate: 3 C/min.
Fig. 6. Viscosity–temperature curves of glasses with three different heating rates.
shoulder already appears at 795 C. However, there seems to be little difference in slope (Dg=DT ) between Tg and the temperature at shoulder beginning shown in three viscosity curves. The apparent viscosity value above Tg increases due to the increase of crystal volume fraction. On the other hand, the viscosity of the residual glass phase except crystalline phase may decrease due to the change of its composition. This opposite effect on the apparent viscosity would result in similar value in Dg=DT . The XRD results of Fig. 5 demonstrate also the nucleation time effect on crystallization for specimens treated thermally at 1050 C for 30 min. The specimen nucleated for 0.5 h exhibits the lower intensity in Gahnite peak than the other two specimens. Viscosity behavior under heating rate of 3, 5 and 7 C/min for the specimen nucleated at 725 C for 1 h is shown in Fig. 6. With increase of heating rate, the position of Tg and the shoulder in viscosity curve shift to higher temperature, and the viscosity value at Tg de-
creases. Although glasses have undergone the same nucleation state, their crystallization process depends on the heating rate.
4.1. Comparison with DTA results Recently, several studies performed by DTA [5,6] reported that the exothermic peak temperature and peak height due to crystallization were important to understand the nucleation and crystallization behavior of a glass. Based on the dependence of the crystallization peak temperature on the nucleation state of glass, they determined the state of maximum nucleation, and the results by DTA technique were confirmed by the microscopy method [3,7]. DTA results of the present glass showed a strong exothermic peak due to the crystallization of h-quartz solid solution. In Fig. 7 the peak temperature (Tp ) and normalized peak height (dTp )
3 Hr
1 Hr
0.5 Hr
7
855
6
840
5
825
4
810
3
795
2
780
1
765
0 650 10
15
20
25
30
35
40
45
50
55
60
65
70
75
Degree (2θ)
Fig. 5. XRD patterns of crystallized glasses treated thermally at 1050 C for 30 min after nucleation at 725 C for 0.5, 1 and 3 h.
675
700
725
750
DTA Peak Temperature (Tp : °C)
Gahnite
Intensity
4. Discussion
Normalized DTA Peak Heigt (δTP )
750 775
Nucleation temperature (°C)
Fig. 7. Peak temperature (Tp : ) and normalized peak height (dTp : ) of glasses determined by DTA as function of the nucleation temperature.
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201 Table 3 DTA peak temperature (Tp ) at various nucleation time of glasses nucleated at 725 C Nucleation time (h)
Peak temperature (Tp : C)
0.5 1 3
827 823 810
of the present glass are depicted as function of the nucleation temperature. Tp is 848 C for the as-quenched glass, but it decreases as increase of nucleation temperature then reaches to a minimum value, 823 C for the specimen nucleated at 725 C. In the case of dTp , it showed a maximum value for the specimen nucleated at 700–725 C. Both results mean that the nucleation rate (or nucleation density) of present glass is a maximum near 725 C. Comparison of the viscosity behavior in Fig. 2 with the peak behavior shown in Fig. 7 indicates that the dependence of Tg on the nucleation temperature is similar to that of Tp . Especially the temperature of 725 C where the maximum nucleation occurs is same in DTA and viscosity technique. Additionally, Table 3 contains results of the nucleation time effect performed by DTA technique. Tp corresponding to the nucleation time of 0.5, 1 and 3 h is 827, 823 and 810 C, respectively. Such a tendency in Tp with increase of nucleation time occurred also in Tg as shown in Fig. 4. The consistency in results of the two techniques demonstrates that the viscosity measurement during heating is also a good method in studying the influence of the initial nucleation state of glasses on the crystallization. 4.2. Microstructure As discussed above, temperature and time for nucleation, and heating rate after nucleation have an influence on nucleation density and crystallization rate. Therefore, the final microstructure would depend on the thermal history of the specimen. Fig. 8 shows the
199
microstructure of two specimens with different thermal history. One specimen (a) was nucleated under the condition regarded as the optimum based on the foregoing viscosity results, namely at 725 C for 1 h, and then heated to 900 C at 3 C/min. The other (b) is nucleated at 650 C for 1 h and then heated to 900 C at 7 C/min. The specimen (a) has a microstructure composed of crystals with relatively uniform size of 0.5–1.0 lm. On the other hand, the specimen (b) shows a nonhomogeneous microstructure in which small crystals less than 0.5 lm and large crystals with size of 1.5–2.0 lm are mixed. Being considered the influence of nucleation temperature and heating rate on crystallization expressed as viscosity behavior, it is clear the microstructure difference between specimens with different thermal history. The nucleation density of the specimen (b) before crystallization would be lower than that of the specimen (a) because its lower nucleation temperature than the optimal one. Tg of the specimen (b) appearing at higher temperature and its lower apparent viscosity at the same temperature as shown in Fig. 1 reflect such effect of the nucleation temperature. This relative low nucleation density of specimen (b) may have some relation with non-uniform crystal growth during heating and finally it shows the heterogeneous microstructure. 4.3. Kinetics of crystallization As observed in experimental results of present work, the volume of crystal contained in specimens seems to be almost reflected by their viscosity value. Fig. 9 shows viscosity behavior of glasses with increase of time under isothermal condition of 795 C where h-quartz solid solution is formed. Prior to the viscosity measurement the three glasses were nucleated at 725 C and the concrete nucleation time is denoted in hour. In Fig. 9, the effect of not only the nucleation time but also the
11.5
logη (dPas)
11.0
10.5
10.0 0
Fig. 8. SEM image of crystallized glasses with different thermal history (·10 000). (a) Heat treatment to 900 C at 3 C/min after nucleation at 725 C for 1 h, (b) heat treatment to 900 C at 7 C/min after nucleation at 650 C for 1 h.
20
40
60
80
100
Time (min)
Fig. 9. Viscosity–time curves under isothermal condition of 795 C for three glasses nucleated at 725 C for 0.5 (), 1 ( ) and 3 h (M).
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201
gapp n ¼ ð1 þ maÞ ; gg
ð2Þ
0.3
0.1
0.0 0
1
2
3
4
5
lnt (min)
Fig. 10. Crystal volume (a)–time (ln t) curves under isothermal condition of 795 C for three glasses nucleated at 725 C for 0.5 (), 1 ( ) and 3 h (M).
occurrence of the time-lag effect under overall crystallization. Based on Eq. (4), the relation between ln½ lnð1 aÞ and lnðtÞ at 795 C can be plotted and the case of specimen nucleated for 3 h is presented in Fig. 11. The determined values of n and k for three nucleation states are summarized in Table 4. The average value of n approaches to 3 and thus the crystal morphology is expected to be sphere as shown in the foregoing microstructure. However, k value consisting of nucleation rate (I) and crystal growth rate (U ) shows an increase with increase of nucleation time. It was already
0.0
-0.5
ð3Þ
where a is the crystal volume fraction at time t, n is the Avrami’s exponent depending on the nucleation mechanism and the crystal morphology, and k is called Avrami kinetic coefficient including the rate of nucleation (I) and crystal growth (U ), namely k IU n . Rearrangement of Eq. (3) allows following Eq. (4): ln½ lnð1 aÞ ¼ ln k þ n ln t:
0.4
ð1Þ
This equation is valid for the concentration range of 0 < a < 0:6 according to the comparison between experimental and theoretical results [14]. The knowledge of a value with increase of time allows the performance of crystallization kinetics under isothermal condition. The isothermal crystallization kinetics in glasses is usually approached by Eq. (3) known as JMA (Johnson, Mehl, Avrami) equation [15]: a ¼ 1 exp½ktn ;
0.5
0.2
where gapp is the apparent viscosity of crystallized glass with volume fraction of crystal phase, gg is the viscosity of homogeneous glass without crystal phase (a ¼ 0) and a is the volume fraction of crystal phase in crystallized glass. m and n are empirical constants depending on the shape of dispersed crystalline solids and their orientation in crystallized glass. As shown in the forgoing microstructure, the shape of crystalline solids in present specimens was similar to sphere. Accordingly, under assumption that spherical crystals are randomly oriented in specimen, Eq. (1) can be altered to gapp 3 ¼ ð1 aÞ : gg
0.6
α
heat treatment time on the crystallization degree in specimen is well depicted by viscosity change. According to several studies [10,14], under the assumption that the partial crystallized glasses are a kind of composite consisting of glass phase and crystalline solids, it is possible to perform the calculation of their viscosity by using following Eq. (1):
-1.0
ln[-ln(1-α)]
200
-1.5
-2.0
-2.5
ð4Þ
The linear relation between ln½ lnð1 aÞ and ln t makes it possible to determine slope (n) and intercept (ln k). By using Eq. (2), the y-axis (g) of Fig. 9 can be altered to a. Fig. 10 represents the crystal volume fraction a at 795 C as function of time (ln t) for h-quartz solid solution. The curves show that a value is constant, nearly zero for initial some minutes. That means that a delay time exists prior to crystallization although the nucleation is already undergone. Especially it is clear for the specimen with short nucleation time. The long delay time may be due to the low nucleation density determined in short nucleation time. The existence of the delay time for crystallization in Fig. 9 implies an
-3.0 2.0
2.2
2.4
2.6
2.8
3.0
3.2
ln(t) (min)
Fig. 11. Relation between ln½ lnð1 aÞ and lnðtÞ at 795 C for glass nucleated at 725 C for 3 h.
Table 4 Summary of Avrami’s exponent (n) and Avrami kinetic coefficient (k) for glasses treated isothermally at 795 C Item
n k (·106 )
Nucleation time (h) at 725 C 0.5
1
3
2.82 2.3
2.43 153
2.48 370
K.-D. Kim et al. / Journal of Non-Crystalline Solids 336 (2004) 195–201
mentioned from Dg=DT of Fig. 4 that the growth rate of initial crystal phase is almost constant irrespective of nucleation time. Therefore, k may be determined only by the nucleation density depending on the nucleation time.
Acknowledgement
5. Conclusion
References
In the present work, the viscosity of lithium aluminosilicate glasses was determined continuously with increase of temperature by fiber elongation method. The viscosity–temperature curve provided useful information, so called a viscosity minimum temperature (Tg ) with respect to the crystallization of glasses. Tg of glass showed a dependence on its nucleation temperature, nucleation time and heating rate. The same results in DTA confirmed that the viscosity measurement during heating is also a good method in studying the nucleation effect of glasses on the crystallization. Nucleation effect was also observed in the microstructure of crystallized glasses with different nucleation state. The crystallization kinetics discussed based on the time dependence of viscosity at Tg showed reasonable results relatively for nucleation effect although the equations for composite were applied to calculate the crystal volume.
201
This work was supported by Korea Research Foundation Grant (KRF-2002-002-D00100).
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