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Analysis of crystallization behavior in Li2O · 2SiO2 glass by DTA method based on a liquid model
JOURNAL OF
1141ffll ELSEVIER
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Journal of Non-CrystallineSolids 197 (1996) 19-24
Analysis of crystallization behavior in Li20.2SiO 2 glass by DTA method based on a liquid model N. Mishima *, R. Ota, T. Wakasugi, J. Fukunaga Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Received 17 April 1995; revised 21 July 1995
Abstract The crystallization behavior of Li20.2SiO 2 glass samples having different melting histories were analyzed by differential thermal analysis (DTA). The glass samples were prepared by remelting a quenched glass at 1300°C or 1035°C for 1 h. The glass melted at low temperature was found to crystallize earlier and to have lower crystallization peak temperature, Tc, than that melted at high temperature. The number density and sizes of crystals precipitated in glasses with the same heat treatment as the DTA run were measured and were found greater in number and larger in the glass melted at lower temperature compared with that at higher temperature. These results are in agreement with the liquid model that crystal embryos are preserved in melts even above liquids and their distribution depends on temperature.
1. Introduction On quenching molten slag from various temperatures above liquids in order to verify crystallization behavior, Ota et al. [1] found that the amount of precipitated crystals is smaller as the melting temperature is higher even if cooling rate is the same. A liquid model was proposed in order to interpret these results. The liquid model includes an assumption that even above the melting point or liquidus tiny crystalline particles (micro-crystals or crystal embryos) could be preserved stably. As temperature is decreased, the number density of the crystal embryos increases and the average size of embryos becomes
* Corresponding author. Tel.: + 81-75 724 7565; fax: + 81-75 724 7580; e-mail: [email protected].
larger. To determine the validity of this model, Mishima et al. [2,3], Ota et al. [4] carried out the following experiments for a L i 2 0 . 2 S i O 2 composition. A glass liquid of L i 2 0 . 2 S i O 2 composition (melting point 1033°C) was prepared by melting a batch at 1300°C and then holding at various temperatures in a range of 1035-1300°C for 3 or 20 h. Then the liquids were water-quenched. The quenched samples were heat-treated at 580 ° to 650°C for 2 0 - 6 0 min. Observation by an optical microscope of the heat-treated glasses revealed that the quantity of crystals was larger in the glass melted at lower temperatures than at higher temperatures. Moreover larger crystals were observed in glass melted at lower temperatures. It can be stated that these results are in agreement with what the liquid model suggests. According to the liquid model, larger number of crystal embryos are preserved in a glass melted at
0022-3093/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-3093(95)00570-6
20
N. Mishima et al. /Journal of Non-Crystalline Solids 197 (1996) 19-24
lower temperature than at higher temperature, so that one may observe more rapid crystallization in the former glass than the latter glass on heating the glasses. It is expected, therefore, that D T A technique will be able to indicate some difference of crystallization behavior such as the shift of the crystallization temperature related to the different thermal history of the glass. In the present study a L i 2 0 - 2SiO 2 glass is targeted and its D T A measurements and optical microscope observation of heat-treated glasses are conducted to prove the validity of the liquid model.
2. E x p e r i m e n t a l
2.1. Sample preparation and Tc measurement by DTA technique Bulk and powder glass samples were used for DTA experiments to see the effect of surface crystallization on peak temperature, Tc. The batches of L i 2 0 . 2 S i O 2 composition from reagent grade lithium carbonate and silica powder were melted at 1300°C in a platinum crucible and quenched to the glass state. Bulk sample was charged in a DTA crucible (platinum, 5 mm wide and 5 mm deep) by remelting the quenched glass at 1300°C or 1035°C for 1 h. Powder glass samples were prepared from the following method. After the batch was once melted at 1300°C for 40 min, the melt was held at 1300 or 1035°C for 1 h and quenched to be glass. In the
present study the particle size was in a range of 4 5 0 - 5 0 0 Ixm. D T A measurement was made (using a model of Rigaku Denki TAS 100) in an air atmosphere with a sample weight of 40 + 0.3 mg. The Tc was determined with heating rates from 3 to 20°C/min.
2.2. Measurement of number density and crystal size by optical microscope Bulk glasses with thermal history as used in D T A measurement were heated to a temperature in a range 600-640°C in an electric furnace with the same heating rates as the D T A run and then quenched. The terminal temperature of the heat treatments was set sufficiently higher than the nucleation temperature range (420-530°C) in L i z O . 2SiO 2 glass [5] so that number densities of nuclei are not affected by the terminal temperatures. The terminal temperatures were, for instance, 600°C at 5 ° C / m i n and 640°C at 20°C/min. Number density of crystals as well as maximum crystal size among the precipitated crystals were measured by an optical microscope in a similar way as described in Ref. [2].
3. R e s u l t s
3.1. Crystallization temperature Tc in bulk samples Typical D T A curves for bulk samples melted at 1300°C (A) and 1035°C (B) with heating rates of
Table 1 Crystallization temperature, Tc, and the Tc difference, Tc(A) - Tc(B), in bulk glass samples and powder glass samples melted for l h at 1300 and 1035°C. Tc was determined from DTA measurement with a heating rates of 3, 5, 10 and 20°C/min Sample Melting Crystallization temperature, Tc (°C) type temperature (°C) Heating rate (°C min - i ) 3
5
10
20
Bulk
1300 (A) 1035 (B) Tc(A) - Tc(B)
635.9 633.5 2.4
654.7 651.2 3.5
683.7 676.8 6.9
710.7 702.7 8.0
Powder
1300 (A) 1035 (B) Tc(A) - rc(B)
619.5 620.9 - 1.4
632.1 630.7 1.4
648.2 648.7 -0.5
666.4 665.9 0.5
N. Mishima et al./Journal of Non-Crystalline Solids 197 (1996) 19-24
5°C/min and 20°C/min are shown in Fig. 1. In the following text (A) and (B) represent glass samples melted at 1300 and 1035°C, respectively. Each sample shows a single crystallization peak; Tc = 651.2°C in sample (B) and 654.7°C in sample (A) at a heating rate of 5°C/min. The Tc difference between sample (A) and (B), Tc(A) - Tc(B) = ATc, is 3.5°C. Obviously sample (B) shows a lower Tc than sample (A). As the heating rate is increased to 20°C/min, Tc shifts toward a higher temperature of 702.7°C in sample (B) and 710.7°C in sample (A), and AT c increased to be 8.0°C. Tc and ATc in bulk samples are summarized in Table 1 for heating rates of 3°C/min to 20°C/min. It is apparent that sample (B) shows a lower Tc than sample (A) for all heating rates. ATc increases as the heating rate increases. It appears that the peak height increases with heating rate but this phenomenon is one of the common phenomena observed on DTA measurements as Matusita et al. [6] reported for L i 2 0 . 2 S i O 2 and 33Li20 • 66.7SiO 2 - 3TiO 2 glasses. 3.2• Tc in powder samples and surface crystallization
Crystallization starts from both interior and surface of the L i 2 0 - 2SiO 2 glass. Though the effect of surface crystallization can be negligible for bulk
I
I
i
Li20.2SiO 2
20*C/min .....
xo
~3oo*c(A)
/
103SOC(B)
//
t%
5*C/min / ! / /,
, I
500
I
~,.,.~ I
700 Temperature/ °C 650
750
Fig. 1. DTA curves of bulk glasses melted at I300°C (A) and 1035°C (B) for 1 h and measured with heating rates of 3 and 20°C/min.
!
21
!
Li20.2SiO 2
5°C/min
13oo°C(A) . . . . . ~0asoC(B)
B u l k sample s¢1
LU
!
550
I
I
I
600 650 700 Temperature/ °C
Fig. 2. DTA curves of powder samples (A) and (B) in comparison with those of bulk samples (A) and (B) with a heating rate of 5°C/rain. (A) and (B) shows the respective glass sample melted at 1300 and 1035°C, respectively.
glass, it becomes predominant as sample surface area increases. In order to see the effect of surface crystallization on Tc shift, crystallization behavior of powder samples by DTA is compared with those of bulk samples in Fig. 2 at a heating rate of 5°C/min. It is shown that the crystallization peak is much broader and lower in height in powder samples than in bulk samples, which is in accordance with that which Ray and Day [7] reported for L i 2 0 . 2 S i O 2 glass. Marotta et al. [8] also showed that DTA peak temperature is less in powder samples of Li20 • 2SiO 2 glass than in bulk sample. In the present measurement Tc shifts toward a higher temperature with increasing heating rate for both sample (A) and (B). Tc and ATc in powder samples are also summarized in Table 1. The Tc difference between samples (A) and (B), ATc, for bulk and powder samples are plotted against heating rate in Fig. 3. It is seen that ATc in bulk samples is increased with an increase in heating rate. Ray et al. [7] stated that bulk crystallization becomes predominant over surface crystallization when the particle size is larger than 300 I~m. However, ATc in powder samples is almost zero within I°C independent of heating rate. This means that the thermal history of glasses has not affected the crystallization behavior in the powder samples of grain size of 450-500 p,m. It is probable that even in this range
22
N. Mishima et al./Journal of Non-Crystalline Solids 197 (1996) 19-24 lt]
.
.
.
.
i
.
.
.
.
U20"2Si02 .o
~ ~-e~ ,.,%,.fl
~,
Table 2 Number density, N, and the largest size of crystal in glass samples (A) and (B) observed by optical microscope. Sample (A) and (B) were prepared by melting at 1300°C (A) and 1035°C (B) for 1 h, respectively, and then heated up to temperatures shown with heating rates corresponding to the DTA run
i
///
. bulk sample
/,0 // 0
0
powdersample
=/
0 i
5 600
10 620
20 640
N (nuclei mm -3)
1035°C (B) 1300°C (A)
404 348
130 110
41 33
Crystal size a (~m)
1035°C (B) 1300°C (A)
0
0 0
Heating rate (°C min- l ): Terminal temperature (°C):
i
i
i
I
10
,
,
i
i
i
20
Heating rate / *C.min-'
7.3 3.2
9.4 3.6
13.5 7.5
Fig. 3. Variation of the Tc differencewith heating rate, Tc(A)Tc(B), between glass (A) and (B) melted at 1300 and 1035°C, respectively,and preparedinto bulk and powderstate.
a Crystal size shows a radius of minor axis of the maximum particle precipitated in a heat-treated glass.
of particle size surface nucleation occurs predominantly.
was chosen and the length of minor axis of particle was measured. It is also found that crystal size of sample (B) is almost double as large as that in sample (A) for all cases of heating rate and temperature range in this experiment.
3.3. Number density and crystal size by optical microscope To confirm the DTA results, the number of crystals precipitated in glass with the same heat-treatment as the DTA run was counted. The results of optical microscope observation are shown in Table 2. In the glasses heated up to 600°C with a heating rate of 5°C/min, the number density of crystal, N, is 4 0 4 / m m 3 in sample (B) and 3 4 8 / m m 3 in sample (A), whereas 4 1 / m m 3 in sample (B) and 3 3 / m m 3 in sample (A), heated up to 640°C with a heating rate of 20°C/min. It is clear that N in sample (B) is always greater than in sample (A). The grown crystals exhibited elliptical sections. The largest crystal
4. Discussion
The liquid model assumes a distribution in number and size of crystal embryos as a function of temperature. At low temperature near the melting point, embryos of greater number density and larger size would exist in a melt, and at high temperature embryos of smaller number density and smaller size. The distribution may be controlled by temperature only. Fig. 4 schematically shows the distribution of crystal size preserved in as-quenched glasses, (A)
Visible by optical microscope
Z
I0
Cryst=-embryo
Nucleus
Crystal-particle
Size Fig. 4. Sehematical distribution of crystal-embryo in as-quenched glasses melted at 1300°C (A) and 1035°C (B) and those of nucleus or crystal-particle in glasses heated up to a transient temperature, T, (AY and (BY; or a crystallization peak temperature, Tc, (A)" and (BY'.
23
N. Mishima et al./ Journal of Non-Crystalline Solids 197 (1996) 19-24
and (B), that of crystal size in the heat-treated glasses at T, (A)' and (BY, and that of crystal size in the heat-treated glasses at Tc, (A)" and (BY', respectively. T indicates one of the temperature corresponding to a transient state from crystal embryo to crystal particle. Nuclei can be barely visible by optical microscope in the glasses, (A)' and (BY. A crystallization peak is observed at Tc in the glasses, (A)" and (BY'. The distribution of nuclei or crystal particles is affected by the structure of the original liquid. During a DTA heating process up to the temperature, Tc, the distribution of crystal embryos preserved in as-quenched glasses, (A) and (B), shifts to those of (A)" and (BY' through the intermediate state of (A)' and (BY. Then the distribution curves of (BY and (BY' would be located always above those of (A)' and (A)" as shown in Fig. 4. Results of optical microscope observation [2-4] convinces us of such distribution of crystal embryos existing in glass, which depends on the thermal history. It was found that the difference between Tc(A) and Tc(B), ATc, decreases with a decrease of heating rate for bulk samples (Fig. 3). The isothermal heat treatment (600°C, 30 min) of bulk glass [2,4] showed that the number density, N, and size, R, of crystals decrease with a increase of temperature of glass melting. However, the effect of thermal history begins to vanish as the period of heat treatment is prolonged (60 min). This result implies that the microstructure of glass is relaxed with time. If glass structure relaxes during DTA run (especially at slower heating rate), the decrease of ATc with a decrease in heating rate may be explained by the above implication. The variation of the number density of crystal particles, N, precipitated in the DTA runs summarized in Table 2 is shown in Fig. 5 as a function of reciprocal of heating rate, l / a . Fig. 5 implies that structure relaxation has not occurred even at the slowest heating rate. It is expected that if structure relaxation occurs, the number density of sample (A), N(A), should approach to that of sample (B), N(B), at slower heating rate and the difference between N(A) and N(B) should become larger as heating rate is increased. In the following paragraph it is shown that the result of Fig. 3 can be explained even if structure relaxation does not occur. During the heating process of DTA measurement, nuclei are formed in the nucleation temperature range
.
.
.
.
I
.
.
.
.
z
•
0
0.1
!
I
0.2
/ min.°C-~
Fig. 5. Number density of crystal, N, plotted against reciprocal of heating rate for glass (A) and (B) melted at 1300 and 1035°C, respectively.
and grow to crystal particles at higher temperature range. Temperature where crystal particles are in contact each other and crystal growth is suppressed can be related to Tc. Assuming that nuclei are of the same radius and uniformly dispersed at an average distance, L = N -1/3, Tc will satisfy the following equation:
L 1 J~rTCudT (1) 7=~To where a is heating rate, U is the crystal growth rate, and TO is a temperature where crystal growth rate becomes apparent. Fig. 5 for sample (A) and (B) indicates that N may be expressed as functions of NO and 1/c~ in Eq. (2): N = No( 1/o~)",
(2)
where NO is a constant characterizing preserved crystal embryos in the glass. The value of n is found to be about 1.7. For a given a, NO for sample (B), N0(B), is always larger than that for sample (A), No(A). The crystal growth rate, U, is generally represented by the bell-shaped curve, which has a peak very close to the melting point. U is supposed to increase monotonously with temperature up to Tc. In the present study, it was assumed, for convenience, that U is simply expressed as Eq. (3) for the temperature range from TO to Tc, where U0 is a constant and rn is greater than unit:
U= Uo(T- To)",
m > 1.
(3)
24
N. Mishima et al./Journal of Non-Crystalline Solids 197 (1996) 19-24
From Eqs. ( 1 ) - ( 3 ) , Tc = T o
+ Col(n+3)/(3m+3)No '/(3m+3),
(4)
where C = {(m + 1 ) / 2 U o } l/(m+ i). Eq. (4) indicates that Tc is increased with increasing a , if TO and U o are constant for samples (A) and (B). Tc(A) - Tc(B) = ATc can be calculated as
that greater number and larger size o f crystals precipitated in the glass melted at lower temperature. The results are in agreement with the proposed liquid model. The variation of Tc and A Tc with heating rate were interpreted by assuming tentative equations.
A T c = C a ( n + 3)/(3m + 3)
X ( N o ( A ) - '/(3m+3) _ No(8)-'/(3m+3))>o.
References
(5) Since N0(A) < N0(B), A Tc will be increased with increasing heating rate, which is in agreement with the experimental variation o f ATc shown in Fig. 3.
5. Conclusion Differential thermal analysis measurements of glass samples subjected to different melting history showed that crystallization peak temperature, Tc, appears at lower temperature in glass melted at lower temperature above the melting point than at higher temperature. Optical microscope observation showed
[1] R. Ota, J. Fukunaga and N. Yoshida, J. Soc. Mater. Sci. Jpn. 39 (1973) 8. [2] N. Mishima, R. Ota, T. Wakasugi and J. Fukunaga, J. Ceram. Soc. Jpn. 101 (1993) 1206. [3] N. Mishima, R. Ota, T. Wakasugi and J. Fukunaga, J. Soc. Mater. Sci. Jpn. 44 (1995) 687. [4] R. Ota, N. Mishima, T. Wakasugi and J. Fukunaga, in: Proc. of the Int. Symp. on Glass Sci. and Techn., Chimica Chronica, New Series, 23 (1994) 385. [5] P.F. James, Phys. Chem. Glasses 15 (1974) 95. [6] K. Matusita, S. Sakka and Y. Matsui, J. Mater. Sci. 10 (1975) 961. [7] C.S. Ray and D.E. Day, J. Am. Ceram. Soc. 73 (1990) 439. [8] A. Marotta, A. Buri, F. Branda and S. Saiello, in: Advances in Ceramics, Vol. 4, Nucleation and crystallization in Glasses (American Ceramic Society, Westerville, OH, 1982) p. 146.
1141ffll ELSEVIER
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Journal of Non-CrystallineSolids 197 (1996) 19-24
Analysis of crystallization behavior in Li20.2SiO 2 glass by DTA method based on a liquid model N. Mishima *, R. Ota, T. Wakasugi, J. Fukunaga Department of Chemistry and Materials Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606, Japan
Received 17 April 1995; revised 21 July 1995
Abstract The crystallization behavior of Li20.2SiO 2 glass samples having different melting histories were analyzed by differential thermal analysis (DTA). The glass samples were prepared by remelting a quenched glass at 1300°C or 1035°C for 1 h. The glass melted at low temperature was found to crystallize earlier and to have lower crystallization peak temperature, Tc, than that melted at high temperature. The number density and sizes of crystals precipitated in glasses with the same heat treatment as the DTA run were measured and were found greater in number and larger in the glass melted at lower temperature compared with that at higher temperature. These results are in agreement with the liquid model that crystal embryos are preserved in melts even above liquids and their distribution depends on temperature.
1. Introduction On quenching molten slag from various temperatures above liquids in order to verify crystallization behavior, Ota et al. [1] found that the amount of precipitated crystals is smaller as the melting temperature is higher even if cooling rate is the same. A liquid model was proposed in order to interpret these results. The liquid model includes an assumption that even above the melting point or liquidus tiny crystalline particles (micro-crystals or crystal embryos) could be preserved stably. As temperature is decreased, the number density of the crystal embryos increases and the average size of embryos becomes
* Corresponding author. Tel.: + 81-75 724 7565; fax: + 81-75 724 7580; e-mail: [email protected].
larger. To determine the validity of this model, Mishima et al. [2,3], Ota et al. [4] carried out the following experiments for a L i 2 0 . 2 S i O 2 composition. A glass liquid of L i 2 0 . 2 S i O 2 composition (melting point 1033°C) was prepared by melting a batch at 1300°C and then holding at various temperatures in a range of 1035-1300°C for 3 or 20 h. Then the liquids were water-quenched. The quenched samples were heat-treated at 580 ° to 650°C for 2 0 - 6 0 min. Observation by an optical microscope of the heat-treated glasses revealed that the quantity of crystals was larger in the glass melted at lower temperatures than at higher temperatures. Moreover larger crystals were observed in glass melted at lower temperatures. It can be stated that these results are in agreement with what the liquid model suggests. According to the liquid model, larger number of crystal embryos are preserved in a glass melted at
0022-3093/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0022-3093(95)00570-6
20
N. Mishima et al. /Journal of Non-Crystalline Solids 197 (1996) 19-24
lower temperature than at higher temperature, so that one may observe more rapid crystallization in the former glass than the latter glass on heating the glasses. It is expected, therefore, that D T A technique will be able to indicate some difference of crystallization behavior such as the shift of the crystallization temperature related to the different thermal history of the glass. In the present study a L i 2 0 - 2SiO 2 glass is targeted and its D T A measurements and optical microscope observation of heat-treated glasses are conducted to prove the validity of the liquid model.
2. E x p e r i m e n t a l
2.1. Sample preparation and Tc measurement by DTA technique Bulk and powder glass samples were used for DTA experiments to see the effect of surface crystallization on peak temperature, Tc. The batches of L i 2 0 . 2 S i O 2 composition from reagent grade lithium carbonate and silica powder were melted at 1300°C in a platinum crucible and quenched to the glass state. Bulk sample was charged in a DTA crucible (platinum, 5 mm wide and 5 mm deep) by remelting the quenched glass at 1300°C or 1035°C for 1 h. Powder glass samples were prepared from the following method. After the batch was once melted at 1300°C for 40 min, the melt was held at 1300 or 1035°C for 1 h and quenched to be glass. In the
present study the particle size was in a range of 4 5 0 - 5 0 0 Ixm. D T A measurement was made (using a model of Rigaku Denki TAS 100) in an air atmosphere with a sample weight of 40 + 0.3 mg. The Tc was determined with heating rates from 3 to 20°C/min.
2.2. Measurement of number density and crystal size by optical microscope Bulk glasses with thermal history as used in D T A measurement were heated to a temperature in a range 600-640°C in an electric furnace with the same heating rates as the D T A run and then quenched. The terminal temperature of the heat treatments was set sufficiently higher than the nucleation temperature range (420-530°C) in L i z O . 2SiO 2 glass [5] so that number densities of nuclei are not affected by the terminal temperatures. The terminal temperatures were, for instance, 600°C at 5 ° C / m i n and 640°C at 20°C/min. Number density of crystals as well as maximum crystal size among the precipitated crystals were measured by an optical microscope in a similar way as described in Ref. [2].
3. R e s u l t s
3.1. Crystallization temperature Tc in bulk samples Typical D T A curves for bulk samples melted at 1300°C (A) and 1035°C (B) with heating rates of
Table 1 Crystallization temperature, Tc, and the Tc difference, Tc(A) - Tc(B), in bulk glass samples and powder glass samples melted for l h at 1300 and 1035°C. Tc was determined from DTA measurement with a heating rates of 3, 5, 10 and 20°C/min Sample Melting Crystallization temperature, Tc (°C) type temperature (°C) Heating rate (°C min - i ) 3
5
10
20
Bulk
1300 (A) 1035 (B) Tc(A) - Tc(B)
635.9 633.5 2.4
654.7 651.2 3.5
683.7 676.8 6.9
710.7 702.7 8.0
Powder
1300 (A) 1035 (B) Tc(A) - rc(B)
619.5 620.9 - 1.4
632.1 630.7 1.4
648.2 648.7 -0.5
666.4 665.9 0.5
N. Mishima et al./Journal of Non-Crystalline Solids 197 (1996) 19-24
5°C/min and 20°C/min are shown in Fig. 1. In the following text (A) and (B) represent glass samples melted at 1300 and 1035°C, respectively. Each sample shows a single crystallization peak; Tc = 651.2°C in sample (B) and 654.7°C in sample (A) at a heating rate of 5°C/min. The Tc difference between sample (A) and (B), Tc(A) - Tc(B) = ATc, is 3.5°C. Obviously sample (B) shows a lower Tc than sample (A). As the heating rate is increased to 20°C/min, Tc shifts toward a higher temperature of 702.7°C in sample (B) and 710.7°C in sample (A), and AT c increased to be 8.0°C. Tc and ATc in bulk samples are summarized in Table 1 for heating rates of 3°C/min to 20°C/min. It is apparent that sample (B) shows a lower Tc than sample (A) for all heating rates. ATc increases as the heating rate increases. It appears that the peak height increases with heating rate but this phenomenon is one of the common phenomena observed on DTA measurements as Matusita et al. [6] reported for L i 2 0 . 2 S i O 2 and 33Li20 • 66.7SiO 2 - 3TiO 2 glasses. 3.2• Tc in powder samples and surface crystallization
Crystallization starts from both interior and surface of the L i 2 0 - 2SiO 2 glass. Though the effect of surface crystallization can be negligible for bulk
I
I
i
Li20.2SiO 2
20*C/min .....
xo
~3oo*c(A)
/
103SOC(B)
//
t%
5*C/min / ! / /,
, I
500
I
~,.,.~ I
700 Temperature/ °C 650
750
Fig. 1. DTA curves of bulk glasses melted at I300°C (A) and 1035°C (B) for 1 h and measured with heating rates of 3 and 20°C/min.
!
21
!
Li20.2SiO 2
5°C/min
13oo°C(A) . . . . . ~0asoC(B)
B u l k sample s¢1
LU
!
550
I
I
I
600 650 700 Temperature/ °C
Fig. 2. DTA curves of powder samples (A) and (B) in comparison with those of bulk samples (A) and (B) with a heating rate of 5°C/rain. (A) and (B) shows the respective glass sample melted at 1300 and 1035°C, respectively.
glass, it becomes predominant as sample surface area increases. In order to see the effect of surface crystallization on Tc shift, crystallization behavior of powder samples by DTA is compared with those of bulk samples in Fig. 2 at a heating rate of 5°C/min. It is shown that the crystallization peak is much broader and lower in height in powder samples than in bulk samples, which is in accordance with that which Ray and Day [7] reported for L i 2 0 . 2 S i O 2 glass. Marotta et al. [8] also showed that DTA peak temperature is less in powder samples of Li20 • 2SiO 2 glass than in bulk sample. In the present measurement Tc shifts toward a higher temperature with increasing heating rate for both sample (A) and (B). Tc and ATc in powder samples are also summarized in Table 1. The Tc difference between samples (A) and (B), ATc, for bulk and powder samples are plotted against heating rate in Fig. 3. It is seen that ATc in bulk samples is increased with an increase in heating rate. Ray et al. [7] stated that bulk crystallization becomes predominant over surface crystallization when the particle size is larger than 300 I~m. However, ATc in powder samples is almost zero within I°C independent of heating rate. This means that the thermal history of glasses has not affected the crystallization behavior in the powder samples of grain size of 450-500 p,m. It is probable that even in this range
22
N. Mishima et al./Journal of Non-Crystalline Solids 197 (1996) 19-24 lt]
.
.
.
.
i
.
.
.
.
U20"2Si02 .o
~ ~-e~ ,.,%,.fl
~,
Table 2 Number density, N, and the largest size of crystal in glass samples (A) and (B) observed by optical microscope. Sample (A) and (B) were prepared by melting at 1300°C (A) and 1035°C (B) for 1 h, respectively, and then heated up to temperatures shown with heating rates corresponding to the DTA run
i
///
. bulk sample
/,0 // 0
0
powdersample
=/
0 i
5 600
10 620
20 640
N (nuclei mm -3)
1035°C (B) 1300°C (A)
404 348
130 110
41 33
Crystal size a (~m)
1035°C (B) 1300°C (A)
0
0 0
Heating rate (°C min- l ): Terminal temperature (°C):
i
i
i
I
10
,
,
i
i
i
20
Heating rate / *C.min-'
7.3 3.2
9.4 3.6
13.5 7.5
Fig. 3. Variation of the Tc differencewith heating rate, Tc(A)Tc(B), between glass (A) and (B) melted at 1300 and 1035°C, respectively,and preparedinto bulk and powderstate.
a Crystal size shows a radius of minor axis of the maximum particle precipitated in a heat-treated glass.
of particle size surface nucleation occurs predominantly.
was chosen and the length of minor axis of particle was measured. It is also found that crystal size of sample (B) is almost double as large as that in sample (A) for all cases of heating rate and temperature range in this experiment.
3.3. Number density and crystal size by optical microscope To confirm the DTA results, the number of crystals precipitated in glass with the same heat-treatment as the DTA run was counted. The results of optical microscope observation are shown in Table 2. In the glasses heated up to 600°C with a heating rate of 5°C/min, the number density of crystal, N, is 4 0 4 / m m 3 in sample (B) and 3 4 8 / m m 3 in sample (A), whereas 4 1 / m m 3 in sample (B) and 3 3 / m m 3 in sample (A), heated up to 640°C with a heating rate of 20°C/min. It is clear that N in sample (B) is always greater than in sample (A). The grown crystals exhibited elliptical sections. The largest crystal
4. Discussion
The liquid model assumes a distribution in number and size of crystal embryos as a function of temperature. At low temperature near the melting point, embryos of greater number density and larger size would exist in a melt, and at high temperature embryos of smaller number density and smaller size. The distribution may be controlled by temperature only. Fig. 4 schematically shows the distribution of crystal size preserved in as-quenched glasses, (A)
Visible by optical microscope
Z
I0
Cryst=-embryo
Nucleus
Crystal-particle
Size Fig. 4. Sehematical distribution of crystal-embryo in as-quenched glasses melted at 1300°C (A) and 1035°C (B) and those of nucleus or crystal-particle in glasses heated up to a transient temperature, T, (AY and (BY; or a crystallization peak temperature, Tc, (A)" and (BY'.
23
N. Mishima et al./ Journal of Non-Crystalline Solids 197 (1996) 19-24
and (B), that of crystal size in the heat-treated glasses at T, (A)' and (BY, and that of crystal size in the heat-treated glasses at Tc, (A)" and (BY', respectively. T indicates one of the temperature corresponding to a transient state from crystal embryo to crystal particle. Nuclei can be barely visible by optical microscope in the glasses, (A)' and (BY. A crystallization peak is observed at Tc in the glasses, (A)" and (BY'. The distribution of nuclei or crystal particles is affected by the structure of the original liquid. During a DTA heating process up to the temperature, Tc, the distribution of crystal embryos preserved in as-quenched glasses, (A) and (B), shifts to those of (A)" and (BY' through the intermediate state of (A)' and (BY. Then the distribution curves of (BY and (BY' would be located always above those of (A)' and (A)" as shown in Fig. 4. Results of optical microscope observation [2-4] convinces us of such distribution of crystal embryos existing in glass, which depends on the thermal history. It was found that the difference between Tc(A) and Tc(B), ATc, decreases with a decrease of heating rate for bulk samples (Fig. 3). The isothermal heat treatment (600°C, 30 min) of bulk glass [2,4] showed that the number density, N, and size, R, of crystals decrease with a increase of temperature of glass melting. However, the effect of thermal history begins to vanish as the period of heat treatment is prolonged (60 min). This result implies that the microstructure of glass is relaxed with time. If glass structure relaxes during DTA run (especially at slower heating rate), the decrease of ATc with a decrease in heating rate may be explained by the above implication. The variation of the number density of crystal particles, N, precipitated in the DTA runs summarized in Table 2 is shown in Fig. 5 as a function of reciprocal of heating rate, l / a . Fig. 5 implies that structure relaxation has not occurred even at the slowest heating rate. It is expected that if structure relaxation occurs, the number density of sample (A), N(A), should approach to that of sample (B), N(B), at slower heating rate and the difference between N(A) and N(B) should become larger as heating rate is increased. In the following paragraph it is shown that the result of Fig. 3 can be explained even if structure relaxation does not occur. During the heating process of DTA measurement, nuclei are formed in the nucleation temperature range
.
.
.
.
I
.
.
.
.
z
•
0
0.1
!
I
0.2
/ min.°C-~
Fig. 5. Number density of crystal, N, plotted against reciprocal of heating rate for glass (A) and (B) melted at 1300 and 1035°C, respectively.
and grow to crystal particles at higher temperature range. Temperature where crystal particles are in contact each other and crystal growth is suppressed can be related to Tc. Assuming that nuclei are of the same radius and uniformly dispersed at an average distance, L = N -1/3, Tc will satisfy the following equation:
L 1 J~rTCudT (1) 7=~To where a is heating rate, U is the crystal growth rate, and TO is a temperature where crystal growth rate becomes apparent. Fig. 5 for sample (A) and (B) indicates that N may be expressed as functions of NO and 1/c~ in Eq. (2): N = No( 1/o~)",
(2)
where NO is a constant characterizing preserved crystal embryos in the glass. The value of n is found to be about 1.7. For a given a, NO for sample (B), N0(B), is always larger than that for sample (A), No(A). The crystal growth rate, U, is generally represented by the bell-shaped curve, which has a peak very close to the melting point. U is supposed to increase monotonously with temperature up to Tc. In the present study, it was assumed, for convenience, that U is simply expressed as Eq. (3) for the temperature range from TO to Tc, where U0 is a constant and rn is greater than unit:
U= Uo(T- To)",
m > 1.
(3)
24
N. Mishima et al./Journal of Non-Crystalline Solids 197 (1996) 19-24
From Eqs. ( 1 ) - ( 3 ) , Tc = T o
+ Col(n+3)/(3m+3)No '/(3m+3),
(4)
where C = {(m + 1 ) / 2 U o } l/(m+ i). Eq. (4) indicates that Tc is increased with increasing a , if TO and U o are constant for samples (A) and (B). Tc(A) - Tc(B) = ATc can be calculated as
that greater number and larger size o f crystals precipitated in the glass melted at lower temperature. The results are in agreement with the proposed liquid model. The variation of Tc and A Tc with heating rate were interpreted by assuming tentative equations.
A T c = C a ( n + 3)/(3m + 3)
X ( N o ( A ) - '/(3m+3) _ No(8)-'/(3m+3))>o.
References
(5) Since N0(A) < N0(B), A Tc will be increased with increasing heating rate, which is in agreement with the experimental variation o f ATc shown in Fig. 3.
5. Conclusion Differential thermal analysis measurements of glass samples subjected to different melting history showed that crystallization peak temperature, Tc, appears at lower temperature in glass melted at lower temperature above the melting point than at higher temperature. Optical microscope observation showed
[1] R. Ota, J. Fukunaga and N. Yoshida, J. Soc. Mater. Sci. Jpn. 39 (1973) 8. [2] N. Mishima, R. Ota, T. Wakasugi and J. Fukunaga, J. Ceram. Soc. Jpn. 101 (1993) 1206. [3] N. Mishima, R. Ota, T. Wakasugi and J. Fukunaga, J. Soc. Mater. Sci. Jpn. 44 (1995) 687. [4] R. Ota, N. Mishima, T. Wakasugi and J. Fukunaga, in: Proc. of the Int. Symp. on Glass Sci. and Techn., Chimica Chronica, New Series, 23 (1994) 385. [5] P.F. James, Phys. Chem. Glasses 15 (1974) 95. [6] K. Matusita, S. Sakka and Y. Matsui, J. Mater. Sci. 10 (1975) 961. [7] C.S. Ray and D.E. Day, J. Am. Ceram. Soc. 73 (1990) 439. [8] A. Marotta, A. Buri, F. Branda and S. Saiello, in: Advances in Ceramics, Vol. 4, Nucleation and crystallization in Glasses (American Ceramic Society, Westerville, OH, 1982) p. 146.