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Stabilisation of crystal densities during nucleation of glass ceramics: A hidden ripening process
Journal of Non-Crystalline Solids 43 (1981) 433-438 North-Holland Publishing Company
433
STABILISATION OF CRYSTAL DENSITIES DURING NUCLEATION OF GLASS CERAMICS: A HIDDEN RIPENING PROCESS
P. LABARBE *, A.K. BANDYOPADHYAY, J. ZARZYCKI Laboratory o f Materials Seience and CNRS Glass Laboratory, University o f Montpellier, France and
A. WRIGHT lnstitut Laue-Langevin, Grenoble, France
Received 16 September 1980
Studies of the interparticle interference effects in the SANS spectra of two different nucleated glass ceramics indicate that the number density of crystaUites saturates with time at different values depending on the temperature of the nucleation heat treatment. The observed saturation levels result from different ripening conditions in one case and from different densities of nuclei in the other. The use of small angle neutron scattering as a measure of the progress of nucleation process has been demonstrated [i] in the case of a cordierite glass ceramic containing TiO2 as nucleating agent. A high density of the precipitating particles is responsible for an interference effect which produces a maximum Qm in the SANS spectrum (Q = 27r sin 20/~). From the maximum we calculate a characteristic wavelength l - o f density fluctuations (l-~--2n/Qm) which we interpret as the mean distance between precipitates, and hence obtain a relative density of precipitates (Np ~--T-3) We report here the effects of nucleation and growth heat-treatment on a basalt glass (composition: 52.0 SiO2, 14.1 A1203, 12.8 Fe203, 9.3 CaO, 6.4 MgO, 3.2 Na20, 1.2 K20, 1.0 TiO2 (wt.%)) for which the primary precipitated phase is magnetite (Fe304). The results very closely resemble those reported for the cordierite glass [2]. A two stage heat treatment is used to develop and then grow the nuclei large enough to give a SANS spectrum. Glasses were nucleated at 610,634 and 665°C for different times from 2 to 16 h. The crystallisation was followed in situ on the D17 spectrometer at the Institut Laue-Langevin, Grenoble [3] at a temperature of 710°C. For this glass, however, the position of the maximum in the SANS curve changes rapidly during the early times of the crystallisation heat treatment with Qm decreasing approximately according to a t 1/3 law. The value of Om appears therefore to be relatively stable at long crystallisation times (t > 80 min). In fig. 1 we show the SANS cu.rves for the base glass, as annealed at 525°C, and for the 0022-3093/81/0000-0000/$02.50 © North-Holland Publishing Company
P. Labarbe et al. / Stabilisation o f crystal densities
434
l(O)
ed glass
610°C
0.0
30
60
90
120
Q(~-I).I0-3
Fig. l. Scattering curves of basalt glass after crystallisation at 710°C for 110 min.
,10-3
50
40
30
o
20
10
I
I
I
I
I
I
I
I
20
30
40
50
60
70
80
90
TIME OF HEAT TREATMENT AT 710°C
~,~-.100
(min)
Fig. 2. Evolution of the maximum of the scattering curve Qm with time at 710°C for nucleated and untreated glass.
P. Labarbe et aL/ Stabilisation o f crystal densities ~-,.. 2~
435
(~,)
(~ rnox
30o
2OO
•
4 hours
x o •
8 hours 16 hours 8 hours
o
8 hours
o 0
o
4 hours
I
I
~
z
/ ~610°C ; 634°C
}6
65'C I
3
4
I
I:V3
v
5
Fig. 3. Variation of interparticle distance l w i t h tl/3 , showing kinetics of an Ostwald ripening process.
glass nucleated at 610°C (Tn) for 16h (t c = 110 min), and in fig. 2 the evolution of Qm with time at Te = 710°C. The crystaUisation is therefore modified by a change in the interparticle distance as the particles grow in size. This interparticle distance T shows a linear variation with t ~3 (fig. 3) for all nucleation conditions. We interpret this apparent growth process as being an Ostwald ripening process [4,5]. From fig. 3 we note the following important points: (i) The base glass is itself highly nucleated (7-= 180 A at t e = 11 min) but during the course of the growth treatment the ripening rate is very rapid and the majority of the crystals redissol,ve. (ii) For nucleated samples the ripening rate is considerably reduced and the treatments at 610°C and 634°C are most effective in retaining the largest number of crystallites. ('tii) If we extrapolate to t e = 0 we obtain values for gwhich indicate that loss of nuclei occurs even during the nucleation heat treatment especially at the higher nucleation temperatures. For Tn = 634°C and 610°C, g(t c = 0) is very similar to that of the annealed glass. We do not observe the increase in the density of nuclei expected as a result of a nucleation treatment. After long crystallization times (e.g. t e = 110 min) the beneficial effects of the nucleation heat treatment are seen. A larger particle density is the result of a very much lower rate of Ostwald ripening in those glasses nucleated at 610°C and 634°C for long periods. After 8 h nucleation at 634°C the Guinier radius (Rg) is 26 A at
436
P. Labarbe et al. / Stabilisation o f crystal densities
0 mox(~-l)I x10-3
[
o 610"C
|
.e .......
I
/ I
/6
o
/ 25
• 634°C
~/o
665°C
t/
5 Nucleotion
10 Time
15 (hrs.)
Fig. 4. Saturation effects in Qm with nucleation treatment. Data taken at t c = 110 rain, 710°C.
the end of the growth treatment, whereas for the base glass Rg grows to 48.5 A in the same time [6]. The nucleation treatment in this case appears to encourage a process of stabilisation of the nuclei already present in the annealed glass, but does not produce new nuclei. Examination of the curves simply after t c = 110 rain would falsely suggest that creation of new nuclei had occurred. In fig. 4 we show the variation of Qm at t e = 110 min for various nucleation conditions. At T n = 61'0°C, Qm and hence the number of stable crystaUites present increases with nucleation time up to 16 h, the maximum time for which measurements were made. We observe an initial increase in Qm for short periods at 634 and 665°C followed by a saturation, after which further nucleation treatment has no apparent effect. The saturation level is higher for the lower nucleation temperatures, and evidently at 610°C saturation has not been reached even in a 16 h treatment, and any final level would probably far exceed that of the 634°C treatment, although the latter is more effective in giving a high density of crystals for treatments of short duration. This behaviour of Qm with nucleation treatments has been reported for a cordierite glass ceramic incorporating TiO: as a nucleating agent [2]. Four nucleation temperatures were studied and a crystaUisation time of 1 h was given in each case (fig. 5). The saturation levels are again inversely related to Tn, and a lower temperature 720°C exists for which no saturation effects have yet been
P. Labarbe et al. / Stabilisation o f crystal densities
Omo,(,8,-~),
./ /
x10-3 20
437
• 720"C
/ f "f/7|-~ -'----'4' 74ooc 10 _/
(:le-,--O. . . . . . . . . "0 760"C
.,..'~/~ ~ 78o'c 9o
~.."
10 20 30 Nucleation Time (hrs.) Fig. 5. S a t u r a t i o n o f t h e n u c l e a t i o n t r e a t m e n t for a c o r d i e r i t e glass. D a t a t a k e n at t c = 6 0 m i n , 8 3 5 ° C (ref. [2]).
found. On the other hand, cordierite glass which has not received a nucleation treatment has no maximum in I(Q) for small angle scattering, indicative of a very low density of stable nuclei in the annealed glass. The strong evidence of ripening shown by fig. 3 brings into question the conventional method of studying nucleation by counting the particle density following a two stage heat treatment. It is normally assumed that the majority of crystallites formed at the nucleation stage do not redissolve on heating to the second stage. Whilst James [7] has shown that this assumption is justified for simple lithium disilicate glass ceramics where the particle density is rather low (Np = 10 e cm-3), in the present case, where Np > 1017 cm -3 this assumption is no longer true. It seems likely that the increase in l ( t c = 0) at the higher nucleation temperature 665~C over a period of 4 to 8 h and the ripening at the growth temperature 710°C are manifestations of the same process. During the nucleation heat treatment there is some reduction in the number of nuclei present and at the same time an increase in the stability of those crystallites remaining. This is consistent with a limited ripening process controlled by the low diffusion rates at these temperatures. Because the concentration of solute is greater around smaller particles than around larger ones, there is a diffusion of solute to the larger from the smaller and these will eventually redissolve. Each particle interacts with its neighbours through the steady state concentration gradients which develop around them, and these in turn are controlled by the bulk diffusion rates and particle growth rates. The ripening process is then most rapid where the gradients are high, that is between closely spaced
438
P. Labarbe et al. / Stabilisation o f crystal densities
particles. In this way, the smallest particles and shortest values of l are progressively eliminated. However, since all particles with similar radii greater than the critical radius draw the solute from a comparable volume of glass determined by the limit of the concentration gradient, no single particle can grow much larger than the mean. Ostwald ripening theory [5] predicts a fairly narrow particle size distribution with a sharp cut-off on the high side for r >~. The results obtained show that during the low temperature heat treatment the system develops increased stability against further ripening. We interpret this as the development of a microstructure which is more homogeneous in its spatial and size distribution. The saturation occurring at smaller values of l a s the treatment temperature is reduced reflects the smaller sphere of influence of one crystallite upon its neighbours due to the lower diffusion rates, whilst the time to reach saturation reflects the longer time to establish the steady state particle size and spatial distribution at the lower temperatures. In other glass systems of course the first step in the nucleation heat treatment may be the creation of new nuclei and this probably occurs in the cordierite system. Where a major component of the glass is being precipitated however, the concentration gradients around each particle rapidly become important in perturbing the creation of further nuclei and the stabilisation of existing ones. The authors would like to thank Drs. G.H. Beall and H.L. Rittler of Coming Glass Works (USA) for supplying the basalt glass.
References [1] A.F. Wright, J. Talbot and B.E.F. Fender, Nature 277 (1969) (5965), 366. [2] Annual Report Institut Laue-Langevin, Grenoble (1979); A.F. Wright and B.E.F. Fender, to be published. [3] Neutron beam facilities at the ILL High Flux Reactor, Institut Laue-Langevin, Grenoble (1974). [4] J. Zarzycki and F. Naudin, Phys. Chem. Glasses 8 (1976) 11. [5] S.C. Jain and A.E. Hughes, J. Mat. Sci. 13 (1978) 1611. [6] P. Labarbe and A.F. Wright, to be published. [7] P.F. James, Phys. Chem. Glasses 15 (1974) 4.
433
STABILISATION OF CRYSTAL DENSITIES DURING NUCLEATION OF GLASS CERAMICS: A HIDDEN RIPENING PROCESS
P. LABARBE *, A.K. BANDYOPADHYAY, J. ZARZYCKI Laboratory o f Materials Seience and CNRS Glass Laboratory, University o f Montpellier, France and
A. WRIGHT lnstitut Laue-Langevin, Grenoble, France
Received 16 September 1980
Studies of the interparticle interference effects in the SANS spectra of two different nucleated glass ceramics indicate that the number density of crystaUites saturates with time at different values depending on the temperature of the nucleation heat treatment. The observed saturation levels result from different ripening conditions in one case and from different densities of nuclei in the other. The use of small angle neutron scattering as a measure of the progress of nucleation process has been demonstrated [i] in the case of a cordierite glass ceramic containing TiO2 as nucleating agent. A high density of the precipitating particles is responsible for an interference effect which produces a maximum Qm in the SANS spectrum (Q = 27r sin 20/~). From the maximum we calculate a characteristic wavelength l - o f density fluctuations (l-~--2n/Qm) which we interpret as the mean distance between precipitates, and hence obtain a relative density of precipitates (Np ~--T-3) We report here the effects of nucleation and growth heat-treatment on a basalt glass (composition: 52.0 SiO2, 14.1 A1203, 12.8 Fe203, 9.3 CaO, 6.4 MgO, 3.2 Na20, 1.2 K20, 1.0 TiO2 (wt.%)) for which the primary precipitated phase is magnetite (Fe304). The results very closely resemble those reported for the cordierite glass [2]. A two stage heat treatment is used to develop and then grow the nuclei large enough to give a SANS spectrum. Glasses were nucleated at 610,634 and 665°C for different times from 2 to 16 h. The crystallisation was followed in situ on the D17 spectrometer at the Institut Laue-Langevin, Grenoble [3] at a temperature of 710°C. For this glass, however, the position of the maximum in the SANS curve changes rapidly during the early times of the crystallisation heat treatment with Qm decreasing approximately according to a t 1/3 law. The value of Om appears therefore to be relatively stable at long crystallisation times (t > 80 min). In fig. 1 we show the SANS cu.rves for the base glass, as annealed at 525°C, and for the 0022-3093/81/0000-0000/$02.50 © North-Holland Publishing Company
P. Labarbe et al. / Stabilisation o f crystal densities
434
l(O)
ed glass
610°C
0.0
30
60
90
120
Q(~-I).I0-3
Fig. l. Scattering curves of basalt glass after crystallisation at 710°C for 110 min.
,10-3
50
40
30
o
20
10
I
I
I
I
I
I
I
I
20
30
40
50
60
70
80
90
TIME OF HEAT TREATMENT AT 710°C
~,~-.100
(min)
Fig. 2. Evolution of the maximum of the scattering curve Qm with time at 710°C for nucleated and untreated glass.
P. Labarbe et aL/ Stabilisation o f crystal densities ~-,.. 2~
435
(~,)
(~ rnox
30o
2OO
•
4 hours
x o •
8 hours 16 hours 8 hours
o
8 hours
o 0
o
4 hours
I
I
~
z
/ ~610°C ; 634°C
}6
65'C I
3
4
I
I:V3
v
5
Fig. 3. Variation of interparticle distance l w i t h tl/3 , showing kinetics of an Ostwald ripening process.
glass nucleated at 610°C (Tn) for 16h (t c = 110 min), and in fig. 2 the evolution of Qm with time at Te = 710°C. The crystaUisation is therefore modified by a change in the interparticle distance as the particles grow in size. This interparticle distance T shows a linear variation with t ~3 (fig. 3) for all nucleation conditions. We interpret this apparent growth process as being an Ostwald ripening process [4,5]. From fig. 3 we note the following important points: (i) The base glass is itself highly nucleated (7-= 180 A at t e = 11 min) but during the course of the growth treatment the ripening rate is very rapid and the majority of the crystals redissol,ve. (ii) For nucleated samples the ripening rate is considerably reduced and the treatments at 610°C and 634°C are most effective in retaining the largest number of crystallites. ('tii) If we extrapolate to t e = 0 we obtain values for gwhich indicate that loss of nuclei occurs even during the nucleation heat treatment especially at the higher nucleation temperatures. For Tn = 634°C and 610°C, g(t c = 0) is very similar to that of the annealed glass. We do not observe the increase in the density of nuclei expected as a result of a nucleation treatment. After long crystallization times (e.g. t e = 110 min) the beneficial effects of the nucleation heat treatment are seen. A larger particle density is the result of a very much lower rate of Ostwald ripening in those glasses nucleated at 610°C and 634°C for long periods. After 8 h nucleation at 634°C the Guinier radius (Rg) is 26 A at
436
P. Labarbe et al. / Stabilisation o f crystal densities
0 mox(~-l)I x10-3
[
o 610"C
|
.e .......
I
/ I
/6
o
/ 25
• 634°C
~/o
665°C
t/
5 Nucleotion
10 Time
15 (hrs.)
Fig. 4. Saturation effects in Qm with nucleation treatment. Data taken at t c = 110 rain, 710°C.
the end of the growth treatment, whereas for the base glass Rg grows to 48.5 A in the same time [6]. The nucleation treatment in this case appears to encourage a process of stabilisation of the nuclei already present in the annealed glass, but does not produce new nuclei. Examination of the curves simply after t c = 110 rain would falsely suggest that creation of new nuclei had occurred. In fig. 4 we show the variation of Qm at t e = 110 min for various nucleation conditions. At T n = 61'0°C, Qm and hence the number of stable crystaUites present increases with nucleation time up to 16 h, the maximum time for which measurements were made. We observe an initial increase in Qm for short periods at 634 and 665°C followed by a saturation, after which further nucleation treatment has no apparent effect. The saturation level is higher for the lower nucleation temperatures, and evidently at 610°C saturation has not been reached even in a 16 h treatment, and any final level would probably far exceed that of the 634°C treatment, although the latter is more effective in giving a high density of crystals for treatments of short duration. This behaviour of Qm with nucleation treatments has been reported for a cordierite glass ceramic incorporating TiO: as a nucleating agent [2]. Four nucleation temperatures were studied and a crystaUisation time of 1 h was given in each case (fig. 5). The saturation levels are again inversely related to Tn, and a lower temperature 720°C exists for which no saturation effects have yet been
P. Labarbe et al. / Stabilisation o f crystal densities
Omo,(,8,-~),
./ /
x10-3 20
437
• 720"C
/ f "f/7|-~ -'----'4' 74ooc 10 _/
(:le-,--O. . . . . . . . . "0 760"C
.,..'~/~ ~ 78o'c 9o
~.."
10 20 30 Nucleation Time (hrs.) Fig. 5. S a t u r a t i o n o f t h e n u c l e a t i o n t r e a t m e n t for a c o r d i e r i t e glass. D a t a t a k e n at t c = 6 0 m i n , 8 3 5 ° C (ref. [2]).
found. On the other hand, cordierite glass which has not received a nucleation treatment has no maximum in I(Q) for small angle scattering, indicative of a very low density of stable nuclei in the annealed glass. The strong evidence of ripening shown by fig. 3 brings into question the conventional method of studying nucleation by counting the particle density following a two stage heat treatment. It is normally assumed that the majority of crystallites formed at the nucleation stage do not redissolve on heating to the second stage. Whilst James [7] has shown that this assumption is justified for simple lithium disilicate glass ceramics where the particle density is rather low (Np = 10 e cm-3), in the present case, where Np > 1017 cm -3 this assumption is no longer true. It seems likely that the increase in l ( t c = 0) at the higher nucleation temperature 665~C over a period of 4 to 8 h and the ripening at the growth temperature 710°C are manifestations of the same process. During the nucleation heat treatment there is some reduction in the number of nuclei present and at the same time an increase in the stability of those crystallites remaining. This is consistent with a limited ripening process controlled by the low diffusion rates at these temperatures. Because the concentration of solute is greater around smaller particles than around larger ones, there is a diffusion of solute to the larger from the smaller and these will eventually redissolve. Each particle interacts with its neighbours through the steady state concentration gradients which develop around them, and these in turn are controlled by the bulk diffusion rates and particle growth rates. The ripening process is then most rapid where the gradients are high, that is between closely spaced
438
P. Labarbe et al. / Stabilisation o f crystal densities
particles. In this way, the smallest particles and shortest values of l are progressively eliminated. However, since all particles with similar radii greater than the critical radius draw the solute from a comparable volume of glass determined by the limit of the concentration gradient, no single particle can grow much larger than the mean. Ostwald ripening theory [5] predicts a fairly narrow particle size distribution with a sharp cut-off on the high side for r >~. The results obtained show that during the low temperature heat treatment the system develops increased stability against further ripening. We interpret this as the development of a microstructure which is more homogeneous in its spatial and size distribution. The saturation occurring at smaller values of l a s the treatment temperature is reduced reflects the smaller sphere of influence of one crystallite upon its neighbours due to the lower diffusion rates, whilst the time to reach saturation reflects the longer time to establish the steady state particle size and spatial distribution at the lower temperatures. In other glass systems of course the first step in the nucleation heat treatment may be the creation of new nuclei and this probably occurs in the cordierite system. Where a major component of the glass is being precipitated however, the concentration gradients around each particle rapidly become important in perturbing the creation of further nuclei and the stabilisation of existing ones. The authors would like to thank Drs. G.H. Beall and H.L. Rittler of Coming Glass Works (USA) for supplying the basalt glass.
References [1] A.F. Wright, J. Talbot and B.E.F. Fender, Nature 277 (1969) (5965), 366. [2] Annual Report Institut Laue-Langevin, Grenoble (1979); A.F. Wright and B.E.F. Fender, to be published. [3] Neutron beam facilities at the ILL High Flux Reactor, Institut Laue-Langevin, Grenoble (1974). [4] J. Zarzycki and F. Naudin, Phys. Chem. Glasses 8 (1976) 11. [5] S.C. Jain and A.E. Hughes, J. Mat. Sci. 13 (1978) 1611. [6] P. Labarbe and A.F. Wright, to be published. [7] P.F. James, Phys. Chem. Glasses 15 (1974) 4.