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Crystallization of sol-gel-derived lithium aluminosilicate (LAS) glass ceramic powders
Journal of Non-Crystalline Solids 116 (1990) 125-132 North-Holland
125
CRYSTALLIZATION OF SOL-GEL-DERIVED L I T H I U M ALUMINOSILICATE (LAS) GLASS CERAMIC POWDERS Gye Song LEE and Gary L. MESSING The Pennsylvania State University, University Park, PA 16802, USA
Frans G.A. DELAAT Litton Guidance and Control Systems, Woodland Hills, CA 91365, USA Received 30 May 1989 Revised manuscript received 3 October 1989
The gel to glass ceramic conversion of a sol-gel-derived LAS powder was characterized by its gel to glass conversion, nucleation, crystallization and microstructural development. At Tg the gel converts to a glass with a dense skeletal structure and at - 800 o C to fully dense, amorphous particles. Crystallization at 860 ° C to a/3-quartz solid solution is independent of nucleating agents, gel powder particle size and heat treatment. In contrast, crystallization of a melt glass derived from the gel was dependent on nucleation heat treatment, reaching a m i n i m u m crystallization temperature of 8 6 0 ° C . As the glass transition temperature and glass structure before crystallization are approximately the same for the gel and melt-derived powders, the lower crystallization temperature of the gel is attributed solely to its surface excess free energy and its effect on nucleation.
1. Introduction
Sol-gel processing has been widely investigated for many reasons; however, a particular interest has come from the possibility of forming novel glasses and crystalline microstructures at low temperature [1,2]. By sol-gel synthesis, glass network formation and chemical homogenization in a glass can be achieved in solution near room temperature. Heat treatment near the glass transition temperature is required to convert the extremely porous gel into dense glass [3-6]. The gel-glass conversion has been examined by various methods such as powder density, specific surface area [5-9], spectroscopy [10-12] and TEM [5,7]. Due to the different thermal histories of gel-derived and melt glasses, fundamental questions have been raised about similarities and differences in their properties with particular interest on nucleation and crystallization [13-19]. Glasses prepared by melting either a gel or mixed oxides had no major differences during crystallization [13-14]. However, when gel powders were heated to obtain 0022-3093/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
dense glass they had a lower crystallization temperature and faster nucleation and crystallization rates relative to melted glasses [19]. These differences were attributed to the lower viscosity of the gel powder due to the higher hydroxyl content [14] and the excess free energy due to the larger interior surface and structural differences [13,18]. Most crystallization studies of gel-derived powders or glasses have been on relatively simple glass compositions, designed to avoid crystallization. Interestingly, there has been no research comparing the crystallization behavior of sol-geland melt-derived glass ceramic powders. This information is important as melt-derived glass ceramic powder crystallization has been shown to directly influence its sinterability [20]. In this study, a gel with a glass ceramic composition in the LizO-A1203-SiO 2 (LAS) system was examined in terms of its gel-glass conversion, nucleation, crystallization kinetics and crystalline microstructures. To elucidate how gel powders effect nucleation and crystallization relative to conventional glass ceramics, their crystallization is
G.S. Lee et al. / Co'stallization of sol-gel-derived L A S glass ceramic powders
126
Table 1 Composition of the gel-derived powder
wt% mol%
SiO 2
A1203
Li 2°
MgO
ZnO
TiO 2
ZrO 2
61.4 67.3
28.0 18.0
4.1 9.0
1.0 1.6
1.4 1.1
2.2 2.0
1.9 1.0
compared with the crystallization of bulk and powdered glasses obtained by melting the glass ceramic gel.
I
a Litton Guidance and Control Systems, Woodland Hills, CA, USA. 2 Quantasorb, Quantachrome Corp., Syosset, NY, USA. 3 Model 1090 Thermal Analyzer, DuPont Co., Wilmington, DE, USA. 4 Model 1090 Thermal Analyzer, DuPont Co., Wilmington, DE, USA.
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2. Experimental The dried gel powders, which were prepared by nitric acid-catalyzed hydrolysis and polycondensation of a multicomponent metal alkoxide solution near room temperature, were provided by Litton G / C S 1 [21] as yellowish gel fragments. The glass composition is close to the B-spodumene ternary compound in the lithium aluminosilicate system and includes ZrO 2 and TiO 2 as nucleating agents (Table 1). For the crystallization study, the as-received gel powder was calcined in flowing air at 0.5 ° C per minute to 640 °C and held for 6 h, the lowest temperature-time condition to obtain organic-free powder. For some experiments, the calcined gel powder was melted in a Pt crucible for 10 h at 1600°C and quenched in air. Particle densities and specific surface areas were measured by pycnometry to + 0.05 g / c m 3 accuracy and single point BET 2 to _+2 m2/g accuracy, respectively. Differential thermal analysis (DTA) 3 and thermogravimetric analysis (TGA) 4 were used to determine thermal reactions, transformation temperatures to _+5°C accuracy and weight loss of the gel-derived powders by heating
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Fig. 1. Thermal analysis of as-received gel-derived LAS powder; (a) D T A curve, (b) T G A curve (10 o C / m i n in air).
40 mg powder samples in flowing air at 10 o C / m i n . Phase identification and quantitative analysis of the calcined gel powders to + 5 vol% accuracy were examined by automated X-ray diffraction (XRD) 5 with a monochromatic C u K a source. The external standard technique [22] with fully crystallized LAS powder as the standard was used for quantitative phase analysis. Microstructural evolution during crystallization of the gel powders was examined by TEM 6
Rigaku USA, Inc., Danvers, MA, USA. 6 Philips 420T, Philips Electronics InstrumentS, Eindhoven, The Netherlands.
G, S. Lee et al. / Crystallization of sol gel-derived LA S glass ceramic powders"
3. Results and discussion
3.1. Gel to glass characterization D T A of the as-received powder shows one relatively broad endothermic peak, two sharp and two broad exothermic peaks (fig. l(a)) at - 150, 200, 370, 420 and 860 o C, respectively. The endothermic peak at - 150 o C is attributed to the evaporation of physically adsorbed water and alcohols. The three exothermic peaks (200-420 o C) are attributed to the oxidation of organics as supported by T G A (fig. l(b)). Gradual weight loss above 600 ° C is attributed to network condensation [23]. The broad exothermic peak initiated at 8 6 0 ° C was identified by X R D as the crystallization of the /3-quartz solid solution and will be discussed below. DTA of the 6 4 0 ° C calcined gel powders showed a glass transition temperature at 755 o C. The original gel has a specific surface area of 350 m2/g and a skeletal density of 1.70 g / c m 3 or 69% of the dense melt glass (2.48 g/cm3)) (fig. 2). Thermal treatment of the gel at 6 4 0 ° C for 6 h resulted in an increase of the skeletal density to 2.38 g / c m 3 (95.6%) and a surface area of 186 m2/g. The powder calcined at 6 4 0 ° C for 6 h, which is used for the crystallization study, was heated for 30 min at 700 and 750 o C. At 750 ° C the specific surface area decreases to 40 mZ/g and a skeletal density of 2.48 g / m 3.
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Fig. 2. Change in skeletal density and specific surface area of the LAS powder as a function of temperature (the solid lines represent the apparent tendency of the data).
127
The above physical property measurements are supported by T E M of the gel-derived powders (fig. 3). The porous microstructure of the as-received gel particles consists of poorly resolved - 8 nm primary units (fig. 3(a)). The porous microstructure is retained at 640 ° C but at 750 o C, near Tg, the particles become dense. However, there is a uniformly distributed surface roughness of - 8 nm still present on the 750 ° C heat treated powder (fig. 3(b)) which is approximately the same size as the primary particles in the as-received gel powder, thus explaining the large surface area. Heat treatment of the 6 4 0 ° C powder for 30 min at 800 ° C of the gel powder resulted in complete densification of the solid glass particles and a reduction of surface area to 4 m 2 / g as a result of surface smoothing by viscous flow and sintering between particles (fig. 3(c)). Infrared spectroscopy demonstrated that this glass had approximately the same structure as the melt glass [24].
3.2. Phase evolution and transformation The crystallization sequence of the gel-derived powders is given in fig. 4 as a function of temperature. The 6 4 0 ° C calcined powder was X-ray amorphous (fig. 4(a)) but begins to crystallize above 800 ° C [24] when heated for 3 h. Between 800 and l l 0 0 ° C , only a single phase crystallizes (fig. 4(b)) whose X-ray peak positions shift as a function of heat treatment temperature [24]. The crystalline phase is a B-quartz solid solution in the L i 2 0 . A1203 • nSiO 2 [25] and L i 2 0 - 1.34A1203 nSiO 2 series [26] which has also been identified as virgilite [21,27]. The B-quartz solid solution is a metastable phase and at l l 0 0 ° C begins to transform to a stable tetragonal B-spodumene solid solution (fig. 4(c)) as supported by the appearance of the (102) peak at 23°(20) of B-spodumene. At 1200 o C the B-quartz solid solution is almost completely transformed to B-spodumene (fig. 4(a)). The isothermal crystallization kinetics of the 640 ° C calcined powder to the B-quartz solid solution are given in fig. 5. The sigmoidal curves are typical of isothermal transformation kinetics controlled by nucleation and growth. The incubation time decreases and the m a x i m u m volume fraction of crystalline phase increases to a maximum crys-
128
G.S. Lee et aL / Crystallization of sol-gel-derived LA S glass ceramic powders
ml
Fig. 3. TEM micrographs of gel-derived LAS powder, showing microstructural development as a function of temperature; (a) at 640 ° C (360 min), (b) at 750 o C (30 min) and (c) at 800 o C (30 min).
tallinity of 0.95 at 1 0 0 0 ° C . A b o v e 1000 ° C, d u e to the a l m o s t i n s t a n t a n e o u s crystallization, a c c u r a t e kinetics could not be o b t a i n e d . T i m e - t e m p e r a t u r e - t r a n s f o r m a t i o n ( T - T - T ) curves were p l o t t e d in fig. 6 by r e a d i n g the i s o t h e r m a l time from fig. 5 for a given a m o u n t of crystallinity (i.e., 0 a n d 0.6) at various t e m p e r a t u r e s . T h e curve V x = 0 is the b o u n d a r y b e t w e e n the X - r a y a m o r p h o u s a n d crystallized states as d e t e r m i n e d from fig. 5 b y e x t r a p o l a t i n g the kinetics to 0 vol.% reaction.
3.3. N u c l e a t i o n
In the previous section, it was d e m o n s t r a t e d that the gel-derived p o w d e r s c a n be crystallized into glass ceramics of fl-quartz solid solution. However, to process glass ceramics with c o n t r o l l e d m i c r o s t r u c t u r e s from s o l - g e l p o w d e r s we m u s t u n d e r s t a n d how the p a r t i c l e characteristics affect n u c l e a t i o n a n d growth. Therefore, the effects of p a r t i c l e size (PS), n u c l e a t i o n t r e a t m e n t a n d its
G.S. Lee et al. / Crystallization of sollgel-deri~ed LAS glass ceramic powders '
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Fig. 4. X-ray diffraction patterns of gel-derived powders heat treated at (a) 640 ° C, (b) 900 ° C, (c) 1100° C and (d) 1200 ° C.
temperature and the nucleating agents on the crystallization temperature were measured to investigate the nucleation behavior of the calcined gel powders. D T A curves of 75 < PS < 150 Ixm and PS < 1 ~xm, 6 4 0 ° C calcined powders exhibited the same crystallization temperature of 860 °C. This indicates that crystallization of the calcined powders is independent of powder surface area. Alternatively, a large number of heterogeneous nuclei could be formed during calcination of the gel --'3-
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Fig. 5. Isothermal crystallization curves for 6 4 0 ° C calcined LAS powders as a function of time and temperature (the solid lines represent the apparent tendency of the data).
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Fig, 6. Time-temperature transformation (T T - T ) curves for the 6 4 0 ° C calcined LAS powder (the solids lines represent the apparent tendency of the data).
powder as they contain nucleating agents, g r O 2 and TiO 2. However, a 6 4 0 ° C calcined powder with no nucleating agents had the same crystallization temperature. Thus, heterogeneous nucleation sites based on the addition of nucleating agents does not account for the 8 6 0 ° C transformation temperature. As discussed earlier, the calcined powders are porous, high surface area aggregates of primary particles. Therefore, the two different-sized particles initially have the same microstructures on a nanometer scale. As crystallization is independent of solid particle size, it is proposed that the micropore surfaces of the 640 ° C calcined particles are active sites for heterogeneous nucleation. To clarify further the nucleation behavior, the calcined gel powders were heat treated at different temperatures below the crystallization temperature. Table 2 shows that heating at 740, 760 and 7 8 0 ° C had no effect on the crystallization temperature. From these observations, it is concluded that the calcined powder already has a large nucleation density and heat treatments do not significantly change this density. The T E M micrograph of the 6 4 0 ° C calcined powder after sintering for 60 min at 9 0 0 ° C (fig. 7(a)) is composed of uniformly distributed and relatively equiaxed crystallites of 30-100 nm. Figure 7(b) is a representative microstructure of the
130
G.S. Lee et aL / Crystallization of sol gel-derit~ed LA S glass ceramic powders
Table 2 Effect of heat treatment (90 rain) on the crystallization temperature (T~) of the calcined gel powders
TN (°C)
T~(°C)
a)
860
740 76O 78O
860 86O 860
treatment on the crystalline microstructures as suggested by the 8 6 0 ° C crystallization temperature. As the scale of the crystallite distribution is much less than the - 1-3 p~m diameter particles, it is concluded that uniformly distributed surface nuclei become internal bulk nucleation sites upon sintering of the calcined gel particles.
a) Calcined at 640 o C, with no further heat treatment.
3.4. Comparison with melt glass
powders heat treated at 730, 750 and 770°C for 90 min and then sintered for 60 min at 900 °C. Clearly, there is no major effect of nucleation heat
Particle size had little effect on the crystallization temperature of gel powders whereas crystallization of the melt glass powder strongly depends on the particle size. As the particle size decreases, the crystallization temperature shifts from 925 ° C for the bulk glass and glass particles of 75 ~m < PS < 150 ~tm diameter. For glass particles of < 38 ~tm and < 3 ~m, the crystallization temperature decreases to 905 and 885 o C, respectively. The particle size dependence of the crystallization temperature clearly indicates that heterogeneous nucleation is dominant and that particle surfaces are active nucleation sites in the melt glass powders. Although direct comparison between calcined gel powders and melt glass powders is difficult due to the smaller particle size of the gel powders, the lower crystallization temperature in the gel powder (865 vs. 885 o C) is attributed to its larger surface area and higher thermodynamic excess free energy. However, the difference cannot be attributed to differences in hydroxyl content as the Tg is 755 °C for both the calcined gel and melt glass powders. A nucleation heat treatment of the bulk melt glass for 90 min at 780 ° C lowered the crystallization temperature from 925 to 860°C. Furthermore, the crystallization temperature of the < 3 ~tm melt glass powder also decreased to 8 6 0 ° C when similarly heat treated. A TEM micrograph (fig. 8) of the bulk melt glass after 60 min at 770 ° C followed by heating at 8 8 0 ° C for 60 min shows crystallites of - 0 . 3 rxm which are approximately 3-10 times larger than obtained with the 640 ° C calcined powder heated at 900 ° C for 60 min. These results clearly demonstrate that the crystallization behavior of the gel powder differs from that of melt glass powder in terms of particle
Fig. 7. TEM micrographs of 6 4 0 ° C calcined powder after sintering for 60 min at 900 ° C: (a) no nucleation heat treatment, (b) heat treated at 730 ° C for 90 min before sintering.
G.S. Lee et al. / Crystallization of sol-gel-derived LA S glass ceramic powders
Fig. 8. TEM micrograph of the melt glass after a nucleation heat treatment of 60 min at 770 ° C , followed by heat treatment for 60 rain at 880 o C.
size and nucleation treatment effects on the crystallization temperature. The melt glass is apparently nucleated as a result of heterogeneous nuclei whereas the gel powder is nucleated on the surface of the gel powder before it becomes dense and, furthermore, it is independent of the heterogeneous nucleating agents. If each grain in the TEM micrographs results from a single nucleation site, then the nucleation frequencies of the calcined gel powder and bulk glass are 3.7 x 1013-+1 and 7.2 x 1015-+1 nuclei/cm 3, respectively. As the transformation temperature is approximately constant at 8 6 0 ° C for the calcined gel powders and reaches a minimum of 8 6 0 ° C when the melt glasses are fully nucleated, it is concluded that this temperature represents a kinetic limitation of the system for crystal growth.
131
(800 °C), well below the crystallization temperature of 860 o C. Crystallization of the glass powder to a /3-quartz solid solution at 8 6 0 ° C was independent of the presence of conventional glass ceramic nucleating agents (e.g. TiO 2 and ZrO2), gel powder particle size and heat treatment conditions. In contrast, the bulk melt glass crystallized to/3-quartz at 925 o C. The crystallization temperature of the melt glass was reduced to as low as 885°C for 3 ~tm particles. Nucleation heat treatment of the melt glass and glass powders lowered the crystallization temperature to 860°C. TEM showed the sol-gel and melt glass powders to have nucleation densities differing by more than an order of magnitude when they had the same crystallization temperature (i.e. 860°C). The higher nucleation density and constant crystallization temperature clearly demonstrates that the surface of the gel powder is an active site for nucleation at < 800°C. Above 8 0 0 ° C the particles densify to distribute uniformly the surface nuclei inside the glass particles. As the glass transition temperature and glass structure before crystallization are approximately the same for the gel and melt powders, the lower crystallization temperature of the gel is attributed solely to its surface excess free energy and its effect on nucleation. The constant crystallization temperature of the gel, coupled with its large nucleation frequency, suggests that 860°C is a kinetic limitation of the LAS system. Financial support from Litton Guidance and Control Systems and the Korean Government is gratefully acknowledged.
References 4. Summary Similar to earlier comparisons of sol gel and melt derived glass crystallization, the LAS gel powder crystallizes at a lower temperature. When heated at 750 ° C the porous gel powder converted to a less porous glass, but with a glass-like skeletal structure, at the gel Tg (750°C) and into amorphous and dense glass particles at Tg + 50 o C
[1] B.J.J. Zelinski and D.R. Uhlmann, J. Phys. Chem. Solids 45 (1984) 1069. [2] D.R. Ulrich, J. Non-Cryst. Solids 100 (1988) 174. [3] C.J. Brinker, E.P. Roth, D.R. Tallant and G.W. Scherer, in: Ultrastructure Processing of Ceramics, Glasses and Composites, eds. L.L. Hench and D.R. Ulrich (NorthHolland, New York, 1984) p. 37. [4] C.J. Brinker, G.W. Scherer and E.P. Roth, J. Non-Cryst. Solids 72 (1985) 345. [5] N. Toghe, G.S. Moore and J.D. Mackenzie, J. Non-Cryst. Solids 63 (1984) 95.
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G.S. Lee et al. / Crystallization of sol-gel-derived LA S glass ceramic powders
[6] D.M. Krol and J.G. van Lierop, J. Non-Cryst. Solids 63 (1984) 131. [7] M. Nogami and Y. Moriya, J. Non-Cryst. Solids 37 (1980) 191. [8] C.A.M. Mulder, J.G. van Lierop and G. Frens, J. NonCryst. Solids 82 (1986) 92. [9] L.C. Klein, T.A. Gallo and G.J. Garvey, J. Non-Cryst. Solids 63 (1984) 23. [10] M. Decottignies, J. Phalippou and J. Zarzycki, J. Mater. Sci. 13 (1978) 2605. [11] C.J.R. Gonzalez-Oliver, P.F. James and H. Rawson, J. Non-Cryst. Solids 48 (1982) 129. [12] A. Bertoluzza, C. Fagnano and M.A. Morelli, J. Non-Cryst. Solids 48 (1982) 117. [13] A.R. Cooper, in: Better Ceramics Through Chemistry II, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (North-Holland, New York, 1986) p. 421. [14] M.C. Weinberg, in: Better Ceramics Through Chemistry II, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (NorthHolland, New York, 1986) p. 431. [15] S.P. Mukherjee, in: Better Ceramics Through Chemistry II, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (NorthHolland, New York, 1986) p. 443. [16] S.P. Mukherjee, J. Zarzycki, J.M. Badie and J.P. Traverse, J. Non-Cryst. Solids 20 (1976) 244.
[17] S.P. Mukherjee, J. Zarzycki and J.P. Traverse, J. Mater. Sci. 11 (1976) 341. [18] D.R. Uhlmann, M.C. Weinberg and G. Teowee, J. NonCryst. Solids 100 (1988) 154. [19] G.F. Neilson and M.C. Weinberg, J. Non-Cryst. Solids 63 (1984) 365. [20] E.M. Rabinovich, J. Mater. Sci. 20 (1985) 4259. [21] J. Covino, F.G.A. DeLaat and R.A. Welsbie, in: Better Ceramics Through Chemistry II, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (North-Holland, New York, 1986) p. 135. [22] H.P. Klug and L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd Ed. (Wiley, New York, 1974). [23] C.J. Brinker and G.W. Scherer, J. Non-Cryst. Solids 70 (1985) 301. [24] G.S. Lee, Ph.D. Thesis, The Pennsylvania State University (1988). [25] S. Ray and G.M. Muchow, J. Am. Ceram. Soc. 51 (1968) 678. [26] K. Nakagawa and T. Izumitani, J. Non-Cryst. Solids 7 (1972) 168. [27] B.M. French, P.A. Jezek and D.E. Appleman, Am. Mineralogist 63 (1978) 461.
125
CRYSTALLIZATION OF SOL-GEL-DERIVED L I T H I U M ALUMINOSILICATE (LAS) GLASS CERAMIC POWDERS Gye Song LEE and Gary L. MESSING The Pennsylvania State University, University Park, PA 16802, USA
Frans G.A. DELAAT Litton Guidance and Control Systems, Woodland Hills, CA 91365, USA Received 30 May 1989 Revised manuscript received 3 October 1989
The gel to glass ceramic conversion of a sol-gel-derived LAS powder was characterized by its gel to glass conversion, nucleation, crystallization and microstructural development. At Tg the gel converts to a glass with a dense skeletal structure and at - 800 o C to fully dense, amorphous particles. Crystallization at 860 ° C to a/3-quartz solid solution is independent of nucleating agents, gel powder particle size and heat treatment. In contrast, crystallization of a melt glass derived from the gel was dependent on nucleation heat treatment, reaching a m i n i m u m crystallization temperature of 8 6 0 ° C . As the glass transition temperature and glass structure before crystallization are approximately the same for the gel and melt-derived powders, the lower crystallization temperature of the gel is attributed solely to its surface excess free energy and its effect on nucleation.
1. Introduction
Sol-gel processing has been widely investigated for many reasons; however, a particular interest has come from the possibility of forming novel glasses and crystalline microstructures at low temperature [1,2]. By sol-gel synthesis, glass network formation and chemical homogenization in a glass can be achieved in solution near room temperature. Heat treatment near the glass transition temperature is required to convert the extremely porous gel into dense glass [3-6]. The gel-glass conversion has been examined by various methods such as powder density, specific surface area [5-9], spectroscopy [10-12] and TEM [5,7]. Due to the different thermal histories of gel-derived and melt glasses, fundamental questions have been raised about similarities and differences in their properties with particular interest on nucleation and crystallization [13-19]. Glasses prepared by melting either a gel or mixed oxides had no major differences during crystallization [13-14]. However, when gel powders were heated to obtain 0022-3093/90/$03.50 © Elsevier Science Publishers B.V. (North-Holland)
dense glass they had a lower crystallization temperature and faster nucleation and crystallization rates relative to melted glasses [19]. These differences were attributed to the lower viscosity of the gel powder due to the higher hydroxyl content [14] and the excess free energy due to the larger interior surface and structural differences [13,18]. Most crystallization studies of gel-derived powders or glasses have been on relatively simple glass compositions, designed to avoid crystallization. Interestingly, there has been no research comparing the crystallization behavior of sol-geland melt-derived glass ceramic powders. This information is important as melt-derived glass ceramic powder crystallization has been shown to directly influence its sinterability [20]. In this study, a gel with a glass ceramic composition in the LizO-A1203-SiO 2 (LAS) system was examined in terms of its gel-glass conversion, nucleation, crystallization kinetics and crystalline microstructures. To elucidate how gel powders effect nucleation and crystallization relative to conventional glass ceramics, their crystallization is
G.S. Lee et al. / Co'stallization of sol-gel-derived L A S glass ceramic powders
126
Table 1 Composition of the gel-derived powder
wt% mol%
SiO 2
A1203
Li 2°
MgO
ZnO
TiO 2
ZrO 2
61.4 67.3
28.0 18.0
4.1 9.0
1.0 1.6
1.4 1.1
2.2 2.0
1.9 1.0
compared with the crystallization of bulk and powdered glasses obtained by melting the glass ceramic gel.
I
a Litton Guidance and Control Systems, Woodland Hills, CA, USA. 2 Quantasorb, Quantachrome Corp., Syosset, NY, USA. 3 Model 1090 Thermal Analyzer, DuPont Co., Wilmington, DE, USA. 4 Model 1090 Thermal Analyzer, DuPont Co., Wilmington, DE, USA.
I
I
I
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2. Experimental The dried gel powders, which were prepared by nitric acid-catalyzed hydrolysis and polycondensation of a multicomponent metal alkoxide solution near room temperature, were provided by Litton G / C S 1 [21] as yellowish gel fragments. The glass composition is close to the B-spodumene ternary compound in the lithium aluminosilicate system and includes ZrO 2 and TiO 2 as nucleating agents (Table 1). For the crystallization study, the as-received gel powder was calcined in flowing air at 0.5 ° C per minute to 640 °C and held for 6 h, the lowest temperature-time condition to obtain organic-free powder. For some experiments, the calcined gel powder was melted in a Pt crucible for 10 h at 1600°C and quenched in air. Particle densities and specific surface areas were measured by pycnometry to + 0.05 g / c m 3 accuracy and single point BET 2 to _+2 m2/g accuracy, respectively. Differential thermal analysis (DTA) 3 and thermogravimetric analysis (TGA) 4 were used to determine thermal reactions, transformation temperatures to _+5°C accuracy and weight loss of the gel-derived powders by heating
I
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0
5 o .9'
IC
J
I
I
I
I
]
0 1200 (b)
t
I
I
I
200 400 600 800 I000 1200 Temperoture (°C)
Fig. 1. Thermal analysis of as-received gel-derived LAS powder; (a) D T A curve, (b) T G A curve (10 o C / m i n in air).
40 mg powder samples in flowing air at 10 o C / m i n . Phase identification and quantitative analysis of the calcined gel powders to + 5 vol% accuracy were examined by automated X-ray diffraction (XRD) 5 with a monochromatic C u K a source. The external standard technique [22] with fully crystallized LAS powder as the standard was used for quantitative phase analysis. Microstructural evolution during crystallization of the gel powders was examined by TEM 6
Rigaku USA, Inc., Danvers, MA, USA. 6 Philips 420T, Philips Electronics InstrumentS, Eindhoven, The Netherlands.
G, S. Lee et al. / Crystallization of sol gel-derived LA S glass ceramic powders"
3. Results and discussion
3.1. Gel to glass characterization D T A of the as-received powder shows one relatively broad endothermic peak, two sharp and two broad exothermic peaks (fig. l(a)) at - 150, 200, 370, 420 and 860 o C, respectively. The endothermic peak at - 150 o C is attributed to the evaporation of physically adsorbed water and alcohols. The three exothermic peaks (200-420 o C) are attributed to the oxidation of organics as supported by T G A (fig. l(b)). Gradual weight loss above 600 ° C is attributed to network condensation [23]. The broad exothermic peak initiated at 8 6 0 ° C was identified by X R D as the crystallization of the /3-quartz solid solution and will be discussed below. DTA of the 6 4 0 ° C calcined gel powders showed a glass transition temperature at 755 o C. The original gel has a specific surface area of 350 m2/g and a skeletal density of 1.70 g / c m 3 or 69% of the dense melt glass (2.48 g/cm3)) (fig. 2). Thermal treatment of the gel at 6 4 0 ° C for 6 h resulted in an increase of the skeletal density to 2.38 g / c m 3 (95.6%) and a surface area of 186 m2/g. The powder calcined at 6 4 0 ° C for 6 h, which is used for the crystallization study, was heated for 30 min at 700 and 750 o C. At 750 ° C the specific surface area decreases to 40 mZ/g and a skeletal density of 2.48 g / m 3.
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127
The above physical property measurements are supported by T E M of the gel-derived powders (fig. 3). The porous microstructure of the as-received gel particles consists of poorly resolved - 8 nm primary units (fig. 3(a)). The porous microstructure is retained at 640 ° C but at 750 o C, near Tg, the particles become dense. However, there is a uniformly distributed surface roughness of - 8 nm still present on the 750 ° C heat treated powder (fig. 3(b)) which is approximately the same size as the primary particles in the as-received gel powder, thus explaining the large surface area. Heat treatment of the 6 4 0 ° C powder for 30 min at 800 ° C of the gel powder resulted in complete densification of the solid glass particles and a reduction of surface area to 4 m 2 / g as a result of surface smoothing by viscous flow and sintering between particles (fig. 3(c)). Infrared spectroscopy demonstrated that this glass had approximately the same structure as the melt glass [24].
3.2. Phase evolution and transformation The crystallization sequence of the gel-derived powders is given in fig. 4 as a function of temperature. The 6 4 0 ° C calcined powder was X-ray amorphous (fig. 4(a)) but begins to crystallize above 800 ° C [24] when heated for 3 h. Between 800 and l l 0 0 ° C , only a single phase crystallizes (fig. 4(b)) whose X-ray peak positions shift as a function of heat treatment temperature [24]. The crystalline phase is a B-quartz solid solution in the L i 2 0 . A1203 • nSiO 2 [25] and L i 2 0 - 1.34A1203 nSiO 2 series [26] which has also been identified as virgilite [21,27]. The B-quartz solid solution is a metastable phase and at l l 0 0 ° C begins to transform to a stable tetragonal B-spodumene solid solution (fig. 4(c)) as supported by the appearance of the (102) peak at 23°(20) of B-spodumene. At 1200 o C the B-quartz solid solution is almost completely transformed to B-spodumene (fig. 4(a)). The isothermal crystallization kinetics of the 640 ° C calcined powder to the B-quartz solid solution are given in fig. 5. The sigmoidal curves are typical of isothermal transformation kinetics controlled by nucleation and growth. The incubation time decreases and the m a x i m u m volume fraction of crystalline phase increases to a maximum crys-
128
G.S. Lee et aL / Crystallization of sol-gel-derived LA S glass ceramic powders
ml
Fig. 3. TEM micrographs of gel-derived LAS powder, showing microstructural development as a function of temperature; (a) at 640 ° C (360 min), (b) at 750 o C (30 min) and (c) at 800 o C (30 min).
tallinity of 0.95 at 1 0 0 0 ° C . A b o v e 1000 ° C, d u e to the a l m o s t i n s t a n t a n e o u s crystallization, a c c u r a t e kinetics could not be o b t a i n e d . T i m e - t e m p e r a t u r e - t r a n s f o r m a t i o n ( T - T - T ) curves were p l o t t e d in fig. 6 by r e a d i n g the i s o t h e r m a l time from fig. 5 for a given a m o u n t of crystallinity (i.e., 0 a n d 0.6) at various t e m p e r a t u r e s . T h e curve V x = 0 is the b o u n d a r y b e t w e e n the X - r a y a m o r p h o u s a n d crystallized states as d e t e r m i n e d from fig. 5 b y e x t r a p o l a t i n g the kinetics to 0 vol.% reaction.
3.3. N u c l e a t i o n
In the previous section, it was d e m o n s t r a t e d that the gel-derived p o w d e r s c a n be crystallized into glass ceramics of fl-quartz solid solution. However, to process glass ceramics with c o n t r o l l e d m i c r o s t r u c t u r e s from s o l - g e l p o w d e r s we m u s t u n d e r s t a n d how the p a r t i c l e characteristics affect n u c l e a t i o n a n d growth. Therefore, the effects of p a r t i c l e size (PS), n u c l e a t i o n t r e a t m e n t a n d its
G.S. Lee et al. / Crystallization of sollgel-deri~ed LAS glass ceramic powders '
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temperature and the nucleating agents on the crystallization temperature were measured to investigate the nucleation behavior of the calcined gel powders. D T A curves of 75 < PS < 150 Ixm and PS < 1 ~xm, 6 4 0 ° C calcined powders exhibited the same crystallization temperature of 860 °C. This indicates that crystallization of the calcined powders is independent of powder surface area. Alternatively, a large number of heterogeneous nuclei could be formed during calcination of the gel --'3-
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powder as they contain nucleating agents, g r O 2 and TiO 2. However, a 6 4 0 ° C calcined powder with no nucleating agents had the same crystallization temperature. Thus, heterogeneous nucleation sites based on the addition of nucleating agents does not account for the 8 6 0 ° C transformation temperature. As discussed earlier, the calcined powders are porous, high surface area aggregates of primary particles. Therefore, the two different-sized particles initially have the same microstructures on a nanometer scale. As crystallization is independent of solid particle size, it is proposed that the micropore surfaces of the 640 ° C calcined particles are active sites for heterogeneous nucleation. To clarify further the nucleation behavior, the calcined gel powders were heat treated at different temperatures below the crystallization temperature. Table 2 shows that heating at 740, 760 and 7 8 0 ° C had no effect on the crystallization temperature. From these observations, it is concluded that the calcined powder already has a large nucleation density and heat treatments do not significantly change this density. The T E M micrograph of the 6 4 0 ° C calcined powder after sintering for 60 min at 9 0 0 ° C (fig. 7(a)) is composed of uniformly distributed and relatively equiaxed crystallites of 30-100 nm. Figure 7(b) is a representative microstructure of the
130
G.S. Lee et aL / Crystallization of sol gel-derit~ed LA S glass ceramic powders
Table 2 Effect of heat treatment (90 rain) on the crystallization temperature (T~) of the calcined gel powders
TN (°C)
T~(°C)
a)
860
740 76O 78O
860 86O 860
treatment on the crystalline microstructures as suggested by the 8 6 0 ° C crystallization temperature. As the scale of the crystallite distribution is much less than the - 1-3 p~m diameter particles, it is concluded that uniformly distributed surface nuclei become internal bulk nucleation sites upon sintering of the calcined gel particles.
a) Calcined at 640 o C, with no further heat treatment.
3.4. Comparison with melt glass
powders heat treated at 730, 750 and 770°C for 90 min and then sintered for 60 min at 900 °C. Clearly, there is no major effect of nucleation heat
Particle size had little effect on the crystallization temperature of gel powders whereas crystallization of the melt glass powder strongly depends on the particle size. As the particle size decreases, the crystallization temperature shifts from 925 ° C for the bulk glass and glass particles of 75 ~m < PS < 150 ~tm diameter. For glass particles of < 38 ~tm and < 3 ~m, the crystallization temperature decreases to 905 and 885 o C, respectively. The particle size dependence of the crystallization temperature clearly indicates that heterogeneous nucleation is dominant and that particle surfaces are active nucleation sites in the melt glass powders. Although direct comparison between calcined gel powders and melt glass powders is difficult due to the smaller particle size of the gel powders, the lower crystallization temperature in the gel powder (865 vs. 885 o C) is attributed to its larger surface area and higher thermodynamic excess free energy. However, the difference cannot be attributed to differences in hydroxyl content as the Tg is 755 °C for both the calcined gel and melt glass powders. A nucleation heat treatment of the bulk melt glass for 90 min at 780 ° C lowered the crystallization temperature from 925 to 860°C. Furthermore, the crystallization temperature of the < 3 ~tm melt glass powder also decreased to 8 6 0 ° C when similarly heat treated. A TEM micrograph (fig. 8) of the bulk melt glass after 60 min at 770 ° C followed by heating at 8 8 0 ° C for 60 min shows crystallites of - 0 . 3 rxm which are approximately 3-10 times larger than obtained with the 640 ° C calcined powder heated at 900 ° C for 60 min. These results clearly demonstrate that the crystallization behavior of the gel powder differs from that of melt glass powder in terms of particle
Fig. 7. TEM micrographs of 6 4 0 ° C calcined powder after sintering for 60 min at 900 ° C: (a) no nucleation heat treatment, (b) heat treated at 730 ° C for 90 min before sintering.
G.S. Lee et al. / Crystallization of sol-gel-derived LA S glass ceramic powders
Fig. 8. TEM micrograph of the melt glass after a nucleation heat treatment of 60 min at 770 ° C , followed by heat treatment for 60 rain at 880 o C.
size and nucleation treatment effects on the crystallization temperature. The melt glass is apparently nucleated as a result of heterogeneous nuclei whereas the gel powder is nucleated on the surface of the gel powder before it becomes dense and, furthermore, it is independent of the heterogeneous nucleating agents. If each grain in the TEM micrographs results from a single nucleation site, then the nucleation frequencies of the calcined gel powder and bulk glass are 3.7 x 1013-+1 and 7.2 x 1015-+1 nuclei/cm 3, respectively. As the transformation temperature is approximately constant at 8 6 0 ° C for the calcined gel powders and reaches a minimum of 8 6 0 ° C when the melt glasses are fully nucleated, it is concluded that this temperature represents a kinetic limitation of the system for crystal growth.
131
(800 °C), well below the crystallization temperature of 860 o C. Crystallization of the glass powder to a /3-quartz solid solution at 8 6 0 ° C was independent of the presence of conventional glass ceramic nucleating agents (e.g. TiO 2 and ZrO2), gel powder particle size and heat treatment conditions. In contrast, the bulk melt glass crystallized to/3-quartz at 925 o C. The crystallization temperature of the melt glass was reduced to as low as 885°C for 3 ~tm particles. Nucleation heat treatment of the melt glass and glass powders lowered the crystallization temperature to 860°C. TEM showed the sol-gel and melt glass powders to have nucleation densities differing by more than an order of magnitude when they had the same crystallization temperature (i.e. 860°C). The higher nucleation density and constant crystallization temperature clearly demonstrates that the surface of the gel powder is an active site for nucleation at < 800°C. Above 8 0 0 ° C the particles densify to distribute uniformly the surface nuclei inside the glass particles. As the glass transition temperature and glass structure before crystallization are approximately the same for the gel and melt powders, the lower crystallization temperature of the gel is attributed solely to its surface excess free energy and its effect on nucleation. The constant crystallization temperature of the gel, coupled with its large nucleation frequency, suggests that 860°C is a kinetic limitation of the LAS system. Financial support from Litton Guidance and Control Systems and the Korean Government is gratefully acknowledged.
References 4. Summary Similar to earlier comparisons of sol gel and melt derived glass crystallization, the LAS gel powder crystallizes at a lower temperature. When heated at 750 ° C the porous gel powder converted to a less porous glass, but with a glass-like skeletal structure, at the gel Tg (750°C) and into amorphous and dense glass particles at Tg + 50 o C
[1] B.J.J. Zelinski and D.R. Uhlmann, J. Phys. Chem. Solids 45 (1984) 1069. [2] D.R. Ulrich, J. Non-Cryst. Solids 100 (1988) 174. [3] C.J. Brinker, E.P. Roth, D.R. Tallant and G.W. Scherer, in: Ultrastructure Processing of Ceramics, Glasses and Composites, eds. L.L. Hench and D.R. Ulrich (NorthHolland, New York, 1984) p. 37. [4] C.J. Brinker, G.W. Scherer and E.P. Roth, J. Non-Cryst. Solids 72 (1985) 345. [5] N. Toghe, G.S. Moore and J.D. Mackenzie, J. Non-Cryst. Solids 63 (1984) 95.
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G.S. Lee et al. / Crystallization of sol-gel-derived LA S glass ceramic powders
[6] D.M. Krol and J.G. van Lierop, J. Non-Cryst. Solids 63 (1984) 131. [7] M. Nogami and Y. Moriya, J. Non-Cryst. Solids 37 (1980) 191. [8] C.A.M. Mulder, J.G. van Lierop and G. Frens, J. NonCryst. Solids 82 (1986) 92. [9] L.C. Klein, T.A. Gallo and G.J. Garvey, J. Non-Cryst. Solids 63 (1984) 23. [10] M. Decottignies, J. Phalippou and J. Zarzycki, J. Mater. Sci. 13 (1978) 2605. [11] C.J.R. Gonzalez-Oliver, P.F. James and H. Rawson, J. Non-Cryst. Solids 48 (1982) 129. [12] A. Bertoluzza, C. Fagnano and M.A. Morelli, J. Non-Cryst. Solids 48 (1982) 117. [13] A.R. Cooper, in: Better Ceramics Through Chemistry II, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (North-Holland, New York, 1986) p. 421. [14] M.C. Weinberg, in: Better Ceramics Through Chemistry II, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (NorthHolland, New York, 1986) p. 431. [15] S.P. Mukherjee, in: Better Ceramics Through Chemistry II, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (NorthHolland, New York, 1986) p. 443. [16] S.P. Mukherjee, J. Zarzycki, J.M. Badie and J.P. Traverse, J. Non-Cryst. Solids 20 (1976) 244.
[17] S.P. Mukherjee, J. Zarzycki and J.P. Traverse, J. Mater. Sci. 11 (1976) 341. [18] D.R. Uhlmann, M.C. Weinberg and G. Teowee, J. NonCryst. Solids 100 (1988) 154. [19] G.F. Neilson and M.C. Weinberg, J. Non-Cryst. Solids 63 (1984) 365. [20] E.M. Rabinovich, J. Mater. Sci. 20 (1985) 4259. [21] J. Covino, F.G.A. DeLaat and R.A. Welsbie, in: Better Ceramics Through Chemistry II, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (North-Holland, New York, 1986) p. 135. [22] H.P. Klug and L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, 2nd Ed. (Wiley, New York, 1974). [23] C.J. Brinker and G.W. Scherer, J. Non-Cryst. Solids 70 (1985) 301. [24] G.S. Lee, Ph.D. Thesis, The Pennsylvania State University (1988). [25] S. Ray and G.M. Muchow, J. Am. Ceram. Soc. 51 (1968) 678. [26] K. Nakagawa and T. Izumitani, J. Non-Cryst. Solids 7 (1972) 168. [27] B.M. French, P.A. Jezek and D.E. Appleman, Am. Mineralogist 63 (1978) 461.