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Effect of methanol concentration on lithium aluminosilicate gels
Journal of Non-Crystalline Solids 109 (1989) 69-78 North-Holland, Amsterdam
69
E F F E C T O F M E T H A N O L C O N C E N T R A T I O N ON L I T H I U M A L U M I N O S I L I C A T E GELS H. de LAMBILLY and L.C. K L E I N Rutgers- The State University of New Jersey, Ceramics Department, PO Box 909, Piscataway, NJ 08855- 0909, USA Received 10 February 1988 Revised manuscript received 27 December 1988
The viscosity of the solution, the diffusion coefficient and the time-to-gel depend on the preparation method used in lithium aluminosilicate gels. The crystallization temperature during heat treatment depends on the preparation method as well. Samples were prepared by adding anhydrous tetraethylorthosilicate (TEOS) to a solution of methanol, water and the nitrates of lithium and aluminum. The amount of methanol was varied to create miscible and immiscible compositions. High methanol compositions took a long time to gel and lost fluidity abruptly. Low methanol compositions took a short time to gel and lost fluidity gradually. Nitrogen sorption techniques were used to determine surface areas in dried and heat treated gels. Low methanol compositions have higher surface areas than high methanol compositions. Thermal analysis was used to identify reaction temperatures. High and low methanol compositions show similar behavior below 4 0 0 ° C but high methanol compositions crystallize beginning around 550 o C while low methanol compositions crystallize above 700 ° C.
1. Introduction
The goal of this study is to determine the effect of the methanol concentration on the texture and thermal stability of lithium aluminosilicate gels. Previous studies of sol-gel processing have emphasized the molar ratio of water to silica precursor [1,2], the nature of the silica precursor [3] and the nature of the catalyst, acid vs. base [4,5]. Also, it was pointed out [6] that there is an effect of solvent concentration in sol-gel processing. Originally, Schwartz et al. [7] added alkali salts to tetraethylorthosilicate (TEOS) solutions to determine the stability of binary silicate gels. They found that lithium was the simplest alkali to incorporate into gels. The method for producing the gels was through acid catalysis of TEOS. No surface crystallization was found on the lithium compositions. The dried lithium gels were analyzed by atomic absorption for chemical homogeneity. The lithium content was uniform throughout the samples. Based on the work by Schwartz et al. [7], it is known that stable lithium silicate gel monoliths can be obtained through low-temperature natural evaporation. Success in obtaining monolithic gels 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
depends on the initial amount of water in the alkoxide solutions, according to Anderson and Klein [8]. A proper amount of water ensures that the crosslinking is sufficient to increase the strength of the gel so that the gel can withstand capillary forces during drying. Separately, lithium silicate gels have been prepared by Wallace and Hench [9,10]. One of their objectives was to produce gels in the shortest period of time by optimizing the gelling process. To keep the gel from breaking during drying, a drying control chemical additive (DCCA) was added to the solution. They used several organic and inorganic precursors for lithium. The gels containing lithium-i-propoxide and lithium methoxide remained monolithic. Lithium nitrate had the best drying characteristics among the inorganic precursors. The optimized process called for a low pH and about a stoichiometric mole ratio of water to silicon alkoxide. Based on these results, acid-catalyzed TEOS solutions with stoichiometric or greater water ratios have been selected for further study. Lithium nitrate is from several accounts the preferred precursor. In order to distribute the lithium in the gel at the time of formulation, the amount of solvent
70
H. de Lambilly, L. C. Klein / Lithium aluminosilicate gels
is controlled to bring about optimum gelling. In the studies on the same compositions, regarding crystallization behavior [6], there is a direct effect of methanol concentration on the distribution of the lithium.
2. Experimental procedure The composition chosen was 83% SiO z - 15% Li20 - 2% A1203 (all in mol%). This choice was the result of work by Schwartz et al. [7] who found that, among the three alkali (lithium, sodium and potassium), lithium was easiest to stabilize inside the gel. Some A1203 helped stabilize the gel as well. The w a t e r / T E O S molar ratio was 8/1 (64.2 ml water for 100 ml TEOS). TEOS was preferred to TMOS (tetramethyl orthosilicate) because of its reduced toxicity and its slower hydrolysis rate. Lithium nitrate and aluminum nitrate were the lithium and aluminum precursors. Six methanol concentrations were used. They are given in vol%. They were 0, 5, 20, 50, 100 and 200% of the volume of TEOS. Solutions with methanol were more stable than with other alcohols. Methanol is not a product of the reaction and may not affect the kinetics of the reactions the way ethanol would. One of the consequences of this choice is the possibility of exchange between TEOS and methanol. It is know to occur but to a lesser extent than between TMOS and ethanol [11]. The solutions were prepared by mixing first the methanol, water, catalyst and salts at 90 ° C for 1 h. The reason for this premixing was to completely dissolve the salts. This improved the stability of the lithium by attaching it to a larger molecule. It is known that lithium, compared to sodium, has a relatively large hydrated radius. The pH of the solution was lowered by addition of hydrochloric acid. Since the function of the acid is to catalyze the reactions, a constant pH of about 0.2 was used for all alcohol dilutions. The solution was then cooled and TEOS, distilled to remove alcohol and impurities, was added. Adding TEOS to the solution did not change the pH. The solutions with a high methanol concentration (greater than 50%) dissolved the volume
of TEOS without turning cloudy. These solutions were homogeneous. With a small volume (0, 5 and 20%) of methanol the solution turned cloudy. After a few hours, they turned clear again. When the alkoxide was added gradually, the solution did not turn completely cloudy. The solutions were then mixed for one half hour. After that, samples were cast into 10 ml polystyrene tubes (cylinder shape) and petri dishes (disk shapes) and the molds were loosely sealed. Half the samples were kept at room temperature while the other half were allowed to gel and dry in an oven at 6 5 ° C for about 1 month. The gelling time was recorded. Gelation was taken as the state where the viscosity was so high that no flow was apparent. Solution densities were measured by weighing a volume of solution in a pipette. Solid densities were measured on powders by helium pycnometry (Micromeritics TM) after a 2 0 0 ° C treatment to remove adsorbed water and volatile species. Viscosity measurements were performed during gelation. A batch was placed in 10 ml glass vials. Samples were then poured into the chamber of the Haake TM rotational viscometer. Viscosities were recorded at 25°C. The shear rate was continuously varied from 0 to 300 s -1. The shear stress was measured on the cylinder with increasing and decreasing shear rate. Diffusion coefficients were calculated on samples from the same batch. A photon correlation spectrometer (Malvern Instruments Model No. 4700C) was used. All measurements were made at 25 o C. Scattered light was measured at 90 ° C from the incident beam. For the 0% case, the medium was assumed to be a mixture of water, TEOS and ethanol and values taken for viscosity and refractive index were 1 m P a - s and 1.33. In the 200% case, 0.758 mPa- s and 1.33 were the values for a mixture of water, methanol and ethanol. As recommended by the manufacturer, the reported values are an average of at least three measurements. Each run was at least 10 counts lasting 10 to 20 s each. Experiment time and measurement settings (apertures) were varied according to the instructions of the manufacturer to increase the signal. The correlator was used to minimize the noise and increase the accuracy of the measurement.
H. de Lambilly, L.C. Klein / Lithium aluminosilicate gels
In dried, powdered samples, the texture was analyzed using nitrogen sorption techniques in an Omnisorb 360 unit from Omicron Technology. Outgassing was done under vacuum at 200°C. For this type of instrument relatively large samples are used since the ASTM standard recommends a measurement on a surface of at least 10 m 2.
The Omnisorp 360 unit allows continuous adsorption/desorption isotherms to be recorded by measuring the pressure change in the measurement cell under a continuous flow of dry nitrogen. Surface areas, pore size distributions and pore shapes can be extracted from the data. The samples were characterized during heat treatment using a DuPont 1090B thermal analyzer. Thermogravimetric analysis was performed on powdered ( - 2 0 0 / + 325 mesh) samples at a heating rate of 5 o C / m i n . For monolithic samples, the heating rate was 1 ° C / m i n . The slower rate prevented disruption of the sample. The experiment was done under a continuous flow of dry oxygen (50 ml/min). On identical samples, differential thermal analysis was performed using an alumina standard with a heating rate of 1 0 ° C / m i n under oxygen flow.
3. Results
Overall, the samples can be divided into two categories: "low alcohol gels" (0, 5 and 20% alcohol content) and "high alcohol gels" (100 and 200% alcohol content). Gel 50%, which was processed close to the boundary between miscible
71
and immiscible solutions, shows transitional behavior. Low alcohol gels show short gelation times; high alcohol gels have long gelation times which increase with increasing alcohol concentration (table 1). Gelling at a higher temperature speeds the process and lessens the difference between high and low alcohol gels. The viscosity versus reduced time is plotted in fig. 1. Two gels, 0 and 200%, are presented to illustrate the difference between the low and high alcohol gels. Reduced time is the actual time divided by the time to gel. For both samples, the viscosity increases with time, slowly at the beginning until the sharp increase at the sol-gel transition. The change of slope occurs earlier for the 0% gel, around reduced time 0.8. The diffusion coefficients calculated from light scattering data measured during gelation are plotted versus reduced time in fig. 2. The diffusion coefficient decreases at the beginning, and is attributed to an increase of polymer size. For the low alcohol case, the diffusion coefficients are about one order of magnitude smaller than the high alcohol case. A smaller diffusion coefficient indicates larger polymers or more interaction between polymeric entities. Light scattering after the sol has gelled is attributed to the movement of free polymers inside the polymeric network. These measurements are not as reliable as those made before gelling, because some drying occurs during the measurement. Surface areas were obtained from the nitrogen sorption experiments. The pore size distributions and pore volumes were determined as well. The surface area is plotted vs. methanol concentration
Table 1 Characteristics of the different alcohol concentration gels before heat treatment Alcohol content
Gelling time (days)
Sol density ( g / c m 3)
Measured density ( g / c m 3) (He pycnometer)
Surface area (m2/g)
0 5 20 50 100 200
3 3 4 4 8 20
1.01 1.01 0.98 0.96 0.94 0.90
1.96 1.96 1.95 1.95 1.92 1.85
230 230 290 145 128 99
Shrinkage ( × 100%) Change in length Original length
(%) 48% 48% 49% 53% 55% 59%
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H. de LambH/y, L C. Klein / Lithium aluminosificate gels 500
like ink bottles (Type E) [12] with a length-to-neck ratio around two. This shape is characteristic of the shape of pores between contacting spheres. Low alcohol gels have pores with diameters less than 4 nm. Figure 5 shows the hysteresis loop for a typical high alcohol gel. The hysteresis is associated with cylindrical pores (Type A) [12] with a high lengthto-neck ratio. The pores in high alcohol gels are larger than for low alcohol gels, with radii between 3 to 7 nm.
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in fig. 3. The upper curve is for samples dried at 65°C, the lower curve is for samples dried at 210°C in a sealed jar with phosphorous pentofide. The low alcohol gels have high surface areas which increase with methanol concentration. The high alcohol gels have surface areas which are smaller and decrease with increasing methanol concentration. The pore texture is interpreted from the hysteresis in the adsorption/desorption isotherms. Figure 4 shows the hysteresis loop for a typical lowalcohol gel. The hysteresis indicates pores shaped
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H. de Lambilly, L C. Klein /Lithium aluminosilicate gels
73
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Fig. 5. Typical hysteresis curve for nitrogen sorption in high alcohol gel.
The change of surface area with heat treatment is plotted in fig. 6. The surface area decreases above 300 ° C and is reduced to 10 m2/g by 400 ° C. By this temperature most organics have been removed and the gel begins to densify by viscous sintering. The primary microporosity has been removed. Above 500°C, an increase in surface area indicates cracks. Complete densification above 600 ° C is prevented because of crystallization.
160
The weight loss vs. temperature for a heating rate of 5 ° C / m i n is plotted in fig. 7. There are four temperature regions. From room temperature to 200 ° C, weight loss is attributed to the removal of physically adsorbed species such as methanol, ethanol and water. From 300 to 400 o C, organics are oxidized and removed. The weight loss in these two regions is smaller for the low alcohol gels than for the high alcohol gels. From 400 to 600 ° C, the weight loss is due to the removal of the nitrate species. Brown fumes were observed during fast heating at these temperatures. The weight loss is more pronounced for gels 0 to 50%, with a slight
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74
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decrease with increasing alcohol concentration. Weight loss above 600°C is attributed to the removal of chlorine from the catalyst HC1. The chlorine removal is more pronounced for the high alcohol gels in which LiC1 crystals have been detected [6]. For high alcohol gels (100 and 200%), the weight loss appears continuous from 200 to 600 o C. The oxidation of the organics and the removal of nitrates are difficult to separate. No gels show the weight loss calculated for complete removal of the nitrates. One reason is that some nitrates escape from the gel with the solvent during drying. Anderson and Klein [8] found some recrystallized nitrates in the liquid expelled during syneresis. It appears that the oxidation of nitrates is controlled by the diffusion of oxygen into the gel. To determine the influence of gas diffusion, the weight loss vs. temperature was compared for a crushed sample and a monolithic sample of gel 200% (fig. 8). The first curve (solid line) shows the weight loss recorded at l ° C / m i n under oxygen flow for the crushed and sieved ( - 2 0 0 / + 325 mesh) sample. The second curve (dashed line) shows the weight loss under the same conditions for a bulk sample. At this slow heating rate the bulk sample remains monolithic. The bulk sample shows a smaller organic weight loss and a larger nitrate weight loss compared to those for the
crushed sample. The measured weight loss for the bulk sample corresponds to the calculated weight loss for the nitrates. For the powdered sample, the weight loss from loss of nitrates is less than the calculated amount. The densities as measured by helium pycnometry were not the same for monolithic and powdered samples for gel 200%. The values of the measured densities are 1.85 for a crushed sample and 2.06 g / c m 3 for a monolithic sample. They differ by about 10%. Crushing the gel presumably exposed organics and nitrates which reacted so that some species were removed before placing in the thermogravimeter for measurements. The differential thermal analyses are shown in fig. 9 for the same gels for which data are shown in fig. 7. For a heating rate of 10 o C/min, below 200 °C all gels show endotherms for the removal of adsorbed species (alcohols and water). Around 200 o C, low alcohol gels show endotherms which are attributed to the removal of unhydrolyzed or partially hydrolyzed momomer. The small exotherms around 400 o C are due to the oxidation of organics and coincide with weight losses recorded at these temperatures. Above 500 °C gels have different patterns due to different crystallization processes [6]. Low alcohol gels show a large endotherm for the removal of nitrates. The endotherm magnitude decreases as
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H. de Lambilly, L.C. Klein / Lithium aluminosilicate gels
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the alcohol concentration increases. These gels have a crystallization exotherm around 700°C. High alcohol gels show a series of crystallization exotherms due to LiC1 (550°C), the nucleation and growth of quartz (between 600 and 700 o C) [6]. These peaks overlap the endotherms for removal of nitrates and chlorine (between 500 and 650 o C). To understand the sharp exotherm at 300 ° C in high alcohol gels, a pure silica gel was first heat treated to 350 o C, then impregnated with a solution of methanol and lithium nitrate. During thermal analysis a large peak was recorded at 300 ° C, similar to that recorded in the high alcohol gel. When the experiment was run in a closed or reducing atmosphere, no peak was recorded. This is the behavior expected for oxidation of dimethyl or diethyl ether [13].
4. Discussion
The viscous flow behavior of the two series of gels differ. The viscosity is plotted versus reduced time to highlight the effect of polymer species (fig. 1). At the beginning, both sols show a slow increase in viscosity. In both cases, the initial polymer particles are small. At a later time, the viscosity starts to increase and finally shows an abrupt change of slope around the gel point.
75
The low alcohol sol (0%) shows Newtonian behavior until reduced time 0.8. This behavior follows the rheology of a dilute solution of particles. As the numbers or sizes of particles increase, so will the viscosity. Around reduced time 0.94, the viscosity values are scattered. Overall, for the low alcohol case, the polymers grow until their concentration and size reach a threshold. Then the polymers start to weakly bond to form the silica network. For the high alcohol case (200%), the sol behaves as a Newtonian fluid until reduced time 0.87 where it deviates gradually. If the gel is cycled through an increase and decrease of shear rate, the sol behaves like a pseudoplastic fluid without a yield value. This reflects the fact that once the bonds between the particles are broken, their reformation is time dependent. The diffusion coefficient obtained from light scattering measurements can be used to determine the sol-gel transition which coincides with a sudden change in viscosity. For example, in organic gels, Ng et al. [14] showed a change of the dependence of the Brillouin width with temperature at the sol-gel transition. Kobayasi et al. [15] found that the scattered light pattern changes when a sample is rotated after its gel point. This change in pattern indicates a change of symmetry after the structure has been "frozen" by the gelation process. Only a few studies have been done in inorganic gels. Using light scattering techniques, Bechtold et al. [16] found that the initial polymer functionality for gelling solutions of T E e S is about 2. This functionality affects the gelling time. KopsWerkhoven et al. [17] studied colloidal silica with photon correlation spectroscopy and found the diffusion coefficient decreases with an increased weight concentration of silica. A typical value for D was 1.1 × 10 -7 cm=/s at 0.1 g / c m 3 concentration. A comparison of the diffusion coefficients for high and low alcohol gels shows that in the 0% case, the diffusion coefficient is about one order of magnitude smaller than that of the 200% case (fig. 2). Since the viscosities differ less than error in the measurement, this difference in diffusion coefficient means a large difference of mass or size
76
H. de Lambilly, L. C. Klein / Lithium aluminosificate gels
of the polymeric species in the sols. The polymers move in the sol due to Brownian motion. The balance between the thermal energy proportional to k T / 2 in each direction and the kinetic energy determines their velocities. At a given temperature, all particles have the same energy and the product m y 2 is constant [18]. Heavy particles will not travel as far as lighter ones. The Stokes-Einstein relation can be used to relate the mass or size of the particle to the diffusion coefficient. This relation gives for each scattering particle an estimated "hydrodynamic radius", r h, the radius of a perfect sphere having the same diffusion coefficient under the same conditions: rh = kT/6(rr)nD,
where n is the viscosity of the medium and D is the diffusion coefficient. This relation is valid only for perfect spheres, but can be used as an indication of the growth stage of the polymers in the sol. The shape of the particle will introduce a deviation, which will increase as one of the dimensions of the particle becomes much larger than the other ones. The Stokes-Einstein relation is valid for dilute systems, assuming that no interaction exists between the particles. From viscosity measurements and for reduced time greater than 0.8, it is known that this is not the case here. As a result, this relation is used more to show trends in the polymerization reaction than to give precise measurements of the particle sizes during the growth stage. For the experiments done at a constant temperature, the product r h D is constant, assuming that the viscosity of the solvent does not change with time. As the diffusion coefficient decreases, the apparent radius of the particle increases and thus the mass or molecular weight of the polymer varies approximately as (D) -3. In this case, the first thing to note is the order of magnitude difference in the coefficients of diffusion between the two series of gels. Transformed into size or mass data, this means that the polymer weights differ by about three orders of magnitude. This difference is only an approximation as the polymer's shape and concentration deviate from the ideal situation. The second thing to note concerns the extent of
the reaction. The gel point is reached when the diffusion coefficient has been decreased roughly by a factor of 8. Using the relation between mass and diffusion coefficient this means that between reduced time 0.2 and the gel point, the polymers have increased about 500 times in mass. When plotted versus real time, the two series of diffusion coefficients vary inversely proportional to time. When transformed into estimated radius versus time, the rate of polymer growth is proportional to time. With time polymers start to condense, which leads to a faster increase of the polymer size. Before gelation, these larger polymers crosslink, showing an apparent doubling or tripling of their size. There is a minimum in the diffusion coefficient at approximately the time of gelation. Then, the only species detected are those small enough to move through the pores of the gel. In light scattering, the signal measured is the scattered intensity and is a function of the number of scattering centers and their scattering cross section. The low alcohol gels, composed of larger polymers than those in high alcohol gels, have higher scattering efficiency. No particles were detected early in the experiment due to the small number of particles. The small number of particles results from the small number of nuclei created by the reaction. When TEOS was added in small amounts over a longer period of time to give a longer gelling time, no signal could be detected during the entire gelation process. Smaller particles, with higher diffusion coefficients, and larger numbers could be detected in the high alcohol case despite their weak scattering. Since the solution filling the experiment cell is diluted about two times in volume by alcohol, it follows that high alcohol sols are composed of more particles per unit volume than the low alcohol ones. In the high alcohol case, the reaction occurs in a homogeneous medium and is triggered by the encounter of the reactants. After gelling, measurements are still possible. They reflect the movement of small polymers through the pores of the network. These pores are in the 5 nm range after drying. To summarize, from light scattering experiments, the low alcohol sols are composed of a few
H. de Lambilly, L. C. Klein / Lithium aluminosilicate gels
large particles which grow and agglomerate quickly. The high alcohol sols, diluted with alcohol, have more particles of smaller size which have a longer time to grow. The thermal behavior and texture of the two series of gels are different. First, there is the change in surface area with temperature (fig. 6). In low alcohol gels there is a rapid decrease of surface area between 6 5 ° C and 210°C. In high alcohol gels, the treatment in a dry oxidizing atmosphere shows a modest decrease in surface area. Thermogravimetry can be used to evaluate the completion of the polymerization reaction. Low alcohol gels lose less weight than high alcohol gels between 200 and 400 ° C indicating that they contain less unreacted monomer or organic groups. Bechtold [19] tried to relate the time to gel, tg, to the functionality, f, of the polymer and its initial concentration, CO, in the sol: tg = - 1 / C o K ( 2 f
- f2),
where K is the condensation rate. When the solution is acidic K should not depend on the alcohol concentration. The functionality of the polymer is related to the number of unreacted bonds after the gel point. The thermal weight loss should be an indication of the remaining organic groups. For low alcohol gels, CO is approximately constant and the time to gel is approximately constant as is the polymer concentration. The weight losses for these gels are all about the same which indicates a similar functionality. For high alcohol gels, both CO and the functionality are changing. Weight losses between 200 and 400 °C for these gels decrease with increasing functionality. This decrease combined with decreasing polymer concentration leads to the large increase of time to gel. Low alcohol gels have more surface area but lose less weight from organic oxidation (300 ° C). A large surface area and small weight loss mean that the residual organics are few or not reactive. It appears that the hydrolysis reaction is more complete than in the high alcohol case, although it is more difficult to initiate. High alcohol gels have larger organic weight losses. Keefer [4] observed that high water, acid
77
catalyzed gels grow preferentially in a linear fashion. In such a case, many organics will remain on the side of the chain. If alcohol increases the dilution, these polymers will grow longer before branching. This will decrease the functionality as the polymer becomes more elongated, increase the weight loss and increase the time to gel as predicted by Bechtold [19]. Low alcohol c o m p o s i t i o n s gel rapidly. Mukherjhee [20] proposed that a preordering occurs in the solution. The shorter the time the solution lasts, the less ordered will be the structure. Fast gelling times should mean less ordered structure or more glass stability. In the present case, low alcohol gels crystallize at higher temperatures. For low alcohol gels, the crystallization exotherm is at a higher temperature than the temperature of the nitrate removal endotherm. The crystalline product is lithium disilicate at 700 ° C [6]. High alcohol gels crystallized 150 ° C below low alcohol gels. In high alcohol gels, lithium reacted with chlorine. In the high alcohol gels lithium is not uniformly distributed. Both high alcohol gels and lithium infiltrated silica gels showed similar behavior. This similarity suggests that lithium in the high alcohol gels is in the same sites as in infiltrated gels. For infiltrated gels, at room temperature, most of the lithium is on the internal surface and not in the silica skeleton itself. As a result, lithium disilicate crystals do not form. One problem of converting lithium aluminosilicate gels to glass is due to a low glass transition temperature. Loss of surface area occurs between 400 and 500 o C, before all volatiles such as nitrates and chlorine are removed. This early densification is a result of small pores with diameter less than 7 nm, and the decrease of viscosity by lithium. Generally, densification is not completed before the onset of crystallization.
5. Summary In summary, the region of the miscibility diagram for w a t e r - T E O S - m e t h a n o l is an important factor in processing acid catalyzed gels. Those gels processed in the miscible region (high alcohol)
78
H. de Lambilly, L.C Klein / Lithium aluminosilicate gels
react i n a h o m o g e n e o u s s o l u t i o n . T h e r e a c t i o n o c c u r s u n i f o r m l y t h r o u g h o u t the s o l u t i o n . T h o s e gels p r o c e s s e d i n the i m m i s c i b l e r e g i o n (low alcohol) react i n a h e t e r o g e n e o u s m e d i u m . R e a c tions occur locally p r o d u c i n g a few largely c o n d e n s e d p o l y m e r s . T h e s e differences i n b e h a v i o r are seen in viscosity m e a s u r e m e n t s , light s c a t t e r i n g a n d time-to-gel. L o w a l c o h o l l i t h i u m silicate gels s y n t h e s i z e d f r o m i m m i s c i b l e c o m p o s i t i o n s q u i c k l y gel a n d are b e t t e r glass formers. H i g h alcohol gels s y n t h e s i z e d f r o m m i s c i b l e c o m p o s i t i o n s gel slowly. T h e l o n g gelling t i m e allows s o m e p r e o r d e r i n g to occur. U p o n heat t r e a t m e n t this leads to c r y s t a l l i z a t i o n at a lower t e m p e r a t u r e i n high a l c o h o l gels t h a n in low alcohol gels. F i n a n c i a l s u p p o r t f r o m the C e n t e r for C e r a m i c s Research, a University-Industry Cooperative C e n t e r , N e w Jersey C o m m i s s i o n o n Science a n d T e c h n o l o g y , is a p p r e c i a t e d .
References [1] L.C. Klein, Ann. Rev. Mater. Sci. 15 (1985) 227. [2] S. Sakka, K. Kamiya, K. Makita and Y. Yamamoto, J. Non-Cryst. Solids 63 (1984) 223. [3] K.C. Chen, T. Tsuchiya and J.D. Mackenzie, J. Non-Cryst. Solids 81 (1986) 227.
[4] K.D. Keefer, Better Ceramics through Chemistry, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (Elsevier, New York, 1984) p. 15. [5] C.J. Brinker et al., J. Non-Cryst. Solids 48 (1982) 47. [6] H. de Lambilly and L.C. Klein, J. Non-Cryst. Solids 102 (1988) 269. [7] I. Schwartz, P.A. Anderson, H. de Lambilly and L.C. Klein, J. Non-Cryst. Solids 83 (1986) 391. [81 P. Anderson and L.C. Klein, J. Non-Cryst. Solids 93 (1987) 415. [9] S. Wallace and L.L. Hench, Better Ceramics through Chemistry, eds. C.J. Brinker, D.R. Ulrich and D.E. Clark (Elsevier-North Holland, New York, 1984) p. 47. [10] S. Wallace and L.L. Hench, Cer. Eng. Sci. Proc. 5 (1984) 568. [11] L.E. Kelts, N.J. Effinger, and S.M. Melpolder, J. NonCryst. Solids 83 (1986) 353. [12] A.J. Lecloux, in: Catalysis - Science and Technology, Vol. 2, Eds. J.R. Anderson and M. Boudant (Springer, New York, 1981) p. 171. [13] B.E. Langner, Appl. Catal. 2 (1982) 289. [14] S.G. Ng, T.J.C. Hosea, H.C. Teh and L.M. Gan, J. Phys. E. Sci. Instr. 18 (1985) 250. [15] S. Kobayasi, Rev. Sci. Instr. 56 (1) (1985) 160. [16] M.F. Bechtold, R.D. Vest and L. Plambeck, J. Amer. Chem. Soc. (1968) 4590. [17] M.M. Kops-Werkhoven and H.M. Fijnaut, J. Chem. Phys. 74 (3) (1981) 1618. [18] N. Ostrowsky, D. Sornette, P. Parker and E.R. Pike, Optica Acta 28 (1981) 1039. [19] M.F. Bechtold, J. Phys. Chem. 59 (1959) 532. [20] S.P. Mukherjee, Better Ceramics through Chemistry II, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (Elsevier, New York, 1986) p. 443.
69
E F F E C T O F M E T H A N O L C O N C E N T R A T I O N ON L I T H I U M A L U M I N O S I L I C A T E GELS H. de LAMBILLY and L.C. K L E I N Rutgers- The State University of New Jersey, Ceramics Department, PO Box 909, Piscataway, NJ 08855- 0909, USA Received 10 February 1988 Revised manuscript received 27 December 1988
The viscosity of the solution, the diffusion coefficient and the time-to-gel depend on the preparation method used in lithium aluminosilicate gels. The crystallization temperature during heat treatment depends on the preparation method as well. Samples were prepared by adding anhydrous tetraethylorthosilicate (TEOS) to a solution of methanol, water and the nitrates of lithium and aluminum. The amount of methanol was varied to create miscible and immiscible compositions. High methanol compositions took a long time to gel and lost fluidity abruptly. Low methanol compositions took a short time to gel and lost fluidity gradually. Nitrogen sorption techniques were used to determine surface areas in dried and heat treated gels. Low methanol compositions have higher surface areas than high methanol compositions. Thermal analysis was used to identify reaction temperatures. High and low methanol compositions show similar behavior below 4 0 0 ° C but high methanol compositions crystallize beginning around 550 o C while low methanol compositions crystallize above 700 ° C.
1. Introduction
The goal of this study is to determine the effect of the methanol concentration on the texture and thermal stability of lithium aluminosilicate gels. Previous studies of sol-gel processing have emphasized the molar ratio of water to silica precursor [1,2], the nature of the silica precursor [3] and the nature of the catalyst, acid vs. base [4,5]. Also, it was pointed out [6] that there is an effect of solvent concentration in sol-gel processing. Originally, Schwartz et al. [7] added alkali salts to tetraethylorthosilicate (TEOS) solutions to determine the stability of binary silicate gels. They found that lithium was the simplest alkali to incorporate into gels. The method for producing the gels was through acid catalysis of TEOS. No surface crystallization was found on the lithium compositions. The dried lithium gels were analyzed by atomic absorption for chemical homogeneity. The lithium content was uniform throughout the samples. Based on the work by Schwartz et al. [7], it is known that stable lithium silicate gel monoliths can be obtained through low-temperature natural evaporation. Success in obtaining monolithic gels 0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
depends on the initial amount of water in the alkoxide solutions, according to Anderson and Klein [8]. A proper amount of water ensures that the crosslinking is sufficient to increase the strength of the gel so that the gel can withstand capillary forces during drying. Separately, lithium silicate gels have been prepared by Wallace and Hench [9,10]. One of their objectives was to produce gels in the shortest period of time by optimizing the gelling process. To keep the gel from breaking during drying, a drying control chemical additive (DCCA) was added to the solution. They used several organic and inorganic precursors for lithium. The gels containing lithium-i-propoxide and lithium methoxide remained monolithic. Lithium nitrate had the best drying characteristics among the inorganic precursors. The optimized process called for a low pH and about a stoichiometric mole ratio of water to silicon alkoxide. Based on these results, acid-catalyzed TEOS solutions with stoichiometric or greater water ratios have been selected for further study. Lithium nitrate is from several accounts the preferred precursor. In order to distribute the lithium in the gel at the time of formulation, the amount of solvent
70
H. de Lambilly, L. C. Klein / Lithium aluminosilicate gels
is controlled to bring about optimum gelling. In the studies on the same compositions, regarding crystallization behavior [6], there is a direct effect of methanol concentration on the distribution of the lithium.
2. Experimental procedure The composition chosen was 83% SiO z - 15% Li20 - 2% A1203 (all in mol%). This choice was the result of work by Schwartz et al. [7] who found that, among the three alkali (lithium, sodium and potassium), lithium was easiest to stabilize inside the gel. Some A1203 helped stabilize the gel as well. The w a t e r / T E O S molar ratio was 8/1 (64.2 ml water for 100 ml TEOS). TEOS was preferred to TMOS (tetramethyl orthosilicate) because of its reduced toxicity and its slower hydrolysis rate. Lithium nitrate and aluminum nitrate were the lithium and aluminum precursors. Six methanol concentrations were used. They are given in vol%. They were 0, 5, 20, 50, 100 and 200% of the volume of TEOS. Solutions with methanol were more stable than with other alcohols. Methanol is not a product of the reaction and may not affect the kinetics of the reactions the way ethanol would. One of the consequences of this choice is the possibility of exchange between TEOS and methanol. It is know to occur but to a lesser extent than between TMOS and ethanol [11]. The solutions were prepared by mixing first the methanol, water, catalyst and salts at 90 ° C for 1 h. The reason for this premixing was to completely dissolve the salts. This improved the stability of the lithium by attaching it to a larger molecule. It is known that lithium, compared to sodium, has a relatively large hydrated radius. The pH of the solution was lowered by addition of hydrochloric acid. Since the function of the acid is to catalyze the reactions, a constant pH of about 0.2 was used for all alcohol dilutions. The solution was then cooled and TEOS, distilled to remove alcohol and impurities, was added. Adding TEOS to the solution did not change the pH. The solutions with a high methanol concentration (greater than 50%) dissolved the volume
of TEOS without turning cloudy. These solutions were homogeneous. With a small volume (0, 5 and 20%) of methanol the solution turned cloudy. After a few hours, they turned clear again. When the alkoxide was added gradually, the solution did not turn completely cloudy. The solutions were then mixed for one half hour. After that, samples were cast into 10 ml polystyrene tubes (cylinder shape) and petri dishes (disk shapes) and the molds were loosely sealed. Half the samples were kept at room temperature while the other half were allowed to gel and dry in an oven at 6 5 ° C for about 1 month. The gelling time was recorded. Gelation was taken as the state where the viscosity was so high that no flow was apparent. Solution densities were measured by weighing a volume of solution in a pipette. Solid densities were measured on powders by helium pycnometry (Micromeritics TM) after a 2 0 0 ° C treatment to remove adsorbed water and volatile species. Viscosity measurements were performed during gelation. A batch was placed in 10 ml glass vials. Samples were then poured into the chamber of the Haake TM rotational viscometer. Viscosities were recorded at 25°C. The shear rate was continuously varied from 0 to 300 s -1. The shear stress was measured on the cylinder with increasing and decreasing shear rate. Diffusion coefficients were calculated on samples from the same batch. A photon correlation spectrometer (Malvern Instruments Model No. 4700C) was used. All measurements were made at 25 o C. Scattered light was measured at 90 ° C from the incident beam. For the 0% case, the medium was assumed to be a mixture of water, TEOS and ethanol and values taken for viscosity and refractive index were 1 m P a - s and 1.33. In the 200% case, 0.758 mPa- s and 1.33 were the values for a mixture of water, methanol and ethanol. As recommended by the manufacturer, the reported values are an average of at least three measurements. Each run was at least 10 counts lasting 10 to 20 s each. Experiment time and measurement settings (apertures) were varied according to the instructions of the manufacturer to increase the signal. The correlator was used to minimize the noise and increase the accuracy of the measurement.
H. de Lambilly, L.C. Klein / Lithium aluminosilicate gels
In dried, powdered samples, the texture was analyzed using nitrogen sorption techniques in an Omnisorb 360 unit from Omicron Technology. Outgassing was done under vacuum at 200°C. For this type of instrument relatively large samples are used since the ASTM standard recommends a measurement on a surface of at least 10 m 2.
The Omnisorp 360 unit allows continuous adsorption/desorption isotherms to be recorded by measuring the pressure change in the measurement cell under a continuous flow of dry nitrogen. Surface areas, pore size distributions and pore shapes can be extracted from the data. The samples were characterized during heat treatment using a DuPont 1090B thermal analyzer. Thermogravimetric analysis was performed on powdered ( - 2 0 0 / + 325 mesh) samples at a heating rate of 5 o C / m i n . For monolithic samples, the heating rate was 1 ° C / m i n . The slower rate prevented disruption of the sample. The experiment was done under a continuous flow of dry oxygen (50 ml/min). On identical samples, differential thermal analysis was performed using an alumina standard with a heating rate of 1 0 ° C / m i n under oxygen flow.
3. Results
Overall, the samples can be divided into two categories: "low alcohol gels" (0, 5 and 20% alcohol content) and "high alcohol gels" (100 and 200% alcohol content). Gel 50%, which was processed close to the boundary between miscible
71
and immiscible solutions, shows transitional behavior. Low alcohol gels show short gelation times; high alcohol gels have long gelation times which increase with increasing alcohol concentration (table 1). Gelling at a higher temperature speeds the process and lessens the difference between high and low alcohol gels. The viscosity versus reduced time is plotted in fig. 1. Two gels, 0 and 200%, are presented to illustrate the difference between the low and high alcohol gels. Reduced time is the actual time divided by the time to gel. For both samples, the viscosity increases with time, slowly at the beginning until the sharp increase at the sol-gel transition. The change of slope occurs earlier for the 0% gel, around reduced time 0.8. The diffusion coefficients calculated from light scattering data measured during gelation are plotted versus reduced time in fig. 2. The diffusion coefficient decreases at the beginning, and is attributed to an increase of polymer size. For the low alcohol case, the diffusion coefficients are about one order of magnitude smaller than the high alcohol case. A smaller diffusion coefficient indicates larger polymers or more interaction between polymeric entities. Light scattering after the sol has gelled is attributed to the movement of free polymers inside the polymeric network. These measurements are not as reliable as those made before gelling, because some drying occurs during the measurement. Surface areas were obtained from the nitrogen sorption experiments. The pore size distributions and pore volumes were determined as well. The surface area is plotted vs. methanol concentration
Table 1 Characteristics of the different alcohol concentration gels before heat treatment Alcohol content
Gelling time (days)
Sol density ( g / c m 3)
Measured density ( g / c m 3) (He pycnometer)
Surface area (m2/g)
0 5 20 50 100 200
3 3 4 4 8 20
1.01 1.01 0.98 0.96 0.94 0.90
1.96 1.96 1.95 1.95 1.92 1.85
230 230 290 145 128 99
Shrinkage ( × 100%) Change in length Original length
(%) 48% 48% 49% 53% 55% 59%
72
H. de LambH/y, L C. Klein / Lithium aluminosificate gels 500
like ink bottles (Type E) [12] with a length-to-neck ratio around two. This shape is characteristic of the shape of pores between contacting spheres. Low alcohol gels have pores with diameters less than 4 nm. Figure 5 shows the hysteresis loop for a typical high alcohol gel. The hysteresis is associated with cylindrical pores (Type A) [12] with a high lengthto-neck ratio. The pores in high alcohol gels are larger than for low alcohol gels, with radii between 3 to 7 nm.
400
300 (L
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High Alcohol
t / tgel Fig. 1. Viscosity vs. reduced time (time/time-to-gel) at shear rate 150 s"] for low alcohol gel (top) and high alcohol gel
(bottom).
in fig. 3. The upper curve is for samples dried at 65°C, the lower curve is for samples dried at 210°C in a sealed jar with phosphorous pentofide. The low alcohol gels have high surface areas which increase with methanol concentration. The high alcohol gels have surface areas which are smaller and decrease with increasing methanol concentration. The pore texture is interpreted from the hysteresis in the adsorption/desorption isotherms. Figure 4 shows the hysteresis loop for a typical lowalcohol gel. The hysteresis indicates pores shaped
% x v tD
~ 0 0.0
012
i 0.4
,
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,
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Fig. 2. Diffusion coefficient calculated from dynamic light scattering vs. reduced time for low alcohol gel (top) and high alcohol gel (bottom). Note difference in order of magnitude.
H. de Lambilly, L C. Klein /Lithium aluminosilicate gels
73
160
280i
High Alcohol
"240 {'l'l h
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Fig. 5. Typical hysteresis curve for nitrogen sorption in high alcohol gel.
The change of surface area with heat treatment is plotted in fig. 6. The surface area decreases above 300 ° C and is reduced to 10 m2/g by 400 ° C. By this temperature most organics have been removed and the gel begins to densify by viscous sintering. The primary microporosity has been removed. Above 500°C, an increase in surface area indicates cracks. Complete densification above 600 ° C is prevented because of crystallization.
160
The weight loss vs. temperature for a heating rate of 5 ° C / m i n is plotted in fig. 7. There are four temperature regions. From room temperature to 200 ° C, weight loss is attributed to the removal of physically adsorbed species such as methanol, ethanol and water. From 300 to 400 o C, organics are oxidized and removed. The weight loss in these two regions is smaller for the low alcohol gels than for the high alcohol gels. From 400 to 600 ° C, the weight loss is due to the removal of the nitrate species. Brown fumes were observed during fast heating at these temperatures. The weight loss is more pronounced for gels 0 to 50%, with a slight
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Fig. 6. Change of surface area with temperature for gel 50%.
74
H. de Lambilly, L.C. Klein / Lithium aluminosilicate gels 100
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85
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,
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Fig. 7. Weight losses vs. temperature for three representative gels.
decrease with increasing alcohol concentration. Weight loss above 600°C is attributed to the removal of chlorine from the catalyst HC1. The chlorine removal is more pronounced for the high alcohol gels in which LiC1 crystals have been detected [6]. For high alcohol gels (100 and 200%), the weight loss appears continuous from 200 to 600 o C. The oxidation of the organics and the removal of nitrates are difficult to separate. No gels show the weight loss calculated for complete removal of the nitrates. One reason is that some nitrates escape from the gel with the solvent during drying. Anderson and Klein [8] found some recrystallized nitrates in the liquid expelled during syneresis. It appears that the oxidation of nitrates is controlled by the diffusion of oxygen into the gel. To determine the influence of gas diffusion, the weight loss vs. temperature was compared for a crushed sample and a monolithic sample of gel 200% (fig. 8). The first curve (solid line) shows the weight loss recorded at l ° C / m i n under oxygen flow for the crushed and sieved ( - 2 0 0 / + 325 mesh) sample. The second curve (dashed line) shows the weight loss under the same conditions for a bulk sample. At this slow heating rate the bulk sample remains monolithic. The bulk sample shows a smaller organic weight loss and a larger nitrate weight loss compared to those for the
crushed sample. The measured weight loss for the bulk sample corresponds to the calculated weight loss for the nitrates. For the powdered sample, the weight loss from loss of nitrates is less than the calculated amount. The densities as measured by helium pycnometry were not the same for monolithic and powdered samples for gel 200%. The values of the measured densities are 1.85 for a crushed sample and 2.06 g / c m 3 for a monolithic sample. They differ by about 10%. Crushing the gel presumably exposed organics and nitrates which reacted so that some species were removed before placing in the thermogravimeter for measurements. The differential thermal analyses are shown in fig. 9 for the same gels for which data are shown in fig. 7. For a heating rate of 10 o C/min, below 200 °C all gels show endotherms for the removal of adsorbed species (alcohols and water). Around 200 o C, low alcohol gels show endotherms which are attributed to the removal of unhydrolyzed or partially hydrolyzed momomer. The small exotherms around 400 o C are due to the oxidation of organics and coincide with weight losses recorded at these temperatures. Above 500 °C gels have different patterns due to different crystallization processes [6]. Low alcohol gels show a large endotherm for the removal of nitrates. The endotherm magnitude decreases as
100
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Temperature (C) Fig. 8. Weight losses vs. temperature for a monolithic and powdered sample of gel 200%.
H. de Lambilly, L.C. Klein / Lithium aluminosilicate gels
i t
Exo
~,
" "~ ":i.
\
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i
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!
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400
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600
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800
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Fig. 9. R e a c t i o n t e m p e r a t u r e s for three representative gels.
the alcohol concentration increases. These gels have a crystallization exotherm around 700°C. High alcohol gels show a series of crystallization exotherms due to LiC1 (550°C), the nucleation and growth of quartz (between 600 and 700 o C) [6]. These peaks overlap the endotherms for removal of nitrates and chlorine (between 500 and 650 o C). To understand the sharp exotherm at 300 ° C in high alcohol gels, a pure silica gel was first heat treated to 350 o C, then impregnated with a solution of methanol and lithium nitrate. During thermal analysis a large peak was recorded at 300 ° C, similar to that recorded in the high alcohol gel. When the experiment was run in a closed or reducing atmosphere, no peak was recorded. This is the behavior expected for oxidation of dimethyl or diethyl ether [13].
4. Discussion
The viscous flow behavior of the two series of gels differ. The viscosity is plotted versus reduced time to highlight the effect of polymer species (fig. 1). At the beginning, both sols show a slow increase in viscosity. In both cases, the initial polymer particles are small. At a later time, the viscosity starts to increase and finally shows an abrupt change of slope around the gel point.
75
The low alcohol sol (0%) shows Newtonian behavior until reduced time 0.8. This behavior follows the rheology of a dilute solution of particles. As the numbers or sizes of particles increase, so will the viscosity. Around reduced time 0.94, the viscosity values are scattered. Overall, for the low alcohol case, the polymers grow until their concentration and size reach a threshold. Then the polymers start to weakly bond to form the silica network. For the high alcohol case (200%), the sol behaves as a Newtonian fluid until reduced time 0.87 where it deviates gradually. If the gel is cycled through an increase and decrease of shear rate, the sol behaves like a pseudoplastic fluid without a yield value. This reflects the fact that once the bonds between the particles are broken, their reformation is time dependent. The diffusion coefficient obtained from light scattering measurements can be used to determine the sol-gel transition which coincides with a sudden change in viscosity. For example, in organic gels, Ng et al. [14] showed a change of the dependence of the Brillouin width with temperature at the sol-gel transition. Kobayasi et al. [15] found that the scattered light pattern changes when a sample is rotated after its gel point. This change in pattern indicates a change of symmetry after the structure has been "frozen" by the gelation process. Only a few studies have been done in inorganic gels. Using light scattering techniques, Bechtold et al. [16] found that the initial polymer functionality for gelling solutions of T E e S is about 2. This functionality affects the gelling time. KopsWerkhoven et al. [17] studied colloidal silica with photon correlation spectroscopy and found the diffusion coefficient decreases with an increased weight concentration of silica. A typical value for D was 1.1 × 10 -7 cm=/s at 0.1 g / c m 3 concentration. A comparison of the diffusion coefficients for high and low alcohol gels shows that in the 0% case, the diffusion coefficient is about one order of magnitude smaller than that of the 200% case (fig. 2). Since the viscosities differ less than error in the measurement, this difference in diffusion coefficient means a large difference of mass or size
76
H. de Lambilly, L. C. Klein / Lithium aluminosificate gels
of the polymeric species in the sols. The polymers move in the sol due to Brownian motion. The balance between the thermal energy proportional to k T / 2 in each direction and the kinetic energy determines their velocities. At a given temperature, all particles have the same energy and the product m y 2 is constant [18]. Heavy particles will not travel as far as lighter ones. The Stokes-Einstein relation can be used to relate the mass or size of the particle to the diffusion coefficient. This relation gives for each scattering particle an estimated "hydrodynamic radius", r h, the radius of a perfect sphere having the same diffusion coefficient under the same conditions: rh = kT/6(rr)nD,
where n is the viscosity of the medium and D is the diffusion coefficient. This relation is valid only for perfect spheres, but can be used as an indication of the growth stage of the polymers in the sol. The shape of the particle will introduce a deviation, which will increase as one of the dimensions of the particle becomes much larger than the other ones. The Stokes-Einstein relation is valid for dilute systems, assuming that no interaction exists between the particles. From viscosity measurements and for reduced time greater than 0.8, it is known that this is not the case here. As a result, this relation is used more to show trends in the polymerization reaction than to give precise measurements of the particle sizes during the growth stage. For the experiments done at a constant temperature, the product r h D is constant, assuming that the viscosity of the solvent does not change with time. As the diffusion coefficient decreases, the apparent radius of the particle increases and thus the mass or molecular weight of the polymer varies approximately as (D) -3. In this case, the first thing to note is the order of magnitude difference in the coefficients of diffusion between the two series of gels. Transformed into size or mass data, this means that the polymer weights differ by about three orders of magnitude. This difference is only an approximation as the polymer's shape and concentration deviate from the ideal situation. The second thing to note concerns the extent of
the reaction. The gel point is reached when the diffusion coefficient has been decreased roughly by a factor of 8. Using the relation between mass and diffusion coefficient this means that between reduced time 0.2 and the gel point, the polymers have increased about 500 times in mass. When plotted versus real time, the two series of diffusion coefficients vary inversely proportional to time. When transformed into estimated radius versus time, the rate of polymer growth is proportional to time. With time polymers start to condense, which leads to a faster increase of the polymer size. Before gelation, these larger polymers crosslink, showing an apparent doubling or tripling of their size. There is a minimum in the diffusion coefficient at approximately the time of gelation. Then, the only species detected are those small enough to move through the pores of the gel. In light scattering, the signal measured is the scattered intensity and is a function of the number of scattering centers and their scattering cross section. The low alcohol gels, composed of larger polymers than those in high alcohol gels, have higher scattering efficiency. No particles were detected early in the experiment due to the small number of particles. The small number of particles results from the small number of nuclei created by the reaction. When TEOS was added in small amounts over a longer period of time to give a longer gelling time, no signal could be detected during the entire gelation process. Smaller particles, with higher diffusion coefficients, and larger numbers could be detected in the high alcohol case despite their weak scattering. Since the solution filling the experiment cell is diluted about two times in volume by alcohol, it follows that high alcohol sols are composed of more particles per unit volume than the low alcohol ones. In the high alcohol case, the reaction occurs in a homogeneous medium and is triggered by the encounter of the reactants. After gelling, measurements are still possible. They reflect the movement of small polymers through the pores of the network. These pores are in the 5 nm range after drying. To summarize, from light scattering experiments, the low alcohol sols are composed of a few
H. de Lambilly, L. C. Klein / Lithium aluminosilicate gels
large particles which grow and agglomerate quickly. The high alcohol sols, diluted with alcohol, have more particles of smaller size which have a longer time to grow. The thermal behavior and texture of the two series of gels are different. First, there is the change in surface area with temperature (fig. 6). In low alcohol gels there is a rapid decrease of surface area between 6 5 ° C and 210°C. In high alcohol gels, the treatment in a dry oxidizing atmosphere shows a modest decrease in surface area. Thermogravimetry can be used to evaluate the completion of the polymerization reaction. Low alcohol gels lose less weight than high alcohol gels between 200 and 400 ° C indicating that they contain less unreacted monomer or organic groups. Bechtold [19] tried to relate the time to gel, tg, to the functionality, f, of the polymer and its initial concentration, CO, in the sol: tg = - 1 / C o K ( 2 f
- f2),
where K is the condensation rate. When the solution is acidic K should not depend on the alcohol concentration. The functionality of the polymer is related to the number of unreacted bonds after the gel point. The thermal weight loss should be an indication of the remaining organic groups. For low alcohol gels, CO is approximately constant and the time to gel is approximately constant as is the polymer concentration. The weight losses for these gels are all about the same which indicates a similar functionality. For high alcohol gels, both CO and the functionality are changing. Weight losses between 200 and 400 °C for these gels decrease with increasing functionality. This decrease combined with decreasing polymer concentration leads to the large increase of time to gel. Low alcohol gels have more surface area but lose less weight from organic oxidation (300 ° C). A large surface area and small weight loss mean that the residual organics are few or not reactive. It appears that the hydrolysis reaction is more complete than in the high alcohol case, although it is more difficult to initiate. High alcohol gels have larger organic weight losses. Keefer [4] observed that high water, acid
77
catalyzed gels grow preferentially in a linear fashion. In such a case, many organics will remain on the side of the chain. If alcohol increases the dilution, these polymers will grow longer before branching. This will decrease the functionality as the polymer becomes more elongated, increase the weight loss and increase the time to gel as predicted by Bechtold [19]. Low alcohol c o m p o s i t i o n s gel rapidly. Mukherjhee [20] proposed that a preordering occurs in the solution. The shorter the time the solution lasts, the less ordered will be the structure. Fast gelling times should mean less ordered structure or more glass stability. In the present case, low alcohol gels crystallize at higher temperatures. For low alcohol gels, the crystallization exotherm is at a higher temperature than the temperature of the nitrate removal endotherm. The crystalline product is lithium disilicate at 700 ° C [6]. High alcohol gels crystallized 150 ° C below low alcohol gels. In high alcohol gels, lithium reacted with chlorine. In the high alcohol gels lithium is not uniformly distributed. Both high alcohol gels and lithium infiltrated silica gels showed similar behavior. This similarity suggests that lithium in the high alcohol gels is in the same sites as in infiltrated gels. For infiltrated gels, at room temperature, most of the lithium is on the internal surface and not in the silica skeleton itself. As a result, lithium disilicate crystals do not form. One problem of converting lithium aluminosilicate gels to glass is due to a low glass transition temperature. Loss of surface area occurs between 400 and 500 o C, before all volatiles such as nitrates and chlorine are removed. This early densification is a result of small pores with diameter less than 7 nm, and the decrease of viscosity by lithium. Generally, densification is not completed before the onset of crystallization.
5. Summary In summary, the region of the miscibility diagram for w a t e r - T E O S - m e t h a n o l is an important factor in processing acid catalyzed gels. Those gels processed in the miscible region (high alcohol)
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H. de Lambilly, L.C Klein / Lithium aluminosilicate gels
react i n a h o m o g e n e o u s s o l u t i o n . T h e r e a c t i o n o c c u r s u n i f o r m l y t h r o u g h o u t the s o l u t i o n . T h o s e gels p r o c e s s e d i n the i m m i s c i b l e r e g i o n (low alcohol) react i n a h e t e r o g e n e o u s m e d i u m . R e a c tions occur locally p r o d u c i n g a few largely c o n d e n s e d p o l y m e r s . T h e s e differences i n b e h a v i o r are seen in viscosity m e a s u r e m e n t s , light s c a t t e r i n g a n d time-to-gel. L o w a l c o h o l l i t h i u m silicate gels s y n t h e s i z e d f r o m i m m i s c i b l e c o m p o s i t i o n s q u i c k l y gel a n d are b e t t e r glass formers. H i g h alcohol gels s y n t h e s i z e d f r o m m i s c i b l e c o m p o s i t i o n s gel slowly. T h e l o n g gelling t i m e allows s o m e p r e o r d e r i n g to occur. U p o n heat t r e a t m e n t this leads to c r y s t a l l i z a t i o n at a lower t e m p e r a t u r e i n high a l c o h o l gels t h a n in low alcohol gels. F i n a n c i a l s u p p o r t f r o m the C e n t e r for C e r a m i c s Research, a University-Industry Cooperative C e n t e r , N e w Jersey C o m m i s s i o n o n Science a n d T e c h n o l o g y , is a p p r e c i a t e d .
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[4] K.D. Keefer, Better Ceramics through Chemistry, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (Elsevier, New York, 1984) p. 15. [5] C.J. Brinker et al., J. Non-Cryst. Solids 48 (1982) 47. [6] H. de Lambilly and L.C. Klein, J. Non-Cryst. Solids 102 (1988) 269. [7] I. Schwartz, P.A. Anderson, H. de Lambilly and L.C. Klein, J. Non-Cryst. Solids 83 (1986) 391. [81 P. Anderson and L.C. Klein, J. Non-Cryst. Solids 93 (1987) 415. [9] S. Wallace and L.L. Hench, Better Ceramics through Chemistry, eds. C.J. Brinker, D.R. Ulrich and D.E. Clark (Elsevier-North Holland, New York, 1984) p. 47. [10] S. Wallace and L.L. Hench, Cer. Eng. Sci. Proc. 5 (1984) 568. [11] L.E. Kelts, N.J. Effinger, and S.M. Melpolder, J. NonCryst. Solids 83 (1986) 353. [12] A.J. Lecloux, in: Catalysis - Science and Technology, Vol. 2, Eds. J.R. Anderson and M. Boudant (Springer, New York, 1981) p. 171. [13] B.E. Langner, Appl. Catal. 2 (1982) 289. [14] S.G. Ng, T.J.C. Hosea, H.C. Teh and L.M. Gan, J. Phys. E. Sci. Instr. 18 (1985) 250. [15] S. Kobayasi, Rev. Sci. Instr. 56 (1) (1985) 160. [16] M.F. Bechtold, R.D. Vest and L. Plambeck, J. Amer. Chem. Soc. (1968) 4590. [17] M.M. Kops-Werkhoven and H.M. Fijnaut, J. Chem. Phys. 74 (3) (1981) 1618. [18] N. Ostrowsky, D. Sornette, P. Parker and E.R. Pike, Optica Acta 28 (1981) 1039. [19] M.F. Bechtold, J. Phys. Chem. 59 (1959) 532. [20] S.P. Mukherjee, Better Ceramics through Chemistry II, eds. C.J. Brinker, D.E. Clark and D.R. Ulrich (Elsevier, New York, 1986) p. 443.