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Effect of hydrolysis on the colloidal stability of fine alumina suspensions
MATEIUALS SarlNE & ENGINEERING ELSEVIER
Materials Science and Engineering A204 (1995) 169 175
A
Effect of hydrolysis on the colloidal stability of fine alumina suspensions J, L i u , L . Q W a n g , B.C. Bunker, G.L. Graft, J.W. Virden, R.H. J o n e s D~7~arlm~'u: ~{/ Materials A'ciem'~,. Pacilic Northwest l.ahoratoJT, Battelle Boule~:ard. Box 999, Rich/and. WA 993.52. USA
Abstract This paper investigates the effect of hydrolysis and the formation of hydrated polycations on the colloidal stability and rheological properties of fine alumina suspensions. This is an important phenomenon in colloidal processing of advanced ceramic materials and nanocomp~site materials from fine particles. The aging process and the formation of large polycations in the solution are monitored by nuclear magnetic resonance, and the aggregation rate and viscosity of the suspension are measured under similar conditions. It is observed that aging under acidic conditions increases colloidal stability against aggregation in dispersed suspensions, and reduces the viscosity in flocculated suspensions. This behavior corresponds well to the formation of large polynuclear species through hydrolysis. It is suggested that the hydrated polycations significantly modify the total interaction energy between two particles at short separation distances.
Kcywords: Hydrolysis; Colloidal stability; Fine alumina suspensions
1. Introduction Colloidal processing methods have been widely used to prepare advanced ceramic materials and nanocomposite materials [1-5]. The packing density and microstructure of the green body are mainly determined by the interactions between the colloidal particles and the conditions under which the suspension is prepared and consolidated. Three mechanisms to control colloidal stability are well documented [6]: electrostatic repulsion due to charge development on particle surfaces, van der Waals attraction through dipole-dipole interactions, and steric interaction from polymers adsorbed onto the particles. These interaction energies can be estimated using a DLVO (Derjaguin-Landau-Verwey Overbeek) theory [6]. Strong electrostatic or steric repulsion will produce a well-dispersed suspension and high packing density. On the other hand, a flocculated suspension in the absence of long-range repulsion will result in poor consolidation and low packing density. The stability, rheology, and consolidation of some common oxide materials, such as alumina, have been Elsevier Science S ,\. ,~SDI 0921-5093195)09955-7
extensively studied in the literature [7--11]. However, many ceramic suspensions, especially those made of fine particles, exhibit time-dependent colloidal properties through aging and hydrolysis [12,13]. The effect of such phenomena on the colloidal stability has not been well studied. For example, metal oxides can undergo extensive hydrolysis in an aqueous solution and produce a variety of solution species, depending on pH, temperature, electrolyte concentrations, mechanical stirring, and reaction time [14]. It has been suggested that the hydrolyzed polynuclear species may have an effect on the colloidal stability of peptized beohmite [15]. Petroff and Sayer have purposely introduced zirconium hydrogel to zirconia, alumina, and titania suspensions to modify the surface chemistry and enhance colloidal stability [16]. There are several recent studies related to "non-DLVO" forces in ceramic suspensions [911,17,18]. Because these non-DLVO forces are generally attributed to the short-range hydration forces [19--22], a better understanding of the influence of surface hydrolysis is important. The purpose of this study is to examine the hydrolysis process of suspensions of fine alumina particles, and
170
J. Liu et al. / Materials Science and Engineering A204 (1995) 169-175
its correlation with colloidal stability and rheological properties. The solution species is studied by magnetic resonance, and the aggregation rate and viscosity of the suspension are monitored at the same time. It is concluded that the hydrolysis process greatly affects the colloidal properties of fine particles: the increased colloidal stability and reduced viscosity can be directly related to the dissolution and formation of large polynuclear species through aging under acidic conditions. These hydrated polycations can significantly modify the total interaction energy between two particles at short separation distances.
2. Experimental procedure Aqueous suspensions of unagglomerated a-alumina powder (AKP 53, 0.2/~m median particle size obtained from Sumitomo Chemical Co., New York) were studied. Nuclear magnetic resonance (NMR) was used to study the hydrolyzed polycation species. The aggregation rates, viscosity, and electrokinetic mobility were measured under controlled experimental conditions. 2.1. Nuclear magnetic resonance spectroscopy
27A1 solid-state NMR was used to identify chemical species in the colloidal aging processes through chemical shifts and line shape of NMR signals. No effort was made to measure the absolute concentration of the soluble species in this study. 27A1 NMR experiments were carried out with a Chemagnetics spectrometer (300 MHz, 89 mm wide bore Oxford magnet) using a CP-MAS probe. Suspensions of 15 vol.% A1203 were prepared with initial pH 1.49, 2.00, 2.57, and 3.79. The suspension was pipetted into 5 mm plastic cylinders, which were then put in the 7 mm zirconia PENCIL T M rotors. NMR measurements were performed during the aging process. Samples were spun at 3-4 kHz; spectra were collected by using a single-pulse Block decay method with a 5 ms aluminum pulse and a 500 ms relaxation delay. For all experiments, 40 ms acquisition times and a 50 kHz spectral window were employed. The number of transients was 500 to 1000. The approach employed in this study is designed to maximize the signal from the dissolved A1 species and minimize the signal from the alumina particles. Because samples were submicrometer A1203 colloidal particles suspended in water, a broad NMR signal was observed for the A1203 particles due to large quadruple moments of A1, having FWHM (full width at half maximum) of 8 to 9 kHZ at nonspin and 1 to 2 kHz at 3 kHz spinning. Both magic angle spinning (MAS) and static Block decay measurements were taken. It was found that static measurements (without spinning) resolved narrow peaks (FWHM of 100 to 200 Hz). Although
with spinning the peaks from AI20 3 particles were greatly narrowed, those narrow peaks were embedded in the A1203 main peak and its side bands. Thus, all aluminum spectra were taken at nonspin and without decoupling because no changes in spectra were observed with and without decoupling. 2.2. Aggregation stud)' of dilute suspensions
Aggregation studies were conducted at pH 2 for diluted suspensions. The samples were aged for different times to study the effect of hydrolyzed clusters formed during aging. A 15 vol.% suspension was made with 11.88 g of AKP 53 powder added to 17 ml deionized water; the pH was preadjusted to 2.00 with dilute HNO3. The suspensions were sonicated for 20 min and pH was readjusted to 2 by dilute HNO3. The samples were then aged for different times: no aging, aged 1 day, and aged 21 days. The pH of the suspension increased slowly with time; therefore, it was necessary to readjust pH to 2 before measuring the particle and aggregate size. About 0.2 ml of the suspension was diluted to 10 ml of 0.1 M NaNO3 solution, with pH preadjusted to 2. The particle size and the aggregate size distribution of the dilute suspension was measured at different times by a Microtrac Series 9200 Ultrafine Particle Analyzer using the Doppler shift laser light scattering principle. The initial particle size distribution was also measured by diluting the suspension with pure deionized water at pH 2. 2.3. Viscosity
Shearing experiments were performed at pH 4. Suspensions of 15 vol.% were prepared with initial pH of 2.00 and 3.79. After the samples had been aged for different times, the suspensions were sonicated for 20 min, and pH of all the suspensions was readjusted to 4. Then NaNO3 was added to the suspension to make a 0.3 M electrolyte solution. The stress-shearing rate relationship was measured by a Haake Viscometer with an SVI sensor. The apparent viscosity was obtained by dividing the stress by the shearing rate. 2.4. Transmission electron microscopy ( T E M )
Transmission electron microscopy was used to study the morphology of the particles before and after aging. Similar suspensions of aged and nonaged suspensions of 15 vol.% solid loading were prepared and 0.3 M NaNO3 was added. A TEM copper grid coated with carbon thin film was gently introduced into the suspension and pulled out. A drying paper was used to remove the excess suspension and liquid. TEM imaging was performed on a Philips 400 TEM at 120 kV.
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J. Liu et al. / Materials Science and Engineering A204 (1995) 169-175
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Fig. 1. NMR results on alumina suspensions aged for different times at different pH. The peak at 0 ppm corresponds to the single AI 3 ÷ ions, and the peak at 63.5 ppm corresponds to the Al~3 clusters; the broad peak comes from the alumina particles themselves. The peak height at 63.5 ppm increased with aging at low pH (a and b), suggesting that the concentration of polycations increases during aging. At a higher pH both the dissolution and formation of the polyclusters were sluggish (c and d).
3. Results The N M R study provided direct information on the hydrolysis process in the suspension. Four suspensions were prepared with an initial pH of 1.49, 2.00, 2.57, and 3.79. During aging, the pH of all the suspensions shifted to a higher value due to hydrolysis. After aging for 3 days, the pH values became 4.05, 4.40, 4.64, and 4.75, respectively. The N M R results are summarized in Fig. 1. Only one sharp peak was observed at 0 ppm for fresh suspensions. This peak corresponds to the single AI(H20) 3 + ions. At a lower pH, this peak was higher, indicating fast dissolution kinetics; at a higher pH, the peak was broad and small, indicating slow dissolution kinetics•
After some time of aging, a second peak appeared at 63.5 ppm at low pH, corresponding to the (AlI304(OH)24(H20)12) v+ (All3 polycation) [15,24]. The All3 cluster contains 12 A106 octahedra joined by common edges. The tetrahedron of the four oxygens surrounds the aluminum atom at the center. This tetrahedrally coordinated aluminum produces a narrow A1 NMR resonance at 63.5 ppm (relative to (AI(H20)6) 3+). Because this is one of the most stable and prominent poly species in the solution [15,24], this narrow resonance can be used as a probe to study the forming of hydrated polycations in the solution. Fig. l(a,b) illustrates how this peak grew with time at low pH, suggesting that the concentration of the polycation increases with time during aging at low pH. The figure also shows that at pH 2.00 the peak height at zero ppm
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J. Liu et al. / Materials Science and EngineerhN A204 (1995) 169 175
decreased after aging while at the same time the peak at 63.5 ppm grew. This occurred because the dissolved A13 + ions combined to form the polyclusters at a later stage. On the other hand, Fig. l(c,d) shows that both the dissolution and the formation of the polyspecies were not significant at a higher p H Only a small peak at 63.5 ppm was detected in Fig. l(c) after 3 days of aging; at even a higher pH, there is only a tiny shoulder (Fig. 1(d)). These results from alumina suspensions are consistent with those reported in the literature [14,23] concerning the hydrolysis and condensation process of aluminum ions: (1) The solubility of aluminum ions is highest at low pH, reaches a minimum at pH 6.5, and increases again at high pH. (2) The mononuclear species forms rapidly and reversibly, while polycation species, such as ( A 1 2 ( O H ) 2 ( H 2 0 ) 8 ) 4+ and ( A 1 3 ( O H ) 4 ( H 2 0 ) 9 ) 5~ form slowly. Larger hydrated polyclusters, ( A l 1 3 0 4 ( O H ) 2 4 ( H 2 0 ) 1 2 ) 7+ (AIj3 cluster), will take longer to grow. (3) The distribution of the polymeric species is pH-sensitive. Under acidic conditions (pH < 4) the large clusters, especially Al13 clusters, dominate, while at a higher pH, most aluminum ions exist as mononuclear species. From this discussion, it is clear that the alumina suspension consists not only of the alumina particles, but also of hydrolyzed polycations. The hydrated polynuclear clusters will certainly have an effect on the colloidal stability and rheological properties of the suspension. Like other organic processing additives (surfactant, polymers), these charged polycation clusters can modify the interaction potentials between the particles when the range of the interaction being considered is comparable to the dimension of these polycations, and can influence the colloidal properties of the suspensions through electrostatic, steric, electrosteric or other mechanisms. The aggregation experiments were conducted on suspensions aged at pH 2 for different times. As discussed in the previous sections, aging at low pH will increase the concentration of polycation clusters. The nonaged sample (fresh sample) has no polycation clusters, the sample aged for 1 day has some polycation clusters, and the sample aged 21 days has more polycation clusters. These experiments will then tell whether the hydrated polycations have any effect on the colloidal stability. The particle size distributions of all the suspensions were first measured to ensure that the starting conditions were the same. As shown in Fig. 2, these suspensions do have the same initial particle size distribution. The mean particle size of 0.2 /~m agrees with the supplied literature data. In Fig. 3 the particle and aggregate size distribution is plotted 1 rain, 8 rain, and
22 min after the dispersed suspension was placed into 0.1 M NaNO3 solutions at pH 2 to start aggregation. Fig. 3(a) shows that significant aggregation had already occurred in the nonaged suspension after 1 min in 0.1 M NaNO3, while the suspensions aged 1 day and 21 days remained mostly dispersed. After 8 min, the suspension aged I day also aggregated. After 22 min all the particles in the nonaged suspension were flocculated, yet the samples aged 21 days were only partially aggregated. In all the cases, the nonaged suspension with least amount of polycation clusters always had a size distribution on the right, indicating large agglomerate size; the suspension aged 21 days with more hydrolyzed polycation clusters had a size distribution on the left, indicating smaller agglomerate sizes. The suspension aged for 1 day was between the two. These experiments prove that aging at low pH makes the suspension more stable. Because it was shown earlier that aging under acidic conditions produces a high concentration of hydrolyzed polycations, these results then suggest that these hydrated polycations help stabilize the suspension. The theological experiment further illustrated the effect of hydrolyzed polycation clusters on the viscosity of the suspension. All the experiments were conducted at pH 4, with some suspensions first aged at pH 2 to create hydrolyzed clusters, and other suspensions prepared fresh. Fig. 4 shows that aging at pH 2 greatly reduced the shearing stress and viscosity. This can be attributed to the existence of hydrolyzed polyclusters to create a weak interaction potential. The fresh suspension prepared at pH 4 had much higher shearing stress and viscosity because the hydrated polycations were not present. Aging under this condition led to even higher
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173
J. Liu et al./ Materials Science and Engineering A204 (1995) 169 175 1
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particles. Fig. 5(a) shows the nonaged alumina particles, and Fig. 5(b) shows the aged alumina particles. Comparison between 5(a) and 5(b) shows that the nonaged suspension consisted of relatively clean particles and particle surfaces; yet the aged suspension had particles with rough surfaces due to dissolution and formation of hydrolyzed clusters. Very fine aluminum hydroxide particles can be observed in the background. Usually fine particles are also seen between larger alumina particles. These very small particles are assumed to be aggregated and condensed products of the polycation clusters in the solutions.
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Size (Micrometer) Fig. 3. Aggregate size distribution in 0.1 M NaNO3 solution at pH 2 after 1 min (a), 8 min (b), and 20 min (c) after the aggregation was initiated. The suspension without the hydrated polycations (nonaged sample) aggregated faster than the suspensions with polycations (aged samples). shearing stress a n d viscosity due to a s t r o n g e r b o n d i n g between particles c r e a t e d d u r i n g aging. O n the o t h e r h a n d , aging at p H 2 i m p r o v e d the rheological p r o p e r ties a n d r e d u c e d the viscosity. T E M was also used to s h o w the m o r p h o l o g i c a l difference between the aged a n d n o n a g e d a l u m i n a
Fig. 5. Comparison of the morphology of aged and nonaged alumina particles. The nonaged particles had smooth clean surfaces; the aged particles showed rough surfaces, and ultrafine particles in the solution and between the particles.
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J. Liu et al. / Materials Science and Engineering A204 (1995) 169 175
4. Summary and discussion
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To evaluate the influence of the polycations on the interaction potential between the particles, the interaction energies between 0.2 mm alumina particles in 0.2 M NaNO3 solutions were estimated through the DLVO theory [6,24] using constant potential approximation. The surface potential was taken as 60 mV, a typical number for fully charged oxide particles, and the effective Hamaker constant was taken as 11.9 kT [7,8]. The final results are plotted in Fig. 6. The net interaction energy remains slightly negative (attractive) over a wide range until the separation distance between the particle surface is less than 5 A or so, below which the attraction energy increases rapidly with decreasing distance. However, if the net interaction is intercepted at about 5 A separation, the total attraction will be small. The radius of the most common aluminum polycation, the Al~3 cluster, is about 3.75 to 3.9 A. These hydrated polycations on the particle surface or between the particles can change colloidal interactions and produce a repulsive potential at about twice their dimensions, as schematically shown by the straight line. This phenomenon will provide an effective steric barrier at close distance similar to adsorbed organic polymers [25] and therefore will reduce the attraction energy between particles and increase the colloidal stability. Due to the steep repulsion from the steric interaction, a shallow energy minimum is introduced to the total energy potential. Unfortunately, the N M R technique used in the current study is not sensitive to the structures on the particle surfaces; therefore the exact conformation on the surface and the adsorption behavior of the hydrated species cannot be studied.
This paper has shown that hydrated polycations greatly affect the colloidal stability and rheological behavior of fine alumina suspensions. Under conditions when the polycations can form, such as aging at low pH, the suspension becomes more stable against aggregation, and has a low viscosity if flocculated. The aggregation rate, rheological properties, and morphological changes are all consistent with this conclusion. It can also be expected that the hydrolysis process will play a more important role for even finer particles (nanometer-sized particles) because of the increased surface area and the range of interactions that need to be considered. In the literature, short-range hydration forces have been used to prepare weakly flocculated alumina suspensions with desirable rheological properties for nearnet-shape forming [9-11]. Such suspensions are produced by the addition of electrolytes into dispersed colloids and subsequent pressure filtration. This procedure produces suspensions with high packing density (58 vol.%), and low viscosity required of plastic forming. The hydration forces are attributed to the absorption of anions (CI , B r - , I , NO3 etc.) on the positively charged particle surface under acidic conditions. Then the questions are why the unusual colloidal behavior in alumina is only observed at pH below 4, not observed at higher pH, and why the short-range forces are more effective in alumina suspensions and less effective in nonoxide suspensions [17]. In latex suspensions and in mica systems [21], the hydration forces are only detected at high pH (above the isoelectric point) with the adsorption of hydrated cations. Below the isoelectric point, the effect of hydration forces on the colloidal stability is not observed since only unhydrated N O r adsorbs to the positively charged surface. Apparently the behavior of the hydration forces discussed in the literature [9-11] is somewhat similar to the hydrolysis and formation of the polycations in alumina suspensions discussed in this paper, which would only have a significant effect at low pH range. Therefore, it is reasonable to speculate that the hydrated polycations formed through aging may have contributed to the hydration forces [9 11] in such oxide suspensions. A better understanding of the origin of such short-range interactions and the effect of surface hydrolysis will be achieved with the application of advanced surface characterization techniques.
Acknowledgments This research is supported by the U.S. Department of Energy under Contract DE-AC06-76RLO 1830. Pacific
J, Liu et al./ Materials Seience and Engineering A204 (1995) 169 175
Northwest Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under contract DE-AC06-76RLO 1830.
References [1] R.G. Horn, J. Am. Ceram. Soc., 73 (1990) lll7. [2] I.A. Aksay, in J.A. Mangels and G.L. Messing (eds.), Advances in Ceramics, 9, Forming o f Ceramics, American Ceramic Society, Westerville, OH, 1984, p. 94. [3] F.F. Lane, J. Am+ Ceram. Soe., 72 (1989) 3. [4] J. Liu, M. Sarikaya, W.Y. Shih, W+-H. Shih and I.A. Aksay, Mat. Res. Soc. Symp. Proe., 175 (1990) 30. [5] Y. Hirata, 1. Haraguchi and Y. Ishihara, J. Mater. Res., 58 (1992) 408. [6] J.T.G. Overbeek, J. Colloid. lnterJace Sei., 58 (1977), 408. [7] L. Bergstrom, J. Chem. Soc. Faraday Trans., 88 (1992) 3201. [8] L. Bergstrom, C.H. Schilling and I.A. Aksay, J. Am. Ceram. Soe., 75 (1992) 3305. [9] B.V. Velamakanni, J.C. Chang, F.F. Lange and D.S. Pearson, Langmuir, 6 (1990) 1323. [10] W.A. Ducker, Z. Xu, D.R. Clarke and J.N. lsraelachvili, J. Am. Ceram. Soe., 77 (1994)437.
]75
[1 t] B.V. Velamakanni, F.F. Lange, F.W. Zok and D.S. Pearson, J. Am. Ceram. Sot., 77 (1994) 216. [12] L. Bergstrom and R.J. Pugh, J. Am. ('er. Sot., 72 (1989)103. [13] L. Bergstrom and E. Bostedt, Coll. SuiJi+ 49 (1990) 183. [14] C.F. Baes and R+E. Mesmer, The ltydroO,sis" ~! Cations+ Krieger Publishing Co., Malabar, FL, 1986. [15] T. Assih, A. Ayral, M. Abenoza and J. Phalippou, J. Mater. Sei.. 23 (1988) 3326. [16] T.E. Petroff and M. Sayer, ('olloids Sur/aee, 78 (1993) 235. [17] E.P. Luther, T.M. Kramer, F.F. Lange and D.S. Pearson, J. Am. Ceram. Soe.+ 77 (! 994) 1047. [18] K. Higashitani, M. Kondo and S. Hatade, J. Colloid. huerjhee Sci., 142 (1991) 204. [19] J.N. Israelachvili and R.M. Pashley, J. Colloid. h~ter/ace Sci., 97 (1984) 446. [20] J. Israelachvili, lntermoleeular and Sur[ace Forces, Academic Press, San Diego, 1991, p. 275. [21] V.E. Shubin and P. Kekicheff, J. Colloid. Inter/ace Sci., 155 (1993) 108. [22] P.M. Pashley, J. Colloid. lnter[aee Sci., 83 (1981) 531. [23] C.J. Brinker and G.W. Scherer, Sol-Gel Seienee, Academic Press, San Diego, 1990, p. 60. [24] D.J. Shaw, h~troduetion to Colloid and Sur/ace Chemistry, Butterworths, Boston, 1980, p. 186. [25] D.H. Napper, Polymeric Stabilization ~1' Colloidal Dispersions, Academic Press, London, 1983.
Materials Science and Engineering A204 (1995) 169 175
A
Effect of hydrolysis on the colloidal stability of fine alumina suspensions J, L i u , L . Q W a n g , B.C. Bunker, G.L. Graft, J.W. Virden, R.H. J o n e s D~7~arlm~'u: ~{/ Materials A'ciem'~,. Pacilic Northwest l.ahoratoJT, Battelle Boule~:ard. Box 999, Rich/and. WA 993.52. USA
Abstract This paper investigates the effect of hydrolysis and the formation of hydrated polycations on the colloidal stability and rheological properties of fine alumina suspensions. This is an important phenomenon in colloidal processing of advanced ceramic materials and nanocomp~site materials from fine particles. The aging process and the formation of large polycations in the solution are monitored by nuclear magnetic resonance, and the aggregation rate and viscosity of the suspension are measured under similar conditions. It is observed that aging under acidic conditions increases colloidal stability against aggregation in dispersed suspensions, and reduces the viscosity in flocculated suspensions. This behavior corresponds well to the formation of large polynuclear species through hydrolysis. It is suggested that the hydrated polycations significantly modify the total interaction energy between two particles at short separation distances.
Kcywords: Hydrolysis; Colloidal stability; Fine alumina suspensions
1. Introduction Colloidal processing methods have been widely used to prepare advanced ceramic materials and nanocomposite materials [1-5]. The packing density and microstructure of the green body are mainly determined by the interactions between the colloidal particles and the conditions under which the suspension is prepared and consolidated. Three mechanisms to control colloidal stability are well documented [6]: electrostatic repulsion due to charge development on particle surfaces, van der Waals attraction through dipole-dipole interactions, and steric interaction from polymers adsorbed onto the particles. These interaction energies can be estimated using a DLVO (Derjaguin-Landau-Verwey Overbeek) theory [6]. Strong electrostatic or steric repulsion will produce a well-dispersed suspension and high packing density. On the other hand, a flocculated suspension in the absence of long-range repulsion will result in poor consolidation and low packing density. The stability, rheology, and consolidation of some common oxide materials, such as alumina, have been Elsevier Science S ,\. ,~SDI 0921-5093195)09955-7
extensively studied in the literature [7--11]. However, many ceramic suspensions, especially those made of fine particles, exhibit time-dependent colloidal properties through aging and hydrolysis [12,13]. The effect of such phenomena on the colloidal stability has not been well studied. For example, metal oxides can undergo extensive hydrolysis in an aqueous solution and produce a variety of solution species, depending on pH, temperature, electrolyte concentrations, mechanical stirring, and reaction time [14]. It has been suggested that the hydrolyzed polynuclear species may have an effect on the colloidal stability of peptized beohmite [15]. Petroff and Sayer have purposely introduced zirconium hydrogel to zirconia, alumina, and titania suspensions to modify the surface chemistry and enhance colloidal stability [16]. There are several recent studies related to "non-DLVO" forces in ceramic suspensions [911,17,18]. Because these non-DLVO forces are generally attributed to the short-range hydration forces [19--22], a better understanding of the influence of surface hydrolysis is important. The purpose of this study is to examine the hydrolysis process of suspensions of fine alumina particles, and
170
J. Liu et al. / Materials Science and Engineering A204 (1995) 169-175
its correlation with colloidal stability and rheological properties. The solution species is studied by magnetic resonance, and the aggregation rate and viscosity of the suspension are monitored at the same time. It is concluded that the hydrolysis process greatly affects the colloidal properties of fine particles: the increased colloidal stability and reduced viscosity can be directly related to the dissolution and formation of large polynuclear species through aging under acidic conditions. These hydrated polycations can significantly modify the total interaction energy between two particles at short separation distances.
2. Experimental procedure Aqueous suspensions of unagglomerated a-alumina powder (AKP 53, 0.2/~m median particle size obtained from Sumitomo Chemical Co., New York) were studied. Nuclear magnetic resonance (NMR) was used to study the hydrolyzed polycation species. The aggregation rates, viscosity, and electrokinetic mobility were measured under controlled experimental conditions. 2.1. Nuclear magnetic resonance spectroscopy
27A1 solid-state NMR was used to identify chemical species in the colloidal aging processes through chemical shifts and line shape of NMR signals. No effort was made to measure the absolute concentration of the soluble species in this study. 27A1 NMR experiments were carried out with a Chemagnetics spectrometer (300 MHz, 89 mm wide bore Oxford magnet) using a CP-MAS probe. Suspensions of 15 vol.% A1203 were prepared with initial pH 1.49, 2.00, 2.57, and 3.79. The suspension was pipetted into 5 mm plastic cylinders, which were then put in the 7 mm zirconia PENCIL T M rotors. NMR measurements were performed during the aging process. Samples were spun at 3-4 kHz; spectra were collected by using a single-pulse Block decay method with a 5 ms aluminum pulse and a 500 ms relaxation delay. For all experiments, 40 ms acquisition times and a 50 kHz spectral window were employed. The number of transients was 500 to 1000. The approach employed in this study is designed to maximize the signal from the dissolved A1 species and minimize the signal from the alumina particles. Because samples were submicrometer A1203 colloidal particles suspended in water, a broad NMR signal was observed for the A1203 particles due to large quadruple moments of A1, having FWHM (full width at half maximum) of 8 to 9 kHZ at nonspin and 1 to 2 kHz at 3 kHz spinning. Both magic angle spinning (MAS) and static Block decay measurements were taken. It was found that static measurements (without spinning) resolved narrow peaks (FWHM of 100 to 200 Hz). Although
with spinning the peaks from AI20 3 particles were greatly narrowed, those narrow peaks were embedded in the A1203 main peak and its side bands. Thus, all aluminum spectra were taken at nonspin and without decoupling because no changes in spectra were observed with and without decoupling. 2.2. Aggregation stud)' of dilute suspensions
Aggregation studies were conducted at pH 2 for diluted suspensions. The samples were aged for different times to study the effect of hydrolyzed clusters formed during aging. A 15 vol.% suspension was made with 11.88 g of AKP 53 powder added to 17 ml deionized water; the pH was preadjusted to 2.00 with dilute HNO3. The suspensions were sonicated for 20 min and pH was readjusted to 2 by dilute HNO3. The samples were then aged for different times: no aging, aged 1 day, and aged 21 days. The pH of the suspension increased slowly with time; therefore, it was necessary to readjust pH to 2 before measuring the particle and aggregate size. About 0.2 ml of the suspension was diluted to 10 ml of 0.1 M NaNO3 solution, with pH preadjusted to 2. The particle size and the aggregate size distribution of the dilute suspension was measured at different times by a Microtrac Series 9200 Ultrafine Particle Analyzer using the Doppler shift laser light scattering principle. The initial particle size distribution was also measured by diluting the suspension with pure deionized water at pH 2. 2.3. Viscosity
Shearing experiments were performed at pH 4. Suspensions of 15 vol.% were prepared with initial pH of 2.00 and 3.79. After the samples had been aged for different times, the suspensions were sonicated for 20 min, and pH of all the suspensions was readjusted to 4. Then NaNO3 was added to the suspension to make a 0.3 M electrolyte solution. The stress-shearing rate relationship was measured by a Haake Viscometer with an SVI sensor. The apparent viscosity was obtained by dividing the stress by the shearing rate. 2.4. Transmission electron microscopy ( T E M )
Transmission electron microscopy was used to study the morphology of the particles before and after aging. Similar suspensions of aged and nonaged suspensions of 15 vol.% solid loading were prepared and 0.3 M NaNO3 was added. A TEM copper grid coated with carbon thin film was gently introduced into the suspension and pulled out. A drying paper was used to remove the excess suspension and liquid. TEM imaging was performed on a Philips 400 TEM at 120 kV.
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J. Liu et al. / Materials Science and Engineering A204 (1995) 169-175
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Fig. 1. NMR results on alumina suspensions aged for different times at different pH. The peak at 0 ppm corresponds to the single AI 3 ÷ ions, and the peak at 63.5 ppm corresponds to the Al~3 clusters; the broad peak comes from the alumina particles themselves. The peak height at 63.5 ppm increased with aging at low pH (a and b), suggesting that the concentration of polycations increases during aging. At a higher pH both the dissolution and formation of the polyclusters were sluggish (c and d).
3. Results The N M R study provided direct information on the hydrolysis process in the suspension. Four suspensions were prepared with an initial pH of 1.49, 2.00, 2.57, and 3.79. During aging, the pH of all the suspensions shifted to a higher value due to hydrolysis. After aging for 3 days, the pH values became 4.05, 4.40, 4.64, and 4.75, respectively. The N M R results are summarized in Fig. 1. Only one sharp peak was observed at 0 ppm for fresh suspensions. This peak corresponds to the single AI(H20) 3 + ions. At a lower pH, this peak was higher, indicating fast dissolution kinetics; at a higher pH, the peak was broad and small, indicating slow dissolution kinetics•
After some time of aging, a second peak appeared at 63.5 ppm at low pH, corresponding to the (AlI304(OH)24(H20)12) v+ (All3 polycation) [15,24]. The All3 cluster contains 12 A106 octahedra joined by common edges. The tetrahedron of the four oxygens surrounds the aluminum atom at the center. This tetrahedrally coordinated aluminum produces a narrow A1 NMR resonance at 63.5 ppm (relative to (AI(H20)6) 3+). Because this is one of the most stable and prominent poly species in the solution [15,24], this narrow resonance can be used as a probe to study the forming of hydrated polycations in the solution. Fig. l(a,b) illustrates how this peak grew with time at low pH, suggesting that the concentration of the polycation increases with time during aging at low pH. The figure also shows that at pH 2.00 the peak height at zero ppm
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J. Liu et al. / Materials Science and EngineerhN A204 (1995) 169 175
decreased after aging while at the same time the peak at 63.5 ppm grew. This occurred because the dissolved A13 + ions combined to form the polyclusters at a later stage. On the other hand, Fig. l(c,d) shows that both the dissolution and the formation of the polyspecies were not significant at a higher p H Only a small peak at 63.5 ppm was detected in Fig. l(c) after 3 days of aging; at even a higher pH, there is only a tiny shoulder (Fig. 1(d)). These results from alumina suspensions are consistent with those reported in the literature [14,23] concerning the hydrolysis and condensation process of aluminum ions: (1) The solubility of aluminum ions is highest at low pH, reaches a minimum at pH 6.5, and increases again at high pH. (2) The mononuclear species forms rapidly and reversibly, while polycation species, such as ( A 1 2 ( O H ) 2 ( H 2 0 ) 8 ) 4+ and ( A 1 3 ( O H ) 4 ( H 2 0 ) 9 ) 5~ form slowly. Larger hydrated polyclusters, ( A l 1 3 0 4 ( O H ) 2 4 ( H 2 0 ) 1 2 ) 7+ (AIj3 cluster), will take longer to grow. (3) The distribution of the polymeric species is pH-sensitive. Under acidic conditions (pH < 4) the large clusters, especially Al13 clusters, dominate, while at a higher pH, most aluminum ions exist as mononuclear species. From this discussion, it is clear that the alumina suspension consists not only of the alumina particles, but also of hydrolyzed polycations. The hydrated polynuclear clusters will certainly have an effect on the colloidal stability and rheological properties of the suspension. Like other organic processing additives (surfactant, polymers), these charged polycation clusters can modify the interaction potentials between the particles when the range of the interaction being considered is comparable to the dimension of these polycations, and can influence the colloidal properties of the suspensions through electrostatic, steric, electrosteric or other mechanisms. The aggregation experiments were conducted on suspensions aged at pH 2 for different times. As discussed in the previous sections, aging at low pH will increase the concentration of polycation clusters. The nonaged sample (fresh sample) has no polycation clusters, the sample aged for 1 day has some polycation clusters, and the sample aged 21 days has more polycation clusters. These experiments will then tell whether the hydrated polycations have any effect on the colloidal stability. The particle size distributions of all the suspensions were first measured to ensure that the starting conditions were the same. As shown in Fig. 2, these suspensions do have the same initial particle size distribution. The mean particle size of 0.2 /~m agrees with the supplied literature data. In Fig. 3 the particle and aggregate size distribution is plotted 1 rain, 8 rain, and
22 min after the dispersed suspension was placed into 0.1 M NaNO3 solutions at pH 2 to start aggregation. Fig. 3(a) shows that significant aggregation had already occurred in the nonaged suspension after 1 min in 0.1 M NaNO3, while the suspensions aged 1 day and 21 days remained mostly dispersed. After 8 min, the suspension aged I day also aggregated. After 22 min all the particles in the nonaged suspension were flocculated, yet the samples aged 21 days were only partially aggregated. In all the cases, the nonaged suspension with least amount of polycation clusters always had a size distribution on the right, indicating large agglomerate size; the suspension aged 21 days with more hydrolyzed polycation clusters had a size distribution on the left, indicating smaller agglomerate sizes. The suspension aged for 1 day was between the two. These experiments prove that aging at low pH makes the suspension more stable. Because it was shown earlier that aging under acidic conditions produces a high concentration of hydrolyzed polycations, these results then suggest that these hydrated polycations help stabilize the suspension. The theological experiment further illustrated the effect of hydrolyzed polycation clusters on the viscosity of the suspension. All the experiments were conducted at pH 4, with some suspensions first aged at pH 2 to create hydrolyzed clusters, and other suspensions prepared fresh. Fig. 4 shows that aging at pH 2 greatly reduced the shearing stress and viscosity. This can be attributed to the existence of hydrolyzed polyclusters to create a weak interaction potential. The fresh suspension prepared at pH 4 had much higher shearing stress and viscosity because the hydrated polycations were not present. Aging under this condition led to even higher
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173
J. Liu et al./ Materials Science and Engineering A204 (1995) 169 175 1
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particles. Fig. 5(a) shows the nonaged alumina particles, and Fig. 5(b) shows the aged alumina particles. Comparison between 5(a) and 5(b) shows that the nonaged suspension consisted of relatively clean particles and particle surfaces; yet the aged suspension had particles with rough surfaces due to dissolution and formation of hydrolyzed clusters. Very fine aluminum hydroxide particles can be observed in the background. Usually fine particles are also seen between larger alumina particles. These very small particles are assumed to be aggregated and condensed products of the polycation clusters in the solutions.
3 21 days 2. Aged 1 day 3. No a gbl g itl
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Size (Micrometer) Fig. 3. Aggregate size distribution in 0.1 M NaNO3 solution at pH 2 after 1 min (a), 8 min (b), and 20 min (c) after the aggregation was initiated. The suspension without the hydrated polycations (nonaged sample) aggregated faster than the suspensions with polycations (aged samples). shearing stress a n d viscosity due to a s t r o n g e r b o n d i n g between particles c r e a t e d d u r i n g aging. O n the o t h e r h a n d , aging at p H 2 i m p r o v e d the rheological p r o p e r ties a n d r e d u c e d the viscosity. T E M was also used to s h o w the m o r p h o l o g i c a l difference between the aged a n d n o n a g e d a l u m i n a
Fig. 5. Comparison of the morphology of aged and nonaged alumina particles. The nonaged particles had smooth clean surfaces; the aged particles showed rough surfaces, and ultrafine particles in the solution and between the particles.
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J. Liu et al. / Materials Science and Engineering A204 (1995) 169 175
4. Summary and discussion
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To evaluate the influence of the polycations on the interaction potential between the particles, the interaction energies between 0.2 mm alumina particles in 0.2 M NaNO3 solutions were estimated through the DLVO theory [6,24] using constant potential approximation. The surface potential was taken as 60 mV, a typical number for fully charged oxide particles, and the effective Hamaker constant was taken as 11.9 kT [7,8]. The final results are plotted in Fig. 6. The net interaction energy remains slightly negative (attractive) over a wide range until the separation distance between the particle surface is less than 5 A or so, below which the attraction energy increases rapidly with decreasing distance. However, if the net interaction is intercepted at about 5 A separation, the total attraction will be small. The radius of the most common aluminum polycation, the Al~3 cluster, is about 3.75 to 3.9 A. These hydrated polycations on the particle surface or between the particles can change colloidal interactions and produce a repulsive potential at about twice their dimensions, as schematically shown by the straight line. This phenomenon will provide an effective steric barrier at close distance similar to adsorbed organic polymers [25] and therefore will reduce the attraction energy between particles and increase the colloidal stability. Due to the steep repulsion from the steric interaction, a shallow energy minimum is introduced to the total energy potential. Unfortunately, the N M R technique used in the current study is not sensitive to the structures on the particle surfaces; therefore the exact conformation on the surface and the adsorption behavior of the hydrated species cannot be studied.
This paper has shown that hydrated polycations greatly affect the colloidal stability and rheological behavior of fine alumina suspensions. Under conditions when the polycations can form, such as aging at low pH, the suspension becomes more stable against aggregation, and has a low viscosity if flocculated. The aggregation rate, rheological properties, and morphological changes are all consistent with this conclusion. It can also be expected that the hydrolysis process will play a more important role for even finer particles (nanometer-sized particles) because of the increased surface area and the range of interactions that need to be considered. In the literature, short-range hydration forces have been used to prepare weakly flocculated alumina suspensions with desirable rheological properties for nearnet-shape forming [9-11]. Such suspensions are produced by the addition of electrolytes into dispersed colloids and subsequent pressure filtration. This procedure produces suspensions with high packing density (58 vol.%), and low viscosity required of plastic forming. The hydration forces are attributed to the absorption of anions (CI , B r - , I , NO3 etc.) on the positively charged particle surface under acidic conditions. Then the questions are why the unusual colloidal behavior in alumina is only observed at pH below 4, not observed at higher pH, and why the short-range forces are more effective in alumina suspensions and less effective in nonoxide suspensions [17]. In latex suspensions and in mica systems [21], the hydration forces are only detected at high pH (above the isoelectric point) with the adsorption of hydrated cations. Below the isoelectric point, the effect of hydration forces on the colloidal stability is not observed since only unhydrated N O r adsorbs to the positively charged surface. Apparently the behavior of the hydration forces discussed in the literature [9-11] is somewhat similar to the hydrolysis and formation of the polycations in alumina suspensions discussed in this paper, which would only have a significant effect at low pH range. Therefore, it is reasonable to speculate that the hydrated polycations formed through aging may have contributed to the hydration forces [9 11] in such oxide suspensions. A better understanding of the origin of such short-range interactions and the effect of surface hydrolysis will be achieved with the application of advanced surface characterization techniques.
Acknowledgments This research is supported by the U.S. Department of Energy under Contract DE-AC06-76RLO 1830. Pacific
J, Liu et al./ Materials Seience and Engineering A204 (1995) 169 175
Northwest Laboratory is operated by Battelle Memorial Institute for the U.S. Department of Energy under contract DE-AC06-76RLO 1830.
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]75
[1 t] B.V. Velamakanni, F.F. Lange, F.W. Zok and D.S. Pearson, J. Am. Ceram. Sot., 77 (1994) 216. [12] L. Bergstrom and R.J. Pugh, J. Am. ('er. Sot., 72 (1989)103. [13] L. Bergstrom and E. Bostedt, Coll. SuiJi+ 49 (1990) 183. [14] C.F. Baes and R+E. Mesmer, The ltydroO,sis" ~! Cations+ Krieger Publishing Co., Malabar, FL, 1986. [15] T. Assih, A. Ayral, M. Abenoza and J. Phalippou, J. Mater. Sei.. 23 (1988) 3326. [16] T.E. Petroff and M. Sayer, ('olloids Sur/aee, 78 (1993) 235. [17] E.P. Luther, T.M. Kramer, F.F. Lange and D.S. Pearson, J. Am. Ceram. Soe.+ 77 (! 994) 1047. [18] K. Higashitani, M. Kondo and S. Hatade, J. Colloid. huerjhee Sci., 142 (1991) 204. [19] J.N. Israelachvili and R.M. Pashley, J. Colloid. h~ter/ace Sci., 97 (1984) 446. [20] J. Israelachvili, lntermoleeular and Sur[ace Forces, Academic Press, San Diego, 1991, p. 275. [21] V.E. Shubin and P. Kekicheff, J. Colloid. Inter/ace Sci., 155 (1993) 108. [22] P.M. Pashley, J. Colloid. lnter[aee Sci., 83 (1981) 531. [23] C.J. Brinker and G.W. Scherer, Sol-Gel Seienee, Academic Press, San Diego, 1990, p. 60. [24] D.J. Shaw, h~troduetion to Colloid and Sur/ace Chemistry, Butterworths, Boston, 1980, p. 186. [25] D.H. Napper, Polymeric Stabilization ~1' Colloidal Dispersions, Academic Press, London, 1983.