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Temperature-induced gelation of concentrated silicon carbide suspensions
Journal of Colloid and Interface Science 277 (2004) 111–115 www.elsevier.com/locate/jcis
Temperature-induced gelation of concentrated silicon carbide suspensions X. Xu a,b , S. Mei a , J.M.F. Ferreira a,∗ , T. Nishimura b , N. Hirosaki b a Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b National Institute for Materials Science, Advanced Materials Laboratory, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Received 19 January 2004; accepted 12 April 2004 Available online 6 May 2004
Abstract Due to the steric barrier provided by the adsorption of the dispersant hypermer KD1 (a polyester/polyamine condensation polymer), stable and low-viscosity suspensions of SiC, Y2 O3 , and Al2 O3 powder mixtures could be prepared in methyl ethyl ketone (MEK)/ethanol (E) solvent with solids loading as high as 60 vol%. The solvency of the dispersant in MEK/E decreased dramatically on cooling. Steady shear viscosity and oscillatory measurements were performed as a function of temperature for suspensions with different solids loading. The viscosity and elastic modulus of suspension increased with decreasing temperature and became more sensitive with the increase of solids loading. The suspensions with solids loading higher than 40 vol% could be solidified with decreasing temperature, but gelation temperature and gelation stiffness decreased with decreasing solids loading. The 60 vol% solid-loaded suspension was a stable and free-flowing fluid at 20 ◦ C and gradually transformed to a very highly viscous and elastic system upon cooling to about 13 ◦ C. Complete solidification occurred when the temperature was decreased to 5 ◦ C. The gelation mechanism was mainly based on the collapse of the adsorbed layer as the temperature decreases, which induced incipient flocculation and formed a stiff network. The gelled body was further strengthened by separation of the dispersant from the suspension. 2004 Elsevier Inc. All rights reserved. Keywords: Temperature-induced gelation; Silicon carbide; Suspension; Rheology; Shape-forming method
1. Introduction Silicon carbide (SiC) is a ceramic material widely used for structural applications at high temperatures. To obtain high-strength, high-reliability SiC components, it is important to minimize the defect size during forming and sintering. Typical processing defects in SiC are agglomerates and pores, which are often related to the forming process [1,2]. Colloidal processing has been realized to be useful for forming ceramic green bodies because of its capability to reduce the strength-limiting defects compared with those from dry powder compaction processing [3–6]. Besides traditional colloidal forming methods [7–9], some new nearnet-shape colloidal methods, such as gelcasting [10,11], direct coagulation casting (DCC) [12], have been introduced into the preparation of green SiC bodies. Gelcasting is based on polymerization of monomers or dimers dissolved in a solvent used as liquid phase to prepare * Corresponding author. Fax: +351-234-425300.
E-mail address: [email protected] (J.M.F. Ferreira). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.04.012
concentrated suspensions of ceramic powders [13]. Upon polymerization, a crosslinked gel network fixes all ceramic particles in their positions, hindering the formation of inhomogeneities during subsequent drying and sintering. Advantages associated with polymerization-based gelcasting techniques include the ability to rapidly produce green bodies that possess high strengths in their wet and dry states, have a wide range of potential shapes and sizes and are readily machinable. However, the use of polymers limits the solids loading of suspension because of the steric barrier built by the polymers between the particles. Gelcasting techniques still present some other disadvantages, such as additive toxicity, extensive drying procedures, and troublesome polymerization reactions. DCC is another useful way to cast components with complex shapes. Its basic principle is to coagulate a stable concentrated suspension using a time-delayed chemical reaction in the suspension, which leads to its destabilization and to a highly homogeneous dense green body. The preparation of highly solids-loaded, electrostatically stabilized suspensions, however, is difficult in many ceramic systems, espe-
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cially in multipowder systems; the drying procedure is still critical; and the strength of the green body is a little low. The reversible destabilization of a sterically stabilized suspension could be used as a novel near-net-shape-forming method called temperature-induced gelation (TIG) [14–16]. Temperature-induced gelation (TIG) originates from the incipient flocculation effect of sterically stabilized suspensions. The magnitude and range of the interactions between polymer layers can be related to the solution properties of the polymer and the conformation of the polymer at the solid–liquid interface [17,18]. Stability is ensured only if the medium is a good solvent for the dispersant [19]; on the contrary, when the solvency decreases to a critical level, the sterically stabilized dispersion incipiently flocculates. The key point of TIG is finding a suitable dispersant, which could achieve a highly solids-loaded suspension and more importantly, the solubility of which changes dramatically in a narrow temperature range near room temperature. The hypermer KD1 could meet the requirements. In this work, hypermer KD1 has been used as a dispersant for preparing a suspension containing SiC powder and oxide additives (Y2 O3 and Al2 O3 ) with solids loading up to 60 vol%. Furthermore, the rheological properties (steady shear and oscillation) of obtained suspensions were studied with changing temperature. The purpose is to develop a novel near-net-shape method for forming SiC green bodies with high homogeneity.
2. Experimental procedure A commercial silicon carbide powder (NF0, Elektroschmelz-Werk-Kempten, GmbH, Germany) was used in this study. Figs. 1 and 2 show the particle size distribution and the morphology of the starting SiC powder respectively. It can be seen that the powder exhibits an obvious bimodal particle size distribution. The sintering additives chosen here were Y2 O3 (Grade C, H.C. Stark, Germany, d50 = 0.75 µm) and Al2 O3 (Alcoa Chemicals, USA, d50 = 0.38 µm). The solvent chosen was an azeotropic mixture of 60 vol% methyl ethyl ketone (MEK) (Reidel–de Haën, Germany) and 40 vol% ethanol (E) (Merck, Germany). The dispersant used was the hypermer KD1 (Imperial Chemical Industries PLC,
Fig. 2. Morphology of the raw SiC powder.
Table 1 Solubility of KD1 in MEK/E solvent at different temperatures Temperature (◦ C)
Solubility (g/100g solvent)
20–25 4 −15
>30 7.3 4.5
England), which is a polyester/polyamine copolymer with an estimated MW of about 10,000 g/mol. It is composed of anchoring groups that absorb onto the particle surface tightly and a polymeric chain with a chemical structure designed to give optimum steric stabilization to the dispersion. The dispersant has proved its efficiency in dispersing a wide range of ceramic powders [20,21]. The solubility of KD1 in MEK/E changed dramatically with temperature, as shown in Table 1 [22]. The suspensions were prepared through planetary ball milling of starting powders, dispersant and solvent, using Al2 O3 jars and Si3 N4 balls, for 4 h. The composition of starting powders was 93 wt% SiC, 4 wt% Al2 O3 and 3 wt% Y2 O3 . The amount of dispersant added was 3 wt%, based on the weight of the starting powders. Steady shear measurements were preformed with a rheometer (C-VOR 150, Bohlin Instruments, UK) under controlled-shear-rate conditions with a core (φ = 40 mm, 4◦ ) and plate geometry. A solvent trap was used to avoid the evaporation of solvent. Viscoelastic measurements were performed using the same rheometer by applying a sinusoidal strain and measuring the stress and the phase shift between the stress and strain. The selected amplitude (0.1%) was sufficiently small to be in the linear viscoelastic range (LVR), where the viscoelastic response is independent of strain.
3. Results and discussions 3.1. Effect of solids loading on the rheological properties
Fig. 1. Particle size distribution of the raw SiC powder.
The use of highly solids-loaded suspensions is a key point in achieving good quality of the ultimate products, espe-
X. Xu et al. / Journal of Colloid and Interface Science 277 (2004) 111–115
Fig. 3. Steady shear viscosity of silicon carbide suspensions with different solids loadings.
cially when consolidated by near-net-shape-forming techniques that do not involve liquid removal. The higher the solids loading (the lower the liquid content in the suspension) the higher the density of the green body (the lower the drying shrinkage) and the higher will be the driving force for densification during the sintering process. However, the suspension with high solids loading should maintain its stability against sedimentation and aggregation of the particles with acceptable viscosity for easy casting. The steady-shear-viscosity curves of suspensions with various solids loadings are shown in Fig. 3. Viscosity always increases with increasing solids loading. Good dispersing properties have been observed, even at high solids loading of 60 vol%, which could be attributed to the chemical structure of the dispersant (as indicated in experimental procedure). The suspensions with 30–55 vol% solids loading exhibit shear-thinning behavior in the low shear-rate (γ˙ ) range, followed by near-Newtonian plateaus. Concentrated colloidal suspensions display shear-thinning behavior because of a perturbation of the suspension structure by shear [23]. At low shear rates, the suspension structure is close to equilibrium because thermal motion dominates over the viscous forces. At higher shear rates, the viscous forces affect the suspension structure, and shear thinning occurs. At very high shear rates, the viscous forces dominate and the plateau in viscosity measures the resistance to flow of a suspension with a complete hydrodynamically controlled structure. Shear thinning might be also an indication of the presence of soft agglomerates, which are expected to break down under shear. At 60 vol% solids similar shear-thinning behavior is still observed, but the Newtonian plateau is too narrow and the rheological behavior of the suspension changes to shearthickening for γ˙ > 100 s−1 . Barnes [24] stated that shear thickening is a typical feature of well-deflocculated suspensions. The onset of shear thickening of the suspension is affected by the solids loading, the particle size and shape, the polydispersity, and the degree of stabilization. Hoffman [25] suggested that shear thickening is a consequence of an order-to-disorder transition. Below the critical shear rate, well-stabilized particles have a tendency to form a layered structure with close packing in the layers but higher sepa-
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Fig. 4. Steady shear viscosity as a function of shear rate for 60 vol% suspensions measured at different testing temperatures.
ration between layers as a result of laminar flow. At high shear rates, the layered arrangement becomes disrupted into a random three-dimensional structure. This structural change requires greater packing space and presents an increase in the apparent viscosity. Although the 60 vol% suspension exhibits shear-thickening behavior at high shear rates, its low viscosity at the low shear rates used in casting operations and the absence of agglomerates make it acceptable for near-net-shape-forming processes. 3.2. Temperature-induced gelation The effect of decreasing temperature on the flow curves of the 60 vol% SiC suspension is shown in Fig. 4. The measurements were performed in the low-shear-rate range because at the lowest testing temperature values (13, 12 ◦ C), high shear rates would completely destroy the gelled suspension. The curves from 20 to 14 ◦ C are smooth and continuous, and the temperature change causes only a small vertical displacement. However, the change of temperature from 14 to 13 ◦ C led to a significant increase of viscosity. Furthermore, the flow curves became less and less smooth as temperature decreased down to 12 ◦ C, suggesting that some discrete structural breakdown events likely occurred under these circumstances. This means that there is a sharp transition in the structure of the suspension from a stable fluid to a volume-filling gel. Oscillatory measurements performed at small deformations in the LVR whenever Hooke’s law applies are more suited to accessing the structural features of the gelling systems, therefore offering a nondestructive method for probing the temperature-induced gelation. At a sufficiently high volume fraction of particles and sufficiently strong interparticle attraction, a space-filling gel will form. The gel transition and the mechanical properties of the particle networks can easily be probed by measuring oscillation properties of suspensions at different temperatures. The elastic modulus (G ) as a function of frequency is shown in Fig. 5 for 60 vol% solids-loaded suspensions at five different temperatures (from 20 to 0 ◦ C). The magnitude of the elastic mod-
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Both properties of the suspensions became more sensitive to temperature with increasing solids loading. The suspensions with solids loading higher than 40 vol% could solidify with decreasing temperature, but the gelation temperature and gelation stiffness decrease with decreasing solids loading. For the lowest-solids-loading suspension (30 vol%), complete gelation is impossible. It seems that there is a solids-loading threshold above which the suspensions can form a continuous-filling network as temperature decreases. Fig. 5. Elastic modulus (G ) as a function of frequency for 60 vol% suspensions measured at different testing temperatures.
(a)
(b) Fig. 6. Evolution of (a) steady shear viscosity (shear rate = 1.37 s−1 ) and (b) elastic modulus (frequency = 1 Hz) on cooling for suspensions with different solids loadings.
ulus, G , strongly increases and becomes less dependent on frequency with decreasing temperature, indicating that the SiC suspension changes from being predominantly viscous at the highest temperatures (20, 15 ◦ C) to displaying strongly elastic behavior at the lowest temperatures (5, 0 ◦ C). The elastic modulus increases more than two orders of magnitude from 15 to 10 ◦ C, which is in good agreement with the results obtained from steady-shear measurements, which revealed a sudden change of flow properties as temperature decreased to about 13 ◦ C. Fig. 6 shows the evolution of steady-shear viscosity (shear rate 1.37 s−1 ) and elastic modulus (frequency 1 Hz) on cooling for suspensions with different solids loadings.
3.3. Gelation mechanism There are two contributions to the total interparticle potential in this study: the attractive van der Waals interactions between the core particles and the steric interactions between the adsorbed polymeric chains. The steric interactions, in turn, can be broken down into osmotic and elastic contributions, among which the latter is always repulsive, while the former is repulsive only in a solvent appropriate to the polymeric chains. The solubility of KD1 in the solvent is appropriate at 20 ◦ C, leading to repulsion forces with enough magnitude to counterbalance the van der Waals forces. Collapse of the adsorbed layer occurs due to the decrease of solubility of KD1 as temperature decreases, where osmotic contributions change sign from repulsive to attractive, and the balance of interparticle forces will change from repulsive to attractive steadily, leading to a kind of gelation among neighboring particles. Separation of KD1 from suspensions with high solids loadings should be another mechanism contributing to the gelation process. For example, less than 40% of added KD1 could be dissolved in the solvent for the 60 vol% suspension at 4 ◦ C. Thus, as the temperature decreases, the free molecules of KD1 will separate from the suspension by rolling up, trapping some amount of solvent, increasing the effective solids loading of the suspension, and promoting the formation of a polymer network that bridges the suspended particles. This mechanism operates better at high solids loadings, near the maximum solids concentration. At low solids loadings, the increment in the effective solids concentration due to the precipitation (by rolling up) of KD1 molecules is not enough to destabilize the suspension. We have studied the temperature-induced gelation of 60 vol% suspensions containing precursor sialon powders [22]; the gelation mechanism was mainly based on the separation of KD1, which caused in situ gelation. However, it seems that the main gelation mechanism of 60 vol% SiC suspensions in this study is based on incipient flocculation due to the collapse of the adsorbed layer, because the gelling temperature is a little bit higher and a more sudden change of viscosity and elastic modulus occurs. This seems reasonable because, as shown in Fig. 1, SiC powder has a higher fraction of small particles, which will increase the importance of steric repulsive (attractive) interactions. Actually, at 20 ◦ C, the 60 vol% SiC suspension has a larger yield stress than the 60 vol% precursor sialon suspension [22]. Thus the
X. Xu et al. / Journal of Colloid and Interface Science 277 (2004) 111–115
lowering of steric repulsive force by decrease of temperature would lead to faster setting of the SiC suspension. Certainly, the continuous increase of the elastic modulus along the temperature range of 10–5 ◦ C could be attributed to separation of KD1, leading to a gelled body with enough strength for demolding and handling without damage and deformation. This result is different from reference [16], where the elastic modulus changes little after flocculation occurs. The rheological properties of the suspensions with lower solids loadings are also related to the balance of the van der Waals attractive force and the repulsive force from adsorbed KD1 layer. So it is easy to understand why rheological properties of the suspensions became more sensitive to temperature changes with the increasing of solids loading. At low solids loadings, the range of van der Waals forces is shorter in comparison with the average distance between suspended particles. Thus, the moderate lower repulsive forces could still counterbalance van der Waals forces. With increasing solids loading, the importance of van der Waals forces also increases due to the shorter distances between suspended particles. Under these conditions, Van der Waals forces will easily dominate the interaction between suspended particles with a small decrease of repulsive force, leading to incipient flocculation. When van der Waals forces dominate the interaction between suspended particles, a large number of flocs appear due to the bridging of suspended particles. Only if the number of particles is high enough, can these flocs connect with each other to form a network. All the solvent was trapped in the network, leading to a solidified gel. If the number of particles is low, flocs remain individual, only resulting in a flocculated suspension with a moderately increased viscosity and elastic modulus. The results also confirm that high solids loading is necessary for the near-net-shape-forming method.
4. Conclusions The rheological properties of concentrated, colloidal stable suspensions of fine silicon carbide powders in a nonaqueous medium have been studied. The results could be summarized as follows:
4. The gelling mechanism is mainly based on incipient flocculation due to the collapse of the adsorbed layer. The separation of KD1 has some contribution to the stiffness of gelled suspension. 5. Based on the results mentioned above, a novel forming method could be developed, which would enable nearnet-shape formation of green SiC bodies with high stiffness and homogeneity. The difference of this method from original TIG is its high stiffness due to the separation of the high-molecular-weight dispersant.
Acknowledgments The authors are very grateful to the Foundation for Science and Technology of Portugal for financial support in the frame of Contract POCTI/CTM/39419/2001, and for the fellowship grant SFRH XXI/BPD/1626/2000 and SFRH XXI/BPD/6648/2001 given to the first two authors.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
1. Using the hypermer KD1 as a dispersant, it is possible to prepare fluid suspensions containing SiC, Y2 O3 , and Al2 O3 powders with solids loadings up to 60 vol%. 2. The 60 vol% suspensions display temperature-induced gelation, changing from a free-flowing fluid to a stiff gel as temperature decreases to 5 ◦ C. 3. The viscosity and elastic modulus of the suspensions become more sensitive to the temperature with increasing solids loading.
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[19] [20] [21] [22] [23] [24] [25]
S.J. Dapkunas, Am. Ceram. Soc. Bull. 65 (1988) 388. Y.S. Takeda, Am. Ceram. Soc. Bull. 67 (1988) 1961. F.F. Lange, J. Am. Ceram. Soc. 72 (1989) 3. W.M. Sigmund, N.S. Bell, L. Bergström, J. Am. Ceram. Soc. 83 (2000) 1557. J.A. Lewis, J. Am. Ceram. Soc. 83 (2000) 2341. C.P. Cameron, R. Raj, J. Am. Ceram. Soc. 73 (1990) 2032. J.M.F. Ferreira, H.M.M. Diz, J. Am. Ceram Soc. 82 (1999) 1993. Y. Hirata, K. Hidaka, H. Matsumura, Y. Fukushige, S. Sameshima, J. Mater. Res. 12 (1997) 3146. M.O. Nieto, R. Moreno, A. Salomoni, Am. Ceram. Soc. Bull. 77 (1998) 62. X. Wang, Z.P. Xie, Y. Huang, Ceram. Int. 28 (2002) 865. E.M.M. Ewais, Y.M.Z. Ahmed, Br. Ceram. Trans. 101 (2002) 255. W.J. Si, T.J. Graule, F.H. Baader, L.J. Gauckler, J. Am. Ceram. Soc. 82 (1999) 1129. A.C. Young, O.O. Omatete, M.A. Janney, P.A. Menchhofer, J. Am. Ceram. Soc. 74 (1991) 612. J.S. Shay, S.R. Raghavan, S.A. Khan, J. Rheol. 45 (2001) 913. L. Bergström, U.S. Patent 534053223, Aug. 1994. L. Bergström, E. Sjöström, J. Eur. Ceram. Soc. 19 (1999) 2117. D.H. Napper, Polymeric Stabilization of Colloidal Dispersions, Academic Press, London, 1982. T.F. Thadros, The Effect of Polymers on Dispersion Properties, Academic Press, London, 1982. X. Xu, M. Oliveira, J.M.F. Ferreira, J. Colloid Interface Sci. 259 (2003) 391. X. Xu, M.I.L.L. Oliveira, R. Fu, J.M.F. Ferreira, J. Eur. Ceram. Soc. 23 (2003) 1525. X. Xu, M.I.L.L. Oliveira, J.M.F. Ferreira, Mater. Sci. Forum 442 (2003) 103. X. Xu, M. Oliveira, J.M.F. Ferreira, J. Am. Ceram. Soc., in press. C.G. de Kruif, Elsevier, Amsterdam, 1990, pp. 79-85. H.A. Barnes, J. Rheol. (NY) 33 (1989) 329. R.L. Hoffman, Trans. Soc. Rheol. 16 (1972) 155.
Temperature-induced gelation of concentrated silicon carbide suspensions X. Xu a,b , S. Mei a , J.M.F. Ferreira a,∗ , T. Nishimura b , N. Hirosaki b a Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b National Institute for Materials Science, Advanced Materials Laboratory, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
Received 19 January 2004; accepted 12 April 2004 Available online 6 May 2004
Abstract Due to the steric barrier provided by the adsorption of the dispersant hypermer KD1 (a polyester/polyamine condensation polymer), stable and low-viscosity suspensions of SiC, Y2 O3 , and Al2 O3 powder mixtures could be prepared in methyl ethyl ketone (MEK)/ethanol (E) solvent with solids loading as high as 60 vol%. The solvency of the dispersant in MEK/E decreased dramatically on cooling. Steady shear viscosity and oscillatory measurements were performed as a function of temperature for suspensions with different solids loading. The viscosity and elastic modulus of suspension increased with decreasing temperature and became more sensitive with the increase of solids loading. The suspensions with solids loading higher than 40 vol% could be solidified with decreasing temperature, but gelation temperature and gelation stiffness decreased with decreasing solids loading. The 60 vol% solid-loaded suspension was a stable and free-flowing fluid at 20 ◦ C and gradually transformed to a very highly viscous and elastic system upon cooling to about 13 ◦ C. Complete solidification occurred when the temperature was decreased to 5 ◦ C. The gelation mechanism was mainly based on the collapse of the adsorbed layer as the temperature decreases, which induced incipient flocculation and formed a stiff network. The gelled body was further strengthened by separation of the dispersant from the suspension. 2004 Elsevier Inc. All rights reserved. Keywords: Temperature-induced gelation; Silicon carbide; Suspension; Rheology; Shape-forming method
1. Introduction Silicon carbide (SiC) is a ceramic material widely used for structural applications at high temperatures. To obtain high-strength, high-reliability SiC components, it is important to minimize the defect size during forming and sintering. Typical processing defects in SiC are agglomerates and pores, which are often related to the forming process [1,2]. Colloidal processing has been realized to be useful for forming ceramic green bodies because of its capability to reduce the strength-limiting defects compared with those from dry powder compaction processing [3–6]. Besides traditional colloidal forming methods [7–9], some new nearnet-shape colloidal methods, such as gelcasting [10,11], direct coagulation casting (DCC) [12], have been introduced into the preparation of green SiC bodies. Gelcasting is based on polymerization of monomers or dimers dissolved in a solvent used as liquid phase to prepare * Corresponding author. Fax: +351-234-425300.
E-mail address: [email protected] (J.M.F. Ferreira). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.04.012
concentrated suspensions of ceramic powders [13]. Upon polymerization, a crosslinked gel network fixes all ceramic particles in their positions, hindering the formation of inhomogeneities during subsequent drying and sintering. Advantages associated with polymerization-based gelcasting techniques include the ability to rapidly produce green bodies that possess high strengths in their wet and dry states, have a wide range of potential shapes and sizes and are readily machinable. However, the use of polymers limits the solids loading of suspension because of the steric barrier built by the polymers between the particles. Gelcasting techniques still present some other disadvantages, such as additive toxicity, extensive drying procedures, and troublesome polymerization reactions. DCC is another useful way to cast components with complex shapes. Its basic principle is to coagulate a stable concentrated suspension using a time-delayed chemical reaction in the suspension, which leads to its destabilization and to a highly homogeneous dense green body. The preparation of highly solids-loaded, electrostatically stabilized suspensions, however, is difficult in many ceramic systems, espe-
112
X. Xu et al. / Journal of Colloid and Interface Science 277 (2004) 111–115
cially in multipowder systems; the drying procedure is still critical; and the strength of the green body is a little low. The reversible destabilization of a sterically stabilized suspension could be used as a novel near-net-shape-forming method called temperature-induced gelation (TIG) [14–16]. Temperature-induced gelation (TIG) originates from the incipient flocculation effect of sterically stabilized suspensions. The magnitude and range of the interactions between polymer layers can be related to the solution properties of the polymer and the conformation of the polymer at the solid–liquid interface [17,18]. Stability is ensured only if the medium is a good solvent for the dispersant [19]; on the contrary, when the solvency decreases to a critical level, the sterically stabilized dispersion incipiently flocculates. The key point of TIG is finding a suitable dispersant, which could achieve a highly solids-loaded suspension and more importantly, the solubility of which changes dramatically in a narrow temperature range near room temperature. The hypermer KD1 could meet the requirements. In this work, hypermer KD1 has been used as a dispersant for preparing a suspension containing SiC powder and oxide additives (Y2 O3 and Al2 O3 ) with solids loading up to 60 vol%. Furthermore, the rheological properties (steady shear and oscillation) of obtained suspensions were studied with changing temperature. The purpose is to develop a novel near-net-shape method for forming SiC green bodies with high homogeneity.
2. Experimental procedure A commercial silicon carbide powder (NF0, Elektroschmelz-Werk-Kempten, GmbH, Germany) was used in this study. Figs. 1 and 2 show the particle size distribution and the morphology of the starting SiC powder respectively. It can be seen that the powder exhibits an obvious bimodal particle size distribution. The sintering additives chosen here were Y2 O3 (Grade C, H.C. Stark, Germany, d50 = 0.75 µm) and Al2 O3 (Alcoa Chemicals, USA, d50 = 0.38 µm). The solvent chosen was an azeotropic mixture of 60 vol% methyl ethyl ketone (MEK) (Reidel–de Haën, Germany) and 40 vol% ethanol (E) (Merck, Germany). The dispersant used was the hypermer KD1 (Imperial Chemical Industries PLC,
Fig. 2. Morphology of the raw SiC powder.
Table 1 Solubility of KD1 in MEK/E solvent at different temperatures Temperature (◦ C)
Solubility (g/100g solvent)
20–25 4 −15
>30 7.3 4.5
England), which is a polyester/polyamine copolymer with an estimated MW of about 10,000 g/mol. It is composed of anchoring groups that absorb onto the particle surface tightly and a polymeric chain with a chemical structure designed to give optimum steric stabilization to the dispersion. The dispersant has proved its efficiency in dispersing a wide range of ceramic powders [20,21]. The solubility of KD1 in MEK/E changed dramatically with temperature, as shown in Table 1 [22]. The suspensions were prepared through planetary ball milling of starting powders, dispersant and solvent, using Al2 O3 jars and Si3 N4 balls, for 4 h. The composition of starting powders was 93 wt% SiC, 4 wt% Al2 O3 and 3 wt% Y2 O3 . The amount of dispersant added was 3 wt%, based on the weight of the starting powders. Steady shear measurements were preformed with a rheometer (C-VOR 150, Bohlin Instruments, UK) under controlled-shear-rate conditions with a core (φ = 40 mm, 4◦ ) and plate geometry. A solvent trap was used to avoid the evaporation of solvent. Viscoelastic measurements were performed using the same rheometer by applying a sinusoidal strain and measuring the stress and the phase shift between the stress and strain. The selected amplitude (0.1%) was sufficiently small to be in the linear viscoelastic range (LVR), where the viscoelastic response is independent of strain.
3. Results and discussions 3.1. Effect of solids loading on the rheological properties
Fig. 1. Particle size distribution of the raw SiC powder.
The use of highly solids-loaded suspensions is a key point in achieving good quality of the ultimate products, espe-
X. Xu et al. / Journal of Colloid and Interface Science 277 (2004) 111–115
Fig. 3. Steady shear viscosity of silicon carbide suspensions with different solids loadings.
cially when consolidated by near-net-shape-forming techniques that do not involve liquid removal. The higher the solids loading (the lower the liquid content in the suspension) the higher the density of the green body (the lower the drying shrinkage) and the higher will be the driving force for densification during the sintering process. However, the suspension with high solids loading should maintain its stability against sedimentation and aggregation of the particles with acceptable viscosity for easy casting. The steady-shear-viscosity curves of suspensions with various solids loadings are shown in Fig. 3. Viscosity always increases with increasing solids loading. Good dispersing properties have been observed, even at high solids loading of 60 vol%, which could be attributed to the chemical structure of the dispersant (as indicated in experimental procedure). The suspensions with 30–55 vol% solids loading exhibit shear-thinning behavior in the low shear-rate (γ˙ ) range, followed by near-Newtonian plateaus. Concentrated colloidal suspensions display shear-thinning behavior because of a perturbation of the suspension structure by shear [23]. At low shear rates, the suspension structure is close to equilibrium because thermal motion dominates over the viscous forces. At higher shear rates, the viscous forces affect the suspension structure, and shear thinning occurs. At very high shear rates, the viscous forces dominate and the plateau in viscosity measures the resistance to flow of a suspension with a complete hydrodynamically controlled structure. Shear thinning might be also an indication of the presence of soft agglomerates, which are expected to break down under shear. At 60 vol% solids similar shear-thinning behavior is still observed, but the Newtonian plateau is too narrow and the rheological behavior of the suspension changes to shearthickening for γ˙ > 100 s−1 . Barnes [24] stated that shear thickening is a typical feature of well-deflocculated suspensions. The onset of shear thickening of the suspension is affected by the solids loading, the particle size and shape, the polydispersity, and the degree of stabilization. Hoffman [25] suggested that shear thickening is a consequence of an order-to-disorder transition. Below the critical shear rate, well-stabilized particles have a tendency to form a layered structure with close packing in the layers but higher sepa-
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Fig. 4. Steady shear viscosity as a function of shear rate for 60 vol% suspensions measured at different testing temperatures.
ration between layers as a result of laminar flow. At high shear rates, the layered arrangement becomes disrupted into a random three-dimensional structure. This structural change requires greater packing space and presents an increase in the apparent viscosity. Although the 60 vol% suspension exhibits shear-thickening behavior at high shear rates, its low viscosity at the low shear rates used in casting operations and the absence of agglomerates make it acceptable for near-net-shape-forming processes. 3.2. Temperature-induced gelation The effect of decreasing temperature on the flow curves of the 60 vol% SiC suspension is shown in Fig. 4. The measurements were performed in the low-shear-rate range because at the lowest testing temperature values (13, 12 ◦ C), high shear rates would completely destroy the gelled suspension. The curves from 20 to 14 ◦ C are smooth and continuous, and the temperature change causes only a small vertical displacement. However, the change of temperature from 14 to 13 ◦ C led to a significant increase of viscosity. Furthermore, the flow curves became less and less smooth as temperature decreased down to 12 ◦ C, suggesting that some discrete structural breakdown events likely occurred under these circumstances. This means that there is a sharp transition in the structure of the suspension from a stable fluid to a volume-filling gel. Oscillatory measurements performed at small deformations in the LVR whenever Hooke’s law applies are more suited to accessing the structural features of the gelling systems, therefore offering a nondestructive method for probing the temperature-induced gelation. At a sufficiently high volume fraction of particles and sufficiently strong interparticle attraction, a space-filling gel will form. The gel transition and the mechanical properties of the particle networks can easily be probed by measuring oscillation properties of suspensions at different temperatures. The elastic modulus (G ) as a function of frequency is shown in Fig. 5 for 60 vol% solids-loaded suspensions at five different temperatures (from 20 to 0 ◦ C). The magnitude of the elastic mod-
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Both properties of the suspensions became more sensitive to temperature with increasing solids loading. The suspensions with solids loading higher than 40 vol% could solidify with decreasing temperature, but the gelation temperature and gelation stiffness decrease with decreasing solids loading. For the lowest-solids-loading suspension (30 vol%), complete gelation is impossible. It seems that there is a solids-loading threshold above which the suspensions can form a continuous-filling network as temperature decreases. Fig. 5. Elastic modulus (G ) as a function of frequency for 60 vol% suspensions measured at different testing temperatures.
(a)
(b) Fig. 6. Evolution of (a) steady shear viscosity (shear rate = 1.37 s−1 ) and (b) elastic modulus (frequency = 1 Hz) on cooling for suspensions with different solids loadings.
ulus, G , strongly increases and becomes less dependent on frequency with decreasing temperature, indicating that the SiC suspension changes from being predominantly viscous at the highest temperatures (20, 15 ◦ C) to displaying strongly elastic behavior at the lowest temperatures (5, 0 ◦ C). The elastic modulus increases more than two orders of magnitude from 15 to 10 ◦ C, which is in good agreement with the results obtained from steady-shear measurements, which revealed a sudden change of flow properties as temperature decreased to about 13 ◦ C. Fig. 6 shows the evolution of steady-shear viscosity (shear rate 1.37 s−1 ) and elastic modulus (frequency 1 Hz) on cooling for suspensions with different solids loadings.
3.3. Gelation mechanism There are two contributions to the total interparticle potential in this study: the attractive van der Waals interactions between the core particles and the steric interactions between the adsorbed polymeric chains. The steric interactions, in turn, can be broken down into osmotic and elastic contributions, among which the latter is always repulsive, while the former is repulsive only in a solvent appropriate to the polymeric chains. The solubility of KD1 in the solvent is appropriate at 20 ◦ C, leading to repulsion forces with enough magnitude to counterbalance the van der Waals forces. Collapse of the adsorbed layer occurs due to the decrease of solubility of KD1 as temperature decreases, where osmotic contributions change sign from repulsive to attractive, and the balance of interparticle forces will change from repulsive to attractive steadily, leading to a kind of gelation among neighboring particles. Separation of KD1 from suspensions with high solids loadings should be another mechanism contributing to the gelation process. For example, less than 40% of added KD1 could be dissolved in the solvent for the 60 vol% suspension at 4 ◦ C. Thus, as the temperature decreases, the free molecules of KD1 will separate from the suspension by rolling up, trapping some amount of solvent, increasing the effective solids loading of the suspension, and promoting the formation of a polymer network that bridges the suspended particles. This mechanism operates better at high solids loadings, near the maximum solids concentration. At low solids loadings, the increment in the effective solids concentration due to the precipitation (by rolling up) of KD1 molecules is not enough to destabilize the suspension. We have studied the temperature-induced gelation of 60 vol% suspensions containing precursor sialon powders [22]; the gelation mechanism was mainly based on the separation of KD1, which caused in situ gelation. However, it seems that the main gelation mechanism of 60 vol% SiC suspensions in this study is based on incipient flocculation due to the collapse of the adsorbed layer, because the gelling temperature is a little bit higher and a more sudden change of viscosity and elastic modulus occurs. This seems reasonable because, as shown in Fig. 1, SiC powder has a higher fraction of small particles, which will increase the importance of steric repulsive (attractive) interactions. Actually, at 20 ◦ C, the 60 vol% SiC suspension has a larger yield stress than the 60 vol% precursor sialon suspension [22]. Thus the
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lowering of steric repulsive force by decrease of temperature would lead to faster setting of the SiC suspension. Certainly, the continuous increase of the elastic modulus along the temperature range of 10–5 ◦ C could be attributed to separation of KD1, leading to a gelled body with enough strength for demolding and handling without damage and deformation. This result is different from reference [16], where the elastic modulus changes little after flocculation occurs. The rheological properties of the suspensions with lower solids loadings are also related to the balance of the van der Waals attractive force and the repulsive force from adsorbed KD1 layer. So it is easy to understand why rheological properties of the suspensions became more sensitive to temperature changes with the increasing of solids loading. At low solids loadings, the range of van der Waals forces is shorter in comparison with the average distance between suspended particles. Thus, the moderate lower repulsive forces could still counterbalance van der Waals forces. With increasing solids loading, the importance of van der Waals forces also increases due to the shorter distances between suspended particles. Under these conditions, Van der Waals forces will easily dominate the interaction between suspended particles with a small decrease of repulsive force, leading to incipient flocculation. When van der Waals forces dominate the interaction between suspended particles, a large number of flocs appear due to the bridging of suspended particles. Only if the number of particles is high enough, can these flocs connect with each other to form a network. All the solvent was trapped in the network, leading to a solidified gel. If the number of particles is low, flocs remain individual, only resulting in a flocculated suspension with a moderately increased viscosity and elastic modulus. The results also confirm that high solids loading is necessary for the near-net-shape-forming method.
4. Conclusions The rheological properties of concentrated, colloidal stable suspensions of fine silicon carbide powders in a nonaqueous medium have been studied. The results could be summarized as follows:
4. The gelling mechanism is mainly based on incipient flocculation due to the collapse of the adsorbed layer. The separation of KD1 has some contribution to the stiffness of gelled suspension. 5. Based on the results mentioned above, a novel forming method could be developed, which would enable nearnet-shape formation of green SiC bodies with high stiffness and homogeneity. The difference of this method from original TIG is its high stiffness due to the separation of the high-molecular-weight dispersant.
Acknowledgments The authors are very grateful to the Foundation for Science and Technology of Portugal for financial support in the frame of Contract POCTI/CTM/39419/2001, and for the fellowship grant SFRH XXI/BPD/1626/2000 and SFRH XXI/BPD/6648/2001 given to the first two authors.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
1. Using the hypermer KD1 as a dispersant, it is possible to prepare fluid suspensions containing SiC, Y2 O3 , and Al2 O3 powders with solids loadings up to 60 vol%. 2. The 60 vol% suspensions display temperature-induced gelation, changing from a free-flowing fluid to a stiff gel as temperature decreases to 5 ◦ C. 3. The viscosity and elastic modulus of the suspensions become more sensitive to the temperature with increasing solids loading.
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