- Email: [email protected]
Influence of temperature on the colloidal processing of electrostatically stabilised alumina suspensions
Journal of Materials Processing Technology 137 (2003) 102–109
Influence of temperature on the colloidal processing of electrostatically stabilised alumina suspensions Giuliano Tarı`a,*, Susana M. Olherob, Jose´ M.F. Ferreirab a
b
CERAM, Queens Road, Stoke-on-Trent ST4 7DS, UK Department of Ceramics and Glass Engineering, UIMC, University of Aveiro, 3810-193 Aveiro, Portugal
Abstract Electrostatically stabilised aqueous alumina suspensions were prepared at various solid loadings. The effects of temperature and solid volume fraction on the rheology and casting rate of the suspensions, and on the drying-shrinkage behaviour, green density and pore size distributions of the consolidated bodies were evaluated. Stress-sweep experiments showed an increase in viscosity, and in the shear- and time-dependence character of the alumina suspensions with increasing temperature, especially at high solid loading. These effects were attributed to a destabilising effect of temperature, derived from a decrease of the surface charge density of the particles and of the dielectric constant of the liquid medium. The flocculating effect of temperature was further confirmed by steady-shear experiments, with an increase of equilibrium viscosity with increasing temperature in the low shear rate range. Concerning the microstructure of slip cast bodies, increasing temperature lead to a decrease in green density (increase in porosity), particularly noticed at high solid loadings. The casting rate was enhanced with increasing temperature as a consequence of the concomitant decrease in water viscosity and the improved permeability of the cake. Overall, good correlation was found among the green microstructure, the rheology of suspensions and the particle interaction forces. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Slip casting; Suspensions; Microstructure-prefiring; Alumina; Rheology
1. Introduction Colloidal techniques in the manufacturing of ceramics provide considerable benefits for the control of the packing uniformity of the consolidated powder form. Compared to powder consolidation in the dry or semi-dry state (e.g. pressing in a die), colloidal methods can lead to better packing homogeneity in the green body, which in turn leads to better microstructure control during firing [1–3]. A basic problem with which this paper is concerned is the stability of colloidal suspensions. A stable colloidal suspension of repulsive interparticle forces may consolidate into a densely packed structure, whereas an unstable one where the attractive interactions between particles dominate may lead to loosely packed structures [4,5]. There exist many factors that can affect the colloidal stability of ceramic powders in water such as the ionic strength [6], the contemporary presence of oxides with different surface charge properties [7,8], the nature and *
Corresponding author. Tel.: þ44-1782-764240; fax: þ44-1782-412331. E-mail address: [email protected] (G. Tarı`).
the amount of surface active agents [9], the solid volume fraction [10,11], and the temperature [12–15]. For electrosterically stabilised alumina suspensions, Uematsu and co-workers [12] observed that the amount of dispersant needed to reach the minimum viscosity increased, while the minimum viscosity achieved at the optimum dispersant concentration decreased, with increasing temperature. The effect was attributed to an increasing amount of dispersant adsorbed onto the surface of particles with increasing temperature. However, changes in the polymer configuration and in the pKa-values of the dissociable groups in the repeating units of the polymer chain, and in other factors related to particle–particle and particle–medium interactions, including the van der Waals forces, with temperature also need to be considered [16]. For instance, the temperature dependence of colloidal stability is one of the main features that distinguish between sterically and electrostatically and/or electrosterically stabilised systems [5] or between enthalpically and entropically stabilised dispersions, characterised by flocculation on heating and on cooling [17]. The influence of temperature on the colloidal stability of electrostatically stabilised alumina suspensions has been also reported in the literature [13–15]. All authors agree
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 9 5 - 6
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
that heating the suspensions would induce the destabilisation of the alumina suspension. Namely, Tarı` et al. [15] have shown that an increase in temperature leads to a decreased degree of ionisation of –OH groups present on the alumina surfaces and to a decrease of the dielectric constant of the liquid medium, both reducing the range of the electrostatic forces. In the industrial production of traditional ceramics such as sanitary ware, porcelain (domestic and electrical), and pottery temperature plays an important role [18]. In these productions, in fact, the environmental conditions of the space in which the conformation of the products occurs (relative humidity and temperature) directly affect the production rates (number of pieces formed per unit time) and the capacity of suction of the plaster moulds. In the case of the slip casting of alumina, very less studies have been carried out aimed to evaluate the effect of temperature. Although there is a general agreement concerning the effect of temperature on colloidal stability, there is still some controversy with respect to its effective role on the green microstructure. The controversial points are mainly related to: (i) the different experimental procedures adopted; and (ii) the use of dispersants to stabilise the alumina powders; which does not allow the establishing of straightforward correlations among the surface properties of the powders and the characteristics of the slips and of the slip cast bodies. In this work, pure electrostatic-stabilised suspensions are used in order to avoid the modification of the surface charge properties of the powder by the dispersant. Therefore, it will be possible to correlate the surface properties of the powder, the rheological behaviour of the slurries, the slip casting performance, and the green microstructure.
2. Experimental procedure 2.1. Materials and slip preparation Aqueous half-micron alumina suspensions (A16 SG, Alcoa Chemicals, USA) with a BET specific surface area of 10 m2/ g, were prepared at various solid loadings from 20 up to 45 vol.%. The suspensions were electrostatically stabilised at pH 4 by adding the required amounts of HCl (AG, Riedel-de Haen, Germany). The alumina powders were first added to the acid solution and stirred for 30 min under controlled pH conditions. De-agglomeration was then completed by ball milling, using alumina balls as the grinding media (f ¼ 15 mm). During the milling step, the pH was checked regularly and HCl was added to maintain the acidity of the medium at pH 4. Next, a de-airing step was performed by gently stirring the slips with a magnetic stirrer under temperature-controlled conditions (ETS D4, IKA, Denmark). 2.2. Rheological characterisation Stress-sweep and steady-shear measurements were carried out with a CS rheometer (Carri-med 500 CSL, UK), to
103
determine the influence of temperature on the viscosity and the shear- and time-dependent flow behaviour of the electrostatically stabilised alumina suspensions. Stress-sweep measurements were performed by increasing and decreasing the shear stress (90 s each branch) in the shear rate range of approx. 0 s1 (the yield point) to 500 s1. Particular attention was paid to ensure that the maximum shear rate attained was the same in all of the experiments. In this way it is possible to compare the time-dependence of the samples by measuring the area enclosed in the hysteresis cycle. Steady-shear measurements were carried out by dividing the shear stress into 10 finite steps and measuring, in each of them, the corresponding equilibrium (or steady state) shearrate value. The maximum equilibrium time was set at 3 min and the measurements were carried out within the shear rate range of approx. 1–500 s1. During both stress sequences, a thermostatic bath ensured the maintenance of the required temperature levels (20, 40 or 60 8C). A double concentric cylinder with a recessed end was used for all of the measurements. The sensor was equipped with a solvent trap to minimise water evaporation. Prior to each experiment, pre-shearing was performed at the highest shear rate for 1 min followed by at rest of 2 min in order to give the same rheological history to all of the suspensions being tested. 2.3. Kinetics of slip casting and green body characterisation Two different sets of casting experiments were carried out to obtain: (i) disk shaped specimens used in the kinetic studies; and (ii) solid bars to study the effect of temperature on the drying-shrinkage behaviour of the green bodies. For the casting rate measurements, the castings were obtained out by pouring the suspensions at different solids loading and temperatures into four plastic rings each set on a small plaster plate. The samples were then placed into an oven set at the same temperature as that of the suspension being used. The moulds were taken off progressively at the end of 4, 9, 16 and 25 min casting times and the excess of slip poured out. After drying, the thickness of the cakes was recorded and the casting rates calculated from the ðthicknessÞ2 ¼ f (time) correlation. For studying the drying-shrinkage behaviour, solid cast bars (length ¼ 110 mm, trapezoidal section with area [15ð15 þ 17Þ=2] mm2) were obtained by slip casting into suitable plaster moulds. The plaster moulds were previously placed into a thermostatic oven set at the same temperature as the slip being used, in order to prevent thermal shocks and/or to ensure the homogeneity of the environmental conditions. De-moulding was performed as soon as the casting was completed. Just after de-moulding, two pieces of approx. 40 mm were cut, weighed and carefully placed in a barelatographe (Adamel Lhomargy, France), a mechanical device that simultaneously measures the variations in weight
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
104
and length of the sample during drying. The natural drying (at room temperature) of the samples was recorded for 1 day and, after that, the drying was completed in an oven at 120 8C for 12 h. The weight and length of the completely dried samples were measured and used as reference for plotting the Bigot curves. The intercept of the linear part of the curves, corresponding to the constant rate period (the critical moisture content, CMC), was determined by linear regression of the data collected in this first stage of drying. The relative densities and pore size distributions of both solid cast bars and cylindrical discs obtained at different casting times, solids loading and temperatures, were measured by the Archimedes method and Hg intrusion porosimetry (PoreSizer 9320, Micromeritics, USA), respectively. In the porosimetry measurements, the high-pressure part of each experiment was carried out in automatic mode with an equilibration time of 10 s at each point. The surface tension and contact angle values adopted were 0.0485 N m1 and 1308, respectively.
3. Results and discussion 3.1. Rheology The influence of particle concentration and temperature on the flow curves of the alumina suspensions is illustrated in Fig. 1. An increase in particle concentration leads to the transition from pseudoplastic to plastic flow behaviour, as usually found in suspensions stabilised with dispersants [19]. The suspensions, with a solid loading equal or higher than 30 vol.%, show a tendency to asymptotic increase in viscosity when the shear stress decreases. At 20 vol.% solids, the suspensions always behave as shear thinning, but at low stresses they tend to attain the first Newtonian plateau [20].
Regarding the influence of temperature, from Fig. 1 it can be seen that the flow behaviour of alumina suspensions is characterised by two regions at low and at high shear stress. This temperature-dependent flow behaviour is due to (i) the increasing tendency of particles towards aggregation with increasing temperature [15], especially at high solid volume fractions; and (ii) the concomitant decrease of water viscosity. In the low shear stress region, surface forces dominate the interactions among particles inducing a flocculating effect that may overcome the effect of reducing the viscosity of the water. This situation is observed at 40 vol.% solids, where the curves appear in a reverse order compared to what happens at 20 vol.%. On the other hand, in the high shear stress region the hydrodynamic forces will predominate over the interactions among particles. In this case, the viscosity decreases slightly with increasing temperature due to the decreased viscosity of water with temperature increase. This aggregation effect is also responsible for the more accentuated time-dependent rheological behaviour of the suspensions as a function of temperature and solid loading, which can be depicted from the area of the hysteresis cycles of the shear stress ¼ f (shear rate) curves. Due to the shearthinning character of the suspensions and according to the accepted definitions, this time-dependence of the suspensions is called thixotropy, even if no reversibility tests were carried out to confirm the effective thixotropic character. From Fig. 2, the area of thixotropy (i.e. the area between the up and down curves) increases with increasing solid loading and temperature. Again, for given solid loading, the variation of thixotropy with to respect temperature was greater higher for most concentrated suspensions. To more correctly characterise the rheological behaviour of such time-dependent alumina suspensions, equilibrium flow measurements were then carried out (Fig. 3). In these measurements, for each shear stress applied, a steady-shear rate value was obtained. Fig. 3 confirms that all suspensions
Fig. 1. Flow curves of pure electrostatic-stabilised alumina suspensions at 20, 40 and 60 8C, as a function of the solid loading.
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
105
Fig. 2. Relative viscosity (i.e. plastic viscosity/viscosity of water adjusted to temperature) as a function of the solid loading of pure electrostatic-stabilised alumina suspensions at 20, 40 and 60 8C.
behave as shear thinning and that for a given solid loading, such shear dependence becomes much more pronounced when the temperature increases. The shear-thinning behaviour is usually associated with the slurry structure. At low shear rates, liquid is immobilised in void spaces within flocs, and the floc network with the surface forces dominating the particulate system. As the shear rate increases, the flocs and floc network break down, the entrapped liquid is released and a more ordered structure in the flow direction is formed. Therefore, the more pronounced shear-thinning character at high temperature can be attributed to a change in state of dispersion towards flocculation, confirming the destabilising effect of temperature. The agitation action can gradually break down this structure, and thus the viscosity of the suspension decreases. At high shear rates, the importance of the hydrodynamic interactions
between particles increases. Under these high shear-rate conditions, the equilibrium viscosity tends to reduce with increasing temperature. This means that the higher frequency of particle collisions and the increased probability for them to overcome the energy barrier against aggregation, which increase with increasing temperature, become less relevant than the decreased viscosity of water. Accordingly, in both stress-sweep and steady-shear experiments, the cross-over shear rate tends to increase when increasing the solid loading, because of the decreased amounts of water present. 3.2. Slip casting Table 1 summarises the slip cast density data for the bodies prepared at different solid volume fractions and
Fig. 3. Equilibrium viscosity curves of pure electrostatic-stabilised alumina suspensions at 20, 40 and 60 8C, as a function of the solid loading.
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
106
Table 1 Density of the slip cast bodies for different solid loading and temperatures 20 vol.%
TD (%)
30 vol.%
40 vol.%
45 vol.%
20 8C
40 8C
60 8C
20 8C
40 8C
60 8C
20 8C
20 8C
40 8C
60 8C
58.8
59.0
59.5
60.1
59.8
59.3
61.3
61.8
60.3
59.0
temperatures. A continuous increase in green density when increasing solids loading can be noticed, whereas an inverse trend was observed with respect to temperature, i.e. the green density decreases with increasing temperature. The poorer packing performance with increasing temperature can be attributed to the flocculating trend of the suspensions under these conditions, in agreement with the previous rheological measurements. In addition to the green density, temperature also has an important role in the slip casting kinetics. For instance, Fig. 4 shows the variations in casting rate with temperature and solids loading. As expected, the casting rate increases with solid loading since a greater proportion of solid material in the slip leads to a higher deposited amount at the surface of the porous mould for the same amount of water absorbed. According to the findings of Ruys and Sorrel [21], an approx. linear dependence of the casting rate with solids loading was found in the range from 20 to 40 vol.%, followed by a deviation from linearity above 40 vol.%. Ruys and Sorrel [21] suggest that two mechanisms were involved in the slip casting kinetics. At low solid loading, a simple dewatering process promoted by the casting mould occurs, explaining the observed linear correlation between the solid loading and the casting time. However, at high solid loading, the trend for particle structuration and the concomitant accentuation of the thixotropic character of the suspensions leads to a higher deposition rates than predicted from the
linear relationship observed at lower solids contents. Fig. 4 also shows that the casting rate increases with increasing temperature. This improvement in the kinetics of the slip casting process can be attributed to two complementary effects: (i) the decrease in the viscosity of water; and (ii) the increase in the permeability of the cake as a direct consequence of the more open structure resulting from the temperature-induced flocculating trend. This latter effect is particularly visible in Fig. 5 which reports the values of porosity and mean pore diameter as a function of casting time for cakes obtained from suspensions at 40 vol.% solids and different temperatures. All of the curves run almost parallel over the casting period: similar results were obtained at 20 and 30 vol.% solid loading. It can be seen that the fraction of porosity and the mean pore diameter of the casting increase with temperature increase for all casting times. Furthermore, these microstructural characteristics also tend to increase with the casting duration, except for the shorter casting times where a diminution is suggested. Some porosimetry experiments on the same sample were repeated three to four times to test its reproducibility, confirming the trend shown in Fig. 5. The results presented in Figs. 4 and 5 can be explained assuming that the kinetics of the casting process are as described by the Darcy’s differential equation for fluid flow through porous media [22]. When integrated with appropriate boundary conditions, this equation establishes that the
Fig. 4. Casting rate of pure electrostatic-stabilised alumina suspensions at 20, 40 and 60 8C, as a function of the solid loading.
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
107
Fig. 5. Porosity (*) and mean pore size (*) of the slip cast bodies, obtained from 40 vol.% electrostatic-stabilised alumina suspension, at 20, 40 and 60 8C as a function of casting time.
thickness of the consolidated layer, L, formed at a given driving force, P, is related parabolically with the elapsed time, t, by the equation: L2 ¼ 2
KfPt Zðfc fÞ
(1)
where Z is the liquid viscosity, and f and fc the solid volume fractions within the slurry and cake, respectively. The permeability, K, is inversely related to the resistance to fluid flow through the consolidated layer. This equation assumes unidirectional filtration, negligible mould resistance, constant rheological properties of the suspension, the absence of particle settling and constant permeability, i.e. the body is incompressible. It has been shown already that the plaster moulds used have permeability values which are 4–5 orders of magnitude higher than the permeability of the cakes formed from well-dispersed slurries [23]. In practice, the cakes are generally compressible, depending on the level of the driving force for the deposition [24]; the interaction forces between the particles [25–27] and particle size distributions [23]. As a consequence, some moisture and/or density gradient and, hence, permeability gradient would be expected, depending on the experimental conditions used. An increase in cake density and a consequent decrease in cake permeability with casting time has already been reported for pressure slip cast SiC bodies [28]. The less permeable microstructures were attributed to an improved particle rearrangement enabled by the slowing down of the deposition process as the cake thickness increases. In these experiments the maximum casting time used was 9 min, while the applied pressure varied from about 0.5 to 5 MPa, i.e. driving forces that are about 3–30 times greater than that used in the present work [23].
From Eq. (1), permeability, K, can be expressed by ZL2 fc f K¼ (2) P 2ft and is sensitive to the ratio ðfc fÞ=P. The pressure drop over the cake thickness increases with casting time and the available P decreases. For given values of Z and f, this means that the evolution of K will depend on the variation in cake density. When very stable (against agglomeration) slurries are use, diminution of the cake permeability along the casting time should be expected, provided that the necessary driving force for the deposition process is available, since the particles have more time to search for the most favourable positions for close packing. However, when there is a trend towards particle structuration within the suspension and the available driving force is limited, as in the present work, the evolution of cake porosity (permeability) would result from these two opposing tendencies. This explains the shape of the curves plotted in Fig. 5, which exhibit an initial decreasing trend up to 9 min casting time, followed by an increasing trend. Fig. 6 shows the results of the drying-shrinkage behaviour obtained at 40 vol.% of solid loading at 20, 40 and 60 8C. Similar results were obtained for other solid loadings tested. It can be seen that all of the curves are practically superimposed, suggesting at first glance that temperature does not influence the CMC and thereby, the green microstructure. These results are supported by the observed small differences in green density for given solid loading. Moreover, the linear shrinkage decreases with increasing temperature. This evolution does not reflect differences in the green microstructure and can be attributed to the higher loss of moisture experienced by the cast bodies obtained at higher temperatures, that occurs along
108
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
Fig. 6. Bigot’s curves of pure electrostatic-stabilised alumina suspensions at 40 vol.%, as a function of temperature.
Fig. 7. CMC of pure electrostatic-stabilised alumina suspensions at different solids loading, as a function of temperature.
the de-moulding operation in the oven and the set up of the drying test. The evolution of CMC as a function of temperature for the different solid loadings used is shown in Fig. 7. It can be seen that the CMC increases with decreasing solid loading, in good agreement with the results for relative density and porosimetry. Therefore, Bigot’s curves still remains an useful and non-destructive technique for the characterisation of the microstructure slip cast bodies [29].
4. Summary and conclusions The results presented in this work show that an increase in temperature of electrostatically stabilised alumina suspensions
induces a flocculation trend that is enhanced with increasing solid loading. In the rheological behaviour, this trend towards particle agglomeration is expressed by an increase in viscosity in the low shear rate range and a more accentuated thixotropic character of the suspensions. The flocculation effect of temperature was also confirmed in the slip casting experiments by a decrease of green density and a concomitant increase in cake porosity, especially for longer casting times where the pressure drop along the cake thickness becomes significant. Particle agglomeration and the slowing down of the depositions rate favouring particle rearrangements are opposing tendencies that can determine an initial decrease in cake permeability (increase in density) followed by an inverse trend for longer casting times.
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
Acknowledgements The authors acknowledge FCT (Portuguese Foundation for Science and Technology) for the financial support.
[13]
[14]
References [1] F.F. Lange, Powder processing science and technology for increased reliability, J. Am. Ceram. Soc. 72 (1989) 3–15. [2] D.-M. Liu, Influence of pore structure in green compacts on densification of SiC–Al2O3–Y2O3, Ceram. Int. 22 (1996) 403–406. [3] Y. Juan, F.J. Oliveira, R.F. Silva, J.M.F. Ferreira, Pressureless sinterability of slip cast silicon nitride bodies prepared from coprecipitation-coated powders, J. Eur. Ceram. Soc. 19 (1999) 433–439. [4] B.V. Velamakanni, F.F. Lange, Effect of interparticle potentials and sedimentation on particle packing density of bimodal particle size distributions during pressure filtration, J. Am. Ceram. Soc. 74 (1991) 166–172. [5] R.J. Pugh, Dispersion and stability of powders in liquids, in: R.J. Pugh, L. Bergstrom (Eds.), Surface and Colloid Chemistry in Advanced Ceramic Processing, Marcel Dekker, New York, 1994, p. 170. [6] J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, San Diego, CA, 1991. [7] G. Tarı`, J.M.F. Ferreira, O. Lyckfeldt, Influence of magnesia on colloidal processing of alumina, J. Eur. Ceram. Soc. 17 (1997) 1341– 1350. [8] E. Liden, M. Persson, E. Carlstrom, R. Carlsson, Electrostatic adsorption of a colloidal sintering agent on silicon nitrate particles, J. Am. Ceram. Soc. 74 (1991) 1335–1339. [9] D.J. Shanefield, Organic Additives and Ceramic Processing, 2nd ed., Kluwer Academic Publishers, Dordrecht, 1996. [10] G. Tarı`, J.M.F. Ferreira, O. Lyckfeldt, Influence of the stabilising mechanism and solid loading on slip casting of alumina, J. Eur. Ceram. Soc. 18 (1998) 479–486. [11] J.M.F. Ferreira, H.M.M. Diz, Effect of solid loading on slip casting performance of silicon carbide slurries, J. Am. Ceram. Soc., in press. [12] T. Tomita, L. Guo, Y. Zhang, N. Uchida, K. Uematsu, Effect of temperature on characteristics of alumina slurry, in: Science,
[15]
[16] [17] [18] [19]
[20] [21] [22] [23]
[24] [25] [26]
[27] [28] [29]
109
Technology and Applications of Colloidal Suspensions, Ceramic Transaction, vol. 54, American Ceramic Society, 1995, pp. 103–108. J.G.P. Binner, A.M. Murfin, The effect of temperature, heating method and state of dispersion on the vacuum filter casting of alumina suspensions, J. Eur. Ceram. Soc. 18 (1998) 791–798. A. Vernon, J. Binner, Microwave enhanced slip casting of advanced ceramics, Ind. Ceram. 16 (1996) 27–30. G. Tarı`, S.M. Olhero, J.M.F. Ferreira, Influence of temperature on stability of electrostatically stabilised alumina suspensions, J. Coll. Interf. Sci., submitted for publication. J.A. Yanez, E. Laarz, L. Bergstro¨ m, Viscoelastic properties of particle gels, J. Colloid Interf. Sci. 209 (1999) 162–172. D.H. Napper, Polymeric Stabilisation of Colloidal Dispersions, Academic Press, 1983, Chapter 3. G. Aliprandi, Principi di Ceramugia e Tecnologia Ceramica, E.c.i.g., Geneva, 1975. A. Zupancic, L. Lapasin, A. Kristoffersson, Influence of particle concentration on rheological properties of aqueous a-Al2O3 suspensions, J. Eur. Ceram. Soc. 18 (1998) 467–477. H.A. Barnes, J.F. Hutton, K. Walters, An Introduction to Rheology, Elsevier, Amsterdam, 1989. A.J. Ruys, C.C. Sorrel, Slip casting of alumina, Am. Ceram. Soc. Bull. 75 (1996) 66–69. F.M. Tiller, C.D. Tsai, Theory of filtration of ceramics. I. Slip casting, J. Am. Ceram Soc. 69 (1986) 882–887. J.M.F. Ferreira, An interface SiC-Soluc¸a˜ o Aquosa e o Enchimento por Barbotina, Ph.D. Thesis, University of Aveiro, Aveiro, Portugal, 1992. L. Frassek, H.W. Hennicke, Some aspects of pressure slip casting with non clay suspensions, Ceram. Forum Int. 67 (1990) 443–446. T.J. Fennelly, J.S. Reed, Mechanics of pressure slip casting, J. Am. Ceram. Soc. 55 (1972) 264–268. T.J. Fennelly, J.S. Reed, Compression permeability of Al2O3 cakes forming by pressure slip casting, J. Am. Ceram. Soc. 55 (1972) 381– 383. F.F. Lange, K.T. Miller, Pressure filtration: consolidation kinetics and mechanics, Am. Ceram. Soc. Bull. 66 (1987) 1498–1504. J.M.F. Ferreira, Influence of pH on the pressure slip casting of silicon carbide bodies, J. Eur. Ceram. Soc. 17 (1997) 259–266. G. Tarı`, J.M.F. Ferreira, Influence of solid loading on dryingshrinkage behaviour of slip cast bodies, J. Eur. Ceram. Soc. 18 (1998) 487–493.
Influence of temperature on the colloidal processing of electrostatically stabilised alumina suspensions Giuliano Tarı`a,*, Susana M. Olherob, Jose´ M.F. Ferreirab a
b
CERAM, Queens Road, Stoke-on-Trent ST4 7DS, UK Department of Ceramics and Glass Engineering, UIMC, University of Aveiro, 3810-193 Aveiro, Portugal
Abstract Electrostatically stabilised aqueous alumina suspensions were prepared at various solid loadings. The effects of temperature and solid volume fraction on the rheology and casting rate of the suspensions, and on the drying-shrinkage behaviour, green density and pore size distributions of the consolidated bodies were evaluated. Stress-sweep experiments showed an increase in viscosity, and in the shear- and time-dependence character of the alumina suspensions with increasing temperature, especially at high solid loading. These effects were attributed to a destabilising effect of temperature, derived from a decrease of the surface charge density of the particles and of the dielectric constant of the liquid medium. The flocculating effect of temperature was further confirmed by steady-shear experiments, with an increase of equilibrium viscosity with increasing temperature in the low shear rate range. Concerning the microstructure of slip cast bodies, increasing temperature lead to a decrease in green density (increase in porosity), particularly noticed at high solid loadings. The casting rate was enhanced with increasing temperature as a consequence of the concomitant decrease in water viscosity and the improved permeability of the cake. Overall, good correlation was found among the green microstructure, the rheology of suspensions and the particle interaction forces. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Slip casting; Suspensions; Microstructure-prefiring; Alumina; Rheology
1. Introduction Colloidal techniques in the manufacturing of ceramics provide considerable benefits for the control of the packing uniformity of the consolidated powder form. Compared to powder consolidation in the dry or semi-dry state (e.g. pressing in a die), colloidal methods can lead to better packing homogeneity in the green body, which in turn leads to better microstructure control during firing [1–3]. A basic problem with which this paper is concerned is the stability of colloidal suspensions. A stable colloidal suspension of repulsive interparticle forces may consolidate into a densely packed structure, whereas an unstable one where the attractive interactions between particles dominate may lead to loosely packed structures [4,5]. There exist many factors that can affect the colloidal stability of ceramic powders in water such as the ionic strength [6], the contemporary presence of oxides with different surface charge properties [7,8], the nature and *
Corresponding author. Tel.: þ44-1782-764240; fax: þ44-1782-412331. E-mail address: [email protected] (G. Tarı`).
the amount of surface active agents [9], the solid volume fraction [10,11], and the temperature [12–15]. For electrosterically stabilised alumina suspensions, Uematsu and co-workers [12] observed that the amount of dispersant needed to reach the minimum viscosity increased, while the minimum viscosity achieved at the optimum dispersant concentration decreased, with increasing temperature. The effect was attributed to an increasing amount of dispersant adsorbed onto the surface of particles with increasing temperature. However, changes in the polymer configuration and in the pKa-values of the dissociable groups in the repeating units of the polymer chain, and in other factors related to particle–particle and particle–medium interactions, including the van der Waals forces, with temperature also need to be considered [16]. For instance, the temperature dependence of colloidal stability is one of the main features that distinguish between sterically and electrostatically and/or electrosterically stabilised systems [5] or between enthalpically and entropically stabilised dispersions, characterised by flocculation on heating and on cooling [17]. The influence of temperature on the colloidal stability of electrostatically stabilised alumina suspensions has been also reported in the literature [13–15]. All authors agree
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 9 5 - 6
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
that heating the suspensions would induce the destabilisation of the alumina suspension. Namely, Tarı` et al. [15] have shown that an increase in temperature leads to a decreased degree of ionisation of –OH groups present on the alumina surfaces and to a decrease of the dielectric constant of the liquid medium, both reducing the range of the electrostatic forces. In the industrial production of traditional ceramics such as sanitary ware, porcelain (domestic and electrical), and pottery temperature plays an important role [18]. In these productions, in fact, the environmental conditions of the space in which the conformation of the products occurs (relative humidity and temperature) directly affect the production rates (number of pieces formed per unit time) and the capacity of suction of the plaster moulds. In the case of the slip casting of alumina, very less studies have been carried out aimed to evaluate the effect of temperature. Although there is a general agreement concerning the effect of temperature on colloidal stability, there is still some controversy with respect to its effective role on the green microstructure. The controversial points are mainly related to: (i) the different experimental procedures adopted; and (ii) the use of dispersants to stabilise the alumina powders; which does not allow the establishing of straightforward correlations among the surface properties of the powders and the characteristics of the slips and of the slip cast bodies. In this work, pure electrostatic-stabilised suspensions are used in order to avoid the modification of the surface charge properties of the powder by the dispersant. Therefore, it will be possible to correlate the surface properties of the powder, the rheological behaviour of the slurries, the slip casting performance, and the green microstructure.
2. Experimental procedure 2.1. Materials and slip preparation Aqueous half-micron alumina suspensions (A16 SG, Alcoa Chemicals, USA) with a BET specific surface area of 10 m2/ g, were prepared at various solid loadings from 20 up to 45 vol.%. The suspensions were electrostatically stabilised at pH 4 by adding the required amounts of HCl (AG, Riedel-de Haen, Germany). The alumina powders were first added to the acid solution and stirred for 30 min under controlled pH conditions. De-agglomeration was then completed by ball milling, using alumina balls as the grinding media (f ¼ 15 mm). During the milling step, the pH was checked regularly and HCl was added to maintain the acidity of the medium at pH 4. Next, a de-airing step was performed by gently stirring the slips with a magnetic stirrer under temperature-controlled conditions (ETS D4, IKA, Denmark). 2.2. Rheological characterisation Stress-sweep and steady-shear measurements were carried out with a CS rheometer (Carri-med 500 CSL, UK), to
103
determine the influence of temperature on the viscosity and the shear- and time-dependent flow behaviour of the electrostatically stabilised alumina suspensions. Stress-sweep measurements were performed by increasing and decreasing the shear stress (90 s each branch) in the shear rate range of approx. 0 s1 (the yield point) to 500 s1. Particular attention was paid to ensure that the maximum shear rate attained was the same in all of the experiments. In this way it is possible to compare the time-dependence of the samples by measuring the area enclosed in the hysteresis cycle. Steady-shear measurements were carried out by dividing the shear stress into 10 finite steps and measuring, in each of them, the corresponding equilibrium (or steady state) shearrate value. The maximum equilibrium time was set at 3 min and the measurements were carried out within the shear rate range of approx. 1–500 s1. During both stress sequences, a thermostatic bath ensured the maintenance of the required temperature levels (20, 40 or 60 8C). A double concentric cylinder with a recessed end was used for all of the measurements. The sensor was equipped with a solvent trap to minimise water evaporation. Prior to each experiment, pre-shearing was performed at the highest shear rate for 1 min followed by at rest of 2 min in order to give the same rheological history to all of the suspensions being tested. 2.3. Kinetics of slip casting and green body characterisation Two different sets of casting experiments were carried out to obtain: (i) disk shaped specimens used in the kinetic studies; and (ii) solid bars to study the effect of temperature on the drying-shrinkage behaviour of the green bodies. For the casting rate measurements, the castings were obtained out by pouring the suspensions at different solids loading and temperatures into four plastic rings each set on a small plaster plate. The samples were then placed into an oven set at the same temperature as that of the suspension being used. The moulds were taken off progressively at the end of 4, 9, 16 and 25 min casting times and the excess of slip poured out. After drying, the thickness of the cakes was recorded and the casting rates calculated from the ðthicknessÞ2 ¼ f (time) correlation. For studying the drying-shrinkage behaviour, solid cast bars (length ¼ 110 mm, trapezoidal section with area [15ð15 þ 17Þ=2] mm2) were obtained by slip casting into suitable plaster moulds. The plaster moulds were previously placed into a thermostatic oven set at the same temperature as the slip being used, in order to prevent thermal shocks and/or to ensure the homogeneity of the environmental conditions. De-moulding was performed as soon as the casting was completed. Just after de-moulding, two pieces of approx. 40 mm were cut, weighed and carefully placed in a barelatographe (Adamel Lhomargy, France), a mechanical device that simultaneously measures the variations in weight
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
104
and length of the sample during drying. The natural drying (at room temperature) of the samples was recorded for 1 day and, after that, the drying was completed in an oven at 120 8C for 12 h. The weight and length of the completely dried samples were measured and used as reference for plotting the Bigot curves. The intercept of the linear part of the curves, corresponding to the constant rate period (the critical moisture content, CMC), was determined by linear regression of the data collected in this first stage of drying. The relative densities and pore size distributions of both solid cast bars and cylindrical discs obtained at different casting times, solids loading and temperatures, were measured by the Archimedes method and Hg intrusion porosimetry (PoreSizer 9320, Micromeritics, USA), respectively. In the porosimetry measurements, the high-pressure part of each experiment was carried out in automatic mode with an equilibration time of 10 s at each point. The surface tension and contact angle values adopted were 0.0485 N m1 and 1308, respectively.
3. Results and discussion 3.1. Rheology The influence of particle concentration and temperature on the flow curves of the alumina suspensions is illustrated in Fig. 1. An increase in particle concentration leads to the transition from pseudoplastic to plastic flow behaviour, as usually found in suspensions stabilised with dispersants [19]. The suspensions, with a solid loading equal or higher than 30 vol.%, show a tendency to asymptotic increase in viscosity when the shear stress decreases. At 20 vol.% solids, the suspensions always behave as shear thinning, but at low stresses they tend to attain the first Newtonian plateau [20].
Regarding the influence of temperature, from Fig. 1 it can be seen that the flow behaviour of alumina suspensions is characterised by two regions at low and at high shear stress. This temperature-dependent flow behaviour is due to (i) the increasing tendency of particles towards aggregation with increasing temperature [15], especially at high solid volume fractions; and (ii) the concomitant decrease of water viscosity. In the low shear stress region, surface forces dominate the interactions among particles inducing a flocculating effect that may overcome the effect of reducing the viscosity of the water. This situation is observed at 40 vol.% solids, where the curves appear in a reverse order compared to what happens at 20 vol.%. On the other hand, in the high shear stress region the hydrodynamic forces will predominate over the interactions among particles. In this case, the viscosity decreases slightly with increasing temperature due to the decreased viscosity of water with temperature increase. This aggregation effect is also responsible for the more accentuated time-dependent rheological behaviour of the suspensions as a function of temperature and solid loading, which can be depicted from the area of the hysteresis cycles of the shear stress ¼ f (shear rate) curves. Due to the shearthinning character of the suspensions and according to the accepted definitions, this time-dependence of the suspensions is called thixotropy, even if no reversibility tests were carried out to confirm the effective thixotropic character. From Fig. 2, the area of thixotropy (i.e. the area between the up and down curves) increases with increasing solid loading and temperature. Again, for given solid loading, the variation of thixotropy with to respect temperature was greater higher for most concentrated suspensions. To more correctly characterise the rheological behaviour of such time-dependent alumina suspensions, equilibrium flow measurements were then carried out (Fig. 3). In these measurements, for each shear stress applied, a steady-shear rate value was obtained. Fig. 3 confirms that all suspensions
Fig. 1. Flow curves of pure electrostatic-stabilised alumina suspensions at 20, 40 and 60 8C, as a function of the solid loading.
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
105
Fig. 2. Relative viscosity (i.e. plastic viscosity/viscosity of water adjusted to temperature) as a function of the solid loading of pure electrostatic-stabilised alumina suspensions at 20, 40 and 60 8C.
behave as shear thinning and that for a given solid loading, such shear dependence becomes much more pronounced when the temperature increases. The shear-thinning behaviour is usually associated with the slurry structure. At low shear rates, liquid is immobilised in void spaces within flocs, and the floc network with the surface forces dominating the particulate system. As the shear rate increases, the flocs and floc network break down, the entrapped liquid is released and a more ordered structure in the flow direction is formed. Therefore, the more pronounced shear-thinning character at high temperature can be attributed to a change in state of dispersion towards flocculation, confirming the destabilising effect of temperature. The agitation action can gradually break down this structure, and thus the viscosity of the suspension decreases. At high shear rates, the importance of the hydrodynamic interactions
between particles increases. Under these high shear-rate conditions, the equilibrium viscosity tends to reduce with increasing temperature. This means that the higher frequency of particle collisions and the increased probability for them to overcome the energy barrier against aggregation, which increase with increasing temperature, become less relevant than the decreased viscosity of water. Accordingly, in both stress-sweep and steady-shear experiments, the cross-over shear rate tends to increase when increasing the solid loading, because of the decreased amounts of water present. 3.2. Slip casting Table 1 summarises the slip cast density data for the bodies prepared at different solid volume fractions and
Fig. 3. Equilibrium viscosity curves of pure electrostatic-stabilised alumina suspensions at 20, 40 and 60 8C, as a function of the solid loading.
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
106
Table 1 Density of the slip cast bodies for different solid loading and temperatures 20 vol.%
TD (%)
30 vol.%
40 vol.%
45 vol.%
20 8C
40 8C
60 8C
20 8C
40 8C
60 8C
20 8C
20 8C
40 8C
60 8C
58.8
59.0
59.5
60.1
59.8
59.3
61.3
61.8
60.3
59.0
temperatures. A continuous increase in green density when increasing solids loading can be noticed, whereas an inverse trend was observed with respect to temperature, i.e. the green density decreases with increasing temperature. The poorer packing performance with increasing temperature can be attributed to the flocculating trend of the suspensions under these conditions, in agreement with the previous rheological measurements. In addition to the green density, temperature also has an important role in the slip casting kinetics. For instance, Fig. 4 shows the variations in casting rate with temperature and solids loading. As expected, the casting rate increases with solid loading since a greater proportion of solid material in the slip leads to a higher deposited amount at the surface of the porous mould for the same amount of water absorbed. According to the findings of Ruys and Sorrel [21], an approx. linear dependence of the casting rate with solids loading was found in the range from 20 to 40 vol.%, followed by a deviation from linearity above 40 vol.%. Ruys and Sorrel [21] suggest that two mechanisms were involved in the slip casting kinetics. At low solid loading, a simple dewatering process promoted by the casting mould occurs, explaining the observed linear correlation between the solid loading and the casting time. However, at high solid loading, the trend for particle structuration and the concomitant accentuation of the thixotropic character of the suspensions leads to a higher deposition rates than predicted from the
linear relationship observed at lower solids contents. Fig. 4 also shows that the casting rate increases with increasing temperature. This improvement in the kinetics of the slip casting process can be attributed to two complementary effects: (i) the decrease in the viscosity of water; and (ii) the increase in the permeability of the cake as a direct consequence of the more open structure resulting from the temperature-induced flocculating trend. This latter effect is particularly visible in Fig. 5 which reports the values of porosity and mean pore diameter as a function of casting time for cakes obtained from suspensions at 40 vol.% solids and different temperatures. All of the curves run almost parallel over the casting period: similar results were obtained at 20 and 30 vol.% solid loading. It can be seen that the fraction of porosity and the mean pore diameter of the casting increase with temperature increase for all casting times. Furthermore, these microstructural characteristics also tend to increase with the casting duration, except for the shorter casting times where a diminution is suggested. Some porosimetry experiments on the same sample were repeated three to four times to test its reproducibility, confirming the trend shown in Fig. 5. The results presented in Figs. 4 and 5 can be explained assuming that the kinetics of the casting process are as described by the Darcy’s differential equation for fluid flow through porous media [22]. When integrated with appropriate boundary conditions, this equation establishes that the
Fig. 4. Casting rate of pure electrostatic-stabilised alumina suspensions at 20, 40 and 60 8C, as a function of the solid loading.
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
107
Fig. 5. Porosity (*) and mean pore size (*) of the slip cast bodies, obtained from 40 vol.% electrostatic-stabilised alumina suspension, at 20, 40 and 60 8C as a function of casting time.
thickness of the consolidated layer, L, formed at a given driving force, P, is related parabolically with the elapsed time, t, by the equation: L2 ¼ 2
KfPt Zðfc fÞ
(1)
where Z is the liquid viscosity, and f and fc the solid volume fractions within the slurry and cake, respectively. The permeability, K, is inversely related to the resistance to fluid flow through the consolidated layer. This equation assumes unidirectional filtration, negligible mould resistance, constant rheological properties of the suspension, the absence of particle settling and constant permeability, i.e. the body is incompressible. It has been shown already that the plaster moulds used have permeability values which are 4–5 orders of magnitude higher than the permeability of the cakes formed from well-dispersed slurries [23]. In practice, the cakes are generally compressible, depending on the level of the driving force for the deposition [24]; the interaction forces between the particles [25–27] and particle size distributions [23]. As a consequence, some moisture and/or density gradient and, hence, permeability gradient would be expected, depending on the experimental conditions used. An increase in cake density and a consequent decrease in cake permeability with casting time has already been reported for pressure slip cast SiC bodies [28]. The less permeable microstructures were attributed to an improved particle rearrangement enabled by the slowing down of the deposition process as the cake thickness increases. In these experiments the maximum casting time used was 9 min, while the applied pressure varied from about 0.5 to 5 MPa, i.e. driving forces that are about 3–30 times greater than that used in the present work [23].
From Eq. (1), permeability, K, can be expressed by ZL2 fc f K¼ (2) P 2ft and is sensitive to the ratio ðfc fÞ=P. The pressure drop over the cake thickness increases with casting time and the available P decreases. For given values of Z and f, this means that the evolution of K will depend on the variation in cake density. When very stable (against agglomeration) slurries are use, diminution of the cake permeability along the casting time should be expected, provided that the necessary driving force for the deposition process is available, since the particles have more time to search for the most favourable positions for close packing. However, when there is a trend towards particle structuration within the suspension and the available driving force is limited, as in the present work, the evolution of cake porosity (permeability) would result from these two opposing tendencies. This explains the shape of the curves plotted in Fig. 5, which exhibit an initial decreasing trend up to 9 min casting time, followed by an increasing trend. Fig. 6 shows the results of the drying-shrinkage behaviour obtained at 40 vol.% of solid loading at 20, 40 and 60 8C. Similar results were obtained for other solid loadings tested. It can be seen that all of the curves are practically superimposed, suggesting at first glance that temperature does not influence the CMC and thereby, the green microstructure. These results are supported by the observed small differences in green density for given solid loading. Moreover, the linear shrinkage decreases with increasing temperature. This evolution does not reflect differences in the green microstructure and can be attributed to the higher loss of moisture experienced by the cast bodies obtained at higher temperatures, that occurs along
108
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
Fig. 6. Bigot’s curves of pure electrostatic-stabilised alumina suspensions at 40 vol.%, as a function of temperature.
Fig. 7. CMC of pure electrostatic-stabilised alumina suspensions at different solids loading, as a function of temperature.
the de-moulding operation in the oven and the set up of the drying test. The evolution of CMC as a function of temperature for the different solid loadings used is shown in Fig. 7. It can be seen that the CMC increases with decreasing solid loading, in good agreement with the results for relative density and porosimetry. Therefore, Bigot’s curves still remains an useful and non-destructive technique for the characterisation of the microstructure slip cast bodies [29].
4. Summary and conclusions The results presented in this work show that an increase in temperature of electrostatically stabilised alumina suspensions
induces a flocculation trend that is enhanced with increasing solid loading. In the rheological behaviour, this trend towards particle agglomeration is expressed by an increase in viscosity in the low shear rate range and a more accentuated thixotropic character of the suspensions. The flocculation effect of temperature was also confirmed in the slip casting experiments by a decrease of green density and a concomitant increase in cake porosity, especially for longer casting times where the pressure drop along the cake thickness becomes significant. Particle agglomeration and the slowing down of the depositions rate favouring particle rearrangements are opposing tendencies that can determine an initial decrease in cake permeability (increase in density) followed by an inverse trend for longer casting times.
G. Tarı` et al. / Journal of Materials Processing Technology 137 (2003) 102–109
Acknowledgements The authors acknowledge FCT (Portuguese Foundation for Science and Technology) for the financial support.
[13]
[14]
References [1] F.F. Lange, Powder processing science and technology for increased reliability, J. Am. Ceram. Soc. 72 (1989) 3–15. [2] D.-M. Liu, Influence of pore structure in green compacts on densification of SiC–Al2O3–Y2O3, Ceram. Int. 22 (1996) 403–406. [3] Y. Juan, F.J. Oliveira, R.F. Silva, J.M.F. Ferreira, Pressureless sinterability of slip cast silicon nitride bodies prepared from coprecipitation-coated powders, J. Eur. Ceram. Soc. 19 (1999) 433–439. [4] B.V. Velamakanni, F.F. Lange, Effect of interparticle potentials and sedimentation on particle packing density of bimodal particle size distributions during pressure filtration, J. Am. Ceram. Soc. 74 (1991) 166–172. [5] R.J. Pugh, Dispersion and stability of powders in liquids, in: R.J. Pugh, L. Bergstrom (Eds.), Surface and Colloid Chemistry in Advanced Ceramic Processing, Marcel Dekker, New York, 1994, p. 170. [6] J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, San Diego, CA, 1991. [7] G. Tarı`, J.M.F. Ferreira, O. Lyckfeldt, Influence of magnesia on colloidal processing of alumina, J. Eur. Ceram. Soc. 17 (1997) 1341– 1350. [8] E. Liden, M. Persson, E. Carlstrom, R. Carlsson, Electrostatic adsorption of a colloidal sintering agent on silicon nitrate particles, J. Am. Ceram. Soc. 74 (1991) 1335–1339. [9] D.J. Shanefield, Organic Additives and Ceramic Processing, 2nd ed., Kluwer Academic Publishers, Dordrecht, 1996. [10] G. Tarı`, J.M.F. Ferreira, O. Lyckfeldt, Influence of the stabilising mechanism and solid loading on slip casting of alumina, J. Eur. Ceram. Soc. 18 (1998) 479–486. [11] J.M.F. Ferreira, H.M.M. Diz, Effect of solid loading on slip casting performance of silicon carbide slurries, J. Am. Ceram. Soc., in press. [12] T. Tomita, L. Guo, Y. Zhang, N. Uchida, K. Uematsu, Effect of temperature on characteristics of alumina slurry, in: Science,
[15]
[16] [17] [18] [19]
[20] [21] [22] [23]
[24] [25] [26]
[27] [28] [29]
109
Technology and Applications of Colloidal Suspensions, Ceramic Transaction, vol. 54, American Ceramic Society, 1995, pp. 103–108. J.G.P. Binner, A.M. Murfin, The effect of temperature, heating method and state of dispersion on the vacuum filter casting of alumina suspensions, J. Eur. Ceram. Soc. 18 (1998) 791–798. A. Vernon, J. Binner, Microwave enhanced slip casting of advanced ceramics, Ind. Ceram. 16 (1996) 27–30. G. Tarı`, S.M. Olhero, J.M.F. Ferreira, Influence of temperature on stability of electrostatically stabilised alumina suspensions, J. Coll. Interf. Sci., submitted for publication. J.A. Yanez, E. Laarz, L. Bergstro¨ m, Viscoelastic properties of particle gels, J. Colloid Interf. Sci. 209 (1999) 162–172. D.H. Napper, Polymeric Stabilisation of Colloidal Dispersions, Academic Press, 1983, Chapter 3. G. Aliprandi, Principi di Ceramugia e Tecnologia Ceramica, E.c.i.g., Geneva, 1975. A. Zupancic, L. Lapasin, A. Kristoffersson, Influence of particle concentration on rheological properties of aqueous a-Al2O3 suspensions, J. Eur. Ceram. Soc. 18 (1998) 467–477. H.A. Barnes, J.F. Hutton, K. Walters, An Introduction to Rheology, Elsevier, Amsterdam, 1989. A.J. Ruys, C.C. Sorrel, Slip casting of alumina, Am. Ceram. Soc. Bull. 75 (1996) 66–69. F.M. Tiller, C.D. Tsai, Theory of filtration of ceramics. I. Slip casting, J. Am. Ceram Soc. 69 (1986) 882–887. J.M.F. Ferreira, An interface SiC-Soluc¸a˜ o Aquosa e o Enchimento por Barbotina, Ph.D. Thesis, University of Aveiro, Aveiro, Portugal, 1992. L. Frassek, H.W. Hennicke, Some aspects of pressure slip casting with non clay suspensions, Ceram. Forum Int. 67 (1990) 443–446. T.J. Fennelly, J.S. Reed, Mechanics of pressure slip casting, J. Am. Ceram. Soc. 55 (1972) 264–268. T.J. Fennelly, J.S. Reed, Compression permeability of Al2O3 cakes forming by pressure slip casting, J. Am. Ceram. Soc. 55 (1972) 381– 383. F.F. Lange, K.T. Miller, Pressure filtration: consolidation kinetics and mechanics, Am. Ceram. Soc. Bull. 66 (1987) 1498–1504. J.M.F. Ferreira, Influence of pH on the pressure slip casting of silicon carbide bodies, J. Eur. Ceram. Soc. 17 (1997) 259–266. G. Tarı`, J.M.F. Ferreira, Influence of solid loading on dryingshrinkage behaviour of slip cast bodies, J. Eur. Ceram. Soc. 18 (1998) 487–493.