JOURNAL OF MATERIALS SCIENCE 33 (1998) 2931—2935

Dependence of the rheological behaviourof electrostatically stabilized alumina slurrieson pH and solid loading

D. J. KIM, H. KIMDepartment of Inorganic Materials Engineering, Seoul National University, Seoul 151-742,South Korea

J. K. LEEDepartment of Materials Engineering, Chosun University, Kwang-ju 501-759, South KoreaE-mail: jklee@mail.chosun.ac.kr

Alumina slurries with different solid loadings were prepared using aqueous solutions ofhydrochloric acid the concentrations of which were predetermined as suitable for dispersingmedia; their rheological behaviour was investigated with respect to the pH of the slurry. Inregions of low pH (below 3.5), the viscosity of the slurries gradually increased withdecreasing pH of the slurry, irrespective of the amount of solid loading. Near the isoelectricpoint of alumina (about pH 8–8.5), the viscosities of the slurries were high and dependent onthe solid loading. The pH range which exhibits the regions with low viscosity narrows onincreasing the solid loading. These results were interpreted using an interparticle potentialenergy diagram for the slurry system. 1998 Kluwer Academic Publishers

1. IntroductionFor the fabrication of a ceramic body through a wet-forming process, it is essential to prepare a homo-geneous slurry. In particular, when the electrostaticstabilization process is involved during the dispersionof ceramic powder, the pH of the slurry should usuallybe adjusted to be far away from the isoelectric point(IEP) of a powder in order for it to possess highsurface charges, because solid particles immersed inliquid media overcome the attractive van der Waalsforce based on the Deryaguin—Landau—Vervey—Over-break theory [1].

The stability of particles is often evaluated from therheological behaviour of the slurry since the rheologyreflects the interaction between particles. In general,a slurry with homogeneous dispersion shows nearlyNewtonian flow behaviour whereas a slurry with floc-culation behaves in a pseudoplastic manner. Rheo-logical behaviour validation by the pH adjustment ofslurry is only found in a restricted pH range includingthe IEP [2, 3]. That is, pseudoplastic behaviour ap-pears not only near the IEP but also in regions of lowor high pH. These behaviours are not expected fromthe electrokinetic data since the magnitude of themeasured zeta potential is still high in regions ofvalidating pH.

On the other hand, mass and/or phase segregationduring the consolidation of the slurry and excessiveshrinkage during the drying of the consolidated bodyare detrimental to the precise design of advancedceramics [4]. Although these problems can be over-

0022—2461 ( 1998 Kluwer Academic Publishers

come by the method of pressure filtration using a hy-dration short-range force [5], net shape forming bysuch methods as a gel casting [6] essentially requiresthe slurry to have maximum solid loading to preventthose problems. Omatete et al. [7] have studied thedependence of maximum solid loading on the pH ofaqueous alumina and zirconia slurries. From the re-sult that the maximum solid loading increases asa slurry pH departs from the IEP of alumina andzirconia powders, they concluded that the elec-trophoretic mobility of the powders determines themaximum solid loading. However, they did not ex-plain why powder loading itself is impossible above orbelow the critical pH corresponding to the maximumsolid loading.

The investigation of particle stability predicted fromthe shape of an interparticle potential energy curvewill be beneficial to understanding the aforementionedrheological phenomena [8]. The interparticle poten-tial energy curve is produced by plotting the net in-terparticle potential energy given as the summation ofthe electrical double-layer potential and van derWaals potential with respect to the separation dis-tance between particles. The net interparticle potentialenergy appropriate for a ceramic system is expressedas follows [9] :

»(H)"2nae3e0(2

$ln[1#exp(!iH)]!

Aa

12H(1)

where H is the separation distance between two spheri-cal particles of radius a, e

3is the relative dielectric

2931

constant of the liquid medium, e0

is the dielectricpermittivity of vacuum, (

$is the surface potential at

the diffuse part of electrical double layer, j is theDebye—Huckel parameter and A is the effectiveHamaker constant. (

$and j are two important fac-

tors dominating the shape of the curve since the choiceof slurry system nearly fixes a, e

3and A. Accordingly,

the reliability of particle stability predicted from thepotential energy curve depends on how the magni-tudes of (

$and j are regulated with respect to the pH

of slurry and solid loading.For real calculations employing Equation 1, the

experimentally inaccessible ($

is replaced by the zetapotential measured using an extremely dilute sus-pension whose ionic atmosphere is similar to that ofthe slurry under investigation. Deliso et al. [10] haveconsidered the dependence of the zeta potential onsolid loading. They concluded that solid loading doesnot affect the zeta potential because results from bothmicroelectrophoresis and the mass transport tech-nique produce similar electrokinetic data. This con-clusion is supported by the electrokinetic dataobtained from the improved mass transport technique[11]. On the other hand, the Debye—Huckel approxi-mation [1] indicates that j should be determined bythe ionic strength of the bulk solution where the elec-trical potential difference between solid particles andliquid media disappears. For a dilute colloidal suspen-sion, the determination of j is irrevelant becausea liquid medium is assumed to be a reservoir of ionicspecies. Meanwhile, for concentrated suspensions suchas a ceramic slurry, it is not simple because a signifi-cant amount of the potential-determining ions addedinitially and the background electrolyte are consumedby the adsorption on the surface of suspended par-ticles [12].

The purpose of present study is to interpret thedependence of the rheological behaviour on the pH ofthe slurry and on the solid loading in an electrostati-cally stabilized aqueous alumina slurry, using the po-tential energy curves. To facilitate the determinationof j, only a HCl electrolyte was used for the adjust-ment of the pH and the slurries were prepared fromloading alumina powder into aqueous solutions ofHCl the concentrations of which were predetermined.

2. Experimental procedureHigh-purity (greater than 99.99%)a-Al

2O

3powder

(AKP 20; Sumitomo Chemical, Osaka, Japan) wasused in the experiment. Particle size analysis (LaserMicron Sizer 24; Seishin Kigyo, Tokyo, Japan) con-firmed that the powder is composed of particles witha diameter of 0.2—1.8lm and an average particlediameter of 0.56lm. The specific surface area meas-ured by the Brunauer—Emmett—Teller method (Quan-tasorb, Quantachrome, Syosset, NY, USA) is4.5 m2 g~1.

Aqueous solutions with concentrations between10~4 and 10~1 M HCl were prepared from concen-trated HCl solution (analytical grade) and the de-ionized water. After the powders equivalent to 20, 50and 58 vol% had been poured into a fixed volume

2932

(typically 30 cm3) of each HCl solution, ultrasonicagitation subsequent to hand mixing was carried outto break down the agglomerates and to obtain a slurrywith homogeneous dispersion. The resultant slurrieswere equilibrated overnight after being sealed to pre-vent evaporation of water. Prior to the powder load-ing, the pH of the solutions was measured with a pHmeter (model 340; Mettler Instrument, Hightstown,NJ, USA), using a combined electrode.

The viscosity of the slurry was measured at variousshear rates with a concentric-cylinder-type viscometer(LVDTII#; Brookfield Engineering, Stoughton, MA,USA) equipped with a small sample chamber anda type 31 spindle. Prior to the viscosity measurement,ultrasonic agitation subsequent to hand mixing wasemployed to disperse the sediments formed during theequilibration process. The pH of the slurries was im-mediately measured as soon as the viscosity measure-ment had been completed.

The electrophoretic mobility of the powder wasmeasured using microelectrophoresis apparatus(Zetameter 3.0; Zetameter, New York, NY, USA) asa function of pH. To identify the IEP of the powder,a mobility measurement was conducted in broadranges of pH in the presence of a 0.01 M NaCl back-ground electrolyte. In this case, HCl and NaOHwere used for pH adjustment. Another mobilitymeasurement was also performed in the absence ofbackground electrolyte in order to retain an ionicatmosphere similar to that of our slurry. So, only HClwas used for the pH adjustment and the samples werealso equilibrated overnight prior to the mobilitymeasurement. The zeta potential was calculated fromthe measured mobility using the Smoluchowski equa-tion [13].

3. Results and discussionThe values of the zeta potential as a function of pH inthis work are presented in Fig. 1. The fact that the IEPappears at pH 8—8.5 coincides with other electro-kinetic data for pure a-Al

2O

3[14, 15]. It should be

noted that the effect of ionic strength on the zetapotential is negligible in the acidic region; this is differ-ent from the double-layer theory [16] which indicatesa strong dependence of the zeta potential on the ionicstrength. This phenomenon may arise from the dis-solution reaction depicted by

12

Al2O

3#3H`PAl3`#3

2H

2O (2)

Once dissolved, alumina remains in solution in theform of Al species or precipitates at the surfaces ofalumina particles; this depends upon the ageing time,the ionic atmosphere and the alumina concentration.The fact that the dissolution reaction affects the sur-face chemistry of solid particles immersed in aqueousmedia has been reported for SiO

2, B

4C and

La0.9

Sr0.1

MnO3

systems [17—19].Fig. 2 shows the relation between the pH values of

the HCl solutions used as the dispersing media andthe pH values of the resultant alumina slurries. In thecase of the 20 vol% a-Al

2O

3slurry the pH of the

Figure 1 Zeta potential of alumina powder as a function of pH. (d),0.01 M NaCl; (O), no electrolyte.

Figure 2 Variation in the pH of an alumina slurry with respect tothe pH of HCl solutions used as the dispersing media. (d), 20 vol%a-Al

2O

3; (m), 50 vol% a-Al

2O

3; (.), 58 vol% a-Al

2O

3.

slurry increases with increasing pH of solution, finallyconverging to pH 7—8. This trend also appeared forthe 50 and 58 vol% a-Al

2O

3slurries. The fact that the

slurry pH converges to the IEP is consistent with theresults for mass titration in an aqueous a-Al

2O

3sys-

tem [20]. On the other hand, the use of HCl solutionswith a low concentration as dispersing media meantthat powder loading could not be achieved. As can beexpected from Fig. 2, the critical pH values of theslurries for solid loading in 50 vol% a-Al

2O

3and

58 vol% a-Al2O

3slurries are about 6.5 and 5.2, re-

spectively. All the slurries obtained in this workshowed pseudoplastic behaviours, but the degree of

this behaviour was dependent upon both the pH of theslurry and solid loading.

Fig. 3 shows the effects of the pH of the slurry andsolid loading on the slurry viscosity which is measuredat a shear rate of 10.2 s~1. At each solid loading,a gradual increase in the viscosity of the slurry appearsin regions of low pH of the slurry, whereas the vis-cosity of the slurry rapidly increases near the IEP ofalumina powder. As the solid loading increases, thepH range with low viscosity narrows, converging atpH 3—4.

If the suspension effect [21] is neglected, the pH ofthe slurry provides information on the H` concentra-tion remaining in bulk solutions after protolytic sur-face reactions which formed the electrical double layerhave been completed. Also, a more accurate deter-mination of j for the slurry system can be obtainedfrom information on the dissolution of alumina par-ticles in aqueous media. Takeda et al. [22] have exam-ined the dissolution of a-Al

2O

3powder in aqueous

media by using the chelation and atomic absorptionmethods. According to their results, the amount ofdissolved alumina increases from 1 to 20 ppm as thepH of the media departs from the IEP of alumina. Thisamount is equivalent to an Al3` concentration ofabout 10~4 M assuming that the dissolved Al speciesexist in the form of Al3`.

A nuclear magnetic resonance study [23] on theaqueous solutions of AlCl

3shows that the dominant

forms of Al species in solution are [Al(H2O)

6]3`

below pH 3.5, [Al13

(OH)28

O4(H

2O)

8]3` at pH be-

tween 3.5 and 4.5, and [Al(OH)3]0 above pH 4.5.

Considering the above results, H` and Cl~ can beregarded as the major ionic species in solution becausethe concentration of Al ionic species below pH 4 islower than that of H` and the presence of non-ionic[Al(OH)

3]0 species above pH 4 reduces the concen-

tration of Al ionic species. The electroneutrality

Figure 3 Apparent viscosity of the alumina slurry with differentsolid loadings as a function of slurry pH. (d), 20 vol%a-Al

2O

3; (m), 50 vol% a-Al

2O

3; (.), 58 vol% a-Al

2O

3.

2933

constraint for the bulk solution is in proportion to theconcentrations of H` and Cl~. Under this condition,it is possible to estimate the parameter j for oursystem from the pH of slurry, which represents theconcentration of H` in the bulk solutions [1] :

j (m~1)"3.29]109](10~1H)1@2 at 25 °C (3)

However, in this case, it is difficult to explain thedependence of the rheological behaviour (as shown inFig. 3) on solid loading at a certain pH of slurry usingthe interparticle potential energy curve obtained fromEquation 1 because both j and the zeta potential areindependent of the solid loading. Also, because of this,the ionic strength of HCl solutions was chosen asa reference for the determination of j.

The interaction between the particles was inves-tigated for particles of different sizes because thealumina powder is composed of particles with variousdiameters from 0.2 to 1.8lm in our slurry system. Tosimplify this, the effective radius, a

%&&, of two particles

with different radii a1

and a2

replaced a in Equation1 according to Hogg et al. [24]:

a%&&"2a

1a2/(a

1#a

2) (4)

In order to obtain interparticle potential energycurves corresponding to a certain pH of the slurry ateach solid loading, an A value of 6.7]10~20 J, wasdeduced for particle sizes a

1" 0.1lm and a

2"

0.5lm by Horn et al. [25]. Also, the zeta-potentialvalue adopted from fitting the curve in Fig. 1 and thej value calculated from Equation 3 was inserted intoEquation 1. The two particle sizes were chosen byconsidering that the density of small particles is higherthan that of large particles.

Fig. 4a, b and c shows the interparticle potentialenergy diagrams obtained at various pH valuesof the slurry for alumina with solid loadings of20 vol%, 50 vol% and 58 vol% a-Al

2O

3, respectively.

As can be seen in Fig. 4, in pH regions with lowviscosity (as shown in Fig. 3 at each solid loading),a high potential energy barrier and a shallow second-ary minimum were observed. The former prevents thealumina particles from falling into a deep primaryminimum and the latter prevents the particles fromstaying at the position of the secondary minimum. Incontrast with the above case, a low potential energybarrier and a deep secondary minimum were observedin regions of the pH where a steep or gradual rise inthe viscosity, respectively, occurred. Although the po-tential energy barrier is a little higher than about10 k¹ (where k is the Boltzmann constant and ¹ is theabsolute temperature) which is generally accepted asthe criterion of Brownian coagulation [26], these re-sults are qualitatively in agreement with viscositydata, as shown in Fig. 3. Also, it observed that thepotential energy barrier disappeared above a pH cor-responding to a steep rise in the slurry viscosity. Atthis point, strong flocs form in the slurry, and furtheraddition of alumina powder to the slurry leads togrowth of the flocs. It is suggested that the eventualpercolation of the flocs reduces the capability forpowder loading.

2934

Figure 4 Interparticle potential energy diagrams obtained at vari-ous pH values for alumina slurries with solid loadings of (a) 20 vol%a-Al

2O

3( ), pH 1.1; ( ), pH 2.4; ( ), pH 4.5; ( ), pH

6.5; ( ), pH 7.1) (b) 50 vol% a-Al2O

3( ), pH 1.9; ( ), pH

3.6; ( ), pH 5.8; ( ), pH 6.4) and (c) 58 vol% a-Al2O

3( ),

pH 2.9; ( ), pH 4.2; ( ), pH 5.1).

4. ConclusionAn interparticle potential energy diagram obtainedfrom the Debye—Huckel parameter which is estimatedfrom the ionic strength of the initial HClsolution used as a dispersing medium is effective forunderstanding the dependence of the rheologicalbehaviour of electrostatically stabilized aqueous a-Al

2O

3slurries upon the pH of the slurry and solid

loading. It is suggested that powder loading is notpossible in regions above a critical pH (correspondingto maximum solid loading) and below pH 3 (in regionsshowing a gradual rise in viscosity). This is attributedto the disappearance of the potential energy barrier inthe former, and the presence of a deep secondaryminimum in the latter.

References1. P. C. HIEMENZ, in ‘‘Principles of colloid and surface chem-

istry’’ (Marcel Dekker, New York, 2nd Edn, 1986) p. 677.2. A. K. NIKUMBH, H. SCHMIDT, K. MARTIN, F. PORZ

and F. THUMMLER, J. Mater. Sci. 25 (1990) 15.3. A. K. NIKUMBH, H. SCHMIDT, K. MARTIN and

F. PORZ, ibid. 26 (1991) 3649.4. F. F . LANGE, J. Amer. Ceram. Soc. 72 (1989) 3.5. B. V. VELAMAKANNI, J . C. CHANG, F. F . LANGE and D.

S. PEARSON, ¸angmiur 6 (1990) 1323.6. O. O. OMATETE, M. A. JANNEY and R. A. STREHLOW,

Amer. Ceram. Soc. Bull. 70 (1991) 1641.7. O. O. OMATETE, A. BLEIER, C . C. WESTMORELAND

and A. C. YOUNG, Ceram. Engng Sci. Proc. 12 (1991) 2084.8. J . W. GOODWIN, Amer. Ceram. Soc. Bull. 69 (1990) 1694.9. R. J . HUNTER, in ‘‘Foundations of colloid science’’, Vol. 1

(Oxford University Press, Oxford, 1987) p. 415.10. E. M. DELISO, A. S. RAO and W. R. CANNON, in ‘‘Ad-

vances in ceramics—ceramic powder processing science’’, Vol.

21, edited by G. L. Messing, K. S. Mazdiyasini, J. W. McCauleyand R. A. Haber (American Ceramic Society, Westerville, OH,1987) p. 525.

11. M. HASHIBA, H. OKAMOTO and K. HIRAMATSU, J.Mater. Sci. 23 (1988) 3649.

12. J . LYKLEMA, in ‘‘Adsorption from solution at thesolid/liquid interface’’, edited by G. D. Parfitt and C. H.Rochester (Academic Press, London, 1983) p. 223.

13. R. J . HUNTER, in ‘‘Zeta potential in colloid science’’ (Aca-demic Press, London, 1981) p. 69.

14. H. J . MODI and D. W. FUERSTENAU, J. Phys. Chem. 61(1957) 640.

15. J . A. YOPPS and D. W. FUERSTENAU, J. Colloid InterfaceSci. 19 (1964) 61.

16. R. J . HUNTER and H. J. L. WRIGHT, ibid. 37 (1971) 564.17. R. J . HUNTER, in ‘‘Zeta potential in colloid science’’ (Aca-

demic Press, London, 1981) p. 282.18. P. D. WILLIAMS and D. D. HAWN, J. Amer. Ceram. Soc. 74

(1991) 1614.19. V. L. RICHARDS and S. C. SINGHAL, J. Mater. Sci. ¸ett. 11

(1992) 1193.20. J . WERNET and D. L. FEKE, J. Amer. Ceram. Soc. 77 (1994)

2693.21. Y. M. CHERNOBEREZHSHII , in ‘‘Surface and colloid

science’’, Vol. 12, edited by E. Matijevic (Plenum, New York,1982) p. 359.

22. S. TAKEDA, M. KUROSE and I . TARI, in ‘‘Ceramic transac-tions—ceramic powder science IV’’, Vol. 21, edited by S.Hirano, G. L. Messing and H. Hausner (American CeramicSociety, Westerville, OH, 1991) p. 203.

23. J . Y. BOTTERO, J. M. CASES, F. FIESSINGER and J. E.

POIRLER, J. Phys. Chem. 84 (1980) 2933.24. R. HOGG, T. W. HEARLY and D. W. FUERSTENAU,

¹rans. Faraday Soc. 62, (1966) 1638.25. R. G. HORN, D. R. CLARKE and M. T. CLARKSON, J.

Mater. Res. 3 (1988) 413.26. E. J . W. VERWEY and J. T. G. OVERBEEK, in ‘‘Theory of

the stability of lyophobic colloids’’ (Elsevier, Amsterdam, 1948)p. 123.

2935