Langmuir 1990,6, 1323-1325
1323
New Method for Efficient Colloidal Particle Packing via Modulation of Repulsive Lubricating Hydration Forces Bhaskar V. Velamakanni,’ Jeanne C. Chang, Fred F. Lange, and Dale S. Pearson Materials Department, College of Engineering, University of California, Santa Barbara, California 93106 Received February 15, 1990 A simple method is described for increasing the viscosity of dispersed (20 vol 95 alumina) suspensions over 4 orders of magnitude through weak aggregation with certain indifferent electrolytes containing hydrolyzable anions. Particles in the viscous slurry, which are covered with short-rangerepulsive hydration layers, can be packed to a high density during pressure consolidation by apparent lubrication-assisted particle rearrangement. A combination of short-range (0.25M NH4C1)and flocced slurries were measured with rheometrics viscometer (Model RMS 800) using either couette- or parallel-plate-type fixtures. Part of the slurries prepared for viscosity measurements were used to make consolidated bodies by using a pressure filtration device4 with an applied pressure of 14.6 MPa. After consolida(6)Cesarano, J., III; Aksay, I. A.; Bleir, A. J. Am. Ceram. SOC.1988, 71, 250. (7) Cesarano, J., 111; Aksay, I. A. J . Am. Ceram. SOC.1988, 71, 1062.
0 1990 American Chemical Society
Letters
1324 Langmuir, Vol. 6,No. 7, 1990 60
I
20 Voi % Alumina Slurry Salt NH4CI
Coagulated
55L i 5 0t 0”
P P
Flocced
0 a a
\ 102
.I
10-2
,
,
...... 0
10
’
. .
.
,
,
.(....I
700
IO’
.
.
}
OM
.
. ‘ . ‘ . ‘ I
20 Vol.% Alumina Slurry Salt: NH 4 CI
D6pelSec pH4
’
.I.-
102
Shear rate, s-’
Figure 1. Viscosity versus shear rate of dispersed, coagulated, and flocced slurries containing 20 vol % alumina powder as a function of NH4Cl concentration. The slope of the curves determines the degree of shear thinning, which increases as the state of the slurry changes from either dispersed to flocced or dispersed to coagulated.
tion, the particle packing density was determined by weighing the body before and after drying, assuming the weight change was only due to evaporated water.
Results Figure 1 shows the influence of NH4C1 on the viscosity of 20 vol 75 alumina slurries. As shown in the figure, the viscosity of flocced slurries is substantially higher than those of the other slurries. However, the behavior of coagulated slurries was dependent on t h e NH4Cl concentration, increasing over 4 orders of magnitude to reach a saturation maximum at 1.70 M NH4C1. What seems to be interesting is while the degree of shear thinning is similar to that of the flocced slurries, the saturation viscosity of the coagulated slurries is about 1 order of magnitude lower. This behavior suggests that a similar attractive network exists within both the coagulated and flocced slurries but that the magnitude of interparticle attraction is altered by the salt concentration. Another important feature of the slurries is their sedimentation behavior. While the flocced slurries did not sediment, coagulated slurries (30.1M NH4Cl) did sediment to produce a clear supernatant. The height of the sediment increased with the salt concentration, reaching a saturation height at approximately 1.50 M NH4C1. The influence of dispersion characteristics of slurries on relative particle packing density of consolidated bodies is shown in Figure 2. Irrespective of the salt content, relative packing densities obtained with dispersed and coagulated slurries appear identical within experimental error at 0.58, whereas the relative packing density produced with flocced slurry was 0.52. Another interesting feature of these saturated consolidated bodies is their rheology. The rheology of consolidated bodies after the compacts were removed from the die was characterized by stress relaxation experiments reported in detail elsewhere.8 The consolidated bodies made from dispersed (no NH4C1) and coagulated slurries were always liquid-like (i.e., very low storage modulus) and (8)Velamakanni, V. B.; Lange, F. F.; Pearson, D. S. J . Am. Ceram. SOC.,to be published.
451 0.0
’ ’
’
0.5
’
’
’
1 .o
’
’ ’
’
1.5
’ ’ ’
2.0
Salt Concentration,M
Figure 2. Effect of NH4Cl concentration in dispersed (pH 4) and flocced (pH 9) slurries containing 20 vol % alumina on the particle packing densities in pressure consolidated bodies.
appeared plastic, while the consolidated bodies made from flocced slurries were always solid-like (i.e., very high storage modulus).
Discussion We have shown that the viscosity of dilute, dispersed alumina slurries can be increased over many orders of magnitude through coagulation in the presence of certain indifferent electrolytes without affecting the high particle packing densities during pressure filtration. The observed viscosity-consolidation behavior of coagulated slurries, especially in the presence of excess salt, is not consistent with the DLVO theory. In addition to van der Waals attraction and electrostatic double-layer repulsion, the existence of a “third” interaction is evident. All experimental results strongly suggest that this “third” interaction is a result of a repulsive hydration potential which only acts at very small interparticle separation distances (
