Effect of pH on rheology of aqueous Al2O3/SiC colloidal system
Journal of Advanced Ceramics
2226-4108
Effect of pH on rheology of aqueous Al2O3/SiC colloidal system
Xiaoqing ZHANG 0
Yanli ZHANG 0
Han CHEN 0
Lucun GUO 0
0 College of Materials Science and Engineering, Nanjing University of Technology , Nanjing 210009 , China
The rheological behavior of aqueous Al2O3/SiC suspensions at different pH values was investigated by rheological measurement. Experimental results showed that at pH = 3-6, Al2O3 and SiC particles have opposite surface charges, and the binary suspensions have lower viscosity than the unary suspensions at shear rates of 0-300 s1. Furthermore, at pH = 3-12, the stability of the Al2O3 component seemed to dominate the overall rheological behavior of the Al2O3/SiC binary suspensions. The tendency mentioned above showed little variations in various ionic strengths, particle diameters and component fractions.
alumina; silicon carbide; mixed colloidal dispersion; rheology
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The dispersion and stability of a colloidal system are
the main issues in colloidal processing. These
properties influence not only the green compacts and
sintering performance, but also the quality of the final
products. Understanding and predicting the stability of
a ceramic colloidal suspension is of particular interest
[
1–5
]. Numerous related studies have been reported in
the literature [
6–10
]. Unary suspension, which is
composed of one kind of particles, has been extensively
studied, but the behavior of multiple suspension (i.e.,
suspension with several types of particles) is much less
understood although dispersion of such system is
commonly found in nature and in various applications
[
11–14
]. One of the major reasons is the complexity of
the phenomena involved. The stability of multiple
system depends not only on many basic parameters,
such as nature of solvent and particles, ionic strength,
and size and charge of particles, but also on the
interactions among the particles, including electrostatic
force and van der Waals force. For a two-component
system containing different particles A and B, the
forces of A–A, B–B and A–B are involved. The two
kinds of particles differ in various factors, such as
Hamaker constant and surface charge. The factors such
as volume fraction and number ratio affect the total
energy of a colloidal system simultaneously. Internal
structures are difficult to observe experimentally at a
microscopic level [
15–19
]. Probing the experimental
system in such detail is not an easy task, specifically
the system with more components and dispersants.
Some interesting experimental results on
multicomponent system have been found in previous
studies. Wang et al. [
20,21
] revealed that the addition
of salt improves the colloidal stability in systems that
contain different particles in a certain pH range
between the isoelectric points (IEPs) of the two types
of particles. Uricanu et al. [22] studied the stability in
colloidal mixtures containing particles with a large
disparity in size. The results supported that the
composite heteroparticles behave in many ways like a
silica sol, as the small silica particles are adsorbed onto
the latex ones and the surface characteristics of latex
cores are modified simultaneously. In recent years,
alumina matrix composite ceramic materials attracted
widespread attention due to their excellent mechanical
properties, especially alumina/silicon carbide ceramics
[
23
]. The study of dispersion properties about
Al2O3/SiC colloidal system is of great significance to
the subsequent process.
The present study aims to investigate the effects of
pH, ionic strength, particle size and component
fraction on the rheological behavior of aqueous
Al2O3/SiC binary suspension, focusing on the
comparison with the unary suspensions of Al2O3 and
SiC and the surprising stability of initially oppositely
charged particle mixtures.
2
Experimental
2. 1
Material
All of the experiments were conducted with Al2O3
(Zhengzhou Aluminum Co., Henan, China, purity >
99% (in mass)) and SiC (Jiangsu Leyuan Factory,
Jiangsu, China, purity > 99% (in mass)). The average
particle sizes (D50) of the powders were 31.63 μm and
0.92 μm, respectively, which were characterized by
laser diffraction (NSKC-IA, Nanjing University of
Technology, China). The Al2O3 powder was ground
into different particle diameters (1.44 μm and 0.55 μm)
for further use. Scanning electron microscopy (SEM)
photographs of 1.44 μm Al2O3, 0.55 μm Al2O3 and
0.92 μm SiC are shown in Fig. 1, observed with a
scanning electron microscope (JSM-6360LV, JEOL
Led., Tokyo, Japan). The IEP values of the powders
were obtained from Zetasizer (3000HSA, Malvern
Instruments Ltd., Malvern, UK).
2. 2
Sample preparation
The ceramic powders were dispersed in deionized
water. The pH was then adjusted to the desired value
using standardized analytical-grade HCl or NaOH
solution. NaCl was used to adjust ionic strength.
Finally, the mixture was ball-milled using an agate
grinding ball as media at a (...truncated)