DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

4.3.2. Preparation of Nickel Ink
Surface chemistry of ENP800 Ni powder in water favors PEI-25K as dispersant.
The isoelectric point (IEP) of nickel particles in water is debatable due to the evolution of
nickel hydroxylates on the particle surface. 135, 137, 142-144 Despite these discrepancies,
nickel ink preparation is performed at a basic pH ≥ 8.0. ENP 800 nickel powder and
water are observed reach an equilibrium pH ≈ 8.0 at ambient temperature (~23 °C). A
rise in pH above neutral conditions with the addition of an insoluble metal oxide indicates
the adsorption of H + ion from solution due to a negatively charged surface. That is, the
change in pH moves in the direction to neutralize the surface. In the presence of PEI-25K
addition, ENP 800 nickel suspension has an equilibrium pH ≈ 8.2. This pH range favors
adsorption of positively charged PEI (IEP = 10.8) on nickel, and then bridging
flocculation by negatively charged PAA (IEP = 3.5 - 4). 105
4.3.3. Rheological measurement
The rheological behavior of Ni gels is shown to depend strongly on the ratio of
Darvan 821A:PEI-25K (or A:C to signify anionic to cationic polyelectrolyte ratio). As
shown in Figure 4.4, the as-prepared Ni colloidal ink with A:C=5:2 displays rheological
behaviors similar to ceramic colloidal gels 32, 105 for Robocasting, while less Darvan
addition leads to weaker elastic response. A plateau region of shear elastic modulus
around 300 kPa with yield stress τy ≈ 154.4 Pa is observed in this Ni ink with φsolids=0.472.
The yield stress is taken to be the stress at which the elastic shear modulus drops to 90%
of its plateau value. Both shear elastic modulus and yield stress decreases as Darvan
concentration deceases, indicating less inter-particle linking in the concentrated particle
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