DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

3.3.2 Gel Preparation
The dispersion, centrifugation, and mixing steps are necessary for achieving a
high φsolids carbon black ink. Sonication and stirring speed up the breaking and
stabilization of carbon black agglomerates in a low φsolids suspension. The subsequent
centrifugation step concentrates carbon black particles prior to further mixing and
dilution. By comparison, direct mixing of the carbon black powder with those polymeric
additives can only reach a maximum φsolids=0.42. At φsolids>0.42, the mixture appears
dilatant.
3.3.3 Rheological Characterization of Carbon Black Ink
Oscillatory measurement indicates the carbon black gel has commensurate
Herschel-Bulkley gel responses to a ceramic ink used in Robocasting. Compared to the
HA ink, the carbon black gel has a lower storage modulus but a higher yield stress.
Polymer additive HPMC, although at a low concentration, strengthens the elastic
response of the carbon black gel. Figure 3.3 shows the oscillatory and steady flow
behavior of carbon black and HA gels. In Figure 3.3a, the HA gel has a storage modulus
of G'=300 kPa, while for carbon black gel G'=17 kPa (with HPMC) and G'=7.2 kPa
(without HPMC). The yield stress for a polymeric system is usually taken as the stress
magnitude where G' drops to 90% of the plateau value during a stress sweep experiment
starting from low stress. 106 For HA ink, τy ≈ 55.4 Pa with strain γ ≈ 0.2×10 -3 . For carbon
black gel with HPMC, τy = 153 Pa with an order of magnitude higher strain γ ≈ 1.0×10 -2
and for carbon black gel without HPMC, τy = 31.3 Pa with strain γ ≈ 4.75×10 -3 .
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