AMERICAN CERAMIC SOCIETY

esearch briefs
Finding the perfect ink for printing ceramic parts
Inkjet printing techniques are applied
with an increasing number of materials.
In particular, solid freeform fabrication
methods for ceramic materials have
attracted great interest in recent years.
Methods such as stereo lithography,
selective laser sintering and threedimensional
printing are used to produce
complex geometries or parts with material
gradients directly from a digital model.
The technological approach for such
production methods can be divided into
two categories: direct ceramic inkjet
printing and three-dimensional printing.
In the former, an ink containing ceramic
particles is printed onto a substrate,
and the part’s buildup is accomplished
through stacking of printed layers. In the
latter, binder solutions are printed into a
powder bed and bind the loose powder.
Now, D.A. Polsakiewicz and W. Kollenberg
at the Bonn-Rhine-Sieg University
of Applied Science in Germany
have investigated inks for application
in a piezoelectric print head with high
solid content. They prepared and characterized
alumina inks and examined
the influence of various solids loadings,
dispersant concentrations and binding
agents. Their paper appears in Materials
Science and Engineering Technology
(doi:10.1002/mawe.201100780).
Polsakiewicz and Kollenberg found
that a binary mixture of water and ethylene
glycol can serve as a that solvent
system for alumina particles and solvent
properties can be adjusted by varying
composition. As their results reveal,
viscosity as well as shear-thinning
behavior increased with raising solids
content. Rheological behavior of the
inks was not affected by small changes
in particle size or specific surface area.
However, surface tension values varied
depending on the powder used. Supplementary
experiments are necessary to
confirm and understand these results.
Moreover, further printing experiments
are essential for clarification of an
interesting assumption. The researchers
suppose that higher solids loadings also
American Ceramic Society Bulletin, Vol. 90, No. 9 | www.ceramics.org
could be possible without increasing the
viscosity. But they say attention must be
given to increasing shear-thinning characteristics
for higher solids loadings.
(This story first appeared in MaterialsViews.com,
a blog by ACerS publishing
partner Wiley.) n
Does fast firing of multilayer
ceramic capacitors help or
hinder?
A group of investigators led by
Penn State University’s Clive Randall
recently published a paper, “Effect
of Firing Rates on Electrode Morphology
and Electrical Properties of
Multilayer Ceramic Capacitors” in
the Journal of the American Ceramic
Society and available via the Early
View option (doi:10.1111/j.1551-
2916.2011.04880.x).
Because of improved processing over
the past decade, the volumetric capacitance
of multilayer ceramic capacitors
has increased significantly. However,
as devices are miniaturized, their usefulness
is limited by high electrical
leakage, which can be attributed to the
morphology of the electrode–dielectric
interface and microstructural defects,
such as porosity and roughness, at the
electrode–dielectric interface.
Part of the problem on firing MLCCs
is that the barium and titanium (from
the BaTiO 3 dielectric layers) and nickel
(from the electrode layers) form a
low-melting Ba–Ti–Ni alloy that leads
to electrode instabilities. One way to
avoid the problematic alloy formation is
“fast-firing,” i.e., using very high heating
rates to reach the firing temperature.
However, even with fast firing,
the electrode interface is not perfectly
continuous and planar, which limits the
device’s performance.
Previous work had established that
differences in heating rates lead to very
different morphologies. Two batches of
MLCC were cofired at approximately
1280°C for two hours under identical
atmospheres, but at heating rates of
either 150°C per hour or 3000°C per
hour. The team used finite element
analysis to predict the outcomes.
As expected, the continuity of the
electrodes is higher for the fast-fired
batch, and measurements show that
the capacitance of the device also is
higher. Also, impedance measurements
showed that the electrode–dielectric
interface and grain boundaries contribute
to the improved resistance of
the MLCC. (However, the interface
contributes more significantly than do
the grain boundaries). The slow-fired
batch exhibited a higher concentration
of electrode defects, leading to a higher
leakage current and confirming the
group’s FEA predictions.
The group concludes that the interface
plays an important role in MLCC
performance and that electrode morphology
can be controlled to a considerable
extent by processing.
Visit www.mri.psu.edu/centers/CDS
n
3D microstructure of a typical MLCC
fabricated by researchers at Penn State
by the (a) slow-fired conditions and (b)
fast-fired conditions.
(Credit: JACerS.)
19