DoD-Printing of Conductive Silver Tracks
Dominik Cibis, Klaus Krüger
Institute of Automation Technology, Helmut-Schmidt-University / University of the German Armed Forces,
Holstenhofweg 85, 22043 Hamburg, Germany
Dominik Cibis: Phone +49/(0)40/6541-2740, Fax +49/(0)40/6541-2004, E-Mail: dominik.cibis@hsu-hh.de
Klaus Krüger: Phone +49/(0)40/6541-2722, Fax +49/(0)40/6541-2004, E-Mail: klaus.krueger@hsu-hh.de
Abstract
The Inkjet-Printing principle shows high potential for new applications besides conventional graphic printing. Based
on a drop-on-demand (DoD) micro feeding system, a colloidal silver ink printer was developed, which makes it
possible to print conductive silver tracks.
The printer has a stationary piezo-driven print head with a nozzle diameter of 100 microns. Its drop-rate can be
adjusted between 50 and 5000 drops per second. Printing of defined structures is realized with an x-y-planar motor
which positions the substrate. As a printing medium, a colloidal silver ink is used. The colloidal suspension consists
of a dispergent (e.g. terpineol or ethylene glycole), a dispersant additive and silver particles. The main particle
diameter of the silver powder is below 1 micron. The percentage of such powder in the colloid reaches up to more
than 20% by weight.
The system reacts sensitively to changes of the colloid properties. Besides the particle load, the viscosity and surface
tension of the dispergent show significant influences. Size and duration of the piezoelectric pulse as well as nozzle
temperature are suitable control parameters. Corresponding to the surface tension of the colloids, the spot size of
drops printed on alumina substrate varies between 125 and 250 microns. By heating the substrate up to 150 °C,
different surface profiles of the conductive tracks can be realized.
Conductive paths are printed and then characterized by conductivity measurement, light microscopy and atomic
force microscopy. The thickness of the fired conductive tracks is about 2 microns when printing single layer.
Key words: piezo print head, drop-on-demand, ink-jet printing, conductive ink, piezo pulse, thick-film
______________________________________________________________________________________________
1. Introduction
Over the last years there has been a great deal of
success in extending the use of ink-jet printers beyond
graphic applications. They have been used for drug
discovery, for multiple automated micro-titrations, for
picoliter fluid dispensers and for combinatorial
devices in medicinal devices. They have also been
used to pattern functional nano- and microstructures
from layers of surfactant and to deposit cell adhesion
proteins in tissue engineering. It has also been
possible to build three-dimensional structures by
using ink-jet printable powders. Ink-jet printer
technology has another application in the production
of flat screen displays. Piezoelectric and thermal
drop-on-demand printers have been used successfully
for forming shapes with ceramic suspensions. High
temperature drop-on-demand nozzles can be used for
depositing molten wax-based ceramic suspensions
with up to 40 vol-% powder onto alumina [1]. The
assessment of these developments leads to the view
that ink-jet printers have a great range of possible
applications and many different inks can be delivered
by the micro print head. Theoretical models suggest
that the printability of a special ink is influenced by
ink viscosity, surface tension and density. In fact, ink
viscosity is the limiting factor for printability and has
to be suitable to the used print head. By heating up
the nozzle the ink viscosity can be adjusted in a
special range. Furthermore, the suspension needs to
be well-dispersed to avoid aggregation and
agglomeration. In all drop-on-demand printers the
shape and form of the electrical piezo pulse that
activates the drop formation is control variable.
Recent work has demonstrated the high potential of
drop-on-demand printers and shows that printing of
metal containing powder suspensions can be achieved
by preparing an ink suitable to the used print head.
An important part for the success of the printing
process is the formulation of the ink, based on an
organic dispergent. The present work shows the
possibility to deposit silver containing ink on alumina
substrate to produce conducting paths.
2. Experimental details
2.1. The printing system
The printer developed at the Institute of Automation
Technology (IfA) consists of a stationary piezodriven print head [2], an x-y-planar motor to position
the substrate and an integrated heating plate such that
it is possible to heat the substrate up to over 150 °C.
A piezo-based ink-jet print head was taken because it
offers a greater range of ink compatibility than for
example thermal ink-jet heads, which are limited in
the use of inks or require a new design for each
different type of ink suspension [3].
For the assembly of defined structures, two
possibilities arise: Moving the print head and leaving
the substrate unmoved or moving the substrate along
with the heating plate. In the case presented here the
moving-substrate choice was preferred to prevent
vibrations on the print head and to gain accuracy.
There are two operation modes for the print head:
Coupled with the planar motor to print defined
structures or observation mode which means, droplet
formation can be recorded. In observation mode the
maximum drop rate of the printer is 5 kHz and can be
adjusted continuously down to 50 Hz.
When operating the printer in structure mode the
droplet frequency goes down to 25 to 50 Hz,
dependend on the center distance between single dots.
Reason for that is the interconnection of the trigger
pulse between the planar motor and the micro print
head. Each trigger pulse signal generates one piezo
pulse signal to form one droplet. Figure 1 shows the
printer in structure mode configuration. The
specifications of the planar motor allow for a
positioning accuracy of 15 µm, a repeat accuracy of
1 µm and a resolution of 1 µm per step. The distance
between nozzle and substrate is generally 1 mm when
printing, but can be varied from 1.5 cm down to less
than 1 mm.
With the specifications described above, different
printing patterns can be realized. The distances
between single droplets can be adjusted as shown
schematically in figure 2. Starting with pattern (a)
center distances are reduced in x and/or y direction to
finally arrive at pattern (e) for conductive paths.
Because of the heating plate underneath the substrate
which enables drying of the printed dots within
seconds, multilayer printing is also possible.
Figure 2: Printing patterns
2.2. Further instruments
To complete the laboratory assembly at the Institue of
Automation Technology further instruments used in
the printing process are an ultrasonic pulse instrument
to stir up the samples being printed, an uv/vis-spectral
photometer for sedimentation tests, a sintering oven
and a light and atomic force microscope for optical
characterization of the printed spots and paths. The
printed conductive paths are finally electrically
characterized with a multimeter.
3. Results and discussion
Figure 1: Printer in structure mode configuration
3.1. Characterization of the dispergent
In usual solutions encountered on chemical level, the
solute and solvent molecules are of comparable size
and it is normally assumed that the solute molecules
are, on average, dispersed uniformly throughout the
solvent. There is an important class of materials,
however, in which the kinetic units that are dispersed
through the solvent are very much larger than the
molecules of the solvent. Such systems are called
colloidal dispersions. If a substance A is insoluble in
substance B, it will usually be possible to break A
down into small particles that can be distributed more
or less uniformly through the substance B. Substance
A is then called the disperse phase and substance B
the dispersion medium or dispergent [4].
To print conductive paths first of all a conductive ink,
a colloid suspension, has to be prepared. Therefore at
least two components are necessary: Particles which
allow for current conduction after the sintering
process and a suitable dispergent as dispersion
medium. Later on, to get a stable suspension with a
homogeneous particle distribution, a dispersant
additive is required. This ingredient acts as a helpful
means to avoid or at least minimize aggregation,
agglomeration and sedimentation of the particles in
the dispergent.
On the way to develop a printable conductive ink for
the used piezo print head initially a dispergent has to
be characterized and selected for the printing process.
Six different dispergents are taken into account being
potentially possible and compatible with the print
head: Ethylene glycol, terpineol, tetradecane,
limonene, ethanole and texanole. Characterization
proceedings are pointing out ethylene glycole,
terpineol and limonene being the dispergents showing
the best printing features. Figure 3 shows the droplet
and filament formation of ethylene glycole, terpineol
and limonene on the print head nozzle.
Figure 3: Droplet formation with pure dispergent
The nozzle diameter is 100 µm. On the left side of
figure 3 the droplet formation of ethylene glycole can
be seen. The printability is rated by optical observation. This is necessary to ensure that drops are
generated always in the same shape and quality which
means no sputtering and no wobbling of the droplet
during the formation process. A stable printing
process, good filament and drop formation and a fast
forming of the drop when detached from the nozzle
are rating points.
The dispergent most suitable for the print head is the
one that ensures establishing a range of print
parameters for the demands described above. Such
print parameters are the voltage UP of the piezo pulse,
the duration tw of the piezo signal and the nozzle
temperature ϑ. With the used 100 µm nozzle the
droplet diameter of ethylene glycole in air is about
95 µm when the drop has formed out completely. All
three pictures are taken at a frequency of 1500 Hz.
The optical observation pointed out limonene being
printable but in comparison with ethylene glycole and
terpineol showing worse results regarding filament
formation and stability. In table 1 the viscosities,
surface tensions and densities together with print
parameter ranges of these three dispergents are given.
Within the parameter range drop formation in case of
the IfA print head is possible.
Table 1: Viscosities, surface tensions, densities and
parameter ranges of ethylene glycole, terpineol and
limonene at 25 °C
Dispergent
Ethylene Glycole
Terpineol
Limonene
Commercial ink
η in mPas σ in mN/m ρ in g/cm3
17.3
46.0
0.8
4.8
46.8
31.4
27.3
38.0
1.11
0.93
0.84
1.06
Dispergent
Ethylene Glycole
Terpineol
Limonene
UP in V
135 - 155
155 - 165
138 - 148
tw in µs
14 - 32
18 - 26
40 - 45
ϑ in °C
25 - 35
37 - 47
27 - 45
The slightly different surface tensions of the
dispergents compaired with the commercial ink [5] do
not seem to impair droplet formation.
Next step is to have a look at the filament length and
droplet speed after detaching from the nozzle. The
measurements mentioned here are for the three
dispergents ethylene glycole, terpineol and limonene.
As already described, ethanole, tetradecane and
texanole do not show an acceptable printing stability,
so they are not subject of any measurements here.
Figure 4 shows the influence of the nozzle
temperature against filament length. By heating up
the print head nozzle the ink viscosity can be reduced
if necessary. Ethylene glycole is the dispergent
offering the best print properties when being printed
at room temperature. Figure 4 shows ethylene
glycole, terpineol as well as limonene having a nearly
linear behaviour, and taking the filament length
ranges of ethylene glycole and terpineol into account,
they are much higher compared to limonene which
means the filament length can be better controlled by
the temperature. Furthermore the printing process can
be affected by the piezo signal, that is the pulse
voltage and the pulse duration.
Figure 5 is depicting the droplet velocity against
pulse voltage. Ethylene glycole offers a maximum
area of droplet velocity ranging from 1.1 m/s to
3.3 m/s linearly from 135 V to 155 V pulse voltage.
The measurements were taken at a frequency of
1500 Hz. Considering the results of figures 3, 4 and 5
ethylene glycole is turning out to offer the best
possibilities for acting as a dispergent in the drop-ondemand printing process described here. By having
the chance to control the droplet velocity, the
question arises, if droplets may splash and be divided
into many little droplets when having a velocity
1000
900
Filament Length in µm
800
700
600
500
400
300
200
100
Ethylene Glycole
Terpineol
Limonene
0
24
25
26
27 28
29
30
31
32
33 34
35
36
37
38
39 40
41
42
43
44
45 46
47
48
Nozzle Temperature ϑ in °C
Figure 4: Influence of the nozzle temperature on the filament length
3,5
Droplet Velocity v in m/s
3
2,5
2
1,5
1
Ethylene Glycole
0,5
0
134
136
138
140
142
144
146
148
150
152
154
Pulse Voltage U p in V
Figure 5: Influence of the pulse voltage on the droplet velocity
Terpineol
156
158
160
Limonene
162
164
166
between 1.1 and 3.3 m/s (ethylene glycole). The
surface energy Esur of a droplet is given by
Esur = 4 ⋅ σ ⋅ π ⋅ r 2 .
(1)
σ is the surface tension of the droplet and r its radius.
After the drop has detached from the nozzle and has
formed out it has a kinetic energy Ekin according to
Ekin = 12 ⋅ m ⋅ v 2 = 23 ⋅ ρ ⋅ π ⋅ r 3 ⋅ v 2 .
(2)
m is the droplet’s mass, v its velocity, ρ its density
and r its radius.
To make a drop break up into two little droplets on
top of the substrate the gain of surface energy in the
transition one drop-two droplets must be gathered up
by the kinetic energy in the moment of impact, that
means the drop can only burst into two droplets when
Ekin has a bigger value than the gain in surface energy
according to
∆Esur = 2 ⋅ Esur, new − Esur, old
the dissipation energy should be even bigger.
Beginning with six different dispergents the number
was reduced to three and from now on focused on
ethylene glycole being the basis for a colloidal silver
suspension for conductive paths.
3.2. Dispergent Ethylene Glycole with particles
After finding out ethylene glycole being suitable as a
dispergent for DoD-printing colloidal suspensions are
prepared. Starting with 7.5 M% solids content in
ethylene glycole percentage is increased to 20 M%
and 30 M%. The suspension is mixed and before
printing homogenized with an ultrasonic pulse
instrument. Figure 6 shows the droplet formation on
the nozzle of the piezo print head. Print medium is a
7.5 M% colloidal suspension with Ag95/Pd5 silverpalladium particles (95 % silver, 5 % palladium).
(3)
and Ekin is completely turned into ∆Esur. The
dispensed volume V of a droplet is
V =
4
⋅π ⋅ r3
3
(4)
and keeps constant during the breakup process, means
2 Vnew = Vold. Table 2 shows the ratio Ekin/∆Esur for
minimum, middle and maximum droplet velocity v
for ethylene glycole. The drop volume and therefore
the drop diameter and radius depend on the
pulsevoltage of the piezo signal [3]. In the observed
experiments with ethylene glycole the diameter of a
droplet varied from 90 µm to 110 µm when increaseing the piezo pulse from 135 V to 155 V (cf.
figure 7).
Table 2: Ratio Ekin/∆Esur of ethylene glycole for
minimum, middle and maximum droplet velocity v
(energies given in nJ)
1.22
∆Esur
0.32
Ekin/∆Esur
0.8
1.54
1.51
0.37
4.2
4.22
1.82
0.48
8.8
velocity
Ekin
Esur, old
1.1 m/s
0.26
2.3 m/s
3.3 m/s
Theoretically the drop should splash for values of
Ekin/∆Esur being greater than 1, but this effect could
not be observed in experiments at the IfA. Reason for
that is that the kinetic energy of a drop is dissipated at
a great deal into thermal energy when attaching and
wobbling on the substrate’s surface, so the remaining
part of Ekin is not enough to let the drop break up into
two droplets. When using particles in the dispergent,
Figure 6: Droplet formation
A PSD (particle size measurement) figured out a d90
of 1.5 microns and a d50 of 0.9 microns for the used
particles Ag95/Pd5. The time τ specifies the point of
observation and shows the time delay after the start of
the piezo signal. By turning up the time τ the droplet
formation can be observed step by step when the
printer is arranged in observation mode.
Watching closely at the droplet formation, a
phenomena can be detected: The droplet filament is
not perpendicular as compared with pure dispergent
(cf. figure 3). Reason for that is an inhomogeneity of
the particles’ allocation in the colloidal suspension.
The better the suspension is homogenized the better
the filament generation is stable and perpendicular.
The skill in that situation is to find the special
dispersant additive that prevents agglomeration and
sedimentation of the particles in the suspension as
well as possible.
Besides it can be observed that a small droplet peeled
off the main drop but this small droplet catches up
with the main drop again. This phenomena is due to a
high pulse voltage UP and its taking place can be
affected by UP. If printing with solid material in the
dispergent in contrast to pure dispergent it can be
realized that the pulse voltage has to be adjusted to a
higher value, but generally it can be said: If the pure
dispergent allows printing then there is a printer
setting that enables printing of a colloidal suspension
as well.
The next experiments presented here were with a
20 M% colloidal suspension of Ag95/Pd5. It has
turned out that 20 M% is printable more stably than
30 M% which means the printing process does not
stop after a certain time as in 30 M% case. Reason for
stopping of the process is an inhomogeneity of the
print medium and therefore nozzle clogging. The
effort in keeping the 30 M% colloid homogeneous is
much higher, but good results in terms of conductive
paths could be achieved with a 20 M% colloid
already. Stable operation of the print head with a
particle loaded dispergent is a basic requirement.
When the colloidal ink is kept homogeneous, there is
almost no difference in printing stability of 7.5 M%
or 20 M%, but the advantage of having a higher
material flow rate when printing 20 M% Ag95/Pd5
gave rise to the idea to print the conductive tracks
with such a percentage of solid material in the colloid.
In figure 7 two spots of colloidal silver on alumina
substrate are mapped, having a diameter of
150 microns. The printed dots were sintered for
15 minutes at 850 °C an then photographed under a
light microscope. When having a look at the spots
under such a light microscope a three-dimensional
structure of the 20 M% spot can be identified, so the
mass fraction 20 % is the basis for the silver tracks
printed at the IfA.
Figure 7: Sintered spots of an Ag95/Pd5 colloidal
suspension with dispergent ethylene glycole on
alumina substrate, 7.5 M% (left) and 20 M% (right)
Another feature in the DoD-printing process is the
drying behaviour of the dots on top of the substrate.
Because of the high fraction of the dispergent
ethylene glycole to keep the colloid liquid and
thereby printable, dispensed dots are wet when the
substrate is unheated. The option to use dispergents
with a high evaporation rate such as e.g. ethanole is
not a good solution because through the high
evaporation rate, printing is possible only for a few
minutes before the nozzle dries and clogs [3]. That is
another point arguing for ethylene glycole as a
dispergent because of its lower evaporation rate.
When using this substance, heating of the substrate
while printing is necessary.
The use of a heating plate during the deposition
process improves the deposit quality. By heating the
substrate, the liquid in the droplets is flash-evaporated
on contact which minimizes the feature size in
comparison to room temperature. Figure 8 shows
printed spots on heated alumina substrate dried
through flash evaporation. The two spots are dried at
different heating plate temperatures and show
different surface profiles. In both cases the same
colloidal suspension was used but there is a significant influence of the substrate temperature.
Figure 8 : Dried spots of an Ag95/Pd5 colloid
(20 M%) with dispergent ethylene glycole on heated
alumina substrate, 100 °C (left) and 140 °C (right)
In case the substrate is heated up to 140 °C, the
dispergent evaporates too fast and so the drop dries
too fast. The drop is slowed down, solid material is
pushed to the edge and does not have the possibility
to build a surface profile according to the interfacial
tension between the colloid and the substrate. The
drop boundaries contain almost the whole solid
material in a ring while in the middle of the drop
there is nearly no silver-palladium any more. When
reducing the substrate temperature to 100 °C this
phenomena could be eliminated. The drop is not
drying and growing stiff that fast and the solid
material can arrange through the whole dot to form a
spot as shown in figure 8.
3.3. Printing of conductive silver tracks
Structural material was fabricated out of a silverpalladium colloidal ink consisting of particles in the
range of 500 nm to 2.5 µm diameter. Figure 9 shows
a 4 cm square alumina substrate on the heating plate
of the planar motor with different printed conductive
paths. Tracks with one, two and three layers were
printed.
For the conductive paths the substrate is heated up to
110 °C and the step size of the planar motor is set to
50 µm. The droplets can flow together before the
dispergent evaporates and can form a conductive line
with a coherent cross section. By varying the pulse
voltage and thus the velocity of the droplets in the air,
conductive paths of different width can be achieved.
Figures 10 and 11 show sintered conductive tracks of
different widths, obtained out of a 20 M% Ag95/Pd5
colloid with ethylene glycole as dispergent.
Figure 9: Alumina substrate with printed conductive
silver tracks on planar motor’s heating plate
The silver lines seen in figure 10 are single layer
(top), double layer (middle) and triple layer printed.
When printing single layer and sintering the line for
15 minutes at 850 °C, part of the silver-palladium
exhales from the substrate and the surface is fragile
and subsequently discontiguous. In case of a single
layer line no conductivity could be established.
The lines in figure 10 were set with a piezo voltage of
190 V and have a width of 270 microns (single layer)
and about 300 to 310 microns (double and triple layer
paths). The triple layer line boundary is more uneven
and not as smooth as the double layer line which
originates in the drying behaviour of the dispergent
on a heated substrate. The surface area of the second
Figure 10 : Sintered conductive silver paths on
heated alumina substrate, 1 layer (top), 2 layers
(middle) and 3 layers (bottom) under a light
microscope
layer has dried already before the third layer is
dispensed and has left back an uneven surface upon
which the third layer is printed. The influence of the
surface roughness seems to have a bigger effect on
the transition second to third layer than first to
second. One remedy in this case to obtain a better
path-boundary is an adaption and lowering of the
pulse voltage to set the droplets with less power.
In figure 11 a conductive silver line printed with the
same colloidal suspension as the lines in figure 10 is
mapped. The difference to the paths before is the
piezo pulse voltage which was chosen to 166 V and
thus 24 V lower than before.
Figure 11 : Sintered conductive silver path (3 layers)
on heated alumina substrate, width 150 µm, with
millimeter paper under a light microscope
Lowering the piezo voltage and reducing the droplet
velocity from 2.9 m/s down to 1.3 m/s reduces the
line width of the silver path substantially. The width
could be halved, measuring now 150 µm, and the
track boundary of a triple layer path looks smoother
than if printing at a high pulse voltage.
For making a rough measurement of the cross
sectional area of a DoD printed line, an atomic force
microscope (AFM) is used. The double layer line as
shown in figure 10 is AFM investigated (see
figure 12). The cross section could not be charted
completely because of the small measurement sector
of only 70 µm, whereas the conductive track has a
width of rough 300 µm. Nevertheless, it can be
noticed that from the boundary towards the center of
the track no gaps seem to appear. The section of the
AFM measured Z-range in the upper part of figure 12
is from outside to outside, middle to middle and
between the two inner marks. Outside to outside
measures a thickness of 2.62 µm, middle to middle
marks a vertical distance of 2.51 µm and the vertical
Z- range between the both inner marks is 2.44 µm.
The conductive path is printed double layer and a
thickness of 2.6 microns is reached before the path
center i.e. not even at the path’s thickest section, so it
can be guessed that the thickness of a double layer
path is at least 3 microns at the path’s center section.
AFM measurement methods enabling the mapping of
a complete cross section of the paths are already
under construction at the IfA. The bottom part of
figure 12 shows the double layer path from above, on
the left as an AFM shot, on the right under a light
microscope with sideways illumination. The tracks
presented here have a length of 25 mm and they
feature the electrical properties illustrated in table 3.
H the track thickness. Best results with variation in RS
appear when printing 3 layers, whereas a conductive
path cannot be achieved with one single layer (only).
4. Conclusions
Figure 12 : AFM picture of the profile of a sintered
conductive silver path (2 layers), and under a light
microscope (right)
With the relationship
RS = R ⋅
W
L
(5)
the sheet resistance RS of the paths can be determined
after measuring R. Based on a rough estimation of the
lines’ thickness, the resistivity ρ calculated according
to
ρ = RS ⋅ H
(6)
is approximately 200 mΩ m.
Table 3: Electrical characterization of DoD-printed
conductive silver tracks (25 mm length)
Track
Layers
R in Ω
W in µm
RS in mΩ/□
1
2
3
4
5
6
7
8
1
1
2
2
2
3
3
3
8.2
4.6
16.9
4.9
1.9
1.9
130
310
270
290
290
150
300
310
88
53
195
30
23
23
In equations (5) and (6) R is the measured track
resistance over the length L, W is the track width and
With a newly developed piezo printing system consisting of a DoD print head and a planar motor,
conductive tracks of different widths and thickness
could be printed. A 20 M% silver-palladium colloidal
suspension with ethylene glycole as dispergent was
used to print lines onto alumina substrate. For a stable
printing process, the colloid has to be well-dispersed
to avoid agglomeration and sedimentation. Through
the possibility of heating the substrate during the
printing process, different surface profiles could be
achieved with the temperature being an evaporation
control parameter for the used dispergent. The piezo
pulse voltage acts as a parameter to control the
droplet velocity, but velocity should not be too high
to obtain a smooth path boundary. Conductive tracks
printed at the IfA, were characterized by conductivity
measurement, light and atomic force microscopy.
Square resistances in a range complying with already
existing values could be measured [6]. Next steps are
to mingle a suitable dispersant additive to the colloid
to obtain a capable printing process.
Thanks to Prof. Rothe and his crew for providing
AFM-measurements and especially to Robert Bosch
GmbH for their support.
References
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tracks”, Journal of Materials Science – Materials in
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liquids in the nano- to picoliter range, Norderstedt,
Germany, http://www.microdrop.de/index.html
[3] S. B. Fuller, “Ink-jet printed nanoparticle microelectromechanical systems”, Journal of Microelectromec. Systems, Vol.11, Issue 1, pp. 54-60, 2002
[4] R. J. Hunter, “Foundations of Colloid Science”,
Oxford University Press, second edition, New York,
Chapter 1, pp. 1-2, 2002
[5] M. Mott, “Microengineering of ceramics by direct
ink-jet printing”, Journal of the American Ceramic
Society Vol. 82, Issue 7, pp. 1653-1658, 1999
[6] J. B. Szczech, “Ink Jet Processing of Metallic
Nanoparticle Suspensions for Electronic Circuitry
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2003