Functional materials for printed sensor structures

Abstract

Today, printing technologies like Ink Jet - printing or Aerosol Jet® - printing are used not only for printing graphics. The printing of electronic structures using metallic nano-particulate dispersions as so called “functional inks” followed by a thermal consolidation process for full functionality is of special interest. For a customized packaging, also ceramic and organic materials are taken into account, requiring flexible technologies for deposition of different materials. For a further miniaturization, for many electronic and
sensorial applications smaller structures are needed. Non-contact printing technologies are often a suitable solution to generate these functional structures of very different materials for various new applications. INKtelligent printing® combines the structuring possibilities of maskless printing technologies with the functionality of micro- and nano-scaled functional materials to generate functional structures like conductors, resistors and even sensors or sensor arrays, respectively. The functional structures are printed onto different flat or non-planar substrates like wafers, glass substrates, polymer foils or non-planar components. Typically, the structures have to be consolidated after printing, i.e. in a furnace, by use of laser treatment or high energy irradiation.

Full text

1
st
Joint International Symposium on System-Integrated Intelligence 2012:
New Challenges for Product and Production Engineering
Functional materials for printed sensor structures
Volker Zöllmer(1), Edit Pál(2,3), Marcus Maiwald(1), Christian Werner(1), Dirk Godlinski(1), Dirk Lehmhus(3), Ingo Wirth(1),
Matthias Busse(1,2,3)
(1) Fraunhofer IFAM, Funktionsstrukturen, Bremen, Germany
(2) University of Bremen, Faculty of Production Engineering, Near Net Shape Technology, Bremen, Germany
(3) ISIS Sensorial Materials Scientific Centre, University of Bremen, Bremen, Germany
Today, printing technologies like Ink Jet - printing or Aerosol Jet
®
- printing are used not only for printing graphics. The printing of
electronic structures using metallic nano-particulate dispersions as so called “functional inks” followed by a thermal consolidation
process for full functionality is of special interest. For a customized packaging, also ceramic and organic materials are taken into
account, requiring flexible technologies for deposition of different materials. For a further miniaturization, for many electronic and
sensorial applications smaller structures are needed. Non-contact printing technologies are often a suitable solution to generate these
functional structures of very different materials for various new applications. INKtelligent printing
®
combines the structuring
possibilities of maskless printing technologies with the functionality of micro- and nano-scaled functional materials to generate
functional structures like conductors, resistors and even sensors or sensor arrays, respectively. The functional structures are printed onto
different flat or non-planar substrates like wafers, glass substrates, polymer foils or non-planar components. Typically, the structures
have to be consolidated after printing, i.e. in a furnace, by use of laser treatment or high energy irradiation.
Keywords: Sensors, Alloy, Deposition
1. Introduction
In recent years, digital printing has become more and more
attractive. In contrast to conventional manufacturing methods
which are mainly subtractive, new classes of manufacturing
techniques have been established offering to manufacturers
significant benefits regarding cost, time and quality across a
broad spectrum of applications. These new techniques are
collectively known as additive manufacture. During additive
manufacture, material is deposited layer-by-layer to build, e.g.,
multilayer or three-dimensional parts or structures. Features of
additive manufacturing processes include direct CAD-driven,
“Art-to-Part” processing.
A much greater design flexibility offers the potential for
revolutionary new end-products with improved performance
based on novel size, geometries, materials and material
combinations. The typical size for drops in Ink Jet - printing
processes is about 50 µm in diameter, resulting in a drop volume
of about 60 pl, restricting the viscosity to a few mPa s. The
viscosity and the size of the generated drops in Ink Jet - printing
limit the possibility of miniaturizing structures down to about 50
µm. Aerosol Jet
®
enables a further miniaturization of deposited
structure below 20 µm, and allows a very flexible packaging with
very different materials on a large variety of substrates [1]:
Figure 1. Aerosol Jet
®
technology (Optomec Inc.)
The Aerosol Jet
®
technology allows deposition of
suspensions and material formulations covering a viscosity range
from 0.7 to 1000mPas and a particle size up to 1µm for metal
particles. An aerosol is produced from the suspension which is
carried by a transport gas to a print head (fig 1). The aerosol is
produced either with the help of an ultrasonic source (as seen in
fig. 1) or with the help of a high velocity air stream. The aerosol
droplet diameter is between 1 and 5 µm, which corresponds to a
volume of some femtolitres. Inside the print head a sheath gas
(e.g. nitrogen) focuses the aerosol beam and also prevents
clogging of the nozzle. Besides, the focused aerosol beam allows
printing on planar and non-planar surfaces with a minimum line
width of about 10 µm. The printing process is followed by a
thermal activation step to evaporate the fluid and to compact the
printed structures. Oven sintering, laser- and UV-curing are
possible solutions to ensure a best functionality and a high
surface adhesion of the deposited structures. A laser or low-level
oven sintering process guarantees faultlessness of the substrate
and electrical conductivity of the printed sensor structure.
Dielectric materials as well as high-viscosity globe-top materials,

for example, can be deposited by aerosol-printing and
consolidated by e.g. UV-curing, if necessary.
2. Material development for maskless printing
2.1. Deposition of conductive silver structures
To highlight the potential of maskless printing, a
commercially available silver ink from Advanced Nano Products
Co., Ltd., (ANP) can easily be deposited on different surfaces.
The ANP material (ink) contains about 50–60 wt.% of silver with
a mean particle diameter of about 20 nm. To prevent
agglomeration of particles, the ink contains about 20-40 wt.%
solvent as well as about 10–20 wt.% additives. Additives are
necessary to avoid agglomeration and sedimentation of the silver
particles as well as to lower the sintering temperature. The
following figures show SEM images of the ink before printing
(Fig. 2.a), after Aerosol Jet
®
deposition (Fig. 2.b) and after
furnace sintering at 350°C (Fig. 2.c) under hydrogen atmosphere
(60 min, heating rate 5 K/min). The ink has a homogeneous
particle size distribution without agglomerates before printing.
After deposition, the particles agglomerate to silver spheres with
a diameter of around 1 µm and form a porous structure. After
sintering at 350°C the silver particles are connected due to
diffusion processes resulting in a condensed structure. Details are
given in ref. [2]
Figure 2. SEM images of (a) silver ink, (b) silver ink after
aerosol printing and (c) silver ink after aerosol printing and
furnace sintering @ 350°C, 60 min, heating rate 5 K/min [2].
Fig. 3 shows the dependence of the conductivity of printed
silver structures on sintering time. For this purpose, test
structures (van-der-Pauw-geometry) have been printed and
sintered at T = 350°C and variation of sintering time from 60 to
600 min. There are no large differences in the measured
conductivities which reach values of about 50–70% compared to
bulk silver (6.25 x 10
7
Sm
-1
).
Figure 3. Left: Printed silver test structures, right: measured
electrical conductivities.
Higher conductivities are difficult to achieve due to
additives remaining in the structures even after a thermal post
treatment.
2.2. Printable alloys
In addition to noble metals, certain alloys have unique
conductive properties. The electrical resistivity of e.g. CuNi44
alloy is almost temperature independent and shows considerable
strain sensitivity, which makes CuNi44 a very interesting
material e.g. for strain gauges, but also for printed resistors.
However, in contrast to the preparation of noble metal inks, the
preparation of an alloy ink is much more complex. Various
chemical methods such as hydrothermal reduction, solution
combustion, sol-gel process, diol- and polyol reduction method,
water-in-oil (w/o) microemulsion technique etc. have been used
for the preparation of alloy or bimetallic nanoparticles.
Fig. 4 presents recent results of electrical characterization of
printed CuNi van-der-Pauw – structures with various Cu:Ni
ratios. Details of the preparation routes are given in reference.
[3].
Figure 4. Resistivity change of printed alloy layers as a
function of sintering temperature [3].
As seen in fig. 4, colloid chemical preparation of CuNi alloy
inks leads to electrical conductivities in printed van-der-Pauw test
structures suitable to facilitate sensor applications[3]. Similar
studies focussing on the alloy composition Cu55Ni44Mn1 are
discussed in reference [4].
3. Printed sensor structures
In the following chapter, strain gauges and thermocouple
sensors deposited by use of Aerosol Jet
®
will be discussed:
3.1. Printed strain gauges
The printed strain gauges consist of three different material
layers, all deposited with Aerosol Jet
®
technology. First, a
polymer layer (Duraseal 1529H) with a thickness of about 5 µm
is printed and cured to isolate the sensor from the metal substrate.
In a second step the silver layer (ANP) with a thickness of about
1–3 µm in form of a meander structure is deposited. Finally the
sensor is encapsulated again with a polymer (Duraseal 1529H).
Fig. 5 shows an example of the three layer printed strain gauge
package on an aluminum surface. Details are discussed in ref. [5].

Figure 5. Printed silver strain gauges on aluminum surface.
For the characterization of printed silver strain gauges, a
second strain gauge was printed on the metal surface in a 90°
rotated position to avoid temperature influences caused by the
high linear temperature coefficient of resistance of silver. In
addition, a reference foil strain gauge (HBM Inc.) was glued on
the substrate for comparison of the results. Fig. 6 shows the
results of periodic stress (1000N tensile stress and 500N
compressive stress) measurements of 1000 cycles with 0.5 Hz.
Results show a constant peak to peak value of the printed strain
gauge indicating reliable and reproducible signal and successful
temperature compensation.
Figure 6. Characterization of printed strain gauges.
3.2 Printed thermocouples
INKtelligent printing
®
also allows the direct deposition of
thermo couples on planar and non-planar surfaces and
components. Thermocouples consist of two different materials,
e.g. two different metals, and they produce an electrical voltage
which is proportional to a temperature difference between a “hot”
and a “cold” junction. A thermoelectric voltage results between
the two junctions which is proportional to the temperature
difference of the materials. Fig. 7.a presents a thermopile
consisting of five silver-nickel thermocouples (details of the
nickel dispersion preparation are given in [2]. The serial
connection increases the voltage output compared to a single
thermocouple. Figure 7.b indicates the measurement results of a
thermally stressed thermopile. The sensor performance was
characterized by using a hotplate to heat up the hot junctions
while the output voltage was recorded in dependence of the
temperature. The black curve shows the thermoelectric voltage
while the red curve displays the temperature of the hot junction.
The measurements show a good performance of the printed
thermopile indicating that the thermoelectric voltage is
proportional to the temperature difference between the hot and
cold junctions. Experimental details are discussed in ref. [5].
Figure 7. Printed thermocouple on glass surface.
4. Conclusion
Printing technologies like Ink Jet and Aerosol Jet
®
, which
are employed for INKtelligent printing
®
, allow a digital,
maskless deposition of a wide range of functional materials.
Nano-particulate metallic and ceramic suspensions as well as
polymer formulations can be used as so called “functional inks”.
The mask- and contact less printing processes are interesting for
deposition of micro- or mesoscaled structures on planar and even
non-planar surfaces. In this paper, several examples have been
presented: Printed strain gauges applied on an aluminum surface,
using a printed polymer as isolating and protective layer have
been characterised. In addition, a printed thermopile was
presented.
Acknowledgements
The authors acknowledge financial support given by the
Innovation Cluster “Multifunctional Materials and Technologies”
(MultiMaT) from the state of Bremen via the European Regional
Development Fund ERDF, the Fraunhofer society and industry in
equal parts. Work on colloid chemical synthesis of CuNi alloy
nanoparticles has been supported by the State and University of
Bremen in the framework of the ISIS (Integrated Solutions in
Sensorial Structure Engineering) Sensorial Materials Scientific
Centre (Fund Nr. 54 416 915 and 54 416 919).
[1] Zöllmer, V., Müller, M., Renn, M., Busse, M., Wirth, I.,
Godlinski, D., Kardos, M., 2006, Printing with aerosols: A
maskless deposition technique allows high definition printing of a
variety of functional materials, European Coatings Journal, 6-7,
46-50.
[2] Maiwald, M., 2010, Untersuchungen zum Einfluss der
Mikrostruktur auf die Eigenschaften aerosolgedruckter
Sensorstukturen, Fortschrittsberichte VDI, Nr. 388.
[3] Pál, E., Kun, R., Schulze, C., Zöllmer, V., Lehmhus, D.,
Bäumer, M., Busse, M., 2012, Composition-dependent sintering
behaviour of chemically synthesised CuNi nanoparticles and their
application in aerosol printing for preparation of conductive
microstructures, Colloid Polym. Sci., DOI: 10.1007/s00396-012-
2612-3.
[4] Pal, E., Zöllmer, V., Lehmhus, D., Busse, M., 2011, Synthesis
of Cu
0.55
Ni
0.44
Mn
0.01
alloy nanoparticles by solution combustion
method and their application in aerosol printing, Colloids and
Surfaces A: Physicochem. Eng. Aspects, 384, 661-667.
[5] Maiwald, M., Werner, C., Zöllmer, V., Busse, M., 2010,
INKtelligent printing® for sensor application, Sensors Review,
30, 19-23.