Characterization of Functional Ink Composites

Rheological characterization
was carried out with an AresG2 rheometer
with a 40 mm flat plate geometry and a 1 mm gap. The mixture was placed
onto the rheometer plate immediately after the complete dissolution
of the polymer and suitable dispersion of the magnetic particles was
obtained. Flow curves were acquired by a three-shear rate sweeps program
(up–down–up), setting a nonstop ramp and a 0 to 300
s^–1 shear rate. The apparent viscosity at 3 s^–1 was studied for the unsteady state (curve 1) once
for this condition the structure was less disturbed. All other shear
rate values (steady state) were evaluated from curve 3. The determined
viscosity (η) value for the developed ink was evaluated at distinct
shear rates (γ), assuming a power-law model^{39 (link),40 (link)} (eq 2) where the coefficients N and K are experimentally determined.
Knowing that the
surface properties of a substrate are among the most important parameters
in the printing of functional materials, determining not only the
printing resolution but also the stability of the printed features,^{41 (link)} contact angle measurements of the different
samples were performed using a Data Physics OCA20 instrument by the
static sessile drop method using different testing mixtures/liquids:
PVA-FO ink, Ag ink, and water. For that, 3 μL drops of different
samples were left on the surface of the different substrates (paper,
PET, and PVA-FO) and the contact angles were determined using the
SCA20 software. The mean contact angle and standard deviation were
obtained after measurments at six different locations.
The morphology
of the developed materials was evaluated by scanning
electron microscopy (SEM) using a NanoSEM-FEI Nova 200 (FEG/SEM) scanning
electron microscope (10 kV). Before experiments, all samples were
coated with Au with a Polaron SC502 sputter coater. The thickness
of the layers was calculated from five images with 15 measurements
in each image using the ImageJ software.
Adhesion of the printed
inks was evaluated with an adapted tape
peel test^{42 (link)} carried out on samples of size
1 cm × 1 cm. Briefly, an adhesive tape (3 M Scotch Magic tape
810) was pressed on the surface of the printed samples with different
forces (100, 200, 300, and 400 N) for 30 s using a Shimadzu AG-IS
universal test setup in compression mode at 2 mm·min^–1. After that, the tape was removed from the sample at the same velocity
while monitoring the required force. The different samples were weighed
before and after each test to determine the mass loss in each experiment.
The samples’ DC surface electrical conductivity was obtained
by measuring the characteristic IV curves at room temperature with
a Keithley 6487 picoammeter/voltage source. Previously, two rectangular
electrodes (4 mm length x 1 mm width, with a spacing of 3 mm) were
deposited using a Polaron, model SC502 sputter coater. From the IV
characteristics of the samples, the electrical resistivity (ρ)
was determined, considering the geometrical characteristics according
to eq 3 where R is the film resistance,
calculated by the inverse of the slope of the IV data, l is the distance among Ag electrodes, w is the length
of the Ag electrodes, and t is the sample’s
thickness, measured with a Digimatic Micrometer MDC-25PX. The electrical
conductivity σ was then determined as ρ^–1.
Magnetization M(H) curves were obtained at room temperature
up
to 1.85 T using a MicroSense EZ7 vibrating sample magnetometer. From
these loops, saturation magnetization and coercive field of the composites
were obtained.
To evaluate the influence of an applied magnetic
field on the paper-based
antennas, an external bias field was applied (in-plane and out-of-plane
to the plane of the antenna’s surface) by an electromagnet
with a maximum value of 0.4 T.