Experimental
2.1. Materials
Commercial medapoxy inject (ER) was obtained from Algerian Granitex.
Tetrahydrofuran (THF), magnetite (Fe3O4) 99.9% of purity, boron trifluoride ethylamine
complex (BF3-complex 95% of purity), hexahydrophtalic anhydride (HHPA)
and polyaniline doped para-toluene sulfonic acid (PANI-PTSA) with conductivity
value of 400–600 S/m and 2–3 m of particle size were purchased from
Sigma–Aldrich.
2.2. Composite preparation
Composites of PANI/epoxy resin and hybrid PANI/Fe3O4/epoxy resin, cured
by HHPA/BF3-complex were prepared by mixing process. PANI and Fe3O4 were
dispersed in THF as solvent separately for 1 h then, the tow dispersions were mixed
using an ultrasonic bath for 3 h at 50 ◦C, followed by mechanical stirring to improve
the dispersion quality and to avoid aggregates. Then, epoxy resin was added to
the above mentioned solution at a continuous moderate mechanical stirring speed
and heating temperature of 65 ◦C for 4 h to remove the solvent residues that
have an evaporation point below the crosslinking temperature and to assure high
dispersion quality and a homogeneous mixture [28]. The weight contents of the
PANI and magnetite in the composites varied from 1 to 25 wt%. When the solvent is
evaporated, 81.5 parts of the curing agent with catalyst was added for 100 parts of
epoxy resin [29]. The obtained mixture was immediately poured into appropriate
molds to provide films with different thicknesses. The films were subjected to
crosslinking steps in a thermostatic oven vacuum, first baking at 80 ◦C for 4 h and
then post-curing at 120 ◦C for 24 h. In the same manner, composites of PANI (15%)
with magnetite loading from 5 to 25% were prepared.
2.3. Instrumental analysis
XRD analysis of the samples were performed at room temperature using a
D8 Advance-Bruker AXS diffractometer with CoK radiation at a wavelength of
1.78897A˚ , operating at 40 kV and 35 mA in the range of 2 = 10–80◦. Morphological
analyses were carried out with a scanning electron microscope type Hitachi
S-4300 SE/N operating between 5 kV and 20 kV and 35–50 A. In order to avoid the
effect of electric charge on the surface and to facilitate the flow of the electrons,
the samples were coated with a thin layer of gold. The electrical conductivity of the
different samples was measured at room temperature with a four-probe method.
Pellets with a thickness of 0.5–0.6 mm of filler powder were prepared, and thin contact
points of gold were deposited to ensure good contact between the sample and
the electrode. The DC conductivity was calculated from the current–voltage dependence.
Electromagnetic reflected and transmitted scattering parameters (S11, S21),
were carried out performing wave guide measurements between 12.4 and 18 GHz
in transmission and reflection mode using WR-62 wave-guide for the Ku band and
a Vector Network Analyzer (VNA) type Agilent technologies, E8362B working from
10 MHz to 20 GHz. Samples were prepared and cured into rectangular holder with
different thicknesses and clamped between two flanges of the input and output
wave guides. The wave-guides were connected to the vectorial network analyzer
with two cables in APC7 standards. To measure the reflection coefficient S11 and
transmission coefficient S21, custom designed TRL calibration kits are applied [30].
Great attention was brought to avoid air gap between the sample and the wave