D.S. Klygach et al.
Journal of Magnetism and Magnetic Materials 526 (2021) 167694
composite and their size, as well as on the properties of the matrix. The
synthesis of MCC can be carried out in one-pot if a common solvent is
used for the precursors of nanoparticles and the matrix [41,42]. The Fe-
based MCC were produced from iron nitrate and phenol–formaldehyde
resin [43]. Co-based MCC were produced from Co and some organic
compounds with further pyrolysis [44–46].
In this paper we present the novel synthetic route to carbon-based
composites with Fe, Co and Ce nanoparticles and the results of investi-
gation of their electrodynamic properties in the frequency band of
30–50 GHz.
2. Materials and methods
Phenol-formaldehyde resin of the novolac type (with 23 wt% mass
fraction of hexamethylenetetramine), citrates of iron, cerium and cobalt,
as well as 95 v% ethanol were used as raw materials for the synthesis of
MCC.
Citrates were obtained by reacting FeCO3, CoCO3, or freshly
precipitated CeO2 with equimolar amounts of citric acid in a concen-
trated aqueous solution at 80 ◦C. After the completion of the reaction,
the citrates were dried for 20 days at 40 ◦C. In this case, the citrates of
cobalt and cerium formed a crystalline mass, while the iron citrate
remained amorphous. The resulting iron-containing product was
Fig. 1. XRD patterns of MCC with 15 wt% of metals (MCC-Fe15; MCC-Co15
and MCC-Ce15).
Table 1
¨
¨
analyzed by Mossbauer spectroscopy. The Mossbauer spectra of iron
citrate can be satisfactorily described by a superposition of three dou-
blets, the isomer shifts of which correspond to iron ions in an octahedral
oxygen environment in the oxidation states +3 (88 wt%) and +2 (12 wt
%).
The results of quantitative full-profile Rietveld analysis of samples with 2, 7, and
15 wt% Fe.
Phase name
Chemical formula
2 wt% Fe
7 wt% Fe
15 wt% Fe
Graphite
Magnetite
Cementite
γ-Iron
C
81(2)
9.5(2)
3.7(2)
2.4(2)
3.5(2)
83(2)
9.5(3)
5.4(3)
2.4(8)
0.1(2)
78(2)
Fe3O4
Fe3C
Fe
9.4(2)
10.4(2)
1.9(2)
0.6(2)
The MCC were prepared by heating or pyrolysis of a mixture of metal
citrate and phenol–formaldehyde resin. To prepare composites with 15
wt% of metal (Fe, Co or Ce), 1.464 g of iron citrate, 1.451 g of cobalt
citrate, or 0.669 g of cerium citrate were added to portions of 1.2 g of
resin. A proportionally smaller mass of citrates was taken to produce
carbon composites with 2 and 7 wt% of metal. For comparison, we also
prepared a composite sample containing 7 wt% iron introduced into the
resin in the form of Fe(NO3)3*9H2O.
α-Iron
Fe
transmission line with the investigated material as a dielectric, a two-
port TRL calibration was used. For uniform filling of the waveguide
insert with material, the measuring line was fixed vertically to reduce
possible errors. The measurements were carried out in the frequency
band of 30 to 50 GHz using an R&S®ZVA 50 vector network analyzer.
The permittivity and permeability of the samples under study were
calculated using the Nicolson-Ross-Weir method. For this, the frequency
dependences of the S-parameters were measured [47,48].
To increase the uniformity of the distribution of the components, the
metal citrates were ground with resin to a homogeneous fine mass, and
then a few drops of ethanol were added to it until a very viscous solution
was formed. Heating was carried out in a muffle furnace without oxygen
access when heated first for 20 h to 500 ◦C, then for 3 h to 800 ◦C, after
which the furnace was switched off and cooled.
3. Results and discussion
After heating, the morphology and composition of the samples were
studied using a Jeol JSM-7001F scanning electron microscope with an
attached Oxford INCA X-max 80 energy dispersive X-ray spectrometer
(EDX). The phase composition was determined using a RigakuUltima IV
powder X-ray diffractometer with CuKα radiation.
After heating, all samples form black carbon mass with a porosity of
40–80% and pore diameters of 5–1000 μm. To study the morphology of
the composites using a SEM, the samples were broken and then their
fragments up to 3 mm in size were glued onto an electrically conductive
substrate, with a fresh cleavage upwards. For X-ray phase analysis and
¨
The Mossbauer absorption spectra were obtained on an MS1104EM
¨
express Mossbauer spectrometer manufactured by CJSC Cordon. The
¨
Mossbauer spectroscopy, the samples were ground in a mortar.
source of γ-radiation was 57Co in a matrix of metallic rhodium with an
activity of 47 mCi produced by RITVERTC GmbH. The spectra were
recorded in an evacuated cryostat both at 77.6 ± 0.3 K and at room
temperature (296 ± 3 K). The spectra were recorded in high resolution
mode (1024 points) with a noise/signal ratio of less than 1%.
Fig. 1 demonstrates XRD patterns of the MCC fixed concentration
(15 wt%) of the different metals. The diffraction pattern of MCC-Fe15
(composite with 15 wt% Fe) represents a superposition of reflections
of graphite, Fe3O4 and Fe3C (Fig. 1, Table 1). The calculation of the
crystal sizes (coherent scattering regions, CSR) of magnetite by the full-
profile Rietveld analysis of the X-ray diffraction pattern leads to a value
of 28 nm. This is consistent with the results of electron microscopy,
which determined that the size of magnetite crystals is in the range from
10 to 200 nm (Fig. 2a). The mass ratio of the phases is shown in Table 1.
Along with magnetite, there appears to be maggemite, having similar
structure, the presence of the latter is indicated by weak reflections (for
example, in the range of 10-25◦ 2θ). X-ray phase analysis of the sample
with iron nitrate [43] also revealed the phases of graphite (main peak
near 26.3◦), Fe3O4 and Fe3C, with the average size of Fe3O4 crystals
being 60–80 nm by the full-profile Rietveld analysis.
¨
Mathematical processing of the Mossbauer spectra was performed
using the SpectrRelax 2.4 software. The spectra were described by
combinations of symmetric doublets and sextets with fixed ratios of
intensity and widths of resonance lines. Chemical shift values were given
relative to α-Fe.
Using an absorption measuring line, the S-parameters of powdered
materials were determined. A WR-22 waveguide with dimensions of 5.7
× 2.8 mm was used as a measuring line. The material sample was placed
in a waveguide insert between two limiters made of material with pa-
rameters
ε ~ 1.1 and μ ~ 1. These limiters do not affect the measured
parameters, since their influence was taken into account when cali-
brating the device. When measuring the S-parameters of a waveguide
¨
The Mossbauer spectra of composites, obtained both at room
2