Corresponding author: Dmitriy G. Muratov (muratovdg@yandex.ru)
FeCoCu ternary nanoparticles distributed and stabilized in the carbon matrix of FeCoCu/C metal-carbon nanocomposites have been synthesized using controlled IR pyrolysis of precursors consisting of the “polymer / iron acetylacetate / cobalt and copper acetates” type system obtained by joint dissolution of components followed by solvent removal. The effect of the synthesis temperature on the structure, composition and electromagnetic properties of the nanocomposites has been studied. By XRD was shown that the formation of the FeCoCu ternary nanoparticles occurs due to the interaction of Fe3С with the nanoparticles of the CoCu solid solution. An increase in the synthesis temperature leads to an increase in the size of the metal nanoparticles due to their agglomeration and coalescence as a result of matrix reconstruction. Furthermore, ternary alloy nanoparticles having a variable composition may form depending on the synthesis temperature and the content ratio of the metals. Raman spectroscopy has shown that the crystallinity of the carbon matrix of the nanocomposites increases with the synthesis temperature. The frequency responses of the relative permittivity and permeability of the nanocomposites have been studied at 3–13 GHz. It has been shown that a change in the content ratio of the metals noticeably increases both the dielectric and the magnetic losses. The former loss is caused by the formation of a complex nanostructure of the nanocomposite carbon matrix while the latter one originates from an increase in the size of the nanoparticles and a shift of the natural ferromagnetic resonance frequency to the low-frequency region. The reflection loss has been calculated using a standard method from the experimental data on the frequency responses of the relative permittivity and permeability. It has been shown that the frequency range and the absorption of electromagnetic waves (from –20 to –52 dB) can be controlled by varying the content ratio of the metals in the precursor. The nanocomposites obtained as a result of the experiment deliver better results in comparison with FeCo/C nanocomposites synthesized under similar conditions.
Muratov DG, Kozhitov LV, Zaporotskova IV, Popkova AV, Tarala VA, Korovin EYu, Zorin AV (2023) Synthesis, structure and electromagnetic properties of FeCoCu/C nanocomposites.
Magnetic nanoparticles (materials) play an important role in the rapidly developing branches of advanced science and industry. Magnetic nanoparticles show good promise for the development of magnetic storage media and the synthesis of ferromagnetic liquids [
Magnetic nanocomposites (e.g. magnetic nanoparticles coated with non-magnetic insulating materials) have been since recently used in the production of electromagnetic radiation absorbers. As compared with conventional ferrite-based absorbers, metals and magnetically soft alloys encapsulated in various non-magnetic shells possess all the required parameters: high magnetic moment and susceptibility, low coercive force and high resistivity.
The synthesis of Fe-, Co- and Ni-containing binary magnetic alloy nanoparticles allows varying their magnetic properties over a wide range [
The high cost of Co gives impetus to studies of the structure and electromagnetic properties of FeCo nanoparticles in which Co is replaced for another metal (Ni, Al or Cu). According to recent publications [
Analysis of reported works suggests that the magnetic parameters of multicomponent alloys depend on the chemical composition of the alloy and the size, morphology and crystallinity of the nanoparticles.
We synthesized FeCoNi/C nanocomposite which is but slightly inferior to FeCo/C nanocomposite in electromagnetic radiation absorption [
The Fe
It is of interest to consider the synthesis and electromagnetic properties of FeCoCu/C nanocomposite.
Cu@FeCo nanoparticles containing Cu cores and FeCo shells were synthesized using chemical reduction [
A correlation was found between the phase composition and the magnetic properties of the Cu–Ni–Co–Fe alloys synthesized by crushing alloy components in a ball mill followed by annealing of the crushed product [
Thus, there are insufficient literary data on the synthesis of Fe–Co-Cu nanoparticles and the properties of FeCoCu/C nanocomposites.
The FeCoCu/C nanocomposites were synthesized using the system containing a polymer (polyacrylonitrile (PAN)), metal salts (iron acetylacetonate, cobalt acetate and copper acetate) and a solvent (dimethylformamide). All the metal salts except iron acetylacetonate were in the form of salt hydrates.
The joint solution principle was used for achieving a uniform distribution of the components in the precursor. Drying at
After that the precursor was subjected to preliminary annealing in air. The process was stepwise, consisting of heating to 150 °C with holding for 15 min and then heating to 220 °C with holding for 15 min. This stage is required for the initial cross-linking of the polymer and the formation of a rigid and stable structure of the polymer chains for hindering metal diffusion. The preliminary heat treatment is also required for the decomposition of metal salts to oxides.
The next stage was the synthesis of the metal-carbon nanocomposites. IR heating was in the 500 to 700 °C range in an inert atmosphere (ultra-high purity nitrogen). The linear heating rate was 50 °C, the holding time being 5 min. The specimens were then cooled down and crushed in a mortar to a uniform fraction.
The nanocomposites were synthesized in a MILA-5000 IR furnace (ULVAC GmbH, Germany) having halogen lamps with a total power of 4 kW. The maximum intensity of radiation delivered by these lamps is in the 0.8–1.2 mm range which is optimum for the acceleration of the polymer carbonization processes and the decomposition of the metal-organic salts. The lamps are separated from the reaction zone as they are installed outside the quartz reactor.
The structural and phase composition of nanocomposites was determinated by XRD at room temperature on a DIFRAY X-ray diffractometer in Cr
Raman spectra were taken with an inVia Raman Microscope (Renishaw plc) Raman spectrometer with 514 nm laser excitation.
The relative permittivity and permeability were measured using a resonance technique with a rectangular multi-mode resonator. The microwave generator and indicator was a E 8363 B vector network analyzer from Agilent Technologies. The measurements were conducted using equipment and methods developed at the Joint Use Center for Radiophysical Measurements, Diagnostics and Parameter Study of Natural and Artificial Materials of the Tomsk State University
The reflection loss was calculated using the following relationships in the assumption that the nanocomposite layer is infinite, planar and positioned on the metal, and that a planar wave is incident from the ambient space.
where
where
The introduction of a diamagnetic metal, i.e., copper, into the Fe–Co system changes both the microstructure and the properties of the FeCoCu alloys.
X-ray phase analysis showed that, unlike the FeCo/C nanocomposites (with equiatomic metal contents), the PAN-based FeCoCu/C nanocomposites do not form single-phase systems. X-ray diffraction patterns of the FeCoCu/C nanocomposites synthesized at 500–700 °C are shown in Fig.
X-ray diffraction patterns of FeCo/C and FeϹoCu/Ϲ nanocomposites synthesized at different temperatures
The introduction of copper in the FeCo alloy affects the phase composition and structural patterns of the nanocomposites. It should be taken into account that the solubility of copper in iron is but a few percents whereas its solubility in cobalt is higher. One should also bear in mind that the reduction of copper and cobalt may occur at about 200 °C while the reduction of iron requires 450 °C or higher (depending on the reducing agent).
Phase analysis of the FeCoCu/C nanocomposite specimens synthesized at
Comparing phase analysis data for FeCoCu/C with those for the FeCo/C nanocomposites one can find that part of iron is in the form of carbide in the FeCoCu/C nanocomposite synthesized at 600 °C. It seems that the formation of the CuCo alloy reduces the probability of cobalt dissolution in iron.
An increase in the FeCoCu/C nanocomposite synthesis temperature to
– an increase in the nanocomposite synthesis temperature leads to a tangible increase in the intensity of the reflections corresponding to the metallic phases, suggesting an increase in the particle sizes;
– the Fe3C reflections are completely eliminated;
– the positions of the bcc phase reflection maxima shift to the low diffraction angles, suggesting an increase in the lattice parameter as a result of iron dissolution.
The iron carbide reflections are eliminated with an increase in the synthesis temperature since the diffusion rate increases at higher temperatures, presumably resulting in the interaction between the nanoparticles having different crystal lattice types (CoCu, FeCo and Fe3C). The interaction of cobalt and copper with the Fe3C nanoparticles leads to partial dissolution of these metals. This in turn causes the decomposition of iron carbide and the formation of FeCoCu nanoparticles. A similar observation was made for Cu-Fe/С nanocomposites [
X-ray diffraction patterns of FeϹo/Ϲ и FeCoCu/C nanocomposites synthesized at 700 °C (2θ = 65–75° angular range)
Calculation of the coherent scattering region (
The growth of the nanoparticle sizes with an increase in the synthesis temperature is attributable to metal agglomeration and coalescence during the structural transformation of the nanocomposite carbon matrix and the reduction of iron oxides.
An increase in the relative copper content in the nanocomposites leads to changes in the phase composition and a redistribution of the reflection intensities of the above-mentioned phases (Fig.
X-ray diffraction patterns of FeCoCu/C nanocomposites with different content ratios of metals
The reflection maximum of the bcc phase shift towards the higher angles and corresponds to the FeCo bcc phase, whereas the fcc peaks shift leftwards, indicating the formation of a CuCo solid solution having an almost equiatomic composition. Accordingly, the remainder of copper is present in the form of oxides, probably due to secondary oxidation in air.
Along with changes to the metallic phases, an increase in the synthesis temperature causes changes in the structure of the carbon matrix. The X-ray diffraction of the composite grown at 600 °C (Fig.
Raman spectra of FeCoCu/C nanocomposites synthesized at different temperatures and with different content ratios of metals: (
Direct comparison between the spectra showed that an increase in the nanocomposite synthesis temperature increases the intensity of the G band relative to that of the D band (Table
Raman data for FeCoCu/C nanocomposites
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500 | 40 : 40 : 20 | 1.43 | 0.35 | 0.88 | 3.1 |
600 | 40 : 40 : 20 | 1.17 | 0.29 | 0.67 | 3.8 |
700 | 40 : 40 : 20 | 0.9 | 0.14 | 0.61 | 4.9 |
700 | 35 : 35 : 30 | 0.92 | 0.15 | 0.63 | 4.8 |
*
On the other hand, an increase in the relative content of copper in the nanocomposite slightly reduces the size of
Thus, analysis of the Raman spectra of the FeCoCu/C metal-carbon composites showed that the carbon matrix of the materials is a mixture of graphite nanocrystallites and amorphous graphite clusters and that an increase in the nanocomposite synthesis temperature increases the cluster sizes of the crystalline fraction in the microstructure of the carbon matrix. An increase in the Cu content in the nanocomposites hinders the structural ordering in the carbon matrix by reducing the solubility of carbon in the Fe-containing particles.
As was shown above for the example of nanocomposites with Ni or Al addition, the structural changes in the nanocomposite matrix and the changes in the size and composition of the nanoparticles or the relative content of copper in the precursor caused by an increase in the nanocomposire synthesis temperature entail tangible changes in the electromagnetic properties of the materials. We showed that the nanocomposites synthesized at
Data on the effect of the metal content ratio on the dielectric and magnetic properties of the nanocomposites in microwave fields are presented in Fig.
Frequency dependencies of relative (
Analysis of the relative permittivity of the two selected FeCoCu alloy compositions showed that the real part of the permittivity of the nanocomposite having the Fe : Co : Cu = 40 : 40 : 20 metal content ratio is 14.5 at 3–14 GHz against 13.0 for the FeCoCu/C nanocomposite having the Fe : Co : Cu = 35 : 35 : 30 metal content ratio. The imaginary part of the permittivity of the nanocomposite having the Fe : Co : Cu = 40 : 40 : 20 metal content ratio is 3.0 at 3–14 GHz against ε’’= 4.0 for the nanocomposite having the Fe : Co : Cu = 35 : 35 : 30 metal content ratio.
An increase in the relative copper content in the nanocomposite reduces the magnetic loss tangent but increases the dielectric loss tangent. The decrease in the magnetic loss tangent is accounted for by a decrease in the imaginary part of the relative permeability which is in turn caused by a decrease in the general magnetization of the alloy. However the imaginary part of the relative permittivity increases, on the one hand, due to a more amorphous carbon matrix and, on the other hand, because of the formation of graphene-like carbon structures as indicated by Raman spectroscopy data. In the meantime the real part of the relative permittivity decreases, probably, because of an increase in the electrical conductivity of the nanocomposite due to the formation of copper-based alloy nanoparticles.
By way of comparison we present relative permittivity data for the FeCo/C nanocomposite [
Figure
Calculations showed that the optimum thickness of FeCoCu/C nanocomposite based absorber layers is greater for specimens with a higher copper content (Fig.
Absorber layer thickness optimization
Analysis of the results shows that, taking into account the maximum magnetic loss, the optimum absorber layer thickness varies but slightly (from 1.4 to 1.5 mm) but the reflection index RL decreases significantly, from –26 to –38 dB.
Calculated data on the reflection loss for different absorber layer thicknesses are shown in Fig.
Frequency dependencies of reflection loss for nanocomposites with different metal content ratios in precursor, Fe : Co : Cu: (
Analysis showed that the FeCoCu/C nanocomposites are of interest since their reflection loss is between –20 and –32 dB (Fe : Co : Cu = 40 : 40 : 20) and between –25 and –52 dB (Fe : Co : Cu = 35 : 35 : 30) at 3–13 GHz. These values correspond to the reflection value of 0.9–0.97 and 0.944–0.997 rel.u. respectively and are higher than those for the FeCo/C nanocomposite (Fig.
An increase in the nanocomposite layer thickness causes a shift of the reflection maximum towards lower frequencies because of resonance phenomena related to the thickness of the material. The magnitude of this shift decreases with an increase in the relative copper content in the nanocomposite. For a 3 mm layer of the nanocomposite having the Fe : Co : Cu = 35 : 35 : 30 metal content ratio, the abovementioned shift is 6.7 GHz (from 13.4 to 6.7 GHz), while for the FeCoCu/C nanocomposite having the Fe : Co : Cu = 40 : 40 : 20 metal content ratio this shift is 7.4 GHz (from 13.8 to 6.4 GHz). The nanocomposite having the Fe : Co : Cu = 35 : 35 : 30 metal content ratio has the lowest reflection value and should therefore be given preference.
Thus, by and large the FeCoCu/C nanocomposites in the frequency range studied in this experiment are the most efficient at the Fe : Co : Cu = 35 : 35 : 30 metal content ratio and hence are more promising, e.g. in comparison with the FeCo/C nanocomposites.
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