Open AccessArticle

Influence of Synthesis Conditions on the Structure, Composition, and Electromagnetic Properties of FeCoSm/C Nanocomposites

Dmitriy Muratov

^1,2

Lev Kozhitov

^1,3,

Irina Zaporotskova

^3,*,

Alena Popkova

¹,

Evgeniy Korovin

⁴,

Sergey Boroznin

³ and

Natalia Boroznina

^3,*

Institute of New Materials, National Research Technological University “MISIS”, Leninsky Prospekt, 4, Moscow 119049, Russia

Laboratory of Chemistry of Polyconjugated Systems, Institute of Petrochemical Synthesis A.V. Topchiev Russian Academy of Sciences, Leninsky Prospekt, 29, Moscow 119991, Russia

Institute of Priority Technologies, Volgograd State University, Universitetskii Prospect 100, Volgograd 400062, Russia

⁴

Faculty of Radiophysics, National Research Tomsk State University, Ave. Lenin, 36, Tomsk 634050, Russia

Authors to whom correspondence should be addressed.

J. Compos. Sci. 2025, 9(2), 62; https://doi.org/10.3390/jcs9020062

Submission received: 24 December 2024 / Revised: 16 January 2025 / Accepted: 23 January 2025 / Published: 1 February 2025

(This article belongs to the Special Issue Recent Progress in Hybrid Composites)

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Figure 1
(a) Change in the mass of precursors of the FeCoSm/C nanocomposite during IR heat treatment, (b) temperature dependences of the derivative of the degree of transformation for precursors relative to PAN. "> Figure 2
Diffractogram of the FeCoSm/C nanocomposite synthesized at T = 300 °C. "> Figure 3
Diffractograms of FeCoSm/C nanocomposites synthesized in the temperature range from 600 to 800 °C (the ratio of Fe:Co:Sm metals is indicated in parentheses). *—The positions of the reflexes of the oxide phase. "> Figure 4
Enlarged areas of diffractograms of FeCoSm/C nanocomposites synthesized in the temperature range from 600 to 800 °C, containing phase composition analysis: peak of the plane (110) of the FeCo phase (a); the area of angles where the reflexes of the oxide phases and carbon are visible (b). "> Figure 5
Dependence of the average size of the BCC of metal nanoparticles, based on the FeCo or Co lattice (300 °C), on the synthesis temperature. "> Figure 6
Diffractograms of FeCoSm/C nanocomposites synthesized in the temperature range from 600 to 800 °C; enlarged diffractogram area with phase composition analysis. "> Figure 7
(a) Diffractograms of two compositions of FeCoSm/C nanocomposites: 50:40:10 and 30:30:40; (b) angle range 2θ from 65° to 75° diffractograms of FeCoSm/C nanocomposites with metal ratio FeCoSm = 30:30:40. "> Figure 8
Raman spectra of PPAN and FeCoSm/C samples synthesized at 700 °C. "> Figure 9
Raman spectra of FeCoSm/C nanocomposites synthesized at various temperatures (for comparison, the spectra of PPAN and FeCo/C nanocomposite are given). "> Figure 10
Deconvolution of Raman spectra of nanocomposites at various temperatures: (a) FeCoSm/C at 600 °C; (b) FeCoSm/C at 700 °C; (c) FeCoSm/C at 800 °C; (d) Fe:Co:Sm = 40:40:20 at 700 °C; (e) PPAN at 700 °C; (f) FeCo/C at 700 °C. Fe and PAN obtained at various temperatures. Deconvolution of Raman spectra of nanocomposites and PPANS obtained at different temperatures. "> Figure 11
Frequency dependences on the ratio of metals in the precursor of the (a) complex dielectric, (b) magnetic, (c) permeability and tangent of dielectric, (d) magnetic losses. "> Figure 12
Optimization of the thickness of the absorber layer (a); frequency dependences of the reflection coefficient of nanocomposites (Tsint. = 700 °C) with a different ratio of metals in the precursor Fe:Co:Sm: 40:40:20 (b), 50:40:10 (c). ">

Versions Notes

Abstract

New materials are actively being developed for use in various fields of electronics, as they can significantly improve the performance of electronic devices and prevent adverse effects. Such materials include nanocomposites, which include nanoparticles of magnetic metals and alloys in a non-magnetic polymer or carbon matrix. For the first time, we synthesized FeCoSm/C nanocomposites and studied the effect of synthesis conditions on their structure, composition, and electromagnetic properties. Thermogravimetric (TG) analysis and differential scanning calorimetry (DSC) analysis of the heating processes of nanocomposite precursors allowed optimizing the mode of IR processing of precursors. X-ray phase analysis (XPA) showed that nanoparticles of a solid-metal solution based on the FeCo structure are formed, and at temperatures above 700 °C, the formation of SmCo5-x alloy nanoparticles is also possible. As the synthesis temperature increases, the average size of nanoparticles of alloys containing Sm increases. The effect of the metal ratio in the precursor on the structure, composition, and electromagnetic properties of FeCoSm/C nanocomposites is analyzed. It has been established that the most promising of all the studied materials are those obtained at a temperature of 700 °C with a metal ratio of Fe:Co:Sm = 50:40:10.

Keywords:

nanocomposites; polyacrylonitrile; FeCoSm nanoparticles; solid solutions; complex dielectric constant; magnetic permeability; reflection losses

1. Introduction

The urgency of developing effective electromagnetic radiation absorbers in the microwave range is due to the growing need to protect electronic devices from electromagnetic interference and ensure electromagnetic compatibility of various systems. The materials used in such systems should have a low specific gravity, low thickness and a wide absorption band.

At the same time, due to the miniaturization and integration of high-frequency microelectronic devices such as thin-film inductors, microtransformers and writing-head core materials, high-frequency soft-magnetic thin films with high saturation magnetization and low coercive force have also become the subject of intensive research. The high-frequency application of soft-magnetic thin films requires that they have a high saturation magnetization and a corresponding large uniaxial anisotropy field to achieve low conductivity and to suppress eddy current loss in the gigahertz range [1,2,3,4,5].

Nanoparticles of magnetic metals such as Fe, Ni, Co and their alloys have higher magnetic permeability and saturation magnetization compared to ferrites. Alloy nanoparticles have greater stability and resistance to oxidation compared to single metals. In addition, alloy nanoparticles, as a rule, have characteristics that are unattainable for individual alloy components. A striking example is FeNi or FeCo alloys. It is known that the FeCo alloy with an Fe content of about 70 at.% has the highest saturation magnetization among magnetic materials. However, for example, thin films of the FeCo alloy are inherently unsuitable for high-frequency applications due to their high coercive force and isotropic magnetic properties [6]. Therefore, the solution to the problem may be the creation of a metal–carbon nanocomposite film, where the nanoparticles of the FeCo alloy are separated by a paramagnetic matrix.

Carbon materials are widely used as a component of absorbers, due to their chemical stability, low density, adjustable properties and variety of shapes. Various carbon materials, for example, carbon nanotubes, carbon nanofibers, graphene, activated carbon, ordered mesoporous carbon, etc., are widely used to create highly efficient EMF absorbers. The creation of composites based on carbon materials by including magnetic components, in addition to dielectric losses, provides absorption due to magnetic losses, which improves impedance matching in the air radio-absorbing-material system, expands the absorption band and increases the absorption intensity. The features and prospects of metal-carbon nanocomposites in the field of radio absorption are considered in great detail in [7].

Carbon coatings (capsules) for metal nanoparticles are often formed by pyrolysis of organic materials of various natures. There are two implementation options: either a carbon carrier is created and nanoparticles are already applied to it, or a carbon matrix is formed during pyrolysis, for example, surfactants that are used to stabilize nanoparticles from agglomeration. The nanoparticles of the FeCo alloy distributed in the volume of the carbon material (matrix) are more stable and resistant to oxidation compared to those deposited on the surface of the carbon carrier. Due to their unique properties, metal–carbon nanocomposites with FeCo-doped nanoparticles can become the basis for the development of new technologies and products in the field of magneto- and radio-electronics and technologies for the creation of radio-absorbing materials.

The introduction of additives of rare-earth elements, for example, samarium, is one of the ways to control the properties of alloys. Due to the spin–orbital interaction between the orbital moment of localized 4f electrons of rare-earth elements and bound 3d electrons of transition metals, the local distribution of Sm atoms relative to Fe and Co, it is possible to change the anisotropy and the saturation magnetization in the FeCoSm alloy, and to control the frequency of natural ferromagnetic resonance [8].

A review of the references showed that there are few publications devoted to the synthesis of nanoparticles, including rare-earth elements. The CoSm systems are mainly considered [9,10,11]. There are also a small number of articles that consider triple alloys based on CoSm and Fe [12,13,14]. There are works where carbon materials were used as a carrier or additive in SmCo alloys [15,16]. In some works, the electromagnetic properties of the obtained materials were studied [17,18]. There are works where the relationship of the conditions of production, heat treatment and magnetic fields on the magnetic and electromagnetic properties of thin films of complex-composition FeCoSm is considered [8].

We have developed methods and technology for the synthesis of metal–carbon nanocomposites, including nanoparticles of various alloys of transition metals of the iron group (FeNi, FeCo, NiCo). The proposed approach to the synthesis of nanocomposites by pyrolysis of precursors based on various polymers and metal salts makes it possible to use a wide range of both polymers and metal compounds. It is shown that FeCo/C and NiCo/C nanocomposites represent a perspective for the field of microwave electronics, since they provide absorption of microwave radiation up to 80% in the range of 3–12 GHz and up to 98.7% in the range of 20–40 GHz [19,20].

Currently, we are working on the synthesis of nanocomposites, including nanoparticles of three or more component alloys. The addition of a third component to the alloy contributes to the creation of more complex nanoparticles, the characteristics of which, taking into account the nanoscale, will be closer to the so-called high-entropy alloys. From the point of view of creating radio-absorbing materials, the introduction of a third metal can allow for the varying of the bandwidth and increase the absorption efficiency of electromagnetic radiation in the microwave range. We have already synthesized nanocomposites FeCoNi/C [21], FeCoAl/C [22], and FeCoCr/C [23], for which this assumption has been confirmed.

2. Experimental Methods

2.1. Synthesis of Nanocomposites

FeCoSm/C Nanocomposites were synthesized from precursors representing the “polyacrylonitrile hydrates of metal nitrates—solvent” system.

Polyacrylonitrile (PAN), with a molecular weight of 100–150 thousand at. units (an amorphous white substance), obtained by redox polymerization, was used in the work.

Metal nitrate hydrates were selected for synthesis, as they have good solubility in dimethylformamide, low decomposition temperatures, relatively low cost and availability. To evenly distribute the metal in the polymer, a joint solution of PAN, iron, cobalt and samarium salts in DMFA was prepared. The total concentration of metals relative to PAN was 20 wt.%. The mass ratio of metals to each other was Fe:Co:Sm = 40:40:20, Fe:Co:Sm = 50:40:10; Fe:Co:Sm = 30:30:40. The choice of the metal ratio is justified by the presence of solid solutions of various compositions on the diagrams of the state of the corresponding systems and a review of the references list.

For each sample, the precursor was prepared separately. First, 1 g of PAN was dissolved in 20 mL of DMFA at 50 °C, stirring periodically until complete dissolution (~4 h). The metal salts were dissolved separately in 10 mL of DMFA. The masses of nitrates were calculated so that the total mass of metals contained in them was 20 wt%, relative to the mass of PAN (Table 1).

The metal salt solution was then added to the PAN solution, and by stirring periodically and heating the solution to 50 °C in a furnace for 2 h, a co-solution was obtained.

The solutions of “PAN—metal salts” were placed in Petri dishes (d = 80 mm) and dried at 70 °C for 4 h in a drying oven, to form a solid residue with constant weight. The obtained precursor samples were subjected to IR pyrolysis (pyrolysis in an IR-heated oven).The temperature regimes of heat treatment were selected, taking into account data on the process of thermal transformations of PAN and metal salts, and taking into account the results of thermogravimetric analysis (TGA) for nanocomposite precursors.

At the first stage of pyrolysis (pre-pyrolysis), stepwise heating was used: heating to 120 °C with an exposure time of 10 min, heating to 150 °C with an exposure time of 15 min, then heating to 200 °C with an exposure time of 15 min. The process is carried out in the air. This is necessary for the smooth decomposition of metal nitrates in order to convert metal compounds into oxide forms. At the same time, a rigid cyclic structure of the PAN is formed, due to the formation of a developed system –C=N– and –C=C– polyconjunction, which leads to the fixation of metal compounds in the structure of the polymer matrix and prevents their diffusion. In addition, it allows for the speeding up of the PAN cyclization process, which leads to less weight loss during the main stage of IR heating.

At the second stage, linear heating was carried out in an atmosphere of N₂ to the required temperature (600–800 °C), with an exposure time of 5 min. The heating rate was 50°/min.

After that, the samples were cooled to room temperature, removed from the IR heating furnace, and ground to a homogeneous powder.

2.2. Investigation Method

The study of the processes of thermal transformations occurring during the formation of nanocomposites was carried out on the thermogravimetric complex (DiscoveryTM (TA Instruments, New Castle, DE, USA). The heating rate during the study was 10 °C/min, and measurements were carried out in a nitrogen stream of 50 mL/min.

The X-ray phase and X-ray diffraction analyzes were performed at room temperature on a DIFRAY X-ray diffractometer, CrKα radiation. The results of the experiment were compared with the standards from the PDF-4 database (International Centre for Diffraction Data, ICDD). According to the data of X-ray phase analysis (XPA), calculations of the average size of synthesized alloy nanoparticles were performed using the Debye–Scherer equation.

Raman spectra were obtained using the inVia Raman Microscope (Renishaw plc, Wotton Under Edge, UK) RAMAN spectrometer, when excited by a laser with a wavelength of 514 nm.

Measurements of the complex values of magnetic and dielectric permittivity were performed using the resonator method on a rectangular multimode resonator. The vector analyzer of the E 8363B circuits from Agilent Technologies was used as a microwave generator and indicator. The measurements were carried out using equipment and techniques developed at the Center for Radiophysical Measurements, Diagnostics and Research of Parameters of Natural and Artificial Materials of Tomsk State University (Russia) [24].

3. Results and Discussion

3.1. Analysis of Thermal Transformations in Precursors of the FeCoSm/C Nanocomposite During IR Heating

The analysis of the physico-chemical properties of various metal compounds for the preparation of a precursor has shown that the most promising is the use of similar metal compounds. It should be borne in mind that when adding rare-earth elements in the form of chlorides as one of the components, the formation of oxychlorides (MeOCl) is possible, stable to temperatures of the order of 1100 °C and recovering at temperatures not lower than 900 °C. For the synthesis of multicomponent FeCoSm nanoparticles, the use of nitrates and/or metal acetates is optimal, since they have a low cost, a high degree of solubility in various solvents, low decomposition temperatures, and perfectly coordinate with PAN to form stable complexes, which ensures a high level of metal distribution in the polymer matrix of nanocomposites and a wide selection of salts and comparable physical properties. The chemical properties of transition metals allow the creation of various precursor compositions.

Precursors of FeCoSm/C nanocomposites were a system containing polyacrylonitrile— nona—and hexahydrates of iron, cobalt and samarium nitrates. Aqueous nitrates were chosen because they have good solubility in DMFA. The total concentration of metals was 20 wt. %. The metal was introduced in the form of salts, so the calculation was performed on the metal. Figure 1 shows the results of the TG analysis of the corresponding precursors.

It is known that oxygen is released during the decomposition of salt nitrates. In this case, the process of oxygen release takes place inside the precursor, which provides stronger oxidation compared to exposure to external air. This accelerates the PAN cyclization process, although the resulting polyconjoint polymer structure is more defective due to the presence of carboxyl, carbonyl and hydroxyl groups. In addition, high mass losses may be due to the fact that metal salts decompose with the release of gaseous products, and these nitrates make up a fairly significant part of the mass of the precursor. For precursors of FeCoSm/C nanocomposites, a mass loss of 64% is observed (Figure 1a). As can be seen from the graphs, the peak in the region of 280–320 °C for precursors of FeCoSm/C nanocomposites corresponds to chemical transformations into PAN, corresponding to the formation of a cyclized structure of molecules and the formation of a developed system –C=C- and –C=N- polyconjunction. Peaks in the differential dependences of the degree of transformation, manifested at temperatures above 320 °C, are responsible for the transformation of the cyclized form of PAN, accompanied by the release of gaseous pyrolysis products (H₂, CO, NH₃). For the precursors of FeCoSm/C nanocomposites, these peaks are absent, due to the strong oxidation of the polymer at the stage of decomposition of metal nitrates, which leads to the formation of a developed polyconjection system and denser crosslinking of PAN macromolecules. Therefore, already at T = 445 °C, a peak appears, characterizing the beginning of the formation of the graphite-like structure of the carbon matrix of the nanocomposite, which is accompanied by intensive removal of oxygen from the precursor in the form of CO. Mass changes in the low-temperature range (up to 270 °C) are quite consistent with the processes of thermal decomposition of metal salts.

The analysis of the temperature ranges of the main processes accompanied by mass loss made it possible to optimize the temperature regime of the first stage of the synthesis of nanocomposites produced in air. Thus, for FeCoSm/C nanocomposites, it is recommended to include two heating stages of the FeCoSm/PAN precursor in the heat-treatment process mode in the air at temperatures of 150 and 220 °C.

3.2. Investigation of FeCoSm/C Nanocomposite Structure

To analyze the possibilities of forming three-component nanoparticles based on an alloy of FeCo and rare-earth metals, FeCoSm/C nanocomposites were synthesized. These nanocomposites were a variation of FeCo/C nanocomposites modified by partial substitution of cobalt or cobalt and iron, simultaneously. Thus, in this case, the effect of samarium on the structure, composition of nanocomposites, magnetic and electromagnetic properties of the obtained materials was analyzed in comparison with FeCo/C nanocomposites.

Nanocomposite samples were synthesized in the temperature range from 300 to 800 °C. The results of the XPA of the FeCoSm/C nanocomposite synthesized at 300 °C are shown in Figure 2.

Based on the results of the diffractogram analysis, it can be concluded that at synthesis temperatures of 300 °C, spinel phases can be traced in the structure of nanocomposites, which most likely relate to magnetite or partially substituted cobalt magnetite, as well as the beginning of the formation of the FCC-cobalt phase. Samarium is present in the form of X-ray amorphous oxides of various compositions.

With an increase in the synthesis temperature to 600 °C, spinel phase reflexes disappear, clearly pronounced reflexes of a solid solution (alloy) of FeCo with a BCC-type crystal lattice appear, and samarium is also present, in the form of X-ray amorphous oxides (Figure 3).

With a further increase in the synthesis temperature to 800 °C, the intensity of the FeCo phase reflexes increases, which indicates an increase in the average size of the BCC of FeCo nanoparticles. In addition, their maximum is shifted to the region of small angles for nanocomposites obtained at 700 °C, regardless of the metal ratio (Figure 4a), which indicates an increase in the crystal-lattice parameter of FeCo nanoparticles. This may be due to several factors: an increase in the relative proportion of iron or a decrease in the relative proportion of cobalt in the FeCo alloy, and partial dissolution of carbon in the alloy without changing the type of crystal lattice.

In addition, reflexes of various oxide forms of samarium appear. It should be noted that the oxides can be of different compositions. So, with the ratio Fe:Co:Sm = 40:40:20 Sm₂O₃ and Sm₃FeO₆ nanoparticles are predominantly present, whereas the Sm₃Fe₅O₁₂ phase begins to appear more clearly for the composition 50:40:10 (Figure 4b).

Such a redistribution can be associated with a change in the Fe:Sm ratio in the precursor. With an increase in the relative iron content with a simultaneous decrease in the samarium content, the interaction of their oxides is more likely to form Sm₃Fe₅O₁₂, and the recovery of these oxides will be more difficult, since samarium is restored under more severe conditions than iron. From this point of view, the increase in the lattice parameter of FeCo nanoparticles is probably associated with a lack of Co in FeCo nanoparticles, relative to the reference values (50:50).

Calculation of the BCC of nanoparticles showed that with an increase in the synthesis temperature from 600 to 800 °C, the average size of BCC-phase nanoparticles based on the FeCo lattice varies from 8–10 to 15–17 nm (Figure 5).

For nanocomposites with metal ratio Fe:Co:Sm = 40:40:20 (700 °C), in addition to oxide reflexes, very weak reflexes of the Co₂Sm₅ phase with a monoclinic crystal lattice can be distinguished (Figure 6).

Analysis of the results showed that the lattice parameter of FeCo nanoparticles increased. Therefore, we can assume the beginning of alloy formation due to the interaction of FeCo nanoparticles, mixed samarium oxides and reducing agent (H₂, CO). Samarium oxide interacts with FeCo, is partially reduced at the interface, dissolves in FeCo, and then, as saturation occurs, the Co₂Sm₅ phase is released, and at the synthesis temperature of 800 °C, SmCo_4.7 begins to be released.

At the same time, the intensity of the FeCo phase reflexes at temperature of synthesis of 800 °C increases significantly, which indicates an increase in the size of the nanoparticles of the corresponding phase. In this case, the maxima shift to the region of large angles, which indicates a decrease in the crystal-lattice parameter, which is apparently associated with the separation of the phase based on the SmCo_4.7 lattice. Since the intensity of the peaks of this phase is small, and iron and samarium, by analogy with cobalt, can also form solid solutions, it is unambiguous to assert that multicomponence nanoparticles of this phase are impossible. Nevertheless, the literature mentions the formation of alloys with samarium where part of the cobalt is substituted with iron.

Thus, the data obtained suggest that cobalt was the first of the oxides to recover, and then iron is reduced by interaction with cobalt, probably on the surface of cobalt nanoparticles at the interface, while forming a solid solution based on the FeCo BCC lattice. With a further increase in the synthesis temperature, samarium oxides interact with CO and FeCo nanoparticles to form a solid solution. When dissolved above the solubility limit, a phase based on the (Co, Fe)₂Sm₅ lattice is formed, and when the synthesis temperature increases, Sm(Co, Fe)_4.7 is formed.

It is known that the percentage of metals in the precursor can significantly affect both the composition of nanoparticles and the phase composition of nanocomposites. Accordingly, we analyzed the effect of changes in the percentage of metals on the composition and structure of FeCoSm/C nanocomposites. To do this, the proportion of samarium in relation to other metals was significantly increased from 10 to 40 rel. % (FeCoSm ratio = 30:30:40). Diffractograms of nanocomposites of these compositions are shown in Figure 7.

It was found that with an increase in the specific content of samarium in the precursor, the composition of the metallic phases of the nanocomposite significantly changes. The formation of the FCC phase based on the cobalt crystal lattice is observed (Figure 7b). But, at the same time, the maximum reflexes of this phase lie much to the left, which indicates an increase in the lattice parameter, i.e., the dissolution of atoms with a large ionic radius, in cobalt. In this case, it is samarium. XRD showed that this position of the maximum is characteristic of the SmCo_4.7 alloy (or solid-solution Sm(Co,Fe)_5−x). At the same time, the intensity of the BCC phase reflexes is significantly lower with a larger half-width, which indicates a smaller size of nanoparticles than for a composition with a low specific content of samarium. In addition, it should be noted that there are no reflexes of the oxide forms of samarium and there is a strong broadening of the halo of the carbon matrix of the nanocomposite.

After analyzing the data obtained using XRD, it can be concluded that the high relative content of samarium in the precursor increases the probability of its dissolution in FeCo nanoparticles to form a solid solution. At the same time, this process greatly reduces the content of samarium oxide which forms, and prevents the formation of large oxide nanoparticles, so they remain X-ray amorphous. The average size of the BCC-phase nanoparticles decreased from 12 to 9 nm, and the size of the FCC lattice nanoparticles was also 9 nm.

3.3. Investigation of the Effect of Synthesis Conditions on the Structure of the Polymer Carbon Matrix of FeCoSm/C Nanocomposites

Changes in the structure and composition of the carbon matrix of nanocomposites can have a significant effect on the dielectric constant. Therefore, in addition to the effect of synthesis conditions on the metal component of FeCoSm/C nanocomposites, we conducted a study of the effect of metals (FeCoSm) on the structure of a polymer carbon matrix. Two precursors were prepared for this purpose: the first was pure polyacrylonitrile (PAN) without metal, and the second was polyacrylonitrile with aqueous nitrates Fe, Co and Sm, with a ratio Fe:Co:Sm = 40:40:20%. Both precursors were pyrolyzed by IR heating at 700 °C.

The Raman spectra were measured using the inVia Raman Microscope, Renishaw plc RAMAN spectrometer, when excited by a laser with a wavelength of 514 nm.

Figure 8 shows the RAMAN spectra of PAN samples subjected to IR heating (the so-called pyrolyzed polyacrylonitrile—PPAN) and FeCoSm/C synthesized at a temperature of 700 °C. Raman-scattering bands with maxima in the range of 1350–1360 cm⁻¹ and 1600 cm⁻¹, which correspond to D- and G-bands in carbon materials, are clearly identified in the samples. Overtones are also observed in the area of displacements of 2500–3200 cm⁻¹, which are usually denoted by 2D stripes. Raman-scattering bands associated with metal carbides or oxides could not be detected, which may be due to the small size of the crystallites or their amorphous state.

It is known that, depending on the microstructure of the carbon material, the position of the maxima of the D- and G-bands, as well as their intensities in the RAMAN spectrum, change. So, in particular, in monocrystalline graphite, the maximum of the G-band is in the region of wave numbers 1580 cm⁻¹ and it is dominant [25]. In nanocrystalline graphite, it is registered in the region of 1600 cm⁻¹, and in amorphous carbon in the region of 1510–1570 cm⁻¹ [26]. In cases of formation of defects in the graphite crystal lattice in the RAMAN spectrum in the range of wave numbers 1350–1370 cm, a D-band appears, the nature of which is associated with the presence of deformed lengths and bond angles between carbon atoms with the sp2 type of hybridization [27]. In nanocrystalline graphite samples, the intensity of the D-peak can exceed the intensity of the G-peak by two times [28], and in carbon nanotube samples, the intensity ratio of these bands is close to one. In amorphous carbon films, due to the broadening of the Raman bands, the D-band manifests itself as a low-frequency shoulder against the background of the G-band [26].

In our opinion, the carbon matrix (CM) formed in this study is a superposition of nanocarbon (graphite nanocrystallites, all kinds of curved carbon structures such as defective carbon tubes, nanosheets, fullerenes, “nanoglobules”) and amorphous carbon. This opinion is supported by the position of the extremum in the region of ~1595–1600 cm⁻¹, which indicates the position of the G-band characteristic of nanocarbon forms. In turn, the high intensity of the Raman spectrum in the range of 1520–1560 cm⁻¹ and the low intensity of overtones in the displacement region of 2500–3200 cm⁻¹ indicate the presence of amorphous carbon. At the same time, the proportion of amorphous carbon clusters in the PPAN sample is higher than in the FeCoSm/C sample. This statement is supported by the fact that the overtones in the displacement range of 2500–3200 cm⁻¹ in the PPAN sample have a strong degeneration. Thus, metals have a clear effect on the structuring of the polymer matrix.

The influence of samarium on the structure of the carbon matrix was of interest. Figure 9 shows the RAMAN spectra in the range of 1000–2000 cm⁻¹ of FeCoSm/C samples synthesized at different temperatures and metal ratios, in comparison with PPAN and FeCo/C nanocomposite.

The analysis of the RAMAN spectra of metal–carbon composites (Figure 9) suggests that their microstructure strongly depends on the synthesis temperature and the metals present.

Deconvolution of the Raman spectra in accordance with the approach proposed in [29] made it possible to identify in more detail the features of the changing structure of the carbon matrix of nanocomposites, depending on the synthesis conditions (Figure 10).

The analysis of the results showed that, with an increase in the synthesis temperature for nanocomposites with a metal ratio of Fe:Co:Sm = 40:40:20, there is an increase in the intensity of the G1 band, which is responsible for the ordered sp2 carbon. In addition, a change in the ratio of metals also leads to an increase in the intensity of this band, all other things being equal, whereas, for the FeCo/C nanocomposite and even more so for IR-PAN, the intensity of this band is lower. A similar trend is typical for the D3 band, which is responsible for the amorphous sp2 carbon, which is part of the nanocomposite matrix. The ratio of the intensities and/or areas of the D and G peaks after deconvolution may allow us to evaluate changes in the structure of the carbon matrix more accurately, as well as to estimate the average lateral size of crystallites (La). In this paper, an approach is used that assumes the decomposition of the “raw” peak D into two—D1 and D2—with the same maximum position. The calculation results are presented in Table 2.

The analysis of the results showed that, with an increase in the synthesis temperature of nanocomposites, the ratio (ID1 + ID2)/IG decreases, which is characteristic of an increase in the crystallinity and ordering of carbon materials. At the same time, the ID3/IG1 ratio also decreases. Thus, it can be argued that at a higher synthesis temperature of nanocomposites (800 °C), the proportion of amorphous carbon is lower.

It should also be noted that the increase in the relative proportion of iron in the composite (Fe:Co:Sm = 50:40:10) also leads to a decrease in the amorphousness of the carbon matrix, all other things being equal. Moreover, in such nanocomposites, the ID3/IG1 ratio is lower than for samples with a composition of 40:40:20 synthesized at 800 °C. Calculations have shown that the average size of graphite crystallites in such nanocomposites is the largest, and amounts to 3.9 nm. This may be determined by the carbide-forming properties of iron, which is abundant in this nanocomposite compared to other metals. However, it is clearly seen that the size of the crystallites and the degree of amorphousness significantly exceed the corresponding values for FeCo/C and IR-PAN nanocomposites synthesized at 700 °C. The latter are characterized by the values of La corresponding to the synthesis temperature of 600 °C of FeCoSm/C nanocomposites.

Thus, the introduction of metals and, in particular, samarium, has a positive effect on the structuring processes of the polymer matrix of FeCoSm/C nanocomposites. In addition, the Fe:Sm ratio also has a significant effect on reducing the amorphousness of the carbon matrix of nanocomposites.

3.4. The Effect of Synthesis Conditions on the Radio-Absorbing Properties of FeCoSm/C Nanocomposites

The complex dielectric and magnetic permeability (including the real and imaginary parts) of FeCoSm/C nanocomposites play an important role in determining its radio-absorption capacity.

In the previous section, we showed that nanocomposites synthesized at T = 700 °C are the most promising. Therefore, we considered the properties of FeCoSm/C nanocomposites, which were obtained precisely at 700 °C, but the ratio of metals in them differed.

The results of the study of the influence of the ratio of metals on electromagnetic characteristics (complex dielectric (relative permittivity) and magnetic permeability (relative permeability)) nanocomposites in the microwave frequency range are shown in Figure 11.

The value of ε’ for a nanocomposite of the composition Fe:Co:Sm = 50:40:10 is almost constant (about 3.8) in the frequency range of 3–13 GHz, and higher than that of FeCo/C (from 3.7 to 1). The real dielectric constant for the FeCoSm/C nanocomposite of the same composition is 14.2, and is constant in the frequency range 3–14 GHz, which is higher than for the FeCo/C nanocomposite (12.5 to 3) in the frequency range 2–18 GHz [30].

In FeCo/C and FeCoSm/C nanocomposites, the polarization of interfacial spatial charges in the core–shell system creates an array of distributed nanoscale capacitors, which leads to an increase in the real part of the dielectric constant. At the same time, the amount of both components of the complex dielectric constant for the composition Fe:Co:Sm = 40:40:20 is also higher than for the composition Fe:Co:Sm = 50:40:10, which is determined by the smaller size of the nanoparticles and the less-ordered structure of the carbon matrix of the nanocomposite. As a result, the dielectric loss tangent for the composition Fe:Co:Sm = 40:40:20 is almost two times higher than for the composition Fe:Co:Sm = 50:40:10.

Calculations have shown that the optimal thickness of the absorber layer based on FeCoSm/C nanocomposites is 1.5 and 2 mm for compositions 50:40:10 and 40:40:20, respectively (Figure 12). At the same time, thicknesses of 4.5 and 5.8 mm, respectively, may also be promising.

For the results of calculations of reflection losses, taking into account the location of the nanocomposite on the metal for Fe:Co:Sm/C nanocomposites, different compositions are shown in Figure 12b,c.

Thus, the most promising materials will be those obtained with the metal ratio Fe:Co:Sm = 50:40:10.

4. Conclusions

Metal–polymer carbon-containing nanocomposites FeCoSm/C based on polyacrylonitrile have been synthesized. It has been established that the formation of systems occurs in the temperature range of 600–700 °C, as samarium is restored. Samarium is not completely recovered from oxides. At a synthesis temperature of 700 °C, in addition to the FeCo-based alloy, the formation of a Co4.7Sm-based alloy with a hexagonal crystal lattice is observed. An increase in the metal content in the precursor does not lead to significant changes in the composition of nanocomposites, but causes an increase in the size of nanoparticles—the Sm3Co phase and the Sm-based solid solution. At the same time, the intensity of the reflexes of the FCC-cobalt phase decreases, which indicates a decrease in its content and a decrease in the size of nanoparticles of this phase.

The introduction of metals and, in particular, samarium, has a positive effect on the structuring processes of the polymer matrix of FeCoSm/C nanocomposites. The Fe:Sm ratio also has a significant effect on reducing the amorphousness of the nanocomposite matrix.

Investigations of the effect of the ratio of metals in the polymer nanocomposite FeCoSm/C on electromagnetic characteristics (dielectric and magnetic permeability) in the microwave-frequency range have shown that with an increase in the relative content of samarium in the FeCoSm/C system, an increase in complex dielectric permeability and a decrease in magnetic permeability are observed. This leads to an increase in the dielectric loss tangent, but the magnetic loss tangent decreases. The minimum reflection loss (RL) is shifted to the low-frequency region, which is determined by the frequency shift of the natural ferromagnetic resonance of the FeCoSm triple-alloy nanoparticles. A comparison of nanocomposites with optimal thicknesses showed that with a 2-fold increase in the relative samarium content in the nanocomposite, the minimum reflection shifts by 2.3 GHz (from 13.8 to 11.5 GHz). In this case, the nanocomposite of the composition Fe:Co:Sm = 50:40:10 demonstrates higher absorption rates, of 95.4–97.6% (RL varies in the range of −27–32.5 dB), than the composition of Fe:Co:Sm = 40:40:20 − 85 ÷ 90.6% (RL varies in the range of −16.5 ÷ −20.5 dB). Thus, a polymer nanocomposite with a metal content of Fe:Co:Sm = 50:40:10 is the most promising for use as a radio-absorbing material.

Author Contributions

Conceptualization, project administration—L.K. and I.Z.; funding acquisition—I.Z.; investigation—D.M. and E.K.; methodology—L.K., I.Z., D.M., E.K. and A.P.; resources—D.M., E.K. and L.K.; original draft—S.B.; visualization, writing—review and editing, N.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Ministry of Science and Higher Education of the Russian Federation (subject “FZUU-2023-0001”).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author. These studies are new and have not been previously published in other articles. The calculation results files are also shown for the first time in this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Change in the mass of precursors of the FeCoSm/C nanocomposite during IR heat treatment, (b) temperature dependences of the derivative of the degree of transformation for precursors relative to PAN.

Figure 2. Diffractogram of the FeCoSm/C nanocomposite synthesized at T = 300 °C.

Figure 3. Diffractograms of FeCoSm/C nanocomposites synthesized in the temperature range from 600 to 800 °C (the ratio of Fe:Co:Sm metals is indicated in parentheses). *—The positions of the reflexes of the oxide phase.

Figure 4. Enlarged areas of diffractograms of FeCoSm/C nanocomposites synthesized in the temperature range from 600 to 800 °C, containing phase composition analysis: peak of the plane (110) of the FeCo phase (a); the area of angles where the reflexes of the oxide phases and carbon are visible (b).

Figure 5. Dependence of the average size of the BCC of metal nanoparticles, based on the FeCo or Co lattice (300 °C), on the synthesis temperature.

Figure 6. Diffractograms of FeCoSm/C nanocomposites synthesized in the temperature range from 600 to 800 °C; enlarged diffractogram area with phase composition analysis.

Figure 7. (a) Diffractograms of two compositions of FeCoSm/C nanocomposites: 50:40:10 and 30:30:40; (b) angle range 2θ from 65° to 75° diffractograms of FeCoSm/C nanocomposites with metal ratio FeCoSm = 30:30:40.

Figure 8. Raman spectra of PPAN and FeCoSm/C samples synthesized at 700 °C.

Figure 9. Raman spectra of FeCoSm/C nanocomposites synthesized at various temperatures (for comparison, the spectra of PPAN and FeCo/C nanocomposite are given).

Figure 10. Deconvolution of Raman spectra of nanocomposites at various temperatures: (a) FeCoSm/C at 600 °C; (b) FeCoSm/C at 700 °C; (c) FeCoSm/C at 800 °C; (d) Fe:Co:Sm = 40:40:20 at 700 °C; (e) PPAN at 700 °C; (f) FeCo/C at 700 °C. Fe and PAN obtained at various temperatures. Deconvolution of Raman spectra of nanocomposites and PPANS obtained at different temperatures.

Figure 11. Frequency dependences on the ratio of metals in the precursor of the (a) complex dielectric, (b) magnetic, (c) permeability and tangent of dielectric, (d) magnetic losses.

Figure 12. Optimization of the thickness of the absorber layer (a); frequency dependences of the reflection coefficient of nanocomposites (Tsint. = 700 °C) with a different ratio of metals in the precursor Fe:Co:Sm: 40:40:20 (b), 50:40:10 (c).

Table 1. Metal salt weight for different compositions of the precursors.

Fe:Co:Sm	Salt Weight, g
Fe:Co:Sm	Fe(NO₃)₃∙9H₂O	Co(NO₃)₂∙6H₂O	Sm(NO₃)₃∙9H₂O
50:40:10	0.719	0.394	0.059
40:40:20	0.576	0.394	0.118
30:30:40	0.432	0.296	0.237

Table 2. Results of the analysis of the Raman spectra of FeCoSm/C, FeCo/C and IR-PAN nanocomposites.

Sample	T_sint., °C	Fe:Co:Sm	ν(I_D1,2), sm⁻¹	ν(I_G1), sm⁻¹	ν(I_D3), sm⁻¹	I_D1+D2/I_G1	I_D3/I_G1	L_a *, nm
FeCoSm/C	600	40:40:20	1357	1596	1536	1.68	0.50	2.6
	700		1353	1599	1545	1.51	0.44	2.9
	800		1351	1595	1519	1.29	0.32	3.4
	700	50:40:10	1351	1588	1500	1.12	0.20	3.9
FeCo/C	700	50:50:0	1349	1594	1542	1.83	0.60	2.4
PPAN	700	0	1355	1606	1543	1.59	0.55	2.7

* average lateral size of crystallites in the direction “a”, calculated from Raman spectra.

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Muratov, D.; Kozhitov, L.; Zaporotskova, I.; Popkova, A.; Korovin, E.; Boroznin, S.; Boroznina, N. Influence of Synthesis Conditions on the Structure, Composition, and Electromagnetic Properties of FeCoSm/C Nanocomposites. J. Compos. Sci. 2025, 9, 62. https://doi.org/10.3390/jcs9020062

AMA Style

Muratov D, Kozhitov L, Zaporotskova I, Popkova A, Korovin E, Boroznin S, Boroznina N. Influence of Synthesis Conditions on the Structure, Composition, and Electromagnetic Properties of FeCoSm/C Nanocomposites. Journal of Composites Science. 2025; 9(2):62. https://doi.org/10.3390/jcs9020062

Chicago/Turabian Style

Muratov, Dmitriy, Lev Kozhitov, Irina Zaporotskova, Alena Popkova, Evgeniy Korovin, Sergey Boroznin, and Natalia Boroznina. 2025. "Influence of Synthesis Conditions on the Structure, Composition, and Electromagnetic Properties of FeCoSm/C Nanocomposites" Journal of Composites Science 9, no. 2: 62. https://doi.org/10.3390/jcs9020062

APA Style

Muratov, D., Kozhitov, L., Zaporotskova, I., Popkova, A., Korovin, E., Boroznin, S., & Boroznina, N. (2025). Influence of Synthesis Conditions on the Structure, Composition, and Electromagnetic Properties of FeCoSm/C Nanocomposites. Journal of Composites Science, 9(2), 62. https://doi.org/10.3390/jcs9020062

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