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Mechanical alloying of iron-fullerite system. Correlation between fullerite content, its structural state and end products
Journal of Alloys and Compounds 829 (2020) 154528
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Mechanical alloying of iron-fullerite system. Correlation between fullerite content, its structural state and end products N.S. Larionova*, R.M. Nikonova, A.L. Ul’yanov, V.I. Lad’yanov, K.G. Mikheev Udmurt Federal Research Center of the Ural Branch of the Russian Academy of Sciences, 34 T. Baramzina Str, Izhevsk, 426067, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 August 2019 Received in revised form 25 February 2020 Accepted 25 February 2020 Available online 26 February 2020
€ssbauer and Raman spectroscopies have been used to study mechanosynthesized ironX-ray analysis, Mo fullerite powders with the initial fullerite С60/70 content of 10, 25 and 75 at%. The structural changes of С60/70 in mechanocomposites have been studied. It has been shown that in conditions of high-energy mechanosynthesis of iron-fullerite powders the disordering of fullerite С60/70 crystalline structure takes place followed by the total fullerene molecules destruction with the formation of amorphous carbon. At content 75e90 at% iron serves as a catalyst of С60/70 destruction. It has been stated that the phase composition of mechanocomposites depends on the amount of fullerite and its structural state. Carbides formation takes place only after the destruction of fullerene molecules as a result of CeC and C]C bonds disrupture. Amorphous carbon, Сam, amorphous phase, Am(FeeC), carbides, Fe3C, Fe7C3 and a paramagnetic P-phase can be the end products of solid-phase reactions. At the initial stage of Fee75C60/ 70 mechanosynthesis, while fullerene structure is preserved, it is possible to obtain iron-fullerene composite. The kinetics of carbide formation in the samples has been analyzed. It has been shown that the speed of solid-phase reactions at С60/70 content 25 at% is higher than at 75 at%. © 2020 Elsevier B.V. All rights reserved.
Keywords: Fullerenes Metal matrix composites Mechanical alloying Crystal structure X-ray diffraction €ssbauer spectroscopy Mo
1. Introduction The development of new nanostructured fullerene-containing construction materials has long been a subject of both fundamental and applied research. High-temperature methods for their synthesis are limited by the thermal stability of fullerenes. Thus, fullerenes remain stable at temperatures no higher than 800e950 С, depending on their purity [1,2]. Taking it into account, a preference can be given to the method of mechanosynthesis (MS) [3]. During mechanical grinding deformation and dispersion of powders reach high degrees [4,5]. It makes it possible to obtain materials in different structural states, such as equilibrium and metastable crystalline, nanocrystalline and amorphous ones, at relatively low milling temperatures. As a result, the solubility limits of the mixed components extend. High-energy ball milling is used both for the preliminary fullerite treatment and for the direct synthesis of metal-fullerite composites [6e11]. Also, it can be followed by such techniques as spark plasma sintering or compression. The application of
* Corresponding author. E-mail address: [email protected] (N.S. Larionova). https://doi.org/10.1016/j.jallcom.2020.154528 0925-8388/© 2020 Elsevier B.V. All rights reserved.
amorphous fullerite obtained by mechanoactivation makes it possible to extend the range of sintering temperatures and temperatures of composites application [6]. In Refs. [7e10] it was demonstrated for the systems FeeC, AleC, MgeC that in comparison with graphite, fullerite-containing mechanocomposites have higher strength characteristics. The composites Cu-С60 are resistant to recrystallization under heating, which solves the problem of thermal stability of nanocrystalline materials [11]. Better properties of composite materials in case of the replacement of graphite by fullerite stimulate researches to study the mechanosynthesis of fullerene-containing materials and alloys, and namely its dependence on the milling parameters and the choice of the optimal ones. It is of interest to study the influence of the initial fullerite content on the structure and phase composition of the mechanocomposites obtained. Being a matter of course, such a correlation has not been adequately addressed to in the literature. Mechanosynthesis of the iron-graphite system has been well studied and examined in detail in many publications [12e16]. Thus, a possibility of obtaining different stable and metastable carbides FexCy with different stoichiometry has been stated. The influence of carbon content on the structure, phase composition, and magnetic properties of the composites obtained is studied, as well as the behavior
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of the latter under continuous heating or annealing [17e19]. In Ref. [17] the solid-phase reactions mechanisms are discussed. The authors in Refs. [20,21] studied how the milling bodies wear and oxygen and nitrogen impurities influence phase formation processes at MS. Considerably fewer papers are devoted to the study of the processes of the mechanosynthesis of Fe-fullerite system. Mainly compositions with the initial fullerite content less than ~15 at% are presented in the literature [6e8,22]. Since there is a direct correlation between the properties of the composites obtained and the structural state of their constituents, much attention is paid to the stability of the crystalline structure of fullerite in the samples in conditions of the MS-induced deformation. It is reported that both Fe-Cfullerene composites may form [8] during the MS, and carbides [22]. Also non-equilibrium structures e low-carbon martensite with high-carbon austenite, which differ from the structures of both the annealed and hardened steel [6] can be present. In Ref. [7] analysis of the interaction of fullerites with metals depending on their location in the periodic table is given. According to the obtained results the authors of [7] attribute Fe to the group of elements, the MS with which results in the destruction of the fullerene molecules without further carbide formation (as for non-carbideforming metals Cu, Ag, et al.). A composite material with up to 95% fullerenes was also reported about [6]. However, it should be mentioned that the authors in Ref. [6] used mechanoactivation as a pretreatment, firstly, to obtain amorphous fullerite, and, secondly, to mix it with carbonyl iron powder. Their main goal was to study how amorphous fullerite interacted with iron at sintering at low pressure e 70 MPa - in the temperature range of 800e1200 С. There are no systematic studies of the effect of the initial fullerites content on the structural-phase composition of the ironbased mechanocomposites. The analysis based on the papers [6e8,22] seems rather complicated. Among the problems arising at the MS is the contamination of the powders with the products of milling bodies wear and their influence on the processes of solid-phase reactions. We have demonstrated in Ref. [23] that the fullerite С60/70 stability during mechanoactivation in an inert medium depends on the milling time and the power density of the process. A question arises here about the influence of iron as a product of milling balls (ballbearing steel) wear on the fullerene destruction processes at mechanoactivation. This paper focuses on the comparative analysis of the structuralphase composition of the iron-fullerite mechanocomposites depending on the initial fullerite content in the sample. One of the tasks solved within the framework of the goal is the investigation of the fullerite crystal structure changes and the stability of fullerenes molecules in iron-containing mechanocomposites. 2. Materials and methods The initial materials for the MS were an iron powder (99.7%) and fullerite С60/70. The latter was prepared in the SCMPMS UdmFRC UB RAS1 by electric-arc evaporation of graphite rods followed by the extraction of fullerenes from fullerene-containing soot by boiling toluene in a Soxlet apparatus and the following fullerite crystallization from the solution in a rotary evaporator. According to the high-performance liquid chromatography data the composition of the initial mixture С60/70 was as follows: 82.18 wt% - С60, 14.08 wt% - С70, 2.81 wt% - С60О, С60О2 and С70О; 0.93 wt% - higher fullerenes
1 Scientific Center for Metallurgical Physics and Materials Science of Federal State Budgetary Institution of Science « Udmurt Federal Research Center of the Ural Branch of the Russian Academy of Sciences».
С76, С78, С82, С84. The residual toluene made up 1.1 wt% (as a crystalline solvate С60eС70eС6Н5СН3) as thermogravimetric analysis demonstrated. The composites Fe-С60/70 with 10, 25 and 75 at% С60/70 were prepared by mechanosynthesis in a planetary ball mill AGO-2S in inert Ar medium (PAr ¼ 0.1 МPа). Hereafter we will note them as Fe10С60/70, Fe-25С60/70, and Fe-75С60/70, respectively. The powders weight was 30 g. Drums from hardened stainless steel and balls 8 mm in diameter from ball-bearing steel were used. The parameters of the MS are presented in Table 1. The mechanosynthesis of Fe-10С60/70 and Fe-25С60/70 systems and Fe-75С60/70 system was performed at the different rotational speeds of v1 ¼ 1090 rpm, and v2 ¼ 890 rpm. The speed of the planetary mill determines the impact energy of the grinding balls and, as a consequence, the speed of the solid state reactions. The latter is higher at the speed v1. It would not be quite appropriate to compare the phase composition of Fe-75С60/70 with those of Fe-10С60/70 and Fe-25С60/ 70 at equal MS times. Nevertheless, the comparison between the intermediate and end products of solid-phase reactions is absolutely justified (to be demonstrated in Section 3.3). X-ray diffractograms were taken on DRON-6 and Bruker D8 Advance (CuKa-radiation) diffractometers. To study the structuralphase composition of the powders MISA-software package was applied. The scanning electron microscopy (SEM) observations were carried out on FEI Inspect S50 apparatus. The structural changes of carbon in the mechanocomposites were measured by Raman spectroscopy on a HORIBA HR800 spectrometer with the excitation wavelength l ¼ 632.81 nm. The input power was not exceeding 5 mW at the surface of the sample in order to exclude its laser modification [24]. An objective with a magnification of 100 ( 100) was used to focus the incident laser €ssbauer spectra beam onto a spot with diameter about 4 mm. Mo were taken using the spectrometer SM2201 DR in a constant acceleration mode equipped with 57Co source in rhodium matrix at room temperature. The distribution functions P(H) of the hyperfine magnetic fields (HFMF) were calculated from the spectra using a regularized algorithm for solving the inverse incorrect problems €ssbauer spectra in the [25]. The mathematical processing of Mo discrete representation was carried out using the least squares technique according to the LevenbergeMarquardt algorithm.
3. Results Fig. 1 shows the SEM micrographs of the initial Fe-С60/70 powders. The С60/70 fullerite particles 200e400 mm in size were flatshaped and had a developed loose surface. The iron particles 0.7e10 mm in size had a spherically shape. Mechanosynthesis leads to the uniformly mixing of the Fe-С60/70 powders and formation of fine dispersed composites. As an example, Fig. 2 illustrated the morphological changes of Fe-25С60/70 after different time of mechanical alloying. The obtained composites are dispersed powders consisting of ellipsoids shaped particles with average sizes of
Table 1 Parameters of mechanosynthesis of the studied samples: v - rotational speed, tMS e milling time, mb/msample e ball-to-powder weight ratio. Sample
v, rpm
tMS, h
mb/msample
Fe-10%С60/70 Fe-25%С60/70 Fe-25%С60/70 Fe-75%С60/70 С60/70 [23] С60/70 [23] Cu-25%С60/70 [30]
890 890 1090 1090 890 1090 890
0.5e32 0.25e32 0.25e5 0.5e8 0.5e28 0.25e3.5 0.5e8
6:1 5:1 6:1 6:1 15:1 15:1 5:1
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€ssbauer presented in Table 2 are equal to the relative area of the Mo subspectra of iron-containing phases. This is not an absolute value. The corresponding histograms are illustrated the changes of the relative content of the iron atoms in phases (Fig. 6). Let us consider the evolution of phase composition for each compared Fe-С60/70 system with the increase of the MS duration. 3.1.1. Fe-10С 60/70 In the X-ray diffraction patterns of the Fe-10С60/70 powders after MS broadened reflections of a-Fe are observed the positions of which do not change as the mechanical treatment time increases up to 32 h (Fig. 3a). No carbide formation was detected. In the €ssbauer spectra and P(H) of the samples at all milling times Mo (0.5e32 h) besides the component attributed to pure a-Fe (H ¼ 330 kOe) there is a component with a broad distribution of HFMF from 100 to 300 kOe (Fig. 3bec). According to Refs. [17,26] this evidences the existence of different nonequivalent local atomic configurations with C atoms located chaotically around Fe atoms, and corresponds to an amorphous phase Am(FeeC). The data of the €ssbauer quantitative phase analysis are presented in Table 2. Mo
Fig. 1. SEM micrographs of the initial Fe-С60/70 powders: (a) e general view of powders, (b) e Fe particles.
~0.3e1 mm, which form agglomerates ~20e40 mm in size (Fig. 2aeb). As seen from the pictures at a long grinding time for 16 h the average particles size is increased to ~3 mm (Fig. 2c). The agglomerates become dense. The latter is typical for mechanocomposites and is observed in the case of completion of solid-phase reactions. 3.1. Structure and phase composition of Fe-С 60/70 The structure and phase composition of Fe-С60/70-composites with the initial fullerite content of 10, 25 and 75 at% after different €ssbauer MS times were studied using X-ray diffraction and Mo €ssbauer spectroscopy. Figs. 3e5 demonstrate diffractograms (a), Mo spectra (b), and corresponding hyperfine magnetic fields distributions P(H) (c) of the samples. The results of the estimates made € ssbauer data for the systems Fe-10С60/70, Fe-25С60/70 from the Mo and Fe-75С60/70 are given in Table 2. The table shows the ironcontaining phases for each of the systems and the relative content of the iron atoms in them. It should be noted, that the values
3.1.2. Fe-25С 60/70 According to the X-ray phase analysis at the initial stage of MS (2 h) the Fe-25С60/70 sample is also characterized by the presence of a-Fe. Also, there are reflexes with the maxima corresponding to carbide Fe3C (Fig. 4a). It should be noted that diffractograms have a complex profile with crystalline and amorphous components. € ssbauer spectra begin as soon as Noticeable changes in the Mo 0.5 h MS (Fig. 4c). Just like in the case with the Fe-10С60/70 sample a broad distribution of HFMF from 100 to 300 kOe corresponds to the amorphous phase Am(FeeC). The component with Н ¼ 202 kOe is typical for cementite Fe3C. Its intensity grows with the increasing of € ssbauer quantitative the milling time to 32 h. According to the Mo phase analysis, the relative content of Fe atoms in cementite increases from 32 at% after 0.5 h MS to 92 at% after 32 h (Table 2). The parameters of hyperfine interactions of cementite (hyperfine magnetic field Н ¼ 202 kOe, 204 kOe, 203 kOe, and isomeric shift with respect to a-Fe at room temperature d ¼ 0.20 mm/s, 0.19 mm/ s, 0.19 mm/s for 0.5, 2 and 32 h, respectively) are in good agreement with the literature data [27,28]. Very wide lines in the cementite component of the spectrum (e.g., 0.75 mm/s after 2 h MS) are due to its strongly distorted structure. For this reason, we will use the notation Fe3CD, which was adopted in Ref. [27]. The Fe3CD structure is characterized by the considerable displacement of Fe and C atoms from their equilibrium positions and by the presence of anti-phase boundaries. Thus, two stages of a solid state reaction can be identified from € ssbauer spectroscopy data (Figs. 4, 6, the X-ray analysis and Mo Table 2) for the system Fe-25С60/70. The initial stage is characterized by the decrease of a-Fe content and the formation of a cementite Fe3CD and an amorphous phase. The content of Am(FeeC) reaches the maximum after 2 h MS. The second stage (4e32 h MS) is marked by a considerable increase of Fe3CD content in the sample with a simultaneous decrease of the amorphous phase Am(FeeC) content. After 4 h MS no a-Fe was observed within the instrumental error. It should be also mentioned that if mechanosynthesis is longer than 32 h, the fraction of the amorphous phase increases again. The reason can be the contamination of the sample by wears products of milling balls (ball-bearing steel) and their solid phase alloying with the sample [20]. We did not use these data for comparative purposes because of their uncertainty. 3.1.3. Fe-75С 60/70 The phase composition of the composite milled with the highest
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initial content of С60/70e75 at% (Fig. 3), much differs from those of the above mentioned powders (Figs. 3 and 4). X-ray analysis indicates the presence of a-Fe in the sample at milling times up to 4 h. At the same time the intensities of the reflexes of carbides formed in the MS interval up to 2 h are at the background level. Carbide lines in the diffractograms after 3e4 h correspond to cementite Fe3C and EckstromeAdcock carbide Fe7C3. The best description of the experimental diffractogram of Fe-75С60/70 after 8 h MS is achieved by taking into account metastable carbide FeC with reflexes close to those for the Fe7C3. However the overlap of the diffraction lines of different carbides hinders the sample phase analysis. It is seen from Fig. 5bec that along with a-Fe (H ¼ 330 kOe) after €ssbauer spectra of the Feas short as 0.5 h of milling in the Mo 75С60/70 sample (approximately at the center) there is a paramagnetic component with H z 0 kOe. The intensity of this component grows with milling time. According to a quantitative evaluation the concentration of Fe atoms in this configuration increases to 28 at% after 8 h of milling (Fig. 6, Table 2). The parameters of the hyperfine interactions have the following value: d ¼ 0.37 mm/s, 0.38 mm/s, 0.37 mm/s, 0.37 mm/s, D ¼ 0.97 mm/s, 0.98 mm/s, 0.97 mm/s, 0.98 mm/s for 0.5, 1, 4 and 8 h of mechanical grinding, respectively (d - isomeric shift relative to a-Fe at room temperature, D - quadrupole splitting). Possible interpretations concerning the nature of the paramagnetic component (from now on called a P-phase for convenience) will be presented in Discussion. After 4 h of milling, a component with H ¼ 203 kOe and d ¼ 0.1 mm/s, corresponding to a distorted cementite Fe3CD, is present in the spectrum (Fig. 5). Mechanosynthesis during 8 h, along with the local environments mentioned above, leads to the formation of other FeeC atomic configurations with Н ¼ 207, 168, 103 kOe, which are in good agreement with literature data for Eckstrom-Adcock carbide, Fe7C3 [28]. Since the most intensive components of Fe7C3 and Fe3CD have close H values, it is difficult to separate the contribution of each. It can be assumed that Fe7C3 is presented in the sample after 4 h MS. In the Table 2 the total relative content of Fe atoms in Fe7C3 and Fe3CD configurations is presented. Absence of pure iron in the spectrum after 8 h milling is the evidence of that the time of MS of 8 h is sufficient for all the Fe atoms to be in the chemically bound FeeC state. The bcc lattice parameter of iron for the composites Fe-10С60/ 70, Fe-25С60/70 and Fe-75С60/70 is within the limits of 2.8665 ± 0.0002 Å at all MS times. Thus, a supersaturated solid solution of carbon in iron isn’t formed. According to the analysis of the profile of diffraction lines the grain sizes of a-Fe decrease from 50 nm to 8e15 nm. 3.2. Structural changes of fullerite during mechanosynthesis
Fig. 2. SEM micrographs of the composites Fe-25С60/70 after milling for 0.5 h (a), 2 h (b) and 16 h (c).
Let us consider the structural changes of fullerite С60/70 crystal structure and the stability of fullerenes molecules in mechanocomposites with iron. At low initial content of С60/70e10 and 25 at% e X-ray analysis turned to be non-informative even at the initial MS stage (0.5e1 h). No carbon reflexes were detected on diffractograms. This can be explained by a low atomic scattering factor for carbon in comparison to a-Fe. The diffractograms fragments (2q ¼ 5e35 deg) of composites with higher fullerite content Fe-75С60/70 after different MS times are presented in Fig. 7a. The structural state of the initial С60/70 (0 h MS), is characterized by the prevalence of fcc-С60, hcp-С70 and crystallosolvate С60-С70-С6H5СH3. The most intensive reflexes correspond to fullerite С60 because of its larger fraction in the sample. Mechanosynthesis results in a considerable broadening of С60 diffraction reflexes and a decrease of their intensities. This evidences the decrease of grain sizes of fullerite along with the
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€ ssbauer spectra (b) and corresponding HFMF distributions P(H) (c) of powders Fe-10С60/70 for different MS times. Fig. 3. X-ray diffraction patterns (a), Room-temperature 57Fe Mo
€ ssbauer spectra (b) and corresponding HFMF distributions P(H) (c) of powders Fe-25С60/70 for different MS times. Fig. 4. X-ray diffraction patterns (a), Room-temperature 57Fe Mo
accumulation of microdistortions in a crystal due to high deformations. As short as after 0.5 h MS fullerite reflexes are inseparable. The appearance of an amorphous halo in their place after longer milling times is evidenced of the disordering of С60/70 fullerite crystalline structure. After 8 h of milling no reflexes are observed in a low-angle region of the diffractogram. The structural components of a fullerite crystal are fullerene molecules. The fullerite С60/70 amorphization under high-energy mechanosynthesis means the change of spatial orientation of CeC bonds. The investigation method most sensitive to such structural changes is Raman spectroscopy. Fig. 8 presents the
results obtained by Raman spectroscopy measurements for the samples Fe-25С60/70 and Fe-75С60/70. A Raman spectrum of the initial fullerite С60/70 (0 h, Fig. 8а), represents a set of bands corresponding to the molecular vibrations of fullerenes С60 and С70. The maxima of the most intense С60 bands are indicated by dashed lines in Fig. 8. The spectra taken from the powders Fe-25С60/70 and Fe-75С60/70 after the MS considerably differ (Fig. 8bec). Two wide bands were detected in the Raman spectra of Fe25С60/70 after 0.5 h MS (Fig. 8b). Their maxima at 1323 and 1543 cm1 can be related to the vibrational frequencies of D and G bands, respectively, for disordered sp2 carbon [29]. The shift from
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€ ssbauer spectra (b) and corresponding HFMF distributions P(H) (c) of powders Fe-75С60/70 for different MS times. Fig. 5. X-ray diffraction patterns (a), Room-temperature 57Fe Mo
Table 2 €ssbauer spectroscopy, the Relative content of Fe atoms (at%) in phases for the Fe-С60/70 samples depending on the initial fullerite content and milling duration (according to Mo error in determining is 1 at%). MS time, h
0 0.5 1 2 4 8 16 32
Fe-10С60/70
Fe-25С60/70
Fe-75С60/70
a-Fe
Am(FeeC)
a-Fe
Am(FeeC)
Fe3C
a-Fe
P-phase
Fe3C þ Fe7C3
100 * * 62 * * * 59
0 * * 38 * * * 41
100 58 30 8 0 * 0 0
0 10 22 41 14 * 11 8
0 32 48 51 86 * 89 92
100 95 80 * 44 0 * *
0 5 13 * 23 28 * *
0 0 7 * 33 72 * *
*- Measurement wasn’t performed.
the peak positions at 1340 and 1590 cm1 for amorphous carbon at laser wavelength 632.8 nm can be explained by occasional strains and local distortions in the structure under the effect of highenergy deformation. A very low signal of the spectrum hinders its detailed analysis. After 2 h MS the Raman spectrum does not include any vibrational bands. Therefore, there is no carbon in a free
state in the sample. From the Fe-75С60/70 spectrum taken after 0.25 h MS (Fig. 8с), it is seen that fullerene bands are preserved. The decrease of their intensity along with their broadening testifies to the С60/70 structural disordering even at the initial stage of the MS. Furthermore, we observe shoulders in the region of characteristic vibrational
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€ssbauer subspectra of iron-containing phases) after different milling times of composites: а e FeFig. 6. Changes of Fe atoms relative content in phases (the relative area of the Mo 10С60/70, b e Fe-25С60/70, c e Fe-75С60/70.
modes of amorphous carbon (1340 and 1590 cm1). Their intensity increases considerably after milling up to 3 h. The corresponding spectrum has a complex structure and represents the superposition of the bands of fullerenes and amorphous carbon. Therefore, we can state partial destruction of fullerene molecules in the sample with the formation of amorphous carbon. This accords well with the Xray analysis data (Fig. 7a). Two broad lines D and G corresponding to amorphous carbon are observed in the spectrum of Fe-75С60/70 powders milled for 4e8 h. A similar spectrum is observed in the case of fullerite milled for 3.5 h without metal at milling speed v1 (Fig. 8a). According previously investigations by X-ray analysis (Fig. 7b), IR- and UVspectroscopies total deformation-induced fullerene destruction occurred in the sample with the formation of amorphous graphitelike phase [23]. Amorphous, highly disordered state of carbon in the composition of Fe-75С60/70 after 8 h MS explains the absence of any reflexes in the low-angle region of the diffractograms (Fig. 7a). Thus, MS of Fe-С60/70 powders results in the partial or total disordering of fullerite С60/70 structure and destruction of fullerene molecules. If a sample consists of fullerene molecules only, the crystalline structure of fullerite is characterized by high defectiveness, however, it remains stable. The sample with an amorphous halo in the area of main fullerite reflexes represents an amorphous fullerite-like phase, which is formed as a result of the partial destruction of fullerene molecules. Its structure is characterized by absence of long-range order, and short-range order similar to fullerite structure. Having compared the data on the fullerite structural state (Figs. 7 and 8) with the data from X-ray analysis and €ssbauer spectroscopy (Figs. 3e5) we have found that MS of FeMo С60/70 results in carbides formation only after the destruction of fullerene molecules, when СeС and С ¼ С bonds disrupt. 3.3. Influence of milling speed on the phase composition of the samples The results obtained for the systems Fe-10С60/70, Fe-25С60/70
and Fe-75С60/70 show that the structure and phase composition of iron-fullerite systems depend on the initial content of С60/70 in the powders. It is important to mention the influence of milling speed on the phase composition of the samples. In compliance with X-ray analysis mechanocomposites iron-fullerite with the initial content of С60/70 25 at% obtained at different milling speeds, v1 and v2 (Fig. 9aeb) have similar compositions of solid-phase reaction products: amorphous carbon, Am(FeeC) and Fe3CD. However at higher speed, v1, cementite is formed considerably faster (Fig. 9b). As short as after 0.25 h its corresponding reflexes appear in the diffractograms, and the intensities of a-Fe lines sharply decrease. The comparison of the diffractograms of the mechanocomposites with different initial fullerite content e 25 at% and 75 at% e milled at the same speed v1 (Fig. 9b-с), showed the differences in their end phase composition similar to those presented above (Figs. 4 and 5). These results make it possible to analyze the investigation results for the systems Fe-10С60/70, Fe-25С60/70, and Fe-75С60/70 in the context of a single approach. It may be of interest to compare the kinetics of carbide formation at the Fe-С60/70 MS depending on the initial content of fullerite С60/70. The information can be obtained by monitoring the change of the fraction of pure components in the samples, namely, of a-Fe, with MS time. It is seen from the comparison of diffractograms and €ssbauer spectra of Fe-25С60/70 powders (Figs. 4, 6 and 9) that Mo even after 0.5 and 4 h MS at milling the speeds v1 and v2, respectively, the composite does not contain pure a-Fe. At the same time in the case of Fe-75С60/70 milled for 3e4 h at the speed v1 the intensities of a-Fe lines are rather high, even in comparison with the intensities of the carbides formed (Fig. 5). It thus follows that at 25 at% С60/70 the speed of solid-phase reactions is higher, even at lower milling speed v2. The observed differences are explained by the structural state of С60/70 in composition of the samples. In accordance with the data of Raman spectroscopy (Fig. 8) after 0.5 h of MS the carbon has a strongly disordered structure. It is observed the almost total fullerenes molecules destruction. In contrast, in Fe75С60/70 powders fullerenes remain stable for as long as 3e4 h MS
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(Figs. 7 and 8). Only amorphous carbon, which is present in the samples as a result of partial fullerene destruction, takes part in the reaction of solid-phase synthesis of carbides. Iron in the sample Fe75С60/70 is present in an entirely carbide phase only as a result of a total С60/70 destruction after 8 h MS (Figs. 5, 7c). 4. Discussion Of particular note is the influence of iron on С60/70 destruction during MS. Detailed investigations of deformation-induced changes of С60/70 structure during MS at v1 and v2 were presented earlier in Ref. [23]. The corresponding diffractograms are shown in Fig. 7b-c. The milling of the powders both without the addition of Fe (Fig. 7b) and in the form of Fe-75С60/70 (Fig. 7а) at the speed v1 for 3 h results in the disordering of the crystalline structure of С60/70 with the formation of amorphous fullerite-like phase. Further milling leads to the total fullerene destruction. At the MS with lower speed v2 without the metal the fullerene molecules remained stable up to 8 h (Fig. 7c). Milling in the period from 8 to 24 h leads to their gradual destruction [23]. After 28 h of MS we observed the total destruction of fullerene molecules with the formation of amorphous carbon. However, as mentioned above (Fig. 8), fullerite in the composition of Fe-25С60/70 undergoes destruction much faster. Even after 0.5 h MS free carbon in the sample was characterized by a strongly disordered structure and represented amorphous carbon and partially preserved fullerene molecules. Note that at the MS of С60/70 a ball-to-powder weight ratio was much higher in comparison with Fe-25С60/70: mb: msample ¼ 15 : 1 vs. mb: msample ¼ 5 : 1, and therefore the power density of the latter was lower. This is another evidence of considerable influence of carbide-forming iron in the composition of Fe-25С60/70 on the deformation stability of fullerite crystal structure and fullerene molecules. As was previously found in the process of Cu-25С60/70 MS at v2 [30] in the presence of carboninactive copper the fullerene destruction was observed after 8 h MS. Based on the results of the present and previous papers [23,30] metal can be regarded as a catalyst of fullerene destruction, with the catalytic action higher in the case of carbide-forming Fe, which has a larger affinity to carbon in comparison with copper. The speed of fullerene destruction depends on the proportion of iron and fullerite in powders (Fig. 7). Turning back to the influence of iron as a product of milling balls wear on the deformation stability of С60/70 at milling without metal [23] we can note the following. As was shown in paper [23], fullerene destruction is induced by deformation. We can only mention catalytic action of iron on the process of fullerene destruction in the case of considerable wear of milling bodies. In the present paper we found this value to be 25 at% Fe. It can thus be stated that the structure and phase composition of mechanocomposites Fe-С60/70 depend on the deformation stability of fullerite and its content in the initial sample. As a result of the disordering of the crystalline structure of fullerite С60/70 and fullerene destruction followed by the formation of amorphous carbon the latter alloys mechanically with nanostructured a-Fe. Depending on the initial content of С60/70 the end composites have the following phase content:
Fig. 7. Disordering of crystallite structure of fullerite С60/70: а e in composition of Fe75%С60/70 (1090 rpm), b-c e during activation without Fe (1090 and 890 rpm respectively) [23], *- crystallosolvate С60-С70-С6H5СH3.
Fee10%C60/70 e amorphous phase Am(FeeC) and nanostructured a-Fe. No carbides are formed. Fee25%C60/70 e corresponds to the eutectic composition of ironcarbon system. Am(FeeC) and cementite Fe3CD are formed. Fee75%C60/70 e high-carbon composition. With the excess of carbon carbides Fe3CD and Fe7C3, P-phase and amorphous carbon are formed. Note that in the case of Fee75%C60/70 with
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9
Fig. 8. Raman spectra of fullerite С60/70 after different milling times: a e initial without Fe, b and c e in composition of powders Fe-25%С60/70 and Fe-75%С60/70 respectively.
Fig. 9. XRD patterns of samples: a-b e Fe-25С60/70 after mechanosynthesis at v2 and v1 respectively, с e Fee75C60/70 at v1.
milling for up to 3e4 h at the speed v1 it is possible to obtain iron-fullerene composite with some quantity of finely-dispersed carbides, P-phase and amorphous carbon. Comparison with the literature [12,17] makes it possible to conclude that after fullerene destruction in process of the MS the system Fe-С60/70 behaves similarly to the iron-graphite system. As was shown in paper [26], in comparison with the equilibrium phase diagram of FeeC, which includes either a ferrite-carbide mixture in concentration regions 0 < x < 25 at% (a meta-stable variant of the diagram) or a mixture a-Fe þ graphite (a stable variant), MS results in the formation of a two-phase or even a three-phase nanocomposite:
a-Fe þ Am(FeeC) (x 15 at%) or a-Fe þ cementite þ Am(FeeC), respectively. This should be taken into account while studying the processes of mechanosynthesis of an iron-fullerite system. Based on the results obtained it can be assumed that the absence of carbide formation during the MS of iron and fullerite powders [7,8] is caused by the lack of carbon (all the papers studied the powders with <15 at% of fullerites). The atomic configurations in a low-field distribution region P(H) for the sample Fe-75С60/70, which correspond to the P-phase, with hyperfine parameters of isomeric shift d z 0.37 mm/s and quadrupole splitting D z 0.97 mm/s (Figs. 5 and 6), disclosed in our paper are different from those cited in the literature. At the
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moment there is no unambiguous interpretation of their nature in the system FeeC. The paramagnetic phase manifested by the existence of a doublet in the spectrum was detected at MS of high€ssbauer carbon mixture Fe1-xCx (x ¼ 0.8e0.9) [12]. Based on Mo spectra recorded at 77 K it was concluded that a dispersed ironenriched phase is disordered finely dispersed carbide. According to Ref. [26] the paramagnetic doublet can be assigned to a metastable carbide FeC. The paramagnetic doublet was also observed at ball milling of chromium-molybdenum steel in the presence of 1 wt % fullerenes [31]. According to the authors in Ref. [31] its presence can be explained by the dissolution in the carbides of big quantities of chrome, and also by high particle dispersion. We assume that the atomic configurations in the low-field distribution region P(H) can be connected with the superparamagnetic state of carbides due to small particle size and/or with the homogeneous amorphous phase with 40 at% of carbon [21]. This, however, requires further study.
appeared to influence the work reported in this paper.
5. Conclusions
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154528.
€ ssbauer and Raman spectroscopies have been X-ray analysis, Mo used to perform a comparative study of the influence of fullerite C60/70 content on the structure and phase composition of mechanosynthesized composites iron-fullerite. It has been shown that: 1. In conditions of high-energy mechanosynthesis of iron-fullerite powders disordering of fullerite С60/70 crystalline structure takes place (at short MS times) followed by the total fullerene destruction with the formation of amorphous carbon. 2. The mechanosynthesis of Fe-С60/70 results in the formation of mechanocomposites with structure and phase composition depending on the fullerite content in the initial sample: At 10 at% С60/70 e Am(FeeC) is formed; no carbides; At 25 at% С60/70 e Am(FeeC) is formed followed by the crystallization of carbide Fe3CD from it. At 75 at% С60/70 e cementite Fe3CD, Eckstrom and Adcock carbide Fe7C3, paramagnetic P-phase, and amorphous carbon are formed. 3. The atomic configurations in a low-field distribution region P(H) for the sample Fe-75С60/70, which correspond to the P-phase (d z 0.37 mm/s, D z 0.97), can be connected with the superparamagnetic state of carbides due to small particle dimensions and/or with the homogeneous amorphous phase with the excess of carbon. 4. At short times of FeeC60/70 MS, while fullerene structure is preserved, it is possible to obtain the iron-fullerene composite. 5. Iron at its content 75e90 at% serves as a catalyst of С60/70 destruction. 6. Only after the total fullerene destruction the phase composition of the products of solid-phase reactions for the powders with the initial С60/70 content of 10, 25 and 75 at% becomes similar to the composition of the iron-graphite system.
Author Contributions Section N.S. Larionova: Conceptualization, Methodology, Investigation R.M. Nikonova: Conceptualization, Methodology, Resources A.L. Ul’yanov: Investigation, Formal analysis V.I. Lad’yanov: Project administration K.G. Mikheev: Investigation, Formal analysis Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have
Acknowledgments This work was performed in the framework of the State Task (project no. AAAA-A17-117022250039-4) using equipment of the Shared Use Centre “Centre of Physical and Physicochemical Methods of Analysis and Study of the Properties and Surface Characteristics of Nanostructures, Materials, and Products” UdmFRC UB RAS supported by Russian Ministry of Science and Education (Grant #RFMEFI62119X0035). The authors express their gratitude to V.V. Mukhgalin, M.I. Mokrushina and S.A. Tereshkina for taking the X-ray diffraction patterns. Appendix A. Supplementary data
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Mechanical alloying of iron-fullerite system. Correlation between fullerite content, its structural state and end products N.S. Larionova*, R.M. Nikonova, A.L. Ul’yanov, V.I. Lad’yanov, K.G. Mikheev Udmurt Federal Research Center of the Ural Branch of the Russian Academy of Sciences, 34 T. Baramzina Str, Izhevsk, 426067, Russia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 28 August 2019 Received in revised form 25 February 2020 Accepted 25 February 2020 Available online 26 February 2020
€ssbauer and Raman spectroscopies have been used to study mechanosynthesized ironX-ray analysis, Mo fullerite powders with the initial fullerite С60/70 content of 10, 25 and 75 at%. The structural changes of С60/70 in mechanocomposites have been studied. It has been shown that in conditions of high-energy mechanosynthesis of iron-fullerite powders the disordering of fullerite С60/70 crystalline structure takes place followed by the total fullerene molecules destruction with the formation of amorphous carbon. At content 75e90 at% iron serves as a catalyst of С60/70 destruction. It has been stated that the phase composition of mechanocomposites depends on the amount of fullerite and its structural state. Carbides formation takes place only after the destruction of fullerene molecules as a result of CeC and C]C bonds disrupture. Amorphous carbon, Сam, amorphous phase, Am(FeeC), carbides, Fe3C, Fe7C3 and a paramagnetic P-phase can be the end products of solid-phase reactions. At the initial stage of Fee75C60/ 70 mechanosynthesis, while fullerene structure is preserved, it is possible to obtain iron-fullerene composite. The kinetics of carbide formation in the samples has been analyzed. It has been shown that the speed of solid-phase reactions at С60/70 content 25 at% is higher than at 75 at%. © 2020 Elsevier B.V. All rights reserved.
Keywords: Fullerenes Metal matrix composites Mechanical alloying Crystal structure X-ray diffraction €ssbauer spectroscopy Mo
1. Introduction The development of new nanostructured fullerene-containing construction materials has long been a subject of both fundamental and applied research. High-temperature methods for their synthesis are limited by the thermal stability of fullerenes. Thus, fullerenes remain stable at temperatures no higher than 800e950 С, depending on their purity [1,2]. Taking it into account, a preference can be given to the method of mechanosynthesis (MS) [3]. During mechanical grinding deformation and dispersion of powders reach high degrees [4,5]. It makes it possible to obtain materials in different structural states, such as equilibrium and metastable crystalline, nanocrystalline and amorphous ones, at relatively low milling temperatures. As a result, the solubility limits of the mixed components extend. High-energy ball milling is used both for the preliminary fullerite treatment and for the direct synthesis of metal-fullerite composites [6e11]. Also, it can be followed by such techniques as spark plasma sintering or compression. The application of
* Corresponding author. E-mail address: [email protected] (N.S. Larionova). https://doi.org/10.1016/j.jallcom.2020.154528 0925-8388/© 2020 Elsevier B.V. All rights reserved.
amorphous fullerite obtained by mechanoactivation makes it possible to extend the range of sintering temperatures and temperatures of composites application [6]. In Refs. [7e10] it was demonstrated for the systems FeeC, AleC, MgeC that in comparison with graphite, fullerite-containing mechanocomposites have higher strength characteristics. The composites Cu-С60 are resistant to recrystallization under heating, which solves the problem of thermal stability of nanocrystalline materials [11]. Better properties of composite materials in case of the replacement of graphite by fullerite stimulate researches to study the mechanosynthesis of fullerene-containing materials and alloys, and namely its dependence on the milling parameters and the choice of the optimal ones. It is of interest to study the influence of the initial fullerite content on the structure and phase composition of the mechanocomposites obtained. Being a matter of course, such a correlation has not been adequately addressed to in the literature. Mechanosynthesis of the iron-graphite system has been well studied and examined in detail in many publications [12e16]. Thus, a possibility of obtaining different stable and metastable carbides FexCy with different stoichiometry has been stated. The influence of carbon content on the structure, phase composition, and magnetic properties of the composites obtained is studied, as well as the behavior
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of the latter under continuous heating or annealing [17e19]. In Ref. [17] the solid-phase reactions mechanisms are discussed. The authors in Refs. [20,21] studied how the milling bodies wear and oxygen and nitrogen impurities influence phase formation processes at MS. Considerably fewer papers are devoted to the study of the processes of the mechanosynthesis of Fe-fullerite system. Mainly compositions with the initial fullerite content less than ~15 at% are presented in the literature [6e8,22]. Since there is a direct correlation between the properties of the composites obtained and the structural state of their constituents, much attention is paid to the stability of the crystalline structure of fullerite in the samples in conditions of the MS-induced deformation. It is reported that both Fe-Cfullerene composites may form [8] during the MS, and carbides [22]. Also non-equilibrium structures e low-carbon martensite with high-carbon austenite, which differ from the structures of both the annealed and hardened steel [6] can be present. In Ref. [7] analysis of the interaction of fullerites with metals depending on their location in the periodic table is given. According to the obtained results the authors of [7] attribute Fe to the group of elements, the MS with which results in the destruction of the fullerene molecules without further carbide formation (as for non-carbideforming metals Cu, Ag, et al.). A composite material with up to 95% fullerenes was also reported about [6]. However, it should be mentioned that the authors in Ref. [6] used mechanoactivation as a pretreatment, firstly, to obtain amorphous fullerite, and, secondly, to mix it with carbonyl iron powder. Their main goal was to study how amorphous fullerite interacted with iron at sintering at low pressure e 70 MPa - in the temperature range of 800e1200 С. There are no systematic studies of the effect of the initial fullerites content on the structural-phase composition of the ironbased mechanocomposites. The analysis based on the papers [6e8,22] seems rather complicated. Among the problems arising at the MS is the contamination of the powders with the products of milling bodies wear and their influence on the processes of solid-phase reactions. We have demonstrated in Ref. [23] that the fullerite С60/70 stability during mechanoactivation in an inert medium depends on the milling time and the power density of the process. A question arises here about the influence of iron as a product of milling balls (ballbearing steel) wear on the fullerene destruction processes at mechanoactivation. This paper focuses on the comparative analysis of the structuralphase composition of the iron-fullerite mechanocomposites depending on the initial fullerite content in the sample. One of the tasks solved within the framework of the goal is the investigation of the fullerite crystal structure changes and the stability of fullerenes molecules in iron-containing mechanocomposites. 2. Materials and methods The initial materials for the MS were an iron powder (99.7%) and fullerite С60/70. The latter was prepared in the SCMPMS UdmFRC UB RAS1 by electric-arc evaporation of graphite rods followed by the extraction of fullerenes from fullerene-containing soot by boiling toluene in a Soxlet apparatus and the following fullerite crystallization from the solution in a rotary evaporator. According to the high-performance liquid chromatography data the composition of the initial mixture С60/70 was as follows: 82.18 wt% - С60, 14.08 wt% - С70, 2.81 wt% - С60О, С60О2 and С70О; 0.93 wt% - higher fullerenes
1 Scientific Center for Metallurgical Physics and Materials Science of Federal State Budgetary Institution of Science « Udmurt Federal Research Center of the Ural Branch of the Russian Academy of Sciences».
С76, С78, С82, С84. The residual toluene made up 1.1 wt% (as a crystalline solvate С60eС70eС6Н5СН3) as thermogravimetric analysis demonstrated. The composites Fe-С60/70 with 10, 25 and 75 at% С60/70 were prepared by mechanosynthesis in a planetary ball mill AGO-2S in inert Ar medium (PAr ¼ 0.1 МPа). Hereafter we will note them as Fe10С60/70, Fe-25С60/70, and Fe-75С60/70, respectively. The powders weight was 30 g. Drums from hardened stainless steel and balls 8 mm in diameter from ball-bearing steel were used. The parameters of the MS are presented in Table 1. The mechanosynthesis of Fe-10С60/70 and Fe-25С60/70 systems and Fe-75С60/70 system was performed at the different rotational speeds of v1 ¼ 1090 rpm, and v2 ¼ 890 rpm. The speed of the planetary mill determines the impact energy of the grinding balls and, as a consequence, the speed of the solid state reactions. The latter is higher at the speed v1. It would not be quite appropriate to compare the phase composition of Fe-75С60/70 with those of Fe-10С60/70 and Fe-25С60/ 70 at equal MS times. Nevertheless, the comparison between the intermediate and end products of solid-phase reactions is absolutely justified (to be demonstrated in Section 3.3). X-ray diffractograms were taken on DRON-6 and Bruker D8 Advance (CuKa-radiation) diffractometers. To study the structuralphase composition of the powders MISA-software package was applied. The scanning electron microscopy (SEM) observations were carried out on FEI Inspect S50 apparatus. The structural changes of carbon in the mechanocomposites were measured by Raman spectroscopy on a HORIBA HR800 spectrometer with the excitation wavelength l ¼ 632.81 nm. The input power was not exceeding 5 mW at the surface of the sample in order to exclude its laser modification [24]. An objective with a magnification of 100 ( 100) was used to focus the incident laser €ssbauer spectra beam onto a spot with diameter about 4 mm. Mo were taken using the spectrometer SM2201 DR in a constant acceleration mode equipped with 57Co source in rhodium matrix at room temperature. The distribution functions P(H) of the hyperfine magnetic fields (HFMF) were calculated from the spectra using a regularized algorithm for solving the inverse incorrect problems €ssbauer spectra in the [25]. The mathematical processing of Mo discrete representation was carried out using the least squares technique according to the LevenbergeMarquardt algorithm.
3. Results Fig. 1 shows the SEM micrographs of the initial Fe-С60/70 powders. The С60/70 fullerite particles 200e400 mm in size were flatshaped and had a developed loose surface. The iron particles 0.7e10 mm in size had a spherically shape. Mechanosynthesis leads to the uniformly mixing of the Fe-С60/70 powders and formation of fine dispersed composites. As an example, Fig. 2 illustrated the morphological changes of Fe-25С60/70 after different time of mechanical alloying. The obtained composites are dispersed powders consisting of ellipsoids shaped particles with average sizes of
Table 1 Parameters of mechanosynthesis of the studied samples: v - rotational speed, tMS e milling time, mb/msample e ball-to-powder weight ratio. Sample
v, rpm
tMS, h
mb/msample
Fe-10%С60/70 Fe-25%С60/70 Fe-25%С60/70 Fe-75%С60/70 С60/70 [23] С60/70 [23] Cu-25%С60/70 [30]
890 890 1090 1090 890 1090 890
0.5e32 0.25e32 0.25e5 0.5e8 0.5e28 0.25e3.5 0.5e8
6:1 5:1 6:1 6:1 15:1 15:1 5:1
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€ssbauer presented in Table 2 are equal to the relative area of the Mo subspectra of iron-containing phases. This is not an absolute value. The corresponding histograms are illustrated the changes of the relative content of the iron atoms in phases (Fig. 6). Let us consider the evolution of phase composition for each compared Fe-С60/70 system with the increase of the MS duration. 3.1.1. Fe-10С 60/70 In the X-ray diffraction patterns of the Fe-10С60/70 powders after MS broadened reflections of a-Fe are observed the positions of which do not change as the mechanical treatment time increases up to 32 h (Fig. 3a). No carbide formation was detected. In the €ssbauer spectra and P(H) of the samples at all milling times Mo (0.5e32 h) besides the component attributed to pure a-Fe (H ¼ 330 kOe) there is a component with a broad distribution of HFMF from 100 to 300 kOe (Fig. 3bec). According to Refs. [17,26] this evidences the existence of different nonequivalent local atomic configurations with C atoms located chaotically around Fe atoms, and corresponds to an amorphous phase Am(FeeC). The data of the €ssbauer quantitative phase analysis are presented in Table 2. Mo
Fig. 1. SEM micrographs of the initial Fe-С60/70 powders: (a) e general view of powders, (b) e Fe particles.
~0.3e1 mm, which form agglomerates ~20e40 mm in size (Fig. 2aeb). As seen from the pictures at a long grinding time for 16 h the average particles size is increased to ~3 mm (Fig. 2c). The agglomerates become dense. The latter is typical for mechanocomposites and is observed in the case of completion of solid-phase reactions. 3.1. Structure and phase composition of Fe-С 60/70 The structure and phase composition of Fe-С60/70-composites with the initial fullerite content of 10, 25 and 75 at% after different €ssbauer MS times were studied using X-ray diffraction and Mo €ssbauer spectroscopy. Figs. 3e5 demonstrate diffractograms (a), Mo spectra (b), and corresponding hyperfine magnetic fields distributions P(H) (c) of the samples. The results of the estimates made € ssbauer data for the systems Fe-10С60/70, Fe-25С60/70 from the Mo and Fe-75С60/70 are given in Table 2. The table shows the ironcontaining phases for each of the systems and the relative content of the iron atoms in them. It should be noted, that the values
3.1.2. Fe-25С 60/70 According to the X-ray phase analysis at the initial stage of MS (2 h) the Fe-25С60/70 sample is also characterized by the presence of a-Fe. Also, there are reflexes with the maxima corresponding to carbide Fe3C (Fig. 4a). It should be noted that diffractograms have a complex profile with crystalline and amorphous components. € ssbauer spectra begin as soon as Noticeable changes in the Mo 0.5 h MS (Fig. 4c). Just like in the case with the Fe-10С60/70 sample a broad distribution of HFMF from 100 to 300 kOe corresponds to the amorphous phase Am(FeeC). The component with Н ¼ 202 kOe is typical for cementite Fe3C. Its intensity grows with the increasing of € ssbauer quantitative the milling time to 32 h. According to the Mo phase analysis, the relative content of Fe atoms in cementite increases from 32 at% after 0.5 h MS to 92 at% after 32 h (Table 2). The parameters of hyperfine interactions of cementite (hyperfine magnetic field Н ¼ 202 kOe, 204 kOe, 203 kOe, and isomeric shift with respect to a-Fe at room temperature d ¼ 0.20 mm/s, 0.19 mm/ s, 0.19 mm/s for 0.5, 2 and 32 h, respectively) are in good agreement with the literature data [27,28]. Very wide lines in the cementite component of the spectrum (e.g., 0.75 mm/s after 2 h MS) are due to its strongly distorted structure. For this reason, we will use the notation Fe3CD, which was adopted in Ref. [27]. The Fe3CD structure is characterized by the considerable displacement of Fe and C atoms from their equilibrium positions and by the presence of anti-phase boundaries. Thus, two stages of a solid state reaction can be identified from € ssbauer spectroscopy data (Figs. 4, 6, the X-ray analysis and Mo Table 2) for the system Fe-25С60/70. The initial stage is characterized by the decrease of a-Fe content and the formation of a cementite Fe3CD and an amorphous phase. The content of Am(FeeC) reaches the maximum after 2 h MS. The second stage (4e32 h MS) is marked by a considerable increase of Fe3CD content in the sample with a simultaneous decrease of the amorphous phase Am(FeeC) content. After 4 h MS no a-Fe was observed within the instrumental error. It should be also mentioned that if mechanosynthesis is longer than 32 h, the fraction of the amorphous phase increases again. The reason can be the contamination of the sample by wears products of milling balls (ball-bearing steel) and their solid phase alloying with the sample [20]. We did not use these data for comparative purposes because of their uncertainty. 3.1.3. Fe-75С 60/70 The phase composition of the composite milled with the highest
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initial content of С60/70e75 at% (Fig. 3), much differs from those of the above mentioned powders (Figs. 3 and 4). X-ray analysis indicates the presence of a-Fe in the sample at milling times up to 4 h. At the same time the intensities of the reflexes of carbides formed in the MS interval up to 2 h are at the background level. Carbide lines in the diffractograms after 3e4 h correspond to cementite Fe3C and EckstromeAdcock carbide Fe7C3. The best description of the experimental diffractogram of Fe-75С60/70 after 8 h MS is achieved by taking into account metastable carbide FeC with reflexes close to those for the Fe7C3. However the overlap of the diffraction lines of different carbides hinders the sample phase analysis. It is seen from Fig. 5bec that along with a-Fe (H ¼ 330 kOe) after €ssbauer spectra of the Feas short as 0.5 h of milling in the Mo 75С60/70 sample (approximately at the center) there is a paramagnetic component with H z 0 kOe. The intensity of this component grows with milling time. According to a quantitative evaluation the concentration of Fe atoms in this configuration increases to 28 at% after 8 h of milling (Fig. 6, Table 2). The parameters of the hyperfine interactions have the following value: d ¼ 0.37 mm/s, 0.38 mm/s, 0.37 mm/s, 0.37 mm/s, D ¼ 0.97 mm/s, 0.98 mm/s, 0.97 mm/s, 0.98 mm/s for 0.5, 1, 4 and 8 h of mechanical grinding, respectively (d - isomeric shift relative to a-Fe at room temperature, D - quadrupole splitting). Possible interpretations concerning the nature of the paramagnetic component (from now on called a P-phase for convenience) will be presented in Discussion. After 4 h of milling, a component with H ¼ 203 kOe and d ¼ 0.1 mm/s, corresponding to a distorted cementite Fe3CD, is present in the spectrum (Fig. 5). Mechanosynthesis during 8 h, along with the local environments mentioned above, leads to the formation of other FeeC atomic configurations with Н ¼ 207, 168, 103 kOe, which are in good agreement with literature data for Eckstrom-Adcock carbide, Fe7C3 [28]. Since the most intensive components of Fe7C3 and Fe3CD have close H values, it is difficult to separate the contribution of each. It can be assumed that Fe7C3 is presented in the sample after 4 h MS. In the Table 2 the total relative content of Fe atoms in Fe7C3 and Fe3CD configurations is presented. Absence of pure iron in the spectrum after 8 h milling is the evidence of that the time of MS of 8 h is sufficient for all the Fe atoms to be in the chemically bound FeeC state. The bcc lattice parameter of iron for the composites Fe-10С60/ 70, Fe-25С60/70 and Fe-75С60/70 is within the limits of 2.8665 ± 0.0002 Å at all MS times. Thus, a supersaturated solid solution of carbon in iron isn’t formed. According to the analysis of the profile of diffraction lines the grain sizes of a-Fe decrease from 50 nm to 8e15 nm. 3.2. Structural changes of fullerite during mechanosynthesis
Fig. 2. SEM micrographs of the composites Fe-25С60/70 after milling for 0.5 h (a), 2 h (b) and 16 h (c).
Let us consider the structural changes of fullerite С60/70 crystal structure and the stability of fullerenes molecules in mechanocomposites with iron. At low initial content of С60/70e10 and 25 at% e X-ray analysis turned to be non-informative even at the initial MS stage (0.5e1 h). No carbon reflexes were detected on diffractograms. This can be explained by a low atomic scattering factor for carbon in comparison to a-Fe. The diffractograms fragments (2q ¼ 5e35 deg) of composites with higher fullerite content Fe-75С60/70 after different MS times are presented in Fig. 7a. The structural state of the initial С60/70 (0 h MS), is characterized by the prevalence of fcc-С60, hcp-С70 and crystallosolvate С60-С70-С6H5СH3. The most intensive reflexes correspond to fullerite С60 because of its larger fraction in the sample. Mechanosynthesis results in a considerable broadening of С60 diffraction reflexes and a decrease of their intensities. This evidences the decrease of grain sizes of fullerite along with the
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€ ssbauer spectra (b) and corresponding HFMF distributions P(H) (c) of powders Fe-10С60/70 for different MS times. Fig. 3. X-ray diffraction patterns (a), Room-temperature 57Fe Mo
€ ssbauer spectra (b) and corresponding HFMF distributions P(H) (c) of powders Fe-25С60/70 for different MS times. Fig. 4. X-ray diffraction patterns (a), Room-temperature 57Fe Mo
accumulation of microdistortions in a crystal due to high deformations. As short as after 0.5 h MS fullerite reflexes are inseparable. The appearance of an amorphous halo in their place after longer milling times is evidenced of the disordering of С60/70 fullerite crystalline structure. After 8 h of milling no reflexes are observed in a low-angle region of the diffractogram. The structural components of a fullerite crystal are fullerene molecules. The fullerite С60/70 amorphization under high-energy mechanosynthesis means the change of spatial orientation of CeC bonds. The investigation method most sensitive to such structural changes is Raman spectroscopy. Fig. 8 presents the
results obtained by Raman spectroscopy measurements for the samples Fe-25С60/70 and Fe-75С60/70. A Raman spectrum of the initial fullerite С60/70 (0 h, Fig. 8а), represents a set of bands corresponding to the molecular vibrations of fullerenes С60 and С70. The maxima of the most intense С60 bands are indicated by dashed lines in Fig. 8. The spectra taken from the powders Fe-25С60/70 and Fe-75С60/70 after the MS considerably differ (Fig. 8bec). Two wide bands were detected in the Raman spectra of Fe25С60/70 after 0.5 h MS (Fig. 8b). Their maxima at 1323 and 1543 cm1 can be related to the vibrational frequencies of D and G bands, respectively, for disordered sp2 carbon [29]. The shift from
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€ ssbauer spectra (b) and corresponding HFMF distributions P(H) (c) of powders Fe-75С60/70 for different MS times. Fig. 5. X-ray diffraction patterns (a), Room-temperature 57Fe Mo
Table 2 €ssbauer spectroscopy, the Relative content of Fe atoms (at%) in phases for the Fe-С60/70 samples depending on the initial fullerite content and milling duration (according to Mo error in determining is 1 at%). MS time, h
0 0.5 1 2 4 8 16 32
Fe-10С60/70
Fe-25С60/70
Fe-75С60/70
a-Fe
Am(FeeC)
a-Fe
Am(FeeC)
Fe3C
a-Fe
P-phase
Fe3C þ Fe7C3
100 * * 62 * * * 59
0 * * 38 * * * 41
100 58 30 8 0 * 0 0
0 10 22 41 14 * 11 8
0 32 48 51 86 * 89 92
100 95 80 * 44 0 * *
0 5 13 * 23 28 * *
0 0 7 * 33 72 * *
*- Measurement wasn’t performed.
the peak positions at 1340 and 1590 cm1 for amorphous carbon at laser wavelength 632.8 nm can be explained by occasional strains and local distortions in the structure under the effect of highenergy deformation. A very low signal of the spectrum hinders its detailed analysis. After 2 h MS the Raman spectrum does not include any vibrational bands. Therefore, there is no carbon in a free
state in the sample. From the Fe-75С60/70 spectrum taken after 0.25 h MS (Fig. 8с), it is seen that fullerene bands are preserved. The decrease of their intensity along with their broadening testifies to the С60/70 structural disordering even at the initial stage of the MS. Furthermore, we observe shoulders in the region of characteristic vibrational
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€ssbauer subspectra of iron-containing phases) after different milling times of composites: а e FeFig. 6. Changes of Fe atoms relative content in phases (the relative area of the Mo 10С60/70, b e Fe-25С60/70, c e Fe-75С60/70.
modes of amorphous carbon (1340 and 1590 cm1). Their intensity increases considerably after milling up to 3 h. The corresponding spectrum has a complex structure and represents the superposition of the bands of fullerenes and amorphous carbon. Therefore, we can state partial destruction of fullerene molecules in the sample with the formation of amorphous carbon. This accords well with the Xray analysis data (Fig. 7a). Two broad lines D and G corresponding to amorphous carbon are observed in the spectrum of Fe-75С60/70 powders milled for 4e8 h. A similar spectrum is observed in the case of fullerite milled for 3.5 h without metal at milling speed v1 (Fig. 8a). According previously investigations by X-ray analysis (Fig. 7b), IR- and UVspectroscopies total deformation-induced fullerene destruction occurred in the sample with the formation of amorphous graphitelike phase [23]. Amorphous, highly disordered state of carbon in the composition of Fe-75С60/70 after 8 h MS explains the absence of any reflexes in the low-angle region of the diffractograms (Fig. 7a). Thus, MS of Fe-С60/70 powders results in the partial or total disordering of fullerite С60/70 structure and destruction of fullerene molecules. If a sample consists of fullerene molecules only, the crystalline structure of fullerite is characterized by high defectiveness, however, it remains stable. The sample with an amorphous halo in the area of main fullerite reflexes represents an amorphous fullerite-like phase, which is formed as a result of the partial destruction of fullerene molecules. Its structure is characterized by absence of long-range order, and short-range order similar to fullerite structure. Having compared the data on the fullerite structural state (Figs. 7 and 8) with the data from X-ray analysis and €ssbauer spectroscopy (Figs. 3e5) we have found that MS of FeMo С60/70 results in carbides formation only after the destruction of fullerene molecules, when СeС and С ¼ С bonds disrupt. 3.3. Influence of milling speed on the phase composition of the samples The results obtained for the systems Fe-10С60/70, Fe-25С60/70
and Fe-75С60/70 show that the structure and phase composition of iron-fullerite systems depend on the initial content of С60/70 in the powders. It is important to mention the influence of milling speed on the phase composition of the samples. In compliance with X-ray analysis mechanocomposites iron-fullerite with the initial content of С60/70 25 at% obtained at different milling speeds, v1 and v2 (Fig. 9aeb) have similar compositions of solid-phase reaction products: amorphous carbon, Am(FeeC) and Fe3CD. However at higher speed, v1, cementite is formed considerably faster (Fig. 9b). As short as after 0.25 h its corresponding reflexes appear in the diffractograms, and the intensities of a-Fe lines sharply decrease. The comparison of the diffractograms of the mechanocomposites with different initial fullerite content e 25 at% and 75 at% e milled at the same speed v1 (Fig. 9b-с), showed the differences in their end phase composition similar to those presented above (Figs. 4 and 5). These results make it possible to analyze the investigation results for the systems Fe-10С60/70, Fe-25С60/70, and Fe-75С60/70 in the context of a single approach. It may be of interest to compare the kinetics of carbide formation at the Fe-С60/70 MS depending on the initial content of fullerite С60/70. The information can be obtained by monitoring the change of the fraction of pure components in the samples, namely, of a-Fe, with MS time. It is seen from the comparison of diffractograms and €ssbauer spectra of Fe-25С60/70 powders (Figs. 4, 6 and 9) that Mo even after 0.5 and 4 h MS at milling the speeds v1 and v2, respectively, the composite does not contain pure a-Fe. At the same time in the case of Fe-75С60/70 milled for 3e4 h at the speed v1 the intensities of a-Fe lines are rather high, even in comparison with the intensities of the carbides formed (Fig. 5). It thus follows that at 25 at% С60/70 the speed of solid-phase reactions is higher, even at lower milling speed v2. The observed differences are explained by the structural state of С60/70 in composition of the samples. In accordance with the data of Raman spectroscopy (Fig. 8) after 0.5 h of MS the carbon has a strongly disordered structure. It is observed the almost total fullerenes molecules destruction. In contrast, in Fe75С60/70 powders fullerenes remain stable for as long as 3e4 h MS
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(Figs. 7 and 8). Only amorphous carbon, which is present in the samples as a result of partial fullerene destruction, takes part in the reaction of solid-phase synthesis of carbides. Iron in the sample Fe75С60/70 is present in an entirely carbide phase only as a result of a total С60/70 destruction after 8 h MS (Figs. 5, 7c). 4. Discussion Of particular note is the influence of iron on С60/70 destruction during MS. Detailed investigations of deformation-induced changes of С60/70 structure during MS at v1 and v2 were presented earlier in Ref. [23]. The corresponding diffractograms are shown in Fig. 7b-c. The milling of the powders both without the addition of Fe (Fig. 7b) and in the form of Fe-75С60/70 (Fig. 7а) at the speed v1 for 3 h results in the disordering of the crystalline structure of С60/70 with the formation of amorphous fullerite-like phase. Further milling leads to the total fullerene destruction. At the MS with lower speed v2 without the metal the fullerene molecules remained stable up to 8 h (Fig. 7c). Milling in the period from 8 to 24 h leads to their gradual destruction [23]. After 28 h of MS we observed the total destruction of fullerene molecules with the formation of amorphous carbon. However, as mentioned above (Fig. 8), fullerite in the composition of Fe-25С60/70 undergoes destruction much faster. Even after 0.5 h MS free carbon in the sample was characterized by a strongly disordered structure and represented amorphous carbon and partially preserved fullerene molecules. Note that at the MS of С60/70 a ball-to-powder weight ratio was much higher in comparison with Fe-25С60/70: mb: msample ¼ 15 : 1 vs. mb: msample ¼ 5 : 1, and therefore the power density of the latter was lower. This is another evidence of considerable influence of carbide-forming iron in the composition of Fe-25С60/70 on the deformation stability of fullerite crystal structure and fullerene molecules. As was previously found in the process of Cu-25С60/70 MS at v2 [30] in the presence of carboninactive copper the fullerene destruction was observed after 8 h MS. Based on the results of the present and previous papers [23,30] metal can be regarded as a catalyst of fullerene destruction, with the catalytic action higher in the case of carbide-forming Fe, which has a larger affinity to carbon in comparison with copper. The speed of fullerene destruction depends on the proportion of iron and fullerite in powders (Fig. 7). Turning back to the influence of iron as a product of milling balls wear on the deformation stability of С60/70 at milling without metal [23] we can note the following. As was shown in paper [23], fullerene destruction is induced by deformation. We can only mention catalytic action of iron on the process of fullerene destruction in the case of considerable wear of milling bodies. In the present paper we found this value to be 25 at% Fe. It can thus be stated that the structure and phase composition of mechanocomposites Fe-С60/70 depend on the deformation stability of fullerite and its content in the initial sample. As a result of the disordering of the crystalline structure of fullerite С60/70 and fullerene destruction followed by the formation of amorphous carbon the latter alloys mechanically with nanostructured a-Fe. Depending on the initial content of С60/70 the end composites have the following phase content:
Fig. 7. Disordering of crystallite structure of fullerite С60/70: а e in composition of Fe75%С60/70 (1090 rpm), b-c e during activation without Fe (1090 and 890 rpm respectively) [23], *- crystallosolvate С60-С70-С6H5СH3.
Fee10%C60/70 e amorphous phase Am(FeeC) and nanostructured a-Fe. No carbides are formed. Fee25%C60/70 e corresponds to the eutectic composition of ironcarbon system. Am(FeeC) and cementite Fe3CD are formed. Fee75%C60/70 e high-carbon composition. With the excess of carbon carbides Fe3CD and Fe7C3, P-phase and amorphous carbon are formed. Note that in the case of Fee75%C60/70 with
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Fig. 8. Raman spectra of fullerite С60/70 after different milling times: a e initial without Fe, b and c e in composition of powders Fe-25%С60/70 and Fe-75%С60/70 respectively.
Fig. 9. XRD patterns of samples: a-b e Fe-25С60/70 after mechanosynthesis at v2 and v1 respectively, с e Fee75C60/70 at v1.
milling for up to 3e4 h at the speed v1 it is possible to obtain iron-fullerene composite with some quantity of finely-dispersed carbides, P-phase and amorphous carbon. Comparison with the literature [12,17] makes it possible to conclude that after fullerene destruction in process of the MS the system Fe-С60/70 behaves similarly to the iron-graphite system. As was shown in paper [26], in comparison with the equilibrium phase diagram of FeeC, which includes either a ferrite-carbide mixture in concentration regions 0 < x < 25 at% (a meta-stable variant of the diagram) or a mixture a-Fe þ graphite (a stable variant), MS results in the formation of a two-phase or even a three-phase nanocomposite:
a-Fe þ Am(FeeC) (x 15 at%) or a-Fe þ cementite þ Am(FeeC), respectively. This should be taken into account while studying the processes of mechanosynthesis of an iron-fullerite system. Based on the results obtained it can be assumed that the absence of carbide formation during the MS of iron and fullerite powders [7,8] is caused by the lack of carbon (all the papers studied the powders with <15 at% of fullerites). The atomic configurations in a low-field distribution region P(H) for the sample Fe-75С60/70, which correspond to the P-phase, with hyperfine parameters of isomeric shift d z 0.37 mm/s and quadrupole splitting D z 0.97 mm/s (Figs. 5 and 6), disclosed in our paper are different from those cited in the literature. At the
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moment there is no unambiguous interpretation of their nature in the system FeeC. The paramagnetic phase manifested by the existence of a doublet in the spectrum was detected at MS of high€ssbauer carbon mixture Fe1-xCx (x ¼ 0.8e0.9) [12]. Based on Mo spectra recorded at 77 K it was concluded that a dispersed ironenriched phase is disordered finely dispersed carbide. According to Ref. [26] the paramagnetic doublet can be assigned to a metastable carbide FeC. The paramagnetic doublet was also observed at ball milling of chromium-molybdenum steel in the presence of 1 wt % fullerenes [31]. According to the authors in Ref. [31] its presence can be explained by the dissolution in the carbides of big quantities of chrome, and also by high particle dispersion. We assume that the atomic configurations in the low-field distribution region P(H) can be connected with the superparamagnetic state of carbides due to small particle size and/or with the homogeneous amorphous phase with 40 at% of carbon [21]. This, however, requires further study.
appeared to influence the work reported in this paper.
5. Conclusions
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2020.154528.
€ ssbauer and Raman spectroscopies have been X-ray analysis, Mo used to perform a comparative study of the influence of fullerite C60/70 content on the structure and phase composition of mechanosynthesized composites iron-fullerite. It has been shown that: 1. In conditions of high-energy mechanosynthesis of iron-fullerite powders disordering of fullerite С60/70 crystalline structure takes place (at short MS times) followed by the total fullerene destruction with the formation of amorphous carbon. 2. The mechanosynthesis of Fe-С60/70 results in the formation of mechanocomposites with structure and phase composition depending on the fullerite content in the initial sample: At 10 at% С60/70 e Am(FeeC) is formed; no carbides; At 25 at% С60/70 e Am(FeeC) is formed followed by the crystallization of carbide Fe3CD from it. At 75 at% С60/70 e cementite Fe3CD, Eckstrom and Adcock carbide Fe7C3, paramagnetic P-phase, and amorphous carbon are formed. 3. The atomic configurations in a low-field distribution region P(H) for the sample Fe-75С60/70, which correspond to the P-phase (d z 0.37 mm/s, D z 0.97), can be connected with the superparamagnetic state of carbides due to small particle dimensions and/or with the homogeneous amorphous phase with the excess of carbon. 4. At short times of FeeC60/70 MS, while fullerene structure is preserved, it is possible to obtain the iron-fullerene composite. 5. Iron at its content 75e90 at% serves as a catalyst of С60/70 destruction. 6. Only after the total fullerene destruction the phase composition of the products of solid-phase reactions for the powders with the initial С60/70 content of 10, 25 and 75 at% becomes similar to the composition of the iron-graphite system.
Author Contributions Section N.S. Larionova: Conceptualization, Methodology, Investigation R.M. Nikonova: Conceptualization, Methodology, Resources A.L. Ul’yanov: Investigation, Formal analysis V.I. Lad’yanov: Project administration K.G. Mikheev: Investigation, Formal analysis Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have
Acknowledgments This work was performed in the framework of the State Task (project no. AAAA-A17-117022250039-4) using equipment of the Shared Use Centre “Centre of Physical and Physicochemical Methods of Analysis and Study of the Properties and Surface Characteristics of Nanostructures, Materials, and Products” UdmFRC UB RAS supported by Russian Ministry of Science and Education (Grant #RFMEFI62119X0035). The authors express their gratitude to V.V. Mukhgalin, M.I. Mokrushina and S.A. Tereshkina for taking the X-ray diffraction patterns. Appendix A. Supplementary data
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