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The formation of graphite encapsulated metal nanoparticles during mechanical activation and annealing of soot with iron and nickel
Journal of Alloys and Compounds 333 (2002) 308–320
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The formation of graphite encapsulated metal nanoparticles during mechanical activation and annealing of soot with iron and nickel B. Bokhonov*, M. Korchagin Institute of Solid State Chemistry, Siberian Branch, Russian Academy of Sciences, Kutateladze 18, 630128 Novosibirsk, Russia Received 19 February 2001; accepted 2 July 2001
Abstract The investigation of morphological and structural changes during high energy ball milling and thermal annealing of the mixtures soot–iron and soot–nickel demonstrated that the activation is accompanied by the formation of nano-sized metal particles (10–50 nm) distributed over the amorphous carbon matrix. Prolonged mechanical activation of the amorphous soot–iron system (for more than 3–5 min) leads to the formation of nano-sized cementite Fe 3 C phase. Moreover, mechanical activation of the soot–metal compositions causes a substantial decrease in graphitization temperature of the amorphous carbon: for the soot–iron system, the temperature at which the amorphous carbon starts to crystallize is 250–3008C while for the soot–nickel system, the minimal temperature at which the crystallization of the amorphous carbon was observed exceeded 6008C. Morphological characteristics of the annealed, mechanically activated soot–metal samples depend on the time of preliminary mechanical activation. The annealing of soot–metal samples obtained after short-time mechanical activation (1–3 min) causes a crystallization of the amorphous carbon as onion-like graphite-metal structures. Annealing of the soot / metal samples after mechanical treatment for more than 5 min leads to the formation of metal nanoparticles (40–50 nm) encapsulated by graphite. The longer preliminary mechanical activation, the smaller the size of encapsulated particles. In-situ electron microscopic studies of the interaction of metal particles with amorphous carbon thin film showed that the interaction starts in these systems at temperatures about 6008C. The interaction in the systems iron–amorphous carbon film and nickel–amorphous carbon film proceeds via the formation of the carbide phases Fe 3 C and Ni 3 C; their decomposition results in the formation of crystal carbon and metal nanoparticles. 2002 Elsevier Science B.V. All rights reserved. Keywords: Metals; Nanostructured materials; Mechanical alloying; X-ray diffraction
1. Introduction At present the interest in carbon–metal systems has substantially increased. This is to a great extent due to the possibility that now exists, to obtain nanocrystalline metal particles encapsulated by crystalline or amorphous carbon. Among the variety of such compositions, much interest is devoted to systems in which iron, nickel or cobalt are used as nanocrystal metal particle. Investigation and preparation of encapsulated magnetic substances is connected with a series of unique properties characteristic of this sort of material defining the possibility of their commercial application. Various carbon encapsulated nanocrystals have been synthesized by an ordinary arc-discharge method in inert
*Corresponding author. E-mail address: [email protected] (B. Bokhonov).
gas atmosphere [1]. Metal–carbon materials of various morphologies were obtained by this technique. The most detailed review of the structural characteristics of nanoparticles formed in such a process is presented in Ref. [1]. Saito et al. [2] investigated the formation of graphite encapsulated nanocrystals of magnetic metal, and metalfullerenes obtained in arc discharge experiments in helium. Under these experimental conditions, along with the formation of graphite encapsulated metal nanoparticles, the formation of carbon as nanotubes was observed, as well as a material with the characteristics similar to those of a web-like material. Nanoparticles of cobalt and cobalt carbide wrapped in multilayered graphitic sheets were synthesized by an electric arc discharge of carbon rods containing cobalt oxide (CoO) [3]. The size of the encapsulated particles was typically in a range from 50 to 200 nm. Three phases
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01741-8
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of wrapped particals: (a)-Co, f.c.c. (b)-Co, and Co 3 C, were identified. It was demonstrated that along with the formation of encapsulated particles of pure metals, arc discharge of electrodes made of Fe–Ni, Fe–Co, Co–Ni alloys and Co leads to the formation of nano-tubes with a diameter of 0.9–3.1 nm and the lengths up to 5 mm. The formation of web-like carbon was observed when a Fe–Ni electrode was arc discharged [3]. Studies of morphological characteristics of materials obtained by arc discharge showed that nano-sized metal (or carbide) particles encapsulated with graphite are formed not only when iron, nickel or cobalt are used as the metal to be arc discharged. For example, in Ref. [4] the arc melting of graphite-cadmium or cadmium oxide Cd 2 O 3 electrodes resulted in the formation of clusters of singlelayer tubes arranged around nuclei of Cd x C y (CdC 2 ) with a length of about 75 nm. Ajayan et al. [5] reported vapor phase growth of partially filled graphitic fibers, 20–30 nm in diameter and up to a micron in length, during a manganese catalyzed carbon electric arc discharge. The formation of nano-sized copper particles encapsulated by graphite was observed in the arc discharge of a copper–graphite electrode [6]. According to the data in Ref. [7], single domain microcrystals of LaC 2 encapsulated inside polyhedral carbon particles are formed during arc discharge of an electrode made of lanthanum oxide, La 2 O 3 . A typical size of these particles was 20–40 nm. The graphite coating formed at the surface prevented the LaC 2 crystal from oxidation in air. It should be noted that, in spite of the increased interest in the preparation processes of nano-sized particles, nanotubes, the mechanism of their formation still remains not quite clear. The interest in the preparation of encapsulated nanosized particles requires search for new, alternative methods of preparation of this sort of material. In our opinion, one of the most promising methods to prepare nano-sized composition materials is the mechanical treatment of solids. The application of mechanochemical methods is known to be helpful in the preparation nano-sized materials for various classes of chemical compounds. The goal of the present study was to investigate the changes of structural, phase and morphological characteristics of the mixtures of metal and amorphous carbon during mechanical treatment and annealing.
2. Experimental Iron and nickel powders of 99.99% purity grade with particle size 5–10 mm were used in mechanochemical experiments. As amorphous carbon we used carbon black powder (lamp soot) of which the carbon content was 95%. The soot globule size was 50–100 nm (Fig. 1). Mechanical
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Fig. 1. TEM micrograph of initial lamp soot globule.
activation was carried out in a planetary centrifugal mill at an acceleration of 600 m / s 2 in stainless steel vials. The ratio of ball mass to the mass of powder to be activated was 6:1. The metal to carbon ratio in mechanochemical experiments was 30 at.% of metal to 70 at.% of carbon, and 70 at.% of metal to 30 at.% of carbon. To prevent oxidation, mechanical activation was carried out in argon atmosphere. During mechanical activation, at different stages of treatment, the material was sampled and annealed in a vacuum furnace. All mechanically activated and annealed carbon–metal samples were studied by means of X-ray analysis and transmission electron microscopy (TEM). In order to carry out model electron microscopic investigations, a thin amorphous carbon film was used. It was produced by arc discharge of graphite electrodes in 10 27 Torr vacuum onto fresh cuts of NaCl single crystals. Then the deposited amorphous carbon film was separated from the crystal by dissolving in distilled water and placed onto a copper electron microscopic grid. Fine particles of metal (iron or nickel) were deposited onto this thin amorphous carbon film. Then the metal particles–amorphous carbon film sample was placed into a special electron microscopic heating holder (EM-SHH4). Electron microscopic studies were carried out using Jeol JEM 2000 FX II electron microscope at the accelerating voltage of 200 kV.
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3. Results
Structural and phase changes in the soot–iron system were observed within the first minutes of mechanical activation. X-ray diffraction (XRD) patterns of soot–iron samples activated mechanically for 1 min (Fig. 2a) show a broadening of the metal reflections. An increase of activation time to 5 min (Fig. 2b) is accompanied by an increase of the width of the metal reflections, a decrease of their intensity and the appearance of weak cementite phase reflections Fe 3 C. After mechanical activation for 10 min (Fig. 2c) the patterns exhibit broadened cementite reflections.
The annealing of the mechanically activated soot–iron mixtures, independent of the time of preliminary mechanical activation, causes the formation of a crystalline graphite phase at temperatures as low as 250–3008C. For example, X-ray diffraction patterns of the annealed mechanically activated (1 and 5 min) soot–iron samples ˚ exhibit the reflections of crystal graphite (d 002 53.39 A) and intensive iron reflections (Fig. 3a,b). Besides these, there are also low-intensity reflections of iron carbide Fe 3 C (cementite). Moreover, changes in the relation between the phases formed during mechanical treatment are observed during annealing of the samples in which a cementite phase is formed during activation. For example, annealing for 1 h at 3008C of the iron–soot samples activated mechanically for 10 min leads to a narrowing of cementite phase reflections and the appearance of crystalline graphite reflections ˚ (Fig. 4a). X-ray diffraction patterns of (d 002 53.401 A) these samples annealed at 4008C contain, along with graphite and cementite phase, broadened iron reflections (Fig. 4b). The intensity of the graphite reflection in this case is somewhat higher than in the sample annealed at 3008C. Heating of the samples at 8008C causes a substantial increase in intensity of the reflections of iron and ˚ and decrease in cementite reflecgraphite (d 002 53.394 A)
Fig. 2. X-ray powder diffraction patterns (CuKa irradiation) of mechanically activated soot–iron samples for (a) 1, (b) 5 and (c) 10 min.
Fig. 3. X-ray powder diffraction patterns (CuKa irradiation) of the annealed (1 h at 3008C) mechanically activated soot–iron samples: (a) 1 and (b) 5 min.
3.1. Structural and morphological changes during mechanical treatment and annealing of soot with iron and nickel The investigations of the changes of morphological and structural characteristics during mechanical activation of metal (iron, nickel)–soot mixture showed that the interaction in these systems had some common features but differences were also observed.
3.2. Mechanical activation of soot with iron
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Fig. 5. TEM microphotograph of iron and soot particles after mechanical treatment for 5 min. Fig. 4. X-ray powder diffraction patterns (CuKa irradiation) of the annealed mechanically activated soot–iron samples (10 min). (a) 1 h at 3008C, (b) 1 h at 4008C and (c) 1 h at 8008C.
tion intensity (Fig. 4c). This sequence of changes in the structural characteristics of the annealed mechanically activated samples, involving the increase in intensity of the iron and crystal carbon reflections and a simultaneous decrease of the cementite phase reflections with increasing annealing time, is characteristic of all soot to iron ratios in the composite material investigated in the present study. In our opinion, this experimental fact is evidence for a substantial role of the carbide phase in the crystallization of amorphous carbon. Electron microscopic studies of the mechanically activated and annealed soot–iron samples showed that after short-time mechanical activation (1–3 min) a rather wide size distribution is observed for metal and soot (amorphous carbon) particles. The size of the metal particles formed during mechanical activation varies from several tens to several hundreds of nanometers (Fig. 5). Soot particles are also substantially dispersed; their size is several tens of nanometers. Besides, coarse metal particles and soot globules that remained non-activated are observed in the samples. An increase in the time of mechanical activation is accompanied by further dispersing of both the metal particles and soot particles. At these stages of mechanical treatment the formation of iron carbide is detected in the samples by means of electron diffraction patterns. The size of iron carbide particles after activation for 10 min exhibits
a broad distribution from 3 to 7 nm to several tens of nanometers while the minimal size of the soot particles is several tens of nanometers. According to electron microscopic data, the morphological characteristics of the annealed mechanically activated samples depend on the time of mechanical activation. Annealing of the 1–3 min activated samples within the temperature range 300–8008C leads to the formation of round onion-like particles (Fig. 6a) (with a diameter about 200 nm). Morphological characteristics of the onion-like particles formed during the annealing of mechanically activated sample in the soot–iron system are periodic layers of concentric rings of crystalline carbon (graphite) 10–20 nm thick and iron (or iron carbide) particle is in the center. Light-field (Fig. 6a) and dark-field electron microscopic images of onion-like particles in carbon (Fig. 6b) and crystalline iron (Fig. 6c) reflections are a good illustration of the above description of onion-like iron– graphite particles. Annealing of the samples activated preliminarily for 5–10 min results mainly in the formation of encapsulated particles in which the surface of iron crystallites is covered with graphite layers of about 5 nm thick. The size of the iron particles varies within a rather wide range from 10 to 20 nm to several hundred nanometers (Fig. 7). An increase of the time of preliminary mechanical activation is accompanied by the formation, after annealing, of smaller metal particles encapsulated by graphite.
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Fig. 6. (a) Bright-field micrograph of onion-like iron-graphite particles. (b) Dark-field electron microscopic images of onion-like iron-graphite particles in crystal carbon reflection. (c) Dark-field electron microscopic images of onion-like iron-graphite particles in iron reflection.
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Fig. 7. TEM images of graphite encapsulated iron particles formed during the annealing of mechanically activated system soot–iron.
3.3. Mechanical activation of soot with nickel The investigations of the changes in the structural characteristics of a soot and nickel powder mixture during mechanical activation followed by annealing showed that there are some common features but also some differences between this system and the above-considered example of phase formation in the carbon–iron system. Substantial changes of the structural characteristics of the mixture of soot with nickel powder are observed within the first minutes of activation. X-ray diffraction patterns of soot– nickel mixtures activated for 1 min exhibit broadened nickel reflections (Fig. 8a). An increase in the activation time to 10 min is accompanied by increasing width of metal reflections and their shift to smaller diffraction angles (Fig. 8b). At the same time, it was observed that the broadening of X-ray reflections depends also on the nickel to soot ratio in the mechanically activated mixture for the same time of mechanical activation. An increase in soot content in the activated mixture leads to smaller broadening of X-ray reflections for the same mechanical activation time. Heating of the mechanically activated soot–nickel mixtures demonstrated that the structure of the composite material depends on annealing temperature. The annealing of mechanically activated samples for 1 h at a temperature up to 5008C leads to narrowing of the nickel reflections (Fig. 9a). Under these annealing conditions, X-ray diffraction patterns do not exhibit the formation of crystalline carbon or carbide phase. An increase of the annealing temperature to above 6008C causes further narrowing of the nickel phase reflections and the appearance of crys˚ (Fig. 9b). The intensity of talline carbon with d 002 53.39 A the crystalline carbon reflection was found to increase with
Fig. 8. X-ray powder diffraction patterns (CoKa irradiation) of mechanically activated soot–nickel samples for (a) 1 and (b) 10 min.
Fig. 9. X-ray powder diffraction patterns (CoKa irradiation) of the annealed mechanically activated soot–nickel samples (10 min). (a) 1 h at 5008C, (b) 1 h at 6008C and (c) 1 h at 9008C.
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increasing the annealing temperature. So, in the sample annealed for 1 h at 9008C the intensity of the crystalline carbon reflection is more than two times higher than for the same sample annealed at 6008C (Fig. 9c). This sequence of the changes of structural characteristics in the annealed mechanically activated samples is typical for all the soot–nickel ratios in the composite investigated in the present study. It should be noted that, unlike the soot–iron system, mechanical activation of soot powder with nickel particles did not lead to the formation of noticeable amounts of carbide phases. Moreover, the crystallization of amorphous carbon in the soot–nickel system took place at higher temperatures (above 6008C) than in the soot–iron system (above 3008C). Electron microscopic investigation of mechanically activated and annealed soot–nickel samples showed that morphological characteristics of this system are in many aspects similar to the characteristics of the soot–iron system and both depend on the time of mechanical activation and on annealing temperature. After short mechanical activation (1–3 min) the formation of nano-sized particles of nickel and soot is observed in the samples. Besides, coarse metal particles occur in these samples, as well as soot globules that remained non-activated. An increase of the time of mechanical activation is accompanied by dispersion of the metal and soot particles. Nano-sized particles of nickel and amorphous carbon form aggregates with a size of 300–1000 nm. Nickel particles after activation for 10 min exhibit a rather broad size distribution ranging from 3 to 7 nm to several tens of nanometers while the minimal size of the soot particles is several tens of nanometers (Fig. 10). It was discovered during the investigation that morphological characteristics of the annealed mechanically activated samples depend on the time of the preliminary mechanical activation and on the annealing temperature. Annealing of the samples (activated for 1–10 min) at a temperature below 5008C did not cause significant changes in the morphology and structure of the activated metal and soot. At the same time, heating of the mechanically activated soot–nickel samples (activated for 1–3 min) at a temperature above 6008C was accompanied by the formation of composite metal–graphite particles with various morphological characteristics. The particles present in the sample include: the particles of initial amorphous soot, rounded crystalline onion-shaped particles 100–200 nm in diameter (Fig. 11); graphite encapsulated metal particles with a size of several hundred nanometers. The onion-like particles formed during the annealing, similar to the annealing in the activated soot–iron system, consisted of periodic layers of concentric rings of crystalline graphite 10–20 nm thick. The annealing at temperatures above 6008C of the samples activated preliminarily for 5–10 min caused the formation of encapsulated metal nanoparticles in which the surface of nickel crystallites was covered with graphite
Fig. 10. TEM microphotograph of nickel and soot particles after mechanical treatment for 10 min.
layers about 5 nm thick (Fig. 12). The size of metal nanoparticles varied within a wide range from 10 nm to several hundred nanometers. An increase of the time of preliminary activation was accompanied by an increase of the fine metal particles encapsulated with graphite.
Fig. 11. TEM micrograph of onion-like nickel-graphite particles.
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carbon film with iron particles showed that the interaction starts when the system is heated to 700–7508C which is substantially lower than the melting point of the lowestmelting eutectics in the iron–carbon system (T511538C). At the initial stages of interaction, a propagation of the reaction front from the particle along the surface of the amorphous carbon film is observed (Fig. 13a). Diffraction patterns of the phases formed during the interaction (Fig. 13b) are characterized by the presence of both the point reflections of the crystalline product and the diffraction
Fig. 12. TEM images of graphite encapsulated nickel particles formed during the annealing of mechanically activated system soot–nickel.
3.4. In-situ electron microscopic investigation of the interaction of metals ( iron, nickel) with the amorphous carbon film As we demonstrated in the above sections, the formation of crystalline carbon takes place during isothermal annealing of mechanically activated carbon–metal mixtures. The formation of phases in the systems under investigation is of rather complicated character. For example, in soot–iron system, mechanical treatment causes the formation of an iron carbide (cementite) phase while no carbide phases could be detected in the soot–nickel system. The features of phase formation during mechanical treatment of iron and nickel with carbon, discovered in the present study, require additional investigations and explanation. However, at present it seems impossible to follow the formation of phases directly during mechanical activation or annealing. In our opinion, additional data on the processes that occur during the interactions in the systems involving amorphous carbon and metal can be obtained with the help of model systems. In the present study we have made an attempt to model directly the in-situ phase formation in nickel particle–amorphous carbon film systems and iron particle–amorphous carbon film systems during isothermal annealing, using an electron microscope. The investigation of structural and morphological characteristics of this composition allowed us to obtain data concerning the interaction of amorphous carbon with metals.
3.5. The changes of morphological and structural characteristics during in-situ annealing of amorphous carbon film with iron particle The investigation of morphological and structural characteristics during in-situ annealing of amorphous
Fig. 13. (a) Electron micrograph of the reaction front formed during in-situ electron microscopic isothermal annealing of iron particle on the surface amorphous carbon thin film. (b) SAD patterns of the carbide (point reflection) and graphite (diffraction rings) phases formed during the interaction.
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rings of polycrystalline graphite (the calculated interstitial distances for ring-type reflections are: 3.38, 2.12, 2.02 and ˚ which is in good agreement with the graphite 1.69 A, parameters). At the same time, the inter-planar distances and the symmetry of the point reflections allow us to state that the interaction of iron with the amorphous carbon film leads to the formation of crystalline iron carbide, Fe 3 C (cementite). Structural characteristics of cementite formed during the interaction of the amorphous carbon film with iron particle are in good agreement with the structural characteristics of cementite reported in Refs. [8–10]. After the reaction front grows till several microns, the formed carbide phase decomposes. This process is accompanied by a decrease in size of the carbide phase and an oriented crystallization of graphite on the surface of the initial iron particle. The graphite layers have crystallized parallel to the surface of the metal particle. Polycrystalline carbon remains in place of the decomposed carbide phase. The formation of graphite layers on the surface of the iron particle during the interaction leads to completion of the crystallization of the amorphous carbon film.
3.6. The changes of morphological and structural characteristics during in-situ annealing of amorphous carbon film with nickel particle Our investigations of the changes of morphological and structural characteristics during in-situ annealing of the amorphous carbon film with nickel particle showed that, independent of heating method (isothermal annealing or heating with a beam of accelerated electrons), the character of the evolution of the solid interaction products remained practically the same, though it exhibited some specific features. In the case where the amorphous carbon film with nickel particle was heated with a special heating holder, the following sequence of morphological and structural changes in the interacting system was observed. The reaction started at 600–6508C, which is much lower than the melting point of the lowest-melting eutectics in the nickel–carbon system (T51310–13208C). At the initial stages of interaction the formation of a reaction front and its propagation along the surface of the amorphous carbon film is observed. The propagation of the reaction front is accompanied by crystallization of the amorphous carbon film and the formation of a carbide phase (Fig. 14a). The diffraction patterns of the phases formed during the interaction exhibit both the point reflections of the crystalline product and the diffraction rings of a polycrystalline substance (Fig. 14b). The resulting inter-planar distances of the polycrystalline product correspond to the parame˚ which coincides exactly ters: 3.38, 2.12, 2.02 and 1.69 A, with the inter-planar distances in graphite. The inter-planar distances and the symmetry of the point reflections allow us to assume that nickel carbide Ni 3 C is the crystalline product of the interaction. After the carbide phase reaches the size of several microns, it decomposed with the
Fig. 14. (a) Electron micrograph of the reaction front formed during isothermal annealing of nickel particle on the surface amorphous carbon thin film. (b) SAD patterns of the nickel carbide (point reflection) and graphite (diffraction rings) phases formed during the in-situ interaction.
formation of crystalline graphite. Graphite phase is observed to crystallize onto the surface of nickel particle. The graphite layers are oriented parallel to the surface of the initial nickel particle. Polycrystalline carbon also remains in the place of the decomposed carbide phase. In cases when the amorphous carbon film–nickel particle system is heated with a high-intensity electron beam, the reaction front moves along the surface of the amorphous carbon film, too. A distinguishing feature of the interaction between the amorphous carbon film and the
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Fig. 15. Electron micrograph of the formation of fine metal (carbide) particles and graphite tracks during in-situ interaction of nickel particles with amorphous carbon film (heating with electron beam).
nickel particle during this heating method is the fact that, along with the formation of the carbide phase and the propagation of the reaction front along the surface of the amorphous carbon film, the formation of nanoparticles occurs. The formed particles move at a high rate along the surface of amorphous carbon film. Traces shaped as periodically located graphite plates remain on their way (Fig. 15). While moving, the particle changes its shape slightly. Under definite conditions it can be dispersed giving rise to particles with a much smaller size. These particles are also highly mobile along the surface of the amorphous carbon. The movement of formed particles leads to crystallization of the carbon film, too. It should be noted that the movement of particles along the surface of the amorphous carbon film is chaotic. Cooling of the nickel–amorphous carbon film system leads to a loss in mobility of the nanoparticles. The diffraction characteristic obtained from these particles correspond with the interplanar characteristics of nickel. In some cases the diffraction characteristics of the fine particles correspond to nickel carbide, Ni 3 C.
4. Discussion The experimental results obtained in our investigations can be outlined as some common and specific features characterizing the sequences of formation during the mechanical treatment and thermal annealing of metalcarbon systems:
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(a) Mechanical treatment of soot with nickel and iron is accompanied by a broadening of X-ray reflections of metals. The activation of soot–iron systems for longer than 3–5 min causes the formation of a cementite phase, Fe 3 C. (b) Electron microscopic studies of mechanically activated soot–metal mixtures showed that the activation of these systems is accompanied by an decrease in size of the soot and metal particles; their aggregation occurs. (c) Mechanical activation of soot–metal systems leads to a substantial decrease of the graphitization temperature of soot. In the soot–iron system, the graphitization of amorphous carbon starts at 250–3008C. The crystallization of soot in this system is accompanied by a decrease of the cementite phase that is formed during mechanical activation and a simultaneous increase of the iron phase. For the soot–nickel system, crystallization of the amorphous carbon was observed at temperatures above 6008C. (d) The morphological characteristics of the annealed mechanically activated soot–metal samples depend on the time of preliminary mechanical activation. For short activation (1–3 min), the soot–metal systems are crystallized with the formation of onion-like structures. Annealing of the soot–metal samples activated for more than 5 min leads to the formation of graphite encapsulated metal nanoparticles. The longer the preliminary mechanical activation, the smaller the size of the metal particles. (e) In situ electron microscopic investigations of the interaction of metal particles with an amorphous carbon film showed that the interaction starts in the systems under investigation at temperatures of about 6008C. The interaction in the iron–amorphous carbon film system and nickel–amorphous carbon film system proceeds via formation of the carbide phases Fe 3 C and Ni 3 C; their decomposition is accompanied by the formation of a graphite phase. For better understanding of the sequence of the phase formation during mechanical activation and annealing in the systems metal (iron, nickel)–soot (amorphous carbon), it is necessary to consider experimental and theoretical data reported in literature. An investigation of structural and morphological characteristics during mechanical activation of carbon was carried out in Ref. [11]. Crystalline graphite was used as carbon phase. According to the data obtained in these investigations, the mechanical activation is accompanied by a destruction of the graphite layers and the formation of disordered structures. Tanaka et al. [12] carried out studies of the processes occurring during mechanical alloying in the systems Fe–C (graphite) and Fe–C–Si. They demonstrated that mechanical activation was accompanied by the formation of an amorphous state while prolonged activation of the Fe–C system caused the formation of a metastable iron carbide, Fe 7 C 3 . It should be noted that the time of mechanical activation necessary for the formation of the Fe 3 C 7 carbide phase in the investigated mixtures was several thousand hours.
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It is known that the metals of the iron subgroup are catalysts for hydrocarbon processing. A scheme proposed in the investigations of hydrocarbon decomposition on iron catalyst (carbide cycle) [13–15] can be represented as follows:
According to this scheme, iron formed during the decomposition of the carbide phase Fe 3 C can interact with hydrocarbons at the very moment of its formation and pass into the carbide phase. It is well known [16] that amorphous carbon materials (soot) can be classified into graphitizing, non-graphitizing and those occupying an intermediate position. The investigation of the kinetics of carbon material graphitizations at 20008C allowed Chesnokov et al. [16] to propose a sequence of the graphitization process involving the formation of graphite crystallites at the first stages of the process due to the migration of large-angle grain boundaries. Much attention has been paid to the investigation of the effect of various additives on the graphitization of amorphous carbon materials. According to the experimental data [16], the effects of impurities on graphitization can be divided into four groups: 1. the dissolution of amorphous carbon in metals or metalcontaining compounds, followed by its separation as graphite; 2. the formation of carbide followed by its decomposition; 3. chemical interaction during carbonization causing structural changes in the formed carbon. According to Chesnokov et al. [16], the major part of the known catalysts of graphitization act according to one or more mechanisms. As a rule, carbide and carbideforming metals are used as graphitization catalysts. Fitzer et al. [17] proposed two mechanisms involving the possibility of dissolution of the amorphous carbon in the carbide melt, the formation of carbide supersaturated with respect to carbon, and its decomposition accompanied by the evolution of graphite. The investigation of the behaviour of the melt of carbon-saturated carbide of vanadium, nickel, iron, zirconium within the temperature range 2650–30008C demonstrated the possibility of rapid (within several minutes) formation of crystalline graphite. Long thermal treatment caused an increase of the size of the formed graphite crystals. A similar mechanism of graphite formation via the dissolution of disordered carbon in metallic melts, according to Fitzer et al. [17], occurs in the graphitization by nickel, cobalt and iron melts above the melting points of the metals. There are also data reported in the literature concerning the process of amorphous carbon graphitization via the formation of intermediate carbides. One example of a
graphitization process proceeding via this mechanism is the system silicon–amorphous carbon [16]. A mixture of silicon with amorphous carbon reacts to form silicon carbide. However, heating at temperatures above 22008C leads to a decomposition of the silicon carbide and the formation of crystalline graphite. According to the data obtained in this investigation, graphite crystals are pseudomorphous with the initial silicon carbide crystals. It is known that the graphitization process is substantially affected also by mechanical action and applied pressure [16]. For example, an increase of pressure to (0.4–0.6)10 8 N / m 2 during carbonization within the temperature range 400–6008C causes graphitization. If thermal treatment is carried out at a temperature above 10008C, a much higher pressure is required (.3310 8 N / m 2 ). According to Fedorov et al. [16], the applied pressure stimulates the destruction of supramolecular structures of amorphous carbon (for example, globules) which helps the formation of layered carbon structures and finally simplifies the crystallization of the graphite phase. Summarizing the literature data, the following features characterizing the graphitization of amorphous carbon materials by metals should be stressed: (i) graphitization of amorphous carbon materials is accompanied by the formation of intermediate carbide phases (ii) the transition from the amorphous state into the crystalline one is observed at temperatures above 10008C and depends both on the structure, composition, morphology of the carbon material to be graphitized, and on the conditions under which the process is carried out, for example, on the presence of the additives of various types acting as graphitization catalysts (iii) the graphitization process is substantially affected by the structure and morphology of the particles of amorphous carbon (soot). The comparison of the above-cited literature data with our experimental results allows us to propose the following sequence of phase formation during the mechanochemical interaction of iron and nickel with amorphous carbon. The formation of crystalline carbon during mechanical activation followed by annealing of carbon / metal samples occurs via the formation of intermediate carbide Me 3 C (Me5Fe, Ni) according to the scheme:
According to the proposed scheme, the formation of the carbide phase or metal–carbon composite occurs during the mechanical activation stage (1). The annealing of the mechanically activated mixture (carbide phase and metal– carbon composite) leads to decomposition of carbide phase and the crystallization of the carbon phase (2).
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It should be noted that mechanical activation leads not only to the formation of the carbide phase and the metalcarbon composite. It is most likely that the particles of the phases formed during mechanical activation not only are small but also contain a large number of defects. The formation of these nano-sized and defect-containing particles stimulates the processes of phase formation both during mechanical activation and during annealing. The above scheme is confirmed by the formation of the cementite phase Fe 3 C during mechanical activation. A decrease in the intensity of carbide phase reflections during isothermal annealing and the evolution of crystalline carbon phase are evidence for the fact that carbon crystallization occurs during decomposition of the iron carbide. Besides, in situ electron microscopic investigations of the interaction of iron particles with an amorphous carbon film proceeding via the formation and decomposition of the iron carbide Fe 3 C with the evolution of crystalline carbon confirm unambiguously the phase formation sequence proposed by us for the interaction of amorphous carbon with iron. Unfortunately, in the investigations of mechanical activation and annealing of the soot–nickel system we did not succeed in detecting the formation of an intermediate nickel carbide Ni 3 C. Nevertheless, we suppose that the crystallization of amorphous carbon during the annealing of mechanically activated compositions also takes place according to the scheme described above. This statement is based both on the literature data characterizing the properties of nickel carbide, and on our experimental data obtained in the in situ studies of a model system (amorphous carbon film-nickel particle). First, it is known that nickel carbide is stable at temperatures below 250–3008C and above 16008C. At temperatures within 250–4008C, nickel carbide decomposes at a noticeable rate [16]. In our opinion, this behaviour of nickel carbide does not allow to detect its formation during mechanical activation and annealing in the system soot–nickel, since under these experimental conditions the rate of nickel carbide decomposition exceeds the rate of its formation. Nevertheless, we detected the formation of an intermediate nickel carbide in the in-situ electron microscopic studies of the interaction of amorphous carbon film with nickel particles. It was discovered that the crystallization of amorphous carbon is preceded by the formation of the nickel carbide Ni 3 C. The decomposition of the latter is accompanied by the formation of crystalline carbon. The whole set of data described above allows us to state that the formation of crystalline carbon during the annealing of mechanically activated soot–nickel mixtures is also accompanied by the formation of an intermediate metastable nickel carbide, Ni 3 C. A special explanation is required for the effect observed by us, of a substantial decrease of the crystallization temperature of amorphous carbon during the annealing of mechanically activated carbon–metal mixtures. In our
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opinion, this decrease of the amorphous carbon graphitization temperature is due to several factors. First of all, mechanical activation leads to the destruction of the supramolecular structure of amorphous carbon (destruction of globules) which, as it was demonstrated in [16], simplifies the crystallization of amorphous carbon. Second, mechanical activation causes dispersion of metal particles to nanometer sizes, formation of various types of defects in the metal structure and formation of metal–carbon and carbide–carbon aggregates. The formation of carbide phases during mechanical activation of carbon with iron and nickel plays a definite part in the crystallization of amorphous carbon during annealing, which is due to the instability of these carbides within the temperature range under consideration.
5. Conclusion The investigation of the phase formation sequence during mechanical activation followed by isothermal annealing demonstrated that this process is available for the synthesis of iron and nickel nano-particles encapsulated by crystalline carbon. Moreover, the dependence of the morphological characteristics of the annealed soot–metal system on the time of preliminary mechanical activation discovered in the present study allows us to perform direct syntheses of graphite encapsulated metal nanoparticles of different shapes. Besides, the application of mechanical activation substantially help to decrease the amorphous carbon crystallization temperature. The experimental results of the in-situ electron microscopic investigations allowed us to propose a scheme describing the formation of crystalline carbon via consequent stages of the formation and decomposition of the metal carbide Me 3 C. In our opinion, the application of mechanical activation and annealing can be a useful and promising method to obtain nano-sized encapsulated particles of various metals, carbides and oxides.
References [1] B.R. Elliot, J.J. Host, V.P. Dravid, M.N. Teng, J.-H. Hwang, J. Mater. Res. 12 (12) (1997) 3328. [2] Y. Saito, T. Yoshikawa, M. Okuda, N. Fujimoto, J. Appl. Phys. 75 (1) (1994) 134. [3] S. Seraphin, D. Zhou, Appl. Phys. Lett. 64 (15) (1994) 2087. [4] S. Subramoney, R.S. Ruoff, D.C. Lorents, R. Malhotra, Nature 366 (6456) (1993) 637. [5] P.M. Ajayan, C. Colliex, J.M. Lambert, P. Bernier, L. Barbedette, M. Tence, O. Stephan, Phys. Rev. Lett. 72 (11) (1994) 1722. [6] X. Lin, X.K. Wang, V.P. Dravid, R.P.H. Chang, B. Ketterson, Appl. Phys. Lett. 64 (1) (1994) 181. [7] R.S. Ruoff, D.C. Lorents, B. Chan, S.R. Malhotra, S. Subramoney, Science 259 (1993) 346.
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[8] V.M. Schaslivtsev, T.I. Tabatchikova, I.L. Yakovleva, D.A. Mirzaev, N.M. Kleinerman, V.V. Serikov, Phisica Metallov i metallovedenie (in Russian) 84 (1) (1997) 61. [9] V.M. Schaslivtsev, T.I. Tabatchikova, I.L. Yakovleva, N.M. Kleinerman, V.V. Serikov, D.A. Mirzaev, Phisica Metallov i metallovedenie (in Russian) 84 (5) (1997) 150. [10] V.M. Schaslivtsev, T.I. Tabatchikova, I.L. Yakovleva, N.M. Kleinerman, V.V. Serikov, D.A. Mirzaev, Phisica Metallov i metallovedenie (in Russian) 82 (6) (1996) 102. [11] F. Salver-Dishma, J.-M. Taraskon, C. Clinnard, J.-N. Rouzaud, Carbon 37 (12) (1999) 1941.
[12] T. Tanaka, S. Nasu, K. Nakagava, K.N. Ishihara, P.H. Shingu, Mater. Sci. Forum 88–90 (1992) 269. [13] R.A. Buyanov, V.V. Chesnokov, A.D. Afanasev, V.S. Babenko, Kinet. Catal. (in Russian) 18 (4) (1977) 1021. [14] R.A. Buyanov, V.V. Chesnokov, A.D. Afanasev, Kinet. Catal. (in Russian) 20 (1) (1979) 207. [15] V.V. Chesnokov, R.A. Buyanov, A.D. Afanasev, Kinet. Catal. (in Russian) 20 (2) (1979) 477. [16] V.B. Fedorov, M.H. Shoshorov, D.K. Hakimova, Carbon and it’s Interaction with Metals, Metallurgy, Moscow, 1978, (in Russian). [17] E. Fitzer, B. Kegel, Carbon 6 (4) (1968) 433.
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The formation of graphite encapsulated metal nanoparticles during mechanical activation and annealing of soot with iron and nickel B. Bokhonov*, M. Korchagin Institute of Solid State Chemistry, Siberian Branch, Russian Academy of Sciences, Kutateladze 18, 630128 Novosibirsk, Russia Received 19 February 2001; accepted 2 July 2001
Abstract The investigation of morphological and structural changes during high energy ball milling and thermal annealing of the mixtures soot–iron and soot–nickel demonstrated that the activation is accompanied by the formation of nano-sized metal particles (10–50 nm) distributed over the amorphous carbon matrix. Prolonged mechanical activation of the amorphous soot–iron system (for more than 3–5 min) leads to the formation of nano-sized cementite Fe 3 C phase. Moreover, mechanical activation of the soot–metal compositions causes a substantial decrease in graphitization temperature of the amorphous carbon: for the soot–iron system, the temperature at which the amorphous carbon starts to crystallize is 250–3008C while for the soot–nickel system, the minimal temperature at which the crystallization of the amorphous carbon was observed exceeded 6008C. Morphological characteristics of the annealed, mechanically activated soot–metal samples depend on the time of preliminary mechanical activation. The annealing of soot–metal samples obtained after short-time mechanical activation (1–3 min) causes a crystallization of the amorphous carbon as onion-like graphite-metal structures. Annealing of the soot / metal samples after mechanical treatment for more than 5 min leads to the formation of metal nanoparticles (40–50 nm) encapsulated by graphite. The longer preliminary mechanical activation, the smaller the size of encapsulated particles. In-situ electron microscopic studies of the interaction of metal particles with amorphous carbon thin film showed that the interaction starts in these systems at temperatures about 6008C. The interaction in the systems iron–amorphous carbon film and nickel–amorphous carbon film proceeds via the formation of the carbide phases Fe 3 C and Ni 3 C; their decomposition results in the formation of crystal carbon and metal nanoparticles. 2002 Elsevier Science B.V. All rights reserved. Keywords: Metals; Nanostructured materials; Mechanical alloying; X-ray diffraction
1. Introduction At present the interest in carbon–metal systems has substantially increased. This is to a great extent due to the possibility that now exists, to obtain nanocrystalline metal particles encapsulated by crystalline or amorphous carbon. Among the variety of such compositions, much interest is devoted to systems in which iron, nickel or cobalt are used as nanocrystal metal particle. Investigation and preparation of encapsulated magnetic substances is connected with a series of unique properties characteristic of this sort of material defining the possibility of their commercial application. Various carbon encapsulated nanocrystals have been synthesized by an ordinary arc-discharge method in inert
*Corresponding author. E-mail address: [email protected] (B. Bokhonov).
gas atmosphere [1]. Metal–carbon materials of various morphologies were obtained by this technique. The most detailed review of the structural characteristics of nanoparticles formed in such a process is presented in Ref. [1]. Saito et al. [2] investigated the formation of graphite encapsulated nanocrystals of magnetic metal, and metalfullerenes obtained in arc discharge experiments in helium. Under these experimental conditions, along with the formation of graphite encapsulated metal nanoparticles, the formation of carbon as nanotubes was observed, as well as a material with the characteristics similar to those of a web-like material. Nanoparticles of cobalt and cobalt carbide wrapped in multilayered graphitic sheets were synthesized by an electric arc discharge of carbon rods containing cobalt oxide (CoO) [3]. The size of the encapsulated particles was typically in a range from 50 to 200 nm. Three phases
0925-8388 / 02 / $ – see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0925-8388( 01 )01741-8
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of wrapped particals: (a)-Co, f.c.c. (b)-Co, and Co 3 C, were identified. It was demonstrated that along with the formation of encapsulated particles of pure metals, arc discharge of electrodes made of Fe–Ni, Fe–Co, Co–Ni alloys and Co leads to the formation of nano-tubes with a diameter of 0.9–3.1 nm and the lengths up to 5 mm. The formation of web-like carbon was observed when a Fe–Ni electrode was arc discharged [3]. Studies of morphological characteristics of materials obtained by arc discharge showed that nano-sized metal (or carbide) particles encapsulated with graphite are formed not only when iron, nickel or cobalt are used as the metal to be arc discharged. For example, in Ref. [4] the arc melting of graphite-cadmium or cadmium oxide Cd 2 O 3 electrodes resulted in the formation of clusters of singlelayer tubes arranged around nuclei of Cd x C y (CdC 2 ) with a length of about 75 nm. Ajayan et al. [5] reported vapor phase growth of partially filled graphitic fibers, 20–30 nm in diameter and up to a micron in length, during a manganese catalyzed carbon electric arc discharge. The formation of nano-sized copper particles encapsulated by graphite was observed in the arc discharge of a copper–graphite electrode [6]. According to the data in Ref. [7], single domain microcrystals of LaC 2 encapsulated inside polyhedral carbon particles are formed during arc discharge of an electrode made of lanthanum oxide, La 2 O 3 . A typical size of these particles was 20–40 nm. The graphite coating formed at the surface prevented the LaC 2 crystal from oxidation in air. It should be noted that, in spite of the increased interest in the preparation processes of nano-sized particles, nanotubes, the mechanism of their formation still remains not quite clear. The interest in the preparation of encapsulated nanosized particles requires search for new, alternative methods of preparation of this sort of material. In our opinion, one of the most promising methods to prepare nano-sized composition materials is the mechanical treatment of solids. The application of mechanochemical methods is known to be helpful in the preparation nano-sized materials for various classes of chemical compounds. The goal of the present study was to investigate the changes of structural, phase and morphological characteristics of the mixtures of metal and amorphous carbon during mechanical treatment and annealing.
2. Experimental Iron and nickel powders of 99.99% purity grade with particle size 5–10 mm were used in mechanochemical experiments. As amorphous carbon we used carbon black powder (lamp soot) of which the carbon content was 95%. The soot globule size was 50–100 nm (Fig. 1). Mechanical
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Fig. 1. TEM micrograph of initial lamp soot globule.
activation was carried out in a planetary centrifugal mill at an acceleration of 600 m / s 2 in stainless steel vials. The ratio of ball mass to the mass of powder to be activated was 6:1. The metal to carbon ratio in mechanochemical experiments was 30 at.% of metal to 70 at.% of carbon, and 70 at.% of metal to 30 at.% of carbon. To prevent oxidation, mechanical activation was carried out in argon atmosphere. During mechanical activation, at different stages of treatment, the material was sampled and annealed in a vacuum furnace. All mechanically activated and annealed carbon–metal samples were studied by means of X-ray analysis and transmission electron microscopy (TEM). In order to carry out model electron microscopic investigations, a thin amorphous carbon film was used. It was produced by arc discharge of graphite electrodes in 10 27 Torr vacuum onto fresh cuts of NaCl single crystals. Then the deposited amorphous carbon film was separated from the crystal by dissolving in distilled water and placed onto a copper electron microscopic grid. Fine particles of metal (iron or nickel) were deposited onto this thin amorphous carbon film. Then the metal particles–amorphous carbon film sample was placed into a special electron microscopic heating holder (EM-SHH4). Electron microscopic studies were carried out using Jeol JEM 2000 FX II electron microscope at the accelerating voltage of 200 kV.
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3. Results
Structural and phase changes in the soot–iron system were observed within the first minutes of mechanical activation. X-ray diffraction (XRD) patterns of soot–iron samples activated mechanically for 1 min (Fig. 2a) show a broadening of the metal reflections. An increase of activation time to 5 min (Fig. 2b) is accompanied by an increase of the width of the metal reflections, a decrease of their intensity and the appearance of weak cementite phase reflections Fe 3 C. After mechanical activation for 10 min (Fig. 2c) the patterns exhibit broadened cementite reflections.
The annealing of the mechanically activated soot–iron mixtures, independent of the time of preliminary mechanical activation, causes the formation of a crystalline graphite phase at temperatures as low as 250–3008C. For example, X-ray diffraction patterns of the annealed mechanically activated (1 and 5 min) soot–iron samples ˚ exhibit the reflections of crystal graphite (d 002 53.39 A) and intensive iron reflections (Fig. 3a,b). Besides these, there are also low-intensity reflections of iron carbide Fe 3 C (cementite). Moreover, changes in the relation between the phases formed during mechanical treatment are observed during annealing of the samples in which a cementite phase is formed during activation. For example, annealing for 1 h at 3008C of the iron–soot samples activated mechanically for 10 min leads to a narrowing of cementite phase reflections and the appearance of crystalline graphite reflections ˚ (Fig. 4a). X-ray diffraction patterns of (d 002 53.401 A) these samples annealed at 4008C contain, along with graphite and cementite phase, broadened iron reflections (Fig. 4b). The intensity of the graphite reflection in this case is somewhat higher than in the sample annealed at 3008C. Heating of the samples at 8008C causes a substantial increase in intensity of the reflections of iron and ˚ and decrease in cementite reflecgraphite (d 002 53.394 A)
Fig. 2. X-ray powder diffraction patterns (CuKa irradiation) of mechanically activated soot–iron samples for (a) 1, (b) 5 and (c) 10 min.
Fig. 3. X-ray powder diffraction patterns (CuKa irradiation) of the annealed (1 h at 3008C) mechanically activated soot–iron samples: (a) 1 and (b) 5 min.
3.1. Structural and morphological changes during mechanical treatment and annealing of soot with iron and nickel The investigations of the changes of morphological and structural characteristics during mechanical activation of metal (iron, nickel)–soot mixture showed that the interaction in these systems had some common features but differences were also observed.
3.2. Mechanical activation of soot with iron
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Fig. 5. TEM microphotograph of iron and soot particles after mechanical treatment for 5 min. Fig. 4. X-ray powder diffraction patterns (CuKa irradiation) of the annealed mechanically activated soot–iron samples (10 min). (a) 1 h at 3008C, (b) 1 h at 4008C and (c) 1 h at 8008C.
tion intensity (Fig. 4c). This sequence of changes in the structural characteristics of the annealed mechanically activated samples, involving the increase in intensity of the iron and crystal carbon reflections and a simultaneous decrease of the cementite phase reflections with increasing annealing time, is characteristic of all soot to iron ratios in the composite material investigated in the present study. In our opinion, this experimental fact is evidence for a substantial role of the carbide phase in the crystallization of amorphous carbon. Electron microscopic studies of the mechanically activated and annealed soot–iron samples showed that after short-time mechanical activation (1–3 min) a rather wide size distribution is observed for metal and soot (amorphous carbon) particles. The size of the metal particles formed during mechanical activation varies from several tens to several hundreds of nanometers (Fig. 5). Soot particles are also substantially dispersed; their size is several tens of nanometers. Besides, coarse metal particles and soot globules that remained non-activated are observed in the samples. An increase in the time of mechanical activation is accompanied by further dispersing of both the metal particles and soot particles. At these stages of mechanical treatment the formation of iron carbide is detected in the samples by means of electron diffraction patterns. The size of iron carbide particles after activation for 10 min exhibits
a broad distribution from 3 to 7 nm to several tens of nanometers while the minimal size of the soot particles is several tens of nanometers. According to electron microscopic data, the morphological characteristics of the annealed mechanically activated samples depend on the time of mechanical activation. Annealing of the 1–3 min activated samples within the temperature range 300–8008C leads to the formation of round onion-like particles (Fig. 6a) (with a diameter about 200 nm). Morphological characteristics of the onion-like particles formed during the annealing of mechanically activated sample in the soot–iron system are periodic layers of concentric rings of crystalline carbon (graphite) 10–20 nm thick and iron (or iron carbide) particle is in the center. Light-field (Fig. 6a) and dark-field electron microscopic images of onion-like particles in carbon (Fig. 6b) and crystalline iron (Fig. 6c) reflections are a good illustration of the above description of onion-like iron– graphite particles. Annealing of the samples activated preliminarily for 5–10 min results mainly in the formation of encapsulated particles in which the surface of iron crystallites is covered with graphite layers of about 5 nm thick. The size of the iron particles varies within a rather wide range from 10 to 20 nm to several hundred nanometers (Fig. 7). An increase of the time of preliminary mechanical activation is accompanied by the formation, after annealing, of smaller metal particles encapsulated by graphite.
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Fig. 6. (a) Bright-field micrograph of onion-like iron-graphite particles. (b) Dark-field electron microscopic images of onion-like iron-graphite particles in crystal carbon reflection. (c) Dark-field electron microscopic images of onion-like iron-graphite particles in iron reflection.
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Fig. 7. TEM images of graphite encapsulated iron particles formed during the annealing of mechanically activated system soot–iron.
3.3. Mechanical activation of soot with nickel The investigations of the changes in the structural characteristics of a soot and nickel powder mixture during mechanical activation followed by annealing showed that there are some common features but also some differences between this system and the above-considered example of phase formation in the carbon–iron system. Substantial changes of the structural characteristics of the mixture of soot with nickel powder are observed within the first minutes of activation. X-ray diffraction patterns of soot– nickel mixtures activated for 1 min exhibit broadened nickel reflections (Fig. 8a). An increase in the activation time to 10 min is accompanied by increasing width of metal reflections and their shift to smaller diffraction angles (Fig. 8b). At the same time, it was observed that the broadening of X-ray reflections depends also on the nickel to soot ratio in the mechanically activated mixture for the same time of mechanical activation. An increase in soot content in the activated mixture leads to smaller broadening of X-ray reflections for the same mechanical activation time. Heating of the mechanically activated soot–nickel mixtures demonstrated that the structure of the composite material depends on annealing temperature. The annealing of mechanically activated samples for 1 h at a temperature up to 5008C leads to narrowing of the nickel reflections (Fig. 9a). Under these annealing conditions, X-ray diffraction patterns do not exhibit the formation of crystalline carbon or carbide phase. An increase of the annealing temperature to above 6008C causes further narrowing of the nickel phase reflections and the appearance of crys˚ (Fig. 9b). The intensity of talline carbon with d 002 53.39 A the crystalline carbon reflection was found to increase with
Fig. 8. X-ray powder diffraction patterns (CoKa irradiation) of mechanically activated soot–nickel samples for (a) 1 and (b) 10 min.
Fig. 9. X-ray powder diffraction patterns (CoKa irradiation) of the annealed mechanically activated soot–nickel samples (10 min). (a) 1 h at 5008C, (b) 1 h at 6008C and (c) 1 h at 9008C.
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increasing the annealing temperature. So, in the sample annealed for 1 h at 9008C the intensity of the crystalline carbon reflection is more than two times higher than for the same sample annealed at 6008C (Fig. 9c). This sequence of the changes of structural characteristics in the annealed mechanically activated samples is typical for all the soot–nickel ratios in the composite investigated in the present study. It should be noted that, unlike the soot–iron system, mechanical activation of soot powder with nickel particles did not lead to the formation of noticeable amounts of carbide phases. Moreover, the crystallization of amorphous carbon in the soot–nickel system took place at higher temperatures (above 6008C) than in the soot–iron system (above 3008C). Electron microscopic investigation of mechanically activated and annealed soot–nickel samples showed that morphological characteristics of this system are in many aspects similar to the characteristics of the soot–iron system and both depend on the time of mechanical activation and on annealing temperature. After short mechanical activation (1–3 min) the formation of nano-sized particles of nickel and soot is observed in the samples. Besides, coarse metal particles occur in these samples, as well as soot globules that remained non-activated. An increase of the time of mechanical activation is accompanied by dispersion of the metal and soot particles. Nano-sized particles of nickel and amorphous carbon form aggregates with a size of 300–1000 nm. Nickel particles after activation for 10 min exhibit a rather broad size distribution ranging from 3 to 7 nm to several tens of nanometers while the minimal size of the soot particles is several tens of nanometers (Fig. 10). It was discovered during the investigation that morphological characteristics of the annealed mechanically activated samples depend on the time of the preliminary mechanical activation and on the annealing temperature. Annealing of the samples (activated for 1–10 min) at a temperature below 5008C did not cause significant changes in the morphology and structure of the activated metal and soot. At the same time, heating of the mechanically activated soot–nickel samples (activated for 1–3 min) at a temperature above 6008C was accompanied by the formation of composite metal–graphite particles with various morphological characteristics. The particles present in the sample include: the particles of initial amorphous soot, rounded crystalline onion-shaped particles 100–200 nm in diameter (Fig. 11); graphite encapsulated metal particles with a size of several hundred nanometers. The onion-like particles formed during the annealing, similar to the annealing in the activated soot–iron system, consisted of periodic layers of concentric rings of crystalline graphite 10–20 nm thick. The annealing at temperatures above 6008C of the samples activated preliminarily for 5–10 min caused the formation of encapsulated metal nanoparticles in which the surface of nickel crystallites was covered with graphite
Fig. 10. TEM microphotograph of nickel and soot particles after mechanical treatment for 10 min.
layers about 5 nm thick (Fig. 12). The size of metal nanoparticles varied within a wide range from 10 nm to several hundred nanometers. An increase of the time of preliminary activation was accompanied by an increase of the fine metal particles encapsulated with graphite.
Fig. 11. TEM micrograph of onion-like nickel-graphite particles.
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carbon film with iron particles showed that the interaction starts when the system is heated to 700–7508C which is substantially lower than the melting point of the lowestmelting eutectics in the iron–carbon system (T511538C). At the initial stages of interaction, a propagation of the reaction front from the particle along the surface of the amorphous carbon film is observed (Fig. 13a). Diffraction patterns of the phases formed during the interaction (Fig. 13b) are characterized by the presence of both the point reflections of the crystalline product and the diffraction
Fig. 12. TEM images of graphite encapsulated nickel particles formed during the annealing of mechanically activated system soot–nickel.
3.4. In-situ electron microscopic investigation of the interaction of metals ( iron, nickel) with the amorphous carbon film As we demonstrated in the above sections, the formation of crystalline carbon takes place during isothermal annealing of mechanically activated carbon–metal mixtures. The formation of phases in the systems under investigation is of rather complicated character. For example, in soot–iron system, mechanical treatment causes the formation of an iron carbide (cementite) phase while no carbide phases could be detected in the soot–nickel system. The features of phase formation during mechanical treatment of iron and nickel with carbon, discovered in the present study, require additional investigations and explanation. However, at present it seems impossible to follow the formation of phases directly during mechanical activation or annealing. In our opinion, additional data on the processes that occur during the interactions in the systems involving amorphous carbon and metal can be obtained with the help of model systems. In the present study we have made an attempt to model directly the in-situ phase formation in nickel particle–amorphous carbon film systems and iron particle–amorphous carbon film systems during isothermal annealing, using an electron microscope. The investigation of structural and morphological characteristics of this composition allowed us to obtain data concerning the interaction of amorphous carbon with metals.
3.5. The changes of morphological and structural characteristics during in-situ annealing of amorphous carbon film with iron particle The investigation of morphological and structural characteristics during in-situ annealing of amorphous
Fig. 13. (a) Electron micrograph of the reaction front formed during in-situ electron microscopic isothermal annealing of iron particle on the surface amorphous carbon thin film. (b) SAD patterns of the carbide (point reflection) and graphite (diffraction rings) phases formed during the interaction.
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rings of polycrystalline graphite (the calculated interstitial distances for ring-type reflections are: 3.38, 2.12, 2.02 and ˚ which is in good agreement with the graphite 1.69 A, parameters). At the same time, the inter-planar distances and the symmetry of the point reflections allow us to state that the interaction of iron with the amorphous carbon film leads to the formation of crystalline iron carbide, Fe 3 C (cementite). Structural characteristics of cementite formed during the interaction of the amorphous carbon film with iron particle are in good agreement with the structural characteristics of cementite reported in Refs. [8–10]. After the reaction front grows till several microns, the formed carbide phase decomposes. This process is accompanied by a decrease in size of the carbide phase and an oriented crystallization of graphite on the surface of the initial iron particle. The graphite layers have crystallized parallel to the surface of the metal particle. Polycrystalline carbon remains in place of the decomposed carbide phase. The formation of graphite layers on the surface of the iron particle during the interaction leads to completion of the crystallization of the amorphous carbon film.
3.6. The changes of morphological and structural characteristics during in-situ annealing of amorphous carbon film with nickel particle Our investigations of the changes of morphological and structural characteristics during in-situ annealing of the amorphous carbon film with nickel particle showed that, independent of heating method (isothermal annealing or heating with a beam of accelerated electrons), the character of the evolution of the solid interaction products remained practically the same, though it exhibited some specific features. In the case where the amorphous carbon film with nickel particle was heated with a special heating holder, the following sequence of morphological and structural changes in the interacting system was observed. The reaction started at 600–6508C, which is much lower than the melting point of the lowest-melting eutectics in the nickel–carbon system (T51310–13208C). At the initial stages of interaction the formation of a reaction front and its propagation along the surface of the amorphous carbon film is observed. The propagation of the reaction front is accompanied by crystallization of the amorphous carbon film and the formation of a carbide phase (Fig. 14a). The diffraction patterns of the phases formed during the interaction exhibit both the point reflections of the crystalline product and the diffraction rings of a polycrystalline substance (Fig. 14b). The resulting inter-planar distances of the polycrystalline product correspond to the parame˚ which coincides exactly ters: 3.38, 2.12, 2.02 and 1.69 A, with the inter-planar distances in graphite. The inter-planar distances and the symmetry of the point reflections allow us to assume that nickel carbide Ni 3 C is the crystalline product of the interaction. After the carbide phase reaches the size of several microns, it decomposed with the
Fig. 14. (a) Electron micrograph of the reaction front formed during isothermal annealing of nickel particle on the surface amorphous carbon thin film. (b) SAD patterns of the nickel carbide (point reflection) and graphite (diffraction rings) phases formed during the in-situ interaction.
formation of crystalline graphite. Graphite phase is observed to crystallize onto the surface of nickel particle. The graphite layers are oriented parallel to the surface of the initial nickel particle. Polycrystalline carbon also remains in the place of the decomposed carbide phase. In cases when the amorphous carbon film–nickel particle system is heated with a high-intensity electron beam, the reaction front moves along the surface of the amorphous carbon film, too. A distinguishing feature of the interaction between the amorphous carbon film and the
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Fig. 15. Electron micrograph of the formation of fine metal (carbide) particles and graphite tracks during in-situ interaction of nickel particles with amorphous carbon film (heating with electron beam).
nickel particle during this heating method is the fact that, along with the formation of the carbide phase and the propagation of the reaction front along the surface of the amorphous carbon film, the formation of nanoparticles occurs. The formed particles move at a high rate along the surface of amorphous carbon film. Traces shaped as periodically located graphite plates remain on their way (Fig. 15). While moving, the particle changes its shape slightly. Under definite conditions it can be dispersed giving rise to particles with a much smaller size. These particles are also highly mobile along the surface of the amorphous carbon. The movement of formed particles leads to crystallization of the carbon film, too. It should be noted that the movement of particles along the surface of the amorphous carbon film is chaotic. Cooling of the nickel–amorphous carbon film system leads to a loss in mobility of the nanoparticles. The diffraction characteristic obtained from these particles correspond with the interplanar characteristics of nickel. In some cases the diffraction characteristics of the fine particles correspond to nickel carbide, Ni 3 C.
4. Discussion The experimental results obtained in our investigations can be outlined as some common and specific features characterizing the sequences of formation during the mechanical treatment and thermal annealing of metalcarbon systems:
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(a) Mechanical treatment of soot with nickel and iron is accompanied by a broadening of X-ray reflections of metals. The activation of soot–iron systems for longer than 3–5 min causes the formation of a cementite phase, Fe 3 C. (b) Electron microscopic studies of mechanically activated soot–metal mixtures showed that the activation of these systems is accompanied by an decrease in size of the soot and metal particles; their aggregation occurs. (c) Mechanical activation of soot–metal systems leads to a substantial decrease of the graphitization temperature of soot. In the soot–iron system, the graphitization of amorphous carbon starts at 250–3008C. The crystallization of soot in this system is accompanied by a decrease of the cementite phase that is formed during mechanical activation and a simultaneous increase of the iron phase. For the soot–nickel system, crystallization of the amorphous carbon was observed at temperatures above 6008C. (d) The morphological characteristics of the annealed mechanically activated soot–metal samples depend on the time of preliminary mechanical activation. For short activation (1–3 min), the soot–metal systems are crystallized with the formation of onion-like structures. Annealing of the soot–metal samples activated for more than 5 min leads to the formation of graphite encapsulated metal nanoparticles. The longer the preliminary mechanical activation, the smaller the size of the metal particles. (e) In situ electron microscopic investigations of the interaction of metal particles with an amorphous carbon film showed that the interaction starts in the systems under investigation at temperatures of about 6008C. The interaction in the iron–amorphous carbon film system and nickel–amorphous carbon film system proceeds via formation of the carbide phases Fe 3 C and Ni 3 C; their decomposition is accompanied by the formation of a graphite phase. For better understanding of the sequence of the phase formation during mechanical activation and annealing in the systems metal (iron, nickel)–soot (amorphous carbon), it is necessary to consider experimental and theoretical data reported in literature. An investigation of structural and morphological characteristics during mechanical activation of carbon was carried out in Ref. [11]. Crystalline graphite was used as carbon phase. According to the data obtained in these investigations, the mechanical activation is accompanied by a destruction of the graphite layers and the formation of disordered structures. Tanaka et al. [12] carried out studies of the processes occurring during mechanical alloying in the systems Fe–C (graphite) and Fe–C–Si. They demonstrated that mechanical activation was accompanied by the formation of an amorphous state while prolonged activation of the Fe–C system caused the formation of a metastable iron carbide, Fe 7 C 3 . It should be noted that the time of mechanical activation necessary for the formation of the Fe 3 C 7 carbide phase in the investigated mixtures was several thousand hours.
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It is known that the metals of the iron subgroup are catalysts for hydrocarbon processing. A scheme proposed in the investigations of hydrocarbon decomposition on iron catalyst (carbide cycle) [13–15] can be represented as follows:
According to this scheme, iron formed during the decomposition of the carbide phase Fe 3 C can interact with hydrocarbons at the very moment of its formation and pass into the carbide phase. It is well known [16] that amorphous carbon materials (soot) can be classified into graphitizing, non-graphitizing and those occupying an intermediate position. The investigation of the kinetics of carbon material graphitizations at 20008C allowed Chesnokov et al. [16] to propose a sequence of the graphitization process involving the formation of graphite crystallites at the first stages of the process due to the migration of large-angle grain boundaries. Much attention has been paid to the investigation of the effect of various additives on the graphitization of amorphous carbon materials. According to the experimental data [16], the effects of impurities on graphitization can be divided into four groups: 1. the dissolution of amorphous carbon in metals or metalcontaining compounds, followed by its separation as graphite; 2. the formation of carbide followed by its decomposition; 3. chemical interaction during carbonization causing structural changes in the formed carbon. According to Chesnokov et al. [16], the major part of the known catalysts of graphitization act according to one or more mechanisms. As a rule, carbide and carbideforming metals are used as graphitization catalysts. Fitzer et al. [17] proposed two mechanisms involving the possibility of dissolution of the amorphous carbon in the carbide melt, the formation of carbide supersaturated with respect to carbon, and its decomposition accompanied by the evolution of graphite. The investigation of the behaviour of the melt of carbon-saturated carbide of vanadium, nickel, iron, zirconium within the temperature range 2650–30008C demonstrated the possibility of rapid (within several minutes) formation of crystalline graphite. Long thermal treatment caused an increase of the size of the formed graphite crystals. A similar mechanism of graphite formation via the dissolution of disordered carbon in metallic melts, according to Fitzer et al. [17], occurs in the graphitization by nickel, cobalt and iron melts above the melting points of the metals. There are also data reported in the literature concerning the process of amorphous carbon graphitization via the formation of intermediate carbides. One example of a
graphitization process proceeding via this mechanism is the system silicon–amorphous carbon [16]. A mixture of silicon with amorphous carbon reacts to form silicon carbide. However, heating at temperatures above 22008C leads to a decomposition of the silicon carbide and the formation of crystalline graphite. According to the data obtained in this investigation, graphite crystals are pseudomorphous with the initial silicon carbide crystals. It is known that the graphitization process is substantially affected also by mechanical action and applied pressure [16]. For example, an increase of pressure to (0.4–0.6)10 8 N / m 2 during carbonization within the temperature range 400–6008C causes graphitization. If thermal treatment is carried out at a temperature above 10008C, a much higher pressure is required (.3310 8 N / m 2 ). According to Fedorov et al. [16], the applied pressure stimulates the destruction of supramolecular structures of amorphous carbon (for example, globules) which helps the formation of layered carbon structures and finally simplifies the crystallization of the graphite phase. Summarizing the literature data, the following features characterizing the graphitization of amorphous carbon materials by metals should be stressed: (i) graphitization of amorphous carbon materials is accompanied by the formation of intermediate carbide phases (ii) the transition from the amorphous state into the crystalline one is observed at temperatures above 10008C and depends both on the structure, composition, morphology of the carbon material to be graphitized, and on the conditions under which the process is carried out, for example, on the presence of the additives of various types acting as graphitization catalysts (iii) the graphitization process is substantially affected by the structure and morphology of the particles of amorphous carbon (soot). The comparison of the above-cited literature data with our experimental results allows us to propose the following sequence of phase formation during the mechanochemical interaction of iron and nickel with amorphous carbon. The formation of crystalline carbon during mechanical activation followed by annealing of carbon / metal samples occurs via the formation of intermediate carbide Me 3 C (Me5Fe, Ni) according to the scheme:
According to the proposed scheme, the formation of the carbide phase or metal–carbon composite occurs during the mechanical activation stage (1). The annealing of the mechanically activated mixture (carbide phase and metal– carbon composite) leads to decomposition of carbide phase and the crystallization of the carbon phase (2).
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It should be noted that mechanical activation leads not only to the formation of the carbide phase and the metalcarbon composite. It is most likely that the particles of the phases formed during mechanical activation not only are small but also contain a large number of defects. The formation of these nano-sized and defect-containing particles stimulates the processes of phase formation both during mechanical activation and during annealing. The above scheme is confirmed by the formation of the cementite phase Fe 3 C during mechanical activation. A decrease in the intensity of carbide phase reflections during isothermal annealing and the evolution of crystalline carbon phase are evidence for the fact that carbon crystallization occurs during decomposition of the iron carbide. Besides, in situ electron microscopic investigations of the interaction of iron particles with an amorphous carbon film proceeding via the formation and decomposition of the iron carbide Fe 3 C with the evolution of crystalline carbon confirm unambiguously the phase formation sequence proposed by us for the interaction of amorphous carbon with iron. Unfortunately, in the investigations of mechanical activation and annealing of the soot–nickel system we did not succeed in detecting the formation of an intermediate nickel carbide Ni 3 C. Nevertheless, we suppose that the crystallization of amorphous carbon during the annealing of mechanically activated compositions also takes place according to the scheme described above. This statement is based both on the literature data characterizing the properties of nickel carbide, and on our experimental data obtained in the in situ studies of a model system (amorphous carbon film-nickel particle). First, it is known that nickel carbide is stable at temperatures below 250–3008C and above 16008C. At temperatures within 250–4008C, nickel carbide decomposes at a noticeable rate [16]. In our opinion, this behaviour of nickel carbide does not allow to detect its formation during mechanical activation and annealing in the system soot–nickel, since under these experimental conditions the rate of nickel carbide decomposition exceeds the rate of its formation. Nevertheless, we detected the formation of an intermediate nickel carbide in the in-situ electron microscopic studies of the interaction of amorphous carbon film with nickel particles. It was discovered that the crystallization of amorphous carbon is preceded by the formation of the nickel carbide Ni 3 C. The decomposition of the latter is accompanied by the formation of crystalline carbon. The whole set of data described above allows us to state that the formation of crystalline carbon during the annealing of mechanically activated soot–nickel mixtures is also accompanied by the formation of an intermediate metastable nickel carbide, Ni 3 C. A special explanation is required for the effect observed by us, of a substantial decrease of the crystallization temperature of amorphous carbon during the annealing of mechanically activated carbon–metal mixtures. In our
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opinion, this decrease of the amorphous carbon graphitization temperature is due to several factors. First of all, mechanical activation leads to the destruction of the supramolecular structure of amorphous carbon (destruction of globules) which, as it was demonstrated in [16], simplifies the crystallization of amorphous carbon. Second, mechanical activation causes dispersion of metal particles to nanometer sizes, formation of various types of defects in the metal structure and formation of metal–carbon and carbide–carbon aggregates. The formation of carbide phases during mechanical activation of carbon with iron and nickel plays a definite part in the crystallization of amorphous carbon during annealing, which is due to the instability of these carbides within the temperature range under consideration.
5. Conclusion The investigation of the phase formation sequence during mechanical activation followed by isothermal annealing demonstrated that this process is available for the synthesis of iron and nickel nano-particles encapsulated by crystalline carbon. Moreover, the dependence of the morphological characteristics of the annealed soot–metal system on the time of preliminary mechanical activation discovered in the present study allows us to perform direct syntheses of graphite encapsulated metal nanoparticles of different shapes. Besides, the application of mechanical activation substantially help to decrease the amorphous carbon crystallization temperature. The experimental results of the in-situ electron microscopic investigations allowed us to propose a scheme describing the formation of crystalline carbon via consequent stages of the formation and decomposition of the metal carbide Me 3 C. In our opinion, the application of mechanical activation and annealing can be a useful and promising method to obtain nano-sized encapsulated particles of various metals, carbides and oxides.
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