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The influence of carbon (fullerite, graphite) on mechanical alloying of Cu-25 at % C composites
R. M. Nikonova, V. I. Ladyanov, N. S. Larionova Physicotechnical Institute, Ural Branch of Russian Academy of Sciences, Izhevsk, Russia
PACS 61.48.-c, 61.72., 63.22. Kn, 68.35.-, 78.30.Na, 81.05.ub, 81.07.Bc, 82.80.Gk DOI 10.17586/2220-8054-2016-7-1-190-197
A comparative study of Cu-C6o/ro and Cu-Cg composites obtained by mechanical alloying has been performed by means of scanning electron microscopy, X-ray diffraction, and Raman spectroscopy. It has been demonstrated that high stress-related effects, which take place during the mechanochemical synthesis of Cu- C60/70 and Cu-Cg composites with a nanocrystalline structure, result in the formation of an oversaturated solid solution of carbon in copper, Cu(C). The morphology and the parameters of the crystal lattice aCu(tMA) and the sizes of the crystallites L(tMA) of the powders obtained depend on the deformational stability of fullerite and graphite and also on their reactivity to adsorbed oxygen.
Keywords: fullerite, graphite, copper, mechanically alloyed composites. Received: 20 November 2015
1. Introduction
At present many of researchers' attention is devoted to investigations on the preparation of nanostructured materials for engineering and functional purposes and on the optimization of their physical and technological properties. An upcoming trend here is the synthesis of metal matrix composites modified by carbon nanomaterials (fullerenes, carbon nanotubes, etc.), which have unique physical and mechanical properties.
One of the most widely commercially applied metals is copper - manufacturers like it for its high electrical and thermal conductivity. However its low hardness, and low corrosion resistance limit its use in pure form. To improve its structural characteristics, researchers suggest considering the prospects of preparing of copper-carbon composites. Carbon can protect copper from oxidation and/or regenerate its already oxidized surface. Also, the problem of electrochemical copper erosion can be resolved.
Different carbon forms are used to prepare the Cu-C composites. In most cases, graphite can be utilized [1, 5-8]. Nanotubes [9-13], nanodiamonds [2,5], and fullerites [3, 14-16] are mentioned in the literature as possible carbon sources. Among the methods used for modification of copper with carbon are sintering [6, 13], the formation of film coatings by the joint deposition of Cu and carbon [15, 16], carbon deposition from gaseous phase via the catalytic decomposition of acetylene and ethylene with subsequent compacting and sintering [12], application of ultrahigh strain [1], and shock-wave loading [14]. Also, a promising technique is mechanical milling in high-energy ball mills [2,3-5,7]. u-C specimens have higher hardness than that of carbon-free copper [8,12], which is attributed primarily to the stabilization of copper grain boundaries [8].
Since under equilibrium conditions carbon is practically insoluble in copper, mechano-chemical synthesis (MS) is considered a promising method for the preparation of such composite materials - it provides the atomic-level mixing of components and thus increases solubility.
Since different forms of carbon have different physico-mechanical and -chemical properties, it is expected that mechanical alloying (MA) of composites with their addition will give different results. In this regard, it would be especially interesting to conduct comparative studies on the effect of allotropic forms of carbon on the formation of the structure of copper-based composites.
So far, comparative studies on the mechanical alloying of copper together with fullerites and graphite have not been performed. However, the authors in [3] assume that the synthesis of composites with fullerene particles in a copper matrix will give better results. The investigations of Fe-C, Al-C, Mg-C systems in papers [14-16] confirm that in contrast with graphite, fullerenes do exert a more significant strengthening effect both immediately after mechanical alloying [16] and after the following spark plasma sintering [14,15].
In this paper, we present the results for the comparative study of mechanically alloyed copper-fullerite and copper-graphite composites.
2. Materials and methods
Copper powders (99.5 %), graphite CgACS (mechanically crushed rods), and fullerite C60/70 were taken as elemental powders for mechanical alloying. Copper powders had a dendrite structure formed by spherically-shaped particles ~ 3-5 ^m in size, with dendrite branches as long as ~ 150 ^m (Fig. 1, 0 h MA). Fullerite C60/70 and graphite particles were of a flat and layered ellipsoid shape, respectively, and had a size of ~100 - 200 ^m. The composition of C60/70, according to HPLC and thermal gravitational analysis, was as follows: 82.18 % C60; 14.08 % C70; 2.81 % C60O, C60O2, and C70O; 0.93 % C76, C78, Cg2, Cg4 %; 1.1 % of residual toluene.
For copper-fullerite and copper-graphite composites, the copper and carbon powders were mixed in the proportion Cu-25 at % C (hereafter referred to as Cu-C60/70 and Cu-Cg, respectively) with the total weight of 30 g. The samples were mechanically alloyed in a planetary ball-mill AGO-2S (balling drum material was hardened stainless steel, balls 8 mm in diameter were made of ball-bearing steel) in a pre-evacuated inert atmosphere PAr = 0.1 MPa (P = 10-3). The duration of mechanical alloying ranged from 15 min to 8 h.
A BRUKER D8 ADVANCE (CuKa radiation) diffractometer was used to perform X-ray diffraction study of the samples. Magnesium oxide (MgO), prepared as described in [17], was taken as a reference specimen to account for instrument broadening. The calculations were
made with an error for lattice parameters (aCu) smaller than 0.002 A, crystallite sizes (L) 0.5
1 /2
nm, and the level of microdistortions ((e2) / ) 0.05 %. To measure the changes in particle morphologies (size and shape) a scanning electron microscope QUANTA 200 3D was used. Raman spectroscopy was used to study the structural changes of carbon in the mechanically alloyed Cu-C60/70 and Cu-C composites. The measurements were taken with a Labram HR800 (HORIBA) spectrometer with an exciting laser wavelength A = 632.81 nm.
3. Results and discussion
Fig. 1 includes microimages of the initial copper powders and Cu-C60/70 and Cu-Cg samples, and also the images taken after their mechanical alloying for 1 and 8 h. It is seen that in the process of mechanical alloying of copper-carbon powders, different deformation kinetics of Cu-C60/70 and Cu-Cg composites are observed, depending on the carbon form. This follows from the different shapes of the resultant particles and their dispersion. After 1 h of milling, the particles of the mechanically alloyed composites represent stone-like agglomerates formed from smaller particles (Fig.1), with Cu-C60/70 powders being more finely-dispersed at the beginning
R.M. Nikonova, V.I. Ladyanov, N. S. Larionova Initial copper
Fig. 1. SEM-images of particles of Cu-25 %C powders in their initial state and after 1 and 8 h of mechanical alloying
than Cu-Cg powders. With an increase of the mechanical alloying time to 8 h, the particle sizes in the composites compared tend to level off (Fig. 1). It should be noted that with a decrease in the composites' carbon content (to 5 at %), the morphological differences become more expressed.
Figure 2 demonstrates X-ray diffraction patterns for Cu-C60/70 and Cu-Cg powders that were mechanically activated for 0.5, 1,2 and 8 h (tMA). Judging by the appearance of an X-ray amorphous halo in the area of C60/70 main reflections at angles 29 — 8 — 35 0 (Fig. 2a, insert) an amorphous fullerite-like phase is formed in the Cu-25 % C60/70 blend after its mechanical alloying for 0.5 - 2 h. At the same time, Cu2O is formed. As concerns Cu-Cg, after as early as 0.5 h of mechanical alloying (29 ~ 26 0), the structural maximum of graphite in the X-ray diffraction pattern totally disappears; also, the formation of Cu2O is no longer observed.
Fig. 2. X-ray diffraction patterns of Cu-C60/7o (a) and Cu-Cg (b) powders at different MA times
Figure 3 shows the Raman spectra of Cu-C60/7o and Cu-Cg samples after 1 and 8 h of mechanical alloying. Since D-band (1332 cm-1) has a higher intensity in comparison with G-band (1582 cm-1) (Fig. 3, a), we can draw a conclusion that the graphite structure in the Cu-Cg sample after 1 h of mechanical alloying is characterized by a higher number of defects. A considerable intensity decrease and broadening of G-band is a consequence of graphite amorphization [18]. The presence of the main bands of fullerenes C60 and C70 in the Raman spectra of the C60/70 composite mechanically milled for 1 h indicates the presence of fullerenes in the sample (Fig. 3, a). Owing to the spherical shapes of C60 molecules, their high compressive strength and weak intermolecular bonds in a crystal, fullerenes in the process of high energy
milling presumably act as "molecular bearings" [19]. Due to this, higher dispersion is attained in the process of mechanical alloying of the Cu-C60/7o system.
Fig. 3. Raman spectra of Cu-C60/70 and Cu-Cg composites after 1 (a) and 8 (b) h of MA
For long periods of mechanical alloying (8 h), the Raman spectra of both Cu-C60/70 and Cu-Cg samples correspond to amorphous carbon (Fig.3, b). This is indicative of the destruction of fullerene molecules and the disordering of graphite's lamellar structure. As a result, particle sizes in the compared Cu-C60/70 and Cu-Cg systems level off (Fig. 1). It is to be noted that smaller particle sizes in Cu-C60/70 powders in comparison with Cu-Cg powders can also be explained by the formation of a fragile copper oxide Cu2O at tMA = 0.5 — 2 h (Fig. 2, a).
Thus, based on the results of Raman spectroscopy, it can be stated that the differences observed in the Cu-C60/70 and Cu-Cg systems in process of mechanical alloying depend on a higher deformation stability of fullerite in comparison with graphite. The latter was previously shown in our research [20].
According to the diffraction maxima of the Cu-C sample profiles, the dependences of copper crystalline lattice parameter aCu and crystalline sizes L on the mechanical alloying time (tMA) for Cu-C60/70 and Cu-Cg are different (Fig. 4). At tMA = 1 — 2 h Cu-C60/70 powders have smaller grains, 12 nm, in contrast with Cu-Cg powders, at a maximum value of aCu = 3.6169 A. In the plot of aCu versus milling time (tMA) we observe the maximum, and in the plot of L versus milling time, the minimum is observed. In the process of milling for 4 - 8 h crystallite sizes increase and reach values close to those for Cu-Cg composites. At the same time, aCu decreases to 3.6162 nm. The change of grain sizes in the samples is seen to correlate with the scanning electron microscopy data (Fig. 1) for particle dispersion.
In compliance with [21], from a thermodynamics point of view, the presence of adsorbed oxygen at Cu-C interfaces should not have resulted in the formation of copper oxide, since the latter should have been reduced by carbon. However, since the energy of attachment for oxygen atoms to a C60 molecule exceeds that of the heat of Cu2O formation (376.8 and 166.6 kJ/mol, respectively [22,23]), it is energetically favorable for oxygen to react with copper. As a result, copper oxide Cu2O is formed in the sample, with part of oxygen additionally dissolved in the copper matrix to form Cu(C, O). This explains larger value of aCu = 3.6169 A in comparison with Cu-Cg (aCu = 3.6162 A). Furthermore, larger aCu value in the Cu-C60/70 system is also determined by smaller grain sizes (L), and hence, larger specific surface area for the particles.
Because of this, the solid phase reactions in the Cu-C60/70 system proceed in two steps. At the beginning (within 2 h) copper oxide (Cu2O) is formed, with fullerene molecules remaining uninvolved. At the same time, an oversaturated solid solution of carbon and oxygen
Fig. 4. Dependence of microstress (a), crystallite sizes (b), and lattice parameter (c) of copper in composites Cu-C60/7o and Cu-Cg on MA time
in copper Cu(C, O) is formed. The second stage is initiated by the destruction of fullerene molecule frameworks caused by high energy deformation. This results in amorphous carbon formation. At 4 - 8 h of MA, the formed copper oxide is reduced:
2Cu2O + C ^ 4Cu + CO2 f -Q
and a partial diffusional decay of the oversaturated solid solution Cu(C, O) occurs with the formation of Cu(C). This decay can be explained by the reaction proceeding between its components:
[C]cu + [O]cu ^ CO f
where [C]Cu and [O]Cu are carbon and oxygen dissolved in copper. As a result, an oversaturated solid solution of Cu(C) is formed with Cu = 3.6161 A, and this is accompanied by the recrystallization of the sample ((L) increases from 12 to 20 nm).
Unlike the case with Cu-C60/70, the mechanical alloying of Cu-Cg is a one stage process. As a result of this process, an oversaturated solid solution Cu(C) is formed. Because of a faster amorphization of graphite and its high reducibility neither oxide formation nor oxygen dissolution in copper is observed here.
Thus, it can be stated that the differences in the dependence aCu(tMC) observed between Cu-C60/70 and Cu-Cg at the initial stage of their MA (up to 2 h) are not only determined by the different deformational stability of fullerite and graphite, but also by their different reactivity with oxygen.
4. Conclusion
Regularities of structural and phase conversions in the mechanical alloying processes of copper-fullerite and copper-graphite composites have been identified. It has been demonstrated that the mechanical alloying of Cu-C60/70 and Cu-Cg composites with a nanocrystalline structure results in the formation of an oversaturated solid solution of carbon in copper, Cu(C). The formation kinetics of this solution, together with the powder morphology has been shown to depend on the deformational stability of fullerite and graphite, as well as their reactivity with adsorbed oxygen.
Acknowledgements
The work has been carried out under financial support of the program the Presidium of Ural Brunch, RAS (grant 15-20-2-22).
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