Influence of the multiwall carbon nanotubes on the thermal properties of the Fe–Cu nanocomposites

Journal of Alloys and Compounds xxx (xxxx) xxx

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Influence of the multiwall carbon nanotubes on the thermal properties of the FeeCu nanocomposites M.C. Bouleklab a, S. Hamamda a, Y. Naoui a, S. Nedilko b, *, T. Avramenko b, K. Ivanenko b, S. Revo b, P. Teselko b, V. Strelchuk c, A. Nikolenko c Laboratoire de Thermodynamique et Traitement de Surfaces des Mat eriaux, Universit e Fr eres Mentouri Constantine 1, B.P. 325 Route, Ain El Bey, Constantine, 25017, Algeria b Physics Faculty, Taras Shevchenko National University of Kyiv, 64/13, Volodymyrska Street, 01601, Kyiv, Ukraine c V. Lashkaryov Institute of Semiconductor Physics, The National Academy of Science of Ukraine, 41, Nauky Pr-t, 03028, Kyiv, Ukraine a

a r t i c l e i n f o

a b s t r a c t

Article history: Received 22 July 2019 Received in revised form 27 September 2019 Accepted 30 September 2019 Available online xxx

The relative linear expansion (DL/L0), coefficient of linear thermal expansion (CTE), differential scanning calorimetry (DSC), thermal conductivity and thermopower (ETP) of the FeeCu-MWCNT nanocomposites (Fe:Cu ratio was 4:1; MWCNT ¼ multi-wall carbon nanotubes of 0, 0.5, 1.0 and 2.0 vol% concentration) were measured and analyzed. It was found that MWCNTs effect on thermal behaviour on these nanocomposites varies in different temperature range and the magnitude of the effect depends on the MWCNTs content. The MWCNTs effect on temperature behaviour is mainly realized via blocking the access of copper atoms to iron grains by the MWCNTs atoms. These processes as well as effect of MWCNTs on the condition of the a-Fe / g-Fe transformation determine temperature behaviour of the nanocomposites in the range 550e700  C. Insignificant dependence of the relative expansion and coefficient of linear thermal expansion on temperature for the nanocomposites with 2.0 vol% of the MWCNTs indicates perceptiveness of this composition for application, particularly in motor vehicle industry. © 2019 Elsevier B.V. All rights reserved.

Keywords: Iron Copper Carbon nanotube Nanocomposite Thermal expansion

1. Introduction The iron-copper (FeeCu) nanocomposite systems drown considerable attention for a long time. This interest is related with attractiveness of the FeeCu systems for many applications due to their high strength, thermal and electrical properties, etc. [1e8]. At the same time, synthesis of FeeCu system encountered a number of obstacles related to the low solubility of components [1,3,9,10]. Mechano - chemical alloying (milling) (MCA) allowed significantly increase solubility of the intermetallic components relatively to equilibrium state and, at the same time, MCA is a rather simple and effective way to produce significant amount of nanocomposite by “cold” treatment [11e16]. In particular, a lot of studies have confirmed opportunity to produce metastable FeeCu nanocomposites by the MCA method [17e19]. The various carbon allotropes were used in the starting mixtures when FeeCu nanocomposites were produced [20,21]. It was shown

* Corresponding author. E-mail address: [email protected] (S. Nedilko).

that the kinetics of solid-state reactions as well as the phase composition of the produced materials depend on the type of used allotropic carbon forms, their content in the initial mixture and on the degree of its degradation under MCA [11,12,22e24]. We first have successfully used carbon nanotubes (CNTs) for production of the FeeCu nanocomposites [25e27]. Strong CeC covalent bonds and the structure of the CNTs walls provide exceptional mechanical properties such as high Young modulus (in excess of 1 TPa) coupled to strong resistance to breaking (more than 100 GPa). The studies of the CueTi-MWCNTs system also confirmed the improvement of their characteristics under addition of the multi-walled carbon nanotubes [28e30]. Some data about thermal behaviour of the FeeCu nanocomposites containing carbon forms (graphite e.g.) can also be found in published works [31]. However, the data on the dilatometric characteristics of such materials are very limited even for the FeeCu samples [21,32e37] and there is no data about thermal properties of the FeeCu nanocomposites containing MWCNTs. At the same time, the thermal behaviour of characteristics of these nanocomposites is of very importance from the view of both

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Please cite this article as: M.C. Bouleklab et al., Influence of the multiwall carbon nanotubes on the thermal properties of the FeeCu nanocomposites, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152525

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scientific and practical use, particularly in the motor vehicle industry. The aim of this work was to study the thermal behaviour of characteristics of the doped with multi-wall carbon nanotubes iron-copper nanocomposites (FeeCu-MWCNT); to clear out the mechanisms determining temperature behaviour of their characteristics and clarifying the effect of carbon nanotubes on these mechanisms. The FeeCu-MWCNTs nanocomposites (Fe:Cu ratio is 4:1; MWCNTs concentration ¼ 0, 0.5, 1.0, and 2.0 vol%) were produced and studied. Mentioned compositions are named here and below as FeeCu, FeeCu-MWCNT (0.5), FeeCu-MWCNT (1.0), and FeeCuMWCNT (2.0), respectively. The Fe:Cu ratio (4:1) was chosen taking into account measured previously by us mechanical characteristics for this content and these characteristics were better than those of the composites with other FeeCu ratios [25e27]. Dilatometric, calorimetric characteristics, as well as thermpower and thermal conductivity were measured and analyzed in terms of the MWCNTs influence on the composition and morphology of the samples. Noted data were obtained by monitoring the nanocomposites by the X-ray diffraction, Raman and IR spectroscopy and electron microscopy. The obtained data allowed us to draw conclusions about origin of the thermal behaviour and physical mechanisms of the MWCNTs influence on the behaviour. The results of this work can be the basis for recommendations concerning technological procedures of the FeeCu-MWCNT nanocomposites production. 2. Sample preparation and methods The multi-walled carbon nanotubes (TU U 24.103291669e009:2009) were synthesized using chemical vapour deposition procedure in a rotating reactor. Characteristics of the produced MWCNTs were: average diameter - between 10 and 20 nm; specific surface area (determined through Ar - adsorption) between 200 and 400 m2/g; bulk density - between 20 and 40 g/ dm3. The iron e copper charge was made from IP-1 (GOST 9849e86) iron and PMS-1 (GOST 4960e2009) copper powders in a 4:1 wt ratio. These powders were mixed with MWCNTs at the concentrations of the last ones of 0, 0.5, 1.0, and 2.0% in volume. After processing in the ball mill the nanocomposites were subjected to a 40% compression followed by annealing at 950  C for 30 min in argon flow. Then, the powder samples were subjected to cold rolling followed with annealing at 900  C in argon flow. The described cycle was repeated three times, thus the samples were ultimately undergone to 80% thinning. Finally, the thickness of the obtained ribbons was near of 0.3 mm and their density was 7.2 ± 0.2 g/cm3 for all of the samples. (The density was measured by hydrostatic (Archimedes) method with three replicates for every sample.) The values of the ultimate tensile stress were 865 ± 43; 1450 ± 65; 900 ± 41; and 930 ± 35 MPa for the FeeCu, FeeCuMWCNT (0.5), FeeCu-MWCNT (1.0), and FeeCu-MWCNT (2.0) nanocomposites, respectively [25,26]. The temperature behaviour of the some physical characteristics of the made nanocomposites, which are named here and below as FeeCu, FeeCu-MWCNT (0.5), FeeCu-MWCNT (1.0), and FeeCuMWCNT (2.0), were studied using dilatometry, calorimetry, thermal conductivity and thermopower technique. The NETZSCH 402C dilatometer (NETZSCH, Selb, Germany) with 3% of accuracy was used to perform dilatometry study. The heating rate was near 10  C/min. The thermal expansion coefficient was measured in the temperature range from 30 to 800  C. Differential scanning calorimetry (DSC) was performed using the Jupiter STA 449 F3 NETZSCH calorimeter (NETZSCH, Selb,

Germany). The same heating rate was applied as under dilatometric measurements. The thermopower method was used to analyse the structural changes in FeeCu-MWCNT nanocomposites after thermal treatments. Measurements of the thermopower characteristic were carried out using potentiometer with an input impedance of 20 Ohm with three replicates for every sample. Temperature at the points of contact between which the ETP was measured was 20 ± 0.1 С and 64 ± 0.050 С. Therefore, value of parasitic ETP arising between the contact points was less than 108 V. Thermal conductivity measurements were performed with use of original device described in the work [38]. The device provides opportunity to measure thermal conductivity in the range 20e400 W/m$K using bridge heat measuring circuit with equilibrium of heat fluxes. Three tests with accuracy 5% were performed for every sample. Structural characteristics and composition of the samples were monitored by the X-Ray diffraction, the transmission (TEM) and the scanning (SEM) electron microscopy, Raman scattering and Infrared spectroscopy (IR). The X-Ray diffraction patterns of nanocomposites were obtained using the automated DRON-4.0 diffractometer with the filtered cobalt radiation Ka ¼ 1.7909 Å under the following scanning parameters: monitoring range 2q ¼ (20e1200), step scan of 0.050, and counting time per step at 3 s. The collecting of the diffraction data was performed by a full-profile analysis using the program providing the experimental diffraction peaks interpolation by the Lorentz function. The average grain size formed after mechanical alloying was estimated by the well-known DebyeeScherrer approach. The TEM images of the MWCNTs and mixtures after milling were obtained using transmission electron microscope SELMI PEM125 K operated at 100 kV. The Tescan Vega 3 electron microscope equipped with Oxford Xmax EDS detector and software Aztec was used for study of the samples morphology and chemical composition. Accelerating voltage was from 1 to 25 kV and maximal resolution riches 3 nm. The IR absorption and Raman (macro e Raman monitoring) spectra were measured using a Jasco FT/130 IR-6300 (Jasco Analytical Instruments, Easton, MD, USA), Bruker SENTERRA (Bruker, Billerica, MA, USA). The triple T64000 Horiba Jobin-Yvon spectrometer equipped with quasi-confocal scanning microscope was used for the microRaman scattering measurements. The scanning and optical systems allowed the movement of the object at XYZ e coordinates with 100 nm accuracy and collection information with submicron spatial resolution. The AreKr Spectra Physics 2018 laser with wavelength of incident light (linc ¼ 488 nm) was used for this measurements. 3. Results 3.1. Dilatometric characteristics The changes of the relative length (DL/L0) and coefficient of thermal expansion (CTE) in accordance with the type of the samples and temperature are shown on Fig. 1 and Fig. 2, respectively. When temperature was higher than 125  C, the behaviour of the DL/L0 values noticeably depended on the MWCNTs concentration. When the MWCNTs concentration reached 2 vol% the DL/L0(T) values did not practically depend on temperature (Fig. 1, curve 4). We distinguished four main temperature ranges on the DL/L0(T) dependence for the FeeCu sample. The first range (~30e170  C) describes decrease of the DL/L0(T) value. A noticeable increase of the FeeCu sample’s relative expansion takes place in the range

Please cite this article as: M.C. Bouleklab et al., Influence of the multiwall carbon nanotubes on the thermal properties of the FeeCu nanocomposites, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152525

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Fig. 1. Dependences of the relative length, DL/L0(T), on temperature for the FeeCu (1); FeeCu-CNT (0.5) (2); FeeCu-CNT (1.0) (3); and FeeCu-CNT (2.0) composites (4).

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FeeCu-MWCNT (0.5). The DL/L0(T) riches the peak (1.9$103) at T z 790  C (Fig. 1, curve 3). When the MWCNTs concentration reached 2 vol%, the DL/L0(T) values did not practically depend on temperature (Fig. 1, curve 4). Thus, the maximal DL/L0 values were 2.1$103, 5.85$103, 2.9$103, and 0.1$103 for the FeeCu, FeeCu-MWCNT (0.5), FeeCuMWCNT (1.0) and FeeCu-MWCNT (2.0), respectively. The shape of the a(T) curves for the all four nanocomposites is different. At the same time, there are some common features that allowed us to select the temperature ranges similar to those noted above for the DL/L0 curves (Fig. 2). The values and behaviour of the a(T) curves for the samples FeeCu and FeeCu-MWCNT (0.5) are similar in the ranges 0 - 125e200  C, but a(T) dependence for the FeeCu-MWCNT (0.5) differs radically in the range 300e600  C from other a(T) curves. Thus, the described situation is similar to that observed for the DL/L0 data. If compared the data for the FeeCu and FeeCu-MWCNT (1.0) samples we can observe similar behaviour of the a(T) curves in the ranges ~ 150e300, 300e600 and 600e800  C. The a(T) values for the FeeCu-MWCNT (2.0) lie in the limits 0.5$107 - 2.0$107 ( C)1 for whole temperature range. Consequently, we can state that the effect of the MWСNT’s is various in different temperature ranges. 3.2. DSC data The measured calorimetric DSC curves are shown on Fig. 3. The shapes of the curves describing the variations of the heat capacities with temperature are similar each other for all the nanocomposites. Each of them reveals the complex exothermic band with close peak position of the main component. At the same time, we have to emphasise that the position and intensity of the band depend on the MWCNTs content and they vary in the ranges 565e590  C and 0.40e0.68 mW/mg, respectively. 3.3. Thermal electric power and thermal conductivity data

Fig. 2. Dependences of the thermal expansion coefficient, a(T), on temperature for the samples FeeCu (1); FeeCu-CNT (0.5) (2); FeeCu-CNT (1.0) (3); and FeeCu-CNT (2.0 (4).

230e580  C. The DL/L0 values also sharply increased when temperature increased from 650 to 740  C. The curve DL/L0(T) goes to saturation at temperature higher than 775  C. Temperature intervals between noted above, namely, 170e230, 580e650 and 740e775  C should be regarded as intermediate temperature ranges (Fig. 1, curve 1). If small amount of the MWCNT (0.5 vol%) was added to the FeeCu sample, the tendency of the DL/L0(T) dependence was practically the same as for the described above (Fig. 1, curve 2). The dependence of the DL/L0 values on temperature in the range 30e375  C is similar to that for FeeCu sample. There is a sharp increase of the DL/L0 value if temperature changes from 375 to 600  C. The next increase of DL/L0 value takes place in the range 650e700  C. In contrast with the case of the FeeCu sample there is a decrease in the DL/L0 value, if temperature was higher than ~720  C. As result, absolute maximum of the DL/L0 temperature dependence occurred at temperature near 715 ± 5  C. The DL/L0 values are larger for the FeeCu-MWCNT (1.0) composite in the 30e~375  C range whereas the DL/L0 values are smaller in the temperature range 400e800  C if compared to

The ETP increase was observed with increasing of temperature annealing. The view of the ETP(T) dependences is different for the samples without MWCNTs and the smallest content of the MWCNTs (Fig. 4, curves 1, 2) compared to nanocomposites with MWCNTs content 1 and 2 vol%. (Fig. 4, curves 3 and 4). As for thermal conductivity measured at room temperature its

Fig. 3. Differential scanning calorimetry data for the samples FeeCu (1), FeeCu-CNT (0.5) (2), FeeCu-CNT (1.0) (3), and FeeCu-CNT (2.0) (4).

Please cite this article as: M.C. Bouleklab et al., Influence of the multiwall carbon nanotubes on the thermal properties of the FeeCu nanocomposites, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152525

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Fig. 4. Dependence of the thermopower, ETP, on the temperature of annealing the samples FeeCu (1); FeeCu-CNT (0.5) (2); FeeCu-CNT (1.0) (3); and FeeCu-CNT (2.0) (4).

values were: 75, 62, 127 and 185 W/m$K for the samples FeeCu FeeCu-MWCNT (0.5), FeeCu-MWCNT (1.0) and FeeCu-MWCNT (2.0), respectively. 4. Discussion 4.1. Analysis of the temperature dependences and MWCNTs effect There are a significant number of reasons for the complex temperature behaviour of composite materials. Those are thermal expansion of the crystalline solid state lattice; transformation of the crystal lattice structure and phase transformation; reduction/ shrinkage due to the temperature-activated release of the initial adsorbed gases and gases formed as a result of thermal decomposition of the adsorbed compounds; sintering and degradation of the composite grains in rolled materials, etc. [1, 4, 11, 17e19,21, 26, 31, 34, 35]. Multiple interactions of the particles (iron-iron; coppercopper, and iron-copper) as well as influence of un-controlled factors (pores, defects, impurities, etc.) complicate temperature and concentration behaviour of the materials under study. It should also be emphasized also that activity of these processes can essentially depend on the size of the grains/particles of the material. It should be noted that measured thermal characteristics could be influenced by anisotropy related with the distribution, shape and size of the MWCNT [39,40], but we have to highlight that ours work was no directed to study anisotropy effects. So, special efforts were not undertaken to reveal anisotropy of thermal behaviour of composites under study. It will be the subject of future research, we suppose. At the same time, we can point that measured relative expansion data (DL/L0) were close to those have been published before about similar systems (see Table 1 and, e.g. Ref. [34]). Table 1 The DL/L0 (T) values for the FeeCu-CNT composites. Sample

FeeCu FeeCu-CNT (0.5) FeeCu-CNT (1.0) FeeCu

Temperature, 0C

Remarks

500

750

0.00178 0.00282 0.00340 0.00279

0.00234 0.00568 0.00203 0.00345

Own data Own data Own data Published [34]

Temperature dependences of the a(T) (CTE) showed that for the FeeCu composite, we can state that temperature behaviour of the a(T) was different on the ranges ~ 65e120, 165e260, 310e525, 575e650 and 725e790  C. The average values of the CTE in these ranges, , have been determined. There were - 2.6, 3.1, 2.7, 8.2 and 7.9∙106 К1 for the noted ranges, respectively. It is interesting to compare values to the values of previously known for components of studied nanocomposites. We can note that for bulk both iron and copper samples the CTE values lie in rather wide limits. It is possible to note the CTE values (12.8e14.4)∙106 К1 and (12.4e14.1)∙106 К1 in the range 100e300  C for a-Fe with carbon content of 0.05 and 0.5 vol%, respectively. The CTE value for copper is 17∙106 К1 in the range 0e100 С. Mentioned values are significantly (in 4.35 and 5.5 times, respectively) higher than the CTE value for the FeeCu sample in the low temperature range 30e300  C. It is common knowledge that the most of solids are expanded if their temperature increases. The temperature range where the contribution of the plain thermal expansion of the crystal lattice is dominated should be that where the DL/L0(T) dependence is close to the linear and, at the same time, the CTE is constant. Taking into account described above and the data on Fig. 2 we concluded that the plain thermal expansion of the FeeCu sample can dominate only in the ~350e550  C temperature range. The sharp change or extremes of the a (T) took place at those temperatures where the slope of the dependence of L/L0(T) was varied. These were the ranges 120e130, 270e280 and 680e690  C. These observations showed that in the vicinity of these temperatures, the chemical composition, structure, and phase composition of the composites under study take place. In fact, in accordance with the simplest model of composite containing two components which do not interact each other, the coefficient of thermal expansion of composite, am, can be described by so-called “rule of mixtures":

am ¼ a1(1- v2) þ a2v2,

(1)

Where a1 is the CTE of the main phase 1 of the material, and a2 and v2 are the CTE and the volume contribution of phase 2 to which the first phase is transformed. It is easy to see that for a case a1 > a2, the v2 increasing leads to a decrease in the value of am at a rate (a1 - a2); whereas in the case of a1 < a2, the v2 increasing is accompanied by the am increase with the same rate: (a1 - a2). Thus, the abovementioned transitions to the ranges of increase (decrease) of the CTE on the a(T) curves can be caused by phase transformations. An existence of more than two components complicates mentioned temperature dependencies. As for DSC data we can state that highest intensity of calorimetric bands is for the FeeCu-MWCNT (1.0) composite and therefore the ratio of its intensity, in comparison with other nanocomposites, is as follows: I0%/I1% ¼ 87%, I0.5%/I1% ¼ 66%, and I2%/I1% ¼ 58%. We could see also three peaks of insignificant intensity in the range 50e175  C; the peak lying on the left of the main (450  C) and the “shoulders” in the ranges of 200e335 and 635e755  C in addition to the main intensive peak. The published data regarding the DSC of FeeCu and FeeCuCarbon nanocomposites are rather limited. Similar data on the FeeCu-MWCNT nanocomposites are completely unknown. Nevertheless, we can see that both temperature dependences and their numerical characteristics are to some extent similar to those already published on different compositions [21,41,42]. In particular, the heat release reached the maximum (~2.25 mW/mg) at ~ 500  C for the sample of 50:50 of the FeeCu ratio [19]. Compared the Textr values with the data concerning the CTE average values, , it was easy to see that the Textr values

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approximately correspond to the temperature ranges in which changes in the values occurred. So, we can conclude that behaviour of dilatometric and calorimetric characteristics was caused by the same reasons. Regarding thermopower data we noted the difference between ETP(T) dependences for the FeeCu, and FeeCu-MWCNT (0.5) samples and for the samples with higher content of the MWCNTs: FeeCu-MWCNT (1.0) and FeeCu-MWCNT (2.0). We see that there was a tendency to saturation for the first two nanocomposites. The effect of MWCNTs was also manifested in a decrease of the ETP values for the FeeCu-MWCNT (0.5) nanocomposite comparing to the FeeCu. However, the ETP values increased with further increasing of the MWCNTs content. The range of the ETP values changing (DETP) for the FeeCu and FeeCu-MWCNT (0.5) samples is larger (DETP z 3.7 mV/K) comparing to the FeeCu-MWCNT (1.0) and FeeCu-MWCNT (2.0) nanocomposites (DETP z 2.4 mV/K). The ETP dependences on Tanneal are close to linear in temperature range 30e300  C. With the Tanneal increasing these dependencies are not linear, though they can also be considered as linear for temperatures higher than 530  C. Similar to the results of dilatometric and thermopower measurements, the effect of MWCNTs on thermal conductivity was also not monotonous and thermal conductivity felled on the step 0 / 0.5 while it increased when the content of MWCNTs roused from 0.5 to 1.0 and then to 2 vol%: 75, 62, 127 and 185 W/m$K for the samples FeeCu, FeeCu-MWCNT (0.5), FeeCu-MWCNT (1.0) and FeeCu-MWCNT (2.0), respectively. So, the data on thermal conductivity also indicated a different type of MWCNTs influence on the structure of the FeeCu-MWCNT nanocomposites. The accomplished analysis of the results showed that MWCNTs affect the temperature characteristics of FeeCu nanocomposites by modifying their composition, structure and defectiveness. It is obvious that the above mechanisms should be interconnected. 4.2. Possible mechanisms of the MWCNTs effect on composition, structure, and defectiveness of the FeeCu-MWCNT nanocomposites The arguments about the role of structural and phase transformations in the thermal behaviour of FeeCu-MWCNT nanocomposites also are from the XRD data (Fig. S1). According to the data on FeeCu powders compacted after MCA we noted that they contain the a-Fe phases with a solid Cu solution (a-Fe,Cu) and their composition varies from 95 wt% of iron and 5 wt% of copper to 83 wt% of iron and 17 wt% of copper [25]. It is obvious that these nanocomposites can contain a significant number of defects, uncontrolled atomic and molecular impurities, too. 4.2.1. Change of the samples composition The IR and Raman spectroscopies were used to study the last question. The maxima of the lines in the IR e spectra are located at the same frequencies for all the FeeCu-MWCNT samples, but the series of additional lines appeared in the range of low frequencies (1400 - 1700 cm1) for the nanocomposites containing carbon nanotubes (Fig. S2, curves 2e4). Intensity of these lines, as well as of the lines near at 2920, 2950 and 2345 cm1 increased markedly with increase of the MWCNTs content. Noted lines can be identified as belonging to some molecular groups: the lines at 1642 and 1523 cm1 are caused by C]C bonds; 2345 and 2355 cm1 e by > C]O bonds; 2850 and 2920 cm1 due to symmetric and antisymmetric valence vibrations of the CH2 group [43]; 3240 cm1 is a manifestation of the OeH bond of the CO2H groups [44]. The study of macro - Raman light scattering in near-surface layers of the samples also revealed a number of lines in the frequency range 150e800 cm1 which correspond to internal

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vibrations of various types of iron and copper oxides (Fig. S3) [45]. Characteristics of the bands in the vicinity ~960, ~1080 and ~1160 cm1 can be corresponded to the methyl groups. The presence of the mentioned molecular and metal-oxide groups in the FeeCu composite should be considered as a result of participation of ambient atmosphere in the MCA and following thermal treatment and rolling. The increase of the IR absorption intensity caused by molecular groups in the nanocomposites containing MWCNTs clearly showed that carbon nanotubes and products of their destruction can adsorb mentioned molecular groups and their radicals from the environment under conditions of high local temperatures which are achieved on the surface of iron and copper grains undergone to MCA. It was obvious that some desorption of these molecular groups occurs under heating the FeeCu-MWCNTs nanocomposites in the temperature range of 30e200  C that is accompanied by shrinkage of the nanocomposites (Fig. 1, curves 1 and 2). The further increase in temperature leads to the plain thermal expansion of the crystal lattice of the composite components, and this plain expansion begins to dominate the effect of shrinkage. The increase of the content of copper solid solution in the a-Fe component, as well as a-Fe / g-Fe structural transformation cause the noticeable increase of the sample’s length and increase of CTE with temperature increasing in the range 550e700  C (see curves 1 on Figs. 1 and 2). The output of the DL/L0(T) dependence on saturation and decrease of the CTE after 700  C can be the result of sintering [33,34]. Similarly, a rise of the rate of the DL/L0(T) increasing with temperature and extremums on the a(T) dependences at temperatures higher than 350  C for the FeeCu-MWCNT (0.5) sample are associated with structural violations produced by carbon atoms. In particular, a diffusion of carbon atoms to the surface layer of metal grains takes place following violations their crystal lattice (the case of a1 < a2 in equation (1)). (See as an illustration of this phenomenon Fig. S4a.) However, these processes counteract the introduction of copper atoms to iron grains, that is accompanied by a decrease in a(T) (the case a1 > a2 un equation (1)). As a result, a large band aroused on the curve a(T) with peak position near 475  C. Further increase and following decrease of the a(T) after T z 650  C are to be related with the a-Fe / g-Fe transformation and subsequent sintering. The further increase of the MWCNTs content up to 1 vol% suppressed the described processes, first of all due to the formation of carbides in the surface layers of the grains, we suppose. This process lead also to a certain separation of the carbides, iron and coppers grains and MWCNTs. An incorporation of higher amount of carbon nanotubes (FeeCu-MWCNT (2.0) composite) enhanced the processes described above and brought to formation of larger amount of the iron carbides. Besides, the MWCNTs when their being added to the nanocomposites in a large amount promote blocking copper atoms incorporation into the iron grains [25]. So, the products of the MWCNTs decomposition form intermediate layers between metallic grains. These layers have very low CTE comparing to one for iron and copper grains and carbides. Indeed, MWCNTs have CTE of opposite sign in the axial and radial directions: 1.2$105 K1 and (1.6e2.6)$105 K1 [46,47]). This means that in the disordered state, which is characteristic state of carbon nanotubes in studied nanocomposites, the MWCNTs have to demonstrate low or close to zero CTE. Thus, formed at significant amount of the MWCNTs (2 vol %) layers and their fragments really damp the expansion of metal fractions of nanocomposites, that is manifested by the behaviour of the DL/L0(T) and a(T) (curves 4 in Figs. 1 and 2). The discussed results are in agreement with the data on some other MWCNTs containing nanomaterials for which CTEs are

Please cite this article as: M.C. Bouleklab et al., Influence of the multiwall carbon nanotubes on the thermal properties of the FeeCu nanocomposites, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152525

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practically equal to the CTE of carbon nanotubes [29,30,46e48]. Such a strong decrease of CTE was also associated with formation of the iron carbides [49,50] or iron and copper oxides and formation of M-O-MWCNT bonds [51,52]. Both types of formations have relatively low CTEs at the grains boundaries and thus they block the possible thermal expansion of the nanocomposites. We have observed formation of the intermediate carbon layers due to direct comparative experiments performed by the CEM method on FeeCu-MWCNTs composites. Thus, a detailed analysis of morphology of the carbon-free samples, FeeCu, practically did not reveal intermediate areas between metallic grains. This image is shown on Fig. 5,b, where the grains of one (bright spots) and other metal (dark spots) are easy to differ. On the contrary, the intermediate regions between the metallic grains can be clearly identified from the images of the surface of the composites with the highest content of the MWCNTs: FeeCuMWCNT (2.0), (Fig. 5c and 5,d). Analysis of chemical elements (EDS procedure) performed along certain directions of this sample showed correlation of the content of iron, сopper and сarbon with the profile of the surface. For an example, one of the following directions is indicated in Fig. 5,d and Fig. 6,a. Measured along this direction the concentration profile (Fig. 6,b) shows an increased content of copper or iron, where metal grains are present, with noticeable anticorrelation of the segments with a higher concentration of iron or copper atoms. The intervals between the grains (dark spots on Fig. 6,a) are just characterized by high carbon content (see Fig. 6,b). The decrease of the DSC and ETP values for the FeeCu-MWCNT (0.5) composite in the entire temperature range compared to FeeCu is also related with increase of defectiveness of nanocomposites that is caused by diffusion of carbon atoms into the surface layer of the grains due to the MCA treatment (Fig. 5,a). In

fact, it is known that ETP(T) characteristic is sensitive to macro and microstructure of materials, to defects in the crystal lattice, to relaxation of structure and defects, etc. [53]. Thus, the increase of structural disorder, the number of defects, etc. reduce the length of free run of charge carriers and phonons resulting in a decrease in the ETP value. Obviously, nanotubes are responsible for changing the ETP(T) curves in the set of the samples FeeCu, FeeCu-MWCNT (0.5), FeeCu-MWCNT (1.0) and FeeCu-MWCNT (2.0). An addition of the small amount of the MWCNTs to the FeeCu composite leads to an increase in the number of defects in the crystalline grate of grains of this material, which also results in a decrease in the values of ETP(T) for the sample FeeCu-MWCNT (0.5) (Fig. 4, curve 1). The increase of the ETP(T) values for FeeCu-MWCNT (1.0) and FeeCu-MWCNT (2.0) nanocomposites should be associated with the formation of the above-mentioned micro/nanoscale extended areas of carbon material formed from MWCNTs fragments. Thus, according to the published data, ETP(T) for multi-wall MWCNTs can reach the values of 80 mV/K [54] that is almost an order of magnitude higher than the ETP(T) of the rolled iron tape (ETP(T) ¼ 12.26 mV/K). Temperature treatment (annealing) is accompanied by a structural relaxation of materials that leads to decrease in the density of defects in the crystalline structure, to redistribution of the phase components and impurities, etc. These factors contribute to increase of ETP with Tanneal increasing. So, combined effect of heat treatment and MWCNTs causes ETP(T) dependencies showed in Fig. 4. It is worth noting here that MWCNTs effect is different on temperature ranges 300e550 and 800e1000  C, and therefore the reasons of the effect also must be different. Obviously, this result is a consequence of the fact that mechanisms of structural transformations on theses temperature ranges are different, while temperature range 550e800  C is the range of junction. These mechanisms also depend on the MWCNTs

Fig. 5. The TEM image of the FeeCNTs (0.5) compacted powder sample (a); the SEM images of the FeeCu (b), and FeeCu-CNT (2.0) composites (c and d).

Please cite this article as: M.C. Bouleklab et al., Influence of the multiwall carbon nanotubes on the thermal properties of the FeeCu nanocomposites, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152525

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Fig. 6. a) Fragment of the SEM image of the FeeCu-CNT (2.0) composite and line along which the concentrations of the Fe, Cu, and C atoms were measured (EDS data). b) Concentration profiles for the Fe (1), Cu (2), and C (3) measured along indicated line.

content and MWCNTs mechanical destroying. It is obvious that a mechanical state of the MWCNTs strongly influences thermal behaviour of FeeCu-MWCNT nanocomposites. Undoubtedly, various fractions of defected and destroyed MWCNTs have to interact differently with other components of nanocomposites forming carbides, countering agglomeration of metal particles and influencing their size, changing the distribution of grain in size, etc. [25]. The role of the size factor in the dilatometry of nanocomposites including FeeCu and FeeCueC has been noted earlier [21]. The absence of a correlation between the contents of the MWCNTs and the average size of grains (see Fig. S1) once again demonstrates the impossibility of allocating only single factor that determines the temperature behaviour of characteristics of nanocomposites under study. Valuable information on morphology and defectiveness of nanocomposites was obtained from their Raman spectroscopy data. It is known, only single so-called G - band (Raman shift ~ 1580 cm1, half width ~ 10 - 15 cm1) caused by vibrations of the double bond C ¼ C in the aromatic cycle of MWCNTs is observed in the spectra of the ideal MWCNTs. Structural violations (defects, impurities, etc.) generate so-called the D - band (Raman shift ~ 1250 - 1450 cm1, half width ~ 20 - 200 cm1) [55,56]. The increased intensity and the considerable width of the D-band may be due to the high content of defective tubes or a large number of shortened nanotubes (MWCNTs fragments) where the boundary effects are more pronounced, high concentration of the amorphous phase, etc. The ratio of the intensity (ID/IG) (ID and IG are intensity integral over

the bands) characterizes the relationship between parts of material in disordered and ordered states [57]. The range of the D and G bands in the macro - Raman spectra of studied nanocomposites is shown in Fig. S3. It is easy to see that the main, in intensity, observed features are the D, G, and T - bands. The half width (170 - 204 cm1 for the D e band and 104e127 cm1 for the G e band) and the ratio of ID/IG (0.86e1.11) of the D and G bands indicate a significant defectiveness and destruction of the MWCNTs. The band in the vicinity of 1100e1160 cm1 is so-called T e band, (half-width is 50e130 cm1), can be attributed to the manifestation of sp3 states in amorphous and glassy - like carbon and in other forms containing deformed carbon layers [58,59]. Described above macro - Raman data were monitored over the area of exciting light beam of cross-section up to ~500 mm. At the same time, temperature behaviour of the characteristics is the result of processes at the micro/nanoscale levels. In order to highlight this fact and the role of heterogeneity the Raman scattering on a micro - level was studied when the cross section of exiting light beam was only ~1e3 mm. The typical micro - Raman spectroscopy data for selected areas (points) are shown on Fig. S4. The main conclusion made from these measurements is about significant heterogeneity in the distribution of MWCNTs on the surface of the samples. This follows, firstly, with a significant difference of intensity of the spectra measured at different areas (points) of the samples, as well as the different ratio of the intensity of the spectrum associated with MWCNTs (spectral range 900e1800 cm1) and the wing of Rayleigh scattering (range

Please cite this article as: M.C. Bouleklab et al., Influence of the multiwall carbon nanotubes on the thermal properties of the FeeCu nanocomposites, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152525

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100e250 cm1) caused by the reflection of light from the metal components of the sample. It is obvious, that the higher values of this ratio correspond to the areas where the content of carbon material is higher (see curves p1 (0.5), p3 (2.0) and p4 (2.0) in Fig. S4). If compared the spectra of the nanocomposites with spectrum of initial carbon nanotubes (Fig. S4, MWCNTs curve), we can see that there are areas of nanocomposites (p1 (0.5) and p4 (2.0)) where the spectra are quite similar to the MWCNTs spectrum. The content of the MWCNTs is hig in these areas, as noted above, but the D and G bands have considerably larger half-widths (39e97 cm1) if compare to “pure” MWCNTs. Moreover, they lie on the background of a very wide band (half-widths are of 272e356 cm1), in contrast to the spectrum of “pure” MWCNTs. Thus, we must state that products of MWCNTs destruction are localized there and they cause this wide band. The spectral components related to different carbon forms can be distinguished at the some areas of the samples. E.g., we can note the band in the vicinity of 1496 cm1 in the spectrum of the FeeCu-MWCNT (2.0) sample (area p3 (2.0)). Additional bands are also observed at 1431 and 1449 cm1 for the areas (p2 (0.5) and p2 (2.0)). Besides, the new bands were identified at frequencies 1278 and 1300 cm1 in the spectra of areas p2 (0.5) and p3 (0.5). Thus, we can argue that wide W band, the D and G bands, etc., in fact, are the superposition of the numerous spectral components associated to various carbon forms. It should be also noted here that, due to the specific nature of the milling processes and the subsequent stages of preparation of the composites an amorphization occurs not only with respect to the “soft” components of these nanocomposites (such as MWCNTs) but amorphization also concerns of the metal constituents. The presence of a rather intense background in the XRD spectra should be considered as a manifestation of amorphous phases (Fig. S1) [25,41]. The results on DSC confirm the above made conclusions about the role of changing the composition (range of low temperatures up to 200  C) and phase transformations at higher temperatures. The origin of the quite wide DSC band is directly related to the role of iron component in the composite. The half width of the band is ~350  C and the band lies from 400 to 750  C. It is known that the iron is in a-Fe allotropic form within this temperature range. However, as we have already shown above, a-Fe / g-Fe structural transformations takes place at the same temperature range for FeeCu and FeeCu-MWCNTs powder nanocomposites and the temperature of transformation depends on the composition of the sample, on the size of grains, etc., that is reflected in the position and intensity of the main DSC peak. Small size and inhomogeneity of particles and grains contribute to the increase of surface energy and energy of interface layers, which are the main sources of energy accumulated under milling. The release of this energy during heating has to increase the activity of composite components by increasing the speed and ways of their atoms transportation, thus affecting the processes of phase transformation, sintering, etc. Here we must add that, in our opinion, the influence of carbon forms on the size of the structural components and on the oxidation processes of the metallic components of nanocomposites is also important. As it had been shown earlier, the kinetics of oxidation on the heating stage depends on the type of carbon allotropic forms and their structure [24]. So, the effect of various carbon forms can be revealed as presence of a number of peaks of varying position and intensity on the DSC curves. 5. Conclusions The iron-copper-multy-wallcarbon-nanotubes (FeeCuMWCNT) micro/nanoscale composite materials (the MWCNT

concentration was 0, 0.5, 1.0, 2.0 vol% and Fe: Cu ratio ¼ 4 : 1) named as FeeCu, FeeCu-MWCNT (0.5), FeeCu-MWCNT (1.0), and FeeCu-MWCNT (2.0) were prepared and studied. The temperature dependences of the relative expansion (DL/L0(T)), of the linear coefficient of thermal expansion (CTE or a(T)), of the thermopower (EPT(T)) and differential scanning calorymetry data (DSC) as well as thermal conductivity were measured. Obtained results were analyzed and discussed together with data on the composition, structure and morphology of the samples. It was found that DL/L0(T) and EPT(T) dependences are not linear, while the a(T) dependences and DSC curves reveal several extremums in the temperature ranges 30e1000  C. These data indicate that changes in the chemical composition, structure and phase composition of nanocomposites occur in mentioned temperature range. An addition of the MWCNTs to the FeeCu and changing MWCNTs concentration results in changes of noted dependences and data. Besides, effect of MWCNTs varies for different temperature ranges and this effect depends on the content of the nanotubes. This is evident both in the manifestation of various mechanisms of thermal behaviours and the various types of carbon nanotubes effects on these mechanisms. The possible mechanisms of the MWCNTs effect on temperature behaviour are a) the blocking of the copper atoms access to the iron grains by the MWCNTs atoms. Such processes as well as effect of MWCNTs on the condition of the a-Fe / g-Fe transformation determine thermal behaviour of the composite in the range 550e700  C. Influence of the MWCNTs on sintering mainly determines measured characteristics of nanocomposites at temperatures higher than 700  C. Described mechanisms of the MWСNTs influence on the properties of the FeeCu-MWCNT nanocomposites operate in the conditions of strong defectiveness of material, destruction of the MWCNTs, heterogeneous distribution of metallic components and MWCNTs over the sample. Their cumulative effect causes “blurry” temperature characteristics of studied nanocomposites. That is why, in order to clarify in detail the mechanisms of the MWCNTs effect on the structure and properties of the FeeCu nanocomposites further research is needed. The obtained results are also interesting in terms of technology. Particularly, it should be pointed out that increasing of the MWCNTs content up to 2 vol% practically eliminates dependences on temperature of the length and coefficient of thermal expansion of the FeeCu nanocomposites. This results is very important for practical uses. Acknowledgements The work was financially supported by the State budgets of Ukraine and Algeria via Ministry of Education and Science of Ukraine and Ministry of Higher Education and Scientific Research of Algeria, respectively. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152525. References [1] C. Ying Yu, S. Rainer, C.Y. Austin, Thermodynamic analysis of the iron-copper system I: the stable and metastable phase equilibria, MTA. A. 15 (1984) 1921e1930. [2] G. Mazzone, M.V. Antisari, Structural and magnetic properties of metastable fcc Cu-Fe alloys, Phys. Rev. B. 54 (1996) 441e446. [3] K. Sumiyama, T. Yoshitake, Y. Nakamura, Magnetic properties of metastable

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Please cite this article as: M.C. Bouleklab et al., Influence of the multiwall carbon nanotubes on the thermal properties of the FeeCu nanocomposites, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152525