Catalytic graphitization behavior of phenolic resins by addition of in situ formed nano-Fe particles

Accepted Manuscript Catalytic graphitization behavior of phenolic resins by addition of in situ formed nanoFe particles H. Rastegar, M. Bavand-vandchali, A. Nemati, F. Golestani-Fard PII:

S1386-9477(18)30095-X

DOI:

10.1016/j.physe.2018.03.013

Reference:

PHYSE 13079

To appear in:

Physica E: Low-dimensional Systems and Nanostructures

Received Date: 20 January 2018 Revised Date:

3 March 2018

Accepted Date: 12 March 2018

Please cite this article as: H. Rastegar, M. Bavand-vandchali, A. Nemati, F. Golestani-Fard, Catalytic graphitization behavior of phenolic resins by addition of in situ formed nano-Fe particles, Physica E: Low-dimensional Systems and Nanostructures (2018), doi: 10.1016/j.physe.2018.03.013. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Catalytic graphitization behavior of phenolic resins by addition of in situ formed nano-Fe particles H. Rastegara, M. Bavand-vandchalia,∗, A. Nematib F. Golestani-Fardc. Department of Materials Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran. b c

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a

Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran.

School of Metallurgical and Materials Engineering, Iran University of Science and Technology, Tehran, Iran.

Abstract

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This work presents the catalytic graphitization process of phenolic resins (PR’s) by addition of in situ nano-Fe particles as catalyst. Pyrolysis treatments of prepared compositions including

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various contents of nano-Fe particles were carried out at 600-1200 °C for 3 h under reducing atmosphere and graphitization process were evaluated by different techniques such as X-Ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), High Resolution Transmission Electron Microscopy (HRTEM), Simultaneous Thermal Analysis (STA) and Raman spectroscopy that mainly performed to identify the phase and microstructural analysis, oxidation resistance and extend of graphitized carbon formation. Results indicate that, in situ

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graphitic carbon development were already observed after firing the samples at 800 °C for 3 h under reducing atmosphere, increasing temperature and amount of nano-Fe led to a more effective graphitization level. In addition, the different nano crystalline carbon shapes such as onion and bamboo like and carbon nanotubes (CNTs) were in situ identified during

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graphitization process of nano-Fe containing samples. It was suggested that formation of these

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different nano carbon structures related to nano-Fe catalyst behavior and the carbon shell growth.

Keywords: Catalytic graphitization, Carbon nanotubes (CNTs), Phenolic resin, Nano-Fe.

∗

Corresponding author. E-mail: [email protected]

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1. Introduction It is well known that thermosetting phenolic resins (PR’s) is an important carbonaceous binder of Oxide-C refractories because of many good properties such as high fixed carbon rate, good wettability with oxide and graphite, high green strength and low hazardous environmental issues[1–4]. Compare

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to above good potential, PR’s are classified as non-graphitizing carbon binders and isotropic glassy carbon derived from these materials would not be spontaneously graphitized, even when Oxide-C refractories heated to high temperatures. In addition, residual glassy carbon has poor oxidation resistance, low thermal stability and high brittleness that will negatively affect some of the main

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refractory properties such as chemical corrosion, thermo-mechanical strength and thermal shock resistance [5–9]. Therefore, some efforts have been performed on PR’s modification by adding various metallic antioxidants in nano/micro sizes and, nano carbon sources such as carbon nano tubes (CNTs),

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carbon nano fibers (CNFs) and carbon black (CB) [10–14]. However, these modification methods still need to be further optimized. Recently, different researches have been reported to induce the crystallization of non-graphitic PR’s binders so-called catalytic graphitization that during pyrolysis of phenolic resin, glassy isotropic carbon convert into graphitic carbon by addition of some catalytic agents. Transition metallic elements (Ni, Co, Cu, and Fe), various compounds including organometallics, soluble metal salt and metal oxides based on transitions metals are the main

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ingredients that can be used as catalytic agents. Furthermore, it is well suggested that in order to higher graphitization level, these additives must be well dispersed in the resin inside structure as micro or nano-powder, suspension or even dissolved in a solvent and available as individualized molecules in the beginning of the resin carbonization process between 400 and 500°C [15–19].

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On the basis of Oya et al [20] studies, interaction of the catalytic agents with glassy carbon during carbonization process accomplish based on two routs “Generation-Decomposition” and “Dissolution-

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precipitation” as main mechanisms. In the first mechanism, graphitic carbon was precipitated at high temperature from carbide compound that formed by reaction of carbon and metallic particles. However, in later mechanism the non-graphitic carbon is continuously dissolved into the metal structure and precipitated as graphite after over saturation. When phenolic resins heat treated, some reducer gaseous consist of H2, C and CO are produced during pyrolysis process. These gases products can have reduced by transition metallic oxide according to Ellingham diagram. Consequently, on the basis of Ni-C, Co-C and Fe-C phase diagrams, carbon has limited solid solution in these metals that leads to fast carbide formation, saturation and precipitation of C with crystalline structure from Ni-C, Co-C and Fe-C solutions [21–25]. 2

ACCEPTED MANUSCRIPT According to Zhao et al. [26], a large variety of controlling factors determine the results of catalytic graphitization such as preparation method, categories and particle size of catalysts, heat treatment temperatures and carbon sources. Bitencourt et al. [27] have studies the role of different catalytic agents such as ferrocene, hematite and nano-Fe2O3 powder and processing parameters in the

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graphitization of phenolic resins. They reported each catalytic agent have effective role to graphitization process and high tendency of nano size powder to agglomeration inhibited carbon crystallization. In addition, heating rate and dwell time are effectively influence on graphitization process and graphitic carbon level. Yu et al. [28], clarified that, compared with a single catalyst, the mixed catalysts of transition metals such as Co:Fe promoted the graphitization process. These authors

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reported that modified phenolic resin have the higher crystallinity, more char yield and higher oxidation resistance.

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Recently, some researchers have realized that to further improve graphitization level of phenolic resins, one of the key issues is to use catalytic agents in nano size. It is well known that due to good dispersion in the phenolic resin structure and available in nano size scale, phenolic resins have been modified by addition of catalyst precursors such as transition metals nitride combination. According to catalytic agent reaction mechanism, some aspects such as higher graphitization rate in lower temperatures, compatibility and higher oxidation resistance and cost should be considered to obtain

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favorable structure and properties in the modified phenolic resin [29–31]. Considering these aspects, in the current research because of some suitable properties of ferric nitrate considered as catalytic agent to modify phenolic resin used as binder in Oxide-C refractories. The effect of catalyst agent and pyrolysis temperature was investigated on the phase, microstructure

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evolution, graphitization level and oxidation resistance of the modified phenolic resins, systematically.

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2. Experimental procedure

2.1. Preparation and pyrolysis of in-situ nano-Fe containing phenolic resins A kind of Iranian novolac type thermosetting phenolic resin was used as base material with 55 % fixed carbon. The ferric nitrate [Fe(NO3)3.9H2O] supplied by Merck and ethanol was chosen as catalyst precursor and dispersant, respectively. Precursor solution was prepared by addition of ferric nitride to 100 ml ethanol solvent under vigorous stirring at 25°C for 3 h to form a homogeneous solution. The weight ratio of Fe to phenolic resin were considered on 3 and 6 wt%). Defined amount of phenolic resin add into prepared solution and mix until a uniform gel obtain. As received gel was cured in 200 o

C for 24 h in the oven and the cured sample include different amounts of ferric nitride grinded into 3

ACCEPTED MANUSCRIPT fine powder. Finally, powdered samples were filled in the alumina crucibles and fired at 3 oC/min in reducing atmosphere (coke bed) to given temperatures 600, 800, 1000 and 1200 oC and held for 3 h. Depending on the content of Fe catalyst in the phenolic resin the samples were named as PR, PRF-3

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and PRF-6 for 0, 3 and 6 Fe wt%, respectively.

2.2. Testing and characterization methods

Phase identification and carbon graphitization in the PR, PRF3 and PRF6 samples after coking at different temperatures was characterized by a powder X-ray diffractometer (XRD). X-ray patterns of fine powdered were recorded over Braggs’ angle (2Ɵ) of 10-90o in Philips X-ray diffractometer using

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Ni-filtered Cu-Kα radiation. The Raman spectroscopy technique (FT-Raman 960 with He-Ne laser, wave-length 633 nm) was also used to investigate structural rearrangement in coking samples. The

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morphology and microstructure of samples were observed by field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS) analyzer (MIRA3 TESCAN) and also high-resolution transmission electron microscopy (HRTEM, TEMCM30 150Kv). Additionally, Thermo gravimetry-differential analysis (TG-DTA, STA449, NETZSCH, Germany) was carried out to investigate the oxidation resistance of the carbon derived from pyrolysis of phenolic resin with a heating rate of 10 oC/min in static air atmosphere.

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(1)

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 =  

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The average crystallite size (LC) was calculated using the Scherrer equation:

Where 0.89 is shape factor, λ is the wavelength of X-ray, β denotes full width at half maximum of the (002) peak in radians, Ɵ denotes the Braggs angle [32].After the Lc calculation degree of graphitization

% =

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(G) was also determined by: .

..

(2)

Where 0.344 nm is the interlayer spacing of turbostratic graphite and 0.3354 nm is the interlayer spacing for single crystal graphite [32].

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3. Results and Discussion 3.1. Phase evolution of different samples after pyrolysis

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The XRD pattern of PR sample cured at 1200 °C is shown in Fig. 1. As can be seen, this pattern indicates that the carbon crystallinity for pure phenolic resin is very limited even heat treated at high temperature which is in agreement with the statements of Rand and McEnancy, who reported that the carbon generated by phenolic resins should mainly show non-graphitic features with low toughness

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and oxidation resistance [33].

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Fig. 1. XRD pattern of PR sample fired at 1200 ºC.

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In Figs. 2 and 3 the XRD patterns of PRF-3 and PRF-6 samples prepared at temperatures of 600 ºC, 800 ºC, 1000 ºC and 1200 ºC are shown. Despite of the formation the amorphous carbon for the PR sample, a graphite crystalline structure is formed in PRF-3 and PRF-6 specimens and the amount of crystallization of phenolic resin has been increased after pyrolysis under the influence of in-situ nanoFe formation. Peaks in the 2θ angles about 26, 43, 54 and 78 degrees refer to (002), (100), (004), (110) graphite planes, respectively. With an increase in temperature from 600 °C to 1200 °C and also by increasing the amount of iron containing additive in the amorphous carbon matrix, the intensity of the peaks in the range of 20 to 30 ° increases, indicating an increase in the amount of graphite crystalline phase resulting from the pyrolysis of phenolic resin. 5

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Fig. 2. XRD patterns of PRF-3 sample fired at different temperatures.

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Fig. 3. XRD patterns of PRF-6 sample fired at different temperatures.

The precise specifications of graphite and crystalline carbon peaks in PRF-3 and PRF-6 specimens are

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shown in Tables 1 and 2. As shown in Tables 1 and 2, using an iron-containing additive in a phenolic resin, the structure of the resin is closer to that of the graphite-carbon structure. That is, the distance between the (002) planes in these compounds is closer to the graphite (002) planes (0.3354 nm) and the length of crystallites formed and the percentage of graphitization is also increased. By examining the results of this test, it can be concluded that by using an iron catalyst, the graphitizing temperature of the carbon obtained from the pyrolysis of the phenolic resin is reduced. At 800 °C, 36.04% and 40.69% of amorphous carbon is graphitized in PRF-3 and PRF-6 samples respectively, which reached 81.39% with increasing curing temperature up to 1200 °C in PRF-6 samples, which is remarkable. Comparing the results of PRF-3 and PRF-6 samples at different temperatures, it can be seen that the 6

ACCEPTED MANUSCRIPT addition of more iron catalyst offers more favorable results. As in PRF-6 samples, graphitization as well as crystallite size are higher than those of PRF-3 at similar temperatures, which indicates a better effect. Table.1. Crystallite size and graphitization level of PRF-3 sample fired at different temperatures. d002 (nm)

LC (nm)

800

26.115

0.3409

5.8

1000

26.2862

0.3388

7.5

1200

26.382

0.33756

8.1

G (%)

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2θ (002) (°)

36.04 60.46 74.8

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T (ºC)

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Table.2. Crystallite size and graphitization level of PRF-6 sample fired at different temperatures. 2θ (002) (°)

d002 (nm)

LC (nm)

G (%)

800

26.214

0.3405

6.1

40.69

1000

26.3359

0.3381

7.7

68.16

1200

26.426

0.337

8.4

81.39

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T (ºC)

3.2. Raman Spectroscopy results

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Raman spectroscopy is a molecular spectroscopy technique that has diverse applications in various research fields. Raman spectroscopy is an extremely valuable analytical tool for studying crystalline

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and amorphous carbon materials. This tool provides good qualitative and quantitative information about various physical properties, such as the amount of crystallization, the amount of order and disorder, and structural defects of carbon materials such as graphite, graphene and etc. Raman peak intensity is sensitive to crystalline structure, and good information such as crystalline size can be obtained by measuring peak intensity. Raman spectroscopy of carbon nanoparticles reveals distinct peaks in the range of 1000 to 2000 cm-1, which represents the amount of order and disorder of the carbon structure [32]. The D peaks near 1340 cm-1 in graphite and carbon nanostructures show the amount of defect and disorder. That is, with increasing the intensity of D peak, the amount of disorder and defects in the 7

ACCEPTED MANUSCRIPT material has increased. The G peaks at about 1600 cm-1 correspond to the stretching vibrations of C ̶ C bonds in the graphite layers. The G peaks show carbon atoms that are spatially coupled in the form of SP2 in graphite layers and, more simply, represent the amount of amorphous carbon graphitization [32].

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G-to-D peaks intensity ratio and vice versa in this analysis can provide appropriate information on the degree of graphitization, the amount of defects, and the size of the crystallites. Based on this, we can use a parameter which shows the ratio between the intensity of these peaks, which is indicated by the R sign, expressing the intensity ratio of the peak D (ID) to the intensity of the peak G (IG). The more the

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R ratio, the less graphitization has occurred. Also, using the Raman test and R results, the (La) crystallite size grown in the direction of the lateral planes can be measured. Namely, La represents the size of crystallites grown in single-layers of graphite or graphene in the direction of the SP2 hybrid.

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 = (2.4 × 10 )! "#

(3)

$

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Equation 3 shows the method of calculating La in nanometers [32].

In this case, λ is the wavelength (633 nm) of the used Raman spectrometer in nanometers.

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Figs. 4 and 5 show the Raman test results for PRF-3 and PRF-6 specimens respectively, and Tables 3 and 4 show the results of quantitative calculations. As can be seen, with increasing temperature in specimens, the intensity of G peaks increases and the intensity of D peaks decreases, which indicates an increase in the percentage of amorphous carbon graphitization from pyrolysis of phenolic resin in

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the presence of an iron catalyst.

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Fig. 4. Raman spectra of PRF-3 sample fired at different temperatures.

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Fig. 5. Raman spectra of PRF-6 sample fired at different temperatures.

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When the intensity of the G peak is much weaker than D in the carbon amorphous sample, that is a small percentage of the amorphous carbon is crystalline, which can be limited to the short range order, so that the X-ray diffraction pattern of such sample fully reflects the amorphous material. The results show that by increasing the curing temperature of the PRF-3 and PRF-6 samples, the process of amorphous carbon graphitization obtained from the phenolic resin pyrolysis improves. So that the R ratio for PRF-6 samples reaches 0.509 at best, this is an acceptable number. Also, with increasing curing temperature, the growth of crystallites in the direction of the SP2 hybrid increases, which indicates the increase of graphite phase in the amorphous carbon matrix resulting from the pyrolysis of the phenolic resin. 9

ACCEPTED MANUSCRIPT Table. 3. Crystallite size and R values of Raman spectra for PRF-3 sample fired at different temperatures. R

La (nm)

600

1.411

27.7

800

0.875

43.99

1000

0.62

62.09

1200

0.59

64.8

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T (ºC)

T (ºC) 600 800 1000

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1200

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Table. 4. Crystallite size and R values of Raman spectra for PRF-6 sample fired at different temperatures. R

La (nm)

1.16

33.1

0.82

46.9

0.58

66.37

0.509

75.6

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The other result observed is the greater effect of the additive, as by increasing the iron amount from 3 to 6 wt%, the crystallite length increases and the amount of R decreases. It is worth noting that by

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increasing the amount of additive as much as possible, it is possible to even improve the amount of amorphous carbon graphitization obtained from the phenolic resin pyrolysis at a lower curing temperature than those containing fewer additives. Therefore, the growth of crystallites and the degree of graphitization in PRF-6 samples cured at 1000 °C is not significantly different with PRF-3 samples cured at 1200 °C. It can be said that all results are consistent with the results of the XRD analysis. 3.3. Thermal Analysis results Due to higher oxidation resistance in crystalline carbon phases compared to their amorphous state, by studying the thermal behavior of phenolic resins pyrolyzed in different temperatures, suitable information can be obtained regarding structural changes and, oxidation resistance of carbon derived 10

ACCEPTED MANUSCRIPT from pyrolysis of Phenolic resin. Figs. 6 and 7 show TG curves of PR samples heat treated at 1200 °C, and PRF-3 and PRF-6 samples cured at 800 °C, 1000 °C and 1200 °C. Figs. 8 and 9 also show DTA curves of PR samples prepared at 1200 °C, and PRF-3 and PRF-6 samples heat treated at 800 °C, 1000 °C and 1200 °C. The results show that the oxidation resistance of PRF-3 and PRF-6 specimens has

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generally improved, so that in the DTA curves, the initial temperature, peak temperature, and oxidation end-point temperature of the samples increased compared to the PR sample. In addition, the effect of 6% iron catalyst was higher than 3% addition. It means that, with the addition of iron to the samples, the oxidation resistance increased, due to the graphitization of the amorphous carbon obtained from the pyrolysis of phenolic resins with an iron catalyst and the formation of graphitic carbon structure with

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content from 3 to 6%, the oxidation resistance improves.

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higher oxidation resistance compared with amorphous carbon. On the other hand, with increasing iron

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Fig. 6. TG curves of a) PR sample fired at 1200 ºC and PRF-3 fired at b)800 ºC, c)1000 ºC, d)1200 ºC.

Fig. 7. TG curves of a) PR sample fired at 1200 ºC and PRF-6 fired at b)800 ºC, c)1000 ºC, d)1200 ºC.

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Fig. 8. DTA curves of a) PR sample fired at 1200 ºC and PRF-3 fired at b)800 ºC, c)1000 ºC, d)1200 ºC.

Fig. 9. DTA curves of a) PR sample fired at 1200 ºC and PRF-6 fired at b)800 ºC, c)1000 ºC, d)1200 ºC.

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3.4. Microstructural Observation by FESEM and HRTEM

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Fig. 10 shows the micrograph images of a PR sample cured at 1200 °C. As you can see, these PR samples have an integrated amorphous structure, mentioning that graphite formation and the formation of crystalline structure did not occur. Fig. 11 shows images of PRF-3 and PRF-6 samples cured at 600°C. The morphology of the structure has not significantly changed compared to the PR specimens, and some of the carbon structure is spherically shaped with a low diameter, which is not significant. Fig. 12 shows the microstructure images of the PRF-3 specimens prepared at 800, 1000, and 1200 °C. In PRF-3 samples cured at of 800oC the carbon microstructure somewhat exceeded the amorphous state and changed to the onion-like structure. The progression of graphitizing in this onion structure is significant according to XRD and Raman results, and up to about 36%, the graphite structure is formed in the amorphous carbon matrix resulting from the pyrolysis of the phenolic resin. Also, a very limited 12

ACCEPTED MANUSCRIPT amount of graphitized carbons grew in tubular shape but their growth and density were very limited. By increasing the curing temperature up to 1000 °C and 1200 °C in PRF-3 samples, the amount of graphitized structure increases, so that the formation of carbon nanotubes is evident at this temperature. As shown in this microstructure, more amounts of carbon nanotubes (CNTs) resulting from pyrolysis

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of phenolic resin are formed alongside graphite onion-like particles and their density has increased in the matrix. The carbon nanotubes are about 50-100 nm thick and over a few micrometers in length. In addition, it can be seen that the presence of iron particles formed between graphitized phases and carbon nanotubes with nanoscale dimensions about 50-100 nm, which is probably formed due to the

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decomposition of iron nitrate during the pyrolysis process of resin samples.

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Fig. 10. FESEM image of PR sample fired at 1200 ºC.

Fig. 11. FESEM image of a) PRF-3 sample and b) PRF-6 samples fired at 600 ºC.

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ACCEPTED MANUSCRIPT The microstructure of the PRF-6 specimens heat treated at 800, 1000, and 1200 °C are shown in Fig. 13. In samples cured at 800 °C, the amount of graphitization has increased and the density of carbon nanotubes has also increased. As seen, CNTs with a diameter of about 50-90 nm and a length of about 500-600 nm are formed in the microstructure. According to the results of XRD and Raman tests,

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graphitization is also evident in these specimens, reaching about 41%. In PRF-6 samples, with an increase in the curing temperature to 1000 °C and 1200 °C, higher percentage of graphitization and

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longer CNTs, as well as the extension of onion-like carbons, is observed.

Fig. 12. FESEM image of PRF-3 sample fired at a)800 ºC, b)1000 ºC, c)1200 ºC.

With regard to the proposed microstructural evolutions, it is clear that increasing the in-situ nano Fe particles and the curing temperature, the graphitization level increases, which is observed both in the 14

ACCEPTED MANUSCRIPT onion-like particles and in the amount and size of CNTs. As the amount of nano iron catalyst and curing temperature increases, the morphology of the specimens changes noticeably. Formation of CNTs with the catalytically effect of nano-Fe particles in PRF3 and PRF6 samples, the growth of carbon nanotubes begins from the iron nanoparticles, and continues in a hollow and tubular shape,

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which the open-end and hollow form of nano tubes are quite distinct in Figs. 12c and 13b.

Fig. 13. FESEM image of PRF-6 sample fired at a) 800 ºC, b) 1000 ºC, c) 1200 ºC.

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Fig. 14. HRTEM image of PRF-3 sample fired at a, b) 800 ºC, c, d)1000 ºC, e, f)1200 ºC and g) Diffraction pattern of PRF3 fired in 1200 ºC.

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Fig. 15. HRTEM image of PRF-6 sample fired at a, b) 800 ºC, c, d)1000 ºC, e, f)1200 ºC and g) Diffraction pattern of PRF6 fired at 1200 ºC.

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ACCEPTED MANUSCRIPT Figs. 14 and 15 show HRTEM images of PRF-3 and PRF-6 samples cured at 800, 1000, and 1200 °C. In order to investigate the shape and dimensions of nanoparticles and carbon nanotubes in larger magnifications and in the nanometer scale, HRTEM images were taken and in order to confirm the XRD and Raman results, diffraction patterns of the samples were prepared (Figs. 14g and 15g). The

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graphitization of carbon is very clear in the images of all specimens, with increasing the curing temperature, the onion-like structure is gradually changed into CNTs, which are generally about 50100 nm thick, and their length has increased with curing temperature. The diffraction patterns of the samples have 4 rings, showing the crystal planes (002), (100), (004) and (110); which indicate the formation of graphite structure in the samples with the aid of Fe nanoparticles derived from the iron

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nitrate as additive. The results of the diffraction patterns are in agreement with the XRD results. At 800 °C, the onion-like graphite structure is observed which CNTs have grown around this structure in low

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amounts. Graphitized onion-like carbon is formed around of Fe nano-particles with a diameter of about 50-70 nm. The distribution of nanoparticles of onion-like graphite is uniformly clustered. This onion structure can normally be formed in phenolic resins at temperatures above 2000 °C [8,15], but by adding Fe nano-particles as catalyst, the graphitization temperature decrease to lower than 1000 °C. The thickness of this -like structure in the respective images is about 100-200 nm. The diffraction pattern also confirms the presence of graphite crystal planes. By increasing the curing temperature to

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1000 °C in PRF-3 and PRF-6 samples, the amount of CNTs and their size has increased. The formed CNTs are observed to have an external diameter of about 60 nm and internal diameter of about 50 nm. As shown in the pictures, the structure is completely hollow and the wall thickness of the nanotubes is about 5-10 nm. In other words, about 25 to 30 graphene layers, together form the CNTs wall for these

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specimens. The rings formed are specified in the diffraction pattern, are clearer and the number of points formed from the crystal planes is higher, that confirm the amount of the graphitization that is

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greater. It is also concluded Fe nano-particles with spherical and pseudo-elliptic morphologies and 50100 nm in diameter present at one end of CNTs in PRF-3 and PRF-6 samples heat treated at 1000 °C and the growth of CNTs begins with the formation of a graphite layer around these nanoparticles and continues in a hollow form.

3.5. Modified Phenolic Resin Graphitization Process Graphitization process in the phenolic resins is very complex process and for controlling and growth of graphitic carbons in phenolic resins structure by addition of nano catalytic agents, need to analysis of some aspects that should be considered. One of the most important factors in facilitating the 18

ACCEPTED MANUSCRIPT graphitization and formation of nanoparticles and graphitic carbon structure is the type of carbon source, choice of suitable transition metal and the size and distribution of homogeneous particles in the microstructure [15]. With using of Fe as a transition element with catalytic properties and microstructural studies of pyrolyzed phenolic resin samples at different temperatures, it was observed

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that the size of these particles in the heat treated resin matrix is about 50-100 nm. The relationship between particle size reduction and melting point can be calculated from equation (4). This relationship shows that by reducing the particle size to the nanoscale, the melting point is significantly reduced [21]. '()*

(4)

+,-

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∆& =

In this equation, Q is the Moore latent heat of fusion, γ is the surface tension, M is the molecular mass,

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T is the melting point, r is the particle diameter and ρ is the density.

To calculate this parameter for iron nano particles with a particle diameter of 50nm we have: 0 3 ' ) × (55.8 145 ) × (1808°7 ) 1 ∆& = = 69 3 0 (50 × 10 1) × (7.86 × 109  ) × (13810 ) 145 1

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2 × (1.862

That is, by Fe size reducing to 50nm, the melting point decreases by 69 oC and reaches a temperature of 1466 °C, which is more than the highest temperature of the curing sample of 1200 °C in our research. However, by looking at the Fe-C phase diagram, the eutectic temperature of this system is

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about 1153 °C. Therefore, it can be said that up to 1200 °C, which has the highest curing temperature in this study, partial melting of the Fe nano-particles containing dissolved carbon, also the part that has

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a lower curing temperature can remain in solid state. Therefore, the following three mechanisms, due to the presence of a variety of carbon sources, can be involved in the formation of carbon nanostructures:

1. Vapor-Solid Mechanism (V-S), when the Fe nano-particles are not melted and the curing temperature is low. In this case, the carbon source includes gaseous hydrocarbons in the system. 2. Vapor-Solid-Liquid mechanism (V-L-S), when the Fe nano-particles are melted and the carbon source includes gaseous hydrocarbons in the system.

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ACCEPTED MANUSCRIPT 3. Solid-Liquid-Solid (S-L-S), when the Fe nano-particles are melted and the carbon source is the amorphous carbon contained in the matrix. In the presence of Fe nano-particles and the presence of gaseous hydrocarbons and amorphous carbon, the process of crystalline nano-carbons formation in the various forms of CNTs and onion shapes is as

II.

Surface adsorption of resin particles by nano metal and metal nano-oxide Resin decomposition and reduce of metal nano-oxides

III.

Carbon diffusion into the nano-metal

IV.

Saturation of carbon inside nano-metal

VI.

Precipitation and formation of graphite layers around nano metal

Growth of graphite layers in the form of onion-like, bamboo shaped and carbon nanotube.

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V.

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I.

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follows [21,26].

To explain this mechanism more precisely, it can be said that during the heating of the samples, first Fe(NO3)3.9H2O available in the composition of the phenolic resin transforms to Fe2O3 under 1-4 reactions at 250 °C [34,35]. Fe(NO3)3.9H2O H2O (v)

2Fe(NO3)3.2H2O Fe2O3.3H2O

(1)

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H2O (l)

Fe(NO3)3.2H2O + 7H2O

Fe2O3.2H2O + 3N2O5 Fe2O3 + 3H2O (v)

(2) (3) (4)

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By continuing the heating process, gases such as CO, CO2, H2, H2O, CH4 and C2H6 are formed in the system. Among existing gases CO and H2 lead to the reduction of iron oxide during the 5-12 reactions

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[36–38] and the rest of the gases such as CH4 and C2H6 and the rest of the hydrocarbon compounds are adsorbed as sources for the formation of carbon nanotubes and graphite on the surface of the iron metal.

3Fe2O3 + CO = 2Fe3O4 + CO2

(5)

∆G ͦ = -32,950-53.86T (J)

Fe3O4 + CO = 3FeO + CO2

(6)

ͦ

∆G = 29,810-38.29T (J) 3FeO + 3CO = 3Fe + 3CO2

(7)

ͦ

∆G = -52,533+64.38T (J) 20

ACCEPTED MANUSCRIPT Fe3O4 + 4CO = 3Fe + 4CO2

(8)

∆G ͦ = -22,723+26.09T (J)

Fe2O3 + H2 = 2Fe3O4 + H2O

(9)

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∆G ͦ = 9,916+18.74TlogT-149.92T (J) Fe3O4 + H2 = 3FeO + H2O

(10)

∆G ͦ = 72,676+18.74TlogT-134.35T (J) FeO + H2 = Fe + H2O

(11)

∆G ͦ = 25,355+18.74TlogT-74.6T (J)

(12)

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Fe3O4 + 4H2 = 3Fe + 4H2O ∆G ͦ = 148,741+74.96TlogT-358.15T (J)

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After the reduction of the iron oxide present in the compound, the volatile hydrocarbon gases adhere to the surface of the Fe nano-particles and decompose into carbon by increasing the pyrolysis temperature (Fig. 16a). The formed carbons diffuse into Fe nanoparticles and dissolve. Carbon diffusion in Fe nano-particles and it is well clarified that the diffusion rate of carbon atoms strongly depends on temperature. Therefore, the rate of diffusion of carbon atoms in Fe nanoparticles is greatly increased

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when the pyrolysis temperature of the resin increased. Thus, the high ability and fast diffusion of carbon atoms into Fe nano-particles can improve the growth of CNTs and graphite layers. It should be noted that in both cases when the added Fe nano-particle is liquid or solid, the graphitization process is equal. The difference between these two states is the amount of carbon diffusion in nano-metal, which

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in both cases the concentration difference resulting from the over saturation of carbon in the nanoparticles of iron is the driving force of the diffusion. Obviously, liquid diffusion is more than solid diffusion in a metal, but by reducing the size of a metal particle to below 0.5 micron, this problem is

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greatly compensated. After dissolution of carbon in iron, the carbon content of the iron is saturated and precipitated rapidly due to the small size of iron particles, and with the continuation of the process, the C-C bonds are formed and expanded, and the layers become larger (Fig. 16b). The growth of the crystalline carbon on the surface of Fe nano-particle is in the direction of the base planes in the graphite structure (covalent bond in the SP2 direction) [32]. The maximum amount of precipitation of graphite layers on the surface of iron nanoparticles depends on three parameters including the size and amount of nano-iron, the amount of carbon containing and reducing gases such as hydrocarbons, and ultimately the temperature. After the formation of graphite layers, some graphite layers overlap and grow together. The reason for this is probably the local agglomeration and the formation of nano21

ACCEPTED MANUSCRIPT particles of iron at high temperatures (Fig. 16d, 15b and d). If some or all of the Fe nano-particles are melted at 1200 °C or above, agglomeration and adhesion of Fe nano-particles and also their growth is likely to take place due to the high surface energy and also the high fluidity of the iron melt. By formation of these agglomerates and increasing their dimensions when CNTs are formed, the diameter

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of the CNTs will increase somewhat, and if the size of these agglomerated nano-particles increases, the formation of the onion like structure may occur due to the overlapping of the graphite layers. With regard to this onion structure, it can be said that by increasing the temperature of the phenolic resin in the reducing atmosphere, the three-dimensional amorphous carbon network is formed. This threedimensional network surrounds iron oxide and forms an island structure. In the next step, iron oxides

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are reduced and Fe nano-particles are formed as agglomerates or adhering particles with a size of approximately 200-300 nm or more (Fig. 14f). By continuing the process and assigning more curing

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time and formation of carbon layers around the Fe nano-particles and their growth, onion-like areas are formed. During the formation of graphite onion like, the process of carbon dissolution in iron and graphite precipitation and the formation of graphite and crystalline layers continue and expand regularly. By forming the van der Waals bonds between the graphite layers, the system moves toward energy reduction. Of course, in some areas of graphite onion-like layers, branches of CNTs also grow, an example of which is shown in Fig. 15a. By extension of structure graphitization in case the structure

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becomes completely crystalline, Fe nano-particles are likely to combine together and become more agglomerated. This would require a long time and a higher temperature [30]. In this case, the dominant mechanism will be vapor-liquid-solid (V-L-S) and solid-liquid-solid (S-L-S).

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In addition to the onion-like structure, it is possible that different shapes of graphite structure will form on the surface of the Fe nano-particles, but most of these shapes have a hollow structure. The process of forming these hollow structures is such that after the encapsulation of Fe nano-particles by graphite

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layers due to the high process temperature expansion occurs in the Fe nano-particles and graphite shells. The amount of nano-particle expansion is higher than that of the graphite shell, which causes stress applied to the graphite shell. On the other hand, increasing the vapor pressure of the Fe nanoparticle at high temperatures is another factor in creating stress and applying pressure on the graphite shell (Fig. 16c). The subsequent phenomena caused by this process depend heavily on the stresses applied on the graphite layer as well as the strength of this layer. When graphite layers are under mechanical stress, changes are caused in the angle and length of the bonds, which may cause bending in the bond, but no change in the structure. Therefore, it can be said that the graphite shell around the Fe nano-particles has an elastic behavior, which is probably due to the homogeneity and flexibility of 22

ACCEPTED MANUSCRIPT the graphite layers [39,40]. According to Kovalevski et al [39] and also Zhu et al [40], if the formed graphite shell is defective, it is removed from the graphite shell due to the stresses applied to the Fe nano-particles, the Fe nano-particles will again be surrounded by the graphite shell of gas hydrocarbons, etc. Also, if the graphite shell is sufficiently homogeneous and elastic, with increasing

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vapor pressure, bursting and opening of the graphite shell is possible which depends on its mechanical properties. When the graphite shell is broken, mechanical strikes are applied to the Fe nano-particles, which cause them to move and exit to the broken area. The rate of this movement, VS, is defined by the <

derivative of the particle distance with graphite shell over time ( <=). At the same time, the process of >

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enveloping nanoparticles with graphite shells begins and expands. The rate of this phenomenon is determined by VP, which is calculated from the derivative of the graphite shell thickness with respect <

to time ( ? ). dS and dH are functions determined by the physical and chemical properties of Fe nano-

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<>

particles and graphitized layers such as nano-particle vapor pressure, iron strength, viscosity during melting and their molecular structure.

VS and VP are parameters which reveal the instant competition of two phenomena which include Fe nano-particles exiting through graphite shells and graphite shell growth. VS shows the rate of separation of Fe nano-particles from the graphite shell and VP shows the rate of graphite shells growth

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from iron nanoparticles.

According to the above, three modes for VS and VP are predictable. First mode

<= <>

>



(VS > VP)

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I.

The rate of separation of Fe nano-particles from the graphite shell is much faster than the formation of

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a new carbon layer. In this case, the gap between the existing atoms of the graphite shell and the new layer is high and there is no interaction between them, which results in the formation of a ring without an iron nanoparticle (Fig. 16e and Figs. 14b and 15d). II.

Second mode

@ A

<

B A

) (VS < VP)

If the growth rate of the graphite shell is much faster than the separation of Fe nano-particles from the shell, the carbon atoms precipitate from the Fe nano-particle and along broken bonds, the graphite react in a short time and a strong bond is formed between them and the graphite shell grows. The 23

ACCEPTED MANUSCRIPT breaking of the new shell is due to the cross-movement of Fe nano-particles, as the lateral walls of the new shell grow stronger. With the repetition and continuation of this process, bamboo-shaped CNTs grow (Fig. 16f, Figs. 14d and 15f). Third mode

<=

≈

<>



) (VS ≈ VP)

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III.

When the difference between VS and VP parameters is close, graphite growth will be tubular. In this case, graphite precipitates on every surface of Fe nano-particles and expands. With the growth of carbon pipes, following the process, graphite layers brake down due to the movement of Fe nano-

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particles, but the broken part applies stretching force to the nano-metal. The hydrocarbons released from the phenolic resin react with the Fe nano-particles and the growth process continues. In other words, after over-saturation of carbon inside the Fe, a stretching force is applied to this solid solution

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(Fe-C) as a result of the growth of graphite layers, which leads to the removal of graphite layers grown from the surface of the nano metal. When this stretching force is greater than the surface tension between the Fe nano-metal and graphite layers, the Fe particles are separated from the interface with the graphite layer and move toward the growth of the nanotubes (Fig. 16g, Figs. 14f and 15e). In certain cases, however, Fe nano-particles are trapped between CNTs and cannot move towards the tip,

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but one end of the nanotubes is definitely open (Fig. 15c and f). At the end of the process, the growth of carbon CNTs is stopped due to reducing carbon sources, as well as the complete coverage of Fe nano-particles [39,40].

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4. Conclusions

Using an iron-containing additive, the temperature of the amorphous carbon graphitization resulting

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from the pyrolysis of the phenolic resin decreased. So that the graphitization began at a temperature of about 800 °C and at 1200 °C in samples with 6 wt% in-situ Fe nano-particles reached to 81.38%. The effect of this additive was stronger by increasing their amount and also by increasing the curing temperature, as the graphitization temperature decreased and the percentage of graphite structure increased. The crystalline carbon structure formed in the presence of in-situ Fe nano-particles in the phenolic resin structure was often found in the form of carbon nano-particles with an onion-like and hollow CNTs morphology, which by increasing the curing temperature as well as the amount of the additive from 3 to 6 wt%, the quantity and size of CNTs increased. Oxidation resistance of phenolic resin specimens showed a significant increase which was more evident by increasing the percentage of 24

ACCEPTED MANUSCRIPT additive from 3 to 6 wt%, as well as rising the curing temperature. The study showed that in the process of graphitizing amorphous carbon obtained from pyrolysis of phenolic resins when different types of gas hydrocarbons are the source of carbon supply, the mechanism of carbon nanotubes growth is vapor-solid (V-S) reactions or vapor-liquid-solid (V-L-S) reactions in general, and if amorphous

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carbon is the source of carbon supply, carbon growth mechanism will be the solid-liquid-solid (S-L-S)

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model.

Fig. 16. Schematic model for the Fe nano-particles catalytic effect and formation of different crystalline carbon structure during graphitization process.

25

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(1998) 963–968.

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ACCEPTED MANUSCRIPT Tables Table.1. Crystallite size and graphitization level of PRF-3 sample fired at different temperatures. Table.2. Crystallite size and graphitization level of PRF-6 sample fired at different temperatures. Table. 3. Crystallite size and R values of Raman spectra for PRF-3 sample fired at different temperatures.

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Table. 4. Crystallite size and R values of Raman spectra for PRF-6 sample fired at different temperatures.

Figures

Fig. 2. XRD patterns of PRF-3 sample fired at different temperatures. Fig. 3. XRD patterns of PRF-6 sample fired at different temperatures.

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Fig. 4. Raman spectra of PRF-3 sample fired at different temperatures.

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Fig. 1. XRD pattern of PR sample fired at 1200 ºC.

Fig. 5. Raman spectra of PRF-6 sample fired at different temperatures.

Fig. 6. TG curves of a) PR sample fired at 1200 ºC and PRF-3 fired at b)800 ºC, c)1000 ºC, d)1200 ºC. Fig. 7. TG curves of a) PR sample fired at 1200 ºC and PRF-6 fired at b)800 ºC, c)1000 ºC, d)1200 ºC. Fig. 8. DTA curves of a) PR sample fired at 1200 ºC and PRF-3 fired at b)800 ºC, c)1000 ºC, d)1200 ºC.

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Fig. 9. DTA curves of a) PR sample fired at 1200 ºC and PRF-6 fired at b)800 ºC, c)1000 ºC, d)1200 ºC. Fig. 10. FESEM image of PR sample fired at 1200 ºC.

Fig. 11. FESEM image of a) PRF-3 sample and b) PRF-6 samples fired at 600 ºC. Fig. 12. FESEM image of PRF-3 sample fired at a)800 ºC, b)1000 ºC, c)1200 ºC.

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Fig. 13. FESEM image of PRF-6 sample fired at a)800 ºC, b) 1000 ºC, c) 1200 ºC. Fig. 14. HRTEM image of PRF-3 sample fired at a, b) 800 ºC, c, d)1000 ºC, e, f)1200 ºC and g) Diffraction pattern of PRF-

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3 fired in 1200 ºC.

Fig. 15. HRTEM image of PRF-6 sample fired at a, b) 800 ºC, c, d)1000 ºC, e, f)1200 ºC and g) Diffraction pattern of PRF6 fired at 1200 ºC.

Fig. 16. Schematic model for the Fe nano-particles catalytic effect and formation of different crystalline carbon structure during graphitization process.

30

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•

Catalytically effect of in-situ Nano-Fe particles formation on graphitization process of phenolic resin has been well illustrated.

•

Nano-Fe catalyst have a positive effect to formation of different nano-shapes of carbon

•

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such as CNTs, bamboo and onion like morphologies. Catalytically modification causes to decrease graphitization temperature in phenolic resins. •

Oxidation resistance of catalytically modified phenolic resins showed significant

The mechanism of different crystalline carbon phases morphologies is generally follow from vapor-solid (V-S) or vapor-liquid-solid (V-L-S) reactions and solid-liquid-solid (S-

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L-S) model can also considered for graphitization process.

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•

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improvement.