- Email: [email protected]
The growth of carbon nanotubes in aluminum powders by the catalytic pyrolysis of polyethylene glycol
CARBON
5 0 ( 2 0 1 2 ) 1 0 5 7 –1 0 6 2
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
The growth of carbon nanotubes in aluminum powders by the catalytic pyrolysis of polyethylene glycol Linlin Cao a, Zhiqiang Li Yang-Soo Kim c a b c
a,* ,
Genlian Fan a, Lin Jiang a, Di Zhang a, Won-Jin Moon b,
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China Korea Basic Science Institute Gwangju Center, Yongbong-Dong, Buk-Gu, Gwangju 500-757, Korea Korea Basic Science Institute, Suncheon Center, Jungangno, Suncheon 540-742, Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
A simple approach based on the catalytic pyrolysis of polyethylene glycol (PEG) was devel-
Received 22 June 2011
oped to grow a uniform dispersion of carbon nanotubes (CNTs) in Al powders and thus sup-
Accepted 7 October 2011
ply raw material for the powder metallurgy fabrication of CNT/Al composites. Al nanoflake
Available online 14 October 2011
powders with quite a large surface area were used to adsorb a homogeneous PEG and citric acid film, and then were impregnated with a cobalt nitrate solution to anchor Co(II) by complexation with citric acid. Then the Al nanoflake powders were heated to 230 C to form Co oxide nanoparticles, and then to 570 C to induce the thermal decomposition of PEG. The pyrolytic products of PEG not only served as the reducing agent to reduce Co oxide to Co nanoparticle catalyst, but also as the carbon source for CNT growth. As a result, 2.13 wt.% graphitic CNTs, with diameters of 10–20 nm and length ranging from sub-micron to a few micrometers, were homogeneously grown in 500 nm thick Al nanoflakes. And the as-obtained CNT/Al composites fabricated by hot-pressing exhibited enhanced strength, which was almost two times that of the matrix. 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon nanotubes (CNTs), owing to their unique and superior mechanical, thermal and electrical properties, have been considered as an ideal reinforcement for metal, ceramic and polymer matrices composites [1]. Especially, the interest to fabricate CNT-reinforced Al composites has been growing remarkably, with the increasing needs to produce novel lightweight materials with high stiffness as well as high strength for potential applications in the aerospace, automotive and sports industries [2–7]. However, till now, the strengthening effect achieved was far below expectation because of the difficulty in obtaining homogeneous distribution and good interfacial bonding of CNTs in Al matrix [4,7]. High energy ball milling, owing to its effectiveness in promoting the dispersion
of CNTs, has emerged as the most widely used technique to supply raw material for the powder metallurgy (PM) fabrication of CNT/Al composites [2,5,7]. However, the ball milling process inevitably causes damages to the morphology and structure of CNTs, which is detrimental to its strengthening effects [8]. Therefore, in situ synthesis of CNTs in Al powders has recently been proposed as an alternative technique to achieve homogeneous distribution of undamaged CNTs and thus, to obtain enhanced mechanical properties. Zhao and co-workers [9,10] made the first attempt by using chemical vapor deposition (CVD) method to grow CNTs in Al powders. Nevertheless, the method they used to load catalyst precursor onto Al powders, the deposition–precipitation route, is not only complicated, but also hard to achieve homogeneous distribution of the catalyst in Al powders. Moreover, the surface
* Corresponding author: Fax: +86 21 3420 3913. E-mail address: [email protected] (Z. Li). 0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.10.011
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area of the used micron-sized spherical Al powders (5 lm in diameter) is so small that it can hardly bear as much as 5 wt.% CNTs homogeneously, as proclaimed by Zhao et al. As a result, CNT clusters should exist among Al powders. Therefore, a simple and effective method needs to be explored that is able to introduce uniform catalyst, leading to a uniform dispersion of CNTs in Al powders and enhanced mechanical properties eventually. In the present study, we make a first attempt to in situ synthesize uniform CNT/Al composite powders by using the catalytic pyrolysis of polymer and Al nanoflake powders. The uniformity of CNTs in the Al powders can be guaranteed by the following reasons. Firstly, Al nanoflakes were chosen as the matrix due to their large surface area, which is very helpful and to increase the amount of homogeneously dispersed CNTs on the Al surface, as proven by our previous work [11]. Secondly, polyethylene glycol (PEG) with low decomposition temperature was used as the carbon-source precursor, which can be cross-linked with citric acid (CA) to form a uniform film on the surface of Al nanoflakes. Thirdly, through forming complex compounds with CA, the catalyst precursor, Co(II), can be homogeneously anchored to the PEG&CA film on the surface of Al nanoflakes. The result did show that the asgrown graphitic CNTs were homogeneously dispersed on the surface of Al nanoflakes and the as-obtained CNT/Al composites at compressive strength of 380 MPa can be achieved.
2.
Experimental
As shown in Fig. 1a–d, the synthesis process consists of four steps: coating the Al nanoflakes with a PEG&CA film (Al@PEG&CA), adsorbing Co(II) as the catalyst precursor through complexation with CA (Al@PEG&CA-Co(II)), low-temperature heating to form Co oxide nanoparticles (Al@PEG&CoOx), high-temperature heating to form Co nanoparticle catalyst and triggered CNT growth on the surface of Al nanoflakes (CNT/Al). In a typical experiment, firstly, 2.0 g PEG (MW:1000) and 1.0 g CA were mixed in 20 ml distilled water through stir-
ring 30 min, and then 2.0 g raw Al nanoflakes (500 nm in thickness) was added and the mixture was further stirred for 30 min and filtered. Then the filtrate was impregnated with a cobalt nitrate solution with a concentration of 0.25 mol/L and stirred for 1 h and filtered. The filtrate was dried at 60 C for 5 h under vacuum and was then heat treated in a 100-mm-diameter tube furnace. The tube furnace was firstly installed to a temperature of 230 C (with heating rate 20 C/min) for 2 h in air, then raised to 570 C (with heating rate 20 C/min) and held in N2 (60 ml/min, 99.99% purity) flow for 3 h, lastly cooled to room temperature under a nitrogen (99.99% purity) atmosphere. Fourier transform infrared spectroscopy (FT-IR) on a Spectrum 100 (Perkin Elmer Inc., USA) was used to characterize the untreated Al and Al@PEG&CA, using KBr pellets from 500 to 2000 cm1 at a resolution of 2 cm1. Scanning electron microscopy (FEI SIRION 200 SEM) was conducted to characterize the morphology of the untreated Al nanoflakes, Al@PEG&CA, Al@PEG&CoOx and CNT/Al composite powders. The morphology and structure of the in situ synthesized CNTs were examined by using a high-resolution transmission electron microscopy (HRTEM) in a Philips CM200 microscope operated at 200 kV. Raman analysis on CNT/Al composite powders was carried out with Raman spectroscopy (SENTERRA) under ambient condition. The purity of CNTs (P) in the as-obtained carbon products and mass fraction of CNTs (Wf ) in the as-obtained CNT/Al composite powders were defined as: P ¼ WCNT =ðWCNT=Al WAl Þ 100%
ð1Þ
Wf ¼ ðWCNT =WCNT=Al Þ 100%
ð2Þ
where WAl , WCNT=Al and WCNT are respectively weight of the Al nanoflakes, weight of the as-obtained CNT/Al composite powders, and weight of the pure graphic CNTs. The weight of graphic CNTs was obtained by hydrochloric acid (HCl) treatment of the as-obtained CNT/Al composite powders, which can remove Al and by products such as amorphous carbon.
Fig. 1 – Fabrication procedures for synthesizing CNT/Al composite powders: (a) PEG&CA film coating on the surface of Al nanoflakes; (b) adsorption of Co(II) by forming complex compounds; (c) formation of CoOx nanoparticles by low-temperature heat treatment; (d) formation of Co nanoparticle catalyst and CNT growth by high-temperature heat treatment.
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Hot-pressing was used to consolidate the as-obtained CNT/Al composite powders and pure Al powders. Prior to hot-pressing, they were compacted to a column (B 40 mm · 30 mm) under high pressure which was then sintered in flowing Ar atmosphere at 550 C for 2 h. Finally, the column was heated to 500 C with a heating rate of 5 C/min within a vacuum hot-press. Hot-pressing was conducted with a pressure of 200 MPa for 30 min.
3.
Results and discussion
In the entire process, CA played triple roles to facilitate the homogeneous growth of CNTs in Al powders. Firstly, CA promoted the cross-linking of the PEG molecules by the hydrogen bonding between the –OH groups of PEG and the –COOH groups of CA and thus forming a stable film on the surface of Al nanoflakes as illustrated in Fig. 1a. Fig. 2 characterizes the change of the Al nanoflakes after the polymer coating process. Compared with the untreated Al nanoflakes (Fig. 2a), the surface morphology of the Al@PEG&CA was found more smooth (Fig. 2b). The successful coating of PEG&CA film could also be proved by the FT-IR measurement in Fig. 2c. The peaks appeared at 1734 and 1632 cm1 were contributed to the carboxyl group of CA, and the peak at 1380 cm1 was owing to the bend vibration of O–H of PEG. Moreover, the peak at 1089 cm1 was related to stretching vibration of C–O. Thus the introduction of the PEG&CA film onto the surface of Al nanoflakes by the polymer coating was verified. Secondly, CA acted as a binder to uniformly anchor Co(II) onto the surface of Al nanoflakes by forming complex compounds with Co(II), as illustrated in Fig. 1b. CA has been widely studied in physiological processes to separate Co(II)
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ions by forming a stable complex [12,13]. Once the stable complex between Co(II) and CA formed, Co(II) is tightly locked in the PEG&CA film on the surface of Al nanoflakes, making it unable to migrate during the subsequent processing such as filtering and drying. As a result, the desorption and agglomeration of the Co(II) was avoided and thus the Al nanoflakes with homogeneously adsorbed Co(II) were achieved, which is determinative to the final uniform distribution of CNTs in Al powders. Thirdly, in the following low-temperature heat treatment, CA served as a fuel to combust Co(II) into CoOx nanoparticles. According to Deganello et al. [14], the citrate–nitrate autocombustion reaction would take place as soon as the temperature was higher than 200 C in the presence of citric acid and metal nitrates and then metal oxide nanoparticles with diameter of tens of nanometers can be obtained. Fig. 3 shows that the as-obtained CoOx nanoparticles were homogeneously dispersed on the surface of Al nanoflakes. From the high-magnification SEM image (Fig. 3b), we can see the diameter of the CoOx nanoparticles is confined to 10–20 nm, and the average distance between adjacent CoOx nanoparticles is about 30– 50 nm. This uniform distribution of CoOx nanoparticles was mainly attributed to the steric effect of PEG [15], which played an important role to prevent the CoOx nanoparticles from agglomeration and growing bigger. The morphology of as-obtained CNT/Al composite powders was investigated by SEM and TEM. From Fig. 4a, we can see that the whole surface of Al nanoflakes was decorated by homogeneously as-grown CNTs, which was believed to originate from the uniform distribution of the catalyst nanoparticles. As shown in the inset of Fig. 4a, with diameters of 10–20 nm and length ranging from sub-micron to a few
Fig. 2 – SEM images of (a) the untreated Al nanoflakes; (b) the Al@PEG&CA; (c) FI-IR curves of the untreated Al nanoflakes and Al@PEG&CA.
Fig. 3 – SEM images of Al@PEG&CoOx at: (a) low-magnification; (b) high-magnification.
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Fig. 4 – Morphology and structure of CNT/Al composite powders: (a) SEM; (b) TEM, showing the in situ CNTs with encapsulated Co nanoparticles; (c) HRTEM and (d) Raman spectrum of CNT/Al composite powders, showing the well-graphitic structure of the as-grown CNTs. micrometers. Therefore the as-grown CNTs have a large aspect ratio to act as reinforcement fiber in composites. When observed under TEM (Fig. 4b), we can see that almost each CNT had a nanoparticle encapsulated in one of its end. These nanoparticles were supposed to be Co that acted as the catalyst for the CNT growth. According to Huang and Li [16], the Co nanoparticles were reduced from the CoOx nanoparticles by the pyrolytic products of PEG, such as H2 and CO. The tubular structure of the CNTs could be further characterized by HRTEM. As seen in Fig. 4c, the as-grown CNTs have clear graphitic layers with a spacing between two neighboring lattice fringes of 0.34 nm, which is consistent with the (0 0 2) plane of graphite [9]. The high graphitization degree of the CNTs could be also proved by the result of Raman spectroscopy of the as-produced CNT/Al composite powders without further purification, as shown in Fig. 4d. The relative intensity ratio of D band (appearing in 1358 cm1) to G band (appearing in 1598 cm1) (ID/IG) is known as a base to determine the CNT structure, the lower the ratio, the higher graphitization degree of CNTs [17]. For the present samples, ID/IG was calculated to be 0.7, lower than most of the reported results [10,17], indicating that the as-grown CNTs were mainly composed of wellcrystallized graphite, in agreement with the HRTEM observation. When held at elevated temperature, PEG was supposed to decompose into gases of H2, CH4, CO, and CO2, among which H2 and CO played a key role for Co catalyst reduction, while CH4 and CO served as carbon source for CNT growth [16]. As shown in Fig. 1d, a tip-growth mechanism was supposed for the growth of CNTs, by which CH4 and CO reacted with Co nanoparticles and form supersaturated solid solution of Co(C), and then the precipitation of supersaturated C atoms
on the surface of Co nucleated into multi-walled CNTs. During this process, the excessive gases could be removed by continued N2 flow, and the only by-product is a little of amorphous carbon on the surface CNTs due to unstable or incomplete reaction. According to Eq. (1) and (2), the purity of CNTs in the as-obtained carbon products was calculated to be 90.5%, while the mass fraction of CNTs in the as-obtained CNT/Al composite was 2.13 wt.%. Thus, the as-grown CNTs are of high purity and the amorphous carbon is no more than 0.25 wt.% in the as-obtained CNT/Al composite powders. In addition, the amount of Co is estimated to be 0.1 wt.% in the as-obtained CNT/Al composite. Since totally encapsulated
Fig. 5 – Compressive stress–strain curves of the unreinforced Al matrix and the as-obtained CNT/Al composites.
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in CNTs (as shown in Fig. 4c), the Co nanoparticles have no direct contact with Al matrix and thus make no contribution to property enhancement. The compressive stress–strain curves of the unreinforced Al matrix and the as-obtained CNT/Al composites were shown in Fig. 5. From the curves we can see that the as-obtained CNT/Al composites exhibited compressive stress about 380 MPa, which was almost two times that of the unreinforced Al matrix. The strengthening effect of CNTs is related to their length (l). The strengthening effect would reach a critical value when the CNT length is above a critical length (lc) defined as [4] lc ¼
rCNT dCNT 2sAl
ð3Þ
where sAl is the shear stress of the matrix (0.5r), dCNT is the average diameter of the as-obtained CNTs, and rCNT is the strength of the CNTs (30GPa [18]). The calculated lc is 2.2 lm and obviously from Fig. 4a, most of the as-grown CNTs are below the critical length. Thus, the yield strength of the composite, for l < lc, was estimated through [4]: l ð4Þ r0CNT=Al ¼ VCNT rCNT þ VAl rAl 2lc where VCNT and VAl are volume fraction of CNTs (4.3 vol.%) and Al matrix (95.7 vol.%). From Eq. (4), the average length of the as-grown CNTs was calculated to be 700 nm, which is longer than that observed in Fig. 4a. This difference mainly comes from the fact that, in Eq. (4), only stress transfer of CNTs is considered while other strengthening mechanisms induced by CNTs, such as grain refinement and dislocation strengthening are neglected. However, it is clearly evidenced that, owing to their large aspect ratio (several tens in magnitude) and homogenous distribution, the as-grown CNTs served effectively as nano reinforcements in the final CNT/Al composites. Compared with the high energy ball-milling method, this novel in situ process is much simpler and the as-grown CNTs could retain their structural integrity. In addition, the most important advantage of the present approach is that both PEG and CA contributed to the homogeneous distribution of Co(II), CoOx and Co nanoparticles on the surface of Al nanoflakes, which finally generate homogeneously dispersed and well-graphitized CNTs. Moreover, the pyrolytic products of PEG served as the reducing agent for Co catalyst formation and the carbon source for CNT growth, thus avoiding the direct usage of explosive and flammable hydrogen and hydrocarbon as in CVD. Therefore, it is a much simpler, safer and effective method to grow homogenous CNT reinforcements in Al powders and thus the reinforced strength of the CNT/ Al composites can be guaranteed.
4.
Conclusions
We have shown that homogeneously dispersed CNTs are synthesized in Al nanoflake powders by the pyrolysis of PEG catalyzed by in situ formed Co nanoparticles and the reinforced compressive strength of CNT/Al composites can be obtained. This process consists of PEG&CA coating on Al nanoflakes, Co(II) adsorption, formation of Co oxides, formation of Co
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nanoparticle catalyst and CNT growth. The advantage of this method is related to the simple processing setup combined with the homogeneously dispersed graphitic CNT products. The results provide an alternative technical pathway to supply high quality CNT/Al powders for composite fabrication via powder metallurgy and it is worthy of further research.
Acknowledgements The authors would like to acknowledge the financial support of the National Basic Research Program of China (973 Program, No. 2012CB619600), the National Natural Science Foundation (Nos. 51071100, 51131004, 50890174), the International S&T Cooperation Program (No. 2010DFA52550) of China, and KBSI Grant (T31903) to W.J. Moon.
R E F E R E N C E S
[1] Thostenson ET, Ren ZF, Chou TW. Advances in the science and technology of carbon nanotubes and their composites: a review. Compos Sci Technol 2001;61:1899–912. [2] Esawi AMK, Morsi K, Sayed A, Taher M, Lanka S. Effect of carbon nanotube (CNT) content on the mechanical properties of CNT-reinforced aluminium composites. Compos Sci Technol 2010;70:2237–41. [3] George R, Kashyap KT, Rahul R, Yamdagni S. Strengthening in carbon nanotube/aluminium (CNT/Al) composites. Scripta Mater 2005;53:1159–63. [4] Choi HJ, Kwon GB, Lee GY, Bae DH. Reinforcement with carbon nanotubes in aluminum matrix composites. Scripta Mater 2008;59:360–3. [5] Pe´rez-Bustamante R, Go´mez-Esparza CD, Estrada-Guel I, Miki-Yoshida M, Licea-Jime´nez L, Pe´rez-Garcı´a SA, et al. Microstructural and mechanical characterization of AlMWCNT composites produced by mechanical milling. Mater Sci Eng A 2009;502:159–63. [6] Deng CF, Zhang XX, Wang DZ, Lin Q, Li AB. Preparation and characterization of carbon nanotubes/aluminum matrix composites. Mater Lett 2007;61:1725–8. [7] Liao JZ, Tan MJ. Mixing of carbon nanotubes (CNTs) and aluminum powder for powder metallurgy use. Powder Technol 2011;208:42–8. [8] Poirier D, Gauvin R, Drew Robin AL. Structural characterization of a mechanically milled carbon nanotube/ aluminum mixture. Compos pt A 2009;40:1482–9. [9] He CN, Zhao NQ, Shi CS, Du XW, Li JJ, Li HP, et al. An approach to obtaining homogeneously dispersed carbon nanotubes in Al powders for preparing reinforced Al-matrix composites. Adv Mater 2007;19:1128–32. [10] He CN, Zhao NQ, Shi CS, Song SZ. An approach for obtaining the structural diversity of multi-walled carbon nanotubes on Ni/Al catalyst with low Ni content. J Alloy Compd 2010;489:20–5. [11] Jiang L, Fan GL, Li ZQ, Kai XZ, Zhang D, Chen ZX. An approach to the uniform dispersion of a high volume fraction of carbon nanotubes in aluminum powder. Carbon 2011;49:1965–71. [12] Matzapetakis M, Dakanali M, Raptopoulou CP, Tangoulis V, Terzis A, Moon N, et al. Synthesis, spectroscopic, and structural characterization of the first aqueous cobalt(II)citrate complex: toward a potentially bioavailable form of cobalt in biologically relevant fluids. J Biol Inorg Chem 2000;5:469–74.
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[13] Zhou ZH, Deng YF, Wan HL. Structural diversities of cobalt(II) coordination polymers with citric acid. Cryst Growth Des 2005;3:1109–17. [14] Deganello F, Marcı` G, Deganello G. Citrate–nitrate autocombustion synthesis of perovskite-type nanopowders: a systematic approach. J Eur Ceram Soc 2009;29:439–50. [15] Tan YW, Dai XH, Li YF, Zhu DB. Preparation of gold, platinum, palladium and silver nanoparticles by the reduction of their salts with a weak reductant-potassium bitartrate. J Mater Chem 2003;13:1069–75.
[16] Huang CW, Li YY. In situ synthesis of platelet graphite nanofibers from thermal decomposition of poly(ethylene glycol). J Phys Chem B 2006;110:23242–6. [17] Chen CM, Dai YM, Huang JG, Jehng JM. Intermetallic catalyst for carbon nanotubes (CNTs) growth by thermal chemical vapor deposition method. Carbon 2006;44:1808–20. [18] Cha SI, Kim KT, Arshad SN, Mo CB, Hong SH. Extraordinary strengthening effect of carbon nanotubes in metal-matrix nanocomposites processed by molecular-level mixing. Adv Mater 2005;17:1377–81.
5 0 ( 2 0 1 2 ) 1 0 5 7 –1 0 6 2
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
The growth of carbon nanotubes in aluminum powders by the catalytic pyrolysis of polyethylene glycol Linlin Cao a, Zhiqiang Li Yang-Soo Kim c a b c
a,* ,
Genlian Fan a, Lin Jiang a, Di Zhang a, Won-Jin Moon b,
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China Korea Basic Science Institute Gwangju Center, Yongbong-Dong, Buk-Gu, Gwangju 500-757, Korea Korea Basic Science Institute, Suncheon Center, Jungangno, Suncheon 540-742, Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
A simple approach based on the catalytic pyrolysis of polyethylene glycol (PEG) was devel-
Received 22 June 2011
oped to grow a uniform dispersion of carbon nanotubes (CNTs) in Al powders and thus sup-
Accepted 7 October 2011
ply raw material for the powder metallurgy fabrication of CNT/Al composites. Al nanoflake
Available online 14 October 2011
powders with quite a large surface area were used to adsorb a homogeneous PEG and citric acid film, and then were impregnated with a cobalt nitrate solution to anchor Co(II) by complexation with citric acid. Then the Al nanoflake powders were heated to 230 C to form Co oxide nanoparticles, and then to 570 C to induce the thermal decomposition of PEG. The pyrolytic products of PEG not only served as the reducing agent to reduce Co oxide to Co nanoparticle catalyst, but also as the carbon source for CNT growth. As a result, 2.13 wt.% graphitic CNTs, with diameters of 10–20 nm and length ranging from sub-micron to a few micrometers, were homogeneously grown in 500 nm thick Al nanoflakes. And the as-obtained CNT/Al composites fabricated by hot-pressing exhibited enhanced strength, which was almost two times that of the matrix. 2011 Elsevier Ltd. All rights reserved.
1.
Introduction
Carbon nanotubes (CNTs), owing to their unique and superior mechanical, thermal and electrical properties, have been considered as an ideal reinforcement for metal, ceramic and polymer matrices composites [1]. Especially, the interest to fabricate CNT-reinforced Al composites has been growing remarkably, with the increasing needs to produce novel lightweight materials with high stiffness as well as high strength for potential applications in the aerospace, automotive and sports industries [2–7]. However, till now, the strengthening effect achieved was far below expectation because of the difficulty in obtaining homogeneous distribution and good interfacial bonding of CNTs in Al matrix [4,7]. High energy ball milling, owing to its effectiveness in promoting the dispersion
of CNTs, has emerged as the most widely used technique to supply raw material for the powder metallurgy (PM) fabrication of CNT/Al composites [2,5,7]. However, the ball milling process inevitably causes damages to the morphology and structure of CNTs, which is detrimental to its strengthening effects [8]. Therefore, in situ synthesis of CNTs in Al powders has recently been proposed as an alternative technique to achieve homogeneous distribution of undamaged CNTs and thus, to obtain enhanced mechanical properties. Zhao and co-workers [9,10] made the first attempt by using chemical vapor deposition (CVD) method to grow CNTs in Al powders. Nevertheless, the method they used to load catalyst precursor onto Al powders, the deposition–precipitation route, is not only complicated, but also hard to achieve homogeneous distribution of the catalyst in Al powders. Moreover, the surface
* Corresponding author: Fax: +86 21 3420 3913. E-mail address: [email protected] (Z. Li). 0008-6223/$ - see front matter 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2011.10.011
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area of the used micron-sized spherical Al powders (5 lm in diameter) is so small that it can hardly bear as much as 5 wt.% CNTs homogeneously, as proclaimed by Zhao et al. As a result, CNT clusters should exist among Al powders. Therefore, a simple and effective method needs to be explored that is able to introduce uniform catalyst, leading to a uniform dispersion of CNTs in Al powders and enhanced mechanical properties eventually. In the present study, we make a first attempt to in situ synthesize uniform CNT/Al composite powders by using the catalytic pyrolysis of polymer and Al nanoflake powders. The uniformity of CNTs in the Al powders can be guaranteed by the following reasons. Firstly, Al nanoflakes were chosen as the matrix due to their large surface area, which is very helpful and to increase the amount of homogeneously dispersed CNTs on the Al surface, as proven by our previous work [11]. Secondly, polyethylene glycol (PEG) with low decomposition temperature was used as the carbon-source precursor, which can be cross-linked with citric acid (CA) to form a uniform film on the surface of Al nanoflakes. Thirdly, through forming complex compounds with CA, the catalyst precursor, Co(II), can be homogeneously anchored to the PEG&CA film on the surface of Al nanoflakes. The result did show that the asgrown graphitic CNTs were homogeneously dispersed on the surface of Al nanoflakes and the as-obtained CNT/Al composites at compressive strength of 380 MPa can be achieved.
2.
Experimental
As shown in Fig. 1a–d, the synthesis process consists of four steps: coating the Al nanoflakes with a PEG&CA film (Al@PEG&CA), adsorbing Co(II) as the catalyst precursor through complexation with CA (Al@PEG&CA-Co(II)), low-temperature heating to form Co oxide nanoparticles (Al@PEG&CoOx), high-temperature heating to form Co nanoparticle catalyst and triggered CNT growth on the surface of Al nanoflakes (CNT/Al). In a typical experiment, firstly, 2.0 g PEG (MW:1000) and 1.0 g CA were mixed in 20 ml distilled water through stir-
ring 30 min, and then 2.0 g raw Al nanoflakes (500 nm in thickness) was added and the mixture was further stirred for 30 min and filtered. Then the filtrate was impregnated with a cobalt nitrate solution with a concentration of 0.25 mol/L and stirred for 1 h and filtered. The filtrate was dried at 60 C for 5 h under vacuum and was then heat treated in a 100-mm-diameter tube furnace. The tube furnace was firstly installed to a temperature of 230 C (with heating rate 20 C/min) for 2 h in air, then raised to 570 C (with heating rate 20 C/min) and held in N2 (60 ml/min, 99.99% purity) flow for 3 h, lastly cooled to room temperature under a nitrogen (99.99% purity) atmosphere. Fourier transform infrared spectroscopy (FT-IR) on a Spectrum 100 (Perkin Elmer Inc., USA) was used to characterize the untreated Al and Al@PEG&CA, using KBr pellets from 500 to 2000 cm1 at a resolution of 2 cm1. Scanning electron microscopy (FEI SIRION 200 SEM) was conducted to characterize the morphology of the untreated Al nanoflakes, Al@PEG&CA, Al@PEG&CoOx and CNT/Al composite powders. The morphology and structure of the in situ synthesized CNTs were examined by using a high-resolution transmission electron microscopy (HRTEM) in a Philips CM200 microscope operated at 200 kV. Raman analysis on CNT/Al composite powders was carried out with Raman spectroscopy (SENTERRA) under ambient condition. The purity of CNTs (P) in the as-obtained carbon products and mass fraction of CNTs (Wf ) in the as-obtained CNT/Al composite powders were defined as: P ¼ WCNT =ðWCNT=Al WAl Þ 100%
ð1Þ
Wf ¼ ðWCNT =WCNT=Al Þ 100%
ð2Þ
where WAl , WCNT=Al and WCNT are respectively weight of the Al nanoflakes, weight of the as-obtained CNT/Al composite powders, and weight of the pure graphic CNTs. The weight of graphic CNTs was obtained by hydrochloric acid (HCl) treatment of the as-obtained CNT/Al composite powders, which can remove Al and by products such as amorphous carbon.
Fig. 1 – Fabrication procedures for synthesizing CNT/Al composite powders: (a) PEG&CA film coating on the surface of Al nanoflakes; (b) adsorption of Co(II) by forming complex compounds; (c) formation of CoOx nanoparticles by low-temperature heat treatment; (d) formation of Co nanoparticle catalyst and CNT growth by high-temperature heat treatment.
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5 0 ( 20 1 2 ) 1 0 5 7–10 6 2
Hot-pressing was used to consolidate the as-obtained CNT/Al composite powders and pure Al powders. Prior to hot-pressing, they were compacted to a column (B 40 mm · 30 mm) under high pressure which was then sintered in flowing Ar atmosphere at 550 C for 2 h. Finally, the column was heated to 500 C with a heating rate of 5 C/min within a vacuum hot-press. Hot-pressing was conducted with a pressure of 200 MPa for 30 min.
3.
Results and discussion
In the entire process, CA played triple roles to facilitate the homogeneous growth of CNTs in Al powders. Firstly, CA promoted the cross-linking of the PEG molecules by the hydrogen bonding between the –OH groups of PEG and the –COOH groups of CA and thus forming a stable film on the surface of Al nanoflakes as illustrated in Fig. 1a. Fig. 2 characterizes the change of the Al nanoflakes after the polymer coating process. Compared with the untreated Al nanoflakes (Fig. 2a), the surface morphology of the Al@PEG&CA was found more smooth (Fig. 2b). The successful coating of PEG&CA film could also be proved by the FT-IR measurement in Fig. 2c. The peaks appeared at 1734 and 1632 cm1 were contributed to the carboxyl group of CA, and the peak at 1380 cm1 was owing to the bend vibration of O–H of PEG. Moreover, the peak at 1089 cm1 was related to stretching vibration of C–O. Thus the introduction of the PEG&CA film onto the surface of Al nanoflakes by the polymer coating was verified. Secondly, CA acted as a binder to uniformly anchor Co(II) onto the surface of Al nanoflakes by forming complex compounds with Co(II), as illustrated in Fig. 1b. CA has been widely studied in physiological processes to separate Co(II)
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ions by forming a stable complex [12,13]. Once the stable complex between Co(II) and CA formed, Co(II) is tightly locked in the PEG&CA film on the surface of Al nanoflakes, making it unable to migrate during the subsequent processing such as filtering and drying. As a result, the desorption and agglomeration of the Co(II) was avoided and thus the Al nanoflakes with homogeneously adsorbed Co(II) were achieved, which is determinative to the final uniform distribution of CNTs in Al powders. Thirdly, in the following low-temperature heat treatment, CA served as a fuel to combust Co(II) into CoOx nanoparticles. According to Deganello et al. [14], the citrate–nitrate autocombustion reaction would take place as soon as the temperature was higher than 200 C in the presence of citric acid and metal nitrates and then metal oxide nanoparticles with diameter of tens of nanometers can be obtained. Fig. 3 shows that the as-obtained CoOx nanoparticles were homogeneously dispersed on the surface of Al nanoflakes. From the high-magnification SEM image (Fig. 3b), we can see the diameter of the CoOx nanoparticles is confined to 10–20 nm, and the average distance between adjacent CoOx nanoparticles is about 30– 50 nm. This uniform distribution of CoOx nanoparticles was mainly attributed to the steric effect of PEG [15], which played an important role to prevent the CoOx nanoparticles from agglomeration and growing bigger. The morphology of as-obtained CNT/Al composite powders was investigated by SEM and TEM. From Fig. 4a, we can see that the whole surface of Al nanoflakes was decorated by homogeneously as-grown CNTs, which was believed to originate from the uniform distribution of the catalyst nanoparticles. As shown in the inset of Fig. 4a, with diameters of 10–20 nm and length ranging from sub-micron to a few
Fig. 2 – SEM images of (a) the untreated Al nanoflakes; (b) the Al@PEG&CA; (c) FI-IR curves of the untreated Al nanoflakes and Al@PEG&CA.
Fig. 3 – SEM images of Al@PEG&CoOx at: (a) low-magnification; (b) high-magnification.
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5 0 ( 2 0 1 2 ) 1 0 5 7 –1 0 6 2
Fig. 4 – Morphology and structure of CNT/Al composite powders: (a) SEM; (b) TEM, showing the in situ CNTs with encapsulated Co nanoparticles; (c) HRTEM and (d) Raman spectrum of CNT/Al composite powders, showing the well-graphitic structure of the as-grown CNTs. micrometers. Therefore the as-grown CNTs have a large aspect ratio to act as reinforcement fiber in composites. When observed under TEM (Fig. 4b), we can see that almost each CNT had a nanoparticle encapsulated in one of its end. These nanoparticles were supposed to be Co that acted as the catalyst for the CNT growth. According to Huang and Li [16], the Co nanoparticles were reduced from the CoOx nanoparticles by the pyrolytic products of PEG, such as H2 and CO. The tubular structure of the CNTs could be further characterized by HRTEM. As seen in Fig. 4c, the as-grown CNTs have clear graphitic layers with a spacing between two neighboring lattice fringes of 0.34 nm, which is consistent with the (0 0 2) plane of graphite [9]. The high graphitization degree of the CNTs could be also proved by the result of Raman spectroscopy of the as-produced CNT/Al composite powders without further purification, as shown in Fig. 4d. The relative intensity ratio of D band (appearing in 1358 cm1) to G band (appearing in 1598 cm1) (ID/IG) is known as a base to determine the CNT structure, the lower the ratio, the higher graphitization degree of CNTs [17]. For the present samples, ID/IG was calculated to be 0.7, lower than most of the reported results [10,17], indicating that the as-grown CNTs were mainly composed of wellcrystallized graphite, in agreement with the HRTEM observation. When held at elevated temperature, PEG was supposed to decompose into gases of H2, CH4, CO, and CO2, among which H2 and CO played a key role for Co catalyst reduction, while CH4 and CO served as carbon source for CNT growth [16]. As shown in Fig. 1d, a tip-growth mechanism was supposed for the growth of CNTs, by which CH4 and CO reacted with Co nanoparticles and form supersaturated solid solution of Co(C), and then the precipitation of supersaturated C atoms
on the surface of Co nucleated into multi-walled CNTs. During this process, the excessive gases could be removed by continued N2 flow, and the only by-product is a little of amorphous carbon on the surface CNTs due to unstable or incomplete reaction. According to Eq. (1) and (2), the purity of CNTs in the as-obtained carbon products was calculated to be 90.5%, while the mass fraction of CNTs in the as-obtained CNT/Al composite was 2.13 wt.%. Thus, the as-grown CNTs are of high purity and the amorphous carbon is no more than 0.25 wt.% in the as-obtained CNT/Al composite powders. In addition, the amount of Co is estimated to be 0.1 wt.% in the as-obtained CNT/Al composite. Since totally encapsulated
Fig. 5 – Compressive stress–strain curves of the unreinforced Al matrix and the as-obtained CNT/Al composites.
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in CNTs (as shown in Fig. 4c), the Co nanoparticles have no direct contact with Al matrix and thus make no contribution to property enhancement. The compressive stress–strain curves of the unreinforced Al matrix and the as-obtained CNT/Al composites were shown in Fig. 5. From the curves we can see that the as-obtained CNT/Al composites exhibited compressive stress about 380 MPa, which was almost two times that of the unreinforced Al matrix. The strengthening effect of CNTs is related to their length (l). The strengthening effect would reach a critical value when the CNT length is above a critical length (lc) defined as [4] lc ¼
rCNT dCNT 2sAl
ð3Þ
where sAl is the shear stress of the matrix (0.5r), dCNT is the average diameter of the as-obtained CNTs, and rCNT is the strength of the CNTs (30GPa [18]). The calculated lc is 2.2 lm and obviously from Fig. 4a, most of the as-grown CNTs are below the critical length. Thus, the yield strength of the composite, for l < lc, was estimated through [4]: l ð4Þ r0CNT=Al ¼ VCNT rCNT þ VAl rAl 2lc where VCNT and VAl are volume fraction of CNTs (4.3 vol.%) and Al matrix (95.7 vol.%). From Eq. (4), the average length of the as-grown CNTs was calculated to be 700 nm, which is longer than that observed in Fig. 4a. This difference mainly comes from the fact that, in Eq. (4), only stress transfer of CNTs is considered while other strengthening mechanisms induced by CNTs, such as grain refinement and dislocation strengthening are neglected. However, it is clearly evidenced that, owing to their large aspect ratio (several tens in magnitude) and homogenous distribution, the as-grown CNTs served effectively as nano reinforcements in the final CNT/Al composites. Compared with the high energy ball-milling method, this novel in situ process is much simpler and the as-grown CNTs could retain their structural integrity. In addition, the most important advantage of the present approach is that both PEG and CA contributed to the homogeneous distribution of Co(II), CoOx and Co nanoparticles on the surface of Al nanoflakes, which finally generate homogeneously dispersed and well-graphitized CNTs. Moreover, the pyrolytic products of PEG served as the reducing agent for Co catalyst formation and the carbon source for CNT growth, thus avoiding the direct usage of explosive and flammable hydrogen and hydrocarbon as in CVD. Therefore, it is a much simpler, safer and effective method to grow homogenous CNT reinforcements in Al powders and thus the reinforced strength of the CNT/ Al composites can be guaranteed.
4.
Conclusions
We have shown that homogeneously dispersed CNTs are synthesized in Al nanoflake powders by the pyrolysis of PEG catalyzed by in situ formed Co nanoparticles and the reinforced compressive strength of CNT/Al composites can be obtained. This process consists of PEG&CA coating on Al nanoflakes, Co(II) adsorption, formation of Co oxides, formation of Co
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nanoparticle catalyst and CNT growth. The advantage of this method is related to the simple processing setup combined with the homogeneously dispersed graphitic CNT products. The results provide an alternative technical pathway to supply high quality CNT/Al powders for composite fabrication via powder metallurgy and it is worthy of further research.
Acknowledgements The authors would like to acknowledge the financial support of the National Basic Research Program of China (973 Program, No. 2012CB619600), the National Natural Science Foundation (Nos. 51071100, 51131004, 50890174), the International S&T Cooperation Program (No. 2010DFA52550) of China, and KBSI Grant (T31903) to W.J. Moon.
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