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Dispersion of carbon nanotubes in hydroxyapatite powder by in situ chemical vapor deposition
Materials Science and Engineering B 166 (2010) 19–23
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Dispersion of carbon nanotubes in hydroxyapatite powder by in situ chemical vapor deposition Haipeng Li a , Lihui Wang b , Chunyong Liang a , Zhifeng Wang a , Weimin Zhao a,∗ a b
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China Automobile Engineering Department, Academy of Military Transportation, Tianjin 300161, China
a r t i c l e
i n f o
Article history: Received 26 July 2009 Received in revised form 15 September 2009 Accepted 23 September 2009 Keywords: Carbon nanotubes Hydroxyapatite Composite Chemical vapor deposition In situ
a b s t r a c t In the present work, we use chemical vapor deposition of methane to disperse carbon nanotubes (CNTs) within hydroxyapatite (HA) powder. The effect of different catalytic metal particles (Fe, Ni or Co) on the morphological and structural development of the powder and dispersion of CNTs in HA powder was investigated. The results show that the technique is effective in dispersing the nanotubes within HA powder, which simultaneously protects the nanotubes from damage. The results can have important and promising speculations for the processing of CNT-reinforced HA-matrix composites in general. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hydroxyapatite (HA) is a principal component of hard tissues and has been of interest in dental and medical fields. The biocompatibility of HA is excellent [1–3], however HA lacks sufficient strength; has low toughness (0.8–1.2 MPa m1/2 ) and low flexural strength (<140 MPa) for application in major load bearing parts of the skeleton. Therefore, secondary constituent materials such as carbon nanotubes (CNTs), alumina (Al2 O3 ), yttria-stabilized zirconia (YSZ), Ni3 Al and Ti-alloys are often added to enhance the fracture toughness of the HA matrix [4–6]. CNTs have recently emerged as materials with exceptional properties exceeding those of any conventional material. Defect-free CNTs have elastic moduli of ∼1 TPa and tensile strengths in the region of 150 GPa [7–10]. These exceptional physical and mechanical properties make CNTs ideal candidates as reinforcements in composite materials to increase both stiffness and strength while also contributing to weight savings. Not surprisingly, CNTs have emerged as new reinforcements for a number of material systems. However, most research efforts in this area have dealt with CNT/polymer composites, which exhibit a tremendous strengthening effect for the composites [11–13]. So far, few researchers have explored CNT–ceramic composites because of the difficulties in homogeneously distributing CNTs in a ceramic matrix by
∗ Corresponding author. Tel.: +86 22 60204477; fax: +86 22 60204129. E-mail address: [email protected] (W. Zhao). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.09.022
traditional methods. Since recent developments in CNT–Al2 O3 composites processed by “in situ synthesis of CNTs in Al2 O3 powder” [14,15], many researchers have been encouraged to pay their attention to the use of CNTs as reinforcement for ceramic matrices [15–19]. And the CNT–ceramic composites prepared by in situ synthesis exhibited a ductile behavior, but the expected improvement in Young’s modulus was only slightly achieved. Therefore, the in situ synthesis technique is promising but extensive investigations are still needed. Furthermore, the dispersion and structure of the nanotubes are the dominating factors for the mechanical improvement of CNT-reinforced composites, but no detailed investigation about the effect of in situ synthesis technique on the dispersion and structure of the nanotubes was conducted. Although some work also referred the in situ synthesis of CNTs in Al2 O3 powder [14,15], to-date in situ synthesis of CNTs in HA powder for investigating the dispersing effect and structure of CNTs within the HA powder has not been conducted, which is the focus of the current paper. 2. Experimental 2.1. Preparation of the catalyst precursor The production process of the catalyst was presented as follows: the right amounts of Fe(NO3 )3 ·9H2 O or Ni(NO3 )2 ·6H2 O or Co(NO3 )2 ·6H2 O and HA powder were mixed in distilled water to yield the final metal (Fe, Ni or Co) weight content in metal/HA catalyst to be 5%. Then NaOH solution was slowly dripped into the mixture with constant stirring till neutral pH. The co-precipitate
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H. Li et al. / Materials Science and Engineering B 166 (2010) 19–23
Fig. 1. SEM images of carbon nanomaterials dispersed in HA powder prepared by in situ CVD of methane over (a–c) Ni/HA, (d–f) Fe/HA, and (g and h) Co/HA.
was aged at room temperature for 24 h, and the Fe(OH)3 /HA, Ni(OH)2 /HA or Co(OH)2 /HA colloid was attained. The formed colloid was filtered for several times with distilled water till neutral pH and calcined in N2 atmosphere at 250 and 500 ◦ C for 2 h, respectively, to yield the Fe2 O3 /HA or NiO/HA or CoO/HA composite powders, which would be used in the following catalytic synthesis experiments. 2.2. Production of CNTs/HA composite powders To synthesize the CNTs, approximately 1.5 g of the Fe2 O3 /HA or NiO/HA or CoO/HA catalyst precursor was sprayed uniformly into a quartz boat, which was inserted in the center of a quartz tube. The quartz tube, mounted in an electrical tube furnace, was heated to reduction temperature (600 ◦ C) in N2 atmosphere. Then hydrogen (100 ml/min, 99.9% purity) was introduced to reduce the catalyst for 2 h. Subsequently, hydrogen flow was stopped and a mixture of CH4 /N2 (420/60 ml/min, v/v) was introduced into the quartz tube at a flow rate of 480 ml/min and was maintained for 60 min, before the furnace was cooled to room temperature under N2 protection. 2.3. Characterization The as-grown powder was sonicated in ethanol and dispersed onto the copper grids for microscopic examination by transmis-
sion electron microscope (TEM) and high-resolution TEM (HRTEM) (Philips Tecnai G2 F20, 200 kV). Field emission scanning electron microscopy (FE-SEM) was carried out by JEOL JSM-6700 field emission scanning electron micrograph. Raman spectroscopy of the composite powders was performed using the 1064 nm line of an Ar+ laser as the excitation source to validate the presence of CNTs in the composite powders. Thermogravimetric analyses (TGA) were performed on TA Instruments SDT Q600 TGA thermogravimetric analyzer. Samples were analyzed in platinum pans at a heating rate of 20 ◦ C/min to 800 ◦ C in an atmosphere of air flowing at 150 ml/min. Sample masses ranged from 8 to 10 mg. 3. Results and discussion Fig. 1 shows SEM images of the CNTs grown on catalyst particles formed on the HA powder using CVD of methane for 60 min at 600 ◦ C. The length of CNTs is about 1000, 2500, and 200 nm for Ni, Fe and Co catalyst supported on HA powder, respectively. The average diameter of CNTs grown on Ni/HA, Fe/HA and Co/HA catalysts is about 18, 16 and 20 nm, respectively. In addition, it can be found that the CNTs in situ synthesized are clean in surface and disperse homogeneously within the HA powder. That is, the CNTs bond well with the HA powder rather than merely on their surfaces. The most important feature of this process is that the CNTs are in situ synthesized and implanted into the HA powder. The morphology of all CNT/HA composite powders shows a good composite microstruc-
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Fig. 2. TEM images of carbon nanomaterials dispersed in HA powder prepared by in situ CVD of methane over (a–d) Ni/HA, (e–h) Fe/HA, and (I–l) Co/HA.
ture, displaying spherical morphologies with CNTs homogeneously implanted into the powder. Furthermore, the density of CNTs is very different for Ni/HA, Fe/HA and Co/HA catalysts. It can be seen that the product synthesized by Fe/HA catalyst has thickest CNTs (as
shown in Fig. 1(d)–(f)), and the product synthesized by Co/HA catalyst has sparsest CNTs (as shown in Fig. 1(g) and (h)), showing that the dispersion state of CNTs within HA powder can be controlled by in situ synthesis technique.
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To better control the dispersion of CNTs in HA powder, the average growth rate of CNTs has been investigated in detail. From several experimental runs, the average growth rate of CNTs on Ni/HA, Fe/HA and Co/HA catalysts is measured as about 17, 42 and 3 nm/min, respectively. The maximum growth rate can be achieved when the Fe/HA catalyst is used. The Co/HA catalyst renders the lowest growth rate among three catalysts. The growth rate is almost constant during the growth time 30 min, but it decreases significantly after about 40 min as the carbonaceous particles cover the surface of metal particles. The growth rate mainly depends on the diffusion rate of carbons. But the growth rate increases as the growth temperature increases regardless of catalyst. To understand the catalyst effect on the growth rate of CNTs, we evaluated a correlation between the diffusion rate of carbon and the relative growth rate of CNT according to the catalysts at 600 ◦ C. In bulk Ni, Fe and Co metals, the diffusion rate of carbon at 600 ◦ C follows the order Ni > Fe > Co. The growth rate of CNTs between Ni and Co catalysts is consistent with the diffusion rate in bulk Ni and Co metals. While the growth rate of CNTs reveals inverse sequence from the diffusion coefficients between Fe and Ni catalysts. This result can be explained by the size effect of catalyst particles on the CNT growth. As the size of catalyst particle decreases, carbons adsorbed at the catalyst surface can arrive at the growth site in a shorter period, resulting in an increasing growth rate of CNTs. In this work, the size of Fe catalyst particle is smaller than that of Ni catalyst particle. We suggest that the size effect of catalyst particles would be another factor to determine the growth rate of CNTs beside the diffusion rate of carbon. TEM was employed to characterize the CNTs/HA composite powders to investigate the structure dependent on the three catalysts. Fig. 2 shows TEM images of the CNTs grown on Ni/HA, Fe/HA and Co/HA catalysts. The CNTs with a closed tip exhibit exclusively a multiwalled structure for all three catalysts. Many small HA nanoparticles, which were observed closely attached to the surface of high-quality CNTs, can stop the slippage of CNTs away from HA matrix. These particle survived the ultrasonic dispersion during TEM sample preparation, indicating an intimate contact between the CNT and the HA particle. According to TEM images, it can also be observed that the graphitic sheets of the CNT grown on Fe/HA catalyst reveal highly ordered crystalline structure (as shown in Fig. 2(e)–(h)). The CNT grown on Ni/HA catalyst has a little lower degree of crystalline perfection than that of Fe/HA catalyst (as shown in Fig. 2(a)–(d)). On the other hand, the CNT grown on Co/HA catalyst shows that the graphitic sheets are waved over a short range and thus have more defective structure compared with Fe/HA or Ni/HA catalysts (as shown in Fig. 2(i)–(l)). The HRTEM images reveal that the crystallinity of CNTs follows a sequence of Fe/HA > Ni/HA > Co/HA. In addition, in the product synthesized over Co/HA catalyst, we find a few carbon coated Co nanoparticles (as shown in Fig. 2(j)), whose sizes are in the range of 10–30 nm. This phenomenon is very different from the product grown over Fe/HA and Ni/HA catalysts. Here, we suggest that the carbon coated Co nanoparticle formation involves first Co metallization during heating in H2 and subsequent graphitic shell formation during methane CVD. No metal-carbide phase is observed in the core, which suggests that the deposited carbon mainly interacts with the surface of the metal particle, or has precipitated out of the core to form an encapsulating graphitic layer during cooling. Our CVD synthesis here is used for CNT growth on metal clusters, thus, the carbon coated Co nanoparticles occur through carbon supersaturation and continuous precipitation from the clusters to form graphitic carbon. TEM is one of the most powerful methods for the evaluation of the crystallinity of CNTs. However, it does not provide the overall information about the structure of the entire CNTs. The use of TGA and Raman spectroscopy compensates such shortcoming of TEM. The temperature-programmed oxidation technique allows one to
Fig. 3. TGA data of the weight loss in % vs. the oxidation temperature for the CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts.
ascertain the relative amounts of defective and crystalline constituents in the CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts. Less ordered crystalline CNTs will react preferentially with the oxidant and lose weight at a lower temperature compared with more highly crystalline CNTs. Fig. 3 is a plot for the weight loss in % vs. the oxidation temperature, measured by heating up the CNTs in a TGA. The weight loss curve between 100 and 800 ◦ C is plotted by adjusting about 100% for the weight loss at 800 ◦ C, in which the actual weight is presumably the weight of catalyst (usually 10% of total weight). The CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts start to gasify at approximately 550, 550, and 500 ◦ C, respectively. For pure graphite, the onset of gasification is observed at 700 ◦ C under the same experimental conditions. The respective weight loss is measured in the range of 560–708, 480–680, and 350–540 ◦ C for the CNTs grown on Fe/HA, Ni/HA, and Co/HA catalysts, respectively. It is well known that the CNTs synthesized can have different defects along the wall and at the end of the tubes, such as single or multiple vacancies and substitutional defects. Along with these defect sites, CNTs can have different defects along the wall and at the end of the tubes before implantation. These defects can be the source of degradation at relatively higher temperatures. If the CNTs have more defects, their structures exhibit less crystallinity. Therefore, above TGA data indicate that the crystallinity of the CNTs grown on Fe/HA catalyst is better than that of CNTs grown on Ni/HA catalyst, but the CNTs grown on Co/HA catalyst exhibit much less crystallinity compared with Fe/HA or Ni/HA catalyst. Fig. 4 shows Raman spectra for the CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts. All spectra show mainly two Raman bands at ∼1345 cm−1 (D band) and ∼1578 cm−1 (G band), which are associated with the vibrations of carbon atoms with dangling bonds for the in-plane terminations of disordered graphite and the vibrations in all sp2 bonded carbon atoms in a 2-dimensional hexagonal lattice, respectively [20,21]. The intensity ratio of D to G band (ID /IG ) is usually employed to evaluate the crystalline of CNTs, the lower relative intensity of D to G band implies that the obtained CNTs are mainly composed of more well-crystallized graphite. In Fig. 4, the values of ID = IG for the CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts are 0.72, 0.78, and 0.85, respectively. It reveals that the degree of crystalline perfection of the CNTs grown on Fe/HA or Ni/HA catalyst is higher than that of CNTs grown on Co/HA catalyst, which is very consistent with the HRTEM images and TGA data. From above results, we suggest that the crystallographic characteristics of catalyst particle play an important role in governing the crystallinity of CNTs, but further extensive
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the best crystallinity among the three catalysts. On the other hand, the CNTs grown on Co catalyst exhibit much lower degree of crystalline character compared with Fe or Ni catalyst. We demonstrate that the growth rate, the diameter, and the crystallinity of CNTs can be manipulated by selecting the catalysts. Thus, the dispersion and structure of CNTs in the HA powder can be controlled by selecting the catalysts. Acknowledgment The authors acknowledge financial support by Tianjin Natural Science Foundation of China (No. 09JCYBJC13900). References
Fig. 4. Raman spectrum for the CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts.
studies are necessary to provide a definite evidence for such possibility. 4. Conclusions One of the key issues in the development of CNT/HA-matrix composites is controlling the agglomeration of the nanotubes. This has been a major impediment facing the development of these new materials. The results presented in this paper demonstrate that in situ synthesis is a promising technique to overcome this problem. Moreover, we have studied the effect of different catalytic metal particles (Fe, Ni or Co) on the morphological and structural development of the powder and dispersion of CNTs in HA powder. The growth rate of CNTs indicates that the performance of catalyst follows in the order of Fe > Ni > Co. The size effect of catalyst particle would be another factor to determine the growth rate of CNT even though the growth rate mainly depend on the diffusion rate of carbon in the catalyst particle. The CNTs grown on Fe catalyst reveals
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Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Dispersion of carbon nanotubes in hydroxyapatite powder by in situ chemical vapor deposition Haipeng Li a , Lihui Wang b , Chunyong Liang a , Zhifeng Wang a , Weimin Zhao a,∗ a b
School of Materials Science and Engineering, Hebei University of Technology, Tianjin 300130, China Automobile Engineering Department, Academy of Military Transportation, Tianjin 300161, China
a r t i c l e
i n f o
Article history: Received 26 July 2009 Received in revised form 15 September 2009 Accepted 23 September 2009 Keywords: Carbon nanotubes Hydroxyapatite Composite Chemical vapor deposition In situ
a b s t r a c t In the present work, we use chemical vapor deposition of methane to disperse carbon nanotubes (CNTs) within hydroxyapatite (HA) powder. The effect of different catalytic metal particles (Fe, Ni or Co) on the morphological and structural development of the powder and dispersion of CNTs in HA powder was investigated. The results show that the technique is effective in dispersing the nanotubes within HA powder, which simultaneously protects the nanotubes from damage. The results can have important and promising speculations for the processing of CNT-reinforced HA-matrix composites in general. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Hydroxyapatite (HA) is a principal component of hard tissues and has been of interest in dental and medical fields. The biocompatibility of HA is excellent [1–3], however HA lacks sufficient strength; has low toughness (0.8–1.2 MPa m1/2 ) and low flexural strength (<140 MPa) for application in major load bearing parts of the skeleton. Therefore, secondary constituent materials such as carbon nanotubes (CNTs), alumina (Al2 O3 ), yttria-stabilized zirconia (YSZ), Ni3 Al and Ti-alloys are often added to enhance the fracture toughness of the HA matrix [4–6]. CNTs have recently emerged as materials with exceptional properties exceeding those of any conventional material. Defect-free CNTs have elastic moduli of ∼1 TPa and tensile strengths in the region of 150 GPa [7–10]. These exceptional physical and mechanical properties make CNTs ideal candidates as reinforcements in composite materials to increase both stiffness and strength while also contributing to weight savings. Not surprisingly, CNTs have emerged as new reinforcements for a number of material systems. However, most research efforts in this area have dealt with CNT/polymer composites, which exhibit a tremendous strengthening effect for the composites [11–13]. So far, few researchers have explored CNT–ceramic composites because of the difficulties in homogeneously distributing CNTs in a ceramic matrix by
∗ Corresponding author. Tel.: +86 22 60204477; fax: +86 22 60204129. E-mail address: [email protected] (W. Zhao). 0921-5107/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2009.09.022
traditional methods. Since recent developments in CNT–Al2 O3 composites processed by “in situ synthesis of CNTs in Al2 O3 powder” [14,15], many researchers have been encouraged to pay their attention to the use of CNTs as reinforcement for ceramic matrices [15–19]. And the CNT–ceramic composites prepared by in situ synthesis exhibited a ductile behavior, but the expected improvement in Young’s modulus was only slightly achieved. Therefore, the in situ synthesis technique is promising but extensive investigations are still needed. Furthermore, the dispersion and structure of the nanotubes are the dominating factors for the mechanical improvement of CNT-reinforced composites, but no detailed investigation about the effect of in situ synthesis technique on the dispersion and structure of the nanotubes was conducted. Although some work also referred the in situ synthesis of CNTs in Al2 O3 powder [14,15], to-date in situ synthesis of CNTs in HA powder for investigating the dispersing effect and structure of CNTs within the HA powder has not been conducted, which is the focus of the current paper. 2. Experimental 2.1. Preparation of the catalyst precursor The production process of the catalyst was presented as follows: the right amounts of Fe(NO3 )3 ·9H2 O or Ni(NO3 )2 ·6H2 O or Co(NO3 )2 ·6H2 O and HA powder were mixed in distilled water to yield the final metal (Fe, Ni or Co) weight content in metal/HA catalyst to be 5%. Then NaOH solution was slowly dripped into the mixture with constant stirring till neutral pH. The co-precipitate
20
H. Li et al. / Materials Science and Engineering B 166 (2010) 19–23
Fig. 1. SEM images of carbon nanomaterials dispersed in HA powder prepared by in situ CVD of methane over (a–c) Ni/HA, (d–f) Fe/HA, and (g and h) Co/HA.
was aged at room temperature for 24 h, and the Fe(OH)3 /HA, Ni(OH)2 /HA or Co(OH)2 /HA colloid was attained. The formed colloid was filtered for several times with distilled water till neutral pH and calcined in N2 atmosphere at 250 and 500 ◦ C for 2 h, respectively, to yield the Fe2 O3 /HA or NiO/HA or CoO/HA composite powders, which would be used in the following catalytic synthesis experiments. 2.2. Production of CNTs/HA composite powders To synthesize the CNTs, approximately 1.5 g of the Fe2 O3 /HA or NiO/HA or CoO/HA catalyst precursor was sprayed uniformly into a quartz boat, which was inserted in the center of a quartz tube. The quartz tube, mounted in an electrical tube furnace, was heated to reduction temperature (600 ◦ C) in N2 atmosphere. Then hydrogen (100 ml/min, 99.9% purity) was introduced to reduce the catalyst for 2 h. Subsequently, hydrogen flow was stopped and a mixture of CH4 /N2 (420/60 ml/min, v/v) was introduced into the quartz tube at a flow rate of 480 ml/min and was maintained for 60 min, before the furnace was cooled to room temperature under N2 protection. 2.3. Characterization The as-grown powder was sonicated in ethanol and dispersed onto the copper grids for microscopic examination by transmis-
sion electron microscope (TEM) and high-resolution TEM (HRTEM) (Philips Tecnai G2 F20, 200 kV). Field emission scanning electron microscopy (FE-SEM) was carried out by JEOL JSM-6700 field emission scanning electron micrograph. Raman spectroscopy of the composite powders was performed using the 1064 nm line of an Ar+ laser as the excitation source to validate the presence of CNTs in the composite powders. Thermogravimetric analyses (TGA) were performed on TA Instruments SDT Q600 TGA thermogravimetric analyzer. Samples were analyzed in platinum pans at a heating rate of 20 ◦ C/min to 800 ◦ C in an atmosphere of air flowing at 150 ml/min. Sample masses ranged from 8 to 10 mg. 3. Results and discussion Fig. 1 shows SEM images of the CNTs grown on catalyst particles formed on the HA powder using CVD of methane for 60 min at 600 ◦ C. The length of CNTs is about 1000, 2500, and 200 nm for Ni, Fe and Co catalyst supported on HA powder, respectively. The average diameter of CNTs grown on Ni/HA, Fe/HA and Co/HA catalysts is about 18, 16 and 20 nm, respectively. In addition, it can be found that the CNTs in situ synthesized are clean in surface and disperse homogeneously within the HA powder. That is, the CNTs bond well with the HA powder rather than merely on their surfaces. The most important feature of this process is that the CNTs are in situ synthesized and implanted into the HA powder. The morphology of all CNT/HA composite powders shows a good composite microstruc-
H. Li et al. / Materials Science and Engineering B 166 (2010) 19–23
21
Fig. 2. TEM images of carbon nanomaterials dispersed in HA powder prepared by in situ CVD of methane over (a–d) Ni/HA, (e–h) Fe/HA, and (I–l) Co/HA.
ture, displaying spherical morphologies with CNTs homogeneously implanted into the powder. Furthermore, the density of CNTs is very different for Ni/HA, Fe/HA and Co/HA catalysts. It can be seen that the product synthesized by Fe/HA catalyst has thickest CNTs (as
shown in Fig. 1(d)–(f)), and the product synthesized by Co/HA catalyst has sparsest CNTs (as shown in Fig. 1(g) and (h)), showing that the dispersion state of CNTs within HA powder can be controlled by in situ synthesis technique.
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H. Li et al. / Materials Science and Engineering B 166 (2010) 19–23
To better control the dispersion of CNTs in HA powder, the average growth rate of CNTs has been investigated in detail. From several experimental runs, the average growth rate of CNTs on Ni/HA, Fe/HA and Co/HA catalysts is measured as about 17, 42 and 3 nm/min, respectively. The maximum growth rate can be achieved when the Fe/HA catalyst is used. The Co/HA catalyst renders the lowest growth rate among three catalysts. The growth rate is almost constant during the growth time 30 min, but it decreases significantly after about 40 min as the carbonaceous particles cover the surface of metal particles. The growth rate mainly depends on the diffusion rate of carbons. But the growth rate increases as the growth temperature increases regardless of catalyst. To understand the catalyst effect on the growth rate of CNTs, we evaluated a correlation between the diffusion rate of carbon and the relative growth rate of CNT according to the catalysts at 600 ◦ C. In bulk Ni, Fe and Co metals, the diffusion rate of carbon at 600 ◦ C follows the order Ni > Fe > Co. The growth rate of CNTs between Ni and Co catalysts is consistent with the diffusion rate in bulk Ni and Co metals. While the growth rate of CNTs reveals inverse sequence from the diffusion coefficients between Fe and Ni catalysts. This result can be explained by the size effect of catalyst particles on the CNT growth. As the size of catalyst particle decreases, carbons adsorbed at the catalyst surface can arrive at the growth site in a shorter period, resulting in an increasing growth rate of CNTs. In this work, the size of Fe catalyst particle is smaller than that of Ni catalyst particle. We suggest that the size effect of catalyst particles would be another factor to determine the growth rate of CNTs beside the diffusion rate of carbon. TEM was employed to characterize the CNTs/HA composite powders to investigate the structure dependent on the three catalysts. Fig. 2 shows TEM images of the CNTs grown on Ni/HA, Fe/HA and Co/HA catalysts. The CNTs with a closed tip exhibit exclusively a multiwalled structure for all three catalysts. Many small HA nanoparticles, which were observed closely attached to the surface of high-quality CNTs, can stop the slippage of CNTs away from HA matrix. These particle survived the ultrasonic dispersion during TEM sample preparation, indicating an intimate contact between the CNT and the HA particle. According to TEM images, it can also be observed that the graphitic sheets of the CNT grown on Fe/HA catalyst reveal highly ordered crystalline structure (as shown in Fig. 2(e)–(h)). The CNT grown on Ni/HA catalyst has a little lower degree of crystalline perfection than that of Fe/HA catalyst (as shown in Fig. 2(a)–(d)). On the other hand, the CNT grown on Co/HA catalyst shows that the graphitic sheets are waved over a short range and thus have more defective structure compared with Fe/HA or Ni/HA catalysts (as shown in Fig. 2(i)–(l)). The HRTEM images reveal that the crystallinity of CNTs follows a sequence of Fe/HA > Ni/HA > Co/HA. In addition, in the product synthesized over Co/HA catalyst, we find a few carbon coated Co nanoparticles (as shown in Fig. 2(j)), whose sizes are in the range of 10–30 nm. This phenomenon is very different from the product grown over Fe/HA and Ni/HA catalysts. Here, we suggest that the carbon coated Co nanoparticle formation involves first Co metallization during heating in H2 and subsequent graphitic shell formation during methane CVD. No metal-carbide phase is observed in the core, which suggests that the deposited carbon mainly interacts with the surface of the metal particle, or has precipitated out of the core to form an encapsulating graphitic layer during cooling. Our CVD synthesis here is used for CNT growth on metal clusters, thus, the carbon coated Co nanoparticles occur through carbon supersaturation and continuous precipitation from the clusters to form graphitic carbon. TEM is one of the most powerful methods for the evaluation of the crystallinity of CNTs. However, it does not provide the overall information about the structure of the entire CNTs. The use of TGA and Raman spectroscopy compensates such shortcoming of TEM. The temperature-programmed oxidation technique allows one to
Fig. 3. TGA data of the weight loss in % vs. the oxidation temperature for the CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts.
ascertain the relative amounts of defective and crystalline constituents in the CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts. Less ordered crystalline CNTs will react preferentially with the oxidant and lose weight at a lower temperature compared with more highly crystalline CNTs. Fig. 3 is a plot for the weight loss in % vs. the oxidation temperature, measured by heating up the CNTs in a TGA. The weight loss curve between 100 and 800 ◦ C is plotted by adjusting about 100% for the weight loss at 800 ◦ C, in which the actual weight is presumably the weight of catalyst (usually 10% of total weight). The CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts start to gasify at approximately 550, 550, and 500 ◦ C, respectively. For pure graphite, the onset of gasification is observed at 700 ◦ C under the same experimental conditions. The respective weight loss is measured in the range of 560–708, 480–680, and 350–540 ◦ C for the CNTs grown on Fe/HA, Ni/HA, and Co/HA catalysts, respectively. It is well known that the CNTs synthesized can have different defects along the wall and at the end of the tubes, such as single or multiple vacancies and substitutional defects. Along with these defect sites, CNTs can have different defects along the wall and at the end of the tubes before implantation. These defects can be the source of degradation at relatively higher temperatures. If the CNTs have more defects, their structures exhibit less crystallinity. Therefore, above TGA data indicate that the crystallinity of the CNTs grown on Fe/HA catalyst is better than that of CNTs grown on Ni/HA catalyst, but the CNTs grown on Co/HA catalyst exhibit much less crystallinity compared with Fe/HA or Ni/HA catalyst. Fig. 4 shows Raman spectra for the CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts. All spectra show mainly two Raman bands at ∼1345 cm−1 (D band) and ∼1578 cm−1 (G band), which are associated with the vibrations of carbon atoms with dangling bonds for the in-plane terminations of disordered graphite and the vibrations in all sp2 bonded carbon atoms in a 2-dimensional hexagonal lattice, respectively [20,21]. The intensity ratio of D to G band (ID /IG ) is usually employed to evaluate the crystalline of CNTs, the lower relative intensity of D to G band implies that the obtained CNTs are mainly composed of more well-crystallized graphite. In Fig. 4, the values of ID = IG for the CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts are 0.72, 0.78, and 0.85, respectively. It reveals that the degree of crystalline perfection of the CNTs grown on Fe/HA or Ni/HA catalyst is higher than that of CNTs grown on Co/HA catalyst, which is very consistent with the HRTEM images and TGA data. From above results, we suggest that the crystallographic characteristics of catalyst particle play an important role in governing the crystallinity of CNTs, but further extensive
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the best crystallinity among the three catalysts. On the other hand, the CNTs grown on Co catalyst exhibit much lower degree of crystalline character compared with Fe or Ni catalyst. We demonstrate that the growth rate, the diameter, and the crystallinity of CNTs can be manipulated by selecting the catalysts. Thus, the dispersion and structure of CNTs in the HA powder can be controlled by selecting the catalysts. Acknowledgment The authors acknowledge financial support by Tianjin Natural Science Foundation of China (No. 09JCYBJC13900). References
Fig. 4. Raman spectrum for the CNTs grown on Fe/HA, Ni/HA and Co/HA catalysts.
studies are necessary to provide a definite evidence for such possibility. 4. Conclusions One of the key issues in the development of CNT/HA-matrix composites is controlling the agglomeration of the nanotubes. This has been a major impediment facing the development of these new materials. The results presented in this paper demonstrate that in situ synthesis is a promising technique to overcome this problem. Moreover, we have studied the effect of different catalytic metal particles (Fe, Ni or Co) on the morphological and structural development of the powder and dispersion of CNTs in HA powder. The growth rate of CNTs indicates that the performance of catalyst follows in the order of Fe > Ni > Co. The size effect of catalyst particle would be another factor to determine the growth rate of CNT even though the growth rate mainly depend on the diffusion rate of carbon in the catalyst particle. The CNTs grown on Fe catalyst reveals
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