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1. Material Science Faculty, Saint-Petersburg State Polytechnical University, Polytechnicheskaya, 29, 195251 Saint-Petersburg, Russia
2. NanoMaterials Group, Department of Applied Physics, Aalto University School of Science and Technology, P.O. Box 15100 Puumiehenkuja 2, FIN-02150 Espoo, Finland
Received: November 22, 2011 / Accepted: December 16, 2011 / Published: April 25, 2012.
Abstract: A few-layered graphene was synthesized on the surface of micron-sized copper powder by chemical vapor deposition technique using ethylene as a carbon source in the temperature range from 700 to 940 °C. As a result, we synthesized graphene with 8-12 layers on the surface of copper particles and examined the effect of this material on the mechanical properties of the compacted materials. The copper/graphene composite showed 39% increase in the hardness compared to pure copper. The improvements of 10% and 70% were obtained for the copper composites materials based on graphite and cartbon nanofibers, respectively.
Key words: Chemical vapour deposition (CVD), сomposite material, copper, graphene.
1. Introduction??
Carbon nanostructures have recently attracted a great interest. Due to their unique physical and chemical properties, carbon nanotubes (CNTs), carbon nanofibers (CNFs) and graphene can be utilized to produce strong and electrically conductive composite materials. Those carbon nanomaterials are especially important for the enhancement of mechanical properties of various metals. Among them copper is one of the most industrially important metals, which for many applications requires improvement in mechanical strength [1]. A number of articles have been devoted to the copper hardening [2-8], wherein authors attempted to add CNTs/CNFs to the metal or to grow carbon nanomaterials on the metal applying additional catalysts. Application of copper particles as a catalyst for carbon nanostructures growth was as well considered. For example, synthesis of octopus-like carbon nanostructures on copper nanoparticles has been recently reported in Ref. [9], however, this method shows relatively low effect on the mechanical properties.
The largest problem to create strong composite is to prepare a good dispersion of the composite materials. Recently, we proposed a novel approach to solve this problem by direct synthesis of carbon nanomaterials on the surface of copper micron-sized particles without additional catalyst [10, 11]. The developed method allowed us to produce well-dispersed CNFs in a copper powder matrix without having to resort additional catalyst preparation and multistep processes of purification, ultrasonication and functionalization for their homogeneous introduction in the metal matrix. Nevertheless, it is expected that a few-layered graphene should enhance the mechanical properties even more due to their intrinsic mechanical strength. Therefore, study we explored the possibility to synthesize graphene on the surface of copper and to improve the composite mechanical properties. The
Fig. 1 shows a dependence of the weight of the samples treated at different synthesis temperature. The results of this study are compared with those obtained when acetylene as a carbon source reported in [10]. As can be seen from the figure a notable ethylene decomposition occurred at temperatures above 750 °C. At lower temperatures neither the sample weight increase nor colour change were observed. The maximum of the mass increase of 6% was found at the temperature of 890 °C. The decrease of the carbon deposited on the powder at the temperatures above 900 °C may be explained by gas phase ethylene decomposition and formation of aerosol carbon particles [16], which were deposited on the cold parts of the reactor.
SEM images of the copper powder treated at temperatures of 780 and 940 °C in the presence of ethylene are presented on Fig. 2. As seen from the pictures the copper particles are covered by a smooth carbon layer.
TEM images of the carbon coatings removed from
the copper particles are presented on Fig. 3. As can be seen disordered carbon films were synthesized on the surface of copper particles at the temperature of 840 °C(Fig. 3a). Increasing the temperature resulted in higher graphene layer crystallinity (Fig. 3b), but the product also contained significant amount of amorphous coatings. As was observed the carbon coatings obtained at temperature of 940 °C contained 8-12 layers of graphene with interplanar distance of 0.373 ±0.05 nm. In an attempt to decrease the number of the layers, we decreased the duration of the treatment from 20 to 10 min. However, neither the weight of the sample nor the number of the layers changed with shorter growth time. Therefore for the mechanical tests we utilized the samples prepared at 10 min growth time.
Assuming that the reaction rate is proportional to the mass increase during 20 min synthesis, we can calculate the activation energy of ethylene decomposition on the surface of copper catalyst for the low temperature region (from 730 to 890 °C, Fig. 1) according to the Arrhenius equitation:
on Fig. 4. As expected any carbon additions to copper led to hardening of the composite and decrease in the ductility compared to pure copper. The best results were obtained for Cu/CNFs composite. Cu/CNF composite prepared by cold pressing and subsequent sintering showed 1.7 times increase in the hardness compared to pure copper. Copper hardened by graphene showed only 39% increase in HB with significant ductility decrease.
In order to better understand the mechanical test results, we examined the microstructure of the produced composite materials and the fracture of the
specimens. As can be seen from Fig. 5a, the Cu/graphite composite has the mean size of copper grains of about 10 μm with a very rough carbon inclusions. The Cu/CNF composite contains finer grains (around 4 μm) with carbon homogeneously dispersed between (Fig. 5b). Apparently a good dispersion of CNFs in the matrix prevented the grain growth during sintering. In case of Cu/graphene the size of the grains was around 7 μm elongated in the rolling direction with fine carbon layers located along the boundaries (Fig. 5c). The plastic deformation of the copper particles during rolling resulted in a disruption of less plastic graphene layers attached to the particles surfaces; therefore, graphene only partially blocked the growth of Cu grains. To confirm these results, the grain size distributions in different products measured on the basis of optical images are shown in Fig. 6.
Therefore, the best hardness for the Cu/CNFs
fractures after the mechanical tests are presented in Fig. 7. As seen from the image the fracture of the sample containing graphite was mostly fragile, there was little or no plastic deformation of the grains observed in the fracture (Fig. 7a). As a result of graphite replacement by CNFs the fracture became more viscous; as it is seen that the grain shape has been distorted under plastic deformation. One can see CNFs on the fracture of the sample (Fig. 7b). A fracture of the sample containing a few-layered graphene had both viscous and fragile component (Fig. 7c).
Thus, from practical point of view carbon nanofibers grown on the surface of copper particles are the most preferable material combination to improve the mechanical properties of copper. First of all, the CNFs synthesis requires lower temperature and cheaper carbon source (acetylene). Second, the produced composite has smaller grain size resulting in the hardest composite. Graphene composite showed worse performance compared to the composite material based on CNFs, but better than traditionally used graphite with the improvement results of 39% versus 10% increase in the hardness.
carbon nanofiber/copper composites, Compos. Sci. Technol. 68 (2008) 1384-1391.
[6] L. Xia, B. Jia, J. Zeng, J. Xub, Wear and mechanical properties of carbon fiber reinforced copper alloy composites, Mater. Charact. 60 (2009) 363-369.
[7] S. Arai, M. Endo, Carbon nanofiber-copper composite powder prepared by electrodeposition, Electrochem. Commun. 5 (2003) 797-799.
[8] J. Kang, P. Nash, J. Li, C. Shi, N. Zhao, Achieving highly dispersed nanofibres at high loading in carbon nanofibre-metal composites, Nanotechnology 20 (2009) 235607-235614.
[9] C. Veríssimo, S.A. Moshkalyov, A.C.S. Ramos, J.L. Gon?alves, O.L. Alves, J.W. Swart, et al., Different carbon nanostructured materials obtained in catalytic chemical vapor deposition, Chem. Soc. 17 (6) (2006) 1124-1132.
[10] L.I. Nasibulina, T.S. Koltsova, T. Joentakanen, A.G. Nasibulin, O. Tolochko, J.E.M. Malm, et al., Direct synthesis of carbon nanofibers on the surface of copper
2. NanoMaterials Group, Department of Applied Physics, Aalto University School of Science and Technology, P.O. Box 15100 Puumiehenkuja 2, FIN-02150 Espoo, Finland
Received: November 22, 2011 / Accepted: December 16, 2011 / Published: April 25, 2012.
Abstract: A few-layered graphene was synthesized on the surface of micron-sized copper powder by chemical vapor deposition technique using ethylene as a carbon source in the temperature range from 700 to 940 °C. As a result, we synthesized graphene with 8-12 layers on the surface of copper particles and examined the effect of this material on the mechanical properties of the compacted materials. The copper/graphene composite showed 39% increase in the hardness compared to pure copper. The improvements of 10% and 70% were obtained for the copper composites materials based on graphite and cartbon nanofibers, respectively.
Key words: Chemical vapour deposition (CVD), сomposite material, copper, graphene.
1. Introduction??
Carbon nanostructures have recently attracted a great interest. Due to their unique physical and chemical properties, carbon nanotubes (CNTs), carbon nanofibers (CNFs) and graphene can be utilized to produce strong and electrically conductive composite materials. Those carbon nanomaterials are especially important for the enhancement of mechanical properties of various metals. Among them copper is one of the most industrially important metals, which for many applications requires improvement in mechanical strength [1]. A number of articles have been devoted to the copper hardening [2-8], wherein authors attempted to add CNTs/CNFs to the metal or to grow carbon nanomaterials on the metal applying additional catalysts. Application of copper particles as a catalyst for carbon nanostructures growth was as well considered. For example, synthesis of octopus-like carbon nanostructures on copper nanoparticles has been recently reported in Ref. [9], however, this method shows relatively low effect on the mechanical properties.
The largest problem to create strong composite is to prepare a good dispersion of the composite materials. Recently, we proposed a novel approach to solve this problem by direct synthesis of carbon nanomaterials on the surface of copper micron-sized particles without additional catalyst [10, 11]. The developed method allowed us to produce well-dispersed CNFs in a copper powder matrix without having to resort additional catalyst preparation and multistep processes of purification, ultrasonication and functionalization for their homogeneous introduction in the metal matrix. Nevertheless, it is expected that a few-layered graphene should enhance the mechanical properties even more due to their intrinsic mechanical strength. Therefore, study we explored the possibility to synthesize graphene on the surface of copper and to improve the composite mechanical properties. The
Fig. 1 shows a dependence of the weight of the samples treated at different synthesis temperature. The results of this study are compared with those obtained when acetylene as a carbon source reported in [10]. As can be seen from the figure a notable ethylene decomposition occurred at temperatures above 750 °C. At lower temperatures neither the sample weight increase nor colour change were observed. The maximum of the mass increase of 6% was found at the temperature of 890 °C. The decrease of the carbon deposited on the powder at the temperatures above 900 °C may be explained by gas phase ethylene decomposition and formation of aerosol carbon particles [16], which were deposited on the cold parts of the reactor.
SEM images of the copper powder treated at temperatures of 780 and 940 °C in the presence of ethylene are presented on Fig. 2. As seen from the pictures the copper particles are covered by a smooth carbon layer.
TEM images of the carbon coatings removed from
the copper particles are presented on Fig. 3. As can be seen disordered carbon films were synthesized on the surface of copper particles at the temperature of 840 °C(Fig. 3a). Increasing the temperature resulted in higher graphene layer crystallinity (Fig. 3b), but the product also contained significant amount of amorphous coatings. As was observed the carbon coatings obtained at temperature of 940 °C contained 8-12 layers of graphene with interplanar distance of 0.373 ±0.05 nm. In an attempt to decrease the number of the layers, we decreased the duration of the treatment from 20 to 10 min. However, neither the weight of the sample nor the number of the layers changed with shorter growth time. Therefore for the mechanical tests we utilized the samples prepared at 10 min growth time.
Assuming that the reaction rate is proportional to the mass increase during 20 min synthesis, we can calculate the activation energy of ethylene decomposition on the surface of copper catalyst for the low temperature region (from 730 to 890 °C, Fig. 1) according to the Arrhenius equitation:
on Fig. 4. As expected any carbon additions to copper led to hardening of the composite and decrease in the ductility compared to pure copper. The best results were obtained for Cu/CNFs composite. Cu/CNF composite prepared by cold pressing and subsequent sintering showed 1.7 times increase in the hardness compared to pure copper. Copper hardened by graphene showed only 39% increase in HB with significant ductility decrease.
In order to better understand the mechanical test results, we examined the microstructure of the produced composite materials and the fracture of the
specimens. As can be seen from Fig. 5a, the Cu/graphite composite has the mean size of copper grains of about 10 μm with a very rough carbon inclusions. The Cu/CNF composite contains finer grains (around 4 μm) with carbon homogeneously dispersed between (Fig. 5b). Apparently a good dispersion of CNFs in the matrix prevented the grain growth during sintering. In case of Cu/graphene the size of the grains was around 7 μm elongated in the rolling direction with fine carbon layers located along the boundaries (Fig. 5c). The plastic deformation of the copper particles during rolling resulted in a disruption of less plastic graphene layers attached to the particles surfaces; therefore, graphene only partially blocked the growth of Cu grains. To confirm these results, the grain size distributions in different products measured on the basis of optical images are shown in Fig. 6.
Therefore, the best hardness for the Cu/CNFs
fractures after the mechanical tests are presented in Fig. 7. As seen from the image the fracture of the sample containing graphite was mostly fragile, there was little or no plastic deformation of the grains observed in the fracture (Fig. 7a). As a result of graphite replacement by CNFs the fracture became more viscous; as it is seen that the grain shape has been distorted under plastic deformation. One can see CNFs on the fracture of the sample (Fig. 7b). A fracture of the sample containing a few-layered graphene had both viscous and fragile component (Fig. 7c).
Thus, from practical point of view carbon nanofibers grown on the surface of copper particles are the most preferable material combination to improve the mechanical properties of copper. First of all, the CNFs synthesis requires lower temperature and cheaper carbon source (acetylene). Second, the produced composite has smaller grain size resulting in the hardest composite. Graphene composite showed worse performance compared to the composite material based on CNFs, but better than traditionally used graphite with the improvement results of 39% versus 10% increase in the hardness.
carbon nanofiber/copper composites, Compos. Sci. Technol. 68 (2008) 1384-1391.
[6] L. Xia, B. Jia, J. Zeng, J. Xub, Wear and mechanical properties of carbon fiber reinforced copper alloy composites, Mater. Charact. 60 (2009) 363-369.
[7] S. Arai, M. Endo, Carbon nanofiber-copper composite powder prepared by electrodeposition, Electrochem. Commun. 5 (2003) 797-799.
[8] J. Kang, P. Nash, J. Li, C. Shi, N. Zhao, Achieving highly dispersed nanofibres at high loading in carbon nanofibre-metal composites, Nanotechnology 20 (2009) 235607-235614.
[9] C. Veríssimo, S.A. Moshkalyov, A.C.S. Ramos, J.L. Gon?alves, O.L. Alves, J.W. Swart, et al., Different carbon nanostructured materials obtained in catalytic chemical vapor deposition, Chem. Soc. 17 (6) (2006) 1124-1132.
[10] L.I. Nasibulina, T.S. Koltsova, T. Joentakanen, A.G. Nasibulin, O. Tolochko, J.E.M. Malm, et al., Direct synthesis of carbon nanofibers on the surface of copper