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natural graphite composites
Composites Science and Technology 68 (2008) 2220–2223
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Preparation and characterization of mesophase-pitch-based foam/natural graphite composites Mei-xian Wang a, Cheng-Yang Wang a,*, Tong-Qi Li b, Zi-Jun Hu b a b
Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China National Key Laboratory of Advanced Functional Composite Materials, Aerospace Research Institute of Materials and Processing Technology, Beijing 100076, PR China
a r t i c l e
i n f o
Article history: Received 1 November 2007 Accepted 7 April 2008 Available online 12 April 2008 Keywords: A. Particle-reinforced composites B. Mechanical properties C. Crack D. Optical microscopy D. Heat treatment
a b s t r a c t Graphite foams were prepared from the mesophase-pitch with addition of natural graphite and the relationship between properties and structure of these foams was investigated in detail. These graphite foams possessed high specific compressive strength and experimental results show that less microcracks appeared on the cell walls of foam by adding of natural graphite. The specific compressive strength increased from 2.0 to 5.84 MPa/g cm3 with the addition of 30 wt% natural graphite, and the inter layer spacing of graphite foams decreased with the increase of the natural graphite content in the pitch. In addition, the optical micrograph shows that the anisotropic domain size of the foam decreased with the addition of natural graphite and it also affects the compressive strength of the foams. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The recent development of a mesophase-pitch-based graphitic foam obtained through the graphitization of carbon foams has attracted attention due to its potential for use in low-density thermal management devices, such as heat sinks, heat exchangers and a core material in sandwich structures, evaporator, etc. [1–5]. The graphitic foam is characterized by highly aligned graphitic structures around the host spherical cells, and has a very high thermal conductivity to weight ratio, graphitic foam is a promising material for applications, particularly in utilizing the high specific thermal conductivity in the aeronautics and aerospace industries. Graphitic foam can be considered to be an interconnected network of graphitic ligaments or struts having the similar structure of carbon fiber, they represent a potential alternative for carbon fiber as reinforcement in the structural composite [6]. The carbon fiber with high strength from mesophase-pitch has been achieved [7], however the graphitic foam appeared to be quite fragile. It is postulated that this may be caused mainly by the presence of defects especially the microcracks in both ligaments and junctions [8]. In this work, natural graphite with high thermal conductivity and graphitic structure was incorporated into the mesophase-pitch precursor prior to the foaming operation in order to produce natu-
* Corresponding author. Tel.: +86 22 27406183; fax: +86 22 27403389. E-mail address: [email protected] (C.-Y. Wang). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.04.007
ral graphite reinforced foam [9–11]. As a result, the imperfections of the graphitic foams such as microcracks are significantly reduced and the mechanical properties of the foams are therefore improved. In this paper the effects of the addition amount of natural graphite on the structure and mechanical properties of the as-prepared foams were investigated. 2. Experimental section 2.1. Raw materials In this research, the Mitsubishi AR naphthalene-based synthetic mesophase-pitch (MP) was used as the precursor of the graphitic foams. The properties of the pitch are listed in Table 1. The SEM micrograph of the natural graphite (NG) is given in Fig. 1. 2.2. Foaming process The powders of the MP were well-proportionally mixed with the NG powders. For the foam production, the powders were introduced into a cylindrical aluminum mold having a predetermined shape and size. The reactor was washed with nitrogen to provide an inert atmosphere. Then the reactor was heated at a heating rate of 4 °C/min up to 500 °C and soaked for 2 h to obtain green foams with different addition amounts of the NG. The as-received foams were oxidized at the temperature of 250 °C in air for 20 h and then the foams were carbonized at 1000 °C for 15 min in a purified nitrogen flow, and finally the carbonized foams were graphitized at 2800 °C. The graphitized foams were named as GFx, where x rep-
M.-x. Wang et al. / Composites Science and Technology 68 (2008) 2220–2223
2221
Table 1 Mesophase pitch properties
2.3. Characterization of foams
Properties
The morphologies of graphite foams were examined by a Philips XL30 scanning electron microscopy (SEM). The samples machined into a solid 10 10 10 mm were used for compressive test at the direction of the foam’s growth which was parallel to the gravitational direction. X-ray wide angle diffraction (XRD) was used to study its crystal structure. The diffractometer utilized Cu Ka radiation (40 kV and 200 mA). The data were collected as continuous scans, with a step size of 0.02° (2h) and a scanning rate of 10° (2h)/min between 3° and 90° (2h). In order to further evaluate the microcracks of the graphitized foams, a polarized light microscope (OM) Nikon E600 POL was used for this purpose. The sample was embedded in an epoxy resin, left overnight, cut and polished for microscopic examination of the particle cross-sections.
3
Bulk density (g/cm ) Softening point (°C) Mesophase content (%) Hydrogen/carbon (atom/atom) Benzene soluble (%) Pyridine insoluble (%)
0.65 275 100 0.59 40 45
3. Results and discussion 3.1. SEM observation of the foam structure
Fig. 1. SEM micrograph of the natural graphite.
resents the addition amount of the NG, such as GF05 means graphitic foam prepared from pitch with addition of NG by 5 wt%.
Depending on the unique properties of the foaming precursor, the cell walls can have different thickness and the bubble sizes, the microcracks can be dramatically affected and the mechanical, thermal properties can also be affected [12]. Fig. 2 shows the SEM micrographs of the graphitized foams in this study. It shows the variation of the content of the NG obviously influenced the cell structures of the foams. As the content of the NG increase, the cell size deceased firstly, and then the cell size increased again. Fig. 2a– c show that the cell size is relatively uniform, but when the content of the NG reached to about 30 wt% the cell size is apparently not uniform. Because the foaming precursor with high content of non-fusible materials, such as NG, the bubble growth is controlled by interface nucleation mechanism and so the bubble size of the
Fig. 2. SEM micrographs of the graphitized foams. (a) GF0, (b) GF05, (c) GF10 and (d) GF30.
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M.-x. Wang et al. / Composites Science and Technology 68 (2008) 2220–2223
as-received foams is comparatively inhomogenous [13]. The microcracks of the graphitized foams are observed by SEM micrographs. A large amount of the microcracks appeared on the cell wall and the junctions in the foams without and with low content of NG. The microcracks appear on almost whole wall of the cell but as the content of the natural graphite increased the microcracks are significantly reduced, and in the Fig. 2d the microcracks are almost disappeared even when the foams are heat-treated above 2800 °C. The amount of the microcracks is an important factor that greatly influences the mechanical properties and thermal conductivity of the graphite foams [14]. The mesophase-pitch-based foams with different amount of NG were foamed by the same process and carbonized, graphitized at the same conditions. The corresponding properties of the foams are listed in Table 2. The specific compressive strength is shown in Fig. 4. It can seen that the ultimate specific compressive strengths of the foams are 2.0, 2.85, 3.1, 5.84 MPa/g cm3 when the additive amount of NG is 0%, 5%, 10%, 30% (wt%), respectively. It shows that the specific compressive strengths of NG reinforced foams are higher than that of the foam without of NG. The strength of pitch-based foam depends not only on the foam structure, such as microcracks, but also on the properties of the precursor, such as the anisotropic domain size, which will be discussed later.
Table 2 Properties of the graphitized foams. Foam
GF0
GF05
GF10
GF15
GF20
GF25
GF30
Bulk density (g/ cm3) d002 (nm)
0.172
0.205
0.203
0.195
0.183
0.152
0.124
0.3373
0.3365
0.3366
0.3370
0.3373
0.3373
0.3378
3.2. Optical characterization During the foaming process, first, a mesophase-pitch precursor is heated in an oxygen-free environment under specific pressure and temperature. While the pitch is molten, it begins to evolve low molecular weight species and then the pitch is foamed. So the viscosity, elasticity and surface tension of the pitch precursor affected the growth and collapse of the bubbles in the molten pitch [15]. The addition of natural graphite in MP changed the viscosity and viscoelasticity of MP. So the content of natural graphite would affect the pore size and the microstructure of the foams. The optical micrographs of the representative foams are given in Fig. 3. Optical microscopy reveals that there are more microcracks in GF0 and GF05 than that in the GF10 and GF30, confirming the SEM observations. The isotropic and anisotropic nature of the pitch significantly affects the properties of the subsequent foam. Isotropic material has similar properties in all the direction, while an anisotropic material has properties which depend on the orientation of the anisotropic flow domain [14]. Foams prepared from the pitch with low content or without addition of NG exhibit large anisotropic domain in both ligaments and junctions. However, foams from the pitch with higher content of NG exhibit smaller anisotropic domain size and tend to be isotropic. This is because the NG prevents the coalescence of the mesophase domains during heat treatment. So it is can be concluded that the microcracks in this foams is possible due to the coefficient of the thermal expansion mismatch between in-plane and out-of-plane graphitic layers developed from the large anisotropic domains. The foams prepared from the pitch with high content of NG exhibit small anisotropic domains and so these foams exhibit less microcracks than that of foams prepared from the pure MP.
Fig. 3. Optical micrographs of the graphitized foams. (a) GF0, (b) GF05, (c) GF10 and (d) GF30.
SPecific compressive strength (MPa/g.cm3)
M.-x. Wang et al. / Composites Science and Technology 68 (2008) 2220–2223
2223
5
ter layer spacing increased with the increase of NG content. The inter layer spacing is the smallest when the NG content is 5 wt%, this maybe because the additive graphite is highly graphitized material.
4
4. Conclusions
6
3
2
1
0 0
5
10
15
20
25
30
Content of graphite (%) Fig. 4. The specific compressive strength of the foams with different amount of the natural graphite.
The manufacturing process and the properties of graphitic foams prepared from the mesophase-pitch with the addition of natural graphite were reported. It was shown that the natural graphite content can significantly affect the structure and performance of the foams such as bubble size, mesophase domain size of the ligaments and junctions and the specific compressive strength. The specific compressive strength of the foams after carbonization and graphitization was significantly increased, and the inter layer spacing decreased firstly and then increased with the increase of natural graphite content in the pitch. It is believed that through condition optimization the microcracks of the foams may be further reduced and so the specific compressive strength can be further increased. Acknowledgements
30000 28000 26000 24000 22000 20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0
002
Intensity
This work was sponsored by The National Basic Research Program 973, No. 2006CB600907 and The Programme of Introducing Talents of Discipline to Universities, No. B06006 and the authors also would like to thank Hui Wang and Hai-Yan Du for SEM test and XRD analysis. References
004
25 20
20
15
40
2θ
10 60
5 80
0
e
hit
ap
r fg
to
ten
n Co
Fig. 5. XRD spectra of the foams with different amount of natural graphite.
3.3. XRD characterization of graphitized foams Based on the visual examination of the optical images of the graphitic foams in Fig. 3, it is expected that there would be some differences of the XRD patterns of these foams. So the XRD measurement was performed on the powders of the graphitized foams. The XRD results were presented in Fig. 5, confirming the above hypothesis. These samples possessed very narrow and asymmetric d002 peaks, which shows that they contained highly ordered graphitic structure. The inter layer spacing of these foams were listed in the Table 2. It can be seen that the inter layer spacing, first, decreased and then increased with the increase of NG content, which means that the graphitization degree, first, increased and then decreased with the increase of the NG content. From Fig. 3 it can be known that the mesophse pitch domain size decreased with the increase of the NG content in the MP precursor. It is known from literature that pitch can develop large anisotropic domain yields highly graphitizable carbon, while pitch develops small anisotropic domain is less graphitizable [16], and this is the reason that the in-
[1] Gallego NC, Burchell TD, Klett JW. Irradiation effects on the graphite foam. Carbon 2006;44:618–28. [2] Yu QJ, Straatman AG, Thompson BE. Carbon-foam finned tubes in air–water heat exchangers. Appl Therm Eng 2006;26:131–43. [3] Wang MX, Wang CY, Zhang XL, Zhang W. Effects of the stabilization conditions on the structural properties of mesophase-pitch-based carbon foams. Carbon 2006;44:3371–2. [4] Klett JW, Trammell M. Parametric investigation of a graphite foam evaporator in a thermosyphon with fluorinert and a silicon CMOS chip. IEEE T Dev Mater Reliab 2004;4:626–37. [5] Barros EB, Demir NS, Filho AGS, Filho JM, Jorio A, Dresselhaus G, et al. Raman spectroscopy of graphite foams. Phys Rev B 2005;71(165422):1–5. [6] Klett JW, Knoxville, Tenn. Process for making carbon foam. US patent 6033506, 2000. [7] Wang MX, Wang CY, Zhang W. Reducing the microcracks of the mesophasepitch-based carbon foams by long-time-coking method. J Mater Sci 2006;41:6100–2. [8] Klett JW, Mcmillan AD, Gallego NC, Walls CA. The role of structure on the thermal properties of graphite foams. J Mater Sci 2004;39:3659–76. [9] Yasmin A, Luo JJ, Daniel IM. Processing of expanded graphite reinforced polymer nanocomposites. Compos Sci Technol 2006;66:1182–9. [10] Zhao H, Liu L, Wu Y, et al. Investigation of were and corrosion behavior of Cugraphite composites prepared by electroforming. Compos Sci Technol 2007;67:1210–7. [11] Cho J, Luo JJ, Daniel IM. Mechanical characterization of graphite/epoxy nanocomposites by multi-scale analysis. Compos Sci Technol 2007;67:2399–407. [12] Klett J, Hardy R, Romine E, Walls C, Burchell T. High-thermal-conductivity, mesophase-pitch-derived carbon foams: effects of precursor on the structure and properties. Carbon 2000;38:953–73. [13] Li K, Luan ZQ, Ye PW, Li Y. Primary research on nucleation mechanism of carbon foams.In: Proceeding of the eighth national symposium on new carbon materials, conference proceedings, Guilin, China; 2007. p. 121–5. [14] Wang Y, Zhong J, Wang Y, Yu M. A study of the properties of carbon foam reinforced by clay. Carbon 2006;44:1560–4. [15] Chen C, Kennel EB, Stiller AH, et al. Carbon foam derived from various precursors. Carbon 2006;44:1535–43. [16] Li TQ, Wang CY. The formation mechanism of carbonaceous mesophase. New Carbon Mater 2005;20(3):278–85.
Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Preparation and characterization of mesophase-pitch-based foam/natural graphite composites Mei-xian Wang a, Cheng-Yang Wang a,*, Tong-Qi Li b, Zi-Jun Hu b a b
Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China National Key Laboratory of Advanced Functional Composite Materials, Aerospace Research Institute of Materials and Processing Technology, Beijing 100076, PR China
a r t i c l e
i n f o
Article history: Received 1 November 2007 Accepted 7 April 2008 Available online 12 April 2008 Keywords: A. Particle-reinforced composites B. Mechanical properties C. Crack D. Optical microscopy D. Heat treatment
a b s t r a c t Graphite foams were prepared from the mesophase-pitch with addition of natural graphite and the relationship between properties and structure of these foams was investigated in detail. These graphite foams possessed high specific compressive strength and experimental results show that less microcracks appeared on the cell walls of foam by adding of natural graphite. The specific compressive strength increased from 2.0 to 5.84 MPa/g cm3 with the addition of 30 wt% natural graphite, and the inter layer spacing of graphite foams decreased with the increase of the natural graphite content in the pitch. In addition, the optical micrograph shows that the anisotropic domain size of the foam decreased with the addition of natural graphite and it also affects the compressive strength of the foams. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction The recent development of a mesophase-pitch-based graphitic foam obtained through the graphitization of carbon foams has attracted attention due to its potential for use in low-density thermal management devices, such as heat sinks, heat exchangers and a core material in sandwich structures, evaporator, etc. [1–5]. The graphitic foam is characterized by highly aligned graphitic structures around the host spherical cells, and has a very high thermal conductivity to weight ratio, graphitic foam is a promising material for applications, particularly in utilizing the high specific thermal conductivity in the aeronautics and aerospace industries. Graphitic foam can be considered to be an interconnected network of graphitic ligaments or struts having the similar structure of carbon fiber, they represent a potential alternative for carbon fiber as reinforcement in the structural composite [6]. The carbon fiber with high strength from mesophase-pitch has been achieved [7], however the graphitic foam appeared to be quite fragile. It is postulated that this may be caused mainly by the presence of defects especially the microcracks in both ligaments and junctions [8]. In this work, natural graphite with high thermal conductivity and graphitic structure was incorporated into the mesophase-pitch precursor prior to the foaming operation in order to produce natu-
* Corresponding author. Tel.: +86 22 27406183; fax: +86 22 27403389. E-mail address: [email protected] (C.-Y. Wang). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.04.007
ral graphite reinforced foam [9–11]. As a result, the imperfections of the graphitic foams such as microcracks are significantly reduced and the mechanical properties of the foams are therefore improved. In this paper the effects of the addition amount of natural graphite on the structure and mechanical properties of the as-prepared foams were investigated. 2. Experimental section 2.1. Raw materials In this research, the Mitsubishi AR naphthalene-based synthetic mesophase-pitch (MP) was used as the precursor of the graphitic foams. The properties of the pitch are listed in Table 1. The SEM micrograph of the natural graphite (NG) is given in Fig. 1. 2.2. Foaming process The powders of the MP were well-proportionally mixed with the NG powders. For the foam production, the powders were introduced into a cylindrical aluminum mold having a predetermined shape and size. The reactor was washed with nitrogen to provide an inert atmosphere. Then the reactor was heated at a heating rate of 4 °C/min up to 500 °C and soaked for 2 h to obtain green foams with different addition amounts of the NG. The as-received foams were oxidized at the temperature of 250 °C in air for 20 h and then the foams were carbonized at 1000 °C for 15 min in a purified nitrogen flow, and finally the carbonized foams were graphitized at 2800 °C. The graphitized foams were named as GFx, where x rep-
M.-x. Wang et al. / Composites Science and Technology 68 (2008) 2220–2223
2221
Table 1 Mesophase pitch properties
2.3. Characterization of foams
Properties
The morphologies of graphite foams were examined by a Philips XL30 scanning electron microscopy (SEM). The samples machined into a solid 10 10 10 mm were used for compressive test at the direction of the foam’s growth which was parallel to the gravitational direction. X-ray wide angle diffraction (XRD) was used to study its crystal structure. The diffractometer utilized Cu Ka radiation (40 kV and 200 mA). The data were collected as continuous scans, with a step size of 0.02° (2h) and a scanning rate of 10° (2h)/min between 3° and 90° (2h). In order to further evaluate the microcracks of the graphitized foams, a polarized light microscope (OM) Nikon E600 POL was used for this purpose. The sample was embedded in an epoxy resin, left overnight, cut and polished for microscopic examination of the particle cross-sections.
3
Bulk density (g/cm ) Softening point (°C) Mesophase content (%) Hydrogen/carbon (atom/atom) Benzene soluble (%) Pyridine insoluble (%)
0.65 275 100 0.59 40 45
3. Results and discussion 3.1. SEM observation of the foam structure
Fig. 1. SEM micrograph of the natural graphite.
resents the addition amount of the NG, such as GF05 means graphitic foam prepared from pitch with addition of NG by 5 wt%.
Depending on the unique properties of the foaming precursor, the cell walls can have different thickness and the bubble sizes, the microcracks can be dramatically affected and the mechanical, thermal properties can also be affected [12]. Fig. 2 shows the SEM micrographs of the graphitized foams in this study. It shows the variation of the content of the NG obviously influenced the cell structures of the foams. As the content of the NG increase, the cell size deceased firstly, and then the cell size increased again. Fig. 2a– c show that the cell size is relatively uniform, but when the content of the NG reached to about 30 wt% the cell size is apparently not uniform. Because the foaming precursor with high content of non-fusible materials, such as NG, the bubble growth is controlled by interface nucleation mechanism and so the bubble size of the
Fig. 2. SEM micrographs of the graphitized foams. (a) GF0, (b) GF05, (c) GF10 and (d) GF30.
2222
M.-x. Wang et al. / Composites Science and Technology 68 (2008) 2220–2223
as-received foams is comparatively inhomogenous [13]. The microcracks of the graphitized foams are observed by SEM micrographs. A large amount of the microcracks appeared on the cell wall and the junctions in the foams without and with low content of NG. The microcracks appear on almost whole wall of the cell but as the content of the natural graphite increased the microcracks are significantly reduced, and in the Fig. 2d the microcracks are almost disappeared even when the foams are heat-treated above 2800 °C. The amount of the microcracks is an important factor that greatly influences the mechanical properties and thermal conductivity of the graphite foams [14]. The mesophase-pitch-based foams with different amount of NG were foamed by the same process and carbonized, graphitized at the same conditions. The corresponding properties of the foams are listed in Table 2. The specific compressive strength is shown in Fig. 4. It can seen that the ultimate specific compressive strengths of the foams are 2.0, 2.85, 3.1, 5.84 MPa/g cm3 when the additive amount of NG is 0%, 5%, 10%, 30% (wt%), respectively. It shows that the specific compressive strengths of NG reinforced foams are higher than that of the foam without of NG. The strength of pitch-based foam depends not only on the foam structure, such as microcracks, but also on the properties of the precursor, such as the anisotropic domain size, which will be discussed later.
Table 2 Properties of the graphitized foams. Foam
GF0
GF05
GF10
GF15
GF20
GF25
GF30
Bulk density (g/ cm3) d002 (nm)
0.172
0.205
0.203
0.195
0.183
0.152
0.124
0.3373
0.3365
0.3366
0.3370
0.3373
0.3373
0.3378
3.2. Optical characterization During the foaming process, first, a mesophase-pitch precursor is heated in an oxygen-free environment under specific pressure and temperature. While the pitch is molten, it begins to evolve low molecular weight species and then the pitch is foamed. So the viscosity, elasticity and surface tension of the pitch precursor affected the growth and collapse of the bubbles in the molten pitch [15]. The addition of natural graphite in MP changed the viscosity and viscoelasticity of MP. So the content of natural graphite would affect the pore size and the microstructure of the foams. The optical micrographs of the representative foams are given in Fig. 3. Optical microscopy reveals that there are more microcracks in GF0 and GF05 than that in the GF10 and GF30, confirming the SEM observations. The isotropic and anisotropic nature of the pitch significantly affects the properties of the subsequent foam. Isotropic material has similar properties in all the direction, while an anisotropic material has properties which depend on the orientation of the anisotropic flow domain [14]. Foams prepared from the pitch with low content or without addition of NG exhibit large anisotropic domain in both ligaments and junctions. However, foams from the pitch with higher content of NG exhibit smaller anisotropic domain size and tend to be isotropic. This is because the NG prevents the coalescence of the mesophase domains during heat treatment. So it is can be concluded that the microcracks in this foams is possible due to the coefficient of the thermal expansion mismatch between in-plane and out-of-plane graphitic layers developed from the large anisotropic domains. The foams prepared from the pitch with high content of NG exhibit small anisotropic domains and so these foams exhibit less microcracks than that of foams prepared from the pure MP.
Fig. 3. Optical micrographs of the graphitized foams. (a) GF0, (b) GF05, (c) GF10 and (d) GF30.
SPecific compressive strength (MPa/g.cm3)
M.-x. Wang et al. / Composites Science and Technology 68 (2008) 2220–2223
2223
5
ter layer spacing increased with the increase of NG content. The inter layer spacing is the smallest when the NG content is 5 wt%, this maybe because the additive graphite is highly graphitized material.
4
4. Conclusions
6
3
2
1
0 0
5
10
15
20
25
30
Content of graphite (%) Fig. 4. The specific compressive strength of the foams with different amount of the natural graphite.
The manufacturing process and the properties of graphitic foams prepared from the mesophase-pitch with the addition of natural graphite were reported. It was shown that the natural graphite content can significantly affect the structure and performance of the foams such as bubble size, mesophase domain size of the ligaments and junctions and the specific compressive strength. The specific compressive strength of the foams after carbonization and graphitization was significantly increased, and the inter layer spacing decreased firstly and then increased with the increase of natural graphite content in the pitch. It is believed that through condition optimization the microcracks of the foams may be further reduced and so the specific compressive strength can be further increased. Acknowledgements
30000 28000 26000 24000 22000 20000 18000 16000 14000 12000 10000 8000 6000 4000 2000 0
002
Intensity
This work was sponsored by The National Basic Research Program 973, No. 2006CB600907 and The Programme of Introducing Talents of Discipline to Universities, No. B06006 and the authors also would like to thank Hui Wang and Hai-Yan Du for SEM test and XRD analysis. References
004
25 20
20
15
40
2θ
10 60
5 80
0
e
hit
ap
r fg
to
ten
n Co
Fig. 5. XRD spectra of the foams with different amount of natural graphite.
3.3. XRD characterization of graphitized foams Based on the visual examination of the optical images of the graphitic foams in Fig. 3, it is expected that there would be some differences of the XRD patterns of these foams. So the XRD measurement was performed on the powders of the graphitized foams. The XRD results were presented in Fig. 5, confirming the above hypothesis. These samples possessed very narrow and asymmetric d002 peaks, which shows that they contained highly ordered graphitic structure. The inter layer spacing of these foams were listed in the Table 2. It can be seen that the inter layer spacing, first, decreased and then increased with the increase of NG content, which means that the graphitization degree, first, increased and then decreased with the increase of the NG content. From Fig. 3 it can be known that the mesophse pitch domain size decreased with the increase of the NG content in the MP precursor. It is known from literature that pitch can develop large anisotropic domain yields highly graphitizable carbon, while pitch develops small anisotropic domain is less graphitizable [16], and this is the reason that the in-
[1] Gallego NC, Burchell TD, Klett JW. Irradiation effects on the graphite foam. Carbon 2006;44:618–28. [2] Yu QJ, Straatman AG, Thompson BE. Carbon-foam finned tubes in air–water heat exchangers. Appl Therm Eng 2006;26:131–43. [3] Wang MX, Wang CY, Zhang XL, Zhang W. Effects of the stabilization conditions on the structural properties of mesophase-pitch-based carbon foams. Carbon 2006;44:3371–2. [4] Klett JW, Trammell M. Parametric investigation of a graphite foam evaporator in a thermosyphon with fluorinert and a silicon CMOS chip. IEEE T Dev Mater Reliab 2004;4:626–37. [5] Barros EB, Demir NS, Filho AGS, Filho JM, Jorio A, Dresselhaus G, et al. Raman spectroscopy of graphite foams. Phys Rev B 2005;71(165422):1–5. [6] Klett JW, Knoxville, Tenn. Process for making carbon foam. US patent 6033506, 2000. [7] Wang MX, Wang CY, Zhang W. Reducing the microcracks of the mesophasepitch-based carbon foams by long-time-coking method. J Mater Sci 2006;41:6100–2. [8] Klett JW, Mcmillan AD, Gallego NC, Walls CA. The role of structure on the thermal properties of graphite foams. J Mater Sci 2004;39:3659–76. [9] Yasmin A, Luo JJ, Daniel IM. Processing of expanded graphite reinforced polymer nanocomposites. Compos Sci Technol 2006;66:1182–9. [10] Zhao H, Liu L, Wu Y, et al. Investigation of were and corrosion behavior of Cugraphite composites prepared by electroforming. Compos Sci Technol 2007;67:1210–7. [11] Cho J, Luo JJ, Daniel IM. Mechanical characterization of graphite/epoxy nanocomposites by multi-scale analysis. Compos Sci Technol 2007;67:2399–407. [12] Klett J, Hardy R, Romine E, Walls C, Burchell T. High-thermal-conductivity, mesophase-pitch-derived carbon foams: effects of precursor on the structure and properties. Carbon 2000;38:953–73. [13] Li K, Luan ZQ, Ye PW, Li Y. Primary research on nucleation mechanism of carbon foams.In: Proceeding of the eighth national symposium on new carbon materials, conference proceedings, Guilin, China; 2007. p. 121–5. [14] Wang Y, Zhong J, Wang Y, Yu M. A study of the properties of carbon foam reinforced by clay. Carbon 2006;44:1560–4. [15] Chen C, Kennel EB, Stiller AH, et al. Carbon foam derived from various precursors. Carbon 2006;44:1535–43. [16] Li TQ, Wang CY. The formation mechanism of carbonaceous mesophase. New Carbon Mater 2005;20(3):278–85.