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carbon composites
Materials Chemistry and Physics 97 (2006) 167–172
Effects of ozone method treating carbon fibers on mechanical properties of carbon/carbon composites Zheng Jin ∗ , Zhiqian Zhang, Linghui Meng Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China Received 15 May 2005; received in revised form 21 July 2005; accepted 1 August 2005
Abstract Ozone method was used to modify surface activity of carbon fiber to improve the compressive strength and flexural strength of carbon/carbon composites. The SEM photos of composites fracture shows that ozone method treatment of carbon fiber improved the interfacial adhesion between fibers and matrix, which was the reason of increasing the compressive strength and flexural strength of the carbon/carbon composites. The untreated and treated carbon fibers were investigated by AFM, XPS and the micro-laser Raman spectroscopy, and the contact angles were used to analyze the surface properties of carbon fibers. The results indicated that oxidation of ozone method increased the carbonyl functional group and the surface roughness, changed the graphitization degree of carbon fiber surface and improved the wettability of carbon fibers, so that the compressive strength and flexural strength of the carbon/carbon composites were enhanced. The stronger wettability decreased the formation of traps in the composites, which was one of the major reasons that improved the compressive strength and flexural strength of carbon/carbon composites. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon fiber; Ozone method; Carbon/carbon composites; Properties
1. Introduction Carbon/carbon composites are often used as structural materials for high temperature applications in electromechanical and petrochemical industries because they exhibit excellent corrosion resistance, structural stability and mechanical performance up to very high temperatures, as well as high electrical and thermal conductivity [1,2]. Carbon/carbon composites are carbon fiber reinforced carbon matrix, and it is generally clear that the physical properties of the fibers influence the performance of the composites. Due to their outstanding mechanical properties, carbon fibers are very attractive materials to be employed as reinforcement in composite materials and are applied in all kinds of industries [3–5]. Carbon fibers are included of high modulus (HM) carbon fibers and the high strength (HT) carbon fibers. High modulus carbon fibers are possibly one of the most impressive reinforcements of composites in terms of specific tensile ∗
Corresponding author. Tel.: +86 45188700111; fax: +86 45182330589. E-mail address: [email protected] (Z. Jin).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.08.002
properties. These properties are related to the high degree of orientation of the crystallites and this highly graphitic character is also responsible for high level of thermal and electric conductivity. On the other hand, the enhanced crystallinity of this type of fiber was often reflected in a lower efficiency of industrial methods for increasing the carbon surface activity, in comparison with the high strength (HT) carbon fibers in that HM carbon fibers were more resistant to oxidation than HT ones [6]. In particular, the interface in carbon fiber/carbon matrix composites not only plays the role of transferring the load between fiber and matrix, but also affects the fracture behavior of the composites. To improve the adhesion between fiber and matrix, it is necessary to increase the surface polarity, create more sites for hydrogen bonding and improve the possibility for mechanical interlocking between the fiber materials and the surrounding matrix materials, which lead to good stress transfer from the matrix materials to the fiber ones [7]. In order to improve the fiber–matrix adhesion, many surface treatments were developed which included ␥ray radiation, electrochemically oxidation, plasma treatment, ozone (O3 ), etc. [8–11]. In this present study the mechanical
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Z. Jin et al. / Materials Chemistry and Physics 97 (2006) 167–172
characteristics of carbon/carbon composites and the surface properties of carbon fibers were investigated and AFM, XPS, the micro-laser Raman spectroscopy and the contact angles were used to appraise the surface structural changes of high modulus carbon fibers treated by a ozone method.
2. Experimental 2.1. Ozone method treatment of carbon fibers Take some carbon fibers and put into ozone calcar. After vacuum handling, the ozone gas was introduced to the reactor at room temperature, and then the reactor was heated to the treatment temperature. After reaction, the samples were cooled to room temperature, and then the reactive gas was purged from the reactor with nitrogen. In the case of the reaction at room temperature, the reactor was cooled and evacuated in a cooling bath prior to charging ozone gas. The reactor was removed from the cooling bath after purging ozone gas with nitrogen. The ozone gas pressure was 0.4 MPa and the nominal reaction time and treatment temperature were adjusted to need. The flow rate of O3 gas was 0.12 m3 h−1 , and the ozone treatment speed and the distance between valves were 5 mm s−1 and 200 mm, respectively. The weight of carbon fibers samples was about 10 g. The ozone method treatment device was shown in Fig. 1. 2.2. Manufacturing of carbon/carbon composites Composites were prepared by unsized pitch-based carbon fiber using ultrasonic disperse technique for manufacturing prepared mixture with subsequent hot pressing. And, the fabrications were fabricated in a hot-press at 50 MPa at 150 ◦ C for 150 min with a mold method in a conventional composites processing. The fabrications were baked at 1000 ◦ C, and they were baked again after were dipped performs pitch. The fiber volume fraction of bulk specimens was about 7% for all composites.
Fig. 1. The structure chart of ozone method treatment apparatus: (1) air tank; (2) generator of ozone; (3) circulation of cold water; (4) flowmeter; (5) thermometer; (6) calcar; (7) thermostat; (8) absorption meter of exhaust gas.
2.3. Analysis method The effect of the ozone method treatment on the composites fracture morphology was observed using Hitachi S4700 scanning electron microscopy (SEM). Typical values of voltage and working distance of operation were 2 kV and 8–10 mm, respectively. Carbon fibers were examined on a Russian solver P47 atom force microscopy (AFM). The Xray photoelectron spectroscopy (XPS), also known as ESCA, measurement of fiber surface was performed with an Xray photoelectron spectrometer (Perkin-Elmer, PHI 5300) equipped with magnesium X-ray source. The base pressure in the sample chamber was controlled in the range 1028–1029 Torr. The contact angles were measured to get surface energy in order to evaluate the wettability of carbon fiber. The Raman spectra were measured by a JY-T64000 triplegrating spectrometer, using the 514.5 nm line of a SP165-09 Ar-ion laser with a power of <30 mW, without heat effect. The micro-laser Raman spectroscopy analysis was carried out on the surface areas of interest for the carbon fiber, more attentions were paid to the fiber before and after ozone method treatment.
3. Results and discussion 3.1. Effects of ozone treatment on the compressive strength and flexural strength of the carbon/carbon composites Carbon/carbon composites were prepared with the untreated and ozone treated carbon fibers. The test results of the flexural strength and compressive strength of carbon/carbon composites, as shown in Fig. 2, indicated that ozone treatment increased compressive strength and flexural strength of carbon/carbon composites. The reason for this would be that the chemical interaction between the carbon fibers and pitch in the matrix and increasing of surface roughness of carbon fibers conduced to the interfaces of carbon fibers and the carbon matrix strengthened. It had been shown
Fig. 2. The compressive strength and flexural strength of the carbon/carbon composites: (a) untreated carbon fiber; (b) the 100 ◦ C for 6 min ozone method; (c) the 120 ◦ C for 3 min ozone method; (d) the 120 ◦ C for 6 min ozone method; (e) the 160 ◦ C for 6 min ozone method.
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Fig. 3. SEM images of the carbon/carbon composites fractures: (a) untreated carbon fiber; (b) after 120 ◦ C for 6 min ozone method treatment.
that chemical bonding is important in the increase of adhesion between the carbon fibers and the carbon matrix. It can be recognized from Fig. 2 that for carbon/carbon composites, the ozone treatment in 120 ◦ C and 6 min improved the adhesion much better than others. When temperature was 160 ◦ C as in Fig. 2(e), the ozone decomposed too fast to oxidize carbon fibers. So the compressive strength and flexural strength of carbon/carbon composites decreased. In conclusion, the best condition of ozone treatment is in 120 ◦ C and 6 min. SEM images of the carbon/carbon composites fractures were shown in Fig. 3. There were some cracks and holes in fractures of untreated carbon/carbon composites. Ruptures happened at interface between carbon fiber and matrix, and surface of carbon fibers exposed were smooth. That was typically interface rupture. After ozone treatment, adhesion was better between carbon fibers and matrix. It would be seen that there was attachment on surface of carbon fiber exposed. Ruptures were engendered in both matrix and interface between fibers and matrix. 3.2. Effect of ozone treatment on wettability of carbon fiber It could be seen from Fig. 4 that surface property of carbon fiber was improved after ozone treatment and its results were shown in Table 1. Table 1 shows that after ozone treatment, the contact angles both of water and glycol on carbon fiber decreased. And surface energy of them increased from 18.04 to 25.35 mJ m−2 . The percentage of increase was 40. Ozone treatment leads to increase in surface energy because ozone treatment changed the property and constitution of car-
Fig. 4. The dynamic curve of wetting of octane on carbon fiber before and after ozone method treatment.
bon fiber. Surface of untreated carbon fiber was composed of gases, oxides and impurities, which decreased surface energy of carbon fiber. After ozone treatment, oxides and impurities on carbon fiber were oxidized and surface of carbon fiber were fluted. After ozone treatment, polar component of surface energy of carbon fiber increased and dispersive component of surface energy decreased. That were the efforts the both increasing of polar function groups and surface roughness of carbon fiber. 3.3. Morphology analysis of the carbon fibers surface Longitudinal surfaces of the carbon fibers under study were examined with AFM before and after the surface treatment. The surface of the untreated fiber was smoother than
Table 1 Surface property of untreated and after ozone method treated carbon fibers Contact angle (◦ )
Surface energy (mJ m−2 )
Water
Octane
Dispersive surface energy, γsd
Polarity surface energy, γs
All surface energy, γs
Untreated
89.90
82.04
6.68
11.36
18.04
Ozone method 100 ◦ C for 6 min 120 ◦ C for 3 min 120 ◦ C for 6 min 160 ◦ C for 6 min
84.86 81.78 78.81 82.28
80.17 77.26 74.39 78.94
7.08 8.18 8.29 7.29
14.14 15.13 17.06 15.02
21.22 23.31 25.35 22.31
Carbon fiber
p
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Fig. 5. AFM images of chopped high modulus carbon fiber: (a) untreated carbon fiber; (b) after 120 ◦ C for 6 min ozone method treatment.
Fig. 6. C 1s XPS spectra: (a) untreated carbon fiber; (b) after 120 ◦ C for 6 min ozone method treatment.
those of treated one. AFM images, which revealed the change on the surface of the carbon fibers in the high resolution after ozone treated, were shown in Fig. 5. It should be noted that after the carbon fibers were ozone treatment, the shallow grooves became manifold hills that result in the increase of surface roughness. In other words, ozone treatment increased the surface area of carbon fiber that was propitious to reinforce the interface between carbon fiber and matrix. 3.4. Functional group analysis of the carbon fibers surface The surface atomic compositions obtained by XPS analysis were presented in Table 2. It could be seen that the surface of the carbon fibers was mainly composed of carbon, oxygen, and silicon. The carbon and oxygen contents of the untreated carbon fiber were 90.29 and 9.36%. Both of the O/C ratio and the Si/C ratio of ozone treated carbon fiber increased compare with the untreated sample due to the increase in Table 2 XPS component content results of untreated and after 120 ◦ C for 6 min treated carbon fibers Carbon fiber
Component content (%)
O/C
C
O
Si
Untreated Ozone treated
90.29 75.73
9.36 22.99
0.35 1.82
0.1037 0.3036
oxygen-containing functional groups on the carbon fiber surfaces. For the detailed studies of functional groups, peaks of C 1s were studied. Fig. 6 shows C 1s peaks of XPS spectra of the carbon fiber before (a: untreated) and after (b: ozone treated). Also, the linearity of the variation confirmed again the validity of our XPS analysis. In order to obtain the best fit between the experimental and the synthesized spectra, a computer simulation estimated the intensity contribution of each functional component peak. Typical XPS spectra of the C 1s peak region at 284.6 eV deconvoluted into surface functional group contributions were shown in Fig. 6a and b for the carbon fiber samples, respectively. It was found that the carbon 1s peaks could be fitted to four line shapes with binding energies at 284.6, 286.2, 287.6 and 289.1 eV [10–13]. As a result, the total area of the C 1s peak region of the untreated carbon fiber sample consisted of 86.01% C–C, 1.93% C–O, 7.52% C O and 4.54% O–C O. The total area of the C 1s peak region of the ozone treated carbon fiber sample consisted of 71.26% C–C, 6.91% C–O, 14.08% C O, and 7.69% O–C O. These data are summarized in Table 3. From this result, it could be seen that the C–C peaks both before and after the surface treatment were the major surface carbon functional component. Due to the ozone treatment process in the carbon fiber, the percentage of C–C was lower and the percentage of C O, C–O and O–C O were higher before the surface treatment. From these results, it could be concluded that the surface carbon functional component was significantly different for the
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Table 3 XPS C 1s curve fit results of untreated and after 120 ◦ C for 6 min treated carbon fibers Carbon fiber
Functional group (%)
Untreated Ozone method
Peak 1, C–C, 284.6 eV
Peak 2, C–OH, 286.2 eV
Peak 3, C O 287.6 eV
Peak 4, COOH/COOR 289.1 eV
86.01 71.26
1.93 6.97
7.52 14.08
4.54 7.69
Table 4 The results of carbon fiber surface Raman curve fit of untreated and 120 ◦ C for 6 min ozone method treated carbon fiber Carbon fiber
νG (cm−1 )
νD (cm−1 )
FWHMG (cm−1 )
FWHMD (cm−1 )
G
D
ν ∼ 1179 cm−1
ν ∼ 1502 cm−1
IG /ID
Untreated Ozone method
1598 1597
1352 1354
83.5 81.8
156.4 160.3
28.86 27.57
52.73 55.33
6.57 6.85
11.84 10.25
0.5473 0.4983
Fig. 7. Raman spectra of carbon fiber: (a) untreated carbon fiber; (b) after 120 ◦ C for 6 min ozone method treatment.
carbon fibers before and after the surface treatment. In other words, the current thermal oxidation process could increase the density of carbonyl functional group on the surface of carbon fiber. 3.5. Crystallinity analysis of the carbon fibers surface Among all the spectra of Fig. 7, the peaks at 1360 and 1580 cm−1 were clear, and especially each of them has different profile and height. This shows the degree of graphitization of the carbon fiber is different before and after ozone method treatment. Nowadays, laser Raman spectroscopy had been used as a very effective technique to evaluate the degree of graphitization, and many studies on various carbon–graphite materials had been carried out by this method [14]. Microlaser Raman spectroscopy, especially, made it possible to analyze a microscopic surface area of interest. Usually, Raman spectra of most of the carbon–graphite materials contained two peaks at around 1580 and 1360 cm−1 , except for natural graphite, which had a single sharp Raman band at 1580 cm−1 . The 1580 cm−1 peak was known to correspond to the graphite structure and the 1360 cm−1 peak was correlated with graphitized carbon structure and the ratio of the integrated intensities of the two peaks, I1580 /I1360 , had been considered to be a good parameter to estimate the degree of graphitization. The smaller the ratio of the I1580 /I1360 , the lower the degree of graphitization of the carbon materials. The 1580 cm−1 peak mainly came from the flex vibration
of chemical bonds in atomic hexagonal net plane, however, the 1360 cm−1 band of the spectra was closely associated with one kind of local unsymmetrical structure, which existed in graphitized carbon or non-integrity graphite crystals containing defects introduced by grinding process, it could be thought that this kind of local structure were obtained by loss of symmetry or conversion from a hexagonal symmetry to a much lower symmetry. From the ratio of integral intensities of two peaks in the Raman spectra of carbon before and after ozone method treatment, some results were concluded. The results of Table 4 shows that the fiber was changed on the degree of graphitization after ozone method treatment, the treated fiber had the lowest crystallinity.
4. Conclusion The results of this study revealed that the compressive strength and flexural strength of carbon/carbon composites increased that the carbon fibers had been ozone treated. And the SEM photos of composites fracture shows ozone method of carbon fiber improved the interfacial adhesion between fibers and matrix. The AFM results indicated that the ozone method treatment changed the surface roughness values of the fibers. The results of Raman shows that the graphitization degree of fiber was changed after ozone method treatment, and the treated fiber has the lowest crystallinity. The surface energy of ozone method treated carbon fibers increased com-
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pared to untreated carbon fibers. Surface functional groups and the surface energy of fibers are considered to be critical properties in predicting the adhesion between fiber and matrix. References [1] Jia-Xiang Zhao, Carbon fibers and composites materials in HEXEL corporation, U.S.A., Thermo Setting Resin 16 (3) (2001) 44–48. [2] Iwashita Norio, Psomiadou Eleni, Sawada Yoshihiro, Effect of coupling treatment of carbon fiber surface on mechanical properties of carbon fiber reinforced carbon composites, Composites 29 (Part A) (1998) 965–972. [3] J.I. Paredes, A. Martınez-Alonso, J.M.D. Tascon, Surface characterization of submicron vapor grown carbon fibers by scanning tunneling microscopy, Carbon 39 (2001) 1575–1587. [4] Oshima Akihiro, Udagawa Akira, Morita Yousuke, Radiation processing for carbon fiber-reinforced polytetra-uoroethylene composites materials, Radiat. Phys. Chem. 60 (2001) 95–100. [5] Miguel A. Montes-Moran, Robert J. Young, Raman spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composites. Part I. Fibre characterization, Carbon 40 (2002) 845–855. [6] Soo-Jin Park, Min-Kang Seo, Kyong-Yop Rhee, Studies on mechanical interfacial properties of oxy-fluorinated carbon fibers-reinforced composites, Mater. Sci. Eng. A 356 (2003) 219–226.
[7] T. Oku, A. Kurumada, K. Kawamata, M. Inagaki, Effects of argon ion irradiation on the microstructures and physical properties of carbon fibers, J. Nucl. Mater. 303 (2002) 242–245. [8] C.U. Pittman Jr., W. Jiang, Z.R. Yue, S. Gardner, L. Wang, H. Toghiani, C.A. Leon y Leon, Surface properties of electrochemically oxidized carbon fibers, Carbon 37 (1999) 1797–1807. [9] Xu-Li Fu, Wei-Ming Lu, D.D.L Chung, Ozone treatment of carbon fiber for reinforcing cement, Carbon 36 (9) (1998) 1337– 1345. [10] Heng-Yi Ma, Yu-Dong Huang, Zhi-Qian Zhang, The effect of ␥ray radiation on carbon fiber structure, High Technol. Lett. 3 (2001) 101–106. [11] Rene I. Gonzalez, Synthesis and in situ atomic oxygen erosion studies of space-survivable hybrid organic–inorganic polyhedral oligomeric silsesquioxane polymers, A Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy, 2002, pp. 48–57. [12] Yu-Wen Liu, Zhi-Qian Zhang, Feng-Mei Li, Effect to febirradiation on the structures of carbon fiber surface, J. Radiat. Res. Radiat. Process 20 (3) (2002) 197–203. [13] L. Dobiasova, V. Stary, P. Glogar, V. Valvoda, Analysis of carbon fibers and carbon composites by asymmetric X-ray diffraction technique, Carbon 37 (1999) 421–425. [14] Lin-Hua Zou, Bai-Yun Huang, Yong Huang, Qi-Zhong Huang, Chang-An Wang, An investigation of heterogeneity of the degree of graphitization in carbon–carbon composites, Mater. Chem. Phys. 82 (2003) 654–662.
Effects of ozone method treating carbon fibers on mechanical properties of carbon/carbon composites Zheng Jin ∗ , Zhiqian Zhang, Linghui Meng Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, PR China Received 15 May 2005; received in revised form 21 July 2005; accepted 1 August 2005
Abstract Ozone method was used to modify surface activity of carbon fiber to improve the compressive strength and flexural strength of carbon/carbon composites. The SEM photos of composites fracture shows that ozone method treatment of carbon fiber improved the interfacial adhesion between fibers and matrix, which was the reason of increasing the compressive strength and flexural strength of the carbon/carbon composites. The untreated and treated carbon fibers were investigated by AFM, XPS and the micro-laser Raman spectroscopy, and the contact angles were used to analyze the surface properties of carbon fibers. The results indicated that oxidation of ozone method increased the carbonyl functional group and the surface roughness, changed the graphitization degree of carbon fiber surface and improved the wettability of carbon fibers, so that the compressive strength and flexural strength of the carbon/carbon composites were enhanced. The stronger wettability decreased the formation of traps in the composites, which was one of the major reasons that improved the compressive strength and flexural strength of carbon/carbon composites. © 2005 Elsevier B.V. All rights reserved. Keywords: Carbon fiber; Ozone method; Carbon/carbon composites; Properties
1. Introduction Carbon/carbon composites are often used as structural materials for high temperature applications in electromechanical and petrochemical industries because they exhibit excellent corrosion resistance, structural stability and mechanical performance up to very high temperatures, as well as high electrical and thermal conductivity [1,2]. Carbon/carbon composites are carbon fiber reinforced carbon matrix, and it is generally clear that the physical properties of the fibers influence the performance of the composites. Due to their outstanding mechanical properties, carbon fibers are very attractive materials to be employed as reinforcement in composite materials and are applied in all kinds of industries [3–5]. Carbon fibers are included of high modulus (HM) carbon fibers and the high strength (HT) carbon fibers. High modulus carbon fibers are possibly one of the most impressive reinforcements of composites in terms of specific tensile ∗
Corresponding author. Tel.: +86 45188700111; fax: +86 45182330589. E-mail address: [email protected] (Z. Jin).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.08.002
properties. These properties are related to the high degree of orientation of the crystallites and this highly graphitic character is also responsible for high level of thermal and electric conductivity. On the other hand, the enhanced crystallinity of this type of fiber was often reflected in a lower efficiency of industrial methods for increasing the carbon surface activity, in comparison with the high strength (HT) carbon fibers in that HM carbon fibers were more resistant to oxidation than HT ones [6]. In particular, the interface in carbon fiber/carbon matrix composites not only plays the role of transferring the load between fiber and matrix, but also affects the fracture behavior of the composites. To improve the adhesion between fiber and matrix, it is necessary to increase the surface polarity, create more sites for hydrogen bonding and improve the possibility for mechanical interlocking between the fiber materials and the surrounding matrix materials, which lead to good stress transfer from the matrix materials to the fiber ones [7]. In order to improve the fiber–matrix adhesion, many surface treatments were developed which included ␥ray radiation, electrochemically oxidation, plasma treatment, ozone (O3 ), etc. [8–11]. In this present study the mechanical
168
Z. Jin et al. / Materials Chemistry and Physics 97 (2006) 167–172
characteristics of carbon/carbon composites and the surface properties of carbon fibers were investigated and AFM, XPS, the micro-laser Raman spectroscopy and the contact angles were used to appraise the surface structural changes of high modulus carbon fibers treated by a ozone method.
2. Experimental 2.1. Ozone method treatment of carbon fibers Take some carbon fibers and put into ozone calcar. After vacuum handling, the ozone gas was introduced to the reactor at room temperature, and then the reactor was heated to the treatment temperature. After reaction, the samples were cooled to room temperature, and then the reactive gas was purged from the reactor with nitrogen. In the case of the reaction at room temperature, the reactor was cooled and evacuated in a cooling bath prior to charging ozone gas. The reactor was removed from the cooling bath after purging ozone gas with nitrogen. The ozone gas pressure was 0.4 MPa and the nominal reaction time and treatment temperature were adjusted to need. The flow rate of O3 gas was 0.12 m3 h−1 , and the ozone treatment speed and the distance between valves were 5 mm s−1 and 200 mm, respectively. The weight of carbon fibers samples was about 10 g. The ozone method treatment device was shown in Fig. 1. 2.2. Manufacturing of carbon/carbon composites Composites were prepared by unsized pitch-based carbon fiber using ultrasonic disperse technique for manufacturing prepared mixture with subsequent hot pressing. And, the fabrications were fabricated in a hot-press at 50 MPa at 150 ◦ C for 150 min with a mold method in a conventional composites processing. The fabrications were baked at 1000 ◦ C, and they were baked again after were dipped performs pitch. The fiber volume fraction of bulk specimens was about 7% for all composites.
Fig. 1. The structure chart of ozone method treatment apparatus: (1) air tank; (2) generator of ozone; (3) circulation of cold water; (4) flowmeter; (5) thermometer; (6) calcar; (7) thermostat; (8) absorption meter of exhaust gas.
2.3. Analysis method The effect of the ozone method treatment on the composites fracture morphology was observed using Hitachi S4700 scanning electron microscopy (SEM). Typical values of voltage and working distance of operation were 2 kV and 8–10 mm, respectively. Carbon fibers were examined on a Russian solver P47 atom force microscopy (AFM). The Xray photoelectron spectroscopy (XPS), also known as ESCA, measurement of fiber surface was performed with an Xray photoelectron spectrometer (Perkin-Elmer, PHI 5300) equipped with magnesium X-ray source. The base pressure in the sample chamber was controlled in the range 1028–1029 Torr. The contact angles were measured to get surface energy in order to evaluate the wettability of carbon fiber. The Raman spectra were measured by a JY-T64000 triplegrating spectrometer, using the 514.5 nm line of a SP165-09 Ar-ion laser with a power of <30 mW, without heat effect. The micro-laser Raman spectroscopy analysis was carried out on the surface areas of interest for the carbon fiber, more attentions were paid to the fiber before and after ozone method treatment.
3. Results and discussion 3.1. Effects of ozone treatment on the compressive strength and flexural strength of the carbon/carbon composites Carbon/carbon composites were prepared with the untreated and ozone treated carbon fibers. The test results of the flexural strength and compressive strength of carbon/carbon composites, as shown in Fig. 2, indicated that ozone treatment increased compressive strength and flexural strength of carbon/carbon composites. The reason for this would be that the chemical interaction between the carbon fibers and pitch in the matrix and increasing of surface roughness of carbon fibers conduced to the interfaces of carbon fibers and the carbon matrix strengthened. It had been shown
Fig. 2. The compressive strength and flexural strength of the carbon/carbon composites: (a) untreated carbon fiber; (b) the 100 ◦ C for 6 min ozone method; (c) the 120 ◦ C for 3 min ozone method; (d) the 120 ◦ C for 6 min ozone method; (e) the 160 ◦ C for 6 min ozone method.
Z. Jin et al. / Materials Chemistry and Physics 97 (2006) 167–172
169
Fig. 3. SEM images of the carbon/carbon composites fractures: (a) untreated carbon fiber; (b) after 120 ◦ C for 6 min ozone method treatment.
that chemical bonding is important in the increase of adhesion between the carbon fibers and the carbon matrix. It can be recognized from Fig. 2 that for carbon/carbon composites, the ozone treatment in 120 ◦ C and 6 min improved the adhesion much better than others. When temperature was 160 ◦ C as in Fig. 2(e), the ozone decomposed too fast to oxidize carbon fibers. So the compressive strength and flexural strength of carbon/carbon composites decreased. In conclusion, the best condition of ozone treatment is in 120 ◦ C and 6 min. SEM images of the carbon/carbon composites fractures were shown in Fig. 3. There were some cracks and holes in fractures of untreated carbon/carbon composites. Ruptures happened at interface between carbon fiber and matrix, and surface of carbon fibers exposed were smooth. That was typically interface rupture. After ozone treatment, adhesion was better between carbon fibers and matrix. It would be seen that there was attachment on surface of carbon fiber exposed. Ruptures were engendered in both matrix and interface between fibers and matrix. 3.2. Effect of ozone treatment on wettability of carbon fiber It could be seen from Fig. 4 that surface property of carbon fiber was improved after ozone treatment and its results were shown in Table 1. Table 1 shows that after ozone treatment, the contact angles both of water and glycol on carbon fiber decreased. And surface energy of them increased from 18.04 to 25.35 mJ m−2 . The percentage of increase was 40. Ozone treatment leads to increase in surface energy because ozone treatment changed the property and constitution of car-
Fig. 4. The dynamic curve of wetting of octane on carbon fiber before and after ozone method treatment.
bon fiber. Surface of untreated carbon fiber was composed of gases, oxides and impurities, which decreased surface energy of carbon fiber. After ozone treatment, oxides and impurities on carbon fiber were oxidized and surface of carbon fiber were fluted. After ozone treatment, polar component of surface energy of carbon fiber increased and dispersive component of surface energy decreased. That were the efforts the both increasing of polar function groups and surface roughness of carbon fiber. 3.3. Morphology analysis of the carbon fibers surface Longitudinal surfaces of the carbon fibers under study were examined with AFM before and after the surface treatment. The surface of the untreated fiber was smoother than
Table 1 Surface property of untreated and after ozone method treated carbon fibers Contact angle (◦ )
Surface energy (mJ m−2 )
Water
Octane
Dispersive surface energy, γsd
Polarity surface energy, γs
All surface energy, γs
Untreated
89.90
82.04
6.68
11.36
18.04
Ozone method 100 ◦ C for 6 min 120 ◦ C for 3 min 120 ◦ C for 6 min 160 ◦ C for 6 min
84.86 81.78 78.81 82.28
80.17 77.26 74.39 78.94
7.08 8.18 8.29 7.29
14.14 15.13 17.06 15.02
21.22 23.31 25.35 22.31
Carbon fiber
p
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Fig. 5. AFM images of chopped high modulus carbon fiber: (a) untreated carbon fiber; (b) after 120 ◦ C for 6 min ozone method treatment.
Fig. 6. C 1s XPS spectra: (a) untreated carbon fiber; (b) after 120 ◦ C for 6 min ozone method treatment.
those of treated one. AFM images, which revealed the change on the surface of the carbon fibers in the high resolution after ozone treated, were shown in Fig. 5. It should be noted that after the carbon fibers were ozone treatment, the shallow grooves became manifold hills that result in the increase of surface roughness. In other words, ozone treatment increased the surface area of carbon fiber that was propitious to reinforce the interface between carbon fiber and matrix. 3.4. Functional group analysis of the carbon fibers surface The surface atomic compositions obtained by XPS analysis were presented in Table 2. It could be seen that the surface of the carbon fibers was mainly composed of carbon, oxygen, and silicon. The carbon and oxygen contents of the untreated carbon fiber were 90.29 and 9.36%. Both of the O/C ratio and the Si/C ratio of ozone treated carbon fiber increased compare with the untreated sample due to the increase in Table 2 XPS component content results of untreated and after 120 ◦ C for 6 min treated carbon fibers Carbon fiber
Component content (%)
O/C
C
O
Si
Untreated Ozone treated
90.29 75.73
9.36 22.99
0.35 1.82
0.1037 0.3036
oxygen-containing functional groups on the carbon fiber surfaces. For the detailed studies of functional groups, peaks of C 1s were studied. Fig. 6 shows C 1s peaks of XPS spectra of the carbon fiber before (a: untreated) and after (b: ozone treated). Also, the linearity of the variation confirmed again the validity of our XPS analysis. In order to obtain the best fit between the experimental and the synthesized spectra, a computer simulation estimated the intensity contribution of each functional component peak. Typical XPS spectra of the C 1s peak region at 284.6 eV deconvoluted into surface functional group contributions were shown in Fig. 6a and b for the carbon fiber samples, respectively. It was found that the carbon 1s peaks could be fitted to four line shapes with binding energies at 284.6, 286.2, 287.6 and 289.1 eV [10–13]. As a result, the total area of the C 1s peak region of the untreated carbon fiber sample consisted of 86.01% C–C, 1.93% C–O, 7.52% C O and 4.54% O–C O. The total area of the C 1s peak region of the ozone treated carbon fiber sample consisted of 71.26% C–C, 6.91% C–O, 14.08% C O, and 7.69% O–C O. These data are summarized in Table 3. From this result, it could be seen that the C–C peaks both before and after the surface treatment were the major surface carbon functional component. Due to the ozone treatment process in the carbon fiber, the percentage of C–C was lower and the percentage of C O, C–O and O–C O were higher before the surface treatment. From these results, it could be concluded that the surface carbon functional component was significantly different for the
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Table 3 XPS C 1s curve fit results of untreated and after 120 ◦ C for 6 min treated carbon fibers Carbon fiber
Functional group (%)
Untreated Ozone method
Peak 1, C–C, 284.6 eV
Peak 2, C–OH, 286.2 eV
Peak 3, C O 287.6 eV
Peak 4, COOH/COOR 289.1 eV
86.01 71.26
1.93 6.97
7.52 14.08
4.54 7.69
Table 4 The results of carbon fiber surface Raman curve fit of untreated and 120 ◦ C for 6 min ozone method treated carbon fiber Carbon fiber
νG (cm−1 )
νD (cm−1 )
FWHMG (cm−1 )
FWHMD (cm−1 )
G
D
ν ∼ 1179 cm−1
ν ∼ 1502 cm−1
IG /ID
Untreated Ozone method
1598 1597
1352 1354
83.5 81.8
156.4 160.3
28.86 27.57
52.73 55.33
6.57 6.85
11.84 10.25
0.5473 0.4983
Fig. 7. Raman spectra of carbon fiber: (a) untreated carbon fiber; (b) after 120 ◦ C for 6 min ozone method treatment.
carbon fibers before and after the surface treatment. In other words, the current thermal oxidation process could increase the density of carbonyl functional group on the surface of carbon fiber. 3.5. Crystallinity analysis of the carbon fibers surface Among all the spectra of Fig. 7, the peaks at 1360 and 1580 cm−1 were clear, and especially each of them has different profile and height. This shows the degree of graphitization of the carbon fiber is different before and after ozone method treatment. Nowadays, laser Raman spectroscopy had been used as a very effective technique to evaluate the degree of graphitization, and many studies on various carbon–graphite materials had been carried out by this method [14]. Microlaser Raman spectroscopy, especially, made it possible to analyze a microscopic surface area of interest. Usually, Raman spectra of most of the carbon–graphite materials contained two peaks at around 1580 and 1360 cm−1 , except for natural graphite, which had a single sharp Raman band at 1580 cm−1 . The 1580 cm−1 peak was known to correspond to the graphite structure and the 1360 cm−1 peak was correlated with graphitized carbon structure and the ratio of the integrated intensities of the two peaks, I1580 /I1360 , had been considered to be a good parameter to estimate the degree of graphitization. The smaller the ratio of the I1580 /I1360 , the lower the degree of graphitization of the carbon materials. The 1580 cm−1 peak mainly came from the flex vibration
of chemical bonds in atomic hexagonal net plane, however, the 1360 cm−1 band of the spectra was closely associated with one kind of local unsymmetrical structure, which existed in graphitized carbon or non-integrity graphite crystals containing defects introduced by grinding process, it could be thought that this kind of local structure were obtained by loss of symmetry or conversion from a hexagonal symmetry to a much lower symmetry. From the ratio of integral intensities of two peaks in the Raman spectra of carbon before and after ozone method treatment, some results were concluded. The results of Table 4 shows that the fiber was changed on the degree of graphitization after ozone method treatment, the treated fiber had the lowest crystallinity.
4. Conclusion The results of this study revealed that the compressive strength and flexural strength of carbon/carbon composites increased that the carbon fibers had been ozone treated. And the SEM photos of composites fracture shows ozone method of carbon fiber improved the interfacial adhesion between fibers and matrix. The AFM results indicated that the ozone method treatment changed the surface roughness values of the fibers. The results of Raman shows that the graphitization degree of fiber was changed after ozone method treatment, and the treated fiber has the lowest crystallinity. The surface energy of ozone method treated carbon fibers increased com-
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pared to untreated carbon fibers. Surface functional groups and the surface energy of fibers are considered to be critical properties in predicting the adhesion between fiber and matrix. References [1] Jia-Xiang Zhao, Carbon fibers and composites materials in HEXEL corporation, U.S.A., Thermo Setting Resin 16 (3) (2001) 44–48. [2] Iwashita Norio, Psomiadou Eleni, Sawada Yoshihiro, Effect of coupling treatment of carbon fiber surface on mechanical properties of carbon fiber reinforced carbon composites, Composites 29 (Part A) (1998) 965–972. [3] J.I. Paredes, A. Martınez-Alonso, J.M.D. Tascon, Surface characterization of submicron vapor grown carbon fibers by scanning tunneling microscopy, Carbon 39 (2001) 1575–1587. [4] Oshima Akihiro, Udagawa Akira, Morita Yousuke, Radiation processing for carbon fiber-reinforced polytetra-uoroethylene composites materials, Radiat. Phys. Chem. 60 (2001) 95–100. [5] Miguel A. Montes-Moran, Robert J. Young, Raman spectroscopy study of HM carbon fibres: effect of plasma treatment on the interfacial properties of single fibre/epoxy composites. Part I. Fibre characterization, Carbon 40 (2002) 845–855. [6] Soo-Jin Park, Min-Kang Seo, Kyong-Yop Rhee, Studies on mechanical interfacial properties of oxy-fluorinated carbon fibers-reinforced composites, Mater. Sci. Eng. A 356 (2003) 219–226.
[7] T. Oku, A. Kurumada, K. Kawamata, M. Inagaki, Effects of argon ion irradiation on the microstructures and physical properties of carbon fibers, J. Nucl. Mater. 303 (2002) 242–245. [8] C.U. Pittman Jr., W. Jiang, Z.R. Yue, S. Gardner, L. Wang, H. Toghiani, C.A. Leon y Leon, Surface properties of electrochemically oxidized carbon fibers, Carbon 37 (1999) 1797–1807. [9] Xu-Li Fu, Wei-Ming Lu, D.D.L Chung, Ozone treatment of carbon fiber for reinforcing cement, Carbon 36 (9) (1998) 1337– 1345. [10] Heng-Yi Ma, Yu-Dong Huang, Zhi-Qian Zhang, The effect of ␥ray radiation on carbon fiber structure, High Technol. Lett. 3 (2001) 101–106. [11] Rene I. Gonzalez, Synthesis and in situ atomic oxygen erosion studies of space-survivable hybrid organic–inorganic polyhedral oligomeric silsesquioxane polymers, A Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy, 2002, pp. 48–57. [12] Yu-Wen Liu, Zhi-Qian Zhang, Feng-Mei Li, Effect to febirradiation on the structures of carbon fiber surface, J. Radiat. Res. Radiat. Process 20 (3) (2002) 197–203. [13] L. Dobiasova, V. Stary, P. Glogar, V. Valvoda, Analysis of carbon fibers and carbon composites by asymmetric X-ray diffraction technique, Carbon 37 (1999) 421–425. [14] Lin-Hua Zou, Bai-Yun Huang, Yong Huang, Qi-Zhong Huang, Chang-An Wang, An investigation of heterogeneity of the degree of graphitization in carbon–carbon composites, Mater. Chem. Phys. 82 (2003) 654–662.