- Email: [email protected]
Calorimetric study of carbon nanotubes and aluminum
Materials Letters 61 (2007) 3221 – 3223 www.elsevier.com/locate/matlet
Calorimetric study of carbon nanotubes and aluminum C.F Deng ⁎, X.X Zhang, D.Z Wang, Y.X Ma School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box433, Harbin 150001, China Received 17 September 2006; accepted 8 November 2006 Available online 28 November 2006
Abstract Differential scanning calorimeter (DSC) was used to investigate apparent activation energy and reaction order of the reaction of carbon nanotubes and aluminum by Kissinger equation and Crane equation under non-isothermal condition. The reaction product was examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results show that carbon nanotubes react with aluminum and form Al4C3 phases with needle shape. The peak temperature of the reaction of carbon nanotubes and aluminum is found to depend on the heating rate during the continuous heating. Apparent activation energy and reaction order of the reaction of carbon nanotubes and aluminum are 194.01 and 0.92 kJ/mol, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Metals and alloys; Differential scanning calorimeter; Kinetics
1. Introduction Carbon nanotubes (CNTs) have been extensively investigated since it was discovered in 1991. One of potential applications of CNTs is the reinforcement of composites to overcome the performance limits of conventional materials [1]. In recent years, CNTs have been regarded as an ideal reinforcement to improve mechanical performance of monolithic materials owing to their extremely high elastic modulus, strength and aspect ratio [2]. Although CNT reinforced polymer [3], and ceramic [4] based composites have been widely investigated, metal matrix composites were limitedly reported [5,6]. One of the reasons could be due to that CNTs can react with metal matrix during fabrication of composites. Despite reaction product formed between CNTs and Al matrix is absent in CNT/Al composites [7,8], however, Xu et al. reported some Al-carbide phases (an atomic ratio of Al: C = 1:1 or 1:2) in CNT/Al composites fabricated by hot pressing technique [9], Zhang et al. found that carbon nanotubes reacted with Al and formed Al4C3 phases in CNT/Al composite when the composite was held at 1073 K for 1 h [10]. Thus the research of reaction kinetics between CNTs and aluminum is of great impor⁎ Corresponding author. Tel.: +86 451 86418836; fax: +86 451 86413922. E-mail addresses: [email protected] (C.F. Deng), [email protected] (X.X. Zhang), [email protected] (D.Z. Wang), [email protected] (Y.X. Ma). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.037
tance because the chemical stability of CNTs in aluminum matrix decides whether or not CNTs can be successfully applied to the Al matrix as reinforcement. 2. Experimental procedures Multi-wall carbon nanotubes (MWNTs) were fabricated by catalysis of hydrocarbon. In order to remove the impurities such as graphitic particles or amorphous carbons and to disperse CNTs, the as-received CNTs were treated with reflux in concentrated nitric acid; subsequently CNTs were washed with distilled water until the washings show no acidity. As a second, 2024Al (wt.%: 4.20 Cu, 1.47 Mg, 0.56 Mn, 0.02 Zr, 0.40 Fe and 0.27 Si) powder with particle size of ∼50 μm were introduced into the CNT– ethanol solution, then mixed powders were dispersed with mechanical stirring assisting ultrasonic shaker for 30 min. Finally, the mixed powders were dried at 393 K in vacuum. For this study, 5.0 wt.% CNTs have been mixed with 2024Al powders. The mixed powders of CNTs and 2024Al powders were compacted into a disc with ∼6 mm in diameter and ~4 mm thick for the samples of DSC. The weight of DSC samples was ∼30 mg. The DSC equipment was Netzsch STA-449 C Thermal analyzer. The scans were made in an argon atmosphere to minimize to oxidation. Four different heating rates were used: 5, 10, 20, 40 K/min, between 303 and 1073 K. Repeated DSC at the different scan rates were carried out to ensure reproducibility.
3222
C.F. Deng et al. / Materials Letters 61 (2007) 3221–3223
Phase analysis and the microstructure of the samples after being tested with DSC were executed using XRD and TEM. 3. Results and discussion Fig. 1 shows DSC analyses carried out at the four heating rates. It is seen from Fig. 1(a) that there is only an endothermic peak for each DSC curve, ascribed to the melting of aluminum. Moreover, as heating rate increases, the peak temperature of aluminum melting shifts to higher temperatures. However, there an exothermic peak appears beside an endothermic peak for DSC curves of the mixed powders at various heating rates as shown in Fig. 1(b). The peak temperature for two peaks in Fig. 1(b) increases with increasing heating rate as similar to Fig. 1(a). But it is observed that melting peak of aluminum is wider and the melting peak temperature is higher for the mixed powders than for the 2024Al powders at the same heating rate as shown in Fig. 1, this can be attributed to that the contacting surfaces of 2024Al particles are reduced when CNTs are introduced into 2024Al powders which leads to the melting temperature of aluminum is postponed, meanwhile the melting of aluminum becomes more difficult. Moreover, an exothermic peak appears in Fig. 1(b), this can be owing to the fact that CNTs react with aluminum and forms Al4C3 phase because the reaction of graphite and aluminum is an exothermic [11]. In order to identify the reaction product, the DSC samples after being tested were examined using XRD and TEM.
Fig. 1. DSC curves with various heating rates for (a) 2024 Al powders; (b) the mixed powders of CNTs and 2024Al.
Fig. 2. XRD patterns of 2024Al powders (a), the mixed powders of 2024Al powders and CNTs (b) and the mixed powders after being tested with DSC (c).
Fig. 2 shows the XRD patterns of three materials. For the pattern of 2024Al powders, there are two major peaks that consisted of Al phase and CuAl2 phase, however, the pattern shows that a carbon peak appears, and the peaks of CuAl2 phase is stronger in relation to the pattern of 2024Al powders when 5.0 wt.% CNTs was introduced into 2024Al powders. But it is be clearly observed from the pattern of the mixed powders after being tested with DSC that the carbon peak disappears, several Al4C3 peaks are formed, and the peaks of CuAl2 phase become relatively stronger than that of mixed powders. These can be attributed to the fact that 2024 Al powder prepared by ultrasonic gas atomization process is metastable due to supersaturated solution of the alloy elements. So, the precipitation phase (CuAl2) is prone to be formed when the powders are heated. The peaks of CuAl2 phases become stronger compared to that of 2024Al powders, attributed to the fact that the mixed powders were dried at 393 K, however, the DSC samples were heated to 1073 K. When they were cooled slowly, the precipitation phase (CuAl2) appears. It is well known that the heated temperature is higher; the cooled time is longer, so
Fig. 3. TEM images of Al4C3 phases.
C.F. Deng et al. / Materials Letters 61 (2007) 3221–3223
3223
exothermic peaks of reaction of CNTs and aluminum. According to formula (2), the value of apparent activation energy of the reaction of CNTs and aluminum is calculated to equal to 194.01 kJ/mol. Moreover, according to the Carne equation [13], d ln b E ¼− 1 nR
d
ð3Þ
Tp
Here n is reaction order. The value of n can be obtained from the plot of ln β versus 1 / Tp . Fig. 4 also shows the plot of ln β versus 1 / Tp for the exothermic peak in the mixed powders. The figure also shows the good linear correlation. The reaction order (n) was obtained from the slope of the curve in Fig. 4. The value of reaction order is calculated to equal to 0.92.
Fig. 4. The curves of (T2p / β) (a) and lnβ (b) versus 1 / Tp for the exothermic peaks in the mixed powders.
the precipitation phase increases with increasing temperature. However, the appearance of Al4C3 phase peak in XRD pattern further confirms the reaction between CNTs and the aluminum, which are also in good agreement with TEM micrographs of the composite as shown Fig. 3. Fig. 3 shows TEM images of Al4C3 phases of the mixed powders after being tested with DSC. It can be seen that most of the Al4C3 phases are implanted into aluminum grains. The Al4C3 phases are needle shape with average diameter of ∼20 nm as similar as purified CNTs. Also CNTs are not found during TEM observation, suggesting that CNTs have reacted with aluminum and changed entirely into Al4C3 phases. So it can be concluded that the reaction between CNTs and aluminum is prone to take place above aluminum melting temperature. In order to obtain reaction kinetics parameters of the reaction of CNTs and aluminum, the Kissinger and Crane equations were carried out to analyze the DSC curves for mixed powders after being tested with DSC. According to the Kissinger equation [12], T 2 d ln bp E ¼ ð1Þ 1 R d Tp
Here Tp represents the exothermic peak temperature of reaction of CNTs and aluminum, β represents heating rate, E represents apparent activation energy and R represents gas constant. Fig. 4 shows a graph of relationship between ln(T 2p /β ) and 1 / Tp. The slope coefficient of that line is tanθ: tanh ¼
E R
ð2Þ
Therefore, the value of E can be obtained from the plot of ln(T 2p / β ) versus 1 / Tp. The figure shows the good linear correlation obtained for
4. Conclusions 1. CNTs react with aluminum and form Al4C3 phases above aluminum melting temperature. 2. The reaction temperature of CNTs and aluminum depends on the heating rate during the continuous heating. The reaction temperature increases with heating rate. 3. Apparent activation energy and reaction order of reaction of CNTs and aluminum calculated using Kissinger and Crane equations equal to 194.01 and 0.92 kJ/mol, respectively. References [1] E.T. Thostenson, Z. Ren, T.W. Chou, Compos. Sci. Technol. 61 (2001) 1899. [2] P.J.F. Harris, Int. Mater. Rev. 49 (1) (2004) 31. [3] M.K. Yeh, N.H. Tai, J.H. Liu, Carbon 44 (2006) 1. [4] G.D. Zhan, J.D. Kuntz, J. Wan, AlKl Mukherhee, Nat. Mater. 2 (2003) 38. [5] S.I. Cha, K.T. Kim, S.N. Arshad, C.B. Mo, S.H. Hong, Adv. Mater. 17 (2005) 1377. [6] K.T. Kim, S.I. Cha, S.H. Hong, Mater. Sci. Eng., A 430 (2006) 27. [7] T. Kuzumaki, K. Miyazawa, H. Ichinose, K. Ito, J. Mater. Res. 13 (1998) 2445. [8] R. George, K.T. Kashyap, R. Rahul, S. Yamdagni, Script. Mater. 53 (2005) 1159. [9] C.L. Xu, B.Q. Wei, R.Z. Ma, J. Liang, X.K. Ma, D.H. Wu, Carbon 37 (1999) 855. [10] X.X. Zhang, C.F. Deng, D.Z. Wang, L. Geng, Trans. Nonferr. Met. Soc. China 15 (2) (2005). [11] J. Grobner, H.L. Lukas, F. Aldinger, J. Alloys Compd. 220 (1995) 8. [12] H.E. Kissinger, Anal. Chem. 29 (1957) 1702. [13] L.W. Crane, J. Polym. Lett. Ed. 11 (1973) 533.
Calorimetric study of carbon nanotubes and aluminum C.F Deng ⁎, X.X Zhang, D.Z Wang, Y.X Ma School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box433, Harbin 150001, China Received 17 September 2006; accepted 8 November 2006 Available online 28 November 2006
Abstract Differential scanning calorimeter (DSC) was used to investigate apparent activation energy and reaction order of the reaction of carbon nanotubes and aluminum by Kissinger equation and Crane equation under non-isothermal condition. The reaction product was examined using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The experimental results show that carbon nanotubes react with aluminum and form Al4C3 phases with needle shape. The peak temperature of the reaction of carbon nanotubes and aluminum is found to depend on the heating rate during the continuous heating. Apparent activation energy and reaction order of the reaction of carbon nanotubes and aluminum are 194.01 and 0.92 kJ/mol, respectively. © 2006 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Metals and alloys; Differential scanning calorimeter; Kinetics
1. Introduction Carbon nanotubes (CNTs) have been extensively investigated since it was discovered in 1991. One of potential applications of CNTs is the reinforcement of composites to overcome the performance limits of conventional materials [1]. In recent years, CNTs have been regarded as an ideal reinforcement to improve mechanical performance of monolithic materials owing to their extremely high elastic modulus, strength and aspect ratio [2]. Although CNT reinforced polymer [3], and ceramic [4] based composites have been widely investigated, metal matrix composites were limitedly reported [5,6]. One of the reasons could be due to that CNTs can react with metal matrix during fabrication of composites. Despite reaction product formed between CNTs and Al matrix is absent in CNT/Al composites [7,8], however, Xu et al. reported some Al-carbide phases (an atomic ratio of Al: C = 1:1 or 1:2) in CNT/Al composites fabricated by hot pressing technique [9], Zhang et al. found that carbon nanotubes reacted with Al and formed Al4C3 phases in CNT/Al composite when the composite was held at 1073 K for 1 h [10]. Thus the research of reaction kinetics between CNTs and aluminum is of great impor⁎ Corresponding author. Tel.: +86 451 86418836; fax: +86 451 86413922. E-mail addresses: [email protected] (C.F. Deng), [email protected] (X.X. Zhang), [email protected] (D.Z. Wang), [email protected] (Y.X. Ma). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.11.037
tance because the chemical stability of CNTs in aluminum matrix decides whether or not CNTs can be successfully applied to the Al matrix as reinforcement. 2. Experimental procedures Multi-wall carbon nanotubes (MWNTs) were fabricated by catalysis of hydrocarbon. In order to remove the impurities such as graphitic particles or amorphous carbons and to disperse CNTs, the as-received CNTs were treated with reflux in concentrated nitric acid; subsequently CNTs were washed with distilled water until the washings show no acidity. As a second, 2024Al (wt.%: 4.20 Cu, 1.47 Mg, 0.56 Mn, 0.02 Zr, 0.40 Fe and 0.27 Si) powder with particle size of ∼50 μm were introduced into the CNT– ethanol solution, then mixed powders were dispersed with mechanical stirring assisting ultrasonic shaker for 30 min. Finally, the mixed powders were dried at 393 K in vacuum. For this study, 5.0 wt.% CNTs have been mixed with 2024Al powders. The mixed powders of CNTs and 2024Al powders were compacted into a disc with ∼6 mm in diameter and ~4 mm thick for the samples of DSC. The weight of DSC samples was ∼30 mg. The DSC equipment was Netzsch STA-449 C Thermal analyzer. The scans were made in an argon atmosphere to minimize to oxidation. Four different heating rates were used: 5, 10, 20, 40 K/min, between 303 and 1073 K. Repeated DSC at the different scan rates were carried out to ensure reproducibility.
3222
C.F. Deng et al. / Materials Letters 61 (2007) 3221–3223
Phase analysis and the microstructure of the samples after being tested with DSC were executed using XRD and TEM. 3. Results and discussion Fig. 1 shows DSC analyses carried out at the four heating rates. It is seen from Fig. 1(a) that there is only an endothermic peak for each DSC curve, ascribed to the melting of aluminum. Moreover, as heating rate increases, the peak temperature of aluminum melting shifts to higher temperatures. However, there an exothermic peak appears beside an endothermic peak for DSC curves of the mixed powders at various heating rates as shown in Fig. 1(b). The peak temperature for two peaks in Fig. 1(b) increases with increasing heating rate as similar to Fig. 1(a). But it is observed that melting peak of aluminum is wider and the melting peak temperature is higher for the mixed powders than for the 2024Al powders at the same heating rate as shown in Fig. 1, this can be attributed to that the contacting surfaces of 2024Al particles are reduced when CNTs are introduced into 2024Al powders which leads to the melting temperature of aluminum is postponed, meanwhile the melting of aluminum becomes more difficult. Moreover, an exothermic peak appears in Fig. 1(b), this can be owing to the fact that CNTs react with aluminum and forms Al4C3 phase because the reaction of graphite and aluminum is an exothermic [11]. In order to identify the reaction product, the DSC samples after being tested were examined using XRD and TEM.
Fig. 1. DSC curves with various heating rates for (a) 2024 Al powders; (b) the mixed powders of CNTs and 2024Al.
Fig. 2. XRD patterns of 2024Al powders (a), the mixed powders of 2024Al powders and CNTs (b) and the mixed powders after being tested with DSC (c).
Fig. 2 shows the XRD patterns of three materials. For the pattern of 2024Al powders, there are two major peaks that consisted of Al phase and CuAl2 phase, however, the pattern shows that a carbon peak appears, and the peaks of CuAl2 phase is stronger in relation to the pattern of 2024Al powders when 5.0 wt.% CNTs was introduced into 2024Al powders. But it is be clearly observed from the pattern of the mixed powders after being tested with DSC that the carbon peak disappears, several Al4C3 peaks are formed, and the peaks of CuAl2 phase become relatively stronger than that of mixed powders. These can be attributed to the fact that 2024 Al powder prepared by ultrasonic gas atomization process is metastable due to supersaturated solution of the alloy elements. So, the precipitation phase (CuAl2) is prone to be formed when the powders are heated. The peaks of CuAl2 phases become stronger compared to that of 2024Al powders, attributed to the fact that the mixed powders were dried at 393 K, however, the DSC samples were heated to 1073 K. When they were cooled slowly, the precipitation phase (CuAl2) appears. It is well known that the heated temperature is higher; the cooled time is longer, so
Fig. 3. TEM images of Al4C3 phases.
C.F. Deng et al. / Materials Letters 61 (2007) 3221–3223
3223
exothermic peaks of reaction of CNTs and aluminum. According to formula (2), the value of apparent activation energy of the reaction of CNTs and aluminum is calculated to equal to 194.01 kJ/mol. Moreover, according to the Carne equation [13], d ln b E ¼− 1 nR
d
ð3Þ
Tp
Here n is reaction order. The value of n can be obtained from the plot of ln β versus 1 / Tp . Fig. 4 also shows the plot of ln β versus 1 / Tp for the exothermic peak in the mixed powders. The figure also shows the good linear correlation. The reaction order (n) was obtained from the slope of the curve in Fig. 4. The value of reaction order is calculated to equal to 0.92.
Fig. 4. The curves of (T2p / β) (a) and lnβ (b) versus 1 / Tp for the exothermic peaks in the mixed powders.
the precipitation phase increases with increasing temperature. However, the appearance of Al4C3 phase peak in XRD pattern further confirms the reaction between CNTs and the aluminum, which are also in good agreement with TEM micrographs of the composite as shown Fig. 3. Fig. 3 shows TEM images of Al4C3 phases of the mixed powders after being tested with DSC. It can be seen that most of the Al4C3 phases are implanted into aluminum grains. The Al4C3 phases are needle shape with average diameter of ∼20 nm as similar as purified CNTs. Also CNTs are not found during TEM observation, suggesting that CNTs have reacted with aluminum and changed entirely into Al4C3 phases. So it can be concluded that the reaction between CNTs and aluminum is prone to take place above aluminum melting temperature. In order to obtain reaction kinetics parameters of the reaction of CNTs and aluminum, the Kissinger and Crane equations were carried out to analyze the DSC curves for mixed powders after being tested with DSC. According to the Kissinger equation [12], T 2 d ln bp E ¼ ð1Þ 1 R d Tp
Here Tp represents the exothermic peak temperature of reaction of CNTs and aluminum, β represents heating rate, E represents apparent activation energy and R represents gas constant. Fig. 4 shows a graph of relationship between ln(T 2p /β ) and 1 / Tp. The slope coefficient of that line is tanθ: tanh ¼
E R
ð2Þ
Therefore, the value of E can be obtained from the plot of ln(T 2p / β ) versus 1 / Tp. The figure shows the good linear correlation obtained for
4. Conclusions 1. CNTs react with aluminum and form Al4C3 phases above aluminum melting temperature. 2. The reaction temperature of CNTs and aluminum depends on the heating rate during the continuous heating. The reaction temperature increases with heating rate. 3. Apparent activation energy and reaction order of reaction of CNTs and aluminum calculated using Kissinger and Crane equations equal to 194.01 and 0.92 kJ/mol, respectively. References [1] E.T. Thostenson, Z. Ren, T.W. Chou, Compos. Sci. Technol. 61 (2001) 1899. [2] P.J.F. Harris, Int. Mater. Rev. 49 (1) (2004) 31. [3] M.K. Yeh, N.H. Tai, J.H. Liu, Carbon 44 (2006) 1. [4] G.D. Zhan, J.D. Kuntz, J. Wan, AlKl Mukherhee, Nat. Mater. 2 (2003) 38. [5] S.I. Cha, K.T. Kim, S.N. Arshad, C.B. Mo, S.H. Hong, Adv. Mater. 17 (2005) 1377. [6] K.T. Kim, S.I. Cha, S.H. Hong, Mater. Sci. Eng., A 430 (2006) 27. [7] T. Kuzumaki, K. Miyazawa, H. Ichinose, K. Ito, J. Mater. Res. 13 (1998) 2445. [8] R. George, K.T. Kashyap, R. Rahul, S. Yamdagni, Script. Mater. 53 (2005) 1159. [9] C.L. Xu, B.Q. Wei, R.Z. Ma, J. Liang, X.K. Ma, D.H. Wu, Carbon 37 (1999) 855. [10] X.X. Zhang, C.F. Deng, D.Z. Wang, L. Geng, Trans. Nonferr. Met. Soc. China 15 (2) (2005). [11] J. Grobner, H.L. Lukas, F. Aldinger, J. Alloys Compd. 220 (1995) 8. [12] H.E. Kissinger, Anal. Chem. 29 (1957) 1702. [13] L.W. Crane, J. Polym. Lett. Ed. 11 (1973) 533.