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Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers
Materials Letters 183 (2016) 117–121
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers Wei Li a,b, Fujun Xu a,b,n, Zhiyong Wang a,b, Jianhua Wu a,b, Wei Liu c, Yiping Qiu a,b a
Key Laboratory of Textile Science & Technology, Ministry of Education (Donghua University), Shanghai 201620, China College of Textiles, Donghua University, Shanghai 201620, China c College of Fashion Technology, Shanghai University of Engineering Science, Shanghai 201620, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 September 2015 Received in revised form 27 June 2016 Accepted 10 July 2016 Available online 12 July 2016
In this study, the structural changes, mechanical and electrical properties of carbon nanotube (CNT) fibers after thermal treatments in air with different cooling conditions have been investigated. The tensile strength of the CNT fibers improves about 50% after heat treatment at 250 °C, due to the increase in crystallinity and oxygen-containing groups. After the thermal treatment followed by air cooling, the CNT fiber shows a 217% increases in its tensile modulus and the tensile failure model changes from ductile behavior to brittle fracture. In addition, Raman spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy and field emission scanning electron microscopy are conducted to observe the structural changes of CNT fibers before and after thermal treatments. & 2016 Elsevier B.V. All rights reserved.
Keywords: Carbon nanotubes Thermal treatment Structural Mechanical properties Electrical properties
1. Introduction Carbon nanotubes (CNTs) have been assembled into macroscopic yarns or fibers mainly by three fabrication processes, namely wet spinning, carpet spinning, and aerogel-spun methods [1]. Among them, the direct aerogel-spun method involves a simple and continuous fabrication process that may be applied in commercial mass production [2]. However, in the aerogel-spun CNT fiber, the nanotubes are relatively poorly aligned with low packing density and weak inter-tube interaction, rendering this fiber high tensile strain but low mechanical properties. Recently, to enhance the structures and performances of CNT fibers, a wide variety of post treatments have been experimented, such as polymer impregnation [3], oxygen plasma treatments [4], solvent densification [5], fiber twisting [1], and thermal oxidization [6]. Thermal oxidization, as an ecologically benign and easily scalable process, can not only create oxygen-containing groups on individual CNTs through the entire fiber structure, but also eliminate the defects of CNTs [6]. Peng et al. have studied effects of heat treatment on structures of carpet spinning CNT fibers in air, and found that the specific strength of the CNT fibers increased 31% after heat treatments at 300 °C [7]. Nevertheless, to our knowledge, little has been reported about the effect of heat n Corresponding author at: Key Laboratory of Textile Science & Technology, Ministry of Education (Donghua University), Shanghai 201620, China. E-mail address: [email protected] (F. Xu).
treatment conditions on properties of aerogel-spun CNT fibers. In addition, the effect of thermal treatments on material properties depends not only on the treatment temperature but also on the entire process of the treatments, especially the cooling conditions, which has not been investigated systematically. In this study, the effects of heat treatment temperature and cooling conditions on structures, mechanical and electrical properties of the aerogel-spun CNT fibers were investigated. Two cooling methods, namely furnace cooling (or annealing) and air cooling (or quenching), were compared.
2. Experimental details The aerogel-spun CNT fibers (as shown in Fig. 1a) were synthesized using the floating catalyst CVD growth method, and the details of CNT fiber synthesis have been reported elsewhere [8]. The thermal gravimetric analysis (TGA, NETZSCH TG 209F1, Fig. 1b) showed that the weight loss of the CNT fiber in air increased sharply after 500 °C. Thermal treatment was carried out using a muffle furnace (Li fen Furnaces, Shanghai, China). The CNT fiber (typically 10 cm) was hanged in the furnace with a 2 g weight to obtain a pre-tension of approximately 10% of the tensile failure load. The final temperatures in the furnace was set at 100, 200, 250, 300, and 400 °C for different treatments, respectively. After the specimens were kept in the furnace for 30 min, they were cooled down either in a closed furnace (furnace cooling) or an open furnace (air cooling).
http://dx.doi.org/10.1016/j.matlet.2016.07.034 0167-577X/& 2016 Elsevier B.V. All rights reserved.
Please cite this article as: W. Li, et al., Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers, Mater Lett (2016), http://dx.doi.org/10.1016/j.matlet.2016.07.034i
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W. Li et al. / Materials Letters 183 (2016) 117–121
Fig. 1. Properties of CNT fibers before and after thermal treatments: (a) pristine CNT fiber collected on a plastic winder; (b) TGA of the pristine CNT fiber; (c) two typical thermal treatment processes: furnace cooling and air cooling; (d) sample for tensile testing; stress–strain curves of CNT fibers with thermal treatments followed by (e) furnace cooling, and (f) air cooling.
As an example, the heat treatment history for 400 °C sample during the thermal treatment was shown in Fig. 1c, in which the CNT fibers were cooled down to room temperature much faster in air than in furnace. The mechanical properties of the fiber were tested on a XQ-2 tensile testing machine (Shanghai Xusai Instrument Co., China) at a crosshead speed of 0.5 mm/min with a gauge length of 6 mm. The tensile testing samples were prepared by gluing the CNT fiber on a cardboard with a rectangular window, as shown in Fig. 1d. Electrical characterizations were carried out using a two-probe method on a multi-meter (Agilent 34410A, USA). The structural changes of CNT fibers before and after the thermal treatments were observed by Raman spectroscopy (Renishaw in Via Raman microscope, 633 nm), Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, KBr pellet method), transmission electron microscopy (TEM, JEOLJEM-2100, and 200 kV) and field emission scanning electron microscopy (FESEM, Hitachi S4800, and 5 kV).
3. Results and discussion The tensile stress-strain curves of the CNT fibers after thermal treatments were shown in Fig. 1e and f. The tensile strengths of the CNT fibers increased with rising treatment temperature till 250 °C and then decreased afterwards as shown in Fig. 2a. When treated at 250 °C, the average tensile strengths of the CNT fibers with furnace cooling and air cooling were 138 MPa and 147 MPa, 44% and 53%, respectively, higher than that of the pristine fiber (96 MPa). The improved tensile strength of the CNT fiber with heat treatment was due to the superposition of oxidation and crystallization, which was evidenced by the Raman spectra and FTIR. As shown in Fig. 2b, the Raman spectra were in-situ measured
after heating at a certain temperature for 5 min, and the ID/IG value was summarized in Fig. 2c. More details about the functional groups induced by thermal oxidization were carried on by FTIR as shown in Fig. 2d. As the treatment temperature increased from 25 to 250 °C, the E1u bonds started to increase, indicating that the defects in CNTs were partially eliminated by graphitization process, which would cause a lower in-situ ID/IG from 0.56 to 0.42. The modified structure of the CNT fiber could be maintained after cooling to room temperature, evidenced by the Raman spectra of the heat treated CNT fiber shown in Fig. 2c. Meanwhile, the amount of the carbonyl group increased, which could enhance the interaction between neighboring CNTs and thus improve the load transfer efficiency as well as the tensile strength of the CNT fibers. As the treatment temperature increased beyond 300 °C, C-O, C-H and C¼ O bonds as well as C-C single bonds increased sharply, indicating that the CNTs were oxidized, resulting in a higher ID/IG value and a deterioration of the tensile strength. Therefore, the CNT fiber heat treated at 250 °C could achieve the best balance between functional groups and crystallization, thus owned the highest tensile strength. The enhanced structure of the CNT fibers could also be evidenced by their morphologies, as shown in Fig. 3. According to the TEM in Fig. 3a, b and c, amorphous carbon and defects of the CNTs were partially removed after 250 °C thermal treatment, leading to better crystallinity and decreased ID/IG values. As shown in Fig. 3d, e and f, the CNT fibers with 250 °C thermal treatment showed a denser structure and more entangled CNTs structure, smaller twist angle and thus short fracture length. The applied axial pre-tension on CNT fiber during the heat treatments was the main factor to obtain better CNTs alignment along the axial direction, a smaller twist angle, leading to a more compact structure and certain degree of diameter reduction in the radial direction. On the other
Please cite this article as: W. Li, et al., Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers, Mater Lett (2016), http://dx.doi.org/10.1016/j.matlet.2016.07.034i
W. Li et al. / Materials Letters 183 (2016) 117–121
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Fig. 2. Tensile strength and structure of CNT fibers before and after thermal treatments: (a) temperature dependent tensile strengths; (b) in-situ Raman spectra; (c) temperature dependent ID/IG, the insert was the Raman spectra of CNT fibers before and after 250 °C thermal treatments plus furnace cooling or air cooling; (d) FTIR spectra of the CNT fibers.
Fig. 3. Morphology of CNT fibers: TEM images of CNTs (a) pristine, (b) after 250 °C thermal treatments plus furnace cooling, (c) that plus air cooling; SEM images of macro, fractured ends and micro structures of CNT fibers (d) pristine, (e) after 250 °C thermal treatments plus furnace cooling, (f) that plus air cooling.
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W. Li et al. / Materials Letters 183 (2016) 117–121
Fig. 4. Mechanical and electrical properties of CNT fibers treated by different temperatures: (a) fiber diameter shrinkage; (b) failure strain; (c) Young's modulus; (d) electrical conductivity.
hand, during the air cooling, the CNT fibers were almost quenched by air and thus could suffer severe cooling compression or larger hoop stress [9], which could further shrink the fiber diameter. For instance, as shown in Fig. 4a, when treated at 250 °C, the diameter shrinkage of the CNT fiber with air cooling was 19%, much larger than 10% of the fibers with furnace cooling. The heat treatment followed by various cooling conditions would influence the failure strain, Young's modulus and electrical conductivities of the CNT fibers. As shown in Fig. 4b. the failure strain of the CNT fibers with furnace cooling kept relatively constant as the heating temperature was below 300 °C, while decreased to 6.7% when treated at 400 °C. However, as for the CNT fibers with air cooling, the failure strain continuously decreased up to 85%, from 17.1% to 2.6% as the treatment temperature increased from 25 to 400 °C. This meant that the fiber transformed from a typical ductile material to a typical brittle material for which the failure mechanism changed from localized deformation to defects dominant fracture. The moduli of CNT fibers after thermal treatments were shown in Fig. 4c. As for the CNT fibers with air cooling, the modulus steadily increased up to 217%, from 2.5 to 6.6 GPa, as the treatment temperatures rose from 20 to 400 °C. The increased modulus of the CNT fibers was mainly attributed to the increased oxygencontaining groups as well as the diameter shrinkage, resulting in a
stronger interaction between CNTs and a more compact structure of the CNT fibers. However, the Young's modulus of the CNT fibers with furnace cooling improved to just a modest extent, due to limited diameter shrinkage. The electrical conductivities of the CNT fibers under different thermal treatments were summarized in Fig. 4d. As for the CNT fibers with furnace cooling, the electrical conductivity kept relatively constant at 630 S/cm when the treatment temperature was below 200 °C, while continuously decreased up to approximately 15% as the treatment temperature increased to 400 °C, due perhaps to the amorphous carbon removal and severe oxidization. As for the CNT fibers with air cooling, the treatment temperature dependent electrical conductivity followed the similar trend but always 5% higher than that of the furnace cooled fibers, due mostly to a larger fiber diameter shrinkage.
4. Conclusions The effects of thermal treatments on structures and properties of aerogel-spun CNT fibers were studied. The mechanical properties of the CNT fibers could be improved due to the enhanced crystallinity and interfacial interaction between CNTs induced by thermal treatments. Moreover, it is found that the failure strain,
Please cite this article as: W. Li, et al., Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers, Mater Lett (2016), http://dx.doi.org/10.1016/j.matlet.2016.07.034i
W. Li et al. / Materials Letters 183 (2016) 117–121
Young's modulus and electrical conductivity of the CNT fibers could be adjusted by changing the cooling rate after heat treatment. Thus, it can be concluded that thermal treatment in air with appropriate conditions could be an effective way to improve mechanical and electrical properties of the aerogel-spun CNT fibers.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51303025 and 51503120), Shanghai Natural Science Foundation (Grant No. 12ZR1440500), Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20120075120016), and Shanghai Science and Technology Committee (Grant No. 14YF1409600), the Fundamental Research Funds for the Central Universities and DHU Distinguished Young Professor Program.
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fibers: opportunities and challenges, Adv. Mater. 24 (2012) 1805–1833. [2] M.F. De Volder, S.H. Tawfick, R.H. Baughman, A.J. Hart, Carbon nanotubes: present and future commercial applications, Science 339 (2013) 535–539. [3] S. Li, X. Zhang, J. Zhao, F. Meng, G. Xu, Z. Yong, et al., Enhancement of carbon nanotube fibres using different solvents and polymers, Composites Sci. Technol. 72 (2012) 1402–1407. [4] O.K. Park, W.Y. Kim, S.M. Kim, N.H. You, Y. Jeong, H.S. Lee, et al., Effect of oxygen plasma treatment on the mechanical properties of carbon nanotube fibers, Mater. Lett. 156 (2015) 17–20. [5] Y. Shao, F. Xu, W. Li, K. Zhang, C. Zhang, R. Li, et al., Interfacial strength and debonding mechanism between aerogel-spun carbon nanotube yarn and polyphenylene sulfide, Composites Part A: Appl. Sci. Manuf. 88 (2016) 98–105. [6] A.P. Pierlot, A.L. Woodhead, J.S. Church, Thermal annealing effects on multiwalled carbon nanotube yarns probed by Raman spectroscopy, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 117 (2014) 598–603. [7] Z. Yang, X. Sun, X. Chen, Z. Yong, G. Xu, R. He, et al., Dependence of structures and properties of carbon nanotube fibers on heating treatment, J. Mater. Chem. 21 (2011) 13772–13775. [8] Y.-L. Li, I.A. Kinloch, A.H. Windle, Direct spinning of carbon nanotube fibers from chemical vapor deposition synthesis, Science 304 (2004) 276–278. [9] Y. Gao, J. Li, L. Liu, W. Ma, W. Zhou, S. Xie, et al., Axial compression of hierarchically structured carbon nanotube fiber embedded in epoxy, Adv. Funct. Mater. 20 (2010) 3797–3803.
References [1] W. Lu, M. Zu, J.H. Byun, B.S. Kim, T.W. Chou, State of the art of carbon nanotube
Please cite this article as: W. Li, et al., Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers, Mater Lett (2016), http://dx.doi.org/10.1016/j.matlet.2016.07.034i
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers Wei Li a,b, Fujun Xu a,b,n, Zhiyong Wang a,b, Jianhua Wu a,b, Wei Liu c, Yiping Qiu a,b a
Key Laboratory of Textile Science & Technology, Ministry of Education (Donghua University), Shanghai 201620, China College of Textiles, Donghua University, Shanghai 201620, China c College of Fashion Technology, Shanghai University of Engineering Science, Shanghai 201620, China b
art ic l e i nf o
a b s t r a c t
Article history: Received 15 September 2015 Received in revised form 27 June 2016 Accepted 10 July 2016 Available online 12 July 2016
In this study, the structural changes, mechanical and electrical properties of carbon nanotube (CNT) fibers after thermal treatments in air with different cooling conditions have been investigated. The tensile strength of the CNT fibers improves about 50% after heat treatment at 250 °C, due to the increase in crystallinity and oxygen-containing groups. After the thermal treatment followed by air cooling, the CNT fiber shows a 217% increases in its tensile modulus and the tensile failure model changes from ductile behavior to brittle fracture. In addition, Raman spectroscopy, Fourier transform infrared spectroscopy, transmission electron microscopy and field emission scanning electron microscopy are conducted to observe the structural changes of CNT fibers before and after thermal treatments. & 2016 Elsevier B.V. All rights reserved.
Keywords: Carbon nanotubes Thermal treatment Structural Mechanical properties Electrical properties
1. Introduction Carbon nanotubes (CNTs) have been assembled into macroscopic yarns or fibers mainly by three fabrication processes, namely wet spinning, carpet spinning, and aerogel-spun methods [1]. Among them, the direct aerogel-spun method involves a simple and continuous fabrication process that may be applied in commercial mass production [2]. However, in the aerogel-spun CNT fiber, the nanotubes are relatively poorly aligned with low packing density and weak inter-tube interaction, rendering this fiber high tensile strain but low mechanical properties. Recently, to enhance the structures and performances of CNT fibers, a wide variety of post treatments have been experimented, such as polymer impregnation [3], oxygen plasma treatments [4], solvent densification [5], fiber twisting [1], and thermal oxidization [6]. Thermal oxidization, as an ecologically benign and easily scalable process, can not only create oxygen-containing groups on individual CNTs through the entire fiber structure, but also eliminate the defects of CNTs [6]. Peng et al. have studied effects of heat treatment on structures of carpet spinning CNT fibers in air, and found that the specific strength of the CNT fibers increased 31% after heat treatments at 300 °C [7]. Nevertheless, to our knowledge, little has been reported about the effect of heat n Corresponding author at: Key Laboratory of Textile Science & Technology, Ministry of Education (Donghua University), Shanghai 201620, China. E-mail address: [email protected] (F. Xu).
treatment conditions on properties of aerogel-spun CNT fibers. In addition, the effect of thermal treatments on material properties depends not only on the treatment temperature but also on the entire process of the treatments, especially the cooling conditions, which has not been investigated systematically. In this study, the effects of heat treatment temperature and cooling conditions on structures, mechanical and electrical properties of the aerogel-spun CNT fibers were investigated. Two cooling methods, namely furnace cooling (or annealing) and air cooling (or quenching), were compared.
2. Experimental details The aerogel-spun CNT fibers (as shown in Fig. 1a) were synthesized using the floating catalyst CVD growth method, and the details of CNT fiber synthesis have been reported elsewhere [8]. The thermal gravimetric analysis (TGA, NETZSCH TG 209F1, Fig. 1b) showed that the weight loss of the CNT fiber in air increased sharply after 500 °C. Thermal treatment was carried out using a muffle furnace (Li fen Furnaces, Shanghai, China). The CNT fiber (typically 10 cm) was hanged in the furnace with a 2 g weight to obtain a pre-tension of approximately 10% of the tensile failure load. The final temperatures in the furnace was set at 100, 200, 250, 300, and 400 °C for different treatments, respectively. After the specimens were kept in the furnace for 30 min, they were cooled down either in a closed furnace (furnace cooling) or an open furnace (air cooling).
http://dx.doi.org/10.1016/j.matlet.2016.07.034 0167-577X/& 2016 Elsevier B.V. All rights reserved.
Please cite this article as: W. Li, et al., Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers, Mater Lett (2016), http://dx.doi.org/10.1016/j.matlet.2016.07.034i
118
W. Li et al. / Materials Letters 183 (2016) 117–121
Fig. 1. Properties of CNT fibers before and after thermal treatments: (a) pristine CNT fiber collected on a plastic winder; (b) TGA of the pristine CNT fiber; (c) two typical thermal treatment processes: furnace cooling and air cooling; (d) sample for tensile testing; stress–strain curves of CNT fibers with thermal treatments followed by (e) furnace cooling, and (f) air cooling.
As an example, the heat treatment history for 400 °C sample during the thermal treatment was shown in Fig. 1c, in which the CNT fibers were cooled down to room temperature much faster in air than in furnace. The mechanical properties of the fiber were tested on a XQ-2 tensile testing machine (Shanghai Xusai Instrument Co., China) at a crosshead speed of 0.5 mm/min with a gauge length of 6 mm. The tensile testing samples were prepared by gluing the CNT fiber on a cardboard with a rectangular window, as shown in Fig. 1d. Electrical characterizations were carried out using a two-probe method on a multi-meter (Agilent 34410A, USA). The structural changes of CNT fibers before and after the thermal treatments were observed by Raman spectroscopy (Renishaw in Via Raman microscope, 633 nm), Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, KBr pellet method), transmission electron microscopy (TEM, JEOLJEM-2100, and 200 kV) and field emission scanning electron microscopy (FESEM, Hitachi S4800, and 5 kV).
3. Results and discussion The tensile stress-strain curves of the CNT fibers after thermal treatments were shown in Fig. 1e and f. The tensile strengths of the CNT fibers increased with rising treatment temperature till 250 °C and then decreased afterwards as shown in Fig. 2a. When treated at 250 °C, the average tensile strengths of the CNT fibers with furnace cooling and air cooling were 138 MPa and 147 MPa, 44% and 53%, respectively, higher than that of the pristine fiber (96 MPa). The improved tensile strength of the CNT fiber with heat treatment was due to the superposition of oxidation and crystallization, which was evidenced by the Raman spectra and FTIR. As shown in Fig. 2b, the Raman spectra were in-situ measured
after heating at a certain temperature for 5 min, and the ID/IG value was summarized in Fig. 2c. More details about the functional groups induced by thermal oxidization were carried on by FTIR as shown in Fig. 2d. As the treatment temperature increased from 25 to 250 °C, the E1u bonds started to increase, indicating that the defects in CNTs were partially eliminated by graphitization process, which would cause a lower in-situ ID/IG from 0.56 to 0.42. The modified structure of the CNT fiber could be maintained after cooling to room temperature, evidenced by the Raman spectra of the heat treated CNT fiber shown in Fig. 2c. Meanwhile, the amount of the carbonyl group increased, which could enhance the interaction between neighboring CNTs and thus improve the load transfer efficiency as well as the tensile strength of the CNT fibers. As the treatment temperature increased beyond 300 °C, C-O, C-H and C¼ O bonds as well as C-C single bonds increased sharply, indicating that the CNTs were oxidized, resulting in a higher ID/IG value and a deterioration of the tensile strength. Therefore, the CNT fiber heat treated at 250 °C could achieve the best balance between functional groups and crystallization, thus owned the highest tensile strength. The enhanced structure of the CNT fibers could also be evidenced by their morphologies, as shown in Fig. 3. According to the TEM in Fig. 3a, b and c, amorphous carbon and defects of the CNTs were partially removed after 250 °C thermal treatment, leading to better crystallinity and decreased ID/IG values. As shown in Fig. 3d, e and f, the CNT fibers with 250 °C thermal treatment showed a denser structure and more entangled CNTs structure, smaller twist angle and thus short fracture length. The applied axial pre-tension on CNT fiber during the heat treatments was the main factor to obtain better CNTs alignment along the axial direction, a smaller twist angle, leading to a more compact structure and certain degree of diameter reduction in the radial direction. On the other
Please cite this article as: W. Li, et al., Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers, Mater Lett (2016), http://dx.doi.org/10.1016/j.matlet.2016.07.034i
W. Li et al. / Materials Letters 183 (2016) 117–121
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Fig. 2. Tensile strength and structure of CNT fibers before and after thermal treatments: (a) temperature dependent tensile strengths; (b) in-situ Raman spectra; (c) temperature dependent ID/IG, the insert was the Raman spectra of CNT fibers before and after 250 °C thermal treatments plus furnace cooling or air cooling; (d) FTIR spectra of the CNT fibers.
Fig. 3. Morphology of CNT fibers: TEM images of CNTs (a) pristine, (b) after 250 °C thermal treatments plus furnace cooling, (c) that plus air cooling; SEM images of macro, fractured ends and micro structures of CNT fibers (d) pristine, (e) after 250 °C thermal treatments plus furnace cooling, (f) that plus air cooling.
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Fig. 4. Mechanical and electrical properties of CNT fibers treated by different temperatures: (a) fiber diameter shrinkage; (b) failure strain; (c) Young's modulus; (d) electrical conductivity.
hand, during the air cooling, the CNT fibers were almost quenched by air and thus could suffer severe cooling compression or larger hoop stress [9], which could further shrink the fiber diameter. For instance, as shown in Fig. 4a, when treated at 250 °C, the diameter shrinkage of the CNT fiber with air cooling was 19%, much larger than 10% of the fibers with furnace cooling. The heat treatment followed by various cooling conditions would influence the failure strain, Young's modulus and electrical conductivities of the CNT fibers. As shown in Fig. 4b. the failure strain of the CNT fibers with furnace cooling kept relatively constant as the heating temperature was below 300 °C, while decreased to 6.7% when treated at 400 °C. However, as for the CNT fibers with air cooling, the failure strain continuously decreased up to 85%, from 17.1% to 2.6% as the treatment temperature increased from 25 to 400 °C. This meant that the fiber transformed from a typical ductile material to a typical brittle material for which the failure mechanism changed from localized deformation to defects dominant fracture. The moduli of CNT fibers after thermal treatments were shown in Fig. 4c. As for the CNT fibers with air cooling, the modulus steadily increased up to 217%, from 2.5 to 6.6 GPa, as the treatment temperatures rose from 20 to 400 °C. The increased modulus of the CNT fibers was mainly attributed to the increased oxygencontaining groups as well as the diameter shrinkage, resulting in a
stronger interaction between CNTs and a more compact structure of the CNT fibers. However, the Young's modulus of the CNT fibers with furnace cooling improved to just a modest extent, due to limited diameter shrinkage. The electrical conductivities of the CNT fibers under different thermal treatments were summarized in Fig. 4d. As for the CNT fibers with furnace cooling, the electrical conductivity kept relatively constant at 630 S/cm when the treatment temperature was below 200 °C, while continuously decreased up to approximately 15% as the treatment temperature increased to 400 °C, due perhaps to the amorphous carbon removal and severe oxidization. As for the CNT fibers with air cooling, the treatment temperature dependent electrical conductivity followed the similar trend but always 5% higher than that of the furnace cooled fibers, due mostly to a larger fiber diameter shrinkage.
4. Conclusions The effects of thermal treatments on structures and properties of aerogel-spun CNT fibers were studied. The mechanical properties of the CNT fibers could be improved due to the enhanced crystallinity and interfacial interaction between CNTs induced by thermal treatments. Moreover, it is found that the failure strain,
Please cite this article as: W. Li, et al., Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers, Mater Lett (2016), http://dx.doi.org/10.1016/j.matlet.2016.07.034i
W. Li et al. / Materials Letters 183 (2016) 117–121
Young's modulus and electrical conductivity of the CNT fibers could be adjusted by changing the cooling rate after heat treatment. Thus, it can be concluded that thermal treatment in air with appropriate conditions could be an effective way to improve mechanical and electrical properties of the aerogel-spun CNT fibers.
Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51303025 and 51503120), Shanghai Natural Science Foundation (Grant No. 12ZR1440500), Specialized Research Fund for the Doctoral Program of Higher Education of China (Grant No. 20120075120016), and Shanghai Science and Technology Committee (Grant No. 14YF1409600), the Fundamental Research Funds for the Central Universities and DHU Distinguished Young Professor Program.
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fibers: opportunities and challenges, Adv. Mater. 24 (2012) 1805–1833. [2] M.F. De Volder, S.H. Tawfick, R.H. Baughman, A.J. Hart, Carbon nanotubes: present and future commercial applications, Science 339 (2013) 535–539. [3] S. Li, X. Zhang, J. Zhao, F. Meng, G. Xu, Z. Yong, et al., Enhancement of carbon nanotube fibres using different solvents and polymers, Composites Sci. Technol. 72 (2012) 1402–1407. [4] O.K. Park, W.Y. Kim, S.M. Kim, N.H. You, Y. Jeong, H.S. Lee, et al., Effect of oxygen plasma treatment on the mechanical properties of carbon nanotube fibers, Mater. Lett. 156 (2015) 17–20. [5] Y. Shao, F. Xu, W. Li, K. Zhang, C. Zhang, R. Li, et al., Interfacial strength and debonding mechanism between aerogel-spun carbon nanotube yarn and polyphenylene sulfide, Composites Part A: Appl. Sci. Manuf. 88 (2016) 98–105. [6] A.P. Pierlot, A.L. Woodhead, J.S. Church, Thermal annealing effects on multiwalled carbon nanotube yarns probed by Raman spectroscopy, Spectrochim. Acta Part A: Mol. Biomol. Spectrosc. 117 (2014) 598–603. [7] Z. Yang, X. Sun, X. Chen, Z. Yong, G. Xu, R. He, et al., Dependence of structures and properties of carbon nanotube fibers on heating treatment, J. Mater. Chem. 21 (2011) 13772–13775. [8] Y.-L. Li, I.A. Kinloch, A.H. Windle, Direct spinning of carbon nanotube fibers from chemical vapor deposition synthesis, Science 304 (2004) 276–278. [9] Y. Gao, J. Li, L. Liu, W. Ma, W. Zhou, S. Xie, et al., Axial compression of hierarchically structured carbon nanotube fiber embedded in epoxy, Adv. Funct. Mater. 20 (2010) 3797–3803.
References [1] W. Lu, M. Zu, J.H. Byun, B.S. Kim, T.W. Chou, State of the art of carbon nanotube
Please cite this article as: W. Li, et al., Effect of thermal treatments on structures and mechanical properties of aerogel-spun carbon nanotube fibers, Mater Lett (2016), http://dx.doi.org/10.1016/j.matlet.2016.07.034i