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Transport of thermal energy in epoxy matrix composites reinforced with a hybrid carbon nanofiller
Results in Physics 14 (2019) 102363
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Microarticle
Transport of thermal energy in epoxy matrix composites reinforced with a hybrid carbon nanofiller
T
⁎
Junjie Chen , Baofang Liu, Longfei Yan Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer nanocomposites Carbon nanotubes Graphene Thermal properties Epoxy matrix Hybrid nanofillers
The transport behavior of thermal energy in an epoxy matrix composite was studied. The composite was reinforced with a hybrid filler consisting of carbon nanotubes and graphene nanoplatelets at the nanometer level. The results indicated that there exists a synergistic effect between carbon nanotubes and graphene nanoplatelets on composite thermal conductivity. The unique structure of hybrid fillers provides functional advantages in relation to thermal conductivity, but only at low loading. The hybrid filler can yield a 28% increase in composite thermal conductivity, compared to graphene nanoplatelets. The synergistic benefit to thermal conductivity derives from the formation of more efficient thermally conducting paths in composites.
Introduction Polymer nanocomposites has attracted tremendous attention recently due to their exceptional properties [1,2]. Nanoparticles have great potential to be used as filler materials to improve the thermal properties of a polymer. Most of the filler materials used in polymers are carbon nanotubes and graphene [3,4], as both of them have unusually high levels of thermal conductivity [5,6]. The incorporation of such fillers into polymer matrices to form composite structures is an effective way to improve the thermal properties of materials, and an accurate understanding of the thermal resistance at filler-matrix interfaces is of great significance in practical applications [7,8]. However, there is still a lack of fundamental understanding of the relationship between composite structures and thermal properties [9,10], which is a major stumbling block to the commercialization of polymer composites. It is therefore of great significance to understand the relationship between nanometer-scale structures and macroscopic thermal performance for polymer composites. Recent studies have demonstrated that hybrid, conductive carbon fillers hold great promise for improving composite thermal properties [11,12]. Considerable progress has been made in understanding how a hybrid filler consisting of carbon nanotubes and graphene can be used to improve the thermal properties of polymer matrix composite materials [13,14]. However, it is still unclear how changes in the intrinsic ability of polymer matrix nanocomposites to conduct heat when there is a dimensional change of these composite materials from two to three dimensions. Furthermore, much effort has been devoted to the problem
⁎
of determining the synergistic effect of different carbon based fillers on composite thermal properties [15,16]. It has been widely reported that there are synergistic effects between carbon nanotubes and graphene on mechanical, thermal, and electrical properties of the resultant composites [13–16], but the mechanisms that lead to the synergistic reinforcement of composite thermal conductivity remain unclear. This basically defines the fundamental challenge for applied research of hybrid filler reinforced polymer composites. An accurate understanding of the synergistic mechanisms is a primary goal of this emerging area of research, which may lead to optimum reinforcement of polymer matrices with hybrid fillers. In this Microarticle, the effect of a hybrid filler consisting of carbon nanotubes and graphene nanoplatelets on the thermal properties of epoxy matrix composites was investigated. Special emphasis was placed on the fundamental relationships between nanometer-scale hybrid filler structures and macroscopic composite thermal properties, the knowledge of which will be of great importance to the understanding of the synergistic mechanisms. Methods The fillers used were (i) multi-walled carbon nanotubes, TNM8, (ii) graphene nanoplatelets, TNGNPs, and (iii) hybrid fillers comprising carbon nanotubes and graphene nanoplatelets, TNIGNP-CNTs, prepared from TNM8 and TNGNPs. All these fillers were obtained from Chengdu Organic Chemicals Co. Ltd. (Chengdu, China). The matrix used was a bisphenol-A epoxy resin (Araldite® LY 1564 SP) and an amine hardener
Corresponding author. E-mail address: [email protected] (J. Chen).
https://doi.org/10.1016/j.rinp.2019.102363 Received 25 April 2019; Received in revised form 16 May 2019; Accepted 17 May 2019 Available online 23 May 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 14 (2019) 102363
J. Chen, et al.
Results and discussion The thermal conductivity of the composite as a function of the content of graphene in the hybrid filler is shown in Fig. 1. The total filler content was always 8% by weight. Clearly, proper selection of total filler loading is very important to maximize the thermal conductivity of the composites prepared, without significantly changing the intrinsic properties of the polymer matrix used. In this context, the total filler content chosen here can ensure that the desired level of improvement in composite thermal conductivity was attained by the hybrid filler. Graphene nanoplatelets can improve thermal conductivity more effectively than carbon nanotubes, as shown in Fig. 1. The hybrid filler can yield at least a twofold increase in composite thermal conductivity, compared to carbon nanotubes. The highest value of thermal conductivity was found for the composite with the 20:80 wt ratio of carbon nanotubes to graphene nanoplatelets. The hybrid filler can improve thermal conductivity more effectively than its two individual fillers. There exists a synergistic effect of the two individual fillers on composite thermal conductivity. To examine the synergistic effect, the theoretical values of thermal conductivity predicted by the rule of mixtures are also included in Fig. 1. Clearly, the synergistic effect is quite considerable from a standpoint of thermal properties. Scanning and transmission electron micrographs of the two individual fillers are presented in Fig. 2(a–d), and a scanning electron micrograph of the hybrid filler dispersed within the epoxy matrix is presented in Fig. 2(e). It might be expected that the thermal properties of the composite depend strongly on its nanometer scale structure. A bridging mechanism between carbon nanotubes and their adjacent graphene nanoplatelets [19] has been presented to describe the considerable reinforcement achieved from such a hybrid filler in a polymer matrix. As shown in Fig. 2(e), thermally conductive networks are formed due to the aggregation of carbon nanotubes and their bridging between graphene nanoplatelets. Long and twisted carbon nanotubes can bridge adjacent graphene nanoplatelets, leading to an increased contact surface area between the carbon nanostructures in the matrix for the formation of thermally conductive networks. The thermal conductivity of a carbon-nanotube polymer composite was found to be limited by the interfacial resistance to thermal flow [20]. As a result, the quality of the interface is a critical factor affecting composite thermal conductivity [21,22]. In the case of hybrid fillers,
Fig. 1. Composite thermal conductivity as a function of the content of graphene in the hybrid filler.
(Aradur® 3486). The hybrid filler was dispersed in acetone, and the epoxy resin was then mixed with the hybrid filler suspension by mechanical stirring. After removing residual solvents, the mixture was subjected to high shear mixing, and the hardener was then added, followed by high shear mixing. Disk shaped samples were cast and subsequently cured in a vacuum oven for 4 h. To gain insight into the mechanisms that lead to the synergistic reinforcement of composite thermal conductivity, a series of epoxy matrix composites reinforced with the hybrid filler were prepared, with the ratio by weight of carbon nanotubes to graphene nanoplatelets being situated within a range from 5:95 to 95:5. Thermal conductivity measurements were then performed by the transient plane source method [17] with the Hot Disk TPS 2500 S thermal constants analyzer (Hot Disk AB, Sweden). Thermal conductivity measurements were conducted in accordance with ISO 220072:2015 [18]. Two different sample thickness measurements were made in order to eliminate the influence of the thermal contact resistance existing between the contacting surfaces.
Fig. 2. Scanning and transmission electron micrographs of (a, b) purified multi-walled carbon nanotubes and (c, d) graphene nanoplatelets. Scanning electron micrograph of (e) the hybrid filler dispersed within the epoxy matrix; the ratio of carbon nanotubes to graphene nanoplatelets was 20:80 by weight, and the total hybrid filler content was 8% by weight. 2
Results in Physics 14 (2019) 102363
J. Chen, et al.
Synergistic enhancement ratio (%)
30
composite thermal conductivity has been demonstrated with hybrid carbon nanofillers, but only at low total filler loading. Combinations at the nanometer scale lead to the formation of more efficient thermally conducting paths in composites.
20
Acknowledgement
10
This work was supported by the National Natural Science Foundation of China (No. 51506048).
0
References
-10 SER: synergistic enhancement ratio
[1] Winey KI, Vaia RA. Polymer nanocomposites. MRS Bull 2007;32:314–22. [2] Balazs AC, Emrick T, Russell TP. Nanoparticle polymer composites: Where two small worlds meet. Science 2006;314:1107–10. [3] Liu C, Chen M, Yu W, et al. Recent advance on graphene in heat transfer enhancement of composites. ES Energy Environ 2018;2:31–42. [4] Qiu L, Guo P, Zou H, et al. Extremely low thermal conductivity of graphene nanoplatelets using nanoparticle decoration. ES Energy Environ 2018;2:66–72. [5] Kim P, Shi L, Majumdar A, et al. Thermal transport measurements of individual multiwalled nanotubes. Phys Rev Lett 2001;87. Article Number: 215502. [6] Balandin AA, Ghosh S, Bao W, et al. Superior thermal conductivity of single-layer graphene. Nano Lett 2008;8:902–7. [7] Hassanzadeh-Aghdam MK, Ansari R. Thermal conductivity of shape memory polymer nanocomposites containing carbon nanotubes: A micromechanical approach. Compos B 2019;162:167–77. [8] Hassanzadeh-Aghdam MK, Mahmoodi MJ. Micromechanical modeling of thermal conducting behavior of general carbon nanotube-polymer nanocomposites. Mater Sci Eng, B 2018;229:173–83. [9] Thostenson ET, Chou T-W. Processing-structure-multi-functional property relationship in carbon nanotube/epoxy composites. Carbon 2006;44:3022–9. [10] Kim H, Macosko CW. Processing-property relationships of polycarbonate/graphene composites. Polymer 2009;50:3797–809. [11] Gaska K, Rybak A, Kapusta C, et al. Enhanced thermal conductivity of epoxy-matrix composites with hybrid fillers. Polym Adv Technol 2015;26:26–31. [12] Agrawal A, Satapathy A. Mathematical model for evaluating effective thermal conductivity of polymer composites with hybrid fillers. Int J Therm Sci 2015;89:203–9. [13] Huang X, Zhi C, Jiang P. Toward effective synergetic effects from graphene nanoplatelets and carbon nanotubes on thermal conductivity of ultrahigh volume fraction nanocarbon epoxy composites. J Phys Chem C 2012;116:23812–20. [14] Yang S-Y, Lin W-N, Huang Y-L, et al. Synergetic effects of graphene platelets and carbon nanotubes on the mechanical and thermal properties of epoxy composites. Carbon 2011;49:793–803. [15] King JA, Hauser RA, Tomson AM, et al. Synergistic effects of carbon fillers in thermally conductive liquid crystal polymer based resins. J Compos Mater 2008;42:91–107. [16] King JA, Barton RL, Hauser RA, et al. Synergistic effects of carbon fillers in electrically and thermally conductive liquid crystal polymer based resins. Polym Compos 2008;29:421–8. [17] Gustafsson SE. Transient plane source techniques for thermal conductivity and thermal diffusivity measurements of solid materials. Rev Sci Instrum 1991;62:797–804. [18] British Standards Institution. ISO 22007-2:2015. Plastics - Determination of thermal conductivity and thermal diffusivity - Part 2: Transient plane heat source (hot disc) method. London, United Kingdom: 2nd ed.BSI Standards Limited; 2015. p. 1–32. [19] Zhou E, Xi J, Guo Y, et al. Synergistic effect of graphene and carbon nanotube for high-performance electromagnetic interference shielding films. Carbon 2018;133:316–22. [20] Shenogin S, Xue L, Ozisik R, et al. Role of thermal boundary resistance on the heat flow in carbon-nanotube composites. J Appl Phys 2004;95:8136–44. [21] Bao H, Chen J, Gu X, et al. A review of simulation methods in micro/nanoscale heat conduction. ES Energy Environ 2018;1:16–55. [22] Zhang Y, Heo Y-J, Son Y-R, et al. Recent advanced thermal interfacial materials: A review of conducting mechanisms and parameters of carbon materials. Carbon 2019;142:445–60.
k: composite thermal conductivity
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10
20
30
40
50
Total weight content of the hybrid filler (%) Fig. 3. Synergistic enhancement ratio as a function of the total content of the hybrid filler. The ratio of carbon nanotubes to graphene nanoplatelets was 20:80 by weight.
carbon nanotubes serve as a bridge linking adjacent graphene nanoplatelets as illustrated in Fig. 2(e), thereby providing additional thermal energy flow paths. Furthermore, such a hybrid structure causes the actual contact area between individual fillers to increase sharply. Both of them can efficiently improve composite thermal conductivity, which is responsible for the synergistic effect of the hybrid filler. The synergistic enhancement ratio as a function of the total content of the hybrid filler is shown in Fig. 3. The ratio of carbon nanotubes to graphene nanoplatelets was 20:80 by weight. The synergistic enhancement ratio refers to the enhancement of the thermal conductivity of graphene-polymer composites due to the presence of carbon nanotubes. This thermal performance indicator firstly increases and then decreases with increasing total hybrid filler content. The maximum synergistic enhancement ratio is 28% for the composites prepared. The highest value of thermal conductivity was found for the composite with the hybrid filler content of about 11% by weight. In this context, the optimum thermal performance can be yielded. However, the synergistic effect seems to disappear after a certain threshold, for example, about 28% by weight in this case, is reached for the total content of the hybrid filler. Under high filler content conditions, the hybrid filler has no advantage over graphene nanoplatelets in terms of composite thermal conductivity. In this context, graphene nanoplatelets enable thermally conducting paths to be formed in composites in a more efficient way. Conclusions The thermal transport properties of epoxy matrix composites reinforced with a hybrid carbon nanofiller were investigated. The results indicated that there is a positive synergistic effect on composite thermal conductivity by combining carbon nanotubes with graphene nanoplatelets. Compared to graphene nanoplatelets, a 28% increase in
3
Contents lists available at ScienceDirect
Results in Physics journal homepage: www.elsevier.com/locate/rinp
Microarticle
Transport of thermal energy in epoxy matrix composites reinforced with a hybrid carbon nanofiller
T
⁎
Junjie Chen , Baofang Liu, Longfei Yan Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, Jiaozuo, Henan 454000, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Polymer nanocomposites Carbon nanotubes Graphene Thermal properties Epoxy matrix Hybrid nanofillers
The transport behavior of thermal energy in an epoxy matrix composite was studied. The composite was reinforced with a hybrid filler consisting of carbon nanotubes and graphene nanoplatelets at the nanometer level. The results indicated that there exists a synergistic effect between carbon nanotubes and graphene nanoplatelets on composite thermal conductivity. The unique structure of hybrid fillers provides functional advantages in relation to thermal conductivity, but only at low loading. The hybrid filler can yield a 28% increase in composite thermal conductivity, compared to graphene nanoplatelets. The synergistic benefit to thermal conductivity derives from the formation of more efficient thermally conducting paths in composites.
Introduction Polymer nanocomposites has attracted tremendous attention recently due to their exceptional properties [1,2]. Nanoparticles have great potential to be used as filler materials to improve the thermal properties of a polymer. Most of the filler materials used in polymers are carbon nanotubes and graphene [3,4], as both of them have unusually high levels of thermal conductivity [5,6]. The incorporation of such fillers into polymer matrices to form composite structures is an effective way to improve the thermal properties of materials, and an accurate understanding of the thermal resistance at filler-matrix interfaces is of great significance in practical applications [7,8]. However, there is still a lack of fundamental understanding of the relationship between composite structures and thermal properties [9,10], which is a major stumbling block to the commercialization of polymer composites. It is therefore of great significance to understand the relationship between nanometer-scale structures and macroscopic thermal performance for polymer composites. Recent studies have demonstrated that hybrid, conductive carbon fillers hold great promise for improving composite thermal properties [11,12]. Considerable progress has been made in understanding how a hybrid filler consisting of carbon nanotubes and graphene can be used to improve the thermal properties of polymer matrix composite materials [13,14]. However, it is still unclear how changes in the intrinsic ability of polymer matrix nanocomposites to conduct heat when there is a dimensional change of these composite materials from two to three dimensions. Furthermore, much effort has been devoted to the problem
⁎
of determining the synergistic effect of different carbon based fillers on composite thermal properties [15,16]. It has been widely reported that there are synergistic effects between carbon nanotubes and graphene on mechanical, thermal, and electrical properties of the resultant composites [13–16], but the mechanisms that lead to the synergistic reinforcement of composite thermal conductivity remain unclear. This basically defines the fundamental challenge for applied research of hybrid filler reinforced polymer composites. An accurate understanding of the synergistic mechanisms is a primary goal of this emerging area of research, which may lead to optimum reinforcement of polymer matrices with hybrid fillers. In this Microarticle, the effect of a hybrid filler consisting of carbon nanotubes and graphene nanoplatelets on the thermal properties of epoxy matrix composites was investigated. Special emphasis was placed on the fundamental relationships between nanometer-scale hybrid filler structures and macroscopic composite thermal properties, the knowledge of which will be of great importance to the understanding of the synergistic mechanisms. Methods The fillers used were (i) multi-walled carbon nanotubes, TNM8, (ii) graphene nanoplatelets, TNGNPs, and (iii) hybrid fillers comprising carbon nanotubes and graphene nanoplatelets, TNIGNP-CNTs, prepared from TNM8 and TNGNPs. All these fillers were obtained from Chengdu Organic Chemicals Co. Ltd. (Chengdu, China). The matrix used was a bisphenol-A epoxy resin (Araldite® LY 1564 SP) and an amine hardener
Corresponding author. E-mail address: [email protected] (J. Chen).
https://doi.org/10.1016/j.rinp.2019.102363 Received 25 April 2019; Received in revised form 16 May 2019; Accepted 17 May 2019 Available online 23 May 2019 2211-3797/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
Results in Physics 14 (2019) 102363
J. Chen, et al.
Results and discussion The thermal conductivity of the composite as a function of the content of graphene in the hybrid filler is shown in Fig. 1. The total filler content was always 8% by weight. Clearly, proper selection of total filler loading is very important to maximize the thermal conductivity of the composites prepared, without significantly changing the intrinsic properties of the polymer matrix used. In this context, the total filler content chosen here can ensure that the desired level of improvement in composite thermal conductivity was attained by the hybrid filler. Graphene nanoplatelets can improve thermal conductivity more effectively than carbon nanotubes, as shown in Fig. 1. The hybrid filler can yield at least a twofold increase in composite thermal conductivity, compared to carbon nanotubes. The highest value of thermal conductivity was found for the composite with the 20:80 wt ratio of carbon nanotubes to graphene nanoplatelets. The hybrid filler can improve thermal conductivity more effectively than its two individual fillers. There exists a synergistic effect of the two individual fillers on composite thermal conductivity. To examine the synergistic effect, the theoretical values of thermal conductivity predicted by the rule of mixtures are also included in Fig. 1. Clearly, the synergistic effect is quite considerable from a standpoint of thermal properties. Scanning and transmission electron micrographs of the two individual fillers are presented in Fig. 2(a–d), and a scanning electron micrograph of the hybrid filler dispersed within the epoxy matrix is presented in Fig. 2(e). It might be expected that the thermal properties of the composite depend strongly on its nanometer scale structure. A bridging mechanism between carbon nanotubes and their adjacent graphene nanoplatelets [19] has been presented to describe the considerable reinforcement achieved from such a hybrid filler in a polymer matrix. As shown in Fig. 2(e), thermally conductive networks are formed due to the aggregation of carbon nanotubes and their bridging between graphene nanoplatelets. Long and twisted carbon nanotubes can bridge adjacent graphene nanoplatelets, leading to an increased contact surface area between the carbon nanostructures in the matrix for the formation of thermally conductive networks. The thermal conductivity of a carbon-nanotube polymer composite was found to be limited by the interfacial resistance to thermal flow [20]. As a result, the quality of the interface is a critical factor affecting composite thermal conductivity [21,22]. In the case of hybrid fillers,
Fig. 1. Composite thermal conductivity as a function of the content of graphene in the hybrid filler.
(Aradur® 3486). The hybrid filler was dispersed in acetone, and the epoxy resin was then mixed with the hybrid filler suspension by mechanical stirring. After removing residual solvents, the mixture was subjected to high shear mixing, and the hardener was then added, followed by high shear mixing. Disk shaped samples were cast and subsequently cured in a vacuum oven for 4 h. To gain insight into the mechanisms that lead to the synergistic reinforcement of composite thermal conductivity, a series of epoxy matrix composites reinforced with the hybrid filler were prepared, with the ratio by weight of carbon nanotubes to graphene nanoplatelets being situated within a range from 5:95 to 95:5. Thermal conductivity measurements were then performed by the transient plane source method [17] with the Hot Disk TPS 2500 S thermal constants analyzer (Hot Disk AB, Sweden). Thermal conductivity measurements were conducted in accordance with ISO 220072:2015 [18]. Two different sample thickness measurements were made in order to eliminate the influence of the thermal contact resistance existing between the contacting surfaces.
Fig. 2. Scanning and transmission electron micrographs of (a, b) purified multi-walled carbon nanotubes and (c, d) graphene nanoplatelets. Scanning electron micrograph of (e) the hybrid filler dispersed within the epoxy matrix; the ratio of carbon nanotubes to graphene nanoplatelets was 20:80 by weight, and the total hybrid filler content was 8% by weight. 2
Results in Physics 14 (2019) 102363
J. Chen, et al.
Synergistic enhancement ratio (%)
30
composite thermal conductivity has been demonstrated with hybrid carbon nanofillers, but only at low total filler loading. Combinations at the nanometer scale lead to the formation of more efficient thermally conducting paths in composites.
20
Acknowledgement
10
This work was supported by the National Natural Science Foundation of China (No. 51506048).
0
References
-10 SER: synergistic enhancement ratio
[1] Winey KI, Vaia RA. Polymer nanocomposites. MRS Bull 2007;32:314–22. [2] Balazs AC, Emrick T, Russell TP. Nanoparticle polymer composites: Where two small worlds meet. Science 2006;314:1107–10. [3] Liu C, Chen M, Yu W, et al. Recent advance on graphene in heat transfer enhancement of composites. ES Energy Environ 2018;2:31–42. [4] Qiu L, Guo P, Zou H, et al. Extremely low thermal conductivity of graphene nanoplatelets using nanoparticle decoration. ES Energy Environ 2018;2:66–72. [5] Kim P, Shi L, Majumdar A, et al. Thermal transport measurements of individual multiwalled nanotubes. Phys Rev Lett 2001;87. Article Number: 215502. [6] Balandin AA, Ghosh S, Bao W, et al. Superior thermal conductivity of single-layer graphene. Nano Lett 2008;8:902–7. [7] Hassanzadeh-Aghdam MK, Ansari R. Thermal conductivity of shape memory polymer nanocomposites containing carbon nanotubes: A micromechanical approach. Compos B 2019;162:167–77. [8] Hassanzadeh-Aghdam MK, Mahmoodi MJ. Micromechanical modeling of thermal conducting behavior of general carbon nanotube-polymer nanocomposites. Mater Sci Eng, B 2018;229:173–83. [9] Thostenson ET, Chou T-W. Processing-structure-multi-functional property relationship in carbon nanotube/epoxy composites. Carbon 2006;44:3022–9. [10] Kim H, Macosko CW. Processing-property relationships of polycarbonate/graphene composites. Polymer 2009;50:3797–809. [11] Gaska K, Rybak A, Kapusta C, et al. Enhanced thermal conductivity of epoxy-matrix composites with hybrid fillers. Polym Adv Technol 2015;26:26–31. [12] Agrawal A, Satapathy A. Mathematical model for evaluating effective thermal conductivity of polymer composites with hybrid fillers. Int J Therm Sci 2015;89:203–9. [13] Huang X, Zhi C, Jiang P. Toward effective synergetic effects from graphene nanoplatelets and carbon nanotubes on thermal conductivity of ultrahigh volume fraction nanocarbon epoxy composites. J Phys Chem C 2012;116:23812–20. [14] Yang S-Y, Lin W-N, Huang Y-L, et al. Synergetic effects of graphene platelets and carbon nanotubes on the mechanical and thermal properties of epoxy composites. Carbon 2011;49:793–803. [15] King JA, Hauser RA, Tomson AM, et al. Synergistic effects of carbon fillers in thermally conductive liquid crystal polymer based resins. J Compos Mater 2008;42:91–107. [16] King JA, Barton RL, Hauser RA, et al. Synergistic effects of carbon fillers in electrically and thermally conductive liquid crystal polymer based resins. Polym Compos 2008;29:421–8. [17] Gustafsson SE. Transient plane source techniques for thermal conductivity and thermal diffusivity measurements of solid materials. Rev Sci Instrum 1991;62:797–804. [18] British Standards Institution. ISO 22007-2:2015. Plastics - Determination of thermal conductivity and thermal diffusivity - Part 2: Transient plane heat source (hot disc) method. London, United Kingdom: 2nd ed.BSI Standards Limited; 2015. p. 1–32. [19] Zhou E, Xi J, Guo Y, et al. Synergistic effect of graphene and carbon nanotube for high-performance electromagnetic interference shielding films. Carbon 2018;133:316–22. [20] Shenogin S, Xue L, Ozisik R, et al. Role of thermal boundary resistance on the heat flow in carbon-nanotube composites. J Appl Phys 2004;95:8136–44. [21] Bao H, Chen J, Gu X, et al. A review of simulation methods in micro/nanoscale heat conduction. ES Energy Environ 2018;1:16–55. [22] Zhang Y, Heo Y-J, Son Y-R, et al. Recent advanced thermal interfacial materials: A review of conducting mechanisms and parameters of carbon materials. Carbon 2019;142:445–60.
k: composite thermal conductivity
-20 0
10
20
30
40
50
Total weight content of the hybrid filler (%) Fig. 3. Synergistic enhancement ratio as a function of the total content of the hybrid filler. The ratio of carbon nanotubes to graphene nanoplatelets was 20:80 by weight.
carbon nanotubes serve as a bridge linking adjacent graphene nanoplatelets as illustrated in Fig. 2(e), thereby providing additional thermal energy flow paths. Furthermore, such a hybrid structure causes the actual contact area between individual fillers to increase sharply. Both of them can efficiently improve composite thermal conductivity, which is responsible for the synergistic effect of the hybrid filler. The synergistic enhancement ratio as a function of the total content of the hybrid filler is shown in Fig. 3. The ratio of carbon nanotubes to graphene nanoplatelets was 20:80 by weight. The synergistic enhancement ratio refers to the enhancement of the thermal conductivity of graphene-polymer composites due to the presence of carbon nanotubes. This thermal performance indicator firstly increases and then decreases with increasing total hybrid filler content. The maximum synergistic enhancement ratio is 28% for the composites prepared. The highest value of thermal conductivity was found for the composite with the hybrid filler content of about 11% by weight. In this context, the optimum thermal performance can be yielded. However, the synergistic effect seems to disappear after a certain threshold, for example, about 28% by weight in this case, is reached for the total content of the hybrid filler. Under high filler content conditions, the hybrid filler has no advantage over graphene nanoplatelets in terms of composite thermal conductivity. In this context, graphene nanoplatelets enable thermally conducting paths to be formed in composites in a more efficient way. Conclusions The thermal transport properties of epoxy matrix composites reinforced with a hybrid carbon nanofiller were investigated. The results indicated that there is a positive synergistic effect on composite thermal conductivity by combining carbon nanotubes with graphene nanoplatelets. Compared to graphene nanoplatelets, a 28% increase in
3