Wei Feng*,
Jianpeng Li,
Yiyu Feng and
Mengmeng Qin
School of Materials Science and Engineering, Tianjin Key Laboratory of Composite and Functional Materials, Tianjin University, Tianjin 300072, P. R. China. E-mail: weifeng@tju.edu.cn; Fax: +86-22-27404724; Tel: +86-22-27404724
First published on 17th December 2013
Three-dimensional hierarchical carbon nanocoil–graphite (CNC–GT) nanocomposite blocks were prepared by the growth of CNCs at the interlayer of expanded GT using chemical vapor deposition followed by hot-pressing. The distribution and density of the CNCs were tuned by vacuum impregnation for catalyst loading and growth time, respectively. Helical CNCs with spring-like structures were observed by scanning electron microscopy and transmission electron microscopy. The CNC–GT blocks showed a higher density and lower porosity than GT due to the intercalation of CNC fillers. The thermal conductivities of the CNC–GT blocks in the cross-plane (λ⊥) and in-plane (λ‖) directions were controlled by the consolidating pressure and growth time of the CNCs. The remarkable increase in λ⊥ and the resilience of the CNC–GT blocks were further optimized using microstructures of CNCs at the interface. The maximum λ⊥ of the CNC–GT blocks (∅ 3 cm × 2 mm) of up to 23.6 W m−1 K−1 was about five-fold higher than that of GT at 4.9 W m−1 K−1. This feature arose from improved phonon transfer in the cross-plane through intercalated CNCs at the interlayer. Moreover, a high resilience ratio of 84.1% and a low compressibility (17%) were also obtained for the CNC–GT blocks due to the excellent elasticity of CNCs. The CNC–GT blocks with high λ⊥, good resilience properties and dimensional stability could be developed to be highly thermally conductive and resilient interface materials for heat sealing.
One of the important methods of improving the thermal conductivity in the cross-plane direction and the mechanical properties of GT-based materials is to design and prepare unique microstructures.
Carbon nanomaterials (CNs), such as nanotubes, nanofibers, and nanorods, with superior thermal and mechanical properties have been regarded as ideal one-dimensional (1D) candidates for the intercalation of GT-based materials with high-performance thermal management.1–20 Recently, a three-dimensional (3D) hierarchical nanostructure built with a 1D nanopillar on GT exhibited enhanced phonon mobility in the cross-plane direction and excellent mechanical properties.19,21–24 Lu et al. demonstrated that GT blocks with carbon nanotubes (CNTs) grown at the interlayer showed an increase in thermal conductivity in the cross-plane direction of 10.3% compared with pure GT blocks.6
Carbon nanocoils (CNCs), including helical nanotubes and nanofibers, are excellent 1D nanofillers for the design of 3D hierarchical blocks due to their unique spring-like shape, super-elasticity and good heat conduction along the axis of the helix. According to previous studies, CNCs not only show comparable thermal conductivity to that of carbon nanofibers,25 but also exhibit an elongation strain up to 42–60% and a compressive strain up to 35% under an external force due to their spiral structures,26,27 and they can regain their original shape when the load is released. Good elasticity and excellent thermal properties enable CNCs to be 1D nanofillers for the fabrication of a 3D hierarchical composite with high thermal conductivity and good resilience by optimizing the nanostructure.
In this paper, 3D hierarchical CNC–GT nanocomposite blocks were prepared by the growth of CNCs at the interlayer followed by hot-pressing. The catalyst loading at the interface of expanded GT (EG) was controlled by vacuum impregnation. Cross-plane thermal conductivity and resilience were improved by the intercalation of CNC into the interlayer of GT. The morphologies of the helical CNCs distributed on the surface and interlayer of EG were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The density and open porosity of the CNC–GT blocks were also studied. Thermal conductivities of the CNC–GT blocks in the cross-plane (λ⊥) and in-plane (λ‖) directions were controlled by the growth time of the CNCs and the consolidating pressure. The increase in λ⊥ was due to enhanced phonon transfer in the cross-plane of GT. The mechanical properties of the CNC–GT blocks were also studied to confirm the low compressibility and high resilience.
Block | Composite | Material | Conditions for catalyst loading | |
---|---|---|---|---|
Pressure (kPa) | Time (h) | |||
GT | — | EG | — | — |
CNC–GT | CNC–EG-1 | EG-1 | 1.013 × 102 | 2 |
— | CNC–EG-2 | EG-2 | <1.013 | 2 |
![]() | (1) |
The density of CNCs on EG was controlled by the growth time. Fig. 2a–d show SEM images of CNC–EG-2 composites with growth times of 15, 60, 120 and 150 min, respectively. It can be seen that an increase in the growth time leads to an increase in the density and length of CNCs throughout EG. At short growth times (15 and 60 min), the low-density and short CNCs at the interlayer are not long enough to connect adjacent layers of GT. When the growth time is 120 min, the high-density and long CNCs at the surface and interlayer of EG are desirable for the fabrication of 3D hierarchical CNC–GT blocks.8,28 The approximate average length of the CNCs is larger than 6 μm. CNCs are overloaded on EG when the growth time is 150 min. Fig. 2d shows many entangled CNC bundles and carbonaceous impurities on EG, which are due to excessive decomposition of the carbon sources.
It can also be seen that, compared with the average distance of EG, CNCs (>5–6 μm long at growth times of 120 and 150 min) are long enough after hot-pressing to connect adjacent layers within a limited space despite the entanglement and independent growth. Yields of CNCs at various growth times are shown in Fig. S3.† The microstructures of CNCs were also observed by high-resolution SEM (Fig. 2e) and TEM (Fig. 2f). A helical multiwalled carbon nanotube with a diameter of about 60 nm is shown.29 The helical conformation is important for the super-elasticity of CNCs and resilience properties of CNC–GT blocks. This 3D hierarchical nanostructure offers great potential for efficient thermal conduction in the cross-plane direction and good resilience of CNC–GT blocks.
CNC–GT and GT blocks were prepared by hot-pressing using CNC–EG-1 and EG respectively. Fig. 3 shows SEM images of the cross-section morphologies of CNC–GT and GT blocks. The arrow indicates the direction of hot-pressing. Both GT and CNC–GT blocks show a typical lamellar structure of the GT sheets due to the preferred orientation of the crystal plane during hot-pressing.8 CNC–GT blocks are more dense and compact than GT because the space at the interlayer is filled with CNCs at a growth time of 120 min.30 The oriented crystal planes of the CNC–GT blocks in the direction perpendicular to the hot-pressing direction is confirmed by XRD (Fig. S4†). The rough surface of the CNC–GT blocks is also observed (Fig. S5†). Compared with GT, the alignment of the CNC–GT blocks is slightly decreased due to randomly disordered CNCs at the interlayer.31,32
The hot-pressing pressure controlled densities and porosities of the CNC–GT and GT blocks are shown in Fig. 4a and b. Both blocks show an increased density and a decreased open porosity when the hot-pressing pressure increases due to more compact stacking of GT sheets. Furthermore, the CNC–GT blocks exhibit a higher density and a lower porosity than GT at the same pressure because the space at the interlayer is filled with dense CNCs. The density of the CNC–GT blocks prepared at a pressure of 40 MPa reaches a maximum of 1.2 g cm−3. Meanwhile, the density of GT varies from 0.8 to 0.9 g cm−3 by consolidating EG under a pressure of 30 to 40 MPa, which is comparable to the densities of previously reported compacted GT plates.8,33,34 Thus, CNC–GT blocks with higher density and lower porosity than GT show great potential for efficient heat conduction due to the CNC connections between adjacent layers.
![]() | ||
Fig. 4 (a) Densities, (b) open porosities, (c) λ⊥ and (d) λ‖ of (1) CNC–GT and (2) GT blocks controlled by the hot-pressing pressure. |
Fig. 4c and d display λ⊥ and λ‖ of the CNC–GT and GT blocks. Both GT and CNC–GT blocks show a clear anisotropy, in that λ‖ is almost one order of magnitude higher than λ⊥ because of efficient phonon mobility through the oriented crystal planes.8,19 Furthermore, an increase in both λ⊥ and λ‖ of the CNC–GT and GT blocks is observed with increasing hot-pressing pressure, which is attributed to the higher density and the lower open porosity (Fig. 4a and b). The λ⊥ of CNC–GT is much higher than that of GT blocks due to the growth of CNCs at the interface. The maximum λ⊥ of CNC–GT is 23.6 W m−1 K−1, which is 5 times higher than that of GT blocks (about 4.9 W m−1 K−1). The increase in λ⊥ of the CNC–GT blocks is attributed to enhanced phonon transfer across the GT layers by intercalated CNCs based on the 3D hierarchical structure.19,20,22 CNC–GT blocks also show slightly lower λ‖ than GT, indicating that CNCs might affect the orientation and size of the crystalline structures during hot-pressing (confirmed by the less intense diffraction peaks in the XRD patterns in Fig. S4† and the rough surface shown in Fig. S5†),8 thus resulting in a decrease in λ‖.
The density, porosity and thermal conductivity of CNC–GT blocks were also controlled by the growth time of CNCs. Fig. 5a and b show that an increase in the CNC growth time leads to an increased density and a decreased open porosity of the CNC–GT blocks (prepared using a hot-pressing pressure of 40 MPa) because thousands of long and dense CNCs effectively fill the open spaces in GT. Interestingly, as shown in Fig. 5c and d, λ⊥ and λ‖ display different trends with changes in the growth time of CNCs. A remarkable continuous increase in λ⊥ and a slight decrease in λ‖ are observed for CNC–GT blocks with increasing growth time. The maximum λ⊥ of CNC–EG blocks of 23.6 W m−1 K−1 observed after a growth time of 120 min is about 2.5 fold higher than that observed after a growth time of 15 min (9.6 W m−1 K−1). Moreover, a slight decrease in λ⊥ (23.3 W m−1 K−1) is observed when the growth time is 150 min. The decrease in λ⊥ might arise from randomly entangled CNC bundles and the formation of carbonaceous impurities (Fig. 2d), which affect phonon mobility in the cross-plane direction.28 This feature indicates that λ⊥ is affected by the microstructures of CNCs at the interface of GT after hot-pressing.34 Meanwhile, for the same growth time, λ‖ (284.6 W m−1 K−1) only shows a slight decrease of 4.56% compared with that for 15 min (298.2 W m−1 K−1) due to the slightly reduced size and larger number of defects of the crystalline structures. The high λ⊥ of the CNC–EG blocks is comparable to or much higher than those of GT-based blocks observed in previous studies.4–6,8,18 In addition, the variation of λ⊥ with the growth time of CNCs indicates that although the CNCs are long (>5–6 μm) and stiff enough to connect adjacent layers of GT after hot-pressing, the random disorder and entanglement of unaligned CNCs inevitably affect the λ⊥ of the CNC–GT blocks. Thus, the alignment of well-distributed and stiff CNCs of a suitable length at the interface of GT needs to be optimized in the future.
![]() | ||
Fig. 5 (a) Densities, (b) open porosities, (c) λ⊥ and (d) λ‖ of (1) CNC–GT and (2) GT blocks controlled by the CNC growth time. |
The mechanical properties of CNC–GT and GT blocks are also important for thermally conductive interface materials. The resilience ratio and compressibility of CNC–GT blocks were controlled by the growth time (Fig. 6). GT blocks show a much lower resilience ratio of 73.9% and a higher compressibility of 20.1% than those of all CNC–GT blocks. This data indicates poor resilience properties of traditional GT-based blocks.
![]() | ||
Fig. 6 (a) Resilience ratios and (b) compressibilities in the cross-plane direction of (1) CNC–GT and (2) GT blocks controlled by the CNC growth time. |
As shown in Fig. 6b, the compressibility of CNC–GT blocks decreases with increasing growth time. CNC–GT blocks show the lowest compressibility of 17% at a growth time of 90 min, which is 3.1% lower than that of GT blocks (20.1%). The decrease in compressibility is confirmed by the increasing resilience ratio (Fig. 6a) when the load is released. CNC–GT blocks exhibit a maximum resilience ratio of 84.1% at a growth time of 90 min, which is 10.2% higher than that of GT blocks (73.9%). The high resilience of CNC–GT blocks is attributed to the storage of mechanical energy in super-elastic helical CNCs at the interface of GT. CNC–GT blocks also exhibit a decrease in resilience and an increase in compressibility when the growth time of CNCs is longer than 90 min. This feature might arise from the randomly entangled CNCs in the limited space at the interface, which is consistent with the growth time controlled results for λ⊥. When a load is applied, the distribution of mechanical energy is affected by disordered and entangled CNC bundles. The resilience of compressed CNCs is limited by intertwined long CNCs in a narrow space at the interface. Thus, CNC–GT blocks show a decreased resilience at a long CNC growth time. CNC–GT shows almost a one-fold increase in resilience and a 50% decrease in compressibility compared with GT-based materials with similar densities,35 which is favorable for thermally conductive interface materials.
The excellent resilience of CNC–GT blocks is attributed to the elasticity of tough CNC nanofillers at the interlayers of GT sheets. As shown in Scheme 1, the intercalation of CNCs forms a “spring-sheet” 3D hierarchical structure of GT, in which CNCs act as the “spring” that facilitates the configuration recovery.29,36 The open interspace between the GT layers is filled with CNCs. The CNCs show compression deformation during hot-pressing and also enable configuration recovery when the pressure is unloaded, resulting in an increase in resilience. However, this recovery can be partially limited by the disordered and entangled CNCs. Thus, the resilience of CNC–GT blocks shows a slight decrease when the growth time is longer than 90 min. The results indicate that the compressibility and resilience of CNC–GT can be improved by controlling the microstructures (alignment, order, density and length) of helical CNCs with super-elasticity. Excellent dimensional stability of the CNC–GT blocks at high temperature is also confirmed by a low coefficient thermal expansion (CTE) in the cross-plane direction (Fig. S6†). CNC–GT blocks with excellent resilience, high λ⊥ and λ‖, and good dimensional stability can be developed for highly thermally conductive interface materials for heat sealing.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra45647a |
This journal is © The Royal Society of Chemistry 2014 |