Journal of Materials Research and Technology 32 (2024) 1395–1415
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Journal of Materials Research and Technology
journal homepage: www.elsevier.com/locate/jmrt
Enhancement mechanisms of mechanical, electrical and thermal properties of carbon nanotube-copper composites: A review
Yilin Jia a, b, Kun Zhou b, Wanting Sun b, Min Ding b, Yu Wang b, Xiangqing Kong b, c, Dongzhou Jia d, Muhong Wu b, e, f, Ying Fu b, * a School of Materials and Energy, Guangdong University of Technology, Guangzhou, 510006, China b Songshan Lake Materials Laboratory, Dongguan, 523808, China c College of Pipeline and Civil Engineering, China University of Petroleum (East China), Qingdao, 266580, China d College of Mechanical Engineering and Automation, Liaoning University of Technology, Jinzhou, 121001, China e International Centre for Quantum Materials, Collaborative Innovation Centre of Quantum Matter, Peking University, Beijing, 100871, China f Interdisciplinary Institute of Light-Element Quantum Materials and Research Centre for Light-Element Advanced Materials, Peking University, Beijing, 100871, China
A R T I C L E I N F O
A B S T R A C T
Handling editor: M Meyers
Keywords: Carbon nanotubes Metal-matrix composites Electrical properties Mechanical properties Thermal properties
Carbon nanotubes (CNTs), as potent reinforcements in composites, have demonstrated excellent strengthening effects when combined with copper in numerous recent studies. Challenges remain in the application of these composites and in fully leveraging the reinforcing capabilities of CNTs to achieve comprehensive performance enhancement. The performance of CNTs/Cu composites can be flexibly regulated owing to the unique structure and properties of CNTs. To achieve the fabrication of high-performance and diverse CNTs/Cu composites, a profound understanding of the reinforcement mechanisms of CNTs in the composites is essential, along with the consideration of key influencing factors on performance. This article provides a comprehensive overview of the reinforcement mechanisms of CNTs on the mechanical, electrical, and thermal properties of CNTs/Cu compos- ites. Factors influencing the effectiveness of CNT reinforcement in composites are discussed, including the at- tributes and dispersion of CNTs, the architectures of composites, and the interface between CNTs and Cu. Furthermore, this study explores the role of CNTs in addressing the trade-off between high strength and high conductivity as well as between high strength and high ductility in the copper matrix.
List of abbreviations
(continued )
ED
SPS
MM
GNDs
TSV
AMM
electrochemical deposition
TCR
spark plasma sintering
CVD
metal matrix
geometrically necessary dislocations through-silicon via
PVD
SD
CPD
acoustic mismatch model
temperature coefficients of resistances chemical vapor deposition physical vapor deposition stretching direction
carbonization of polymer dots
CNTs
SWCNTs
MWCNTs
m-SWCNTs
s-SWCNTs
SPD
SEM
TEM
carbon nanotubes
single-walled carbon nanotubes multi-walled carbon nanotubes metallic single-walled carbon nanotubes semiconductor single- walled carbon nanotubes severe plastic deformation scanning electron microscope transmission electron microscope
DMM
TTM
PDOS
LFPs
GNRs
TDP
CTE
UTS
diffuse mismatch model two-temperature model phonon density of states low-frequency phononss graphene nanoribbons thermal distortion parameter coefficient of thermal expansion ultimate tensile strength
(continued on next column)
- Corresponding author.
E-mail address: fuying@sslab.org.cn (Y. Fu).
https://doi.org/10.1016/j.jmrt.2024.07.181 Received 22 May 2024; Received in revised form 14 July 2024; Accepted 24 July 2024 Available online 31 July 2024 2238-7854/© 2024 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/ ).
Y. Jia et al.
- Introduction
Based on different formation conditions, carbon atoms exhibit various hybridization states, yielding distinct essential properties. Under the high-temperature or high-pressure formation condition, carbon ex- hibits a thermodynamically favorable trigonal sp3 configuration forming diamonds [1]. As the formation heat decreases, carbon assumes a planar sp2 and π configuration forming graphene, as shown in Fig. 1a. CNTs, exhibiting a one-dimensional structure, can be envisioned as rolled-up graphene sheets with hollow cylindrical structures. Depending on the number of layers, CNTs can be classified as single-walled carbon nano- tubes (SWCNTs) or multi-walled carbon nanotubes (MWCNTs), as shown in Fig. 1b. Typical SEM and TEM images of SWCNTs can be seen in Fig. 1c and d [2,3]. On a graphene sheet, a chiral vector connects the centers of two hexagons and can be written as C = na1 + ma2, where a1 and a2 are basis vectors on the graphene lattice (Fig. 1e and f). Rolling up the graphene sheet to superimpose the ends of the chiral vector re- sults in the formation of CNTs. When n equals m, the CNTs are referred to as the armchair structure, exhibiting metallic properties. When m is 0, the CNTs are known as the zigzag structure, displaying semiconductor behavior. In other cases, if the difference between n and m is a multiple of 3, the CNTs exhibit metallic properties; otherwise, the CNTs behave as quasi-metallic. Thus, SWCNTs can be divided into metallic single-walled
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carbon nanotubes (m-SWCNTs) and semiconductor single-walled carbon nanotubes (s-SWCNTs) [4]. Both experimental and theoretical studies have shown that MWCNTs exhibit Young’s Modulus and tensile strength on the order of 1 TPa and 63 GPa [5], respectively. Additionally, MWCNTs are capable of carrying currents as high as 109 A/cm2. On the other hand, SWCNTs have an electrical conductivity as high as 108 S/m, and their thermal conductivity at room temperature can reach 3500 W/mK [6–9]. Consequently, CNTs have attracted widespread attention due to their excellent mechanical, electrical, and thermal properties since their discovery. Currently, CNTs show potential applications in various fields such as nanomedicine, transistors, vacuum electronic devices, biosensors, membranes, and capacitors [10–15].
Copper is widely used in various fields owing to its excellent prop- erties. Current research has found that CNTs, with their exceptional properties such as high strength, electrical conductivity, and thermal conductivity, hold promise for the comprehensive enhancement of copper matrix performance. Utilizing CNTs as reinforcement to surpass the performance limits of copper materials and thereby obtain superior alternatives to copper is a major research focus today. The combination and fabrication processes of the anisotropic one-dimensional CNTs with the three-dimensional copper matrix exhibit a certain multiplicity. The mainstream methods for fabricating CNTs/Cu composite include pow- der metallurgy, chemical deposition, melting and solidification, and
Fig. 1. (a) Carbon allotropes and their bonding [1]; (b) Structure diagrams of the SWCNTs and the MWCNTs; (c) SEM image [2] and (d) TEM image [3] of SWCNT networks; (e, f) Armchair SWCNT with chiral index (4,4) produced from a graphene sheet. The chiral vector C (brown vector) is formed by summing the two basis vectors 4a1 and 4a2 (red vectors). Then the graphene sheet is rolled into a cylinder so that the chiral vector start point (0,0) meets the vector endpoint (4,4). Zigzag SWCNT with chiral index (8,0). C is equal to the base vector a1. Then the graphene sheet is rolled into a cylinder so that the chiral vector start point (0,0) meets the vector endpoint (0,8) [4]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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spray techniques. The CNTs/Cu composites with desired properties can be obtained by adjusting the preparation processes and parameters. In addition, rolling [16–19], hot pressing [20–22] and severe plastic deformation (SPD) [23–26] can be employed to strengthen the com- posite and organize the randomly dispersed CNTs within the copper matrix, fully exploiting the one-dimensional characteristics of CNTs. Furthermore, the type and size of CNTs are controlled by the fabrication process of CNTs, which greatly influences the ultimate performance of the composite. Additionally, the dispersion of CNTs as well as the interface between the CNTs and the copper matrix directly impact the ultimate performance of the composite. As a result, the composite can be selectively designed to exhibit superior performance for specific needs. Many studies have shown the effective role of CNTs in enhancing the performance of copper matrix. For instance, Wang et al. [27] prepared the MWCNTs/Cu nanocomposite with an ultra-high yield strength of 692 MPa using electrochemical deposition (ED) and spark plasma sin- tering (SPS), which was twice the strength of nanocrystalline copper samples. Xiong et al. [28] fabricated super-aligned carbon nanotube reinforced copper matrix composite through SPD. The resulting sample achieved a high tensile strength of 470 MPa while maintaining ultra-high electrical conductivity (98% IACS). Tian et al. [29] utilized ultrasonic chemical synthesis to prepare silver-modified CNTs, which were then used to create the Ag-CNTs/Cu composite via powder met- allurgy. The resulting sample exhibited high electrical conductivity (94.9% IACS) and tensile strength (315 MPa), along with thermal con- ductivity surpassing that of the copper matrix, reaching 416 W/m⋅K. These performance levels indicate that CNTs/Cu composites, with their superior properties, are highly compatible with applications in copper-related fields, such as electronics, thermal management, trans- portation, energy, communications, and medical fields. They hold promising potential to meet the growing application demands for copper materials. Currently, the demonstrated performance of CNTs/Cu com- posites holds the potential to be utilized as interconnect interfaces or thermal interface materials in conventional electronic devices, as well as conductor materials for applications involving data and power trans- mission [30]. However, the comprehensive methods to fully leverage the reinforcing effect of carbon nanotubes have not yet been systemat- ically summarized. Therefore, to precisely achieve a high-quality com- posite that meets specific requirements, it is crucial to thoroughly understand the reinforcing mechanisms of CNTs within the copper matrix and the factors that affect their effectiveness.
This article provides a comprehensive review of the enhancement behavior of CNTs on the properties of the copper matrix. The first section summarizes the enhanced mechanisms of CNTs in the mechanical, electrical, and thermal properties. The next section discusses four key issues: the attributes of CNTs, the dispersion of CNTs within the copper matrix, the architectures of CNTs/Cu composite, and the interface be- tween CNTs and copper matrix. Finally, the roles played by CNTs in resolving the trade-off relation between strength and conductivity, as well as strength and ductility in the copper matrix, along with the key factors influencing their effectiveness were revealed. From the micro- scopic level of the enhancement mechanisms of CNTs in the copper matrix and the factors influencing CNTs to the macroscopic performance of the composite, this article explores the intrinsic connections with the aim of providing references for obtaining high-performance composites.
- Enhancement mechanisms
2.1. Mechanical properties
CNTs possess excellent mechanical properties due to their strong sp2 bonds and unique one-dimensional structure [9]. Introducing CNTs into the copper matrix as reinforcements theoretically allows for a significant enhancement of the mechanical performance of the composite. In cur- rent research, by employing appropriate modulation methods, CNTs can effectively enhance the tensile strength, hardness, ductility, and
tribological properties of the copper matrix [31–35]. The diverse enhancement mechanisms of CNTs on the mechanical properties of the copper matrix offer opportunities for tailoring the performance of the composite.
For composites, the performance is strongly affected by the syner- gistic interaction between the matrix and the reinforcing agent. In this review, the enhancement mechanisms of mechanical properties are categorized as either direct enhancement or indirect enhancement, based on the leading or participatory role played by CNTs in the rein- forcement process. The excellent mechanical properties of CNTs directly enhance the load-bearing capacity and tribological properties of the composite through load transfer and self-lubrication mechanisms, respectively. CNTs can also indirectly enhance the strength of the copper matrix through mechanisms such as grain boundary strengthening, Orowan strengthening, and dislocation density strengthening.
Load transfer has been adopted as the most important enhancement mechanism of CNTs in the copper matrix. A large number of theoretical calculations have demonstrated that the load transfer mechanism con- tributes the most to the strength enhancement of CNTs/Cu composite [36,39,40], as shown in Fig. 2a. Specifically, when CNTs/Cu composite undergo plastic deformation, microcracks are initiated and propagate within the composite. As these cracks extend to the CNTs, a portion of the CNTs becomes exposed from the copper matrix (Fig. 2b and c). The load is transmitted through the interface to the outermost layer of the tube wall, where CNTs inhibit the further expansion of the cracks. With the increase in load, the outermost tube wall of CNTs begins to fracture, subsequently leading to the rupture of all tube walls. This phenomenon relies on effective load transfer between the walls of the CNTs. However, the van der Waals forces between walls of perfect or low-defect-density MWCNTs are relatively weak [9]. These weak bonding forces may struggle to support effective load transfer, potentially resulting in failure modes of outermost-wall fracture and inner-wall sliding. Contrary to theoretical expectations, the actual performance of composite during deformation deviates from this pattern. The irregular walls of MWCNTs might cross-link adjacent walls together, effectively enhancing inter-wall bonding [37,41]. It is evident that the key to the load transfer process lies in the number of CNTs effectively bearing the load and the interface between CNTs and Cu. Efficient load transfer is contingent upon the outstanding dispersion of CNTs and the high interface bonding strength between CNTs and Cu [42]. Furthermore, the load transfer models established by Deng et al. have shown a positive correlation between the size and aspect ratio of CNTs and the efficiency of load transfer [43].
Grain boundary strengthening is a typical enhancement mechanism of metallic materials and their composites, and the Hall-Petch equation intuitively demonstrates the remarkable strengthening effect of grain refinement on mechanical properties. Plastic deformation, such as roll- ing, extrusion, and drawing, is a commonly employed processing tech- nique in composites, and during the process of plastic deformation, CNTs located at the grain boundaries play a crucial role in inhibiting the movement of dislocations and grain boundaries, making contributions to grain refinement. Researchers often find that the contribution of the grain boundary strengthening [44] is not particularly prominent when evaluating the various reinforcement mechanisms using simulation calculations (Fig. 2a). However, Akbarpour et al. [26] introduced the factor of nanocopper raw material in calculations, ultimately discov- ering that the grain refinement mechanism constitutes the primary means for achieving enhanced strength in CNTs/Cu nanocomposites. Zhang et al. [45] established a self-developed theoretical model to investigate the temperature-dependent evolution of four common strengthening mechanisms in CNTs/Cu composites. The results show that the proportion of grain boundary strengthening contributing to the total strength of the composite increases with temperature, within the range of 300 K–450 K, signifying the significant role of grain boundary strengthening in maintaining the high strength of the composite at elevated temperatures.
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Fig. 2. (a) The strength contributed by different strengthening mechanisms of the CNTs/Cu composites prepared by different processes [36]; (b) Fracture behavior of CNTs subjected to in-situ tensile test in the metal matrix (MM) [37]; (c) Schematic of the CNT behavior in the MM/CNT during tensile testing [38].
Orowan strengthening is another common strengthening mechanism in CNTs/Cu composites [46], in which the CNTs acted as the second phases with high shear strength, contributing positively to the rein- forcement of the matrix. During the deformation of the composite, the dislocation motion could be hindered by CNTs, and the dislocation line bows to balance the external load. As a result, under sustained external driving forces, dislocations can only bypass the CNTs, leading to the formation of dislocation loops. The entangled and bent dislocation loops increase the lattice distortion energy near the loops, thereby elevating the resistance to dislocation motion. Consequently, dislocation motion becomes more challenging, ultimately enhancing the mechanical prop- erties of the composite. The contribution of Orowan strengthening to the copper matrix enhancement is relatively modest. Zheng et al. found that it accounted for only 16.58% of the total strengthening in the Ni@CNTs/Cu composite [40].
The dislocation strengthening introduced by CNTs in the CNTs/Cu composites is mainly attributed to the generation of geometrically necessary dislocations (GNDs) [36,39,45,47]. During the deformation process of CNTs/Cu composites, the copper matrix undergoes plastic deformation while the CNTs experience elastic deformation, resulting in mechanical incompatibility within the composites. To maintain the continuity of the material, GNDs are formed in the copper matrix. The formation of GNDs can be attributed to two main sources. Firstly, during the cooling process after deformation, the mismatch in the coefficient of thermal expansion between the CNTs and Cu leads to the accumulation stress and strain, generating dislocations. Secondly, of deformation-induced plastic strain gradients during the plastic defor- mation of the composite also contribute to the formation of dislocations [48]. localized areas with high-density dislocations are formed. These dislocations impede the movement of other dislocations, thus enhancing the overall strength of
In the regions surrounding the CNTs,
thermal
the composite. Moreover, the presence of these high-density dislocations improves the efficiency of load transfer during the tensile process [36]. The direct enhancement of the frictional properties of copper matrix by CNTs is also noteworthy. Numerous studies have demonstrated that the friction coefficient and wear rate of the copper matrix composites can be reduced due to the addition of CNTs [49–51]. The main wear mechanism of Cu is serious adhesive wear. During the friction process, a significant amount of heat is generated on the contact surface, leading to minor plastic deformation of the copper surface accompanied by melting and micro-welding. This results in the formation of grooves and debris on the contact surface, increasing the friction coefficient (Fig. 3a), and the friction coefficient of copper will increase with the rise in wear rate. CNTs play a beneficial role in the tribological performance of CNTs/Cu composite. The uniformly dispersed CNTs exhibit a reinforcing effect on the matrix, enhancing the hardness of the composite while reducing plastic deformation and wear losses [50]. Additionally, at the initiation of the friction process, CNTs exposed on the contact surface break to form a carbonaceous film. The carbonaceous film possesses a self-lubricating ability, averting direct metal-to-metal contact and consequently reducing heat generation and adhesive wear. The main mechanisms during friction for CNTs/Cu composite gradually transition from plow wear mechanism [52] to flake formation-spalling mechanism [53,54] with the increase of CNTs content, as shown in Fig. 3b–d. In the friction process, plow wear occurs initially in the composite, leading to the appearance of neatly arranged grooves on the contact surface. This is attributed to the reciprocating motion of micro-convex bodies and a small amount of wear debris on the contact surface. Subsequently, inherent defects in the composite, such as voids, can induce internal cracks. Under frequent load cycling, these cracks eventually propagate to the surface, forming flake-like wear debris and undergoing spalling. Moreover, an excessively high CNT content increases the clusters of
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Fig. 3. SEM images of the worn surfaces of pure copper and the CNTs/Cu composites with different CNT contents. (a, b) The pure copper and the CNTs/Cu composites containing 0.6 vol% CNTs under dry friction conditions [52]; (c, d) The CNTs/Cu composites containing 5 vol% and 20 vol% CNTs [53].
Fig. 4. (a) Schematic representation of electron pathway in Cu with and without CNTs; (b) Plot of resistivity versus current density for the CNTs/Cu composite (red trace) and Cu (black trace) confirming approximately 100 times higher ampacity for nano-scale CNTs/Cu current pathways [70]; (c) Plots of void growth length as a function of the stressing time for pure Cu and CNTs/Cu composites stripes [71]; (d) Normalized current density versus position along the TSV diameter for SWCNT bundles, Cu/SWCNT composites, and Cu [72]. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
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CNTs in the composite, adversely affecting mechanical properties, resulting in decreased density, and weakening tribological performance [32,52,55].
2.2. Electrical properties
The electrical conductivity of individual CNT, as a unique one- dimensional material, can be explained by electron hopping [56] and electron tunneling [57] mechanisms. Charge flows through the shortest and least resistive pathways between CNTs while disregarding indirect connections. The conductivity of materials is determined by the distance traveled by electrons and the carrier density. Before further discussing the conduction mechanism in CNTs/copper composites, it is necessary to clarify the concept of mean free path. When electrons move in crystal materials, any deviation from a perfect crystal results in collisions, causing electrons to scatter from one state to another and lose some momentum. The mean free path is the average distance traveled by an electron before losing all its original momentum. Therefore, a larger mean free path corresponds to higher conductivity [58].
Although Cu already exhibits excellent conductivity among metals, the presence of small-angle scattering within the lattice reduces the mean free path of Cu to approximately 40 nm. Due to the structural characteristics of CNTs, electron transport along CNTs is predominantly unidirectional, and the CNTs themselves have mean free paths on the order of tens of micrometers, which means that the CNTs/Cu composites have the potential to be the ultralow resistivity materials in theory. Overall, the conductive network CNTs formed within the copper matrix greatly enhances the average free path of electron motion in the com- posite, as shown in Fig. 4a. Under these circumstances, both the inter- tube and intra-tube resistances of the CNT network significantly influ- ence the overall electrical conductivity of the composite [59]. The intra-tube resistance is naturally regulated by the attributes of the CNTs, such as their size and number of walls, while the inter-tube resistance is controlled by the arrangement and dispersion of the CNTs, and, importantly, the strength of the interfacial bonding between the CNTs and copper. Both CNTs and the copper matrix are excellent conductors themselves, so electron transport at the interface is particularly crucial. The weak coupling and hybridization between the electron orbitals of copper and carbon atoms greatly limit electron transport. Meanwhile, the imperfection of interfaces (such as interfacial gaps and impurities) can significantly affect the electron transport pathways. Any defects at the interface will increase electron scattering, thereby reducing the overall conductivity [60,61]. The interface between CNTs and Cu directly determines certain inevitable resistance generation effects, such as Coulomb blockades, resonant tunneling, and Fano resonances, and more complete accounts can be found in various articles [59,62–65]. It is worth noting that in ceramic-based and polymer-based CNTs composites [57,66–69], electron tunneling resulting from close contact between conductive fillers is the primary mechanism for electron conduction.
CNTs exhibit excellent electrical properties, including high carrier density, superior electromagnetic resistance, and low skin effect. The introduction of CNTs to enhance the electrical performance of Cu rep- resents a promising reinforcement technique [70,73,74]. The combi- nation of the outstanding conductivity of Cu and the diverse electrical properties of CNTs positions CNTs/Cu composites as compelling candi- date materials for electronic applications.
The increasing miniaturization of electronic devices demands high current-carrying capacity which is also known as current density. It is worth noting that high current density and high electrical conductivity are mutually exclusive properties. The former requires a strong bonding system, while the latter relies on free electrons from weak bonding systems. Subramaniam et al. [70] reported a method using a two-stage electrodeposition of Cu onto pre-patterned porous CNT films as tem- plates for electrodes. This approach enabled the fabrication of CNTs/Cu composites with nanoscale features of various sizes and shapes. All the complex structures obtained using this process, ranging from 500 nm to
failure
primary
electromigration
20 μm in width, exhibited current densities 100 times higher than any known metal while maintaining electrical conductivity comparable to Cu, as shown in Fig. 4b. They contended that such high current-carrying capacity can be primarily attributed to the inhibitory effect of CNTs on the and grain-boundary diffusion) of copper, Cu diffusion was thus smaller in composite compared with bulk Cu [75]. Fig. 4c illustrates the variation of void growth length with stress time during electromigration testing for pure Cu and CNTs/Cu composite. It can be observed that the average electromigration void growth rate in CNTs/Cu composite is approxi- mately four times smaller than that in pure Cu, indicating superior electromagnetic resistance in the composite. This is primarily attributed to CNTs serving as trapping centers for electrons, leading to a reduction in the diffusion of electromigration-induced migrating atoms [71].
pathways
(surface
CNTs are insensitive to the skin effect. In the case of through-silicon via (TSV) interconnects, where copper interconnects are severely affected by the skin effect, the edge current density is 2.4 times higher than the central density. In the case of pure SWCNTs, the observed current density is only 1.1 times the edge current density [72]. There- fore, incorporating pure SWCNTs into TSV results in a more even current distribution. The greater the proportion of added Cu, the more pro- nounced the sensitivity of the interconnect to the skin effect, as shown in Fig. 4d. In conclusion, CNTs/Cu composites can thus be considered promising materials for advanced interconnect applications that require high conductivity and electron migration resistance.
2.3. Thermal properties
Due to the strong covalent bonds between carbon atoms, heat transfer in CNTs is primarily governed by phonons rather than electrons, with thermal transport predominantly attributed to lattice vibrations that effectively propagate heat from the hot region to the cold region. In contrast, heat transfer in copper depends on the collisions between free electrons and metal cations. CNTs exhibit significantly higher thermal conductivity (ranging from 3000 to 6000 W/mK along the tube axis) compared to copper with thermal conductivity of 400 W/mK. The thermal conductivity of CNTs/Cu composites depends not only on the intrinsic thermal properties of the individual constituents but also on the interface, which is a key factor in determining the overall thermal conductivity. The interfacial thermal resistance significantly impacts heat transfer efficiency between the CNTs and the copper matrix. Currently, various models have been developed to describe the thermal conductivity behavior at the interfaces and to estimate interfacial thermal resistance. Three common models used to describe interfacial thermal transport are the Acoustic Mismatch Model (AMM) [76], the Diffuse Mismatch Model (DMM) [77,78], and the Two-Temperature Model (TTM) [79]. The AMM model assumes phonons at the interface behave as acoustic waves that are either fully transmitted or reflected. This model is based on the premise that the interface is ideally smooth, leading to specular reflection, meaning that phonons do not experience energy loss at the interface. Although the model was later improved by incorporating anharmonic atomic vibrations at the interface, it still has limitations due to its idealized assumptions. In contrast, the DMM as- sumes the interface is completely disordered, causing phonons to scatter diffusely and lose the memory of their incident wave vector and polar- ization upon reaching the interface. This model can establish an upper limit for interfacial thermal transport, constrained by the overlap of phonon density of states (PDOS). The TTM treats electrons and phonons as two interacting subsystems, effectively capturing electron-phonon coupling in the interface region. In metals and dielectrics, electrons and phonons are the primary heat carriers, respectively. The mismatch between these carriers leads to electron-phonon energy redistribution at the interface. Beyond traditional models, recent advancements have included modeling transient thermal transport in heterogeneous media using Brownian motion of walkers, which offers new insights into heat transfer mechanisms at interfaces [80]. Additionally, heat transfer in
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CNTs/Cu composites is not limited to the interaction between CNTs and the copper matrix but also includes the thermal transport between CNTs themselves. Low-frequency phonons (LFPs) play a crucial role in this process [81]. LFPs refer to lattice vibration modes with lower fre- quencies, longer wavelengths, and lower energy, which directly affect the thermal conductivity of materials. In nanocarbon materials such as CNTs, graphene, and graphene nanoribbons (GNRs), high phonon ve- locities and/or long mean free paths result in thermal conductivity primarily attributed to LFPs.
However, certain studies investigating the thermal conductivity of CNTs/Cu composites through simulations and experiments have observed that CNTs provide minimal enhancement to the thermal con- ductivity of composites. This is mainly due to the poor interface bonding and high interfacial thermal resistance caused by randomly oriented CNTs [82,83]. In CNTs/Cu composites where interface bonding and the arrangement of CNTs have been improved, efficient heat exchange be- tween CNTs and the copper matrix is enabled, allowing effective con- duction of heat within the copper matrix and resulting in a slight enhancement of the composite thermal conductivity [29,84].
The introduction of CNTs also can enhance the thermal stability of the copper matrix composites. CNTs exhibit minimal expansion during the heating process and may even undergo contraction [85]. When there is a high shared contact area between CNTs and the copper matrix, the low thermal expansion and high bulk modulus of CNTs contribute to offsetting the thermal expansion of Cu. Thermal Distortion Parameter (TDP) which represents the ratio of the coefficient of thermal expansion (CTE) of the material to its thermal conductivity serves as an indicator of the level of distortion due to temperature. A lower TDP signifies greater thermal stability. The CNTs-reinforced composite achieves a minimum TDP value of 0.037 compared to 0.055 for pure copper, highlighting its significant potential as a thermal management material [86]. The stra- tegic utilization of CNTs with their ultra-low thermal expansion coeffi- cient can address a major challenge in the application of copper matrix materials in the field of electronics, particularly the vulnerability of copper with its high thermal expansion coefficient to device damage. Sundaram et al. [87] reported a macroscopic CNTs/Cu composite in the form of a wire with an extremely low coefficient of thermal expansion. The uniform blend of 40 vol% CNTs in a continuous copper matrix –1, which is resulted in the composite with a CTE of 4.42 × 10 merely 26% of the CTE of copper.
(cid:0) 6 ◦
C
- Key factors
The ideal scenario where CNTs fully manifest their inherent prop- erties within a composite is uncommon. The attributes of CNTs them- selves, their dispersion within the composite, the overall structure of the composite, and the interface between copper and CNTs all play roles in influencing the overall performance of the composite. This chapter ex- plores the mechanisms by which these interrelated factors impact the performance of composite, along with common methods for their modulation, providing an in-depth investigation into the entire process of how CNTs function within the composites.
3.1. Attributes of CNTs
As mentioned earlier in the article, CNTs can be classified into various types, and each type of CNT exhibits distinct structures and properties [88–91]. Additionally, the size and number of walls in a specific type of CNT also impact its intrinsic performance [72,92,93]. Therefore, the diverse structures and types of CNTs determine the versatility of their performance, which influences the final properties of the CNTs/Cu composite.
As the wall number of CNTs continues to increase, the mechanical strength of the composites decreases. Nayan et al. prepared the copper matrix composite reinforced with 5 vol% and 10 vol% SWCNT, and MWCNT using high-energy ball milling and SPS. The ultimate tensile
strength (UTS) of the composite peaked at 463 MPa with 5 vol% SWCNT, whereas the UTS for 5 vol% MWCNT-reinforced copper composite was only 395 MPa. This trend of decreasing UTS with an increasing number of CNT walls continued at 10 vol%, with UTS values of 302 MPa and 209 MPa, respectively. This is attributed to the lower bonding strength be- tween the concentric walls of MWCNTs [94]. Since it is the inner walls that attract the outer walls towards the center of the MWCNT chain, which is referred to as inter-tube effects, and makes the outer walls move away from the copper matrix. Therefore, the interface bonding strength between MWCNTs and the copper matrix is weaker than that between the SWCNTs and the copper matrix [95], and thus, separating MWCNT chains from copper requires less loading. In addition, by decreasing the diameter of MWCNTs, the gap between the inner and outer walls is subsequently reduced, thus reducing the inter-tube effect on the inter- face. It can be confirmed that reducing the diameter of MWCNTs will enhance the bonding between the CNTs and the copper matrix and in- crease the strength of the composites [96,97]. It is worth noting that the pull-out force of SWCNTs in the copper matrix is increased with the increase of the SWCNT diameter, due to the increasing contact area between the tube walls and the copper. Some studies utilize simulations to determine the pull-out force as shown in Fig. 5a, the pullout force is found to be proportional to SWCNT diameters.
It has been proposed that the strengthening mechanism of CNTs- reinforced metal matrix composites varies with the aspect ratio or the length of the CNTs, and when the aspect ratio exceeds 40, the strengthening mechanism of the composites is dominant by load transfer [98]. However, when the aspect ratio decreases to less than 10, the strengthening mechanism induced by the CNTs in the metal matrix shifts towards the Orowan mechanism (Fig. 5b). For the long CNTs with a length exceedingly approximately half the length of the dislocations, it becomes difficult for the dislocations to interact with the CNTs in Oro- wan mechanism, and load transfer becomes the primary strengthening mechanism. Meanwhile, due to the accumulation of dislocations, stress concentration is easy to form and the ductility of the composite is generally reduced as the strength increases when incorporated with long CNTs. On the other hand, for the short CNTs with a length much shorter than the dislocations, the CNTs play the role of secondary phase particles that can create dislocation loops when dislocations pass by. In the case of the Orowan mechanism, dislocations can bypass short CNTs and continue moving within the matrix, resulting in the possibility of further deformation of the metal matrix, thus being beneficial for improving the ductility of the composites.
The electrical conductivity of CNTs is influenced by their chirality, number of walls, and diameter. Several studies have reported that the electrical conductivity of s-SWCNTs and m-SWCNTs is approximately 105 S/m and 108 S/m, respectively, while MWCNTs exhibit an electrical conductivity of around 2.2 × 104 S/m [89,90]. The intrinsic electrical conductivity of CNTs is essential, however, our primary interest lies in their behavior within the copper matrix. Notably, in the composites utilizing the semiconductor-type and metallic-type SWCNTs with small diameters as reinforcements, only metallic-type CNTs contribute to the conduction of the composite. When using CNTs with large diameters, both metallic and semiconductor-type CNTs need to be considered for their joint contribution [88]. Furthermore, the SWCNTs with small di- ameters and MWCNTs with large diameters present superior electrical performance [72], and experimental evidence indicates that the per- formance of SWCNTs/Cu wires surpasses that of the MWCNTs/Cu wires, exhibiting an approximately threefold higher four-probe electrical conductivity at room temperature. Meanwhile, SWCNTs/Cu wires display four times lower temperature coefficients of resistances (TCR) [92].
The friction coefficient and wear rate of CNTs/Cu composite decrease with an increase in the number of CNT walls. During the fric- tion process, the ellipticity and collapse of double-wall CNTs are more common phenomena, whereas the CNTs with a larger number of walls exhibit lower shear resistance. Consequently, the CNTs with a larger
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Fig. 5. (a) Pullout force for CNTs/Cu nanocomposites with various CNT diameters, controlling the diameter by chirality [99]; (b) Different enhancement behaviors of small- and large-length CNTs in grains [98].
number of walls are more prone to fracture, which undoubtedly con- tributes to the formation of a self-lubricating carbon film in the com- posite during the friction process [100].
The diameter of CNTs has a minimal impact on the thermal con- ductivity of the CNTs/Cu composite. Instead, the structural integrity of CNTs significantly influences the thermophysical properties of the composite. CNTs with fewer structural defects contribute to both phonon and electron conduction, facilitating heat transfer [101,102]. Wang et al. numerically designed a class of sandwich-like CNT/Cu/CNT nanotubes and conducted molecular dynamics simulations to investi- gate the thermal conductivity of the composite (Fig. 6a). PDOS is commonly employed to comprehend the thermal transport behavior across interfaces. The same sp2 bonding and negligible diameter factor dictate the resemblance of PDOS frequency curves among CNTs with varying numbers of walls (Fig. 6b). However, as the number of walls in CNTs within the copper matrix composite changes from monolayer to trilayer, more substrate interactions occur between MWCNTs and the copper matrix. Consequently, an increased number of phonons partici- pate in the thermal transport process, leading to the enhancement of the phonon density, thereby augmenting the thermal transport capability of the composite (Fig. 6c). This study solely considers lattice thermal
conductivity, neglecting the influence of electrons, hence introducing a certain degree of error in the simulation results. However, noteworthy is the unique advantage of investigating the thermal conductivity of composite using atomic stress analysis and PDOS analysis.
In conclusion, to tailor the properties of CNTs/Cu composites to meet the demand for applications, there is a requirement for various types and sizes of CNTs. The attributes of CNTs can be controlled by selecting proper preparation methods and process parameters, and the commonly used preparation methods include electric arc discharge method, laser ablation method, thermal synthesis process, chemical vapor deposition (CVD), vapor-phase growth, flame synthesis method, and plasma enhanced chemical vapor deposition. Among these techniques, CVD is widely utilized to produce CNTs with different diameters, lengths, and chiralities. During the CVD process, the carbon source sample is heated to around 700 C inside a quartz tube, and the nucleation of CNTs is initiated at the position of a metal catalyst (such as cobalt, nickel, and iron) in a controlled gas environment. Both metallic and non-metallic catalysts are employed in the CVD method to manipulate the CNT structure. For instance, small-diameter catalysts like iron nanoparticles are utilized for producing CNTs with small diameters. Catalyst material, reaction gas, substrate quality, system temperature, and pressure are
◦
Fig. 6. (a) Atomic configurations of the sandwich-like CNT/Cu/CNT tubular nanocomposite cases with varied outer CNT layers; (b) Phonon density of states of pristine copper nanowire, isolated single-walled, double-walled and triple-walled CNTs, and the sandwich-like CNT/Cu/CNT tubular nanocomposite cases with varied outer CNT layers at 300 K; (c) The enhancement factor and lattice thermal conductivity of the tubular nanocomposite structure cases with varied outer CNT layers at 300 K [105].
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among the factors that influence the CNT structure and purity, while the length of CNTs depends on the growth time. By controlling these con- ditions, CNTs that fulfill the desired applications can be obtained [103]. Furthermore, physical cutting, chemical cutting, and physical/chemical cutting techniques have proven effective in reducing the length of CNTs [104]. These approaches provide researchers with a robust theoretical and experimental foundation for flexibly manipulating the properties of CNTs.
3.2. Dispersion of CNT
Lightweight conductors with low density are certainly requirements in electrical applications. Therefore, it is crucial to achieve the com- posite with a well-defined macroscopic and microscopic structure, even along with a high content of CNTs (>90 wt%). It is essential to maximize the volume fraction of nanotubes while ensuring their uniform disper- sion and homogeneous mixing with copper to attain the objective.
As a reinforcing material, CNTs possess exceptional performance and exhibit unique physical properties, and a high concentration of CNTs is beneficial for enhancing the strength, electrical properties, and tribo- logical properties of the CNTs/Cu composites [50,106]. In most studies, the performance of the composite initially increases and then decreases with increasing CNT concentration. Deng et al. [43] reinforced the copper matrix composite with 0.3, 0.5, 1.0, and 2.0 vol% MWCNT. The UTS of the composite peaked at 307 MPa with 0.5 vol% MWCNT. However, further addition of MWCNT up to 2.0 vol% reduced the UTS to 256.6 MPa. It is recognized that this is due to the severe agglomeration of CNTs in high-concentration composite powders that result in the properties reduction (Fig. 7). In the investigations of CNT agglomeration phenomena, it has been observed that the agglomeration of CNT in saturated Ca(OH)2 solution is a spontaneous process highly sensitive to time parameters [107]. Specifically, over the course of time, CNTs exhibit a rapid tendency to form small, nearly parallel bundles, which subsequently evolve into three-dimensional network agglomerates. Within CNTs/Cu composites, the primary factors contributing to CNT agglomeration are the mismatch in density and van der Waals forces. Notably, CNT density ranges from 1.4 to 1.8 g/cm3, while copper density is 8.9 g/cm3, thus there is a significant density difference between CNTs and copper. This disparity in physical properties results in the pro- pensity of CNTs to agglomerate, and this phenomenon cannot be avoi- ded in most fabrication methods. Furthermore, the substantial specific surface area of CNTs and the pronounced van der Waals forces between the tubes drive the aggregation of CNTs into bundles. Currently, the issue of CNT agglomeration remains challenging for achieving high-performance CNTs/Cu composites with high CNT concentrations
[35,43,94,102].
An abundance of research showcases the favorable influence of a good dispersion of CNTs on the strength effect of CNTs/Cu composites. The dispersion of CNTs leads to a pronounced effect on improving the load transfer efficiency, consequently bolstering the strength of the composites [36,108]. The agglomeration and entanglement of CNTs in the composite with a high CNT concentration significantly affect the load-carrying capacity, thereby reducing the reinforcing effect on the composite. CNT aggregation is not the primary factor contributing to conductivity loss. The main cause is the inadequate bonding between CNTs and the copper matrix, resulting in more significant conductivity deterioration. However, the CNT aggregation decreases the average free path of electrons and phonons, which indeed results in the weakening of the ballistic transport ability of CNTs [59]. Consequently, the thermal conductivity and the electrical conductivity of high-concentration CNTs/Cu composites are compromised.
A high CNT content proves advantageous as the CNTs promote the formation of substantial carbon film during the frictional process, and the composites featuring well-dispersed CNTs exhibit effective preven- tion of the alloy particle detachment, both of which optimize the tribological performance of CNTs/Cu composites [50].
Employing targeted preparation methods can effectively address the issue of CNT aggregation. Commonly utilized methods include electro- plating [109], chemical plating [86], and molecular-level mixing [108]. In practical fabrication processes, pre-treatment of CNTs is a widely employed strategy to optimize their dispersion in CNTs/Cu composites, and different techniques are often combined to ensure the dispersion of CNTs. Considering the inherent tendency of CNTs to spontaneously aggregate and the density difference between CNTs and copper that may lead to stratification, an approach involving ED in conjunction with SPS proves effective. Firstly, a uniform copper coating is applied to the surface of CNTs using ED. Subsequently, copper powder is mixed with the copper-coated CNTs to obtain a composite powder with varying volume fractions of CNTs. Finally, the composite is rapidly consolidated using the SPS process. This set of preparation methods addresses both issues mentioned earlier simultaneously and the preparation of copper matrix composite containing uniformly dispersion high-volume fraction CNTs can be efficiently achieved [110]. Besides, using the acid treat- ment method can introduce oxygen-containing functional groups at CNT defect sites [111]. These oxygen-containing functional groups on the surface generate negatively charged surface charges, thereby facilitating the improved dispersion of CNTs through electrostatic repulsion. How- ever, excessive acid treatment can result in significant structural damage to CNTs, thereby impacting their reinforcing effect on the copper matrix composites [112].
Fig. 7. SEM micrographs of the fractured CNTs/Cu composite surface after tensile tests: (a) Cu–0.3 vol% CNTs, (b) Cu–0.5 vol% CNTs, (c) Cu–1.0 vol% CNTs, (d) Cu–2.0 vol% CNTs; (e) The corresponding stress–strain curves of the CNTs/Cu nanocomposites [43].
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3.3. Architectures in composites
Based on the different distributions and arrangements of CNTs, the properties of CNTs/Cu composites with different architectures are significantly different and exhibit strong controllability. The three typical architectures of the CNTs/Cu composites are homogeneous, alignment, and laminate structure, as shown in Fig. 8a. Homogeneous structure refers to an isotropic structure that CNTs are uniformly distributed in the copper matrix with random orientations, and the properties of the composite exhibit isotropy (Fig. 9a and c). Alignment architecture is an anisotropic structure, the corresponding CNTs present continuous alignment in a single direction in three-dimensional space and are aligned in the same direction, forming interconnected connec- tions. This structure allows for the full exploitation of the one- dimensional properties of CNTs, and the composite generally exhibits remarkable performance in a specific direction (Fig. 9b and d). The laminate structure is a typical composite material architecture inspired by the “brick and mortar” structure of nacre. In CNTs/Cu composite, CNTs and Cu alternate in layers, with continuous CNT networks in the carbon nanotube layers, which can fully exploit the performance po- tential of CNTs (Fig. 10).
Each of these architectures offers unique properties and performance characteristics, and the choice of the structure depends on the specific requirements of the composites for the applications. In the composite with a laminate structure, the reinforcement mechanisms primarily involve load transfer strengthening, grain refinement mechanism, and dislocation blocking effect induced by the laminated structure, while the Orowan mechanism plays a minor role. CNTs uniformly dispersed along the plane direction in the laminate structure exhibit robust load-sharing capability and high efficiency in load transfer [114]. Shuai et al. [113] investigated the performance of composite with two different laminate structures featuring distinct CNT distribution patterns (Fig. 8b). The study revealed that the composites with a cross-dispersed distribution of CNTs exhibit higher strength. At a CNT content of 5 vol%, the composite achieved peak strength, with tensile strength and yield strength reach- ing 336 MPa and 246 MPa, respectively. In contrast, the composite with the same CNT concentration but without aligned CNTs showed tensile and yield strengths of approximately 260 MPa and 180 MPa,
Fig. 8. Schematic of (a) homogeneous, laminate, and alignment structure of CNTs/Cu composites and (b) laminar structure with unidirectional and cross- ply (/0
/) of CNT orientation [113].
◦ /90
◦ /90
/0
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respectively. During the process of plastic deformation, the CNTs exert a pronounced refining effect on their surrounding copper matrix, and compared with unidirectional stacking, the refinement effect of CNTs with the arrangement of orthogonal stacking in the composite has been proposed to be more significant. The presence of fine grains and twin boundaries in the copper layers further contributes to the enhanced strength and ductility of the composite [113]. In the meantime, the CNTs with high alignment between the copper layers act as bridges, enabling the full exploitation of their ballistic transport mechanism. Conse- quently, in current-carrying capacity, electrical conductivity, and thermal conduc- tivity [115].
improvements,
significant
composite
exhibits
the
The alignment structure composites enable the full exploitation of the one-dimensional nature of CNTs, compared to the laminate struc- ture. Extensive studies have demonstrated that copper matrix compos- ites with aligned CNTs exhibit remarkable tensile strength and elongation along the longitudinal direction (parallel to the nanotube axis) [40,117–119]. The excellent shear strength along the axis of CNTs cannot contribute to the load transfer efficiency in the longitudinal di- rection of alignment structure composites. The reduction in hindrance to dislocation movement results in the preservation of a favorable level of longitudinal elongation in the composite [42]. Simultaneously, the grain refinement mechanism ensures the high strength of the alignment structure CNTs/Cu composites. By combining physical vapor deposition (PVD) and stretching treatment, the tensile strength of CNTs/Cu com- posite fibers can be increased from 258 MPa to 515 MPa [118]. Incor- straight, and small-diameter aligned CNTs can porating long, significantly further enhance the mechanical properties of the metal matrix composites with an alignment structure.
CNTs aligned in a specific orientation can significantly influence the frictional properties and induce the anisotropic friction behavior of the composite [33,34,119]. In the case of CNTs/Cu composites, as discussed in Section 1.1, CNTs undergo detachment during friction and create a self-lubricating film on the material surface, thereby enhancing the friction coefficient and reducing the wear rate of the composite. In copper matrix composites with aligned CNTs, when the frictional surface is perpendicular to the axial direction of CNTs, the CNTs experience radial cutting or point-by-point grinding during high-friction processes. Conversely, when the frictional surface is parallel to the axial direction of the CNTs, a significant number and surface of CNT walls contact with the frictional surface. Consequently, the formation rate and thickness of the self-lubricating film on the frictional surface perpendicular to the CNT axial direction are considerably lower than those on the surface parallel to the axial direction. This discrepancy results in the distinct friction coefficients and wear rates of the composites in the two di- rections (Fig. 11). The friction coefficient and wear rate of the 5 vol% CNTs/Cu composites were 74.2% and 77.8% lower, respectively, when measured parallel to the SD plane compared to perpendicular to it [119]. The previous discussion has expounded on the mechanisms by which CNTs enhance the electrical properties of copper matrix composites. CNTs play a beneficial role in improving the overall mean free path of the composite, and the enhancement is fully realized when the CNTs are aligned in a specific orientation. Through such alignment, the CNTs form continuous and interconnected one-dimensional channels that effec- tively exploit this enhancement. Furthermore, when CNTs are arranged in an oriented manner and connected end-to-end, the dominant contact mode between the CNTs and the copper matrix is the end-contact mode, during electron movement along the direction of CNT alignment. Compared to the side-contact mode, the end-contact mode with free radicals is highly effective in reducing the contact resistance [120]. Moreover, highly aligned CNTs facilitate the achievement of ultrahigh current-carrying capacity. A multitude of charge carriers tunnel through the copper matrix and inject into the CNTs, where they undergo ballistic transport without scattering. This profoundly enhances the electrical conductivity and current-carrying capacity of the composite [118,121]. It is indeed factual that the thermal conductivity of CNTs along their
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Fig. 9. SEM and TEM images of the SWCNTs/Cu composites: (a, c) homogeneous structure (forged) and (b, d) alignment structure (die-stretched). The arrows show the SWCNTs [116].
Fig. 10. (a) SEM images and (b) metallographic graphs at the parallel cross-section of the Cu–0.52 vol% SACNTs composite [115].
Fig. 11. (a) Friction coefficients and (b) wear rates of pure Cu and CNTs/Cu composite samples after forging, die-stretching on the plane perpendicular to the stretching direction (SD), and die-stretching on the plane parallel to the SD [119].
longitudinal axis is nearly two orders of magnitude higher compared to the perpendicular direction [122]. Consequently, the composites embedding highly aligned CNTs can manifest exceptional thermal properties along a specific direction [115]. Nevertheless, certain in- vestigations have revealed that the thermal conductivity of CNTs/Cu
composites, irrespective of the arrangement of CNTs within the copper matrix, falls short of that exhibited by pure copper. This observation signifies that attaining elevated thermal performance in the composite necessitates robust interfacial bonding strength to synergistically enhance the thermal properties [116].
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Currently, the deposition of copper onto a highly oriented CNT substrate proves to be an effective approach to obtain highly aligned CNTs/Cu composites. Techniques such as electron beam evaporation [121], electroplating [115,123], physical vapor deposition [118], elec- trodeposition [113], and others are specifically utilized for the deposi- tion process. Through precise control of parameters, aligned CNTs can be generated on a metallic substrate, and various deposition methods are employed to combine CNTs with copper. This approach enables control over CNT alignment and the achievement of a favorable interface bond. However, the resulting composite often exhibits relatively low density, necessitating subsequent processes such as mold stretching or compression treatment. Plastic deformation such as drawing [116,118, 119], hot extrusion [34,40], rolling [23,28], and especially severe plastic deformation is common to be utilized to redistribute the orien- tation of CNTs within the composite, through which randomly dispersed CNTs within the copper matrix are compelled to reorient and align along the direction of strain, as shown in Fig. 12. To obtain compact CNTs/Cu composite and overcome the challenges of achieving a perfectly aligned structure, the deposition of copper and plastic deformation methods are often combined. It is well known that the severe deformation process is a commonly used method for the strengthening of copper. Owing to the pinning effect of CNTs, the generations of low-angle grain boundaries and high-density twins are promoted [113,124], which can contribute to the refinement of the deformation grains and the enhancement of the mechanical properties of the composites.
It is worth mentioning that the utilization of magnetic fields to enhance the alignment of polymer-based CNT composites has attained a high level of maturity, and it has also been applied to prepare CNTs/Cu composites. However, significant differences exist between polymer and copper matrices, and the applicability of the magnetic field method in copper matrix composites is somewhat limited [125], with relatively limited research conducted in this domain. Currently, remarkable studies have been carried out to prepare CNTs/Cu composites through magnetic field treatment. Introducing pre-electrodeposited nickel layer on the CNTs and then followed by the magnetic field treatment, the composite presents a commendable overall performance, e.g., high tensile strength of 292 MPa, longitudinal elongation of approximately 34%, longitudinal electrical conductivity of 93% IACS, and longitudinal thermal conductivity of 339.8 W/mK [117].
In addition to the aforementioned three structures, researchers have explored many intriguing structures in CNTs/Cu composites. This paper highlights two such structures: CNT–Cu core-shell wires [126,127] and leaf-like CNT–GNRs Cu-based composites [128,129]. The CNT–Cu core-shell wires were obtained by depositing copper atoms onto CNT fibers, followed by deformation or heat treatment, resulting in a struc- ture similar to aluminum-clad copper wires (Fig. 13a and b). Chen et al. [126] utilized copper pre-seeding and thiol functionalization methods, achieving the composite wires with a current carrying capacity of (1.12
Fig. 12. Schematic representation of the structural transformation of CNTs/Cu composites during the rolling process.
± 0.03) × 105 A/cm2 and a tensile strength of 1.2 GPa. The leaf-like CNT–GNRs mixture is a novel reinforcing material where CNTs and GNRs form a leaf-like structure (Fig. 13c and d). The “midrib” CNTs facilitate uniform dispersion, while the “leaf blade” GNRs provide ample interfacial contact area. Cao et al. [129] prepared the composite via electrodeposition, achieving a thermal conductivity of 527 W/mK and a hardness of 226.8 HV.
3.4. Interface
◦
Interfaces are essential for connecting the CNTs and the copper matrix, and thus the microstructure, interaction, and properties of the interfaces are always the central topics issues in the existing researches for developing high-performance CNTs/Cu composites [59]. The wet- ting between CNTs and copper is remarkably poor with a contact angle of 145 . Meanwhile, the significant difference in the thermal expansion coefficient between CNTs and copper gives rise to void formation at the interfaces, leading to weak interface bonding in the CNTs/Cu compos- ites. The weak mechanical bonding hampers effective load transfer across the interface when the composite is subjected to loads, and even allows the CNTs to potentially slide out of the copper matrix. At present, many of the prepared CNTs-reinforced copper composites do not exhibit an ideal excellent electrical conductivity, and the main reason lies in the inadequate bonding at the interface between CNTs and the copper ma- trix, resulting in significant conductivity deterioration. Moreover, the weak interface bonding amplifies the potential barrier at the interface, impeding the transfer of electrons between CNTs and the matrix. This, in turn, hinders electron-phonon coupling and increases electron and phonon scattering at the interface [60,130]. Therefore, improving the ability of the copper to wet CNTs to enhance the interfacial bonding can substantially improve both the strength and the conductivity of the composites.
Seamless integration serves as an ideal solution to address interface issues by enhancing load transfer efficiency and reducing energy losses during electron and phonon transport. Commonly employed methods for achieving seamless integration between CNTs and copper include surface alloying of CNTs [131–133], surface functionalization of CNTs [117], and in-situ growth techniques [134–136]. These approaches aim to promote strong bonding and intimate contact at the interface, thereby improving the overall performance of the composite.
In general, the interactions between CNTs and metals can be cate- gorized into two major types. The first type includes metals such as Cu and Al, where only weak cohesive forces exist at the interface region. However, another type includes metals like Ni and Co, which exhibit strong bonding between their d orbitals and carbon atoms. Therefore, utilizing surface alloying techniques with appropriate metals (such as Co, Ni, and Ti) to modify the surface of carbon nanotubes can effectively improve the interface bonding between CNTs and copper [99,133,137]. The originally weak van der Waals forces between CNTs and copper are transformed into strong attractive forces between Ni atoms and CNTs as well as Ni atoms and Cu atoms. Duan et al. [99] utilized molecular dy- namics to investigate the pull-out force of CNTs in CNTs/Cu composite. The results revealed that the pull-out force of CNTs in Ni-coated com- posites is five times higher than in non-Ni-coated ones, as shown in Fig. 14a. Furthermore, since the doped metals are added locally at the interface, their influence on the lattice parameters of the copper matrix can be disregarded. In the meantime, the improved interface bonding achieved through the doped metals is beneficial for maintaining a high level of electrical conductivity of the composite [138]. Besides, the presence of a metal interlayer between CNTs and copper contributes uniquely to thermal conductivity. The intensity of the PDOS for CNTs is much higher than that of Cu atoms, resulting in a considerable PDOS mismatch at the Cu–CNT interface, which contributes to the interface thermal resistance. In contrast, the intermediate layer of Co atoms ex- hibits greater overlap with the PDOS of CNTs, as shown in Fig. 14b. This
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Fig. 13. (a, b) SEM images of the fatigue fracture surface of CNTs/Cu core-shell wire [126]; (c, d) TEM images of GO–CNTs hybrid reinforcement in the GO–CNTs/Cu composite [129].
Fig. 14. (a) Pullout force curves of CNTs/Cu and Ni-coated CNTs/Cu nanocomposites [99]; (b) PDOS of the Cu–M–CNT interfaces. (M: Co, Cu and Au) [139].
increased overlap indicates effective phonon coupling at the Co–CNT interface, leading to an improvement in interface thermal conductivity. In numerous studies, it has been observed that the presence of oxy- gen functional groups not only facilitates electronic exchange between Cu and carbon atoms but also strengthens the interface bonding strength between Cu and CNT, playing a crucial bridging role. The functionalized interface between carbon nanotubes and copper exhibits stronger bonding interfaces, such as Cu–O–C and Cu–N–C. To establish robust bonding, functionalization of CNTs can be applied in conjunction with alloyed CNTs, as shown in Fig. 15a. This significantly contributes to the enhancement of the mechanical performance of composite [117,140, 141]. There are various methods for functionalizing CNTs, such as oxidation, amination, N-doping, and thiol functionalization (Fig. 15b). Milowska et al. combined experiments and calculations to investigate the effect of different CNT surface modifications on the properties of
indicate that oxidation, amination, and composites. The results N-doping can all enhance interface binding forces. However, oxygen-containing functional groups act as charge carrier traps at the interface, reducing carrier density in the system and increasing back-scattering. In contrast, N-doping reduces the back-scattering of Cu around CNTs. This allows N-doping to effectively improve both the mechanical and electrical properties of the composite simultaneously [142]. Daneshvar et al. investigated three common functionalization groups, namely, carboxyl, N-doping and thiol. The results indicate that thiol-activated CNTs in CNTs/Cu films exhibit conductivity 4 times and 7 times higher than systems with carboxyl and N-doped surface groups, respectively. The reason is that thiol groups have lone pair electrons that can be deprotonated, forming strong bonds with Cu [143]. Furthermore, compared to polymer-functionalized CNTs, covalently functionalized improving the interface thermal CNTs are more beneficial
for
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Fig. 15. (a) The schematic diagram of the oxygen-mediated bonding (Cu–O–C and Ni–O–C) and the formation of Cu2O [117]; (b) A schematic describes the ap- proaches and processes for preparing the CNTs/Cu composites [134].
conductivity of the composite. This is primarily attributed to the strong coupling between nano-copper ions covalently attached to CNTs and the copper matrix [144]. Functionalized CNTs have significant advantages in improving interface binding; however, it’s essential to note that the
degree of CNT modification is not strictly positively correlated with the final performance of the composite. As processing time increases, the performance reaches a peak, emphasizing the importance of selecting an appropriate degree of modification to achieve better composite
Table 1 – Summary of the CNTs/Cu composites: composition, methods, properties, and strategies associated with the key factors of CNTs.
Composite composition
Fabrication method
Mechanical properties
Electrical properties
Key strategies
5 vol% SWCNT/Cu SWCNT/Cu wire
High-energy ball-milling, SPS Two-step Cu ED
UTS: 463 MPa
Electrical conductivity (EC): 2.1 × 105 S/cm
5.0 vol% CNT/Cu
ED
Cu–CNT
Spray pyrolysis, flake powder metallurgy
MWCNT/Cu wires
Cu–Cu/CNTs–Cu composite foil
MWCNTs/Cu
nanocomposite
3.0 vol% MWCNTs/Cu
5 vol% Ni/Cu coated
SWCNTs–Cu
Ni@MWCNTs/Cu
Acid free wet spinning, periodic pulsed reversed electroplating ED
ED, SPS
High-energy ball-milling and high-ratio differential speed rolling Electroless plating, ultrasonic-assisted mechanical agitating, hot forging and die- stretching Surface alloying of MWCNTs, sintering, hot extrusion and cold drawing
1.04 vol% MWCNT/Cu
Electroplating
CNT/Cu wires
Cu–CNT films
1 wt%
CNT–COOH@Cu
0.2 wt% CNTs/ Cu–0.5Ti
CNT/Cu core-shell
fibers CNT/Cu–Cr
Plasma-enhanced chemical vapor deposition, electron beam evaporation, inductively coupled plasma Thermal chemical vapor deposition technique
Functionalized nanotubes, reduction of copper ions, ball-mill, cold press and SPS Surface alloying of CNTs, flake powder metallurgy Electroless deposition, functionalize with thiol groups High-energy ball-milling, hot pressing
CNTs/Cu
Pre-press shaping, CVD and SPS
Tensile strength (TS): 336.3 MPa Yield strength (YS): 246.0 MPa UTS: 353.9 MPa
TS: 582 Mpa Vickers hardness: 2.38 GPa YS: 692 MPa
YS: 417 MPa UTS: 500 MPa Hardness: 174 HV TS: 371 MPa
UTS: 381 MPa compressive strength: 463 MPa COF: 0.21 Wear rate: 0.41 × 10 (cid:0) 1m (cid:0) 3N mm TS: 287.2 MPa YS: 213.6 MPa
(cid:0) 1
(cid:0) 5
EC: 5.5 × 107 S/m specific EC: 9.38 × 104 Scm2/g
Electrical resistivity: 2.1341 μΩ cm Ampacity: 11,667 A/cm2 Ampacity: (1.7–2.6) × 107 A/cm2
Preferred types of CNTs Preferred types of CNTs
Homogenous dispersion, controllable orientation, laminated structure
Uniform dispersion and strong interface bonding Uniform dispersion
Uniform dispersion
Uniform dispersion and strong interface bonding Directional alignment and uniform dispersion Directional alignment and strong interface bonding
Directional alignment, uniform dispersion and strong interface bonding
Superaligned MWCNT arrays
[115]
Super-aligned CNTs
Current density: 20 mA/ cm2
Vertically aligned CNTs
Functionalized nanotubes and strong coupling Strong interface bonding
UTS: 407 MPa
TS: 1 GPa
EC: 3.7 × 107 S/m
Functionalized nanotubes
Hardness: 157 HV TS: 388 MPa TS: 421.2 MPa
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Strong interface bonding
EC: 84.6% IACS
Strong interface bonding
Ref.
[94] [92]
[113]
[135]
[73]
[111]
[27]
[23]
[119]
[33]
[121]
[147]
[144]
[138]
[127]
[132]
[148]
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performance [145,146]. It is worth noting that Sreekanth et al. con- ducted a study on the field emission characteristics of vertically aligned CNTs modified with copper oxide. Ultimately, they achieved a current density of the order of 20 mA/cm2 at a field of 5 V/μm. This achievement is attributed to the itinerant electrons between CuO and CNT and the small barrier [147]. This discovery underscores the potential of CNTs/Cu composite as electron sources for field emission in future vacuum microelectronic devices.
Overall, in many conventional processing methods, the functional- ized interfaces introduced by functional groups (e.g., Cu–O–C and Cu–N–C) is beneficial due to poor interfacial wettability. These func- tionalized interfaces enhance interface bonding, facilitate electron ex- change, and reduce electron and phonon scattering, thereby improving the mechanical performance and electrical conductivity of the com- posite. Modified with Co and Ni metals can form strong covalent bonds with carbon instead of Cu and enhance phonon coupling, increasing load transfer efficiency and interface thermal conductivity. This leads to a comprehensive improvement in the mechanical, electrical, and thermal properties of the composite.
al.
[126]
In Section 2.2, it is mentioned that the deposition methods or in-situ growth methods can improve the dispersion of CNTs, and this method can also enhance the interface bonding between the CNTs and copper matrix. Chen et employed pre-seeded copper on thiol-functionalized CNTs as nucleation sites for subsequent copper ion deposition and prepared a composite wire with continuous thin crys- talline copper coating. The composite wire exhibited a 120% increase in specific current-carrying capacity, a 30% higher electrical conductivity, and a tensile strength is 1.2 GPa which is six times that of regular copper wire. Chen et al. [148] have exploited a novel approach involving a three-dimensional structure of the composite. The Cu–Cr alloy powder was compacted into a three-dimensional porous structure and then fol- lowed by the deposition of CNTs, resulting in the fabrication of a three-dimensional The three-dimensional porous structure enhances the quality and interface bonding strength of CNTs while also preventing contamination during the synthesis of CNTs.
interconnected
composite.
CNTs/Cu
Researchers have employed various fabrication strategies to achieve superior composite performance. Table 1 summarizes the properties of the CNTs/Cu composites obtained through different fabrication methods and highlights the key strategies emphasized in each study. Several trends can be observed from the summarized studies. When optimizing carbon nanotube properties, a moderate amount of well-integrated SWCNTs provides the best reinforcement effects. Additionally, uni- form dispersion and strong interfacial bonding are the most common and effective reinforcement strategies. Finally, the designed alignment of CNTs imparts significant anisotropy to the composites. Notably, one study utilized nanoscale raw materials to prepare MWCNTs/Cu nano- composites, achieving a yield strength of 692 MPa [27], which is among the highest strengths reported, second only to CNTs/Cu fibers.
- Trade-off of properties
The strengthening effect of metal materials depends on dislocation accumulation, while good conductivity relies on unimpeded electron mobility, and favorable plasticity necessitates reducing stress concen- trations. An increase in the number of dislocations is bound to lead to significant electron scattering. The introduction of a reinforcement phase to increase the number of dislocations also inevitably results in stress concentrations. Therefore, trade-offs between strength and conductivity as well as strength and plasticity are perma- nent challenges for researchers in improving and developing new metal and metal matrix composites. CNTs/Cu composites, as innovative so- lutions in material design and engineering, offer new opportunities to overcome the limitations of copper matrix. Integrating CNTs and copper, the composites can fully exploit the advantages of each component, thus potentially achieving synergistic enhancement of performance.
the inherent
4.1. Strength-conductivity trade-off strategy
The strengthening of copper matrix is often accompanied by the generation and accumulation of numerous lattice defects, which act as scattering centers for conducting electrons within the material, increasing the likelihood of electron scattering during their motion and reducing the average free path of electron movement. As a result, the conductivity of the material decreases [149]. Thus, the trade-off rela- tionship between strength and conductivity in materials cannot be evaded. Section 1.1 in this review elucidates that the mechanism through which CNTs enhance the strength of the composites follows this pattern. However, as stated in Section 1.2, the mechanism by which CNTs enhance the electrical properties of the composite is associated with their ability to provide a high average electron-free path. This mechanism presents an opportunity to strike a balance between the strength and conductivity of the CNTs/Cu composites. In theoretical calculation research, the conductivity of copper matrix composites with CNT reinforcement far exceeds that of pure copper. Therefore, the investigation of preparation processes that can effectively harness the synergistic reinforcement mechanisms inherent in CNTs/Cu composites represents a focal pursuit among researchers.
Addressing the issues of gaps and interface bonding between CNTs and copper effectively mitigates the trade-off relationship between the strength and conductivity of the composites. Rolling processing plays a crucial role in effectively addressing the issue of porosity between CNT bundles and copper matrix, thereby promoting the denser stacking of CNTs and enhancing the interactions among them [17,109]. Xiong et al. [28] demonstrated the ability to adjust the distribution of dislocations through SPD at room temperature, resulting in the composite exhibiting high TS (470 MPa) and excellent conductivity (98% IACS) after the rolling process. The achievement of such outstanding electrical con- ductivity while maintaining excellent TS can be attributed to the com- bined effects of various factors introduced by the rolling process and CNTs. These factors include grain elongation, suppression of dislocation generation, and adjustment of dislocation distribution.
At present, the utilization of interface improvement methods such as surface functionalization, CNT alloying, and carbonization of polymer dots (CPD) effectively harnesses the synergistic enhancement of CNTs and copper, resulting in a CNTs/Cu composite with high strength and conductivity [127,150]. Through the surface modification of CNTs, a strong interface binding is achieved in the composite. The functional groups on the CNT surface can form chemical bonds, acting as bridges between carbon and Cu atoms, thereby facilitating electron transfer and ultimately achieving a harmonious balance between material strength and conductivity. Tian et al. [29] prepared a composite of Ag–CNTs reinforced copper matrix, and the composite exhibited exceptional TS (315 MPa) and thermal conductivity (416 W/mK) while maintaining high electrical conductivity (94.9% IACS).
4.2. Strength-ductility trade-off strategy
The ductility of the copper matrix is governed by the extent of uni- form deformation and the dislocation mobility during the straining process [151]. The inherent high stiffness of CNTs in CNTs/Cu com- posites inevitably introduces a discontinuity in the deformation of the copper matrix, resulting in localized stress concentration that reduces the ductility. While pure copper can achieve an elongation of over 40%, existing CNTs/Cu composites struggle to reach this level.
CNTs with high strength and high aspect ratios possess the capability not only to impede dislocation motion but also to absorb dislocations and prevent their accumulation. As mentioned in section 2.1, the length of CNTs influences the reinforcement mechanisms within the composite. In composites featuring shorter CNTs, the dominant reinforcement mechanism is attributed to the Orawan strengthening [98]. In this mechanism, CNTs cause the multiapplication of dislocations without hindering their mobility, thereby effectively mitigating the detrimental
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impact of CNTs on the ductility of the composite. Simultaneously, this mechanism preserves a favorable enhancement effect.
The presence of voids between CNTs and copper, as well as the clustering of CNTs, often act as crack initiation sites and are critical factors affecting the ductility of the composites. Hence, the elevated densification of the composite coupled with the heightened dispersion of CNTs can effectively improve the ductility of composites [152]. The strong bonding between CNTs and the copper matrix often leads to the formation of an interdiffusion region at the interface, which plays a crucial role in enhancing plasticity by serving as a dislocation generation source. Simultaneously, it acts as a high-capacity region for dislocation pinning and accumulation during material deformation. The multipli- cation and annihilation of dislocations reach a long-term dynamic equilibrium in this region, enabling the composite to sustain uniform strain and exhibit favorable ductility [153]. The favorable bonding conditions alter the behavior of dislocation initiation, propagation, movement, rearrangement, and annihilation at the CNTs–Cu interface. Consequently, effective synergistic strengthening between the rein- forcing phase and the matrix is achieved, leading to high strength and ductility in the composite.
The interfacial bonding situation plays a significant role in the syn- ergistic strengthening effects between the CNTs and copper matrix. A robust interfacial bonding is advantageous for enhancing the strength,
conductivity, and ductility of CNTs/Cu composite. This also provides theoretical possibilities for achieving these three properties simulta- neously in the composite. Wang et al. [154] conducted a study using a composite composed of copper foam as the reinforcement skeleton and Ni-decorated CNTs as the reinforcing phase. This material achieved a nanoscale interfacial transition zone with a high load-bearing capacity and tight interface bonding, effectively enhancing the interface strength between CNTs and copper. As a result, the composite exhibited a TS of 364.9 MPa, approximately 54.6% higher than that of pure copper samples, while maintaining a conductivity of 95.6% IACS and an elon- gation rate of 40.6%, both at desirable levels. In fact, numerous studies have successfully achieved a good balance between strength, ductility, and conductivity [124,155–158]. These studies have focused on estab- lishing strong interfacial bonding and ensuring the good dispersion of CNTs within the copper matrix. This indicates that with well-controlled fabrication processes, enabling the full utilization and synergistic com- bination of the excellent properties of CNTs and copper, the composite can achieve comprehensive performance improvement.
Table 2 summarizes various studies that have employed different fabrication strategies to overcome the strength-conductivity and strength-ductility trade-offs in CNTs/Cu composites. Some studies have achieved simultaneous enhancements in strength, ductility, and con- ductivity. Most of the research indicates that the combined effects of
Table 2 – Summary of the CNTs/Cu composites: composition, methods, properties and trade-off strategies.
Composite composition
Fabrication method
Strength
Elongation
CNT–Ni–Cu fiber
Quick online electrodeposition
Effective TS: 830 MPa
Cu–CNT
Ag–CNTs/Cu
2.5 vol% CNT/Cu CNT/Cu
composite fibers
ED, hot pressing and cold rolling
TS: 418 MPa
Ultrasonic chemical synthesis and powder metallurgy SPD Physical vapor deposition and rolling
TS: 315 MPa
TS: 470 MPa Effective TS: 1.01 ± 0.13 GPa
Vickers hardness: 1.3 GPa YS: 142.2 MPa Vickers hardness: 102.5 HV YS: 275 MPa Vickers hardness: 130 HV TS: 457 MPa YS: 383 MPa TS: ~350 MPa
UTS: 344 MPa YS: 195 MPa TS: 306 MPa UTS: 307.4 MPa TS: 395 MPa
Electrical properties
EC: >2 × 107 S/m Ampacity: >1 × 105 A/cm2 EC: 90 ± 0.16% IACS EC: 94.9% IACS
EC: 98% IACS EC: (2.6 ± 0.3) × 107 S/m
EC: 90.9% IACS
Key strategies
Strong interface bonding
Uniform dispersion
Uniform dispersion and strong interface bonding Super-aligned CNT Copper film (~2 μm) was coated on the surface of the CNT fibers
Uniform dispersion and strong interface bonding
EC: 92.9% IACS
Strong interface bonding
EC: 92% IACS
Uniform dispersion and strong interface bonding
40%
19.60% 29%
38.44% ~20.0%
EC: ~90% IACS
Uniform dispersion, strong interface bonding and suitable CNTs content Strong interface bonding Strong interface bonding
Uniform dispersion Nanolaminated structure and uniform dispersion Strong interface bonding
Uniform dispersion and strong interface bonding Surface and intratube decoration of CNTs, strong interface bonding Laminated structure and strong interface bonding Strong interface bonding
Ref.
[159]
[109]
[29]
[28] [17]
[110]
[36]
[146]
[152]
[160] [161]
[43] [156]
[35]
[124]
[155]
[134]
[154]
[157]
[158] [117]
Molecular-level mixing, microwave sintering and rolling Flake powder metallurgy, high pressure torsion Surface functionalization of CNTs
TS: 218 MPa
37.75%
EC: >98% IACS
TS: 474 MPa
11%
EC: 82.5% IACS
UTS: 272 MPa
14.30%
EC: 93.6% IACS
1 vol% CNTs/Cu
Electroless deposition, SPS and hot-rolling
YS: 264 MPa
29%
EC: 96.6% IACS
Ni–CNTs/Cu
0.75 wt%
CNT–Ag/Cu 2.5 vol% CNT/Cu 0.5 vol%
Ni–CNTs/Cu
Electroplating Ni–CNTs/Cu, cold pressing, SPS ED, ball milling and SPS
Wet mixing, ball milling and SPS Depositing nickel, flake ball milling, magnetic alignment treatment and SPS
TS: 364.9 MPa
40.60%
EC: 95.6% IACS
TS: 314 MPa
24.80%
EC: 93.6% IACS
Surface alloying of CNTs
UTS: 280 MPa TS: 292 MPa
41.70% 34%
EC: 91.6% IACS EC: 93% IACS
Uniform dispersion Alignment structure and strong interface bonding
1410
0.5 vol% CNT/Cu
ED and SPS
CNTs/Cu
Alloying method, CVD and SPS
CNTs/Cu
Powder metallurgy with hetero-aggregation mixing and SPS
0.5 wt% CNT/Cu
SPS
CNT/Cu CPD–CNT/Cu
Spraying pyrolysis, SPS Powder metallurgy
High-energy ball-milling, SPS Flake powder metallurgy
0.5 vol% CNT/Cu 1.0 vol%
MWCNTs/Cu 0.5 wt% CNT/Cu
4 vol% CNT/Cu
Cu@CNTs/Cu
Y. Jia et al.
Journal of Materials Research and Technology 32 (2024) 1395–1415
uniform dispersion and strong interfacial bonding can effectively improve all three properties of CNTs/Cu composites concurrently.
- Summary
By comprehensively reviewing the impact of CNTs on the properties
of CNTs/Cu composites, the following conclusions have been drawn:
(1) The mechanisms contributing to the improved mechanical properties of CNTs/Cu composites can be categorized into load transfer mechanism, self-lubrication mechanism of CNTs, grain boundary strengthening mechanism, Orowan mechanism, and geometrically necessary dislocation strengthening. These enhancement mechanisms collectively improving the performance of the composite. CNTs/Cu compos- ites can to some extent inherit the ultra-high electrical and thermal conductivity of CNTs. Demonstrating high current- carrying capacity, high electron migration resistance, low skin effect, electromagnetic shielding effectiveness, and ultra-low coefficient of thermal expansion of CNTs, CNTs/Cu composite are highly promising for practical applications.
synergize,
often
(2) The number of walls, length, and diameter of CNTs directly affect their performance. Reasonable control of carbon nanotube properties can result in higher performance composite re- inforcements. The spontaneous aggregation behavior, random spatial distribution, and weak interface bonding with Cu contribute to an overall decline in the reinforcing effect of CNTs. Appropriate preparation and pre-treatment methods can effec- tively address these issues. However, the introduction of defects, i.e., compromising the integrity of CNTs, is unavoidable in this process.
(3) CNTs/Cu composites show promise in addressing the trade-off relationship between strength and conductivity. Focused studies on ensuring the reinforcing effect of directionally aligned CNTs and investigating the recrystallization process of the copper ma- trix after rolling treatment are deemed crucial. Simultaneously, effective Orowan strengthening and high-density solutions have the potential to resolve the trade-off relationship between strength and ductility. In summary, in the process of CNTs rein- forcement, effectively utilizing enhancement mechanisms, and reasonably controlling factors influencing performance, CNTs/Cu composites exhibit strong application potential across a wide range of fields, even achieving simultaneous development of multiple properties.
- Outlook
Currently, researchers have conducted comprehensive studies on the properties of CNTs/Cu composites, but there are still areas worth exploring. It is known that nanoscale twin boundaries can accommodate more dislocations and have a lower resistivity approximately one order of magnitude lower than conventional high-angle grain boundaries. This characteristic proves advantageous for enhancing the strength, con- ductivity, and ductility of the composites [162]. Some studies have found the presence of twinned copper around CNTs in composite after severe plastic deformation, but the research remains in its early stages. A crucial focus for future investigations lies in controlling the twinning process within the copper matrix. Furthermore, how to impart the ultra-high electrical and thermal conductivity of CNT to composite has been a focal point of research. Currently, adjusting the spatial distri- bution of CNTs and enhancing interface binding emerge as promising research directions for achieving favorable outcomes. With the rapid development of computers, there is significant exploration potential in using numerical simulations and neural networks to predict the effective transfer modes of phonons and electrons in CNTs/Cu composites during operation. While innovative research contributes to scientific and
technological advancement, a deeper understanding of the interactions between components in practical applications of CNTs/Cu composites requires more foundational research. CNTs/Cu composites with excel- lent performance, coupled with mature industrial manufacturing pro- cesses, should be the next pursuit in this field. The potential utilization of CNTs/Cu composites in manufacturing lightweight conductors that meet the high flexibility [163] and conductivity [164] requirements of electric vehicles and wearable electronic devices holds great promise.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The Project was supported by the Guangdong Basic and Applied Basic Research Foundation, China (2022A1515140003), National Nat- ural Science Foundation of China, China (52172035), Guangdong Major Project China Applied (2021B0301030002), Innovative Model Factory Project of Songshan Lake Materials Laboratory, China (Y1D1051C511/Y1Q1011C511).
Research,
Basic
Basic
and
of
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Muhong Wu is a Research Assistant Professor at the Interna- tional Center for Quantum Materials at Peking University and is also the Head of the High Conductivity Thin Films Subgroup within the Light Element Materials Team at the Songshan Lake Materials Laboratory. His main research interests include large- scale single-crystal metal foil and 2D materials, the growth of low-dimensional materials, and electronic and optoelectronic devices made entirely of 2D materials.
Yilin Jia received his bachelor’s degree in June 2022 from the Faculty of Materials Metallurgy and Chemistry at JiangXi University of Science and Technology. He is currently pursuing a master’s degree at Guangdong University of Technology. His research, primarily conducted at the Songshan Lake Materials Laboratory under the guidance of Professor Ying Fu, focuses on high-performance copper alloys and copper-based composites.
Ying Fu is a professor at Dalian Jiaotong University and also holds the position of Senior Engineer at the Songshan Lake Materials Laboratory’s Advanced Materials and Devices Team. Her main research interests include the preparation technology of graphene metal-based composite materials, metal purifica- tion technology, and continuous casting technology of metal layered composite materials.
Yu Wang received her Ph.D. from Northeastern University in 2023 and is currently conducting postdoctoral research with the Advanced Materials and Devices Team at the Songshan Lake Materials Laboratory. Her main research focuses on the preparation of high-performance copper-based composites and the growth of large-size single-crystal metal materials.
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