Abstract
Since their discovery in 1991, carbon nanotubes (CNTs) have found widespread use in various industries, from aerospace to energy, due to their excellent mechanical, thermal, electrical, and tribological properties. Their lightweight nature, small size, incredible tensile strength, and conductivity have made them very popular as reinforcements in metals, polymers, and even finding employment in additive manufacturing. In this review, we provide a rundown of these structures and discuss in detail the numerous methods used to process CNT-reinforced materials, such as chemical vapor deposition (CVD), ball milling, hot pressing, and selective laser melting. The limitations of manufacturing and processing these composites are also discussed, strengthened by the support of different published works. To understand the changes in the properties of these composites in terms of varying parameters such as temperature, CNT length, diameter, etc., an extensive summary is provided, describing several techniques to perform experimental analysis and giving plausible reasons for attributing these changes. Consequently, we explore the specific areas of applications for these CNT-reinforced composites in fields such as aerospace, energy, biomedical, and automobile, and how they can be further processed and changed to allow for more affordable and efficient solutions in the future.
1 Introduction
Carbon-based research started gaining traction between 1970 and 1980, during which period it was discovered that the decomposition of hydrocarbons produced carbon filaments with very small diameter, at high temperatures in the presence of catalyst particles [1]. A breakthrough occurred with the discovery of fullerene in 1985, when graphite was vaporized by laser irradiation leading to the formation of C60 atoms [2]. Following the discovery of carbon nanotubes (CNTs) in 1991, [3] there have been tremendous advancements in the domain of carbon-based research, finding applications in various fields such as electronics, healthcare, additive manufacturing, and more.
CNTs are a kind of tubular structure typically having a diameter in nanometer and length in micrometer [4]. They can be categorized into three types, single-walled CNTs (SWCNTs), double walled CNTs (DWCNTs), and multi-walled CNTs (MWCNTs) [5]. Their properties, such as extremely high surface areas and large aspect ratios, provide them with extraordinary mechanical strength (100 times greater than steel) and thermal and electrical conductivity similar to that of copper. The chirality of these structures also has a profound impact on the electronic properties of CNTs [6]. Various ways to synthesize these structures include the carbon-arc discharge method, laser ablation carbon, and chemical vapor deposition (CVD) [7,8], which will be explored in detail in this review.
CNTs have a wide variety of applications spanning various fields such as materials science, energy, electronics, sensors, and more. The excellent mechanical properties possessed by CNTs are used extensively in carbon composites and composites having CNTs as fillers. In contrast, the thermal properties can be utilized for heat dissipation, among other applications. A high surface-to-volume ratio enables CNTs to be used as sensors, especially for biomedical applications. CNTs are used in electronics as supercapacitors and actuators due to their high electrical conductivity. Apart from these, CNTs are also used as scanning probe tips for hydrogen storage, nanoelectromechanical devices, and more [9,10,11]. The biomedical applications of CNTs range from tissue engineering, promoting neuronal outgrowth, to developing robust and lightweight prosthetics and neuro-prosthetics because of their ability to interact with electrically active tissues and outstanding flexibility [12,13].
However, CNTs come with their fair share of challenges that need further research to be resolved [14]. Due to their tendency to stick together in a matrix, it is difficult to align the tubes with the matrix. CNTs need to be manufactured with longer lengths and in larger quantities economically with efficient distribution in the matrix.
With the rapidly advancing industrialization, there is an urgent need for material composites that can resolve the issues mentioned above and are light, have excellent mechanical and thermal properties, and have a varied field of applications. Graphene is a possible substitute for CNTs in composites but lacks the superior mechanical strength of the latter [15]. Over the past decades, there has been an exponential growth in the development of CNT Metal Matrix Composites (CNT-MMCs), which allow us to benefit from the properties of CNTs, such as their high tensile strength and thermal and electrical conductivity. With this review, we hope to provide a comprehensive overview of CNTs, the numerous material processing methods, their manufacturing methods, different kinds of experimental analysis and their impact on the use of CNTs in today’s world. Figure 1(a) and (b) show the structure of SWCNTs and MWCNTs [13].
(a) Structure of SWCNT and (b) structure of MWCNT.
2 Materials processing
2.1 MMCs
2.1.1 CVD and infiltration
This popular process comprises forming a thin film on the surface of a heated substrate when treated with gaseous vapors in a vacuum. It can be used to fabricate CNT/Al composite foams. Using a C2H2/Ar mixture flow, CNTs can be manufactured using Co/Al as the catalyst at 600°C. An Ar atmosphere can cool down the system to finally produce CNT/Al composite powders [16,17].
In a similar process, chemical vapor infiltration can be utilized to form composites by diffusing reactant gases into a heated fibrous preform, followed by the reaction to a solid phase on the fiber’s surface. CNT/SiC composites can be formed using this, where methyl trichlorosilane acts as a precursor gas, hydrogen as a carrier gas, and argon as diluting gas, and the CNT is placed into the chemical vapor infiltration furnace for infiltration [18].
2.1.2 Spray pyrolysis
Another method to form CNT-metal composites is spray pyrolysis, where a fine film is deposited on a heated surface by spraying or injecting the precursor on the heated surface. This method can be used to prepare the precursor CNT–Cu2O. Initially, Cu powders and CNT dispersions are dissolved in deionized water and magnetically stirred, and the resultant solution is broken down in an ultrasonic nebulizer, and the droplets are deposited in a heated reactor where solvent evaporation, solute precipitation, precursor decomposition, and sintering are carried out to give rise to the composite powder [19].
2.1.3 Ball milling
One of the most common methods of fabricating CNT-metal composites is ball milling which consists of grinding the mixture in a hollow, rotating cylindrical chamber into an excellent powder. It is usually carried out in combination with other processes such as CVD, spray pyrolysis, and cold pressing [20,21,22].
2.1.4 Mechanical alloying
The mechanical alloying process is a dry, high-energy ball milling method that involves recurring cold welding, followed by fracturing and then welding the blended powder again to create a homogenous composite. One of the ways this can be done, taking the case of Al6061 powders as an example, is by alloying zirconia balls, Al6061 powder, and CNTs by placing them in a zirconia jar and mixing them at speeds as high as 1,200 rpm. Following this, alcohol is added, and the mixture is ultrasonicated for 60 min. Numerous drops of the solution were taken separately and diluted until the CNTs dispersed. The processed material was then placed on an aluminum foil and pressurized at 50 MPa for half an hour to amalgamate the composite [23]. The exact process can also be done in a tungsten carbide jar using Toluene as a process control agent, rotating at 250 rpm for 6 h. The amalgamation can be done by spark plasma sintering (SPS) at a pressure of 50 MPa at a temperature of 550°C [24].
2.1.5 Powder metallurgy
Fine powders are blended and pressed into the required shape and heated just below their melting points. This technique can synthesize CNT-Al composites; Al powder, fly ash, and MWCNTs can be milled with stainless steel balls for 2 h at 250 rpm using methanol as a control agent. The resultant product can then be cold compacted at 280 MPa and sintered for an hour in an Argon environment for 500°C [25,26]. Alternatively, the milled mixture can also be hot-pressed in a vacuum at 50 MPa for 30 min at 580°C [27]. SPS can also consolidate the milled sample at 550°C at a pressure of 30 MPa. After preheating at 500°C for 3 min in an Argon environment, hot extrusion can be performed using a 2,000 kN hydraulic press [28].
2.1.6 Electrophoretic deposition
This technique makes use of the process of electrophoresis to disperse and cumulate electrolyte particles toward the anode surface in the presence of a high electric field to form a coating. CNTs can be incorporated in TiO2 films (with ultrasonically purified titanium foil acting as the substrate) using a voltage of 350 V for 40 s, keeping the electrolyte at 20°C [29].
2.1.7 Stir casting process
Stir casting involves mechanical stirring to mix reinforcement in the material matrix. This process is based on the Taguchi method, and it is used to form aluminum alloy and CNT composites. The stir casting process results in an increase in the hardness values compared to the normal casting method and a decrease in the porosity of the cast metal [30].
2.1.8 Friction stir processing (FSP)
Based on the friction stir welding principle, this method modifies the properties of the metal through severe deformation. CNT/Al composites can be formed by combining the processes of powder metallurgy and FSP. CNT/Mg composites can also be produced using this process which considerably reinforces the microhardness of the product. Other compounds such as polyethylene, AlSi10Mg, and Al–Mg alloy can also be reinforced with CNTs using FSP [16,31].
2.1.9 Colloidal mixing
This blending method develops hot uniaxially mixed composites with CNT addition. This process can be used to make CNT-Nickel MMCs with required fractions of CNT. The process includes dispersion of CNT mixed with metallic powder in an ultrasound bath which is then evaporated to get the desired product [32].
2.1.10 Equal-channel angular extrusion
This process aids in the production of ultra-fine refined materials and is extensively used for Al composites. A die with a 20 mm diameter is used with the outer corner angle as 20° and channel angle as 120° on the route Bc with a ram speed of 0.8 mm s−1 [33].
2.1.11 Molecular level mixing (MLM)
This method constitutes mixing CNTs and the metal matrix uniformly in an aqueous solution at a molecular level. Cu composites can be manufactured using this, when MLM is performed with microwave sintering and rolling technology. To enhance the composite structure further, sintered CNT/Cu composites can be cold rolled up to 70% and annealed at 600°C for 2 h [34]. Another option can be to disperse CNTs in deionized water to get an ink-like solution, and copper acetate monohydrate can be dissolved in water, followed by magnetic stirring for 30 min. The two solutions can be mixed and heated up to 75°C for 5 min, and NaOH and 2 M glucose can then be added. After the color turns brick red, stirring has to be stopped, and the final composite can be attained by filtering and vacuum drying [35].
2.2 Polymers, ceramics, and composites
2.2.1 Pickering emulsion method
This method involves extracting cellulose nanocrystals from micro-fibrillated cellulose using acid hydrolysis followed by the addition of CNTs to the CNC suspension. The obtained suspension is ultrasonicated, and polylactic acid (PLA) is added for preparing the Pickering emulsion, which is used to prepare the PLA/CNT/CNC composites by compression molding [36].
2.2.2 Polymer infiltration
In this process, CNT films developed using CVD are immersed in polyvinyl alcohol solution and nitric acid followed by extraction of CNT fibers by shrinking the CNT film [37].
2.2.3 Hot pressing
Hot pressing is a densification process for forming powders by applying heat and pressure simultaneously in a simple die. This process is performed at temperatures high enough to enable creep processes and sintering. CNT-reinforced WC-Al2O3 cemented carbides can be produced using this technique [38].
2.2.4 Hot isostatic pressing (HIP)
This process involves densifying materials under very high temperature and pressure in a gas medium. For processing CNT/Si3N4 composites, a two-step sinter-HIP process can be used with high purity nitrogen as the pressure medium [39].
2.2.5 Vacuum bag oven process
This process involves using a flexible bag and vacuum to assist in holding layers together while the curing process takes place. CNT nanocomposites can be created using this process to ensure uniformity in the process [40].
2.2.6 Layer by layer (LBL) drafting
LBL drafting is used to prepare homogeneously distributed CNTs effectively. The process involves depositing two interactive materials onto the surface of the substrate alternatively. LBL drafting can be used for any shape or size and has the distinct advantage of being an easy to control process without any need for specialized tools [41].
2.2.7 SPS
This sintering method involves passing pulsed direct current in electrically conducting die under uniaxial pressure. SPS enables the densification of materials at comparatively lower temperatures and shorter holding times. B4C composites can be densified using this process using graphite punch rods and die [42].
2.3 Additive manufacturing
2.3.1 Fused deposition modeling (FDM)
CNTs can be combined with polymers such as PLA, thermoplastic polyurethane (TPU), and others to obtain reinforced filaments for FDM. This process can be used to three-dimensional (3D) print CNT-yarn-based components [43], CNT-graphene-based conductive polymer nanocomposites [44], functionalized nanocomposite filaments to produce multiaxial force sensors [45], and CNTs/PLA composites [46]. Polybutylene terephthalate powder is mixed with CNT and graphene and then extruded to obtain the filament in one such process. The FDM process can also produce CNT-enabled electrodes for Li-ion batteries, which have enhanced ion and electron transport capabilities [47].
2.3.2 Selective laser melting (SLM)
SLM uses a power source in the form of a laser to melt and fuse the metal powder to obtain the product, as shown in Figure 2. SLM can be used effectively for additive manufacturing of metals reinforced with CNTs. In one such process, NiCrAlY-CNT powder is processed via SLM for producing substrates that are further coated with lanthanum zirconate using plasma sprayed coating deposition. Figure 2 demonstrates the process of SLM [48]. Metallic components like CNT-decorated titanium alloy powders are also manufactured using laser powder bed fusion processes [49].
Selective laser melting process.
2.3.3 Selective laser sintering (SLS)
Figure 3 shows the SLS process which involves a laser power source to sinter the material in powder form to create the final product. This technique can be used to create products with complex geometry easily, as shown in Figure 3. CNT-reinforced composites such as CNT/alumina composites and high-performance polymers like polyether ether ketone (PEEK) can be produced via SLS process [50,51,52].
Selective laser sintering process.
2.3.4 Digital light processing (DLP)
This additive manufacturing process is based on curing a photopolymer resin using a directed light source to obtain the product. DLP enables 3D printing of photocurable formulations consisting of CNTs to develop composites with improved electrical properties [53].
2.3.5 Direct write printing
In this process, a viscoelastic liquid is used as the 3D ink for the manufacturing process without requiring a heat or light source. Epoxy nanoclay CNT composites can be created by sonicating the dispersion of CNTs in acetone along with the addition of curing agents and nanoclay, resulting in the production of ink that can be used in direct write printing [54].
2.4 Disadvantages concerning the preparation methods of CNT composites
Mechanical processing methods can induce a high propensity for reactions that take place simultaneously with other processes at the mesoscopic and macroscopic scales, such as structural disordering and mixing [55]. The dry ball milling method poses various disadvantages such as high non-uniformity in reaction and the size distribution along with noise, the additional material loss of powders, and additional impurities from the grinding media and the grinding jar in final products [56]. Spray pyrolysis is difficult to control and has a fairly poor homogeneity of the film thickness distribution over the substrate’s area [57]. Powder metallurgy has its own set of drawbacks – the composite parts have low strength and ductility and there is a high cost associated with powder materials and the equipment used [58]. Stir casting also has some weaknesses: (a) a homogeneous distribution of reinforcements is requisite for achieving a high-strengthening effect, but a uniform distribution is relatively hard to obtain in stir casting; (b) reinforcing particles may be segregated by the surfacing of settling of the reinforcement in melting and casting process; and (c) gases and unwanted inclusions may be entrapped during stir casting [59]. SLS can include cool time up to 12 h, which means longer production time. It also causes low mechanical characteristics in the samples and comes with the possibility of distortion of geometry in case of non-observance of technological procedures for impregnation with wax [60]. In SLM, it is difficult to obtain single-phase material with the annealing step and quenching after annealing causes internal strains [61]. For SPS, only simple symmetrical shapes may be prepared and expensive pulsed DC generators are required [62].
3 Challenges in the fabrication of composites
CNT reinforcement provides numerous benefits, in terms of increasing the mechanical strength, thermal conductivity, or electrical conductivity. However, the process is not easy as it comes with its own set of challenges which have been outlined below:
Chemical functionalization may warp the bonding of graphene sheets
The presence of functionalities such as carbonyl, carboxyl, and hydroxyl groups increase the interface strength between CNTs and the polymer matrices. Moreover, they also cause the CNTs to be dispersed more effectively. Nevertheless, chemical functionalization can reduce the mechanical properties in the CNT composite as they disrupt the bonding of graphene sheets [63].
Size difference between CNT and matrix powders
When CNTs larger than 1% in weight are used, they impede the achievement of required properties. The clusters present in such composites lead to reduced ductility, strength, and stiffness. Effective dispersion of CNTs can be done by milling, but those conditions lead to severe strain hardening of the matrix powders, making it difficult to process the composite further by conventional powder metallurgy [64].
Ineffective dispersion of CNT in the matrix due to van der Waals forces
Due to strong van der Waals forces, a large specific surface area, and a high aspect ratio, CNTs tend to agglomerate, entangle, and form clusters [65]. This phenomenon is highly undesirable as it prevents effective homogeneous dispersion of CNTs and can act as defects. This agglomeration leads to poor solubility [66], poor adhesion [50,67], and a considerable increase in the porosity and viscosity of the composite [68]. Inhomogeneous distribution might adversely affect the thermal and electrical properties due to interconnections among CNTs in the 3D networks. There have been proposals to use simple ultrasonic dispersion to overcome the strong van der Waals force [69], but this might create more significant problems. The high-energy ball milling can ruin the structure of CNTs, and the irreversible formation of metal oxides will weaken the beneficial mechanical properties of the composite [23]. Densification, which has to be performed at high temperatures in CNT/ceramic composites, might also cause CNT degradation [35,70].
Improper bonding at CNT–matrix interface
The proper bonding at the CNT–matrix interface plays a significant role in effective load transfer between phases [21,71]. However, the wettability of metal alloys poses a considerable problem and lowers the mechanical strength of the composite [68,69]. It can cause the CNTs to pull out from the metal matrix or fracture the CNT–metal interface [71].
Delamination of composites
Carbon-fiber-reinforced PEEK (CF/PEEK) is thermally stable, has high chemical resistance, and thermal stability and is widely used in aviation [72]. However, delamination, i.e., the material fracture, can significantly qualify these composites’ applications. The poor interfacial strength between the fiber reinforcements and the polymeric matrix is caused by the homophobic and chemically inert nature of CF and PEEK. The introduction of oxygen-containing functional groups improves the shear strength of these composites but at the expense of their tensile strength.
4 Experimental analysis
Tables 11–14 give a detailed overview of the effect of fabricating CNT-composites on their mechanical, thermal, electrical, and tribological properties by studying their methods of preparation, testing methods, the properties affected and changed, and the possible reasons for their change.
4.1 Mechanical properties
Table 1 shows the comparative analysis of the mechanical properties of CNT composites.
Comparative analysis of mechanical properties of CNT composites
| Sl. no. | Matrix | Method of preparation | Testing method | Property affected | Change observed | Possible reason for change |
|---|---|---|---|---|---|---|
| 1 | Alumina [73] | (i) Tape casting, lamination, and hot pressing | Ball-on-reciprocating wear tester under an unlubricated condition at room temperature | Weight loss | (i) 12% decrease | Grain size effect and reinforcement effect of CNT |
| (ii) Hot pressing | (ii) 4% decrease (increases after further CNT addition) | |||||
| 2 | Polymer (PET, polypropylene (PP), and PE) [74] | Catalytic decomposition of propylene on Fe/Al2O3 catalyst followed by compounding the polymer using a twin-screw extruder | Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (TEM) | (i) (a) Loss factor of composites | (i) (a) Increases | Dielectric loss is much higher than that of magnetic loss especially in frequencies ranging from 6 to 18 GHz |
| (b) reflectivity peak moves | (b) moves to a lower frequency | |||||
| (ii) Absorbing peak changes | (ii) Peaks occur at 7.6 and 15.3 GHz | |||||
| 3 | Cu [75] | Molecular level Process | Scanning electron microscopy. high-resolution TEM, Vickers hardness test, and pin-on-disk type wear tests | Hardness changes | Linear increase up to 1.1 GPa | The strength of the bond at the CNT/Cu interface, the homogeneous distribution of CNTs within Cu matrix, and attained high relative densities.are possible reasons for the mentioned change. |
| 4 | Ni–P–CNT coated copper composite [76] | Electroless composite plating process | X-ray spectrometer | Young’s modulus and tensile strength affected | (i) Increase if CNT is non-activated | Different processing methods of CNTs before electroless composite plating can lead to totally different effects on the mechanical properties of Ni–P–CNTs coated copper composite materials |
| (ii) Decrease if CNT is activated | ||||||
| 5 | Aluminum [64] | Ball milling, cold compaction, and hot extrusion | Field emission scanning electron microscope (FE-SEM) and TEM | (i) Young’s modulus affected | (i) 50% strength increased after adding 2 wt% CNT; no increase observed on further CNT addition | The better dispersion of CNTs provided by ball milling in addition to the strain-hardened powders contributes as a strengthening mechanism to the final strength of the composites |
| (ii) Tensile strength affected | (ii) 23% strength increased after adding 2 wt% CNT, drops slightly at 5 wt% | |||||
| 6 | Diamond-like carbon (DLC) films [77] | Dispersion, spin coating, and DLC deposition | Nanoindentation | (i) Elastic modulus and hardness affected | (i) Increases with CNT doping | Anisotropic property and the orientation effect of CNT |
| (ii) Friction coefficient affected | (ii) Decreases with the increase in the CNT doping | |||||
| 7 | CNT-reinforced basalt/epoxy composites [78] | Impregnation of woven basalt fibers into epoxy resin mixed with CNTs | Dynamic mechanical analysis | Young’s modulus and tensile strength affected | 60 and 34% increase, respectively, compared to unmodified composite | Improved dispersibility and strong interfacial interaction between the silane functionalized CNTs and the epoxy in the basalt fabric/epoxy composites |
| 8 | Magnesium alloy ZK60A [79] | Disintegrated melt deposition technique and hot extrusion | (i) 0.2% tensile yield strength (YS) and ultimate tensile strength (UTS) affected | (i) 10% increase observed | (i) Overall positive effect derived from lower grain size and well-known factors of reinforcement | |
| (ii) 0.2% compressive YS and ultimate compressive strength affected | (ii) −14% and +5%, respectively | (ii) Overall negative effect derived from (a) significantly reduced precipitation of intermetallic phase(s) in the nanocomposite matrix compared to monolithic material and (b) compressive shear buckling of CNT in the nanocomposite. | ||||
| (iii) Microhardness affected | (iii) 17% decrease observed | |||||
| 9 | Carbon fiber/epoxy composite [67] | Fabricated by incorporating woven-type carbon fibers into epoxy matrices modified with 2 wt% acid-treated and silane-treated MWCNTs | Three-point bending and ball-on-disk wear tests | (i) Flexural moduli and strengths affected | (i) 34 and 20% greater, respectively, than those of the acid-treated carbon/CNT/epoxy composites | Improved dispersibility and interfacial interaction between the silane-modified MWCNTs and the epoxy in the carbon/epoxy composites. |
| (ii) Wear properties changed | (ii) Alson improved | |||||
| 10 | Ni/CNTs hybridization on CNTs/epoxy nanocomposites [80] | CVD and electroless Nickel plating | SEM, X-ray spectroscopy, and inductively coupled plasma mass spectrometry | Shear viscosity changes | Small amount (0.8 wt%) of Ni/MWCNTs induced a slight increase in the viscosity | Good dispersion behaviors of the Ni-coated MWCNTs in the matrix |
| 11 | Epoxy/CNT [81] | Magnetically stirred, ultrasonically treated, sprayed, and curated | SEM and TEM | Critical stress intensity factor (K IC) changes | A 38% increase in K IC was observed when compared to neat epoxy resins | Functionalization of CNTs significantly increased the fracture properties of the DGEBA/CNT composites, which was attributed to the improved dispersion of CNTs in an epoxy matrix and interfacial interactions between the functional groups on the CNT surfaces and the epoxy matrix |
| 12 | CNT/Al–Cu composites [82] | MLM and a high energy ball milling process | Artificial aging heat treatment | YS and UTS changes | YS increases from 110 to 384 MPa, while the UTS doubles | Homogeneous dispersion of CNTs in Al–Cu matrix and the fracture surface of CNT/Al–Cu composites |
| 13 | CNT/2009Al composites [65] | Powder metallurgy (PM) followed by 4-pass FSP | Elevated temperature tension and coefficient of thermal expansion (CTE) measurements | YS affected | (i) At 293–573 K, enhanced compared with that of the 2009Al alloy | (i) Resulted from load transfer mechanism |
| (ii) Exhibited significantly increased YS compared to the 2009Al alloy and 1.5 vol% CNT/2009Al composite, and decreases on further temperature increase | (ii) The dimples became shallow, and when the temperature reached 573 K, the composites showed an obvious intergranular fracture | |||||
| 14 | CNTs-reinforced Sn−58Bi composites [83] | Ball milling in an argon atmosphere | Three-point bending tests and SEM | Bending strength and toughness change with respect to Sn−58Bi alloy | 10.5 and 48.9% increase observed, respectively. | Bending strength increases because of the refinement of the Bi-rich phase. Also, during the fracture process, CNTs were pulled out from the matrix alloy leading to the formation of pseudo-dimple, which can elongate the micro-crack and increase the fracture area dramatically |
| 15 | Ni–P–CNT composite [84] | CVD, ball milling, and electroless deposition | SEM, atomic force microscopy (AFM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), and Vickers measurements | Microhardness values of coating changed | Significantly increases | Precipitation hardening effect generated by the precipitation of the Ni3P phase in the nickel matrix |
| 16 | CNT-reinforced porous CuSn oil bearings [85] | Powder metallurgy method | TEM, SEM, and digital light optical microscopy | Microhardness and radical crushing strength affected | Steadily improve with the increase in the content of CNTs from 0 to 2.0 wt%; decline when the content of CNTs is higher than 1.0 wt% | Increase caused by even dispersion of CNTs; too much CNT will degrade pore structure |
| 17 | Reactive oligomers and epoxy–MWCNT composite [86] | Dried poly(ethylene oxide) (PEO) was added to isophorone diisocyanate for 30 min in a N2 atmosphere, followed by defluxion for 2 h at 80°C. The reactive oligomer, after mixing with dried epoxy, was stirred at 80°C for 4 h | (i) Impact strength at room temperature and cryogenic temperature affected | (i) Increased by 23.6 and 69.5%, respectively, compared to the unmodified epoxy | The presence of the oligomer in the modified epoxy facilitates the dispersion and interfacial interaction of the O-CNTs with the epoxy, which improves the toughness of the epoxies | |
| (ii) Tensile strength mostly unaffected | ||||||
| 18 | B4C/CNT composites with Al additive [87] | Fabricated by hot pressing using Al additive | SEM, Vickers hardness test, and X-ray diffractometry | (i) Vickers hardness | (i) Almost constant, decreases with increase in CNT after 5 vol% CNT | (i) This can be explained by the presence of higher CNT quantities at grain boundaries (ii) Due to the effects of mismatched interfaces between B4C and CNT and the orientation of the CNTs |
| (ii) Elastic modulus | (ii) For 0.5 vol% CNT, comparable to the value of B4C; for more than 2 vol% CNT, decreases with a linear increase in CNT content | |||||
| 19 | CNTs/Mg–6Zn composites [88] | Fabricated by CVD with different solidification rates of the composites | SEM and TEM | YS and UTS affected | YS and UTS increase by 18 and 13% for lower solidification rate composite. For a higher solidification composite rate, YS and UTS increase by 50 and 36%, respectively | Good dispersion of CNTs in the Mg metal matrix and good interfacial bonding between CNTs and matrix. Also, CNTs can act as solidification nuclei and restrict grain growth by the pinning effect during solidification |
| 20 | CNT/Al composite [89] | Ball milling | SEM and TEM | Tensile elongation | (i) Increases by 1.4–2.5 times | The strain rate sensitivity intensifies at high strain rates |
| (ii) Decreases when compared to milled pure Al | ||||||
| 21 | CNTs/Mg–6Zn composite [90] | Hot extrusion | SEM, TEM, and electron backscatter diffraction analysis | YS, UTS, modulus, and elongation | (i) 10, 8, 7.5 and 22% increase observed, respectively | The grain refinement effect, load transfer mechanism, and Orowan mechanism of CNTs |
| (ii) 33, 18, 30, and −22% increase observed, respectively | ||||||
| (iii) 25.4, 13, 56, and −54% increase observed, respectively | ||||||
| 22 | Graphene nanoplatelets (GNPs) and CNTs [91] | Semi-powder metallurgy method with vacuum sintering technique followed by hot extrusion | XRD, SEM, and TEM | Elastic modulus, 0.2% YS, UTS, and failure strain | +17%; +19%; +15% and +137% increase, respectively | Dislocation generation due to mismatch in CTE and elastic modulus between matrix and reinforcement. Efficient load transfer and absence of intermetallic phase also significantly contribute to basic strengthening mechanism |
| 23 | CNT/epoxy composites [92] | Drawing, winding, and pressing | FE-SEM and Raman spectroscopy | Tensile strength and elastic modulus | 12 and 14% increase in CNTs with a diameter of 22 nm when compared to CNTs with a diameter of 38 nm | Larger diameter CNTs show a lower effective modulus and occupy a more significant volume fraction in the composite than smaller diameter CNTs do; minimal load transfer due to weak Van der Waal’s interaction |
| 24 | CNT/epoxy composites[93] | CNTs, intermixture of epoxy resin, and the corresponding diamine-based hardener were dissolved in a large portion of solvent phase | Nano indentation | Young’s modulus | Almost identical for 20% CNT content, an increase by 330% for CNT content of 60%, decreases on further increasing CNT content | Air inclusions caused by an insufficient molding pressure during composite processing |
| 25 | CNT-filled glass fiber/epoxy composite [94] | MWCNT was first dispersed in 150 mL acetone which was then stirred at 1,000 rpm for 1 h followed by 1 h of bath sonication. After adding preweighed epoxy, the suspension was stirred at 70°C at 1,000 rpm till all the acetone evaporated. Sonication was then performed at 70°C for 1 h | Short beam shear (SBS) test and flexural test | Modulus and strength affected | 11.5 and 32.8% increase, respectively, when tested at room temperature; gradually decreases when CNT content exceeds 0.1% | Efficient stress transfer from the soft polymer matrix to the stiff MWCNT through the subtle CNT/polymer interface |
| 26 | Glass- and carbon-fiber-reinforced plastic (GFRP and CFRP) composites with CNT fibers [95] | Vacuum infusion | Average tensile strength changes | 6.9% higher strength in GFRP than those without CNT fibers and a 2.5% increase in CFRP | The load carried by the CNT fibers and the epoxy resin coating layer applied to the surface of the specimens to fix the position of the CNT fibers | |
| 27 | CNT-Cu composites [96] | Young’s modulus, yielding strains, tensile strength, and compressive strength | Young’s modulus increases by 32%, yielding strain decreases, tensile and compressive strength increase by 14 and 66%, respectively | The inclusion of CNT into the Cu, matrix precipitates the formation of dislocations and slip planes in a different way, depending on the loading nature | ||
| 28 | TiC-modified CNT-reinforced Al [97] | Ultrasonication, wet milling, and tip sonication | Raman spectroscopy, XRD, TEM, SEM, and nanoindentation | Vickers hardness, nano hardness, and Young’s modulus affected | 176, 82, and 48% increase observed when compared to pure Al sample | A combination of abrasive and oxidation wear contribute to the mechanisms of the samples |
| 29 | Functionalized carbon nanotubes (FCNTs) with fiberglass/epoxy laminates [98] | Synthesized CNTs were oxidized by refluxing at 130°C in an acid mixture for 30 min, diluted with water and filtered. The filtered samples were washed by distilled water and dried in an oven at 50°C for overnight, thus obtaining the FCNTs | SEM | Tensile strength and elasticity modulus (E) | Increased by ∼7 and 6%, respectively, correlated with the original sample. On adding CNT up to 0.35 wt%, increases by 30 and 31%, respectively; decreases on further addition | Mixing of FCNTs with epoxy restricted the molecular rearrangement of epoxy chains during the mixing, preparation, and solidification processes |
| 30 | CNTs with concrete [99] | Sonification, dispersion, mixing, compacting, and demolding | SEM | Compressive, flexural, and tensile strength | 23, 29, and 20% increase, respectively | |
| 31 | CNT-(Mg-6Zn) [100] | CVD, semisolid stirring assisted with ultrasonic vibration method | SEM and optical microscopy | E, YTS, UTS, elongation and residual stress affected | increased by 9.6, 14, 20, 18.2%, respectively, while residual stress drops from 74 to 35 MPa | CNTs-(Mg-6Zn) interface layer is formed, which allows the grain refinement and the low residual stress, which contributes to enhancing the mechanical properties of composites |
| 32 | CNT and GNPs reinforced PEEK composites [52] | Stir mixed prior to melt compounding | SEM and Vickers hardness test | Young’s and storage moduli affected | (i) Increase by 20 and 66% | Influenced by factors such as the bead–bead interfacial strength, nanostructure–PEEK interfacial strength, the dispersion state of the nano reinforcement, interfacial voids between beads, and the voids within the beads |
| (ii) Increase by 23 and 72% | ||||||
| 33 | CNTs/Cu matrix [101] | Ball milling | FE-SEM and TEM | Strength and ductility change | 18 and 14.3% increase | Uniform distribution of reinforcements, good interfacial bonding, and extrusion pressure acting on CNTs due to the filling of Cu inside the tubes |
4.2 Thermal properties
Table 2 shows the comparative analysis of thermal properties of CNT composites.
Comparative analysis of thermal properties of CNT composites
| Sl. no. | Matrix | Method of preparation | Testing method | Property affected | Change observed | Possible reason for change |
|---|---|---|---|---|---|---|
| 1 | Aluminum and graphite with CNT thermal interface material [102] | (i) Tape casting, lamination, and hot pressing | SEM and light flash apparatus | Thermal conductivity | There was a 300% increase with graphite coating for Al samples and 350% with CNT interface. There was a 250% increase with graphite coating and 350% with CNT interface for graphite samples. | Without any interface material, air trapped in the micro-gaps created a thermal resistance bringing down the thermal conductivity. The thin CNT sheet thermal interface material helped provide a high conductivity heat transfer path, thus minimizing the effect of micro-air gaps resistance |
| (ii) Hot pressing | ||||||
| 2 | CNT-reinforced basalt/epoxy composites [78] | Impregnation of woven basalt fibers into epoxy resin mixed with CNTs | Dynamic mechanical analysis | Glass transition temperature | Gradually increases | The difference in the extent of cross-linking reactions of epoxy and the silane molecules make more covalent bonds with CNTs and a higher cross-linking network than the unmodified and oxidized samples |
| 3 | CNTs and intumescent flame retardant (IFR) embedded in PP [103] | Melt blending | Thermogravimetry (TG) and cone calorimetry tests | Thermal stability and flammability | Improvement in thermal stability compared to PP/IFR composite before the Tmax but severe deterioration of flame retardancy | The efficient barrier effect of carbonaceous char formed by the incorporation of CNTs and IFR at relatively low temperatures and the decrease in the protective effect of intumescent char on underlying polymer materials |
| 4 | Epoxy/CNT [81] | Magnetically stirred, ultrasonically treated, sprayed, and curated | Thermogravimetric analyzer | Glass transition temperature | Increases by about 11°C compared to neat epoxy resins | Reduced mobility of the epoxy matrix around the nanotubes by the interfacial interactions |
| 5 | CNT/2009Al composites [65] | Powder metallurgy (PM) followed by 4-pass FSP | Elevated temperature tension and CTE measurements | CTE | Decreases upon increasing the volume fraction of the CNTs | A large number of interfaces between the CNTs and aluminum matrix |
| 6 | CNT/Bi2Te3 composites [104] | SPS | Properties measured using a ZEM-3 unit | Seebeck coefficient and thermal conductivity | Seebeck coefficient changes from −83 to −113 µV/K upon CNT addition; thermal conductivity decreases | Decreased carrier concentration and the newly formed CNT/Bi2Te3 interface cause a lattice phonon dissipation and hot carrier scattering |
| 7 | CNT/bismaleimide (BMI) composites [105] | CVD and spray winding | Thermal diffusivity measured with laser PIT device | Thermal conductivity changes with CNT diameter | 1.1 mmlong CNTs show 112% improvement over the composites containing 0.65 mm long CNTs; no change upon further change in length and diameter | Weak tube–tube coupling, dangling tube ends, misalignment, and structural defects |
| 8 | Calcium ferrite-CNTs/PET nano-composite [106] | Melt compounding | Thermogravimetric (TGA) analysis | Thermal stability affected | Thermal stability shows a marked improvement | Either the barrier effect of the nano-filler dispersed into the PET matrix, respect to the volatile decomposed products, as well as the air gases permeating through the nanocomposites, or to an effect of the trap action exercised by CNTs on the polymer peroxyl radical that prevents their recombination |
4.3 Electrical properties
Table 3 displays the comparative analysis of the electrical properties of CNT composites.
Comparative analysis of electrical properties of CNT composites
| Sl. no. | Matrix | Method of preparation | Testing method | Property affected | Change observed | Possible reason for change |
|---|---|---|---|---|---|---|
| 1 | CNT/polymer nanocomposites [66] | CNTs were filtered and washed with deionized water until the filtrated solution had a pH of 7. The CNTs were heated for 4 h at 292°C before the Fenton Chemistry purification in order to remove amorphous carbon impurities. The solution was then briefly sonicated (30 min) and stirred overnight. The composite solutions were cast onto glass plates that were cleaned by sequential washing with ethanol and acetone under sonication | Four probe method | Electrical conductivity | Increases with increase in CNT concentration | Dependent on various factors such as the surface functionality, tube diameter, length, wall thickness, and bundle size |
| 2 | CNT/Bi2Te3 composites [104] | SPS | Properties measured using a ZEM-3 unit | Electrical resistivity | Increases with CNT addition at 298 K and keeps on rising until 498 K | Due to the newly formed CNT/Bi2Te3 interface |
| 3 | CNT/BMI composites [105] | CVD and spray winding | Four probe method | Electrical conductivity | Increases with increase in CNT length and diameter | Longer and thicker CNTs ensure a more effective electron conduction path along the individual nanotubes, similar to the thermal conduction mechanism, and CNTs with larger diameters favor more compact structures, so the separation between CNTs decreases |
| 4 | CNT/epoxy composites [93] | Material testing machine and universal calibrator | Electrical conductivity | Increases strongly with increasing solid content, reaching a maximum value of 838 S/m at a solid content of 60 wt%, decreases again with further increased solids content | Due to the air inclusions caused by the composite processing | |
| 5 | CNT/polyaniline (PANI) hybrid nanoparticles [107] | In situ polymerization of aniline in the presence of MWNT, with ammonium peroxidisulfate as the oxidant | DC Kelvin bridge and a standard, four-probe technique | (i) Carrier mobility | (i) Increases with increased CNT content | (i) The π-π interaction reduces the threshold for inter-chain hopping |
| (ii) Electrical conductivity | (ii) Increases with increased CNT content and then plateaus | (ii) With increased doping content, MWNT/PANI particles gain proximity, at which point MWNT interconnects with the aid of a conductive PANI layer. Expanded channels for transporting electrons are formed, which induces an apparent rise in carrier concentration | ||||
| 6 | CNT and PU/PVC nanocomposites [108] | Casting technique | The nanocomposite was tested by utilizing the programmable automatic | (i) Permittivity affected | (i) Decreases with increase in frequency | (i) Due to direction dipoles of applied electric field |
| (ii) Electrical conductivity | (ii) Increases with increased CNT content | (ii) Effective conductive network is formed when CNTs load in polymer blend due to the electronic and impurity contributions arising from the CNTs (percolation theory) | ||||
| (iii) Dielectric loss | (iii) Increases progressively with CNTs content | (iii) There is a higher charge carrying capacity of loaded films compared to pure blend film | ||||
| 7 | PP as a polymer matrix and Graphite (G), Carbon Black (CB) and CNTs as reinforcements [109] | Ball milling, hot and cold pressing | Four-point probe method | Electrical conductivity | Increases initially with CNT addition from 1.0 to 6.0 wt%, decreases slightly with the CNTs filler loading concentration increased | The gaps between the synthetic graphite particles were filled effectively with CNTs, thus conducting networks were formed between the CNTs/G/CB and PP matrix. higher CNTs loading in the polymer composites may cause critical CNTs aggregation, and the electrical conductivity will be leveled off, even decreased |
| 8 | CNT/polymer matrix [110] | Probe sonication, magnetic stirring, and refluxing of CNT | Four-point probe method | Electrical conductivity | (i) Field-assisted PVDF-aligned CNT films have shown a 28% increase in conductivity in parallel direction of alignment, while transverse directions have shown a 58% decrease in electrical conductivity | Partial melting and distortion of the film due to high passage of current at 500 V during the fabrication process, which might have caused an extraordinary increase in temperature at places |
| (ii) Alternating pulsed current passage assisted PVDF-aligned CNT film shows a 360% increase in conductivity in a parallel direction (over random sample). The transverse direction becomes almost insulating | ||||||
| 9 | GFRP and CFRP composites with CNT fibers [95] | Vacuum infusion | Two-probe method | Maximum electrical resistance change rate | Exceeded 3.15%, which corresponds to a value 10 times higher than those reported in previous studies | Both the decrease in the cross-sectional area and the formation of gaps may lead to an increase in the electrical resistance of the fiber |
| 10 | CNTs/silica composite ceramics [111] | Sonication, ball milling, vacuum-rotary evaporation | Four-probe Van der Pauw method | (i) Electrical resistivity and absorption ratio | (i) Decreased with increasing CNTs amount | (i) The formation of the electro-conductive network contributed to ensure electrons move freely in the material e |
| (ii) Morphology | (ii) Denser and superior microstructure compared to composites with lower CNT content | (ii) Homogenous dispersion of carbon nanotubes in the silica matrix without affecting the formation and solidification of the ceramic bas | ||||
| 11 | CNTs/Reduced graphene oxide (RGO) composite [112] | Freeze-drying and in situ catalytic grown methods. | Two probe method | Conductivity and EM absorption | Increases with CNT addition | The CNTs layer, which is around the surface of the interconnection of the RGO frame, act as the EM absorbing reinforcement |
| 12 | CNT polycarbonate composite [113] | Hot pressing and microinjection molding | Lock-in thermography | Electrical conductivity | Increased up to over 100 times after changing the melt temperature from 250 to 280° C | Due to a lower mold temperature and faster cooling in the skin region, the melt temperature decreased and increased melt viscosity and injection pressure loss resulting in a larger skin layer thickness (percolation threshold gives rise to step wise change in resistivity) |
| 13 | CNT-reinforced industrial waste fly ash based polymer nanocomposites [114] | Compressive molding and ball milling | Measured using Keysight LCR meter | (i) Dielectric constant | (i) Drastically decreases from 38 to 10 for 5% CNT filled nanocomposites | (i) Due to the conducting nature of CNT and the creation of space charge distribution in polymer matrix |
| (ii) AC conductivity | (ii) Increases | (ii) Due to the formation of CNT-induced conductive network and may also be related to the percolation threshold of CNT filler | ||||
| 14 | MWCNTs/phenolic nanocomposites [115] | Hot pressing | SEM and Raman spectroscopy | Electrical conductivity of the nanocomposite | Rapid increase by 10 orders of magnitude | Due to the formatting of the percolating network (i.e., percolation threshold – between 0 and 0.5 wt% of MWCNT concentrations) |
| 15 | MWCNTs/glass/epoxy composite [116] | Vacuum-assisted resin infusion | Gauge factor | Increase in the percentage of MWCNTs results in the reduction in electrical resistance and strain sensitivity (Gauge factor) | Strain sensitivity of the MWCNTs-GE composites reduces due to a decrease in the distance between neighboring MWCNTs | |
| 16 | GFRP and CFRP composites with CNT fibers [97] | Vacuum infusion | Gauge factor | Gauge factor is higher by 3.1 | Formation of gaps and a decrease in the cross-sectional area |
4.4 Tribological properties
Table 4 displays the comparative analysis of the tribological properties of CNT composites.
Comparative analysis of tribological properties of CNT composites
| Sl. no. | Matrix | Method of preparation | Testing method | Property affected | Change observed | Possible reason for change |
|---|---|---|---|---|---|---|
| 1 | Alumina–CNT composites [75] | Tape casting, lamination, and hot pressing | Ball-on-reciprocating wear tester under an unlubricated condition at room temperature | Wear rate | (i) For hot-pressed samples, it decreased with increasing up to 4 wt%, but increased with further addition of CNT | Agglomeration of CNTs frequently observed with hot-pressed specimens was significantly reduced, and a relatively uniform distribution of CNTs was obtained. The effective dispersion of CNTs contributed to maintaining the densification of composites and superior mechanical properties |
| (ii) For tape casted composites decreased steadily with increasing CNT addition up to 12 wt% | ||||||
| 2 | CNT reinforced Cu matrix nanocomposites [64] | Molecular level process | SEM, high-resolution TEM, Vickers hardness test, and pin-on-disk type wear tests | Wear loss changes | Reduced to 1/3 of the value of pure Cu matrix | The dispersed CNTs in Cu-matrix nanocomposite retards the peeling of Cu grains during the sliding wear process |
| 3 | CNT-coated carbon fiber/polyester composites [117] | CVD and electroless dip coating method | SEM, dynamic mechanical thermal analysis, and single fiber pull out test | Interfacial shear stress | Increases monotonically till a growth time of 20 min and drops abruptly after that | Attributed to the larger contact area and mechanical interlocking between the fiber and matrix due to the presence of CNTs on the fiber surface |
| 4 | Carbon fiber/epoxy composites [118] | Resin transfer molding | X-ray and SEM | (i) Formation of transverse cracks (ii) Surface of CF | (i) Significantly reduced (ii) Formation of dense CNT networks on the surfaces of CF, making the morphology of surface crowded and flocky | The mechanism of (localized) energy dissipation through transverse microcracking is replaced by another mechanism that promotes (distributed) damage through fiber debonding |
| 5 | Ni–P–CNT composite [86] | CVD, ball milling, and electroless deposition | SEM, AFM, XRD), EDS, and Vickers measurements | Wear and friction behavior | Improves significantly | CNTs act as very intense obstacles to the movement of dislocations through the Ni–P matrix, hindering the occurrence of the plastic deformation |
| 6 | B4C/CNT composites with Al additive [89] | Fabricated by hot pressing using Al additive | FE-SEM | (i) Fracture toughness | (i) Slightly improved by CNT addition, and was ranged between 2.37 and 3.10 MPa m1/2 | |
| (ii) Bridging of matrix | (ii) In the case of the composite containing 10 vol% CNT, the cracks were produced from the tip of the indentation, and much bridging by the CNT was observed in cracks | |||||
| 7 | CNT-epoxy based composites [119] | Sonication and hot curing | SEM and centrifugal accelerator erosion tester and surface profilometry | Erosive wear resistance | The vertically aligned CNT arrays/epoxy erosion rate is decreased by 30, 27, 20, and 13% at impingement angles of 20, 30, 45, and 90, respectively, compared to neat epoxy | CNT alignment configurations strongly influence the erosion resistance of the composites with identical CNT loading fractions |
| 8 | Cu-based composites with Cu-coated NbSe2 and CNT [120] | Powder metallurgy technique | SEM and a multi-functional friction and wear tester | (i) Friction coefficient | (i) Increases slightly | NbSe2 with laminated structure significantly improved the friction-reducing property, while CNT with superior mechanical strength enhanced the wear resistance by increasing the load-carrying capacity |
| (ii) Wear rate | (ii) Decreases with the increasing content of copper-coated CNT from 1 to 3 wt%. When the range of copper-coated CNT reaches 4 wt%, the wear of copper-based composite increases | |||||
| 9 | CNTs with epoxy resin and carbon fiber composites [121] | Ultrasound mixing and sonication. The hardener was then added to the modified resin and mixed using a mechanical agitator for about 10 min. The mixture was degassed in vacuum for 10 min. Finally, the mixture was transferred to open silicone rubber molds and cured for 2 h at 60°C | SEM and Vickers hardness tester | Erosion rate | 50% decrease at high impact angles, slight increase at low impact angles | A positive synergy between the carbon fibers and the CNTs, which resulted in a reduced amount of broken and detached fibers in the case of CFRPs |
| 10 | MWCNT and polyurethane (PU) [122] | The substrates were cleaned in an ultrasonic bath with acetone for 10 min, air-dried, and subsequently coated with PU coatings. The prepared coating samples were cured at room temperature overnight and were then postcured at 80°C for 1 h in an oven | Electrochemical impedance spectroscopy, polarization curve measurement, salt spray tests, pull-off tests, and dynamic mechanical analysis | Corrosion resistance and adhesion strength | Enhanced by adding 0.4 wt% CNTs. When CNTs rose to 1 wt%, the corrosion rate of the coating increased | Adding CNTs modified the physical properties of the PU coating so that the internal stress induced in the layer–substrate interface attributable to temperature variations during the curing process could be relaxed and reduced |
| 11 | TiC-modified CNT-reinforced Al [99] | Ultrasonication, wet milling, and tip sonication | XRD and high-resolution TEM | Mean friction coefficient and wear weight loss | A reduction of around 68 and 61% in mean friction coefficient as well as a reduction of about 88 and 84.5% in wear weight loss was achieved for 1.5 wt% modified-CNT/Al composite compared to pure Al and 1.5 wt% unmodified sample, respectively | Nanotubes can fill microvoids of the metal particles |
4.5 Raman and XRD spectrum analysis
Table 5 shows the Raman and XRD spectrum analysis.
Raman and XRD spectrum analysis
| Sl. no | Matrix | Method of preparation | Testing method | Property affected | Change observed | Possible reason for change |
|---|---|---|---|---|---|---|
| 1 | MWCNTs/polymer nanocomposites [123] | Sonication followed by melt bending | SEM and Raman spectroscopy | Raman spectra | Raman spectroscopy shows 3 prominent bands – G band at 1,580 cm−1, the D band at 1,360 cm−1, and the D’ band at 1,620 cm−1 with a considerable reduction in the R values | Intermolecular interactions and internal stresses |
| 2 | Self-sensing MWCNTs/polymers nanocomposites [115] | Hot pressing technique | SEM and Raman spectroscopy | Raman spectra | Sample containing 2.0 wt% MWCNT shows a Raman shift of 0.13, 0.24, and 0.31% when compared with the filled 1.5, 1.0, and 0.5 wt% MWCNTs samples | Strong interfacial interaction between the MWCNTs and the phenolic |
| 3 | CNT/polymers composites [124] | Fused filament fabrication and melt mixing | SEM and Raman spectroscopy | Raman spectra | Two characteristic “D” and “G” were found in the Raman spectra | G-band peaks are obtained due to strong interfacial interaction between MWCNTs and the matrix material |
| 4 | CNTs grown CFRP matrix composites [125] | Thermal CVD | SEM, HRTEM, and Raman spectroscopy | Raman spectra | The sample shows “D” and “G” bands with D peak centered at 1,334 cm−1 and G peak centered at 1,581 cm−1 | Disordered carbon and graphite as carbon nanofibers and CNTs |
| 5 | Self-sensing nanocomposites [126] | Direct mixing method | SEM and XRD | Diffraction pattern | Matrix with 2.0 wt% of MWCNTs shows higher diffraction peaks compared to the pure sample | Diffraction peaks in the pure sample are due to the short-range regular ordered structure and amorphous disordered structure |
4.6 Modeling of CNT composites to theorize change in the parameters
4.6.1 Modeling based on CNT waviness
On analyzing wavy CNTs using 3D representative volume element using pullout technique, it was found that the interfacial shear stress of wavy CNTs is higher than straight ones and increases with the increase in waviness [127]. For CNT/shape memory polymer composites analyzed using analytical micromechanical methods, predicted effective mechanical properties were found to be significantly decreased [128]. In the case of carbon nanotube reinforced polymer (CNRP) on the basis of multi-scale modeling, wavy CNTs result in Young’s modulus reduced by 25 to 50%, while the Poisson’s ratio for wavy CNTs was found out to be slightly higher than that of straight CNTs [129]. When analyzed using simplified unit cell methods, CNT-reinforced polymer hybrid nanocomposites demonstrate that the wavy CNT nanocomposites have higher thermal conductivity than straight CNT ones. A multi-stage micromechanical analysis indicates that it enhanced the transverse thermal conductivity of hybrid nanocomposites with straight CNTs and increased in CNT length and volume fraction [130]. Another micromechanical method estimates that thermal conductivity increases nonlinearly with increasing CNT volume fraction [131].
4.6.2 Modeling based on other properties of CNTs
Molecular dynamic simulations for CNT-reinforced metallic glass nanocomposites conclude that adding CNTs result in significant changes in the lateral and axial mechanical properties. The use of long CNTs leads to an increase in the strength and stiffness of metallic glass and changes the mechanism for elastic and plastic deformation [132]. Another molecular dynamic simulation for CNT reinforced aluminum composites shows that toughness, Young’s modulus, and other mechanical properties are enhanced significantly with an increase in CNT reinforcement, thus enabling the material to support extra loading and prevent fracture in the metal matrix [133]. Shear lag and Schapery models for determining the stiffness of CNT-reinforced metal matrix nanocomposites show that the addition of CNTs leads to an increase in stiffness and initial yield surface size. Decreasing the CNT diameter, while increasing the volume fraction and length results in improved transverse elastic modulus and size of the initial yield surface [134]. Estimation of effective elastic properties of CNT/polymer composites demonstrates remarkable enhancement in elastic moduli due to changes in matrix and filler interface of polymer composites [135]. A unified model developed for free vibration analysis of CNT-reinforced laminated cylindrical shells shows that the addition of CNTs improves the natural frequency of the shell, registering a maximum increase of 50% in frequency [136].
5 Applications of CNT-reinforced composites
CNTs, with their lightweight structure, thermal, electrical, and mechanical properties, find use in a variety of applications. We have outlined below the different domains where CNT-reinforced composites have bearing depending on their diverse range of properties:
5.1 Aerospace, automobile, and military applications
CNT/polymer composites such as polyethylene terephthalate (PET), PP, and polyethylene (PE) have broad frequency ranges for absorbing values exceeding 5 dB and have potential applications as radar absorbing materials and can be extensively used in commercial and military applications [74]. Their fracture resistance and damping characteristics also make them an excellent candidate to be used in the aerospace and automotive industry [127]. CNT/Al composites with high structural strength and functional abilities are universally employed in aircraft structures [82]. When SWCNTs are added to IM7 prepreg composites, there is a considerable increase in the thermal conductivity of the composite when compared to the original material, which makes them favorable for use in the low-temperature environment of space. The efficient heat dissipation of CNTs necessary in the pipes and components of aerospace vehicles make them a good fit for rockets such as the Space Launch System [40]. A small amount of CNT can drastically increase the water resistance and interlaminar properties of CNT/epoxy composites and thus find prospective implementation in the automobile industry [137]. Likewise, CNT/SiC composites meet the requisites of advanced structural vehicles such as space vehicles due to their high strength and EMI shielding performance [138]. By combining different amounts of CNTs with nanoclay, ink can be 3D printed with enhanced mechanical and electrical properties, making them a promising composite prospect [54]. Water sustainable fly ash polymer nanocomposites are also widely used in automobile body parts due to their dielectric and water-absorbing characteristics [114].
5.2 Biomedical industry
The electromechanical sensitivity of TPU composite with ionic liquid modified CNTs is significantly high compared to dielectric elastomers, which shows application prospects in artificial muscles, prostheses, bionic robots, and wearable tactile devices [139]. The surface charge of oxidized CNT leads to improved local ionic strength during collagen fibrillogenesis and can be used in tissue engineering and for manipulating collagen’s properties to direct cells’ fates [140]. Muscle-based biohybrid actuators with aligned CNT forest microelectrode arrays can be integrated into scaffolds for cell stimulation and can be used to create electric fields using low potentials to give rise to cell polarization [110]. TiC-reinforced titanium matrix composites can also be tailored for biomedical applications [49].
5.3 Electrical and electronics industry
Silver nanoparticles decorated with CNT show prospective applications in electronics packaging and assemblies, such as isotropic conductive adhesives, owing to their electrical conductivity being four times greater than pristine CNTs [141]. DLCs find use in numerous fields such as optics, microelectronics, and tooling. The addition of CNT also increases fracture toughness and reduces their internal stress, thus minimizing their limitations [77]. The evolution of electrically conducting polymers with decent air stability can bring organic materials into a degenerate semiconductor or metallic regimes and can be used to create thermoelectric materials [66]. Ni/CNTs hybridized with CNT/epoxy nanocomposites have low weight, very high strength with moderate electrostatic discharge properties, and can be used in nanoelectronic devices, flat-panel field-emission displays, and chemical sensors [80]. Polymers with carbon and metal fibers find several uses in structural reinforcement, EMI shielding, electronic packaging, radar absorption, and high-charge storage capacitors [142]. The high aspect ratio and large specific surface area of MWCNTs make them suitable for developing biosensors, thermoelectric devices, functional membranes, capacitors, and artificial muscles [50]. The conductive and highly porous nature of s-CNT-reinforced polymeric powders makes them a good fit for manufacturing electronic packaging and actuation [20]. The high performance of Li-ion batteries can also be enabled by converging nanoscale additive manufacturing [47]. CNT fibers and their derived functional materials also find fiber- or fabric-based applications such as supercapacitors, batteries, intelligent sensors, actuators, and artificial muscles [37].
5.4 Miscellaneous
Assimilating vertically aligned CNT arrays into the existing conventional structural composites makes them appropriate for erosive wear applications [119]. Ni–P-based composite coatings display remarkable wear resistance and are universally used for applications based on tribological performance [76]. Moreover, incorporating CNTs into cementitious composites makes them suitable for piezoresistive and crack sensors. They can also be a heating composite for deicing on the roads and applied to an accelerated curing method for the cementitious materials in cold weather conditions [143]. The enhanced thermal conductivity qualifies them to be used as heat interface materials and for heat enhancement applications [102]. MWCNT/polymer composites can be used as strain sensors, as their piezoresistive sensitivity is 3.5 times higher than that of a conventional metallic strain gage [144]. Multifunctional CNT yarn reinforced components can be 3D printed and employed in adaptive structures and structural health monitoring [43]. CNT/TPU filaments can also be 3D printed for direct manufacturing of multi-axial force sensors [44]. CNTs incorporated with a mesoporous TiO2 film on a titanium foil find dye-sensitized solar cells [29]. At low concentrations, epoxy resin-based CFRP with a surface coating of CNTs demonstrates improved energy concentration [145]. Incorporating ZnO and CNTs in a PANI matrix creates nanocomposites that can be employed in solar cells and other energy-related devices [146]. The high aspect ratio of CNTs, high elastic modulus, and strength make them appropriate for fabricating sports goods and energy [147]. Insertion of CNTs can reduce fuel consumption and greenhouse gas consumption by 16% and 26%, respectively [98]. Flexible organic and perovskite solar cells can be manufactured using PEDOT: PSS and CNTs [148]. For light shuttering applications, electrically switchable light absorbers can be fabricated by dye and MWCNTs-doped liquid crystal droplets [149]. CNTs/PLA composites are also implemented in scaffolds for tissue, water treatment, textile, packaging, and even flexible electronic devices [85].
6 Conclusion
The presented review illustrates the basics of CNTs, how they can be fabricated and processed, and the obstacles encountered when they are manufactured. Ball milling seems to be one of the most popular and efficient techniques currently in use, as it gives homogeneous dispersion of CNTs in the metal matrix. The review also demonstrates an extensive study highlighting how different properties of these composites can be observed and experimentally analyzed. Techniques such as SEM, TEM, X-ray spectroscopy, and nanoindentation are commonly used to study and estimate the properties of the composite. Their excellent mechanical, thermal, tribological, and electrical properties make them a strong candidate as reinforcement materials, and they find use in all kinds of industries, including aerospace, electronics, and biomedical. Over the past decades, there has been considerable improvement in the manufacturing and synthesis of CNTs, but several areas still require significant research, such as structure control and growth techniques. Advancement in these areas will have essential impacts on the applications of CNTs and will also reduce the cost of fabrication. Tackling the challenges outlined in the article, such as improper bonding at matrix interface, delamination of composites, and ineffective dispersion of CNTs will further streamline the CNT-composite processing, and thereby, their utilization in different industries.
Acknowledgments
The authors would like to thank Universiti Putra Malaysia for financial support through the Geran Putra Berimpak, GPB 9668200. The authors would also like to acknowledge the Department of Aerospace Engineering, Faculty of Engineering, Universiti Putra Malaysia for the close collaboration in this research. The authors are thankful for Vellore Institute of Technology (VIT), Vellore, India for providing seed grant (SG20210290) for conducting preliminary research work.
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Funding information: This work was supported by Universiti Putra Malaysia through the Geran Putra Berimpak, GPB 9668200.
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Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.
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Conflict of interest: Authors state no conflict of interest.
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Data availability statement: Data sharing is not applicable to this article as no datasets were generated or analyzed during the current study.
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