High Performance Fibers from Carbon Nanotubes: Synthesis

Review Cite This: Ind. Eng. Chem. Res. 2017, 56, 12407-12437

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High Performance Fibers from Carbon Nanotubes: Synthesis, Characterization, and Applications in CompositesA Review Manishkumar D. Yadav,† Kinshuk Dasgupta,‡ Ashwin W. Patwardhan,† and Jyeshtharaj B. Joshi*,†,§ †

Department of Chemical Engineering, Institute of Chemical Technology, Mumbai 400019, India Materials Group, Bhabha Atomic Research Centre, Mumbai 400085, India § Homi Bhabha National Institute, Mumbai 400094, India ‡

ABSTRACT: A carbon nanotube fiber is formed by interlocking carbon nanotubes that are perfectly aligned to give a continuous long fiber. The production of neat carbon nanotube fibers has paved the way for realization of superior mechanical and physical properties of individual carbon nanotubes at macroscopic scale. In this review paper, we elucidate the state of the art advances in fabrication methods, characterization, modeling of mechanical properties, and applications of carbon nanotube fibers as next generation high performance fibers for composites. Recommendations have been made on the aspects of design and scale-up for large scale manufacturing of carbon nanotube fibers using (a) a stepwise protocol for the determination rate controlling step(s) and the estimation of overall rate of reaction, and (b) computational fluid dynamics. Finally, the challenges and opportunities in carbon nanotube fiber research have been clearly brought out, and suggestions have been made for future work.

1. INTRODUCTION Since at least 4000 BC, natural fibers (silk, cotton) have been used in human life. However, rayon, the first man-made fiber, was manufactured only at the end of the 19th century. Later in 1939, nylon was introduced. Since then, many synthetic fibers were developed and commercialized. The synthetic fibers are mainly classified on the basis of performance such as conventional textile fibers and high performance fibers. Current advancement of fibrous materials has focused on performance by maximizing the mechanical and physical properties. Key properties of high performance fibers such as Kevlar, Dyneema, and carbon nanotube fibers are high flexibility, high strength, lightweight, high thermal and electrical conductivity, etc. These properties open doors to a wide range of applications such as composites, bulletproof vests, electromagnetic interference shielding (EMI), energy storage, and many others. Carbon nanotubes (CNTs) have been brought to scientific attention in 1991 by Iijima.1 Since then, CNTs have been one of the representative materials in nanotechnology. It combines the best properties of metals and polymers, due to unique atomistic structure and strong carbon−carbon covalent bonds. Many experimental and theoretical works have reported CNTs to be strong and stiff with tensile strength in the ranges of 11−63 GPa,2,3 which is much higher than all existing materials. In addition, the electrical conductivity is as high as 106 S cm−1 for single walled CNTs4(SWCNTs) and 3 × 104 S cm−1 for multiwalled CNTs5 (MWCNTs). However, the nanoscale of CNTs possess deficiencies of their own virtues in handling and control of their use in particular application(s). The simplest effect of © 2017 American Chemical Society

formation of macrostructures is ease of use in a variety of applications. However, it is important that the macrostructures are formed in such a way as to retain the exceptional properties of individual CNTs. Macrostructures based upon CNTs are CNT fibers,6 films,7−9 and arrays10,11 that provide ease at handling and processing. Long fibers comprising only CNTs (neat fibers) can be woven in the form of a fabric and can be easily used in composites. In addition, such fibers can be used for replacing copper wires in cables for electricity transmission with negligible losses. The alignment of CNTs into fibers is a valuable one having the same symmetry (uniaxial) as its nanoscale building blocks. Some of the mechanical properties of various high performance fibers are listed in Table 1. Here, carbon nanotube fibers exhibit the highest tensile strength in comparison to other high performance fibers. A wide range of potential applications of CNT fibers are anticipated, such as reinforcements for composites,12−19 supercapacitors,20−25 electrochemical sensors,26−29 transmission lines,30−33 etc. In order to have products of CNT fibers in the market, production rates needs to be increased dramatically. The real challenge to materials scientist and chemical engineers is to achieve, as far as possible perfect alignment of individual CNTs in order to match the axis of fiber.34 To date, three methods for continuous production of CNT fibers are Received: Revised: Accepted: Published: 12407

June 5, 2017 October 14, 2017 October 17, 2017 October 17, 2017 DOI: 10.1021/acs.iecr.7b02269 Ind. Eng. Chem. Res. 2017, 56, 12407−12437

Review

Industrial & Engineering Chemistry Research

or carbon dioxide) is impinged with a high power laser beam under inert atmosphere. It results in soot formation consisting of pristine CNTs.72 SWCNTs and MWCNTs can be selectively synthesized by operating the system with or without transition metal catalyst such as Ni or Co, respectively. A better control over properties of carbon nanotubes such as diameter and chirality can be obtained, but it is expensive to carry out. Chemical vapor deposition (CVD) technique is the most suited method for scalable industrial applications of CNTs due to mild operations and low cost.73−76 The introduction of a catalyst divides CVD technique into two main approaches, namely floating catalyst chemical vapor deposition (FCCVD) and fixed catalyst chemical vapor deposition. In the floating catalyst CVD technique, catalyst is present in the gas phase. In case of fixed catalyst CVD technique, a thin film of catalyst material usually a transition metal is deposited onto a support layer such as alumina(Al2O3). For the synthesis of CNT fibers, several CVD methods have been reported in the published literature, which includes fluidized bed,77−82 substrate growth,11,83,84 etc. Depending upon the carbon sources used (aromatic or non aromatic), catalyst composition and operating conditions (temperature and pressure) it is possible to tune the composition of type of CNTs (MWCNTs or SWCNTs) formed along with their length and chirality.85,86 It is desirable to know the sensitivity of these parameters on the characteristics of CNTs is of utmost important for the formation of macroscopic assemblies such as CNT fibers, films, etc.

Table 1. Comparison of CNT Fiber with Other High Performance Fibers Fibers

Density (g/cm3)

Tensile strength (GPa)

Gauge length (mm)

CNT Fiber6 Graphene fibers53 Kevlar 4954 Dyneema SK 7655 UHMW-PE56 HS carbon fiber57

0.897 0.2−1.8 1.44−1.47 0.970 0.980 1.73−1.93

8.8 0.14 3.4−4.1 3.6 2.2−3.9 3.4−7

1 − 50 5 − 1.4−0.1

reported. The first method is spinning from CNT solutions,35−39 the second method is spinning from vertically aligned CNT arrays,40,41 and the third method is spinning from a CNT aerogel.6,42 A number of review articles on carbon nanotube fibers have been published recently that summarize the fabrication, performance, and properties (together with applications) of CNT fibers.43−52 However, a comprehensive review on the synthesis and application in composites of high performance fibers from carbon nanotubes is still lacking. The emphasis of this review is to illustrate the present status of the subject under consideration and to highlight the possible future directions for further understanding of the subject. In sections 2 and 3, we address the methods (major) existing for the fabrication of CNT and CNT fibers, respectively. In section 4 we have critically analyzed the published literature and presented the current status of CNT fiber synthesis. We have compared the various methods reported for processing CNTs into CNT fibers and lacunae in the available literature have been delineated. The characterization and analysis of CNT fibers, including their physical and mechanical properties, are highlighted in section 5. In section 6, design and scale-up of CNT fibers has been discussed, and recommendations have been made on the aspect of design and scale-up for large scale manufacture. In section 7, the mechanical properties such as tensile and compressive properties of CNT fibers are discussed. Section 8 summarizes the potential applications of CNT fibers in composites. This paper will be helpful to get more insight about CNT fibers, and to find the most appropriate processing techniques and conditions.

3. CARBON NANOTUBE FIBERS/YARNS/ROPES CNTs are mostly synthesized in powder form, and are effectively used as fillers in nanocomposites enhancing the mechanical properties of polymer matrices. However, reinforcement of CNT powders in polymer is limited by the difficulty in maintaining higher concentrations, due to cohesive forces or van der Waals forces.87−91 In order to effectively utilize the outstanding properties of individual CNTs, they must be aggregated into macrostructures such as fibers or films so as to retain the excellent properties at larger scale. Fiber is a basic filament from which yarn is spun. In the literature, these terminologies are used interchangeably. CNT fibers are easier to disperse in polymer in comparison to CNT powders providing enhanced electrical and mechanical properties. In practice, two principal methods have been employed for the large scale synthesis of CNT fiber, i.e., “wet spinning” and “dry spinning”. Both methods (wet and dry spinning) can be employed for neat as well as polymer-infiltrated CNT fibers. Most of the synthetic fibers are formed by wet spinning where fibers are extruded from concentrated, viscous liquid of the starting material, which are further aligned and converted into a fiber through solvent removal or cooling or using electrical fields. In the case of CNT fibers, the wet spinning approach can be further classified as spinning (1) using superacids; (2) using surfactant details of these approaches are mentioned in a later section. Natural fibers like cotton and wool are formed by dry spinning approaches; here discrete fibers are assembled into a yarn. Carbon nanotubes have been used in a similar fashion to produce CNT fibers, direct spinning of CNT aerogel formed in the reaction zone into a fiber is done whereas a number of different research groups use a different technique that involves spinning of fiber from a vertically grown CNT array. Table 2 shows that the method of direct spinning of CNT aerogel gives excellent properties.

2. CARBON NANOTUBES The structure of CNT fibers consists of a network of carbon nanotube bundles relatively aligned parallel to the fiber axis. Obviously, the excellent properties are directly correlated with the properties of constituting CNTs such as long length and excellent alignment. It is also desired that the CNTs are packed in defect free arrangement.6 Hence, research efforts on CNT fibers and CNT synthesis are interdependent.45 Here, we review the various techniques utilized for synthesis of CNTs. Arc discharge,58−61 laser ablation,62−64 and chemical vapor deposition65−68 are three main strategies used for producing carbon nanotubes (both single walled carbon nanotubes (SWCNTs) and multiwalled carbon nanotubes (MWCNTs)). In addition to it, electrolysis69 and solar energy70 methods have also been proposed. In arc discharge technique, a vapor is created between two carbon electrode by an arc discharge in the presence or absence of catalyst71 resulting in formation of self-assembled CNTs. Along with CNTs, a huge amount of impurities and/or amorphous carbon are also produced. In laser ablation technique, carbon containing feedstock gas (most commonly methane 12408

DOI: 10.1021/acs.iecr.7b02269 Ind. Eng. Chem. Res. 2017, 56, 12407−12437

Review

Industrial & Engineering Chemistry Research Table 2. Comparisons of Strength and Modulus of CNT Fibers Fabricated by Wet and Dry Spinning Methods Method Spinning using super acids92 Spinning using surfactant solutions93 CNT array spinning94 Direct spinning of CNT aerogel6


Young’s modulus (GPa)

Gauge length (mm)

1.3

200

20

0.15

142

10

3.3 8.8

263 357

10 1

4. STRATEGIES TO FABRICATE CARBON NANOTUBE FIBERS 4.1. Wet Spinning. Wet spinning is a generic process consisting of injecting a concentrated solution of a dissolved macromolecular material through a spinneret immersed in liquid bath. Further, a solid filament is obtained by coagulation, precipitation or gelation of the dissolved macromolecular material. Polymers that cannot be melted or get degraded at high temperatures are appropriately processed using wet spinning methodology. Many synthetic fibers such as polyaramid, polyacrylonitrile(PAN) and poly(vinyl alcohol)(PVA) are produced by wet spinning approach. Wet spinning technology is industrially viable for large scale production of both composite fibers (i.e., fibers composed of CNTs and polymer) and neat CNT fibers (i.e., fibers solely composed of CNTs). In the literature, two important wet spinning approaches for fabrication of CNT fibers are reported: (1) spinning of surfactant-stabilized suspensions of CNTs; (2) liquid crystalline spinning of SWCNTs in superacids (fuming sulfuric acid or chlorosulfonic acid). Other wet spinning methods are spinning of dispersions prepared with DNA, hyaluronic acid or chitosan, i.e., biomolecules.95,96 4.1.1. Spinning Using Superacids. It is one of the most popular methods of spinning from super acid liquid crystalline phase. Here, the liquid crystal phase is prepared from SWCNTs and superacids, which lead to the formation of homogeneous SWCNTs solution. The liquid crystal phase is extruded through a capillary into the coagulation bath and directly wound on a rotating spindle. CNTs are chemically inert and possess strong intertube van der Waals attractions, which limit their solubility in aqueous, acidic or organic media. Hence, it becomes extremely difficult to control the dispersion and alignment of CNTs. In order to circumvent this problem, covalent functionalization of CNTs is done to increase its solubility in a given medium. Nevertheless, such covalent functionalization downgrades key physical properties and limits the ordering and packing of CNTs in the fibers. The major roadblock in the production of neat CNT fibers is dispersion of CNTs at high concentrations suitable for effective coagulation and efficient alignment. In 2004, Ericson et al.37 first successfully synthesized macroscopic fibers with perfect alignment, without any supporting surfactant or polymer structure, i.e., neat CNT fibers. In superacids, SWCNTs spontaneously dissolve without any aid of mechanical energy and form a thermodynamically stable dispersion. SWCNTs are protonated in the presence of superacids forming individual charged nanotubes surrounded by number of sulfate anions as shown in Figure 1. The electrostatic repulsion due to the positive charges on each SWCNTs counteracts the attractive van der Waals interactions present between individual CNTs. Depending upon the concentration of SWCNTs, three distinct regimes are observed as shown in Figure 2.97 Rai et al.97 have demonstrated that at very low concentration (4 wt %) as shown in the Figure 2, protonation of the ordered phase increases until the CNTs coalesce and form ordered domains behaving similar to nematic liquid crystalline rod-like polymers. Table 3 summarizes the synthesis conditions given by different investigators for the formation of CNT fibers by using super acids. Maximum 8% by wt SWCNT dispersion in 100+% sulfuric acid are reported in the literature. SWCNTs dissolved in super acids are injected in a solvent in which CNTs are not soluble also known as coagulants (water, acetone, diethyl ether or dilute sulfuric acid solutions are reported coagulants). CNTs instantly coagulate in such coagulants or antisolvents that can be drawn in the form of fibers and subsequently dried by removing residual coagulant or antisolvent. A schematic of the process is depicted in Figure 3. The density of fibers coagulated in ether yields a low density(0.87 ± 0.08 g/cm3) collapsed structure also referred as “dog bone structure” whereas fibers coagulated in water (1.11 ± 0.07 g/cm3) possessed a circular shape. Since, the Young’s modulus is principally affected by orientations/alignments of CNTs within these fiber. It may be emphasized that for the improved alignment of SWCNTs 12409


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Table 3. Synthesis Conditions and Mechanical Properties of CNT Fibers Manufactured Using Super Acids Reported in the Literature Reaction Conditions Carbon nanotube

Carbon nanotube wt %

MWCNT

2−6

SWCNT

0.5

SWCNT SWCNT

6−8 8

Solvent Chlorosulfonic acid Chlorosulfonic acid 100% H2SO4 100% H2SO4

Results Coagulant

CNT fiber diameter (μm)



Fiber density (g/cm3)

ref

Acetone/water

9

1.3

200

1.3

92

Water

13

0.32

120

−

98

Ether Diethyl ether 5 wt % aq H2SO4 Water

− 52.7−87.4

− 0.11

− 120 ± 10

− 0.87 ± 0.08 1.11 ± 0.07

99 37

4.1.2. Surfactant-Based Coagulation Spinning. In 2000, Vigolo et al.35 were the first to propose semicontinuous wet spinning process to obtain CNT fibers. Employing this coagulation spinning technique for the fabrication of CNT fiber, the CNTs need to be dispersed into a liquid solution at the molecular level so that they can be manipulated and aligned well. Surfactant possess ability to form micellar structures around individual CNTs. Hence, CNTs are dispersed in a surfactant solution followed by recondensation in the flow of a polymer solution to form a nanotube mesh, and subsequently collating this mesh to a nanofiber35 as shown in Figure 4. Here, SWCNTs Figure 3. Schematic of apparatus used to spin carbon nanotube fiber from liquid crystalline phase containing 8.5 vol % SWNTs in pure chlorosulfonic acid and coagulated in 96% aqueous sulfuric acid. (Figure reproduced with permission from ref 98. Copyright 2009, Nature Publishing Group.)

(and hence superior properties of fibers), the governing parameter is the shear rate within the extrusion orifice. Young’s modulus is a good indicator of mechanical performance, and probably the highest value has been reported using for acid-spun fibers. The as-spun fibers exhibit a low modulus due to the acid doping that can be improved via postprocessing treatments such as annealing at very high temperature, i.e., 1000−1500 °C in the presence of inert gases like nitrogen or argon. The best tensile strength reported for CNT fiber using this method is 1.3 GPa, and the Young’s modulus approaches 200 GPa with SWCNT concentration between 2 and 6% by wt. However, this process (SWCNT/acid system) is very sensitive to water, the presence of even minimal amount of moisture can cause phase separation and eventually precipitation of discrete needle-like crystal solvates. In addition, super acid route is not found to be effective for MWCNTs. Because MWCNTs possess limited solubility in superacids, this methodology is limited to usage of SWCNTs. In order to circumvent this problem, Zhang et al.93 proposed a novel coagulation process where the MWCNTs are not dissolved in thermodynamically stable forms; instead, they are spun from liquid crystalline solution of ethylene glycol. Here, MWCNTs (1 to 3 wt %) are dispersed in ethylene glycol with the aid of ultrasound. An increase in concentration of ethylene glycol results in phase change; the dispersion went from isotropic to biphasic and finally to nematic phase as observed in the case of dispersion using superacids. The fibers are spun by injecting the dispersions into a coagulating bath of diethyl ether. This results into rapid equimolar counter diffusion of ethylene glycol and ether. The MWCNT fibers synthesized using this methodology possess a Young’s modulus of 69 ± 41 GPa and tensile strength of 0.15 ± 0.06 GPa.

Figure 4. Schematic of the experimental setup used to make nanotube ribbons. Flow-induced alignment of the nanotubes took place at the tip of the capillary. The ribbons could be drawn in the third dimension and formed a helical structure when the polymer solution was slowly pumped out from the bottom of the container. (Figure reproduced with permission from ref 35. Copyright 2000, AAAS.)

produced using an arc-discharged method are dispersed in water using sodium dodecyl sulfate (SDS) as surfactant and tip sonication for breaking CNT bundles and allowing the surfactant micelle to encase the CNTs. The dispersion is injected in the coflowing stream of a rotating bath containing 5 wt % of poly vinyl alcohol(PVA). PVA possessing amphiphilic character, gets adsorbed at CNT interface and simultaneously displaces the surfactant molecules.100 Here, concentration of surfactant is one of the crucial parameter for obtaining good dispersion of CNTs.101 Vigolo et al.101 have observed that at very low concentration of surfactant, the stabilization of CNT fiber is inadequate, which in turn induces electrostatic repulsion that could counterbalance van der Waals attractions, whereas if it is too high surfactant aggregates, i.e., micelles formation takes place in aqueous solution. Micelles are unable to fit in between two bundles that are close to each other, as a result the osmotic pressure of the 12410


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Table 4. Synthesis Conditions and Mechanical Properties of CNT Fibers Manufactured Using Surfactants as Coagulation Medium Reported in the Literature Reaction Conditions Carbon nanotube

Carbon nanotube (wt %)

Dispersant


ref

25−30

−

2

102

LDS (1.2 wt %)

20

−

−

38

−

Ethylene glycol (1−3 wt %)

Diethyl ether

80−100

SDS (1 wt %) SDS

Poly(vinyl alcohol) 5 wt % Poly(vinyl alcohol)

100 35

0.35

SDS/tetratrimethly ammonium bromide

Poly(vinyl alcohol) 2−6 wt %

20

69 ± 41 142 ± 70 9−15 10a 40b 10−20

93

0.35

0.15 ± 0.06 0.17 ± 0.07 − 0.12a 0.23b −

0.8

SDS (1 wt %)

SWCNT/ MWCNT MWCNT N-MWCNT SWCNT SWCNT

0.6

a

Coagulant


Ethanol/glycerol (1:1) Ethanol/glycol (1:3) HCl (37%)

SWCNT

SWCNT

Results Tensile strength (GPa)

35 101 103

Raw fiber. bStretched fiber.

Figure 5. Spinning continuous yarns from superaligned CNT arrays. (a) SEM image of CNT array with a length of 0.65 mm. (Figure reproduced with permission from ref 106. Copyright 2007, John Wiley and Sons.) (b) Pulling yarn model (Figure reproduced with permission from ref 105. Copyright 2006, John Wiley and Sons.)

ethanol/glycol mixture (1:3 v/v) as coagulants is found to be 2 GPa, whereas that of SWCNT fibers coagulated in presence of PVA was found to be approaching 10 GPa for raw fiber and 40 GPa for stretched fiber. The mechanical properties of PVACNT composite fibers largely depend upon the treatments and composition of the fibers. Type of CNTs (SWCNT or MWCNT) and method used to make these CNT largely affect the mechanical properties of the composite fibers. Surfactant based coagulation spinning is a simpler and faster and potentially scalable for large scale production process used to assemble CNTs into indefinitely long fibers with very high loading of CNTs (up to 60 wt %) possessing tensile strength as high as 1.8 GPa. The main challenges existing in this method include lower processing rate and efficient dispersion of CNTs at high concentrations, which can be easily achieved by various modifications in spinning apparatus and usage of CNTs with high purity and minimal defects. 4.2. Dry Spinning. Dry spinning is done for most of the natural fibers such as wool and cotton, where discrete fibers are assembled into yarn. Dry spinning for CNT fibers is further classified into one step and two step processes. CNTs synthesized from a precursor material and carrier gas (H2 or Ar) in the form of an aerogel during chemical vapor deposition and subsequently pulled out of the furnace to form a yarn/fiber is known as a onestep spinning method. Two-step techniques rely on spinning of CNT fibers from other macroscopic assemblies such as CNTs arrays,104−106 CNT cotton,107,108 and CNT films.109,110 4.2.1. Forest Spinning. In 2002, Jiang et al.40 were the pioneer group to report spinning of the continuous CNT fibers from vertically aligned CNT arrays up to 30 cm in length resembling the process of drawing silk out of a cocoon. CNT arrays are also known as CNT forests or brushes or carpets. Typically, vertically

micelles around the bundles creates an effective attraction also known as depletion attraction. At intermediate concentrations of surfactants CNTs are homogeneously dispersed and form one phase. This process has been successfully used for MWCNTs, and other surfactants such as tetra-trimethylammonium bromide (TMB), lithium dodecyl sulfate (LDS), and amphiliphilic polymers such as denatured DNA have also been used. The dispersant used has to be displaced by the coagulant during fiber spinning, so as bridging coagulation mechanism can take place. It implies that adsorption energy of the dispersant at the CNT interface must be lower than that of coagulant used. The flow of coagulant must be faster than the gel-fiber in order to stretch the fiber along the axis direction and promote alignment of CNTs in the fiber. The findings of Vigolo et al.35 have suggested a way for attaining such conditions by rotating the coagulant container as shown in Figure 4. Table 4 summarizes the synthesis conditions given by different investigators for the formation of CNT fibers using surfactant coagulation method. In order to produce polymer-free fibers, coagulation baths other than PVA/water have been utilized such as ethanol/glycerol(1:1 v/v), ethanol/ glycol mixture(1:3 v/v) or polymer free acids or bases that promotes the flocculation of the initial CNT dispersion. Here, nearinstantaneous flocculation occurs with coagulants when the pH is changed from almost neutral to strong acidic(pH < 1) or alkaline(pH > 13). The as-spun fiber retains much of coagulants, which lead to a hollow fiber morphology affects the mechanical properties such as tensile strength and tensile modulus resulting in fibers that are flexible when wet but become brittle upon drying.38 Table 4 lists the mechanical properties of CNT fibers obtained by surfactant based coagulation spinning. Young’s modulus of SWCNT fibers obtained from ethanol/glycerol (1:1 v/v), 12411


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Figure 6. (a) Nonspinnable, tortuous CNT array produced from catalyst layer of 5 nm Fe. (b) Spinnable, straight CNT array produced from catalyst layer of 2.5 nm Fe. (Figure reproduced with permission from ref 111. Copyright 2010, Elsevier.)

Table 5. Synthesis Conditions and Mechanical Properties of CNT Fibers Manufactured Using CNT Forests Reported in the Literature Reaction conditions Carbon source Ethylene (100 sccm) Acetylene (500 sccm) Ethylene (100 sccm) Ethylene (100 sccm) Ethylene (150 sccm) Acetylene (500 sccm) Ethylene Acetylene Ethylene (300 sccm)

Catalyst Fe (0.3−1 nm) Fe (3−5.5 nm) Fe (0.3−1 nm) Fe (1 nm) Fe (2 nm) Fe (3−5.5 nm) Fe FeCl2 Fe+Co

Buffer layer

Temperature (°C)

Al2O3 (10 nm) −

750

Al2O3

750

Al2O3

750

Al2O3 (15 nm) −

750

Al2O3 − Al2O3 (5 nm)

660−750 820 750

680−720

680−720

Results Carrier gas flow rate (sccm)

CNT fiber diameter(μm)

CNT yarn strength (GPa)

Ar (94%) H2 (6%) Ar (90%) H2 (10%) Ar (94%) H2 (6%) Ar (94%) H2 (6%) Ar+H2

100

2−10

1.91−3.3

−

114

475

20−30

0.564

74

105

100

4−5.8

1.35−3.3

100−263

115

100

5−13

0.17−1.91

89−330

106

530

10

0.3−0.6

7−30

116

Ar (90%) H2 (10%) Ar+H2 Ar+H2 Ar

475

10

0.63−1.1

48−56

117

− − 1000

7.7−8 − 33−44

1.17−1.23 0.38 −

53.5 − −

118

Carrier gas

aligned CNT arrays are grown by CVD method on an catalyst and thermal oxide coated silicon substrate placed in a reaction furnace. The hydrocarbons (methane, acetylene, etc.) along with carrier gas, for example argon or helium, are introduced into the furnace hot zone possessing temperature above 600 °C. As the interactions between microfilaments increase friction between each CNTs also increase; hence, once the CNTs at the edge of a CNT forest are pulled out, the neighboring CNTs are successively pulled out and thus forming a continuous and long CNT fiber as shown in Figure 5. In the literature as shown in Figure 6, many articles conclude that not all CNT arrays can be spun into fibers and morphology of CNT arrays is closely related to spinnability of CNT arrays.111 Various key parameters involved in growth of spinnable CNT arrays are reported such as (1) catalyst thickness and deposition process, (2) substrate oxide thickness, (3) growth temperature, (4) carrier and reactant gas flow rate and ratio, and (5) reaction time. The synthesis conditions of CNT arrays and details of CNT fibers drawn in published reports are summarized in Table 5. Huynh and Hawkins111 have reported a detailed and comprehensive investigation of the effects of key parameters using acetylene as carbon source, helium as carrier gas, and 1−3.5 nm thick iron catalyst layer on a silicon substrate with 3−900 nm of thermal oxide.

CNT yarn stiffiness (GPa)

ref

119

The effect of reaction temperature on CNT length and spinnability was studied and it was found that CNT growth rate and spinnability increases with temperature. However, at higher temperature formation of amorphous carbon and rate of deactivation of catalyst also increases as shown in Figure 7. CNT length

Figure 7. Effect of reaction temperature on CNT length and spinnability. (Figure reproduced with permission from ref 111. Copyright 2010, Elsevier.) 12412


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Industrial & Engineering Chemistry Research increases with increase in temperature, but spinnability initially increases but at very high temperatures, i.e., 710 °C, spinnability steadily declines as temperature is increased despite the nanotubes being longer. A possible reason asserted for decline in spinnability is the formation of amorphous carbon and presumably on the CNTs, which may account for the decrease in spinnability. The spinnability of CNT forest is highly sensitive to the substrate characteristics; its primary role is to isolate the catalyst from the silicon substrate in order to obviate noncatalytic silicides. Silicon dioxide (SiO2) and alumina (Al2O3) are commonly used substrates due to their inert nature. Thickness of substrate also plays an important role in spinnability of CNT forests. Huynh and Hawkins111 have found that for SiO2 substrate highly spinnable forest were grown between 25 to 50 nm oxide coatings while as the thickness was increased from 100 to 900 nm poor spinnability of CNT forest was observed. Probably it is due to incorporation of impurities such as water, hydroxyl, etc. during the oxide growth process, which eventually affects the silicon substrate integrity and leads to fragmentation of catalyst. Surface structure and surface chemistry dictates the CNT morphology and spinnability fundamentally appears to be related to the interactions of CNTs with each other. Very few reports are available on computational modeling related to CNT array formation. Computational fluid dynamics is one of the technique preferred by few researchers in order to model the growth pattern of CNTs. Kim et al.112 recently reported that the principal factor for the formation of vertically aligned CNT forest during the CVD process is the gas flow (related to reactor geometry). Effects of parameters such as flow rate of the reactants and carrier gas, direction of the carrier gas, and growth time were studied in-depth. In comparison to flow rates, the direction of flow was found to be dominating for the creation of microchannels. Nonlinear gas flows induced by microchannels were found to be controlling the growth directions of CNTs. In order to explain the directionality of CNTs, a computational fluid dynamics (CFD) model was developed by Kim et al.112 and it was reported that microchannels induce variations in local pressure that largely affect the growth of CNT forest. Samandari-Masouleh et al.113 have developed a mathematical model for the growth of MWCNT arrays. For this purpose, they have made several assumptions such as (a) plug flow of gas phase, (b) the controlling step to be a first-order intrinsic chemical reaction for the formation of CNTs as well as amorphous carbon. The model predicts the height of CNT arrays as a function of concentration of carbon source and catalyst and also the temperature. The authors have also incorporated the deactivation kinetics in the mathematical model. The predictions of the model (Figure 8) shows the model validation by comparing the model predictions and the experimental measurements of growth rates. It can be observed that the CNT array height calculated via model is in good agreement with the experimental values when deactivation of catalyst is taken into account. 4.2.2. Direct Spinning of CNT Aerogel. Direct spinning of CNT aerogel from a floating catalyst chemical vapor deposition (FC-CVD) is a one-step process capable of producing CNT fibers without any length limitations. Table 6 summarizes the synthesis conditions of CNT aerogels and details of produced fibers reported in the literature. Here, the feedstock comprising carbon source, catalyst, promoter, and carrier gas is introduced into the hot zone of a CVD furnace (temperature > 1000 °C). Ferrocene is utilized as a source of iron catalyst along with some sulfur containing compound that acts as a promoter. The role of

Figure 8. Comparison between experiment and calculated height of carbon nanotubes with and without deactivation of catalyst. (Figure reproduced with permission from ref 113. Copyright 2011, American Chemical Society.)

sulfur is most important in high temperature floating catalyst CNT synthesis because it acts as a promoter in enhancing the addition of carbon atoms to the growing ends. The carbon source (hydrocarbons) undergoes decomposition in reducing atmosphere (H2) at high temperature (>1200 °C) allowing growth of carbon nanotubes on the catalyst clusters. The grown CNTs form an aerogel consisting of thousands of individual CNTs that are drawn out of the furnace using a metal rod inserted inside the furnace. The van der Waals forces between these CNTs hold them together forming a monolithic entity. A pioneering contribution has been made by Zhu et al.,42 who synthesized 20 cm long strands of ordered SWCNTs after the pyrolysis of hexane, ferrocene, and thiophene using a floating catalyst CVD method in a vertical furnace. The CNT fiber formed possessed good strength of about 1.2 GPa. Adopting a similar method in 2004, Li et al.120 explained a process for the direct spinning of long pure CNT fibers by mechanically drawing CNT aerogel from the gaseous reaction zone, and directly winding it onto a rotating rod. In the direct spinning process, the CNTs grow tangentially on the catalyst particles and form a porous cluster, i.e., aerogel also called as “elastic-sock” as shown in Figure 9. Here, the formation of “elastic-sock” is a crucial step in the process of producing CNT fibers. The sock exhibits a peculiar property of nonadherence to a hot reactor surface and the possible reason is asserted due to the thermophoretic effect.121 In 2010, Conroy et al.121 developed a mathematical model for the synthesis of CNT fibers. The proposed mathematical model describes the formation as well as the growth of iron particles in a CVD reactor. Further, the model has been built to include the effects of practically all the design parameters such as reactor geometry and operating parameters such as ferrocene concentration at the injector, gas flow rate, and the wall temperature. The authors brought out a very important point that the formation of aerogel needs to happen above 1000 °C so that enough quantities of nanotubes are formed. The authors further reported below 1000 °C, the rate of cracking is insufficient for providing enough carbon for CNT formation. The reduction of rate was attributed to the growth of catalyst particle (when temperature is below 1000 °C). A temperature above 1265 °C promotes self-pyrolysis, and hence a huge amount of soot particles are formed when ethanol is used as the carbon source, ferrocene as catalyst, and thiophene as promoter. Current advancement in research on CNT fibers synthesized via the CNT aerogel process has reached more than 1 km122 long fibers, and 12413


1050−1200

1250

1050−1200 1300 1180

1170 1200 1150 >1300 1150−1300

−

Thiophene Thiophene Thiophene

Thiophene Thiophene Thiophene Thiophene Thiophene

Methane/ethanol

Ethanol

Ethanol Ethanol Ethanol/ethylene glycol/hexane Ethanol Acetone Ethanol Hexane Ethanol

1300 1200 1200

Thiophene − Thiophene/ carbon disulfide Thiophene

Ethanol/hexane Toluene Methane

1250

Thiophene

Promoter

Butanol

Carbon source

Temperature (°C)

Water Water Water − Water

H2 H2 H2 H2 N2

H2 H2 H2

− Acetone −

H2

− H2

H2 He H2

− − Acetone

Acetone

H2

Acetone

Densification

Carrier gas

1000 1000 1000 250 250−600

400−800 − 400−1000 8 10 10 30 2−10 (mL/min)

4.8−15 − 2−7.5

−

−

− −

2−12 − −

5

Reaction injection rate (mL/h)

400−4000 − −

−

Carrier gas flow rate (mL/min)

DWCNT − DWCNT SWCNT MWCNT

PMMA/ SWCNT SWCNT/ DWCNT MWCNT DWCNT MWCNT

SWCNT/ MWCNT DWCNT SWCNT SWCNT

CNT type

0.1−1 8.8 1.46 0.4−1.25 0.25−0.6 N/tex − 0.8 4.34

− 38−44 1−3 mm 0.3−0.5 mm 5−9

−

1 GPa/SG

>1 N/Tex − −

−

CNT yarn strength (GPa)

− 7 10−100

10

2−20 − 10

−


Table 6. Synthesis Conditions and Mechanical Properties of CNT Fibers Manufactured Using CNT Aerogels Reported in the Literature

− − − 49−77 −

− 357 5−30

−

50−100 GPa/SG

∼60 N/Tex − −

−

CNT yarn stiffiness (GPa)

20 7.5−9 3 − 2−20

− −

20−30

5−25 20 20

7−9

Winding rate (m/min)

139 140 132 42 34

120 6 138

137

136

125 135 128

122

ref

Industrial & Engineering Chemistry Research Review

12414


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Figure 9. Overview schematic of the CVD process for the direct spinning of CNT fibers and detailed schematic of the decomposition of the precursors leading to the formation of an elastic CNT aerogel (inset shows the image of “sock” formed in the reactor). (Figure reproduced with permission from ref124. Copyright 2014, Royal Society of Chemistry.)

process at a winding rate of 20 m/min, fibers mainly consisting of DWCNTs and the strength of fiber reached around 10 GPa.6 Testing of fiber involving small gauge lengths (∼1 mm) may result in misleading values of specific strength. This is because the yarn failure mechanism would be effectively based on breaking of individual CNTs. The specific strength of CNT fiber reported by Koziol et al.6 was obtained by testing a fiber at a very small gauge length of 1 mm, which was found to be in the range of 8.8− 10 GPa. At gauge lengths to 20 mm the specific strength drops down to much lower value, i.e., 0.86 GPa. In order to have a proper comparison between the strength of CNT fibers synthesized by various groups, standards provided by ASTM must be followed. 4.2.2.1. Role of Sulfur. Sulfur plays a crucial role in the synthesis of spinnable carbon nanotubes. The role of sulfur in the growth of vapor grown carbon fibers obtained by catalytic decomposition of hydrocarbons is known. Considerable evidence exists in the role of sulfur on the activity of the catalyst particles when used in a minute quantity. Addition of sulfur to iron particles enhances their ability to grow fibers; in addition, it also causes melting of the iron particle due to formation of Fe/S eutectic. Motta et al.123,127 showed that sulfur plays two major roles, i.e., enhancing carbon diffusion on the surface of the iron particles and aiding the assembly of the carbon atoms onto the graphene tube edge resulting in high growth rate of carbon nanotubes. Sudaram et al.128 synthesized CNT fibers composed of metallic SWCNTs using methane and CS2 as carbon source and promoter, respectively. Predominant formation of SWCNTs was attributed to the presence of sulfur, which helps in decreasing the activation energy required for Stone−Wales dislocation motion. The role of sulfur appears to be similar to that of oxygen, which if added in minute quantity enhanced the growth of CNTs by preventing blockage of catalyst, widely known as supergrowth of nanotubes. Oxygen and sulfur appears to impart similar effects for the growth of CNTs due to similar structures. Recently, Mas et al.129 exhibited spinning of CNT fibers using Se and Te as promoters, which in many ways are similar to sulfur and oxygen. Even at very low concentrations, Se or Te resulted in reduction of

hence it is a scalable and continuous process with extremely high throughput. It is known that the microstructure of fibers control their mechanical properties. In turn, the microstructure of CNT fibers is heavily dependent on the carbon source used and processing conditions such as winding rates and postprocessing treatments such as densification, twisting and so on, which is discussed in detail in later sections. Few recent reports mention usage of oxygen-containing carbon sources (i.e., diethyl ether, acetone, ethanol, etc.) for continuous production of CNT fibers; it has been suggested that the presence of oxygen in the molecular chain of the organic solvent enhances the spinnability of aerogel.123 Li et al.120 pointed out that the use of aromatic hydrocarbons results in amorphous carbon deposition making the process unviable for producing fibers. On the contrary, Gspann et al.124 found that spinnability of CNT aerogel is not dependent on whether the carbon source is aromatic or nonaromatic: they compared two hydrocarbon toluene and methane, aromatic and nonaromatic compounds, respectively, and found that availability of carbon in the case of toluene is much more than methane, especially at lower temperatures. Hence, if carbon content is maintained constant for both the hydrocarbons, stable spinning conditions are achievable in both cases. As shown in Table 6, thiophene is found to be a necessary additive in order to achieve stable spinning conditions, whereas lower concentrations of iron lead to an increase in strength of CNT fibers obtained thereof. Motta et al.125 reported that, at low concentration of iron, greater proportions of SWCNTs and DWCNTs are obtained, which is reflected in strength of CNT fibers obtained. The authors also reported that large diameter DWCNTs do not retain structural stability and subsequently collapse to form a “dog-bone” shape.125 The “dog-bone” shape helps in increasing contact area leading to an increase in friction between individual CNTs, which increases the strength of CNT fibers tremendously.126 Though higher winding rates (around 20 m/min) improve the alignment of CNTs and fiber density, which in turn improves the fiber stiffness and strength. The strongest ever neat CNT fiber reported was indeed produced by direct spinning 12415


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methods used for CNT fiber production with respect to their potential for large scale production is shown in Table 7.

surface tension by strong adsorption of the catalytic surface. In addition, enhanced stabilization of the edge of the nascent CNTs is observed that promoted the growth of graphitic tubular layers. 4.2.2.2. Role of Hydrogen. It can be seen in Table 6 almost all the groups have reported usage of hydrogen as carrier gas. Highly flammable gases such as hydrogen pose a great amount of risk, specifically when high temperature reactions are undertaken. The process of CNT growth by CVD is closely related to the amount of H2 present in the system. The process can be divided into three regimes: pyrolysis, growth, and inactiveness.130 In the pyrolysis regime, the decomposition of hydrocarbon is vigorous due to the limited inhibition by H2. Low CNT yield is observed in the pyrolysis regime; in addition, catalysts are poisoned due to the amorphous carbon formed during pyrolysis. In the growth regime, the concentration of hydrogen present in the system retains a perfect balance between the hydrocarbon decomposition and formation of amorphous carbon resulting in catalytic growth of CNT. In the inactive regime, the concentration of H2 reduced the decomposition of hydrocarbon that results in insufficient carbon feedstock to the catalyst. Presence of hydrogen reduces the rate of carbon production by dehydrogenation so that the more ordered and thermodynamically stable CNTs are formed. In addition it helps in keeping the catalyst active for longer time.131 It would be better if hydrogen is replaced with other inert gases such as N2 or Ar which are much safer to handle at higher temperatures provided the reaction chemistry inside the reactor remain unaffected. Reactor arrangement also plays an important role in continuous spinning of CNT fibers. Zhong et al.132 have recommended use of a horizontal reactor because it was found to give more stable gas flow. However, a detailed study is still required before final recommendations can be made. Direct spinning of CNT fibers is easily scalable and can be made continuous. The key requirement for continuous spinning of the CNT fibers are the formation of CNT aerogel and continuous removal of the product from the reaction zone. 4.2.2.3. Purification of CNT Fibers. CNT aerogel spinning into fibers is the most attractive method due to its scale-up potential. In order to produce CNT fibers of high quality and exceptional tensile strength, impurities present in the macroscopic structure must be minimized. Non-CNT impurities such as amorphous carbon, catalyst particles, etc. are formed inherently during synthesis. Presence of such impurities deteriorates the fiber quality and limits the intertube stress transfer, which results in lower strength. Few researchers have reported posttreatments such as acid treatment, sonication in the presence of acetone, etc., for as-synthesized CNT fibers in order to remove the impurities. Tran et al.133 reported usage of 65% HNO3 at room temperature and subsequently washed with deionized water for purification. They found that the mass density of CNT fibers measured before (1.8 g/cm3) and after (1.53 g/cm3) purification differed by ∼15% depicting oxidation and subsequent removal of impurities. Acid concentration and time for treatment must be optimized in order to avoid destruction of CNT structures. Recently, Sundaram et al.134 reported a novel technique for purification of CNT fibers using postproduction sonication in the presence of acetone. They found that impurity level decreased by ∼42% in 1 min.

Table 7. Comparison of CNT Fiber Production Methods Wet spinning Process/ property Process nature Per unit cost Reactor design

Spinning using superacids

Spinning using surfactants

Dry spinning Forest spinning

Aerogel spinning

Batch

Batch

Batch

Continuous

High Difficult

High Difficult

Moderate Difficult

Low Easy and can be designed for large scale process

The FC-CVD process for the spinning of CNT fibers combines the nucleation, growth, and aggregation of CNTs in the form of an aerogel with fiber spinning. The optimization of process requires understanding of the impact of parameters such as the choice and flow rates of carbon and catalyst precursors, the nature and ratio of sulfur containing compounds to other reactants, the reaction temperature as well as hydrogen flow rate. In addition, the reactor geometry and reactor tube type have to be carefully selected in order to create the required conditions for spinning. For CNT morphology, the sensitivity analysis of the effects of carbon source, sulfur source, and bulk flow rates have been well documented. The reports indicate that interactions of these variables with the catalyst nanoparticles is a key to the product purity and the formation of spinnable CNT sock. Reguero et al.122 recently reported that, for some CVD systems, CNT growth results in the presence of low concentration (120 GPa) as compared with those known for the neat CNT fiber as well as for CNT/PVA composite fiber. Jung et al.161 explored the effects of polymer infiltration into CNT fibers, specifically the interactions between the CNTs and the polymer. Polystyrene (PS), polyacrylonitrile (PAN), and poly(vinyl alcohol) (PVA) were studied possessing different Hansen affinity parameters toward CNTs were impregnated into the CNT yarn spun from CNT aerogel using methane, ferrocene, and hydrogen as carbon source, catalyst, and carrier gas respectively. Figure. 27 illustrates the experimental setup of this process. CNT fibers were continuously spun from aerogel and subsequently passed through water bath and further passed through second bath containing polymer (PS, PAN, PVA) dissolved in DMSO. The Hansen affinity parameter (MPa1/2) with CNTs and polymers PS, PAN, and PVA are 2.5, 9.5, 18.1, respectively. The low affinity parameter of PS indicated that the PS and CNTs had similar characteristics and hence, PS infiltrated more into CNT yarns in comparison to PAN and PVA at same concentration. However, CNT/PS yarn exhibited lowest specific strength and stiffness and CNT/PVA showed better specific strengths regardless of the solution concentration and the amount of infiltrated polymer into CNT yarns. Detailed characterization of internal structures of polymers-infiltrated CNT yarns revealed that CNT/PS composite yarn had the smallest bundle size, whereas CNT/PVA composite yarn possessed the largest bundle size. Further, the intrabundle shear strength increases with an increase in the bundle size; this was attributed to an increase in the contact area between the individual CNTs in the bundles. Hence, the key factor for enhancing the mechanical properties of

7.1.4. Effects of Surfactants. Carbon nanotube diameter is directly affected by the size of the catalyst particles, which in turn is affected by the feed composition and process conditions. Sulfur sources such as thiophene and carbon disulfide are widely used in order to reduce agglomeration of iron particles formed from ferrocene. Recently, few reports on usage of surfactants have been published.140,182 A surfactant is used to reduce the agglomeration of iron catalysts, which helps in enhancing the spinnability of the CNT fibers. Song et al.182 reported usage of nonionic surfactant polysorbate-20, which is selected on the basis of its stability with the carbon source. Acetone as carbon source, ferrocene as catalyst, and thiophene as promoter were used. Hydrogen gas was used as carrier gas and a reaction temperature of 1200 °C was maintained. Surfactant concentration was varied from 0 to 3 wt %, where 1 wt % was found to be most favorable concentration in order to achieve higher spinnability and strength. At concentrations higher than 1%, the amount of amorphous carbon formed increases degrading, hence the CNT fiber formed decreases. Enhancement in spinnability was observed in the presence of surfactant. Winding rate was observed to be 7.5 m/min in the absence of any surfactant, whereas in the presence of polysorbate-20 spinnability increases up to 9 m/min. Higher winding speed is desirable in order to have higher cohesive forces among the CNTs due to alignment of the molecular chains along the fiber axis at the higher level of orientation. Higher spinnability in the presence of surfactant is mainly due to increased crystalline perfection achieved by reduction in agglomeration of iron catalysts during the CNT synthesis.140 As shown in Figure 26, the addition of surfactant affects the tensile strength of CNT fibers. The CNT fibers synthesized at the same winding speed depicts approximately 160% increase in specific strength when surfactant was present in the synthesis solution, compared to the case when no surfactant was present in the synthesis solution. At higher winding rate, the specific strength of CNT fibers in the presence of surfactant increased by 114%, which is likely caused by decrease in linear density of CNT fibers and the increased cohesive force between CNT due to high winding speed.140 7.1.5. Effect of Polymer Impregnation. In a neat CNT fiber, the interactions between the CNTs are via weak van der Waals forces, resulting in inefficient load transfer among neighboring 12428


Review


Figure 28. Compressive stress−strain curves of HMCF and CNT fibers. (Figure reproduced with permission from ref 191. Copyright 2010, John Wiley and Sons.)

because of compression which helps CNT fibers in retaining flexibility of individual CNT. Hence, CNT fibers can deform at large strain without fracture or permanent deformation. On the other hand, in the case of HMCF, due to the anisotropic nature and presence of defects it results in failure.192 7.3. Modeling and Simulation. In order to get significant insights regarding the effects of various structural parameters on the performance of CNT fibers, modeling and simulation studies are a must. Depending upon the range of length and time scales, mechanical properties of CNT fibers have been investigated. A variety of approaches such as micromechanical modeling, Monte Carlo simulations, finite element method, etc. have been employed. Beyerlein et al.193 analyzed the effects of fiber diameter and gauge length on the statistical strength of CNT fibers. A micromechanical model for an CNT fiber consisting of n-twisted CNTs has been developed as shown in Figure 30. The modeling analysis was performed in 3 steps: (1) Weibull distrubution is assumed in order to quantify CNT strength, and in order to understand the failure process in a fiber, a 3D statistical Monte Carlo model of Porwal et al.194 has been used in order to simulate the failure process in a fiber of a characteristic size along with frictional slip between CNTs. (2) The statistical strength of the longer fibers was determined based on the principle of weakest link. (3) The calculated CNT fibers distribution are incorporated into another Monte Carlo simulation model in order to estimate their effects on the statistical strength of a composite structure reinforced with unidirectionally aligned CNT fibers. Results obtained by simulations depicts decrease in strength (both statistical and mean) as the number of CNTs in cross section increases. Similar results were observed in case of surface twist angle and gauge length of the fiber. Zhang and Li126 focused on enhancement of friction between carbon nanotubes as an strategy in order to improve the mechanical properties of CNT fibers. Molecular dynamic simulations were used in order to study the effect of pressure on the friction within the carbon nanotube bundles. They found that the intertube frictional force could be enhanced by a factor of 1.5−4, depending on the tube chirality and radius, when all tubes collapse above a critical pressure. The collapse of CNTs help in decreasing the cross sectional area of the CNT bundle thus improving the packing density and in turn enhancing the strength of the tube bundle. In order to calculate the axial strength of CNT fiber Vilatela et al.195 have developed a analytical model. The model comprises parallel, rigid rods as shown in Figure 31. The model assumes that there is no correlation between the position of the ends of

Figure 27. (a) Schematic diagram showing the CNT yarn spinning process using the aerogel method. (b) TEM image of the synthesized CNTs that composed the CNT yarn. (c) SEM image of the CNT assemblies prior to passage through the water bath. (Figure reproduced with permission from ref 161. Copyright 2015, Elsevier.)

composite CNT yarns is the type of polymer rather than amount of polymer. 7.2. Compressive Properties of Carbon Nanotube Fibers. To date, several methods exist to carry out compressive behavior of fibers in micrometer scale such as tensile-recoil,54 bending-beam,187 elastic-loop,188 and single-fiber-composite.189 Wu et al.190 employed a fiber-recoil method in order to estimate the compressive strength of CNT fibers. Aerogel-spun fibers exhibit kinks at multiple regions along the length when subjected to quasi-static tensile loading. Occurrence of kinking is mainly due to stress wave propagation along the fiber length. Aerogelspun CNT fibers exhibited compressive strength in the range of 172−177 MPa, which was further confirmed by elastic-loop method. Gao et al.191 studied axial compression of hierarchically structured CNT fiber embedded in an epoxy matrix along with high modulus carbon fibers (HMCF) for comparison using a single-fiber composite method. The single-fiber composite method is best suited for intrinsic brittle material with opaque characteristics. Because carbon fibers are brittle in nature as compared to CNT fibers, they exhibited higher flexibility and resilience even under large compressive strength. The compression stress is applied to the fiber by thermal shrinkage, which occurred due to mismatch of coefficient of thermal expansion between the fiber and the polymer matrix. The relationship between strain and compressive stress is shown in Figure 28. In situ Raman spectroscopy analysis provided the compressive stresses, whereas the compressive strains are obtained via coefficients of thermal expansions of the polymer matrix and fibers. Although the original Young’s modulus of CNT fiber is much lower than that of HMCF, the apparent compression modulus of the CNT fibers embedded in polymer is around 350 GPa, whereas that of HMCF is 223 GPa. Figure 29 depicts deformation mechanism of the CNT bundles within CNT fibers along with microbuckling and kinking process of crystals in HMCF when the fibers are compressed in an epoxy matrix. CNTs exhibit extraordinary flexibility hindering its damage 12429


Review


Figure 29. Schematic diagram depicting the deformation process of the CNT bundles within CNT fiber (a) and the microbuckling and kinking process of crystals in high modulus carbon fiber (HMCF) when the fibers are compressed in an epoxy matrix. (Figure reproduced with permission from ref 191. Copyright 2010, John Wiley and Sons.)

σ=

1 Ω1Ω 2τFl 6

(14)

where σ is the specific area in N/tex; Ω1 is the fraction of the total number of graphene layer(s) on the outside of the fibrous element; Ω2 is the fraction of the surface of the outer graphene wall(s) of the element in contact with neighboring elements; τF is the interfacial shear strength; l is the mean length of the fibrous elements, and the prefactor 1/6 has the units of m2/kg. In the case where basic fibrous elements are CNTs, the estimated bundle strength was 3.5 N/tex, which is comparable to a CNT bundle intrinsic strength of 5 N/tex. The model provided a rationale for improvements in yarn properties by increasing the degree of contact using larger diameter nanotubes with fewer walls that possibly collapse under lateral pressure as determined by transmission electron microscopy images, continuum elasticity theory, and atomistic simulations. Gspann et al.196 reported use of finite element analysis(FEM) in order to simulate the stress distribution in CNT fibers. The model of the fiber was derived by dividing the fiber into small elements each forming a CNT bindles as shown in Figure 32. Results demonstrate that

Figure 30. Model for designing structural composites from CNTs: (a) micrometer-scale CNT fiber consisting of n-twisted CNTs and (b) unidirectional composite reinforced with CNT fibers. (Figure reproduced with permission from ref 193. Copyright 2009, IOP Publishing Ltd.)

Figure 32. (A) Model of hexagonal elements, each depicting bundles of CNTs with perfect internal contact. (B) Model of concentric cylinders, under the assumption that the stress within each cylindrical shell of bundles is constant. (C) Structure is further reduced to one of planar layers here; shear force transmitted from layer to layer is proportional to the circumference. (Figure reproduced with permission from ref 196. Copyright 2015, Elsevier.)

Figure 31. (a) Simple model of the fiber as a collection of fibrous elements, (b) tensile fracture of fiber involving failure in shear between the fibrous elements, and (c) SEM micrograph of actual CNT fiber having undergone failure in tensile test (scale bar is 10 μm). (Figure reproduced with permission from ref 195. Copyright 2011, American Chemical Society.)

length-diameter ratios of the order 103 are required before the stress becomes uniform across the fibers. The results obtained are in accordance with the St Venant’s principle, which states that the difference between the effects of two different but statically equivalent loads become very small at sufficiently large distances from the load. The presence of particulates such as carbonaceous particles, catalyst particles, etc. might affect the degree of alignment, but it helps in improving stress transfer by acting as a inter

the fibrous elements and they are perfectly aligned(axially) and all of the same length and diameter. It has been found that strength of CNT fiber depends upon CNT length, intertube contact area, and the shear strength and intertube contact area between CNTs and the neighboring, which is given by the following equation 12430


Review


Figure 33. Scanning electron micrographs of a CNT-fiber (a) and the fiber after integration into the epoxy matrix (b) pristine fiber (c) stress−strain curves of pure epoxy (black) and a specimen with 14% volume fraction of CNT-fiber (red) in compression. (Figure reproduced with permission from ref 14. Copyright 2009, Elsevier.)

bundle adhesive reducing the detrimental effect of the longitudinal crack.

8. POTENTIAL APPLICATIONS OF CARBON NANOTUBE FIBERS IN COMPOSITES Carbon nanotube fibers appear to be a ready component for composite material. They provide strengthening, stiffening and flexibility that can meet demands for advanced composites. CNT fibers appear to be a key material in the aviation, automobiles, sports, energy, and defense industries in the near future. Very few reports are available on CNT fiber reinforced composites. Mora et al.14 fabricated high volume fraction (>25%) CNT fiber/ polymer composites, with extraordinary mechanical performance. The epoxy matrix was reinforced with CNT fibers because fibers are yarn-like: the matrix infiltrates in the fiber itself, ensuring an excellent fiber/matrix bond. Tensile and compression tests of the composites exhibit increment in strength and stiffness as compared with pure epoxy. A fracture surface from a tensile sample is depicted in Figure 33. The crack propagated through the epoxy matrix leaving a flat surface, indicating increase in toughness. Recently, Mikhalchan et al.148 reported 35% CNT volume fraction for the epoxy composites filled with CNT fibers used without any structural modifications. The tensile strength and stiffness of the epoxy composite filled with CNT fibers reached 445 MPa and 26.5 GPa, opening a new field for further enhancement of properties of such composites. Vilatela et al.197 reported synthesis of composite CNT fibers made by direct polymer infiltration of an array of aligned fibers. Figure 34a shows a schematic of a CNT fiber that appears to be like the twisted structure of the composite yarn. T300 carbon fiber(CF)/epoxy composites were used as comparator while fiber mass fractions varied from 25 to 33% as confirmed by thermogravimetric analysis (TGA). Figure 34b,c shows SEM micrographs of fractured surfaces of CNT fiber/epoxy and CF/ epoxy composites. Authors reported enhanced adhesion of the thermoset polymer to the fiber with absence of any visible gaps in between on the other hand, CF/epoxy composite exhibit poor

Figure 34. (a) Schematic showing the hierarchical structure of CNT fiber composites due to the yarn-like structure of the fibers. SEM micrographs of fracture surfaces (b) of CNT fiber/epoxy composite (c). Inset shows the fiber subunits, the nanotube bundles. (Figure reproduced with permission from ref 197. Copyright 2012, Elsevier.)

penetration of matrix in the fiber. Discrepancy in adhesion for CNT fibers and CF epoxy composites is mainly due to limited accessibility of polymer molecules to the internal surface of fiber. BET measurements of CNT fibers (200 m2/g) and CF(0.2 m2/g) supports the results obtained. Recently, usage of composite CNT fibers has been reported for composite overwrapped pressure vessels (COPVs).198 COPVs are used for tension dominated applications where thin walled liners are wrapped with composites and commonly used for gas and propellent storage in spacecraft and launch vehicles. Improvement of strength-to-weight ratio of composites used in COPVs 12431


Review


respectively.200 Figure 36 shows the values of U for armor made from various high performance fibers. It can be clearly observed

could yield tremendous savings in weight, which in turn help in minimizing the mass of launch vehicles and spacecraft. Kim et al.198 reported overwrapping of aluminum rings with thermoset CNT yarns as shown in Figure 35a. Mechanical properties of

Figure 35. Photographs of (a) Epon828/CNT yarn composite overwrapped pressure vessel. (Figure reproduced with permission from ref 198. Copyright 2016, Elsevier.) (b) Quadcopter frame printed using CNT yarn/Ultem filaments. (Figure reproduced with permission from ref 199. Copyright 2016, Elsevier.)

Figure 36. Values of ballistic figure of merit for ballistic protection of armor made of different high performance fibers.

the CNT composite overwrapped aluminum rings (CCOARs) were measured under static and cyclic loads at room, elevated, and cryogenic temperatures. Wrapping of about 10 wt % of CNT/Epon 828 over Al rings resulted in a ∼200% increase in hoop tensile properties compared to that of the bare Al ring in addition the composite overwrap sustained load during stress− rupture cycle without mechanical degradation. CNT yarns are also been used as feedstock in latest technologies such as 3D printing, which are widely used for fabrication of geometrically complex parts. Gardner et al.199 reported usage of CNT yarns as multifunctional feedstock for additive manufacturing. As shown in Figure 35b, a quadcopter frame is manufactured using continuous CNT yarn reinforced polyetherimide (Ultem) by additive manufacturing. The development of printable CNT yarn filament with high performance polymer matrix depicts potential to fabricate tailored components taking advantages of CNT yarn properties. The ever increasing threats such as terrorism, counterinsurgency, etc. have created an urgent need for the reliable armor protection to presonnel from armed forces deployed for national security. Role of armor is to protect person, structure, or device by absorbing kinetic energy of the projectile. Kinetic energy primarily gets absorbed by plastic deformation and may result in fracture. Thus, an armor plate has to fulfill two roles: (1) protective and (2) structural. Both the roles can be fulfilled if armor possesses sufficient strength at high strain rate and appropriate thickness. High performance fibers such as aramids (Kevlar 29, Twaron, etc), S-glass, Dyneema (UHMWPE), etc. are being increasingly exploited as soft, flexible materials for the manufacturing of armor. Researchers from Military University of Technology, Poland have reported numerical modeling for determination of ballistic performance of CNT fiber reinforced 7017 aluminum alloy. Their numerical model was proposed for the impact of sharp nosed projectile on the metal matrix composite plate using finite element method (FEM). They concluded that CNT fibers reinforcement played a major role in the overall ballistic resistance of the composite plate. The ballistic figure of merit U (m/s) for ballistic applications is defined as 0.5 σf εf ⎛ E ⎞ U= ⎜ ⎟ 2ρ ⎝ ρ ⎠

that CNT fibers outperforms all the high performance fibers currently used for ballistic applications.129,201 In spite of the significant improvements in the recent decade toward the development of high performance CNT fibers, there still exists some major challenges. The mechanical and physical performance of CNT fibers are at a level far below those of individual CNTs. In addition, the scale-up of current processes to produce continuous fibers is still imperative.

9. CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK (a) This paper makes an attempt to analyze the published literature on CNT fibers. The analysis has been presented in a manner for the aspects such as laboratory and large scale synthesis and characterization. The review is expected to be useful to researchers as well as practitioners. The development on CNT fibers is currently at a rapid pace, with abundant research achievements emerging every day. This is evident from Figure 37, which shows steady increase of research publications and patents on CNT fibers (scopus). (b) Recommendations have been made on how the synthesis can be achieved on the large scale in processes akin to the continuous production of kilometer long CNT fibers.

(15)

where σf, εf, E, and ρ is fracture stress, fracture strain, elastic stiffness, and density of fiber used for manufacturing of armor,

Figure 37. Frequency of papers and patents on CNT fibers for the period of 1993−2016. 12432


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(c) There has been paucity of fundamental data for the operating conditions such as temperature and flow rates of inert and carbon source. Therefore, in the present paper, a systematic procedure has been given for understanding the rate controlling step(s). A procedure has also been given for determining the intrinsic kinetics of the reaction. The knowledge of rate controlling step(s) and the kinetics is useful for the estimation of overall rate of reaction at a given set of carbon source partial pressure, catalyst concentration and activity, temperature, etc. A procedure for design and scale-up has been described in the present paper using computational fluid dynamics.

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AUTHOR INFORMATION

Corresponding Author

*J. B. Joshi. Tel.: +91 2225597625; fax: +91 2233611020; email: [email protected]. ORCID

Jyeshtharaj B. Joshi: 0000-0002-4945-0788 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the J. C. Bose Fellowship-Department of Science and Technology (DST), to carry out this work.

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REFERENCES

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