aerospace applications

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Multifunctional hierarchical nanocomposite laminates for automotive/aerospace applications

15

Mehrdad N. Ghasemi Nejhad Department of Mechanical Engineering, University of Hawaii at Manoa, Honolulu, Hawaii, USA

15.1  INTRODUCTION The automotive industry faces many challenges, including increased global competition, the need for higher performance vehicles, a reduction in weight and costs, and tighter environmental and safety requirements. Ultimately lighter and stronger/ stiffer materials mean lighter vehicles and hence lower fuel consumption and lower emissions. Composites are being used increasingly in the automotive industry due to their excellent quality, specific strength (i.e., strength divided by density), and specific stiffness (i.e., stiffness divided by density) [1]. A comprehensive explanation of how advanced composite materials, including fiber-reinforced polymers (FRPs), reinforced thermoplastics, carbon-based composites, and many others, are designed, processed, and utilized in vehicles is given by Elmarakbi [1], which includes technical explanations of composite materials in vehicle design and analysis and covers all phases of composite design, modeling, testing, and failure analysis. It also sheds light on the performance of existing materials including carbon composites and future developments in automotive material technology which work toward reducing the weight of the vehicle structure. In addition, it presents the state of the art in composite materials and their application in the automotive industry and considers energy efficiency and environmental implications, and hence presents a useful source of information for those considering using composites in automotive applications in the future. Additional automotive applications of composite materials such as body panels, bumpers, driveshafts, engine exhaust manifold, and other components can be found in Refs. [1–4]. Composite materials are becoming more important in the construction of aerospace structures as well. Aircraft parts made from composite materials, such as fairings, spoilers, and flight controls, were developed during the 1960s for their weight savings over aluminum parts. New generation large aircraft are designed with all Multifunctionality of Polymer Composites. ISBN: 978-0-323-26434-1 DOI: http://dx.doi.org/10.1016/B978-0-323-26434-1.00015-5 © 2015 2014 Elsevier Inc. All rights reserved.

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composite fuselage and wing structures, and the repair of these advanced composite materials requires an in-depth knowledge of composite structures, materials, and tooling. The primary advantages of composite materials are their high strength, relatively low weight, and corrosion resistance. Comprehensive explanations of how advanced composite materials are designed, processed, and utilized in aerospace are given in Refs. [5–8]. The aerospace industry, like the automotive industry, also faces many challenges, including increased global competition, the need for higher performance airplanes, a reduction in weight and costs, and tighter environmental and safety requirements. Ultimately lighter and stronger/stiffer materials mean lighter airplanes and hence lower fuel consumption and lower emissions. [9,10]. However, conventional composite materials in the past have traditionally suffered from two main issues: first, brittleness of the matrix, and second susceptibility to delaminations. If the matrices in composites can be toughened, the fracture toughness and damage tolerance of composites will increase, resolving the brittleness issue, and hence cracks initiations and growths in the matrices can be prevented or delayed leading to higher performance composites. This technology is called “nanoresin” and is explained in detail in this chapter. In addition, in a conventional composite material, in-between the layers are filled by the matrix and hence is the weakest link in composites and susceptible to delaminations. If a “reinforcement” can be placed in-between the composite layers, then the weakest link will also be reinforced and hence the crack initiation and growth in-between the layers will be eliminated or delayed to eliminate or improve the delamination susceptibility of the composites. This technology is called “nanoforest” and is explained in detail in this chapter. Therefore, this chapter explains how the two main issues in composites, i.e., the rather brittleness of the matrices/resins and the composites laminates susceptibility to delaminations can be resolved by two technologies explained here. The resin brittleness can be resolved by proper use, integration, and processing of nanomaterials within a resin system to produce high-performance nanoresin. The delamination issues of composite materials can be resolved by proper use, integration, and processing of carbon nanotubes (CNTs) onto the fibers (or in-between the fiber layers) to produce nanoforest fibers, and then the combination of these nanoforest fibers with nanoresins matrices can produce super-performing MHNs that could potentially solve the brittleness and delamination issues in composites. Of course, the large-scale industrial productions of such super-performing nanocomposites are still under research and development; however, it is expected that the development of MHNs with their advantages in resolving the brittleness and delamination issues of conventional composites to a large extent as well as their multifunctionality in improving mechanical, thermal, electrical, chemical, and other performances of the resulting nanocomposites will expand the applications of such nanocomposites in automotive and aerospace industries. Nanotechnology can be broadly defined as “the creation, processing, characterization, and utilization of materials, devices, and systems with dimensions of the order of 0.1–100 nm, exhibiting novel and significantly enhanced physical, chemical, and biological properties, functions, phenomena, and processes due to their nanoscale size” [11]. Nanocomposites are of significant importance in the rapidly developing

15.1  Introduction

field of nanotechnology. Recently, researchers have investigated nanocomposites containing polymer nanomaterials to improve their physical, mechanical, and chemical properties. Nanomaterials embedded in polymer matrix have attracted increasing interest because of the unique properties displayed by nanomaterials and their inclusion in polymers. Due to the nanometer size of these particles, their physicochemical characteristics differ significantly from those of micron size and bulk materials. When two or more phases are mixed together to make a composite, one can often obtain a combination of properties, in the resulting composite, that are not available in either one constituent. The field of nanocomposites involves the study of multiphase materials where at least one of the constituent phases has one dimension less than 100 nm. This is the range where the phenomena associated with the atomic and molecular interaction strongly influence the macroscopic properties of materials. Since the building blocks of nanocomposites are at nanoscale, they have an enormous surface area, and there are numerous interfaces between the two intermixed phases. The special properties of the nanocomposite arise from the interaction of its phases at the interface and/or interphase regions. By contrast, in a conventional composite based on micrometersized filler such as carbon fibers, the interfaces between the filler and matrix constitutes have a much smaller surface-to-volume fraction of the bulk materials and hence influence the properties of the host structure to a much smaller extent. The optimum amount of nanomaterials in the nanocomposites depends on the nanomaterials’ size, shape, homogeneity, distribution, and the interfacial bonding properties between the fillers and matrix. The promise of nanocomposites lies in their multifunctionality, i.e., the possibility of realizing unique combination of properties unachievable with traditional materials. The challenges in reaching this promise are tremendous. They include control over the distribution in size and dispersion of the nanosize constituents, and tailoring and understanding the role of interfaces between structurally or chemically dissimilar phases on bulk properties. Motivated by the recent enthusiasm in nanotechnology, development of nanocomposites is one of the rapidly evolving areas of composites research and development. Scientists and engineers working with fiber-reinforced composites have practiced this “bottom-up” approach in processing and manufacturing, at micron level, for decades. When designing a composite, the material properties are tailored for the desired performance across various length scales. From the selection and processing of matrix and fiber materials and architecture, to the layup of laminae in laminated composites, and finally to the net-shape forming of the macroscopic composite parts, the integrated approach used in composites processing is a remarkable example in the successful use of the “bottom-up” approach (even prior to the development of nanocomposites—albeit at the micron level). In this chapter, the current state of knowledge in processing, performance, and characterization of nanocomposites and hierarchical (bottom-up) multifunctional nanocomposites for polymer matrix composites (PMCs) and ceramic matrix composites (CMCs) are addressed. Dispersion, functionalization, exfoliation, and optimization are of utmost importance in processing of nanocomposites using the

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bottom-up approach. Techniques used to develop nanocomposites and hierarchical nanocomposites will be discussed with major emphasis on the multifunctionality of such nanocomposites. Figure 15.1 shows the flowchart for the development of MHNs. The conventional composites are produced when resins are combined with fibers. The integration/combination of various nanomaterials with various resins gives nanoresin nanocomposites with multifunctionality. When these nanoresin nanocomposites are combined with fibers hierarchical nanocomposites are produced with multifunctionality. On the other hand, when CNTs are grown on the surface of fibers, nanoforest fibers are produced. When resins are combined with nanoforest fibers MHNs are produced. When nanoresin nanocomposites are combined with nanoforest fibers, multiscale MHNs are produced. In Figure 15.1, nanomaterials are placed inside round-angle square and rectangle to separate them from other materials, i.e., resins, fibers, composites, and nanocomposites, which are placed inside right-angle rectangles. In addition, in Figure 15.1, as explained in the legend, a “thin-line solid arrow” means “integrate” one or more materials from the box at the beginning of the arrow into Legends: : Integrate : Combine : Results

Nanomaterials: • Nanoparticles • Nanoclays • Nanofibers • CNTs • GNSs

Resins

CNTs

Fibers/

Composites

Fabrics

Hierarchical nanocomposites

Multifunctional hierarchical nanocomposites

Nanoresin nanocomposites

FIGURE 15.1 Development flowchart for MHNs.

CNT-based Nanoforest Fibers/ Fabrics

Multiscale multifunctional hierarchical nanocomposites

15.2  Nanoresin Nanocomposites

the material system of the box at the end of the arrow. “Thin-line dotted arrows” are inside the boxes and mean “combine/manufacture” the materials taken in the box from the beginning of the “thin-line solid arrow” with the material system inside the box the thin-line dotted arrow is. A “thick/bold-line solid arrow” means that the integration and combination of the materials coming from the beginning of this arrow “results” in the material system given in the boxes located at the end of this arrow. The remainder of this chapter explains the mechanisms as shown in Figure 15.1 and explains the multifunctional properties that can be produced for various nanocomposites as shown in Figure 15.1.

15.2  NANORESIN NANOCOMPOSITES The composites and nanocomposites of the future will offer many advances over the composites of today. Recent developments in production and characterization of various nanoparticles have created numerous new opportunities to develop nanocomposites for different applications. The progress made in the production and purification of nanotubes in the last few years has made it practical to use nanotubes as reinforcements in polymer composites [12–14]. The potential to develop CNTreinforced nanocomposites looks promising for a wide range of applications including high mechanical damping, strength, fracture toughness, and electrically and thermally conductive polymer nanocomposites [15–19]. However, the CNT applications as structural reinforcements depend on their ability to transfer load from the matrix to the nanotubes [15].

15.2.1  NANORESIN NANOCOMPOSITES’ CHALLENGES Nanocomposite materials hold the potential to redefine the field of traditional composite materials both in terms of performance and potential applications. There is little doubt that polymer nanocomposites have tremendous market potential both as replacements for current composites and in the creation of new markets through their outstanding properties. But developing the processing–manufacturing technologies in terms of scale-up and value for commercialization will be one of the biggest challenges. For example, dispersion of nanomaterials within the resin systems is the important issue. A homogeneous dispersion of nanomaterials in a polymer by using existing/traditional compounding techniques is very difficult due to the strong tendency of nanomaterials to agglomerate [20,21]. Degassing is another critical problem while processing parts with nanocomposites. The air trapped while pouring the highly viscous material in the mold (e.g., for 5–10 wt% of nanoclay or other nanomaterials in the matrix) initiates crack, and hence the failure of specimen can take place under low strains [21]. Therefore, a low concentration of nanomaterials to avoid agglomeration and prevent an increase in viscosity is more desirable. As mentioned earlier, to improve dispersion and compatibility in polymer matrices, nanomaterials are being functionalized. In addition, it is desirable that while a targeted property of

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composites is improved, the nanocomposite does not face degradation in other properties, as compared with the pristine resin.

15.2.2  NANOMATERIALS FOR NANORESIN NANOCOMPOSITES Common nanomaterials used in nanocomposites are nanoparticles, nanoclays, nanofibers, nanotubes, and nanosheets. While nanoparticles, nanoclays, and nanofibers can improve the properties of nanocomposites to some extent, CNTs and graphene nanosheets (GNSs) can improve those properties to a larger extent. CNTs and GNSs have tensile modulus and strength values ranging from 270 GPa to 1 TPa and 11 to 200 GPa, respectively [22]. Also, there is a general concern that nanomaterials may have negative health and environmental impacts [23,24]. Few studies have been published on the effects of inhaling free manufactured nanomaterials to date. It has been found that toxicity is related to inhalation of nanomaterials, which can penetrate into deep lung tissue. Of these nanomaterials, especially, single-walled carbon nanotubes (SWCNTs) [25,26] and carbon nanofibers (CNFs) due to their very high aspect ratios (>1000), resembling asbestos, have been very alarming. The original graphite flakes with a thickness of 0.4–60 mm may expand up to 2–20,000 mm in length [27]. These sheets/layers can be separated down to 1 nm thickness, forming a high aspect ratio (200–1500) and high modulus (~1 TPa) graphite nanosheets (i.e., graphenes). Furthermore, when dispersed in the matrix, the nanosheet exposes an enormous interface surface area (2630 m2/g) and plays a key role in the improvement of both physical and mechanical properties of the resultant nanocomposite [28].

15.2.2.1  Nanoparticle–nanoresin nanocomposites Nanoparticle nanocomposites are being used increasingly for enhancement of structural, thermal, electrical, and optical properties. Researchers [29–32] developed polypropylene (PP)-based nanocomposites by the incorporation of a novel nanosized precipitated calcium carbonate (NPCC) filler and polyethylene–octane elastomer (POE). Talc and calcium carbonate (CaCO3) are the most common mineral fillers used for PP due to their availability in readily useable form and low cost. Adding 60 wt% of stearic acid treated CaCO3 particles of 0.7 μm diameter into a PP matrix increased the impact strength and the modulus significantly [33]. Also considerable improvement in impact performance was obtained using 0.7 μm diameter stearic acid-coated CaCO3 particles at 30 vol% loading with PP matrix [34]. In addition, Chan et al. [35] successfully prepared PP/CaCO3 nanocomposites and reported an impact enhancement of about twice that of the neat PP matrix. Zhang et al. [36] carried out further studies to improve the dispersion of CaCO3 nanoparticles in the PP matrix by adding a nonionic modifier. Studies on phase structure of ternary PP/elastomer/CaCO3 composites were carried out by Premphet and Horanont [37]. The NPCC used in this study had a measured average primary particle size of 50 nm. CaCO3 filler is known to have a spherical shape with an aspect ratio of close to one. Without any POE, the inclusion of up to 15 wt% of NPCC improved

15.2  Nanoresin Nanocomposites

the flexural modulus of PP by about 30% and increased the impact strength by 1.5 times. At both 10 and 15 wt% NPCC levels studied, the impact strength of the neat PP increased substantially with increasing POE content. However, when comparing the compositions with the same POE content, the nanocomposite with lower NPCC loading of 10 wt% showed better impact enhancement as compared to the nanocomposite loaded with higher loading of 15 wt% NPCC (indicating some agglomerations at higher loading). Thus, although the main contributor to the impact enhancement in PP appears to be from the POE phase, the addition of NPCC provided further synergistic toughening effect on the PP. Additions of NPCC and POE have resulted in improvements in mechanical properties of interest of the ternary PP/POE/CaCO3 nanocomposites. The impact strength has been greatly improved albeit at the expense of the tensile strength and flexural modulus. Although the main contributor to the impact enhancement in PP was from POE, the addition of NPCC provided further synergistic toughening effect on the PP. The observed decrease in stiffness with increasing NPCC content in the presence of POE could possibly be due to the encapsulation of NPCC particles by the POE phase. The size of the dispersed POE domains within the continuous PP matrix influenced the impact strength of the nanocomposites, and the average size of about 0.6 μm was found to be the most optimum for the impact enhancement. Ghasemi Nejhad et al. [38,39] employed SiC nanoparticles with the size range of 45–55 nm to develop nanoresin nanocomposites using a high-temperature epoxy. They obtained remarkable improvements in fracture toughness.

15.2.2.2  Nanoclay–nanoresin nanocomposites Over the last decade, the mechanical and thermal properties of nanoclay/epoxy nanocomposites have been investigated extensively. Enhanced mechanical and thermal performances of conventional polymer-based composites reinforced with a relatively small amount of layered silicates typically in the range of 3–5 wt% has been reported [40,41]. Researchers from Toyota [42–47] pioneered the utilization of nanoclay in thermoplastics. Improved properties are obtained only when nanoclay is intercalated and/or exfoliated into the polymeric matrix [41–43]. Alexandre and Dubois [48], for instance, showed that mechanical and barrier properties, transparency, and toughness are directly proportional to the degree of exfoliation. Nanoclay dispersion into epoxies also induces significant enhancements in mechanical properties. For example, Advani and Shonaike [49] observed more than 100% and 120% increase in tensile modulus and strength, respectively, after the addition of 5 wt% nanoclay into an epoxy adhesive. Furthermore, Shah et al. [50] reported a reduction in moisture absorption diffusivity after the introduction of Closite®10A nanoclay into a molded Derkane epoxy part. A nanoclay load as low as 0.5 wt% reduced moisture diffusivity by more than 50%, while a 5 wt% clay load resulted in an 86.4% reduction in moisture diffusivity. Increases in the glass transition temperature (Tg) and tensile modulus with increasing nanoclay contents were also reported [50]. Kinloch and Taylor [51] studied Tg improvements in an epoxy due to the introduction of 10 wt% of

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nanoclay. The authors reported a small improvement from 78°C to 79°C for exfoliated Nanomer®I30E and a higher Tg of 85°C for intercalated Closite®25A. Akkapeddi [52] studied both short and continuous glass-fiber (GF)-reinforced clay–polyamide 6 nanocomposites manufactured by a melt compounding technique. The author reported improved flexural modulus, strength, and heat distortion temperature under load as well as improved moisture resistance at 2 and 5 wt% nanoclay contents. Haque et al. [53] reported significant improvements in mechanical and thermal properties of S2-glass/epoxy composites with low nanoclay contents manufactured by vacuum-assisted resin infusion molding (VARIM). The authors observed that dispersing 1 wt% clay resulted in a 26°C increase in Tg as well as 44%, 24%, and 23% improvement in interlaminar shear strength (ILSS), flexural strength, and fracture toughness, respectively. Hussain and Dean [54] utilized the VARIM process to fabricate a series of S2-glass/vinylester nanocomposites containing 0.5, 1, 2, 5, and 10 wt% clay. They reported significant improvements in Tg, ILSS, flexural strength, flexural modulus, and fracture toughness [54]. Becker et al. [55] investigated intercalated clay–epoxy nanocomposites reinforced with 49% unidirectional carbon fibers. The addition of layered silicate to the prepregs resulted in tougher composites with more than 50% increase in fracture energy reported for composites containing 2.5, 5, and 7.5 wt% nanoclay. Manufacturing industries have also started to investigate the development of this type of nanocomposites to fabricate materials with high strength and thermal stability [44,47,56]. However, it is found that a fully exfoliated interplanar structure of nanoclay platelets inside polymer matrices is needed to achieve the desired properties of the composites [57–59]. Six types of samples were made in this study, namely, pure epoxy with 0 wt% nanoclay and nanoclay/epoxy nanocomposites with 2, 4, 6, 10, and 15 wt% nanoclays. It was found that the hardness increases from pure epoxy to the nanoclay/epoxy nanocomposite with 4 wt% nanoclay content and decreases with further increase in the nanoclay content. The average diameter of the nanoclay clusters measured at different locations was about 125 nm, whereas at 15 wt% of nanoclay loading the cluster size was about 400 nm. At higher weight percentage of nanoclay in polymer nanocomposites, the tendency to form agglomerates is increased [60], and hence larger clusters of nanoclay would be easily formed.

15.2.2.3  CNFs–nanoresin nanocomposites Zhu et al. [61] functionalized the surface of CNFs using an amine terminated functional group via silanization, which in situ reacts with epoxy monomers and provides a decreased resin viscosity and improved dispersion of CNFs within the epoxy system. The authors [61] showed enhanced tensile properties and presented recommendations for the control of electrical properties for the produced nanocomposites. Ma et al. [62] compounded CNFs (5 wt%) with poly-ethylene-terephthalate (PET) resin. Compounding methods included ball-milling, high shear mixing in the melt, as well as extrusion. The resulting fibers had good dispersion of CNFs in PET matrix with considerably higher compressive strength compared to the conventional PET fiber.

15.2  Nanoresin Nanocomposites

15.2.2.4  CNTs–nanoresin nanocomposites Significant improvements in the in-plane mechanical properties of CNT-reinforced nanocomposites compared to their unreinforced counterparts have been reported [63,64]. For example, Qian et  al. [65] dispersed multiwalled carbon nanotubes (MWCNTs) homogeneously throughout polystyrene matrices by a simple solution-evaporation method. Tensile tests on composite films show that 1 wt% nanotube additions result in 36–42% and 25% increases in elastic modulus and break stress, respectively. Yeh et al. [66] found improvements in tensile strength (25%) of MWCNTs/phenolic composites compared to pure phenolic. Similarly, Schadler et al. [67] studied the mechanical behavior of MWCNT/epoxy nanocomposites in both tension and compression. It was found that the compression modulus is higher than the tensile modulus, indicating that the load transfer to the nanotubes in the composite is much higher in compression. Ghasemi Nejhad et  al. [38,39] employed SWCNTs to develop nanoresin nanocomposites using a high-temperature epoxy. They obtained remarkable improvements in fracture toughness. Much of the previous work was performed in the area of nitrile substituted polymers including polyacrylonitrile (PAN) [68,69], poly(vinylidenecyanide vinylacetate) (PVDCN/VAc) [70,71], polyphenylethernitrile (PPEN) [72], and poly(1bicyclobutanecarbonitrile) [73]. Development of multifunctional materials using SWCNT in polymers is best demonstrated by the work of Ounaies and colleagues [74–76]. In these papers [74–76], the authors studied the effects of incorporating lead zicronate titanate (PZT) powder and SWCNT to the polyimide matrix. The authors have developed amorphous polyimides containing polar functional groups and investigated their potential use as high-temperature piezoelectric sensors. In these studies, a small volume fraction of SWCNT was added to the polyimide for obtaining improvements in the thermal, dielectric, mechanical, and piezoelectric properties of the resulting SWCNT–polyimide nanocomposites. Later, the threephase SWCNT–PZT–polyimide nanocomposites were manufactured and assessed for resulting mechanical, dielectric, and piezoelectric properties. Both SWCNT and PZT showed good dispersion in polyimide matrix without any bundling or percolation. Improvement in the mechanical and thermal properties of the SWCNT– polyimide nanocomposites resulted from the incorporation of SWCNT. Dielectric measurements as a function of SWCNT content revealed a low percolation threshold of 0.06 vol%. The overall dielectric constant increases with increasing inclusion content. The presence of the SWCNT allows for a better poling of the polyimide and yields higher Pr [66–68] values at the same poling conditions. In addition, a small amount of SWCNT facilitates the simultaneous poling of the polyimide and PZT inclusions, resulting in a value of Pr one order of magnitude higher than that of the pristine polyimide. This increase in Pr was achieved at a relatively low loading of PZT particles (20 vol%). The mechanical properties of the two-phase and three-phase composites were measured by dynamic mechanical analysis (DMA) to assess the effects of SWCNT and PZT inclusions on the modulus of the polyimide. At 2.0 vol% SWCNT loading, both modulus and strength increased with the same elongation at

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break. At 5.0 vol% SWCNT, the modulus increased by 44%. PZT loading did not affect the mechanical properties. These properties demonstrate the multifunctionality of the developed nanoresin nanocomposites.

15.2.2.5  CNT/nanoparticle–nanoadhesive nanocomposites The optimal amount of nanofillers inclusion, CNTs, and alumina nanopowder for enhanced tensile debonding and shear properties in hierarchical nanocomposites is best demonstrated by the work of Meguid and Sun [77] for composite adherends made of carbon fiber/epoxy laminate and aluminum alloy 6061-T6. In view of their importance and utility in aerospace, automotive, and communication fields, carbonfiber-reinforced polymers (CFRPs) are currently being extensively studied. This is because this class of materials possesses admirable properties, low weight, high fracture toughness, and relatively high strength. The influence of the incorporation of nanoscale particles into epoxy adhesives for the purpose of joining two dissimilar composite materials has not been investigated thoroughly. This may be due to the large variance in function, intricacy of geometry, incompatible materials, and operating conditions. Structural bonded joints can fail at different locations and by a variety of failure modes. Failure can occur or initiate in the adhesive or in the adherend, depending on the geometrical configuration, the materials of the adherends, the adhesive, as well as the manufacturing process. It is difficult to describe and define all the possible failure modes of adhesive bonded joints. The authors [77] classify the failure modes of adhesive bonded joints into the following four general categories: (i) adherend failure due to tension, (ii) interfacial failure due to shear, (iii) debonding cohesive failure, and (iv) out-of-plane failure due to delamination in composite adherends. However, the presence of the interface in adhesively bonded joints governs the strength of that joint. Therefore, a strong interface, which possesses high toughness, is highly desirable. Carbon-fiber-reinforced laminates were fabricated by autoclave at specified pressure and temperature. CNTs and alumina nanopowders were dispersed into an epoxy adhesive. The dispersion was carried out by stirring the mixture for 30 min at 50°C followed by the addition and blending of the remaining components of the adhesive. This technique ensured homogeneous dispersion of the nanofillers into the adhesive. The authors obtained uniform dispersion of CNTs and alumina nanopowder within the epoxy adhesive. The weight concentration in these two cases was 2.5%. Additional nanomixtures with nanofillers at nominal weight percentages of 1.5%, 5%, 7.5%, 10%, 12.5%, and 15% were also prepared. The substrates were then bonded together and carefully cured at a controlled room temperature for 7 days. The authors utilized test setups used for the tensile debonding and shear testing of the new interface. Tensile debonding was employed for determining the tensile stress– strain characteristics, modulus of elasticity, and ultimate tensile strength of the nanoreinforced interface. Shear testing was employed for determining the shear modulus and shear strength of the nano-reinforced interface. Three different cases, i.e., epoxy adhesive with alumina nanopowder (EANP), epoxy adhesive with CNTs (EANT), and epoxy adhesive (EA) were considered. The authors demonstrated that the

15.2  Nanoresin Nanocomposites

bonding strength increases as a result of the presence of uniformly dispersed CNTs and alumina nanoparticles. In the tensile tests, the critical load for EANT and EANP was 1.2 and 1.4 times of that for EA, respectively. In addition, the stiffnesses for the EANT and EANP were about 50% and 100% better than that for the EA samples, respectively. The sample size was three in these experiments. Remarkable improvement was found in Young’s modulus as well as the ultimate tensile strength for the cases involving different weight fractions of homogeneously dispersed nanofillers used in the study. The increase continues with the increase in the weight percentage of the nanofillers. However, for percentages above 10%, the properties degrade to below the EA level, possibly due to agglomerations. The same three different cases, i.e., EANP, EANT, and EA were also considered for the shear lap tests, and again, the weight fraction ratio of the filler concentration was 2.5% for EANP and EANT. Once again, the authors demonstrated the strengthening of the shear resistance as a result of dispersed CNTs and alumina nanoparticles. The critical shear load for EANT and EANP was 1.1 and 1.3 times of that for EA, respectively. Again, remarkable improvement was found in the shear modulus as well as the shear strength for the cases involving different concentrations of dispersed alumina nanopowders and CNTs. Analogues to the tensile tests, the results also revealed the sensitivity of the shear properties to the concentration of the nanofillers. An increase in the weight fraction of the nanofillers beyond 7–8% resulted in a reduction in the shear properties of the adhesive. The authors [77] attributed the dependence of the tensile and shear properties of nano-reinforced interface adhesive to the presence of nanoparticles, which play a major role in determining the strength of the interface at a given weight (volume) percent. The experimental results show that there is a limit to the amount of dispersed nanofillers beyond which a drop in the properties is observed. It is also believed that agglomeration of the nanoparticles could act as failure initiation sites, which could result in lowering the strength and stiffness of the adhesive. The results reveal that at a given weight (volume) percent, the presence of nanoparticles plays a major role in determining the strength of the interface, and for the most favorable response, this amount should be optimized, often experimentally.

15.2.2.6  GNSs–nanoresin nanocomposites Nanocomposites with exfoliated (well-dispersed) nanomaterials (such as GNSs) can enhance the properties of nanocomposites [78]. Some factors such as length and number of modifier chain, nanomaterial structure, curing agent, curing conditions (i.e., temperature and time), viscosity, functionalization, and resin matrix need to be considered to process the exfoliated thermoset polymer nanocomposites. It is a technical challenge to achieve full exfoliation of nanomaterials, such as GNSs, due to the large lateral dimensions of the layers, high intrinsic viscosity of the resin (especially, when large percentage of nanomaterials are used), and strong tendency of nanomaterials to agglomerate [79]. The speed of diffusion of curing agents and increased curing temperatures also influences the degree of exfoliation [78–82]. Messermith and Giannelis [83] used anhydride species curing agents to prepare exfoliated nanocomposites, whereas the use of amine curing agents allowed them

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to prepare only intercalated nanocomposites. Each GNS is theoretically separated by a 3.354 Å space. Vaia et  al. [84] mention that the degree of exfoliation can be improved by the use of conventional shear devices such as extruders, mixers, and sonicators. Improved impact, and flexural and tensile properties have been achieved from exfoliated epoxy nanocomposites by using ball milling [85]. A combination of both using higher shear forces and improved cure temperature during the curing process or the use of swelling agents [60] and functionalization are the effective tools for better exfoliation. In addition, improvement in electrical conductivity is achieved by adding GNSs in a polystyrene or epoxy polymer. It basically represents the sharp transition of the polymer from an electrical insulator to an electrical semiconductor with the addition of a graphite nanoplatelets (or GNSs), with a rather low percolation threshold (i.e., 1.8 wt%) [86]. Fukushima and Drazal [87] showed better flexural and tensile properties of chemically functionalized graphite nanoplatelets (or GNSs) in the epoxy matrix. In addition, they achieved lower coefficient of thermal expansion (CTE) and electrical resistivity compared to other carbon materials like carbon fiber and carbon black. These properties of GNSs combined with its low costs make them useful in electromagnetic interference (EMI) shielding, thermal conductors, electrical conductors, etc., demonstrating the multifunctionality of the developed nanoresin nanocomposites.

15.2.2.7  Nanoresin nanocomposites processing and manufacturing As mentioned earlier, dispersion of nanomaterials into a polymer matrix has been one of the main challenges to date [13] due to the aggregation of nanomaterials as a result of the van der Waals forces and interactions between the nanomaterials. Consistent dispersion of reinforcing nanomaterial throughout the resin leads to consistent load transfer from resin to nanomaterials and vice versa. Moreover, a good dispersion can assist with the realization of a network for conductivity of electrical and thermal energy. SWCNTs and thinner GNSs with a few layers agglomerate more easily than their multiwalled or multilayered counterparts due to their size differences (i.e., greater surface area for thinner nanomaterials). On the other hand, the SWCNTs and thin GNSs with a few layers, when dispersed well, have been found to demonstrate higher mechanical, electrical, and thermal properties. Researchers have used many different techniques in an attempt to disperse nanotubes (such as CNTs) and nanosheets (such as GNSs) in polymer matrices, including solution chemistry to functionalize the nanomaterials’ surfaces [88–92], the use of polymers to coat the nanomaterials’ surface [93], ultrasonic dispersion in a solution [65,94], and the use of surfactants [95,96]. Functionalizations allow nanomaterials to bond better to the resin and overcome the van der Waals interactions between nanomaterials [92], yielding better interfaces and interphases. Of course, these improvements have been shown to increase until an optimal loading level and then decrease above this optimum concentration [38,39,67,97–107]. Although most polymer/nanotube nanoresin nanocomposite research had utilized thermoplastic matrices in early years, many research work have been conducted with thermosetting materials [38,39,67,98– 106] in recent years. The change in viscosity as a function of cross-linking can be

15.3  Hierarchical Nanocomposites

problematic for optimizing dispersion and orientation. Of the various thermosetting polymers that have been reported in the literature, epoxies have been the most commonly used.

15.3  HIERARCHICAL NANOCOMPOSITES Through nanotechnology, it is envisioned that nanostructured materials will be developed using a bottom-up approach, where materials and products are made from the bottom-up, i.e., by building them from atoms, molecules, nanomaterials, and micromaterial to produce macromaterials. A “hierarchical composite” is a composite the constituents of which are increased in size hierarchically, i.e., a nanostructure material such as a nanoparticle, nanoclay, nanofiber, nanotube, and/or nanosheet is manufactured first. Next, this nanomaterial is mixed with a resin to give a “nanoresin nanocomposite.” Finally, a micron-size fiber is mixed with this “nanoresin nanocomposite,” as one would do in traditional FRP composites to yield “hierarchical nanocomposites” (see Figure 15.1). Here, an attempt is made to provide a brief overview of advances in hierarchical nanocomposites research, and critical issues in hierarchical nanocomposites research are discussed. Subramaniyan and Sun [108] used vacuum-assisted wet layup (VAWL) process for the inclusion of dispersed nanoclay in resin as reinforcement in conventional fiber-reinforced composites. The addition of 5% nanoclay produced a substantial increase in longitudinal compressive strength (33.9%) of GF-reinforced composites. Iwahori et al. [109] studied the effects of reinforcing CNF (5 and 10 wt% with aspect ratio of 10 and 50) in epoxy for the manufacture of fiber-reinforced composites. The addition of CNFs and CNTs in epoxy-reinforced fiber composites increased flexure and compression properties considerably [109,110]. Miyagawa et  al. [111] reported mechanical and thermophysical properties of bio-based epoxy nanocomposites reinforced with organomontmorillonite (MMT) clay and PAN-based carbon fibers. DMA was conducted and yielded an increase of 0.9 GPa for the storage modulus of bio-based epoxy at 30°C with the addition of 5.0 wt% exfoliated clay nanoplatelets. It was observed that the ILSS of the CFRP improved by adding 5.0 wt% intercalated clay nanoplatelets [111].

15.3.1  NANOPARTICLE–NANORESIN HIERARCHICAL NANOCOMPOSITES CMCs are being developed for applications that require lightweight structural materials, oxidation resistance, and high-temperature resistance capabilities including high strength and modulus for automotive and space/aerospace engine parts and structural applications [112–114]. An important type of CMC that is currently being investigated is the continuous fiber-reinforced ceramic composite (CFCC) employing the polymer infiltration and pyrolysis (PIP) technique [115–117]. In a PIP technique, a preceramic polymer (which has a ceramic backbone) is used in conjunction with a ceramic fiber to first combine the two and then cure (B-staging/C-staging) the

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resulting composite at relatively low temperatures (e.g., 100–150°C) employing traditional polymer composites processing equipment such that the composite takes the shape of the mold. The cured part is then taken out of the mold and is placed in a hightemperature furnace under the inert environment to pyrolyze the preceramic polymer at relatively high temperatures (e.g., 1000°C), upon which the majority of the preceramic polymer converts to ceramic but some portion of it is exhausted from the furnace as combustion products, leaving some voids and cracks in the part. Since not all of the preceramic polymer is converted to ceramic, the part is taken out of the hightemperature furnace and is infiltrated with the preceramic polymer under the vacuum and cured again (B-staging/C-staging) and then pyrolyzed at high temperature in an inert environment one more time. At each iteration, the weight gain percentage of the resulting CFCC is calculated and recorded. This reinfiltration/pyrolysis cycle will be repeated a number of iterations until a weight gain convergence is achieved (e.g., an incremental or cumulative weight gain of < 1%). This technique normally gives good quality CFCCs with less than 1% porosity [115–117]). Since the matrix for the CFCC systems is a preceramic polymer (and is liquid at room temperature), it opens the possibility of including nanomaterials within it and hence producing nanoresins and nanocomposites. However, one of the important areas of concern is that even the best processed ceramic materials pose many unsolved problems; among them, relatively low fracture toughness and strength, degradation of mechanical properties at high temperatures, and poor resistance to creep, fatigue, and thermal shock. Attempts to solve these problems may involve incorporating secondary phases such as particulates, platelets, whiskers, and fibers in the micron- and nanometer-size range at the matrix grain boundaries. However, the results obtained have been generally disappointing when micron-size fillers are used to achieve these goals [115]. By scaling the particle size to the nanometer scale, it has been shown that novel material properties can be obtained. The optimal amount of nanoparticle inclusion for enhanced mechanical performance in hierarchical nanocomposites is best demonstrated by the work of Gudapati et al. [116,117] in CFCCs. These authors initially reported considerable (up to 25%) improvements in flexural strength of CFCCs with nanoparticle inclusions. It has been reported that the mechanical properties of the composites could be improved by adding an optimal amount of nanoparticles into polymer-based composites. In Ref. [116], the authors investigate the effects of various nanoparticles (such as silicon carbide, SiC; carbon, C; titanium oxide, TiO2; yttrium oxide, Y2O3; and zinc oxide, ZnO) at 5% inclusion by weight within CFCCs with plainweave SiC woven fabric. In this work, Ni/CE referred to a CFCC that was not reinforced with nanoparticles and was considered as a base/control material, where Ni stood for Nicalon™ silicon carbide fiber and CE stood for KiON CERASET® preceramic polymer [116]. The effects of various nanoparticle reinforcements on the processing and flexural mechanical performance of CFCCs using the PIP technique were investigated. Glycerol monooleate was used as surfactant agent to provide good dispersion of nanoparticles in the KiON CERASET. Ni/CE-n referred to

15.3  Hierarchical Nanocomposites

a manufactured base/control composite reinforced with n nanoparticle (referring to the nanoparticle type) in the polymer, and with preceramic polymer reinfiltration/ pyrolysis route, where the initial matrix had nanoparticles but the matrix for the reinfiltrations did not (i.e., plain reinfiltration). Ni/CE-n-R-n referred to a Ni/CE composite that used matrix reinforced with n nanoparticle for both fabrication and subsequent reinfiltrations (i.e., nanoparticle reinfiltration). In this study, the weight percentage of all nanomaterials is at 5%, and hence n represents the type of nanomaterial. Detailed description of the manufacturing of various nanocomposites and materials used can be found in the work reported by Gudapati et al. [116]. It was shown, in this study, that yttrium oxide, Y2O3, performed the best and increased the flexural strength by 25% as compared to the base/control CFCC without any nanoparticle. In addition, it was shown that the reinfiltration using preceramic polymer with 5% by weight of nanoparticle inclusion performed better than the reinfiltration using preceramic polymer without nanoparticle inclusions. In a follow-up work, Gudapati et  al. [117] experimentally investigated the effects of different weight percentages of yttrium oxide, Y2O3, nanoparticle inclusions on the processing and in-plane flexural properties of the CFCCs. The CFCCs were manufactured using a SiC fabric and KiON CERASET preceramic polymer with increasing weight percentage of nanoparticles to find the optimum amount of these nanoinclusions. Five different types of CFCCs were manufactured with varying weight percentage of nanoparticles at 0%, 5%, 10%, 15%, and 20%. The nanoparticle used was yttrium oxide with an average size of 29 nm. The effects of varying weight percentage of nanoparticle reinforcements on processing and flexural mechanical performance of CFCCs using the PIP technique was investigated. Glycerol monooleate was used as surfactant agent to provide good dispersion of nanoparticles in the KiON CERASET. Ni/CE-n referred to a base composite reinforced with yttrium oxide nanoparticles, with n referring to the percentage of nanoparticles in the polymer, manufactured with preceramic polymer reinfiltration/pyrolysis route, where the initial matrix had nanoparticles but these were not present in the reinfiltration matrix. Ni/CE-n-R-n referred to a Ni/CE composite that used matrix reinforced with n percentage of yttrium oxide nanoparticles for both fabrication and corresponding reinfiltration. Detailed description of the manufacturing of various nanocomposites and materials used can be found in the work reported by Gudapati et al. [117], where the authors show that the cumulative percentage weight gain for both Ni/CE-n and Ni/CE-n-R-n type CFCCs, after each reinfiltration/pyrolysis step is lower than that of the base/control Ni/CE CFCCs. In addition, a consistent decrease in cumulative percentage weight gain was observed with increasing nanoparticle weight percentage, which the authors attributed to a better matrix densification at B-staging/curing for composites reinforced by the nanoparticles as approaching the convergence of less than 1% weight gain [117]. The authors attributed the decrease in cumulative percentage weight gain for Ni/CE-n-R-n samples with increasing nanoparticle weight percentage to the difficulties in reinfiltrations with increasing weight content of the nanoparticles. It is apparent that the viscosity of the nanoparticle filled

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FIGURE 15.2 SEM micrographs of nanoparticles-reinforced CERASET preceramic polymer matrix: (a) CERASET reinforced with 10% nanoparticles, (b) CERASET reinforced with 15% nanoparticles, (c) CERASET reinforced with 20% nanoparticles. All figures are at the same magnification and the size of the individual particles in (c) is about 1 µm.

polymer increases with increasing weight percentage of nanoparticles; however, this increase was insignificant in terms of processing [117]. Dispersion is an important factor in the performance of nanoparticle-reinforced nanocomposites. Improper dispersion leads to agglomeration of nanoparticles and the agglomerated inclusions act as defects with high stress concentrations, instead of acting as uniform reinforcements. Ghasemi-Nejhad and coworkers [117] used 5 wt% of glycerol monooleate as surfactant agent to disperse the nanoparticles in the preceramic polymer followed by a mechanical stirring system. In this study, the type of nanomaterial is fixed (i.e., Y2O3) and its weight percentage varies, and hence n represents the percentage of Y2O3. The scanning electron microscope (SEM) studies have revealed uniform dispersion up to a weight percentage of 15% without any agglomeration (e.g., see Figure 15.2(a) and (b)). As shown in Figure 15.2(c), particles started to agglomerate at a threshold weight percentage of 20% suggesting the inability of the surfactant used in dispersing the nanoparticles. From Figure 15.3, it is observed that the flexural strength of the base/control material is increased by approximately 5–34 wt% by adding nano-sized particles to it. It should also be mentioned that the CFCC flexural strength improves significantly with nanoparticle-reinforced preceramic polymer reinfiltration route, when the reinfiltration polymer contains 5 wt% of nanoparticles. Further increase in weight fraction of nanoparticles in the reinfiltration polymer results in problems during reinfiltration stage leading to the degradation of flexural properties, as compared with Ni/CE-5-R-5. Figure 15.3 shows that, in general, the nanoparticle addition to CFCC enhances its strength, and the optimum strength enhancement is reached with lower weight percentage of nanoparticle inclusion for the Ni/CE-n-R-n samples (i.e., at 5%), whereas the best result is obtained for the Ni/CE-n samples at 15%. Figure 15.3 results are also consistent with Figure 15.2 which shows a threshold for agglomeration at 15%.

15.3  Hierarchical Nanocomposites

Strength enhancement (%)

40 35 30 25

Plain reinfiltration Nanoparticle reinfiltration

20 15 10 5 0

5%

15% 10% Weight % of nanoparticles

20%

FIGURE 15.3 CFCC strength enhancement for different types of nanocomposites.

15.3.2  NANOCLAY–NANORESIN HIERARCHICAL NANOCOMPOSITES Reinforcing polymer matrices with inorganic fillers in the nanometer-size range, exemplified by exfoliated MMT clay platelets, in addition to improving the mechanical performance, also broadens their range of functionality to include flame resistance and modified transport properties [48,118,119]. Hence, it is of significant importance to study the resulting “nanocomposites” as matrices for conventional fiber-reinforced thermoplastic composites. In this feasibility study for integrating nanocomposite matrices into conventional composites, two processing routes for hybrid isotatic PP (iPP)/MMT nanocomposite fiber-reinforced composites have been considered: long GF-reinforced composites based on co-woven yarns, as a step toward the fabrication of nanocomposite-based comingled yarns [120], and glass-mat-reinforced thermoplastic (GMT) composites [121]. IsotaticPP/MMT nanocomposite granulates were prepared from an iPP homopolymer and a commercial iPP/40–50 wt% organically modified MMT concentrate (Nanocor), which also contained a PP-based compatibilizer, by melt compounding at 200°C in a twin-screw extruder. Melt spun iPP/MMT fibers were prepared from the nanocomposites. Precursor fabrics were prepared by co-weaving as-spun iPP/MMT yarns with GF yarns or with poly(ether ketone) (PEEK) yarns. The iPP/ MMT precursors for the GMT were films of about 1 mm in thickness, produced by extrusion through a planar die at 180°C of granulates produced as described above from a low-viscosity iPP homopolymer grade specifically tailored for GMT applications. Transmission electron microscope images of an as-compounded nanocomposite extrudate with an MMT content of 3.4 wt% showed that the MMT was well dispersed within the polymer. However, exfoliation could not be achieved and many of the MMT

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particles consisted of stacks of individual MMT layers rather than isolated layers. Undrawn fibers showed far less orientation of both the MMT stacks and the iPP lamellae than the drawn fibers. In the case of the oriented films, the orientation and aspect ratios of the MMT particles were similar to those of the undrawn fibers. The reinforcing effect of the MMT on the axial stiffness, as inferred from static tensile tests, was generally greater in the drawn fibers than in the undrawn fibers, in which the MMT layers showed less orientation. Unidirectional hybrid fiber-reinforced nanocomposites were prepared by compression molding a precursor fabric. In initial tests, GF yarns and iPP yarns containing either 0 or 3.6 wt% MMT were wound consecutively onto a preform to give an overall GF content in the final composite of about 60 vol%. Simple co-weaving of the iPP/MMT and GF yarns was considered as a means of obtaining a more controlled distribution of the GF in the precursor. IsotaticPP/3.6 wt% MMT fibers were melt spun with GF yarns using a tabletop loom to produce a simple woven textile containing about 50 vol% GF. The warp direction consisted of the iPP/MMT yarn and the weft direction was made up of both iPP/MMT and the reinforcing fibers. Impregnation was significantly improved with respect to that in the co-wound yarn and 10 min at 220°C and 2 MPa was sufficient to reduce the porosity to 1 vol%. Next, a similar composite was processed under identical conditions where the GF tows were replaced by PEEK filament tows. In spite of the relative coarseness of the MMT particles present in the manufactured specimen, MMT was clearly present within the PEEK tows, i.e., in the interstices between the PEEK filaments, which is encouraging for this type of processing. Sections, taken perpendicular and parallel to the fiber direction, confirmed the MMT to have lost any orientation acquired during fiber spinning. Three-point bend tests were carried out to investigate the mechanical properties in the presence of the MMT [121]. The flexural strength of the glass matbased composites (glass mat fraction of 30%) for various MMT contents was measured at 0%, 3.6%, and 6% of MMT at room temperature, 50°C and 90°C. The results show that while the flexural strength increases by about 20–25% for room temperature and 50°C by the MMT percentage increase, these strengths for the samples tested at 90°C did not change at various MMT loading. In addition, at any given MMT weight percentage, the flexural strength dropped by increasing the temperature [121].

15.3.3  NANOTUBE/NANOSHEET–NANORESIN HIERARCHICAL NANOCOMPOSITES In the past decade, CNT nanocomposites have been extensively used to investigate their mechanical, thermal, electronic, and optical properties, which demonstrate their multifunctionality. A novel area, where nanotube nanocomposites can have a major impact, is in the damage detection and health monitoring of fiber-reinforced composites. Breakthroughs have demonstrated the myriad of opportunities that can be explored by dispersing nanotube networks in the host matrix. Studies by Thostenson and Chou [122] have shown the ability to detect damage in the composite by using electrical conductivity measurements. Such studies show promise for research in the multifunctionality area of nanotube-reinforced hierarchical nanocomposites.

15.4  Multifunctional Hierarchical Nanocomposites

Electrical techniques have been used in the past to detect damage in carbon-fiberreinforced composites both under static and dynamic loading [123–126]. However, this technique does not apply to composites reinforced with nonconductive fibers. In Ref. [58], GFs which are nonconductive are used as fiber reinforcements. CNTs dispersed in the epoxy are used to detect the damage mechanisms. It is well established that nanotubes can form a conducting network in polymers at low concentrations similar to the nervous system in human beings. Studies have shown that MWCNTs show highest potential for enhancement of electrical conductivity as compared to its SWCNT counterpart [127]. In Ref. [58], unidirectional and bidirectional composites were manufactured and tested. Both mechanical and electrical readings were recorded concurrently during tensile and fracture tests. The resistance of the sample tested in tension increased linearly with initial deformation. A sharp increase in resistance was observed when delamination initiated. Similar trends were observed for both unidirectional and bidirectional composites. Similar relationship between resistivity and mechanical parameters was observed. In summary, in situ monitoring of CNTs offers tremendous potential for damage sensing, self-healing, and in situ health monitoring techniques in nanocomposites, demonstrating the multifunctionality of such nanocomposites. As explained in Section 15.2.2.6, a great deal of research has been conducted on the use of GNSs within various matrices to produce nanoresin nanocomposites. However, since such research has started in recent years, there is little systematic work on the development of GNS-based nanoresin hierarchical nanocomposites.

15.4  MULTIFUNCTIONAL HIERARCHICAL NANOCOMPOSITES In previous sections, nanocomposites with the inclusion of nanomaterials such as nanoparticles, nanoclays, nanotubes, and nanosheets as reinforcing materials are discussed. In this section, MHNs employing a bottom-up approach by in situ manufacturing of nanoscale materials (such as MWCNT) onto microscale materials (such as carbon, glass, kevlar, spectra, or SiC fibers) and finally combining these with resins to give MHNs are discussed. In this section, a brief overview of recent advances in “bottom-up” MHNs research is presented. Critical issues in this research area as well as promising techniques for processing precursors for macroscopic nanocomposites are discussed.

15.4.1  MHNs WITH FUZZY FIBERS The controlled surface growth of CNTs is best illustrated by the work of Thostenson et al. [128] using carbon fibers. Advances in the synthesis of CNTs have enabled their growth on carbon fibers using chemical vapor deposition (CVD) [129,130]. With CNTs, the change in reinforcement scale relative to carbon fibers offers opportunity to combine potential benefits of nanoscale reinforcement with well-established fibrous composites to create hybrid, multiscale, hierarchical micro/nanocomposites.

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By varying the reinforcement scale, it is possible to tailor the mechanical and physical properties of the composites. The CNTs grown on carbon fibers, in this work [128], could potentially be used to create hierarchical CNT/carbon fiber composites where individual carbon fibers, which are several microns in diameter, are surrounded by nanotubes. Also, the authors [128] show the surfaces of a single carbon fiber before and after the growth of CNTs as “fuzzy fibers” by CVD, as a typical hierarchical manufacturing technique for microfibers to create MHNs. The thickness/length of the nanotube region surrounding the fiber ranged between 250 and 500 nm [128]. In the synthesis of CNTs on the surfaces of carbon fibers, a thin layer of stainless steel was applied to bundles of carbon fiber using magnetron sputtering. Prior to the application of the catalyst, the carbon fiber bundles were heat-treated at 700°C in a vacuum to remove any polymer sizing applied to the fiber. For nanotube growth, the flowing hydrogen was replaced with acetylene (C2H2). The growth time for the nanotubes was 1/2 h. After the CVD, the fibers were examined with an SEM to verify the nanotube growth. The surfaces of the fibers after each step in the synthesis process for growing CNTs (heat treatment, catalyst application, exposure to growth conditions) were examined with the SEM, and no pitting of the fiber surfaces was observed. The authors [128] used single-fiber fragmentation test method to assess the properties of the fiber/matrix interface. The fiber fragmentation experiments for four different fiber specimens (i.e., catalyst, CVD/no catalyst, unsized, and nanotube) were carried out. The results show that the critical aspect ratio (lc/d) is inversely proportional to the interfacial shear strength. The growth of CNTs on the surface of the fibers resulted in the strongest interface, and both the application of stainless steel catalyst and exposure to CVD conditions without nanotube growth resulted in a significant degradation of the fiber/matrix interface and increased scatter in experimental data as compared with the unsized (baseline) fiber and the CNT-modified fiber. Compared to the unsized fiber, the nanotube-modified fiber shows a 15% improvement in interfacial strength while the catalyst and CVD (no catalyst) fibers show a 37% and 32% degradation in the interface strength, respectively. Before preparing fiber fragmentation specimens with the nanotube-coated fibers, the authors [128] viewed the individual filaments under an optical microscope. It was clear that there were variations in the surface of the fiber. During the catalyst application, the catalyst layer may not always be deposited uniformly on the surface of the individual fiber. As a result, there will be local areas on the fiber where amorphous carbon is deposited instead of nanotubes. Individual nanotube-modified fibers are also quite difficult to extract from the fiber bundle because of nanotube entanglement. Thus, some CNTs are stripped from the fiber bundle as they are extracted. This surface variation could clearly be seen when viewing the carbon fiber under an optical microscope [128]. Carbon fiber surfaces are relatively smooth and, therefore, reflect light. In contrast, the presence of CNTs on the fiber surface results in light scattering. Therefore, in a micrograph, the dark areas would correspond to fibers where CNTs are attached to the surface, and the bright areas would correspond to bare fiber. However, it should be noted that factors such as fiber sizing removal conditions, coating material, uniform fiber coating, nanotube growth length on the fibers, and maintaining the integrity of the

15.4  Multifunctional Hierarchical Nanocomposites

base fibers throughout these processes are important parameters that could affect the performance of the resulting MHNs.

15.4.2  MHNs WITH NANOBRUSHES The controlled surface growth of CNTs on silicon carbide fibers to demonstrate nanobrush nanocomposites for multifunctional applications is best illustrated by the work of Ghasemi-Nejhad and coworkers [131,132]. Here, the authors [131] demonstrate an innovative approach to construct multifunctional, conductive brushes with CNT bristles grafted on fiber handles, and perform several unique tasks such as cleaning of nanoparticles from microtrenches of micro/nanoelectronics, coating of the inside of microcapillaries, selective chemical adsorption with environmental cleaning applications, and as movable electromechanical brush contacts and microswitches. The nanotube brush consists of a silicon carbide fiber (SiC, diameter 16 μm) as the handle and aligned MWCNTs grafted on the fiber ends as bristles. The nanotubes (average diameter 30 nm) were grown by selective CVD with ferrocene and xylene as the precursors. Before CVD, individual SiC fibers were partially masked by a 15 nm Au layer except for the top ends and placed vertically in the furnace. Figure 15.4(a) shows the schematic steps for the partial masking and growth of CNTs on SiC fibers to develop nanobrushes. Figure 15.4(b) shows the SEM image of the top morphology of typical as-grown nanotubes on a group of microfibers. Here, the nanotubes grew in three prongs symmetrically distributed around the center fiber (like a dust sweeper) and have a uniform length (~60 μm after 30 min growth) along the fiber axis. Figure 15.4(c) shows various views of an individual nanobrush of Figure 15.4(b). GhasemiNejhad and coworkers [131] showed individual brush with 60-μm-long nanotube bristles spanning over 75 μm of the microfibers. They also showed that the trim length could be varied from hundreds down to a few micrometers depending on the growth time. The authors [131] obtained brushes with bristle spans ranging from several micrometers to millimeters. For example, they showed a brush with 200 μm span and 70 μm trim length formed from nanotubes grown for a rather short time of approximately 35 min. The geometry of the bristles can also be made different, such as three prongs like a dust sweeper (e.g., see Figure 15.4), two prongs resembling a handheld fan, a one-prong toothbrush, and lengthwise alternating bristle brush [131]. The authors [131] also performed tensile tests to measure the adhesion of MWCNT to the SiC microfibers by mechanically pulling away nanotubes from the handle, where the nanotubes experienced a shear stress at the nanotube/SiC interface, which eventually strips their ends away from the SiC fiber. The stress vs. strain curve of an as-grown brush showed a maximum stress of 0.28 MPa before the bristles detached from the handle (for the 10 brushes tested, this stress ranged over 0.2–0.3 MPa). The authors demonstrated that the adhesion strength can be improved by a subsequent annealing of the brush at 950°C for several hours in argon, where the failure shear stress nearly doubles from 0.28 to 0.50 MPa [131]. Therefore, the annealing strengthened the interaction between carbon and the underlying silicon (SiC bonding), thereby substantially enhancing the bristle–handle adhesion. The

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Nanotubes

(a)

Partial masking

Au

SiC fiber (c)

(b)

CVD

Nanotube brush

20 µm

100 µm

100 µm

FIGURE 15.4 (a) Illustration of partial masking of SiC fibers to grow nanotubes only on the fiber top, (b) as-grown nanotubes on top of SiC fibers creating multiple nanobrushes, and (c) various views of a single nanobrush.

authors performed extensive contact-brush operations to evaluate the brush lifetimes; for example, the rotating brush, contacting a metal surface in every rotation, remains robust without shedding the nanotube bristles after over 0.1 million cycles [131]. The flexibility of nanotubes can relieve the contact stresses as the brush touches a solid surface on each cycle. In addition, the nanotube brush was glued to the shaft of a micromotor and then immersed into a solution housed in a capillary and stirred for 5 min at 2000 r.p.m., and no shedding of the nanotube bristles was observed [131]. The developed nanotube brushes are multifunctional and possess a number of useful functions, among which cleaning of nanomaterials and nanoparticles from surfaces and microtrenches (e.g., 10 μm wide and 100 nm deep), painting and cleaning inside microcapillaries, chemical and metallic ion adsorption using brush MWCNT bristles’ chemical functionalizations, and electrical contact and switching are described

15.4  Multifunctional Hierarchical Nanocomposites

by the authors [131]. These durable nanotube brushes could also serve as versatile, antistatic, heat-tolerant tools in many industrial and environmental applications. It should be noted that if the fiber is a non-SiC fiber, such as carbon, glass, kevlar, and spectra, the fiber sizing needs to be removed in an inert environment in a furnace heated at about 650–700°C, and then a preceramic polymer coating needs to be applied on the surface of the fiber and cured before CNTs can be grown on the non-SiC fibers in a CVD furnace [133,134]. The nanobrush technology described in this section can be employed to create nanobrushes with various numbers of prongs, prongs lengths, and fiber lengths. These nanobrushes, as nanotubes-grown shortfibers, can then be included into a resin to produce nanotube-grown short-fiber nanoresins as MHNs.

15.4.3  MHNs WITH NANOFORESTS The controlled surface growth of CNTs on fibers to manufacture 3D nanocomposite is best illustrated by the work of Ghasemi-Nejhad and coworkers [133,134] using silicon carbide fibers. For many decades, advanced FRP and ceramic matrix composites have been used as viable primary load-bearing structures. Although the in-plane loading and stresses have been handled by various configurations of fiber architectures, such as 1D (i.e., unidirectional tapes) and 2D (i.e., woven fabrics), the interlaminar and intralaminar stresses have remained major issues, resulting in relatively weak interlaminar fracture toughness. Despite huge promise of studies using CNT reinforcements in polymer composites, the real use of nanotubes in composites for structural applications has often been rather disappointing, due to issues such as dispersion, alignment, and interfacial strength. In this study [133,134], the authors demonstrate an innovative approach using nanotubes in composites, to influence and increase the 3D composite interlaminar properties, using unique bottom-up multiscale hierarchical manufacturing of CNT forests grown on fibers present in-between the plies to give 3D nanocomposites. To optimize the interfacial effects, well-aligned MWCNTs were grown perpendicular to 2D woven fabrics of SiC to produce 3D fabrics. A brief manufacturing route for this type of nanocomposites is shown schematically in Figure 15.5. Figure 15.6(a) shows a scanning electron micrograph of a piece of SiC fiber cloth consisting of 16 μm fibers (see inset) woven into a 2D plain-weave fabric. The SiC fiber cloth after growing MWCNTs on its surface, by the CVD process, is shown in Figure 15.6(b), and its close-up view is shown in Figure 15.6(c). MWCNTs are grown uniformly on all of the exposed fibers on the surface of the fiber cloth. The nanotubes grown on the fibers were about 40–80 μm long. MWCNTs were successfully grown uniformly on SiC fiber cloths of up to 120 × 40 mm. The nanotube-grown fabrics were infiltrated with a high-temperature epoxy, then stacked to yield a “sandwich” structure, and the laminated structure was cured in an autoclave. Generally, the proposed fabric reinforcement can be infiltrated with polymers or preceramic polymers with appropriate viscosities to produce polymer or ceramic matrix composites, respectively.

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1

3

2 Polymer matrix

Press

Woven cloth CNTs

FIGURE 15.5 Schematic diagram of the steps involved in the hierarchical nanomanufacturing of multifunctional 3D nanocomposites. (1) Aligned nanotubes grown on the fiber cloth. (2) Stacking of matrix-infiltrated MWCNT-grown fiber cloth. (3) 3D MHN laminated plate fabrication by layup technique.

FIGURE 15.6 The growth of MWCNTs on the SiC woven cloth: (a) plain-weave SiC fabric cloth. (Inset: individual bare fibers of the woven cloth.) (b) The cloth with MWCNTs grown perpendicularly on its surface. (c) Close-up view of the MWCNTs grown on the SiC woven cloth.

Several experiments such as double-cantilever beam (DCB [133]) and endnotched flexure (ENF [133]) were carried out to demonstrate the transverse mechanical properties’ improvement, and the effects of the MWCNT nanoforests bridging the plies [133]. To characterize the local behavior of the 3D-reinforced composite, the hardness and indentation modulus were measured using a nanoindenter. At an approximate indentation depth of 2.5 nm, the percentage of improvements in the

15.4  Multifunctional Hierarchical Nanocomposites

Table 15.1  Multifunctional Properties of the 3D Multifunctional Hierarchical Nanoforest Nanocomposite Material Property Type Fracture toughness test results Flexure test results

Structural dynamic properties

2D Composite

3D Nanocomposite

GIC (kJ/m2) (DCB)

0.95

GIIC (J/m2) (ENF)

91

Flexural modulus (GPa) Flexural strength (MPa) Flexural toughness (N mm) ζ (damping ratio)

23.1 ± 0.3

4.26 (348% enhancement) 140 (54% enhancement) 24.3 ± 0.2 (5% enhancement) 150.1 ± 1.4 (140% enhancement) 30.4 (424% enhancement) 0.0731 (669% enhancement) 601.4

fn (Hz) (natural frequency) fnζ (damping characteristic)

62.1 ± 2.1 5.8 0.0095 753.9 7.162

43.963 (514% enhancement)

Average CTE (α) over 0–150°C (ppm/°C)

123.9 ± 0.4

Through-thickness thermal conductivity at 125°C (W/mK) Through-thickness electrical conductivity (S/cm)

0.33

47.3 ± 0.3 (62% enhancement) 0.50 (51% enhancement) 0.408 (conductive)

0.075 × 10E–6 (insulating)

indentation modulus and hardness of the 3D nanocomposite compared with those for the base/control 2D composite samples were 30% and 37%, respectively. The authors [133] attributed this to partial misalignment of the MWCNTs during the fabrication of the 3D nanocomposites. Table 15.1 gives the multifunctional properties of the 3D multifunctional hierarchical nanoforest nanocomposite as compared with their 2D woven composite counterpart. From the fracture toughness values, the 3D MHN shows a Mode I GIC of 4.26 kJ/m2, i.e., a 348% improvement compared with the base/control 2D composite with a GIC of 0.95 kJ/m2 (see Table 15.1). The interlaminar shear fracture toughness of the 3D nanocomposite for Mode II GIIC was 140 J/m2 which showed an improvement of about 54% as compared with that for the base 2D composite at 91 J/m2 (see Table 15.1). An examination of the scanning electron micrographs of the fracture surface of the base 2D composite and the 3D composite reveals that the superior fracture performance of the 3D nanocomposite results from a mechanical interlocking between the fibers and the matrix by means of the nanotubes [133,134]. The interlocking effect between the adjacent plies due to the MWCNT nanoforests makes it difficult to open the plies, leading to the high interlaminar fracture toughness, GIC, of the 3D nanocomposite. These composite specimens were also tested under a

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three-point flexural loading to measure the in-plane mechanical performance. The measured in-plane modulus, strength, and toughness of the 3D nanocomposite show 5%, 140%, and 424% enhancements compared with the base 2D composite (see Table 15.1), indicating that the enhancement of the through-the-thickness mechanical properties does not compromise the in-plane properties, and in fact it enhances these properties, potentially due to the alignments of some of the CNT nanoforests in the in-plane direction (see Figure 15.6(c)). In addition to the large-scale improvements in mechanical properties, the 3D nanocomposite also shows superior multifunctional performances such as damping, CTE, and thermal and electrical conductivities. The natural frequencies and damping ratios of the 3D nanocomposite were measured and compared with those of its 2D counterpart using a cantilevered-specimen experiment. The data in Table 15.1 compare the results of structural dynamic properties of the 3D nanocomposite with the base 2D composite, where fn and ζ are the natural frequency and damping ratio, respectively. It is shown that the nanotube nanoforest 3D nanocomposite improves ζ by 669% compared with its 2D composite counterpart (see Table 15.1). In addition, the damping characteristic, fnζ, enhancement is more than 5 times (i.e., 514%) for the 3D nanocomposite as compared with the base 2D composite counterpart. These multifunctional property improvements are very encouraging for the use of 3D nanotube nanoforest hierarchical nanocomposites in many structural areas where such properties are highly desired. In addition, the through-the-thickness CTE was measured in the range of 0–150°C and was found to be decreased (i.e., enhanced) by 62% for the nanoforest-based 3D nanocomposites (see Table 15.1), due to negative CTE of the CNTs [133]. This is significant particularly where a low CTE and dimensional stability is required for an environment with large temperature variations. Furthermore, the through-thethickness thermal conductivities of the base 2D and the 3D nanocomposites were also measured for comparison. The thermal conductivity of the 3D nanocomposites showed consistent improvements compared with the base 2D composite over the range of 0–150°C. The high thermal conductivity of the MWCNTs grown in the thickness direction improves the transverse thermal conductivity of the base 2D composite by about 51% (see Table 15.1) [133]. The electrical conductivities of the 3D and 2D composite were also measured. The average through-the-thickness electrical conductivity of the 3D nanocomposite (i.e., 0.408 S/cm) was significantly higher than that of its 2D composite counterpart (i.e., 0.075 × 10−6 S/cm). The measured average in-plane electrical conductivities of the 3D nanocomposite and base 2D composite were 3.44 and 0.75 × 10−6 S/cm, respectively, indicating that the interfacial nanoforest nanotubes provides conducting paths along all directions in the 3D nanocomposite structure [133]. The much-improved through-the-thickness electrical conductivity observed in the 3D nanocomposites would potentially impart to these structures an electrical sensing capability for structural health monitoring during delaminations (i.e., crack initiations and/or propagations) for automotive and aerospace applications, as well as a potential lightning strike protection for aerospace structures. For the 3D nanocomposites fabricated with nanotube nanoforests, the authors [133,134] demonstrated remarkable improvements in the interlaminar fracture

15.5  Multiscale MHNs

toughness, hardness, delamination resistance, in-plane mechanical properties, damping, thermoelastic behavior, and thermal and electrical conductivities making these structures truly multifunctional. The multifunctional hierarchical 3D nanoforest nanocomposite introduced by the authors [133,134] is the first real instance where the nanotubes are used in a value-added proposition to effectively improve traditional composite performance by manyfolds. The vertical arrays of nanoforest nanotubes in the thickness direction of the composites improve the out-of-plane mechanical properties without compromising the in-plane properties and also alleviate the problem of agglomeration when nanotubes are randomly introduced in the composites. Such 3D nanocomposites could pave the way for the application of CNTs in structural and MHNs. It should be noted that if the fiber (be it 1D unidirectional, 2D woven, or 3D textile) is a non-SiC fiber, such as carbon, glass, kevlar, and spectra, the fiber sizing needs to be removed in an inert environment in a furnace heated at about 650–700°C, and then a preceramic polymer coating needs to be applied on the surface of the fiber and cured before CNTs can be grown on the non-SiC fibers in a CVD furnace [133,134]. It should also be mentioned that instead of growing CNT nanoforest directly onto the fibers, the CNT nanoforest can be grown in a CVD furnace on an appropriate substrate such as a silicon oxide substrate [135] and then be removed and interleaved in-between the layers of a composite laminate [136] to again give multifunctional hierarchical nanoforest nanocomposite, with very similar multifunctional properties’ improvements as seen in the as-grown nanoforest fibers explained in this section [133,134]. One advantage of these alternative multifunctional nanoforest-based hierarchical 3D nanocomposites [136] is that they have applications in prepreg-based composites as well as wet layups (which are more suitable for the as-grown multifunctional nanoforest-based hierarchical 3D nanocomposites [133,134]).

15.5  MULTISCALE MHNs When nanoresins reinforced by nanoparticles, nanoclays, nanofibers, nanotubes, and/or nanosheets are combined as matrix material with nanoforest fibers/fabrics, they produce multiscale MHNs (see Figure 15.1) to yield super-performing MHNs. This class of nanocomposites is called multiscale, here, since both composite constituents, i.e., the resin and the fiber, are reinforced by nanomaterials. This class of super-performing MHN materials can solve the brittleness of the matrix in a composite using a toughened nanoresin as well as the delamination susceptibility of the fiber-reinforced composite laminates using a nanoforest fiber/fabric system. It is expected that the performance enhancement of nanoresin (compared to base resin) and that of nanoforest fibers/fabrics (compared to base fibers/fabrics) be cumulative in the resulting super-performing MHNs; however, the materials’ properties enhancement may not be additive and most likely will exhibit a nonlinear behavior by combining nanoresin and nanoforest technologies in terms of materials’ properties

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enhancements. This technique can produce MHNs for both PMCs and CMCs as explained in this chapter. Due to the novelty of this area and needs for multidisciplinary skills, expertise, and facilities in both nanoresin and nanoforest technologies, little to no work has been reported in this area to date. However, since this is a fundamentally different category than others in the MHNs flowchart introduced here (see Figure 15.1) and opens a new area of research in super-performing MHNs, it is presented in a separate section to stress its importance as a separate MHN category.

15.6  CONCLUSIONS The automotive and aerospace industry face many challenges, including increased global competition, the need for higher-performance vehicles, a reduction in weight and costs, and tighter environmental and safety requirements. Ultimately lighter and stronger/stiffer materials mean lighter vehicles and hence lower fuel consumption and lower emissions. Currently, advanced composite materials, including FRPs, reinforced thermoplastics, and carbon-based composites, are designed, processed, and utilized in vehicles [1]. In addition, composite materials are becoming more important in the construction of aerospace structures as well. New generation large aircrafts are designed with all composite fuselage and wing structures, and the repair of these advanced composite materials requires an in-depth knowledge of composite structures, materials, and tooling. The primary advantages of composite materials are their high strength, relatively low weight, and corrosion resistance [5]. However, conventional composite materials in the past have traditionally suffered from two main issues: first, brittleness of the matrix, and second susceptibility to delaminations. If these two primary issues can be resolved to a large extend, the applications of composite materials in automotive and aerospace industries will substantially increase. It is shown in this chapter that it is possible to resolve composites’ brittleness issues to a large extend using nanoresin technology explained here to improve the toughness of the resin and hence composite systems. If the matrices in composites can be toughened, the fracture toughness and damage tolerance of composites will increase, resolving the brittleness issue, and hence cracks initiations and growths in the matrices can be prevented or delayed leading to higher performance composites. It is also shown in this chapter that it is possible to resolve composites’ delamination issues to a large extend using nanoforest technology explained here. In a conventional composite material, in-between the layers are filled by the matrix and hence is the weakest link in composites and susceptible to delaminations. If a nanoforest “reinforcement” is placed in-between the composite layers, then the weakest link will also be reinforced and hence the crack initiation and growth in-between the layers will be eliminated or delayed to eliminate or improve the delamination susceptibility of the composites. Therefore, this chapter explains how the two main issues in composites, i.e., the rather brittleness of the matrices/resins and the composites’ laminates susceptibility to delaminations can be resolved by two technologies explained here. The resin

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brittleness can be resolved by proper use, integration, and processing of nanomaterials within a resin system to produce high-performance nanoresin. The delamination issues of composite materials can be resolved by proper use, integration, and processing of CNTs onto the fibers (or in-between the fiber layers) to produce nanoforest fibers, and then the combination of these nanoforest fibers with nanoresins’ matrices can produce super-performing MHNs that could potentially solve the brittleness and delamination issues in composites. Of course, the large-scale industrial productions of such super-performing nanocomposites are still under research and development; however, it is expected that the development of MHNs with their advantages in resolving the brittleness and delamination issues of conventional composites to a large extent as well as their multifunctionality in improving mechanical, thermal, electrical, chemical, and other performances of the resulting nanocomposites will expand the applications of such nanocomposites in automotive and aerospace industries. Figure 15.1 summarizes the development of MHNs. This chapter first explains the developments of nanocomposites (nanoresins) where a matrix material is reinforced by various nanomaterials such as nanoparticles, nanoclays, nanofibers, nanotubes, and/ or nanosheets. Second, it explains the developments of hierarchical nanocomposites, where the nanoresins, instead of pure resins, are used for the manufacture of composite materials. Third, it explains the developments of MHNs, where the fiber structure is reinforced by the in situ growth of CNTs (nanoforests) on their surfaces and the resulting CNT-grown fibers are used with a resin system to produce MHNs. In this case, alternatively, the CNT nanoforests can also be grown separately on a substrate, be removed from the substrates, and be interleaved in-between the layers of the composite laminates. Finally, multiscale MHNs are explained, where both of the composite constituent materials, i.e., the fiber and the matrix, are reinforced by nanomaterials first and are then combined to produce super-performing nanocomposites.

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