11

Multifunctional materials and nanotechnology for assessing and monitoring civil infrastructures

K.J. Loh and D. Ryu,    University of California-Davis, USA

Abstract:

Multifunctional materials, or material systems intentionally encoded with a multitude of engineering functionalities, are revolutionizing civil engineering practice by offering new materials that can resist loads and self-sense for damage. These new developments have been made possible by technological breakthroughs in the nanotechnology domain. New nanomaterials with impressive properties and scalable manufacturing methods have enabled the design of these multifunctional materials or composite structures. The objective of this chapter is to showcase three types of nanotechnology-enabled multifunctional materials that have impacted, and will continue to impact, the civil engineering community. Particular attention will be paid to materials assembled using two unique nanomaterials, namely carbon black and carbon nanotubes.

Key words

carbon nanomaterials; fiber-reinforced polymer composite; multifunctional; nanocomposite; smart concrete; sensing

11.1 Introduction

Multifunctional materials, or material systems intentionally encoded with a multitude of engineering functionalities, have been regarded as potential candidates for revolutionizing the current state-of-practice in structural health monitoring (SHM). Unlike traditional technologies that only perform a specific task, multifunctional systems are tailored to simultaneously perform combinations of sensing, actuation, energy harvesting, mechanical reinforcement, thermal exchange, and energy dissipation, among many others.1 For instance, conventional coatings seek to achieve multifunctionality by stacking individual functional layers to obtain a multilayered architecture.2 An example is using separate coatings for thermal insulation, resisting wear, and corrosion protection of aircraft components.3–5 On the other hand, functionally graded materials are being considered as alternatives to address issues of compatibility and adhesion between disparate layers,6–8 although it remains challenging to incorporate drastically different functionalities in one material. In contrast, the new domain of multifunctional materials seeks to encode diverse performance attributes in what is often a homogeneous structure.9–12

Technological breakthroughs over the last few decades in the nanotechnology domain have brought forth a plethora of new materials, tools, and methods that have enabled the assembly of revolutionary multifunctional materials.11,13 Nanomaterials such as quantum dots, nanoparticles, fullerenes, nanotubes, and nanowires offer drastically unique material properties as compared to their bulk counterparts. On another front, the atomic force microscope, scanning tunneling microscope, and focused ion beam lithography, to name a few, have permitted imaging and manipulation of individual atoms and molecules. In essence, nanotechnology offers new ways for isolating, controlling, and assembling nanostructured morphologies for attaining high-performance functional systems.14 ‘Bottom-up’ assembly by building materials starting at molecular length scales to achieve precise properties in bulk-scale systems is now possible.

The remainder of this chapter is organized into five sections. Section 11.2 begins with the introduction of two types of nanomaterials that will be the focus of this chapter, namely carbon black (CB) and carbon nanotubes (CNT). The next three sections discuss how these carbon-based nanomaterials can be integrated in different composite architectures to improve structural performance and to encode strain sensitivity or piezoresistivity. Strain sensing is of focus here, because these measurements are critical for SHM. Localized large strain measurements point to the possibility of excessive deformation or cracks, or measured strains can be directly correlated to stress fields in structural components. Section 11.3 showcases how carbon nanomaterials can be incorporated to form smart concrete or cementitious composites (e.g., mortar, cement mixtures, and reinforced concrete) as next-generation civil infrastructure materials. Section 11.4 is on fiber-reinforced polymer (FRP) composites, which are used primarily for aerospace, naval, and specialized civil structures (e.g., column retrofits, wind turbine blades, etc.). Section 11.5 focuses on nanocomposite coatings or thin films that can be applied onto structural surfaces. Finally, the chapter concludes with a brief summary and discussion of future trends in Section 11.6.

11.2 Properties of carbon nanomaterials

Among the diverse selection of nanomaterials, CB and CNT have attracted considerable attention in academic research and commercial developmental realms. First, CB is produced by the partial combustion of hydrocarbons.15 While they are similar to soot, the main difference is that CB exhibits greater order, often characterized by a spherical shape with diameters ranging between 100 nm and 1 μm. Its physical morphology represents that of randomly oriented graphitic layers aggregated to form a spherical structure. It is this unique microstructure that also provides CB with interesting material properties that have motivated their use for various applications. For instance, CB density ranges from 2.05 to 2.11 g-cm− 3, and electrically, they are also semiconducting.15,16 Their unique properties, wide availability, and relative low costs have promoted their use in applications such as improving the tear resistance in automobile tires, printer/copier toner pigments, and UV protection for plastics, among others.15,16

Second, CNTs are specific examples within a new class of nanomaterials characterized by nanometer dimensions with superior intrinsic electronic, mechanical, chemical, thermal, and optical properties that greatly differ from bulk carbon.17,18 It is their spectacular properties that have motivated many to take advantage of single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT) for various applications.19 In short, SWNTs physically comprise a single rolled sheet of graphite (or graphene) to form a cylindrical structure, and MWNTs are typically characterized by SWNTs stacked together concentrically.19 An illustration of an SWNT is shown in Fig. 11.1. The diameter of a typical SWNT ranges between 0.4 and 2.0 nm, and they also possess high aspect ratios (greater than 104) that are ideal for surface functionalization.19 Mechanically, SWNTs’ Young’s modulus (E) and their ultimate tensile strength (σf) have been estimated to be E ≈ 1.1 TPa and σf ≈ 75 GPa, respectively; for MWNTs, they are E ≈ 1.2 TPa and σf ≈ 150 GPa.20 The electrical properties of SWNTs can be either metallic or semiconducting depending on chirality (i.e., the direction of rolling the graphene sheet).,21 Khare and Bose22 and Kang et al.23 have compiled comprehensive reviews of CNT synthesis methodologies and their impressive material properties. Despite the fact that CNTs have only been recently discovered (in 1991 by Iijima18), they are readily available and can be purchased from companies around the world. Table 11.1 summarizes some of the different types of nanotubes (as well as their properties) offered by select companies (note that this is only a small subset of all companies that sell CNTs).

image
11.1 An illustration of a single-walled carbon nanotube shows that it physically represents a single sheet of graphene rolled to form its cylindrical structure. (Source: Image provided courtesy of Dr James Hedberg.)

Table 11.1

The properties of various types of carbon nanotubes sold by different companies

CompanyHelix Material Solutions (TX, USA)Nanocyl (Belgium; MA, USA)Cheap tubes (VT, USA)Unidym (CA, USA; Japan)Nanostructured andAmorphous (TX, USA)SouthWest NanoTechnologies (OK, USA)
CNT typeSWNTSWNTMWNTSWNTMWNTSWNTMWNTSWNTSWNTSWNTMWNTSWNTMWNT
Product # or grade1HP 2ARC - NC1100 NC3100 SKU-0101 SKU-030102 3R 4P 1283YJ 1204YJ SG65i SMW100
Synthesis technique 5CVD ARC CVD 6CCVD  CCVD  7HIPCO  CCVD  8CoMoCAT®  
Outer diamete (nm) ≤ 1.3 1.2 10 2 9.5 1 8 0.8 0.8 1 8 0.76 6
  ~ 1.5 ~ 30   ~ 2 ~ 15 ~ 1.2 ~ 1.2 ~ 2 ~ 15 ~ 0.78 ~ 9
Length (μm) 0.5 0.5 0.5 Several 1.5 5 10 0.1 0.1 5 10 - 1~13
 40 ~ 3 ~ 40   ~ 30 ~ 50 ~ 1 ~ 1 ~ 30 ~ 50   
Specific surface area (m2-g− 1) 300 300 40 1000 N/A 407 233 1000 1000 360 180 – –
 ~ 600 ~ 600 ~ 300       ~ 400 ~ 240   
CNT purity ((%)) 90 50 95 70 90 90 95 65 85 90 95 95 98
Price (US $ /10 g) 1600 700 190 – – 975 75 5500 7700 350 170 4500 50

Image

Note: The information contained in this table is obtained from each company’s website or by contacting a sales représentative. The data is accurate as of November 2012.

1HP: High purity;

2ARC: Arc discharge growth;

2R: Regular;

4P: Purified;

5CVD: Chemical vapor deposition;

6CCVD: Catalytic chemical déposition;

7HiPCO: High pressure carbon monoxide conversion;

8CoMoCAT®: CO catalytic disproportionation with Mo.

In order to harness the impressive material properties offered by nano-materials such as CBs and CNTs at macro-length scales, these materials have been incorporated in devices such as nanoelectronics,24 ultra-strong nanocomposites,25,26 and various sensors,27,28 among others.21 However, incorporation of nanomaterials is met by two primary challenges, namely the dispersion of them into solution and the assembly of homogeneous percolated nanocomposites. Dispersion of nanomaterials is difficult due to their hydrophobic nature and strong van der Waals force interactions that promote aggregation and clumping.29 Many studies have demonstrated success with dispersing nanotubes via covalent surface modification (e.g., oxidation and functionalization)30 and steric stabilization (i.e., using surfactants, DNA, or organic solvents).31 Even with CNTs dispersed in solution, the second challenge is to incorporate them effectively in composite architectures for its intended application. The remainder of this chapter illustrates how CBs and CNTs have been used for various composite architectures, namely cementitious-based composites in Section 11.3, fiber-reinforced polymer (FRP) composites in Section 11.4, and thin film coatings in Section 11.5.

11.3 Cementitious-based composites

To date, one of the most widely adopted construction materials for bridges, buildings, dams, and roadways, among others, is concrete.32 In general, concrete exhibits excellent compressive strength but cannot withstand substantial tensile loads and deformation, which can result in sudden catastrophic failure due to its brittle nature.33 Although steel reinforcement bars provide enhanced tensile load carrying capacity for this composite structure, the cementitious matrix still undergoes large cracks during applied tensile loads. By embedding fibers that can bridge these large cracks, fiber-reinforced concrete (FRC) benefits from enhanced ductility and strain capacity.34 Further improvements to this technology have been achieved by the development of high-performance fiber-reinforced cementitious composites (HPFRCC) or engineered cementitious composites (ECC).35,36 Similar to concrete, the material is fabricated from a mixture of cement, silica, sand, and water but with the addition of small amounts of steel, glass, carbon, or polymeric fibers.37 Designed from a theoretical micromechanics principle,38 these HPFRCC and ECC materials can undergo significant strain hardening via the formation of multiple and distributed micro-cracks due to fiber internal friction, deformation, and pull-out mechanisms.39 The end result is improved mechanical and life-cycle properties.

Cementitious materials are inherently multifunctional. Not only are they used for carrying load, they are also piezoresistive by nature (i.e., their electrical properties change in response to applied strain).12,40,41 However, their inherent piezoresistive strain sensitivity (or gage factor) is fairly low; their electrical properties do not change significantly in response to even large applied strains. Thus, by embedding conductive fillers such as fibers or particles within cement-based materials, experimental research by Chung42 and theoretical calculations by Garas and Vipulanandan43 have shown that cementitious composites such as FRC possess enhanced piezoresistivity. Hou and Lynch44 have also validated the self-sensing performance of ECC composed of short poly(vinyl alcohol) (PVA) fibers. In general, Garas and Vipulanandan43 report that the FRC’s gage factor can double when the conductive filler weight concentration is increased from 3% to 6%. A study conducted by Chung42 also explores how different concentrations and types (i.e., steel or carbon) of short and continuous fibers modify the material’s inherent piezoresistivity. Most of these aforementioned studies have focused on using macro-sized fillers/fibers. It has been envisioned that nanomaterials embedded in cementitious composites can further enhance mechanical and sensing properties to achieve next-generation smart concrete structures.

11.3.1 Smart concrete fabrication and nanomaterial dispersion

The fabrication of smart concrete (i.e., ones with embedded nanomaterials) is similar to conventional concrete manufacturing procedures and will be described in this section. The main ingredients are cement, water, sand/silica, and the nanomaterial of choice. First, Portland cement and sand/silica are mixed in a rotary mixer. Second, the appropriate amount of water is measured in a separate container; typical water-to-cement ratios used range from 0.22 to 0.60.45–48 Then, CB, or CNTs can be dispersed in water, or they can be added in their dry state to the cement-sand/silica mixture. Regardless, cement, sand/silica, water, and nanomaterials are then mixed thoroughly in a mixer for several minutes. The mixture is then poured into oiled molds and left to cure for 24 h. Finally, the specimens are demolded and cured in a humid environment, typically for 28 days. These specimens are often dried in an oven if smart concrete specimens are to be tested in the laboratory.

Three different methods can be used to disperse CBs or CNTs in water. First, Manzur and Yazdani46 have mixed MWNTs in water by hand, followed by subjecting the mixture to 30 min of bath ultrasonication to disperse the nanotubes. A second alternative is to dissolve a water-reducing or superplasticizer agent in water, adding the nanomaterial, followed by the manual mixing or sonication of the solution to achieve dispersion.48–50 The third technique is to disperse nanomaterials in dilute aqueous surfactant solutions, again via ultrasonication. For example, Han et al.45 have used sodium dodecylbenzene sulfonate (NaDDBS), Yu and Kwon51 have used sodium dodecyl sulfate (SDS) and Wille and Loh48 have employed poly(sodium 4-styrenesulfonate) (PSS) as dispersing agents. It should be mentioned that this technique often also requires the addition of a defoamer or superplasticizer. However, the dispersion or the separation of carbon nanomaterials into its individual units is extremely difficult to accomplish.52 They tend to aggregate and agglomerate due to their high surface energies and strong van der Waals forces.29 Issues with regards to dispersing CNTs in cement matrices have been reported by Wansom et al.53

11.3.2 Strain sensing

To date, several works have been accomplished to enhance the inherent piezoresistivity of cement-based materials by the addition of CBs49,54 and CNTs.41,53 In fact, Li et al.49 have shown that the inherent electrical resistivity of cementitious composites can decrease by two orders of magnitude when the volume fraction of CB is increased from 3% to 14%. In addition, the percolation threshold has been estimated to be between 7% and 11%, and tunneling is suspected to be the primary mechanism that enhances the composite’s electrical conductivity and piezoresistivity. Similar enhancements in electrical conductivity and strain/pressure sensing performance have been obtained by Li et al.; 41 instead of using CB, 0.5 wt.% of untreated and treated CNTs (i.e., chemically oxidized) have been incorporated within the cementitious matrix. It has been determined that the high strain sensitivity of the composite is due to: nanotube’s inherent piezoresistive behavior,55 number of nanotube-to-nanotube junctions, and the effect of field emission during mechanical load testing.41 Han et al.45 have proposed the use of smart concrete modified with 0.1 wt.% MWNT for traffic monitoring applications such as vehicle detection, weigh-in-motion, and speed measurements (Plate VII in the color section between pages 294 and 295). The technology has been implemented in a test roadway, and sensor readouts are noticeably different depending on the size of the vehicle driven over the self-sensing concrete (i.e., passenger vehicle versus mini-van). Figure 11.2 shows two resistance time histories as measured from the self-sensing concrete, and it is clear that the size and number of vehicles can be differentiated based on these measurements. Saafi56 has shown that strain measured by smart concrete can also be interrogated wirelessly using a commercial off-the-shelf system. These studies show that this technology is at the cusp of field and commercial adoption, provided that larger-scale and long-term studies are conducted to assess their viability over time.

image
11.2 Han et al.45 have measured the resistance time history of a self-sensing CNT/cement composite embedded in a test roadway. The sensor has successfully detected the (a) passing of two mid-sized passenger vehicles and (b) a mini-van. (Source: Image provided courtesy of IOP)

11.3.3 Mechanical reinforcement

In addition to the enhancement of electronic properties, carbon nanomaterials have also been studied with the goal of improving the mechanical response of cementitious composites. Early investigations have suggested that significant improvements in the mechanical properties (i.e., compressive strength and failure strain) of MWNT-reinforced cement may be difficult to achieve.57 However, this is likely due to poor dispersion of nanotubes. Musso et al.50 have studied how different types of MWNTs (i.e., as-grown, annealed, and functionalized) in a concentration of 0.5 wt.% in plain cement can influence the composite’s bulk compressive properties. It has been found that the modulus-of-rupture and compressive resistance decreased significantly when functionalized nanotubes have been used, which is primarily due to their strong hydrophilic nature and thus limited cement hydration. On the other hand, annealed and pristine MWNTs show enhancements in mechanical properties.50 Similar conclusions have also been reached by Manzur and Yazdani,46 where higher water-to-cement ratios are needed for proper hydration and strength enhancement. It has also been observed that 0.3 wt.% MWNTs and smaller-sized MWNTs provide greatest compressive strengths based on the results obtained from a parametric study.

11.4 Fiber-reinforced polymer composites

FRP composites fabricated from epoxy-bonded carbon-, aramid-, or glass-fiber laminates are widely adopted for various engineering applications, particularly for aerospace/naval vessels and civil infrastructure repair, upgrade, retrofit, or new construction.58–60 Their low density, high strength-to-weight ratio, corrosion resistance, cost effectiveness, and conformability make them ideally suited for these applications.60–62 For example, FRP-retrofitted concrete beams exhibit a 46–90% strength increase (as well as stiffness enhancements) as confirmed by laboratory validation studies.63 Examples of real-world implementations of FRP jacketing and new constructions are witnessed worldwide, including the Palazzo Elmi-Pandolfi historical building (Foligno, Italy),58 the Interstate-80 State Street Bridge (Salt Lake City, UT),64 and the Kings Stormwater Channel Composite Bridge’s FRP-based bridge deck (Riverside County, CA),59 and others.65 In addition, the American Composites Manufacturers Association (ACMA) reports that 256 FRP composite civilian bridges (i.e., 121 pedestrian and 135 vehicular bridges) are in service as of 2003.66 Besides civil structures, recent aerospace vessels (e.g., Airbus A350, Boeing 787, and F-35) and wind turbines blades have up to half of their structural weight fabricated from carbon- or glass- FRP composites.67

Despite the performance improvements offered by FRPs, these materials are susceptible to damage such as matrix cracking, interlaminar fracture, delamination, and debonding that have the potential to cause catastrophic failure.68,69 Similarly, FRP-retrofitted structures have experienced premature failure prior to attaining their theoretical strength.61 In fact, numerous sources are responsible for FRP damage, including excessive loading (e.g., wind and earthquakes), fatigue, environmentally induced deterioration, material defects, and improper construction.70,71

One approach to using carbon fiber-reinforced polymer (CFRP) composites as a multifunctional material for SHM is to monitor its changes in electrical properties due to damage formation. In fact, in addition to being a structural material, CFRPs have been shown to be piezoresistive.72–76 Change in resistance in response to applied strain is usually linear, although it has been shown by Ogi73 that transverse cracking can cause deviations to this linear response. Delamination between layers can also cause changes in CFRPs’ electrical resistance.77,78 Many studies have also been conducted for detecting fatigue damage.79–81 Measurements of electrical resistance or conductivity can be obtained in-plane or through-the-thickness of the composite, and the orientation of measurements can be used as a way to distinguish between the different possibilities of damage occurring in the CFRP structure.

11.4.1 FRP manufacturing

FRP composites are most commonly manufactured using two techniques: namely wet layup and vacuum-assisted resin-transfer molding (VARTM). In short, wet layup begins by putting an initial layer of peel ply on a mold. A first layer of fabric is then placed in the mold, and an adhesive is spread evenly until the fabric is fully wetted. The adhesive used usually consists of an epoxy resin mixed with a hardener (at an appropriate volume ratio). Then, the procedure of laying the fabric and applying epoxy/adhesive is repeated until the desired number of layers has been reached. On the other hand, VARTM FRP manufacturing follows a slightly different procedure. Here, the fabric is layered based on the desired orientations, order, and number of layers. The layered fabric is then transferred and sealed into a vacuum bag, and resin infiltration takes place. The flow of adhesive through-out the panel and bag is assisted by the use of a vacuum, for example, based on the procedure by Govignon et al.82 In general, VARTM permits greater control and less variability from sample to sample, but wet layup is pre-dominantly used for civil structures. Both manufacturing techniques are illustrated in Fig. 11.3.

image
11.3 The (a) wet layup and (b) VARTM FRP composite manufacturing techniques are illustrated.

Carbon nanomaterials, primarily CNTs, are introduced in FRP composites via two techniques: either as an additive to the epoxy/polymer matrix or as a thin film within or coated on the surface of the composite. This section will focus on the former case, and thin films will be discussed in Section 11.5. A popular method for dispersing nanotubes in the epoxy resin is by the manual mixing of CNTs with the resin. Then, the mixture is subjected to intense shear mixing using a three-roll mill, as has been used by Gojny et al.83 and Thostenson and Chou.84 The gap between the rolls can be adjusted from a larger spacing to a smaller gap during processing to facilitate nanomaterial dispersion. Once adequate suspension has been achieved, the nanomaterial-enhanced resin is then combined with the curing agent, heat-treated, and degassed in a vacuum oven.84

11.4.2 Improving FRP electromechanical properties

To date, several studies have attempted to enhance the electromechanical performance of FRPs, mainly by employing nanomaterials as conductive fillers in the epoxy matrix. A state-of-the-art overview of nanomaterial-modified FRP composites can be found in Loh and Azhari.85 An earlier study conducted by Gojny et al.83 and Wichmann et al.86 have successfully showed that a 0.3 wt.% amino-functionalized double-walled CNT-modified epoxy resin-based glass fiber reinforced polymer (GFRP) possessed improved ultimate strength (σf ≈ 67 MPa), Young’s modulus (E ≈ 2.94 GPa), and fracture toughness (KIC ≈ 0.92 MPa-m1/2); anisotropic electrical conductivity has also been verified, but its piezoresistivity has not been explored. An extension to these studies by Boger et al.69 has explored strain sensing by VARTM-manufactured CFRPs, where their epoxy matrices have been modified with CBs and CNTs. The results are promising and have illustrated that CFRPs’ electrical resistance changes in tandem with applied incremental tensile strains, but an irreversible non-linear electrical drift exists. Figure 11.4 shows a set of representative strain sensing results obtained by Thostenson and Chou.84 On the other hand, Anand and Mahapatra87 have used models and experiments to show that CNT/epoxy composites that also incorporate CBs are characterized by non-linear piezoresistive properties under quasi-static loading.

image
11.4 Thostenson and Chou84 have reported that FRPs modified with CNTs exhibit great promise for strain sensing and SHM. The overlay of the resistance and applied strain time histories validate the FRPs piezoresistive performance. (Source: Image provided courtesy of IOP)

Besides sensing, Rajoria and Jalili88 have shown that CNT/epoxy composites also possess improved energy dissipation, where MWNT/epoxy composites are characterized by damping ratios up to 700% higher than their pristine epoxy-based counterparts. CNT/epoxy composites have also been demonstrated for actuation applications, as has been discussed by Yun et al.89 Instead of using dispersed CNTs, Abot et al.90 have proposed the use of CNT-wound thread sensors for strain monitoring. These CNT threads are wound from as-grown CNT forests to form 15–30 μm-thick threads, which are also coated with a dielectric insulation layer. The CNT threads exhibit linear piezoresistivity after the application of 0.3% pre-strains. They have been stitched into CFRP composites and have been validated for delamination and crack detection.90

11.5 Polymer-based thin films

Unlike smart concrete or FRPs where CBs and CNTs can be incorporated within structural materials, these nanomaterials have also been investigated for creating advanced structural coatings or thin films. In many cases, nano-materials are embedded in polymer matrices to form conformable coatings that can be applied onto structural surfaces, whether it is for concrete, steel, FRPs, or other types. The thin film’s properties can be controlled by judiciously selecting the type of nanomaterial, polymer matrix, nanomaterial-matrix interaction, and fabrication procedure.91 In particular, this section will present an overview of how electromechanical sensing capabilities can be encoded in these polymer-based thin films for civil infrastructure monitoring.92 Specific attention will be on CNTs, simply due to the extensive amount of research and applications in this area.

11.5.1 Thin film fabrication

Numerous thin film fabrication techniques exist and have been employed for assembling CNT-enhanced structures. For example, extensive work has been conducted on using dispersed CNT solutions for vacuum filtration and then dried to form films called ‘buckypaper.’ The electrical and mechanical properties of selected works have been summarized in Tables 11.2 and 11.3, respectively. It should be mentioned that Tables 11.2 and 11.3 are not meant to be an exhaustive literature review, but rather just an overview of the wide variety of published work. In addition, buckypapers have been tested for strain sensing, and Dharap et al.93 and Kang et al.94 have reported their linear piezoresistivity up to 500 με.

Table 11.2

The electrical properties of CNT buckypapers

References 1F-CNT-P Fabrication technique Addition Further treatment CNT concentration Conductivity (S·m− 1)
122 2EBA-3FWNT Filtration – – – 29 400
123 4O-SWNT-5A Filtration – – 0.1 wt./vol% 30 000
  Filtration – 10 M HNO3 0.1 wt./vol% 12 000
124 SWNT-6p Filtration 9SDS Chemical treatment in SOCl2 – 70 000 350 000
124 O-SWNT-A/7H Filtration 9SDS – – 24 000
  Filtration  Chemical treatment in SOCl2 – 96 000
125 8C-MWNT-A Filtration – – 0.086 wt./vol% 10 000
  Filtration 10PEO Soaking  2630
  Filtration 11PVA Vacuum filtration for 24 h 0.086 wt./vol% 826
126 SWNT-p Filtration 12PS – 22% 16 Mf 100
  Filtration PS – 100% Mf 9000
127 MWNT Filtration 13IC – 6% 17 Vf 2880
 SWNT Filtration IC – 17.9% Vf 11 700
128 SWNT Filtration Triton X-100 – 0.05 wt./vol% 24 700
  Filtration 14DNA – 0.05 wt./vol% 30 600
  Filtration Chitosan – 0.05 wt./vol% 29 000
129 SWNT-p Filtration SOCl2 Soaking - 300 000
  Filtration Aniline 15DI rinsing - 2400

Image

1F-CNT-P: functionalization-type of CNT-purification;

2EBA-: 4-ethoxybenzoic acid;

3FWNT: few-walled carbon nanotube;

4O-: oxidized;

5-A: acid-treated;

6-p: purified;

7-H: heat-treated;

8C-: carboxylated;

9SDS: sodium dodecyl sulfate;

10PEO: polyethylene oxide;

11PVA: poly(vinyl alcohol);

12PS: polystyrene;

13IC: biopolymer t-carrageenan;

14DNA: deoxyribonucleic acid;

15DI: deionized water;

16Mf: mass fraction in composite;

17Vf: volume fraction in composite.

Table 11.3

The mechanical properties of CNT buckypapers

References 1F-CNT-P Fabrication technique Addition Further treatment CNT
concentration		 Tensile
strength
(MPa)		 Young’s
modulus
(GPa)		 Exte
(%)
122 2EBA-3FWNT Filtration – – – 80 15 –
123 4O-SWNT-5A Filtration – – 0.1 wt./vol% 10 0.8 5.6
  Filtration – 10M H2NO3 0.1 wt./vol% 74 5 3
124 SWNT-6p Filtration 10SDS – – 11 0.66 1.7
  Filtration SOCI2 – – 37 0.95 4.3
130 O-SWNT-A Filtration – – – 10.5 1.5 –
  Filtration – E-beam – 80 3.5 –
131 SWNT Filtration SDS – 0.01 wt./vol% 15 – 1.15
 7C-SWNT-A Filtration SDS – 0.01 wt./vol% 24 – 1.16
125 C-MWNT-A Filtration 11PEO Soaking 0.086 wt./vol% 3.5 0.323 –
  Filtration 12PVA Vacuum filtration for 10 h 0.086 wt./vol% 96.1 6.23 –
132 C-SWNT-A Filtration – – – 10 1.2 2
133 SWNT-8H Filtration – – – 6.29 2.3 –
  Filtration PVA Soaking – 57 6.9 –
127 MWNT Filtration 13IC – 6% 16Vf 7.2 1.415 1.2
  Filtration  – 12% Vf 23.9 2.665 3.4
128 SWNT Filtration Triton X-100 – 0.05 wt./vol% 16 4 –
  Filtration 14DNA – 0.05 wt./vol% 76 3.3 –
  Filtration Chitosan – 0.05 wt./vol% 149 3.4 –
134 SWNT Filtration Water Soaking 0.1 wt.% 15.1 1.32 –
  Filtration 15BMI.Bf4 Soaking 0.1 wt.% 1.9 0.28 –
135 9VA CNT Shear pressing Epoxy Soaking 27% Vf 300 15 –
    Soaking /5% stretched 27 %Vf 402 22.3 –

Image

1F-CNT-P: functionalization-type of CNT-purification;

2EBA-: 4-ethoxybenzoic acid;

3FWNT: few-walled carbon nanotube;

4O-: oxidized;

5-A: acid-treated;

6-p: purified;

7C-: carboxylated;

8-H: heat-treated;

9VA: vertically aligned;

10SDS: sodium dodecyl sulfate;

11PEO: polyethylene oxide;

12PVA: poly(vinyl alcohol);

13IC: biopolymer t-carrageenan;

14DNA: deoxyribonucleic acid;

15BMI.Bf4: 1-butyl-3-methyl-imidazolium tetrafluoroborate;

16Vf: volume fraction in composite.

Besides buckypapers, examples of other fabrication methods include polymer casting,95 epoxy molding,26 spin coating,96 layer-by-layer (LbL),97 evaporation,98 and spraying,99 among many others.100 For instance, Kang et al.94 have assembled SWNT/polymethyl methacrylate (PMMA)-casted films. Quasi-static and dynamic cantilevered beam tests have been conducted, and the results show that these films are capable of monitoring strains while possessing high gage factors up to ~ 5. This study has also been extended for crack detection and corrosion monitoring using a sprayed-on CNT-neuron thin film.99 Even higher strain sensitivities (up to ~ 15) have been achieved by Pham et al.95 Tables 11.4 and 11.5 summarize the electrical and mechanical properties of CNT/polymer-based nanocomposites. Many of these studies also describe strain sensing using these thin films. More detailed reviews of CNT-based nanocomposite sensors can be found in Sinha et al.,101 Mahar et al.,102 and Li et al.103

Table 11.4

The electrical properties of carbon nanotube-polymer nanocomposites

References 1F-CNT-P Fabrication Polymer CNT concentration Electrical conductivity (S-m− 1)
136 2O-SWMT-3A Coagulation 4PMMA 0% 10 Wf 1 E-13
    2% Wf 0.01
137 MWMT Epoxy infusion Exposy 3266 0% Wf 1E-12
    10% Wf 3527
    10% Wf 7715
    17% Wf 13 084
138 MWMT Hot pressing/mold casting 5NR 0 11 phr 4E+14 (Ω-cm)
    10 phr 1000 (Ω-cm)
   6SBR 0 phr 3E+13 (Ω-cm)
    10 phr 1000 (Ω-cm)
   7EPDM 0 phr 2E+13 (Ω-cm)
    10 phr 1E+5 (Ω-cm)
139 MWMT Solvent evaporation 8PS 0% 12 Vf 1E+12 (Ω/ent)
    2.49% Vf 1000 (Ω/ent)
127 MWMT Solvent evaporation 9IC 2.29% Vf 36
    4.66% Vf 82
    7.75% Vf 558
    6.65% Vf 420
    5.79% Vf 260
 SWNT Solvent evaporation  3.18% Vf 0.057
    6.45% Vf 25
    10.5% Vf 118
    8.94% Vf 82
    7.90% Vf 6
139 MWMT Spin casting PS 0% Vf 1E+12 (Ω/ent)
    2.49% Vf 1000 (Ω/ent)

Image

1F-CNT-P: functionalization-type of CNT-purification;

2O-: oxidized;

3-A: acid-treated;

4PMMA: poly(methylmethacrylate);

5NR: natural rubber;

6SBR: styrene-butadiene rubber;

7EPDM: ethylene propylene diene monomer;

8PS: polystyrene;

9IC: biopolymer t-carrageenan;

10W4: weight fraction in composite;

11phr: parts per hundred parts of rubber;

12Vf: volume fraction in composite.

Table 11.5

The mechanical properties of carbon nanotube-polymer nanocomposites

References 1F-CNT-P Fabrication Polymer CNT Concentration Tensile strength (MPa) Young’s
modulus
(GPa)		 Extensibility
(%)
137 MWNT Epoxy infusion Epoxy 3266 0%Wf 89 2.5 5.2
    17% Wf 231 20.4 1.2
140 CNT Epoxy infusion 15BMI – 2082 169 –
 4E-CNT   – 3081 350 –
138 MWNT Hot pressing/Mold casting 16SBR 0 26phr 1.08 0.0005 326
    10 phr 6.3 0.00494 149
141 5B-MWNT 13LbL 17PEI/18PAA – 150 4.5 0.0375
 6H-MWNT   – 110 2 0.045
142 7C-SWNT-A 13LbL 19PSS/20PDDA 70% 27 Vf – 460 –
143 SWNT 13LbL PSS/PVA 0.50 mg-mL− 1 180 10.4 2.12
144 8f-MWNT-A Mold casting PMMA 0%Wf 29.65 – 1.13
    1.5% Wf 52 – 0.9
145 9F-SWNT-10HO Roll casting 21PEO 0%Wf 3.5 0.0595 –
    1%Wf 10 0.147 –
146 11n-MWNT-12p Solvent evaporation 22CPP 0% Vf 13 0.22 –
    0.6% Vf 49 0.68 –
139 MWNT Solvent evaporation 23PS 0% Vf 19.5 1.53 –
    2.49% Vf 30.6 3.4 –
127 MWNT Solvent evaporation 24IC 2.29% Vf 15 0.859 4
    5.79% Vf 39.5 2.602 6.5
147 MWNT Solvent evaporation Morthane 0% Vf 2.1 0.013 1.19
    10.2% Vf 5.2 0.09 1.08
139 MWNT Spin casting PS 0% Vf 19.5 1.53 –
    2.49% Vf 26 3.37 –

Image

2O-: oxidized;
3-A: acid-treated;
14PMMA: poly(methylmethacrylate);

1F-CNT-P: functionalization-type of CNT-purification;

4E-: epoxide;

5B-: bamboo;

6H-: hollow;

7C-: carboxylated;

8f-: functionalized;

9F-: fluorinated;

10-HO: H202;

11n-: n-butyl lithium;

12-p: purified;

13LbL: layer-by-layer;

15BMI: bismaleimide resin solution;

16SBR: styrene-butadiene rubber;

17PEI: poly(ethyleneimine);

18PAA: poly(acrylic acid);

19PSS: poly(sodium 4-styrenesulfonate).

In general, most of these film fabrication methods assemble nano-composites characterized by a random percolated morphology of CNTs. Exceeding the percolation threshold is critical since CNTs enable electrical conductivity while also reinforcing bulk film mechanical properties, as will be shown later in this section. Two representative scanning electron microscope images of LbL-based films are shown in Fig. 11.5. Both these images show that only individual or at least small bundles of nanotubes are deposited during nanocomposite assembly.97,104 Due to the diversity of research in this area, the remainder of this section will focus on CNT-based films fabricated using LbL.

image
11.5 Scanning electron microscope images of the (a) surface and (b) cross-section of a LbL SWNT-based thin film are shown.

11.5.2 Layer-by-layer (LbL) fabrication

A schematic illustrating the LbL self-assembly process is shown in Fig. 11.6, and the fabrication procedure is described in this section.97,105 First, a cleaned charged substrate such as glass, silicon, metal, or polymer is immersed in a positively charged solution such as PVA. Electrostatic and van der Waals force interactions drive the adsorption of PVA onto the substrate surface. After several minutes, the substrate and its adsorbed PVA layer is removed from the solution and then rinsed with deionized water and dried with air, thereby completing the assembly of the first monolayer. Next, the substrate is immersed in a negatively charged solution, such as CNTs dispersed in PSS solution. As mentioned in Loh et al.,97,106 SWNT-PSS dispersions are achieved by subjecting the mixture to bath sonication and high-energy tip sonication.97,106 Adsorption of CNTs and PSS onto the previous monolayer is again based on electrostatic and van der Waals forces. The film is then rinsed and dried, and this procedure completes the fabrication of one bilayer. The procedure is repeated multiple cycles for fabricating homogeneous nanocomposites. Films can be left on the substrate, or chemical etching using hydrofluoric acid can be performed to obtain freestanding nanocomposites. These films are denoted as (A/B)n, where A and B are the film constituents (e.g., SWNT-PSS and PVA), and n is the number of bilayers.

image
11.6 A schematic of the LbL fabrication method for assembling CNT-PE thin films is presented.

11.5.3 Mechanical properties of LbL nanocomposites

Stress–strain testing has revealed that the average modulus of elasticity, ultimate tensile strength, and ultimate failure strain for LbL (SWNT-PSS/PVA)200 thin films are approximately 10.4 GPa, 180 MPa, and 21 200 με, respectively. As compared to the stress–strain performance of films without the addition of CNTs, these SWNT-based nanocomposites exhibit an order of magnitude improvement in bulk Young’s modulus and ultimate tensile strength.9 Although the ultimate failure strain is significantly lower than its pure polymeric counterpart, they remain fairly flexible. Zhao and Loh107 have also shown that additional mechanical property enhancements can be achieved by adjusting post-fabrication parameters, such as by performing thermal annealing.

11.5.4 Strain sensing

Characterization of thin film piezoresistivity has been performed by applying tensile-compressive cyclic loads to (SWNT-PSS/PVA)n specimens while simultaneously measuring their electrical resistance (or resistivity) using a two-point probe method. The resistance time history response of a representative thin film strain sensor subjected to multiple cycles of tensile-compressive loading to ± 5000 με is shown in Fig. 11.7. It can be clearly seen that the nanocomposite exhibits piezoresistive behavior. In addition, film resistance increases in tandem with increasingly applied strain, and the opposite is true when the films are subjected to compression. Similar results are also obtained for films fabricated with different CNT and polyelectrolyte (PE) concentrations and number of bilayers.97,106 However, closer inspection of these results show that their electromechanical responses are distorted and show rounded peaks, despite the fact that a saw-tooth cyclic load pattern has been applied. The resistance time history shown in Fig. 11.7 actually approaches that of a sinusoidal signal. It is possible that measurement of bulk film time-domain resistance is insufficient since the films also possess inherent capacitance to cause such distortions. Their capacitive nature has been identified through electrical impedance spectroscopic studies conducted by Loh et al.106 A parallel resistor–capacitor equivalent circuit model has been derived for modeling these thin films’ electromechanical properties.

image
11.7 The resistance time history response of an SWNT-PE LbL thin film subjected to a 10-cycle tensile-compressive load pattern to ± 5000 με is overlaid with the applied load pattern.

11.5.5 ‘Sensing skins’ for spatial damage detection

Structural damage such as cracks, impact, and delamination are inherently localized phenomena that are difficult to detect and monitor using global-based techniques such as modal analysis. Conventional sensors (such as strain gages and accelerometers) only measure structural response parameters at discrete and instrumented locations. Detecting the presence, magnitude, and location of damage thus requires instrumenting a dense network of these discrete transducers and combining them with methods of interpolating between sensor nodes. Often, a dense sensor network may be cost prohibitive, and the estimation of structural response over large spatial domains may be inaccurate.

In this section, the LbL CNT-PE thin films described in Section 11.5.2 are used as the platform for assembling multifunctional sensing skins capable of direct spatial damage detection and strain mapping. While other types of skin-like sensors have been developed and have been summarized by Loh and Azhari,85 this section will showcase one particular technology. Here, spatial sensing is enabled by employing the SWNT-PE thin films, instrumenting them with a set of boundary electrodes, and then applying a technique known as electrical impedance tomography (EIT).108 The EIT algorithm utilizes boundary current input and voltage output measurements for reconstructing the spatial conductivity distribution of the film (or any conductive or semi-conductive body).108,109 Due to the impracticality of obtaining a continuum spatial conductivity distribution and an applied boundary potential function, solving the EIT inverse problem relies on using a finite element model (FEM) of the film.110 The FEM model utilizes discrete elements for describing the film’s spatial conductivity distribution, and each element possesses constant properties (i.e., in this case, electrical conductivity). The true spatial conductivity of the sensing skin is determined by solving this inverse problem in an iterative fashion. During each iteration, the electrical conductivity of each FEM element is updated, and the model’s boundary voltage estimations are compared to experimental ones. The inverse problem continues until the differences between the numerical and experimental results are within a predefined error threshold. The entire EIT procedure is illustrated in Fig. 11.8.111

image
11.8 A schematic illustrating the procedure for solving the EIT inverse problem for spatial conductivity mapping is shown.111 (Source: Image provided courtesy of Springer.)

EIT is advantageous, as compared to a dense sensor instrumentation strategy, because sensing skin measurements are only obtained along the film boundaries. This eliminates the need to physically probe every location on the film surface, or the need for instrumenting a dense network of transducers. Instead, the only requirement is that the film can be coated or painted onto the structural surface of interest (or embedded within structural materials like FRP). Since thin film conductivity (or resistivity) is calibrated to strain or damage other phenomena,9,97,106 the spatial conductivity maps are directly related to spatial damage in the structure. The end result is a 2D strain map (or stress map) of the structure. It has been shown that the EIT-estimated spatial conductivities are within 2% error, as compared to two-point probe experimental measurements of the same film and spatial region.112

Loh et al.111 have assessed the performance of sensing skins for identifying the severity and location of impact damage on metallic structures. LbL (SWNT-PSS/PVA)50 thin films have been coated onto aluminum plates. An impact apparatus has been used to impart different degrees of damage (i.e., permanent deformation) at various locations on the skin-coated plate (Plate VIIIa in the color section between pages 294 and 295). Using EIT, the sensing skin conductivity maps have been obtained and are shown as a 2D strain map in Plate VIIIb or as a 3D contour plot in Plate VIIIc. These results clearly show dramatic changes in film electrical conductivity at their points of impact. The relative change in conductivity (as compared to its baseline undamaged case) also decreases in tandem with the severity of impact. Based on these results, it is clear that this technology is viable for detecting the location and severity of structural damage. Other work has also been conducted for using this technique for crack damage detection in cementitious composites,113 corrosion formation on steel specimens,114 and for sensing pH changes on film surfaces.112 Based on these results, this sensing skin technology has the potential for field and commercial adoption.115

11.6 Conclusion and future trends

It is evident from the discussions in this chapter that nanotechnology-enabled multifunctional materials are entering the commercial and field implementation domains. Since the discovery of CNTs in 1991, multiple industries have developed and advanced technology to manufacture these nanomaterials at high throughput and low costs. Decades of intensive basic research have allowed this field of study to mature in a relatively short amount of time. In fact, one can find a plethora of issued patents related to nanotechnology and multifunctional systems. Agencies such as the US Department of Defense has active projects investigating the use of nanomaterials for enhancing warfighter capabilities, and these projects are often conducted by joint partnerships between academia and industry. Another example is that sprayed CNT/polymer-based thin films are being integrated with GFRPs for in situ sensing and damage detection;116,117 they will be integrated with wind turbine blades and validated as a next-generation multifunctional composite material through a study sponsored by the US National Science Foundation.118

There is no doubt that as some of these existing multifunctional nano-composite systems make their way into practice, more advanced systems will continue to be developed. An area with tremendous room for technological innovation is in the domain of biologically inspired sensing systems. Such bio-inspired technologies extend beyond mimicking biological sensing functionalities, but the focus is to learn how nature assembles and organizes to form those sensory features that outperform the manmade counterparts. A review by Bar-Cohen119 has highlighted advancements in sensor technologies inspired by the various biological senses. Another example is the development of photosynthesis-inspired structural coatings by Ryu and Loh.120 Unlike conventional sensors that require a constant power supply for operations, the coating is photoactive and can generate electrical energy in response to broadband light illumination (Plate IXa in the color section between pages 294 and 295). The magnitude of current also changes in tandem with applied strains (Plate IXb), thereby making them ideally suited for remote monitoring of civil structures (i.e., for applications where a constant energy supply may be difficult to achieve). All in all, these different bio-inspired sensors that are currently still in their research phases have the potential to revolutionize and find applications for civil SHM/damage detection in the very near future,121

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