Nanocomposite Sensing Skins for Distributed
Structural Sensing
J.P. Lynch, K.J. Loh, T.-C. Hou, and N. Kotov1
Abstract. The operational safety of civil engineered structures can be jeopardized
by structural deterioration (e.g., corrosion) and damage (e.g., yielding, cracking).
Structural health monitoring has been proposed to provide engineers with sensors
and algorithms that can detect structural degradation in a timely manner for costeffective correction. In this paper, a thin film material engineered at the nanoscale is proposed for distributed sensing of metallic structures. Assembled from
single wall carbon nanotubes (SWNT) and polymers, the thin film’s electrical
properties are designed to change in response to external stimulus such as strain or
tearing. Electrical impedance tomographic (EIT) conductivity imaging is adopted
to make a measurement of the film conductivity over its complete surface area.
The end result is a true distributed sensor offering engineers with impressive twodimensional maps from which strain and damage can be observed in fine detail.
1 Introduction
During the past decade, an infusion of expertise and high-technologies from related engineering domains have dramatically improved the state-of-art in sensors
and actuators used for monitoring and controlling large-scale civil structures.
Concurrent to these advances has been an ongoing revolution in the emerging field
J.P. Lynch
Department of Civil and Environmental Engineering, Universtiy of Michigan
e-mail: [email protected]
K.J. Loh
Department of Civil and Environmental Engineering, Universtiy of California Davis
e-mail: [email protected]
T.-C. Hou
Department of Civil and Environmental Engineering, University of Michigan
e-mail: [email protected]
N. Kotov
Department of Chemical Engineering, University of Michigan
e-mail: [email protected]
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of nanotechnology [1]. The transcendent advances of the nanotechnology field
introduce a paradigm shift in how the next generation of smart structures will be
designed. Specifically, it is now possible to design materials with specific macroscopic mechanical, electrical and chemical properties by controlling structure and
assembly at the nano-scale [2].
This paper explores the design of a novel thin film assembled at the nano-scale
using single wall carbon nanotubes (SWNT) and polyelectrolytes (PE) to create a
homogenous SWNT-PE composite with superior mechanical strength and with
electrical conductivities that change in response to stimulus (in this paper, strain
and tearing). This multifunctional material can therefore sense the stimulus everywhere the material is; hence, it is capable of true distributed sensing. To unleash
the distributed sensing capabilities of the thin film, electrical impedance tomography (EIT) is explored. EIT inversely solves for the distribution of film conductivity using only electrical measurements made along the film boundary. To highlight
the film’s functionality as a distributed sensor, laboratory experiments are made using the skin to detect strain fields and cracking in steel plates.
2 Multifunctional Nanocomposite Films
While individual carbon nanotubes (single and multi wall) undoubtedly offer impressive mechanical and electrical properties, they must be processed to offer
macroscopic materials endowed with similar properties. To date, the processing
of SWNT to attain desired macroscopic material properties (e.g., strength, bulk
conductivity) has been a major challenge. Early approaches used vacuum filtration of an aqueous solution of suspended SWNT to form a thin carbon nanotube
mat on filtration paper. Unfortunately, this “buckypaper” is brittle and incapable
of high strain due to the weak van der Waals interaction between the nanotubes
[3]. In response to this limitation, polymer-carbon nanotube composites have
been proposed. These composites have only shown moderate strength enhancements when compared to other carbon fiber composite materials [4-6] due to a
lack of uniform nanotube connectivity throughout the polymer matrix.
In this study, sequential layering of chemically-modified SWNT and polymers
is proposed to fabricate a homogenous polymer-carbon nanotube composite [7].
The layer-by-layer (LBL) approach offers SWNT-polyelectrolyte (SWNT-PE)
composites with outstanding phase integration and homogeneity [7; 8]. LBL assembly entails the dipping of a solid substrate (e.g., glass or silicon) in solutions of
the individual components (in this study, SWNT and polyelectrolytes). First,
SWNT are dispersed using non-covalent methods. A high molecular weight
polyelectrolyte, poly(sodium 4-styrene-sulfonate) (PSS) is non-covalently bonded
to the surface of individual SWNT providing them with an overall negative
charge. Similarly, a positively charged solution of poly(vinyl alcohol) (PVA) is
prepared as a conjugate polymeric material for the LBL assembled composite. A
substrate is then prepared by cleaning its surface using a piranha solution. LBL
Nanocomposite Sensing Skins for Distributed Structural Sensing
(a)
(b)
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(c)
Fig. 1 (a) SWNT-PE film on glass slides; (b) SEM image of a 200-layer film; (c) resistance
time history under a saw-tooth +/- 5000 με load pattern (100 cycles) [9]
assembly begins by dipping the treated substrate for 5 minutes in the PVA solution. Next, the substrate is rinsed using deionized water and dried. After a PVA
monolayer is deposited, the substrate is dipped in the SWNT-PSS solution to deposit negatively charged SWNT to the surface of the positively charged PVA
monolayer. This process is repeated to build up films (Fig. 1a) of any number of
bilayers. The resulting film is referred to as (SWNT-PSS/PVA)n where n is the
number of bi-layers. The film can be either left in place or released into its freestanding form using etchants such as hydrofluoric acid. When viewing the film
using a scanning electron microscope (SEM), it is evident that SWNT are uniformly distributed in the film with a well interdigitated morphology (Fig. 1b).
The mechanical properties of SWNT-PE thin films have been extensively
tested in the laboratory to characterize their stress-strain properties. The homogenous distribution of SWNT in polyelectrolyte matrices allows the strength of the
SWNT to be transferred to the composite. Tensile strain testing of multiple
(SWNT-PSS/PVA)n thin film specimens reveal tensile strengths of more than 250
MPa with Young’s modulus of 10 GPa or greater [9]. In addition to incredible
strength, the film is also inherently piezoresistive. In general, SWNT-composite
materials exhibit a piezoresistive behavior under applied strain which motivates
their use as strain sensors [10; 11]. Prior work fundamentally explored a methodical approach to optimizing the fabrication parameters of the SWNT-based film to
derive high strain, high gage factor strain sensors [12]. Parameters such as the
SWNT dispersive agent, polyelectrolyte conjugate pair, dipping time, annealing
process, among other parameters have been varied to produce SWNT-PE LBL
films exhibiting linear changes in conductivity under high strain levels (∈ > 4%)
with high gage factors (GF) in excess of 5. For example, Fig. 1c presents the
measured change in resistivity of a SWNT-PSS/PVA thin film axially loaded in
tension; the resistivity change is reversible and nearly linear.
The beauty inherent to sensing-based multifunctional materials is that measurement of the material conductivity (which in this case is correlated to strain) can
be made anywhere the material is. Therefore, such materials intrinsically offer the
capability of distributed sensing. Distributed sensing is realizable if the material
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can be probed repeated over its full area to develop a conductivity map. Repeated
probing is labor intensive and rules out its use in an automated system. In this
study, conductivity mapping will be conducted in an indirect manner through the
use of electrical impedance tomography.
3 Electrical Impedance Tomography
The development of EIT evaluation represents a major step-forward in the further
development of nanoengineered materials for distributed sensing. EIT is essentially an inverse problem intended to estimate the spatial distribution of conductivity of a body based on boundary electrical measurements (i.e., voltages) made
during stimulation (i.e., regulated current injection) of the boundary. When current is applied to a conductive thin film material, the flow of electrical current
can be described by the 2D Laplace vector equation:
∇ ⋅ [σ( x, y )∇φ( x, y )] = − I ( x, y )
(1)
where σ is the material conductivity, φ is the electrical potential (voltage), and I is
the applied current at a point source. The two in-plane dimensions of the thin film
are designated by the position variables, x and y. Conductivity, σ, measures how
easy it is for electrical current to flow normal to two faces of a unit volume of material. Two boundary conditions are specified for the Laplace equation including
the Neuman (the sum of current crossing the film boundary, S, is zero) and
Dirichlet (electric potential v along S is equal to the internal potential, φ, at the
boundary) conditions [13]. If the conductivity distribution, σ(x,y), is known, the
internal electrical potential, φ(x,y), can be found from a known current across the
film boundary; this approach is often termed the forward problem [13]. In contrast, the inverse problem attempts to find a mapping of conductivity, σ(x,y), based
upon voltage measurements at the boundary based on a regulated applied current
(DC or AC). However, this inverse problem is ill-posed and requires a set of
boundary measurements corresponding to multiple applied current distributions.
(a)
(b)
(c)
Fig. 2 (a) Impact test apparatus; (b) four impacts upon the sensing skin coated plate element; (c) corresponding EIT conductivity map with percentage change in conductivity
shown [9]
Nanocomposite Sensing Skins for Distributed Structural Sensing
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In recent years, a number of researchers have proposed effective solutions to
the non-linear inverse problem providing the tools necessary to perform accurate
EIT. In the study reported herein, the inverse problem is solved using a finite
element (FE) model of the thin film for repeated solution of the forward problem
[14]. An iterative Gauss-Newton algorithm is used for modification of finite element conductivity until convergence. In this case, convergence is defined by
minimization of the difference of the experimentally obtained boundary voltage,
vexp, and the boundary voltage predicted by each solution of the FE forward
model, φFE.
4 Experimental Validation
To validate the distributed sensing properties of the nanocomposite thin film, a
simple set of experiments in which impacts are used to introduce damage are
described. A100-layer (SWNT-PSS/PVA)100 thin film is deposited on a primercoated thin steel plate (110 mm by 110 mm and 0.75 mm thick) to produce a large
structural specimen to which damage can be introduced. After deposition of the
sensing skin, 32 copper electrodes are attached to the plate along its boundary with
8 electrodes on each of the four sides. The sensing skin-coated steel plate is then
clamped into an impact apparatus in which a pendulum is used to impact the plate.
The apparatus (Fig. 2a) can control the amount of energy imparted to the plate by
changing the height from which a sharp-tipped weight is dropped. Damage is introduced in two forms: permanent residual deformation and plate/skin penetration.
The plate is impacted four time with controlled energy input of 0.09, 0.38,
0.81, 1.17 J. The four impacts are sequentially numbered in terms of the energy as
shown in Fig. 2b with the lowest energy given the index “i”. Evident in Fig. 2b is
the residual deformation of the plate with locations ii, iii, and iv clearly dented. At
location iv, the plate is mildly cracked due to excessive deformation. As shown in
Fig. 2c, the EIT-derived conductivity map clearly identifies the location and extent
of “damage” introduced in the plate. The percentage change in conductivity
(when comparing before and after) is correlated to the degree of damage.
5 Conclusions
A powerful, new nanocomposite assembled from SWNT and PE has been proposed as a distributed sensing skin. Controlled of the assembly of the composite
at the nano-scale allows for the development of a multifunctional material with
impressive mechanical properties and self-sensing functionality. This work
largely explored the piezoresistive properties of the SWNT-PE sensing skin. In
particular, EIT was adopted to provide a means of automated spatial conductivity
mapping. Conductivity maps are capable of presenting the spatial distribution of
strain and deformation in a structure as illustrated using a simple impact test on a
steel plate. With sensing skins in their infancy, more work is needed to refine the
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skin for field use. Current work is also exploring the embedment of other sensing
mechanisms including the sensing of corrosion using the SWNT-PE sensing skin.
Acknowledgments. This research is supported by the National Science Foundation (NSF)
under grant number CMS-0528867 (program manager: Dr. S. C. Liu). The authors would
also like to express their gratitude to NSF and Prof. Jeffrey Schweitzer (University of Connecticut) for sponsoring the travel of the authors to the NICOM3 conference.
References
1. Chong, K.P.: Nanotechnology in civil engineering. Adv. Struct. Eng. 8, 325–330
(2005)
2. Nalwa, H.S.: Nanostructured Materials and Nanotechnology. Academic Press, San
Diego (2002)
3. Kang, I., Heung, Y.Y., Kim, J.H., Lee, J.W., Gollapudi, R., Subramaniam, S., Narasimhadevara, S., Hurd, D., Kirikera, G.R., Shanov, V., Schulz, M.J., Shi, D., Boerio,
J., Mall, S., Ruggles-Wren, M.: Introduction to carbon nanotubes and nanofiber smart
materials. Compos. Part B-Eng. 37, 382–394 (2006)
4. Shaffer, M.S.P., Windle, A.H.: Fabrication and characterization of carbon nanotube/poly(vinyl alcohol) composites. Adv. Mat. 11, 937–941 (1999)
5. Haggenmueller, R., Gommans, H.H., Rinzler, A.G., Fischer, J.E., Winey, K.I.:
Aligned single-wall carbon nanotubes in composites by melt processing methods.
Chem. Phys. Lett. 330, 219–225 (2000)
6. Watts, P.C.P., Hsu, W.K., Chen, G.Z., Fray, D.J., Kroto, H.W., Walton, D.R.M.: A
low resistance boron-doped carbon nanotube-polystyrene composite. J. Mater.
Chem. 11, 2482–2488 (2001)
7. Mamedov, A.A., Kotov, N.A., Prato, M., Guldi, D., Wicksted, J., Hirsch, A.: Molecular design of strong SWNT/polyelectrolyte multilayers composites. Nat. Mat. 1, 190–
194 (2002)
8. Decher, G.: Fuzzy nanoassemblies toward layered polymeric multicomposites. Science 277, 1232–1237 (1997)
9. Loh, K.J.: Development of multifunctional carbon nanotube nanocomposite sensors
for structural health monitoring. Ph.D. Thesis, University of Michigan, Ann Arbor, MI
(2008)
10. Dharap, P., Li, Z., Nagarajaiah, S., Barrera, E.V.: Nanotube film based on single-wall
carbon nanotubes for strain sensing. Nanotechnology 15, 379–382 (2004)
11. Kang, I., Schulz, M.J., Kim, J.H., Shanov, V., Shi, D.: A carbon nanotube strain sensor
for structural health monitoring. Smart Mater Struct. 15, 737–748 (2006)
12. Loh, K.J., Kim, J.H., Lynch, J.P., Kam, N.W.S., Kotov, N.A.: Multifunctional layerby-layer carbon nanotube-polyelectrolyte thin films for strain and corrosion sensing.
Smart Mater. Struct. 16, 429–438 (2007)
13. Barber, D.C.: A review of image reconstruction techniques for electrical impedance
tomography. Med. Phys. 16, 162–169 (1989)
14. Hou, T.C., Loh, K.J., Lynch, J.P.: Spatial conductivity mapping of carbon nanotube
composite thin films by electrical impedance tomography for sensing applications.
Nanotechnology 18, 315501 (2007)