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Sensors and Materials, Vol. 32, No. 7 (2020) 2453–2462
MYU Tokyo
S & M 2274

Fabrication and Mechanical Properties of Carbon-fiber-reinforced
Polymer Composites with Lead-free Piezoelectric Nanoparticles
Hiroki Kurita, Zhenjin Wang, Hiroaki Nagaoka, and Fumio Narita*
Department of Materials Processing, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan(Received February 3, 2020; accepted February 27, 2020)

Keywords: piezoelectricity, polymer-matrix composites, bending strength, energy harvesting

Wireless sensor networks (WSNs) are one of the key factors in realizing an Internet ofThings (IoT) society. Piezoelectric materials that can convert mechanical energy into electricenergy directly are highly promising as energy-harvesting materials for supplying power towireless sensors. However, it is well known that lead zirconate titanate (PZT), which has thehighest piezoelectric performance, is harmful to the environment. Moreover, the high densityand brittleness of the piezoceramics hinder the fabrication of a long-life energy-harvesting
device. In this study, we fabricated 0-3 structure piezoelectric composite materials consistingof lead-free piezoceramic nanoparticles, epoxy resin, and carbon-fiber-reinforced polymer (CFRP)
and evaluated their mechanical properties. The maximum strain energy of the piezoelectricresin/CFRP composites was 20–40% larger than that of piezoelectric/epoxy composites. The
maximum stress and flexural modulus of the lead-free piezoelectric potassium sodium niobate(KNN) nanoparticles/CFRP composite were approximately 5–10% larger than those of thebarium titanate (BTO) nanoparticles/CFRP composite. Consequently, it is likely that better
energy harvesting performance and mechanical properties can be obtained by using KNN
nanoparticles than by using BTO nanoparticles.

1.

Introduction

The construction of wireless sensor networks (WSNs) has led to the development of theInternet of Things (IoT) society. To realize true WSNs, the method of supplying energy mustbe established. Batteries have long been utilized as the main power source of WSN nodes,
where the usage time of the node is determined by the remaining amount of energy in thebattery. Moreover, batteries must be changed or recharged, and these operations decrease theefficiency of the network and increase the cost in many applications.(1) Several strategies havebeen studied to extend the lifespan of WSNs, such as switching between active and sleep modes
to reduce energy consumption and introducing energy harvesting technology to supply power tothe nodes.(2,3) Energy harvesting technology in particular has attracted considerable attention as
an alternative method of powering nodes in wireless communication, and ways to collect energy*

Corresponding author: e-mail: narita@material.tohoku.ac.jp
https://doi.org/10.18494/SAM.2020.2820
ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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from direct force input such as vibrations and movements have been studied.(4–7) Narita andFox summarized the recent progress in piezoelectric ceramics and polymers, magnetostrictive
alloys, and magnetoelectric multiferroic composites for energy harvesting applications.(8) The
challenges presented by the materials in fabrication, characterization, modelling and simulation,and durability and reliability have also been studied.
Piezoelectric material is one of the solutions for energy harvesting, which is the directconversion of mechanical energy to electric energy.(9) There are several forms of piezoelectricmaterial, such as quartz, polymers, and ceramics. The piezoelectric ceramics (piezoceramics)
have been widely used as sensors and actuators. The piezoelectricity of piezoceramics generally
comes from their perovskite structure. In this structure, the charge in the positional relationshipwith positive or negative charge causes the generation of an electric field. In addition, thepiezoelectricity is characterized by the piezoelectric constant d33. In a one-dimensional
problem, the relationship between d33 and electric displacement D is given by
D = dσ + εE,(1)where σ is the stress, E is the electric field intensity, and ε is the dielectric constant. d33 is
the piezoelectric constant when mechanical loading and poling are in the same direction.
Piezoceramics normally have high d33. For example, lead zirconate titanate (PZT) shows
d33 = 225–590 pC/N and barium titanate (BTO) shows d33 = 191 pC/N, whereas polyvinylidene
fluoride (PVDF) shows d33 = 24–37 pC/N.(10,11) However, there are several problems regarding
the application of piezoceramics. The use of PZT with the largest d33 is restricted because of its
environmental toxicity.(12) On the other hand, the high density and brittleness of piezoceramics
result in the shortening of the device lifetime in WSNs.
To overcome these problems, two approaches have mainly been considered. The first
approach is to use lead-free piezoceramics such as BTO and potassium sodium niobate(KNN).(13,14) These piezoceramics also have a perovskite structure and show relatively
large piezoelectric constants. They have widely been considered as the main alternative
piezoceramics to lead-based ones.(15,16) Another approach is to use a combination of epoxy
resin and carbon-fiber-reinforced polymer (CFRP).(17) Epoxy resin has outstanding mechanicalproperties, thermal stability, chemical resistance, adhesion properties, and dimensional stability
arising from the highly crosslinked structures of the cured epoxy resin.(18) CFRP is a composite
material that effectively exploits the advantages of carbon fibers framed by a polymer matrix.
Its superior strength-to-weight ratio and good corrosion resistance and fatigue properties
accelerate its application in engineering, such as in reinforcing, strengthening, and retrofitting
structures.(19) In previous studies, Tanimoto combined CFRP and bulk PZT material to damp
the vibration of CFRP beams.(20) However, PZT and CFRP were bonded, not fabricated as acomposite. Saidina et al. studied a BTO/epoxy composite thin film for capacitor application.(21)Sundar et al. developed a PZT/epoxy composite that showed a maximum d33 of 0.56 pC/N whenthe volume fraction of PZT was equal to 60%.(22) Nevertheless, the fabrication and polarizationof the piezoelectric composites combined with CFRP have not yet been studied to the best of
our knowledge.

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In this study, we fabricated 0-3 structure piezoelectric composite materials consisting of leadfree piezoceramic nanoparticles, epoxy resin, and CFRP. After confirming the polarization of
the piezoelectric resin composite material via electrical testing, we evaluated their mechanicalproperties.

2.

Experimental Procedure

Figure 1 shows the fabrication process of the piezoelectric nanoparticle-dispersed epoxy
resin composites (piezoelectric/epoxy composites). KNN nanoparticles (Nippon Chemical
Industrial Co., Ltd.) and BTO nanoparticles (Nippon Chemical Industrial Co., Ltd.) were
employed as piezoelectric filler. Piezoelectric nanoparticles, bisphenol-F epoxy resin (Daido
Co., Ltd.), and hardener (Mitsubishi Chemical Co., Ltd.) were mixed for 10 min and defoamedfor 10 min. The volume fraction of piezoelectric nanoparticles was controlled to be 30 vol.% inthe final composite.
The mixture was spread on a mold and cured at 80 °C for 180 min. After curing, thespecimen was cut and polished to the dimensions as shown in Figs. 2(a) and 3(a). The specimen
was polarized using a corona poling system (ELC-01N, Element Co., Ltd.) for 30 min at
an electric field of 16 kV/mm. In addition, the poling temperatures were 75 °C for KNN
nanoparticles/epoxy and 65 °C for BTO nanoparticles/epoxy, respectively.(13,14) Poling was
performed along the thickness direction, and Au electrodes were coated on the upper and lower
surfaces using a sputtering machine (SC-701 Mk II, Sanyu Electron Co., Ltd.).
Electrical properties of the specimens were measured as shown in Figs. 2(b) and 3(b), and thepiezoelectric constant d33 and output power density W were estimated. Output power density
was calculated as

Fig. 1.

(Color online) Fabrication method of piezoelectric resin specimen.

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(a)

(b)

Fig. 2. (Color online) d33 measurement: (a) piezoelectric resin specimen dimensions and (b) measurement
conditions and equipment.

(a)

(b)

Fig. 3. (Color online) Output power measurement: (a) piezoelectric resin specimen dimensions and (b)measurement conditions and equipment.

W=

2
Vout
,
R ⋅V

(2)

where Vout is the output voltage, R is the load resistance, and V is the volume of the specimen.
Figure 4 shows the fabrication process of CFRP composites with piezoelectric resin(piezoelectric resin/CFRP composites). The piezoelectric nanoparticles/epoxy composites were
sandwiched by glass-fiber-reinforced polymer (GFRP) and CFRP laminates. CFRP laminate
was prepared by laminating CFRP prepregs in the cross-ply (0°/90°/0°/90°) format. Afterlamination, the specimen was cured at 125 °C for 45 min in an oven.
The modified small punch (MSP) test(23) and the three-point bending test were carriedout. Figure 5(a) shows the cross section of the piezoelectric resin/CFRP composite andthe piezoelectric nanoparticles/epoxy composite for comparison. The specimens had thedimensions of 10 × 10 × 1.5 mm3. Figure 5(b) shows the punch and specimen holder designed
for MSP tests. The specimen holder consisted of an upper and a lower die. A universal testingmachine (Autograph AG-50kNXD, Shimadzu Corporation) with a 5 kN load cell was usedSensors and Materials, Vol. 32, No. 7 (2020)

Fig. 4.

(Color online) Dimensions and fabrication of piezoelectric resin/CFRP composite specimen.

(a)Fig. 5.

(b)(Color online) Schematic of MSP: (a) piezoelectric resin/CFRP composite and (b) test conditions.

(a)Fig. 6.

2457(b)(Color online) Three-point bending test: (a) schematic of test and (b) equipment and specimen.

for the MSP test with the machine crosshead speed of 0.1 mm/min. The strain energies of
piezoelectric resin/CFRP composite and piezoelectric nanoparticles/epoxy composite werecompared to clarify the mechanical reinforcement resulting from the combination with CFRP.
The three-point bending test was performed to investigate the bending properties
and fracture process of piezoelectric resin/CFRP composite. The specimens were cut to
40 × 6.0 × 1.5 mm3 as shown in Fig. 6(a). The loading nose and support roller diameters were

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2.0 mm. The support span length was chosen to be 32 mm. The three-point bending test was
carried out with reference to JIS K 7017. The test condition was the 2/5 size of JIS K 7017
except for the thickness of the specimen (see Fig. 6). A universal testing machine (Autograph
AG-50kNXD, Shimadzu Corporation) was used for the three-point bending test with a machinecrosshead speed of 0.1 mm/min.

3.

Results and Discussion

Table 1 shows the density of the piezoelectric nanoparticles/epoxy specimen. The densities
of KNN nanoparticles/epoxy and BTO nanoparticles/epoxy composites were 2.34 and2.69 g/cm3, respectively. These densities were approximately half that of the bulk material,(24,25)
confirming weight saving as a result of combining piezoelectric nanoparticles and epoxy.
The piezoelectric constant d33 was measured and the results are shown in Table 2. The d33
values of KNN nanoparticles/epoxy and BTO nanoparticles/epoxy composites were 13.6 and12.3 C/N, respectively. Table 2 also shows the d33 values for the bulk of each piezoelectricmaterial.(10,20) Because the piezoelectric nanoparticles/epoxy composite has the 0-3 structure (inthis case, the piezoelectric particle is the 0-dimensional filler and epoxy is the three-dimensional
matrix), the piezoelectric properties were decreased compared with those of the bulk.
The output power density against variable resistance (1.0–10 kΩ) is shown in Fig. 7.
The optimum resistance was approximately 5 kΩ, and the output power densities of KNN
nanoparticles/epoxy and BTO nanoparticles/epoxy composites were approximately 0.606 and
0.371 μW/cm3, respectively.

Table 1
Densit y of piezoelect r ic na nopa r t icles/e poxycomposites and bulk.
Density (g/cm3)
Piezoelectric material
Composites
Bulk
KNN
2.34
4.51
BTO
2.69
6.02

Fig. 7.

Table 2
Piezoelectric constant of piezoelectric nanoparticles/
epoxy composites and bulk.
d33 (pC/N)
Piezoelectric material
Composites
Bulk
KNN
13.6
127
BTO
12.3
191

(Color online) Output power density vs resistance for the piezoelectric resin composite.

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Furthermore, the mechanical properties of piezoelectric/CFRP composites were also
evaluated. The load-displacement curves obtained by the MSP tests are shown in Fig. 8.
According to the curve, the piezoelectric resin/CFRP specimen displayed a temporary reductionin the load gradient when the displacement was approximately 0.2 mm. This decrease in thegradient was caused by the FRP layer. To elucidate the failure mechanism, the piezoelectricresin/CFRP composite was prepared again and its cross section was observed under loads of
1000, 1200 (before failure), and 1200 N (after the failure). The cross section of the specimens
is shown in Fig. 9. At the load of 1000 N, an interlaminar crack occurred at the interface

(a)

(b)

Fig. 8. (Color online) Load-displacement curve: (a) KNN nanoparticles/CFRP and KNN nanoparticles/epoxy
composites and (b) BTO nanoparticles/CFRP and BTO nanoparticles/epoxy composites.

(a)

(c)

(b)

(d)

Fig. 9. (Color online) Cross section with loads of (a) 1000 N, (b,c) 1200 N before failure, and (d) 1200 N after
failure.

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between the CFRP layers. At the load of 1200 N (before failure), small cracks were found in thepiezoelectric layer. Obviously, the two kinds of cracks extended after failure [see Fig. 9(d)].
Overall, the piezoelectric resin/CFRP composite can be utilized with the displacement
smaller than 0.2 mm. Therefore, to prove the strengthening by CFRP, strain energy wascalculated by the integration of load from the displacement of 0 to 0.2 mm. An example of theintegrated area is presented in Fig. 10. Figure 11 gives the calculated maximum strain energyof the composites. According to the results, the strain energy increased about 20–40% when
CFRP was incorporated, indicating the enhancement of the strength of the composite by CFRP.
The stress–strain curve obtained by the three-point bending test and the bending properties
are shown in Fig. 12 and Table 3, respectively. The maximum stress σf values of KNN
nanoparticles/CFRP and BTO nanoparticles/CFRP composites were approximately 854 andFig. 10. (Color online) Region used to calculatestrain energy.

Fig. 11. (Color online) Strain energies up to thedisplacement of 0.2 mm.

Fig. 12. (Color online) Bending stress–strain curve of CFRP composites.
Table 3
Bending properties of piezoelectric resin/CFRP.
Piezoelectric material
Measured physical properties
KNN
BTO
850
804
Maximum stress σ f (MPa)
1.69
1.87
Maximum strain εf (%)
Flexural modulus (GPa)
46.7
41.6

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(a)

(b)

Fig. 13. (Color online) Cross-sections of (a) KNN nanoparticles/CFRP composite and (b) BTO nanoparticles/CFRP
composite.

804 MPa, respectively. In addition, the flexural moduli of KNN nanoparticles/CFRP and BTO
nanoparticles/CFRP composites were approximately 46.7 and 41.6 GPa, respectively. There
was an approximately 10% difference between using KNN and BTO piezoelectric material.
The cross section was observed and the failure process of the piezoelectric/CFRP composite
was discussed. The images observed are shown in Fig. 13. They show that CFRP first breaks
on the compression side, and then the cracks propagate along the interface of CFRP for the
KNN nanoparticles/epoxy composite. For the BTO nanoparticles/CFRP composite, the cracks
extend from the interface to the piezoelectric layer. We believe that this is because higher
compressive stress is generated in the CFRP layer owing to the lower flexural modulus of BTO
nanoparticles/epoxy composite (see Fig. 12).

4.

Conclusion

We fabricated CFRPs with lead-free piezoelectric nanoparticles and evaluated the mechanicalproperties of these piezoelectric resin/CFRPs by performing MSP and three-point bending tests.
The maximum strain energy of the piezoelectric resin/CFRP composites was approximately
20–40% larger than that of piezoelectric nanoparticle/epoxy composites. On the other hand,
the maximum stress and flexural modulus of the KNN nanoparticles/CFRP composite wereapproximately 5–10% larger than those of the BTO nanoparticles/CFRP composite. It seems
that KNN nanoparticles contribute to the higher piezoelectric and mechanical properties to
CFRPs compared with BTO nanoparticles.

Acknowledgments
The authors greatly acknowledge the support of this work by the Japan Society for the
Promotion of Science (JSPS), KAKENHI under Grant No. 19H00733 and the Japan Science andTechnology Agency (JST), Matching Planner Program. They would also like to thank Nippon
Chemical Industrial Co. Ltd. for providing piezoelectric nanoparticles.

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