Piezoelectric characteristics of urethane composites incorporating piezoelectric nanomaterials

Journal Pre-proofs Piezoelectric characteristics of urethane composites incorporating piezoelectric nanomaterials J.S. Kim, I.W. Nam, H.K. Lee PII: DOI: Reference:

S0263-8223(19)33525-1 https://doi.org/10.1016/j.compstruct.2020.112072 COST 112072

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Composite Structures

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Please cite this article as: Kim, J.S., Nam, I.W., Lee, H.K., Piezoelectric characteristics of urethane composites incorporating piezoelectric nanomaterials, Composite Structures (2020), doi: https://doi.org/10.1016/j.compstruct. 2020.112072

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1

Piezoelectric characteristics of urethane composites incorporating

2

piezoelectric nanomaterials

3

J.S. Kima, I.W. Namb,c*, and H.K. Leea

4 5 6 7 8 9 10 11 12 13 14

a

Department of Civil and Environmental Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, South Korea b College

of Civil Engineering, Nanjing Tech University, 30 Puzhu Road(S), Nanjing, Jiangsu Province 211800, China c

School of Spatial Environment System Engineering, Handong Global University, 558 Handong-ro, Buk-gu, Pohang, Gyeongbuk 37554, South Korea

15

Abstract

16

30

In the present study, piezoelectric energy generating composites were fabricated using three types of piezoelectric nanomaterials (lead zirconate titanate (PZT), zinc oxide, and barium titanate), along with two types of urethane matrices. The piezoelectric performance of the composites was evaluated in terms of the output voltage generated by external stamping loads. Based on the output voltage results, the effects of the type of piezoelectric nanomaterials, their content ratio, and the type of urethanes on the piezoelectric performance were systematically analyzed. The PZT nanomaterial-incorporated composites showed the best piezoelectric performance followed by ZnO nanomaterials- and BaTiO3 nanomaterials-incorporated composites. Vibrathane 6060-based composites were found to exhibit a greater output voltage than the Adiprene LF 900A-based composites due to their low hardness, bashore rebound, and compressive modulus. In addition, the effect of incorporating a multi-wall carbon nanotube (MWNT) on the piezoelectric performance was examined and the MWNT addition led to an adverse effect on piezoelectric performance due to the increase in the relative permittivity and the deterioration in the flexibility of the composites.

31

Keywords: Piezoelectric composite; Piezoelectric nanomaterial; Poly-urethane; Multi-walled

32

carbon nanotube; Energy harvesting

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Submitted to Composite Structures for possible publication.

17 18 19 20 21 22 23 24 25 26 27 28 29

*

Corresponding author. Tel.: +86 131 8291 2040; E-mail address: [email protected]

1

1

1. Introduction

2

The piezoelectric effect was first discovered by French scientists, Jacques and Pierre

3

Curie in 1880 [1]. They found the two types of scientific phenomena which were later

4

referred to as direct and indirect piezoelectric effects [2]. The two types of piezoelectric

5

effects were classified depending on the direction of the conversion between mechanical

6

energy and electrical energy. Direct piezoelectric effect refers to the conversion of the applied

7

mechanical energy into electrical energy, and indirect piezoelectric effect refers to the

8

conversion of electric energy into mechanical energy.

9

The wide application of the piezoelectric characteristics fits in a variety of fields. A

10

diaphragm micro-pump with a piezoelectric device [3], a piezoelectric biosensor for the

11

immobilization of antibodies [4], laser ablation for piezoelectric applications [5],

12

piezoelectric energy harvesting [6], and structural damage detection [7] have all been

13

investigated in various fields. In particular, among these applications, piezoelectric energy

14

harvesting has attracted the attention of a number of global researchers [6, 8-12].

15

Electric energy generation in piezoelectric energy harvesting stems from the unique

16

crystal structure of piezoelectric materials embedded in the harvesters. Representative

17

piezoelectric materials are lead zirconate titanate (PZT) [8], zinc oxide (ZnO) [9], barium

18

titanate (BaTiO3) [10], zinc stannate (ZnSnO3) [11], sodium niobate (NaNbO3) [12], and so

19

forth. Based on piezoelectric materials, piezoelectric energy harvesters have been fabricated

20

in forms of film, patch, composite, and others, and the energy harvesting characteristics have

21

been examined in numerous studies.

22

Cauda et al. [13] experimentally examined piezoelectric output peak voltage of

23

PDMS composites fabricated with ZnO mirco-particles or ZnO microwires. Under 75 Hz of

24

oscillating frequency, PDMS composites exhibited approximately 2.5 V due to incorporation

2

1

of ZnO micro-particles (2 μm in diameter) of 20 ~ 40 wt% and approximately 8.5 V was

2

obtained by PDMS composites incorporating ZnO microwire of 40 wt%. The authors found

3

that the higher aspect ratio of ZnO microstructure, the larger the output peak voltage was.

4

Hao et al. [14] synthesized micro-/nano-ZnO of different shapes such as sphere, ellipsoid,

5

cube, urchin, and others by means of microfluidics-based reactors. The synthesized ZnO in

6

the diversified forms were examined in terms of piezoelectric characteristics. It showed that

7

ZnO structure having greater surface area produced higher output peak voltage. However, the

8

correlation between scale of the ZnO structure and the piezoelectric characteristics was not

9

distinct. Li et al. [15] fabricated the flexible and ultrasensitive piezoelectric composited based

10

on BaTiO3 microplatelets with P(VDF-TrEE)BT single-crystal microplatelets. The BaTiO3-

11

P(VDF-TrEE)BT piezoelectric composite generated an output voltage of up to 6.32V at

12

19.33kPa pressure. The authors also found that the output voltage increased with increasing

13

bending angle of the BaTiO3 composite, generating a maximum output voltage of 2.87V at

14

the 90 degree bending angle of the composite. Alumusallam et al. [16] investigated flexible

15

PZT-polymer composites with PZT particle. Different weight ratios of PZT particle (0.1~2

16

μm in diameter) was incorporated in polymer composites and printed on piezoelectric shoe-

17

insoles used as self-powdered sensor. An open-circuit voltage of 3V was measured under

18

compressive load of 70kg. Muralt et al. [17] investigated a micro-power generator fabricated

19

from laminated PZT thin film with a cantilever structure. Interdigitated electrodes were

20

attached to a 2μm PZT thin film on 5μm of silicon adhesive. At the optimum load impedance,

21

a voltage of 1.6V and an output power level of 1.4μW were measured at a size of

22

0.8mm×1.2mm under a load condition of 870Hz and 2g. Peigney and Siegert [18] fabricated

23

a piezoelectric energy harvester with bimorph PZT patches. The bimorph PZT patches were

24

bonded to both sides of a 40mm × 220mm × 0.8mm steel plate, and the fabricated energy

25

harvester was attached to a bridge superstructure. The piezoelectric harvester generated the

3

1

output voltage by means of vibration-induced traffic on the bridge. A power of 0.03mW and

2

an output voltage of 1.8V to 3.6V were measured under peak traffic conditions.

3

Furthermore, several studies have reported on the role of MWNT in piezoelectric

4

composites. Wang et al. [19] fabricated a nanocomposite generator using barium titanate

5

(BaTiO3) nanowires and MWNT in polydimethylsiloxane (PDMS). A maximum output

6

voltage density of 7.3V/cm2 and a stable output current density level of 3.3nA/cm2 were

7

generated from the nanocomposite. The output voltage was significantly improved by adding

8

MWNT, which revealed that MWNT can be an effective enhancer to electrical conductivity.

9

Specifically,

conductivity

was

enhanced

by

MWNT

networks

formed

in

the

10

nanocomposites, and the networks were able to reduce the resistance and improve the

11

generated voltage and current. Souri et al. [20] fabricated a nanocomposite generator using

12

ZnO nanoparticles and MWNT incorporated in polyurethane. In a foot stamping and vehicle

13

load test, the average output voltage of the specimen containing only ZnO nanoparticles was

14

greater than that of the nanocomposite generator containing MWNT. The authors discussed

15

that the performance of the MWNT-incorporated nanocomposite was inferior to the

16

nanocomposite that solely incorporated ZnO, and was ascribed to the reduced flexibility that

17

led to inhibiting the deformation of the composites. In view of the inconsistent correlation

18

between the incorporation of MWNT and the piezoelectric performance obtained in the

19

literature, the influence of MWNT on piezoelectric properties remains a subject to explore.

20

In the present study, piezoelectric energy generating composites were fabricated

21

using three types of piezoelectric nanomaterials (PZT, ZnO, BaTiO3), along with two types of

22

urethane matrices. PZT, ZnO and BaTiO3 are the most frequently used piezoelectric

23

nanomaterials in piezoelectric harvesting studies, since they exhibit high piezoelectric

24

coefficients and harvesting performance. Among various forms of piezoelectric

25

nanomaterials, particle-type piezoelectric nanomaterials (e.g. PZT, ZnO and BaTiO3)

4

1

reinforced urethane nanocomposites were fabricated in this study. The harvesting

2

performance of each of the three type nanocomposites was systematically investigated. In

3

particular, the piezoelectric characteristics of lead-free nanocomposites containing ZnO or

4

BaTiO3 was compared with those of lead-containing PZT nanocomposites. The experiments

5

of piezoelectric harvesting under the identical manufacturing conditions and loading

6

conditions show the effect of the characteristics of the three types of piezoelectric

7

nanomaterials on the piezoelectric performance. The piezoelectric performances of the

8

composites were evaluated in terms of the output voltage generated by external stamping

9

loads. In addition, the effect of the MWNT addition on the piezoelectric performance was

10

examined.

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12

2. Experimental program

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Three types of piezoelectric nanomaterials and two types of urethane matrices were

14

used in this study to fabricate piezoelectric composites. The three piezoelectric nanomaterials

15

used were ZnO (Sigma Aldrich Co., Ltd.), BaTiO3 (Sigma Aldrich Co., Ltd.) and PZT

16

nanomaterial (IT Co., Ltd.), respectively, which are typically used in a nanocomposite

17

harvester. The two urethanes were Adiprene LF 900A and Vibrathane 6060, which are

18

proprietary products of Kangshin Industrial Co., Ltd. The properties of the nanomaterials and

19

the urethanes are provided in Tables 1 and 2, respectively. The piezoelectric nanomaterials

20

have different piezoelectric constants, and the two urethanes have different mechanical

21

properties. The piezoelectric performance capabilities were compared in relation to these

22

differences.

23

The mix proportions of the piezoelectric composite specimens are summarized in

24

Tables 3 and 4. Note that 10.32% of MOCA(4,4’-methylenebis) was added to each urethane

5

1

sample to enhance the formability. The independent variables are the types of piezoelectric

2

nanomaterials and urethanes, as well as the content of the piezoelectric nanomaterial and

3

MWNT. The following notations are used to indicate the samples: VB and AP denote the

4

Vibrathane 6060 and Adiprene LF 900A urethanes, and Z, B, P, and C denote the ZnO,

5

BaTiO3 and PZT nanomaterials and MWNT, respectively. The numbers indicate the

6

volumetric mixing ratio of the piezoelectric nanomaterials, i.e., 0.5, 1.5, and 2 times the

7

volume of the respective nanomaterial indicated in the reference (e.g., P1).

8

The specimens were prepared as shown in Figure 1(a). First, the urethane was

9

preheated at 70° C for 16 h prior to specimen preparation. Similarly, a Teflon-coated mold

10

(50 × 70 × 7 mm3) was preheated at 115° C for 1 h ahead of time to avoid the hardening of

11

the composite specimen when the mixture was decanted immediately to the mold. The

12

urethane and nanomaterial were mixed at the designated volume ratios using a mechanical

13

stirrer for 5 min and then poured into the mold. The specimens were cured at room

14

temperature for seven days. Subsequently, the specimens were demolded, and the copper tape

15

was applied to the top and bottom faces of each specimen, thereby, providing an electrode.

16

The applied copper tape was made to protrude by 4 cm for connection to a multimeter.

17

Finally, the device was coated with insulating tape.

18

A foot stamping test was conducted to measure the generated voltage from a

19

vibrational load caused by a human step applied to the specimen (Fig. 1(b)). A human body

20

load (70 kg) was repeatedly applied for 30 sec at an interval of 2 sec. To minimize the error at

21

each step, a load of the same intensity was applied throughout the test. The voltage generated

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by the specimens during the foot stamping test was recorded by a multimeter (Keysight

23

DMM 34410A). The multimeter was connected to copper tape acting as an electrode for the

24

specimens. Voltage exceeding 0.5V was chosen for analysis of the test results since 0.5V was

25

used as a reference for the threshold voltage of a complementary metal-oxide-semiconductor

6

1

(CMOS) transistor [21].

2 3

3. Results and discussion

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3.1 Effect of the type of piezoelectric nanomaterials on the piezoelectric performance

5 6 7 8 9

The efficiency of converting mechanical energy into electrical energy is determined by electrical coefficients, such as piezoelectric constant and permittivity [22]. In a general case of mechanical stress in direction p and an induced electric field in direction i, the open-circuit voltage of a piezoelectric device can be expressed as follows [6 ] (5)

𝑉 = 𝑇𝑝𝑔𝑖𝑝𝑙 𝑑𝑖𝑝

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where, 𝑇𝑝 is the mechanical stress, 𝑔𝑖𝑝 is the voltage coefficient (𝑔𝑖𝑝 = 𝜀𝑟,𝑖𝑝, in

11

which 𝑑𝑖𝑝 is the piezoelectric charge constant and 𝜀𝑟,𝑖𝑝 is the relative permittivity of the

12

transducer), and 𝑙 is the displacement due to external stress [6]. The aforementioned

13

equation implies that the generated voltage is proportional to the piezoelectric charge

14

constant and displacement between the electrodes and is inversely proportional to the relative

15

permittivity. Piezoelectric performance was affected by the mechanical characteristic of the

16

circuit, which is the piezoelectric composites in the present study. The mechanical

17

characteristics, such as stiffness, limit the displacement produced in the piezoelectric

18

composites when the composites are subjected to external forces, and the limited

19

displacement is closely related to change in piezoelectric performance. For example, if

20

stiffness is enlarged, the displacement is lowered, and it leads to the decrease in the

21

piezoelectric performance. This is due to that the magnitude of the output voltage is

22

proportional to the displacement of the composite [6]. For this reason, the effects of the

23

electrical coefficient of the piezoelectric nanomaterial and the mechanical characteristics of

24

composites on the magnitude of the output voltage were investigated.

7

1

Figure 2(a) and (b) exhibit the voltage produced by the Vibrathane 6060-based

2

piezoelectric composites. Figure 2(a) presents the voltage generated by types VB-B1, VB-Z1,

3

and VB-P1 versus the time. The measured output voltage value exhibited repeatedly changed

4

signs of the voltage in accordance with the direction of the load. A compressive load was

5

applied to the inside of the piezoelectric composites when the composite was pressed, and the

6

tensile load was produced in the piezoelectric composites when the pressure was released. A

7

positive sign of the voltage was generated as the compressive load acted on the piezoelectric

8

composite, and a negative sign of the voltage was produced as the tensile load acted on the

9

piezoelectric composite [6]. Because of this, the positive and negative signals of the voltage

10

were repeated in the figure. Other grounds for the change of the voltage sign are relevant to

11

the polarity of the piezoelectric nanomaterial randomly oriented in the urethane matrix. The

12

polarization in this piezoelectric material may lead to a different sign under pressure applied

13

on the composite when the polarity of the nanomaterial is not aligned in the composite. As a

14

result, the process described above generated a random pattern of output voltage [23].

15

In the measured values for the VB-Z1 sample, the average maximum voltage was

16

4.68V and the average output voltage was 0.96V. The average maximum voltage was defined

17

as an average value of maximum voltage peaks produced from each replicated sample, and

18

the average output voltage was defined as an average value of all output voltages produced

19

from all replicated samples. The voltage generated by the VB-Z1 sample was higher than that

20

of the composites incorporating ZnO and PF-359/E-145 urethanes examined in Souri et al.

21

[20]. Specifically, the maximum voltage of the VB-Z1 type was four times that of the ZnO-

22

incorporated PF-359/E-145 urethane composite. The difference in the piezoelectric

23

performance of the composites was attributable to the experimental environment and

24

measurement method. Few differences were found in samples containing Vibrathane 6060

25

and the PF-359/E-145 urethane when their physical properties, such as hardness and

8

1

elongation values were regarded. However, in both experiments, errors can be caused by the

2

individual experimenter and different sampling rates adopted.

3

Energy harvesting experiments showed large differences in outcomes depending on

4

the experimental environment and the method used [24]. Hence, the experimental

5

environment and the method should be identical when conducting this type of harvesting

6

experiment [25]. In the present study, the identical experimental environment and

7

measurement method were applied on all of the samples in an effort to reduce the errors and

8

ensure reliable comparisons in the output voltage of each specimen. According to the

9

measurement results from the VB-B1 sample, the average maximum voltage was 2.16V and

10

the average output voltage was 0.79V. The VB-Z1 and VB-B1 samples contained ZnO and

11

BaTiO3 nanomaterials, respectively. Due to the different piezoelectric constants between the

12

two nanomaterials, the voltage generated by the VB-B1 sample was expected to be higher

13

than that of the VB-Z1 sample. However, the average output voltage of type VB-B1 was

14

0.17V lower than that of type VB-Z1. This result stems from the difference in the flexibility

15

of the two piezoelectric composites. The flexibility of the piezoelectric composite has a

16

positive correlation with the piezoelectric performance [26]. The flexibility of type VB-Z1

17

was higher than that of type VB-B1. The BaTiO3 nanomaterial appears to bring adverse

18

effects to the flexibility of the composite. Accordingly, it can be said that the difference in the

19

flexibility between the two piezoelectric composites has a greater influence on the

20

piezoelectric performance than the difference in the piezoelectric constant of the piezoelectric

21

material.

22

Type VB-P1 is a Vibrathane-6060-based composite in which PZT nanomaterial was

23

incorporated. Among the three nanomaterial types discussed here, PZT has the highest

24

piezoelectric constant. Therefore, the VB-P1 type was expected to show the highest

25

piezoelectric performance among the three types of piezoelectric composites. According to

9

1

the measured values from the VB-P1 sample, the average maximum voltage was 6.83V, and

2

the average output voltage was 1.63V. The average maximum voltage for type VB-P1 was

3

45.94% and 216.20% higher than that of types VB-Z1 and VB-B1, respectively. The average

4

output voltage of type VB-P1 was 69.79% and 106.33% higher than that of types VB-Z1 and

5

VB-B1. The flexibility of type VB-P1 was similar to that of type VB-Z1. In other words, the

6

piezoelectric constant, which showed a difference of approximately 35 times, had more effect

7

on the difference in piezoelectric performance than the flexibility (refer to Eq.(5)).

8

Figure 2(b) shows the voltage generated by the VB-P0.5, VB-P1, VB-P1.5, and VB-

9

P2 samples over time. The piezoelectric composites were fabricated with Vibrathane 6060

10

and varied contents of PZT nanomaterial. The effect of the piezoelectric nanomaterial content

11

on piezoelectric performance was investigated by comparing the piezoelectric performance

12

outcomes of the piezoelectric composites. The output voltage generated by the piezoelectric

13

composite is expected to increase as the content of the piezoelectric nanomaterial increases

14

[27].

15

In the measured values for type VB-P0.5, the average maximum voltage was 6.49V,

16

and the average output voltage was 1.52V. Compared to the measured results of type VB-P1,

17

the average maximum voltage decreased by 4.98%, and the average output voltage was

18

decreased by 6.75%. In the measured values of type VB-P1.5, the average maximum voltage

19

was 8.02V and the average output voltage was 1.77V, whereas, in the measured values of

20

type VB-P2, the average maximum voltage was 7.5V, and the average output voltage was

21

1.92V. The average maximum voltages for the VB-P1.5 and VB-P2 samples increased by

22

17.42% and 9.81%, and the average output voltages for these two samples increased by

23

8.59% and 17.79%, respectively, compared to the results of type VB-P1. As the content of

24

the PZT nanomaterial increases, the output voltage of the piezoelectric composite tends to

25

increase.

10

1

Figure 3 displays the comparison of the piezoelectric performance of the composites

2

fabricated with the three different piezoelectric materials and the two different urethane

3

matrices when the volumetric ratio of the piezoelectric materials was fixed. As shown in the

4

figure, the best piezoelectric performance was accomplished by the composites incorporating

5

the PZT nanomaterial, and the piezoelectric performance results showed higher output

6

voltages in order of PZT, ZnO, and BaTiO3 nanomaterial-incorporated composites regardless

7

of the matrix type. Figure 4 presents the comparison of the piezoelectric performance of the

8

composites fabricated with two different urethane matrices when a single type of the

9

piezoelectric material, the PZT nanomaterial, was used, and the volumetric ratio of the PZT

10

was varied. As shown in the figure, an increase in the content of the PZT nanomaterial

11

enhanced piezoelectric performance. However, when the content of PZT nanomaterial was

12

increased from the volume ratio 1 to the ratio 2, the average output voltage increased by

13

17.79% for the piezoelectric composites based on Vibrathane 6060 and by 16.41% for the

14

piezoelectric composites based on Adiprene LF 900A. The efficiency with regard to

15

improving piezoelectric performance was low compared to an increase in the PZT

16

nanomaterial content. Therefore, it is necessary to use an appropriate amount of PZT

17

nanomaterial in a piezoelectric composite considering cost efficiency.

18

19

3.2 Effects of the type of urethane matrices on the piezoelectric performance

20

Figure 5 presents comparisons on the average output voltage of 12 pairs of sample

21

types. Each pair of the sample type incorporates an identical type of the piezoelectric

22

nanomaterial and its volumetric ratio but is fabricated with a different type of matrix. Among

23

the 12 pairs of sample types, the average output voltage of seven pairs of the piezoelectric

24

composites, where the type of the incorporated piezoelectric nanomaterial and its volumetric

11

1

ratio were varied, are shown in Figure 5(a), and the average output voltage of the other five

2

pairs of composites, where the volumetric ratio of the PZT nanomaterial and the mixing ratio

3

of MWNT were varied, are shown in Figure 5(b).

4

The average output voltage results indicated that the Vibrathane-based piezoelectric

5

composites exhibited higher average output voltages than those fabricated with Adiprene LF

6

900A. This result can be attributed to the differences in the physical properties of urethane

7

matrices. Table 2 shows the differences in the physical properties of the two urethanes.

8

Vibrathane 6060 has a lower hardness, a lower bashore rebound, and a lower compressive

9

modulus than Adiprene LF 900A. If the hardness of a matrix is low, the piezoelectric

10

composite fabricated with the matrix is relatively flexible, which leads to an enhancement in

11

piezoelectric performance. In a similar way, a low compressive modulus of a matrix leads to

12

an increase of displacement in the matrix when an identical load is applied. The increase of

13

the displacement leads to an increase in output voltage of the piezoelectric composite, which

14

means the low compressive modulus contributes to an enhancement in piezoelectric

15

performance. Moreover, if the bashore rebound of a matrix is low, the repulsive force of the

16

matrix against the external load is lowered, which means the load exerted in the composite

17

becomes larger. Due to the enlarged load exerted in the composites, the relatively low

18

bashore rebound characteristics enhance the piezoelectric performance. As a result, the

19

differences in the physical properties of urethane matrices result in variations in piezoelectric

20

performance [6, 28].

21

Figure 6 presents the experimental results of the tensile test of the piezoelectric

22

composites. During the foot stamping test, the tensile load was produced in the composites in

23

the course of releasing the load, and the tensile modulus of the composites affects

24

piezoelectric performance in that process. However, the tensile strength and modulus were

25

not included in the mechanical characteristic information of urethane. The results shown in

12

1

Figure 6 demonstrate that the tensile strength and modulus of the composites based on

2

Adiprene LF 900A were higher than those of the composites based on Vibrathane 6060. This

3

tendency was constant regardless of the content ratios of the piezoelectric nanomaterial and

4

MWNT. The higher the tensile modulus, the lower the strain for the same tensile load. In

5

other words, an increase in the modulus leads to a decrease in the displacement of

6

piezoelectric composites when the tensile load is applied. Accordingly, the output voltages of

7

the composite based on Adiprene LF 900A were lower than those of the composite based on

8

Vibrathane 6060. On the other hand, the increase in the tensile strength is interpreted as an

9

enhancement in the mechanical properties of the composites.

10

11

3.3 Effect of the MWNT addition on the piezoelectric performance

12

Experimental results on piezoelectric performance of piezoelectric composites

13

containing MWNT, which can be found in the literature, were inconsistent. This means the

14

MWNT produced positive or negative effects with regard to piezoelectric performance

15

depending on the experimental program [19, 20]. In the present study, the piezoelectric

16

performance of the piezoelectric composite was examined with regard to the contents of PZT

17

and MWNT, and the influence of MWNT on piezoelectric performance was explored on the

18

basis of the experimental results.

19

Figure 7(a) presents the voltages generated by the VB-P1, VB-P1(C2), VB-P1(C3),

20

and VB-P1(C4) samples over time. The mass of MWNT incorporated into the Vibrathane

21

6060-based piezoelectric composites with PZT nanomaterial did not exceed 0.5g in an effort

22

to prevent the deterioration of flowability of the fresh mixture. With regard to the measured

23

values for the VB-P1(C2), VB-P1(C3) and VB-P1(C4) samples, the average maximum

24

voltages were 4.47V, 1.83V, and 2.73V, and the average output voltages were 1.09V, 0.77V,

13

1

and 0.76V, respectively. Comparisons among the Vibrathane 6060-based piezoelectric

2

composites with the PZT nanomaterial revealed that the average maximum voltage of the

3

three types of piezoelectric composites containing MWNT reduced 34.55% to 73.21%, and

4

the average output voltage reduced 33.13% to 53.37%, compared to that of the control type,

5

VB-P1. These results indicate that MWNT does not contribute to an enhancement in

6

piezoelectric performance, which was also supported by the experimental results of Souri et

7

al. [17]. In addition, the average output voltage tended to decrease as the MWNT content

8

increased. The deterioration of the average output voltage was likely due to two reasons.

9

First, it can be ascribed to the relative permittivity of MWNT, which is relatively higher than

10

that of the matrix material. According to Micheli et al. [27], the relative permittivity of

11

MWNT/epoxy composites increased with increasing incorporation of MWNT. The increase

12

of the relative permittivity leads to a decrease of the voltage coefficient (refer to Eq.(5)) [6].

13

As a result, the increase in the relative permittivity results in the decrease of the generated

14

output voltage (refer to Eq.(5)). Second, it was likely due to the deterioration of flexibility of

15

the MWNT-incorporated piezoelectric composites. After completion of the fabrication of the

16

piezoelectric composites, it was found that the composites incorporating MWNT were less

17

flexible than the composite fabricated without MWNT. The reduced flexibility led to

18

inhibiting the deformation of the composites, and this, in turn, produced adverse effects on

19

the piezoelectric performance.

20

Figure 7(b) presents the voltages generated by the VB-P2, VB-P2(C1) and VB-

21

P2(C2) samples over time. The average maximum voltages of the two types of piezoelectric

22

composites which incorporated MWNT were 11.87 to 49.6% lower than that of type VB-P2,

23

and the average output voltage was 41.15 ~ 50% lower than that of type VB-P2. This result

24

also showed the adverse effect of the MWNT incorporation on piezoelectric performance.

25

Figure 8(a) and (b) present the average output voltage and maximum output voltage of the

14

1

Vibrathane 6060- or Adiprene LF 900A-based piezoelectric composites incorporating PZT

2

nanomaterial when the mixing ratio of MWNT was varied. The VB-P1(C2) and VB-P2(C2)

3

composites in Figure 8(a), which incorporated MWNT with the identical volume ratio,

4

showed a similar level of average output voltage, although the PZT nanomaterial content was

5

doubled. A similar relationship can be found in the AP-P1(C2) and AP-P2(C2) composites in

6

Figure 8(b). Accordingly, it can be said that the effect of MWNT on the piezoelectric

7

performance of the composite is more dominant than the effect of the piezoelectric material

8

on the piezoelectric performance.

9

Figure 9 shows the average noise in the output voltage of the control samples and the

10

piezoelectric composites incorporating MWNT. The average noise means the average value

11

of the ratio of the voltage less than 0.5 V to the total output voltage. 0.5V was used as a

12

reference for the threshold voltage of a complementary metal-oxide-semiconductor (CMOS)

13

transistor [18]. The average noise levels in the output voltage for the VB-P1(C2), VB-P1(C3),

14

VB-P1(C4), and VB-P1(C5) samples were reduced by 8.36 ~ 62.93% compared to that of the

15

VB-P1 sample. Specifically, the VB-P1(C3), VB-P1(C4), and VB-P1(C5) samples mixed

16

with no less than 0.3g of MWNT all showed noise reductions exceeding 55%. The average

17

noise levels in the output voltage for the VB-P2(C1) and VB-P2(C2) samples were decreased

18

by 9.37% and 62.49%, respectively, compared to that of the VB-P2 sample. In addition, the

19

reduction of average noise was demonstrated in the Adiprene LF 900A-based piezoelectric

20

composites. The average noise levels in the output voltage of the AP-P1(C3), AP-P1(C4),

21

AP-P2(C1), AP-P2(C2), and AP-P2(C3) decreased by 9.16%, 47.56%, 38.52%, 30.90% and

22

12.96% respectively, compared to the control samples. As a result, it was found that the

23

incorporation of MWNT reduces the noise in the output voltage [29].

24

Sun et al. [22] investigated the role of nano-electrical bridges of MWNT between

25

piezoelectric materials, and the investigated results were in good agreement with earlier

15

1

results, showing that a noise reduction of the output voltage was due to the MWNT

2

incorporation. MWNT acted as a nano-electric bridge between piezoelectric materials to

3

smooth the flow of electricity in the piezoelectric composite, and this led to a reduction in

4

noise. Considering that the piezoelectric composite can be utilized in sensing applications, the

5

reliability of the sensing data can be enhanced by noise reduction. Accordingly, it can be said

6

that MWNT incorporated into the piezoelectric sensor plays a role in enhancing the reliability

7

of the sensing system.

8 9

4. Concluding remarks

10

In the present study, the piezoelectric composites were fabricated using three types of

11

piezoelectric nanomaterials and two types of urethane matrices. The effects of the type of

12

piezoelectric nanomaterials, urethane matrices and the MWNT addition on the piezoelectric

13

performance were investigated. The relationship between the characteristics of each material

14

and the generated output voltage can be summarized as follows:

15 16

(1) The piezoelectric performance capabilities in relation to changes in the type and

17

contents of the piezoelectric nanomaterials of the composites were evaluated as the

18

compressive stamping load was applied. PZT nanomaterials-incorporated composites

19

showed the greatest average maximum output voltage followed by ZnO

20

nanomaterials- and BaTiO3 nanomaterials-incorporated composites. As the content of

21

the PZT nanomaterials increased, the output voltages of the piezoelectric composites

22

also tended to increase. However, the enlargement rate of the generated voltage was

23

not as high as the increment of the PZT nanomaterial.

24

(2) The effects of differences in the mechanical properties of the urethane matrices on the

25

piezoelectric performances were examined. Vibrathane 6060-based composites

16

1

showed higher generated voltages than Adiprene LF 900A-based composites due to

2

their low hardness, bashore rebound and compressive modulus.

3

(3) The effects of the MWNT addition on the piezoelectric performance of the

4

composites were investigated. As the MWNT contents increased, the output voltages

5

of the piezoelectric composites tended to decrease. The adverse effect of the MWNT

6

addition on piezoelectric performance was likely due to two reasons: the relative

7

permittivity of MWNT was superior to that of the matrix material, and the flexibility

8

of the resultant composites was deteriorated. However, the MWNT addition reduced

9

the noise in the output voltage by forming an electrically conductive network in the

10

composites.

11 12

As mentioned earlier, the same volume fraction of the three different types of

13

nanomaterials were used when mixing with urethane so that the effect of the reinforcement

14

type on the piezoelectric performance can be directly evaluated. The influences of different

15

types of matrix materials and addition of MWNT on the piezoelectric performance were

16

thoroughly investigated. The possible contribution of the MWNT addition to the reduction of

17

sensor noise was also briefly discussed.

18

19 20 21

Acknowledgments This study was supported by a grant from the National Research Foundation of Korea (NRF) (2018R1A2A1A05076894) funded by the Korean government.

22 23

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