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공학석사학위논문

High Performance Acoustic Actuator of

Urchin-like ZnO Nanostructure/

Polyvinyldienefluoride/Graphene Assembly

나노 성게모양 징크옥사이드와

폴리비닐리덴플로라이드를 이용한 박막형 음향기

2016년 2월

서울대학교 대학원

화학생물공학부

정 욱 재

i

Abstract

High Performance Acoustic Actuator of

Urchin-like ZnO Nanostructure/

Polyvinyldienefluoride/Graphene Assembly

Oug Jae Cheong

School of Chemical and Biological Engineering

The Graduate School

Seoul National University

A bass frequency response enhanced flexible PVDF based thin film

acoustic actuator was successfully fabricated. High concentration of various

zinc oxide (ZnO) was embedded in PVDF matrix, enhancing the β phase

content and the dielectric property of the composite thin film. ZnO acted as a

nucleation agent for the crystallization of PVDF. A chemical vapor deposition

ii

(CVD) grown graphene was used as electrodes, enabling high electron

mobility for the distortion free acoustic signals. The frequency response of the

fabricated acoustic actuator was studied as a function of the film thickness and

filler content. The optimized film had the thickness of 80 μm with 30 wt%

filler content, and showed 72% and 42 % frequency response enhancement in

bass and midrange, respectively. Also, the total harmonic distortion decreased

82 % and 74 % in the bass and midrange regions, respectively. Most of all, it

is demonstrated that acoustic actuator performance is strongly influenced by

degree of PVDF crystalline.

Keywords: Thin Film Acoustic actuator, Zinc oxide, PVDF,

Crystallinity, dielectric constant

Student Number: 2014-20586

iii

List of Abbreviations

ZnO: Zinc Oxide

PVDF: Polyvinylenefluoride

PMMA: Poly(methylmethacrylate)

DMF: Dimethylformamide

APS: (3-aminopropyl)trimethoxysilane

PEDOT:PSS: poly(3,4-diethylenedioxythiophene)-poly(styrenesulfonate)

CVD: Chemical vapor deposition

FE-SEM: Field-emission scanning electron microscopy

TEM: Transmission electron microscopy

HRXRD: High resolution X-ray diffraction

POM: Polarized optical microscopy

FT-IR: Fourier-transform infrared

AFM: Atomic force microscopy

THD: Total harmonic distortion

iv

List of Figures

Figure 1. An illustration of the thin film acoustic actuator consistng of

ZnO urchin/PVDF with graphene electrodes.

Figure 2. A photograph of the thin film acoustic actuator consistng of

ZnO urchin/PVDF with graphene electrodes.

Figure 3. (a) HR-TEM (inset: Raman spectrum) and (b) AFM of

fabricated graphene electrode.

Figure 4. Low and high magnification of SEM images for fabricated (a)

urchin-like and (b) nanorods ZnO. (c) XRD patterns to investigate the

crystal structure of the ZnO nanofillers.

Figure 5. Polarized optical microscopy image of (a) Pristine PVDF

film, PVDF hybrid films with (b) ZnO rod, and (c) ZnO urchin by

isothermally crystallized at 170 °C and maintained for 240 s then

cooled down to observe crystal growth morphology.

Figure 6. IR spectrum of pristine PVDF film and different types of

ZnO with PVDF thin films.

Figure 7. Permittivity and loss factor of fabricated thin films

Figure 8. Cole-Cole fitting curves by permittivity verse dielectric loss.

v

Figure 9. (a) The frequency responses and (b) total harmonic distortion

of different PVDF-based thin film loudspeakers.

Figure 10. The frequency response as a function of film thickness.

Figure 11. Relative efficiency of the actuators performance compare to

pristine PVDF acoustic actuator at 100 Hz as a function of filler content.

vi

List of Tables

Table 1. β phase content, F(β), of commercial, pristine, ZnO rod

embedded, and ZnO urchin embedded PVDF thin films.

Table 2. Dielectric constant and relevant dielectric parameter for

pristine PVDF and ZnO/PVDF composite thin films.

vii

Contents

Abstract...….............................................................................i

List of Abbreviations...…......................................................iii

List of Figures...….................................................................iv

List of Tables...…..................................................................vi

Contents...….........................................................................vii

Chapter 1. Introduction.........................…..............................1

1.1. Piezoelectric acoustic actuator................................................1

1.2. Piezoelectric materials.............................................................2

1.3. Nanofiller composites..............................................................4

1.4. Objective of this study............................................................5

Chapter 2. Experimental.........................…............................6

2.1. Materials..................................................................................6

2.2. Synthesis of urchin-like ZnO..................................................7

2.3. Synthesis of rod ZnO..............................................................7

2.4. Preparation of ZnO/PVDF films.............................................7

2.5. Fabrication of CVD graphene.................................................8

2.6. Fabrication of ZnO/PVDF acoustic actuator..........................9

viii

2.7. Frequency response and THD testing of fabricated

film speaker.............................................................................9

2.8. Vocal receiving test as film microphone...............................10

2.9. Characterization....................................................................10

Chapter 3. Results and Discussion...….................................12

3.1. Fabrication of ZnO/PVDF film.............................................12

3.2. Characterization of ZnO........................................................17

3.3. Crystallinity of various ZnO/PVDF films.............................19

3.4. Permittivity and loss factors of various

ZnO/PVDF films...................................................................24

3.5. Performance of ZnO/PVDF thin film acoustic actuator.......30

Chapter 4. Conclusion...…...................................................35

References...…......................................................................36

국문초록...….......................................................................39

1

Chapter 1. Introduction

1.1 Piezoelectric acoustic actuator

Recently, portable devices have attracted great attention. Due to the

limited allowed space and weight, all components are compulsory to be small,

lightweight, as well as power-efficient. Traditionally, electrodynamic

loudspeaker, which operates based on the movement of a voice coil, have

been used in many audio devices. However, its use in small portable devices

has been limited due to the relatively heavy and thick device structure [1-5].

Thus, new forms of loudspeaker have been extensively studied as an

alternative electrodynamic loudspeaker. Among them, piezoelectric acoustic

actuator has have received attention as the most remarkable option.

Piezoelectric loudspeaker can be prepared into a film that is thinner than 1

mm, whereas electrodynamic loudspeakers have 3 mm or greater thickness.

Also, piezoelectric loudspeakers are more power-efficient than

electrodynamic types due to its high impedance characteristics, generating a

large sound pressure at a relatively low operating voltage [6].

2

1.2 Piezoelectric materials

Piezoelectric acoustic actuator is a type of traducers that generates sound

wave in response to the electrical audio signal inputs, which is in a form of

electrical potential. Piezoelectric acoustic actuators operate based on the

piezoelectric effect. Piezoelectric effect is a reversible process, in which

electrical potential is transformed into the mechanical stress and vice versa [7].

Representative piezoelectric materials are quartz, BiTiO3, Pb(Zr,Ti)O3 (PZT)

(lead zirconate titanate), ZnO, PVDF, and so on [8-11]. Although inorganic

materials exhibit large piezoelectric moduli, the use of these materials as thin

film applications is limited because of brittleness and difficulty in shaping

these materials to complex and flexible structures [12, 13]. Thus, organic

piezoelectric materials such as polyvinyleneflouride (PVDF) have received a

considerable attention [14]. As the semicrystailline polymer, PVDF exists in

several phases according to the chain conformation as trans or gauche

linkages: alpha(α), beta(β) and gamma(γ) [15, 16]. Especially, the

piezoelectric property is strongly depended on the β phase content origianted

from a high net diepole moment due to its all-trans structure [17]. Therefore,

improving the β phase content of PVDF film has been in need for applications

in actuators [18-21]. In spite of much progess in increasing the β phase

3

content, the challenge still remains to improve the piezoelectric effect of

pristine PVDF thin film for practical acoustic actuator applications.

4

1.3 Nanofiller composites

Various fillers have been introduced into PVDF matrix to enhance the

piezoelectric property. Martins et al. loaded dielectric BaTiO3 nanoparticle in

PVDF thin film increasing dielectric pertimittivity, resulting from the

increased β phase content [22, 23]. In spite of the dielectric filler loading

increased the piezoelectric effect of pristine PVDF, they have limited the

maintenance well-dispersed fillers in matrix. Namely, incorporating high

concentration of nanofillers to the matrix caused the particle–particle

aggregation that result in the reduction of dielectric permittivity [24-27]. Our

previous attempt to enhance the piezoeletric response by incorporating Ba-

doped SiO2-TiO2 hallow nanoparticles as nanofillers into the PVDF matrix

showed a promising increase in the low frequecny reponse [28, 29]. However,

nanofillers’ realatively low dieletric constant may have limited the low bass

sound improvement. Thus, the development of a well-dispersed high

dielectric nanofiller has been focused on recently in order to fabricate flexible

acoustic actuators with high bass frequecy response.

5

1.4 Objective of this study

Herein, we report the behavior of a PVDF based thin film acoustic

actuator which the performance of every frequency range has been increased

by the incorporation of urchin-like ZnO as nanofiller and CVD-grown

graphene as electrodes. Especially, high dielectric urchin-like ZnO nanofillers

have prevented filler aggregation and allowed nanospike-to-nanospike current

path, while they also acted as a nucleation agent augmenting the β phase

content of the fabricated composite film. To investigate the composite film,

the crystalline behavior and dielectric permittivity parameters were studied.

Furthermore, the fabricated composite film was applied in loudspeaker. The

fabricated PVDF/ZnO thin film demonstrated the enhanced bass frequency

response and decreased total harmonic distortion compared to other PVDF

based thin films.

6

Chapter 2. Experimental

2.1 Materials

Zinc nanopowder (size ca. 250 nm; 99.9%; Nano Technology Inc., South

Korea) was used in the fabrication of urchin-like ZnO. Zinc acetate dihydrate,

zinc nitrate hexahydrate, hexamethylenetertramine, and ethanol were obtained

from Sigma-Aldrich, and they were used in the fabrication of rod ZnO. Gases

(H2, CH4, Ar; 99.99%; Daesung Industrial Gases Co., South Korea), Cu foil

(Sigma-Aldrich), and poly(methylmethacrylate) (950 PMMA A4, 4% in

anisole, MicroChem Corp., USA) were used in the fabrication of the graphene

films. PVDF pellets (molecular weight ca. 275,000 by gel permeation

chromatography), (3-aminopropyl)trimethoxysilane (APS), and

dimethylformamide (DMF) were obtained from Sigma−Aldrich. Commercial

PVDF film was purchased from the Films Corporation (South Korea).

2.2 Synthesis of urchin-like ZnO

First, an appropriate amount of high purity Zn (purity, 99.99%) was

ground in an agate mortar for 20 min. Then, 1 g of power was loaded on an

7

alumina boat and placed in a 1.2 m long quartz tube furnace. The growth

process was divided into three steps. Initially, the furnace purged with Ar gas

for 20 min. The furnace was heated to a certain temperature in the presence of

Ar gas. Soon after the growth temperature is reached, a flow of oxygen is

introduced into the quartz tube and the growth process was maintained for 30

min. On a cooling to room temperature, a white woolly deposit was observed

on the alumina boat.

2.3 Synthesis of rod ZnO

A seed solution of zinc acetate in ethanol was spin-coated onto the silicon

wafer, placed in a 100 ˚C oven, and dried for 30 min to create a seed layer for

further growth of ZnO nanostructures. The growing process was carried out

by immersing seeded wafer in a growing solution of 0.05 M

hexmethylenetetramine and 0.05 M zinc nitrate hexahydrate in de-ionized

water at 90 ˚C for 3-7 h to control the morphology of ZnO nanostructures.

2.4 Preparation of ZnO/PVDF films

PVDF pallets were dissolved in a 1:1 mixture of DMF and acetone and

ZnO nanostructures were added into solution. The mixture was sonicated at an

8

elevated temperature for 2 h to uniformly disperse the particles. The

homogenous solution is drop-casted on the polyimide film. The dried mat on

the polyimide film was then hot pressed at 220 ˚C and 800 psi for 30 min and

cooled to room temperature. The resulting film was uniaxially drawn at 10

mm/min at an elevated temperature to induce the α-to-β phase transformation,

and then it was poled at a constant electric field at 30 kV/min to further

increase the β phase content.

2.5 Fabrication of CVD graphene

Cu foil (7 x 7 cm2) was placed in a quartz tubular furnace and the furnace

was vacuumed to 30 mTorr. Then, the furnace was heated to 1000 ˚C under an

8 sccm flow rate of H2 gas at 90 mTorr and maintained for 30 min to

maximize the size of Cu domains. To grow graphene on the Cu substrate, a 20

sccm flow of CH4 gas was injected for 30 min at 560 mTorr. After the

growing process, CH4 gas was shut and cooled to room temperature under H2

atmosphere. To transfer the graphene, 4 % PMMA solution in anisole was

spin coated at 5,000 rpm for 1 min onto the Cu foil. The pristine graphene was

isolated after Cu foil was etched in a Cu etchant.

9

2.6 Fabrication of ZnO/PVDF acoustic actuator

ZnO/PVDF thin film (7 x 7 cm2) was first treated with a 0.5 wt% APS

solution for 12 h to reduce hydrophobicity of the PVDF film. The film was

dried in a 60 ˚C oven overnight. Two CVD grown graphene/PMMA films

were used to sandwich the ZnO/PVDF film. The PMMA layer was then

removed from the graphene by washing the film with acetone several times.

The Cu tape was attached on the corner of fabricated thin film speaker and

used as contact electrode. The sound signal is connected to an amplifier and it

produced time variable voltage (AC electric field) to generate sound wave

with attractive and repulsive forces.

2.7 Frequency response and THD testing of fabricated

film speaker

The fabricated film speaker is placed in a soundproof chamber to isolate

noise and reflection interference. The dynamic microphone, which is

connected to an audio analyzer, is placed 10 cm above the fabricated film.

The frequency response acoustic output of the film was received by the

microphone, and the swept signals were measured by audio analyzer.

10

2.8 Vocal receiving test as film microphone

The fabricated film speaker is located in a soundproof chamber to isolate

noise and reflection interference as same as speaker test method. The

electrical resistance was measured using a Keithley 2400 source meter.

2.9 Characterization

Field-emission scanning electron microscopy (FE-SEM) images were

taken using JEOL JSM-6700F. Transmission electron microscopy (TEM)

images were acuqied with a JEM-2100 (JEOL, Japan). Raman spectra

measurement was recorded with LabRAM HR (Horiba, Japan) with 1064 nm

laser excitation. Fourier-transform infrared (FT-IR) spectra were measured

using FT-IR spectrometer (Bomen MB 100, USA), utilizing universal-ATR

mode. Atomic force microscopy (AFM, Nanoscope IIIa, Digital instruments,

USA) were used in the tapping mode with silicon tips at a resonance

frequency of 330 kHz. A dScope Series IIIA (Prism Sound Co., UK) audio

analyzer was used to quantity the acoustic actuator performance from 10 Hz

to 20 kHz. Polarized optical microscopy images were carried out by Lv100

microscope, Nikon, Japan. To investigate the crystal structure of the ZnO rod

11

and urchin, high resolution X-ray diffraction (HRXRD, Bruker D8

DISCOVER, Germany) was used.

12

Chapter 3. Results and Discussion

3.1 Fabrication of PVDF-based thin film acoustic

actuator

Frist, urchin-like ZnO and rod ZnO were synthesized by chemical vapor

deposition method and hydrothermal method, respectively [10, 28, 29]. The

prepared ZnO was mixed a PVDF solution, which was then drop casted to

form a ZnO/PVDF mat. The ZnO/PVDF films were drawn at an elevated

temperature and poled under a strong constant electric field at 30 kV/mm to

prompt the transformation of α phase to β phase. The resultant film was

transparent and polarized. Subsequently, an acoustic actuator electrode was

fabricated with few layered ca. 2 nm thick graphene, which was synthesized

by chemical vapor deposition method. Prior to applying graphene as an

electrode, the fabricated ZnO/PVDF film surface was treated with 3-

aminopropyltriethoxysilane (APS) to improve the adhesion with graphene [28,

29]. Hence, a sandwich of the ZnO/PVDF film and graphene electrodes was

prepared as an acoustic actuator, which was connected to a sound source and

amplifier (Figure 1 and 2).

13

From our previous work by Lee et al., we have discovered that the

electrode conductivity greatly influenced acoustic actuator performance,

especially at midrange and treble frequency [8]. Compared to the commercial

thin film loudspeaker which uses PEDOT:PSS electrode, using chemical

vapor deposition (CVD) grown graphene electrodes increased the frequency

response in midrange and treble by 19 % and 22 %, respectively. TEM image

(Figure 3a) reveals CVD graphene with 3 layers and Raman spectroscopy

unveils a graphitic structure with the ration of IG/I2D of ca. 2 (Figure 3a inset).

An atomic force microscopy result exhibit a 2 nm multilayered graphene

(Figure 3b).

14

Figure 1. An illustration of the thin film acoustic actuator consistng of ZnO

urchin/PVDF with graphene electrodes.

15

Figure 2. A photograph of the thin film acoustic actuator consistng of ZnO

urchin/PVDF with graphene electrodes.

16

Figure 3. (a) HR-TEM (inset: Raman spectrum) and (b) AFM of fabricated

graphene electrode.

17

3.2 Characterization of ZnO

The SEM images reveal that the urchin-like ZnO was composed of several

ca. 400 nm nanorods extending from the core with the overall size of ca. 1 μm

(Figure 4a). Whereas, the fabricated rod ZnO had the hexagonal base diameter

of ca. 85 nm with the length of ca. 300 nm (Figure 4b). The XRD analysis for

ZnO indicates that all of the diffraction peaks can be exactly indexed to the

hexagonal ZnO with lattice constant c = 0.52 nm, which is in good alignment

of the ZnO along the c-axis direction (Figure 4c).

18

Figure 4. Low and high magnification of SEM images for fabricated

(a) urchin-like and (b) nanorods ZnO. (c) XRD patterns to investigate the

crystal structure of the ZnO nanofillers.

19

3.3 Crystallinity of various ZnO/PVDF films

The polarized optical microscopy (POM) morphology of the crystal

growth shows that the amounts of spherulites have significantly increased

with the addition of ZnO nanofillers, which means that added ZnO nanofiller

act as nuclei for PVDF crystallization (Figure 5a-c). Furthermore, it can be

inferred that the reinforced crystallization behavior is intimately proportional

to the content of β phase. To achieve in depth insight into the crystallization

behavior by ZnO nanofiller in PVDF matrix, Fourier-transform infrared

(FTIR) spectra of various PVDF films of urchin-like ZnO embedded, rod ZnO

embedded, homemade pristine PVDF, and commercial PVDF films were

carried out (Figure 6). Among different crystalline phases of PVDF, the β

phase exhibits piezoelectric and pyroeletric properties due to its large

spontaneous polarization, arising from aligned molecular chain configuration

[30]. The representative peak of α phase, trans-gauche conformation,

appeared at 766 cm-1, while β phase peak, all trans planar zigzag conformation,

appeared at 840 cm-1. The relative intensity of representative α and β phase

peaks were then calculated in order to calculate the β phase content in the film,

using the Lambert-Beer law [23, 31].

Assuming that the IR absorption follows the Beer-Lambert law, the β

phase content in the PVDF can be measured by

20

( ) =

=

( / ) (1)

where and are the absorbance at 766 and 840 cm-1, respectively, and

= 6.1 × 104 cm2/mol and = 7.7 × 10

4 cm2/mol are the absorption

coefficients of α and β phases at the respective wavenumbers.

As a result, urchin-like ZnO embedded PVDF film had a relative intensity

of β and α phase (Iβ/Iα ratio) of 5.74 with 88 % β phase, and incorporating rod

ZnO led to the ratio of 4.31 with 84 % β phase. This is 18 % and 40 %

enhancement with the incorporation of urchin-like ZnO compared to the

homemade PVDF film and commercial PVDF film, respectively. Whereas rod

ZnO nanofillers have shown 12 % and 36 % enhancement with respect to the

homemade PVDF and commercial PVDF films (Table 1). Judging from these

results, the introduction of ZnO nanofillers into PVDF matrix has successfully

increased the β phase content.

21

Figure 5. Polarized optical microscopy image of (a) Pristine PVDF film,

PVDF hybrid films with (b) rod ZnO, and (c) urchin-like ZnO by isothermally

crystallized at 170 °C and maintained for 240 s then cooled down to observe

crystal growth morphology.

22

Figure 6. IR spectrum of PVDF films and different types of ZnO with

PVDF thin films.

23

Table 1. The β phase content, F(β), of commercial, pristine, rod ZnO

embedded, and urchin-like ZnO embedded PVDF thin films

Sample F(β) a

Commercial PVDF 62

Pristine PVDF 74

PVDF/ZnO rod 84

PVDF/ZnO urchin 88

a Data calculated using the Lambert-Beer law.

24

3.4 Permittivity and loss factors of various ZnO/PVDF

films

The enhanced permittivity, which is related to the polarization and dipole

moment of PVDF, is the key factor for improving the piezoelectric property

of PVDF. For this reason, the addition of inorganic semiconductor ZnO

nanofillers with high dielectric constant into PVDF matrix makes it possible

to enhance the permittivity of PVDF. The fitting lines plotted in Figure 7 and

8 were obtained based on the Cole-Cole equation below [32].

∗ = + " = +

{ ( ) } (2)

Where ε is the dielectric constant, ε" is the dielectric loss; the permittivity is

therefore given by

Δε = - (3)

where is the static permittivity (i.e., when lim → ∗( )), and is the

high-frequency permittivity (i.e., when lim → ∗( )).

In addition, Cole-Cole fitting curve (Figure 8) is a particular case of the

Havriliak-Negami relaxation and the equation is follow:

∗ = +

( ) (0 ≤ < 1) (4)

where Δε is associate with the electrostatic interaction between the dielectric

materials. Also, the relaxation time (λ) is correlated with the proper interfacial

25

polarization response and is related to the maximum value of dielectric loss

(ε"). The relaxation time is determined by following equation:

=

(5)

where is the frequency of the peaks in the loss spectrum. The measured

all permittivities are listed in Table 2

The deflection of the ZnO generates a positive strain field on the stretched

surface along the z-direction of electric field. The piezoelectric potential can

be created by the relative displacement of the Zn2+ cations with respect to the

O2- anions, resultant in the wurtzite crystal structure [32]. Therefore, these

ionic charges can freely move and recombine with each other as releasing the

strain. The urchin-like ZnO embedded PVDF showed the greatest

enhancement in the permittivity. It is generally accepted that the dielectric

properties of composites are strongly influenced by interfacial region. From

our previous work by Lee et al., we have proved the interfacial polarization in

rod or disk ZnO nanostructure and PVDF composites attribute to the higher

dielectric constant of their composites [10].

Therefore, the increase in the permittivity could be explained by two

factors: the added current paths of nanospike-to-nanospike contacts among

adjacent urchin-like ZnO and high interfacial surface area per structure of the

urchin-like ZnO and PVDF. It is known that a larger aspect ratio of a

polarizable material leads to a greater dielectric constant [18]. Each nanospike

26

extending form the core, forming an urchin-like structure, has a larger aspect

ratio compared to the rod ZnO, and its 3-dimentioanl structure induces greater

non-uniform electric dipole moments upon an external electric field, leading

to a greater polarization and thus a greater dielectric constant. Also, the added

current paths can facilitate the transport of charges by transforming the

arrangement of charges. In addition, due to the especial hierarchical structure

of urchin-like ZnO, a lot of empty space between adjacent urchins and

consequently a considerably larger surface area is conceivable compared to

the closely-packed rod ZnO. Consequently, the added nanospike-to-nanospike

connections and increased surface area arising form the 3-dimential

hierarchical structure of urchin-like ZnO contributed to uplifting the

permittivity.

27

Figure 7. Permittivity and loss factor of fabricated thin films.

102

103

104

105

106

107

0

50

100

150

200

250

300

350

400 Commercial PVDF Pristine PVDF PVDF/ZnO Rod PVDF/ZnO Urchin

Frequency (Hz)

Pe

rmit

tiv

ity (e'

)

0

5

10

15

20

25D

iele

ctric

Lo

ss

(e'')

28

Figure 8. Cole-Cole fitting curves by permittivity verse dielectric loss.

29

Table 2. The values of β content and dielectir parameter for pristine PVDF

and ZnO/PVDF hybrid films.

Sample εS ε∞ Δεa λ[μs]b

PVDF/ZnO urchin 414 211 200 101

PVDF/ZnO rod 260 157 103 120

Pristine PVDF 22 13 9 157

a Data calculated using the Havriliak-Negami and Fourier transform relationship. b Data were obtained via the interfacial polarization response relaxation time.

30

3.5 Performance of ZnO/PVDF thin film acoustic

actuators

Figure 9a illustrates the acoustic actuator performance of various

fabricated PVDF films as a function of frequency. Notably, the urchin-like

ZnO embedded PVDF film showed the greatest improvement in all measured

frequency regions: approximately a 29 dB full-scale (72 %) increase in the

low bass region (below 100 Hz). From our previous study with the Ba-doped

SiO2/TiO2 nanoparticle embedded PVDF film, the overall frequency response

at a low frequency was limited to above 200 Hz [8, 14]. However, with the

introduction of urchin-like ZnO, the bass frequency response significantly

improved. In addition, the total harmonic distortion in the bass region

decreased by 82 % (Figure 9b). We suggest that this effect is due to the

increased dielectric constant and enhanced crystallinity of PVDF because of

the space charge interaction between the PVDF interfaces with embedded

various ZnO particles. The improved midrange and treble frequency response

is due to the decreased sheet resistance by replacing PEDOT:PSS electrode

with graphene electrode and by embedding urchin-like ZnO which induced

the added nanospike-to-nanospike current paths. This result implies that the

fabricated urchin-like ZnO embedded PVDF film had the significant

enhancement as the thin film acoustic actuator for the maximum sound

31

pressure generation with reduced distortion. Importantly, not only the high β

phase content but also the added nanospike-to-nanospike current paths play

the key role in the improvement for the acoustic actuator. With more number

of current paths present in the film, more electrons are transferred to the

surface of ZnO which gets transformed to the vibrational force, generating

more stable sound. Figure 10 shows frequency response of film as a function

of film thickness. The output sound level of 60 μm film decreased at lower

range because of relatively low piezoelectricity, and the acoustic performance

of 100 μm film showed the diminished frequency response above middle and

treble ranges since thick and heavy surface of film obstruct the vibration of

the film surface to generate the acoustic signal. Figure 11 illustrates the

acoustic performance as a function of ZnO urchin filler contents. Loading

ZnO filler enhanced frequency response at the bass frequency range until

30 wt%. However, exceeding the maximum concentration of filler might have

caused aggregation of fillers and thus reduced the sound level. Therefore,

evidently 80 μm thick PVDF film with 30 wt% of ZnO urchin filler is

optimum for the thin-film acoustic actuator for the maximum sound

performance as represented by the frequency response and total harmonic

distortion.

32

Figure 9. (a) The frequency responses and (b) total harmonic distortion of

different PVDF-based thin film loudspeakers.

33

Figure 10. The frequency response as a function of film thickness.

34

Figure 11. Relative efficiency of the actuators performance compare to

pristine PVDF acoustic actuator at 100 Hz as a function of filler content.

35

Chapter 4. Conclusion

The PVDF based acoustic actuator performance can be effectively boosted

by incorporating various high dielectric ZnO structures using CVD graphene

electrode. Most of all, it is confirmed the increased dielectric constant and the

high degree of crystallinity of PVDF, arising from the 3-dimentional

hierarchical structure of urchin-like ZnO, certainly enriched bass frequency

response of loudspeaker, supporting our previous work done by Lee et al..

This further enhancement in thin film acoustic actuator performance has great

possibilities for future thin film applications.

36

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39

초 록

본 실험은 간단하며 효과적인 기상증착성장 방법과 수열 반응을

이용하여 여러 모양의 산화아연을 제조하고, 이를

폴리비닐리덴다이플로라이드 (Polyvinylidenefluoride, PVDF) 에

충진제로 사용하여 저주파수에서도 뛰어난 음압을 갖은 박막형 음향

작동기를 제조하였다. 고분자 기반에 박막형 압전 음향 작동기는 구조가

간단할 뿐만 아니라, 구동전압이 진동형 음향 작동기에 비해 더 낮고,

작고 가벼우며, 플렉시블(flexible)하다는 장점을 가지고 있다. 이에

압전 특성을 띄는 베타상의 폴리비닐리덴다이플로라이드와 성게

모양의 산화아연를 이용해 압전성 박막을 제조하였다. 전극으로는

기상증착성장 방법으로 제조된 그래핀을 사용함으로써 플렉시블하며

투명한 박막형 음향 작동기를 제조하였다. 제조된 박막형 음향작동기는

기존 폴리비닐리덴다이플로라이드 박막형 음향작동기의 최대 단점으로

알려진 저주파수 음압을 72 % 향상 시켰으며 전체 고조파 왜곡율을 82 %

감소시켰다. 산화아연의 높은 유전율과 성게모양을 통해 무기물간의

응집을 방지하고 고분자/무기물 간에 증가된 표면적으로 인해

폴리비닐리덴다이플로라이드의 베타상 결정구조를 유도하여,

40

최종적으로 제조된 박막의 분극을 증가시켜 향상된 음질과 음향을 얻을

수 있었다. 이 논문에서와 같은 신규 접근은 다양한 모양의 나노 충진제

응용을 위한 저 주파수에서 고성능의 음압을 갖은 박막형 스피커 제조를

위한 새로운 방법을 창출 할 것으로 사료된다.

주요어 : 박막형 음향 작동기, 산화아연, 폴리비닐리덴다이플로라이드,

결정성, 유전율

학번 : 2014-20586

Chapter 1. Introduction1.1. Piezoelectric acoustic actuator1.2. Piezoelectric materials1.3. Nanofiller composites1.4. Objective of this study

Chapter 2. Experimental2.1. Materials2.2. Synthesis of urchin-like ZnO2.3. Synthesis of rod ZnO2.4. Preparation of ZnO/PVDF films2.5. Fabrication of CVD graphene2.6. Fabrication of ZnO/PVDF acoustic actuator2.7. Frequency response and THD testing of fabricated film speaker2.8. Vocal receiving test as film microphone2.9. Characterization

Chapter 3. Results and Discussion3.1. Fabrication of ZnO/PVDF film3.2. Characterization of ZnO3.3. Crystallinity of various ZnO/PVDF films3.4. Permittivity and loss factors of various ZnO/PVDF films3.5. Performance of ZnO/PVDF thin film acoustic actuator

Chapter 4. ConclusionReferences국문초록

11Chapter 1. Introduction 1 1.1. Piezoelectric acoustic actuator 1 1.2. Piezoelectric materials 2 1.3. Nanofiller composites 4 1.4. Objective of this study 5Chapter 2. Experimental 6 2.1. Materials 6 2.2. Synthesis of urchin-like ZnO 7 2.3. Synthesis of rod ZnO 7 2.4. Preparation of ZnO/PVDF films 7 2.5. Fabrication of CVD graphene 8 2.6. Fabrication of ZnO/PVDF acoustic actuator 9 2.7. Frequency response and THD testing of fabricated film speaker 9 2.8. Vocal receiving test as film microphone 10 2.9. Characterization 10Chapter 3. Results and Discussion 12 3.1. Fabrication of ZnO/PVDF film 12 3.2. Characterization of ZnO 17 3.3. Crystallinity of various ZnO/PVDF films 19 3.4. Permittivity and loss factors of various ZnO/PVDF films 24 3.5. Performance of ZnO/PVDF thin film acoustic actuator 30Chapter 4. Conclusion 35References 36±¹¹®ÃÊ·Ï 39