Materials Transactions, Vol. 45, No. 2 (2004) pp. 240 to 243
Special Issue on Materials and Devices for Intelligent/Smart Systems
#2004 The Japan Institute of Metals
Ionic Conductivity Enhancement of YSZ Film Induced by Piezoelectric Vibration
Hiroshi Masumoto and Takashi Goto
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan
To improve electric properties of oxide ion conducting yttria-stabilized zirconia (YSZ) films at low temperatures, a mechanical distortion
was induced by a piezoelectric actuator. YSZ films containing 8 mol%Y2 O3 were prepared by metalorganic chemical vapor deposition. The
YSZ film was placed on a PZT (lead zirconate titanate) multilayer piezoelectric actuator. The effect of piezoelectric vibration on electric
properties of the YSZ film was investigated. The resistivity of the YSZ film decreased with increasing amplitude of the piezoelectric vibration.
Electrical conductivity of the YSZ film at 353 K vibrated by the actuator was 2 106 Sm1 , 103 times greater than that without vibration.
(Received July 31, 2003; Accepted October 24, 2003)
Keywords: yttria-stabilized zirconia film, lead zirconate titanate, piezoelectric vibration, metalorganic chemical vapor deposition, electric
property
1.
Introduction
It is known that yttria-stabilized zirconia (YSZ) has
excellent thermal and chemical stability, and oxide ion
conduction. Therefore, YSZ is expected to be used as oxide
ion conducting devices such as oxide sensors and solid-oxide
fuel cells. However, YSZ does not show enough ionic
conductivity around room temperature. Then, YSZ should be
heated to about 700 K for the sensor application.1) To
improve the sensitivity of YSZ sensors at low temperatures,
developments of electrodes,2,3) modifying the principle of
sensing mechanism4–6) and the enhancement of ionic conductivity of YSZ have been conducted. It is well-known that
the ionic conductivity of YSZ could be increased by doping
aliovalent elements.7) On the other hand, the ionic conductivity of YSZ may be increased by distorting the YSZ lattice.
Suzuki et al. reported that the ionic conductivity of YSZ films
increased with increasing the lattice parameter by using the
thermal expansion difference between substrates and YSZ
films.8)
A piezoelectric actuator may be applicable to induce the
distortion of YSZ lattice yielding an increase in ionic
conductivity of YSZ films. In the present study, YSZ films
were prepared by metalorganic chemical vapor deposition
(MOCVD), and the YSZ film was placed on a multilayer
piezoelectric PZT (lead zirconate titanate) actuator. The
effect of piezoelectric vibration on electric properties of the
YSZ film was investigated.
2.
Experimental Procedure
A horizontal hot-wall type CVD chamber was employed to
prepare the films. Zr(dpm)4 and Y(dpm)3 (dpm: dipivaloymethanato) precursors were used as Zr and Y source
materials, respectively. The precursor vapors were transported to the reaction chamber with Ar gas, and mixed with
O2 gas around the substrate. The Zr and Y source temperatures were kept at 503 K and 403 K, respectively. The
deposition reaction was conducted at 973 K at 400 Pa for
3.6 ks. (100)MgO single crystals were used as substrates. Pt
films 100 nm thick were deposited by dc sputtering on
(100)MgO substrates. The YSZ films were deposited on the
(100)Pt/(100)MgO substrates.
The phase and preferred orientation were examined by Xray diffraction (XRD: Rigaku RAD-C). The composition was
analyzed by electron probe X-ray microanalysis (EPMA:
JEOL, JXA-8621) and X-ray fluorescence spectroscopy
(XRF: Kevex, model 920). The surface and cross-sectional
microstructures were observed by scanning electron microscopy (SEM: Hitachi, S-3100H). Ag top-electrodes
(0.1 mm thick, 0.4 mm2 ) were attached to the YSZ film by
printing Ag paste for electrical measurements. Electric
properties were measured by impedance spectroscopy (Toyo
Technica, Solartron 1260, Solartron 1294) at temperatures
(Tm ) of 92 to 973 K in the frequency ( fm ) range of 102 to
107 Hz and at an ac voltage (Em ) of 0.1 V.
The YSZ film and the multilayer piezoelectric actuator
were glued with an epoxy resin. Figure 1 shows a schematic
configuration of YSZ film and PZT actuator for the electric
measurement. The multilayer piezoelectric actuator consisted
of seven sheets of PZT (lead zirconate titanate) 17 mm 17 mm 4 mm in size. The actuator vibrated the YSZ film in
contour extensional vibration mode. The resonance amplitude of the actuator was generated at the frequency of
115 kHz. Effect of the vibration on the electric property of
YSZ film was measured by impedance spectroscopy. The
electric power was applied to the piezoelectric actuator by a
power amplifier (Hewlett-Packard, HP33120A) and a function generator (NF, NF4015) at voltages (Ev ) of 0 to 40 V and
frequencies ( fv ) of 0 to 127 kHz.
(a)
(b)
(c)
(d)
(e)
(f)
Fig. 1 A schematic configuration of YSZ film and PZT actuator for the
electric measurement. (a) Ag top-electrode (thickness d ¼ 0:1 mm,
0.4 mm2 ), (b) YSZ film (d ¼ 1:4 mm), (c) Pt bottom-electrode
(d ¼ 100 nm), (d) MgO substrate (d ¼ 0:5 mm), (e) epoxy resin and (f)
PZT piezoelectric actuator (17 mm 17 mm 4 mm).
Ionic Conductivity Enhancement of YSZ Film Induced by Piezoelectric Vibration
241
Fig. 2 XRD pattern of YSZ (ZrO2 -8 mol%Y2 O3 ) film.
(a)
Fig. 4 The relationship between the real part of impedance (Z 0 ) and the
imaginary part of impedance (Z 00 ) for the YSZ film measured at
Tm ¼ 584 K without actuator vibration.
2 µm
(b)
Fig. 5 Effect of actuator voltage (Ev ) on the frequency ( fm ) dependence of
resistivity for the YSZ film at applied frequency ð fv Þ ¼ 115 kHz.
2 µm
Fig. 3
3.
SEM images of YSZ film. (a) surface and (b) cross-section.
Results and Discussion
Figure 2 shows the XRD pattern of YSZ film, showing
highly crystallized cubic-ZrO2 and a significant (200)
orientation. The Y2 O3 content of the film was 8 mol%.
Figure 3 shows the surface and cross-sectional microstructures of the film. The film was 1.4 mm in thickness
consisting of fine grains about 400 nm in diameter and a
columnar structure.
Figure 4 shows the relationship between the real part of
impedance (Z 0 ) and the imaginary part of impedance (Z 00 ) for
the YSZ film measured at Tm ¼ 584 K without actuator
vibration. One semicircle and so-called spike were observed
at high and low measurement frequency, respectively. The
capacitance (C) associated to the semicircle was about
1011 F, suggesting a bulk response. The spike and its
associated large capacitance of 107 F imply the ionic
conduction in the YSZ film.
Figure 5 shows the effect of actuator voltage (Ev ) on the
frequency ( fm ) dependence of resistivity for the YSZ film at
applied frequency ð fv Þ ¼ 115 kHz. The vibration amplitude
increased with increasing Ev . The resistivity of YSZ film
decreased with increasing the vibration amplitude. The
resistivity decreased with increasing Ev at low frequencies
below fm ¼ 103 Hz. The resistivity without vibration after the
measurement at Ev ¼ 40 V (Ev ¼ 0 V(2nd)) was in agreement with that before applying vibration (Ev ¼ 0 V(1st)).
This implies that the resistivity change due to the vibration is
repeatable being not resulted from some structural change
242
H. Masumoto and T. Goto
Fig. 6 Effect of fv on fm dependence of the resistivity of YSZ film at
Ev ¼ 40 V.
such as permanent deformation or crack formation.
Figure 6 shows the effect of fv on fm dependence of the
resistivity of YSZ film at Ev ¼ 40 V. The resistivity of the
YSZ film at fv ¼ 80 kHz and 127 kHz were in agreement with
that at fv ¼ 0 kHz. The actuator has no vibration at fv ¼ 80
and 127 kHz. The decrease of resistivity has appeared only at
the resonance frequency of 115 kHz. The mechanical strain
due to the vibration could yield the decrease of resistivity.
The temperature of the YSZ film increased from room
temperature to 353 K during the measurement at Ev ¼ 40 V
and fv ¼ 115 kHz. This was caused by the increase in
actuator temperature. The resistivity of the YSZ film without
vibration at 353 K was too large to measure being far
different from that measured with vibration. Therefore, the
decrease of resistivity measured with the vibration was not
caused by the increase in temperature of YSZ film.
Figure 7 shows the relationship between Z 0 and Z 00 for the
YSZ film measured with and without vibration at 353 K. One
semicircle and spike were observed in the YSZ film with
Fig. 8 Temperature dependence of electrical conductivity for YSZ films.
(a) and (b): this work, (9)–(14): reported data of reference numbers.
vibration. The capacitance associated to the semicircle was
about 1010 F, probably implying the bulk response. The
semicircle for the YSZ film without vibration was too large to
determine the resistivity and capacitance value.
Figure 8 shows the temperature dependence of electrical
conductivity for YSZ films measured in this study and
reported values.9–14) The activation energy (Ea ) for the film
prepared was about 1.0 eV that was in good agreement with
reported values. The electrical conductivity of YSZ film
measured with actuator vibration ( fv ¼ 115 kHz and
Ev ¼ 40 V) is indicated in Fig. 8. The value of about
2 106 Sm1 was about 103 times greater than that without
vibration.
The actuator showed the displacement of about 500 nm
according to the laser displacement tester at Ev ¼ 40 V. This
would correspond to the strain about 0.003% in the YSZ film.
Although the distortion of YSZ lattice would be very small,
the mobility of ionic conductivity was enhanced by accelerating the hopping probability of oxide ions. The strain
caused by the actuator vibration to the YSZ film might
promote hopping conduction of oxide ions.
4.
Fig. 7 The relationship between Z 0 and Z 00 for the YSZ film measured (a)
with and (b) without vibration at 353 K.
Summary
Ionic conducting YSZ films containing 8 mol%Y2 O3 were
prepared at 973 K by MOCVD. The YSZ film was placed on
a PZT (lead zirconate titanate) multilayer piezoelectric
actuator. The effect of piezoelectric vibration on electric
properties of the YSZ film was investigated. The mechanical
vibration induced by the piezoelectric actuator enhanced the
ionic conduction of YSZ film. The electrical conductivity of
the YSZ film at 353 K with vibration was about
2 106 Sm1 . That was 103 times greater than that without
vibration.
Ionic Conductivity Enhancement of YSZ Film Induced by Piezoelectric Vibration
Acknowledgments
This research was partly supported by a Grant-in-Aid for
Scientific Research (No. 12555174 and 13650729) from the
Japan Society for the Promotion of Science. This work was
performed as a part of Nanostructure Coating Project carried
out by New Energy and Industrial Technology Development
Organization (NEDO), Japan. The X-ray fluorescence measurements were performed at the Advance Materials Laboratory, Institute for Materials Research, Tohoku University.
The authors thanks to Mr. Mamiya of NEC Tokin Co. Ltd. for
supplying the piezoelectric actuator.
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