INSTITUTE OF PHYSICS PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 37 (2004) 804–812
PII: S0022-3727(04)70380-0
The influence of CeO2 on the
microstructure and electrical behaviour of
ZnO–Bi2O3 based varistors
Ming Lei1 , Shengtao Li1 , Xiaodong Jiao1 , Jianying Li1 and
Mohammad A Alim2
1
Multi-disciplinary Materials Research Center and State Key Laboratory of Electrical
Insulation for Power Equipment, Xi’an Jiaotong University, Xi’an 710049,
People’s Republic of China
2
Department of Electrical Engineering, Alabama A & M University, PO Box 297, Normal,
Alabama 35762, USA
E-mail: leiming@mailst.xjtu.edu.cn, sli@mail.xjtu.edu.cn, jxd9@hotmail.com,
lijy@mail.xjtu.edu.cn and malim@aamu.edu
Received 14 October 2003
Published 11 February 2004
Online at stacks.iop.org/JPhysD/37/804 (DOI: 10.1088/0022-3727/37/5/024)
Abstract
The processing–microstructure-property relations have been studied in order
to understand the role of the addition of CeO2 (up to 0.9 mole%) in the
ZnO–Bi2 O3 based varistor recipe. The microstructural investigation
suggests that CeO2 is segregated at the corners of the ZnO grains in addition
to the existence of the Zn7 Sb2 O12 spinel phase. However, the α-spinel phase
was observed instead of the β-spinel phase that is usually found in most
commercial and laboratory ZnO–Bi2 O3 based varistors. The α-spinel phase
is more stable than the β-spinel phase and does not transform to the
pyrochlore phase during the cooling process. The most significant effect of
the CeO2 particles is the ZnO grain refinement owing to the pinning effect
of the grain growth. The average grain size decreases from 7.8 to 5.7 µm
when compared to the 0.9 mole% CeO2 -added sample against the CeO2 -free
sample. This grain refinement results in a significantly enhanced breakdown
field when compared to the CeO2 -free sample. The coefficient of
nonlinearity of the current–voltage (I –V ) characteristics is found to be
nearly identical for the CeO2 added varistor materials. However, when a
slower cooling cycle (1˚C min−1 instead of 4˚C min−1 ) is used in the
sintering process, these varistor materials exhibited a high nonlinear
coefficient (α = 29 ± 5) as extracted from the I –V behaviour.
1. Introduction
The zinc oxide (ZnO) based varistors are polycrystalline
devices that are widely used as surge absorbers in electrical
power systems, including appliances, due to their excellent,
symmetric, nonlinear current–voltage (I –V ) characteristics
and tremendous surge withstanding capabilities [1–5]. In
conventional ZnO–Bi2 O3 based varistors Bi2 O3 is used as the
liquid-phase sintering promoter [6] which segregates at ZnO
grain boundaries and forms electrostatic potential barriers,
whereas other concurrent additives such as Sb2 O3 , Co2 O3 ,
MnO2 , Cr2 O3 , etc are added in small amounts to further
0022-3727/04/050804+09$30.00
© 2004 IOP Publishing Ltd
enhance or control the desired electrical properties of the
resulting device. Both spinel and pyrochlore phases are formed
during the sintering cycle, and inhibit the ZnO grain growth
[7]. In these varistors the microstructural study reveals, in
general, three distinct phases: ZnO grains, Bi-rich phases,
and the Zn7 Sb2 O12 spinel in conjunction with the occasional
Bi3 Zn2 Sb3 O14 pyrochlore type phase. It is commonly accepted
that the nonlinear I –V characteristics of the ZnO varistors
originate from the electrical barriers arising from the liquidphase sintering [8]. These electrical barriers are known as
the back-to-back (double) Schottky barriers at the electrically
active grain boundaries and are responsible for the symmetrical
Printed in the UK
804
ZnO–Bi2 O3 based varistors
nonlinear I –V characteristic. The electrical response of the
varistors can be represented as the total response of ‘m’
junctions in parallel with ‘n’ junctions in series between the
electrodes. The number of junctions, either in parallel or
in series, is not identical for every current flowing thread
between the electrodes placed on opposite faces with respect to
each other [8–12]. Each of these non-Ohmic grain boundaries
has a breakdown voltage [4, 13–15] Vb of about 3 V. Therefore,
the overall varistor breakdown voltage VB of the bulk in a single
current thread direction can be approximated as the product of
Vb and the number of grain boundaries N (=n in series) across
the two opposite electrodes of the varistor. Thus,
VB = NVb .
(1)
In general, the ZnO varistor, which has a typical
composition arising from the ZnO–Bi2 O3 –Sb2 O3 –Co2 O3 –
MnO2 –Cr2 O3 recipe, has a breakdown field Eb ranging
between 100 and 200 V mm−1 . Eb is measured as the
breakdown voltage VB per unit varistor thickness [1, 16]
corresponding to the current density 1 mA cm−2 . The so-called
high-field varistors possessing a high value of Eb are of
specific interest for their promising applications in overvoltage protection purposes [17]. Since Vb is relatively fixed
for a combination of reasonably ‘good’ and ‘bad’ nonlinear
microjunctions [13–15], VB may be enhanced either via
increase in varistor thickness or decrease in the average grain
size, giving a large value of N. The first approach is contrary
to the concept of miniaturization of the surge protective device,
considering industrial cost-effectiveness. Thus, the latter
approach is a plausible concept as long as the resulting device
provides service in the applications arena having a reasonable
withstanding capability. To achieve this goal, the varistor
ceramics must have uniform grains in the microstructure,
emphasizing a narrow distribution of the grain size. Such
a distribution is likely to provide a large number of ‘good’
microjunctions. The ‘good’ microjunctions result in a highly
nonlinear I –V characteristic possessing a very high leakage
resistance while ‘bad’ microjunctions give rise to considerably
worse varistor electrical characteristics [13–15].
The decreases in sintering temperature and sintering
time are quite well known [8, 18] for their ability to
reduce the grain size, thus, yielding higher breakdown field
varistors. But a device obtained via such a sintering cycle
may cause deterioration in electrical properties due to the
presence of more pores and ‘bad’ microjunctions arising from
the incomplete sintering via the low nonlinear coefficient
Ai et al
(α) value and a large leakage current, IL .
[19] have reported high-field varistors of Eb in the range
between 300 and 400 V mm−1 by modifying the basic recipe
comprising Sb2 O3 , Co3 O4 , MnO2 and Cr2 O3 . Several
attempts have been documented in the literature to fabricate
varistors having high breakdown electric fields. Thus, using
the combinations of nano-sized ZnO and additive powders,
there have been attempts to achieve an average grain size
of about 2–3 µm at which high breakdown field varistors
may be achieved. The literature [20–22] provides examples
showing that this processing concept works for Eb up to
1000 V mm−1 . Recently, it has been reported that the
introduction of various rare-earth oxides (REO) such as Pr6 O11
or Y2 O3 can significantly increase the breakdown field without
deterioration in the performance of the varistors [22–24].
However, so far, there has been no mention of CeO2 (a widely
known REO) added ZnO–Bi2 O3 based varistors in the existing
literature. Since the general tendency of the investigators (also
manufacturers) is to focus on achieving high voltage gradient
varistors having an improved electrical response, it is important
to concentrate on the addition of the REO in the ZnO–Bi2 O3
based varistor recipe followed by subsequent evaluation.
The aim of this paper is to investigate the effect of addition
of CeO2 to a basic ZnO–Bi2 O3 based recipe, identical to
those in the published literature, to obtain improved electrical
response for potential applications. The amount of CeO2
in the recipe used goes up to 0.9 mole%. The resulting
microstructure, densification, I –V , and capacitance–voltage
(C–V ) characteristics of these varistors are examined in a
systematic fashion.
2. Experiment
2.1. Sample preparation
We prepared ZnO varistor samples of nominal composition:
95.216 mole% ZnO, 0.5 mole% Bi2 O3 , 1.2 mole% Sb2 O3 ,
0.5 mole% Co2 O3 , 1 mole% MnO2 , 0.5 mole% Cr2 O3 ,
0.5 mole% Ni2 O3 , 0.5 mole% SiO2 , 0.08 mole% B2 O3 , and
0.004 mole% Al2 O3 and x mole% CeO2 for x = 0, 0.1, 0.3, 0.5
and 0.9 mol%. The extra decimal places for ZnO and Al2 O3
are given to show the accuracy involved in the recipe. The
samples prepared using each recipe are labelled C0, C1, C3,
C5 and C9, respectively. Thus, C0 is the reference varistor
sample.
The reagent grade additives were wet balls milled with
agate balls for 24 h and then mixed with ZnO powder. About
1.2 wt% polyvinyl alcohol (PVA) was added to the mixture as
binder, and then dried at 100˚C and pulverized using agate
mortar/pestle. It has been checked and confirmed [25]3 that the
instrumentation does not significantly contaminate the recipe,
as it already contains 0.5 mole% SiO2 . The dried powders
were sieved using a 60-mesh screen. These powders were
pressed to discs of 10 mm in diameter and 2 mm in thickness
using a bidirectional coaxial pressing machine at a pressure
of 60 MPa. For all samples, the sintering operation was
performed in normal ambient air using a constant heating rate
of 3˚C min−1 up to 600˚C from room temperature followed by
a slow ramping of 2˚C min−1 to 1000˚C. Then, a 1˚C min−1
ramping was used to achieve a temperature of 1175˚C. After
holding at 1175˚C for 2 h (known as hold-time or soak-time)
[8], the samples were cooled to 720˚C using a cooling rate of
1˚C min−1 , and then furnace cooled. The resulting varistor
samples ranged between 8.5 and 8.6 mm in diameter and
1.6–1.7 mm in thickness.
2.2. Densification process
Selected samples were sintered at temperatures varying from
800˚C to 1175˚C, with intervals of 50˚C, and quenched in air
to obtain the densification characteristics. The densification
3 It is a question of the level of the contamination contributed by the agate
mortar, but verified by this author without further publishing. It is quite
difficult to obtain minute trace amount (usually seem to be below ppm level)
contamination of SiO2 while 0.5% mole% SiO2 already exists in the recipe.
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M Lei et al
behaviour is defined as radial shrinkage via L/L0 , where L
is the change in diameter (i.e. before and after sintering) and L0
is the initial diameter. The final densities of the samples were
determined by weight and dimension measurements based on
at least eight samples for each composition and exceed 95%
of the theoretical density.
2.3. Microstructure investigation and phase identification
The surfaces of the sintered samples were lapped with
SiC grinding paper and polished with Cr2 O3 powder to
obtain mirror-like surfaces. After ultrasonic cleaning, the
samples were lightly etched with HCl solution at room
temperature. The microstructure was examined by a scanning
electron microscopy (SEM, Philips XL-20, Netherlands) in
the backscattered electron (BSE) mode. The compositional
analyses were assisted by an attached energy dispersion x-ray
analysis system (EDX, EDAX DX-4, USA). The average grain
size D was determined by the Mendelson intercept method
[26], based on the SEM micrographs using D = [1.56 ×
C]/(MN), where C is the total length of test lines adopted, N
is the number of intercepts and M is the magnification of the
photograph. The crystalline phases present were determined
by powder x-ray diffraction (XRD; Cu Kα radiation: Rigaku
D/MAX-3C, Japan) analysis.
2.4. I–V and C–V measurements
Both surfaces of the samples were coated with conductive Ag
paste and fired at 530˚C for 20 min to obtain Ohmic contact. It
is known that such a contact is always Ohmic as it satisfies
the work function matching of the metal with the varistor
surfaces from previous work [27, 28]. In this context, it is
worth noting that the work function matching is a pre-requisite
to the Ohmic contact, besides minimized surface states at the
semiconductor surfaces. Nevertheless, the surface states play
a minimized role for several large work function metals and
alloys for the varistor samples as demonstrated in [27, 28].
Another aspect of these surface states may be demonstrated as
the frequency dependence of the Mott–Schottky behaviour of
varistors. But it was ascertained by subsequent experiments
that variation of the metals and alloys did not alter the
nature of the frequency dependent Mott–Schottky behaviour,
as demonstrated by Alim et al [29, 30]. This indicates that the
selection of the metallization (electroding) process is not out
of the scope of this study due to the presence of the surface
states. The electrode area was approximately 0.6 cm2 . The
I –V behaviour was obtained using a dc power supply, and
measuring current through the sample and voltage across the
sample by inserting digital multi-meters in series and parallel,
respectively. The nonlinear coefficient α is defined by the
empirical formula [1]:
I = KV α ,
(2)
where K is a constant [4]. The exponent α is calculated from
−1
V1 mA cm−2
,
(3)
α = log
V0.1 mA cm−2
where V1 mA cm−2 and V0.1 mA cm−2 are the voltages corresponding to the current density 1 mA cm−2 and 0.1 mA cm−2 ,
806
respectively. The definition of leakage current, IL , is defined
as 0.8V1 mA cm−2 (=80% of V1 mA cm−2 ). The C–V measurement
was conducted using an in-house built circuit that worked in
the frequency range from 100 Hz to 10 kHz at dc voltages up
to 400 V. This is an ac small-signal superimposed with the
dc voltage measurement where the amplitude of the ac signal used was 120 mV. The measurement frequency range was
established based on the time constant of the circuit, similar
to that used by Richmond [28]. The C–V measurement was
as reasonable as the previously published data [29, 30], showing the presence of the back-to-back Schottky barrier via the
linearity in the Mott–Schottky plane (C −2 versus VDC ).
3. Results
3.1. Microstructure
3.1.1. Scanning electron microscopy. Figure 1 shows the
SEM micrographs of four selected samples. In the C0 sample
(without CeO2 ), two phases designated as the ZnO phase and
the spinel phase are observed, whereas in the CeO2 -added
samples the existence of an additional phase is evident. Since
the atomic number of Ce (58 compared to atomic number 30
for Zn) is large, the new Ce-rich (high Ce content) phase
looks brighter in the image than other areas in the BSE
mode. It distributes mainly at the tri-grain and tetra-grain
(or multi-grain) intersections containing ZnO grains and is
rarely observed along the grain boundaries bounded by two
successive ZnO grains. This is probably due to the atomic
dimension of Ce and/or the solubility of CeO2 in the rest of the
recipe. The existence of the Ce-rich phase becomes prominent
with increasing CeO2 content.
3.1.2. Energy dispersive spectroscopy. The compositional
identification of Ce-rich particles was assisted by EDX and
depicted in figure 2(a). It is found that peaks of Ce, Zn and
O are intense while peaks of all other constituting elements
in the starting recipe such as Bi, Sb, Cr, Mn, Co, Si, etc are
weak. Ce was not observed in the interior of the ZnO grains
as well as in the spinel phase above the detection limit (about
0.1 wt%), for EDX analysis as shown in figures 2(b) and (c).
Again, figure 2(b) refers to the interior of the ZnO grain and
figure 2(c) represents the interior of the spinel phase. Mn and
Co existed at the interior of the ZnO grains. Besides Zn, Sb
and O, the spinel also contains Cr, Mn, Co, etc. Although
the exact value of the intensity may vary slightly from one
measuring point to another, these spectra generally reflect the
typical intensity contrasts of each element.
3.1.3. X-ray diffraction. The powder XRD patterns of
selected samples are shown in figure 3. The JCPDS cards
(ZnO: 5-664; Zn7 Sb2 O12 : 15-687 and CeO2 : 4-593) were
used in the analysis of the phases within the sample. The
XRD data reveal only two pronounced phases in the C0
sample: (1) the ZnO phase and (2) the Zn7 Sb2 O12 spinel phase.
However, in the CeO2 -added samples (C5 as a representative),
then additional peak is evident. Qualitative analysis of XRD
attributes the new phase to the addition of CeO2 in the recipe.
No crystallographic Bi2 O3 peak was observed in any of these
samples. Also there is no evidence of structural complications
ZnO–Bi2 O3 based varistors
Figure 1. SEM micrographs of various ZnO–Bi2 O3 based varistors: (a) C0, (b) C1, (c) C3 and (d) C9. (Zn: ZnO phase; Sp: spinel phase;
Ce: Ce-rich phase.)
via overlapping of Bi and Ce. However, the SEM studies
reveal that the Bi-rich phase is scattered at the ZnO–ZnO grain
boundary interfaces as well as in the tri-grain and four-grain
intersections. This is consistent with the observations in most
commercial varistors.
3.1.4. Densification. Figure 4 shows the densification
curves for various varistor samples as a function of sintering
temperature. It is clear that as the amount of CeO2 increased
the curves moved to the high temperature side. This means
that the densification for the CeO2 -added samples is occurring
at high temperatures while the reference sample is dense at the
same radial shrinkage level at low temperatures.
3.2. Electrical properties
3.2.1. I –V behaviour. The variation of the breakdown field
[31] Eb with CeO2 content is shown in figure 5. The grain size
variations are also plotted for comparison. It can be seen that
the average ZnO grain size decreased from 7.8 to 5.7 µm, while
the breakdown field (Eb ) increased substantially from about
235 to about 425 V mm−1 . The current density versus electric
field (J –E) characteristics of the investigated samples is shown
in figure 6. The calculated parameters α and IL are displayed
in figure 7 with increasing CeO2 content. The varistor samples
of figures 6 and 7 were sintered at 1175˚C. The leakage current
IL decreased with the addition of CeO2 below 0.3 mole% and
then steadily increased above it. A drastic change is observed
in the values of Eb while the nonlinear coefficient α did not
change substantially and remained around 30. The values for
α, Eb and IL are obtained for at least eight samples for each
composition. Therefore, the values presented are a statistical
representation of the devices.
3.2.2. C–V response. The C–V characteristics of the C1
sample for the measurement frequency range 100 Hz through
10 KHz is depicted in figure 8, wherein the corresponding
I –V behaviour is also provided for the measurement in
the same voltage range. Each measurement frequency
yielded a Mott–Schottky curve containing three distinct
regions reflecting: (1) the Ohmic range, (2) the non-Ohmic
(intermediate) range and (3) the breakdown range. The
linear region of the Mott–Schottky response corresponds to
the non-Ohmic voltage range of the device. For the C1
sample the nonlinear coefficient α in this non-Ohmic voltage
range is low compared to the devices’ Mott–Schottky result
reported earlier [29, 30]. In these early works, commercial
varistors possessing α values of 35–60 were reported. Each
of these curves corresponding to the measurement frequency
appears nearly parallel but non-converging, i.e. not converging
upon extrapolation on the voltage-axis. Thus, frequencydependent straight lines are obtained in the low, nonlinear
I –V region for the C1 sample. The frequency-dependent
Mott–Schottky behaviour was discussed earlier [29, 31] for
most laboratory and commercial devices. In the breakdown
region, at low frequencies, it usually shows an increase in
capacitance with applied voltage, referring to the decrease
807
M Lei et al
Grain size, D (µm)
8
450
D
Eb
400
7
350
300
6
250
5
0.0
0.2
0.4
0.6
0.8
200
1.0
Breakdown field, Eb (V/mm)
Figure 4. Densification of the varistor as a function of sintering
temperature with CeO2 content.
CeO2 content (mol%)
Figure 5. Variation of the ZnO grain size D and the breakdown field
Eb with CeO2 content.
Figure 2. EDAX patterns of (a) Ce-rich phase, (b) ZnO grain and
(c) Zn7 Sb2 O12 spinel phase for sample C1.
Electrical field, Eb (V/mm)
1000
100
10
C0
C1
C3
C5
C9
1 –9 –8 –7 –6 –5 –4 –3 –2 –1
10 10 10 10 10 10 10 10 10
ZnO
Zn7Sb2O12
CeO2
Current density, J (A/cm2)
Figure 6. Current density versus electric field (J –E) characteristics
of the ZnO–Bi2 O3 based varistors added with CeO2 . C0 stands for
the reference sample while C1, C3, C5 and C9 stands for
0.1 mole%, 0.3 mole%, 0.5 mole% and 0.9 mole% CeO2 varistors,
respectively.
(b)
(a)
20
30
50
40
2-THETA (degrees)
60
70
Figure 3. XRD patterns of ZnO–Bi2 O3 based samples: (a) without
CeO2 (C0) and (b) with 0.5 mole% CeO2 (C5).
in the overall depletion region caused by the destruction of
the electrical barriers, while at high frequencies the trapping
response becomes transparent to the conduction processes,
thereby eliminating the contribution of the trapping, causing
808
a decrease in capacitance with increasing applied voltage.
The low frequency Mott–Schottky response is an effect of the
geometric capacitance corresponding to the net grain boundary
depletion region width. The high frequency Mott–Schottky
response is an effect of the reduction of the trapping effect as
the conduction becomes transparent across the grain boundary
electrical barrier. The transparent conduction implies a
decrease in the trapping response and the terminal capacitance
gradually approaches a minimum.
ZnO–Bi2 O3 based varistors
40
α
35
34
Il
30
32
25
30
20
28
15
26
10
24
5
22
0.0
0.2
0.4
0.6
Leakage current, Il(µA)
Nonlinear coefficient,α
38
36
0
1.0
0.8
CeO2 content (mol%)
Figure 7. Nonlinear coefficient α and leakage current IL versus
CeO2 content.
10KHz
24
1000
5 KHz
1 KHz
500Hz
800
2
1/C
100Hz
20
600
18
400
o
16
T= 18 C
I
I (µA)
1/C2 ( x 1018F–2)
22
the interface with the ZnO grains. It is unlikely that significant
solid-state reactions between them and surrounding materials
have taken place. The ZnO–ZnO interfaces and other phases
are distinct in the microstructure. The spinel phase is broader at
the edges of the ZnO grains. It is speculated that the sequential
incorporation of Sb2 O3 , ZnO, Bi2 O3 , and other additives play
a role in the formtion of the spinel during the sintering cycle.
Nevertheless, the Zn peaks in the EDX pattern in the CeO2
regions (figure 2(a)) do not necessarily mean that there is a
substantial amount of Zn in CeO2 , because the diameter of
the electron beam of EDX is about 1 µm. Therefore, if the
size of the CeO2 phase is close to this size (as is the case
from SEM micrographs) signals from the surrounding area
(ZnO grains or grain boundaries rich in elements in the basic
recipe) will also enter into the spectrum. This will cause
an erroneous assessment of the microstructure. Based on
the foregoing discussion, we can conclude that the solid-state
reactions between CeO2 and ZnO, if they did occur, were too
weak to form a new Ce–Zn–O phase, or the Ce-rich regions
are essentially CeO2 .
200
14
0
100
200
300
400
VDC (Volts)
Figure 8. Mott–Schottky behaviour and I –V response of the
ZnO–Bi2 O3 based varistor containing 0.1 mole% CeO2 (C1).
4. Discussion
4.1. Compositional analysis
The exact structure of the Ce-rich region is not precisely
known at this moment. However, some concepts may be
discussed based on the results. It is known that the ionic radius
2+
4+
4+
mismatch [32] between Zn2+ and Ce4+ [(rZn
− rCe
)/rCe
) is
2+
stands for the ionic radius
about −7.22%. The parameter rZn
4+
of Zn2+ and rCe
stands for the ionic radius of Ce4+ . Based
on the mismatch value, ZnO may undergo solid–solid solution
with CeO2 via two mechanisms: (1) vacancy compensation
and (2) cation interstitial compensation. Thus, a limited solid
solution of Ce1−z Znz O2−z may be formed by the following
solid-state reactions:
CeO2
••
ZnO −→ ZnCe
+ VO
+ OzO ,
CeO2
+ Zni•• + 2OzO .
2ZnO −→ ZnCe
(4)
4.2. Effect of grain size refinement
Considering the high melting point of CeO2 (about 2600˚C),
it is unlikely that there will be a liquid phase, as Bi2 O3
has in the recipe. Actually, the ratio of final density
(ρ) and theoretical density (ρt ) of the samples decreased
monotonically from 96% to 95% (of the theoretical density)
as the CeO2 content increased. At the same time, the
ZnO varistor microstructures were refined with the grain size
decreasing consistently from 7.8 to 5.7 µm. These results,
when combined with the distribution of the CeO2 phase, have
allowed one to postulate that the CeO2 particles pinned at
the grain boundaries during sintering. Thus, it hindered the
mass transportation during the sintering cycle and restricted
the grain growth substantially. The observed behaviour of the
grain growth inhibition incorporating CeO2 in the recipe is an
additional behaviour of the grain refinement caused usually by
the addition of Sb2 O3 . The role that Sb2 O3 plays has been
confirmed in many previous publications.
Although the addition of CeO2 provided a sharp increase
in Eb , the concurrent impact on the nonlinear coefficient
α is less. Considering α and IL concurrently, it appears
that the C9 sample is somewhat superior among all the
samples investigated. This comparative assessment is based
on considerations of suitability for application. Since the
geometry of the investigated samples is kept the same,
each parameter is, thus, normalized while describing their
behaviour. The values of α, Eb and IL for the C9 sample
are 35, 425 V mm−1 , and 4.8 µA, showing similar nonlinear
characteristics but a much higher breakdown field than the
reference C0 sample with α, Eb and IL of 36, 230 V mm−1 , 36
and 13.1 µA, respectively.
(5)
The value of z depends on the solubility of these two
oxides (ZnO and CeO2 ) and may vary from sample to sample.
However, from the morphology of the Ce-rich regions observed
in the SEM micrographs it can be seen that these reactions
might not be a significant plausible concept. The shape of the
Ce-rich region is short or small in terms of the formation of
4.3. Effect of slow cooling
It is worth noting that when the cooling rate is accelerated,
specifically from 1 to 4˚C min−1 , the varistor performance
suffered an obvious degradation via decreasing α and
increasing IL . From the application stand point this is a critical
aspect concerning a well-formed varistor referring to a stable
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M Lei et al
behaviour that is usually used in the commercial sector [8–15].
Thus, the cooling cycle plays an important role in the resulting
performance of the varistors.
A few investigators [33–35] noted that the addition of
the transition metals (Co, Mn, etc) strongly enhances the
nonlinear characteristics and also affects the leakage current.
Li [36] and Song [37] demonstrated that the origin of high
nonlinearity is attributed to the segregation of non-saturated
transition metal oxides at the grain boundary regions that occur
primarily during the cooling process. In an earlier study,
Sletson et al [8] pointed out that the change in chemistry affects
partial elemental distribution such as Mn and Ni at the grain
boundaries. Thus, for a specific recipe, each sintering cycle
provides a new distribution of these cations both at the grain
boundaries as well as in the ZnO grains. At this juncture it is
relevant to note that the immittance (impedance or admittance)
measurements resolved the concept of the role of the traps
(defect states capable of holding and releasing charges) via a
new (or modified) equilibrium concentration of the traps as the
sintering profile is modified and then used with the same recipe.
This concept was extensively evaluated by Alim [10–12] using
immittance measurements and subsequent analytical processes
on a variety of varistor samples to understand the role of
the trapping behaviour at the grain boundary interfaces. In
particular, the regions where the voltage (or the electric field) of
the ac small-signal drops at the interfaces provides an effective
contribution to the terminal immittance. A large portion of the
amplitude of the ac small-signal voltage drops across the grain
boundaries because it is the most resistive part of the device
compared to the lumped ZnO grains. Such a contribution is
attributed to the sintering profile in addition to other processing
variables. Keeping other processing variables fixed for a
specific recipe, the variation in the sintering profile affects the
behaviour of the intrinsic trapping (defined as τ3 -relaxation), as
reported earlier [8, 10–12, 29, 30], and confirmed by a unique
method of using lumped parameter/complex plane analysis
(LP/CPA). Levinson and Philipp (see [29, 30]) originally
called the τ3 -relaxation the intrinsic trapping behaviour. This
was because a common response around 10−6 Hz for every
varistor, regardless of the content and constituent of the recipe,
was obvious whether prepared in the laboratory or in the
manufacturing set-up. Thus, the primary contribution to the
electrical response is attributed to the ZnO properties.
The trapping is the behaviour of the defect states that
respond to the applied ac small-signal in the form of immittance
data. The symbol τ3 , obtained in the complex plane plot
formalism (complex capacitance plane, C∗ ) using these data,
noted earlier [30, 31], denote the intrinsic trapping behaviour
of all types of ZnO–Bi2 O3 based varistors. Using immittance
data, Sletson et al [8] noted that the inherent intrinsic
defect states in ZnO-based varistors reach a new equilibrium
concentration for each sintering cycle, which ultimately affects
the response of the intrinsic trapping behaviour. Based on
these reports, it is noted that the intrinsic trapping substantially
differs with every variation in the processing route. Invariably,
this includes the variation in the cooling rate. However,
based on the available information in the literature it is
strongly believed that during the sintering process Co and Mn
cations are dispersed into the ZnO grains. These dispersions
are attributed to the quantified intrinsic traps called the
810
τ3 -relaxation [8, 10–12]. A slow cooling rate allows enough
time for Co and Mn cations to equilibrate at the surfaces or
inter-granular interfaces of the ZnO grains. This process would
vary the interface state density Ns greatly, which in turn causes
a variation in the barrier height b and nonlinear coefficient α.
Therefore, at least in the present system a slow cooling cycle
is beneficial to the nonlinearity of the CeO2 added varistors.
Nevertheless, it is known that the spinel phase would
transform into pyrochlore during the cooling process, given
below as indicated by Olsson et al [35, 38–41].
3Zn7 Sb2 O12 + 3Bi2 O3 (L)
+17ZnO.
650˚C∼950˚C
−→
2Zn2 Bi3 Sb3 O14
(6)
They mentioned that the pyrochlore phase is detrimental
to the nonlinear response of the ZnO varistors. A slow cooling
rate also renders enough time for pertinent reactions [20] that
allows a variation in the intrinsic species as well as other
trapping species. The role of these traps on the performance
of the stable and well-formed varistors have been summarized
by Alim [10–12]. However, pyrochlore was not identified via
XRD in the samples studied. This may be explained by the
combined effect of the inadequate role of Bi2 O3 due to the
presence of CeO2 , which inhibits grain growth in conjunction
with Sb2 O3 despite allowing a finite soak-time (2 h) and
having Cr2 O3 in the recipe. Zn7 Sb2 O12 can dissolve a fairly
large amount of Cr5+ , [42, 43], resulting in the formation of the
α-spinel rather than the β-spinel. The α-spinel is more stable
and does not transform to pyrochlore during cooling [44–48].
Further examination of the XRD patterns shows that the spinel
peaks appear only at 2θ ≈ 29˚, 35˚, and 52˚ (figure 3) and
are absent at least in the region 2θ = 21˚ through 28˚. This
suggests strongly that the observed spinel phase in the resulting
ZnO varistor samples is an α-spinel [44]. Thus, the presence
of the β-spinel in the microstructure is ruled out. Based on the
foregoing discussion, it is reasonable to say that in the present
system containing Ce and Cr, only a trace amount of spinel
will transform into pyrochlore, although the cooling rate is
very slow. Therefore, highly nonlinear characteristics of these
varistors are guaranteed.
In contrast to the above description it may be noted that
this study confirms that the increased breakdown field has been
achieved due to the addition of CeO2 to a ZnO–Bi2 O3 based
recipe. It is observed that the addition of CeO2 causes no
change in the level of the theoretical density or porosity of
the sintered body. It is clear that CeO2 does change the grain
size in an identical way to that of Sb2 O3 via the formation of
a Ce-rich phase. Again, the Ce-rich phase is not identical to
what is obtained usually by Sb. Therefore, the role of Sb and
Ce is not identical except for the charge of the defect states
caused by Ce. This is presumably due to the valence state of
Ce which is not the same as that of Sb.
4.4. C–V characteristics
Initial C–V studies have been conducted to comprehend the
nature of the frequency dependence of the interfacial traps
resulting from the grain surface chemistry and interaction
of the grain boundary interfaces with the ac small-signal
amplitude. These results are presented, and they display the
fundamental basis for the future comprehensive immittance
ZnO–Bi2 O3 based varistors
studies of these samples. The existence of the Schottky
barriers at the grain boundary regions has been confirmed
via the voltage dependence of the capacitance measurement.
Inspection of the measured ac small-signal C–V data reveals
the strong influence of charge transport, as shown in the
frequency-dependent Mott–Schottky plot in figure 8. The
frequency-dependence of the Mott–Schottky slope strongly
suggests that the single-frequency C–V analysis can invariably
lead to an erroneous set of device-related parameters such
as built-in potential (i ), barrier height (b ), carrier density
(for ZnO grain: donor density Nd ), etc. Extracting devicerelated parameters was not emphasized in this study except
for comprehending the nature of the trapping and the Schottky
barrier response. In this context, it may be noted that Mukae
et al [49] used a modified Mott–Schottky equation for the
analysis of the measured C–V data, and extracted frequencydependent device-related parameters for ZnO–Pr6 O11 based
varistors using an arbitrarily selected measurement frequency,
e.g. 1 MHz. In contrast, it is asserted in earlier works
[29, 30] that a meaningful insight satisfying a reasonable
physical basis is lost when Mukae et al’s equation is used
regardless of the varistor recipe. Based on this fact, Mukae
et al’s modified Mott–Schottky equation is not used but
a conventional frequency-dependent Mott–Schottky plot is
achieved. Like other ZnO varistors, the CeO2 added varistors
exhibited a strong frequency dependence of the C–V data
as shown in figure 8.
The frequency-dependent C–V
behaviour is indicative of the complexity of the trapping states
within the depletion regions constituted by grain boundaries.
Additionally, the breakdown region C–V response is also
complex, as was observed for the usual commercial varistors
[29, 30].
Obviously the low-voltage region is attributed to the
Ohmic region, where the net change in the barrier width
(depletion region) is nearly unchanged, or very little change
takes place due to the symmetric configuration of the electrical
barrier across the grain boundaries. This is further explained
using the fact that the net change in the depletion region
is near zero at the Ohmic voltage range. The straight line
segment is referred to the low nonlinear range of the I –V
behaviour for the C1 sample. At these biasing ranges the
applied dc voltage is experienced only on the reverse bias
side while the forward bias side is pinned. Therefore, a net
change in the depletion is experienced only on the reverse bias
side of the electrical barrier across the grain boundaries. The
third region is observed in the high voltage domain wherein the
device experiences breakdown and the measured capacitance
loses linearity due to the gradual destruction of the electrical
barriers in the microstructure. Such a behaviour was reported
by Morris [50] and others [29, 30]. Indeed the LP/CPA is
a technique for analysing such frequency-dependent C–V
behaviour. This technique has been effectively demonstrated
earlier for polycrystalline systems on several occasions.
5. Conclusions
The CeO2 added to the ZnO–Bi2 O3 based varistor recipe
remained very much unchanged chemically but plays a
significant role physically via pinning at the ZnO grain
boundaries and, thus, inhibited the grain growth during the
sintering process. The average grain size decreased with
increasing amount of CeO2 resulting in a substantial increase
in the breakdown field Eb of the samples, while the density
of all the well-formed varistor samples was above 95% of the
theoretical value.
The influence of CeO2 addition on the nonlinear
coefficient α is very little but the leakage current IL was
affected. However, this finding allowed a demarcation
or a cut-off in the amount of CeO2 added, with respect
to a reference ZnO–Bi2 O3 based varistor composition.
The nonlinear response of the varistor samples showed
potential improvement when a slower cooling rate is used.
The detrimental pyrochlore phase was not found in any of the
CeO2 -added samples. Experimental data also suggest that the
α-spinel phase is predominant in the microstructure, attributed
primarily to the existence of Cr and Ce. The frequencydependent C–V data indicate that these varistor samples
possess back-to-back Schottky barriers across the grain
boundaries possessing a complex nature of the trapping states.
Thus, the precise extraction of the grain-boundary parameters
needs further clarification via additional experiments.
Acknowledgment
The authors would like to acknowledge Dr Xiaobing Ren of
National Institute for Materials Research of Japan for valuable
suggestions in the preparation of this manuscript.
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