IOP PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 41 (2008) 122002 (5pp)
doi:10.1088/0022-3727/41/12/122002
FAST TRACK COMMUNICATION
Comparative degradation of ZnO- and
SnO2-based polycrystalline non-ohmic
devices by current pulse stress
M A Ramı́rez1 , W Bassi2 , P R Bueno1 , E Longo1 and J A Varela1
1
2
Instituto de Quı́mica, Universidade Estadual Paulista, UNESP, CEP. 14800-900, Araraquara, SP, Brazil
Instituto de Eletrotécnica e Energia, Universidade de São Paulo, USP, 05508-010, São Paulo, SP, Brasil
E-mail: margbrasil@yahoo.com
Received 5 February 2008, in final form 24 April 2008
Published 19 May 2008
Online at stacks.iop.org/JPhysD/41/122002
Abstract
The degradation behaviour of SnO2 -based varistors (SCNCr) due to current pulses (8/20 µs) is
reported here for the first time in comparison with the ZnO-based commercial varistors (ZnO).
Puncturing and/or cracking failures were observed in ZnO-based varistors possessing inferior
thermo-mechanical properties in comparison with that found in a SCNCr system free of
failures. Both systems presented electric degradation related to the increase in the leakage
current and decrease in the electric breakdown field, non-linear coefficient and average value
of the potential barrier height. However, it was found that a more severe degradation occurred
in the ZnO-based varistors concerning their non-ohmic behaviour, while in the SCNCr system,
a strong non-ohmic behaviour remained after the degradation. These results indicate that the
degradation in the metal oxide varistors is controlled by a defect diffusion process whose rate
depends on the mobility, the concentration of meta-stable defects and the amount of
electrically active interfaces. The improved behaviour of the SCNCr system is then inferred to
be associated with the higher amount of electrically active interfaces (85%) and to a higher
energy necessary to activate the diffusion of the specific defects.
(Some figures in this article are in colour only in the electronic version)
those having a SnO2 · CoO · Nb2 O5 · Cr2 O3 composition,
named SCNCr) in the 1990s, which has also been extensively
studied and it is suggested to possess some advantages when
compared with the ZnO-based compositions. The advantages
claimed by SCNCr compositions seem to be intimately related
to its simple microstructure and relatively low concentration
of additives necessary to attain electrical non-ohmic properties
[5]. A special advantage pertains to the high refractivity
of the SCNCr system [6, 7], which minimizes losses due to
evaporation and is allied to a higher thermal conductivity
(0.5 W K−1 cm−1 ), which is almost double that found for the
ZnO-based varistors [8]. Finally, the SCNCr system presents
excellent mechanical properties [9]. All these advantages
justify the analysis of its degradation behaviour with pulses,
1. Introduction
Non-ohmic devices (i.e. devices presenting non-linear voltage–
current characteristics) have been employed in electrical
and electronic circuits as over-voltage protection and surge
absorbers.
Due to their excellent non-linear electrical
characterisitics, non-ohmic electrical ceramics, known as
varistors, are often used as surge arrestors against switching
spikes or lightning strikes [1]. The varistor protects a circuit
by ‘clamping’ the voltage across it to a safe level [2–3].
The most studied varistor system, being currently used
for commercial purposes, is based on a ZnO composition, as
initially developed by Matsuoka around 1970 [4]. Pianaro et al
[5] developed a SnO2 -based varistor composition (specifically
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J. Phys. D: Appl. Phys. 41 (2008) 122002
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a phenomenon already sufficiently studied in ZnO-based
varistors, but still lacking for the SCNCr system. Therefore,
the main purpose of this work is to study degradation behaviour
of the SCNCr systems and to compare them with traditional
ZnO-based varistors.
The voltage of lightning strikes is very high and when it
reaches electronic or power circuits, a great amount of damage
may occur due to the burning of devices and/or the interruption
of the electric energy leading to great economic losses. Due
to this fact, the study of the degradation mechanisms of
new varistor compositions is justified to improve the devices.
Generally, the degradation phenomena are studied under stress
using alternating current (ac), direct current (dc) or pulse
electric fields, thus giving rise to changes in both the nonlinear coefficient and the leakage current [10,11]. In this work,
the degradation of the ZnO- and the SnO2 -based varistors
(particularly the one referred to as SCNCr) is analysed by
applying 8/20 µs pulses. This pulse behaviour represents an
atmospheric lighting discharge well [12]. Several mechanisms
of degradation for ZnO-based varistors have been suggested
such as electron trapping, dipole orientation, ion migration
and oxygen desorption [13, 14]. Among these, the ion
migration mechanism finds strong support in the light of
experimental evidence [2, 15–17]. Many techniques have
been proposed to investigate the degradation mechanisms
of the varistors, including deep-level transient spectroscopy,
thermally stimulated current, impedance spectroscopy (IS),
C–V characteristics curves and so on [13, 18–19]. In this
work, J –E measurements and C–V characteristics were used
in order to monitor the electrical degradation of the potential
barrier in ZnO- and SnO2 -based varistors.
Figure 1. Oscillographs of the current and voltage wave used in the
degradation tests for ZnO system (a) before and (b) after
degradation.
a predetermined number of pulses was applied until either
degradation occurred or the voltage wave fell to zero (short
circuit), as shown in figure 1(b). The total energy absorbed by
the samples was calculated according to the equation:
t
vi dt,
(1)
E=
2. Experimental procedure
The ZnO system has a composition corresponding to 95.4%
ZnO + 1.5% Sb2 O3 + 1% NiO + 0.1% SiO2 + 0.5% (Bi2 O3 ,
SnO2 , Co2 O3 and MnO). The SnO2 system has a composition
corresponding to 98.9% SnO2 + 1% CoO + 0.05% Nb2 O5 +
0.05% Cr2 O3 (named here as SCNCr). All these proportions
are given in mol%. The pellets were prepared in an adequate
area/volume ratio (A/V ) [6, 20] from analytical grade raw
materials employing mixed oxide procedures as described in
previous works [4, 5]. After the sintering process, the pellet
samples were polished until the faces became flat and parallel.
Silver electrodes were deposited onto these faces and treated
at 400 ◦ C for 30 min. A critical event is the encapsulation of
the ceramic disc since the selected material might influence
the electrical response. In this work, the encapsulation was
done according to the procedure related in [21]. The high
voltage circuit to generate the pulses of 8/20 µs was based
on three capacitors having a total capacitance of 2.25 µF
and a maximum voltage and energy of 200 kV and 45 kJ,
respectively. The response of the devices was registered with a
Tektronix (8 bits, 100 MHz) digital oscilloscope. The sample
blocks of both systems were degraded by directly applying a
series of individual current pulses as shown in figure 1(a) with
different magnitudes of current (from 100 A up to 5 kA). The
time between the pulses was fixed at 20 s. For each sample,
0
where v is the voltage, i is the current and t is the time for which
current is applied. The capacity to absorb energy is usually
expressed as the maximum energy absorbed per volume unit
before the sample fails and it is measured in J cm−3 .
Before and after the degradation process, each sample was
electrically characterized by J –E and impedance spectroscopy
(IS) analysis. The electrical parameters that determine the
varistor’s behaviour, i.e. the electrical breakdown field (Eb ),
the non-linearity coefficient (α) and the leakage current
(IL ) were obtained from the J –E characteristic curves [1].
The measurements were carried out under room temperature
conditions with a Keithley 237 (60 Hz) source-measurement
unit with a maximum current value of 10 mA. The IS was
carried out using a frequency response analyzer 4192 LF HP
covering frequencies from 5 Hz to 110 MHz with an oscillation
amplitude of 0.5 V. There was also a continuous current
potential (Vcc up to 38 V) superimposed over the alternating
potential. Considering the variation of the capacitance as a
function of the applied dc voltage for each varistor system,
the parameters of the potential barrier such as height (φb ),
width (ω), donor (ND ) and superficial state density (NIS )
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J. Phys. D: Appl. Phys. 41 (2008) 122002
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were calculated according to the model of the methodology
proposed in [22].
3. Results and discussion
Figures 1(a) and (b) show current and voltage waves
(oscillographs) before degradation and for the last current
pulse that generated the degradation in the ZnO system.
Note that in the last voltage pulse, a drastic decrease was
observed related to the short-circuit mode reached by the
system due to the atmospheric dielectric breakdown (surface
flashover phenomena), i.e. due to the encapsulation failure
or, in other words, due to thermo-mechanical failure which
is classified (or best known) as puncturing and/or cracking
failure. Equation (1) was applied to each oscillograph to
calculate the total energy for the degradation and the average
value is given in table 1. The energy absorbed by the SCNCr
system is about twice the energy value absorbed by the ZnO
system. This result is closely and accordingly related to the
fact that the SCNCr system exhibits approximately double the
thermal conductivity [8] compared with the ZnO system. As a
result, the SCNCr system can dissipate heat more easily, which
avoids the thermal avalanche regime.
Figure 2(a) shows the J –E behaviour for the ZnO system
before and after the degradation. From this figure, it can
be observed that the systems change from a highly nonohmic feature (α = 68) to a completely ohmic feature
(α = 1) after degradation. The breakdown electric field Eb
(V cm−1 ) was taken as the applied field at a current density
of 1 mA cm−2 , as is commonly used. Strictly, Eb is defined
as the point of maximum change in a slope of a log J –log E
plot. Nevertheless, values at 1 mA cm−2 provided a consistent
comparison between the different samples. In the degraded
ZnO system, electrical breakdown, which is one of the main
characteristics of a varistor, was not observed. The slope of
the J –E characteristic curves gives information on the total
resistivity of the system. The resistivity of the degraded ZnO
system was estimated as ρ = 1.0 cm. This value is in
agreement with recent values found [21] according to the slope
of the J –E curves in high-current regions, i.e. a J –E region
named and known as region III (see [21] for more details). In
fact, this region III is an upturn that corresponds to the high
current region where the electrical conduction behaviour is
governed by the resistivity of the grains. The ohmic feature
observed in the ZnO system after degradation, in which the
total resistivity of the system coincides with the value of the
grain resistivity, is an important indication that there is total
degradation of the grain-boundary potential barrier. After
degradation, the grain boundary does not contribute to the total
resistivity, which is then controlled by the bulk.
The degradation of the non-ohmic property occurs due to
multiple causes and the main ones are: (1) macrostructural
failures whose occurrence depends on the microstructure
and the thermo-mechanical features of the sample, (2)
local structural changes governed by bulk compositional
rearrangement and/or by grain-boundary chemistry changes,
which determine the characteristics of the potential barrier.
To elucidate the degradation mechanisms at a macrostructural
Figure 2. J –E plot before and after the degradation. (a) ZnO
system (photographs of the sample before and after degradation are
shown in the inset and they show the thermo-mechanical failures)
and (b) the SCNCr system (photographs of the sample before and
after degradation are shown in the inset and they show the insulation
failure).
level, photographs of the ZnO varistor surface were taken
before and after degradation and are shown in the inset of
figure 2(a). After the degradation, the partial fusion of the
silver electrodes (black regions) is shown and the thermomechanical puncturing and cracking failures are also observed.
These are usually observed in ZnO degradation tests [1–3]. It is
important to stress that the ZnO-based varistors here presented
concomitantly exhibited both types of failures, which is in
disagreement with the observations made by Eda [23] and
Vojta [24]. Both authors reported a correlation between the
duration of the pulse and the type of failure. According to them,
short pulses typically generate cracking failures, while long
pulses (100 µs) lead to puncturing failure. Recently [9], it
was proposed that the type of failure is associated with thermomechanical behaviour instead of the pulse-duration type.
Figure 2(b) shows the J –E behaviour before and after
the degradation in the SnO2 -based system, i.e. the SCNCr
composition. Similarly to the ZnO system, the SCNCr system
degradation was monitoring by following the values of Eb , α,
as well as the increase in the IL parameter. More details are
given in table 1. It is clearly observed that the degradation
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J. Phys. D: Appl. Phys. 41 (2008) 122002
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Table 1. Non-ohmic electrical features of ZnO and SCNCr (SnO2 -based) systems before and after degradation.
Sample
ZnO before
ZnO after
SCNCr before
SCNCr after
E (J cm−3 )
5560
13 800
Eb (V cm−1 )
2945
—
4720
2590
IL (µA)
α
1.0
2620
1.2
235
68
1
75
4
in the SCNCr system is less severe than that seen in the ZnO
system. After the degradation in the SCNCr system, nonohmic behaviour is still present, which was not observed in
the case of ZnO system degradation. The IL value after
degradation is 235 µs for SCNCr. One of the most relevant
changes observed after the degradation of the SCNCr system
is the increasing range of the pre-breakdown regions (elbow),
which gives information regarding the effectiveness of the
potential barrier and the amount of leakage current. According
to non-ohmic parameters, it can be concluded that the potential
barriers have lost their efficiency after degradation. However,
as there are non-ohmic features after degradation, it is possible
to conclude that there are still either an active potential
barrier or electrically active grain–grain junctions, which are
particularly evident due to the presence of a breakdown field
after degradation (see table 1). To confirm this, electrostatic
force microscopy was performed according to the procedures
described in [21, 27].
In figure 2(b), the photograph of the SCNCr samples is
shown before and after the degradation stress. There is no
puncture and/or cracking failure in the SCNCr system. This
feature can be related to the fact that the SnO2 -based varistor
system has a very good thermo-mechanical behaviour [9].
Indeed, the only macroscopic failure observed in this system is
the rupture of the lateral insulation, as indicated in figure 2(b).
The potential barrier degradation mechanism can be
inferred by means of complex impedance measurements,
which allow us to separate the electrical features of the grains
themselves and the grain boundaries [22]. Figure 3(a) shows
the Nyquist diagram at 0 V bias at room temperature before
and after the current stress pulses. The Nyquist diagram
gives evidence that before degradation, the total resistance
of the system is high, which is in agreement with the nonohmic feature under this condition. After degradation, the total
resistance is lower, as is expected and this is in agreement with
the increase in the leakage current. In the inset of figure 3(a) is
shown the Mott–Schottky pattern constructed according to [22]
and the capacitance of the grain boundary was obtained at
1 MHz for both systems. The potential barrier parameters
obtained are indicated in table 1. The values found here are in
agreement with those found for this kind of system [15, 22].
After degradation, there is no Mott–Schottky pattern for this
system, and hence it is impossible to calculate the potential
barrier parameters.
Figure 3(b) shows the complex impedance pattern for the
0 V bias of the SCNCr system before and after degradation
measured at room temperature. It can be observed that the
complex impedance pattern changes after applying the pulses.
The changes indicate a decreasing total resistance and this is
associated with the increasing leakage current. In comparison
φb (eV)
ND (m−3 )
1.03
—
1.65
0.52
6.86 × 10
—
1.50 × 1024
1.10 × 1024
23
NIS (m−2 )
ω (nm)
2.50 × 10
—
6.19 × 1016
2.98 × 1016
18.2
—
20.6
13.5
16
(a)
(b)
Figure 3. Nyquist plot at 0 V bias and room temperature before and
after the degradation for (a) ZnO system. A typical Mott–Schottky
behaviour after degradation is shown in the inset. (b) SCNCr
(SnO2 -based) system. A typical Mott–Schottky behaviour after
degradation is shown in the inset.
with previous result for the ZnO system, it can be seen that
the degradation of the potential barrier in the SCNCr system is
partial. In contrast to what is observed in the ZnO system,
the Mott–Schottky pattern of the degraded SCNCr system
still remains, which indicates the presence of space charges
in the grain boundaries. This means that there is still an active
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J. Phys. D: Appl. Phys. 41 (2008) 122002
Fast Track Communication
potential barrier in the grain-boundary region. In this case, it is
possible to compare the potential barrier parameters before and
after degradation, as indicated in table 1. It can be observed
from table 1 that the potential barrier height and width and the
density of the superficial state decrease.
The ac and dc results are consistent and in agreement
with the models proposed to explain the degradation of the
barriers. The most accepted models in the literature [2, 15–
17] are based on the electro-migration of meta-stable defects
(defect positively charged with high mobility and located in the
•
••
•
••
depletion layer, Zni , Zni , VO , VO in ZnO-based varistors [16]
•
•
••
and VO , VO , NbSn in the SCNCr system [26]). These defects
migrate to the grain boundaries where they are capable of
recombining with negatively charged defects (Oads , Oads in
both systems). Therefore, the electro-migration model is valid
for the traditional ZnO non-ohmic system as well as for the
SCNCr system.
The better performance of the SnO2 non-ohmic devices
compared with the ZnO system may be related to the greater
number of active potential barriers. The proportion of active
grain–grain junctions was 85% for SnO2 [25, 26] against
35% for the ZnO [21]. Also up to this moment, the metastable interstitial defects in SnO2 had not been identified. For
••••
example, Sni is capable of helping in the degradation of the
potential barrier. Furthermore, the high thermal conductivity
of the SCNCr system is capable of avoiding the diffusion
processes related to the degradation. A more detailed study
of the comparative degradation in SnO2 and ZnO systems in
terms of the degradation mechanism will be the subject of
a future work. The main goal of this paper was to report
degradation features of the SCNCr compositional varistor
system with 8/20 µs pulses. In particular, this system showed
slow degradation kinetics that favour the stability of the
material when compared with ZnO under the same stress
conditions.
of its superior thermo-mechanical properties when compared
with the traditional ZnO system.
Acknowledgment
The authors gratefully acknowledge the financial support of
the Brazilian financing agency FAPESP.
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4. Conclusions
After applying the 8/20 µs current pulses, the ZnO-based
varistors changed from a highly non-ohmic behaviour (α ∼ 68)
to an essentially ohmic one (α ∼ 1) having a low-resistance
behaviour. On the other hand, the SCNCr system also presents
a degradation of its non-ohmic behaviour; but after the stress,
the system still had a non-ohmic characteristic. The change in
the non-ohmic behaviour of the system was associated with the
degradation of the potential barrier. The observed changes in
the non-ohmic behaviour were associated with the fact that the
current stress pulses have sufficient energy to active diffusion
via electro-migration of the meta-stable defects adjacent to the
grain boundary region where they are capable of combining
with negative defects.
Finally, we can infer that the degradation kinetics are
different and slower in the SCNCr system when compared
with the ZnO system. Furthermore, the SCNCr system does
not present failures due to puncturing and/or cracking because
5