Study on the electrical properties and defect structures of a high voltage gradient ZnO varistor Xuetong Zhaol*, Ruijin Liaol, Kanglin Liul, Feipeng Wangl, Jianying Li2 IState Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, Shapingba District, Chongqing, 400044, China
2State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, No.28, Xianning West Road, Xi'an, Shaanxi, 710049, China *E-mail:
[email protected]
Abstract: In this research, ZnO-based varistor ceramics with
different recipes were obtained. It was found that the grain
growth can be effectively restrained with the addition of Ba2+ and the decrease of sintering temperature. SEM results indicate that
the grain size of ZnO varistor ceramics sintering at 950 ·C was
reduced to 1.28 �m, which is about 8 times smaller than that of
ordinary ZnO varistor ceramics. Correspondingly, the voltage gradient
of
the
ZnO
varistor
samples
were
also
extremely
enhanced to 3845 V/mm. The dielectric properties of the ZnO varistor
ceramics
were
measured
by
Broadband
Dielectric
Spectroscopy. The results showed that two dielectric loss peaks
with activation energy at about 0.22 eV and 0.36 eV observed in 163-203 K are common for all the ZnO varistor samples, which were considered to be consistent with the electronic levels of intrinsic defect interstitial zinc
and vacancy oxygen in
the
depletion layer. However, for sample C, the dielectric loss was reduced greatly in low frequency range with the addition of Ba2+ and
the
resistance
of
grain
boundary
increased
128
to
MO at 473 K. Therefore, it was proposed that the addition of Ba2+
and the sintering procedure may play a great role in improving
the electrical properties of ZnO varistors.
I. Zinc devices
oxide with
characteristics,
varistor
ceramics
excellent
voltage-current
and
been
have
widely
applications. Conventional ceramic processing technology is employed in manufacturing these cylindrical ceramic blocks [5-6]. The varistor blocks are usually sintered between 1000 °C and 1400 °c [5-7]. The range of temperature for sintering is known for specific compositions since Matsuoka [5] revealed the application of this device. The early investigation using impedance
measurements
are
semiconductor
(I-V)
used
in
This investigation further indicated that a balance between the desired device characteristics satisfying electrical performance in the application
the questions that one genuine or scholarly reader may have the
processing
and it is not clear which microstructural parameters are of paramount importance for their electrical properties. Common for all proposed models for their electrical behavior is the concept that grain boundaries play a vital role. It is considered that the transport properties in these multiphase ceramics are electrically
active
grain
boundaries between two semiconducting ZnO grains [3]. And properties
conditions to
obtain
controlled
In this work, microstructure of ZnO varistor ceramics were modified by doping Ba2+ and improving the sintering procedure. The electrical properties and ac responses of the ZnO varistor
ZnO varistor materials have a complicated microstructure
electrical
patents
ceramics were investigated in detail.
of the manufacture of metal oxide varistors in ultra-high
excellent
varistor papers or
electrical
voltage arresters [1-2].
the
field. Most
available till to date do not describe precisely all the answers of
nonlinear
voltage in power system, the voltage gradient of traditional
the
minImum
sintering profile and the hold-time may be necessary to obtain
concerning
ZnO-based varistors is not high enough to meet to the demands
by
a
temperature are necessary for a specific composition or a
II.
absorbers. However, with the increase of the transmission
controlled
that
formulation or a recipe to obtain a better performing device.
apparatus and electronic circuits as over-voltage and surge
completely
suggested
temperature for sintering and a minimum time for hold at that
properties of the resulting device.
INTRODUCTION
(ZnO)
form of various lengths and diameters depending on the type of
of ZnO
ceramics
are
A.
EXPERIMENTAL
Samples preparation Three types of ZnO ceramic samples with different recipes
and sintering temperature were prepared for the present study. Sample A is ZnO polynary system with small amount of additives Bh03, Sb203, MnC03, Cr203 and C0203 and the content of the additives are the same of 0.5 mol%. Sample B was
multi-doping
commercial ZnO
ceramics
with
small
amounts of additives of Biz03, Sb203, C0203, Mn02, and BaC03• Sample C with the same recipe as sample B was sintering at a lower temperature. These samples were produced by traditional electronic ceramic processes and sintered at 1050 °C
for
sample
A
and
B,
950°C
for
sample
C.
The
attributed to grain boundary phenomena related to dopant
sintering holding time for all the samples was 2 hours. The
induced defects and the phase formation in sintering [4].
final samples were trimmed into disks of 10.6 mm in diameter
Commercial varistor ceramics are made in the cylindrical
and 1.4 mm in thickness, and silvered on both sides for the
TABLE I. The electrical property parameters of sample A, B and C
electrical measurements. Commercial Ag-paste was used in the electroding, in which the disks were heat-treated at 800 °C for 30 min in air. B.
samples
Test items The current-voltage (1- II) characteristic of samples A, B
and C are measured at room temperature by using HP 34401A multimeter and WJl 000 ID precision linear high-voltage dc power. The voltage gradient was calculated by
E1mA=VlmA/h (h
is the thickness of the sample) and the nonlinear coefficient was got by
a= I1log(VIrnA/VOlrnA) ,
where
VIrnA
and
VOlrnA
are
the voltages at I mA and 0.1mA, respectively.
B.
ElmA (V/mm)
a
d(�m)
Egb (V19b)
A
275
31.4
10.05
2.0
B
873
37.3
4.72
4.)
C
3845
55.0
1.28
4.9
Microstructure Analyses
The microstructure of sample A, B and C were observed by means of scanning electron microscopy (SEM, Jeol JSM6390A, Japan) micrographs. The dielectric properties were measured
using
a
Novocontrol
broadband
dielectric
spectrometer (Concept 80, made in Germany) with an Alpha-A high performance frequency analyzer in the frequency range of 0.1 Hz�106 Hz from 163 K to 473 K. III. TEST RESULTS AND DISCUSSION
J-E Property
A.
4000 ,------, . .. .. ....... ...........---..,-----
l tJ.
3500
]'� "1
3000
_-sampIeB
-.A.-sample C
'e_e-e-e-e----e
800
400
e
...- .--- .--- .--- . --- . 0.0
Figure 1.
-.-sampleA
.01.
0.2
0.4
0.6
.l(rnA/em')
0.8
1.0
1.2
J-E characteristics of ZnO varistor samples at room temperature.
As far as we know, the pure ZnO ceramics without any
J-E characteristics are J-E characteristics of the sample the samples show a nonlinear J-E
additives are semiconducting and their linear. As shown in Fig. 1, the A, B and C were given. All
property, which means that the thermionic emission current crosses the Schottky barrier in the grain boundary region will be achieved, and the current density can be expressed in the low-current pre-breakdown region of the
J-E characteristics by
the following equation[8]: (c)
(1) where
A
Figure 2. Scanning electron micrographs of ZnO varistor
is the Richardson constant, qJ is the Schottky barrier
height, k is the Boltzmann constant, fJ is a constant, electric field, and
T
E
is the
is the absolute temperature.
The voltage gradient and nonlinear coefficient are all greatly
affected
by
the
additives
of
Ba2+
and
sintering
temperature. Just as shown in TABLE I, it is evidently revealed that
the
voltage
gradient
and nonlinear coefficient were
enhanced to 3845 V/mm and 55 respectively, from sample A to sample C.
samples samples: (a)-sample A, (b)-sample B, (c)-sample C In order to investigate the role of the additives and sintering temperature
on
the
microstructure
of
the ZnO
ceramics,
examination with scanning electron microscopy (SEM) was carried out as shown in Fig. 2. The average grain size (d) is determined
by
the
lineal
intercept
method,
given
by
d=1.56L/MN, where L is the total line length on the micrograph,
M
is the magnification of the photomicrograph, 1.56 is the
proportionality constant, and N is the number of the grain boundaries by lines[9]. The SEM observation reveals that sample C have smaller average grain size than that of sample A and B as depicted in Fig. 2(a), (b) and (c), which demonstrates that the addition of Ba2+ and lower sintering temperature have played an effective role
in
controlling
the
growth
of
grain.
The
detailed
microstructural parameters are summarized in TABLE I. It is clear that the breakdown field
ElrnA
increases significantly in a
wide range from 275 to 3845 V/mm from sample A to sample C. This is mainly attributed to the increase in the number of grain boundaries caused by the decrease in the ZnO grain size. In addition, the breakdown electric field per grain boundary increases to 4.9 V for sample C, which suggested that the additives of BaC03 may induce a significant effect on the improvement of grain boundary behaviors. C.
Relaxation response in dielectric loss plot The frequency dependence of dielectric loss (tano=c"IE:') at 1�
different temperatures for the sample A, B and C is described
l�
ld
l� Id f(Hz)
in Fig. 3. At low temperatures, sample A, B and C all illustrate two relaxation peaks named peak 1 and peak 2, as shown in
1� -.-163K --
