ZnO-Bi203-based varistor ceramics prepared by direct high-energy ...
2011 International Conference on Electronic & Mechanical Engineering and Information Technology
ZnO-Bi203-based varistor ceramics prepared by direct high-energy ball milling of the dopants Dong Xu1'2'*, Biao Wang lb , Mingshuang Li1'0, Xiao Ye l d 1.School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, P. R. China 2.Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P.R. China [email protected]; b. [email protected]; c. [email protected]; d. [email protected] electrical
addition of Sb 2 0 3 results in the formation of the
properties of ZnO-Bi203-based varistor ceramics
Zn7Sb20i2 spinel-type phase, to which the inhibition of
prepared by direct high-energy ball milling of the
the grain growth is generally assigned. Oxides of Co, Mn
dopants were studied. The varistor ceramics samples
and Ni incorporate into the grains of ZnO and increase
were characterized by XRD and SEM analysis, as
their conductivity. Hence, an optimum distribution of the
well as by dc electrical measurements, such as the
dopants has to be achieved in the ZnO matrix, both along
nonlinearity
the grain boundaries and in the grains in order to obtain
Abstract—The
microstructure
coefficients,
and
leakage
current
and
threshold voltage. The best electrical characteristics
the best functional
were found in sample P2, which exhibited the
ceramics. The homogeneity of the starting oxide mixture
threshold
voltage
was
347
V/mm,
is therefore of key importance for the preparation of
nonlinear
high-quality varistor ceramics[4_7].
coefficient was 39.2 and leakage current was 0.18 uA. Keywords-varistors; ZnO; high-energy ball milling;
Typically, the standard varistor ceramic procedure
microstructure; electrical properties I.
characteristics of the varistor
of mixing ZnO with varistor dopants is used for the processing of varistor ceramics. The high-energy ball
INTRODUCTION
milling of varistor powder can improve ZnO varistor
ZnO varistor ceramics show a highly nonlinear
characteristics,
providing
the
narrow
grain
size
current-voltage characteristic with a highly resistive
distribution, homogenous distribution of phases and high
state in the prebreakdown region and large non-linearity
sintering activity of the powders[ ' . It was also reported
2]
coefficient^' . The zinc oxide based varistors are widely
that mechanical activation reduces the weight loss due to
used in electronic appliances and especially in high
the volatilization of B12O3 and Sb203[ . In this work the
voltage lines as voltage surge protection devices.
dopants of varistor ceramics by high-energy ball milling
Commercial varistors are usually made by solid state of
firstly, then milled with ZnO powder, the influence of
ZnO particles with doping agent oxides such as Bi 2 0 3 ,
the different ball milling process on the microstructure
Sb203, C02O3, Mn02 and Cr203, the mixed powder then
development,
being pressed and sintered at higher temperatures^
current-voltage
.
the
phase
characteristics
composition of varistor
and
the
ceramics
Each of the dopants added to the ZnO powder play a
prepared by the high-energy ball milling method was
distinctive role in providing the varistor characteristics to
thoroughly analyzed.
the sintered ZnO-based ceramics. B12O3 is essential for II.
inducing the nonlinearity to the ZnO ceramics. Sb203 is a standard additive for controlling the grain growth. The
978-l-61284-088-8/ll/$26.00 ©2011 IEEE
EXPERIMENTAL PROCEDURE
The varistor dopant powder mixture, marked as DP,
713
12-14 August, 2011
which were used in reagent-grade raw materials in
III.
RESULTS AND DISCUSSION
proportions of 0.7 mol% Bi 2 0 3 , 1.0 mol% Sb 2 0 3 , 0.8
Fig. 1 shows the microstructures of the samples (PI, P2,
mol% Co 2 0 3 , 0.5 mol% Cr 2 0 3 and 0.5 mol% Mn0 2 , and
P3 and P4) sintered at 1100 °C for 2 h. A similar
the mixture were homogenized in absolute ethanol media
microstructure of the samples obtains in all cases with
in a polyethylene bowl with zirconia balls for 5 h at 500
the only appreciable difference coming from the average
rpm by planetary high-energy ball milling. Then the
size of ZnO grains, the microstructure of varistor
powder samples (3.5 mol% DP + 96.5 mol% ZnO) were
ceramics typically consists of three phases: ZnO grain,
ball milling with 0 h at 500 rpm (instead withl h at 100
spinel, and an intergranular Bi-rich phase as determined
rpm), 1 h at 500 rpm, 5 h at 500 rpm, 10 h at 500 rpm by
by X-ray diffraction (XRD) analysis. The 5h-milling
marked as PI, P2, P3 and P4, respectively.
sample has the smallest grains, and its grain interstice
The slurry was dried at 70 °C for 24h, it was then
phase (spinel phase) as well as grain boundary phase
pulverized by an agate mortar/pestle and after 2 wt.%
(Bi-rich phase) are more refined.
polyvinyl alcohol (PVA) binder had been added, it was
The influence of the high-energy ball milling on the
granulated by sieving through a 100-mesh screen to
characteristics, including the density D, the threshold
produce the starting power. The powders were then dried
voltage Vj, the nonlinear coefficient a, and the leakage
and pressed into discs with a diameter of 12 mm and a
current 7L of the ZnO-Bi203-based varistor ceramics
thickness of 2.0 mm. The pressed disks were sintered in
sintered at 1100 °C for 2 h are presented in Table 1.
air at 1100 °C (2h dwell time), using a heating rate of 5 °C/min and then cooled in the furnace. The sintered
Table 1 Density and electrical properties of
samples were lapped and polished to 1.0 mm thickness.
ZnO-Bi 2 0 3 -based varistor ceramics
The final samples were about 10 mm in diameter and 1.0 mm in thickness. The surface microstructure was examined by a scanning electron microscope (SEM, FEI QUANTA 400). The crystalline phases were identified by an X-ray diffractometry (XRD, Rigaku D/max 2200, Japan) using a Cu Ka radiation. The bulk density of the samples was measured in terms of their weight and
D (%)
IL (uA)
VT (V/mm)
q
PI
94.9
0.08
232
36.2
P2
97.4
0.18
347
39.2
P3
96.7
0.07
290
29.6
P4
96.6
0.05
279
36.6
Differential thermal and high temperature X-ray
volume. For the characterization of DC current-voltage,
analysis have recently suggested the following reactions
the silver paste was coated on both faces of samples and
for the microstructure development of ZnO varistor
the silver electrodes were formed by heating at 600 °C
during reactive liquid phase sintering in the temperature
for 10 min. The electrodes were 5 mm in diameter. The voltage-current (V-I)
Sample
range 500-1050°C[1415].
characteristics were measured
Sb 2 0 3 (s)+0 2 ^Sb 2 0 5 (l)
using a V-I source/measure unit (CJP CJ1001). The
Sb 2 0 5 (l)+ZnO(s)^ZnSb 2 0 6 (s)
nominal varistor voltages (KN) at 0.1 and 1.0 mA were
527 °C
700-800 °C
ZnSb 2 0 6 (s)+6ZnO(s)^Zn 7 Sb 2 Oi 2 (s)
measured and the threshold voltage Vj (V/mm) (Vj =
(1) (2)
6^800°C (3)
3ZnSb 2 0 6 (s)+3Bi 2 0 3 (s)+ZnO(s)^2Zn 2 Bi 3 Sb 3 Oi 4 (s)
V^(lmA)/d; d is the thickness of the sample in mm) and
700°C
ZnO-Bi203-based varistor ceramics prepared by direct high-energy ball milling of the dopants Dong Xu1'2'*, Biao Wang lb , Mingshuang Li1'0, Xiao Ye l d 1.School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, P. R. China 2.Key Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P.R. China [email protected]; b. [email protected]; c. [email protected]; d. [email protected] electrical
addition of Sb 2 0 3 results in the formation of the
properties of ZnO-Bi203-based varistor ceramics
Zn7Sb20i2 spinel-type phase, to which the inhibition of
prepared by direct high-energy ball milling of the
the grain growth is generally assigned. Oxides of Co, Mn
dopants were studied. The varistor ceramics samples
and Ni incorporate into the grains of ZnO and increase
were characterized by XRD and SEM analysis, as
their conductivity. Hence, an optimum distribution of the
well as by dc electrical measurements, such as the
dopants has to be achieved in the ZnO matrix, both along
nonlinearity
the grain boundaries and in the grains in order to obtain
Abstract—The
microstructure
coefficients,
and
leakage
current
and
threshold voltage. The best electrical characteristics
the best functional
were found in sample P2, which exhibited the
ceramics. The homogeneity of the starting oxide mixture
threshold
voltage
was
347
V/mm,
is therefore of key importance for the preparation of
nonlinear
high-quality varistor ceramics[4_7].
coefficient was 39.2 and leakage current was 0.18 uA. Keywords-varistors; ZnO; high-energy ball milling;
Typically, the standard varistor ceramic procedure
microstructure; electrical properties I.
characteristics of the varistor
of mixing ZnO with varistor dopants is used for the processing of varistor ceramics. The high-energy ball
INTRODUCTION
milling of varistor powder can improve ZnO varistor
ZnO varistor ceramics show a highly nonlinear
characteristics,
providing
the
narrow
grain
size
current-voltage characteristic with a highly resistive
distribution, homogenous distribution of phases and high
state in the prebreakdown region and large non-linearity
sintering activity of the powders[ ' . It was also reported
2]
coefficient^' . The zinc oxide based varistors are widely
that mechanical activation reduces the weight loss due to
used in electronic appliances and especially in high
the volatilization of B12O3 and Sb203[ . In this work the
voltage lines as voltage surge protection devices.
dopants of varistor ceramics by high-energy ball milling
Commercial varistors are usually made by solid state of
firstly, then milled with ZnO powder, the influence of
ZnO particles with doping agent oxides such as Bi 2 0 3 ,
the different ball milling process on the microstructure
Sb203, C02O3, Mn02 and Cr203, the mixed powder then
development,
being pressed and sintered at higher temperatures^
current-voltage
.
the
phase
characteristics
composition of varistor
and
the
ceramics
Each of the dopants added to the ZnO powder play a
prepared by the high-energy ball milling method was
distinctive role in providing the varistor characteristics to
thoroughly analyzed.
the sintered ZnO-based ceramics. B12O3 is essential for II.
inducing the nonlinearity to the ZnO ceramics. Sb203 is a standard additive for controlling the grain growth. The
978-l-61284-088-8/ll/$26.00 ©2011 IEEE
EXPERIMENTAL PROCEDURE
The varistor dopant powder mixture, marked as DP,
713
12-14 August, 2011
which were used in reagent-grade raw materials in
III.
RESULTS AND DISCUSSION
proportions of 0.7 mol% Bi 2 0 3 , 1.0 mol% Sb 2 0 3 , 0.8
Fig. 1 shows the microstructures of the samples (PI, P2,
mol% Co 2 0 3 , 0.5 mol% Cr 2 0 3 and 0.5 mol% Mn0 2 , and
P3 and P4) sintered at 1100 °C for 2 h. A similar
the mixture were homogenized in absolute ethanol media
microstructure of the samples obtains in all cases with
in a polyethylene bowl with zirconia balls for 5 h at 500
the only appreciable difference coming from the average
rpm by planetary high-energy ball milling. Then the
size of ZnO grains, the microstructure of varistor
powder samples (3.5 mol% DP + 96.5 mol% ZnO) were
ceramics typically consists of three phases: ZnO grain,
ball milling with 0 h at 500 rpm (instead withl h at 100
spinel, and an intergranular Bi-rich phase as determined
rpm), 1 h at 500 rpm, 5 h at 500 rpm, 10 h at 500 rpm by
by X-ray diffraction (XRD) analysis. The 5h-milling
marked as PI, P2, P3 and P4, respectively.
sample has the smallest grains, and its grain interstice
The slurry was dried at 70 °C for 24h, it was then
phase (spinel phase) as well as grain boundary phase
pulverized by an agate mortar/pestle and after 2 wt.%
(Bi-rich phase) are more refined.
polyvinyl alcohol (PVA) binder had been added, it was
The influence of the high-energy ball milling on the
granulated by sieving through a 100-mesh screen to
characteristics, including the density D, the threshold
produce the starting power. The powders were then dried
voltage Vj, the nonlinear coefficient a, and the leakage
and pressed into discs with a diameter of 12 mm and a
current 7L of the ZnO-Bi203-based varistor ceramics
thickness of 2.0 mm. The pressed disks were sintered in
sintered at 1100 °C for 2 h are presented in Table 1.
air at 1100 °C (2h dwell time), using a heating rate of 5 °C/min and then cooled in the furnace. The sintered
Table 1 Density and electrical properties of
samples were lapped and polished to 1.0 mm thickness.
ZnO-Bi 2 0 3 -based varistor ceramics
The final samples were about 10 mm in diameter and 1.0 mm in thickness. The surface microstructure was examined by a scanning electron microscope (SEM, FEI QUANTA 400). The crystalline phases were identified by an X-ray diffractometry (XRD, Rigaku D/max 2200, Japan) using a Cu Ka radiation. The bulk density of the samples was measured in terms of their weight and
D (%)
IL (uA)
VT (V/mm)
q
PI
94.9
0.08
232
36.2
P2
97.4
0.18
347
39.2
P3
96.7
0.07
290
29.6
P4
96.6
0.05
279
36.6
Differential thermal and high temperature X-ray
volume. For the characterization of DC current-voltage,
analysis have recently suggested the following reactions
the silver paste was coated on both faces of samples and
for the microstructure development of ZnO varistor
the silver electrodes were formed by heating at 600 °C
during reactive liquid phase sintering in the temperature
for 10 min. The electrodes were 5 mm in diameter. The voltage-current (V-I)
Sample
range 500-1050°C[1415].
characteristics were measured
Sb 2 0 3 (s)+0 2 ^Sb 2 0 5 (l)
using a V-I source/measure unit (CJP CJ1001). The
Sb 2 0 5 (l)+ZnO(s)^ZnSb 2 0 6 (s)
nominal varistor voltages (KN) at 0.1 and 1.0 mA were
527 °C
700-800 °C
ZnSb 2 0 6 (s)+6ZnO(s)^Zn 7 Sb 2 Oi 2 (s)
measured and the threshold voltage Vj (V/mm) (Vj =
(1) (2)
6^800°C (3)
3ZnSb 2 0 6 (s)+3Bi 2 0 3 (s)+ZnO(s)^2Zn 2 Bi 3 Sb 3 Oi 4 (s)
V^(lmA)/d; d is the thickness of the sample in mm) and
700°C
