From the SelectedWorks of Kevin Lee
Summer July 12, 2016
ZnO fillers doped polymer composites with
dependent nonlinear conductive and dielectric
characteristics in electrical fields
Lei Gao, Tsinghua University
Xiao Yang, Tsinghua University
This work is licensed under a Creative Commons CC_BY International License.
Available at: https://works.bepress.com/kevin-lee/18/
Materials Letters 171 (2016) 1–4
Contents lists available at ScienceDirect
Materials Letters
journal homepage: www.elsevier.com/locate/matlet
ZnO fillers doped polymer composites with dependent nonlinear
conductive and dielectric characteristics in electrical fields
Lei Gao, Xiao Yang, Jun Hu, Jinliang He n
State Key laboratory of Power Systems, Department of Electrical Engineering, Tsinghua University, Beijing 100084, China
ar t ic l e i n f o
ab s t r act
Article history:
Received 22 June 2016
Received in revised form
28 June 2016
Accepted 4 July 2016
Available online 12 July 2016
ZnO microvaristor/rubber composites with nonlinear conductive and dielectric properties were fabricated and investigated for the first time. The composites exhibit nonlinear conductive behaviors only
when the filler concentration is above percolation threshold (39 vol%), but show nonlinear dielectric
properties with various filler concentration (viz. 30–60 vol%). Both the nonlinear conductive and dielectric properties can be tuned by adjusting the filler concentration. Percolation threshold theory was
utilized to explain the nonlinear electrical properties, and an internal barrier layer capacitor (IBLC) model
was proposed to successfully explain the novel nonlinear dielectric properties, which demonstrates that
the hole injection might be the key to the dramatic increase of the permittivity.
& 2016 Elsevier B.V. All rights reserved.
Keywords:
Polymeric composites
Electrical properties
Grain boundaries
Interfaces
1. Introduction
2. Experimental
In power systems, the unexpected local enhancements of
electric field often appear in various electrical devices, such as the
accessories of cables and wall bushing. Field grading materials are
an attractive method to reduce the local enhancements due to
their outstanding nonlinear properties [1,2]. Based on the applications (AC or DC), field grading materials can be divided into
capacitive field grading and resistive field grading.
For resistive field grading, polymeric composites based on fillers such as SiC or carbon black have been studied for decades [3].
Many effects have also been made to develop materials with
nonlinear dielectric properties on account of their potential applications in HVAC, which mainly employ carbon tubes with
high di-electric permittivity as functional fillers [4–6]. Although
the di-electric permittivity of these composites varies with the
electric field, the nonlinear dielectric properties are relatively
poor, which makes it less attractive to capacitive field grading.
ZnO varistor is a kind of ceramic with excellent nonlinear
conductive properties and has been widely used to limit the
overvoltage in power systems [7]. In this paper, ZnO microvaristors
were fabricated and employed as fillers to tailor both the conductive and dielectric properties of the composites for the first
time. The ZnO microvaristor/rubber composites exhibit excellent
nonlinear conductive and dielectric properties, which may further
broaden the application of nonlinear composites.
The ZnO fillers were prepared by the formula as 95 mol%
ZnO þ 1.0 mol% Bi2O3 þ 0.5 mol% MnO2 þ 1.0 mol% Co2O3 þ 0.4 mol%
Cr2O3 þ 1.0 mol% Sb2O3 þ 1.0 mol% SiO2 þ 0.1 mol% Al2O3. The
mixed powders were blended in anhydrous alcohol by ball milling
for 8 h. Then, organic binder was added into the mixture, and
spherical particles were processed by spray granulation. After that,
the microspheres were sifted and then sintered at the temperature
of 1200 °C for 4 h with the heating rate of 0.55 °C/min and cooling
rate of 1.6 °C/min to get the microvaristors.
Silicone and vulcanizing agent were mixed in tetrahydrofuran
solvent and blended at a high torque blender for 20 min. After
silicone was fully dissolved in the solvent, ZnO microvaristors
were poured into the liquor and the blending continue for 40 min.
Then, the mixture was dried in a vacuum oven for 10 h to remove
the solvent. Finally, the composites were pressed by the vulcanizing machine at 15 MPa and 170 °C for 15 min and then naturally
cooled to room temperature. The ZnO microvaristor/silicone rubber composite samples are about 0.5 mm in thickness and 20 mm
in diameter.
n
Corresponding author.
E-mail address: hejl@tsinghua.edu.cn (J. He).
http://dx.doi.org/10.1016/j.matlet.2016.07.012
0167-577X/& 2016 Elsevier B.V. All rights reserved.
3. Results and discussion
Fig. 1a–c shows the SEM micrographs of the ZnO microspheres,
where ZnO grains and spinels could be observed clearly. The size
distribution of the ZnO microspheres and the grains was calculated, which shows that the average size of the microspheres and
grains is 120 and 9.5 mm, respectively. Each microvaristor is
2
L. Gao et al. / Materials Letters 171 (2016) 1–4
Fig. 1. Scanning electron microscope (SEM) images of (a) ZnO microvaristor fillers, (b) a microvaristor filler and (c) ZnO microvaristor surfaces. (d) X-ray diffraction patterns
of ZnO microvaristor.
composed of hundreds of grains,which endows the microvaristor
with nonlinear electrical properties just as the bulk ZnO varistors
do. XRD is further employed to detect the structure of microvaristors. As shown in Fig. 1d, the main phases are ZnO, spinel and
Bi2O3, which is in accord with the bulk ZnO varistors [8].
The J–E characteristics of ZnO/silicone composite samples with
filler content from 30 vol% to 60 vol%.
are shown in Fig. 2a. To evaluate the nonlinear properties of
them, a typical way is to compare the nonlinear conductive coefficient αj of the samples [9], which can be calculated by Eq. (1),
α j=
log ( J2 /J1 )
log ( E2/E1)
(1)
where J2 ¼ 1.5 mA/cm2, J1=0. 5J2 , E2 and E1 are the corresponding
Fig. 2. (a) The conductivity J of the composites versus applied electric field E, and (b) the dielectric permittivity Ɛ of the composites versus applied dc bias electric field E. The
frequency of the measurement is 100 Hz.
L. Gao et al. / Materials Letters 171 (2016) 1–4
electric field of J2 and J1 according to the measured J–E curve of the
samples, and E2 is defined as the switching field Eb. Similarly, the
nonlinear dielectric coefficient αε of the samples are calculated by
Eq. (2),
( )
α=
log ( )
log
ε
ε2
ε1
E2
E1
(2)
where E2 and E1 are the same parameters in Eq. (1), ε2 and ε1 are
the corresponding permittivity of E2 and E1 according to the
measured ϵ–E curve of the samples. Results demonstrate that
samples with 30 vol% fillers fail to exhibit stable nonlinear conductive behaviors, but successfully show nonlinear dielectric
properties. With larger filler concentration, the composites show
an αj of 12.5, 15.8 17.1 19.0 and an αε of 6.9, 8.4 9.1 10.2 with
39 vol%, 46.5 vol%, 52 vol% and 60 vol% filler concentration, respectively. Former results shows that typical SiC/Silicone composites only exhibit an αj of 4.1 [3], and typical BaTiO3/resin composites only show an αε of 2.1 [4], which are much smaller than
that of the ZnO microvaristor/rubber composites. In a word, the
ZnO microvaristor/rubber composites shows excellent nonlinear
conductive and dielectric properties, which increase with the fillers concentration, indicating a promising way to tailor the nonlinear properties.
Fig. 3 is the schematic diagram of fillers distribution in ZnO
microvaristor/rubber composites, which is widely used to explain
the mechanism of composites’ nonlinear conductive properties.
With low filler concentration, like 30 vol%, there is no conduction
path in the samples, resulting in the low conductivity of them
(Fig. 3a). However, when the filler concentration is larger than the
percolation threshold, conduction paths come into being (Fig. 3b),
leading to stable nonlinear conductive behavior. In samples with
relatively high filler concentration, several conduction paths coexist and most of the current is likely to choose the shortest path.
Thus, the switching field decreases dramatically, while the nonlinear conductive coefficient increases.
Since interface barrier layer capacitor (IBLC) is the key to the
nonlinear properties of the ZnO microvaristors, an IBLC model was
proposed to analyze the nonlinear dielectric properties of the
composites. As shown in Figs. 1c and 3d, there are lots of grains in
the ZnO microvaristor fillers, and each grain boundary serves as an
internal capacitor. The novel thing for the ZnO microvaristor fillers
is that the capacitance varies with the bias DC voltage (Fig. 3e). The
internal barrier layer capacitance C first shows a slight decrease
3
and then shows a sharply increase at the breakdown strength
(Fig. 3f). Typically, a double-depletion-layer model is employed to
analyze the internal barrier layer (Fig. 3g). And when a bias voltage
is applied on the junction, it causes VR to increase and VL to decrease (Fig. 3h). The junction capacitance C can be calculated by
Eq. (3) [10]:
C≈
C0
1
V 2
VB
( 1+ )
(3)
where C0 is the capacitance at zero bias voltage, VB is the barrier
height, V is the applied bias voltage. Thus, the capacitance shows a
slight decrease first. However, when the bias voltage is around
breakdown voltage, injected holes come into being at the interfaces, and the capacitance is calculated by finding the second derivation of the total electrostatic energy with respect to V [10]:
C=
d2
( ξL +ξR+ξhole )
dV 2
(4)
where the three terms in the parentheses are from the left depletion layer, the right depletion layer, and the injected hole region, respectively. When the applied voltage is bigger than the
breakdown strength, the injected hole shows a sharp increase
with the applied voltage (Fig. 3i). Consequently, the hole creation
provides numerous charges and decreases the separation between
them and the electron charges in the other side of the interface.
Since capacitance is charge divided by distance, the largely increased charge and much smaller distance leads to a very large
capacitance. The significant increase of the interface capacitance
finally leads to the large increase of the permittivity of the composites at high bias voltage.
An obvious difference between the nonlinear conductive and
dielectric properties is that although samples with 30 vol% filler
concentration show no nonlinear conductivity, it demonstrates
stable nonlinear dielectric properties. This can be easily explained
by the models above, which is that the nonlinear conductive behavior needs conduction path, while the nonlinear dielectric behavior has no such requirement.
4. Conclusion
Composites with nonlinear conductive and dielectric properties
were fabricated by doping ZnO microvaristor fillers with rubber
Fig. 3. The schematic diagram of the filler distribution and possible conduction path with (a) 30 vol%, (b) 39 vol%, (c) Z 46.5 vol%. (d) Schematic diagram of the microstructure inside ZnO/silicone rubber composite responsible for the nonlinearity electric properties. (e) Equivalent circuit of the grains. (f) Varistor-junction capacitance with
applied dc bias voltage V. (g) Double-depletion-layer model at zero applied voltage. Two ZnO grains are separated by an interface of a different oxide, thickness 2a. The
barrier height is VB. The depletion regions extend a distance χR0 −a= − a−χL0 . (e) The application of a voltage V causes VR to increase and VL to decrease. (f) The holes are
created on the forward side of junction when the hole chemical potential is below the valence band edge.
4
L. Gao et al. / Materials Letters 171 (2016) 1–4
matrix. The results indicate that the composites can only show a
nonlinear conductive behavior with filler concentration above
percolation threshold, while the nonlinear dielectric properties
can be observed with various filler concentration. Both the nonlinear conductive and dielectric coefficients increase filler concentration, while the switching field decreases with that. Percolation threshold theory is utilized to explain the nonlinear electrical properties, while IBLC model are proposed to explain the
nonlinear dielectric properties, indicating that hole injection at the
breakdown region is the key to the novel dielectric properties.
Acknowledgement
This work was supported by the National Basic Research Program of China under Grant 2014CB239504.
References
[1] T. Christen, L. Donzel, F. Greuter, Nonlinear resistive electric field grading part 1:
theory and simulation, IEEE Electr. Insul Mag. 26 (2010) 47–59.
[2] L. Donzel, F. Greuter, T. Christen, Nonlinear resistive electric field grading part 2:
materials and applications, IEEE Electr. Insul Mag. 27 (2010) 18–29.
[3] F. Wang, P. Zhang, M. Gao, X. Zhao, J. Gao, Research on the non-linear con-ductivity
characteristics of nano-SiC silicone rubber composites, IEEE CEIDP (2013) 535–538.
[4] B.R. Varlow, K. Li, Non-linear AC properties of filled resin, IEE Proc. Sci. Meas. Technol.
150 (2003) 75–82.
[5] H. Gu, J. Wang, C. Yu, Three-dimensional modeling of percolation behavior of electrical
conductivity in segregated network polymer nanocomposites using Monte Carlo method,
Adv.in Mater. 5 (2016) 1-8.
[6] V. Narayanunni, H. Gu, C. Yu, Monte Carlo simulation for investigating influence of
junction and nanofiber properties on electrical conductivity of segregated-network
nanocomposites, Acta Mater. 59 (2011) 4548-4555.
[7] J. He, J. Liu, J. Hu, W.A.C. Long, Ageing characteristics of Y2O3-doped ZnO
varistors with high voltage gradient, Mater. Lett. 65 (2011) 2595–2597.
[8] J. Li, G. Chen, C. Yuan, Microstructure and electrical properties of rare earth doped ZnObased varistor ceramics, Ceram. Int. 39 (2013) 2231–2237.
[9] X. Yang, J. He, J. Hu, Tailoring the nonlinear conducting behavior of silicone composites
by ZnO microvaristor fillers, J. Appl. Polym. Sci. (2015) 40.
[10] G.D. Mahan, L.M. Levinson, H.R. Philipp, Theory of conduction in ZnO varistors,
J. Appl. Phys. 50 (1979) 2799–2812.