This document summarizes a study on the effect of Gd2O3 substitution on the microstructure and electrical properties of Zn-V-Mn-Nb-O varistor ceramics for low voltage applications. XRD and SEM analysis showed the formation of secondary phases like GdMnO3 and GdVO4 at grain boundaries. Gd2O3 substitution decreased grain size from 3.85 to 3.06 μm and increased density from 5.12 to 5.19 g/cm3. Samples with 0.03 mol% Gd2O3 exhibited the optimal nonlinear coefficient of 9.91, highest breakdown field of 88.48 V/mm, and lowest leakage current density of 0.

1. Microstructural and Nonlinear Properties of Zn-V-Mn-Nb-O Varistor
Ceramics with Gd2O3 Substitution for Low Voltage Application
Nor Hasanah Isa1,a *
, Azmi Zakaria1,2,b
, Raba’ah Syahidah Azis1,2,c
and Zahid Rizwan3,d
1
Department of Physics, Faculty of Science, Universiti Putra Malaysia, 43400 UPM Serdang,
Selangor, Malaysia
2
Institute of Advanced Materials, Universiti Putra Malaysia, 43400 UPM Serdang,
Selangor, Malaysia
3
Department of Applied Sciences, National Textile University, Faisalabad (37610), Pakistan
a
hasanah5152@yahoo.com, b
azmizak@upm.edu.my, c
rabaah@upm.edu.my,
d
zahidrizwan64@gmail.com
Keywords: ZnO, Gd2O3, Low voltage Varistor, Electrical properties, Microstructure
Abstract. The effect of Gd2O3 substitution on the microstructural and electrical properties of Zn-V-
Mn-Nb-O varistor ceramics sintered at 900°C was investigated. XRD, SEM, and EDAX results show
that the GdMnO3 and GdVO4 phases formed at the grain boundaries and triple point junctions. Gd2O3
substitution inhibited the grain growth from 3.85 to 3.06 μm and increased the sintered ceramics
density from 5.12 to 5.19 g/cm3. The samples containing the amount of 0.03 mol% Gd2O3 exhibit an
optimum nonlinear coefficient α value which is 9.91, highest breakdown electrical field which is
88.48 V/mm and lowest leakage current density which is 0.11 mA/cm2 in low voltage application.
Introduction
A varistor is an electronic component used to protect electrical devices from harmful overvoltage that
caused from either by a manmade or environmental factor. ZnO varistor ceramics are well known as
a surge protection device made by sintering ZnO powder modified with selected additives such as
CoO, Sb2O3, MnO2 and Cr2O3 together with Bi2O3 as main additives [1-3]. The nonlinear properties
of ZnO based varistors ceramics are ascribed to a double Schottky barrier formed at active grain
boundaries containing many trap states [4, 5]. The ZnO-V2O5 ceramic systems show a good nonlinear
properties and strongest accelerated degradation characteristics at 900 °C [6-10].
Numerous studies on the effect of rare earth oxide additives such as Gd2O3, Dy2O3, and Er2O3 on the
Zn-V-Mn-Nb-O varistor ceramic systems shows that it exhibit good varistor properties in the amount
as low as 0.05 mol% at 900 °C [8-10]. The typical range of Gd2O3 concentration used in these
previous works was more than 0.05 mol% Gd2O3 [10]. Interestingly, it possessed a high breakdown
voltage about 536.5 V with 1 mm thickness for high voltage application at 0.05 mol% Gd2O3 [10].
The breakdown voltage is an important parameter to determine the practical application of varistor.
For commercial low voltage application, the breakdown voltage is in the range between 82 V to 120
V with bulk thickness range between 5 to 10 mm. Although the ceramic systems have been studied
in many aspects for high voltage application, the studies of Gd2O3 substitution effect on the varistor
properties of the Zn-V-Mn-Nb-O varistor ceramic systems for low voltage varistor application have
not yet been attempted. So it is useful to find the specific concentration in low voltage varistor
application by focusing on Gd2O3 low concentration below 0.05 mol%. The substitution of Gd2O3
below 0.05 mol% is expected to improve the microstructure and nonlinear properties of ZnO based
varistors in low voltage varistor application. In this paper, the effect of Gd2O3 incorporation at a

2. concentration below 0.05 mol% on the microstructure and electrical properties of ZnO-based
ceramics varistors were discussed to determine its potential in low voltage varistor application.
Experiment
High-purity (> 99.9%, Alfa Aesar) reagent-grade raw materials composed of (97.4-x)mol% ZnO, 0.5
mol % V2O5, 2.0 mol % MnO2, 0.1 mol % Nb2O5 and x mol% Gd2O3 were prepared as x is 0.01,
0.02, 0.03, 0.04 and 0.05 mol%. Raw materials were ground using distilled water by ball milling with
zirconia balls for 24 hours. The slurry was dried at 70 °C for 12 hours. Polyvinyl alcohols 1.75 wt%
was added as a binder to avoid cracks in the samples and then granulated by sieving through the 75
μm mesh screen. The powder from each ceramics combination was pressed into pellets with 10 mm
diameter and 1 mm in thickness at a pressure of 4 tonne/m2. The pellets were sintered at a temperature
of 900 °C in air for 2 hours at the heating and cooling rates of 5 °C /min.
The crystalline phase was identified using Cu Kα radiation (λ = 1.540598 Å) with PANalytical X’Pert.
XRD software X’Pert high score software Pro PW3040/60) was used to analyse the date for
crystalline phases. The surface microstructure was examined by a scanning electron microscope
(model: LEO 1455 VPSEM) attached with energy dispersive X-ray (EDX). One side of the samples
was lapped and ground with SiC paper, and then polished with 0.3 micron-Al2O3 powder to make a
mirror-like surface. The average grain size (d) was determined by the linear intercept method [15].
The density (ρ) of sintered sample was measured by Archimedes method.
The current density (J) ‒ electric field (E) characteristics at room temperature were measured using a
low voltage source-measure (Keithley Model 2410) unit to obtain their non-linear coefficient (α). All
samples were coated with silver conductive paint and cured at 550 °C for 10 min. to make the
electrodes. The α value was determined through the expression α = (log J2 – log J1)/ (log E2 – log E1),
where E1 and E2 are the electric fields corresponding to J1= 1.0 mA/cm2 and J2=10 mA/cm2,
respectively. The breakdown electrical field (E1mA) was measured at 1 mA/cm2 in the current density
and the leakage current density (JL) was measured at 0.8.E1mA. It is well accepted that thermionic
emission is the predominant conduction mechanism in the pre-breakdown region. For this reasons,
the potential barrier height ϕB could be estimated according to J=AT2 exp((βE1/2 - ϕB)( kB T)-1) where
kB is the Boltzmann constant (8.167 × 105 eV/K), A is the Richardson’s constant (30 A/cm2K2) for
ZnO, T is the absolute temperature, β is a constant related to the relation as β ~ (rω)1, where r is
grains per unit length and ω is the barrier width [16].
Results and discussion
Figure 1 shows the XRD patterns of the various Gd2O3 amount. The major diffraction peaks belong
to hexagonal ZnO [8-10] while the minor diffraction peaks show the existence of GdMnO3, GdVO4,
MnV, Zn2Nb2Mn2O9, and ZnV2O4 as a secondary phase. The ZnV2O4 phase was detected in 0.02 and
0.05 mol% Gd2O3 at position 2θ = 30.01°, [10, 11]. Among secondary phase, GdMnO3,
Zn2Nb2Mn2O9 and MnV almost detected at all concentration. The XRD peak of ZnO shifts to the high
diffraction angle slightly from 2θ = 47.53 to 47.54° and interplanar space decreased slightly from
1.913 to 1.912 Å with increasing Gd2O3 concentration. It means that the substitution of Zn+2 by Gd+3
occurs since the ionic radius of Gd+3 (1.07.Å) is larger than that of Zn+2 (0.74.Å).

3. Fig 1. XRD patterns of Gd2O3 substitution at various concentration
The sintered ceramics density increases from 5.12 to 5.22 g/cm3 as the Gd2O3 concentration increases
up to 0.03 mol%, but the grain size decreased remarkably in the range of 3.85 to 3.06 μm with
increasing Gd2O3 concentration up to 0.05 mol%. The addition of Gd2O3 increase the number of
grains and here it act as a grain inhibitor (Fig 2). It is expected that the vanadium-rich liquid phase
ZnV2O4 enhances the densification mechanism by the solution and re-precipitation of ZnO at 900 °C
[10-12]. The grain boundary containing Mn and V element shown in Fig 3 confirmed the segregation
of excess element due to limited Zn interstitial/substitution as the ionic radii of V+5 (0.59.Ǻ) and
Mn+4(0.53.Ǻ) smaller than that of Zn+2 (0.74.Ǻ).
Fig. 2. SEM micrograph of different Gd2O3 concentration (a) 0.01 mol% (b) 0.05 mol%
(a) (b)

4. Table 1. The density (ρ), grain size (D), breakdown field (E1mA), nonlinear coefficient (α), leakage
current density (JL) and Schottky barrier height (ϕB) of Gd2O3 at different concentration
0 20 40 60 80 100 120
0
2
4
6
8
10 0.01mol%
0.02mol%
0.03mol%
0.04mol%
0.05mol%
Currentdensity,J(mA/cm
2
)
Electric field, E(V/mm)
Fig. 4. J-E curve of Gd2O3 substitution at different concentration
The nonlinear properties were determined from J-E curve in Fig. 4 and presented in Table 1. The
nonlinear coefficient α value has been used to estimate nonlinear properties quantitatively and can be
related to the variation of the Schottky barrier height ϕB. The α value and ϕB increases from 6.40 to
9.91 and 0.62 to 0.67 eV with increasing Gd2O3 concentration up to 0.03 mol%. The E1mA also
increased from 74.34 to 88.48 V/mm. While the JL decreases from 0.24 to 0.11 mA/cm2. It is assumed
that the appearance of GdMnO3, MnV, Zn2Nb2Mn2O9 and ZnV2O4 promoted the density of interface
Sample
(mol%)
ρ
(g/cm3)
D
(μm)
E1mA
(V/mm)
α JL
(mA/cm2)
ϕB
(eV)
0.01 5.12 3.85 74.34 6.40 0.24 0.62
0.02 5.15 3.67 75.79 9.55 0.12 0.64
0.03 5.22 3.56 88.48 9.91 0.11 0.67
0.04 5.11 3.03 77.14 7.26 0.20 0.63
0.05 5.19 3.29 74.62 5.86 0.27 0.60
Fig. 3. EDAX spectra of Gd2O3 substitution

5. states at the grain boundaries and consequently increase the ϕB. Beyond that concentration, the barrier
height value decreases to 0.60 eV and directly reduces the α value about 5.86. The E1mA also decreased
to 74.62 V/mm because of secondary phase disappearance. The reduction in α value had caused an
increment in JL value which is 0.27 mA/cm2. The future work is going to focus on the degradation
behaviour of this systems ceramics varistor.
Conclusion
In the Gd2O3 substituting of the ceramics, there is an increasing number of secondary phase that
inhibited the grain growth and reduced the porosity, thus improving the sample microstructure. The
formation of this secondary phases in the grain boundary caused an increasing value of α, hence
improving the nonlinear properties of the varistor ceramic in low voltage operation.
Acknowledgements
The authors are grateful to the Universiti Putra Malaysia for supporting this work under the Universiti
Putra Malaysia Grant No. GP-IPS/2016/9493000.
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