Preparation of ZnO-glass varistor from tetrapod ZnO nanopowders

Wu Jun, Xie Changsheng *, Bai Zikui, Zhu Bailin, Huang Kaijin, Wu Run

Faculty of Materials Science and Engineering, Huazhong University of Science and Technology, 430074 Wuhan, People’s Republic of China

Abstract

Varistors based on ZnO�/lead zinc borosilicate glass were prepared from tetrapod ZnO nanopowders, every one of which had four

needle-like legs and each one was about 20 nm or less in diameter and from several hundreds of nanometers to several micrometers

in length by the method of direct co-sintering synthesis of oxides instead of adding amorphous lead zinc borosilicate frit. The

compact green disks were conventionally sintered in air for 2 h at a temperature of 900�/1170 8C. The varistors with nonlinear

coefficient a�/38.7 and leakage current IL�/1.7 mA were obtained. The results showed that the sintering temperature was lowered to

900 8C, and there was very little influence of the sintering temperature on the nonlinear coefficient a at a range 900�/1170 8C.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Tetrapod; ZnO; Nanopowder; Varistor

1. Introduction

ZnO varistors are a kind of sintering semiconductor

ceramic device with high nonlinearity primarily com-

posed of polycrystalline ZnO and additives of other

oxides. They are widely used in electric power systems

and circuits as a type of protect devices because of their

strong surge current-absorbed capacity. Gupta [1]

suggests that the additives found in ZnO varistors can

be classified into three general categories. It is believed

that the effective main additives, termed ‘varistors

formers’, consisted of oxides without which varistor

behavior can not be obtained, are Bi2O3 [2,3], Pr6O11

[4,5], V2O5 [6,7] or lead Zinc borosilicate glass [8�/10],

the assistant additives are metal oxides, such as Co, Mn,

Cr, Al, Ni, Sb and Ti oxides [11], etc. ZnO varistors are

the typical devices, whose microstructure dominates

over properties. Generally, the microstructure consists

of three phases: homogeneous ZnO, high resistive

intergranular phase and spinel [12]. The high resistive

intergranular phase is dependent on the main additives,

for example, Bi-rich phase for Bi2O3 [13,14], praseody-

mium oxide for Pr6O11 [15,16] and crystallized glass

phase for lead zinc borosilicate glass [17,18]. Although

ZnO�/Bi2O3 varistors possess very excellent properties,

Bi2O3 easily reacts with the internal electrode of multi-

player varistors, such as Ag/Pd, hence the multiplayer

structure is destroyed and the varistor properties are

lost. Therefore, it is important to search for a suitable

additive to replace Bi2O3 from the various additives.

Shohata et al. [8,19] have reported that varistors based

on ZnO and lead zinc borosilicate glass instead of

partial Bi2O3 exhibit excellent nonohmic behavior and

lead zinc borosilicate glass is a potential candidate for

multilayer varistors. In previous work [20], the amor-

phous lead zinc borosilicate frit must be prepared in

advance as raw material, however, this method is

relatively complicated and it is difficult to maintain

uniformity of intergranular phases, spinel and ZnO

grains, which plays a very important role in electronic

properties of varistor.

It is well known that ZnO powders normally exist in

two shapes: one is spheroid and the other is tetrapod

[21�/23]. Compared with spheroid ZnO powders, the

tetrapod ZnO nanopowders have higher activity because

of their large surface area, thus the sintering tempera-

ture is markedly decreased. At the same time, it is worth

noticing that all the ZnO powders applied for varistors

are spheroid. To our knowledge, there is no report on

the ZnO varistors prepared from tetrapod ZnO pow-

ders, especially from tetrapod ZnO nanopowders. Some

researchers prepared ZnO varistors from nanometer

powders by the method of sol�/gel [24], and chemical

co-precipitation and plasma pyrolysis [25], but it still* Corresponding author. Tel.: �/86-27-8754-3840

E-mail address: csxie@mail.hust.edu.cn (X. Changsheng).

Materials Science and Engineering B95 (2002) 157�/161

www.elsevier.com/locate/mseb

0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 2 2 7 - 1

belongs to ZnO�/Bi2O3 system and the powders are still

spheroid. Kong et al. [26] reported the sol�/gel glass-

coated zinc oxide for varistor applications, however, its

nonlinear coefficient is too small to be used in practice.In this paper, we give a report on the ZnO�/lead zinc

borosilicate glass varistors prepared from tetrapod ZnO

nanopowders by the method of direct co-sintering

synthesis of oxides and investigate their current�/voltage

characteristics.

2. Experimental procedure

According to the traditional process, the amorphous

lead zinc borosilicate frit must be prepared in advance;

the typical manufacture method was introduced in

previous work [19,20]. In this work, tetrapod ZnO

nanopowders used as raw material were prepared by a

vaporization condensation technique; the other raw

materials such as Sb2O3, Cr2O3, Co2O3, MnO2, PbO,SiO2 and B2O3 were reagent grade. Foresaid materials

with appropriate proportion were mixed by milling for

6�/8 h, then 2% ethyl cellulose was added as binder, the

mixture was pressed at a pressure of 300 MPa into discs

20.3 cm in diameter. The discs as-prepared were placed

in SiO2 crucible, sintered in air for 2 h at the

temperature range from 900 to 1170 8C, and then

furnace-cooled. Silver paste was printed on both sidesof the sintered discs (8 mm in diameter), and then

sintered at 500 8C in air for 15 min so as to prepare for

the electronic characteristic experiment. Breakdown

voltage is the voltage value when current is 1 mA,

leakage current value is the current when voltage value

is 0.83 times breakdown voltage, and the nonlinear

coefficient a is estimated by the following equation:

a�1=log(V1mA=V0:1mA)

where V1mA and V0.1mA represent the voltages at 1 and

0.1 mA, respectively.

The fracture surface microstructures of the varistors

were examined by JEOL JSM-35A scanning electron

microscope. Crystalline phases of the varistors were

examined by D/max-IIIC X-ray diffraction (XRD) with

Cu target (l�/0.15406 nm). The morphology of the

tetrapod ZnO nanopowders was observed with JEOLJEM-2000EX transmission electron microscope.

The sizes of the samples were measured before and

after sintering, the densification degree of samples was

signified with the shrinkage ratio [25] in this study.

3. Results and discussion

The tetrapod ZnO nanopowders prepared by vapor-

ization condensation technique have very special geo-

metric morphology; their typical morphology is shown

in Fig. 1. Every nanopowder has four needle-like legs,

every one of which is about 20 nm or less in diameter

and from several hundreds of nanometers to several

micrometers in length. The specific surface area oftetrapod ZnO nanopowders is about 20% larger than

that of spheroid ZnO nanopowders from the measure-

ment results of specific surface area, thus the tetrapod

ZnO nanopowders have higher activity.

XRD patterns of samples sintered at different tem-

perature are shown in Fig. 2. It is easy to identify the

main phase ZnO and Zn7Sb2O12 spinel by fitting d-

spacing data. Some researchers found that 5ZnO �/2B2O3 might exist in the ZnO-glass system [17], but

recent research results showed that it was Zn2SiO4,

which crystallized at the ZnO grain boundary from

amorphous zinc lead borosilicate glass during furnace

cooling [20]. The results show that Zn2SiO4 generates

during sintering, but the d-spacing data do not exactly

match with those of JCPDS card. On the other hand,

B2O3, SiO2 and PbO are directly added instead ofamorphous zinc lead borosilicate frit. So a reaction

may occur during furnace heating as follows:

2ZnO�SiO2�Zn2SiO4 (2)

Then Zn2SiO4 crystallized at the ZnO boundary

during furnace cooling and formed potential barrier.

There is no evidence that borate exists, and there is a

little difference in d-spacing between Zn2SiO4 in the

samples and JCPDS card. Therefore, the crystallizedZn2SiO4 is a solid solution dissolving B and other

elements. SiO2 does not react completely with others

at 900 8C because of the evidence of SiO2 existing in the

sample sintering at 900 8C showed in the XRD

patterns.

Spinel phase plays a very important role in determin-

ing the electronic properties of ZnO varistors. A spinel-

like phase, pyrochlore phase formed before the forma-tion of spinel phase, the spinel phase was formed by the

decomposition of the pyrochlore phase and SiO2 and

Fig. 1. Morphology of tetrapod ZnO nanoparticles.

W. Jun et al. / Materials Science and Engineering B95 (2002) 157�/161158

Cr2O3 decreased the decomposition temperature below

900 8C [27]. On the other hand, Si-rich glass at the grain

boundary layer transformed into crystalline zinc silicate

phase after heat treatment, strong pinning of the barrier

height was found in ZnO-glass with crystalline inter-

granular layer and ultimately caused an increase in the a

values, grain boundary barrier height, breakdown

voltage per grain and device stability [16]. So there are

two main functions for SiO2: one decreases the decom-

position temperature of pyrochore phase; the other

forms the intergranular layer and potential barrier.

SEM images of ZnO-glass varistors prepared from

tetrapod ZnO nanopowders are shown in Figs. 3�/5.

Zn7Sb2O12 spinel and Zn2SiO4 intergranular are much

smaller at lower sintering temperature but they grow

rapidly at higher sintering temperature, such as

1170 8C. The ZnO grains are homogeneous, the

Fig. 2. XRD patterns of samples sintering at different temperature.

Fig. 3. SEM image of sample sintering at 900 8C.

Fig. 4. SEM image of sample sintering at 1050 8C. Fig. 5. SEM image of sample sintering at 1170 8C.

W. Jun et al. / Materials Science and Engineering B95 (2002) 157�/161 159

Zn7Sb2O12 spinel and Zn2SiO4 intergranular are very

small and uniformly distributed at the boundary of ZnO

grains at 1050 8C. These results show that the optimum

sintering temperature is 1050 8C. During the sintering,

the tetrapod ZnO nanopowders may grow rapidly

before the intergranular layer forms. The main reasons

are that the tetrapod ZnO nanopowders possess high

activity, and the grain growth inhibition induced by the

intergranular layer [20] is delayed, which is the result of

the reaction between ZnO and SiO2 caused by the

method of co-sintering synthesis of oxides instead of

adding zinc lead borosilicate glass. Additionally, the

integrality and amount of Zn7Sb2O12, especially

Zn2SiO4 increase with increasing sintering temperature

according to relative strength of the peaks in XRD

patterns.

Average grain size, sintering temperature, shrinkage

ratio and electronic properties of the samples sintered at

different temperature with the same component are

summarized in Table 1. The values of the nonlinear

coefficient a of the samples are all larger than 35, while

the values of the leakage current of the samples are all

less than 5 mA. The varistor has higher nonohmic

property even at the lowest sintering temperature,

900 8C. It is also found that the shrinkage ratio reached

12.3% when the sintering temperature is as low as

900 8C. The results show that while the sintering

temperature increases at the range from 900 to

1170 8C, the breakdown voltage monotonically de-

creases and the grain-sizes monotonically increase but

the shrinkage ratio almost remains unchanged. These

prove that the tetrapod ZnO nanopowders are very

efficient for sintering at low temperature and the

breakdown voltage of the varistors can be adjustable

not only by sintering temperature but also by the

thickness of the varistors, and the other electronic

properties remain in an excellent level at the same time.

It is worth noticing that there is almost no difference

of leakage current and nonlinear coefficient a between

1C1 and 5C1 samples. However, SiO2 is found in 5C1

but not in 1C1 from the XRD patterns. These results

suggest that there is almost no influence of SiO2 on the

leakage current and nonlinear coefficient a of ZnO-glass

varistors in this work, the details regarding why and

how SiO2 does not influence the electronic properties of

ZnO-glass varistors need further study.

4. Conclusion

(1) The ZnO-glass varistors are prepared by the

method of directly adding the oxides that are used as

raw materials for fabricating amorphous lead zinc

borosilicate frit.

(2) Because of the higher activity of tetrapod ZnO

nanopowders, the sintering temperature is evidently

lowered to 900 8C. Sintering temperature markedly

influences the grain-growth behavior and the break-down voltage, but does not significantly affect the

nonlinear coefficient and the densification rate at the

range from 900 to 1170 8C.

(3) There is almost no influence of SiO2 on the leakage

current and nonlinear coefficient a of ZnO-glass var-

istors in this work.

Acknowledgements

The work above is supported by the key project of thestate educational ministry of People’s Republic of

China. The authors gratefully thank Professor Liu

Xinlang and Zeng Dawen, who give this paper some

valuable suggestions.

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Table 1

Average grain size, sintering temperature, shrinkage ratio and electronic properties of the samples

Sample Average grain size

(m)

Sintering temperature

(8C)

Shrinkage ratio

(%)

Breakdown voltage

(V1mA/mm)

Leakage current

(mA)

Nonlinear coefficient a

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5C1 3 900 12.3 909 4.4 36.0

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