Electrical and Microstructural Properties of Varistors Based on Nanostructured Tetra-Needle Like Zinc Oxide Powders

Felipe Antonio Lucca Sánchez1,a, Diego Pereira Tarragó2,b, A.S. Takimi1,c, V.C. Sousa2,d and C.P. Bergmann1,e 1 UFRGS – Laboratório de Materiais Cerâmicos.

Av. Osvaldo Aranha, 99 sala 705c, Porto Alegre - RS – Brazil. CEP: 90035-190 2 UFRGS - Laboratório de Biomateriais e Cerâmicas Avançadas.

Av. Bento Gonçalves, 9500, Setor 4, Prédio 74, sala 118, Porto Alegre - RS – Brazil. CEP: 91509-900

a felipe.lucsan@gmail.com, b dptarrago@gmail.com, c astakimi@ufrgs.br, d vania.sousa@ufrgs.br and e bergmann@ufrgs.br

Keywords: ZnO varistor, nanostructured ZnO, tetra-needle like

Abstract. It is well known that nanostructured materials have relevant influences in properties

behavior that can be achieved when compared with conventional materials. In this study is proposed

an investigation of the electrical and microstructural properties of zinc oxide based varistors

prepared with nanostructured zinc oxide powder obtained by a thermal evaporation process. Zinc

oxide powder morphology was investigated by scanning and transmission electron microscopy

(SEM and TEM, respectively) and the specific surface area evaluated by adsorption of N2. The

varistors were prepared by the mixture of typical dopants with zinc oxide powders in a ball mill.

The surface area of zinc oxide powder used was 17.4 m2/g with tetra-needle like morphology. After

powder mixture process it was observed by TEM micrographs that most of the tetrapod shaped zinc

oxide broke into needles well mixed with dopant particles. The compressed powders were sintering

at 1050, 1150 and 1250ºC for 1.5 h and densification over 94% were achieved in all tested

temperatures. Preliminary electrical characterization reveals that nanostructured zinc oxide

compositions have interesting varistor properties.

Introduction

Ceramic materials based on ZnO can be doped in order to induce a highly non-ohmic electrical

behavior, allowing their application as varistors in a great range of electric and electronic devices

[1]. This electrical characteristic is intimately related to a specific microstructure formation in

which a liquid phase is formed during sintering that, after cooling, is insulating and tends to remain

in the grain boundaries [2]. The grain size of nanostructured ZnO plays an important role in its

electrical conductivity, since this property is higher in the grain boundaries, interfering then in the

electrical properties of varistor based on such oxide [3,4]. As nanostructured materials can have

very distinct characteristics of those microstructured [5], this work intends to analyze the

microstructure and the electrical properties of varistors made with a nanostructured tetra-needle like

ZnO powder, obtained by an adapted thermal evaporation process. A commercial grade of a

microstructured polyhedral shaped ZnO powder was used to obtain the same varistor composition

for comparison.

Materials and Methods

It this work a nanostructured ZnO powder was obtained by an adaptation of a thermal evaporation

process [6-9]. Thanks to a innovative design which allows a precise control of temperature and gas

composition in the reactive atmosphere, metallic zinc granulates, as raw material, were evaporated

and driven to a controlled oxidation zone in where a nanostructured ZnO powder was produced.

Materials Science Forum Vols. 727-728 (2012) pp 533-538Online available since 2012/Aug/24 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.727-728.533

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The specific surface area of both powders was measured by B.E.T. technique (Quantachrome

Instrument - Nova 1000) using N2. Transmission electron microscopy (TEM) was performed in a

Jeol JEM 2010 and the sample was prepared by dispersing the powder in isopropanol in a ultrasonic

bath (40kHz) for 15 min. Also scanning electron microscopy (SEM) was used to analyze the

starting powder morphology in a SEM - Jeol JEM 6060.

The ZnO varistors were prepared using both ZnO powders: the nanostructured and the commercial

grade (denominated Var Nano and Var Com, respectively) and metal oxides as dopants in a molar

proportion of 92.5% ZnO, 3% Bi2O3, 3% Sb2O3, 0.5% CoO, 0.5%, MnO and 0.5% Cr2O3. These

formulations were mixed in a ball mill with isopropanol for 4 h. PVB (10% in isopropanol) was

used as binder only in the Var Com composition since the samples in its first batch laminated. The

powder mixture was sieved (mesh # 200) and pressed under 140 MPa in discs (12 mm diameter)

and sintered at 1050, 1150 and 1250°C for 1.5 h under a heating and cooling rate of 5°C/min. The

microstructure of the sintered samples was revealed by thermal etching with annealing polished disc

in a furnace at about 100°C below the sintering temperature for 20 min. The morphology and and

the local chemical composition, in the grains cores and boundaries, was investigated using energy

dispersive spectroscopy (EDS) analyzers coupled with SEM (Jeol JEM 5800).

For the electrical characterization both faces of the samples were coated with silver paste and ohmic

contacts were formed by a heating treatment at 400 ºC for 15 min. Electric field–current density (E–

J) characteristic of varistors was measured at room temperature using a Keithley 237 unit. The

breakdown electric field (Eb) was then determined for a current density of 1mA/cm2 and the leakage

current density (JL) was defined as the current density at 0.8 Eb. The nonlinear coefficient (α) was

calculated by a linear regression of E-J curve on a logarithmic scale starting from 1mA/cm2

[2, 10].

Results and discussion

With the procedure used was possible to obtain a nano powder with a predominant morphology of

tetra-needle with submicron size core and needles with a few nanometers of diameter, as is shown

in the micrographs of Fig. 1a. For comparison a commercial grade of a ZnO powder (LABSYNTH)

was used and Fig. 1b shows a microstructured polyhedral particle shape.

Fig. 1: Scanning electron micrographs of (a) nanostructured tetra-needle shaped ZnO and (b)

commercial polyhedral shaped ZnO.

After the ball milling process most of the tetrapods of the ZnO powder broke into needles and

mixed with the dopant particles, as seen in the TEM micrograph in Fig. 2.

The specific surface area of nanostructured and commercial ZnO grades were 17.4 m²/g and 6.5

m²/g respectively. After mixture the specific surface area of Var Nano composition decreased to

15.8 m²/g which might be related to the presence of dopants with larger particles size.

b a

534 Advanced Powder Technology VIII

Fig. 2: TEM micrograph of Var Nano composition after ball mill process.

The curves obtained in electrical characterization are shown in Fig. 3, where it can be observed that

all samples showed varistor properties and those sintered at 1150ºC had a more sudden transition of

the insulating-conductor behavior.

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

6500

0 5 10 15 20 25 30

J [mA/cm2]

E [

V/c

m]

Var Nano 1050ºC

Var Nano 1150ºC

Var Nano 1250ºC

Var Com 1050ºC

Var Com 1150ºC

Var Com 1250ºC

Fig. 3: E – J characterization of different ZnO varistor composition tested.

According to results of Tab. 1 the samples burned at 1050ºC were those that supported higher

electrical fields and the Var Com composition had the higher Eb value. However, the leakage

current was smaller in samples sintered at 1150ºC and the non-linearity coefficient was

considerably better in these samples. When the sintering temperature was increased to 1250ºC the

samples still had non-ohmic behavior, nevertheless the varistor properties drastically degraded.

From technological characterization it was observed that the higher densification was achieved in

Var Nano and Var Com samples sintered at 1150°C also the lower apparent porosity was registered

for these samples. Varistors sintered at 1250°C had the lower densification attributed to bismuth

and antimony oxides evaporation and by the increase of the grain size. These oxides have a large

Materials Science Forum Vols. 727-728 535

vapor pressure which leads to some evaporation during sintering. Additional gradients in the

components with higher concentrations of volatile species in the interior of the parts can be

produced by this evaporation promoting an increase in the porosity [11-13]. Summarizing, the best

electrical properties were achieved in samples with higher sintering densification and consequently

lower apparent porosity that are samples sintered at 1150°C. At sintering temperature of 1250ºC the

evaporation of volatiles caused an increase in the open porosity and demoted the electrical

properties.

Table 1: Results of electrical and technological characterization of ZnO varistors sintered at

different temperature

Sample

Eb -

Breakdown

electrical

field [V/cm]

JL

Leakage

current

[mA/cm²]

α

(nonlinearity

coefficient)

Sintered

densification

[%]

Apparent

porosity

[%]

Var Nano 1050ºC 3749 0,29 ± 0,01 9,35 ± 0,21 97,18 ± 1,40 1,04 ± 0,13

Var Nano 1150ºC 3633 0,11 ± 0,06 19,61 ± 1,4 97,42 ± 1,01 0,70 ± 0,48

Var Nano 1250ºC 1783 0,39 ± 0,08 6,38 ± 0,89 94,23 ± 0,26 1,41 ± 0,91

Var Com 1050ºC 4567 0,28 ± 0,02 9,18 ± 0,4 96,64 ± 0,86 1,06 ± 0,75

Var Com 1150ºC 3867 0,15 ± 0,03 18,58 ± 1,27 98,66 ± 0,39 0,82 ± 0,33

Var Com 1250ºC 1767 0,38 ± 0,02 7,66 ± 1,48 96,08 ± 0,75 1,23 ± 0,28

The microstructural analysis of both varistors compositions revealed a tendency to increase the ZnO

grain size as the sintering temperature increased, as shown in Fig. 4. For samples burned at 1150°C

which were considered with the best electrical properties it was found that a composition made with

nanostructured ZnO powder shown a smaller grain size than varistors prepared with the commercial

powder. By EDS analysis it was possible to see a greater presence of a second phase rich in

bismuth, antimony and zinc formed at the grain boundaries of the sample Var Nano 1150C,

confirming the results presented in Tab. 1.

Although the microstructure of Var Nano 1150°C seem better formed, with secondary phases

distributed along the grain boudaries, it did not reflect in the electrical properties, when compared to

the Var Com 1150°C composition. This fact suggests that the showed micrograph (Fig. 4b) might

not be representative of the overall microstructure.

536 Advanced Powder Technology VIII

Fig. 4: SEM micrographs of Var Nano composition sintered at (a) 1050°C, (b) 1150°C and (c)

1250°C and Var Com composition sintered at (d) 1050°C, (e) 1150°C and (f) 1250°C. Below each

micrographs the respective EDS analysis with a yellow arrow indicating the analyzed point.

Conclusions

The use of nanostructured ZnO powders had a very discrete influence over electrical properties of

the studied varistor compositions. The best electrical properties were achieved in samples with

higher sintering densification and consequently lower apparent porosity that are samples sintered at

1150°C. The microstructure of Var Nano 1150°C seem to have a better formation when compared

to the Var Com 1150°C, however the electrical results were not influenced by the ZnO used,

suggesting some microstructural heterogeneity. Although, it was observed that the use of

nanostructured powder facilitated processing without the need to use any kind of additive. Also

better control in the grain growth and morphology of the grains was noticed when the

nanostructured ZnO powder was used.

Acknowledgement

The authors would like to thanks to CNPq (National Counsel of Technological and Scientific

Development) for the financial support.

5µm

b a

5µm

5µm

f

5µm

d

5µm

e

5µm

c

Materials Science Forum Vols. 727-728 537

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538 Advanced Powder Technology VIII

Advanced Powder Technology VIII 10.4028/www.scientific.net/MSF.727-728 Electrical and Microstructural Properties of Varistors Based on Nanostructured Tetra-Needle Like

Zinc Oxide Powders 10.4028/www.scientific.net/MSF.727-728.533

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http://dx.doi.org/10.1016/j.vacuum.2009.10.031