Impedance performances of SnO2–Zn2SnO4 composite ceramics

Journal of Alloys and Compounds 580 (2013) 611–613

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Impedance performances of SnO2–Zn2SnO4 composite ceramics Guo-Zhong Zang a,b,⇑, Li-Ben Li a, Huan-Huan Liu a, Xiao-Fei Wang a, Zhi-Gang Gai b a b

School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471003, China State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China

a r t i c l e

i n f o

Article history: Received 24 June 2013 Accepted 22 July 2013 Available online 31 July 2013 Keywords: Varistors Electroceramics Grain boundaries

a b s t r a c t To investigate the formation of grain boundary barriers, composite SnO2–Zn2SnO4 varistor ceramics were prepared by traditional ceramic processing and, the electrical properties of grains and grain boundaries were evaluated by the measurement of complex impedances spectra. Calculated using the low frequency impedance data for each sample, a lower value about 0.09–0.32 eV and a higher value about 0.50–0.83 eV of activation energy Ea was obtained in the temperature range of 50–110 and 140–200 °C, respectively. The simulated results of complex impedances spectra by ‘‘Zsimpwin’’ software indicate that the equivalent circuit for SnO2–Zn2SnO4 composite ceramics is connected in series by one RC and two QR parallels. The relaxations in the dielectric dispersion spectra at low frequency of 1000 Hz suggest that one of the QR parallels is corresponding to oxygen vacancy. With increasing sintering temperature, the Ea values related to oxygen vacancy decreased whereas, the values related to grain boundary barriers increase obviously. The results suggest that the oxygen vacancy is the key factor to the semi-conductance of SnO2 grains and formation of grain boundary barriers. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction A varistor is a voltage-dependent resistor whose primary function is to sense and limit transient-voltage surges and hence, protect sensitive-state components [1]. To facilitate the miniaturization and densification of integrated electronic components or devices, the recent trend in electrical appliance design requires varistors contain more functions and have relatively low breakdown voltage. Although ZnO is the most widely studied varistor material [1–5], the relative low permittivity of ZnO varistor weakens its ability to absorb the sparks, and therefore limits its protection for electrical micro-machines. Since the discovery of SnO2-based varistors in 1995 [6], the systems of SnO2–Co–Nb (Ta) [7,8], SnO2–Zn–Nb (Ta) [9] and SnO2–Cu–Nb (Ta) [10] have been studied as high-voltage varistors widely. Recently, our research group has discovered that without any doping, the composite SnO2–ZnO ceramics have nonlinear electrical properties characterized by low breakdown voltage and high permittivity [11,12]. Further study revealed that the varistor behavior is also a grain boundary barrier effect and ZnO has synthesized with SnO2 during sintering, and the composite ceramics are virtually composed of SnO2, Zn2SnO4. However, the electrical properties of SnO2 and Zn2SnO4 grains, the varistor and high permittivity properties are not clearly understood. In this work, we attempt to

⇑ Corresponding author at: School of Physics and Engineering, Henan University of Science and Technology, Luoyang 471003, China. Tel.: +86 379 65626260. E-mail address: [email protected] (G.-Z. Zang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.07.152

investigate the formation mechanism of grain boundary barriers using complex impedances spectra. 2. Experimental procedure The ceramic samples with the compositions (1  x) mol SnO2 + x mol Zn2SnO4 (x = 0.20, 0.25) were prepared using analytical grades of SnO2 and ZnO. At 1000 °C using SnO2 and ZnO powders, Zn2SnO4 was synthesized and crushed to be used. The raw chemicals were mixed in a nylon bottle for 12 h using ZrO2 milling media in anhydrous ethanol. The dried powders were mixed with 5% weight of polyvinyl alcohol (PVA) binder and pressed into 15 mm diameter disks with 1.5 mm thickness at 200 MPa. After burning out the PVA binder at 650 °C for 2 h, the disks were sintered at 1300 and 1400 °C for 2 h. To measure the electrical properties, silver electrodes were made on both surfaces of the sintered disks. Using the impedance analyzer (Agilent 4294A), the permittivity spectra was measured from 40 Hz to 1 MHz and the complex impedances spectra was measured during the temperature range of 50–200 °C. The microstructure of the sample surfaces was analyzed by scanning electron microscopy (SEM) (JSM-5900).

3. Results and discussion Usually, for varistor ceramics, the equivalent electrical circuit is composed by two series circuits of a resistance and capacitor in parallel and accordingly, the Nyquist diagram of the impedance is composed of two semicircles [13]. However, the two time constants model sometimes exhibits only one semicircle because of the randomly distributed defects in the grain boundary region [14]. Fig. 1 shows the complex impedances spectra for the sample of x = 0.25 sintered at 1400 °C. As can be observed, only one partial semicircle was traced out and the radius decreased rapidly with

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G.-Z. Zang et al. / Journal of Alloys and Compounds 580 (2013) 611–613

2.0

Table 1 Activation energies Ea calculated from the Arrhenius plot at low and high frequencies.

(a)

1.5

Samples

1.0

x = 0.20, x = 0.20, x = 0.25, x = 0.25,

o

80 C

Z '' (MΩ . cm)

o

110 C

0.0 0.0

0.2

0.4

0.6

0.8

1.0

(b) o

140 C

0.05

o

170 C o

200 C

0.00 0.00

0.05

0.10

0.15

0.20

Z ' (MΩ . cm)

0.25

0.30

Fig. 1. Impedance spectra diagrams for the sample of x = 0.25 sintered at 1400 °C.

increasing measure temperature from 50 to 200 °C. Thus, the one time constant and two time constants models were used to fit the experimental results. Unexpectedly, both models can not fit the data well. This indicates that the equivalent electrical circuit for composite SnO2–Zn2SnO4 ceramics cannot be viewed traditionally. Fig. 2 presents the low (40 Hz) and high (1 MHz) frequencies Arrhenius plots. The low frequency data present two linear regions with different slopes whereas, the high frequency data present only one linear region above 140 °C. The results indicate that at least two kinds of defects exist at grain boundary and the conductance mechanism of grains is so complex below 140 °C that the activation energies Ea may be superimposed by different defects. The obtained Ea values are listed in Table 1 and the following results can be observed. (1) At 40 Hz, the Ea values measured above 140 °C are much larger than those below 110 °C; (2) The Ea values at 1 MHz are much lower than those at 40 Hz; (3) At 40 Hz below 110 °C, the Ea values for the samples sintered at 1300 °C are lower

(a)

R =Aexp (Ea /KT )

14 o

x=0.20 1300 C o x=0.20 1400 C o x=0.25 1300 C o x=0.25 1400 C

12 40Hz

ln R

10

1300 °C 1400 °C 1300 °C 1400 °C

50–110 °C

140–200 °C

50–110 °C

140–200 °C

0.22 0.14 0.32 0.09

0.49 0.72 0.53 0.85

– – – –

0.09 0.35 0.14 0.06

1.2

0.10

16

1 MHz, Ea (eV)

o

50 C

0.5

0.15

40 Hz, Ea (eV)

(b) 9

than those sintered at 1400 °C and on the contrary, above 140 °C, they are higher than those for the samples sintered at 1400 °C. Kim et al. [15] observed that the activation energies for the O0 and O00 species on the SnO2 surface are 0.6 and 1.0 eV, respectively. For SnO2 varistor system, Bueno et al. [14] reported the Ea values for O2 O00 and O0 are about 0.39 and 0.77 eV, respectively. Because the Ea values measured at 40 Hz above 140 °C varied between 0.49 to 0.85 eV in present study, it is possible that these three species all predominate at grain boundary for the SnO2–Zn2SnO4 composite system. In addition, at 40 Hz below 110 °C, the low values varied between 0.09 and 0.32 eV may be related with the presence of V O at depletion layer since this defect exhibits low frequency time constant as widely described in the literature [14,16–18]. With increasing sintering temperature, more V O were produced and a larger quantity of electrons would diffuse to the grain boundary. Thus, the electrons were captured by oxygen based on Eqs. (1)–(4) and higher grain boundary barriers were formed. 0 VO ! V O þ 2e

ð1Þ

e0 þ O2 ! O02

ð2Þ

e0 þ O02 ! 2O0

ð3Þ

e0 þ O0 ! O00

ð4Þ

It is worthy to note that the activation energies measured at high frequency are also as low as 0.06–0.35 eV which is similar to the values at 40 Hz below 110 °C. For SnO2 varistors, donor doping such as Nb2O5, has usually been adopted to improve the conductivity of grains which is necessary for the formation of Schottky-type barriers. For SnO2–Zn2SnO4 composite varistor system, the low Ea values at 1 MHz suggest that most of the grains are semi-conducted. Recently, for SnO2–Zn2SnO4 composite ceramics, we have reported that the growth of SnO2 grains is accompanied with the mass transport of Zn2SnO4 and, the diffusion of Sn4+ and Zn2+ ions improve the ceramic sintering. Moreover, because no Zn elements were detected by the energy dispersive spectroscopy (EDS) in SnO2 grains [12], it is reasonable to suppose that a larger number of interstitial tin atoms are produced during sin tering and besides V O , the Sni which has the activation energy about 0.05–0.09 eV with high frequency time constant in SnO2 grains [14,19] is another important defect for the grain semiconducting. Based on the above results and analysis, as shown in Fig. 3, a model composed of three polarization processes for SnO2–Zn2SnO4 composite ceramics were proposed: thermal ionic polarization of

8 7 2.0

2.2

2.4

2.6

2.8

1MHz

Qgb

QO

Cg

3.0

Rgb

RO

Rg

3.2

-1

1000/T (K ) Fig. 2. Arrhenius plots for low and high frequencies region: (a): low frequency of 40 Hz; and (b) high frequency of 1 MHz.

Fig. 3. Equivalent circuit used to fit the impedance data. QgbRgb, QORO and CgRg represent the relaxations of grain boundaries, oxygen vacancies and grains, respectively.

G.-Z. Zang et al. / Journal of Alloys and Compounds 580 (2013) 611–613

1.6

(a)

1.2 0.8

o

1300 C, x=0.20 o

Z'' (MΩ . cm)

0.4

1300 C, x=0.25 fitted

0.0 0.0

0.6

0.5

1.0

1.5

2.0

(b)

0.4

613

are also affected by oxygen vacancy concentration since they varied with Zn2SnO4 content and the relation between oxygen vacancy and Zn2SnO4 needs to be further studied. Fig. 5 shows the dielectric spectra and SEM photos for all the samples. As can be seen, with increase frequency from 40 Hz to 1 MHz, er of all the samples decreased rapidly below 1 kHz and than slowly to the constants at higher frequency. The existence of oxygen vacancy is confirmed again by the low frequency dielectric relaxations since it has low frequency time constant as discussed above. At 40 Hz, the relative dielectric constants are higher than 104 for 1400 °C sintered samples whereas, lower than 2000 for 1300 °C sintered samples. This indicates that er strongly depends on the grain size and the dielectric behavior can be explained by the internal barrier layer capacitance (IBLC) model [20].

o

1400 C, x=0.20

0.2

o

1400 C, x=0.25 fitted

0.0 0.0

0.2

0.4

Z' (MΩ . cm)

0.6

0.8

Fig. 4. Impedance spectra diagrams and fitted curves for all the samples measured at 110 °C, (a): samples sintered at 1300 °C; and (b): samples sintered at 1400 °C.

o

12

1300 C, x=0.20 o

10

o

1400 C, x=0.20

3

Permittivity (10 )

1300 C, x=0.25 o

1400 C, x=0.25

8 6

4. Conclusions Densified SnO2–Zn2SnO4 composite ceramics were prepared and the electrical properties of the grains and grain boundaries were evaluated by the measurement of complex impedances spectra. The electrical conduction of the grains may attribute to the donor defects of interstitial tin atoms and the oxygen vacancies. Because of the high concentration in SnO2 grains, the electrons generated by the shallow donor defects with activation energy about 0.06–0.35 eV will diffuse to the grain boundaries and be captured by oxygens. The negative charged oxygens O2 O00 , O0 and O00 are all predominate at grain boundaries and thus, form the grain boundary Schottky barriers. For SnO2–Zn2SnO4 composite system, the varistor and high dielectric behaviour can be explained by the Schottky barrier and IBLC model, respectively.

4

Acknowledgements

2

This work was supported by China Postdoctoral Science Foundation (No. 2012M511499) and National Natural Science Foundation of China (No. 51002087).

0 102

103

104

105

106

Frequency (Hz) Fig. 5. Relative dielectric constant spectra of the samples. The inset is the SEM photos for the samples, top left: x = 0.20, 1300 °C; top right: x = 0.25, 1300 °C; bottom left: x = 0.20, 1400 °C; bottom right: x = 0.25, 1400 °C.

oxygen vacancies, interface polarizations of grains and grain boundaries. Because of the segregating of smaller Zn2SnO4 grains between SnO2 grains and the decrease of effective barrier number [12], the relaxations of grain boundaries and oxygen vacancy layers are so complex that the equivalent circuit of a resistance and a capacitor in parallel cannot fit the process well. By using the constant phase angle elements Qgb and QO substitute the capacitances, all the complex impedances spectra were fitted with minor errors about 104. Fig. 4 shows the 110 °C measured impedance spectra diagrams and the fitted curves for all the samples. The well fitting curves demonstrate the exact existence of oxygen vacancies in the composite ceramics as widely reported in CCTO varistor ceramics [17,18]. From Fig. 4 it can also be found that the radius at low frequency of the samples sintered at 1300 °C are much larger than those sintered at 1400 °C. Because lower sintering temperature resulted in smaller grain size, the higher concentration of grain boundary is the chief factor of the larger radius. In addition, the semi diameters

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