Microstructure and varistor properties of ZnO–V2O5–MnO2 ceramics with Ta2O5 addition

Journal of Physics and Chemistry of Solids 73 (2012) 834–838

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Microstructure and varistor properties of ZnO–V2O5–MnO2 ceramics with Ta2O5 addition Choon-W. Nahm n Semiconductor Ceramics Laboratory, Department of Electrical Engineering, Dongeui University, Busan 614-714, Republic of Korea

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 April 2011 Received in revised form 20 December 2011 Accepted 14 February 2012 Available online 19 February 2012

The effect of Ta2O5 addition on microstructure, electrical properties, and dielectric characteristics of the quaternary ZnO–V2O5–MnO2 vaistor ceramics was investigated. Analysis of the microstructure indicated that the quaternary ZnO–V2O5–MnO2–Ta2O5 ceramics consisted of mainly ZnO grain and minor secondary phases such as Zn3(VO4)2, ZnV2O4, TaVO5, and Ta2O5. As the amount of Ta2O5 increased, the sintered density increased from 94.8 to 97.2% of the theoretical density (5.78 g/cm3 for ZnO), whereas the average grain size decreased from 7.7 to 6.0 mm. The ceramics added with 0.05 mol% Ta2O5 exhibited the highest breakdown field (2715 V/cm) and the highest nonlinear coefficient (20). However, further increase caused a to abruptly decrease. The Ta2O5 acted as a donor due to the increase of electron concentration in accordance with the amount of Ta2O5. The donor concentration increased from 1.97  1018 to 3.04  1018cm  3 with increasing the amount of Ta2O5 and the barrier height exhibited the maximum value (0.95 eV) at 0.05 mol% Ta2O5. & 2012 Elsevier Ltd. All rights reserved.

Keywords: A. Ceramics D. Microstructure D. Electrical properties D. Dielectric properties

1. Introduction Zinc oxide (ZnO) is a non-stoichiometric material containing excessive Zn ions. ZnO added with several different metal oxides, such as excessive metal, metal vacancies, excessive oxygen, and oxygen vacancies, are smart semiconducting electroceramics applicable to the nonlinear devices, which exhibit abruptly increasing current in accordance with increasing voltage. This nonlinearity of voltage–current properties is due to the presence of a double Schottky barrier (DSB) formed at active grain boundaries containing many trap states. Owing to highly nonlinear properties, these ceramic devices are widely used in the field of overvoltage protection ceramics from electronic circuits to electric power ceramics [1,2]. In the light of research results up to now, ZnO ceramics cannot exhibit a strong nonlinear behavior without adding heavy elements with large ionic radii such as Bi, Pr, Ba, etc. Commercial ZnO/Bi2O3-based ceramics and ZnO/ Pr6O11-based ceramics cannot be co-fired with a silver innerelectrode (m.p. 961 1C) in a multilayered chip component because of their high sintering temperature above 1000 1C [3,4]. Therefore, the new varistors require further research in order to use a silver inner-electrode. Among the various possible ceramics, one candidate is the ZnO–V2O5 system [5]. These ceramics can be sintered at a relatively low temperature of approximately 900 1C. This is important for multilayer chip component applications, because it

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can be co-sintered with a silver inner-electrode without using expensive palladium or platinum metals. Binary ZnO–V2O5 ceramics exhibits low nonlinear properties with a nonlinear coefficient below 10 [5–8]. In order to develop high performance nonlinear ceramics, ZnO–V2O5 ceramics containing a few additives have been actively studied [9–16]. However, a study on ZnO–V2O5 ceramics is in its early stages in terms of materials’ composition and sintering process. Therefore, to develop useful ZnO–V2O5 ceramics, it is very important to understand the influence of diverse additives. MnO2 is often added to ZnO– Bi2O3 system to improve the varistor properties [17,18]. Also, MnO2 was found to restrict the abnormal grain growth of ZnO and to improve varistor properties in ZnO–V2O5 system [9,12]. A study on the influence of Ta2O5 addition on the electrical properties of the ternary ZnO–V2O5–MnO2 ceramics has not been reported. In this study, the influence of Ta2O5 addition on the microstructure, electrical properties, and dielectric characteristics of the ternary ZnO–V2O5–MnO2 varistor ceramics was investigated and some new results were obtained.

2. Experimental procedure 2.1. Sample preparation Reagent-grade raw materials were used in the proportions of (97.5 x) mol% ZnO þ0.5 mol% V2O5 þ2.0 mol% MnO2 þx mol% Ta2O5 (x ¼0.0, 0.05, 0.1, and 0.25). Raw materials were mixed by ball milling with zirconia balls and acetone in a polypropylene

C.-W. Nahm / Journal of Physics and Chemistry of Solids 73 (2012) 834–838

835

20 μm

20 μm

20 μm

20 μm

Fig. 1. SEM micrographs of the samples with different amounts of Ta2O5: (a) 0.0 mol%, (b) 0.05 mol%, (c) 0.1 mol%, and (d) 0.25 mol%.

bottle for 24 h. The mixture was dried at 120 1C for 12 h. The dried mixture was mixed into a container with acetone and 0.8 wt% polyvinyl butyral (PVB) binder of powder weight. After drying, the mixture was granulated by sieving through a 100-mesh screen to produce starting powder. The powder was uniaxially pressed into disks of 10 mm in diameter and 1.5 mm in thickness at a pressure of 100 MPa. The disks were sintered at 925 1C in air for 3 h and furnace cooled to room temperature. The heating and cooling rates were 4 1C/min. The final samples were about 8 mm in diameter and 1.0 mm in thickness. Silver paste was coated on both faces of the samples and the electrodes were formed by heating at 550 1C for 10 min. The electrodes were 5 mm in diameter. 2.2. Microstructure characterization

1:56L MN

2.3. Electrical measurement The E–J characteristics of the samples were measured using a high voltage source measure unit (Keithley 237, Keithley Instruments Inc., Cleveland, OH, USA). The breakdown field (EB) was measured at a current density of 1.0 mA/cm2 and the leakage current density (JL) was measured at 0.80EB. In addition, the nonlinear coefficient (a) was determined from the following expression:

a¼

One side of the samples was lapped and ground with SiC paper and polished with 0.3 mm-Al2O3 powder to a mirror-like surface. The polished samples were chemically etched with 1HClO4  1000 H2O for 25 s at 25 1C. The surface microstructure was examined by a scanning electron microscope (FESEM, Quanta 200, FEI, Brno, Czech). The average grain size (d) was determined by the lineal intercept method using the following expression [19]: d¼

Panalytical, Almelo, Netherlands) with Ni filtered CuKa radiation. The density (r) of sintered pellets was measured using a density determination kit (238490) attached to balance (AG 245, Mettler Toledo International Inc., Greifensee, Switzerland).

ð1Þ

where L is the random line length on the micrograph, M is the magnification of the micrograph, and N is the number of the grain boundaries intercepted by the lines. The crystalline phases were identified by powder X-ray diffraction (XRD, X0 pert-PRO MPD,

log J 2 log J 1 log E2 log E1

ð2Þ

where E1 and E2 are the electric fields corresponding to J1 ¼ 1.0 mA/cm2 and J2 ¼10 mA/cm2, respectively. The capacitance– voltage (C–V) characteristics of the samples were measured at 1 kHz using an RLC meter (QuadTech 7600, Marlborough, MA, USA) and an electrometer (Keithley 617, Keithley Instruments Inc., Cleveland, OH, USA). The donor concentration (Nd) and the barrier height (Fb) were determined by the following expression [20]:   1 1 2 2ðFb þV gb Þ  ¼ ð3Þ qe Nd C b 2C b where Cb is the capacitance per unit area of a grain boundary, Cbo is the value of Cb when Vgb ¼0, Vgb is the applied voltage per grain boundary, q is the electronic charge, and e is the permittivity of

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ZnO (e ¼ 8.5e0). The density of interface states (Nt) at the grain boundary was determined by the following expression [20]:   2eNd Fb 1=2 Nt ¼ ð4Þ q

Zn

and the depletion layer width (t) of the either side at the grain boundaries was determined by the following expression [21]: Nd t ¼ Nt

ð5Þ

O

2.4. Dielectric measurement

Zn3(VO4)2

ZnV2O4

The dielectric characteristics, such as the apparent dielectric constant (eAPP0 ) and the dissipation factor (tan d) of the samples were measured in the range from 100 Hz to 2 MHz using an RLC meter (QuadTech 7600, Marlborough, MA, USA).

3. Results and discussion Fig. 1 shows SEM micrographs of the samples with different amounts of Ta2O5. It can be easily seen that the grain structure is very homogeneously distributed through the entire ceramics. As the amount of Ta2O5 increased, the average grain size decreased from 7.7 to 6.0 mm. Excessive Ta2O5 segregates at the grain boundaries and hinders the movement of the grain boundaries due to a decrease of interface energy at the grain boundaries. As a result, the average grain size decreases. The density of sintered pellets increased from 94.8 to 97.2% of the theoretical density (TD) (pure ZnO, TD¼5.78 g/cm3) with increasing the amount of Ta2O5. As a result, the incorporation of Ta2O5 slightly enhanced the densification. The XRD patterns of the samples with different amounts of Ta2O5 are shown in Fig. 2. These patterns reveal in common the presence of Zn3(VO4)2 and ZnV2O4 as the secondary phases, in addition to a primary phase of hexagonal ZnO. Moreover, the Ta2O5-added samples revealed the presence of TaVO5, Ta2O5, and an unidentified phase related to Ta2O5 as a secondary phase. No secondary phase related to MnO2 is detected. No peak for V species is found at the grain interior within EDS detection limit though the ion radius of V is smaller than that of Zn. This means the V species are not dissolved into the ZnO grain [15]. The EDS analysis in Fig. 3 shows that V species are segregated at grain boundaries and triple points, and form Zn–V phases there. The EDS analysis in Fig. 4 shows that a part of Ta species forms Ta–V phases within ZnO grain and at triple points. It can be seen that the samples possess various minor phases. Therefore, the

Fig. 2. XRD patterns of the samples with different amounts of Ta2O5: (a) 0.0 mol%, (b) 0.05 mol%, (c) 0.1 mol%, and (d) 0.25 mol%.

V Mn

0

1

2

3

4

5

6

7

Energy (KeV) Fig. 3. EDS microanalysis for Zn–V phases of ZnO–V2O5–MnO2–Ta2O5 ceramics.

Zn

TaVO5

O Ta

0

1

2

3

4

V

Mn

5

6

7

Energy (KeV) Fig. 4. EDS microanalysis for Ta–V phases of ZnO–V2O5–MnO2–Ta2O5 ceramics.

decrease of average grain size with increasing the amount of Ta2O5 is attributed to the generation of several secondary phases related to Ta2O5. The detailed microstructure parameters are summarized in Table 1. Fig. 5 shows the electric field–current density (E–J) characteristics of the samples with different amounts of Ta2O5. The nonlinear properties are characterized by the nonlinearity in the E–J characteristics. The curves show that the conduction characteristics are divided into two regions: an ohmic region before breakdown field and a nonlinear region after breakdown field. The sharper the knee of the curves between the two regions, the better the nonlinear properties. It can be seen from curve shapes that the incorporation of Ta2O5 has a significant effect on the nonlinear properties. The breakdown field (EB) increased from 2350 to 2715 V/cm by increasing the amount of Ta2O5 up to 0.05 mol%. Further increase caused EB to decrease to 800 V/cm at 0.25 mol%. The behavior of EB in accordance with Ta2O5 amount can be explained by the

C.-W. Nahm / Journal of Physics and Chemistry of Solids 73 (2012) 834–838

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Table 1 Microstructure, E–J, and C–V characteristic parameters of the samples with different amounts of Ta2O5. Ta2O5 amount (mol%)

d (mm)

r a (%)

EB (V/cm)

vb (V/gb)

a

JL (mA/cm2)

Nd (1018 cm  3)

Nt (1012 cm  2)

Fb (eV)

t (nm)

0.0 0.05 0.1 0.25

7.7 7.5 7.3 6.0

94.8 95.5 96.2 97.2

2350 2715 2533 800

1.80 2.03 1.85 0.48

13 20 9 2

0.4 0.4 0.5 0.8

1.97 2.42 2.70 3.04

3.91 4.64 3.83 2.45

0.81 0.95 0.58 0.21

19.8 19.2 14.2 8.1

Theoretical density 5.78 g/cm3 for ZnO.

Electric field, E (V/cm)

a

3750

3750

3000

3000

2250

2250

1500

1500

750

750 0.0 mol%

0

Electric field, E (V/cm)

0

2

4

6

8

10

12

0

3750

3750

3000

3000

2250

2250

1500

1500

750

750 0.1 mol%

0 0

2

4

6

8

10 2

0.05 mol%

0 2

4

6

8

12

0.25 mol%

0 12

10

0

2

4

6

8

10

12

2

Current density, J (mA/cm )

Current density, J (mA/cm )

Fig. 5. E–J characteristics of the samples with different amounts of Ta2O5.

following expression: 4

2

where d is the grain size and vb is the breakdown voltage per grain boundaries. This expression indicated that EB is directly determined by d and vb. In general, the decrease of EB results from the decrease in the number of grain boundaries owing to the increase of the average ZnO grain size. However, this experiment exhibits a different result. Although the average ZnO grain size decreases with increasing the amount of Ta2O5, the decrease of EB is due to a decrease of the vb. The a-value of Ta2O5-free samples was 13, whereas the a value of 0.05 mol% Ta2O5-added samples significantly increased to the maximum value of 20. Further increase resulted in bad nonlinear properties. The a value of the samples added with 0.25 mol% Ta2O5 was only 2. The JL value increased in the 0.4–0.8 mA/cm2 with increasing the amount of Ta2O5. It can be seen that the behavior of JL in accordance with the amount of Ta2O5 is inversely proportional to the behavior of a. The detailed E–J characteristic parameters are summarized in Table 1. Fig. 6 shows the capacitance–voltage (C–V) characteristics of the samples with different amounts of Ta2O5. It is forecasted from curve shapes that the incorporation of Ta2O5 will have a significant effect on nonlinear properties due to a variation of the donor concentration and barrier height with Ta2O5 amount. The detailed C–V characteristic parameters, such as donor concentration (Nd), barrier height (Fb), density of interface states (Nt), and

0.8

13

ð6Þ

0.6

2

vb d

(1/Cb-1/2Cb0) (10 cm /F )

EB ¼

1.0

0.4

0.0 mol% 0.05 mol% 0.1 mol% 0.25 mol%

0.2 0.0 0.0

0.1

0.2

0.3

Voltage/grain boundary, V gb (V/gb) Fig. 6. C–V characteristics of the samples with different amounts of Ta2O5.

depletion layer width (t) are summarized in Table 1. The Nd increased from 1.97  1018 to 3.04  1018 cm  3 with increasing the amount of Ta2O5. Ta5 þ ions can be easily substituted in the Zn2 þ site, because the Ta5 þ ionic radius (0.070 nm) is smaller than that (0.074 nm) of the Zn2 þ ion. Therefore, the Ta5 þ ion increases the electron carrier concentration by donating an electron to the conduction band according to the following chemical-defect reaction: ZnO

0 Ta2 O5 !2Ta Zn þ4OO þ 1=2O2 þ 6e

ð7Þ

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C.-W. Nahm / Journal of Physics and Chemistry of Solids 73 (2012) 834–838

Dielectric constant, εApp'

2500

Table 2 Dielectric parameters of the samples with different amounts of Ta2O5.

0.0 mol% 0.05 mol% 0.1 mol% 0.25 mol%

2000

Ta2O5 amount (mol%)

eAPP0 (1 kHz) tan d (1 kHz)

1500

1296.8 0.365

0.05 1315.6 0.336

0.1 1365.9 0.377

0.25 2229.7 1.103

1000 500 2

10

3

10

4

10

5

10

6

10

Frequency, f (Hz) 1.2

Dissipation factor, tanδ

0.0

0.0 mol% 0.05 mol% 0.1 mol% 0.25 mol%

1.0 0.8 0.6

where eg is the dielectric constant of ZnO (8.5), d is the average grain size, and t is the depletion layer width of the both sides at the grain boundaries. The increase of d/t ratio in eAPP0 ¼ eg(d/t) gives rise to the increase of the eAPP0 . The eAPP0 at 1 kHz increased in the range of 1296.8–2229.7. On the other hand, the tan d decreased until the vicinity of 30 kHz with the increasing frequency. This exhibits weak absorption peak in the vicinity of 100 kHz and thereafter again decreased. The tan d at 1 kHz increased in the range of 0.365–1.103. The detailed dielectric parameters are summarized in Table 2.

0.4 0.2

4. Conclusions

0.0 2

10

3

10

4

10

5

10

6

10

Frequency, f (Hz) Fig. 7. Dielectric characteristics of the samples with different amounts of Ta2O5.

As a result, the Ta5 þ ion acted as a donor on the basis of the above data. The density of interface states (Nt) in accordance with the amount of Ta2O5 was a maximum value (4.64  1012 cm  2) at 0.05 mol% Ta2O5 and on further increase decreased to 2.45  1012 cm  2 at 0.25 mol% Ta2O5. The barrier height (Fb) at the grain boundaries increased in the range of 0.81–0.95 eV by increasing the amount of Ta2O5 up to 0.05 mol%. Further increase caused Fb to decrease to 0.21 eV at 0.25 mol% because of the decrease in the density of interface states (Nt) at the grain boundaries. This coincides with the variation of a in the E–J characteristics. Really, the higher barrier gives rise to the higher nonlinear coefficient. The depletion layer width (t) on either side of depletion region between grains decreased from 19.8 to 8.1 nm with increasing the amount of Ta2O5 and it exhibited the dependence of donor density with Ta2O5 amount, with a semiconductor theory, which the depletion region extends farther into the side with a low concentration. Fig. 7 shows the dielectric characteristics of the samples with different amounts of Ta2O5. As the amount of Ta2O5 increased, the apparent dielectric constant (eAPP0 ) decreased with less sharp dispersive drop with increasing frequency, which is closely associated with the polarization of dielectrics. It is assumed that this is attributed to the decrease of the number of dipole, which can follow the test frequency. On the whole, the eAPP0 in the measuring frequency increased by increasing the amount of Ta2O5 up to 0.1 mol%, whereas the eAPP0 of the sample added with 0.25 mol% increased sharply up to 30 kHz. This is directly related to the average grain size and depletion layer width, as can be seen in the following expression:   d ð8Þ eAPP 0 ¼ eg t

The microstructure and varistor properties of the quaternary ZnO–V2O5–MnO2–Ta2O5 ceramics were investigated with different amounts of Ta2O5. For the samples added with Ta2O5, the microstructure of the quaternary ZnO–V2O5–MnO2–Ta2O5 ceramics consisted of mainly ZnO grain and various secondary phases such as Zn3(VO4)2, ZnV2O4, TaVO5, and Ta2O5. The incorporation of Ta2O5 to the ternary ZnO–V2O5–MnO2 ceramics was found to restrict the grain growth and enhance the densification. The breakdown field (EB) the nonlinear coefficient (a) exhibited a maximum value at 0.05% with increasing the amount of Ta2O5. Ta2O5 additives acted as a donor due to the increase of donor concentration. The amount of Ta2O5 was optimized at 0.05 mol% by considering the nonlinear properties.

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