Effect of sintering temperature on microstructure and electrical properties of ZVMCDN ceramics

Effect of sintering temperature on microstructure and electrical properties of ZVMCDN ceramics

Materials Letters 64 (2010) 830–832 Contents lists available at ScienceDirect Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e ...

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Materials Letters 64 (2010) 830–832

Contents lists available at ScienceDirect

Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Effect of sintering temperature on microstructure and electrical properties of ZVMCDN ceramics Choon-W. Nahm ⁎ Semiconductor Ceramics Laboratory, Department of Electrical Engineering, Dongeui University, Busan 614-714, Republic of Korea

a r t i c l e

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Article history: Received 15 November 2009 Accepted 10 January 2010 Available online 15 January 2010 Keywords: Zinc oxide Sintering Microstructure Electrical properties Varistors

a b s t r a c t The microstructure and electrical properties of ZnO–V2O5–MnO2–Co3O4–Dy2O3–Nb2O5 (ZVMCDN) ceramics were investigated in accordance with sintering temperature (850–950 °C). The microstructure of the samples consisted of mainly ZnO grain as a main phase, and Zn3(VO4)2, ZnV2O4, and DyVO4 as the minor secondary phases. The sintered density decreased from 5.69 to 5.52 g/cm3 due to the volatility of V2O5 in accordance with increasing sintering temperature. The maximum nonlinear coefficient (57) was obtained at 925 °C. The donor concentration increased from 1.15 × 1018/cm3 to 11.1 × 1018/cm3 in accordance with increasing sintering temperature and the barrier height exhibited the maximum value (1.03 eV) at 925 °C. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Zinc oxide varistor is a II–VI defect oxide semiconductor device made by sintering ZnO powder doped with minor additives, such as Bi2O3, Pr6O11, CoO, MnO, Cr2O3, etc. ZnO varistor is a multi-junction grain boundary device with an electrostatic potential barrier between grains in the sintered body, compared with back-to-back Zener diode using single pn-junction for Si. This possesses a much higher energy handling capability than the Zener diode. It exhibits a highly nonlinear voltage–current (V–I) characteristics expressed by I = kVα, where k is a constant and α is a nonlinear coefficient. This means that the varistor acts as an open-circuit when they are subjected to normalvoltage and as a short-circuit when they are subjected to over-voltage. Owing to highly nonlinearity, ZnO varistors are widely used in the field of over-voltage protection systems [1,2]. Commercial Bi2O3- and Pr6O11-based ZnO varistors cannot be cofired with a silver inner-electrode in multilayered chip components because of the relatively high sintering temperature [3,4]. Therefore, new varistor ceramics are required in order to use a silver innerelectrode. Among the various ceramics, one candidate is the ZnO– V2O5 system [5–11]. This system can be sintered at relatively low temperature. This is important for multilayer chip component applications, because it can be co-sintered with a silver inner-electrode without using expensive Pd or Pt. To develop ZnO–V2O5-based varistors of high performance, it is very important to comprehend the effects of sintering process on

⁎ Tel./fax: +82 51 890 1669, +82 51 890 1664. E-mail address: [email protected] 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.01.030

varistor properties. Until now, ZnO–V2O5-based varistors are only quaternary system and 30 in nonlinear coefficient [12]. In this report, the effect of sintering temperature on microstructure and electrical properties of ZnO–V2O5–MnO2–Co3O4–Dy2O3–Nb2O5 (ZVMCDN) ceramics was studied, and this experimental result provides high nonlinear coefficient exceeding 50. 2. Experimental procedure Reagent-grade raw materials were used in the proportions of 96.8 mol% ZnO, 0.5 mol% V2O5, 2.0 mol% MnO2, 0.5 mol% Co3O4, 0.1 mol% Dy2O3, and 0.1 mol% Nb2O5. Raw materials were mixed by ball-milling with zirconia balls and acetone in a polypropylene bottle for 24 h. The dried mixture was mixed into a container with acetone and 0.8 wt.% polyvinyl butyral (PVB) binder of powder weight. After drying at 120 °C for 12 h, the mixture was granulated by sieving with a 100-mesh screen to produce starting powder. The powder was pressed into discs 10 mm in diameter and 1.5 mm in thickness at a pressure of 100 MPa. The discs were sintered at four fixed sintering temperatures (850, 900, 925, and 950 °C) in air for 3 h, and furnacecooled to room temperature. The heating and cooling rates were 4 °C/min. The final samples were 8 mm in diameter and 1.0 mm in thickness. Silver paste was coated on both faces of the samples and the ohmic contacts were formed by heating it at 550 °C for 10 min. The electrodes were 5 mm in diameter. The microstructure was examined by a scanning electron microscope (SEM, Hitachi S2400). The average grain size (d) was determined by the linear intercept method [13], given by d = 1.56 L/MN, 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

C.-W. Nahm / Materials Letters 64 (2010) 830–832

831

Fig. 1. SEM micrographs of the samples at different sintering temperatures.

boundaries intercepted by the lines. The crystalline phases were identified by an X-ray diffractometry (XRD, X′pert-PRO MPD, Netherland) with Ni filtered CuKα radiation. The sintered density (ρ) of the ceramics was measured by the Archimedes method. The electric field–current density (E–J) characteristics of the samples were measured using a V–I meter (Keithley 237). The breakdown field (EB) was measured at a current density of 1.0 mA/cm2 and the leakage current density (JL) was measured at 0.80 EB. In addition, the nonlinear coefficient (α) was determined from α = (logJ2 − logJ1)/(logE2 − logE1), where J1 = 1.0 mA/cm2, J2 = 10 mA/cm2, and E1 and E2 are the electric fields corresponding to J1 and J2, respectively. The capacitance–voltage (C–V) characteristics of the samples were measured at 1 kHz as a test frequency using an RLC meter (QuadTech 7600) and an electrometer (Keithley 617). The donor concentration (Nd) and the barrier height (Φb) were determined by the equation (1/ Cb − 1/2Cbo)2 = 2(Φb + Vgb)/qεNd [14], where Cb is the capacitance per unit area of a grain boundary, Cbo is the value of Cb when Vgb = 0, Vgb is the applied voltage per grain boundary, q is the electronic charge, ε is the permittivity of ZnO (ε = 8.5εo).

addition to the primary phase ZnO grain. No secondary phase related to MnO2, Co3O4, and Nb2O5 was detected. Therefore, it is assumed that other oxides are dissolved into the interior or exterior of ZnO grain. The detailed microstructure parameters are listed in Table 1. Fig. 2 shows the electric field–current density (E–J) characteristics of the samples at different sintering temperatures. The varistor properties are characterized by non-ohmicity in the E–J characteristics. It is clearly shown that the conduction characteristics divided into two regions, which are of very high impedance and very low impedance. The sharper the knee of the curves between the two regions, the better the nonlinear properties. The knee gradually becomes more pronounced in accordance with increasing sintering temperatures of up to 925 °C, whereas further elevated temperatures gradually reduce the nonlinear Table 1 properties. The detailed E–J characteristic parameters are summarized in Table 1. The EB decreased from 5604 V/cm to 1476 V/cm in accordance with increasing sintering temperatures. The behavior of EB in accordance with the sintering temperatures can be explained by the grain size: EB = vgb/d, where vgb is the breakdown voltage per grain boundaries and d is the grain size. This expression indicated that EB is directly determined by

3. Results and discussion Fig. 1 shows the SEM micrographs of the samples at different sintering temperatures. It can be seen that the grain size increased greatly from 2.7 µm to 11.5 μm in accordance with increasing sintering temperatures. The sintered density decreased from 5.69 g/cm3 to 5.52 g/cm3 (theoretical density = 5.78 g/cm3 in ZnO) for all the Fig. 1. samples. It is assumed that the decrease of sintered density is attributed to the volatility of the V-species for V2O5 with low melting point. Thus, it can be seen that the sintering temperature has a significant effect on the densification process. XRD analysis reveals the presence of Zn3(VO4)2, ZnV2O4, and DyVO4 as the secondary phases, in

Table 1 Microstructure, E–J, and C–V characteristic parameters of the samples at different sintering temperatures. Sintering d ρ Eb vgb α temp. (°C) (μm) (g/cm3) (V/cm) (V/gb)

JL Nd Φb (mA/cm2) (1018/cm3) (eV)

850 900 925 950

0.5 0.3 0.05 0.03

2.7 5.6 9.1 11.5

5.69 5.56 5.54 5.52

5604 3181 1972 1476

1.5 1.7 1.8 1.7

8 19 57 45

1.15 2.38 7.14 11.1

0.48 0.54 1.03 0.96

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Fig. 2. E–J characteristics of the samples at different sintering temperatures.

Fig. 3. C–V characteristics of the samples at different sintering temperatures.

the d and vgb value. However, the d value absolutely affects the EB. Therefore, the decrease of EB in accordance with increasing sintering temperature is attributed to the increase of the average ZnO grain size. The vgb was almost constant in the range of 1.5–1.8 V/gb in accordance with increasing sintering temperature. The nonlinear properties were remarkably improved at a sintering temperature beyond 925 °C and a maximum nonlinear coefficient (α = 57) was obtained at 925 °C. This is the maximum value in the ZnO–V2O5-based varistors reported until now. However, further elevated temperatures caused it to decrease, reaching 45 at 950 °C. This is also a considerably high value. The behavior of α in accordance with sintering temperature can be related to the Fig. 2 variation of a Schottky barrier height in accordance with the variation of the electronic states at the grain boundaries. The sintering temperature will vary the density of the interface states with the transport of the defect ions moving toward the grain boundary and will make more active grain boundaries. Therefore, the increase of α in accordance with increasing sintering temperature is attributed to the increase of potential barrier height at grain boundaries. The leakage current abruptly decreased at sintering temperature beyond 925 °C. The minimum value (0.034 mA/cm2) of leakage current density (JL) was obtained at 950 °C. Fig. 3 shows the C–V characteristics at different sintering temperatures. The C–V parameters obtained are shown in Table 1. The barrier height (Φb) increased from 0.48 eV to 1.03 eV in accordance with increasing sintering temperature of up to 925 °C. Further elevated temperatures caused it to decrease, reaching 0.96 eV at 950 °C. The behavior of Φb in accordance with the sintering temperature coincides with that of α. The increase of Φb in accordance with increasing sintering temperature is deeply related to the transport of the defect ions moving toward the grain boundary. On the other hand, the donor density (Nd) increased from 1.15 × 1018/cm3 to 11.1 × 1018/cm3 in accordance with increasing sintering temperature. The increase of Nd value is assumed to be due to the dissociation

of zinc oxide in the following chemical reaction expression, ZnO→Znxi + 1/2O2, Znxi →Zni +e′, where Znxi is a neutral zinc of interstitial site, Zni is a positively charged zinc ion of interstitial site [14]. 4. Conclusions The effect of sintering temperature on microstructure and electrical properties of ZnO–V 2 O 5 –MnO 2 –Co3 O 4 –Dy 2 O 3 –Nb 2 O 5 (ZVMCDN) ceramics were investigated. The microstructure of the samples consisted of Zn3(VO4)2, ZnV2O4, and DyVO4 as the secondary phases together with ZnO as the main phase. Increasing sintering temperature decreased the sintered density due to the volatility of V2O5. The nonlinear coefficient remarkably increased from 8 to 57 in accordance with increasing sintering temperature of up to 925 °C. Further elevated temperatures also caused it to decrease to 45. The donor concentration increased from 1.15 × 1018/cm3 to 11.1 × 1018/ cm3 in accordance with increasing temperature and the barrier height exhibited a maximum value (1.03 eV) at 925 °C. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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