Microstructure and electrical properties of Ho2O3 doped Bi2O3-based ZnO varistor ceramics

Microstructure and electrical properties of Ho2O3 doped Bi2O3-based ZnO varistor ceramics

Physica B 405 (2010) 3770–3774 Contents lists available at ScienceDirect Physica B journal homepage: www.elsevier.com/locate/physb Microstructure a...

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Physica B 405 (2010) 3770–3774

Contents lists available at ScienceDirect

Physica B journal homepage: www.elsevier.com/locate/physb

Microstructure and electrical properties of Ho2O3 doped Bi2O3-based ZnO varistor ceramics M.A. Ashraf a,n, A.H. Bhuiyan b, M.A. Hakim c, M.T. Hossain d a

Department of Physics, National University, Gazipur, Bangladesh Department of Physics, Bangladesh University of Engineering and Technology, Dhaka-1000, Bangladesh c Material Science Division, Atomic Energy Center Dhaka, Dhaka-1000, Bangladesh d Industrial Physics Division, BCSIR Laboratories, Dhaka-1205, Bangladesh b

a r t i c l e in f o

a b s t r a c t

Article history: Received 17 May 2010 Received in revised form 27 May 2010 Accepted 28 May 2010

The microstructure and electrical properties of Ho2O3 doped Bi2O3-based ZnO varistor ceramics were investigated. The bulk density varies between 5.41 and 5.47 g cm  3 with the maximum value of 5.47 g cm  3 for 0.50 mol% Ho2O3 content. The average grain size for all the samples was calculated from the scanning electron micrographs and were found between 5.1 and 7.1 mm. The microstructure of the prepared samples shows a decrease in grain size of ZnO phase with Ho2O3 doping. The energy dispersive X-ray analysis and X-ray diffraction analysis of the samples show the presence of ZnO, Bi-rich, spinel Zn7Sb2O12 and Ho2O3-based phases. The nonlinear coefficient, a, obtained from electric field–current density plots has a maximum value of 78 for the ceramics with 0.50 mol% Ho2O3 content. The leakage current, IL, has a minimum value of 1.30 mA for the 0.50 mol% Ho2O3 doped ZnO varistor ceramics. The breakdown field, Eb, was found to increase with Ho2O3 content. & 2010 Elsevier B.V. All rights reserved.

Keywords: Holmium oxide ZnO varistor ceramics Microstructure Spinels Nonlinear coefficient

1. Introduction Zinc oxide (ZnO) varistors are polycrystalline semiconducting ceramic devices, which are widely used for voltage stabilization and transient surge suppression in electric power systems and electronic circuits [1–5]. These ceramic devices exhibit highly nonlinear current–voltage (I–V) characteristics with a high resistivity below a breakdown/threshold voltage. This nonlinearity of I–V characteristics is believed to be due to the presence of a double Schottky barrier (DSB) formed at active grain boundaries containing many trap states [6–8]. Varistor action is controlled by depletion layers situated within the ZnO grains at the grain–grain interfaces. The current density–electric field (J–E) characteristics of ZnO ceramics are expressed by the following empirical equation: J ¼ KEa

ð1Þ

where K is a constant and a is the nonlinear coefficient. Most of the varistors are based on polycrystalline ZnO with small amount of Bi2O3 and other dopants. The microstructure of ZnO varistors is complex. It is a polycrystalline multiphase, with each phase having different dopants, dopant concentrations, shape and size. It consists predominantly of ZnO grains, secondary phases including spinel (Zn7Sb2O12) and pyrochlore (Zn2Bi3Sb3O14) n

Corresponding author. Tel.: +880 2 9665086; fax: + 880 2 8613046. E-mail address: [email protected] (M.A. Ashraf).

0921-4526/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2010.05.084

surrounded by a three dimensional network of Bi-rich phases i.e., grain boundaries and pores [9,10]. As the nonlinear electrical behavior occurs at the boundary of each semiconducting ZnO grain, the varistor can be defined as a multijunction device composed of series and parallel connections of grain boundaries. An ideal varistor should consist only of homogeneously distributed ZnO grains with highly resistive grain boundaries without secondary phases. Thus to achieve a high breakdown voltage, one can change the varistor thickness or vary the grain size to increase the number of barriers or grain boundaries. To increase the breakdown voltage, it is necessary to decrease the average size of the ZnO grains. Previous report [11] revealed that addition of antimony oxides in the starting composition decreases the average grain size. Recently, many studies [12–17] have been made in order to understand the influence of different rare earth oxides (such as Y2O3, Nd2O3, Er2O3, Ce2O3 and Dy2O3) on the microstructure and electrical properties of the ZnO varistor ceramics. These investigations indicate that rare earth oxides may significantly increase the breakdown field without deterioration in the performance of the varistor. It can be noticed from the review of the research work on the investigation of Bi2O3-based ZnO varistor ceramics that the rare earth oxides play an important role in controlling different operation parameters of these kinds of varistor devices. To address the influence of rare earth oxides on the ZnO varistor ceramics, this paper reports the microstructure and electrical characteristics of Ho2O3 doped Bi2O3-based ZnO varistor ceramics. The objective of the work is also to understand how the composition controls the microstructure

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and electrical properties of the ZnO varistors doped with Ho2O3. The relation between the electrical characteristics of the Bi2O3-based ZnO varistor ceramics with various Ho2O3 content was investigated and the results are analyzed.

2. Experimental details Reagent-grade raw materials were used to prepare Bi2O3-based ZnO varistor ceramics with the compositions of (96-X) mol% ZnO+0.5 mol% Bi2O3 +1 mol% Sb2O3 +1 mol% MnO2 +0.5 mol% Co3O4 +0.5 mol% Cr2O3 +0.5 mol% SiO2 +X mol% Ho2O3 (X¼ 0.00, 0.25, 0.50, 0.75, 1.00 mol%). Raw materials were mixed by ball milling for 12 h using alumina balls in de-ionized water and the mixture was dried at about 120 1C for 12 h followed by calcination at 700 1C for 2 h. The calcined powder was ground and sieved through 200-mesh screen to produce the starting powder. The starting powder was pressed into discs of 12 mm in diameter and 2.0 mm in thickness at a pressure of 100 MPa. The sintering operation was performed in ambient air for all samples in a furnace (Carbolite, Eurotherm-2408, England) using an average heating rate of 2 1C/min up to 1200 1C. After holding 1 h at 1200 1C the samples were cooled to 720 1C using a cooling rate of 2 1C/min and then furnace cooled. The resulting dark-green ceramic bodies were approximately 10 mm in diameter and 1.2 mm in thickness. The bulk density of the samples was measured using the Archimedes method in distilled water. Selected disc shape was polished using alumina powder of 1.0, 0.3 and 0.05 mm sizes. These samples were cleaned via ultrasonic cleaning using de-ionized water for at least 30 min so that the tiny particles were removed from the surfaces of the varistor samples, yielding clean surfaces. Then these samples were dried in air. The polished samples were thermally etched at the temperature 150 1C below the sintering temperature for 10 min. The surface of microstructure was analyzed using a scanning electron microscope (SEM, Hitachi S-3400, Japan) equipped with an energy dispersive X-ray (EDX) spectrometer in Bangladesh Council of Scientific and Industrial Research (BCSIR), Dhaka-1205. The grain size (D) was calculated by using the linear intercept method [18], following the relation: 1:56L MN



ð2Þ

where L is the random line length on the micrograph, M is the magnification of the micrograph and N is the number of grain boundaries intercepted by lines. The compositional analysis of the selected areas was performed by an EDX analyzer attached to the SEM. The crystalline phases of the sample were identified by a Philips PW3040 X’pert PRO X-ray diffractometer, Germany, in the Atomic Energy Centre Dhaka (AECD), using CuKa radiation. For the electrical measurements, silver paste was coated on both faces of the samples and ohmic contacts with samples formed by heating at 600 1C for 10 min. The dc electrical measurements at room temperature were performed with a Keithley 614 electrometer (Keithley Instruments, USA) and a 5 kV dc (digital) power supply (Leybold-Heraeus, Germany). The current was recorded by increasing the applied voltage step by step manually. To characterize the electric field–current density (E–J) behavior of different Bi2O3-based ZnO varistor ceramics, the breakdown field, Eb, and a were calculated using Eqs. (3)–(5). Eb ¼



I A

Vb d

  log JJ21   a¼ log EE21

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ð5Þ

where Vb is the breakdown voltage (breakdown voltage is the voltage at which the varistor switches from a highly resistive state to a highly conductive state), d is the sample thickness, I is the current, A is the electrode area, and E1 and E2 are the electric fields measured at the current densities J1 ¼0.5 mA and J2 ¼1.5 mA, respectively. The breakdown field (Eb) was measured at a current density of 1.0 mA cm  2 and the leakage current (IL) was measured at 0.80 Eb.

3. Results and discussion 3.1. Bulk density The bulk density of Ho2O3 doped Bi2O3-based ZnO varistor ceramics fired at 1200 1C for 1 h is shown in Fig. 1. The result represent that the bulk density increases with the Ho2O3 content up to 0.50 mol% and then decreases. Ho3 + ions have a larger radius (0.0901 nm) than that of Zn2 + ions (0.074 nm). Atomic weight of Zn (65.39 g) is less than the atomic weight of Ho (164.93 g). Thus initial addition of Ho2O3 affects the grain distribution and develops different phases in the ceramic matrix thereby increasing the bulk density initially. Further increase of the Ho2O3 content may mainly contribute to the change in grain size and phase distribution. So bulk density increases up to 0.50 mol% Ho2O3 content then decreases. The decrease in bulk density of the varistor ceramics with higher Ho2O3 content may be due to the increase of intragranular porosity. 3.2. Microstructural analyses The SEM micrographs of all the samples and the representative EDX spectra of a sample of Bi2O3-based ZnO varistors doped with 0.50 mol% of Ho2O3 sintered at temperature 1200 1C are shown in Figs. 2 and 3, respectively. The micrographs in Fig. 2 are composed of ZnO grains, Bi2O3-rich phase, Zn7Sb2O12 spinel-type phase and also a Ho2O3-based phase. The average grain size of ZnO grain depends on the content of Ho2O3 and on the amount of spinel phase located at the grain boundaries. It is seen from the micrographs of the five types of ZnO varistor ceramics that the size of spinels becomes smaller and the quantity of spinels are increased with the increment of Ho2O3 additive, which indicates that the composition and the behavior of these spinels are different from those without Ho2O3 dopant. These new types of spinels are in Ho2O3-based (Bi–Zn–Sb–Ho–O) phase. Ho2O3-based phase is distributed mainly at the tri-grain

ð3Þ

ð4Þ

Fig. 1. Variation of bulk density with different contents of Ho2O3 in doped Bi2O3-based ZnO varistor ceramics.

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Fig. 2. SEM micrographs of varistor ceramics doped with different amounts of Ho2O3: (a) 0.00 mol% (b) 0.25 mol% (c) 0.50 mol% (d) 0.75 mol% and (e) 1.00 mol%.

Fig. 3. EDX spectra of the Bi2O3-based ZnO varistor ceramics doped with 0.50 mol% Ho2O3 sintered at 1200 1C with marked points.

and tetra-grain intersections of ZnO matrix and is rarely observed along the grain boundaries bounded by two successive ZnO grains. The existence of the Ho-based phase becomes prominent with increase in Ho2O3 content. Compositional analyses of different phases by EDX analyses have evidenced that the Bi2O3-based ZnO varistor ceramics doped with Ho2O3 are composed of ZnO grains,

Bi2O3-rich phase, Zn7Sb2O12 spinel-type phase containing Cr, Co, Mn, etc. and also with a Ho2O3-based phase containing Bi, Sb, etc. as displayed in the EDX spectra of Fig. 3. The XRD patterns of Bi2O3-based ZnO varistor ceramics doped with Ho2O3 sintered at 1200 1C are given in Fig. 4. It appears in the patterns that ZnO is the predominant compound in these

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Fig. 4. XRD patterns of the Bi2O3-based ZnO varistor ceramics doped with different amount of Ho2O3 sintered at 1200 1C.

materials. These samples consist of the main ZnO phase, spinel phase of Zn7Sb2O12 and Bi-rich phase. However, as the Ho2O3 content increases in the samples, additional peaks are evident due to the formation of a Ho-based phase in the ceramic containing 0.50 mol% Ho2O3. The appearance of these XRD peaks may indicate the development of a Ho-based phase in the form of Bi–Zn–Sb–Ho–O as observed in EDX, which may mainly disperse in the grain boundary. The intensity of these new peaks increases as Ho2O3 content increases. At the same time some of the peaks corresponding to spinel phase of Zn7Sb2O12 diminish with higher content of Ho2O3. So, 0.50 mol% Ho2O3 doped Bi2O3-based ZnO varistor appears to have an optimal composition of spinel phase of Zn7Sb2O12 and Ho-based phase, which results in homogeneous grain size of ZnO in this sample as observed by SEM. The Bi-rich phase is a d-Bi2O3 type phase having a fundamental composition of 12Bi2O3  Cr2O3 and a solid solution of the system Bi2O3–ZnO–Sb2O3. Considering both XRD and EDX analyses, it can be attributed that the white regions are Bi-rich phase, the off-white regions are Zn7Sb2O12, Ho-rich spinel phase, the gray regions are ZnO phases and the black regions are pores (please see Fig. 3).

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Fig. 5. E–J curves of the Bi2O3-based ZnO varistor ceramics doped with different amounts of Ho2O3 sintered at1200 1C.

Fig. 6. Variation of nonlinear coefficient a and leakage current IL with different contents of Ho2O3 doped ZnO varistors (sintering temperature ¼1200 1C).

3.3. Electrical properties The dc E–J curves of different samples sintered at 1200 1C are shown in Fig. 5. It is observed that the J for all the varistor samples increases slowly with E up to about 2  103 V cm  1 and above this electric field there is a sudden rise in J. It is noticed that the E–J curve of 0.50 mol% Ho2O3 sample has shifted towards the high electric field and low current density values. The a, Eb and IL were determined from the E–J curves. Fig. 6 shows the variation of a as a function mol% of Ho2O3 along with leakage current, IL, of Bi2O3-based ZnO varistor ceramics. It is observed that the a initially increases and then decreases with increase in Ho2O3 content. The a as a factor of characterizing nonlinearity of a varistor, its value varied from a maximum of 78 in 0.50 mol% Ho2O3 to a minimum of 29 in 1.00 mol% Ho2O3 containing varistor ceramics. Ho2O3 is involved in the formation of interfacial states and deep bulk traps, both of which contribute to the highly nonlinear properties. It is seen that an increase of Ho2O3 content above 0.50 mol% deteriorates the nonlinear properties. The variation of IL value is observed to be opposite to a. The minimum value of leakage current IL was found for 0.50 mol% Ho2O3 varistor ceramic. Fig. 7 shows the variation of the breakdown field Eb and grain size D (obtained from the SEM micrographs of Fig. 2) as a function of Ho2O3 mol% content of Bi2O3-based ZnO varistor ceramics. It is

Fig. 7. Dependence of grain size D and breakdown field Eb on Ho2O3 content.

seen that the average grain size decreases with Ho2O3 mol% content and is varying from the value of 5.10 to 7.10 mm. The stabilizing effect of segregated Ho3 + cations on ZnO grain can inhibit the growth of ZnO grains, resulting in smaller grain size. When the amount of Ho2O3 is less than 0.5 mol%, most of Ho3 + cations are segregated at the grain boundaries which might lead to the smaller grain size. The minimum value of average grain size of ZnO grain occurs for sample of ZnO varistor doped with 0.75 mol% Ho2O3. It is observed that the Eb increases with increase in the content of Ho2O3. It is noticed in Fig. 7 that the Eb increases with Ho2O3 mol% content in ZnO varistor ceramic.

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It is seen from Figs. 5–7 that the 0.50 mol% Ho2O3 doped Bi2O3based ZnO varistor shows optimum characteristics of a, Eb and IL corresponding to the minimum ZnO grain size and optimal composition of spinel Zn7Sb2O12 phase and Ho2O3-based phase.

National University, Bangladesh and for granting deputation from the Ministry of Education, Govt. of Bangladesh, to pursue the research work. Authors are thankful to the authorities of BUET, BCSIR and AECD, Dhaka, Bangladesh, for extending the laboratory facilities during this research work.

4. Conclusions The maximum bulk density of 5.47 g cm  3 is observed for 0.50 mol% Ho2O3 doped Bi2O3-based ZnO varistor ceramics. A minimum grain size of ZnO phase has been found in the ZnO varistor ceramics doped with 0.50 mol% Ho2O3. The EDX and XRD analyses of the samples show the presence of ZnO, Bi-rich, spinel Zn7Sb2O12 and Ho2O3-based phases. The nonlinear coefficient is observed to be a maximum of 78 and the IL has a minimum value of 1.30 mA in 0.50 mol% Ho2O3 doped ZnO varistor ceramics. The Eb increases with Ho2O3 content. Finally it can be inferred from the above results that the 0.50 mol% Ho2O3 doped ZnO varistor ceramics is the optimum composition for the best performance of these Ho2O3 doped Bi2O3-based ZnO varistor ceramics. Acknowledgements One of the authors, M.A. Ashraf, gratefully acknowledges financial grant for research work through a fellowship from

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