Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors

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Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors K. Hembram a, b, *, T.N. Rao a, M. Ramakrishana a, R.S. Srinivasa b, A.R. Kulkarni b a Center for Nanomaterials, International Advanced Research Center for Powder Metallurgy & New Materials (ARCI), Balapur PO, Hyderabad, 500005, A.P, India b Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 31 May 2019 Received in revised form 7 October 2019 Accepted 15 October 2019 Available online xxx

In present study, impact of CaO doping on microstructure and phase electrical properties of a ZnO varistor is being reported. The doped ZnO nanopowders were prepared by solution combustion followed by conventional sintering. The ZnO particle size of powders decreased from 22 to 17 nm with CaO doping was noted. Decrease in grain size of the sintered samples from 4.54 to 1.19 mm with CaO doping was observed. This is attributed to Ca4Bi6O13 and Ca0.89Bi3.11O5.58 secondary phase formations apart from pyrochlore and spinel phases. A breakdown field as high as 21 kVcm
1 in X ¼ 1.00 sample was obtained. The Coefficient of nonlinearity (a) decreased from 95 to 42 is due to decrease in carrier concentrations. Increase in resistivity with CaO doping and decrease in resistivity beyond X ¼ 2.5 were noted. The electric modulus plots yielded two relaxations at a temperature in the range 100e300  C and frequency in the range 0.1 Hz-1MHz. Activation energy (Ea) decreased from 0.65 to 0.50 eV for a region I and from 0.89 to 0.75 eV for region II with CaO doping was observed. © 2019 Elsevier B.V. All rights reserved.

Keywords: ZnO Doping Sintering Electrical properties Varistor Dielectric

1. Introduction Polycrystalline ZnO ceramic varistor behaves like Zener diode, however with a better coefficient of nonlinearity and much higher energy absorption capacity. It is used in wide range of surge protection applications; starting from electronic circuit to large current transmission line [1e3]. In general, commercial ZnO varistors breakdown field are reported in the range 2-3kVcm
1 and coefficient of non-linearity in the range 30e90, these devices are prepared by conventional powder metallurgy route having a composition containing more than five oxides such as Bi, Sb, Co, Cr and Mn oxides to name a few, and are used in limited to the relatively low-voltage applications [3e5]. When these varistors are used in high surge voltage applications which needed higher numbers of discs resulting increase in residual voltage in the varistor discs leading to catastrophic failure of the device. A varistor having breakdown field of >10 kVcm
1 is recommended to overcome such problems [4,5].

* Corresponding author. Center for Nanomaterials, International Advanced Research Center for Powder Metallurgy & New Materials (ARCI), Balapur PO, Hyderabad, 500005, A.P, India. E-mail address: [email protected] (K. Hembram).

The high performance varistors are achieved by getting a homogeneous microstructure and uniform dopant distribution, small grain size with a narrow size distribution and highly dense discs. It is well documented that the varistor made from nanopowders exhibited superior properties compared to commercial [6e9]. Breakdown voltage is proportional to the number grain boundaries in a given volume. There are reported of controlling of grain size and size distribution by using advanced sintering methods e.g. SPS [10], hot press [11]. However, controlling of grain growth by advanced sintering techniques is quite expansive and also required post heat treatment to get required varistor properties [10]. Engineering the grain growth by addition of dopants is comparatively inexpensive. Dopants play an important role to control most of the electrical properties of the varistor; Sb2O3 [12], SiO2 [13]ZrO2 [14] and Al2O3 [15] are added as a grain growth inhibiter in ZnO varistor. ZnO grain growth is retarded by pinning mechanism. Generally, antimony oxide is used as a grain growth inhibiter in a ZnO varistor, however, regulation of antimony oxide due to environmental concern, novel dopants are needed to be identify for grain growth inhibiter which could provide above challenges and possible better properties of a varistor. There is hardly any literature available exclusively study on effect of CaO doping on ZnO varistor system to best of our knowledge. In our previous study simple

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Please cite this article as: K. Hembram et al., Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152700

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2. Experimental and characterization techniques

with emery papers of different grades (600 and 1000). A dimple of 3 mm diameter was produced by an ultrasonic cutter (GATAN, Model 601) followed by thinned down to 50e60 mm by polishing with 2000 emery paper. The sample was kept in an ion milling (GATAN, Model No # 691) for 8e10 h, with an energy at 4 KeV (current 11 mA) for both the guns and an angle at 11 till small hole was formed. Before putting into TEM for analysis, sample was cleaned with 1.5 KeV energy for 15 min. Selective elemental composition analysis of the microstructure was studied by energydispersive X-ray spectroscopy attached to TEM (EDS, EDAX, USA).

2.1. Processing of the powder

2.3. Electrical properties measurement

Zn (NO3)2.6H2O (99%, Alfa Asear), Cr(NO3)3.9H2O (99%, Loba Chemical), Co (NO3)2 .6H2O (99%, Sigma-Aldrich), Mn(NO3)2.6H2O (99%, Sigma-Aldrich) and Ca (NO3)2 .4H2O (99.9%, Alfa-Asear) were dissolved in de-mineralized water followed by heating at 50e100  C to make a solution (Solution 1). Bi (NO3)3 .5H2O (99%, Sigma-Aldrich) was mixed in dilute-nitric acid (99.9%, Loba) (Solution 2) and Sb2O3 (99%, Loba) was dissolved in citric acid (99%, SD Fine) (Solution 3). The solution 2 and 3 were mixed into the solution 1. Sucrose was added to the solution as a fuel. The solution was heated ~200  C and stirred till the solution completely dried up and catch fire. As-synthesized powders were calcined at 750  C for 1 h. The composition used for present study was ZnO: 88 wt%, Bi2O3: 5.00 wt%, Sb2O3: 3.50 wt%, Co3O4: 1.50 wt%, MnO2: 1.00 wt% and Cr2O3: 1.00 wt%. To understand the impact of CaO doping on phase, microstructure and properties of a varistor, a different concentrations of CaO 0.00 (X ¼ 0.00), 0.25 (X ¼ 0.25), 1.00 (X ¼ 1.00), 2.50 (X ¼ 2.50) and 5.00 (X ¼ 5.00) wt. % were synthesized. The calcined powders were blended by pot milling for 4 h. A 2.0 wt% PVA dissolved in water was used a binder and powder to ball ratio was kept 1:4 during the bending. The mixture slurry was dried in an oven at 150  C for 6 h in open atmosphere. The pellets was made in 10 mm stainless steel die with pressure of 160 MPa. The pellets (~50% of the theoretical density) were debineded at 600  C-3h (heating rate 1  Cmin-1). Two-step sintering cycle was used for sintering the pellets: first heated to 1000  C and held for 0.5 h followed by cooled to 925  C and held for 4 h. The rates of cooling and heating was kept at 2  Cmin-1 till 925  C followed by furnace cooling.

2.3.1. IeV characteristics The sintered pellets having diameter of ~8 mm and thickness of ~1 mm were studied IeV characteristics. Silver was applied both the faces of a pellet as a contact electrode. The IeV characteristics of the pellets were measured by breakdown voltage tester (5 kV DC, Rectifier Electronics, India). The coefficient of nonlinearity (a) was estimated by using equation: a ¼ (log J2-log J1)/(log E2-log E1), where E1 is the field at current density J1 (0.1 mAcm
2) and E2 is field at current density J2 (1 mAcm
2). The breakdown field (Eb) was estimated at a reference current density of 1 mAcm
2. A voltage drop per barrier (Vg) per grain boundary was calculated using the equation: Vg ¼ V (d/t-1), where, V is the breakdown, d is the average grain size, t is the thickness of the sample. The leakage current density (Lc) was estimated at 75% of the Eb (1mA).

ZnOeBi2O3eCaO and ZnOeBi2O3eCaOeCo3O4 varistor system, an excellent combination of electrical properties was obtained [16]. In this study, ZnO varistors are fabricated from doped ZnO nanpowders, and the effect of CaO doping on varistor properties are studied. The density, microstructure and phase of ZnO varistor are studied and structure-property correlation is established. The electrical properties of ZnO varistors are studied by VeI and complex impedance measurement.

2.2. Phase and microstructure characterization Phase of powder samples were analysed by X-ray diffractometer (XRD, Bruker D8 Advance) using Cu Ka radiation. The crystallite size of powder samples was calculated by using Scherrer equation. The composition of the calcined powder samples was studied by ICPOES (Varian, Liberty Series II) whereas surface chemistry of the powders was analysed by XPS (Omicron, Nano Technology, UK). Differential scanning calorimetry (DSC) analysis of the powder samples were carried using thermal analyser (Netzsch, STA 449 F3 Jupiter, Germany) in air. The morphology and particle size of the powder samples were studied by transmission electron microscopy (TEM, TECNAI, 200 KV, FEI, Netherlands). The powder was dispersed by sonication in an ethanol solvent around 10e15 min, a drop of dispersed nanopowders was put on a copper grid for TEM analysis. An average particle size of the powders was estimated from dark field TEM images. Porosity and density of sintered samples were measured by ASTM-C373-88. The phase of sintered samples was confirmed by X-ray diffractometer. The microstructure of thermally etched sintered samples was studied by SEM (Se3400 N, Hitachi, Japan). An average grain size was estimated by the linear intercept technique [17]. The TEM sample was prepared of sintered (X ¼ 0.00 and X ¼ 1.00) samples which were thinned down by Isomet and then reduced thickness to 200 mm by polishing

2.3.2. Dielectric and impedance study AC electrical properties of the sintered pellets at different temperature and frequency were studied with an impedance analyzer (Alpha-High Resolution Novocontrol, Germany) in the wide range frequencies (mHz-MHz) and temperatures (25e500  C at 25  C intervals). Silver was used as an electrode. 3. Results and discussion 3.1. X-ray diffraction of powders Fig. 1 shows the XRD patterns of undoped (UDVP) and CaO doped varistor ZnO nanopowders (CDVP) samples. The red vertical lines are the Bragg position of the ZnO phase (JCPDS No.: 36-1541, P63mc/186). In all the samples a negative shift in 2q is observed

Fig. 1. Comparison of XRD patterns for with and without CaO doped ZnO varistor powders calcined at 750  C for 1 h.

Please cite this article as: K. Hembram et al., Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152700

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compared to the Bragg position of ZnO. A small peak corresponding to Ca4Bi6O13 (JCPDS No.: 13-7122) was observed in CDVP samples having X  1.00 (a weak single peak is observed in the present case, identification has been substantiated later in phase study of sintered pellet samples). The lattice parameters and crystal volume decreased up to X ¼ 1.00 CaO doping, after that it remains constant (Tables Se1). A decrease in lattice constant and crystal volume is attributed to the formation of Ca4Bi6O13 precipitates on the powders for the higher concentration of CaO doped samples. XPS analysis of X ¼ 1.00 CDVP sample shows a presence of Zn2þ, Sb3þ and Bi3þ but no Ca species spectrum is observed, indicating that ZnO powder surface is coated with Bi2O3 and Sb2O3; ZnO is doped with Co2þ, Cr2þ, Mn2þ and Ca2þ ions species (Figure S-1). It has been demonstrated in earlier HRTM studies that the big ionic diameter species like Sb3þ and Bi3þ precipitates out on the surface of ZnO during the calcination and sintering process [5,60]. Increase in lattice constant and crystallite volume by ~0.30% for varistor powder samples compared to pure ZnO. The lattice expansion of ZnO crystal is due to doping of higher ionic diameter species Sb3þ ¼ 0.76 Å, Bi3þ ¼ 1.03 Å and Ca2þ ¼ 1.06 Å [18]. The Ca2þ may occupy the interstitial site of Zn2þ in the lattice of ZnO. It is known that the solubility limit of dopants is improved due to the size reduction of the materials [19]. Solid solubility of calcium in ZnO is reported in nano and bulk by a few researchers [20]. The ZnO lattice volume expansion of ~0.2 mol % is demonstrated with calcium doping in earlier work [20]. However, the solubility of calcium in ZnO is contradicted by a few other researchers. Immiscibility of calcium in ZnO is explained by the fact that the ionic diameter of Ca2þ (1.06 Å) is too large compared to Zn2þ (0.63 Å). Secondly, the crystal structure of these two species is not the same [21,22]. Till date, solid solubility of calcium in ZnO is a debatable issue. 3.2. TEM study of the powders To understand better, the influence of CaO doping on morphology and crystallite size, dark field TEM on powder samples were taken; the crystallite size is compared with XRD. Fig. 2 (a) and (b) show dark field TEM images and insets are crystallite size distribution histograms of UDVP and X ¼ 1.00 CDVP samples, respectively. A log-normal crystallite distribution was observed in all powder samples with an unnoticeable difference in morphology. Effect of CaO doping on ZnO crystallite size (XRD and TEM) of the powder samples is depicted in Fig. 2 (c). The graph clearly shows decrease in ZnO crystallite size with CaO doping. Mean particle/ crystallite size (TEM) was decreased from 19.60 ± 0.30 nm to 17.00 ± 0.30 nm (X ¼ 5.00) with CaO doping. A similar trend was observed crystallite size measured using Scherrer equation (decrease from 24 ± 0.4 nm to 17 ± 0.4 nm) with CaO doping. ZnO Wurtzite phase was confirmed by both TEM electron diffraction patterns and XRD techniques. The particle size measured from dark field TEM is very close to crystallite measured from the Scherrer equation which suggest that each particle corresponds to a crystal. A decrease in crystallite size may be due to segregation of Ca4Bi6O13 and Bi3Zn2Sb3O14 phases on the ZnO surfaces of the powders. The grain growth retardation is due to the Zener drag effect as observed in classical alumina-zirconia system [23,24]. 3.3. Compositional and thermal analysis of the powders The quantitative compositions (ICP-OES) of powder samples are given in the Tables Se2. It can be seen that the compositions were retained to an experimental error limit in all powder samples. However, deviation of Bi2O3, CaO, and Sb2O3 additives was observed to some extent in the powder samples which is attributed to segregation of Bi2O3 and Sb2O3. A comparative DSC analysis of

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X ¼ 0.00 and X ¼ 1.00 samples is shown in Figure S-2. It can be seen a small peak (first endothermic reaction) for X ¼ 0.00 sample at ~ 600  C which could be corresponding to melting of ZnO, Bi2O3 and Sb2O4 phases. The pyrochlore (Bi3Zn2Sb3O14) phase formation takes place at >500  C by reacting ZnO, Bi2O3 and Sb2O4 phases and an eutectic temperature of ZnOeBi2O3 is 740  C [2]. The peaks at ~875  C and ~980  C are corresponding to Zn2.33Sb0.67O4 and Bi2O3 phases formation, and peak at ~1110  C is indication of evaporation of Bi2O3 for X ¼ 0.00 sample. The first peak at ~830  C could be corresponding to Ca4Bi6O13 phase formation for X ¼ 1.00 sample. The peaks at ~905  C and ~1000  C are for phase formation of Zn2.33Sb0.67O4 and Ca0.89Bi3.11O5.58 (Figure S-2). The detail discussion will be carried out again in following section. During the calcination process a liquid phase forms by reaction among all these phases resulting in segregation of the oxides. A small deviation of dopants of transition metal oxides e.g. Co3O4, Cr2O3 and MnO2 were observed in the powder samples. This indicates that these transition oxides are homogeneous distributed throughout the matrix phase of ZnO. 3.3.1. Phase analysis of sintered pellets Comparison of overall XRD patterns of undoped (UDV) and CaO doped ZnO varistor (CDV) samples is shown in Fig. 3 (a) and close view of secondary phases formation in Fig. 3 (b). ZnO (JCPDS No.: 36-1451) along with secondary phases of d-Bi2O3 (JCPDS No.: 521007), Zn2.33Sb0.67O4 (JCPDS No: 15-0687) and Bi3Zn2Sb3O14 (JCPDS No.: 36-0114) were yielded in UDV and X ¼ 0.25 CDV samples. For X ¼ 1.00 CDV sample, Ca0.89 Bi3.11O5.58 (JCPDS No.: 040-0317) and Ca4Bi6O13 (JCPDS No.: 048-0217) phases were observed apart from the above phases. However, for X ¼ 2.50 and X ¼ 5.00 CDV samples the majority of grain boundary phases were of Ca4Bi6O13 and Ca0.89Bi3.11O5.58 instead of d-Bi2O3, Bi3Zn2Sb3O14 and Zn2.33Sb0.67O4 phases (Fig. 3 (b)). It is necessary to understand the phase formation in commercial ZnO varistors before going into a discussion of CDV samples. During the initial stages of sintering, Sb2O4 and Sb2O5 phase formation takes place from Sb2O3 in the temperature range 600e800  C. Bi3Zn2Sb3O14 and Zn2.33Sb0.67O4 formation take place by decomposition of ZnO, Sb2O5 and Bi2O3 in the temperature range 700e900  C, while orthorhombic spinel (Zn7Sb2O12) and bismuth oxide formation takes place by decomposition of the ZnO, cubicspinel and pyrochlore in the temperature range 900e1050  C [2,25,26]. The spinel composition of Zn2.33Sb0.67O4 and Zn7Sb2O12 exists in ZnO varistor, and high temperature orthorhombic Zn7Sb2O12 phase is thermodynamically stable [27]. Bi2O3 helps in densification and wetting of ZnO grains. Bi2O3 exists in five polymorphic phases viz. a, g, d, b and ε. In ZnOeBi2O3 varistors only g, d and b Bi2O3 phases have been reported by researchers [28]. The dBi2O3 is known to be the fastest ionic conductor among all the other Bi2O3 phases. Pyrochlore and spinel (both cubic and orthorhombic) phases are known as ZnO grain growth retarder [29],122]. Generally, spinel are present near the triple junction and grain boundary as well as inside the ZnO grains in the form of inverse grain boundary [3,26]. In the present study, major contributions to decrease in ZnO grain size is due to the formation of Ca4Bi6O13 and Ca0.89Bi3.11O5.58 phases along with Zn2.33Sb0.67O4 and Bi3Zn2Sb3O14 phases in the CDV samples. To understand the phase formation in CDV samples the ½ Bi2O3eCaO phase diagram was studied closely in detailed [30,31]. It can be seen in the phase diagram that for 10 mol % CaO composition the CaBi2O4 and Bi2O3 phases are formed, and above 10 mol% the Ca3Bi8O15, Ca4Bi6O13, and Ca2Bi2O5 phases are reported. Non-stoichiometric Ca0.89 Bi3.11O5.58 phase formation is favored for the specific composition and heating above 1000  C followed by quenching to room temperature (JCPDS No. 040-0317) [69]. Stoichiometric Ca4Bi6O13 phase is favored at a temperature below 800  C (JCPDS No 048-021) which could be seen

Please cite this article as: K. Hembram et al., Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152700

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Fig. 2. Dark field TEM images and insets are the crystallite size distribution of ZnO (a) (X ¼ 0.00) and (b) (X ¼ 1.00) ZnO varistor powders. (c) Influence of CaO doping on crystallite size of the ZnO powders.

in DSC as well (Figure S-2). Above phases are within the range of present compositions and processing temperatures. The above phases are similar to that of SiO2 and WO3 doped ZnO varistors. The grain boundary secondary phases of ZnSiO4, Bi2SiO5, and Bi24Si2O40 were reported in case of SiO2 doped ZnO varistor samples [13,32]. The secondary phases of ZnWO4, Bi2WO6 for WO3 doped ZnO varistor were reported [33]. These secondary phases are known to be oxygen generators at grain boundary interfaces and grain growth retarders resulting in better electrical properties in ZnO varistors.

3.4. Microstructure analysis

Fig. 3. (a) Comparison of XRD patterns, (b) close view of secondary phase with and without CaO doping sintered samples.

3.4.1. SEM study of sintered pellet SEM images of thermally etched and grain size distribution plots of X ¼ 0.00 UDV and X ¼ 1.00 CDV samples are shown in Fig. 4. It can be seen that a wide grain size distribution (2e7 mm) with an average grain size of 4.54 mm was obtained for (X ¼ 0.00) UDV sample, whereas a narrow grain size distribution in the range 0.5e2.0 mm with an average grain size of 1.25 mm for (X ¼ 1.00) CVD samples was obtained. Density of 5.62 and 5.49 ± 0.05 gcc
1 (9997% of theoretical) for X ¼ 0.00 and X ¼ 1.00 samples was obtained, respectively. Effect of CaO doping on density and grain size is depicted in Fig. 4(c). It can be seen in the plot that the grain size and density decreased with CaO doping. A decrease in density could be due to the presence of CaO whose theoretical density is 3.34 gcc
1 and is the lowest among all the dopants and matrix phase. Another factor for decrease in density of sintered samples could be the presence of higher amount of pyrochlore and lesser liquid Bi2O3 phase formation during sintering [34]. However, it is difficult to estimate the actual theoretical density of the sintered samples due to the presence of complex phases, the theoretical density of 5.67 gcc
1 is considered of the varistor samples for approximation [35]. The porosities in these samples were less than the 1e3% which

Please cite this article as: K. Hembram et al., Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152700

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Fig. 4. SEM images and insets are the grain size distribution of (a) pure (X ¼ 0.00) and (b) CaO doped (X ¼ 1.00) varistor samples. (c) Influence of CaO doping on grain size of the sintered samples.

means that the samples densities were greater than 97% of theoretical. 3.4.2. TEM study of sintered pellet To confirm the phase formation and to locate the calcium rich phase in the microstructure TEM, SAED and EDS studies were carried out on (X ¼ 0.00) UDV and (X ¼ 1.00) CDV samples. EELS mapping was carried out on a (X ¼ 1.00) CDV sample. Bright field TEM image along with EDS and SAED patterns at different locations namely matrix, triple point, and grain boundary areas of (X ¼ 0.00) UDV sample are shown in Fig. 5(aee). EDS studies confirmed that ZnO is doped with Co2þ whereas bismuth rich and spinel phases are heavily doped with transition ions of Co, Mn, and Cr as shown in Fig. 5(b). SAED patterns confirmed a Wurtzite ZnO matrix phase along with secondary phases of d-Bi2O3 and Zn2.23Sb0.67O4 in the UDV sample as shown in Fig. 5(c)(d) and (e), respectively which are in good agreement with XRD result. Phases obtained in the above samples are well known in ZnO varistors [1,3,4]. Bright field TEM image, EDS spectrum at different location of the microstructure, bright and dark (insets) field TEM images at grain boundary and SAED pattern of grain boundary phase for X ¼ 1.00 CDV sample are shown in Fig. 6(aef). EDS spectrum confirm presence of Zn and Co at core ZnO grain and Zn, Ca, Bi, Sb, Cr, and Mn at triple point; however, overlapping of Sb and Ca energy peaks were observed, as shown in Fig. 6 (b), (c) and (d). EDS at grain boundary interface and grain boundary area shows the presence of Bi, Ca and Sb species. The twin/stacking faults within the ZnO matrix grains were discerned. Bi3Zn2Sb3O14 phase formation at grain boundary area was confirmed. Surprisingly, spinel Zn2.23Sb0.67O4 phase could not be detected by SAED patterns in X ¼ 1.00 CDV sample; it may be because the amount of spinel is less than pyrochlore phase. Even after repeated efforts, the Ca0.89Bi3.11O5.58 and Ca4 Bi6O13 phases could not be established by SAED technique; it is because more than one phases are present at the grain boundary and triple

junction points and the SAED diffraction patterns are interfering among neighboring phases resulting in mixed electron diffraction patterns. Another experimental problem faced was detecting calcium species by EDS, and it was because EDS energy spectra of Sb and Ca are very close to each other, making it difficult to differentiate between them. EELS mapping at the grain boundary and the triple junction was carried out to locate calcium in the microstructure. Bright field TEM image, EDS spectrum and EELS mapping at the triple junction are shown in Fig. 6(gei) for X ¼ 1.00 CDV sample. EELS mapping confirming the calcium species (white) segregated at the triple junction which is shown in Fig. 6(h). EELS mapping has an energy resolution of ~1 eV whereas for EDS it is over 20 eV; therefore, it is easy to detect low atomic weight elements by EELS compared to EDS. However, SAED studies are unable to confirm the phases present at triple points due to the complexity and limitation of experiments involved. At this point, XRD, EDS, and EELS mapping studies indicated that Ca0.89Bi3.11O5.58 and Ca4Bi6O13 phases are located at triple point and grain boundary area. These phases are responsible for ZnO grain growth retardation due to Zener drag [23,24]. 3.5. Electrical properties 3.5.1. Varistor properties Fig. 7 (a) shows the comparison of IeV characteristics for UDV and CDV samples, and Table 1 summarizes their electrical properties. Effect of CaO doping on ZnO varistor properties are displayed in Fig. 7 (b-e): (b) breakdown field, (c) barrier voltage, (d) leakage current density and (e) coefficient of nonlinearity. Breakdown field increased with CaO doping, reaches a maximum at 20.3 ± 1 kVcm
1 in X ¼ 1.00 CDV sample, after that decreased and then remain almost constant. Increase in breakdown field is due to a decrease in ZnO grain size. The barrier voltage was found to be in the range 2.51-1.82 ± 0.25 V for all the samples. Barrier voltage decreased

Please cite this article as: K. Hembram et al., Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152700

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Fig. 5. TEM studies of sintered (X ¼ 0.00) sample. (a) Bright field TEM image, (b) EDS at different locations, (c) SAED on ZnO grain (d) triple junction and (e) secondary phase.

slightly with CaO doping. However, it does not change significantly. In general, for ZnO varistor, the barrier voltage per grain boundary is in the range 2e4 V [2,3,36,37]. Leakage current density increases from 1.15 to 170 ± 0.5 mA/cm2 with CaO doping, it is due to decrease in the relative density of the samples. The coefficient of nonlinearity decreased from 95 to 42 ± 5 with CaO concentration and it is due to reduction in carrier concentration, increase in resistivity of ZnO grain and decrease in bulk density for the higher CDV samples [38]. It is well known that the breakdown field is inversely proportional to ZnO grain size of a varistor [5,6,8]. Bi2O3 rich liquid phase wets the ZnO grain boundaries during sintering and forms a few nanometer thin insulating layer between the ZnO grains which is responsible for Schottky barrier formation. Bi2O3 and Sb2O3 segregate at the depletion layer lead to an increase in charge carrier density of the ZnO varistor. The best electrical properties is found in a device where an excess of oxygen with a monolayer of Bi2O3 are present at the grain boundaries [25,39,40]. Abnormal increase in breakdown field in CDV samples compared to UDV sample is due to ZnO grain refinement and presence of oxygen-rich secondary grain boundary phases of Ca0.89 Bi3.11O5.58 and Ca4Bi6O13 at the interface between grains apart from the spinel and pyrochlore phases [33,41]. As demonstrated by XRD and SAED patterns, secondary phases of d-Bi2O3, Zn2.23Sb0.67O4, Bi3Zn2Sb3O14 were observed in the X ¼ 0.00 UDV and X ¼ 0.25 CDV samples. The d-Bi2O3 is known to be a highly oxygen-ion conductor due to the presence of oxygen vacancies which is responsible for a better coefficient of nonlinearity. A decrease in coefficient of nonlinearity is due to doping of CaO into ZnO matrix and Bi2O3 phases resulting in a reduction in carrier concentration (equations (1)e(3)). However, XRD, TEM, EDS, and EELS analysis suggested that the major grain boundary phases are Ca0.89Bi3.11O5.58, Ca4Bi6O13 along with d-Bi2O3, Zn2.23Sb0.67O4

and Bi3Zn2Sb3O14 for X  1.00 CDV samples which are responsible for altering electrical properties. In the case of monovalent metal oxide doping, the metal ions substitute the Zn2þ site in ZnO lattice, acts as acceptors and leads to a decrease in the coefficient of nonlinearity [42]. For trivalent metal oxides, the metal ions substitute the Zn2þ site of ZnO lattice releasing electrons by ionization and acting as donors, increasing a [15,43]. In case of CaO doping, the ionic diameter of Ca2þ (1.06 Å) is too big compared to Zn2þ (0.63 Å) to substitute, therefore, it preferable moves to Bi3þ site position of Bi2O3. Substitution and interstitial doping of Ca2þ into Zn2þ ion in the lattice of ZnO at the vicinity of the depletion region are possible. On CaO doping, three possible defect reactions can occur as described by [44,45]:

. Bi2 O3 2CaO
















!2Ca’Bi þ V::o þ 1 2O2 ZnO

CaO




!Ca::i þ V }Zn þ 2OXo ZnO

CaO




!Ca}Zn þ V ::o

(1)

(2) (3)

The first reaction is Ca2þ substituting Bi3þ in the Bi2O3 lattice and generating O2 and oxygen vacancies V::o which act as an acceptor resulting in decreasing electron trap density at the grain boundary interface (reaction 1). The second reaction is Ca2þ going at interstitial of Zn2þ in ZnO lattice, with generation of two extra i holes, as shown in defect reaction 2. These holes in the valence band behave like acceptors. The third possible reaction is the Ca2þ substituting Zn2þ in ZnO lattice and generating V ::o in the vicinity of depletion regions of grains, as described in reaction 3. The V ::o acts as acceptor which decrease the carrier density resulting in a

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Fig. 6. TEM studies of sintered (X ¼ 1.00) sample. (a) Bright field TEM image, (b-d) EDS at different locations, (e) Bright field TEM image and inset is the dark field TEM image of secondary phase and (f) SAED of secondary phase; (g) Bright field TEM image, (h) EELS mapping and (i) EDS at triple point of sintered (X ¼ 1.00) sample.

decrease in coefficient of nonlinearity for CDV samples [44,46]. In particular, for X ¼ 0.25 CDV sample, reduction in the coefficient of nonlinearity is due to a decrease in density of traps at the interface as explained in Eqn 1. For X  1.00 CDV samples, the trap density at the interface is more due to the presence of oxygen absorption Ca0.89 Bi3.11O5.58 and Ca4Bi6O13 phases at grain boundary interface. However, the carrier concentration at the vicinity of depletion is less which reduce the net charge carrier concentration resulting in a decrease in coefficient of nonlinearity. 3.5.2. Impedance, dielectric and conductivity studies To verify the IeV results and to understand the effect of CaO doping on grain and grain boundaries resistance, dielectric properties of ZnO varistor the complex impedance results are analysed. A complex impedance is written as [16,47].


1  
1 1 1 þ juCg þ juCgb1 þ Rg Rgb1   n 
1 þ 1 Rgb2 þ Ao ðjuÞ

Z* ¼ Z’
Z} ¼



(4)

where u is the angular frequency, n represents the depression of

impedance graph, Cg is grain capacitance and Rg is grain resistance. Rgb1and Cgb1 are the capacitance and resistance of grain boundary type 1. Rgb2 and Cgb2 ¼ YCPE ¼ Ao ðjuÞn are the resistance and capacitance of grain boundary type 2. Relaxation take place when product of angular frequency, resistance and capacitance is unity, uRC ¼ 1. Grain boundary type 1 is allocated to the lowest resistance component whereas grain boundary type 2 to the highest resistance component. Fig. 8 shows complex impedance plots; the experimental data are fitted to two parallel RC circuits (insert of Fig. 8) for UDV and CDV samples. Two parallel RC circuit models have been used for data interpretation in ZnO varistors by several researchers [48e50]. Tables Se3 summarized dielectric properties at 200  C and 50 Hz for all the samples. The high-frequency intercept (1 MHz) values of the Z0 axis were found to be ~100 U-cm at room temperature for all the samples which are designed as a resistivity of core ZnO (Rg). The Rg was found to be marginally higher in higher CDV samples compared to the UDV sample. The trend of Rg is consistent with the coefficient of nonlinearity values. The impedance of grain may be influenced by the inductive effect associated with the experimental arrangement; care is needed to obtain realistic values from the high-frequency intercept. In a few

Please cite this article as: K. Hembram et al., Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152700

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Fig. 7. Influence of CaO doping on varistor properties of sintered samples: (a) IeV characteristics, (b) breakdown field, (c) voltage drop per barrier, (d) leakage current density and (e) coefficient of non-linearity.

Table 1 Effect of CaO doping on varistor properties of sintered samples. Sample ID

Density (gcc
1)

Grain Size (mm)

Bf (kVcm
1)

a

Leakage Current Density (mAcm
2)

Barrier Voltage (Volt)

X ¼ 0.00 X ¼ 0.25 X ¼ 1.00 X ¼ 2.50 X ¼ 5.00

5.59 ± 0.05 5.59 ± 0.05 5.49 ± 0.05 5.43 ± 0.05 5.26 ± 0.05

4.54 ± 0.50 1.65 ± 0.25 1.25 ± 0.25 1.32 ± 0.25 1.19 ± 0.25

05.53 ± 0.5 13.83 ± 1.0 20.30 ± 1.0 16.16 ± 1.0 15.85 ± 1.0

95 ± 5 65 ± 5 49 ± 5 50 ± 5 42 ± 5

01.1 ± 0.15 48 ± 5.0 87 ± 5.0 171 ± 5.0 170 ± 5.0

2.51 ± 0.50 2.30 ± 0.25 2.52 ± 0.25 2.13 ± 0.25 1.80 ± 0.25

instances in literature, the capacitance Cg (10
12-10
13 F) is neglected, since the frequency needed to resolve is very high for a small sample [51]. It is well reported that ZnO grain relaxation peak is a representation of the intrinsic Zni and Vo defects [52e55]. The extracted Rgb, Cgb1 Cgb2 values are given in Tables Se3. The grain core resistivity was neglected here since Rg « Rgb1þRgb2. The

capacitance values of Cgb1 was increased from 235 to 530 pF and Cgb2 decreased from 262 to 101 pF (200  C) with CaO doping. The Rgb increased with increasing CaO doping up to X ¼ 1.00 CDV (5.16 MU-cm) and then decreased to 0.593 MU-cm. The trend of Rgb is consistent with the breakdown field (IeV measurement) of the samples. Increase in resistivity with increasing CaO doping up to

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Fig. 8. Complex impedance plots at different concentration of CaO for sintered samples: symbols are experiment data and solid lines are fitted data to two parallel RC circuit model (inset).

1.00 wt % is due to a decrease in charge carriers which is explained by defect equations (1)e(3). A decrease in resistivity for X ¼ 2.50 CDV sample and beyond is due to the increase in mobility of the charge carrier due to the decrease in carrier concentrations. This observation is in good agreement with earlier studies on CaO doped ZnO sputtered films [38]. Frequency dependent real part of dielectric constant and tan d for UDV and CDV samples are presented in Fig. 9 (a) and (b). The dielectric constant gradually decreases with frequency, however, beyond 105 Hz the value remains almost constant. The dielectric constant was found to be in the range 465e3878 ± 5 at 50 Hz and 200  C. The dielectric constant of a boundary-layer capacitor in bulk ceramic is inversely proportional to the thickness of grain boundary and directly proportional to the grain size of the material [56]. The liquid phase at the grain boundary is more in case of X ¼ 0.00 UDV and X ¼ 0.25 CDV compared to X ¼ 1.00 and X ¼ 2.50 CDV samples. Therefore, the boundary layer thickness for X ¼ 1.00 CDV sample would be less compared to X ¼ 0.00 UDV sample. The factors affecting dielectric constant in the present study are the average

9

grain size of ZnO and thickness of grain boundary. The phase formation is not the same for UDV and CDV samples, hence directed correlation cannot be justified for the present study. As it can be seen that Tan d decreases drastically with frequencies ~103 Hz and remains constant thereafter. Tan d value in the range 4.5e39.5 ± 0.5 were obtained at 50 Hz and 200  C. Tan d decreases with CaO doping up to X ¼ 1.00 and increases after that. Tan d is due to electric dipole rotation which cause Joule heating by friction [57]. The tan d and leakage current obtained from IeV graphs are complementing each other. Tan d in the range of 5e30 and dielectric constant of 1000e4000 were reported at 50 Hz and temperature 100e175  C [51,57]. The present study dielectric results are within the range of reported values. The frequency dependent AC conductivity temperature in the range 125e500  C at 25  C interval for all samples. The experimental data were fitted to Jonscher’s power law. The conductivity spectrum shows two regions viz. (i) The low-frequency plateau and (ii) the mid-frequency dispersion. This phenomenon is explained by a modified Jonscher’s power law [58]. The temperature dependent exponent “n” values were found to be in the range 0.34e0.90 as shown in Fig. 10(a). The “n” value in the range 0e1 is attributed to translational hopping of charge carriers [58]. The charge carrier movement could be explained by thermally activated hopping between two grains separated by insulation barrier which is a typical conduction mechanism observed in dielectric materials [59]. The activation energy of DC conductivity is estimated using equation:



sdc ¼ so exp

Ea kT

 (5)

where so is defined as the pre-exponential factor, Ea is defined as the activation energy and k is defined as the Boltzmann’s constant. It can be seen clearly in the Arrhenius plot (Fig. 10(b)) that there are two stages of DC conductivity, region I (<175  C) and rejoin II (>200  C) for all the samples which are attributed to shallow and deeper traps. It may be noted that the activation energy of DC conductivity decreases with CaO doping which is attributed to a decrease in barrier strength and carrier concentration (equations (1)e(3)). The d-Bi2O3 phase forms Schottky barrier between grains for X < 0.25 CDV samples whereas Ca0.89 Bi3.11O5.58 and Ca4Bi6O13 phases are found to be responsible for barrier formation for X  1.00 CDV samples [1,60,61]. Decrease in grain size and increase in breakdown field and resistivity are attributed to the

Fig. 9. Effect of CaO doping on (a) dielectric constant and (b) Tan d of sintered samples at 200  C.

Please cite this article as: K. Hembram et al., Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152700

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Fig. 10. Impact of CaO doping on (a) conductivity Exponent “n” and (b-c) activation energy of DC conductivity of sintered samples.

Ca0.89Bi3.11O5.58 and Ca4Bi6O13 phases at the grain boundary interfaces. Fig. 10(c) shows the Arrhenius plot of t2 and t1 for all the samples. Activation energy of relaxation peaks were found in the range 0.79-0.60 for t2 and for t1 ¼ 0.59e34 in all samples. The activation energy for t2 was found in the range 0.79 eV is due to good Schottky junction (R2C2: homo-junction). The activation energy for t1 was found in the range 0.34 eV is due to Ca0.89 Bi3.11O5.58 and Ca4Bi6O13 phases between the two grains (R1C1: heterojunction) for X  1.00 CDV samples [61]. The activation energy of t1 and t2 of X ¼ 0.25 and X ¼ 1.00 CVD samples are comparable to X ¼ 0.00 UDV sample. However, as CaO doping increased to X ¼ 2.50 and X ¼ 5.00 the barrier strength decreased drastically was due to a decrease in bulk density and carrier concentrations.

CaO doping. This is attributed to Ca4Bi6O13 and Ca0.89 Bi3.11O5.58 secondary phase formation. Breakdown field as high as 21 kVcm
1 in X ¼ 1.00 CDV sample was obtained in present study. The Coefficient of nonlinearity (a) decreased from 95 to 42 is due to a decrease in carrier concentration. From IeV and impedance measurements, it can be concluded that the increase in resistivity with CaO doping is due to the reduction in carrier concentration; decrease in resistivity beyond X ¼ 2.50 CDV samples is attributed to increasing in carrier mobility. The electric modulus plots yielded two relaxations at a temperature in the range 100e300  C and frequency in the range 0.1 Hz-1MHz. Ea decreased from 0.65 to 0.50 eV for a region I and from 0.89 to 0.75 eV for region II with CaO doping is attributed to decrease in carrier concentration and trap density at the interface.

4. Conclusion Declaration of competing interest Influence of CaO doping on phase, microstructures and electrical properties of ZnO varistor were studied. Decrease in crystallite size of the powder from 22 to 17 nm with CaO doping was observed. Grain size of the sintered body decreased from 4.54 to 1.19 mm with

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Please cite this article as: K. Hembram et al., Influence of CaO doping on phase, microstructure, electrical and dielectric properties of ZnO varistors, Journal of Alloys and Compounds, https://doi.org/10.1016/j.jallcom.2019.152700

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Acknowledgements This work was partially supported by DST-SERB, Govt. of India (Grant No. SB/EMEQ-221/2013). Authors would like thanks the Director, ARCI for giving permission to publish this work.

[29] [30] [31]

Appendix A. Supplementary data

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Supplementary data to this article can be found online at https://doi.org/10.1016/j.jallcom.2019.152700.

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