Revisiting the effects of Co 2 O 3 on multiscale defect structures and relevant electrical properties in ZnO varistors

: Element doping is an effective method to improve the performance of ZnO varistors. Previous studies mainly focused on the variation of microstructures and Schottky barriers. In this study, the effects of Co dopant on electrical properties are investigated from the aspect of multiscale defect structures, including intrinsic point defects, the heterogeneous interface of depletion/intergranular layers, and interface states at grain boundaries. Combining with analysis of phase composition and energy dispersive spectroscopy, it is found that Co tends to dissolve into ZnO grains when slightly doped. It substitutes Zn 2+ with the same valence and affects little on densities of donors. Segregation of Co at grain boundaries would result in the formation of spinel phase Co(Co 4/3 Sb 2/3 )O 4 and transformation of the intergranular phase from α-Bi 2 O 3 to δ-Bi 2 O 3 . Meanwhile, densities of point defects are indirectly affected by oxygen ambient during sintering, resulting in abnormal variation of grain resistivity. And interface states are enhanced, leading to improved barriers at grain boundaries. Therefore, reduced leakage current, enhanced grain resistivity, and improved non-linear coefficient in Co-doped ZnO varistor blocks are understood from the underlying multiple defect structures. This presents a potential approach to explore short-term performance and long-term stability of ZnO varistors from the aspect of defect responses.


Introduction
ZnO varistor ceramics with proper addition of a small amount of cations are widely employed in overvoltage protection and surge absorption in electric power systems, because of their excellent non-linear current-voltage characteristic and large energy absorption capability [1][2][3]. Since it was discovered by Matsuoka in 1971, the initial non-linear material SiC was quickly replaced by ZnO and gapless surge arresters became the mainstream of surge arresters [4]. In the past half-century, ZnO varistors with superior performance have been continuously expected and developed with the continuous increase of voltage grade of power systems [5,6]. Remarkably, the development of ultra-high voltage (UHV) dc power transmission in recent years raises the higher requirement for ZnO blocks due to the complex voltage waveform, high chargeability, high temperature, etc. [7]. Therefore, further understanding of microstructure, Schottky barrier, electrical properties and their inside correlations, which is the fundamental issue for developing both short-term electric performance and long-term stability, is of great necessity [8][9][10].
Primarily, the remarkable non-linear properties of ZnO varistor ceramics arise from the double Schottky barriers (DSBs) at the grain boundaries. Electron transport is in charge of the DSB, which follows assisted thermal electron emission under low electric field and tunneling under a high electric field. Currently, the DSB is acknowledged consisting of negatively charged interface states and positively charged depletion layers [11,12]. That is, the overall electrical properties of ZnO varistor blocks depend on the dynamics of the DSB under external electric field, including both short-term performance and long-term stability. Besides the barrier height being determined by the ratio of the density of interface states (N s ) and density of donors (N d ), the remarkable non-linearity is determined by the electron filling of interface states, as well [13]. In addition, the reduction of interface charge from both decomposition of absorbed oxygen and migration of moveable zinc interstitial is generally known as the origin of degradation of ZnO varistor. They are all closely related to the DSB and underlying defect structures.
Doping is one of the most effective methods to modify the DSB by manipulating its basic defect structures, e.g. point defects of zinc interstitial and oxygen vacancy. Cobalt, as an example, is one of the commonly used transition elements to modify the DSBs at grain boundaries. It is presented that the addition of Co could reduce grain boundary resistivity, enhance the non-linearity, prevent Bi 2 O 3 evaporation and improve stability [3]. Cobalt ions in ZnO grains exit as Co 2+ , which replaces Zn at the lattice sites in the manner of tetrahedral coordination [14]. It forms a deep energy level of about 1.9 eV below the conduction band edge in ZnO [15]. It is suggested that Co 2 O 3 can increase the oxygen partial pressure in the sintering processing and decrease the density of the donor, which decreases the conductivity of the ZnO grains [16]. In addition, Yano et al. suggested that the transition metals, e.g. Mn, Co and Cu, were conducive to forming interface states between the conduction band and the valence band of ZnO, which is attributed to the 3d character of the transition metals and the π character of excessively adsorbed oxygen [17]. Cobalt can also form a spinel phase (Co(Co 4/3 Sb 2/3 )O 4 ), which has a limited impact on the average grain size of ZnO [18]. The research of Co-doped ZnO varistors has been mainly focused on the improvement of the performances, the phase composition and the defect structure. However, the effects of Co dopant on the multiscale defect structures have not been systematically presented yet. It is necessary to investigate how Co at different content distributes in ZnO varistors and influences multiscale defect structures. Combining both, the impacts of element dopant on ZnO varistors can be analysed more reasonably, as well as any defect related phenomenon.
In this paper, a series of ZnO varistor blocks with different Co contents were prepared based on a commercial formula. The phase composition and micro-morphology were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. Electrical current-voltage characteristics were measured in both small and large current regions. The relation between microstructure and overall electrical performance was demonstrated by electron trapping behaviours of multiscale defects, including shallow and deep point defects in grains, intergranular phase, and interfaces at grain boundaries.

Experimental
A series of ZnO varistor block samples were prepared via the solidstate reaction method with the following raw materials: (94.52-x) mol% ZnO, 1.2 mol% Bi 2 O 3 , 1 mol% Sb 2 O 3 , 0.5 mol% MnCO 3 , 1.3 mol% NiO, 1.48 mol% SiO 2 and x mol% Co 2 O 3 (x = 0, 0.28, 0.55, 0.83, 1.1, 1.38). The raw materials were mixed by ballmilling in polyethylene bottles for 12 h with deionised water and an appropriate amount of polyvinyl alcohol (PVA). The slurry was processed into particles with size at 80-120 μm via spray granulation. The particles were added 3 wt% water to be stale for 24 h. Then, stale particles were pressed into blocks at 100 MPa, and pre-sintered at 600°C to discharge PVA. Then, the blocks were sintered in air at 1150°C for 2 h, and naturally cooled. Finally, ZnO varistor block samples with a diameter of 40 mm and a thickness of about 9 mm were obtained. They were designated as S1, S2, S3, S4, S5 and S6 for short, with the increase of Co content.
The crystalline phases of samples were examined by an X-ray diffractometer (XRD, D8 Advance, Bruker, German) at room temperature. Morphologies of polished surfaces were characterised by scanning electron microscope (SEM, VE-8600S, Keyence, Japan) and the relevant element analyses were measured by an energy dispersive spectrometer (EDS, JSM-6390A, JEOL, Japan).
Al electrodes were prepared on both sides of the samples for electrical measurements. The current density-electrical field (J-E) characteristics in the small current region were measured at room temperature by multi-meter (HP 34401A, America) and precision linear high-voltage DC power (WJ10001D, China). In contrast, those in the high current region were measured under 8/20 μs lightning current pulse by impulse current generator (Keytek EMC Pro, USA) and oscilloscope (Tektronix 7245B, USA). The breakdown field E 1mA = U 1mA /d and non-linear coefficient α = 1/ lg(U 1mA /U 0.1mA ) were calculated from the J-E curves, where U 1mA and U 0.1mA are voltages under a current density of 1 and 0.1 mA/cm 2 , respectively, and d is the sample thickness. The leakage current density J L was also acquired at 0.75U 1mA . Dielectric properties were measured by an impedance analyser (Novocontrol, Concept 80, Germany) in the temperature range of −110 to 200°C and frequency range of 10 −1 -10 7 Hz with an AC signal of 1 V in magnitude.  [18,19]. Notably, both the Bi-rich phase and spinel phase are different in different samples. On the one hand, the α-Bi 2 O 3 phase (JCPDS Card No. 41-1449) is detected in samples S1-S4 while δ-Bi 2 O 3 phase (JCPDS Card No. 27-0052) is detected in samples S5 and S6. δ-Bi 2 O 3 phase trends to appear in those samples with relatively high Co content. That is, a phase transition of Bi 2 O 3 from the α-monoclinic phase to the δ-cubic phase is revealed.

Phase composition and surface morphology
On the other hand, a similar phenomenon is also found for the spinel phase. Two spine phases Zn(Zn 4/3 Sb 2/3 )O 4 (JCPDS Card No. 15-0687) and Co(Co 4/3 Sb 2/3 )O 4 (JCPDS Card No. 78-0718) are identified in those samples. The latter phase can hardly be detected when Co content is relatively small, i.e. in samples S1, S2 and S3. The intense diffraction peak of it is observed when Co content exceeds 0.55 mol%, i.e. in samples S4, S5 and S6. It is, therefore, reasonable to deduce that Co tends to dissolve into ZnO grains at low doping rate, while it is likely to segregate at grain boundaries with higher content. Zn(Zn 4/3 Sb 2/3 )O 4 and Co(Co 4/3 Sb 2/3 )O 4 have a similar stoichiometric proportion. It is implied that some Zn atoms in Zn(Zn 4/3 Sb 2/3 )O 4 phase are replaced by Co in those Co-doped samples, because of the similar radius of Co 2+ (72 pm) and Zn 2+ (74 pm). This also accounts for the detection of other spinel phases, e.g. Zn 0.6 Co 1.73 Sb 0.67 O 4 , in Co-doped ZnO varistor ceramics [19]. Fig. 2 is back-scattering electron micrographs of polished surfaces of ZnO varistor blocks. Well grown equiaxed ZnO grains, grey-coloured parts of SEM pictures, are observed in all the samples. ZnO grains are pinned by those sharp spinel phases and surround by light-coloured Bi-rich phases. The grain size distribution is statistically measured, as shown in the insets, following the log-normal distribution [20,21]. Average grain sizes were further calculated to be 9.98, 10.27, 10.85, 10.19, 10.24 and 10.71 μm for samples S1, S2, S3, S4, S5 and S6, respectively, changing little with the addition of Co element.
To further study the effects of Co dopant on the microstructure of ZnO varistors, EDS was conducted along the red lines in Fig. 3. Element distribution was measured traversing several ZnO grains, grain boundaries and intergranular phases. The majority of Zn distributes in grains so that it can be regarded as an indicator of grain's boundary line. Also, Bi exists in light-coloured intergranular phases, while Sb mainly exists between ZnO grains and seemingly in the spinel phase. The distribution of Co is relatively homogeneous when its content is small in samples S1-S3. However, clear peaks, accomplished with the agglomeration of Sb, are observed at grain boundaries in samples S4-S6, which arises from the segregation of Co and the formation of the spinel phase (Co(Co 4/3 Sb 2/3 )O 4 ) detected by XRD in Fig. 2. Fig. 4 shows the current density-electrical field (J-E) characteristics of the ZnO block samples. As presented in Fig. 4a, clear electrical non-linearity is observed in all the samples in the small current region, from which breakdown field (E 1mA ), leakage current density (J L ) and non-linear coefficient (α) were calculated and summarised in Table 1. In this region, carrier transport in the samples is dominated by the DSBs at grain boundaries. However, DSB is collapsed in the high current region so that the external voltage is almost withstood by the grains, leading to linear J-E curves in Fig. 4b. Grain resistivity (ρ g ) is, in consequence, calculated as the slope and listed in Table 1, as well. As shown in Table 1, decreased E 1mA , J L , ρ g and improved α were acquired after Co was added into the sample, although these electrical parameters did not change monotonously. By other reports, the addition of Co results in enhanced electrical properties of ZnO varistors [3]. It is worth noting that the morphology shown  in Fig. 2 varied little in different samples while largely improved properties are obtained. In this case, the DSBs, along with their defect structures, should take the major responsibility for it [22,23].

Frequency-domain dielectric responses
The frequency-domain dielectric spectroscopy (FDS) is a powerful method to explore the inside electron trapping behaviours of defects in ZnO varistor ceramics. FDS of sample S5, as an example, at varying temperatures is shown in Fig. 5. Two explicit permittivities (ε′) steps are shown at low temperatures in Fig. 5a. They are correlated with two dielectric relaxations, which are marked as Relax. A and Relax. B, respectively. Only one clear ε′step, marked as Relax. C, is detectable in the high-frequency range (>10 2 Hz) at high temperatures in Fig. 5b. Moreover, a blurry ε′plateau also appears in the low-frequency range (<10 2 Hz), which is marked as Relax. D. The results shown in Fig. 5 indicate that four relaxation peaks should appear in an imaginary part of permittivity (ε″) spectra. However, the entire peak of Relax. D and the partial peak of Relax. C are commonly covered by the intense dc conductance, which is even mistaken as a relaxation process in commonly used impedance and modulus spectra [24,25].
Facing the difficulty of clearly exhibiting the relaxation processes and relating electron trapping dynamics in ZnO varistor blocks, an optimised (∂ε′/∂lnω)/ε′ spectroscopy is employed [26,27]: where ε′ is a real part of complex permittivity. ε ∞ and ε s are optical and static permittivity, respectively. ω is the angular frequency and τ is relaxation time. Only relaxation processes are exhibited as loss peaks in this spectroscopy. Two loss peaks are observed at low temperatures in Fig. 5c, which is accordant with ε′-steps in Fig. 5a.
Remarkably, two-loss peaks are also detectable at high temperatures in Fig. 5d, indicating two relaxation processes. All those peaks move toward a higher frequency range, showing thermally activated processes. A relaxation peak in (∂ε′/∂lnω)/ε′ spectroscopy reaches its maximum only when ( In this situation, the dependence of peak frequency f m on temperature, as plotted in Fig. 6, follows the Arrhenius equation.
The corresponding activation energies of all the ZnO block samples are listed in Table 2.
Besides the relaxation processes, temperature-dependent conductivity can also be calculated from FDS, as demonstrated in Fig. 7. According to Jonscher's universal relaxation law [28]: where σ′ is a real part of complex conductivity and σ dc is dc conductivity. B and s (0 < s < 1) are constants. σ dc is approximate to σ′ at the lowest frequency and its activation energy (E dc ) shown in Table 2 is thereby calculated from lnσ dc ∼1/T curve plotted in the inset of Fig. 7. As clearly shown in Table 2, activation energies for Relax. A and Relax. B are about 0.22 and 0.33 eV, respectively. Both of them are independent of Co content, indicating intrinsic processes. Generally speaking, Relax. A and Relax. B are supposed to originate from electron trapping of intrinsic point defects Zn i¨ and V O˙, respectively, with their relaxation activation energies similar to the trap depths [29][30][31]. In addition, activation energies of Relax. C (E c ) and Relax. D (E d ) depend on the addition of Co content, indicating extrinsic processes, which are ascribed to the intergranular phase and trapping behaviours of interface states, respectively [25]. E c and E d are gradually increasing with Co content. E dc is the activation energy of dc conductance, which is not influenced by Co content and then indicates that the dc conductance mechanism of different samples does not change with Co content.

Effects of intrinsic defects:
The Cole-Cole model is employed to fit the measured results [32,33]: where ε h is the high-frequency permittivity. Δε i , τ i and α i are magnitude, relaxation time and depression angle of the ith relaxation, respectively. At − 100°C, dc conductance is negligible in the measured frequency range. Two entire Relax. A and Relax. B and partial Relax. C are involved in this range. S5 was taken as an example to exhibit the fitting results, as shown in Fig. 8. Fitting results of all the ZnO block samples are shown in Table 3. The overall permittivity in Fig. 8 is the superposition of ε h , Δε a and Δε b , which arises from electron trapping of intrinsic point defects. It should be noted that ε h is related to shallow donors in ZnO, which is generally acknowledged to be Zn i ⋅ [34]. Its relaxation time is so far shorter than 10 −8 s that the relating loss peak is unable to be detected in the current frequency range. Fortunately, its magnitude is approximate to the value of ε h as the optical permittivity of ZnO is only 8. In ZnO grains, almost all of Co exists in the form of Co 2+ [35], which substitutes Zn 2+ with tetrahedral coordination [14]. It forms a deep level about 1.9 eV below the conduction band [15], which hardly changes the concentration of free electrons. Based on the valence and coordination structure, it is deduced that the Co dopant is not directly correlated with intrinsic point defect concentration. Therefore, the reduction of grain resistivity (ρ g ) shown in Table 1 in Co-doped ZnO varistor blocks might originate from increased carrier mobility, which is accordant with previous reports [16]. The only abnormal increase of ρ g in sample S4 corresponds to the dramatic decrease of the shallow donor Zn i ⋅ . However, it has to be pointed out that variations of point defects cannot be well explained by themselves. The effects of Co dopant on the extrinsic defects should also be taken into consideration.

Effects of extrinsic defects:
Extrinsic defects related relaxations are always observed at high temperatures due to their long relaxation times. Taking sample S5 as an example, the FDS at 180°C is exhibited in Fig. 10. By previous analyses, only the loss peak from Relax. C can be found, while Relax. D is completely covered by dc conductance. Keeping in mind that there are two relaxations existing in ZnO block samples, the measured FDS is fitted by (4), as well. Fitting curves of sample S5 are plotted in Fig. 10 and detailed parameters of all the samples are summarised in Table 4.
A large number of researches show that Relax. C is closely correlated with the intergranular phases in ZnO varistors [22,36]. The heterogeneous interface between intergranular and depletion layers of DSB acts as a hurdle for electron transport, resulting in space charge polarisation, which can be expressed by the frequency-dependent capacitance where ΔC is the additional capacitance that arose from Relax. C and τ C is the relaxation time. R b and C b are the overall resistance and capacitance of DSB and R i and C i are the resistance and capacitance of the intergranular phase, respectively. Based on  [25], the additional capacitance and relaxation time can be simplified as According to (8), ΔC contributed by Relax. C should be equal to the capacitance of DSB. As presented in Table 4, its permittivity (Δε c ) is indeed similar to ε h , which is the apparent permittivity of DSB consisting of polarisations caused by all the intrinsic point defects. From this point of view, intense of Relax. C is just another indicator of DSBs and point defects in grains. Furthermore, its R i related relaxation time τ C makes it possible to study the effects of Co dopant on intergranular phases in ZnO varistors. R i is calculated according to (9) and its dependence on Co content is plotted in Fig. 11. On the basis that Δε c is approximately independent on temperature, the activation energy of R i is equal to the relaxation activation energy (E c ), whose dependence on Co content is also shown in Fig. 11. As demonstrated in Fig. 11, R i dramatically decreases from ∼0.65 to ∼0.45 MΩ when the Co content is over 0.83 mol%. Meanwhile, its activation energy rapidly increases from ∼0.52 to 0.65 eV. Both of two results indicate that Co content of about 1 mol % (around samples S4 and S5) would be a critical point, at least, for ZnO varistor blocks in this paper. As derived from XRD patterns in Fig. 1, Co tends to dissolve into ZnO grains when lightly doped, so that both R i and E c only slightly changed in samples S1-S3. With increasing Co content, Co gradually segregates at grain boundaries leading to the formation of resistive spinel phase Co(Co 4/3 Sb 2/3 )O 4 [37]. Therefore, both R i and E c increase in samples S4. When the Co content is further increased, δ-Bi 2 O 3 intergranular phase is formed, which is more conductive than α-Bi 2 O 3 phase. Generally, δ-B 2 O 3 is stable in the temperature range of 729-824°C [38]. However, the substitution of some oxide additives stabilises δ-B 2 O 3 , down to room temperature [19]. It is supposed that Co segregation in the intergranular phase may stabilise δ-B 2 O 3 in a wider temperature range. Therefore, dramatic variations of R i and E c appear in samples S5 and S6.
Besides the variations of intergranular resistivity itself, intrinsic point defects in the grains were also affected by the transformation of intergranular phases. As presented in Fig. 9, intensities of intrinsic point defects are almost changeless in lightly doped samples (S1-S3), where Co mainly distributes in ZnO grains in the valence of +2. Dramatic decrease of intensity of Zn i ⋅ , which results in a rapid increase of grain resistivity in Table 1, is just accompanied by segregation of Co at the grain boundary in sample S4: Extra oxygen is produced during the formation of spinel phase Co(Co 4/3 Sb 2/3 )O 4 detected by XRD in Fig. 1. The generation of donors is inhibited by the oxygen-enriched environment, leading to an abnormal increase of grain resistivity. With further increasing Co content in samples S5 and S6, the δ-Bi 2 O 3 phase was detected even at room temperature, which, as a fast oxygen transport channel [38], allowed the produced extra oxygen to escape into ambient during the entire sintering process [39]. In this case, intensities of intrinsic point defects in samples S5 and S6 recovered to the level of that in samples S1-S3. In addition, interface states are influenced by the addition of Co and its reactions at grain boundaries, as well. Fig. 12 shows the dependence of the intensity of Relax. D (Δε d ), activation energies of dc conductivity (E dc ) and Relax. D (E d ) on Co content. Δε d increases with Co content, reflecting the increased density of interface states. As analysed above, extra oxygen was released due  to the addition of Co 2 O 3 . According to Yano et al. [17], Co is conducive to forming interface states because of the 3d character of the transition metals and the π character of excessively adsorbed oxygen. In addition, E d is equal to the energy level of interface states, by which barrier height is pinned, and E dc is similar to the barrier height. In consistency with the increase of intensity of Relax. D, both E dc and E d increase with the addition of Co. All those results indicate well-developed barriers at grain boundaries are formed in Co-doped ZnO varistor blocks. This is macroscopically exhibited as reduced leakage current density and improved non-linear coefficient.

Conclusion
In conclusion, the effects of Co 2 O 3 on phase composition, micromorphology, electrical performance and dielectric properties of ZnO varistor blocks are investigated in this paper. Great discrepancies of electrical properties are observed, although the morphologies of the samples are similar, indicating the differences of Schottky barriers at grain boundaries, which is closely related to the underlying defect structures. Although Co tends to dissolve into ZnO grains and substitutes Zn 2+ , it affects little on densities of point defects because of iso-valence doping. Alternatively, the formation of the spinel phase of Co(Co 4/3 Sb 2/3 )O 4 and transformation of intergranular phase from α-Bi 2 O 3 to δ-Bi 2 O 3 would indirectly affect the densities of point defects in ZnO grains by modifying the oxygen ambient at grain boundaries. This was characterised as simultaneous changes of intergranular and grain resistivity. In addition, interface states are also enhanced due to the addition of Co 2 O 3 , resulting in well-developed barriers and improved electrical non-linearity. From the view of multiscale defect structures of intrinsic point defects in grains, the heterogeneous interface of depletion/intergranular layers and interface states at grain boundaries, effects of Co dopant, as an example, on ZnO varistor are systematically understood.