Microstructural Evaluation of Rare-Earth-Zinc Oxide-Based Varistor Ceramics

Zinc oxide varistors are nonlinear voltage dependent ceramic resistors used to suppress and limit transient voltage surges. The work reported in this paper involves the relationship between microstructural characteristics and the varistor performance of ZnO ceramics doped with rare-earth oxides. Samples of these ceramics with different nonlinear current-voltage characteristics, according to the specific chemical composition and sintering parameters, were prepared and microstructurally analyzed by scanning electron microscopy, X-ray energy dispersive spectroscopy, X-ray fluorescence spectroscopy and X-ray diffraction. The results denote that intergranular phase is rich in rare-earth elements, but its morphology, obtained by selective leaching of ZnO grains (which are only doped with Co), provides evidence that ZnO grains are not completely surrounding by the intergranular phase, also existing ZnO grains are in direct contact with each other, as well as it occurs in conventional varistor system.


Introduction
Zinc oxide (ZnO) varistors (variable resistors) are polycrystalline ceramic devices exhibiting highly nonlinear (nonohmic) electrical behavior and greater energy absorption capabilities 1,2 .The fabrication of ZnO varistors is done by mixing semiconducting ZnO powder with other oxide powders such as Bi, Co, Mn, and Pr, and subjecting the powder mixture to conventional ceramic pressing and liquid-phase sintering techniques 3 .The sintering results in a polycrystalline ceramic with a singular grain boundary property which produces the nonlinear current-voltage (I-V) characteristics of the device 4 .Microstructurally, the ZnO varistors are comprised of semiconducting n-type ZnO grains, surrounded by very thin (1-10 -3 µm) insulating intergranular layers 2,5 .Several conduction mechanisms for the varistor have been proposed based on this ceramic microstructure 1,2,6 , which has led to varistor behavior being widely interpreted as resulting from the series-parallel network formed by the ZnO-intergranular-phase junctions 6,7 .Capacitance measurements as a function of voltage have supported the model of Schottky barriers at grain boundaries 8 .Electrically, the varistors show highly nonlinear I-V characteristics similar to the back-to-back Zener diode, but with much higher voltage, current, and energy handling capabilities 9 .As a result, they are widely used as surge absorbers in electronic circuits and core elements of surge arresters in electric power systems 10 .
ZnO varistors are divided generally into two categories, called Bi 2 O 3 -based and Pr 6 O 11 -based varistors, in terms of varistor-forming oxides inducing the nonlinear properties of varistors 11 .Most of the commercial ZnO varistors are Bi 2 O 3 -based varistors, which have been mainly studied in various aspects since ZnO varistors were discovered by Matsuoka et al. 12 .However, Bi 2 O 3 -based ZnO varistors are not suitable to be used in multilayer chip varistors manufacture, due to Bi 2 O 3 having a high volatility and reactivity 13 .Furthermore, in general, Bi 2 O 3 -based ZnO varistors possess four phases, namely ZnO grains, Bi-rich intergranular layers, an additional insulating spinel phase, which does not play any role in electrical conduction, and pyrochlore phase.On the other hand, in Pr 6 O 11 -based ZnO varistors only two phases are present in a sintered body, namely, ZnO grains and the intergranular phase composed mainly of praseodymium oxide 14 .The absence of a spinel phase increases the active grain boundary through which the electrical current flows.Therefore, the effective cross-section area of the element is increased 13 .Earlier studies about Pr 6 O 11 -based varistors have been limited to the ternary system ZnO-Pr 6 O 11 -CoO and dissimilarities between Bi 2 O 3 -based and Pr 6 O 11 -based ZnO varistors 11,[15][16][17][18] .Recently, many works have been made in order to study the influence of other rare-earth oxides (such as Y 2 O 3 , Nd 2 O 3 , Er 2 O 3 , and Dy 2 O 3 ) on the microstructural and electrical properties of the ternary system ZnO-Pr 6 O 11 -CoO 5,13,19 .The varistors produced have exhibited a relatively good electrical performance 13,19 .
Earlier studies in ZnO varistors have shown that the narrow regions, where the sintered grains have grown together, control the resistance of the entire sample 12,20 .In those regions, the surface/volume ratio is sufficiently high for the acceptor concentration (which occurs because of adsorbed oxygen) to exceed the donor concentration inside the ZnO grains 21 .More recent works have shown that Schottky barriers result from interface states because of the chemisorbed oxygen ion at the ZnO-ceramic grain boundaries 1,2,[22][23][24][25] .Although the microstructure of varistors exhibit considerable variation from one manufacturer to another, they all exhibit the characteristics of a typical ceramic prepared by liquid-phase sintering, consisting mainly of large ZnO grains with a varistor former-rich second phase at the nodal points (triple juntions) and/or intergranular layer (IGL) regions.
In the present work an evaluation of the microstructural features of ZnO-Pr 6 O 11 -Nd 2 O 3 -Co 3 O 4 -Cr 2 O 3 based varistor ceramics (ZPNC) was made and applied to the understanding of the varistor performance.

Experimental
Appropriate molar reagent grades of ZnO (Merck), Co 3 O 4 (Riedel), Pr 6 O 11 (Aldrich), Nd 2 O 3 (Aldrich), Cr 2 O 3 (Merck) powders were used to prepare the ZnO based ceramics.The sample compositions are shown in Table 1.The powders with adequate compositions were ball milled with zirconia balls in isopropyl alcohol media inside of a zirconia jar for 24 hours.The resultant mixture was dried at 110 °C for 12 hours and calcined in air at 750 °C for 2 hours.The calcined mixture was granulated in a 200 mesh sieve and pressed into discs of 12.4 mm in diameter and 2.1 mm in thickness at a pressure of 80 MPa.The discs were sintered at 1300-1350 °C with interval by 25 °C in air atmosphere for 1 hour.The heating and cooling rates were 6 °C/min.The average size of the final samples was 10.2 mm in diameter and 1.1 mm in thickness.The sintered bodies were sanded and polished, silver paste was coated on both faces of the samples and the silver electrode was formed by heating at 600 °C for 10 min.The area of the electrodes was approximately 0.212 cm2 .The I-V characteristics of ceramics were measured using a curve tracer sourcemeasurement unit (Tektronix 577).The breakdown electric field (E B ) was measured at 1.0 mA/cm 2 and the leakage current density (J L ) was measured at 80% of breakdown electric field.In addition, the nonlinear coefficient (α) was estimated for current-density ranges of 1.0-10.0mA/cm 2 .
The sample microstructures were examined by scanning electron microscopy (SEM, ZEISS DSM 960) applied on the polished and 6 M-NaOH aqueous solution-etched (5 minutes) surface of samples, as well as on the fractured surface of ZPNC samples.The grain sizes were estimated by linear intercept method 26 .The compositional analysis of the selected areas was determined by an attached X-ray energy dispersive spectroscopy (EDS, Oxford ISIS) system.The density (ρ) of the ZPNC ceramics was measured by the Archimedes method.The crystalline phases were identified by powder X-ray diffraction (XRD, Diano XRD-8545, λCuKα radiation).The chemical composition of the residue of leached ZPNC samples was determined by X-ray fluorescence (XRF, 3070 Rigaku spectrometer).The leaching procedure consisted of placing each sample into a beaker containing 400 mL of solution (cf.Table 3 for solution systems used) under constant agitation.After each specific dissolution time, the residue was passed through a preweighed millipore filter, dried and reweighed.The weight percent of the residue was calculated.

Results and Discussion
The Table 2 presents the results obtained (varistor characteristics) from electrical measurements of the ZPNC samples and the Figure 1 shows the microstructural parameters evaluated for this varistor ceramic system in function of the chemical composition and sintering temperature employed.Figure 2 shows microstructure of  the fractured surface examined by SEM of ZPNC samples sintered at 1325 °C.Among all the samples analyzed, the ZPNC-2 sintered at 1325 °C exhibited better varistor performance, i. e., α > 60 and low leakage current in the prebreakdown regime.As the sintering temperature increases, the average grain size (d G ) was increased in the range of 5.1-13.3µm for ZPNC-1 samples, 4.8-12.9µm for ZPNC-2, and 5.0-13.7 µm for ZPNC-3.This increase of grain size resulted in the decrease of the breakdown electric field (cf.Table 2).The relative density of ZPNC ceramics was increased in the range of 5.03-5.56g/cm 3 for ZPNC-1 subsystem, 5.05-5.58g/cm 3 for ZPNC-2, and 5.05-5.57g/cm 3 for ZPNC-3.In general, all the samples present satisfactory densification level (in the range of 89.7% to 99.5% of the theoretical density, 5.61 g/cm 3 ) and stability of the nonlinear electrical characteristics.The increase of Cr 2 O 3 content improves the nonlinear coefficient and the electric breakdown field, in all sintering temperature evaluated.For the samples sintered at 1300 °C, the increase of Cr 2 O 3 content resulted in increase of the leakage current, but for the samples sintered at 1325 °C and 1350 °C resulted in the decrease of the leakage current.In fact, the added Cr 2 O 3 (0.2 mol%) resulted in the improving of the varistor behavior of the quaternary system ZnO-Pr 6 O 11 -Nd 2 O 3 -Co 3 O 4 proposed by Nahm et al. 5 .In all sintering temperature studied, the increase of rare-earth oxides content resulted in the degeneracy of the varistor properties.
The Figure 3 shows the XRD patterns of ZPNC ceramic samples, identifying the ZnO phase (grains), Pr 6 O 11 and Nd 2 O 3 rich phase peaks.The Pr and Nd oxides coexist in the grain boundaries, intergranular layers (IGL) and in nodal points, behaving as a single phase 5,13 .Figure 4a shows a SEM micrograph of the polished and chemical etched surface (leached) of the ZPNC-2 ceramic sintered at 1300 °C and Figure 4b shows in detail the X-Z-marked region.The X, Y and Z marks (Figure 4) indicate the specific regions to qualitative EDS analyses (Figure 5), denoting, respectively, a ZnO grain, a very thin IGL (grain boundaries region) and an IGL region, whose EDS patterns are shown, respectively, in the Figures 5a, 5b  and 5c.In Figure 5c can be seen that, in fact, the IGL region contains a high concentrations of rare-earth elements (Pr and Nd) and smaller amounts of Cr and Co.However, as can be seen in the Figures 5  and 6, the intergranular layer distribution is heterogeneous, because regions exist that ZnO grains are in direct contact with each other (cf. Figure 6).The microstructures present in Figures 2 and 4 are not quite uniform but a bimodal grain size distribution was not observed in any of the samples.The heterogeneous distribution of the Pr-Ndrich intergranular material observed, mainly in Figure 4, suggests    that processing conditions are not optmized yet, and the ZnO grains were not suffered an uniform wetting by liquid phase-varistor former rare-earth mixture, although the resultant varistor ceramic samples have presented excellent electrical performance (cf.Table 2).In fact, the uniform distribution of grain boundary materials (varistor formers and dopants) can be expected to improve significantly the performance of ceramics which are liquid-phase sintered and this is believed to result in the proper conditioning required to form the electrically active junctions in the large majority of the grain boundary regions such as shown the enlarged SEM micrograph of the triple point of the ZPNC-2 sample (Figure 6) 2,16 .Thus, from these observations, it is reasonable that the nonlinear electrical behavior of ZPNC ceramics can be considerablly improved by means of the increase of its microstructural homogeneity.
A quantitative residue analysis, by XRF, using perchloric acid solutions (cf.Table 3), which are most effective in the selective leaching of the ZnO grains, showed that the residue is formed mainly by rare-earth oxides (about 80 wt.(%)), which exhibited, by EDS analysis, the typical composition of a solidified eutectic liquid charac-teristic of the ZnO-Pr 6 O 11 -Co 3 O 4 system (82 wt.(%) Pr 2 O 3 -10 wt.(%) CoO-8 wt.(%) ZnO, consisting in the praseodymium rich phase) 16,17 doped with Nd 2 O 3 .Pr 6 O 11 and Co 3 O 4 present variable stoichiometry with oxygen, depending on oxygen partial pressure and temperature conditions, so that Pr 2 O 3 is formed during the sintering process by reduction of the Pr 6 O 11 11 .

Conclusions
The microstructure of ZnO-Pr 6 O 11 -Nd 2 O 3 -Co 3 O 4 -Cr 2 O 3 based varistor ceramics (ZPNC) consists of two phases: ZnO grains (doped with Co) and rare-earth elements (Pr and Nd)-rich phase (doped with Cr and Co) segregated at grain boundaries.The distribution of the intergranular layer is heterogeneous and exist ZnO grains that are in direct contact.Additionaly, ZPNC ceramics sintered at 1325 °C with 0.2 mol% Cr 2 O 3 exhibited the most excellent varistor characteristics.

Figure 1 .
Figure 1.Average Grain Size and Relative Density (in percentage of the theoretical density) of ZPNC varistor ceramics as a function of the sintering temperature.

Figure 6 .
Figure 6.SEM micrograph of magnified detail of the ZPNC-2 microstructure showing that ZnO grains are in direct contact with each other.

Figure 4 .
Figure 4. a) SEM micrograph of the polished and etched surface of the ZPNC-2 ceramic sintered at 1300 °C; and b) X-Z-marked region in detail.

Table 3 .
Results of residue analysis (XRF) of the leached ZPNC samples 1 .

Table 1 .
Composition of investigated samples of the ZPNC system.

Table 2 .
Varistor characteristics of investigated samples of the ZPNC system.