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Microstructural and nonohmic properties of ZnO.Pr₆O₁₁ CoO polycrystalline system

Miguel Angel Ramírez^I,* * e-mail: margbrasil@yahoo.com ; José Francisco Fernández^II; José de Frutos^III; Paulo Roberto Bueno^I; Elson Longo^I; José Arana Varela^I

^IInstituto de Química, Universidade Estadual Paulista – UNESP, 14801-970 Araraquara - SP, Brazil

^IIInstituto de Cerámica y Vidrio, Departamento de Electrocerámica, 24049, Madrid, España

IIIETSIT, Universidad Politécnica de Madrid, Ciudad Universitaria, 28040, Madrid, España

ABSTRACT

The microstructure and electrical properties of varistors composed of (95-x) ZnO + x Pr₆O₁₁ + 5 CoO (ZPC), (x = 0.1, 0.5 and 1.0) and sintered at 1300 and 1350 °C, were investigated. According to X-ray diffraction, several phases (ZnO, Pr₂O₃ and Pr₂CoO₄) are present when x = 1.0. Using Scanning Electron Microscopy, all of these compositions contain precipitates. These phases are important regarding the development of the microstructure and the electrical properties. The samples with x = 0.1 introduce the best nonohmic behavior (α = 9.0), however when x = 0.5 the electrical properties are highly degraded due to the small quantity of effective barriers. The density of superficial states N_IS and donor concentration Nd decreases with Pr₆O₁₁ addition. The decrease in the donor concentration is attributed to the annihilation of the donor defects according to the transformation of praseodymium oxides from Pr₆O₁₁ to Pr₂O₃.

Keywords: electrical properties, varistors, grain boundary, microstructure

1. Introduction

ZnO varistor ceramics are resistor devices manufactured by sintering ZnO powder containing various minor additives¹. It has been recognized that at least two different classes of additives must be added to the ceramic varistors in order to obtain good nonohmic characteristics². The first class of such additives are varistor-forming oxides (VFO), such as Bi₂O₃, Pr₆O₁₁, La₂O₃, BaO, V₂O₅ and glass frits. The second class of additives are 3d transition-metal oxides, which improve significantly the nonohmic characteristics³. Zinc oxide ceramics, containing Pr₆O₁₁ and CoO, which exhibit current nonohmic characteristics, were first reported by Mukae et al.⁴. In fact, the history of Pr₆O₁₁-based ZnO varistors is similar to the Bi₂O₃-based ZnO varistors⁵, but they have not been adequately studied. However, the Pr₆O₁₁-based ZnO varistors don't have volatilization of components³ and they own a simpler microstructure when compared with Bi₂O₃-based varistors, which is an important fact for the stability and degradation phenomena^6-9. Most of commercial ZnO varistors are ZnO-Bi₂O₃-based varistor ceramics, possessing as major additive Bi₂O₃. Although they exhibit excellent varistor performance, they possess few drawbacks due to the high volatility and reactivity of Bi₂O₃ at a sintering temperature above 1000 °C¹⁰. Another flaw of Bi₂O₃-based varistors is that they need many additives to obtain high nonlinearity and stable electrical properties. ZnO varistors generally contain three phases, such as ZnO grains, Bi₂O₃-rich intergranular layer and spinel particles¹⁰. To overcome the problems in Bi₂O₃-based varistors, ZnO varistors using praseodymium oxides as VFO have been reported^{3-5, 11-12}. On the other hand, Pr₆O₁₁-based ZnO varistors are reported to have only two-phases detected by X-ray diffraction, namely, ZnO grains and intergranular layers enriched with praseodymium¹³. The absence of a spinel phase increases the active grain boundary area through which the electrical current flows. Therefore, the effective cross-section area of the elements is increased¹⁴. Several studies describe the dependence of varistor properties on processing conditions of Pr-doped ZnO varistors, especially in the field of grain boundary formation^{2-4, 13}. It is very important to understand the structure, composition and properties of the boundaries since they have a strong influence on electrical properties. Our purpose is to investigate the microstructure, the current density-electric field (J vs. E) characteristics, and the capacitance-voltage (C-V) characteristics of ZPC varistor systems.

2. Experimental Procedure

The varistor systems (95-x) mol% ZnO + x mol% Pr₆O₁₁ + 5 mol% CoO (x = 0.1, 0.5 and 1.0) were prepared from the mixture of oxide precursors method, all of them being analytical grade oxides: ZnO (Aldrich-99.99%), Pr₆O₁₁ (Aldrich-99.9%) and CoO (Riedel-99.9%). The starting materials were attrition-milled in water for 3 hours. Then the dry slurry was calcined at 750 °C in air for 2 hours. The calcined powders were pulverized using agate mortar and pestle and after 2 wt. (%) polyvinyl alcohol (PVA) binder addition, granulated by sieving 200-mesh screen to produce the starting powder. The powder was uniaxially pressed into discs of 12 mm diameter and 1 mm thickness at a pressure of 80 MPa. The pellets were sintered at 1300 and 1350 °C for 2 hours with a heating rate of 5 °C/min followed by furnace cooling. The samples were characterized by X-ray diffraction (XRD, Rigaku, 20-2000), 40 kV and 150 mA from 2θ (20 to 80°), Δ2θ = 0.02°, with Cukα wavelength monocromatized by a graphite crystal. To obtain the Scanning Electron Microscopy (SEM) micrographies (ZEISS^® DSM 940 A) the sintered samples were polished and thermally etched by heating at 100 °C below the sintering temperature during 30 minutes The average grain size (d) was determined by the linear intercept method, given by d = 1.56 L/MN, where L is the random line length on the micrograph, M the magnification of the micrograph, and N is the number of the grain boundaries intercepted by lines¹⁵. Gold contacts were deposited on the samples surfaces by sputtering in order to measure the nonohmic properties. Current-voltage measurements were taken by using a High Voltage Measure Unit (Keithley Model 237). The breakdown electric field (Eb) was obtained at a current density from 1 mA.cm^–2 and the leakage current I_L was measured at 0.80 Eb. In addition, the nonlinear coefficient (α) was determined from linear regression of log J vs. log E in the 1-10 mA.cm^–2 region. The impedance measurements were made with a frequency response analyzer (HP 4294 A LF) using frequency ranging from 40 Hz up to 110 MHz. The capacitance-voltage (C-V) characteristics of ZPC varistors were measured as a function of frequency with the variable applied bias voltage (0-38 V). The donor concentration (Nd) of ZnO grains and the barrier height (_B) at the grain boundary were determined from the slope and intercept of straight line, respectively, using the Equation 1:

proposed by Mukae et al.¹⁶. In this equation C is the capacitance per unit area of grain boundary, C₀ is the capacitance when V = 0, V is the voltage applied per grain, q is the electronic charge, ε₀ is the permittivity of free space, k is the dielectric permittivity of ZnO (~8.5) and p is the number of barriers between grains (obtained by calculating the distance between electrodes divided by average grain size). The density of the interface states N_IS at the grain boundary was determined using the Equation 2:

Once the donor concentration and barrier height are known, the depletion layer width (ω) of either side at the grain boundaries was determined by the Equation 3.

3. Result and Discussion

Figure 1a shows XRD pattern for the samples of the ZPC system sintered at 1300 °C. The samples with x = 0.1 and 0.5 contain only a wurtzite ZnO phase (card PDF 36-1451), while when x = 1.0, the sample contents a praseodymium sesquioxide phase, Pr₂O₃ (card PDF 47-1111), and exhibits a low intensity peak of Pr₂CoO₄ (card PDF 34-1282). XRD pattern for the samples sintered at 1350 °C are shown on Figure 1b. The difference concerning the sample sintered at 1300 °C is that for x = 1.0 the Pr₂CoO₄ phase is not observed. At high (1350 °C) sintering temperature, a densification occurs through the liquid phase and the temperature where the eutectic appears depends strongly on the system's composition.

The Figures 2a, 2b and 2c show micrographs of the samples sintered at 1300 °C. In all cases dense ceramics (>95% of theoretical density) were obtained with presence of inter- and intragranular pores, in small amounts. For x = 1.0 the intragranular pores were completely eliminated, indicating that Pr₆O₁₁ helps densification. Pr₆O₁₁ strongly influences the grain size, which gets its maximum average value of 30 µm for x = 0.5 (see Table 1). Figures 2d, 2e and 2f show micrographs of the samples sintered at 1350 °C. The presence of a larger amount of precipitates in the grain boundary is an obvious indication for the liquid phase of all compositions, at the temperature interval between 1300 and 1350 °C. Micrographs show that the average grain size reaches a maximum when x = 0.5. The increase of the average grain size by addition of a small amount of dopant (x = 0.5) may be caused by some lattice defects, similar to that related to the La doped BaTiO₃¹⁷, which is reported elsewhere. Further addition of Pr₆O₁₁ above 0.5 mol% causes the suppression of grain growth (except for the sample with x = 1.0 mol% and sintered at high temperature). Such suppression has been also reported for 2 mol% rare-earth (Sm, Eu, Y and Er) doped ZnO. The reason for this suppression has been attributed to the pinning effect during the movement of grain boundaries^18-19. In the light of variations of intergranular material, the liquid phase formation temperature is expected to occur between 1300 and 1350 °C depending on the system's composition. The crystals generation for the sample with x = 1 and sintered at 1350 °C suggests that the liquid phase has the lowest temperature formation in this case. EDS analysis (not shown) determined that the precipitated phase is Pr-rich.

Figure 3a and 3b show the nonohmic behavior (E vs. J) of ZPC varistors sintered at 1300 and 1350 °C, respectively. More detailed concerning the E vs. J characteristic parameters, including the breakdown electric field (Eb), leakage current (I_L), nonlinear coefficient (α), are summarized in Table I. The amount of 0.5 mol% Pr₆O₁₁ is not appropriate for varistors properties. This can be associated with the presence of a small number of effective barriers in the grain boundaries, besides the grain growth, generating few barriers among electrodes. The nonohmic properties have a significant increase for x = 1.0, as the highest value of the breakdown electric field since this dopant level inhibits grain growth, as shown by SEM.

It can be observed from Table I, that the nonohmic behavior of 0.1 mol% Pr₆O₁₁ doped system sintered at 1300 °C is superior when compared with the sample with the same composition sintered at 1350 °C. This indicates, that the liquid phase is important for homogenizing dopants, but the electric properties of this phase is as well an important aspect. For instance, praseodymium has several oxidation states in the ZnO-based varistors, which can change dramatically the electrical behavior. The valence change is very sensitive to the applied process, as well as the Pr₆O₁₁ amount. This aspect should be carefully evaluated. In ZnO.Bi₂O₃-based varistor systems, changes also occur in both phases, the amorphous and crystalline phase of Bi₂O₃ during the processing or its useful life^{7, 20}. The varistors submitted to different stresses during use can modify its crystalline structure, which can cause the degradation process⁷.

Figures 4a and 4b show Nyquist complex impedance diagrams for ZPC system with 0.1 and 1 mol% of Pr₆O₁₁ and sintered at 1300 and 1350 °C respectively. The inset of the region shows high frequencies. Two semicircles are observed which indicate the presence of two time constants in the systems. The samples sintered at 1350 °C showed more conductivity, this result occurs according to the leakage current values. The semicircles arcs in the complex moved with the center displaced below the real axis, because of the presence of distributed elements and relaxation process resulting from the trapped states.

Figures 5a and 5b show Nyquist complex capacitance behavior of the ZPC system with different content of Pr₆O₁₁ and sintered at 1300 and 1350 °C, respectively. The capacitive complex diagrams for semiconductor polycrystalline devices are discussed in detail in reference²¹ and reviewed in reference²². The samples sintered at 1350 °C possess a larger grain size, and therefore exhibit a higher capacitance than samples sintered at 1300 °C. The decrease of the amount of the grain boundary, resulting from a faster grain growth at higher liquid sintering temperature, makes a contribution to the increase of the capacitance of the ZnO varistors, a capacitance of grain boundary barrier-layer type, which depends on the grain size²³. The accumulate defects in the grain boundary region can lead to this widening thus decreasing the grain boundary capacitance. The grain boundary capacitance is located in the transition region, while the electrical manifestation of the trapping states is evident in intermediate frequencies when the Mott-Schottky behavior is not satisfied. At low frequencies exists a conductive component (G/ ω) related to the grain boundary resistance. A good linear relationship can be seen between the (1/C – 1/2C₀)² vs. dc voltage (except the samples with x = 0.5 and sintered at 1350 °C) indicating that the barriers formed at the grain boundaries of highly dense ZnO are of Schottky nature.

The corresponding Mott-Schottky plot is shown in Figures 6a and 6b. The observed frequency, depending on capacitance in conjunction with the information on grain size, reflects the averaged Mott-Schottky response. The _B, Nd, N_IS and ω values calculated from this averaged Mott-Schottky response is displayed in Table 1. It is observed in this table that increasing the temperature from 1300 to 1350 °C, Nd decrease due to oxidation of Zn donor defects². Nd decrease is accompanied by an increase of interfacial states density N_IS and height barrier _B, as well as decrease of depletion layer ω. The values of barrier height _B are calculated based on the fact that all the grains are considered electroactive. The obtained values are small, indicating that not all the barriers are active. In ZnO-Bi₂O₃ based varistor the active potential barrier ranges from 15-35%^24-25 and SnO₂-based varistor 85% of the grain-to-grain junctions were active potential barriers^26-27. For the sample with x = 0.5, the increase in the capacitance with bias voltage can be related to degradation in the nonohmic properties involving a distortion of the Schottky barriers at intergrains. A change in the donor concentration profile would directly affect the conductivity and capacity of the samples.

4. Conclusions

The nonohmic and microstructural properties of the ZPC systems are very sensitive to Pr₆O₁₁ contents and the sintering temperature. The best behavior is obtained for the system with 0.1 mol% of Pr₆O₁₁, sintered at 1300 °C. However, samples doped with 0.5 mol% of Pr₆O₁₁ showed to be very conductive (specifically the sintered sample at 1350 °C which does not follow a Mott-Schottky behavior), this aspect is related to a lower number of electrically active barriers. According to DRX, doping with 1.0 mol% Pr₆O₁₁ generates secondary phases of Pr₂O₃ and the properties have been recovered, showing similar electric parameters like the samples doped with 0.1 mol% Pr₆O₁₁. SEM analyses showed that all compositions contain precipitates Pr-rich, which should be carefully studied, specifically regarding the oxidation state and their influence on the nonohmic properties. ZPC systems own a simpler microstructure than ZnO.Bi₂O₃ traditional varistors and a potential application for low-tension varistors. However new dopants should be used in order to increase the potential barrier and decrease the leakage current.

Acknowledgements

The authors gratefully acknowledge the financial support of the Brazilian agency FAPESP and the CYTED Program (Protect PI-VIII.13 PROALERTA).

Received: May 6, 2009;

Revised: December 7, 2009

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