Microstructural and electrical properties of varistors ... - Springer Link

J Mater Sci: Mater Electron (2010) 21:571–577 DOI 10.1007/s10854-009-9959-3

Microstructural and electrical properties of varistors prepared from coated ZnO nanopowders Seyyed Ali Shojaee Æ Mohammad Maleki Shahraki Æ Mohammad Ali Faghihi Sani Æ Ali Nemati Æ Abbas Yousefi

Received: 29 May 2009 / Accepted: 4 August 2009 / Published online: 1 September 2009 Ó Springer Science+Business Media, LLC 2009

Abstract This paper describes a solution-based technique for fabrication of varistor grade composite nanopowders. The method consists of coating major varistor dopants on the surface of the ZnO nanoparticles. As a result, a homogenous mixture of dopants and ZnO nanoparticles will be achieved. TEM results indicated that a composite layer of dopants with the average particle size of 9 nm on the surface of ZnO nanoparticles has been successfully prepared. Sintering of the coated powders was performed in temperatures as low as 850 °C and final specimens with average particle size of 900 nm and density of 98.5% were achieved. In comparison to conventional mixing, varistors prepared from coated nanopowders exhibited superior electrical properties and microstructure homogeneity. The improvement of electrical properties can be attributed to small grain size, homogenous distribution of dopants and elimination of large Bi-Pockets. In addition, the processing route of schottky barrier formation is quite different from what is generally considered as the method of barrier formation in ZnO grain boundaries.

1 Introduction Zinc oxide nanoparticles have been excessively used in cosmetics, rubbers, varistors, and catalysts [1]. Varistors S. A. Shojaee M. Maleki Shahraki M. A. Faghihi Sani (&) A. Nemati Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave, P.O Box: 11365-9466, Tehran, Iran e-mail: [email protected] A. Yousefi Par-e-Tavoos Institute, P.O Box: 91375-5395, Mashhad, Iran

prepared from ZnO nanoparticles as the main component exhibit excellent properties [2–4]. Zinc Oxide varistor is a polycrystalline ceramic, which has become technologically important because of their highly nonlinear electrical characteristics enabling them to be used as reversible, solidstate switches with large-energy-handling capabilities [5]. Nevertheless, to achieve the superior electrical properties, ZnO must be doped with Bi2O3 and other transition or rare earth metal oxides. Electrical properties of varistors result from the formation of schottky barrier in the grain boundaries of the zinc oxide grains [6]. It has been discovered that precipitation of minute amount of dopants in the grain boundaries can create higher barriers and accordingly, enhance the breakdown voltage and non-linear properties of varistors. However, the exact condition of dopants and the grain boundaries are not completely obvious [7]. It is believed that during cooling from sintering temperatures, due to an increase in wetting angle, liquid phase slowly contracts from the boundaries and leaves a trace of dopants, necessary for schottky barrier formation. Regarding the fact that almost all dopants are present in concentrations smaller than 1%, conventional mixing method can hardly lead to homogenous composite powders, which is necessary for the occurrence of superior electrical properties. This concern becomes more serious if using the nanopowders as the starting materials. Recently, some chemical methods have been introduced to overcome the homogeneity challenge in varistors like coprecipitation [8], microemulsion [9], polymerized complex method [10], and sol–gel [11]. However, all of these methods have their own disadvantages. Coprecipitation and sol–gel are strongly influenced by the pH values and there is a risk of variation of powder composition. Microemulsion is not suitable for large-scale industrial manufacturing and polymerized complex method is complicated and costly.

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J Mater Sci: Mater Electron (2010) 21:571–577

Considering the fact that dopants are mainly needed in the grain boundaries of zinc oxide grains, coating the zinc oxide powders with a layer of dopants looks like a promising way to achieve high homogeneity and enhanced electrical properties. This method has led to high level of homogeneity in many materials for structural [12] and functional [13] properties, both in the form of powders and bulk devices. Previous studies have utilized a composition consisting of zinc, bismuth, cobalt, chromium and manganese oxides for studying the coating process of powders [14]; however, they prepared the mixture of zinc, cobalt and manganese oxide via coprecipitation, then coated bismuth oxide on the surface of the prepared powders and improved the electrical and microstructural properties of varistors. Others [15–17] have effectively utilized coating of all dopants on micron and sub-micron ZnO powders of varistors. In this study, we aimed at preparing a homogenous layer of dopants on the surface of ZnO nanoparticles through a precipitation from an aqueous solution and then study the microstructural and electrical properties of prepared varistors. Fig. 1 TEM image of the starting ZnO nanopowders. The scale bar is 50 nm

2 Experimental procedure ZnO nanopowders (Nanofaravaran Catalyst, Iran) with the average particle size 100 nm (Fig. 1, TEM, EM208 Philips) have been used as the main ingredient. Reagent grade of bismuth nitrate (Sigma-Aldrich), cobalt acetate (SigmaAldrich), chromium nitrate (Acros), and manganese acetate (Acros) has been used as the starting materials. The varistor formulation is presented in Table 1. In the first step, an aqueous solution of all metal salts has been prepared in a magnetic stirrer. In the case of bismuth nitrate, a few drops of nitric acid were added to complete the solution of bismuth nitrate in water. Then dispersed ZnO nanopowders have been added to the batch. The batch was heated up to 60 °C and stirred continuously until it changed to slurry. The slurry was dried in 90 °C for 16 h in oven. The resulting powder was crushed using pestle and mortar and then was calcined for 3 h in 650 °C. In addition, samples with the same composition were also prepared by conventional mixing method. The starting materials were ZnO nanopowders with Bismuth Oxide (100 nm, 99.9%, Sigma Aldrich), Chromium Oxide (50 nm, 99% Aldrich), Manganese Oxide (99.99%, Aldrich), and Cobalt Oxide (50 nm, 99.8%, Aldrich). The powders were mixed for 1 h in Jar mill with zirconia balls. The powder mixtures were then granulated with aqueous solution of 5% PVA. Green bodies of varistors were

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Table 1 Chemical composition of the prepared varistors Compounds

ZnO

Bi2O3

Cr2O3

MnO

Co3O4

Mole percent

98

0.5

0.5

0.5

0.5

prepared by pressing the powders under the pressure of 200 Mpa. The samples were then preheated at 600 °C for 3 h. Sintering was performed in air at various temperatures for 20 h. Sample from conventional mixing method was sintered at 900 °C, since in lower temperature it did not achieve high levels of density. The accomplishment of the coating process was evaluated with TEM (LEO-912AB). The thermal decomposition of salts was studied with DTA/TG (PL 1640) in air, with heating rate of 5 °C/min. The microstructure and phase composition of the prepared varistors were studied with SEM (VegaÓTscan). The density of sintered samples was measured by Archimedes method. For DC current–voltage characterization, silver paste was painted on both surfaces of the discs and then samples were fired at 670 °C in air. Electrical properties were measured with Automated 5 kV Insulation Tester (Megger BM25). I–V characteristics were calculated by taking current and voltage readings. The breakdown voltages of the samples were measured at V1 mA=cm2 . The varistor


voltages at 1 and 10 mA/cm2 were measured and nonlinear coefficients were determined by the following equation: a¼ log

V

1

10 mA=cm2

V1 mA=cm2

3 Results and discussions Figure 2 shows the DTA/TG curve of the prepared coated powders. As it can be seen, the weight loss will reach its ending at temperatures around 500 °C. Therefore, 650 °C is a suitable temperature for complete thermal decomposition. The peaks in the DTA curves are in good agreement with the previous data on the decomposition of bismuth nitrate [18], manganese acetate [19], chromium nitrate [20], and cobalt acetate [21], as shown in Fig. 2. Figure 3 represents the TEM images of the coated nanopowders after calcination. In comparison with Fig. 1, it is obvious that the surfaces of ZnO nanopowders are suitably covered with a layer of dopants. The mechanism of the coating process lies in the presence of minute amounts of nitric acid in the batch. As it has been stated previously [14], the coating process has originated from a hydrolysis reaction. Dissolution of ZnO consumes HNO3, resulting in localized high pH value area near the surface of nanopowders. This phenomenon will lead to hydrolysis and precipitation of hydrated Bi and other dopants on the surface of nanopowders. Figure 4 shows the particle size distribution of synthesized dopants, derived from the TEM images. As it can be seen, the synthesized particles are in the range of 5–12 nm, with an average size of about 9 nm. In comparison with the previous results [15], the synthesized powders are much smaller. This can be attributed to utilization of nanopowders with higher available surface, more nucleuses of dopants and therefore finer particles. Figure 5 represents the SEM image and EDX analysis of the triple point junctions of the sintered samples at 850 °C

Fig. 2 DTA/TG analysis of powders after drying at 90 °C

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for soaking time of 20 h. As it can be seen from backscattered SEM image, at least two different phases are present in the sample after sintering. The most abundant phase is ZnO, which appears like rounded grains throughout the bulk of the solid matrix. The white phase in the triple point junctions is liquid phase. As it can be seen from EDX analysis, liquid phase is highly rich of Bi, with small amount of Co, Cr and Mn. The homogenous microstructure is a result of initial homogenous distribution of dopants through the powder coating process. Figure 6 represents the SEM images of the sintered samples at various temperatures for 20 h and variation of average grain size and density with sintering temperature are presented in Fig. 7. As it can be seen, all SEM images of samples prepared from coated nanopowders exhibit high levels of homogeneity in the microstructure, which is a result of initial homogenous distribution of dopants. Besides, there is no sign of large-Bi pocket phases in the microstructure of these samples. Although the average grain size of the sintered sample at 750 °C is very small, its relative density is low. This can be attributed to small amount of liquid phase in this temperature (which is almost near the eutectic temperature in ZnO–Bi2O3 binary system [22]) and low dissolution of ZnO in liquid phase (Fig. 6a). As it can be seen in Fig. 6b, liquid phase has been formed and contracted to the triple point junctions at 800 °C. Higher amount of the formed liquid phase at this sintering temperature as well as its lower viscosity led to an increase in densification through particle rearrangement [10] and SDP (solution-diffusion-precipitation) processes. Also the grain growth is not considerable at this sintering temperature. This can also be inferred from Fig. 7. Wetting behavior of liquid phase is found to be temperature dependent. Around 800 °C, wetting angle of liquid phase and ZnO grain interface is about 60° [23]. It has also been found that higher surrounding of ZnO grains with liquid phase will increase the grain growth [24]. So, the wetting behavior can be considered as the key factor for the simultaneous presence of small grain size and high densities. The utilized coating process can affect the grain size by assuring the uniform distribution of liquid phase and elimination of large liquid phase localized areas. As it is obvious from Fig. 7, the sample sintered at 850 °C (Fig. 5) has the highest density without considerable grain growth. All the previous discussions for samples sintered at 800 °C are also valid here. In addition, lower viscosity of liquid phase has led to higher densities. Large grain size in sintered sample at 900 °C (Fig. 6c) can be related to formation of higher amount of liquid phase as well as higher solution of ZnO in the liquid phase. Figure 6d shows the SEM images of samples prepared from conventional mixing powders. In temperature as high

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Fig. 3 TEM image of ZnO nanopowders coated with dopants after calcination

Fig. 4 Particle size distribution of synthesized dopants on the surface of ZnO nanopowders

as 900 °C, the density of the sintered bodies was only 95% and in lower temperatures the density was lower than 90%. Due to van der waals forces, the appropriate mixing of nanopowders cannot be achieved via conventional mixing method [25]. Thus, only limited number of ZnO grains will be in direct contact with liquid phase during sintering. Also, as it can be seen in the SEM images (Fig. 6d), the grain size distribution is extremely un-uniform and large

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Bi-rich pockets are present in the microstructure of the sintered bodies. I–V diagrams of varistors prepared from conventional and coating techniques are presented in Fig. 8 and the summary of electrical properties is exhibited in Table 2. It is clear that there is a dramatic increase in the electrical properties of varistors prepared from coated nanopowders. All the major electrical properties, breakdown voltage, nonlinear coefficient and leakage current have been improved. The only exception is the samples sintered at 750 °C. As it has been mentioned before [22] the liquid phase will not form in the binary system of ZnO–Bi2O3 until 740 °C. So, one can conclude that in this sintering temperature liquid phase has not been formed yet and bismuth diffusion to the grain boundaries has not been fully achieved and thus suitable schottky barriers have not been formed. As a result, the schottky barrier has not been formed properly in the grain boundaries, which can be considered as the reason for the poor electrical properties of this sample. Considering the mentioned microstructural properties, three main factors can lead to superior electrical properties of varistors prepared from coated nanopowders. The first


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Fig. 5 SEM image and EDX analysis of triple point junctions for samples sintered at 850 °C

Fig. 6 SEM image of samples prepared from coated nanopowders sintered at a 750 °C b 800 °C c 900 °C and d SEM image of sample prepared from conventional mixing method sintered at 900 °C

reason is elimination of large Bi-Pockets. Bi-Pockets can degrade electrical properties of varistors because of their electrical conductivity that offers easy paths for electrical current and leads to fast thermal runaway of varistors [26].

The elimination of large Bi-pockets highly enhances the breakdown voltage and leakage current of varistors. Another factor is the homogenous distribution of dopants in the microstructure. As a result, the total area of

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Fig. 7 The variation of density and grain size as a function of temperature for samples prepared from coated ZnO nanopowders

Fig. 8 Electrical properties of samples from powder-coating and conventional mixing method sintered at different temperatures Table 2 Summary of electrical properties of prepared varistors a

VB (V/cm)

4 Conclusion

IL (lA)

Conventional

12.9

2600

300

Solution-coating-750 Solution-coating-800

8 30.2

19800 15400

400 75

Solution-coating-850

30.6

15800

77

Solution-coating-900

30.4

7200

120

effective boundaries will increase, resulting in higher VB and a. In addition, since most grain boundaries possess similar concentration of dopants, their breakdown voltage will have a narrow distribution and thus, they break in similar voltages. Finally, the fine grain size of the prepared varistors can lead to higher breakdown voltage, nonlinear coefficient and leakage current. Reduced grain size means more boundaries in a constant volume. As a result, it offers more barriers for current which leads to improved breakdown voltage and lower leakage current. It has been stated previously [27] that smaller grain size will increase the nonlinear coefficient. Since, oxygen diffusion through Bi-rich phase is essential for formation of barriers with higher heights, smaller grain size offers more available diffusion

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paths and as a result, enhanced barriers with higher nonlinearities will form in the boundaries. The formation of schottky barrier may be a little different from what is generally considered as the main mechanism of barrier formation in varistors during sintering [5, 7, 23, 24]. It is believed that in temperatures higher than 1100 °C, the liquid phase will completely wet the ZnO grain boundaries [23]. Then, during cooling and contraction of liquid phase due to the increase in wetting angle, a trace of dopants either in form of separate atoms or layers [7] will cover the grain boundaries, leading to formation of schottky barrier. In this case, since the temperature is lower than 850 °C, the wetting angle is not sufficient to wet all the boundaries. However since dopants are already present in the grain boundaries of ZnO, the schottky barriers still occur in the boundaries. Here, as the liquid phase contract to triple points, it leaves a trace of dopants in grain boundaries. Therefore, although the liquid phase does not wet the boundaries, the barriers will form in the ZnO–ZnO boundaries. Higher sintering temperature (900 °C) has led to lower breakdown voltage and higher leakage current in samples prepared from coated powders. This decline results from larger ZnO grain size. Also higher temperatures will lead to lower wetting angle and thus reduces the total direct contacts between ZnO grains.

In this study, a solution based technique for synthesis of dopants on the surface of nanopowders has been studied. TEM studies confirm the success of the method to prepare a layer of dopants in the range of 5–12 nm. SEM studies revealed that this method would lead to a uniform microstructure of ZnO varistors. The microstructure of varistors prepared from solution-coating method showed high uniformity and fine grain size, resulted from the homogenous distribution of dopants. Electrical properties of these varistors were drastically higher than conventional varistors. Elimination of large Bi-pocket, homogenous supply of dopants to all boundaries and reduced grain size were the major reasons for enhancement of electrical properties. In addition, possible route for schottky barrier formation during sintering were also proposed.

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