Electrical and Microstructural Properties of Varistors Based on Nanostructured Tetra-Needle Like Zinc Oxide Powders

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<ul><li><p>Electrical and Microstructural Properties of Varistors Based on Nanostructured Tetra-Needle Like Zinc Oxide Powders </p><p>Felipe Antonio Lucca Snchez1,a, Diego Pereira Tarrag2,b, A.S. Takimi1,c, V.C. Sousa2,d and C.P. Bergmann1,e 1 UFRGS Laboratrio de Materiais Cermicos. </p><p>Av. Osvaldo Aranha, 99 sala 705c, Porto Alegre - RS Brazil. CEP: 90035-190 2 UFRGS - Laboratrio de Biomateriais e Cermicas Avanadas. </p><p>Av. Bento Gonalves, 9500, Setor 4, Prdio 74, sala 118, Porto Alegre - RS Brazil. CEP: 91509-900 </p><p>a felipe.lucsan@gmail.com, b dptarrago@gmail.com, c astakimi@ufrgs.br, d vania.sousa@ufrgs.br and e bergmann@ufrgs.br </p><p>Keywords: ZnO varistor, nanostructured ZnO, tetra-needle like </p><p>Abstract. It is well known that nanostructured materials have relevant influences in properties </p><p>behavior that can be achieved when compared with conventional materials. In this study is proposed </p><p>an investigation of the electrical and microstructural properties of zinc oxide based varistors </p><p>prepared with nanostructured zinc oxide powder obtained by a thermal evaporation process. Zinc </p><p>oxide powder morphology was investigated by scanning and transmission electron microscopy </p><p>(SEM and TEM, respectively) and the specific surface area evaluated by adsorption of N2. The </p><p>varistors were prepared by the mixture of typical dopants with zinc oxide powders in a ball mill. </p><p>The surface area of zinc oxide powder used was 17.4 m2/g with tetra-needle like morphology. After </p><p>powder mixture process it was observed by TEM micrographs that most of the tetrapod shaped zinc </p><p>oxide broke into needles well mixed with dopant particles. The compressed powders were sintering </p><p>at 1050, 1150 and 1250C for 1.5 h and densification over 94% were achieved in all tested </p><p>temperatures. Preliminary electrical characterization reveals that nanostructured zinc oxide </p><p>compositions have interesting varistor properties. </p><p>Introduction </p><p>Ceramic materials based on ZnO can be doped in order to induce a highly non-ohmic electrical </p><p>behavior, allowing their application as varistors in a great range of electric and electronic devices </p><p>[1]. This electrical characteristic is intimately related to a specific microstructure formation in </p><p>which a liquid phase is formed during sintering that, after cooling, is insulating and tends to remain </p><p>in the grain boundaries [2]. The grain size of nanostructured ZnO plays an important role in its </p><p>electrical conductivity, since this property is higher in the grain boundaries, interfering then in the </p><p>electrical properties of varistor based on such oxide [3,4]. As nanostructured materials can have </p><p>very distinct characteristics of those microstructured [5], this work intends to analyze the </p><p>microstructure and the electrical properties of varistors made with a nanostructured tetra-needle like </p><p>ZnO powder, obtained by an adapted thermal evaporation process. A commercial grade of a </p><p>microstructured polyhedral shaped ZnO powder was used to obtain the same varistor composition </p><p>for comparison. </p><p>Materials and Methods </p><p>It this work a nanostructured ZnO powder was obtained by an adaptation of a thermal evaporation </p><p>process [6-9]. Thanks to a innovative design which allows a precise control of temperature and gas </p><p>composition in the reactive atmosphere, metallic zinc granulates, as raw material, were evaporated </p><p>and driven to a controlled oxidation zone in where a nanostructured ZnO powder was produced. </p><p>Materials Science Forum Vols. 727-728 (2012) pp 533-538Online available since 2012/Aug/24 at www.scientific.net (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.727-728.533</p><p>All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID:, University of North Carolina at Chapel Hill, Chapel Hill, USA-30/11/14,14:20:02)</p><p>http://www.scientific.nethttp://www.ttp.net</p></li><li><p>The specific surface area of both powders was measured by B.E.T. technique (Quantachrome </p><p>Instrument - Nova 1000) using N2. Transmission electron microscopy (TEM) was performed in a </p><p>Jeol JEM 2010 and the sample was prepared by dispersing the powder in isopropanol in a ultrasonic </p><p>bath (40kHz) for 15 min. Also scanning electron microscopy (SEM) was used to analyze the </p><p>starting powder morphology in a SEM - Jeol JEM 6060. </p><p>The ZnO varistors were prepared using both ZnO powders: the nanostructured and the commercial </p><p>grade (denominated Var Nano and Var Com, respectively) and metal oxides as dopants in a molar </p><p>proportion of 92.5% ZnO, 3% Bi2O3, 3% Sb2O3, 0.5% CoO, 0.5%, MnO and 0.5% Cr2O3. These </p><p>formulations were mixed in a ball mill with isopropanol for 4 h. PVB (10% in isopropanol) was </p><p>used as binder only in the Var Com composition since the samples in its first batch laminated. The </p><p>powder mixture was sieved (mesh # 200) and pressed under 140 MPa in discs (12 mm diameter) </p><p>and sintered at 1050, 1150 and 1250C for 1.5 h under a heating and cooling rate of 5C/min. The </p><p>microstructure of the sintered samples was revealed by thermal etching with annealing polished disc </p><p>in a furnace at about 100C below the sintering temperature for 20 min. The morphology and and </p><p>the local chemical composition, in the grains cores and boundaries, was investigated using energy </p><p>dispersive spectroscopy (EDS) analyzers coupled with SEM (Jeol JEM 5800). </p><p>For the electrical characterization both faces of the samples were coated with silver paste and ohmic </p><p>contacts were formed by a heating treatment at 400 C for 15 min. Electric fieldcurrent density (E</p><p>J) characteristic of varistors was measured at room temperature using a Keithley 237 unit. The </p><p>breakdown electric field (Eb) was then determined for a current density of 1mA/cm2 and the leakage </p><p>current density (JL) was defined as the current density at 0.8 Eb. The nonlinear coefficient () was </p><p>calculated by a linear regression of E-J curve on a logarithmic scale starting from 1mA/cm2 </p><p>[2, 10]. </p><p>Results and discussion </p><p>With the procedure used was possible to obtain a nano powder with a predominant morphology of </p><p>tetra-needle with submicron size core and needles with a few nanometers of diameter, as is shown </p><p>in the micrographs of Fig. 1a. For comparison a commercial grade of a ZnO powder (LABSYNTH) </p><p>was used and Fig. 1b shows a microstructured polyhedral particle shape. </p><p> Fig. 1: Scanning electron micrographs of (a) nanostructured tetra-needle shaped ZnO and (b) </p><p>commercial polyhedral shaped ZnO. </p><p>After the ball milling process most of the tetrapods of the ZnO powder broke into needles and </p><p>mixed with the dopant particles, as seen in the TEM micrograph in Fig. 2. </p><p>The specific surface area of nanostructured and commercial ZnO grades were 17.4 m/g and 6.5 </p><p>m/g respectively. After mixture the specific surface area of Var Nano composition decreased to </p><p>15.8 m/g which might be related to the presence of dopants with larger particles size. </p><p>b a </p><p>534 Advanced Powder Technology VIII</p></li><li><p> Fig. 2: TEM micrograph of Var Nano composition after ball mill process. </p><p>The curves obtained in electrical characterization are shown in Fig. 3, where it can be observed that </p><p>all samples showed varistor properties and those sintered at 1150C had a more sudden transition of </p><p>the insulating-conductor behavior. </p><p>0</p><p>500</p><p>1000</p><p>1500</p><p>2000</p><p>2500</p><p>3000</p><p>3500</p><p>4000</p><p>4500</p><p>5000</p><p>5500</p><p>6000</p><p>6500</p><p>0 5 10 15 20 25 30</p><p>J [mA/cm2]</p><p>E [</p><p>V/c</p><p>m]</p><p>Var Nano 1050C</p><p>Var Nano 1150C</p><p>Var Nano 1250C</p><p>Var Com 1050C</p><p>Var Com 1150C</p><p>Var Com 1250C</p><p> Fig. 3: E J characterization of different ZnO varistor composition tested. </p><p>According to results of Tab. 1 the samples burned at 1050C were those that supported higher </p><p>electrical fields and the Var Com composition had the higher Eb value. However, the leakage </p><p>current was smaller in samples sintered at 1150C and the non-linearity coefficient was </p><p>considerably better in these samples. When the sintering temperature was increased to 1250C the </p><p>samples still had non-ohmic behavior, nevertheless the varistor properties drastically degraded. </p><p>From technological characterization it was observed that the higher densification was achieved in </p><p>Var Nano and Var Com samples sintered at 1150C also the lower apparent porosity was registered </p><p>for these samples. Varistors sintered at 1250C had the lower densification attributed to bismuth </p><p>and antimony oxides evaporation and by the increase of the grain size. These oxides have a large </p><p>Materials Science Forum Vols. 727-728 535</p></li><li><p>vapor pressure which leads to some evaporation during sintering. Additional gradients in the </p><p>components with higher concentrations of volatile species in the interior of the parts can be </p><p>produced by this evaporation promoting an increase in the porosity [11-13]. Summarizing, the best </p><p>electrical properties were achieved in samples with higher sintering densification and consequently </p><p>lower apparent porosity that are samples sintered at 1150C. At sintering temperature of 1250C the </p><p>evaporation of volatiles caused an increase in the open porosity and demoted the electrical </p><p>properties. </p><p>Table 1: Results of electrical and technological characterization of ZnO varistors sintered at </p><p>different temperature </p><p>Sample </p><p>Eb - Breakdown </p><p>electrical </p><p>field [V/cm] </p><p>JL Leakage </p><p>current </p><p>[mA/cm] </p><p>(nonlinearity </p><p>coefficient) </p><p>Sintered </p><p>densification </p><p>[%] </p><p>Apparent </p><p>porosity </p><p> [%] </p><p>Var Nano 1050C 3749 0,29 0,01 9,35 0,21 97,18 1,40 1,04 0,13 </p><p>Var Nano 1150C 3633 0,11 0,06 19,61 1,4 97,42 1,01 0,70 0,48 </p><p>Var Nano 1250C 1783 0,39 0,08 6,38 0,89 94,23 0,26 1,41 0,91 </p><p>Var Com 1050C 4567 0,28 0,02 9,18 0,4 96,64 0,86 1,06 0,75 </p><p>Var Com 1150C 3867 0,15 0,03 18,58 1,27 98,66 0,39 0,82 0,33 </p><p>Var Com 1250C 1767 0,38 0,02 7,66 1,48 96,08 0,75 1,23 0,28 </p><p>The microstructural analysis of both varistors compositions revealed a tendency to increase the ZnO </p><p>grain size as the sintering temperature increased, as shown in Fig. 4. For samples burned at 1150C </p><p>which were considered with the best electrical properties it was found that a composition made with </p><p>nanostructured ZnO powder shown a smaller grain size than varistors prepared with the commercial </p><p>powder. By EDS analysis it was possible to see a greater presence of a second phase rich in </p><p>bismuth, antimony and zinc formed at the grain boundaries of the sample Var Nano 1150C, </p><p>confirming the results presented in Tab. 1. </p><p>Although the microstructure of Var Nano 1150C seem better formed, with secondary phases </p><p>distributed along the grain boudaries, it did not reflect in the electrical properties, when compared to </p><p>the Var Com 1150C composition. This fact suggests that the showed micrograph (Fig. 4b) might </p><p>not be representative of the overall microstructure. </p><p>536 Advanced Powder Technology VIII</p></li><li><p>Fig. 4: SEM micrographs of Var Nano composition sintered at (a) 1050C, (b) 1150C and (c) </p><p>1250C and Var Com composition sintered at (d) 1050C, (e) 1150C and (f) 1250C. Below each </p><p>micrographs the respective EDS analysis with a yellow arrow indicating the analyzed point. </p><p>Conclusions </p><p>The use of nanostructured ZnO powders had a very discrete influence over electrical properties of </p><p>the studied varistor compositions. The best electrical properties were achieved in samples with </p><p>higher sintering densification and consequently lower apparent porosity that are samples sintered at </p><p>1150C. The microstructure of Var Nano 1150C seem to have a better formation when compared </p><p>to the Var Com 1150C, however the electrical results were not influenced by the ZnO used, </p><p>suggesting some microstructural heterogeneity. Although, it was observed that the use of </p><p>nanostructured powder facilitated processing without the need to use any kind of additive. Also </p><p>better control in the grain growth and morphology of the grains was noticed when the </p><p>nanostructured ZnO powder was used. </p><p>Acknowledgement </p><p>The authors would like to thanks to CNPq (National Counsel of Technological and Scientific </p><p>Development) for the financial support. </p><p>5m </p><p>b a </p><p>5m </p><p>5m </p><p>f </p><p>5m </p><p>d </p><p>5m </p><p>e </p><p>5m </p><p>c </p><p>Materials Science Forum Vols. 727-728 537</p></li><li><p>References </p><p>[1] M. Matsuoka: Jap. J. of App. Phys. Vol. 10 (1971), p. 736-746 </p><p>[2] V. 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