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Projects / Programmes source: ARIS

Low-doped ZnO-based ceramics for energy varistors

Research activity

Code Science Field Subfield
2.09.01  Engineering sciences and technologies  Electronic components and technologies  Materials for electronic components 

Code Science Field
T153  Technological sciences  Ceramic materials and powders 
Keywords
ZnO, varistors, energy varistors, electrical characteristics, grain growth, microstructure, composition, processing
Evaluation (rules)
source: COBISS
Researchers (4)
no. Code Name and surname Research area Role Period No. of publicationsNo. of publications
1.  06627  PhD Slavko Bernik  Materials science and technology  Head  2007 - 2009  629 
2.  19029  PhD Nina Daneu  Materials science and technology  Researcher  2007 - 2009  428 
3.  27843  PhD Matejka Podlogar  Materials science and technology  Technical associate  2007 - 2009  287 
4.  10083  PhD Aleksander Rečnik  Chemistry  Researcher  2007 - 2009  651 
Organisations (1)
no. Code Research organisation City Registration number No. of publicationsNo. of publications
1.  0106  Jožef Stefan Institute  Ljubljana  5051606000  91,921 
Abstract
The aim of the proposed project is to develop low-doped ZnO-based varistor ceramics with low, medium or high breakdown voltage, for energy varistors. The scientific fundamentals of the proposal are laid by our recent discovery about the controlling influence of inversion boundaries (IBs) on the grain growth in ZnO ceramics: in the early stage of sintering IBs nucleate in some ZnO grains, which exaggeratedly and anisotropically grow at the expense of normal grains till they prevail in the microstructure. IBs-induced grain-growth mechanism enables tailoring of either a coarse- or fine-grained microstructure using very low amounts of dopant that triggers their formation and hence the preparation of varistor ceramics with desired breakdown voltage at significantly lower additions of varistor dopants to ZnO. The formation of IBs is triggered by Sb2O3 and TiO2, which are standard varistor dopants for grain-growth control. Addition of TiO2 enhances grain growth – ¬it role was explained only by the identification of IBs – and results in a coarse-grained microstructure of low voltage varistor ceramics; however, non-uniform grain growth and strong donor effect of TiO2 gives inferior nonlinear characteristics of varistor ceramics in comparison to the Sb2O3-doped. Sb2O3 doping results in fine-grained high-voltage varistor ceramics and inhibited grain growth is, according to a conventional understanding, a consequence of the Zn7Sb2O12 spinel phase formed at the grain boundaries, which reduces their mobility. Such an approach to the grain growth control requires large additions of Sb2O3 and the formed spinel phase bounds a significant share of oxides of Co, Mn and Ni, which therefore has to be added in large excess to an optimal amount needed for doping the ZnO phase. Due to these reasons, as well as the addition of typically 1 mol.% of Bi2O3, which is much in excess of the amount required to create non-linear ZnO-ZnO grain boundaries, about 10 wt.% of varistor dopants is added to the ZnO in standard varistor ceramics. This results in significant excess of secondary phases at the grain boundaries of ZnO, which aggravate the characteristics of varistor ceramics. We are expecting to make, based on the IBs-induced grain-growth mechanism with Sb2O3 addition, low, medium or high voltage varistor ceramics for energy varistors with less than 3 wt.% of varistor dopants. Its characteristics at high currents (energies) will be optimized by the addition of Al, which contributes to the conductivity of ZnO grains and stabilizes performance. The development of low-doped varistor ceramics for energy varistors is very important for domestic producers of varistors and surge protection devices (SPDs) and supports their aspirations on the standard production program as well as on the development of new products to penetrate the newly opening areas for SPDs application, power supply and information/telecommunication technologies. Reduced consumption of metal oxides, some of which are toxic, gives besides economic also ecological advantages to such technology. Results of the project will strongly contribute to strengthening the competitiveness of domestic producers on the world market of SPDs.
Significance for science
Grain-growth is among the key element in microstructure development of polycrystalline ceramic material and tailoring of their characteristics. The influence of grain size on the characteristics is especially straightforward in the ZnO-based varistor ceramics; its breakdown voltage, which results from the current-voltage nonlinearity of grain boundaries, is inverse proportional to the grain size. The results and findings obtained within the project, about the influence of inversion boundaries (IBs), add new fundamental knowledge to the intensively studied field of the grain growth and enhance it. The driving force for the grain growth is decrease in the surface energy via reduction of the specific surface of the grains and follows the mechanism of the Ostwald ripening. In the ZnO-based varistor ceramics, Bi2O3 and Sb2O3 are the key dopants that influence grain-growth. Bi2O3 influences grain growth by the formation of the Bi2O3-rich liquid phase at the grain boundaries. Sb2O3 is standard dopant in fine-grained varistor ceramics with high breakdown voltage. The inhibition of the grain growth is generally attributed to the reduced grain boundary mobility caused by Zener pinning effect of the spinel phase formed at the grain boundaries. Doping with Sb2O3 results also into the formation of inversion boundaries (IBs) in ZnO grain. Grain infected by IBs in the early stage of sintering grows exaggeratedly and anizotropically on the expense of normal grains till it impinge to the other IB containing grains, which prevail in the microstructure, and grain-growth is strongly slowed down. Growth of the grain is dictated by incorporation of Sb2O3 into the structure of planar fault, inversion boundary, which is, at temperature below the temperature of formation of the binary phase between ZnO and Sb2O3, thermodynamically more stable than the reactants. In the case of smaller number of grains infected by IB, they can grow larger before they impinge to each other, which results into coarse grained microstructure. In opposite case, when most of the grains are infected by IB, they can grow very little before they collide to each other, and the microstructure remains fine-grained. Fraction of the grains infected by IB can be influenced by the amount of IBs triggering dopant (Sb2O3, TiO2, SnO2) which enable tailoring of either coarse- or fine-grained microstructure. All these dopants result also in formation of the spinel phase and in accordance to the classical understanding of the grain growth, coarse grained microstructure would be impossible when they are added to ZnO. New fundamental knowledge on the grain growth under the influence of IBs reveals true nature of the grain-grow and microstructure development in ZnO ceramics when IBs are present. Based on the real understanding of actual mechanism that control growth of the grains we were able to be the first to prepare also coarse grained ZnO ceramics doped with Bi2O3 and Sb2O3. Exploiting IBs induced grain growth mechanism we succeeded also in preparation of low doped varistor ceramics, at additions of only 2.7 to 4 wt.% of varistor dopants to ZnO (standard additions are 8 to 10 wt.%), with broad range of grain sizes from 6 to 35 ?m and consequently corresponding breakdown voltages ranging from 350V/mm to 60V/mm, and with coefficient of nonlinearity in the range from 30 to50. Obtained results therefore in a very direct way show the significance of the fundamental research which only one can reveal a true nature of the processes and show the key elements which influence and control them. The fundamental knowledge we obtained in the ZnO-based ceramics is not limited to it, but has strong significance also for other polycrystalline ceramic materials where the presence of special grain boundaries such as inversion boundaries is observed.
Significance for the country
There are several producers of components and devices for overvoltage protection in Slovenia and the results obtained within the project provide significant support to their research and development plans and activities. Company VARSI is a producer of varistors which are used as active core of the surge protection devices (SPDs) for voltage protection against damaging electromagnetic surges caused by lightnings. Iskra Zaščite produces SPDs based on the varistors and also combination/spark gap. Hence, the development of SPDs is closely related to the development of varistors. Already now, both companies export most of their production which are based on the domestic knowledge, to the most demanding world markets and the competition in their fields is very tough. The obtained results of development of the low-doped varistor ceramics with low, medium and high breakdown voltages for energy varistors are of great importance for both producers. According to the current state of the art in the world low doped varistor ceramics represent a major breakthrough which opens possibilities for significant improvements of current roducts and to expand the use of varistors to new areas of obvervltage protection in energetic and information technologies. Processing of low-doped varistor ceramics is important also due to savings of raw materials, which contributes to a reduction of production costs and gives an advantage over competitors. A reduced consumption of metal oxides, some of which are toxic, also gives ecological advantages to such a technology. Results of the project will strongly contribute to strengthening the competitiveness of domestic producers on the world market of SPDs. The project represents a continuation in the collaboration between partners from industry and the institute and will contribute to a further flow of knowledge, experience and expertise among partners, which is very important in areas of electronic materials and components, new materials and related technologies.
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