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Projects / Programmes source: ARIS

Materials and technologies for applications of ZnO-based thick-film varistors and oxide thermoelectrics

Research activity

Code Science Field Subfield
2.09.00  Engineering sciences and technologies  Electronic components and technologies   

Code Science Field
T150  Technological sciences  Material technology 

Code Science Field
2.05  Engineering and Technology  Materials engineering 
Keywords
ZnO, varistor ceramics, thick film varistors, oxide thermoelectric materials, thermoelectric modules, screen printing, tape casting, characterisation
Evaluation (rules)
source: COBISS
Researchers (13)
no. Code Name and surname Research area Role Period No. of publicationsNo. of publications
1.  06627  PhD Slavko Bernik  Materials science and technology  Head  2011 - 2014  629 
2.  04329  Mirjam Cergolj  Materials science and technology  Researcher  2011 - 2014  64 
3.  19029  PhD Nina Daneu  Materials science and technology  Researcher  2011 - 2014  428 
4.  32783  PhD Sandra Gardonio  Materials science and technology  Researcher  2011 - 2014  82 
5.  06963  MSc Andrej Pirih  Telecommunications  Researcher  2011 - 2014  45 
6.  27843  PhD Matejka Podlogar  Materials science and technology  Technical associate  2011 - 2014  287 
7.  31815  PhD Mojca Presečnik  Materials science and technology  Junior researcher  2011 - 2014  26 
8.  10083  PhD Aleksander Rečnik  Chemistry  Researcher  2011 - 2014  651 
9.  30149  Saša Rustja-Kosec  Materials science and technology  Researcher  2011 - 2014  15 
10.  15597  PhD Zoran Samardžija  Materials science and technology  Researcher  2011 - 2014  583 
11.  10618  Željko Šket  Materials science and technology  Researcher  2011 - 2014 
12.  05652  MSc Alojzij Tavčar  Materials science and technology  Researcher  2011 - 2014  70 
13.  11991  PhD Matjaž Valant  Materials science and technology  Researcher  2011 - 2014  611 
Organisations (2)
no. Code Research organisation City Registration number No. of publicationsNo. of publications
1.  0106  Jožef Stefan Institute  Ljubljana  5051606000  91,921 
2.  1540  University of Nova Gorica  Nova Gorica  5920884000  14,355 
Abstract
The objective of the proposed project is the development of materials, devices and technologies for applications involving ZnO-based thick-film varistors (TFV) and oxide TE materials (OTE). While the focus will be on the preparation of thick-film and LTCC ZnO-Bi2O3-based varistor ceramics by screen-printing, other technologies such as tape casting and direct-writing fabrication will be developed as well, in accordance with the specific demands of the application. The TE modules based on the improved n- and p-type oxide TE materials will be developed either in the thick-film technology by screen printing on substrates or in multilayer technology by lamination of the tapes made by tape casting. The development of the thick-film varistors is driven by the increasing needs for the integration of overvoltage protection into hybrid circuits and for possible new elements, highly integrated with other active/passive elements, for which bulk varistors are not suitable. The current state-of-the-art is much below the required characteristics. The development of thermoelectric (TE) modules is motivated by their potential in harvesting waste heat which is widely dispersed and its sources amounts strongly vary. TE systems offer the only viable method of overcoming these problems by converting heat energy directly into electrical energy irrespective of the source size and without the use of moving parts or the production of environmentally deleterious wastes. Oxide TE materials have the potential for a high conversion efficiency and are, in comparison to classical thermoelectric, nontoxic, composed of abundantly available elements, highly chemically stable in air, even at temperatures of 800oC. The development of thick-film varistors will be based on our previous results on the preparation of low-doped varistor ceramics. We found that inversion boundaries (IBs) in the ZnO grains, which are triggered by dopants like Sb2O3, TiO2, SnO2, In2O3 or Ga2O3, play a key role in the microstructure development of the varistor ceramics. Using the IBs-induced grain-growth mechanism we can prepare varistor ceramics with a broad range of breakdown voltages from 70V/mm up to 350V/mm and high nonlinearity for the addition of only 3 to 4 wt.% of varistor dopants to ZnO. Based on the varistor compositions with such low amounts of dopants and with the use of pre-reacted, highly doped ZnO with a high conductivity, we will be able to prepare high-quality thick films by firing at temperatures above 1000oC. At lower firing temperatures such compositions give us additional freedom to add Bi2O3-based frit for improved densification and connectivity among the ZnO grains. Our goal is to develop thick-film varistor inks for the preparation of high-quality thick-film varistors by firing in the temperature range between 750oC and 950oC. The realisation of thermoelectric modules for harvesting waste energy requires n-type and p-tape thermoelectric materials, combined in the device. ZnO-based ceramics doped with the same dopants, which are used for tailoring the current-voltage characteristics of the ZnO-based varistor ceramics, has high potential as n-type TE material. The TE properties will be enhanced by doping with selected dopants for optical electrical conductivity, which also enable tailoring of the structural defects (single or multiple inversion boundaries) that result in phonon scattering and reduced thermal conductivity in the ZnO-based ceramics. As the p-type TE materials, selected compounds in the M-Co-O system with a layered structure, which showed so far the most promising characteristics, will be developed. While the proposed project will bring new fundamental knowledge, its significant values are also in the application potential of the obtained results for the development of new products and technologies at the Slovenian producers of electronic components and devices, which will also participate in the project and financially support it.
Significance for science
In the project we developed materials, processing technologies and devices, which would enable the integration of overvoltage protection based on thick-film varistors into hybrid circuits and the exploitation of the thermoelectric (TE) effect for harvesting the waste heat. For integration into hybrid circuits the varistors have to be realised in thick-film technology in a way that they are compatible with other components. The project also focused on the oxide TE ceramics and technologies for making the TE modules. Waste heat equal to about 70% of the total primary energy and TE systems offer the only viable method for its harvesting directly into electrical energy. Oxide TE materials have the potential for a high conversion efficiency and are, in comparison to classic thermoelectrics, nontoxic, composed of abundantly available elements and highly stable in air up to about 1000oC. The results of the project contribute to a comprehensive understanding of the synthesis of compounds with the desired characteristics and the influence of temperature, sintering method and dopants on tailoring their microstructure, electrical and thermal characteristics. The development of ZnO-based thick-film varistors fired at temperatures below 1000oC (varistor ceramics are usually sintered at temperatures around 1200oC for several hours) was based on low-doped varistor ceramics with the addition of only 3–4 wt% of varistor dopants to ZnO (standard additions are 7–10 wt.%), which was possible only because of our understanding of the key influence of the Sb2O3-triggered inversion boundaries (IBs) on the grain growth and the microstructure development in the ZnO-based ceramics. New knowledge and understanding about the nucleation of IBs trigged by TiO2 and, consequently, the development of a coarse-grained microstructure gives new possibilities for the development of thick-film varistors with better control of their break-down voltage, and also for the development of bulk, low-voltage varistors. Understanding the influence of composition, heat treatment regime and selected dopants like the oxides of W and Mo on the microstructure development in varistor ceramics at low sintering temperatures is important for the successful preparation of thick-film varistors and also enables the preparation of bulk varistor ceramics with applicable break-down voltages and good I-U characteristics at temperatures below 1000 oC, which means significant energy savings. The obtained knowledge about the rheological characteristics of screen-printing ink in regard to the organic vehicle, its components and the amount of the solids load is important for the preparation of printable pastes with the highest solids load; it enabled the preparation of thick-films with a high green density for good sintering behavior and microstructure development, which is important for the preparation of any electroceramics in thick-film technology and in particular for varistor ceramics, the unique electrical characteristics of which result from the nature of the grain boundaries and the ZnO grains. In the development of oxide TE materials significant new knowledge about the influence of the sintering method (classic, hot-pressing, microwave-MW, pulse electric current – PECS) with specific effects on the diffusion processes, on the sintering, the formation of the ZnkIn2O3+k (k=5,9,11,18,…) homologous phases in the n-type In2O3-doped ZnO ceramics or the p-type Ca3Co4O9-based ceramics, the grain growth, the microstructure development and, consequently, the TE characteristics was obtained. It gives a foundation for the further improvement of their TE characteristics through structural and microstructural engineering. The results and knowledge obtained on the thick-film TE microgenerators from the n-type Zn5In2O8 and p-type Ca3Co4O9 prepared by the screen printing showed the promising advantages of this technology and the possibilities for further development.
Significance for the country
A breakthrough in the integration of overvoltage protection based on thick-film varistors into hybrid circuits would certainly be highly recognized and appreciated by the electronics community. The development of oxide thermoelectric (TE) materials and TE modules, which started about 15 years ago in Japan and a major enhancement in the world in the recent years, is highly important as it represents the only viable way to harvest waste heat, by converting it directly into electrical energy, irrespective of the source size and without the use of moving parts or the production of environmentally deleterious wastes. Hence, it is important that the research in Slovenia also starts in this field and contributes with knowledge and results to the efforts in the world. Research activities and the obtained results of the project are closely related to the development programs and ambitions of Slovenian companies (VARSI and KEKON), which are partners in the project. They are important for their standard programs as well as for their future attempt to develop new products and technologies. The company VARSI produces various types of varistors that were developed within 35 years of the collaboration between the company and the institute. The results of the development of the thick-film varistors and the ability to tailor the microstructure and I-U characteristics of low-doped varistor ceramics at a much lower temperature than typically used will also be very beneficial for the further development of bulk varistors based on the low-doped varistor ceramics. Both companies already collaborated on the development of some special types of varistors. The developnet of thick-film varistors further enhanced their collaboration for the development of new devices. As a company, KEKON produces multilayer “chip” capacitors, the development of thick-film varistors based on low-doped varistor ceramics opens up new possibilities for the successful integration of capacitors and varistors. Both companies are also looking for the development of new devices and technologies, which are very promising for future applications, and the oxide TE materials and TE modules certainly are. The results of the project greatly contribute to strengthening the competitiveness of domestic producers on the world market of electronic components and devices. The project results have application potential for the development of new products and technologies at the companies VARSI and KEKON. These are all products with a higher added value, which are based on a domestic knowledge and will strengthen the position of both producers on the world market and secure jobs. Already now, both companies export most of their production to the most demanding world markets and the competition in their fields is very tough. The project represents a continuation of 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 the areas of electronic materials and components, new materials and related technologies. However, the results of the project also opens up potentials for completely new devices and applications in waste-energy harvesting, which could also be of great interest for other companies in Slovenia.
Most important scientific results Annual report 2011, 2012, 2013, final report, complete report on dLib.si
Most important socioeconomically and culturally relevant results Annual report 2011, 2012, 2013, final report, complete report on dLib.si
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