Low-temperature fully dense and electrical properties of doped-ZnO varistors by a polymerized complex method
Introduction
Electronic circuits and electronic power systems can be subject to several impulse voltage transients generated by lightning, switching and electrostatic discharge accumulated on the human body, therefore, a protection for such devices becomes necessary. Although several kinds of transient suppressers, as for example, silicon carbide and Zener diode can be used, however, the zinc oxide, due to its highly nonohmic behaviour in voltage-current characteristics, is more widely used as a protector in small current electronic circuits as well as for large current transmission lines. ZnO-based varistors contain ZnO as the main component (typically ⩾90%) with several metal oxides (up to 10) added as dopant. Although each additive controls one or several parameters, Bi2O3 and Sb2O3 can be considered as the most important. For example, Bi2O3 plays a significant role in the basic structure of the bulk varistor, Sb2O3 enhances its stability, and others like Al2O3, MnO, CoO, etc. strongly influence their nonohmic electrical properties. Because of the multi-components present in a ZnO-based varistor, several reactions are to take place during sintering. This leads to a final microstructure, which, in the ideal situation, is constituted on the uniform grain size without porosity, conducting ZnO surrounded by a Bi2O3 phase, resulting from the liquid phase at the sintering temperature, segregated at grain boundaries, and homogeneously distributed crystalline secondary phases such as the spinel type Zn7Sb2O12 and pyrochlore type Zn2Bi3Sb3O14. Therefore, the kind, the concentration, and the distribution of both the dopant and the corresponding created phases on sintering, determines the final microstructure and the electrical properties of the obtained varistor material. To fulfil all those specification becomes difficult using the conventional powder preparation method, and albeit the cost of the non-conventional method being quite higher, the advantages of a better chemical homogeneity, a higher powder purity, and a more uniform grain size can decide its use if the final electrical properties are significantly enhanced. In the last decade attempts to achieve this have been carried out.1, 2, 3, 4, 5, 6, 7, 8, 9
The polymerized route, based on a modification of the Pechini method,10 was used to prepare doped-ZnO powders which included the four most important Bi, Sb, Mn, and Co metal oxide additives as raw material for achieving ceramic bodies with controlled microstructure (denoted as unconventional powders). For comparison, a ceramic powder with identical composition, 98 mol% ZnO, 2 mol% ( Bi2O3+CoO+MnO+Sb2O3), was prepared using the conventional mixed oxide method (denoted as conventional powders).
Section snippets
Experimental procedure
Doped-ZnO powders were synthesized by the polymerized route as shown in Fig.1. Individual aqueous solutions containing the required amounts of metal nitrates (except in the case of the Sb2O3 which was dissolved in melted citric acid) were prepared. Then they were mixed together to produce solutions for gel production. Separately, citric acid (CA) was added to ethylene glycol (EG) in a 1:4 CA/EG molar ratio, and this mixture was stirred at about 80°C to form a transparent solution. The aqueous
Powder characterisation
Fig. 2 shows the crystallisation process studied by TG–DTA. It is clearly seen in the TG curve, that with increasing temperature the weight loss increases up to 620°C and beyond that temperature the weight remains constant. Therefore, it can be concluded that at 620°C all the involved reactions in the weight loss process are finished, i.e. the thermal decomposition of all organic products of the precursor (between room temperature and about 400°C), their combustion (from 400 to ∼460°C), and the
Conclusions
Homogeneous and nanosized doped-ZnO ceramic powders were prepared by the metal citrate-based polymeric organic Zn, Co, Mn, Bi, Sb precursor method. After calcining at 400°C the nanosized powders (∼28 nm in size) led to green compacts with a very uniform powder packing, a narrow pore size distribution and pore size in the nanoscale (∼19 nm), i.e. with a low pore-coordination-number. During sintering it is believed that the formed liquid-phase rapidly filled the small pores, giving rise in a very
Acknowledgements
The present work was supported by the Spanish CICYT MAT 97-0679-C02-01.
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