Thick film ZnO based varistors prepared by screen printing

doi:10.1016/j.jeurceramsoc.2006.02.016

Journal of the European Ceramic Society

Volume 26, Issue 14, 2006, Pages 2985-2989

https://doi.org/10.1016/j.jeurceramsoc.2006.02.016 Get rights and content

Abstract

Thick film varistors based on the system ZnO–Bi₂O₃–Sb₂O₃ have been prepared by screen printing technology on dense alumina substrates. Different processing strategies have been designed in order to control the excessive volatilization of Bi₂O₃ in varistor films during the sintering, due to the high area–volume ratio, and as a means to improving their electrical response. Starting powders were selected and pre-treated in different ways to obtain different phases and control the Bi-rich liquid phase formation. Significant differences have been observed in the electrical properties which are related to the selection of the starting powders.

Introduction

Zinc-oxide based varistors are multiphase ceramic devices which exhibit highly non-linear current-voltage characteristics.¹ This extensive non-ohmic characteristic results in the widespread application of varistors as voltage surge protectors in electrical circuits.²

Screen printing has been developed in the fields of microelectronics for hybrid and integrated circuit manufacture. The advantages of this technology are; low cost, versatility in the design, miniaturization and high reproducibility. One of the main problems, inherent to this technology, is the lack of compaction of the films. In general in this technology to reach a satisfactory level of densification usually involves the addition of a large amount of glass-frit which melts during the firing process. However, in the case of the varistor material, a significant amount of glass-frit will modify the electrical performance of the thick films.

Menil et al.³ proposed a method to improve the compaction by applying mechanical pressure, either uniaxial or isostatic, to the calcined screen printed samples with composition 95 mol% ZnO–5 mol% additives (CoO, Cr₂O₃, Mn₂O₃, Sb₂O₃, Bi₂O₃). They obtained nonlinear coefficient values between 17 and 19 for thick films sintered at 1150 °C. Unpressed screen-printed varistors were mostly short-circuited. Tovher et al.⁴ prepared thick-film varistors by direct-write techniques in highly integrated, multifunctional electroceramic devices, with chemical composition in mol%: 98.94 ZnO, 0.25 CoO, 0.25 MnO, 0.56 Bi₂O₃.

One of the main problems associated with varistor thick film manufacture, as well as the lack of compaction of the films, is the high volatilization of Bi₂O₃ at the sintering temperatures due to the high area–volume ratio of the thick films. In recent work Peiteado et al.⁵ have measured a loss in weight of Bi₂O₃ up to 60% in bulk varistors with area–volume ratio of 4.1 cm⁻¹. De la Rubia et al.⁶ reported that the partial volatilization of Bi₂O₃ limits the highest area–volume ratio in a bulk varistor, exhibiting good varistor behaviour with high and low sintering temperatures to an area–volume ratio of 5 cm⁻¹. For higher area–volume ratios than 5 cm⁻¹, this volatilization damages the electrical response invalidating its application as a varistor.

Bi₂O₃ volatilization takes place from the Bi₂O₃-rich liquid phase that forms during sintering. Varistor functional microstructure is achieved through the following reactions: $2 ZnO+ \frac{3}{2} S b_{2} O_{3} + \frac{3}{2} B i_{2} O_{3} + \frac{3}{2} O_{2} \overset{< 900 ° C}{⟶} \underset{(pyrochlore)}{Z n_{2} S b_{3} B i_{3} O_{14}}$ $2 Z n_{2} S b_{3} B i_{3} O_{14} + 17 ZnO \overset{900 - 1050 ° C}{⟶} 3 Z n_{7} S b_{2} O_{12} + \underset{(liquid)}{3 B i_{2} O_{3}}$

Appropriate densification of the varistor material is obtained when a Bi₂O₃ liquid phase appears leading to liquid phase sintering. However, if the temperature is increased, bismuth volatilization is also increased and has a significant effect on the final electrical properties. Such an effect is even more marked for the thick film geometry, with a very high area–volume ratio (two orders of magnitude higher than for bulk ceramics). Within this framework, a reasonable approach to overcome such difficulties might be based on the formation of the Bi₂O₃ rich liquid phase at low temperatures, so that Bi volatilization kinetic is hindered and Bi loss avoided.

The objective of this work is to obtain thick film varistors with good electrical response, controlling the Bi₂O₃ volatilization and improving densification. For this purpose, different processing strategies leading to the formation of a Bi₂O₃ rich liquid phase at low temperatures have been studied.

Section snippets

Powder preparation

The nominal composition of the varistor powder is 95.5 mol% ZnO, 1.5 mol% Sb₂O₃, 0.5 mol% Bi₂O₃, 0.5 mol% Co₃O₄, 1.25 mol% NiO and 0.75 mol% MnO. Three different processing strategies with this nominal composition have been designed:

(I)
Batch SCM: Prepared by a classical mixed-oxide route including ball milling for 2 h in ethanol, calcination treatment at 950 °C–1 h and milling.
(II)
Batch SP: Classical mixed-oxide route but replacing the Sb₂O₃ by the equivalent amount of a previously synthesized Zn₇Sb₂O₁₂

Results and discussion

For batch SCM, reaction 2 was partially completed since the calcination step was carried out at 950 °C–1 h, therefore this system initially exhibits Zn₇Sb₂O₁₂ spinel phase, Zn₂Bi₃Sb₃O₁₄ pyrochlore phase and ZnO. The Zn₇Sb₂O₁₂ phase is found in the grain boundaries and triple points and inhibits grain growth, hindering the movement of the grain boundaries.⁷ In batch SP, the spinel phase Zn₇Sb₂O₁₂ is used as Sb precursor instead of Sb₂O₃, avoiding the reactions 1 and 2. The release of the Bi-rich

Conclusions

By means of different processing strategies, thick films of ZnO-based varistors have been prepared with α values as high 20. Between the strategies suggested, the incorporation of the previously synthesized spinel phase allows for a lower sintering temperature, 900 °C, and so prevents excessive Bi₂O₃ volatilization, which due to the extremely high area–volume ratio of the films represents the major challenge in manufacturing these devices. Together with the incorporation of the spinel phase the

Acknowledgements

This work has been carried out within the Marie Curie Training Site Ceramos (HPMT-CT-2001-00372) program and CICYT MAT 2004-04843-C02-01 project.

References (11)

F. Menil et al.
Screen-printed thick-films: from materials to functional devices
J. Eur. Ceram. Soc.
(2005)
M. Peiteado et al.
Bi₂O₃ vaporization from ZnO-based varistors
J. Eur. Ceram. Soc
(2005)
M.A. De la Rubia et al.
Compact shape as a relevant parameter for sintering ZnO–Bi₂O₃ based varistors
J. Eur. Ceram. Soc.
(2004)
M.A. De la Rubia et al.
Equilibrium phases in the Bi₂O₃-rich region in the binary system ZnO–Bi₂O₃
J. Eur. Ceram. Soc.
(2005)
D.R. Clarke
Varistor ceramics
J. Am. Ceram. Soc.
(1999)

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Thick film ZnO based varistors prepared by screen printing

Abstract

Introduction

Section snippets

Powder preparation

Results and discussion

Conclusions

Acknowledgements

J. Eur. Ceram. Soc.

J. Eur. Ceram. Soc

J. Eur. Ceram. Soc.

J. Eur. Ceram. Soc.

Varistor ceramics

J. Am. Ceram. Soc.