Effect of microwave sintering on the microstructure and electric properties of (Zn,Mg)TiO3-based multilayer ceramic capacitors

doi:10.1016/j.jeurceramsoc.2011.12.025

Journal of the European Ceramic Society

Volume 32, Issue 8, July 2012, Pages 1725-1732

https://doi.org/10.1016/j.jeurceramsoc.2011.12.025 Get rights and content

Abstract

In this study, the effects of microwave sintering on the sintering behaviour, microstructure and silver diffusion of (Zn,Mg)TiO₃-based multilayer ceramic capacitors (ZMT MLCCs) with pure silver electrodes were investigated. The energy dispersive spectroscopy results showed that the silver ions diffused into the dielectric layer significantly when the ZMT MLCC was sintered with conventional processing at 900 °C. However, sintering of ZMT MLCC at 900 °C using microwave processing was found to effectively suppress silver ion diffusion into the dielectric layer. The concentration of silver ions was identified by wavelength dispersive spectroscopy, which showed that the Ag ion concentrations for conventional and microwave sintering are approximately 1.0 at.% and below 0.4 at.%, respectively. The observed difference may be due to different kinetics between conventional and microwave sintering.

Introduction

Miniaturisation, performance, and cost savings are the major drivers in the manufacture of multilayer ceramic capacitors (MLCCs), although many manufacturers have switched over from Ag–Pd electrodes to base metal electrode systems.1, 2 Pure silver inner electrodes have a cost advantage when compared to base metal systems, and silver is also the most conductive element, facilitating lower ESR (equivalent series resistance) and higher frequency requirements. However, multilayer ceramic capacitors are one of the most widely used discrete electronic components, playing a critical role in the electronics industry. The use of silver rather than pure palladium as the conductor for MLCCs is one way to reduce costs.3, 4, 5 Therefore, due to the development of many low-sintering ceramic formulations, Ag pastes have been widely used as internal electrodes in the metal–ceramic cofiring step during the fabrication of these multilayer ceramic devices.⁶ However, during the cofiring of the ceramic layers and silver inner electrodes, the potential for chemical reactions and inter-diffusion at the interfaces must be considered along with their influence on the cofiring behaviour of MLCCs. The interaction and inter-diffusion may change the sintering behaviour and final properties of the MLCCs.7, 8It is well known that zinc titanates (ZnTiO₃) can be sintered at 1100 °C without the use of sintering aids.9, 10 When a sintering aid is added, it can be sintered at temperatures below 900 °C.9, 10, 11 ZnTiO₃ has a perovskite-type oxide structure and should be useful as a microwave resonator material.¹⁰⁾ ZnTiO₃ material has a permittivity (ɛ_r) of 19, a Q value of 3000 at 10 GHz, and a temperature coefficient of resonant frequency (τ_f) of −55 ppm/°C.11, 12

Microwave sintering is a method of internal self-heating by absorption of microwave power. Therefore, comparing the internal microwave sintering is possible with external sintering by thermal conduction or radiation. When this method is utilised in the processing of ceramics, the material is expected to display fine grain, uniformity and high densification. Additionally, the electrical and mechanical properties can be improved.¹³

It is well known that silver migration into ceramics in the cofiring process of low sintering temperature MLCCs significantly influences the reliability and dielectric characteristics.14, 15, 16, 17, 18 In a previous study,¹⁹ the effect of heating rates and two-step sintering on the diffusion of Ag into the ZMT dielectric were investigated. It was found that two-step sintering can effectively prevent the Ag ions from diffusing into the dielectric layer. However, the influence of microwave sintering on the Ag diffusion of MLCCs was not investigated. In this paper, the effects of microwave sintering on the sintering behaviour, microstructure and silver diffusion of ZMT MLCCs with pure silver electrodes were investigated.

Section snippets

Preparation of the ZMT powders

ZMT powders were synthesised by conventional solid-state methods from individual high-purity oxide powders: ZnO (99.8% Umicore Zinc Chemicals, France), TiO₂ (99.9% Showa Denko Inc., Japan) and MgO (99% Pharmacie Central Inc., France). They were mixed and ground in deionised water with 2 mm zirconia beads for 24 h; the mean particle size (D₅₀) of the milled powder was approximately 0.35 μm. The powders were calcined in air at 900 °C for 5 h after ball milling. 3ZnO–B₂O₃ (ZnBO) glass with 1 μm particle

Effect of microwave sintering on the density and microstructure of ZMT dielectric

Fig. 1 shows the bulk density of the ZMT ceramic pellets from each sintering process as a function of the sintering temperature. The density of ZMT ceramics sintered at different temperatures depends on the heating mode. For the ZMT ceramics sintered by the conventional process, the density is much lower than those sintered by microwave processing at temperatures <880 °C. The theoretical density of the ZMT ceramic is ∼4.95 g/cm³.¹⁴ The ZMT ceramics produced by microwave processing can reach over

Conclusion

ZMT MLCCs prepared by different sintering methods were investigated. The results are summarised below.

1.
Microwave sintering can achieve a high-density ZMT ceramic at lower heating temperature (840 °C), while in the conventional sintering, there was no significant densification below 880 °C. The microwave sintering temperature was 60 °C lower than the conventional sintering.
2.
The effect of microwave sintering on Ag diffusion into the ZMT dielectric was investigated. According to the WDS analysis, the

Acknowledgement

The authors would like to acknowledge the financial support of this research by the National Science Council of Taiwan under contract No. 99-2221-E-020-018.

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Effect of microwave sintering on the microstructure and electric properties of (Zn,Mg)TiO3-based multilayer ceramic capacitors

Abstract

Introduction

Section snippets

Preparation of the ZMT powders

Effect of microwave sintering on the density and microstructure of ZMT dielectric

Conclusion

Acknowledgement

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