Elsevier

Materials Research Bulletin

Volume 47, Issue 12, December 2012, Pages 4510-4513
Materials Research Bulletin

Short communication
Microstructure and microwave dielectric properties of Bi12SiO20 ceramics

https://doi.org/10.1016/j.materresbull.2012.08.075Get rights and content

Abstract

Bi12SiO20 ceramics were well sintered at 800 °C after calcination at 700 °C. A liquid phase of composition Bi2O3 was formed during the sintering at temperatures ≥800 °C and assisted the densification of the Bi12SiO20 ceramics. When the sintering temperature exceeded 800 °C, however, the relative density, ɛr, and Q × f values of the Bi12SiO20 ceramics decreased, probably due to the formation of a large amount of the liquid phase. The Bi12SiO20 ceramics sintered at 800 °C for 5.0 h exhibited excellent microwave dielectric properties with a high ɛr of 43, a high Q × f of 86,802 GHz and a small τf of −10.39 ppm/°C.

Highlights

► Bi12SiO20 ceramics can be sintered at 800 °C with a dense microstructure. ► Bi12SiO20 ceramics show a high Q × f = 86,802 GHz, ɛr = 43, and τf = −10.39 ppm/°C. ► Bi12SiO20 ceramics can be good candidate materials for LTCC devices.

Introduction

Mobile systems, such as laptop computers and cellular phones, require the miniaturization of microwave devices. Low temperature co-fired ceramic (LTCC) multilayer devices have been extensively investigated in an attempt to miniaturize microwave devices. LTCC multilayer devices are composed of alternating dielectric ceramics and internal metallic electrode layers [1]. With a melting temperature of 961 °C, Ag is generally used as the metallic electrode layer on account of its high conductivity and low cost. Therefore, microwave dielectric ceramics with a low sintering temperature less than 960 °C need to be developed for co-firing with the Ag electrode. Furthermore, microwave dielectric materials require a high quality factor (Q × f) to increase the frequency selectivity and a near-zero temperature coefficient of resonance frequency (τf) to stabilize the frequency against temperature.

A large amount of glass is generally used to decrease the sintering temperature of microwave dielectric materials for application to LTCC multilayer devices [2], [3], [4], [5]. However, the added glass deteriorates the Q × f value of the specimens by developing an amorphous phase in the specimens [6], [7], [8], [9], [10], [11]. Moreover, even those materials with a high Q × f value had a τf value that was too negative, or required special techniques such as the cold isostatic press method to produce [12], [13], [14]. Therefore, new materials with a high Q × f and an almost zero τf, as well as the ability to be sintered at low temperatures (≤900 °C), are needed for application to LTCC devices. According to the phase diagram, the solid solutions of Bi2O3 and SiO2 such as Bi4(SiO4)3 and Bi12SiO20 ceramics are expected to be sintered below 900 °C without the addition of glass or oxide additives due to their low melting temperature [15]. The detailed formation process and the microwave dielectric properties of the Bi4(SiO4)3 ceramics have been investigated [16]. Although the microwave dielectric properties of these Bi12SiO20 ceramics have been reported [17], the detailed formation process and microstructural variation with respect to the process conditions have not been investigated. Moreover, since the Q × f value of the Bi12SiO20 ceramics has been reported to be low, further research has not been conducted. Therefore, in this work, the Bi12SiO20 ceramics were produced under the various process conditions for the first time to elucidate their formation process and determine the optimum process conditions. Furthermore, their microwave dielectric properties and microstructural variation with respect to the process condition were also investigated to evaluate their potential use as a microwave dielectric material for LTCC devices.

Section snippets

Experimental

Using conventional solid-state synthesis, Bi12SiO20 ceramics were prepared from high purity (>99%) oxides. For the synthesis of these materials, Bi2O3 (High Purity Chemicals Osaka, Japan) and SiO2 (Junsei Chemical Co. Ltd., Tokyo, Japan) powders were ball-milled in a nylon jar with zirconia balls for 24.0 h. After drying, the mixed Bi12SiO20 powders were calcined at various temperatures for 5.0 h. The powders were subsequently ball-milled again for 24.0 h and dried. Then, the hydraulically pressed

Results and discussion

Fig. 1(a) shows the XRD patterns of the Bi12SiO20 powders calcined at various temperatures for 5.0 h. The Bi12SiO20 phase was formed for the specimen calcined at 600 °C but a large amount of Bi2O3 phase was also formed. The amount of the Bi2O3 phase decreased with increasing calcination temperature but a small amount of Bi2O3 was still observed in the specimen calcined at 800 °C. Moreover, the specimen calcined 800 °C was agglomerated, which made it difficult to grind the specimen to make the

Conclusions

The Bi12SiO20 phase was formed for the specimens calcined at temperatures above 600 °C with the unreacted Bi2O3 phase. Although the amount of Bi2O3 phase decreased with increasing calcination temperature, some still remained in the specimen calcined at 800 °C. A dense microstructure with a relative density of 93% of the theoretical density was developed for the specimen sintered at 800 °C after calcination at 700 °C. Moreover, a liquid phase with a composition of Bi2O3 was observed in the specimens

Acknowledgments

This work was supported by the Industrial Strategic Technology Development Program, 10041232, ‘Development of synthesis method of exfoliated inorganic nanosheets with a high dielectric constant of >300 and the corresponding thin films applicable for the fabrication of high performance MLCC’ funded by the Ministry of Knowledge Economy (MKE, Korea).

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