Microwave dielectric properties of Li2ZnTi3O8 ceramics doped with Bi2O3

doi:10.1016/j.ceramint.2013.05.095

Ceramics International

Volume 39, Issue 8, December 2013, Pages 9829-9833

https://doi.org/10.1016/j.ceramint.2013.05.095 Get rights and content

Abstract

The effects of Bi₂O₃ on the sintering temperature, phase composition, microstructure and microwave dielectric properties of Li₂ZnTi₃O₈ ceramics are investigated. The addition of Bi₂O₃ can effectively reduce the sintering temperature of Li₂ZnTi₃O₈ ceramic from 1075 to 950 °C. Only a single phase Li₂ZnTi₃O₈ forms in the Li₂ZnTi₃O₈ ceramic with less than 1.0 wt% Bi₂O₃ addition sintered at 950 °C. However, when the addition of Bi₂O₃ exceeds 1.0 wt%, the second phase Bi₂Ti₂O₇ forms in the Li₂ZnTi₃O₈ ceramics. Typically, 2.0 wt% Bi₂O₃-doped Li₂ZnTi₃O₈ ceramic sintered at 950 °C can reach a maximum relative density of 97.4% and exhibits good microwave dielectric properties of ε_r =27.8, Q×f=36386 GHz, τ_f=−19.5 ppm/°C.

Introduction

In recent years, along with the rapid growth in wireless communications industry, a variety of microwave devices such as filters, duplexers, resonators and antennas were well developed [1]. Nowadays increased attention has been paid to develop microwave dielectric ceramics with low sintering temperatures for low-temperature cofired ceramics (LTCC) application [2]. To realize cofiring with Ag electrode, the sintering temperatures of these ceramic materials should be lower than 950 °C [3]. However, the sintering temperature of most commercial dielectric ceramic is much higher than the melting point of silver [4], [5], [6]. Therefore, it is important to develop the microwave dielectric ceramic with low sintering temperature and good microwave dielectric properties.

Recently, Sebastian et al. [7], [8] reported that Li₂ZnTi₃O₈ ceramics sintered at 1075 °C for 4 h exhibited good microwave dielectric properties of ε_r=25.6, Q×f=72000 GHz, τ_f=−11.2 ppm/°C. Li₂ZnTi₃O₈ ceramics are expected as very competitive ultra-low loss microwave materials compared with the previous materials such as Ba(Mg_1/3Ta_2/3)O₃ [9] and Zr_0.8Sn_0.2TiO₄ [10], which have a low cost of raw materials and low bulk density. In order to realize the LTCC applications of Li₂ZnTi₃O₈ ceramic, it is essential to reduce the sintering temperature of Li₂ZnTi₃O₈ ceramic to 950 °C.

As a common sintering aid, Bi₂O₃ has been reported as a promising sintering aid for the densification at relatively low sintering temperature [11], [12], [13]. Tay et al. [12] reported that the addition of 15 mol% Bi₂O₃ can reduce the sintering temperature of the La(Mg_0.5Ti_0.5)O₃ ceramics from 1600 to 1325 °C without degradation of the microwave dielectric properties. However, the effect of Bi₂O₃ addition on the sintering temperature and microwave dielectric properties of the Li₂ZnTi₃O₈ ceramics has not been studied.

In the present study, the effects of Bi₂O₃ addition on the sintering temperature, densification, phase composition, microstructure and microwave dielectric properties of the Li₂ZnTi₃O₈ ceramics were thoroughly investigated. Furthermore, the relationships among densification, microstructure and microwave dielectric properties of the Bi₂O₃-doped Li₂ZnTi₃O₈ ceramics were discussed.

Section snippets

Experimental procedure

The Li₂ZnTi₃O₈ ceramics were prepared by the conventional solid-state ceramic route. Li₂CO₃ (99%), ZnO (99.5%), TiO₂ (99.5%) were used as starting powders. The powders were weighed according to the stoichiometric ratio of Li₂ZnTi₃O₈, and milled with zirconia balls in ethanol for 24 h. The slurries were dried and sieved and calcined at 900 °C for 4 h, then re-milled with different amounts of Bi₂O₃. With 5 wt% PVA solution as a binder, the granulated mixtures were pressed into disks with 20 mm in

Results and discussions

The X-ray diffraction patterns of the Li₂ZnTi₃O₈ ceramics doped with different Bi₂O₃ additions sintered at 950 °C are shown in Fig. 1. Only a cubic spinel structure Li₂ZnTi₃O₈ phase (space group P4332, a=8.3710 Å, JCPDS file No. 86-1512) appears in the XRD patterns of the specimens with 1.0 wt% Bi₂O₃ addition. However, when the addition of Bi₂O₃ exceeds 1.0 wt%, the Li₂ZnTi₃O₈ and Bi₂Ti₂O₇ phase which is formed at 800 °C [15] co-exist in the specimens sintered at 950 °C. With increasing Bi₂O₃

Conclusion

The sintering temperature, densification, microstructure and microwave dielectric properties of the Li₂ZnTi₃O₈ ceramics with Bi₂O₃ addition are investigated. Only a single phase Li₂ZnTi₃O₈ forms in the Li₂ZnTi₃O₈ ceramic with less than 1.0 wt% Bi₂O₃ addition sintered at 950 °C. However, when the addition of Bi₂O₃ exceeds 1.0 wt%, the second phase Bi₂Ti₂O₇ forms in the Li₂ZnTi₃O₈ ceramic. The lattice parameters and cell volumes of Li₂ZnTi₃O₈ ceramics with Bi₂O₃ additions are larger than that of

Acknowledgments

This work was supported by the National Natural Science Foundation of China (NSFC-51174118), “333” High-level Talents Training Project of Jiangsu Province and “Blue Project” of Jiangsu Province.

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Over the past few years, some researchers have focused on reducing the sintering temperature and dielectric loss of LZT ceramics by using low-melting-point oxides or doped glass and composition-control methods [8–10]. Lu et al. studied the effect of Bi2O3 addition on LZT ceramics, which have excellent microwave dielectric properties when 2 wt% Bi2O3 doped in LZT are sintered at 950 °C: εr = 27.8, Q × f = 36 386 GHz, and τf = −19.5 ppm/°C [11]. Xiao et al. reduced the proportion of Ti in Li2ZnTi3O8 ceramics by composition-control method, thereby successfully improving the Q × f of Li2ZnTi2.92O8 ceramics: εr = 25.4, Q × f = 108 000 GHz, and τf = −10.5 ppm/°C [6].
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Comparing with other microwave dielectric ceramic materials, Li2ATi3O8 (A = Mg, Zn) ceramics with sintering temperature of about 1075 °C and high Qf value of 72000 GHz, which was reported by S. George and M. T. Sebastian in 2010, are very attractive for multilayer capacitors and LTCC applications [2]. Several sintering aids, such as H3BO3, Bi2O3, B2O3, and glass, have been successfully used to lower down the sintering temperature of Li2ZnTi3O8 ceramic to about 900 °C, the co-firing compatibility between Li2ZnTi3O8 ceramic and Ag was also discussed in these literatures [3–11]. However, the reliability of low temperature sintered Li2ZnTi3O8 ceramic under high D.C. voltage and high temperature has not been investigated.
The effects of acceptor dopant (typical Mn²⁺) and donor dopant (typical V⁵⁺) on the reliability of Li₂ZnTi₃O₈ based low temperature co-fired ceramics (LTCC, in short) have been investigated. Large difference occurs among different types of dopants. V⁵⁺ doping decreases the breakdown voltage of the multilayer capacitors which were manufactured by low temperature sintered Li₂ZnTi₃O₈ ceramics, however, Mn²⁺ doping increases the breakdown voltage and lowers down the leakage current significantly. This may very likely due to the fact that donor induces the generation of weak bound electrons while acceptor capture the weak bound electrons. The results suggest that Mn²⁺ is a very effective additive for improving the reliability of low temperature sintered Li₂ZnTi₃O₈ ceramics.

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Microwave dielectric properties of Li2ZnTi3O8 ceramics doped with Bi2O3

Abstract

Introduction

Section snippets

Experimental procedure

Results and discussions

Conclusion

Acknowledgments

Ceramics International

Ceramics International

Journal of the European Ceramic Society

Materials Chemistry and Physics

Ceramics International

Journal of the European Ceramic Society

Materials Letters

Journal of the European Ceramic Society

Ceramics International

Materials Letters

Microwave dielectric properties of Li₂ZnTi₃O₈ ceramics doped with Bi₂O₃