Microwave properties of Ba(Zn1/3Ta2/3)O3 dielectric resonators

https://doi.org/10.1016/j.jallcom.2010.09.069Get rights and content

Abstract

Ba(Zn1/3Ta2/3)O3 (BZT) dielectric resonators were prepared by solid-state reaction. The starting materials were BaCO3, ZnO, and Ta2O5 powders with high purity. The double calcined BZT pellets were sintered in air at temperatures of 1575, 1600, 1625, and 1650 °C for 4 h. The X-ray diffraction data allowed the study of the unit cell distortion degree and the presence of the secondary phases. A long-range order with a 2:1 ratio of Ta and Zn cations on the octahedral positions of the perovskite structure was observed with the increase of the sintering temperature. The dielectric constant of BZT resonators measured around 6 GHz was between 26 and 28. High values of Q × f product (120 THz) were obtained for BZT resonators sintered at 1650 °C/4 h. The temperature coefficient of the resonance frequency exhibits positive values less than 6 ppm/°C. The achieved dielectric parameters recommend BZT dielectric resonators for microwave and millimeter wave applications.

Research highlights

▶ Ba(Zn1/3Ta2/3)O3 dielectric resonators with bulk densities up to 95% were obtained by solid-state reaction. ▶ The morpho-structural results obtained by using XRD and SEM were correlated with the microwave dielectric properties. ▶ The temperature coefficient of the dielectric permittivity τɛ was estimated to be −37 ppm/°C by measurements at 1 MHz. ▶ The temperature coefficient of the resonance frequency τf  6 ppm/°C measured around 6 GHz is in a good agreement with the value estimated from temperature dependence of the capacitance at low frequency. ▶ BZT resonators with dielectric constant around 28 and a Q × f product up to 120,000 GHz were achieved.

Introduction

Dielectric materials continue to have a decisive influence on the evolution of the electrical and electronic engineering, communications and information technology. These materials, which exhibit high dielectric constant, low dielectric losses, and good temperature stability are required to reduce the size and weight of equipment, enhance its reliability, and lower the manufacturing and operational costs [1], [2], [3], [4], [5], [6], [7], [8], [9]. Ceramics are from far the most utilized as they offer cost-effective solutions for applications.

The Ba(Zn1/3Ta2/3)O3 (BZT) dielectric resonators are known to exhibit a high dielectric constant (ɛr), a small temperature coefficient of resonance frequency (τf) and a high quality factor (Q). All of these properties are important for the applications of BZT ceramics to microwave devices, in satellite broadcasting and as a high Q dielectric resonator, in mobile phone base stations or combiner filter for Personal Communication System applications [2], [10], [11].

The factors influencing Q values of BZT have been considered to be long-range ordering (LRO) of cations, zinc oxide evaporation, point defects and stabilization of microdomain boundaries [12], [13]. This explained the high Q values from the point of view of its hexagonal superstructure. Sagala and Nambu [14] calculated the dielectric loss tangent at microwave frequencies from the equation of ion motions, which was a function of B-site ordering. Gallasso and Pyle [15] concluded that the B-site ordering increased as the difference in charge and size between B′ and B″ atoms increased. Reaney et al. [16], [17] studied the order–disorder transition in BZT using XRD and TEM. They found that the reversible order–disorder phase transition in BZT occurs between 1600 and 1625 °C. The cation ordering in Ba(Zn1/3Ta2/3)O3 complex perovskite is important because the 1:2 ordering along 〈1 1 1〉 direction is closely related to the high-Q property of BZT. There is a strong correlation between LRO, domain growth, zinc loss and microwave dielectric parameters.

As a result of our previous investigations, high-Q BZT dielectric resonators (Q  17,000 at 5.6 GHz) were achieved by solid-state reaction [18], [19]. The aim of this work is a further increase of the quality factor. Four sintering temperatures (1575, 1600, 1625, 1650 °C) were used in order to determine the structural and morphological changes that occur in BZT resonators as a function of sintering temperature, and to use the data obtained to explain the changes in the microwave dielectric properties.

Section snippets

Experimental

Ba(Zn1/3Ta2/3)O3 samples were prepared by solid-state reaction. The starting materials were high purity BaCO3, ZnO and Ta2O5 powders. Stoichiometric quantities were weighted, ground, homogenized and milled in an agate mill in water for 5 h. The powders were calcined at T = 1200 °C for 2 h. Then the powders were milled for 3 h and calcined at 1250 °C/2 h. The double calcined powders were mixed with 12% polyvinyl alcohol (PVA) and dried at 80 °C, then were pressed into cylindrical samples of 12 mm diameter

Results and discussion

The bulk densities of the fired BZT ceramics were measured after grinding and polishing. The temperature dependence of the densification after sintering treatment in air for 4 h is shown in Fig. 1. The X-ray density of BZT compound with stoichiometric composition was considered as ρ = 7.94 g/cm3 [4]. The BZT samples sintered between 1575 and 1650 °C exhibit bulk density greater than 90% of theoretical density. The bulk density slightly increases with the increase of sintering temperature Ts. The

Conclusions

Ba(Zn1/3Ta2/3)O3 ceramic materials with high dielectric constant and low loss in microwave domain were achieved by solid-state reaction in the temperature range 1575–1650 °C for 4 h.

The XRD patterns confirm the formation of the BZT materials with hexagonal structure. For sintering temperatures higher than 1625 °C, the XRD patterns reveal the formation of a secondary phase with low Zn content.

Low frequency measurements at 1 MHz showed a slow decrease of the dielectric constant with the increase of

Acknowledgments

This work was supported by the Ministry of Education and Research of Romania through the Core Program PN09-45 and the project PNII 12-078/2008. The authors thank to Dr. Sorin Jinga from the University “Politehnica” of Bucharest for SEM investigations. The authors are also grateful to Paul Ganea from the National Institute of Materials Physics for his help in low frequency measurements.

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