Nickel-doped (Zr0.8, Sn0.2)TiO4 for microwave and millimeter-wave applications

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Abstract

(Zr0.8, Sn0.2)TiO4 ternary compounds (ZST) have been prepared by conventional solid-state reaction from raw materials. The effects of such sintering parameters as sintering temperature, sintering time, and NiO addition on structural and dielectric properties were investigated. The material exhibits a dielectric constant ɛr ∼36.0 and high values of the product Qf of the intrinsic quality factor Q and the frequency f from 32,170 to 50,000 at microwave frequencies. The dielectric loss tan δ values of ZST ceramics are decreased by low-level doping of NiO, while the temperature coefficient of the resonance frequency τf takes values in the range −2 to +4 ppm/°C. Investigations on whispering gallery modes revealed low dielectric loss in millimetre-wave domain. An intrinsic quality factor of 480 was measured at 115.6 GHz. Dielectric resonators and substrates of ZST material were manufactured. The dielectric properties make the ZST material very attractive to microwave and millimeter-wave applications, such as dielectric resonators, filters, planar antennas, hybrid microwave integrated circuits, etc.

Introduction

The development of modern microwave and millimeter-wave communication systems requires new materials with appropriate properties [1] in specific frequency ranges. For many applications, dielectric materials with low loss and high dielectric constant [2], [3] offer a high degree of miniaturization and improved performances [4].

Dielectric materials based on the ZrO2–SnO2–TiO2 ternary system [5] are very attractive for applications in the microwave domain due to their high dielectric constant, low loss, and controlled temperature coefficient of the permittivity [1], [3]. Such compounds as (Zr0.8, Sn0.2)TiO4 exhibit an almost zero temperature coefficient and offer a great temperature stability to the specific applications such as frequency discriminators [4], low phase-noise dielectric resonator oscillators (DRO) [6], duplexers, and filters [1].

These materials are generally difficult to sinter without additives, especially at lower temperatures. The sinterability increases with additions, such as La2O3, ZnO, NiO, but no mechanisms have been proposed for the improved densification kinetics or for the addition effect on the dielectric loss. The La2O3 addition has been reported as an improving sintering factor, which promotes the grain growth [7].

Section snippets

Experimental

The (Zr0.8, Sn0.2)TiO4 ceramic materials were prepared by standard solid-state reaction technique. Powder oxides ZrO2, SnO2, and TiO2 (equivalent to a weight ratio of 47:15:38) with purity higher than 99.9% were mixed according to the (Zr0.8, Sn0.2)TiO4 stoichiometry. In order to reduce the sintering temperature, 2 wt.% La2O3 and 1 wt.% ZnO were added. The powders were milled in the distilled water for 24 h in a mill with agate balls. All mixtures were dried and treated at 1200 °C for 2 h. The

Results and discussions

The effect of the sintering temperature Ts on ZST samples was analyzed. The investigated samples were generally multiphase. The X-ray diffraction patterns, which are presented in Fig. 4, Fig. 5, showed that the (Zr0.8, Sn0.2)TiO4 compound was the majority phase, corresponding to the standard crystalline data [14]. At lower sintering temperatures, e.g. Ts = 1330 °C, the minority phases, such as ZrTiO4, and a very small amount of unreacted TiO2, were also present. For sintering temperatures Ts equal

Conclusions

The investigations on (Zr0.8, Sn0.2)TiO4 revealed that the increase of the sintering temperature does not affect the dielectric constant essentially, but it results in the decrease of the dielectric loss. Moreover, the dielectric loss is further decreased by the sintering addition (0.2 wt.%) of NiO, while the other dielectric parameters, such as the dielectric constant ɛr and the temperature coefficient τf, do not show significant changes.

The investigations in the millimeter-wave range showed

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