Optimized microwave dielectric properties of Co- and Ca-substituted Mg0.6Zn0.4TiO3
Introduction
High frequency passive components such as microwave dielectric resonator and antenna have been rapidly developed in the past decades, particularly for telecommunication systems including cellular phones and global positioning systems [1], [2]. An advantage of using dielectric ceramic materials is basically from the potential size reduction of the microwave components with improved performance. Requirements for these dielectric materials must be the combined, excellent dielectric properties such as a high dielectric constant, a low dielectric loss and a near-zero temperature coefficient of resonant frequency (TCF) [3], [4], [5], [6], [7], [8]. These required dielectric parameters are correlated directly to size, frequency selectivity and temperature stability of the given high frequency system.
Numerous material candidates have been introduced to meet the demands of the property requirements [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. The system of MgTiO3–CaTiO3 (MCT) is one of the well-known promising candidates in the field of high frequency components, particularly in that raw materials corresponding to Mg, Ca and Ti are relatively cheaper compared to other Ta or Nb-based materials [12], [13]. The MCT material system has been well documented with demonstration of good microwave dielectric properties as a result of composition optimization based on two end members of modified-ilmenite MgTiO3 (ɛr = 17, Q = 11,000 GHz and TCF = −45 ppm/°C) and perovskite CaTiO3 (ɛr = 170, Qf = 8000 GHz and TCF = +800 ppm/°C) [1], [14], [15]. As one of the best examples, 0.95MgTiO3–0.05CaTiO3 gives ɛr = 21, Qf = 56,000 GHz and TCF = 0 ppm/°C after sintering at 1400 °C [15].
On the other hand, the (MgxZn1−x)TiO3 system have been studied as another promising titanate candidate in this high frequency dielectric field [5], [6]. Compositional investigations to understand the effects of relative contents between Mg and Zn revealed that subsequent microwave dielectric properties strongly depended on the content of Mg. Particularly when focusing on obtaining a higher quality factor, for instance, a potential promising candidate can be found as the composition of (Mg0.6Zn0.4)TiO3 since it possesses a dielectric constant of ∼22.5, a Qf of −10,000 GHz and a TCF value of −75 ppm/°C [5]. Partial utilization of a chemical route for preparing the (MgxZn1−x)TiO3 system showed that firing temperature can be reduced below 1000 °C but there were still limits of the Qf value for the compositions having near-zero TCF [6].
This work intends to further improve the (MgxZn1−x)TiO3 system, particularly in terms of the Qf and TCF values, by starting with one composition of (Mg0.6Zn0.4)TiO3 and then by modifying it sequentially with certain contents of Co and Ca. The subsequent property changes of the material system were primarily investigated to find ultimately an optimal composition. The effects of sintering at excessively high temperatures were also studied.
Section snippets
Experimental
Compositions based on the system of (1 − y)(Mg0.6Zn0.4)1−xCoxTiO3–yCaTiO3, where x = 0–0.25 and y = 0–0.1, were prepared by using the conventional solid state reactions. Reagent-grades MgCO3, ZnO, CaCO3, CoO and TiO2 were used as raw materials. They were weighed and then milled for 24 h in ethyl alcohol. The mixture slurry was dried and calcined at 900 °C for 4 h. The calcined powder was re-milled and then pressed into pellets with a diameter of ∼12 mm and a thickness of ∼6 mm. These pellets were sintered
Effects of Co addition
The effects of Co addition on microstructures and dielectric properties were first investigated in the composition of (Mg0.6Zn0.4)1−xCoxTiO3 that does not contain Ca. It should be mentioned that there has been no second phase observed in the XRD phase analysis within the composition range of x = 0.0–0.25 studied. Only a pure ilmenite structure was observed. Fig. 1 shows the variations in linear shrinkage of (Mg0.6Zn0.4)1−xCoxTiO3 sintered at either 1150 or 1200 °C as a function of x. The linear
Conclusions
The compositional modifications of a microwave dielectric system of (1 − y)(Mg0.6Zn0.4)1−xCoxTiO3–yCaTiO3 were successfully demonstrated with the subsequent optimization of Co and CaTiO3 content for best performance. The addition of Co improved densification at 1200 °C with an accompanying increase in quality factor, while CaTiO3 was effective in enhancing dielectric constant and TCF. The improved quality factor with Co was affected adversely with subsequent addition of CaTiO3, which can be
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