High-Q microwave dielectric in the (1 − x)MgTiO3–xCa0.6La0.8/3TiO3 ceramic system with a near-zero temperature coefficient of the resonant frequency
Introduction
Low loss dielectrics with dielectric constants in the 20 s have become most popular materials used for today's GPS patch antennas, WLAN band-pass filters and even for 5.8 GHz ISM band filters [1], [2], [3]. It was attributed to the fact that increasing carrier would render materials with high dielectric constant a less of interest. On the other hand, high quality factor (inverse of the dielectric loss, Q = 1/tan δ) would play a more prominent role instead [3]. Being one of the leading microwave dielectric materials, magnesium titanate (MgTiO3) exhibits a high Q × f of 160,000 GHz (at 7 GHz), a εr of 17, and a τf of − 50 ppm/°C [4]. One of the most important properties for a resonator to contain is a near-zero τf. This can be achieved by combining a positive-temperature-coefficient material with a negative one [5], [6], [7]. With the addition of CaTiO3, ceramic 0.95MgTiO3–0.05CaTiO3 possesses a εr of ~ 21, a Q × f of ~ 56,000 GHz (measured at 7 GHz), and a zero τf value. Similar to CaTiO3, Ca0.6La0.8/3TiO3 was reported to have a εr of 109, a Q × f of ~ 17,600 GHz (at 4.5 GHz), and a large positive τf of ~ 213 ppm/°C [8] and has been shown as an effective τf compensator to other materials [9]. Therefore, a (1 − x)MgTiO3–xCa0.6La0.8/3TiO3 system is likely to give a temperature-stable resonant frequency. In the present study, the microwave dielectric properties of the (1 − x)MgTiO3–xCa0.6La0.8/3TiO3 system were investigated and proposed as a candidate material for ISM band filters and GPS antennas.
Section snippets
Experimental procedure
The starting materials were high-purity oxide powders (> 99.9%): CaCO3, La2O3, TiO2, and MgO. The powders were separately prepared according to the desired stoichiometry Ca0.6La0.8/3TiO3 and MgTiO3, and ground in distilled water for 12 h in a ball mill with agate balls. The prepared powders were dried and calcined at 1100 °C for 4 h in air. The calcined powders were mixed according to the molar fraction (1 − x)MgTiO3–xCa0.6La0.8/3TiO3 and then remilled for 12 h. A fine powder with 3 wt.% of a 10%
Results and discussion
Table 1 illustrates the microwave dielectric properties of the (1 − x)MgTiO3–xCa0.6La0.8/3TiO3 ceramic system at 1275 °C for 4 h. As the x increased from 0.1 to 0.9, the τf of the ceramic system varied from − 23 to 203 ppm/°C. Because the τf curves went through zero, this indicates that a zero τf can be obtained by appropriately adjusting the x value of the (1 − x)MgTiO3–xCa0.6La0.8/3TiO3 ceramic system. With x = 0.15, the ceramic system shows good temperature stability (τf = 0.5 ppm/°C).
Fig. 1 shows
Conclusion
(1 − x)MgTiO3–xCa0.6La0.8/3TiO3 ceramics showed mixed phases of MgTiO3 as the main phase with some minor phases of Ca0.6La0.8/3TiO3 and MgTi2O5. The presence of the MgTi2O5 phase decreased the Q × f value. The microwave dielectric properties strongly depended on density. With x = 0.15, a near-zero τf value can be obtained for the (1 − x)MgTiO3–xCa0.6La0.8/3TiO3 ceramics. A εr of 25.45, a Q × f of 82,500 GHz (at 8.5 GHz), and a τf of 0.5 ppm/°C were obtained for 85MCLT ceramics sintered at 1275 °C for
Acknowledgement
This work was supported by the National Science Council of the Republic of China under grant NSC 96-2221-E-006-117.
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Processing of low-fired glass-free Li<inf>2</inf>MgTi<inf>3</inf>O<inf>8</inf>microwave dielectric ceramics
2016, Journal of Alloys and CompoundsCitation Excerpt :The comparison of properties between Li2MgTi3O8 (LMT1, LMT2) ceramics and various material systems is listed in Table 1. Some niobate, tantalate and Ti-based compounds demonstrated good microwave dielectric properties with appropriate permittivity (εr) and high quality factor (Q × f) [33–38]. But higher sintering temperatures (≥1200 °C) and larger τf values restricted their further applications in LTCC devices.