Characterization and dielectric behavior of a new dielectric ceramics MgTiO3–Ca0.8Sr0.2TiO3 at microwave frequencies
Introduction
The growing importance of ceramic dielectrics for applications as microwave oscillators, filters, etc., has led to great advances in the material research and development of dielectric ceramic systems [1]. Miniaturization of microwave circuits using low loss and temperature stable dielectric ceramic resonators has spurred the wireless communication industry enormously.
Basically, a dielectric resonator is a ceramic compact with high dielectric constant (ɛr > 25), low dielectric loss or high quality factor (Q > 2000) and good temperature stability (near-zero temperature coefficient of resonant frequency, τf) at microwave frequencies [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13].
Most of the ceramic dielectrics developed so far for microwave applications are composed of mixed phases consisting of different compounds in the multi-component systems. In order to meet the requirements for use in microwave resonators and filters, dielectric materials must satisfy stringent physical properties. These requirements greatly restrict the number of materials that can be considered for use in actual devices. With this objective, many dielectric ceramic compositions such as BaTi4O9/BaTi9O20 [14], (Zn,Sn)TiO4 [15], Ba(Mg1/3Ta2/3)O3 [16], [17], Ba(Zn1/3,Ta2/3)O3 [18], etc., have been developed and successfully integrated with microwave circuits.
Two conventional approaches are usually employed in the development of excellent dielectric ceramics; one is to create a new dielectric ceramic material and the other is to combine more than two materials with characteristic compensation. The most popular method is to mix two or more compositions with different dielectric properties. In other words, to adjust the temperature coefficient (τf) to zero, two or more compounds having negative and positive τf values are used to form a solid solution or mixed phases [19].
MgTiO3-based ceramics has been widely applied to dielectrics in resonators, filters and antennas for communication, radar, and global positioning systems operated at microwave frequencies. MgTiO3–CaTiO3 (MCT hereafter) ceramics have an ilmenite-type structure, which belongs to the trigonal space group . In the microwave frequency range, MgTiO3 ceramics show good dielectric properties: dielectric constant (ɛr) ∼17, quality factor (Q × f value) ∼160,000 (GHz), and temperature coefficients of resonant frequency (τf) approximately −51 ppm/°C [20]. Instead of CaTiO, Ca0.8Sr0.2TiO3 ceramics having much higher dielectric properties of ɛr ∼181, Q × f value ∼8300 (GHz) and a large positive τf value ∼991 ppm/°C [21] than that of CaTiO3 (Table 1) was chosen as a τf compensator for MgTiO3.
In this study, Ca0.8Sr0.2TiO3 was added to MgTiO3 to make a ceramic system of (1 − x)MgTiO3–xCa0.8Sr0.2TiO3, which demonstrated an effective compensation of its τf value. The resultant microwave dielectric properties were analyzed using densification, X-ray diffraction patterns, and the microstructures of the ceramics. The correlation between the microstructure and the Q × f value was also investigated.
Section snippets
Experimental procedures
The raw materials MgO and TiO2 were mixed according to the composition of MgTiO3 and the purity of these powders was higher than 99.9%. They were milled with ZrO2 balls in distilled water for 24 h, then dried and calcined in air at 1100 °C for 4 h. CaCO3, SrCO3 and TiO2 were mixed aside according to the stoichiometry of Ca0.8Sr0.2TiO3. They were milled with ZrO2 balls in distilled water for 24 h, then dried and calcined in air at 1100 °C for 4 h. After that, the two kinds of calcined powders were
Results and discussion
Table 2 demonstrates the microwave dielectric properties of (1 − x)MgTiO3–xCa0.8Sr0.2TiO3 ceramics sintered at 1275 °C for 4 h. Significant variation in the dielectric properties can be observed due to a different compositional ratio. It was mainly a result from a difference in the dielectric properties for each composition. Since the specimen using 0.94MgTiO3–0.06Ca0.8Sr0.2TiO3 ceramic shows a good temperature stability with τf ∼0.7 ppm/°C a more comprehensive and closer investigation on the
Conclusions
Microwave dielectric properties of (1 − x)MgTiO3–xCa0.8Sr0.2TiO3 ceramics were investigated. For (1 − x)MgTiO3–xCa0.8Sr0.2TiO3 ceramics sintered at 1200–1325 °C for 4 h, as the amount of Ca0.8Sr0.2TiO3 (x value) increased from 0.02 to 0.1, the dielectric constant increased from 19.8 to 25.1, the temperature coefficient of resonant frequency (τf) increased from −35.7 to 65.5 ppm/°C, and the Q × f value decreased from 150,000 (GHz) to 107,000 (GHz). At the composition of x = 0.06, the 0.94MgTiO3–0.06Ca0.8Sr
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(Ba<inf>x</inf>Mg<inf>1-x</inf>) (Ti<inf>0.95</inf>Sn<inf>0.05</inf>)O<inf>3</inf> (x = 0.025, 0.05, 0.075 and 0.1) solid solutions as effective Ku-band (12.4–18 GHz) shielders
2020, Ceramics InternationalCitation Excerpt :Therefore, these materials can also be investigated for EMI shielding applications. MgTiO3 is an important dielectric material with a wide range of applications in GPS, WLAN, telecommunication industry, integrated optical devices and optical waveguides as antenna substrates, resonators, capacitors, buffer layers and protective coatings [32–35]. According to earlier reports, the properties of pure MgTiO3 could be improved further by doping and/or addition of other materials in small amounts [36–41].