High-Q dielectrics using ZnO-modified Li2TiO3 ceramics for microwave applications
Introduction
The development of microwave dielectric materials for applications as substrates, resonators, filters, and patch antennas in communication systems has received much more attention in the last two decades. A material with a high dielectric constant for volume efficiency is a major requirement in modern wireless communication technology. In addition, a low-dielectric-loss for better selectivity and a near-zero temperature coefficient of resonant frequency () for stable frequency stability is also critical requirements for practical applications.1, 2 Dielectric materials subject to these requirements have been reported for microwave and millimeter wave applications and research on new microwave dielectrics is still ongoing and has become a primary issue in the last few years.3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15
Ternary rock-salt oxide ceramic system AaBbOa+b (where A+ = Li, Na; B4+ = Ti, Sn, Zr; B5+ = Nb and Ta) have been reported due to their excellent microwave dielectric properties.16, 17, 18, 19, 20 Lithium titanium (Li2TiO3), one of the rock-salt type ceramics with a general formula of A2BO3, undergoes an order-disorder phase transition at 1213 °C, and having a high melting point at 1547 °C.21 Moreover, it also possesses a high-dielectric-constant (), a high quality factor (Q × f ∼ 63,500 GHz) and positive value (20.3 ppm/°C).16 In the Li2TiO3–MgO ceramic system, it formed complete solid solution with MgO and order-disorder phase transition with increasing MgO content. Solid solution compositions may also be written as Li2/3(1−x)Ti1/3(1−x)MgxO proposed by Castellanos and West21 In addition, the (1 − x)Li2TiO3–xMgO solid solution replacement mechanism could be proposed as 2Li++Ti4+ ↔ 3Mg2+, where charge balance was maintained.22 When x = 0.24, an excellent combination of microwave dielectric properties (, Q × f = 106,226 GHz, and a ) can be obtained.
In the present work, an inexpensive, easy to process ceramic system is proposed for applications in today's HIPERLAN (high-performance radio local area network, 5150–5350 MHz) antennas. The (1 − x)Li2TiO3–xZnO solid solution (can be written as Li2(1−x)Ti(1−x)ZnxO(3−2x)) was synthesized to investigate its microwave dielectric properties because the ionic radii of Zn2+ (0.74 Å, CN = 6)23 are similar to that of Li+ (0.76 Å, CN = 6)23 and Ti4+ (0.605 Å, CN = 6).23 The resultant microwave dielectric properties analysis were based on the densification, X-ray diffraction (XRD) patterns, and microstructures of the ceramics. The correlation between the microstructure and the Q × f value was also investigated.
Section snippets
Experimental procedure
Sample of Li2TiO3 was synthesized by conventional solid-state methods from individual high-purity oxide powders (99.9%): Li2CO3 and TiO2. The initial oxide powders were mixed and ground in an agate ball mill together with distilled water for 24 h. The wet mixtures were dried at 100 °C, thoroughly milled before they were calcined 800 °C for 2 h. The calcined powders were mixed according to the molar fraction (1 − x)Li2TiO3–xZnO (x = 0.1–0.5). The fine powder with 3 wt% of a 10% solution of PVA as a
Results and discussion
Fig. 1 illustrates the room temperature XRD patterns recorded from the (1 − x)Li2TiO3–xZnO ceramic system sintered at different temperatures for 2 h. A rock-salt monoclinic phase of Li2TiO3 type (ICDD-PDF#00-033-0831), belonging to the space group C2/c (15), was identified as the main phase implying a forming of solid solution. Additional phase formation was not detected throughout the complete range of mixtures under test. However, some ZnO and Zn2Ti3O8 were identified for specimen with x = 0.5,
Conclusion
The microstructures and the microwave dielectric properties (1 − x)Li2TiO3–xZnO (x = 0.1–0.5) ceramic system were investigated. In order to achieve temperature stability and low dielectric loss for microwave antenna applications, the pure Li2TiO3 using ZnO modified. The increase of dielectric loss at high-level ZnO addition (x > 0.3) was owing to the intensity of the (0 0 2) superstructure reflection decreased and became disordered rock-salt structure. The dielectric constant is mainly controlled by
Acknowledgement
This work was financially sponsored by the National Science Council of Taiwan under grant NSC 100-2221-E-006-124-MY3.
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2022, Ceramics InternationalCitation Excerpt :As shown in Fig. 6(b), porous structures are displayed in SEM images of the x = 0.02 composition. Similar phenomena were reported in the literature [22,23,27,28]. These pores of sintered samples were mainly distributed at the grain boundaries or inside the grains.