High-Q dielectrics using ZnO-modified Li2TiO3 ceramics for microwave applications

https://doi.org/10.1016/j.jeurceramsoc.2012.03.030Get rights and content

Abstract

The microwave dielectric properties of the (1  x)Li2TiO3xZnO (x = 0.1–0.5) ceramic system prepared by mixed oxide route have been investigated. The rock-salt structured (1  x)Li2TiO3xZnO were characterized by using X-ray diffraction spectra, scanning electron microcopy (SEM). The dielectric properties are strongly dependent on the compositions, the densifications and the microstructures of the specimens. The decrease of Q × f value 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. For practical applications, a new microwave dielectric material 0.7Li2TiO3–0.3ZnO is suggested and it possesses a good combination of dielectric properties with an εr of ∼22.95, a Q × f of ∼99,800 GHz (measured at 8.91 GHz), and a τf of ∼0 ppm/°C. A low-loss dielectric resonant antenna using aperture-coupled cylindrical dielectric resonant was designed and fabricated using the proposed dielectric to study its performance.

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 (τf) 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 (εr22.0), a high quality factor (Q × f  63,500 GHz) and positive τf 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)Li2TiO3xMgO 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 (εr=19.2, Q × f = 106,226 GHz, and a τf=3.56ppm/°C) 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)Li2TiO3xZnO 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)Li2TiO3xZnO (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)Li2TiO3xZnO 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)Li2TiO3xZnO (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.

References (34)

  • J.F. Dorrian et al.

    Refinement of the structure of Li2TiO3

    Mater Res Bull

    (1969)
  • H. Ogawa et al.

    Crystal structure of corundum type Mg4(Nb2−xTax)O9 microwave dielectric ceramics with low dielectric loss

    J Eur Ceram Soc

    (2003)
  • H. Tamura

    Microwave dielectric losses caused by lattice defects

    J Eur Ceram Soc

    (2006)
  • C.L. Huang et al.

    Low dielectric loss ceramics in the ZnAl2O4–TiO2 system as a τf compensator

    J Am Ceram Soc

    (2009)
  • C.L. Huang et al.

    High dielectric constant and low-loss microwave dielectric ceramics using (Zn0.95M2+0.05)Ta2O6 (M2+ = Mn, Mg, and Ni) solid solutions

    J Am Ceram Soc

    (2010)
  • C.L. Huang et al.

    Phase relation and microwave dielectric properties of (Zn1−xCox)Ta2O6 system

    J Am Ceram Soc

    (2010)
  • Y. Zhou et al.

    Microwave dielectric properties of Ba2Ca1−xSrxWO6 double perovskites

    J Am Ceram Soc

    (2011)
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