Effect of annealing time on structural and microwave dielectric characteristics of Li 2 ZnTi 3 O 8 ceramics
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In the present work, the Li2ZnTi3O8 ceramics were prepared via the solid-state reaction method, afterward annealed at 800 °C in a time variation from 4 to 20 h. The ordering, microstructures and dielectric properties were investigated using x-ray diffraction, scanning electron microscopy, network analyzer, and Raman spectroscopy. The most significant enhancement of quality factor is obtained in the sample annealed for 20 h, while the dielectric constant and temperature coefficient of resonant frequency change slightly. This result mainly attributes to the enhancement of ordering, which could be related to the increase in the Zn–O bond strength in ZnO4 tetrahedra. Meanwhile, the full-width at half-maximum of A1g mode decreased with higher annealing time, which suggested less variation in the Zn–O bond length and a higher degree of ordering. The best combination of microwave dielectric characteristic is obtained in the sample annealed at 800 °C for 20 h: Q f 5 112,400 GHz, er 5 24.500, and sf 5 11 ppm/°C.
I. INTRODUCTION
Nowadays, with the rapid development of wireless communication and satellite systems, low dielectric loss materials with a high dielectric constant (er) and a nearly zero temperature coefficient of resonant frequency (sf) have been more required for the production of passive microwave components such as dielectric resonator, dielectric filter, and dielectric waveguides.1–3 Until now, some dielectric materials, such as (Zr,Sn)TiO4 (ZST), Ba(Zn1/3Nb2/3)O3 (BZN), and Ba(Zn1/3Ta2/3)O3 (BZT) with excellent microwave dielectric properties, have been used in manufacturing dielectric resonators for the wireless communication system.1–7 Newly, the lithium-based spinel structure ceramics have impressed scientists because of the promising dielectric properties to fabricate passive microwave components.8,9 George and Sebastian10,11 reported that Li2ZnTi3O8 (LZT) cubic spinel ceramics sintered at 1075 °C for 4 h exhibited good microwave dielectric properties of er 5 25.6, Q f 5 72,000 GHz, and sf 5 11.2 ppm/°C. The LZT structure is a cubic spinel in which the octahedral sites are ordered into two sets with full occupancy by Ti 41 (12d) and Li11 (4b), respectively, and the tetrahedral sites are occupied by a disordered mixture of Li11 and Zn21(8c).12 Furthermore, the ordering of Li11 and Ti 41in the octahedral sites with the 1:3 ratio is completed, as represented by the formula (Li 0.5 Zn 0.5 ) tet [Ti1.5 Li0.5 ] oct O4 .12
Contributing Editor: Ian M. Reaney a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2015.107 J. Mater. Res., Vol. 30, No. 10, May 28, 2015
In addition, the benefits of the LZT ceramic are its dielectric properties, cost efficiency, and low preparation temperature compared to the mentioned compounds. Meanwhile, it could be a good candidate for the production of passive microwave components if its dielectric loss decreased.9 Since the properties of LZT ceramics depend on the ordering and occupancy of cations in tetrahedral (8c) and octahedral (4b and 1
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