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Detailed Program
Paper Number : EL-P10
Time Frame : 12:00~13:30
Presentation Date : Friday, 28, November
Session Name : Electronic Ceramics
Session Chair 1# : -
Session Chair 2# : -
Microwave Dielectric Properties of (Zn1-xMgx)1.918GeO3.918 Ceramics
Mr. Young jun Eoh
Kyonggi University
Recently many interests have been paid to the dielectric materials applicable to the several types of microwave and millimeter-wave systems. For these applications, the dielectric materials should have a low dielectric constant (K) to minimize the cross-coupling effect with conductors, a high quality factor (Qf) to increase their selectivity, and a near-zero temperature coefficient of resonant frequency (TCF) to ensure the stability of the frequency against temperature changes. Zn2SiO4 ceramics have been widely investigated as the typical candidates for microwave integrated circuits and advanced ceramic substrate materials due to their low K (6.6) and high Qf value (219,000GHz). However, they show a large negative TCF (-61ppm/oC), which force the addition of material with large positive TCF value such as TiO2. Therefore, the zero TCF was obtained while the Qf of the specimens rapidly decreased due to the TiO2 with poor Qf value. In this study, the microwave dielectric properties and crystal structural characteristics of new microwave dielectrics (Zn1-xMgx)1.918GeO3.918 (0.2 ≦ x ≦ 0.6) were investigated as alternative microwave dielectric material of Zn2SiO4. With increasing of Mg2+ substitution, the average grain size of the specimens continuously decreased while the single rhombohedral Zn2GeO4 phase was observed with Mg2+ substitution up to x = 0.4. For the specimens with x = 0.5 and 0.6, the secondary phase of Mg2GeO4 with orthorhombic symmetry was observed due to the phase transition. Generally, Qf value of the microwave dielectric ceramics was affected by microstructure and/or cation ordering of crystal structure. In this case, the microwave dielectric properties of (Zn1-xMgx)1.918GeO3.918 ceramics was significantly dependent on the crystal structural characteristics. Raman spectroscopy and Rietveld refinement were used to evaluate specific crystal structural characteristics such as bond valence and tetrahedral distortion of (Zn,Mg)O4 and GeO4 tetrahedron. Maximum value of Qf(280,000GHz) was observed for the specimens with x = 0.4. This result could be attributed to the cation ordering of crystal structure due to the phase transition from low symmetry (rhombohedral) to high symmetry (orthorhombic). From the Rietveld refinement of crystal structure, the specimens with x = 0.4 showed equilateral hexagonal ring composed of GeO4 and (Zn,Mg)GeO4 tetrahedra, which showed the lowest value of tetrahedral distortion. Also, the cation ordering of the specimens could be evaluated by Raman spectroscopy. With increase of Mg2+ substitution up to x = 0.4, the relative intensity ratio (I779/I748) of Raman spectra increased, which indicated the increase of cation ordering. The K of the specimens decreased with Mg2+ substitution due to the small polarizability of Mg2+ than Zn2+. The TCF of the specimens increased slightly to x = 0.4, while those of x = 0.5 and 0.6 specimens were changed remarkably due to the secondary phase of Mg2GeO4.
Acknowledgements : This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.