Microwave dielectric properties of RETiTaO 6 (RE = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, Al, and In) cerami
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Pezholil Mohanan Department of Electronics, Cochin University of Science and Technology, Cochin-682 022 India
Mailadil Thomas Sebastiana) Regional Research Laboratory, Trivandrum-695 019 India (Received 22 February 2002; accepted 10 July 2002)
Microwave dielectric ceramics based on RETiTaO6 (RE ⳱ La, Cc, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Y, Er, Yb, Al, and In) were prepared using a conventional solid-state ceramic route. The structure and microstructure of the samples were analyzed using x-ray diffraction and scanning electron microscopy techniques. The sintered samples were characterized in the microwave frequency region. The ceramics based on Ce, Pr, Nd, Sm, Eu, Gd, Tb, and Dy, which crystallize in orthorhombic aeschynite structure, had a relatively high dielectric constant and positive f while those based on Ho, Er, and Yb, with orthorhombic euxenite structure, had a low dielectric constant and negative f. The RETiTaO6 ceramics had a high-quality factor. The dielectric constant and unit cell volume of the ceramics increased with an increase in ionic radius of the rare-earth ions, but density decreased with it. The value of f increased with an increase in RE ionic radii, and a change in the sign of f occurred when the ionic radius was between 0.90 and 0.92 Å. The results indicated that the boundary of the aeschynite to euxenite morphotropic phase change lay between DyTiTaO6 and HoTiTaO6. Low-loss ceramics like ErTiTaO6 (⑀r ⳱ 20.6, Quxf ⳱ 85,500), EuTiTaO6 (⑀r ⳱ 41.3, Quxf ⳱ 59,500), and YTiTaO6 (⑀r ⳱ 22.1, Quxf ⳱ 51,400) are potential candidates for dielectric resonator applications.
I. INTRODUCTION
Dielectric resonators (DRs) have gained a prominent position as the key components of microwave communication systems. This is due to the capability of DRs to reduce the size of microwave components in temperature-stable filters and oscillators. The important characteristics required for a ceramic to be used for dielectric resonator applications1 are (i) low dielectric loss, which enhances the selectivity of the low-noise oscillators and narrow-band filters with low insertion loss; (ii) high dielectric constant (⑀r), which facilitates miniaturization since the size of the resonator is inversely proportional to (⑀r)1/2; and (iii) low or zero temperature coefficient of resonant frequency (f), which is mandatory for the temperature stability of the microwave components. A large number of ceramic dielectric materials have been developed1–7 over the years, though there has been not even a single material that can satisfy all the a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 17, No. 10, Oct 2002
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requirements. The search for new materials with an optimum balance of microwave dielectric properties is one of the most challenging problems in material science. It is interesting to note that most of the useful microwave dielectric ceramic materials developed in recent years are complex oxides of titanates, tantalates,
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