Microwave dielectric properties and applications of rare-earth aluminates
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Microwave dielectric properties and applications of rare-earth aluminates Seo-Yong Cho School of Materials Science and Engineering, Seoul National University, Seoul, 151-742, Korea
In-Tae Kim Ceramics Division, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea
Kug Sun Hong School of Materials Science and Engineering, Seoul, National University, Seoul, 151-742, Korea (Received 6 October 1997; accepted 16 April 1998)
Rare-earth aluminates, LnAlO3 (Ln Dy, Er, Gd, La, Nd, Pr, Sm, and Y) were prepared using the mixed oxide method, and their microwave dielectric properties were examined at X-band. Most rare-earth aluminates have suitable permittivities and quality factors for applications as dielectric resonators, but a modification of tf is necessary due to the coefficient’s large negative value. When considering dielectric properties and lattice matching, YAlO3 , rather than LaAlO3 , was suggested as a promising substrate material for microstrip antennas utilizing high-temperature superconductor thin films. Rare-earth aluminates with a rhombohedral structure exhibited larger permittivities than those with an orthorhombic structure. This difference was attributed to the difference in ionic size and coordination number. It was demonstrated that a nonzero magnetic susceptibility of rare-earth aluminates has an adverse effect on their quality factor. An abrupt variation in the temperature coefficient of permittivity was discussed in terms of oxygen octrahedra tilting.
I. INTRODUCTION
Recently many efforts have focused on developing dielectric materials with very high quality factors, Q 3 f (Q ø tan d 21 ; f: measuring frequency), for the use as dielectric resonators and as substrates for hightemperature superconducting (HTSC) microwave devices.1,2 The quality factor of dielectric resonators determines their frequency selectivity. In addition to a high quality factor, a high relative permittivity (er ) and very small temperature coefficient of resonant frequency (tf ) are necessary for the size reduction and stability of resonators, respectively. Many compounds such as (Ba, Pb)O–Nd2 O3 –TiO2 , (Zr, Sn)TiO4 , Ba(Zn1/3 Ta2/3 )O3 , Ba(Mg1/3 Ta2/3 )O3 , and (Mg, Ca)TiO3 have therefore been developed.3 Although these compounds exhibit good dielectric properties, a large demand exists for new materials. In the case of HTSC microwave devices, the major function of superconducting thin films is generally as an electrode with ultralow loss. The total loss of these devices is the sum of the conduction loss and the substrate dielectric loss. Substrates with a very high quality factor are required in order to prepare very sensitive HTSC filters, resonators, and antennas since the surface resistivity of YBa2 Cu3 O72x (YBCO) is on the order of 1026 Vcm corresponding to a quality factor of 105 .4 Quality factor of LaAlO3 , the most popular 114
http://journals.cambridge.org
J. Mater. Res., Vol. 14, No. 1, Jan 1999
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