Microwave Dielectric Properties of the (1&minus;<I>x</I>)La<SUB>2&frasl;3</SUB>TiO<SUB>3</SUB>&ndash;<I>x</I>La(Ti<SUB>1&frasl;2</SUB>Mg<SUB>1&frasl;2</SUB>)O<SUB>3</SUB> System

Abstract

(1 À x)La 2=3 TiO 3 -xLa(Mg 1=2 Ti 1=2 )O 3 ceramics with x ranging from 0.1 to 0.9 were prepared by conventional solid-state reaction. La 2=3 TiO 3 and La(Mg 1=2 Ti 1=2 )O 3 were found to form a solid solution over the whole compositional range. However, the second phase La 2 Ti 2 O 7 existed for x ¼ 0:1 and 0.3. As the x value increased from 0.1 to 0.9, the dielectric constant " r decreased from 65.1 to 28.4. The oxygen vacancies were the main factor affecting the Q Â f values of the (1 À x)La 2=3 TiO 3 -xLa(Mg 1=2 Ti 1=2 )O 3 ceramics. A f value of À5 ppm/ C can be obtained at x ¼ 0:3. Keywords: microwave ceramic, dielectric properties, La 2=3 TiO 3 , La(Mg 1=2 Ti 1=2 )O 3 InstructionRecently, the development of microwave applications, including mobile telephones and satellite communication systems, makes rapid progress due to the improved performance of dielectric resonators and filters. Requirements of microwave dielectric ceramics used as dielectric resonators and filters are combined with high dielectric constant (" r ), high quality factor (Q) and near-zero temperature coefficient of resonant frequency ( f ).1,2) These three parameters are correlated to the size, frequency selectivity and temperature stability of the microwave electronic component.La 2=3 TiO 3 is a perovskite compound with general formula ABO 3 . It is unstable in its pure form due to the high content of vacancies in the A cationic sublattice.3) La 2=3 TiO 3 has become an interesting dielectric material after it was stabilized by Ni, 4) Pb, 5) Al, [6][7][8] or Nb 9) et al. For example, La 2=3 TiO 3 doped with 4% LaAlO 3 has attractive microwave properties: " r ¼ 61:9, Q Â f ¼ 23900 GHz and f ¼ 123 ppm/ C. 7) The value of f of such materials can be adjusted to zero by preparation of solid solution with compounds having negative temperature coefficient of resonant frequency. 10,11) As it has reported, La(Mg 0:5 Ti 0:5 )O 3 ceramics was a promising microwave material for application of dielectric resonators and filters.12,13) It exhibits dielectric constant of 28, Q Â f of 631000 GHz and negative f of À79 ppm/ C. The purpose of the present work thus is to stabilize La 2=3 TiO 3 and adjust the f of La 2=3 TiO 3 by introducing La(Mg 0:5 -Ti 0:5 )O 3 . The sintering and microwave dielectric properties of (1 À x)La 2=3 TiO 3 -xLa(Mg 1=2 Ti 1=2 )O 3 were also investigated. ExperimentIn this work, the La 2 O 3 , TiO 2 and MgO powders were mixed according to the compositions (1 À x)La 2=3 TiO 3 -xLa(Mg 1=2 Ti 1=2 )O 3 (x ¼ 0:1; 0:3; 0:5; 0:7; 0:9) and milled with ZrO 2 balls for 24 h in ethanol and then dried at 120 C for 10 h. The dried powders were calcined at 1250 C for 3 h. A small mount of Al 2 O 3 was added as sintering aid. The calcined powders were milled again for 24 h and dried at 120 C for 10 h. After the dried powders doped with 7 mass% polyvinyl alcohol (PVA) were preformed using cylindrical mold of 16 mm diameter, they were pressed at 2000 kg/cm 2 with cold-isostatic pressing. The pellets were sintered at 1400-1600 C for 3 h in air w...