Seo-Yong Cho

MICROWAVE DIELECTRIC PROPERTIES AND APPLICATIONS OF RARE-EARTH ALUMINATES

Rare-earth aluminates, LnAlO 3 (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 τ f is necessary due to the coefficients large negative value. When considering dielectric properties and lattice matching, YalO 3 rather than LaAlO 3 , 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.

Influence of ZnO evaporation on the microwave dielectric properties of La(Zn12Ti12)O3

Microwave dielectric properties of the complex perovskite compound La(Zn12Ti12)O3 (LZT) and the effect of ZnO evaporation on the quality factor were examined. The results show that ZnO evaporation was related with the increase in quality factor. The comparison between weight loss and XRD data shows that defects were induced in the samples sintered in air. To investigate the correlation between quality factor and ZnO evaporation, samples sintered in both air and a ZnO atmosphere were examined. The characterization techniques used for this investigation were XRD and TEM. Samples sintered in a ZnO atmosphere have lower quality factor than those sintered in air. The results were compared to the previously reported case of Ba(Zn13Ta23)O3. The variation of quality factor of LZT samples sintered in a ZnO atmosphere was not related to the degree of cation ordering, lattice distortion and second phase.

Mixture-like behavior in the microwave dielectric properties of the (1-x)LaAlO3-xSrTiO3 system

Abstract Compositions in the (1−x)LaAlO 3 –xSrTiO 3 system were prepared in order to compensate the negative temperature coefficient of the resonant frequency (τ f ) of LaAlO 3 . Their microwave dielectric properties and crystal structure were investigated. Powder X-ray diffraction patterns showed that LaAlO 3 and SrTiO 3 form a complete solid solution, but the permittivity and τ f values exhibited nonmonotonic mixture-like behavior. This peculiar behavior is discussed in terms of the structural trends in the system. The low permittivity and negative τ f of LaAlO 3 appear to dominate the dielectric properties of the (1−x)LaAlO 3 –xSrTiO 3 system for SrTiO 3 concentrations as high as 60%.

Twin structures in lanthanum, praseodymium, and neodymium aluminate ceramics

Abstract Ferroelastic twins in LaAlO3 (LAO), PrAlO3 (PAO), and NdAlO3 (NAO) ceramics were investigated using transmission electron microscopy. In all three materials, (100) twin formation was favored over (110) twins. To confirm the predominance of (100) twins, strain tensors for the two twins were calculated. These calculations demonstrated that (100) twins relieve more strain inherent in the twinned grain than (110) twins. It was observed that, from LAO to NAO, the twin shape was changed from wedge to lamellar, and that the twin width was decreased for a given grain. These results were attributed to the increased lattice distortion from LAO to NAO.

A new microwave dielectric ceramics based on the solid solution system between Ba(Ni1/3Nb2/3)O3 and Ba(Zn1/3Nb2/3)O3

Microwave dielectric properties of the solid solution between Ba(Ni{sub 1/3}Nb{sub 2/3})O{sub 3} and Ba(Zn{sub 1/3}Nb{sub 2/3})O{sub 3} was investigated. Samples were prepared by the conventional mixed oxide method as well as the Pechini method. Sintered samples were analyzed using x-ray diffraction and scanning electron microscopy, and their microwave dielectric properties were measured by the post resonator method. The sample prepared by the Pechini method revealed an excellent dielectric properties, i.e., permittivity of 35.6, quality factor of 5700 at 10 GHz, and very small temperature coefficient of resonant frequency. Order-disorder behavior of this solid solution system was found to depend on processing parameters. Variation in microwave dielectric properties was discussed in terms of order-disorder behavior, chemical composition, mixture rule, etc. {copyright} {ital 1997 Materials Research Society.}

The variation of phase transition behavior on substituting Pb2+ and Sr2+ for a site cations in (Na1/2Bi1/2)TiO3 system

Abstract The phase transition behavior in (Na1/2Bi1/2)1-xPbxTiO3[NBPT(x)] and (Na1/2Bi1/2)1-xSrxTiO3[NBST(x)] with respect to changes in composition and temperature was investigated using X-ray analysis and dielectric characterization. Morphotropic phase boundary(MPB) are located in the composition range of 0.13 < × < 0.15 and 0.25 < × < 0.26 in NBPT(x) and NBST(x), respectively. The phase transition exhibited more 1st order behavior and the dielectric loss could be suppressed at high temperatures with the increase of Pb2+ substitutions. NBST(x) showed diffuse phase transition and the degree of the shift in permittivity peak with the measuring frequency increased with the increase of SrTiO3 content. This increase was correlated with the symmetry at room temperature and decrease of the temperature of maximum permittivity