Atsushi Yokoi

Low-Temperature Sintering and Microstructure of Mg4(Nb2-xVx)O9 Microwave Dielectric Ceramic by V Substitution for Nb

The effects of microstructure on the microwave dielectric properties of Mg4Nb2O9 (MN) were investigated and the possibility of low-temperature sintering of Mg4(Nb2-xVx)O9 (MNV) by V substitution for Nb was also discussed in this study. The quality factors (Qf) of MN ranged from 10338 to 217390 GHz with increasing sintering temperatures up to 1300°C, resulting in the grain growth and densification of the samples. In the case of V substitution for Nb, it was found that the limit of the solid solution is lower than x=0.125; the improvements in the sinterability and apparent density of MNV at x=0.0625 were recognized at the sintering temperature of 1025°C. The dielectric constants (er) and the Qf values of MNV at x=0.0625 sintered at 1025°C were 11.6 and 160256 GHz, respectively. Thus, it was clarified that a small amount of V substitution for Nb is effective in reducing the sintering temperatures and improving the microwave dielectric properties of MNV.

Microwave Dielectric Properties of Perovskite-Like Structured Ba8Ta6(Ni1-xMx)O24 (M=Co, Cu, and Zn) Solid Solutions

Perovskite-like structured Ba8Ta6(Ni1-xMx)O24 (M=Co, Cu, and Zn) solid solutions were prepared by solid-state reaction method and the crystal structure and microwave dielectric properties of the solid solutions were characterized in this study. The structural refinements of all the solid solutions in the composition range of 0–1 fit well with the hexagonal structure with the space group P63cm (No. 185). All the lattice parameters of the solid solutions linearly increased, depending on the composition x and, such variations in lattice parameters were attributed to an increase in the (Ni/M)–O bond. No remarkable differences between the dielectric constants were observed with M substitution for Ni and these dielectric constants ranged from 27 to 29. On the other hand, the Qf value of the solid solutions was significantly influenced by M substitution for Ni. In the case of Ni substitution for Cu, the Qf value decreased from 81000 to 8500 GHz. The temperature coefficient of resonant frequency (τf) varies from 33 to 77 pm/°C with Cu substitution for Ni and such a variation in τf may be related to the increase in the covalency of M–O bonds in an MO6 octahedron caused by an increase in temperature.

Crystal structure and microwave dielectric properties of aeschynite-type R(W0.5Ti1.5)O6 (R = Nd, Sm, Eu, Gd and Dy) ceramics

The microwave dielectric properties and crystal structure of aeschynite-type structured R(W0.5Ti1.5)O6 (R=Nd, Sm, Eu, Gd and Dy) ceramics were characterized to investigate a microwave dielectric ceramic that is suitable for use in the base station. A single phase of R(W0.5Ti1.5)O6 ceramics was obtained; it was recognized that the lattice parameters of the ceramics vary linearly, depending on the ionic radii of the R ions. The dielectric constants of R(W0.5Ti1.5)O6 ceramics were also related to the ionic radii of the R ions and changed linearly with the ionic radii. From the result of crystal structure analysis, a decrease in the covalency of the (W,Ti)-O bond was observed with increasing the ionic radii of the R ions; these variations in covalency corresponded to the linear increases in the dielectric constants of the R(W0.5Ti1.5)O6 ceramics. The Qf and τf values significantly depended on the R ions; the highest Qf value with a near zero τf value was obtained for the Sm(W0.5Ti1.5)O6 ceramic (Qf =35000 GHz and τf =-0.7 ppm/°C).

Effects of Ca Substitution for Sr on Ferroelectric Properties and Crystal Structure of (Sr1-xCax)Bi3Ti2NbO12

The effects of substituting Ca for Sr in (Sr1-xCax)Bi3Ti2NbO12 compounds on the ferroelectric properties and the weight fraction of the orthorhombic phase were investigated in this study. Mixtures of orthorhombic and tetragonal phases of (Sr1-xCax)Bi3Ti2NbO12 compounds were obtained in the composition ratio range of x=0–0.4. The weight fraction of the orthorhombic phase calculated by Rietveld analysis increased from 89.66 to 92.83 wt % as the composition ratio x increased from 0 to 0.25. The remanent polarization (Pr) of the samples in the composition ratio range of 0–0.25 was greatly improved by the substitution of Ca for Sr, and the maximum value of Pr was 9.31 µC/cm2 at x=0.25. These results suggest that the Pr values of (Sr1-xCax)Bi3Ti2NbO12 compounds are related to the weight fraction of the orthorhombic phase, because the variation in remanent polarization showed the same tendency as the variation in the weight fraction of the orthorhombic phase. Moreover, the Curie temperature (Tc) increased as the tolerance factor (t) of the pseudo-perovskite block (SrBiTi2NbO10)2- decreased; it is suggested that the increase in Tc is connected to the structural distortion of the pseudo-perovskite block.