Tohru Moriyama

Low-Temperature Sintering and Microwave Dielectric Properties of MgO Ceramic with LiF Addition

MgO ceramics with a nominal composition of (1-x)MgO–xLiF (x = 0.02–0.10) were synthesized and the effects of LiF addition and sintering conditions on the microwave dielectric properties of MgO ceramics were investigated in this study. With LiF addition, the MgO ceramics were well-sintered at 950 °C in a covered crucible, and the Qf value of the LiF-added MgO ceramic strongly depended on the amount of LiF and sintering time. As a result, the MgO ceramics with a nominal composition of 0.96MgO–0.04LiF (x=0.04) exhibited the highest Qf value of 751,500 GHz (er = 9.6 and TCf= - 56 ppm/°C) when they were sintered at 950 °C for 100 h in a covered crucible. The endothermic reaction of the MgO ceramics was observed with LiF addition by differential thermal analysis and thermograviometry (DTA–TG), and it was considered that the partial melting of MgO with LiF addition to form the liquid phase enhances the sinterability of the MgO ceramics. The crystal structure analysis of the LiF-added MgO ceramics showed a slight increase in the lattice parameters of the MgO ceramics, suggesting the Li substitution for Mg in the MgO structure.

Cation Distributions and Microwave Dielectric Properties of Spinel-Structured MgGa2O4 Ceramics

The Mg2+ and Ga3+ cation distributions in the MgGa2O4 lattice were characterized by the refinement of the crystal structure and the firing temperature dependence of microwave dielectric properties was described in this study. The crystal structure refinement of MgGa2O4 ceramics fired at different temperatures indicated that the degree of inversion x, which represents the Mg2+ and Ga3+ cation distributions in the 8(a) and 16(d) sites in (Mg1-xGax)[MgxGa2-x]O4, slightly decreases from 0.88 to 0.84 with increasing firing temperature from 1500 to 1600 °C. This implies that the Mg2+ cation preferentially occupies the 8(a) site, i.e., the tetrahedral site, with increasing firing temperature. The dielectric constant (er) of the MgGa2O4 ceramics fired above 1520 °C was almost constant (er = 9.2), whereas their Qf significantly increased from 92,000 to 298,000 GHz, depending on the firing temperature. Such an increase in the Qf may be related to the Mg2+ and Ga3+ cation distributions in the MgGa2O4 lattice.

Influence of Zn substitution for Mg on microwave dielectric properties of spinel-structured (Mg1−xZnx)Ga2O4 solid solutions

The microwave dielectric properties and crystal structure of spinel-structured (1 − x)MgGa2O4–xZnGa2O4 ceramics were characterized in this study. From the linear variation in the lattice parameters, the formation of solid solution was obtained over the whole composition range. On the basis of the refined atomic coordinates, the variations in the volume of tetrahedron and octahedron were estimated and the expansion of the MO4 (M = Mg, Zn, and Ga) tetrahedron was recognized. The covalency of the cation–oxygen bond in the 8a site decreased with increasing composition x, suggesting the preferential Zn substitution for Mg in 8a site. The er values of Zn-substituted ceramics were higher than that of MgGa2O4 ceramic. The Q f values higher than 1.8 × 105 GHz were obtained when the ceramics were sintered at temperatures higher than 1475 °C.

Influence of Ag-Doping on Ferroelectric Properties of Bi7Ti4NbO21 Ceramics

The crystal structure and ferroelectric properties of Ag-doped Bi7Ti4NbO21 (BTN-xAg, x = 0–0.015) ceramics were investigated in this study. The small amounts of Ag dopant extremely exerted an influence on the stability of crystal structure of BTN-xAg ceramics sintered at 1160°C and was effective in coarsening the grain structures of BTN ceramics without partial melting. Based on the lattice parameters of Ag-doped BTN ceramics, the solubility limit of Ag substitution for Bi was found to be approximately x = 0.01 and the highest P r value of 14μC/cm2 was obtained.

Synthesis and Ferroelectric Properties of Bi4.5Sr0.5Ti4Fe0.5-x V x O15 Ceramics

The Bi4.5Sr0.5Ti4Fe0.5-x V x O15 (BSTFV) solid solutions which consist of the mixture of Bi5Ti3FeO15 and SrBi4Ti4O15 Aurivillius phases were synthesized and the ferroelectric properties of the solid solutions were characterized in this study. The formation of solid solutions in the composition range of 0-0.05 was recognized on the basis of the variations in the lattice parameters and the small amounts of V substitution also exerted an influence on the ferroelectric properties and grain growth of BSTFV. As a result, the well-saturated P-E hysteresis loop was obtained at x = 0.03 (P r = 16μC/cm2 and E c = 96 kV/cm).

Ferroelectric properties and crystal structure of mixed-layer Bi7-xSrxTi4Nb1-xWxO21 ceramics

ABSTRACT The ferroelectric properties and crystal structure of Bi7-xSrxTi4Nb1-xWxO21 (BSTNW) ceramics were investigated in this study. From the variation in the lattice parameters, the solubility limit of the solid solution was x = 0.5. In the single phase region, the structural distortions of orthorombic and oxygen octahedra significantly varied, depending on the composition x; the distortion of the octahedron increased from 2.0 × 10−2 to 3.8 × 10−2. The remnant polarization (Pr) of the ceramics also increased from 10.4 to 15.1 μC/cm2 in the composition range of 0 – 0.5; therefore, it was considered that such the enhancement of Pr value relates with the structural distortion of octahedra caused by the Sr and W substitutions for Bi and Nb. Moreover, the influence of grain orientation on the ferroelectric properties was also characterized for the ceramics at x = 0.5, using hot-forging method. As a result, the Pr value of grain-oriented BSTNW ceramics was higher than that of randomly oriented ceramic; the Pr value of 20.5 μC/cm2 was obtained in this study.

Temperature dependence of dielectric and ferroelectric properties for (1−x)Bi0.5(Na0.8K0.2)0.5TiO3-xBaZn0.5W0.5O3 lead-free piezoelectric ceramics

ABSTRACT The ferroelectric and piezoelectric properties of (1−x)Bi0.5(Na0.8-K0.2)0.5TiO3-xBaZn0.5W0.5O3 ((1−x)BNKT-xBZW) ceramics have been studied to clarify the temperature dependence of the polarization-electric field (P-E) hysteresis and dielectric properties. Based on the temperature dependence of the dielectric constant, it was found that the depolarization temperature (Td) shifted to lower temperatures, depending on the composition x. The crystal structure refinement of (1−x)BNKT-xBZW ceramics with rhombohedral and tetragonal phases revealed that the fraction of tetragonal phase to rhombohedral phase increased in the composition range of 0-0.05, which implies a formation of a morphotropic phase boundary (MPB). The piezoelectric constant (d33) of the (1−x)BNKT-xBZW ceramics increased with the composition x up to x = 0.05 and the maximum value was 168 pC/N. Such an increase in d33 is attributed to the increase in the dielectric constant at room temperature which is due to the shift of Td; a pinched P-E hysteresis loop was obtained at x = 0.05. The temperature dependence of the P-E hysteresis loops indicated that the shape of the hysteresis loop is closely related to the coexistence of ferroelectric and antiferroelectric phases.

Dielectric and piezoelectric properties of (1-x)(K0.474Na0.474Li0.052)(Nb0.948Sb0.052)O3-xBaMO3 (M = Ti or Zr) ceramics

ABSTRACT The effects of Ba and M (M = Ti or Zr) substitutions for K, Na, Li, Nb and Sb on the ferroelectric and piezoelectric properties of (1-x)(K0.474Na0.474Li0.052)(Nb0.948Sb0.052)O3 (KNLNS)-xBaMO3 ceramics were investigated. X-ray powder diffraction profiles showed the coexistence of rhombohedral-orthorhombic-tetragonal phases, representing a morphotropic phase boundary (MPB), in both (1-x)KNLNS-xBaTiO3 (BT) and (1-x)KNLNS-xBaZrO3 (BZ) ceramics. DSC measurement of (1-x)KNLNS-xBZ ceramics indicated that the Ba and Zr substitutions for A- and B-sites in KNLNS led to the further increase in the dielectric constant in comparison with that of (1-x)KNLNS-xBT ceramics. Moreover, the rhombohedral-orthorhombic phase transition temperature was decreased to room temperature by Ba and Zr substitutions; as a result, a d33 value of 337 pC/N was obtained at 0.94KNLNS-0.06BZ (x = 0.06) ceramic.

Microwave Dielectric Properties of LiF- and CaTiO3-Added MgO with Low Dielectric Loss and Near-Zero Temperature Coefficient of Resonant Frequency for Low-Temperature Cofired-Ceramic

The influences of LiF and CaTiO3 addition on the microwave dielectric properties of MgO ceramics were investigated in this study. The LiF was effective in reducing the sintering temperature of MgO without a detrimental effect on the quality factor (Q?f). The 0.05 mol LiF-added MgO ceramics were sintered at a temperature of 950 ?C which resulted in dielectric constant (?r) and Q?f values of 9.6 and 215,000 GHz, respectively, though the temperature coefficient of resonant frequency (TCf) had a large negative value of -63.5 ppm/?C. In order to achieve near-zero TCf values, (0.95-x)MgO?xCaTiO3?0.05LiF ceramics were also investigated and the formation of a secondary phase in MgO?CaTiO3?LiF ceramics was not observed in the X-ray powder diffraction profiles. Although the Q?f values of the ceramics were lowered by CaTiO3 addition, a near-zero TCf was obtained at x = 0.05 (TCf= -3.2 ppm/?C). The ?r and Q?f values of the 0.90MgO?0.05CaTiO3?0.05LiF (x = 0.05) ceramic were 12.7 and 93,500 GHz, respectively. From these results, it is considered that CaTiO3-added (0.95-x)MgO?xCaTiO3?0.05LiF ceramics are appropriate candidates as high-Q low-temperature cofired-ceramics (LTCCs).

Effects of SrZrO3 addition on piezoelectric properties of Bi0.5(Na0.8K0.2)0.5TiO3 ceramic

(1 − x)Bi0.5(Na0.8K0.2)0.5TiO3–xSrZrO3 [(1 − x)BNKT–xSZ] ceramics were synthesized by conventional solid-state reaction methods, and their ferroelectric and piezoelectric properties were investigated in this study. The X-ray powder diffraction profiles of the ceramics showed a single phase in the composition range of 0–0.07; the formation of morphotropic phase boundary between rhombohedral and tetragonal phases was suggested at compositions lower than x = 0.01. From the temperature dependence of dielectric properties, both the depolarization temperature and transition temperature from a rhombohedral phase to a tetragonal phase decreased, depending on the composition x. A pinched polarization–electric field (P–E) hysteresis loop was obtained for x = 0.005, suggesting the coexistence of ferroelectric and antiferroelectric phases. The remanent polarization and coercive field of the ceramics decreased with increasing composition x, whereas d33 improved from 116 to 191 pC/N in the composition range of 0–0.005.