Tohru Moriyama

Bio: Tohru Moriyama is an academic researcher from Meijo University. The author has contributed to research in topics: Ferroelectricity & Crystal structure. The author has an hindex of 8, co-authored 20 publications receiving 140 citations.

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

Abstract: 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

Abstract: 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.

Crystal structure and microwave dielectric properties of low temperature sintered MgO ceramic with LiF addition

Abstract: The (1−x)MgO–xLiF ceramics (x = 0.02–0.08) were successfully sintered when the ceramics were sintered at 950 °C for 4 h in covered crucible. From the crystal structure analysis, it was found that a small amount of Li+ cation occupied Mg2+ site in MgO ceramic; the formation of oxygen vacancy induced by Li substitution for Mg was suggested by the evaluation of the bulk conductivity and the calculation of density of state (DOS) for the (Mg13O43)−60 and (Mg11Li2O42)−58 cluster models. As for the microwave dielectric properties of the (1−x)MgO–xLiF ceramics, the dielectric constant er and the temperature coefficient of resonant frequency values of the ceramic were independent of the lithium fluoride (LiF) content, and these values were approximately 9.5 and −62 ppm/°C. On the other hand, the quality factor $$(Q \cdot f)$$ values strongly depended on the LiF content. As a result, the highest value of 282,230 GHz was obtained at x = 0.04. From these results, it is determined that the LiF addition is effective in reducing the sintering temperature of MgO without any detrimental effect on the microwave dielectric properties of MgO ceramics.

Synthesis and ferroelectric properties of bismuth layer-structured (Bi7−xSrx)(Fe3−xTi3+x)O21 solid solutions

Abstract: Bismuth layer-structured (Bi 7− x Sr x )(Fe 3− x Ti 3+ x )O 21 (BSFT) ceramics were synthesized and the ferroelectric properties and crystal structure were investigated. X-ray powder diffraction profiles and refinement of the lattice parameters indicated single phase BSFT was obtained in the composition range 0–1.5. The lattice parameter b of BSFT remained almost constant, while a slight decrease in the lattice parameter a was observed by the Sr and Ti substitution for Bi and Fe, respectively, which indicated an increase in the orthorhombicity. The dependence of the BSFT lattice parameter on temperature implied a phase transition from the orthorhombic to the tetragonal phase, which was in good agreement with the Curie temperature. The remnant polarization P r , of BSFT was significantly improved by the Sr and Ti substitution for Bi and Fe, and ranged from 9 to 16 μC/cm 2 , although no remarkable variation in the coercive field E c was observed. As a result, a well-saturated P – E hysteresis loop of BSFT ceramic was obtained at x =0.5 with a P r of 30 μC/cm at an applied voltage of 280 kV/cm.

Crystal structure and ferroelectric properties of Ca(Cu3−xMx)Ti4O12 (M = Fe and Ni) ceramics

Abstract: M-substituted Ca(Cu3−xMx)Ti4O12 (CCMTO) ceramics, where M = Fe and Ni, were synthesized and the influence of M substitutions for Cu on the crystal structure and ferroelectric properties of CCMTO ceramics were investigated in this study. From the variations in the lattice parameters of CCMTO ceramics, the solubility limit of Ni substitution for Cu in CaCu3−xNixTi4O12 (CCNTO) ceramics was x = 0.2, whereas that of CaCu3−xFexTi4O12 (CCFTO) ceramics was x = 0.05. The crystal structural analysis of CCMTO ceramics revealed that the single phase of CCMTO ceramics belongs to the I23 non-centrosymmetric space group of I23; as a result, the Pr and Ec values of CCFTO ceramics at x = 0.05 were 1.8 μC/cm2 and 40 kV/cm, respectively. The ferroelectric behavior of CCMTO ceramics by the M substitutions for Cu may be related to the displacement of a Ti4+ cation in the TiO6 octahedra and tilting of the Ti–O–Ti angle because of the non-centrosymmetric space group.

Materials development and potential applications of transparent ceramics: A review

Abstract: Transparent ceramics have various potential applications such as infrared (IR) windows/domes, lamp envelopes, opto-electric components/devices, composite armors, and screens for smartphones and they can be used as host materials for solid-state lasers. Transparent ceramics were initially developed to replace single crystals because of their simple processing route, variability in composition, high yield productivity, and shape control, among other factors. Optical transparency is one of the most important properties of transparent ceramics. In order to achieve transparency, ceramics must have highly symmetric crystal structures; therefore, the majority of the transparent ceramics have cubic structures, while tetragonal and hexagonal structures have also been reported in the open literature. Moreover, the optical transparency of ceramics is determined by their purity and density; the production of high-purity ceramics requires high-purity starting materials, and the production of high-density ceramics requires sophisticated sintering techniques and optimized sintering aids. Furthermore, specific mechanical properties are required for some applications, such as window materials and composite armor. This review aims to summarize recent progress in the fabrication and application of various transparent ceramics.

Electric field-induced ultrahigh strain and large piezoelectric effect in Bi1/2Na1/2TiO3-based lead-free piezoceramics

Abstract: Lead-free piezoelectric ceramics, (Bi 1/2 Na 1/2 ) 0.935 Ba 0.065 Ti 1− x (Fe 1/2 Nb 1/2 ) x O 3 , were prepared by the conventional solid-state reaction method. The room temperature ferroelectric P–E loops illustrated the transition of ferroelectric domains. The composition and electric field dependent strain behavior of this system were investigated. A highest unipolar strain of ∼0.422% and corresponding normalized strain, d * 33 ( = S max / E max ) of 844 pm/V under an applied field of 50 kV/cm were observed at x = 0.020, due to the destabilization of the ferroelectric order. It is observed that the unipolar strain of BNBT-0.02FN is temperature insensitive, and the d * 33 maintains a high value of ∼600 pm/V at 90 °C. The observed large strain behavior can be attributed to the transformation from the ferroelectric phase at zero electric field into a relaxor ferroelectric phase under an applied electric field. The large strain response with good temperature stability would be quite suitable for environmental-friendly solid-state actuator applications.

Microwave dielectric properties and crystal structures of spinel-structured MgAl2O4 ceramics synthesized by a molten-salt method

Abstract: The microwave dielectric properties and crystal structures of MgAl2O4 ceramics synthesized using either molten-salt (MA-M) or solid-state reaction (MA-S) methods were characterized in this study. Raman and 27Al solid-state nuclear magnetic resonance spectra indicated that Al3+ cations primarily occupied in tetrahedral sites of the MA-M ceramic. The degree of inversion x, i.e., degree of cation disorder in tetrahedral and octahedral sites, of the MA-M was higher than that of MA-S; such the preferential site occupation of Al3+ cations enhanced the covalency of the M O bonds in the MO4 tetrahedra (M = Mg and Al), leading to a decrease in the lattice parameters. The Q·f of the MA-M fired at 1600 °C was 201,690 GHz, while the MA-S synthesized at 1600 °C exhibited a Q·f of just 85,100 GHz. Based on these results, an intermediate spinel structure with a greater x evidently has a higher Q·f, and therefore the cation distribution is closely related to the Q·f of the ceramic.

High-throughput determination of structural phase diagram and constituent phases using GRENDEL

Abstract: Advances in high-throughput materials fabrication and characterization techniques have resulted in faster rates of data collection and rapidly growing volumes of experimental data. To convert this mass of information into actionable knowledge of material process-structure-property relationships requires high-throughput data analysis techniques. This work explores the use of the Graph-based endmember extraction and labeling (GRENDEL) algorithm as a high-throughput method for analyzing structural data from combinatorial libraries, specifically, to determine phase diagrams and constituent phases from both x-ray diffraction and Raman spectral data. The GRENDEL algorithm utilizes a set of physical constraints to optimize results and provides a framework by which additional physics-based constraints can be easily incorporated. GRENDEL also permits the integration of database data as shown by the use of critically evaluated data from the Inorganic Crystal Structure Database in the x-ray diffraction data analysis. Also the Sunburst radial tree map is demonstrated as a tool to visualize material structure-property relationships found through graph based analysis.