Tsuguto Takeuchi

Piezoelectric Properties of Bismuth Layer-Structured Ferroelectric Ceramics with a Preferred Orientation Processed by the Reactive Templated Grain Growth Method

Dense CaBi4Ti4O15 (CBT) and Na0.475Ca0.05Bi4.475Ti4O15 (NCBT) ceramics with a highly preferred {001} orientation were prepared by the reactive templated grain growth (RTGG) method. Plate-like Bi4Ti3O12 (BIT) particles were synthesized by a molten salt technique and used as the reactive template. The template particles were mixed with other oxide and carbonate powders and aligned by tape-casting. During the sintering, oriented CBT and NCBT were formed in situ topotaxially on the oriented BIT particles, and textured CBT and NCBT ceramics were eventually fabricated by the templated grain growth and densification. The Lotgering {001} orientation degree of the textured ceramics exceeded 90% for secondary-laminate sintered specimens. Textured CBT and NCBT ceramics poled in the perpendicular direction to the preferred axis exhibited electromechanical coupling coefficient (k33) and piezoelectric coefficients (d33 and g33) three times higher than the values for nontextured ceramics with the same composition.

Unidirectionally Textured CaBi4Ti4O15 Ceramics by the Reactive Templated Grain Growth with an Extrusion.

Dense CaBi4Ti4O15 (CBT) ceramics with a unique texture were prepared by the reactive templated grain growth (RTGG) method using an extrusion technique. Plate-like Bi4Ti3O12 (BIT) particles, a reactive template for CBT, were mixed with other oxide and carbonate powders and unidirectionally aligned by extrusion. During sintering, oriented CBT grains were formed in situ topotaxially on the oriented BIT particles, and the templated grain growth and their densification eventually fabricated textured CBT ceramics. X-ray diffraction measurements and scanning electron microscopic observations revealed that the plate-like CBT grains were unidirectionally oriented parallel to the extruding direction without uniaxial alignment. Textured CBT ceramics poled in the extruding direction exhibited electromechanical coupling coefficients (k33) and piezoelectric coefficients (d33 and g33) more than two times as large as one of nontextured ceramics with the same composition.

Microcomposite particles Sr3Ti2O7–SrTiO3 with an epitaxial core–shell structure

Abstract Microcomposite particles Sr3Ti2O7–SrTiO3 with an epitaxial core–shell structure were prepared by the hydrothermal treatment of Sr3Ti2O7 core particles. The Sr3Ti2O7 core particles synthesized in a fused salt had a plate-like morphology with a well developed {001} plane of the Ruddlesden–Popper type crystal structure. The particles after hydrothermal treatment have an anisotropic morphology and a core–shell microcomposite structure: the plate-like Sr3Ti2O7 particles covered with SrTiO3 fine particulates and thin layers. TEM analysis revealed that the SrTiO3 shell on a Sr3Ti2O7 core particle had an epitaxial relationship with the core such that {100} SrTiO3//{001} Sr3Ti2O7 and [010] SrTiO3//[010] Sr3Ti2O7. The possible formation mechanisms of the SrTiO3 phase were through solution–precipitation and the extraction of Sr–O layers from the Sr3Ti2O7 structure under the hydrothermal conditions. The processing method will be extended to the other type of microcomposite with an anisotropic morphology and an functional surface phase.

Texture design for microwave dielectric (Ca0.7Nd0.3)0.87TiO3 ceramics through reactive-templated grain growth

Abstract Plate-like Ca3Ti2O7 (CT) and Nd2Ti2O7 (NT) particles were synthesized in molten salts and used as reactive templates for the preparation of highly textured (Ca0.7Nd0.3)0.87TiO3 bulk ceramics (CNT) with preferred pseudocubic 〈100〉 and 〈110〉 orientations, respectively. During flux growth CT and NT particles developed facets parallel to the pseudocubic {100} and {110} planes, respectively, in a perovskite unit cell, since those planes correspond to the interlayers of the layered perovskite-type crystal structures. Complementary reactants for the CNT stoichiometry were wet-mixed with the reactive templates and the slurries were tape-cast. Then stacked tapes were heat-treated for dense single-phase CNT ceramics with a distorted and A-site deficient regular perovskite-type structure. The CNT ceramics prepared with CT and NT reactive templates exhibited strong pseudocubic 100- and 110-family x-ray diffraction peaks, respectively, with other peaks drastically suppressed when non-perovskite sources were used as complementary reactants. The textured ceramics possess unique microstructures; as either parallel or obliquely stacked block structures with a pseudocubic {100} plane faceted. The pseudocubic {100}-and {110}-textured CNT ceramics exhibited ∼10 and ∼20% higher products of the dielectric quality factor and frequency, Q · f, respectively, than conventional ceramic sintered at the same temperature. When Q · f is compared based on the same grain size, the {100}-textured CNT exhibited 27% higher values than non-textured while relative permittivity and temperature coefficient of resonant frequency were of similar values. Simple geometrical relationships between electric field and penetrated pseudocubic {hk0}-type grain boundaries must lead to the reduced scattering and dielectric loss.