Toshihiko Tani

Lead-free piezoceramics.

Lead has recently been expelled from many commercial applications and materials (for example, from solder, glass and pottery glaze) owing to concerns regarding its toxicity. Lead zirconium titanate (PZT) ceramics are high-performance piezoelectric materials, which are widely used in sensors, actuators and other electronic devices; they contain more than 60 weight per cent lead. Although there has been a concerted effort to develop lead-free piezoelectric ceramics, no effective alternative to PZT has yet been found. Here we report a lead-free piezoelectric ceramic with an electric-field-induced strain comparable to typical actuator-grade PZT. We achieved this through the combination of the discovery of a morphotropic phase boundary in an alkaline niobate-based perovskite solid solution, and the development of a processing route leading to highly 〈001〉 textured polycrystals. The ceramic exhibits a piezoelectric constant d33 (the induced charge per unit force applied in the same direction) of above 300 picocoulombs per newton (pC N-1), and texturing the material leads to a peak d33 of 416 pC N-1. The textured material also exhibits temperature-independent field-induced strain characteristics.

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.

Thermoelectric performance of Bi- and Na-substituted Ca3Co4O9 improved through ceramic texturing

Plate-like particles of Ca3Co4O9 have been prepared in which Bi and Na were partially substituted for Ca to improve the thermoelectric performance. Successfully fabricated dense and highly textured ceramics have been obtained by combining the templated grain growth technique (TGG) with hot-pressing (HP). Ca3Co4O9 single crystals, which have alternating layers of Co–O and Ca–Co–O in the direction of the c-axis, show high electrical conductivity along the layer compared with that across the layer. Hence, a highly oriented TGG + HP specimen showed high electrical conductivity compared with the specimen sintered under uniaxial pressure (UP + PLS). The improved electrical conductivity with high Seebeck coefficient of the highly oriented specimen (TGG + HP) gave a high thermoelectric power factor of 5.9 × 10−4 W m−1 K−2 at 1073 K. The figures-of-merit at 773 K and 1073 K were calculated to be 8.54 × 10−5 K−1 (ZT = 0.066 at 773 K) and 1.69 × 10−4 K−1 (ZT = 0.18 at 1073 K), respectively. These values are quite high among Ca–Co–O polycrystalline systems reported so far.

Fabrication of textured thermoelectric layered cobaltites with various rock salt-type layers by using β-Co(OH)2 platelets as reactive templates

Textured bulk ceramics have been prepared of various cobaltites including the common CdI2-type CoO2 layer (i.e., [Ca2(Co0.65Cu0.35)2O4]0.624[CoO2]: CCO-4-layer, [Bi2M2−xO4]p[CoO2] (M2 = Sr2: BSCO, M2 = Ca1Sr1: B(SC)CO, M2 = Ca2: BCCO)) by using β-Co(OH)2 (CdI2-type) platelets as reactive templates, and their thermoelectric (TE) properties have been examined. All the specimens were found to be highly c-axis aligned ceramics. Especially for CCO-4-layer, B(SC)CO and BCCO, the present study is the first report on the fabrication of textured ceramics. Our textured ceramic specimens exhibited larger values of electrical conductivity (σ) than the ceramics prepared by a conventional solid-state reaction (SSR specimens): e.g., σab = 0.46 × 104 S m−1 for the textured specimen and σSSR = ∼0.26 × 104 S m−1 at 1060 K for the SSR specimen for BCCO. This study indicated that our preparation method using β-Co(OH)2 templates is a versatile and effective technique to fabricate cobaltites, including CoO2 layers, with a high degree of orientation and improved TE properties.

Hidden magnetic transitions in the thermoelectric layered cobaltite [Ca2CoO3]0.62[CoO2]

A positive muon spin rotation and relaxation

Thermoelectric properties of highly textured NaCo2O4 ceramics processed by the reactive templated grain growth (RTGG) method

({\ensuremath{\mu}}^{+}\mathrm{SR})

Dome-shaped magnetic phase diagram of thermoelectric layered cobaltites.

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Thermoelectric Performance of Yttrium-substituted (ZnO)5In2O3 Improved through Ceramic Texturing

[{\mathrm{Ca}}_{2}{\mathrm{CoO}}_{3}{]}_{0.62}[{\mathrm{CoO}}_{2}]

Topotactic synthesis of highly-textured thermoelectric cobaltites

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