Hisashi Kaga

Preparation and Thermoelectric Property of Highly Oriented Al-Doped ZnO Ceramics by a High Magnetic Field

Highly oriented Al-doped ZnO ceramics were prepared by a high magnetic field method and their anisotropic thermoelectric properties were examined. The c-axis oriented specimen along the ab-plane showed a higher electrical conductivity compared to the a-axis and non-oriented specimens as a result of high electron mobility. On the other hand, the differences in the Seebeck coefficients and thermal conductivities between oriented and non-oriented specimens were a small. Consequently, the dimensionless figure of merit of the c-axis oriented specimen increased by about 30% compared to the other specimens.

A New Fabrication Process for High-Tc Superconducting Oxide Ceramic Fibers

A new fabrication process for superconducting oxide ceramic fibers, the alginate method, is proposed. This process is based on a gelation of a sodium alginate aqueous solution by sodium ion exchange with protons or multivalent metal ions. Dense Y-Ba-Cu-O superconducting fibers have been successfully fabricated by firing Y-Ba-Cu-alginate fibers at temperatures above 900°C. The fibers have shown a tensile strength of 192 MPa and an end-point critical temperature of 85 K.

Fabrication of c-axis oriented higher manganese silicide by a high-magnetic-field and its thermoelectric properties

Fabrication of c-axis oriented higher manganese silicide by a high-magnetic-field and its thermoelectric properties

Thermoelectric Properties of Doped (ZnO)mIn2O3

The thermoelectric properties of layer-structured homologous compounds of (ZnO)mIn2O3 (m is an integer) were investigated in terms of the detailed dependence of the molar ratio of ZnO to In2O3, n (=ZnO/In2O3), and of doping, such as with Ba2+, Ca2+, Sr2+, and Sn4+, for an In site. Except for the Ba and Sr dopings, all sintered specimens with n≥3 were found to be in the phases of (ZnO)mIn2O3. The highest power factor apparently existed around n=3 while the thermal conductivity was minimum at the range from n=3.5 to 5. The Ca doping effectively reduced the thermal conductivity, resulting in a dimensionless figure of merit of 0.23 (at 1053 K) for n=3.5, which was relatively high among n-type oxides.

Thermoelectric Properties of Highly Textured Ca-Doped (ZnO)mIn2O3Ceramics

Highly textured Ca-doped (ZnO)mIn2O3 (m is an integer) ceramics were fabricated by the reactive templated grain growth (RTGG) method and their thermoelectric properties were examined. Platelike ZnSO43Zn(OH)2 particles were used as reactive templates and mixed with In2O3 and CaCO3 powders into a stack of tapes. In situ formation and subsequent sintering resulted in textured Ca-doped (ZnO)mIn2O3 ceramics. The electrical conductivity of the textured specimen along the ab-plane was almost two times larger than that of the textured specimen along the c-axis and about 30% larger than that of a nontextured specimen. On the other hand, the Seebeck coefficients of the textured specimen exhibited a small anisotropy. The thermal conductivity of the RTGG specimen along the ab-plane was higher than that of the RTGG specimen along the c-axis. However, both specimens showed similar values at high temperatures. As a result, the Ca-doped specimen along the ab-plane with a composite phase of (ZnO)3In2O3 and (ZnO)4In2O3 showed a ZT value of 0.31 (at 1053 K), compared with 0.23 (at 1053 K) for the nontextured specimen.

Physical preparation of metal-oxide composite particles under vacuum

Abstract Using a physical processing apparatus capable of composite particle preparation under vacuum, the effect of vacuum on the preparation of two sets of mixed powders, Al2O3 guest particles-Cu core particles and Cu guest particles-Al2O3 core particles, was investigated. Variations in the yield of formation of the composite particles, the surface flatness and the compactness of the coating layer were discussed. The temperature rise of the particles during processing was also discussed. Moreover, the surface oxygen content of the coating layer under vacuum or under a low pressure flow of Ar or H2 gas was considered.

Fabrication of a- and c-axis oriented Zn0.98Al0.02O by a high magnetic field via gelcasting

Highly a- and c-axis oriented Zn0.98Al0.02O ceramics were prepared by a combination of the high magnetic field and gelcasting techniques, and their thermoelectric properties were examined. The gelcasting made it possible to align the particles preferentially along the a- and c-axis within a short exposure time in the high magnetic field. The particle orientation was not degraded and disturbed by the gelation and subsequent processing. The c-axis oriented specimen along the ab-plane showed a higher electrical conductivity compared with the a-axis and non-oriented specimens as a result of high electron mobility. The differences in the Seebeck coefficients between oriented and non-oriented specimens were a very small. This study indicated that applying the magnetic alignment via gelcasting method provides effective and versatile techniques to fabricate the grain oriented materials without hindering the thermoelectric properties

Zinc Oxide Ceramics with High Mobility as n-Type Thermoelectric Materials

Thermoelectric properties of zinc oxide ceramics are largely influenced by the mobility variation because of the formation of double Schottky barrier at grain boundary. It was demonstrated that magnetic texturing enabled to fabricate highly c-axis oriented ceramics with orientation degree of 100 MRD. This high orientation of grains resulted in periodic grain boundary structures along ab -plane, which was confirmed by edge-on HRTEM images. Hall measurement revealed that the mobility was enhanced at 80% by the magnetic texturing. Furthermore, it was estimated that the effect of magnetic texturing was equivalent to the reduction in the density of trap by 5.9×1012 cm-2.

Fabrication of Bi2-xSbxTe3 (x=0-1.5) Thick Film by Centrifugal Deposition

A Bi2-xSbxTe3(x=0-1.5) thick film thermoelectric element was fabricated using centrifugal deposition and its Sb content-dependent thermoelectric properties were investigated. When the Sb content was low (x=0.5), two types of fine structure along the direction of the thickness were observed. Pole figure measurements revealed that the vicinity of the film surface was composed of single crystal layers oriented along the c-axis and the vicinity of the interface with the substrate was composed of randomly orientated layers. As the content of Sb increased, the degree of orientation improved, and at x=1.5 the entire film was close to a single crystal. A Bi0.5Sb1.5Te3 thick film showed p-type thermoelectric properties and a thermoelectric power factor of 3.5 ×10-3W/mK2. It was thus demonstrated that centrifugal deposition can be used to fabricate thermoelectric elements with high efficiency.