Hirotsugu Takizawa

Thermoelectric properties of Sn-filled skutterudites

Thermal conductivity, resistivity, Seebeck coefficient, and structure measurements of CoSb3 with tin interstitially placed in the voids are reported. These tin-filled skutterudites were synthesized under high pressure and temperature conditions; they cannot be synthesized under “normal” synthesis approaches. The tin atoms exhibit very large atomic displacement parameters indicating a large “rattling” motion inside their atomic “cages.” The disorder induced by the Sn atoms is a very good phonon scattering mechanism. The thermal conductivity of these compounds is very low with a temperature dependence that is atypical of simple solids. The tin-filled compounds exhibit n-type semiconducting behavior with relatively high Seebeck coefficients for compounds whose electronic properties have not been optimized. The potential for thermoelectric applications is discussed.

Metal-insulator transition and thermoelectric properties in the system (R1−xCax)MnO3−δ (R: Tb, Ho, Y)

(R{sub 1-x}Ca{sub x})MnO{sub 3-{delta}}(R: Tb, Ho, Y) solid solutions with the orthorhombic perovskite-type structure were synthesized and their electrical resistivity and thermoelectric power were measured in the temperature range 80 to 1000 K. At low temperature, the electrical properties of (R{sub 1-x}Ca{sub x})MnO{sub 3-{delta}} are explained by the variable range hopping of electrons model. At high temperature, hopping conduction occurs in composition range 0.4{le}x{le}0.6, while metal-insulator transition occurs in the range 0.7{le}x{le}0.9. Transition temperature increases with increasing average Mn-O distance. It is concluded that the increase in Mn-O distance enhances the magnitude of the electrostatic field, and consequently the transition temperature is raised. (R{sub 1-x}Ca{sub x})MnO{sub 3-{delta}}(R: Tb, Ho, Y, 0.7 {le}x{le}0.9) are candidates as high temperature thermoelectric conversion materials.

Atom insertion into the CoSb3 skutterudite host lattice under high pressure

Abstract CoSb3 with the skutterudite-type structure was synthesized by solid-state reaction and the atom insertion into the body-centered vacant site was attempted under high pressure and temperature conditions. Various 14-group elements were selected for atom insertion under pressure of 30 MPa to 5 GPa. It was found that various elements could be inserted in the body-centered vacant site, resulting in the formation of new filled-skutterudite phase MxCo4Sb12 (M=Si, Ge, Sn, Pb). The degree of insertion (x) increased with increasing the applied pressure. In the case of the Sn–CoSb3 system, nearly 40% of the body-centered vacant site was filled by Sn atom at 5 GPa. The crystal structure of resulting Sn0.4Co4Sb12 was refined by the Rietveld analysis of the X-ray diffraction data.

Synthesis of Gd1 − x Eu x Al3 ( BO 3 ) 4 ( 0 < x ⩽ 1 ) and Its Photoluminescence Properties under UV and Vacuum Ultraviolet Regions

Single phase europium-activated gadolinium aluminum borate, Gd 1 x Eu x Al 3 (BO 3 ) 4 (0 ≤ x ≤ 1) was obtained by the evaporation of its nitrate solution, and calcination at 900 to 1100°C in air. All the solid solution was identified as the isomorphs of huntite. Eu 3+ activated GdAl 3 (BO 3 ) 4 showed intense red emission with CIE chromaticity coordinates of (0.645, 0.330) under 147 nm excitation. According to the excitation and emission spectra, the red emission of Eu 3+ was decreased under excitation of the vacuum ultraviolet (VUV) region (158-160 nm), and increased under excitation UV region (258-260 nm) with the increase of Eu 3+ concentration. In comparison with the absorption data of borates, the 158 nm excitation peak was assigned to the energy level of BO 3 groups. In excitation spectra, the sharp 274 nm peak, corresponding to the 8 S 72 → 6 I 2/11 transition of Gd 3+ in addition to the f-f transitions of Eu 3+ were observed. Consequently, the emission of Eu was induced by the energy transfer of VUV excitation to the activator Eu 3+ ions via the co-activator Gd 3+ in GdAl 3 (BO 3 ) 4 .

Low-temperature growth of high-quality lead zirconate titanate thin films by 28 GHz microwave irradiation

Pb(ZrxTi1−x)O3 (PZT) thin films were coated on Pt/Ti/SiO2/Si substrates by a sol-gel method and then crystallized by 28 GHz microwave irradiation. The elevated temperature generated by microwave irradiation to obtain the perovskite phase is only 480 °C, which is significantly lower than that of conventional thermal processing. X-ray diffraction analysis indicated that the PZT films crystallized well in the perovskite phase. A scanning electron microscopy image showed that the film has a spherulite grain structure and most of the grains are approximately 2 μm in size. The average values of the remanent polarization, coercive field, dielectric constant, and loss of the PZT films are 40μC∕cm2, 50 kV/cm, 1100, and 004, respectively. It is clear that microwave irradiation is effective for obtaining well-crystallized PZT films with good properties at low temperatures.

Transport properties of germanium-filled CoSb3

We report the transport properties of dense polycrystalline Ge filled skutterudites Ge0.25Co4Sb12 and Ge0.05Co4Sb12 perepared by a high-pressure synthesis approach. Low temperature electrical resistivity, Seebeck coefficient, Hall coefficient, and thermal conductivity measurements were performed and compared with those of Co4Sb11Ge and CoSb3. The Ge atoms residing inside the interstitial voids of the skutterudite crystal structure act as electron donors. The lattice thermal conductivity of these compounds is lower than that of CoSb3 but higher than that of other filled skutterudites. The potential for thermoelectric applications is also discussed.

Polarized Raman-scattering study of Ge and Sn-filled CoSb3

Raman-scattering spectra of Ge-filled and Sn-filled CoSb3 were studied as a function of polarization. Polarized Raman-scattering spectra of CoSb3 were used to help distinguish the symmetry of each vibrational mode observed in the filled skutterudite specimens. Seven out of the eight Raman-active Sb vibrational modes of CoSb3 and the Ge-filled skutterudites were identified for each specimen. We also compare our experimental assignments with theoretical calculations for CoSb3. The Sn atoms “rattle” inside the voids of the crystal structure and interact strongly with the lattice vibrations to shift and broaden the Sb-vibrational modes as compared to the spectra of CoSb3. The smaller and lighter Ge atoms however do not have such an effect on the lattice vibrations, indicating that the interaction of Ge with the lattice is relatively weak in comparison.

Synthesis and long-period phosphorescence of ZnGa2O4:Mn2+ spinel

Abstract Novel long-period phosphor ZnGa 2 O 4 :Mn 2+ was prepared by a conventional solid reaction of ZnO, Ga 2 O 3 and Mn(NO 3 ) 2 under condition of > 1000°C for 24 h in flowing Ar gas. The phosphor had green intense emission peaking at 503.6 nm. The improvement of emission intensity was brought out by the higher temperature processing. This implied that long lifetime phosphorescence was closel related to the formation of vacancies at the Zn 2+ sites of the host lattice. The carrier trap level was positioned at 0.71 eV higher than the valence band of host lattice. The possible mechanism for prolonging the emission for a long time was discussed.

Microwave synthesis of Fe-doped β-rhombohedral boron

Iron-doped β-rhombohedral boron was synthesized by 28 GHz microwave irradiation on a powder mixture of iron and β-boron. β-Boron strongly absorbs 28 GHz microwaves, and this strong coupling with microwave energy can be used to promote a reaction with iron dopant. The powder mixture was heated to 1800°C within 2 min by microwave irradiation, resulting in the formation of β-rhombohedral boron interstitially doped with iron. The reaction proceeded rapidly without accompanying grain growth. The XRD analysis and the electrical conductivity measurements revealed successful incorporation of iron into two doping sites of β-boron.

High-pressure synthesis and electrical and magnetic properties of MnGe and CoGe with the cubic B20 structure

Abstract MnGe and CoGe with the cubic B20 structure were synthesized at 4–5.5 GPa and 600–1000°C for 1–3 hr using the belt-type high-pressure apparatus. Both MnGe and CoGe are antiferromagnetic with Neel temperatures of 197 and 120 K, respectively. One expects an electron transfer of 0.7 electron from germanium to transition metal in these compounds. The anomalous temperature dependences of the electrical resistivity, the thermoelectric power, and the magnetic susceptibility of MnGe and CoGe are due to a “temperature-induced local moment.”