Kazunori Oshiro

Fabrication of ultrathin Ni-Zn ferrite films using electron cyclotron resonance sputtering method

Well-crystallized Ni–Zn ferrite (Ni0.4Zn0.6Fe2O4) highly oriented ultrathin films were obtained at a substrate temperature of 200 °C by a reactive sputtering method utilizing electron cyclotron resonance microwave plasma, which is very effective to crystallize oxide or nitride materials without heat treatment. Thin films of Ni–Zn ferrite deposited on a MgO (100) underlayer showed an intense X-ray-diffraction peak of (400) from the Ni–Zn ferrite as compared to similar films deposited directly onto thermally oxidized Si substrates. A 1.5-nm-thick Ni–Zn ferrite film, which corresponds to twice the lattice constant for bulk Ni–Zn ferrite, crystallized on a MgO (100) underlayer.

Fabrication of circulator with coplanar wave guide structure

A circulator with a coplanar wave-guide structure, whose external form is rectangular, was designed and analyzed by using a three-dimensional finite-element method. In this analysis, the nonreciprocal transmission characteristics with an insertion loss of 1.0 dB and an isolation of 33 dB appeared at around 7 GHz. The bandwidth defined as the band with isolation of over 20 dB was 50 MHz. A circulator was fabricated based on the design, and the transmission characteristic of the circulator was measured. The fabricated circulator showed an insertion loss of 4.9 dB and an isolation of 28 dB at around 8 GHz. The bandwidth at isolation over 20 dB was 70 MHz.

Electronic structure and thermoelectric properties on transition-element-doped clathrates

Transition element(TM) doping for group IV clathrates is very interesting from the viewpoint of p-type thermoelectric materials. Group IV clathrates are candidates of high performance thermoelectric materials, because they show low thermal conductivity and high carrier mobility. But almost all clathrate semiconductors show n-type conduction due to excess electrons brought by alkali or alkaline-earth metal elements. We have studied the doping effects of noble metal elements on the electronic structure and thermoelectric properties by means of computational approaches. The electronic structure is calculated by the Full-potential Linearized Augmented Plane Wave (FLAPW) method with the Generalized Gradient Approximation (GGA) based on the density functional theory. The calculated electronic structure shows that TM-substituting clathrates Ba/sub 8/M/sub 6/X/sub 40/(M=Cu, Ag, Au; X=Si, Ge) are p-type semiconductors and have large thermoelectric power(/spl alpha/) at room temperature. The calculated energy of the band gap E/sub g/ is 302 meV in Ba/sub 8/Au/sub 6/Ge/sub 40/, which is smaller than that in Ba/sub 8/Ga/sub 16/Ge/sub 30/(E/sub g/=513 meV ). When La atoms are doped at guest sites, the band gap becomes large: E/sub g/=353 meV(La/sub 2/Ba/sub 6/Au/sub 6/Ge/sub 40/). By using a rigid band and a constant relaxation time approximation, we have calculated the thermoelectric properties. For La/sub 2/Ba/sub 6/Au/sub 6/Ge/sub 40/, we obtained /spl alpha/=240 /spl mu/V/K at a hole concentration n/sub h/=10/sup 20//cm/sup 3/ and at 300 K.

Photocatalytic Performance of Barium-Doped Strontium Tantalate

Sr2Ta2O7, a layered perovskite compound, has been reported to possess most excellent photocatalytic properties among the layered perovskite materials. Recently, we have successfully demonstrated that Ba5Ta4O15 that was prepared under Ta rich atmosphere has high photocatalytic performance as well as Sr2Ta2O7. In this study, several amount of Ba was doped into Sr2Ta2O7, and the photocatalyst samples with a mol ratio of Sr: Ba: Ta = (1-x): x: 1 were prepared by the polymerized complex method to investigate the effect of Ba substitution for the Sr site on photocatalytic activity. The maximum photocatalytic performance was obtained for x = 0.2 near the solid solubility limit, which is three times as high as that of undoped Sr2Ta2O7 and is in the highest level in a series of tantalum photocatalysts reported so far. The increase of photocatalytic activity would be caused by crystal distortion due to doping of Ba ion.

Electronic structure and thermoelectric properties of Skutterudites

Thermoelectric properties on Yb filled skutterudite antimonides are calculated by using a realistic band structure and discussed from the point of view of electronic structure. The electronic structure is calculated by the full-potential linearized augmented plane-wave (FLAPW) method with the local density approximation (LDA). In the band structure calculation of YbFe/sub 4/Sb/sub 12/ the 4f electron is treated by two ways: one is as itinerant electron, another is as localized electron in the Yb ion. In the itinerant case Yb ion shows mixed valence between divalent and trivalent, in the localized case trivalent. In both cases the band structure near Fermi level is characterized by Fe: 3d orbital. The band structure of YbCo/sub 4/Sb/sub 12/ is calculated as Yb-filling CoSb/sub 3/, which is a n-type material. The f orbital of the Yb ion is strongly hybridized with the valence band, the conduction band near the bottom affects 6s and 5d of Yb atomic orbital. Electronic transport coefficients are calculated within relaxation time approximation using the linearized Boltzmann equation. Then the electron velocity is calculated by the Fourier interpolation method. The calculated coefficient of thermoelectric power /spl alpha//T is 0.07 /spl mu/V/K/sup 2/ at room temperatures, which is about 1/2 of the experimental value.

Exciton–LO phonon interaction in a quantum dot

Abstract Exciton–LO phonon interaction in a spherical quantum dot embedded in non-polar matrix is studied using the dielectric continuum model and the intermediate-coupling-type theory. The interaction contribution can be divided into the self-energy-type and the screening-type terms. Both terms depend on the size of the quantum dot and tend to cancel especially much in the smaller dot. In the limit of the smaller dot both terms approach to zero and then effects of the exciton–LO phonon interaction vanish. These properties reflect the nature of the exciton–LO phonon interaction in the quantum dot.

Thermoelectric properties on antimonide skutterudites and filled skutterudites

We study thermoelectric properties on IrSb/sub 3/ and the electronic structure of YbFe/sub 4/Sb/sub 12/. The band structure of IrSb/sub 3/ is calculated by the full-potential linearized augmented plane wave (FLAPW) method with and without the consideration of the spin-orbit(SO) interaction. The calculated valence and conduction band structures near the band gap are fitted well by a simple band model, i.e., the Kanes nonparabolic bands and the parabolic bands. Using the simple band model, thermoelectric properties are calculated easily. In the calculation the relaxation time is dealt with the two types of the approximation, i.e. a constant relaxation time and a relaxation time which is inversely proportional to the electronic density of states (DOS). The results indicate that the detailed consideration of the scattering mechanism is necessary. In YbFe/sub 4/Sb/sub 12/ the DOS is obtained by the ab initio band calculation. The DOS at the Fermi energy is in good agreement of the experimental value, and thus the correlation effect of electron-electron interaction due to the localized f-orbital seems to be weak in the system.

Fabrication of deep-sub-millimeter-thick compacts using spark plasma sintering

Nd-Fe-B type powder was sintered using spark plasma sintering method. Fabricated compact sintered at the temperature of 700 °C, is found to be a composite magnet with Nd-Fe-Co-B and α-Fe. The compact sintered at 700 °C shows slightly low coercivity and large remanent magnetization comparing to the compact sintered at 600 °C due to the formation of α-Fe phase, resulting in the large maximum energy product. Maximum energy product tends to decrease with decreasing thickness of sintered compacts below 0.5 mm in thickness.

Thermoelectric properties of Sn-based clathrates

Inorganic clathrates consisting in group-IV elements have crystal structures with large cages. Electronic structure and transport properties of Sn-based clathrate semiconductors have been studied by the ab-initio band structure calculating method. In this calculation the FLAPW method with the GGA is used and the Ga configuration of occupied sites is determined by the comparison of the total energy E/sub tot/ among various configurations. The band gap of the calculated electronic structure E/sub g/ is 0.16 eV, which agrees with the experimental result, estimated by the temperature dependence of the electric conductivity. However the band gap E/sub g/=0.16 eV does not explain the experimental Seebeck coefficient /spl alpha/. When we assume a lager band gap E/sub g/=0.45 eV and a carrier concentration n/sub e/=8/spl times/10/sup 18//cm/sup 3/, the calculated /spl alpha/ reasonably explains the experimental data.

Possibirity of drastic miniaturization of microstrip Y-isolator [Possibirity read Possibility]

Recently miniaturization of electronic parts is strongly required for miniaturization and multi-hnctionarization of mobile communication devices. The current isolator products controlling the propagation direction of electromagnetic wave using gyro-magnetic effect of a ferrite, have minimum size of 4 mm x 4 mm x 1.6 mm which is larger than the other electronic parts. Thus the miniaturization of the isolators is urgently demanded.