Hironori Asada

Synthesis and thermoelectric properties of type-VIII germanium clathrates Sr8AlxGayGe46−x−y

Nominal Sr8AlxGa16−xGe30 samples with x=6, 8, and 10 crystallized in the type-VIII clathrate structure (I4¯3m, No. 217), while the sample with x=4 crystallized in the type-I clathrate structure (Pm3¯n, No. 223). While a large number of the type-I thermoelectric clathrates exist, only three type-VIII clathrates of Ba8Ga16Sn30, Eu8Ga16Ge30, and Sr8AlxGa16−xSi30 had been synthesized before. The type-VIII Sr8AlxGayGe46−x−y samples (6≤x≤7 and 10≤y≤11) with various carrier concentrations were prepared to investigate their thermoelectric properties. They exhibited the temperature dependences of electrical conductivities and the Seebeck coefficients typical of n type degenerate semiconductors, which almost depended on their carrier concentrations systematically. A relatively large dimensionless figure-of-merit ZT of 0.56 at 800 K was obtained for the type-VIII Sr8Al6.3Ga10.3Ge29.4 sample with a carrier concentration of 3.0×1020 cm−3. This ZT value is comparable to that of 0.62 at 800 K for the type-I Sr8Ga16.5Ge2...

Low Current Magnetic-ram Memory Operation With A High Sensitive Spin Valve Material

Bistable bit state changes of a magnetic RAM memory cell (2/spl times/5 /spl mu/m/sup 2/) made with a highly sensitive spin valve material were performed with pulsed conductor currents I/sub W/ of 5.5 mA//spl mu/m for parallel to antiparallel and 3.0 mA//spl mu/m for the reverse switching. An evaporated spin valve of NiFe(6 nm)/Co(1/spl sim/2 nm)/Cu(5 nm)/Co(6 nm) was used for the material. Output voltage change of 1 mV was obtained for a sense current of 0.5 mA due to the MR change of 3%. A threshold of I/sub W/ for the state change was decreased 20% by applying a transverse external field of 30 Oe, which confirms the selective write operation with the coincident current selection scheme.

Thermoelectric and transport properties of sintered n-type K8Ba16Ga40Sn96 with type-II clathrate structure

This clathrate had a maximum dimensionless figure-of-merit, ZT, of 0.93 at 637 K, which was slightly higher than that of 0.83 for the sintered type-VIII clathrate Ba8Ga16Sn30. We investigated the high-temperature thermoelectric properties, transport properties, electronic structures, and thermal stabilities of the clathrates. The type-II clathrate was found to be superior to the type-VIII clathrate as a thermoelectric material; it had a high thermal stability and melting point, 859 K, high mobility, 141 cm2V−1s−1 at 300 K, because of its low inertial mass, and low high-temperature lattice thermal conductivity, approximately 4 mW cm−1K−1, resulting from a larger unit cell and weaker bipolar thermal conduction. We discuss these properties in terms of the electronic structure and the differences between the two types of clathrate.

Preparation and thermoelectric properties of sintered n-type K8M8Sn38 (M = Al, Ga and In) with the type-I clathrate structure

The maximum dimensionless figures-of-merit ZTmax were 0.13 at 410 K for K8Al8Sn38, 0.25 at 440 K for K8Ga8Sn38 and 0.21 at 420 K for K8In8Sn38, where their room temperature (RT) mobilities were 4.1 cm2 V−1 s−1, 31.0 cm2 V−1 s−1 and 9.8 cm2 V−1 s−1, respectively, their RT effective masses were 4.3, 1.8 and 3.2 me, respectively, and their RT lattice thermal conductivities were 12 mW cm−1 K−1, 14 mW cm−1 K−1 and 13 mW cm−1 K−1, respectively. The analysis of the measured transport properties suggested that these samples suffered from grain boundary scattering at lower temperatures and from alloy disorder scattering and acoustic phonon scattering at higher temperatures. In particular, such a low mobility of the K8Al8Sn38 sample was considered to be caused by strong alloy disorder scattering. Band structure calculations demonstrated that, in order of K8Ga8Sn38, K8In8Sn38 and K8Al8Sn38, their conduction band edges were sharper and their band structures near the gaps were more similar to that of K8Sn46. These features were well consistent with their measured effective masses and mobilities.

Local environment of Mn atoms in IV-VI ferromagnetic semiconductor Ge1−xMnxTe

MnL2,3 x-ray absorption and magnetic circular dichroism study has been performed on IV-VI ferromagnetic semiconductor Ge1−xMnxTe with different Mn compositions x=0.12, 0.32, 0.51, and 1. The absorption and dichroism line shapes are found to be almost independent of the Mn composition, suggesting that the local chemical environment of Mn atoms is the same for Ge1−xMnxTe with the wide range of x. On the basis of the configuration interaction analysis for a MnTe6 cluster, the electronic structure parameters such as the p‐d hybridization and the p‐d exchange constant have been estimated.

Electrochemical synthesis of a nanohybrid film consisting of stacked graphene sheets and manganese oxide as oxygen evolution reaction catalyst

A new electrochemical process yielded a thin film consisting of stacked graphene (GR) sheets and manganese oxide. The process involved anodic deposition of manganese oxide nuclei on an electrode substrate and on the surface of GR sheets through electrolysis of aqueous Mn2+ ions in the presence of cationic GR colloids modified with poly(diallydimethylammonium) (PDDA+), followed by extraction of the PDDA. The catalytic activity of Mn oxide for the oxygen evolution reaction in an alkaline electrolyte was dramatically enhanced by hybridizing it with GR; an onset overpotential of 335 mV and a Tafel slope of 58 mV per decade were observed with excellent stability. These results can be attributed to a synergistic effect of the catalytic activity of nanosized Mn oxide deposits and the ability of GR to transfer electrons from the catalysts.

Improvement of polymer type EB resist sensitivity and line edge roughness

In order to improve sensitivity and line edge roughness (LER) for electron beam (EB) lithography, the positive-type polymer resists with various molecular weights and controlled dispersion were newly synthesized and examined. The synthesized resists have the same composition as ZEP520A (Nippon Zeon). With the low molecular and the narrow dispersion resist, improvements of both the sensitivity and LER are confirmed by obtaining the SEM images of line and space resist patterns exposed by EB writing system at an acceleration voltage of 100 kV. The polymer resist with molecular weight (Mw: 30k) and dispersion (1.4) exhibited 22 nm hp resolution, 20% improved LER and 15 % improved sensitivity compared with original ZEP520A.

Local Structure around Mn Atoms in IV–VI Ferromagnetic Semiconductor Ge0.6Mn0.4Te Investigated by X-ray Fluorescence Holography

The local atomic structure around Mn atoms in Ge1-xMnxTe thin-film single crystal has been investigated by X-ray fluorescence holography (XFH) at room temperature. The obtained atomic image suggests that the Mn atoms replace the Ge atoms in the host GeTe, and the Mn position is stable in the exact positions of the anion fcc sublattice. The XFH result also suggests the fluctuation of the Ge positions or the cation vacancies.

Preparation and thermoelectric properties of sintered type-I clathrates K8Ga(x)Sn(46-x).

Polycrystalline clathrate samples of nominal K(8)Ga(x)Sn(46-x) were prepared by the spark plasma sintering method to investigate their crystal structures, mobilities and thermoelectric properties. The samples almost had a single-phase type-I clathrate structure, and their relative densities reached as high as 98%. The room-temperature mobility of the K(8)Ga(8)Sn(38) sample was 25 cm(2) V(-1) s(-1), which substantially exceeded the reported mobilities of Rb- and Cs-containing Sn clathrates. Moreover, the mobility was larger than those of the type-I Ba(8)Ga(16)Sn(30), which had more Ga substituting atoms in unit cell. A higher mobility might accordingly be achieved in thermoelectric clathrates with a smaller number of substituting atoms. Their electrical conductivities and Seebeck coefficients in the temperature range 300-450 K were typical of n-type doped semiconductors in the extrinsic region while their room-temperature lattice thermal conductivities were as low as approximately 11 mW cm(-1) K(-1). The maximum dimensionless figure of merit ZT was estimated to be 0.27 at 490 K from the Seebeck coefficient of -262 microV K(-1) and the electrical conductivity of 96 S cm(-1) for the K(8)Ga(8)Sn(38) sample with a carrier concentration of 2.9 x 10(19) cm(-3).

Material dependence of magnetic force microscopy performance using carbon nanotube probes: Experiments and simulation

We investigated the dependence of the performance of magnetic force microscopy (MFM) on magnetic coating materials by using carbon nanotube (CNT) probes. The magnetic structures of CNT-MFM probes with CoFe(80:20), CoFe(30:70), and CoCr(80:20) coatings were essentially the same in the simulations, but their magnetic properties, such as stability after magnetic reversal, varied slightly in the experiments. It was found that a larger saturation magnetic moment (Ms) is of advantage for attaining high spatial resolution. Although the distinction was hard to see in the MFM images, the experiments and simulations showed that CNT probes coated with high Ms materials gave stronger perturbations to soft magnetic materials than probes coated with low Ms materials.