Kazuo Ueno

Field-effect transistor on SrTiO3 with sputtered Al2O3 gate insulator

A field-effect transistor has been constructed that employs a perovskite-type SrTiO3 single crystal as the semiconducting channel. This device functions as an n-type accumulation-mode device. The device was fabricated at room temperature by sputter-deposition of amorphous Al2O3 films as a gate insulator on the SrTiO3 substrate. The field-effect (FE) mobility is 0.1u2009cm2/Vu200as and on-off ratio exceeds 100 at room temperature. The temperature dependence of the FE mobility down to 2 K shows a thermal-activation-type behavior with an activation energy of 0.6 eV.A field-effect transistor that employs a perovskite-type SrTiO3 single crystal as the semiconducting channel is revealed to function as n-type accumulation-mode device with characteristics similar to that of organic FETs. The device was fabricated at room temperature by sputter-deposition of amorphous Al2O3 films as a gate insulator on the SrTiO3 substrate. The field-effect(FE) mobility is 0.1cm2/Vs and on-off ratio exceeds 100 at room temperature. The temperature dependence of the FE mobility down to 2K shows a thermal-activation-type behavior with an activation energy of 0.6eV.

Synthesis and thermoelectric properties of the new oxide ceramics Ca3-xSrxCo4O9+δ (x = 0.0-1.0)

Abstract A new series of oxides Ca3−xSrxCo4O9+δ, (x=0.0–1.0) were synthesized, and their structures, electrical properties and Seebeck coefficients were measured from room temperature to 700°C. All the oxides have Ca2Co2O5 type structure. For all the oxides, both the electrical conductivity and the Seebeck coefficients increase with increasing temperature. The Seebeck coefficients are all positive. The values of Seebeck coefficient decrease with increasing Sr content for the Sr substituted samples. The power factor decreases with increasing Sr content for the samples with x 0.5. The Sr substituted sample (x=1.0) has lower thermal conductivity than Ca3Co4O9+δ due to its lower electrical conductivity. At 600°C, the figure of merit for Ca2SrCo4O9+δ is 0.9×10−4 K−1, and the value increases toward higher temperature.

Field-effect transistor based on KTaO3 perovskite

An n-channel accumulation-type field-effect transistor (FET) has been fabricated utilizing a KTaO3 single crystal as an active element and a sputtered amorphous Al2O3 film as a gate insulator. The device demonstrated an ON/OFF ratio of 104 and a field-effect mobility of 0.4 cm2/Vu200as at room temperature, both of which are much better than those of the SrTiO3 FETs reported previously. The field-effect mobility was almost temperature independent down to 200 K. Our results indicate that the Al2O3/KTaO3 interface is worthy of further investigations as an alternative system of future oxide electronics.An n-channel accumulation-type field effect transistor (FET) has been fabricated utilizing a KTaO3 single crystal as an active element and a sputtered amorphous Al2O3 film as a gate insulator. The device demonstrated an ON/OFF ratio of 10^4 and a field effect mobility of 0.4cm^2/Vs at room temperature, both of which are much better than those of the SrTiO3 FETs reported previously. The field effect mobility was almost temperature independent down to 200K. Our results indicate that the Al2O3 / KTaO3 interface is worthy of further investigations as an alternative system of future oxide electronics.

Structural phase transition accompanied by metal-insulator transition in PrRu4P12

A structural phase transition has been found using electron diffraction technique in PrRu4P12 accompanied by a metal-insulator (M-I) transition (TMI = 60 K). Weak superlattice spots appeared at (H,K,L) (H + K + L = odd) position at a temperature of T = 12 K and 40 K. Above T = 70 K, the spots completely vanished. The space group of the low-temperature phase is probably Pm. This is the first observation of a symmetry other than Im in skutterudite compounds.

Thermoelectric Properties of Bi2Te3-Based Thin Films with Fine Grains Fabricated by Pulsed Laser Deposition

Bi2Te3-based thin films were fabricated on quartz glass substrates by the pulsed laser deposition (PLD) method. By adjustment of the substrate temperature, films with c-axis orientation to the surface of the substrates were grown, and their in-plane electric resistivity and Seebeck coefficient were comparable with those of the bulk material. In contrast, their cross-plane thermal conductivity (evaluated using a thermoreflectance system with nanosecond pulsed laser heating) was comparatively much lower, possibly due to the grain size being as small as 40 nm (estimated based on X-ray diffraction measurement). The dimensionless figure of merit, ZT, for in-plane direction of n-type Bi2Te3 thin films was estimated based on the measurement results.

Mechanism of Bi2Sr2CaCu2Ox crystallization and superconducting properties for Bi2Sr2CaCu2Ox/Ag tapes prepared using isothermal partial melting method

Abstract The mechanism of Bi2Sr2CaCu2Ox (Bi-2212) crystallization from a partially molten state has been studied by means of thermal analysis, X-ray diffraction (XRD) measurement, and microstructural observation in Bi-2212/Ag tapes. Tapes have been prepared using the isothermal partial melting (IPM) method, under which partial melting and solidification are carried out at the same temperature in an nitrogen atmosphere and 20% oxygen partial pressure (p(O2)), respectively. The Bi-2212 phase crystallizes by peritectic reaction between (Sr, Ca)CuO2 (1:1 phase) and the liquid phase at processing temperatures (Tp)≥855°C. At the beginning of Bi-2212 crystallization, only the 1:1 and the liquid phases are present in tapes. The Bi-2212 nucleation occurs in the vicinity of 1:1 grains (homogeneous nucleation). This peritectic reaction progresses by recovery of oxygen in the liquid phase. In contrast, the nucleation of the Bi-2212 phase occurs heterogeneously at Tp≤850°C. The Bi-2212 phase co-exists with four Bi-free phases ((Sr, Ca)14Cu24O41 (14:24 phase), (Sr, Ca)2CuO3 (2:1 phase), (Sr, Ca)O (1:0 phase), and the 1:1 phase) and the liquid phase in the early stage of the Bi-2212 crystallization step. Since the precipitation temperature of the Bi-2212 phase is higher than 850°C, the Bi-2212 crystallization progresses by direct precipitation. The crystallization paths affect microstructure and transport critical current density (Jc) of Bi-2212 tapes.

In situ observation of Bi2Sr2CaCu2Ox superconducting whisker growth

Abstract In situ observation of Bi2Sr2CaCu2Ox (Bi-2212) superconducting whisker growth has been performed. It is shown that the whiskers grow from the bottom, which suggests not the vapor-liquid-solid or the spiral growth mechanisms, but a self-top-seeding mechanism for the Bi-2212 whisker growth.

Preparation and properties of SiC fibre reinforced SiAION ceramic composite

Abstract Polymer-derived SiC fibre (Nicalon)/SiAlON(Si3Al3O3N5)-matrix composite was fabricated by a filament-winding technique, followed by hot-pressing at 1350 °C. SiC fibres with and without carbon coating were used and the effect of carbon coating on the composite properties was investigated. The composite reinforced with non-coated SiC fibre indicated improved bending strength (600 MPa) and apparent fracture toughness (12.8 MPam0.5) compared with those of monolithic SiAlON (389 MPa, 3.6 MPam0.5), while the composite with carbon-coated fibre showed delamination fracture. The composites with non-coated and carbon-coated fibres both exhibited a remarkably higher fracture energy (9.8 kJ/m2 and 11.1 kJ/m2, respectively) than that of monolithic SiAlON (0.1 kJ/m2).

Overcoming weak links at grain boundaries in Bi2Sr2CaCu2Ox thick films by incorporation of superconducting whiskers

Bi2Sr2CaCu2Ox (Bi‐2212) superconducting thick films incorporating Bi‐2212 superconducting whiskers have been prepared using a partial melting method. Hysteresis behavior of critical current density (Jc) for magnetic field is observed in a film without whiskers (monolithic film), but not noticeably in composite film (whisker content: 20 wtu2009%). It is found from ac susceptibility measurement that a peak in χ″, resulting from weak links, appears at a higher temperature for the composite film than for the monolithic film. Jc decreases with increasing magnetic field more rapidly for the monolithic film than for the composite film less than 1 T. These results indicate that the incorporation of the whiskers is effective for overcoming weak links. In consequence Jc for the composite film is about three times higher than for the monolithic film. It is clear from SEM observation that a microstructural improvement leads to an increase in Jc.

Critical current density for Bi2Sr2CaCu2Ox/Ag tapes prepared using an isothermal partial melting method under controlled atmospheres

Abstract Bi 2 Sr 2 CaCu 2 O x (Bi-2212)/Ag tapes have been prepared using an isothermal partial melting method under controlled atmospheres, in which partial melting and solidification have been carried out at the same temperature but under different atmospheres. After melting the Bi-2212 powder in an N 2 gas flow (O 2 partial pressure, P (O 2 ) = 0%)_for 5 min, soliddification has been carried out in an atmosphere of P (O 2 ) = 20% for 6–36 h. The critical current density ( J c ) is as high as 3.0 × 10 5 A/cm 2 at 4.2 K under 0 T for tapes of t s ≥ 24 h at 865°C, but is limited to 1.0 × 10 5 A/cm 2 for tapes of t s = 12 and 18 h. It is found that t s is the most important factor in the preparation of pure Bi-2212 tapes with high J c .