Haruhiko Obara

Thermoelectric Properties of Y-Doped Polycrystalline SrTiO3

The thermoelectric properties of Y-doped polycrystalline SrTiO3 (Sr1-xYxTiO3) are measured from 10 K to 900 K. The Y component is doped up to the solubility limit of Sr1-xYxTiO3, x=0.1. The Y-doped polycrystalline samples show a low resistivity, ~1 ×10-5 Ωm and a large absolute value of the Seebeck coefficient, ~150 µV/K, at room temperature. Consequently, they have a high power factor, 1.2 ×10-3 W/mK2, defined as PF=S2/ρ, where S is the Seebeck coefficient and ρ is the resistivity. Furthermore, we observe a decrease in the thermal conductivity of SrTiO3 due to phonon scattering by the Y dopant.

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.

Flux pinning centres correlated along the c-axis in PLD-YBCO films

Flux pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ? dependence of the critical current density JC and by observing the microstructure. First, c-axis oriented YBCO films were prepared on four different substrates at three different target-to-substrate distances D by using a pulsed-laser-deposition method. Films deposited on LaAlO3 (100) and CeO2/Al2O3 with large D (112 or 142 mm) showed narrow peaks in the JC versus ? curves when the magnetic field was applied parallel to the c-axis (B//c). In these films, a high density (up to 5.5 ?m?1) of planar defects, most probably stacking faults parallel to the c-axis, were evident. The ratio of JC (B//c) to JC (B//a?b) significantly increased as the density of the planar defects increased. In contrast, films deposited on SrTiO3 (100), LaAlO3 (100) and NdGaO3 (110) with small D (50?60 mm) showed high, broad JC peaks around B//c, and had a high density (up to 430 ?m?2) of nanometre-sized precipitates that were elongated along the c-axis. The ratio of JC (B//c) to JC (B//a?b) increased as the density of these small elongated precipitates increased. In conclusion, planar defects such as stacking faults parallel to the c-axis, and the small elongated precipitates are effective as c-axis correlated flux pinning centres.

Thermoelectric properties of SiC thick films deposited by thermal plasma physical vapor deposition

Abstract SiC thick films of about 300 µm could be prepared with a deposition rate above 300 nm/s by thermal plasma physical vapor deposition (TPPVD) using ultrafine SiC powder as a starting material. The thermoelectric properties were investigated as a function of composition and doping content. The nondoped films showed n-type conduction. Although the Seebeck coefficient reached as high as -480 µV/K, the power factor was only around 1.6 × 10-4 Wm-1 K-2 at 973 K due to the relatively high electrical resistivity. In order to reduce the electrical resistivity and to deposit layers with n-type and p-type conduction, N2, B and B4C were selected as the dopants. Nitrogen-doped samples exhibit n-type characterization, B and B4C-doped samples exhibit p-type characterization, and the electrical resistivity decreased from 10-2–10-3 to 10-4–10-5 Ωm after doping. The maximum power factor of the nitrogen-doped SiC and the thick films deposited with B4C powder reached 1.0 × 10-3 and 6.4 × 10-4 Wm-1 K-2 at 973 K, respectively.

Microcrack-free thick YBCO/CeO2/Al2O3 films prepared by a large-area pulsed laser deposition system

Abstract High-quality YBa 2 Cu 3 O 7− δ (YBCO) films were deposited on CeO 2 -buffered sapphire substrates using a large-area pulsed laser deposition system. Interplay of parameters such as thickness and target-to-substrate distance on the surface morphology evolution and superconducting properties of the films were examined. In particular, YBCO films with thicknesses up to ∼1.8 μm were successfully deposited with no observable microcracks on the surface. Characterization of the films revealed the following unique features: (1) a porous morphology, consisting of interconnected islands of varied heights and deep holes; (2) higher normal-state resistivity value, which is attributed to the presence of various defects in the film; and (3) Y-rich composition. These factors are considered to contribute to the strain-relieving mechanism responsible for the increase in film thickness without microcracking. Using this system, a 2 ″ YBCO film with thickness of ∼370 nm and no observable microcracks was deposited. A microwave surface resistance value of R s ∼1.4 mΩ was obtained at 77.3 K at a resonant frequency of 10.7 GHz. Mapping of the critical current density ( J c ) by inductive measurement showed values of ∼1.4–1.6 MA/cm 2 at 77.3 K.

Flux pinning properties of c-axis correlated pinning centres in PLD-YBCO films

Flux pinning properties of pinning centres having correlation along the c-axis in epitaxial YBCO films were investigated by measuring the magnetic-field angle ψ-dependence of the critical current density JC and the E–J-characteristics. YBCO films were prepared by using the pulsed-laser-deposition method on four different substrates at three different target-to-substrate distances D. The ψ-dependence of JC showed large peaks when magnetic field B was applied parallel to the c-axis (), and we observed two types of JC-peak: that is, a broad peak for the films deposited at small D (50–60 mm), and a narrow peak for the films deposited at large D (112 and 142 mm). The E–J-characteristics followed the power law, , and the ψ-dependence of the n-value also showed broad peaks around for the films deposited at D = 50–60 mm, and narrow peaks for the film deposited at D = 112 mm. Based on these results and our previous microstructural observations by AFM and TEM, we confirm that the broad-angle flux pinning effect around may be attributed to a high density of elongated precipitates, and the narrow-angle pinning effect around may be attributed to dense planar defects parallel to the c-axis.

Field-sweep rate dependence of magnetization and current–voltage characteristics in superconducting disks

We investigated distributions of electric field E and current density J in disk-shaped superconductors exposed to a perpendicular magnetic field Ba that is swept at a constant rate β=dBa/dt. The expressions of the dependence of the magnetization M on the field-sweep rate β were derived for arbitrary E–J characteristics taking into account the distributions of E and J accurately. We measured the dependence of M on β in a YBa2Cu3O7 disk, and estimated the E–J characteristics using the theoretical analysis. Furthermore, distribution of J(r) was estimated from the E(r) distribution and E–J characteristics.

Microcrack-free epitaxy of thick YBa2Cu3O7−δ films on vicinal r-cut sapphire buffered with CeO2

YBa2Cu3O7−δ (YBCO) films were fabricated by pulsed laser deposition on deliberately miscut Al2O3(11¯02) buffered with CeO2. Scanning electron microscopy observations demonstrated that 1-μm-thick YBCO films were microcrack free. Characterization of the films revealed porous morphology and high crystalline quality of YBCO with CuO planes tilted vertically and terminated frequently. These features are considered to be a contributing factor to the strain-relieving mechanism responsible for the increase in film thickness without microcracking. Microcrack-free thick YBCO films revealed Tc=90.5±0.5K, Jc(77.3K,0T)=2.0–3.0×106A∕cm2, and a substantial enhancement of Jc×t(77.3K,0T) up to 246 A/cm.

Superconducting Properties of Grain-Oriented Samples of Y1Ba2Cu3Oy

Highly oriented Y1Ba2Cu3Oy (YBCO) bulk samples more than 2 cm3 in volume have been prepared by a field-induced orientation method. The ratio of the resistivity along the c axis to that in the ab plane at a room temperature was about six. The critical current density Jc was larger in the ab plane than along the c axis. In the case of current I//ab plane, it was larger for field H//ab plane than for H//c axis. From the Jc-H property and the growth morphology of the samples, the possibility of grain boundary pinning in the YBCO superconductors has been discussed.

Magnetic Susceptibility of High-Tc Superconducting Oxides (La, A)2CuO4 (A=Ba, Sr)

Magnetic properties of high-Tc superconducting oxides (La, A)2CuO4 (A=Sr, Ba) are investigated in both normal and superconducting states. The results indicate that these oxides belong to type II superconductor and that lower critical field Hc1 is estimated to be about 400 Oe at 4.7 K from magnetization measurements. It is found that these oxides show a paramagnetism of the order of 10-7 emu/g in the normal state. Density of states at Fermi level is estimated from the magnetic susceptibility in the normal state.