Abstracts of published papers in 1999

Lattice Gas Modeling in Thermal Spraying M.Jeandin, F.Borit, V.Guipont, L.Decker, D.Jeulin, M.Suzuki and S.Sodeoka [Surface Engineering, Vol.15, No.3, pp.191, 1999]
V-shape pore formed in metal-wire/ceramic composite and the simulation of pore formation by LGM.	A two-dimensional original model developed to simulate the deposition of droplets by plasma spraying onto a substrate. The new model is based on a lattice gas automaton which can reproduce the hydrodynamic behavior of fluids. Various applications of model are discussed, especially to composites.

Preparation of Textured YBCO Films Using All Iodide Precursors I. Matsubara, M. Paranthaman, A. Singhal, C. Vallet, D. F. Lee, P. M. Martin, R. D. Hunt, R. Feenstra, C. Y. Yang, and S. E. Babcock [Physica C, Vol. 319, pp127, 1999]

The temperature dependence of resistivity for YBCO film on SrTiO3 (100) substrate.

We developed an all iodide precursor approach to grow epitaxial YBCO films on single crystal substrates. The respective metal iodides were used as the starting materials. Initially, yttrium and barium iodides were dissolved in 2-methoxyethanol, whereas copper iodide was dissolved in ammonium iodide/dimethylformamide solution. The solutions were mixed together and concentrated to give a 2.5-M total cation precursor solution. The precursor solutions were spin coated on SrTiO3 (100) substrates and heat-treated at 800-830oC in 300 ppm oxygen atmospheres. XRD results from the theta-2theta scan, omega and phi scans for the films revealed a (100) cubic texture. The full-width-at-half-maximum values for YBCO (105) and YBCO (005) were 1.3o(in-plane epitaxy, Df) and 0.42o(out-of-plane epitaxy, Dw), respectively. The YBCO films had a Tc of 90.3 K. The measured transport Jc, critical current density was 1.3x105 A/cm2 at 77 K and self-field.

Preparation of Epitaxial YbBa2Cu3O7-d on SrTiO3 Single Crystal Substrates using a Solution Process I. Matsubara, M. Paranthaman, T. G. Chirayil, E. Y. Sun, P. M. Martin, D. M. Kroeger, D. T. Verebelyi, and D. K. Christen [Jpn. J. Appl. Phys., Vol. 38, pp. L727, 1999]

X-ray diffraction pattern of the Yb-123 film on SrTiO3 (100) substrate. Closed circles indicate Yb-124 impurities.

We have prepared YbBa2Cu3O7-d (Yb-123) epitaxial films on SrTiO3 (100) single crystal substrates by a metal organic decomposition (MOD) method. Precursor solution was prepared by dissolving ytterbium acetylacetonate, barium neodecanoate, and copper (II) 2-ethylhexanoate in a mixture of solvents containing toluene/pyridine/propionic acid. The precursor solutions were spin coated on the substrates and fired at 730-770oC in 100-ppm oxygen atmospheres followed by 1 atm O2 annealing. X-ray diffraction results from the theta-2theta, phi, and omega scans for the films revealed a (100) cubic texture. The full-width-at-half-maximum values for Yb-123 (103) and Yb-123 (005) were 1.5o(in-plane epitaxy, Df) and 0.73o(out-of-plane epitaxy, Dw), respectively. The highest Tc obtained for Yb-123 films was 87.2 K. The measured transport Jc was 6.4x105 A/cm2 at 77 K and self-field.

Angle-resolved tunneling study on the anisotropic gap of Bi-2212 superconducting whisker K. Morikawa, H. Enomoto, I. Matsubara, N. Mori and H. Ozaki [Applied Surface Science, Vol. 144-145, pp534-537, 1999]

Rotating angle dependence of the superconducting gap energy of 2. The solid line is a sine curve with 90 periodicity.

A tunneling study was performed on the c-axis direction of the Bi-2212 superconducting whiskers with the superconducting transition temperature of 72 K. The contact junction method was adopted to fabricate the nanometer scale tunnel junction by rotating the probe whisker over the sample whisker. We find that the superconducting gap energy of Bi-2212 superconductor is strongly dependent on the rotating angle, reflecting the d-wave symmetry of the Cooper pair wave function.

High temperature thermoelectric properties of oxide Ca9Co12O28 S. Li, R. Funahashi, I. Matsubara, K. Ueno and H. Yamada [J. Mater. Chem., Vol. 9, pp.1659, 1999]
Figure of merit Z vs. temperature for Ca9Co12O28.	The electrical conductivity, Seebeck coefficient and thermal conductivity of oxide Ca9Co12O28 with Ca2Co2O5-type structure are 84 S cm-1, 118mVK-1 and 1.73 W m-1K-1 respectively at 700oC, and its figure of merit is 0.67¡ß10-4K-1, showing that Ca9Co12O28 is a potential material for high temperature thermoelectric energy conversion.

Preparation of Textured YbBa2Cu3O7-d Films using a Solution Process I. Matsubara, M. Paranthaman, T. G. Chirayil, E. M. Sun, P. M. Martin, D. M. Kroeger, D. V. Verebelyi, and D. K. Christen [Proceeding of the 16th International Japan-Korea Seminar on Ceramics, pp420, 1999]

Surface Morphology of La2Zr2O7/SrTiO3 film.

We have prepared YbBa2Cu3O7-d (Yb-123)/SrTiO3 and Yb-123/La2Zr2O7 (buffer layer)/SrTiO3 films using a solution technique. Yb-123 and La2Zr2O7 were deposited by a metal organic decomposition (MOD) method and a sol-gel method, respectively. Precursor solutions were spin coated on the SrTiO3 single crystal substrates. X-ray diffraction results from the q-2q, j, and w scans for the films revealed a (100) cubic texture for the Yb-123/SrTiO3 films. The highest Tc obtained for Yb-123/SrTiO3 films was 87.2 K. The measured transport Jc was 6.4x105 A/cm2 at 77 K and self-field. The Yb-123/La2Zr2O7/SrTiO3 films showed Tc = 87.6 K with a narrow transition width and Jc = 1.0x105 A/cm2 (77 K, 0 T).

Thermoelectric properties of Ru-based oxide materials I. Matsubara, R. Funahashi, S. Li, K. H. Yamada, T. Inoue, and S. Sodeoka [Proc. of TEC '99, pp 80, 1999]

Temperature dependence of Seebeck coefficients of Ru-based oxide materials.

We have prepared three kinds of Ru-based oxides, SrRuO3, Sr3Ru2O7, RuSr2GdCu2O8, and evaluated their thermoelectric properties. SrRuO3 and Sr3Ru2O7 have a perovskite and a layered perovskite structure, respectively. All the parameters of electrical conductivity, thermal conductivity, and Seebeck coefficient are reduced in Sr3Ru2O7 compared with SrRuO3. Two-dimensional Ru-O layers are separated by double layers of square-pyramidal cuprate in the RuSr2GdCu2O8 compound, indicating that it has a reduced dimensionality compared with the SrRuO3 with a three dimensional perovskite structure. The Seebeck coefficients of RuSr2GdCu2O8 are larger than those of SrRuO3. A low dimensional structure is preferable to realize a high thermoelectric performance in the viewpoint of Seebeck coefficient and thermal conductivity.

High Temperature Thermoelectric Properties of Oxides Ca3-xSrxCo4O9+d S. Li, R. Funahashi, I. Matsubara, K. Ueno and H. Yamada [Proc. of TEC '99, pp108, 1999]

Thermal conductivity vs temperature of Ca3-xSrxCo4O9+d

A new series of oxides Ca3-xSrxCo4O9+d (x=0.0-1.0) were synthesized, and their structures, electrical properties, Seebeck Coefficients and thermal conductivity were measured. The electrical conductivity of Ca3Co4O9+d is 84 S cm-1 at 700oC. The value of Seebeck coefficient for Ca3Co4O9+d is 118 mVK-1, and the calculated value of the Power Factor is 1.16¡ß10-4 WK-2m-1 at 700oC. The thermal conductivity of Ca3Co4O9+d at room temperature is 1.93Wm-1K-1 and decrease slightly with the increasing of temperature. At 700oC, the figure of merit is 0.67¡ß10-4K-1.

Mechanism Of Bi2Sr2CaCu2Ox Crystallization And Superconducting Properties For Bi2Sr2CaCu2Ox/Ag Tapes Prepared Using Isothermal Partial Melting Method R. Funahashi, I. Matsubara, K. Ueno, and H. Ishikawa [Physica C, vol. 311, pp. 107-121, 1999]

Variation of phase diagrams of Bi-(Sr, Ca)-Cu-O system during IPM processing. At melting step (a), an early stage of solidification step (b), in the middle of peritectic reaction (c). Abbreviations indicate as follows; L : liquid phase, 2212 : Bi-2212 phase, 2201 : Bi-2201 phase, and 1:1 : 1:1 phase, respectively.

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) > 855oC. 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¡î. The Bi-2212 phase coexists 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 850oC, the Bi-2212 crystallization progresses by direct precipitation. The crystallization paths affect microstructure and transport critical current density (Jc) of Bi-2212 tapes.

Isotropic Pinning In Heavily Pb-doped Bi-2212/Ag Tapes R. Funahashi, I. Matsubara, K. Ueno, and K. Mizuno [Physica C, vol. 315, pp. 247-253, 1999]

Heavily Pb-doped Bi-2212/Ag tapes have been prepared using an isothermal partial melting (IPM) method. Under heavy Pb doping, irreversibility field (Hirr) for magnetic fields parallel to the c-axis (H//c-axis) is elevated at temperatures higher than 20 K and is comparable to Hirr for H//ab-plane. As a result, the original anisotropic pinning properties disappear in Pb-doped tape. At 30 K, the isotropic pinning force suppresses the anisotropy of transport critical current density (Jc) under magnetic field which is the most serious problem for power applications of Bi-2212 materials. Transmission electron microscopic (TEM) observation shows a disordered stripe structure and stacking faults along the c-axis in the bc-plane of Bi-2212 grains in Pb-doped tape. Such a stripe structure or stacking faults seem to be related to the production of strong pinning centers by heavy Pb doping.

Magnetic field dependence of Jc at 30 K under magnetic fields at 0o (¡û), 45o(¢¤), and 90o(¢¢) to the c-axis of Pb-free tape (a) and Pb-doped tape (b).

Preparation And Thermoelectric Properties Of Layered Co-based Oxides R. Funahashi, I. Matsubara, S. Li, H. Yamada, T. Inoue, and S. Sodeoka [Proc. Thermoelectric Symposium 99, pp. 76-77, 1999]

Crystal structure of Bi2Sr2Co2Ox (BC-2202) phase

Layered Co-based oxide materials with high durability at temperatures higher than 400oC have been prepared using solid sintering and partial melting methods. The composition of the oxide materials are Bi2Sr2Co2Ox (BC-2202). Power factor for the sample prepared by solid sintering is 0.68x10-4 Wm-1K-2 at 600oC. On the other hand, power factor for the sample prepared by partial melting is 1.7x10-4 Wm-1K-2 and 2.2x10-4 Wm-1K-2 at 600oC and 700oC, respectively. Higher power factor for the sample prepared by partial melting than that prepared by solid sintering is due to the microstructural difference, grain size and degree of grain alignment.

Fabrication and Mechanical Properties of Si3N4-Based Prismatic Material T. Inoue, M. Suzuki, S. Sodeoka and K. Ueno [Proceedings of 16th Japan-Korea International Seminar on Ceramics, pp285-288, 1999]

Work-of-fracture of Si3N4-based prismatic material with graphite inter-fiber layer

TiN/Si3N4-based prismatic structure ceramics composite (prismatic material) was fabricated by stacking of green fibers, followed by hot pressing at 1800oC. The prismatic material was composed of uni-directionally oriented Si3N4-fibers reinforced by in situ formed TiN particles and thin graphite inter-fiber layers. The composite with continuous graphite inter-fiber layers showed non-brittle fracture manner, and its work-of-fracture significantly improved as 2 - 3kJ/m2.

Mechanical Properties and Fracture Behavior of SiTiCO Fibre/SiAlON Composite T. Inoue and K. Ueno [Ceramics International, 24, pp565-569 1998]

Load-deflection curves of SiTiCO(f)/SiAlON and monolithic SiAlON recorded during the SENB test.

Unidirectional SiTiCO fiber (Tyranno fiber(R)) reinforced SiAlON (Si2Al4O4N4) was fabricated by the filament winding method. The fiber/matrix green sheets were stacked and hot-pressed at 1773 K for 1.8 ks under a nitrogen atmosphere. The composite with a fiber content of 42 vol% showed significantly higher flexural strength (800 MPa) than that of monolithic SiAlON ceramics (570MPa). A strength decrease due to matrix degradation was observed at high temperature (1273 K and 1473 K). The fracture toughness of the composite measured by SENB (single edge notched beam) method was 8.0 MPa¡¦m0.5, being two times higher than that of monolithic ceramics (4.0 MPa¡¦m0.5). The work-of-fracture of the composite measured by static and dynamic method was 9.62 and 8.29 kJ/m2, respectively.

Fabrication of Dense Zircon Coating by Plasma Spray M. Suzuki, S. Sodeoka, T. Inoue, K. Ueno, S. Oki and K. Shimosaka [Proc. of the 16th Int. Japan-Korea Seminar on Ceram., pp. 97-101, 1999]

Relationship between spray distance and open porosity (Substrate temperatures are shown in figure.)

The effects of spray parameter and heat treatment on the plasma-sprayed zircon coating were evaluated. Zircon (ZrSiO4) was totally dissociated by plasma spray; the coatings were composed of tetragonal zirconia (t-ZrO2) and amorphous silica (a-SiO2). Porosity of the as-sprayed coatings was highly affected by both of substrate temperature and spray distance. In all range of the spray distance, higher substrate temperature resulted in lower porosity; the coating with porosity of 2% was obtained at 1573K. Porosity also decreased with decrease of spray distance. By the heat treatment at 1473K, t-ZrO2 transformed to monoclinic zirconia (m-ZrO2) and a-SiO2 crystallized to cristobalite, respectively. Cracks in the coating disappear, and open porosity decreases. This can be attributed to sintering of SiO2 and phase transformation of ZrO2. When heat-treated at 1673K, the coating was composed of ZrSiO4 with dispersed fine m-ZrO2 particle. Open porosity of all the coatings increased up to 10% at this temperature. This is because of volume shrinkage during the formation of zircon.