Abstracts of published papers in 2000

Plasma-Spray-Forming of Alumina Matrix Composite Reinforced with Metal Thin Wire S.Sodeoka, T.Inoue and M.Suzuki [Thermal Spray: Surface Engineering via Applied Research, Ed. C.C.Berndt, Pub. ASM International, OH. USA., pp.619 (2000)]

Load-displacement curve of PSF-CMC SENB specimen: CMC specimens show the non-linear fracture behaviors and consume larger fracture energy.

Alumina matrix composites reinforced with metal thin wire (Inconel-600) were successfully fabricated by plasma spray forming (PSF). The atmospheric plasma sprayed matrix layers and wire layers arranged by filament-winding technique were piled up alternately. Though the matrix and the wire were partially bonded only on the side which sprayed particles came flying to, a solid structure was obtained by this technique. Spraying in one direction perpendicular to the substrate made peculiar V-shape pores around the wires, but tilting the torch was effective to reduce the pores. The flexural strength of composite didn't increase in spite of some crack deflections on the fracture surface. Owing to the wire pull-out, however, the composite exhibited a remarkably higher apparent fracture energy than that of monolithic alumina ceramics.

Thermoelectric Properties of Ru-based Oxides with a Layered Structure I. Matsubara, R. Funahashi, H.Yamada, and K. Ueno [Proc. of 18th International Conference of Thermoelectrics, pp. 411, 2000]

XRD patterns of RuSr2GdCu2O8 and SrRuO3.

RuSr2GdCu2O8 (Ru-1212) has a layered structure in which insulating CuO2 plane and conductive RuO2 plane are piled up alternatively. Thermoelectric properties of Ru-1212 have been evaluated from room temperature to 600oC and these results are compared with those of SrRuO3 with a three dimensional perovskite structure. SrRuO3 shows a metallic s-T behavior, whereas a semiconducting-like transport is observed in the Ru-1212. At temperatures T < 400oC, the Seebeck coefficient values of Ru-1212 are larger than those of SrRuO3 by more than twice. The thermal conductivity of Ru-1212 is lower than that of SrRuO3 whole the measured temperature range. In the viewpoint of Seebeck coefficient and thermal conductivity, making an insulating/conducting superlattice structure is effective to enhance the thermoelectric performance.

Paraconductivity analysis for superconducting Bi-Sr-Ca-Cu-O H. Enomoto, N. Mori, I. Matsubara, and H. Ozaki [Physica B, Vol.284-288, pp. 579, 2000]

A typical R(T) for whiskers, indicating a two-step transition. A temperature interval D T ford ' analysis is also shown.

By means of the paraconductivity analysis, we characterize superconducting Bi2Sr2CaCu2O8+d whiskers which show a sharp resistive transition corresponding to the 2223 phase while identified to be mostly in the 2212 phase (99% or more) from XRD and SQUID measurements. The paraconductivity along the a-axis follows the behavior of the Aslamazov-Larkin theory, showing a dimensional crossover from 0D to 1D near Tc. This implies that small islands of the 2223 phase distributed randomly in the 2212 phase could form quasi-1D dimensional percolation structure just above Tc.

Preparation of Cr-doped Y3Al5O12 phosphors by heterogeneous precipitation methods and their luminescent properties I. Matsubara, M. Paranthaman, S. W. Allison, M. R. Cates, D. L. Beshears, and D. E. Holcomb [Mater. Res. Bull., Vol.35, pp. 217, 2000]

Emission spectra of (a) YAG-Cr0.5% heated at 1600��C for 2 h, (b) YAG-Cr0.75% heated at 1600��C for 2 h, and (c) commercial YAG-Cr powder from Nichia.

Chromium-doped Y3Al5O12 (YAG-Cr) phosphor powders were prepared by a heterogeneous precipitation method. Hydroxide precursor powders were precipitated from their respective metal sulfate solutions with urea. The formation of the YAG phosphors was investigated by means of XRD. Phase pure YAG-Cr was formed by heating the precursors at 1300oC, which is more than 300oC lower than that required for the conventional solid-state reaction method. The emission intensity of YAG-Cr phosphors increased with increasing firing temperature, and the intensities observed for phosphor powders were brighter than that of the commercial YAG-Cr phosphors.

Preparation of long Bi-2212 whiskers and their superconducting properties I. Matsubara, R. Funahashi, K. Ueno, and H. Ishikawa [Mater. Res. Bull., Vol.35, pp. 441, 2000]

Growth time dependence of maximum whisker length under oxygen gas flow rates of 150 ml/min (open circle) and 1000 ml/min (open triangle).

The effects of oxygen gas flow rate on the growth of Bi2Sr2CaCu2Ox (Bi-2212) superconducting whiskers have been studied. Increasing the gas flow rate inhibits whisker growth and reduces both maximum and average whisker lengths. Flow rate should be kept as low as possible to obtain a long whisker. Apart from the top part, chemical composition and Tc are uniform in a whisker of 14 mm length. It is confirmed that the superconductivity of whiskers is not suppressed during growth of long duration.

Synthesis and thermoelectric properties of the new oxide materials Ca3-xBixCo4O9+d (0.0 < x < 0.75) S. Li, R. Funahashi, I. Matsubara, K. Ueno, S. Sodeoka, and H. Yamada [Chem. Mater., Vol.12, pp. 2424, 2000]

Figure of merit vs temperature of Ca3-xBixCo4O9+d (x = 0.0 and 0.5)

A new series of oxides Ca3-xBixCo4O9+d, (x = 0.0-0.75) with Ca2Co2O5-type structures were synthesized, and their structures, electrical properties, Seebeck coefficients, and thermal conductivities were measured. The values of Seebeck coefficients of the new oxides are all positive, showing that they are p-type conductors. Both the electrical conductivity and Seebeck coefficients increase with the increasing Bi contents which can be attributed to the increase of carrier mobility due to the larger size of Bi ion. The electrical conductivity, Seebeck coefficient, and the calculated value of the power factor of Ca3-xBixCo4O9+d (x = 0.5) are 105 Scm-1, 160 mu V K-1, and 2.7 x 10-4 W K-2 m-1 at 700oC, respectively. The thermal conductivity of Ca3-xBixCo4O9+d (x = 0.5) at room temperature is 1.14 W m-1K-1 and increase slightly with the increasing temperature. At 700oC, the figure of merit of Ca3-xBixCo4O9+d (x = 0.5) is 2.0 x 10-4 K-1.

Thermoelectric properties of oxides Ca2Co2O5 with Bi substitution S. Li, R. Funahashi, I. Matsubara, K. Ueno, S. Sodeoka, and H. Yamada [J. Mater. Sci. Lett., Vol.19, pp. 1339, 2000]

Figure of merit vs temperature of Ca2-xBixCo2O5

We have synthesized a new series of oxide materials Ca2-xBixCo2O5, (x = 0.0-0.75), they have high electrical conductivity, large Seebeck coefficient and low thermal conductivity, indicating that they are promising p-type thermoelectric materials. The second phase Ca2Bi2Co2Ox with 2202-type structure exists in the Bi containing samples, and its amount increases with Bi contents. Because of the low thermoelectric performance of Ca2Bi2Co2Ox, it can be concluded that the large power factor and figure of merit of Ca2-xBixCo2O5, (x = 0.3, 0.5, and 0.75) samples is resulted from the substitution of Bi for Ca in the phase with Ca2Co2O5 structure.

Fabrication and Fracture Behavior of TiN Particles/Si3N4-Based Prismatic Ceramic Composite with ZrO2 Boundary Layer Takahiro Inoue, Masato Suzuki, Satoshi Sodeoka, Yoshihiro Sawada and Kazuo Ueno [Journal of the Ceramic Society of Japan]

Load-displacement curves of prismatic ceramic composite with YSZ inter-fiber layer. Thickness of YSZ layer are A: 1.94mm, B: 3.04mm, C: 4.55mm, respectively.

Uni-directional oriented TiN/Si3N4-based prismatic ceramic composite was fabricated by a root of extrusion. The prismatic material composed with prismatic TiN particle reinforced Si3N4 fiber and thin inter-fiber layer. Y2O3 stabilized ZrO2 powder was coated on the surface of green fiber as inter-fiber layer. These green fibers were stacked and hot-pressed at 2073 K for 3.6ks. Though bending strength of prismatic ceramic composite was lower than TiN/Si3N4, the work-of-fracture was improved significantly due to its non-brittle fracture behavior. It was found the most appropriate thickness for boundary layer. The composite with 3 mm thick inter-fiber layer showed relative maximum work-of-fracture as 1.5kJ/m2.

Structure and Properties of Plasma-Sprayed Zircon Coating Masato Suzuki, Satoshi Sodeoka, Takahiro Inoue, Kazuo Ueno, S. Oki and Keiji Shimosaka [Proceedings of the 1st International Thermal Spray Conference : Thermal Spray -Surfce Engineering via Applied Research pp.333, 2000]

Relationship between substrate temperature and open porosity of the plasma-sprayed zircon coatings.

Effects of spray parameters, such as spray distance, SD, and substrate temperature, TS, and post heat treatment on the structure and properties of plasma-sprayed zircon coatings were investigated. Zircon was totally decomposed by plasma spray; the coatings were composed of tetragonal zirconia (t-ZrO2) and amorphous silica (a-SiO2), because of the rapid cooling of molten particle right after the impingement to the substrate. Porosity of the as-sprayed coatings was highly affected by both of substrate temperature and spray distance. In all range of the spray distance which had been tried in this study, higher substrate temperature resulted in lower porosity of the coatings; the coating with porosity of 2% was obtained at TS=1573K with SD=95mm. 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 disappeared, and open porosity decreased. This can be attributed to sintering of SiO2 and phase transformation of ZrO2. After the heat treatment 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.

Structure control of Plasma Sprayed Zircon Coating by Substrate Temperature Masato SUZUKI*, Takahiro INOUE*, Satoshi SODEOKA* Sachio OKI** and Keiji SHIMOSAKA** [Journal of Institute of Applied Plasma Science, Vol 8, 2000]

Adhesive strength of the as-sprayed zircon coatings onto graphite substrate. "C" and "H" stand for "Cooled" and "Heated" substrate, respectively.

The effects of substrate temperature (Ts) and heat treatment on the plasma-sprayed zircon coating were evaluated. Zircon (ZrSiO4) was dissociated by plasma spray; the coating was composed of tetragonal zirconia (t-ZrO2) and amorphous silica. Porosity of the as-sprayed coatings was highly affected by substrate temperature. In all range of the spray distance, higher substrate temperature resulted in low porosity; the coating with porosity of 2% was obtained at Ts=1573K. The high Ts coating also indicated high adhesive strength to graphite substrate. Even after the heat treatment at 1473K, open porosity of high Ts coatings was maintained as low as 2%, while that of low Ts coatings was high. However, when heat-treated at 1673K, open porosity of all the coatings increased up to 10%. This is due to volume shrinkage during the formation of zircon from zirconia and silica.

Thermoelectric properties of Bi2Sr2Co2Ox polycrystalline materials R. Funahashi, I. Matsubara, and S. Sodeoka [Appl. Phys. Lett., vol. 76, pp. 2385-2387, 2000]

Figure of merit Z (= S2/rk) for 2202 (��), Bi-1.8 (��), and Sr-1.8 (��)samples

Bi2Sr2Co2O9 (BC-2202) polycrystalline materials with a layered structure have been prepared by partial melting. The chemical compositions of the samples are Bi2Sr2Co2Ox (2202), Bi1.8Sr2Co2Ox (Bi-1.8), and Bi2Sr1.8Co2Ox (Sr-1.8). The BC-2202 grains are well grown with high c-axis alignment. All three samples are p-type conductors. The electric properties, namely Seebeck coefficient (S) and electric resistivity (r), of the samples are dependent on chemical composition. Temperature dependence of r reflects semiconducting-like behavior (dr/dT < 0) at up to 973 K in the 2202 sample, whereas metallic-like behavior (dr/dT > 0) is observed in other two samples at T < 400 K (Bi-1.8 sample) or 500 K (Sr-1.8 sample) and T > 750 K. S values increase with temperature at T > 673 K and, at 973 K, reach 100, 110, and 150 mVK-1 for the 2202, the Bi-1.8, and the Sr-1.8 samples, respectively. Thermal conductivity (k) for all samples is lower than for ordinary conducting oxides. At T > 773K, k values increase with temperature in the 2202 and the Bi-1.8 samples, but are almost constant in the Sr-1.8 sample. The figure of merit (Z) increases with temperature for all samples. Z values at 973 K are 0.77x10-4, 0.61x10-4, and 2.0x10-4 K-1 for the 2202, the Bi-1.8, and the Sr-1.8 samples, respectively. The thermoelectric properties depend on the chemical composition of the BC-2202 phase. The BC-2202 material thus appears to be a promising thermoelectric material due to its high performance at high temperature (~ 1000 K).

An Oxide Single Crystal with High Thermoelectric Performance in Air R. Funahashi, I. Matsubara, H. Ikuta, T. Takeuchi, U. Mizutani and S. Sodeoka [Jpn. J. Appl. Phys., vol. 39, pp. L1127-1129 2000]

Temperature dependence of figure of merit ZT of Co-225 whiskers calculated using k of polycrystalline sample (��) and k estimated by Wiedemann-Franz's low (kelec) and phonon conduction (kphon) (��). Actual ZT is present between two lines (in the region of oblique lines).

An oxide single-crystalline whisker with high thermoelectric properties at temperatures (T) higher than 600 K in air has been discovered. This whisker is assigned to Ca2Co2O5 phase (abbreviated to Co-225 whiskers) and has a layered structure in which Co-O layers of two different kinds alternate in the direction of the c-axis. Seebeck coefficient of the whiskers is higher than 100 mVK-1 at 100 K and increases with temperature up to 210 mVK-1. Temperature dependence of electric resistivity shows a semiconducting-like behavior. These results indicate that the electric carriers are transported via hopping conduction. Using thermal conductivity of a Co-225 polycrystalline sample, figure of merit (ZT) of the Co-225 whiskers is estimated 1.2-2.7 at T > 873 K. This compound is characterized with regard to low mobility and high density of carriers, which contradicts the conventional materials with high thermoelectric properties.

Thermoelectric properties of Co-based oxide single crystals with layered structure. R. Funahashi, H. Ikuta, T. Takeuchi, I. Matsubara, U. Mizutani, and S. Sodeoka [Proc. The 17th International Korea-Japan Seminar on Ceramics, pp. 61-65.]

Crystal structure of Co-225 and BC-222 phases.

Single crystalline whiskers of two Co-based oxide phases have been prepared by heating glassy melt-quenched plates. The average compositions of the whiskers are Ca1.3Sr0.3Bi0.3Co2O5-d (Co-225) and (Bi, Pb)2.2(Sr, Ca)2.8Co2O9-d.(BC-222). Seebeck coefficient (S) of both whiskers increases with temperature at 373-973 K and is about 200 mVK-1 at temperatures higher than 873 K. On the other hand, electric resistivity (r) decreases with increasing temperature. This result could be explained by transport of electric carriers via hopping conduction between inequivalent sites. r for Co-225 and BC-222 whiskers is 1.4 and 4.0 mW cm at 973 K, respectively. As thermal conductivity for Co-225 and BC-222 phases is very low because of their layered structure, these compounds are promising thermoelectric materials at high temperature in air.

Thermoelectric properties of Co-based oxide single crystals with layered structure R. Funahashi, T. Takeuchi, H. Ikuta, I. Matsubara, U. Mizutani, and S. Sodeoka [Recent Res. Devel. Appl. Phys., vol. 2, pp. 601-627, 2000.]
Temperature dependence of Seebeck coefficient for the Co-225 whiskers..	Oxide single crystals in which two kinds of Co layers alternate have been synthesized. These single crystals show p-type thermoelectric properties and Seebeck coefficient is higher than 200 mVK-1 at 873 K.

Crystallization mechanism and superconductivity of Bi2Sr2CaCu2Ox/Ag tapes prepared by isothermal partial melting Ryoji Funahashi and Ichiro Matsubara [Recent Res. Devel. Appl. Phys., vol. 2, pp. 601-627, 2000.]

In order to obtain Bi2Sr2CaCu2Ox (Bi-2212)/Ag tapes with high transport critical current density (Jc) under magnetic fields, controlling both inter- and intra-grain microstructure, the former is related to superconductive coupling at grain boundaries and the latter to pinning centers, has been attempted. The mechanism of Bi-2212 crystallization from a partially molten state has been clarified in an isothermal partial melting (IPM) method. The Bi-2212 phase crystallizes by peritectic reaction between (Sr, Ca) CuO2 (1:1 phase) and a liquid phase at processing temperatures (Tp) higher than precipitation temperature (Tpre). At the beginning of Bi-2212 crystallization, only the 1:1 and the liquid phases are present in tapes. The Bi-2212 nucleation occurs homogeneously in the vicinity of 1:1 grains. This peritectic reaction progresses by recovery of oxygen in the liquid phase. In contrast, the nucleation of the Bi-2212 phase occurs heterogeneously by direct precipitation at Tp < Tpre. The Bi-2212 phase coexists with the liquid phase and 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 in the early stage of the Bi-2212 crystallization step. The crystallization paths affect on microstructure and Jc of Bi-2212 tapes. Maximum Jc at 4.2 K under zero magnetic field is 3.0x105 A/cm2 and 1.9x105 A/cm2 for the tapes prepared by peritectic reaction and direct precipitation, respectively. This difference in Jc is attributed to microstructure: Bi-2212 grain size and alignment. The preparation parameters governing the microstructure in Bi-2212 tapes have been made clear. Because there are only two preparation parameters, Tp and solidification time (ts), in the IPM method, the microstructure is easily controlled by these parameters. The higher Tp leads to the larger grains. The c-axis alignment of Bi-2212 grains is higher in tapes prepared by the peritectic reaction alone than direct precipitation. Amounts of impurity phases decrease with increasing ts. Grain coupling is dominated by the microstructure. Namely, it becomes stronger with increasing degree of grain alignment and decreasing amounts of impurity phases. In order to achieve high Jc, Tp as high as possible and enough ts to complete the Bi-2212 crystallization are necessary in the IPM method. Schematic model of Bi-2212 crystallization from partially molten state in IPM method. Intra-grain microstructure can be improved using IPM method. Introduction of new pinning centers has been succeeded in heavily Pb-doped Bi-2212 tapes. Under heavy Pb doping, irreversibility field (Hirr) for magnetic fields parallel to the c-axis (H//c-axis) is elevated to that for H//ab-plane at temperatures higher than 20 K. As a result, the original anisotropic pinning properties disappear in Pb-doped tape. At 30 K, the isotropic pinning force suppresses the anisotropy of Jc under magnetic field which is the most serious problem for wire application 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 the stripe structure or stacking faults seem to be related to the production of strong pinning centers by heavy Pb doping.

Damage resistance and R-curve behavior of multilayer Al2O3/SiC ceramics Jihong She, Takahiro Inoue, Kazuo Ueno [Ceramic International, 26 (2000) p801-805]

Fracture toughness as a function on notch depth for multiplayer Al2O3/SiC ceramics.

Damage resistance and R-curve behavior of multiplayer Al2O3/SiC ceramics were evaluated in bending by the indentation-strength and single-edged-notched-beam methods. Due to the crack deflection at the Al2O3/SiC interface, a plateau indentation strength response was achieved, suggesting an exceptional resistance to contact-induced damage. Moreover, fracture toughness was observed to increase from 8.0 to 15.5 MPa m1/2 with increasing notch depth from 0.5 to 2.0 mm, indicative of a strong R-curve behavior.

Mechanical properties and fracture behavior of Al2O3 laminates with different architectures Jihong She, Takahiro Inoue, Masato Suzuki, Satoshi Sodeoka, Kazuo Ueno [Materials Letters 46 (2000) p65-69]

Load-displacement curves of fibrous, multilayered and hybrid Al2O3 laminates under three-point bending tests.

Fibrous, multilayered and hybrid Al2O3 laminates were fabricated by extrusion and hot-pressing techniques. The effects of different architectures on the mechanical properties and fracture behavior are investigated. It is shown that hybrid laminates may provide a considerable flexibility and tailorability in mechanical behavior. An alternative arrangement of the fibrous and monolithic layers in hybrid laminates gives an average strength of up to 450 MPa, an apparent toughness of �`11 MPa m1/2 and fracture energy in excess of 2600 J/m2, with a non-catastrophic fracture behavior.