THERMEC 2006

Paper Title Page

Authors: S.Y. Luo, Y.C. Chen, W.H. Wen
Abstract: The paper was to investigate the development of the graphite/phenolic resin composite materials that were fabricated by compression molding for the bipolar plate of the fuel cell. Effect of the different amounts of resin and the graphite powder size on the porosity, density, bending strength, and electrical resistance of the composites would be analyzed. In addition, the composites of the bipolar plates would be machined into the groove channel using high speed milling, which then assembles into single fuel cell. The I-V performance of the single fuel cell would be also discussed. The experimental results showed that when the larger amount of resin was employed, the resulting composites had a lower porosity, a larger electrical resistance, and a higher bending strength. The graphite/resin composites showed a good machinability during the high-speed milling. Finally, the graphite/resin composites of bipolar plates containing 25-30 vol. % phenolic resins were fabricated into a single fuel cell, which displays a relatively better I-V performance.
1431
Authors: Toshiyuki Mori, John Drennan, Ding Rong Ou, Fei Ye
Abstract: Rare earth doped ceria compounds are fluorite related oxides which show oxide ionic conductivity higher than yttria stabilized zirconia in oxidizing atmosphere. As a consequence of this, considerable interest has been shown in application of these materials for ‘low (below 500°C)’ temperature operation of solid oxide fuel cells (SOFCs). In this study, the nano-sized powders of DyxCe1-xO2-x/2 (x=0.15 and 0.2) were prepared using ammonium carbonate co-precipitation method. To design the nano-structure in aforementioned materials, the round shape particles were prepared in nano-scale. The combined process of Spark Plasma Sintering (SPS) and Conventional Sintering (CS) was examined for fabrication of nano-structured doped CeO2 solid electrolytes. The nano-structural features in the (SPS+CS) specimen and CS specimen were observed using transmission electron microscopy (TEM). This micro-analysis suggested that the micro-domain with distorted pyrochlore structure exists in the grain of these materials. The conducting properties in the specimens were strongly influenced by the micro-domain size. It is found that the present combined process minimized the micro-domain size and maximized the conductivity in the specimens. Also nano-structured Dy doped CeO2 sintered bodies in the present study had wide ionic domain and high transport number of oxygen. This suggests that fabricated sintered bodies are suitable for the solid electrolyte in low temperature operated SOFCs. It is concluded that a control of micro-domain size is a key for development of high quality doped CeO2 electrolytes for fuel cell application. It is expected that advanced solid electrolytes for clean energy production will be produced by a design of nano-structure in rare earth doped CeO2 solid electrolyte.
1437
Authors: E. Garlea, V.O. Garlea, Hahn Choo, C.R. Hubbard, Peter K. Liaw
Abstract: Neutron incoherent scattering measurements were conducted on Zircaloy-4 round bars. The specimens were charged in a tube furnace at 430 °C, using a 12.5 vol. % hydrogen in an argon mixture for 30, 60, and 90 minutes at 13.8 kPa pressure. The volume-average neutron diffraction measurements showed the presence of the face-centered-cubic delta zirconium hydride (δ-ZrH2) phase in the hydrogenated specimens. The assessment of the background in the diffraction profiles due to the incoherent scattering from the hydrogen atoms was carried out by performing inelastic scans around zero energy transfer and at a fixed two-theta value for which there was only flat background and no coherent scattering. To estimate the relative amount of hydrogen in the Zircaloy-4 samples, the increase in incoherent scattering intensities with hydrogen content was calibrated using samples for which the hydrogen content was known. Measurement of the background scattering from locations within the round bar was also performed to map the distribution of hydrogen content.
1443
Authors: Z.X. Li, Xia Huang, L.C. Qi, Chun Xiao Cao
Abstract: The beneficial effects of boron addition on microstructure transformations and mechanical properties of γ-TiAl alloys were investigated. Two growth mechanisms of boride (TiB2) in γ-TiAl alloy were confirmed, the curved flaky borides are products of irregular eutectic reaction growing coupled with matrix, while some faceted blocky borides in boron-rich alloy are primary TiB2 phase growing directly in melt. The core of flaky TiB2 is ultra-fine B2 phase and there has an orientation relationship [1210] TiB2//[001]B2, (1010) //(010)B2. In addition to the well-known grain refinement effect, boron addition can suppress the formation of metastable feathery and Widmastätten structure and broadens cooling-rate-range for the formation of fully lamellar structure, consequently, it improves thermal stability of the lamellar structure and accordingly prolongs the creep rupture life significantly. Another beneficial effect of boron addition is that boride can restrain discontinuous coarsening on lamellar grain boundary by pinning action and accelerates recrystallization of γ grain by introducing TiB2/matrix interfaces as nuclear sites during homogeneous treatment at 1150°C. Therefore, compared with boron-free alloy more homogeneous and refined near γ microstructure can be obtained in boron modified alloy.
1451
Authors: Haruyuki Inui, Katsushi Tanaka, Kyosuke Kishida, Satoshi Fujio
Abstract: A CBED (convergent-beam electron diffraction) method proposed by the present authors for chiral identification of enantiomorphic crystals has been successfully applied to intermetallic compounds with the point groups of 23, 422, 432 and 321. The intensity asymmetry of ZOLZ and/or FOLZ reflections of the Bijvoet pairs is easily recognized in CBED patterns with the incidence along the appropriate zone-axis orientations for each of the two members of the enantiomorphic pair and the intensity asymmetry with respect to the symmetry line is reversed upon changing the space group (handedness) from one to the other. Thus, the generality of the proposed method in identifying the chirality for all crystallographycally possible enantiomorphic crystals is verified.
1457
Authors: Antonín Dlouhý, Kateřina Dočekalová, Ladislav Zemčík
Abstract: The present study focuses on vacuum induction melting and investment casting of neargamma TiAl intermetallic alloys. The attention is mainly given to a cost-effective melting process in which a primary alloy ingot is re-melted in a ceramic crucible and cast into a ceramic shell mould. Two types of crucibles (based on Al2O3 and Y2O3) are considered. The most detrimental reactions that govern the contamination of the molten alloy with ceramic particles were determined. Results suggest that the crucible wall attack can be considerably limited by using either the Y2O3 (with no SiO2-type binder) or Al2O3 crucibles with a suitable coating. After pouring, a mechanical interaction associated with different thermal expansions of TiAl casts and ceramic shell moulds can result in serious product damage. A simple 1D-1D model of the cooling process was formulated and the heat flow as well as stress states in the cast-mould system were numerically solved. Process parameters (melt superheat, initial mould temperature, cooling kinetics and mould composition) were optimized in order to reduce the stress in the casts. The optimized parameters delimited a processing window in which complex-shaped TiAl castings like turbocharger wheels can be fabricated.
1463
Authors: T.D. Reynolds, M. Acosta, David R. Johnson
Abstract: Alloys of Ru-Al-Cr with compositions between Ru-10Al-35Cr and Ru-3Al-39Cr (at.%) were directionally solidified and heat treated to produce single phase hcp-Ru(Cr,Al) and two phase B2-hcp microstructures. The room temperature fracture toughness, tensile behavior, and cyclic oxidation behavior at 1100°C were investigated and compared to previous results measured from RuAl and Ru-Al-Mo alloys. For microstructures consisting of a Ru(Cr,Al) matrix with fine RuAl precipitate, a good room temperature fracture toughness, tensile ductility, and oxidation resistance at 1100°C were measured.
1469
Authors: Michael Oehring, V. Küstner, Fritz Appel, Uwe Lorenz
Abstract: Gamma titanium aluminide alloys often solidify peritectically and show a coarse, dendritic microstructure, which can lead to unacceptable mechanical properties in the as-cast condition. In view of the development of improved cast alloys, the dependence of the solidifying microstructure on the aluminum content and other alloying elements was investigated. The formation of the observed solidification microstructures is discussed in comparison with microstructure formation maps calculated by the NCU (nucleation and constitutional undercooling) model developed by Hunziker et al. [1].
1475
Authors: Alain Jacot, Amin Rostamian
Abstract: A phenomenological modeling approach has been developed to describe the massive transformation and the formation of lamellar microstructures during cooling in gamma titanium aluminides. The modeling approach is based on a combination of nucleation and growth laws which take into account the specific mechanisms of each phase transformation. Nucleation of both massive and lamellar γ is described with classical nucleation theory, accounting for the fact that nuclei are formed predominantly at α/α grain boundaries. Growth of the massive γ grains is calculated with a mathematical expression for interface-controlled reactions. A modified Zener model is used to calculate the thickening rate of the γ lamellar precipitates. The model incorporates the effect of particle impingement and rapid coverage of the nucleation sites due to growth. The driving pressures of the phase transformations are obtained form Thermo-Calc based on actual temperature and matrix composition. The model permitted investigating the influence of alloy chemistry, cooling rate and average α grain size upon the amount of massive γ and the average thickness and spacing of the lamellae. Calculated CCT diagrams were compared with experimental data collected from the literature and showed good agreement.
1481
Authors: Jörg M.K. Wiezorek
Abstract: Alloys based on FePd in the vicinity of the equiatomic composition are good model systems to study the microstructrual and phase transformation behavior of the class of technologically interesting ferromagnetic L10-intermetallics that includes also FePt and CoPt. Here, thermo-mechanical processing, involving cold-work and annealing at temperatures below the ordering temperature was used to control microstructures and to improve properties of FePd. L10- ordered FePd with ultra-fine grain sizes and up to about 8-fold increased coercivity compared with conventionally processed material has been obtained. Relationships between processing condition, microstructure (scale, morphology and texture) and the magnetic properties are discussed.
1487

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