Defect and Diffusion Forum Vols. 323-325

Abstract: nterdiffusion in the Cu|Cu30Zn diffusion couples under a strong gravitational field was investigated at 400. The Cu-Brass diffusion couples were prepared by electrolytic plating method. Two samples were centrifuged at 0.4×10⁶ G for 60 hours. The initial contact interface was perpendicular to the gravitational force direction. The two different orientations of the samples were studied: with Cu 1) downside and 2) upside of the gravitational force direction. It was found that interdiffusion was markedly affected by the gravity field. This was caused by the sedimentation of atoms where heavy Cu atoms move in the direction of the strong gravitational field. A numerical simulation of the sedimentation process is self-consistent with experimental results.

Abstract: An in situ observation experiment of faceted cellular growth was carried out using transparent organic alloy, salol - t-butyl alcohol, in microgravity conditions on the International Space Station. The temperature and solute concentration fields in the vicinity of the solid-liquid and the growth rate were simultaneously measured by microscopic interferometers.

Abstract: As a preliminary experiment for the growth of InGaSb alloy crystals under microgravity at International Space Station (ISS), bulk crystal was grown under terrestrial condition using the same gradient heating furnace (GHF). Czochralski grown GaSb <111>B single crystal was used as a seed and feed crystals for the growth of InGaSb bulk crystals. During the growth, heat pulses were intentionally introduced periodically to create the growth striations. From the striations, the growth rate of the grown crystal was estimated. The results show that the growth rate was gradually increased from the beginning of the growth and became stable. On the other hand the In composition of the grown crystal decreased along the growth direction. From the In composition, the temperature gradient in the solution was estimated and it was almost the same of that fixed during the growth.

Abstract: Homogeneous Bi-Pb alloy samples in the ε-phase are treated under mega-gravity centrifugal acceleration field. Atomic sedimentation in solid and associated partial crystallization occurs during the treatment. Small differences of densities in the ε-phase are enhanced under the mega-gravity, and they cause the separation of the ε-phase between fully packed hcp structure of the Bi₃Pb₇ and partially defected hcp structure of the Bi₃Pb_7-δ. The compositional graded superconducting alloy is obtained in the latter. Partially melt-growth occurs in the latter and microstructure of Pb is formed along the gravity field associating with the formation of highly oriented the (211) phase of Bi₃Pb_7-δ. The superconductivity parallel to gravity is Pb, and perpendicular to gravity is Bi₃Pb_7-δ as the result. Superconducting properties differs between the separated two phases.

Abstract: Growing high quality crystals is a bottleneck in the multi-stepped process of three-dimensional structural analyses of protein. It is known that a microgravity environment may maintain ideal depletion zones of protein and impurity around a growing crystal and the filtering effect of these depletion zones may contribute to obtaining high-resolution X-ray diffracting crystals with superior internal order. The effects of these depletion zones around growing crystals are thought to be the main mechanisms for the improvement of crystal quality in microgravity. A competition between the diffusion of protein molecules in the solution (indexed by the diffusion coefficient, D) and the adsorption of those into the growing crystal (indexed by the kinetic coeffcient, β) decides the extent of depletion zones. Lower D values and higher β values indicate that these effects are more obvious in numerical analyses. Therefore we use the D/β value as an index for these effects. The most effective method of lowering the D/β value is using viscous precipitant reagents, such as a high molecular weight polyethylene glycol (PEG) to decrease the D value and using highly homogenous protein samples to increase the β value. In this report, we briefly introduce simple yet practical methods of estimating D and β values followed by a numerical analysis to understand the filtration effects, and the results of crystallization experiments in microgravity when controlling the diffusive field around the growing crystals using the D/β value as an index.

Abstract: A silica colloidal crystal obtained by centrifugation at 9 G for 2 days in water was annealed by additional stronger centrifugation at 50 G for 5 days. The number of the striations observed in the colloidal crystal under crossed polarized light decreased at some parts in a growth container after the additional centrifugation, while the number also increased at the other parts. The decrease probably shows the shrinkage of the stacking disorders under high gravity, while the increase probably shows the production of new stacking disorders.

Abstract: A compositionally graded thin film of Fe/Si was fabricated by a gravity-assisted pulsed laser ablation (GAPLA) system. By this method, a compositionally graded structure along gravity direction was successfully produced under a gravity field of 5,400 G. Systematic experiments were conducted by several parameters, including gravity, distance between target and substrate, and laser fluence in case of typical target material of iron disilicide (FeSi₂). We demonstrate that the atomic fraction of Fe, the heavier component of the thin film, showed increasing spatial distribution with the direction of gravity. Relatively high laser fluence as well as a very narrow space between the target and the substrate are found to be essential to the compositionally gradient of thin film.

Abstract: t is believed that a microgravity environment may maintain ideal depletion zones of protein (PDZ) and impurity (IDZ) around growing crystals and may contribute to growing high-quality crystals. This can lead to an X-ray diffraction data collection of higher resolution with lower mosaicity, because of the better internal order and fewer defects in the crystals when compared to ground-grown crystals. The extent of these depletion zones are dependent on a competition between the diffusion of the molecules in the solution (indexed by the diffusion coefficient, D) and the adsorption of those into the growing crystal (indexed by the kinetic constant, β). If we use the D/β value as an index of the extent of PDZ and IDZ, a lower D/β value is ideal for maintaining PDZ and IDZ. Using experimental results, we could easily obtain the D/β value. When we combined the D/β value with the quality of protein crystals obtained in microgravity experiments provided by Japanese Space Agency (JAXA), we found that the effects of microgravity contributed to obtaining superior crystals especially if the D/β value was less than 3 mm. The numerical analysis of the PDZ and IDZ shows that the radius of the crystal (R) is also related to the PDZ and the IDZ. If the Rβ/D value is large, both the PDZ and the IDZ provide a filtration effect, but if the Rβ/D value is small, only the IDZ does.

Abstract: Interdiffusion in Zn/InSb system has been investigated under high (0.59×10⁶ G) and low (1 G) gravity conditions at 673, 593 and 573K, respectively. Samples annealed at 0.59×10⁶ G, 673K for 60 hours, indicated the formation of a periodic reaction layer structure. Since such structures can be formed in solid state reactions of ternary systems [, the effect of high gravitational field and high hydrostatic pressure (approximately 3kbar) in the formation of periodic patterns was investigated. Systematic investigations at ambient pressure and low gravitational field were carried out at 593 and 573 K in sandwich geometry. SEM and EDX analysis had shown that there are different phases between the initial components. Starting from the Zn side of the specimen there is a very thin single-phase with high (about 90%) In content. Next to it is a thin two-phase layer, containing mainly 50-50% InSb and some elongated Zn particles and then there is a thick phase with the composition of Zn₅In₂Sb₄ which is followed by a similar two-phase mixture (InSb+Zn) similar to the Zn side of the sample. Although the diffusion zone is not a well developed periodic structure, but every layer (clearly distinguishable form the others and was either a single-or multiphase layer) grows with the time.

Abstract: Alloy 230 (also named Haynes® 230) was tested as interconnect for production of hydrogen via High Temperature Vapor Electrolysis (HTVE). Samples were oxidized at 800°C in the both atmospheres representative of the HTVE operating conditions: Ar-1%H2-9%H2O (for cathode side) and air (for anode side). The high temperature oxidation behaviour was studied in both atmospheres together with the electrical conductivity of the thermally grown oxide scales. Oxidation kinetics indicated lower oxidation rate in H2/H2O compared to air (kp = 3.8 .10-15 g2.cm-4.s-1 in H2/H2O and kp = 1.6 .10-14 g2.cm-4.s-1 in air). The corrosion products were characterized by scanning electron microscopy associated with X-ray diffraction analyses and energy dispersive X-ray analyses. The sample electrical behaviour was evaluated by determining the Area Specific Resistance (ASR). The ASR was higher in H2/H2O (ASR = 1 ohm.cm2) than in air (ASR = 0.04 ohm.cm2). The diffusion of proton or hydrogen containing species through the oxide scale is proposed to be responsible for the increase of the electrical conductivity in cathode side.