Materials Science Forum Vols. 561-565

Abstract: The cell-structure of highly porous aluminum material prepared by foaming of aluminum alloy melt with titanium hydride was investigated nondestructively with fine-focus X-ray 3D-CT at several interrupt steps during slow compressive deformation. The foamed highly porous aluminum has anisotropic shape of each cell inevitably because of gravity force during solidification of foamed material and mechanical properties especially the dependence on the deformation direction of highly porous aluminum is analyzed well from the size and shape of each void composing the porous material. The statistic anisotropic distribution of these form factors such as three axial lengths and directions at the time of ellipsoidal approximation of each cell was found to be less important to improve the mechanical properties of this type of material.

Abstract: It is very possible for the foam zinc materials to take the place of zinc powder and become a new ideal material of electrode for zinc-air battery, because of its excellent three dimensional reticulated structure, high porosity, high specific surface area and uniform quality, and was widely used in many other fields. A novel method for preparing very porous zinc foam was proposed, in which the polyurethane foam as substrate was processed by degreasing process, roughening process, activation, electroless plating and zinc electrodeposition. Then, experiments were carried out to explore the effects of the solution composition（concentration of ZnSO4）and the operation conditions （such as the interelectrode distance; current density, temperature, pH value of electroplating solution and so on) on the morphology of the foam zinc and the current efficiency. The optimum experimental conditions were determined by optimizing the factors. On these conditions the foam zinc presents a three-dimensional reticular structure with high porosity, and uniformity, and the outward appearance of the coating is bright.

Abstract: In order to prepare the functional porous glasses, fabrication of foaming glass materials was achieved utilizing the hydrothermal treatment. In the present paper, SiO2-Na2O-B2O3 glass was hydrated by the hydrothermal hot pressing of the glass powder mixed with pure water under the condition of loading pressure of 40 MPa and heating up to 523K. The hydrothermally treated glass contained 10.8 mass% of water. By heating up to 473 K, it started to foam by releasing water and the porous structure was obtained. The minimum apparent density was obtained to be 0.25 g /cm3 after the heat treatment at 673 K for 300 s. It was clarified that the prepared hydrated glass with the hydrothermal treatment would be promising for fabricating the porous glass materials by firing at low temperature.

Abstract: Three-dimensional zinc mapping based on X-ray K-edge scanning has been performed. By microtomographies with energies above and below the K-absorption edges of the elements, the concentration distribution of the elements is evaluated during in-situ experiments, respectively. It is found that the Zn concentration distribution during the heat treatment was changed inside the cell wall of the aluminum foams and it has been homogenized. Also several precipitated phase transformation can be three-dimensionally visualized by the CT-method tuning X-ray energies.

Abstract: We studied the fatigue strength of lotus-type porous magnesium with cylindrical pores aligned unidirectionally, which was fabricated through unidirectional solidification in pressurized hydrogen atmospheres. The fatigue strength shows anisotropy; the fatigue strength in the direction parallel to the longitudinal axis of pores is higher than that in the perpendicular direction. Not only anisotropic pores but also fiber texture grown along the pore direction contributes to the anisotropy in the fatigue strength.

Abstract: Tin/copper (Sn/Cu) coatings on Cu leadframes (CUFE), in which Fe atoms are doped as a minor element, showed whisker initiation at room temperature over a long period of 47 months. By means of the planar slice method and electron back scattering pattern (EBSP) measurement, the whisker roots were consistently found to be located at the intersections of grain boundaries in the coating. Whisker roots were also located above the peaks and ridge lines of intermetallic compound (IMC) Cu6Sn5, which was formed with a pyramid-shaped configuration between the Sn/Cu coating and Cu leadframe. Using finite element analysis (FEA), we calculated stress distribution in the coating. The results indicated that compressive stress normal to the grain boundary was induced with a gradient toward the surface in the coating. Therefore, the compressive stress gradient induced by the pyramid-shaped IMC is thought to be the root cause of whisker initiation in Sn/Cu coatings on Cu leadframes. When the Cu leadframe with a minor doped element of Cr atoms (CUCR) was used as the substrate with the same Sn/Cu coating, no whisker initiation was observed even after a longer storage time of 65 months. Through field-emission scanning transmission electron microscopy (FE-STEM) and field-emission transmission electron microscopy (FE-TEM) microstructural observations of vertical sections of each sample, the shape of the IMCs formed between the coating and the leadframe in the Sn/Cu-CUFE sample was found to be different from that in the Sn/Cu-CUCR sample. The difference in whisker initiation tendency can therefore be explained by the difference in compressive stress depending on the shape of the Cu-Sn IMCs, because stress distribution in the coating of the Sn/Cu-CUCR sample calculated using FEA revealed a smaller stress gradient than that in the Sn/Cu-CUFE sample.

Abstract: Ti-26 at.%Nb (hereafter Ti-26Nb) alloy foams were fabricated by space-holder sintering process. The porous structures of the foams were characterized by scanning electron microscopy (SEM). The mechanical properties of the Ti-26Nb foam samples were investigated using compressive test. Results indicate that mechanical properties of Ti-26Nb foam samples are influenced by foam porosity. The plateau stresses and elastic moduli of the foams under compression decrease with the increase of their porosities. The plateau stresses and elastic moduli are measured to be from 10~200 MPa and 0.4~5.0 GPa for the Ti-26Nb foam samples with porosities ranged from 80~50 %, respectively.

Abstract: A method to fabricate the metallic closed cellular material has been developed. Powder particles of polymer or ceramics coated with a metal layer using electro-less plating were pressed into pellets and sintered at high temperatures by heating and spark plasma sintering (SPS) method. A metallic closed cellular material containing different materials from that of cell walls was then fabricated. The mechanical and Thermal properties of this material were measured. The results of the compressive tests show that this material has the different stress-strain curves among the specimens that have different thickness of the cell walls and the sintering temperatures of the specimens affect the compressive strength of each specimen. Also, it seems that the results of the compressive tests show that this material has high-energy absorption and Young’s modulus of this material depends on the thickness of the cell walls. Also thermal properties of this material were measured.

Abstract: Unusual wetting behavior of liquid Cu was found on a surface-oxidized iron substrate in reducing atmosphere. Liquid Cu wetted and spread very widely on the iron substrate when a droplet was attached with the substrate in Ar-10%H2 after the surface oxidation of the substrate. The oxidationreduction process fabricates a porous layer at the surface of the iron substrate. The pores in the porous iron layer are 3-dimensionally interconnected. Thus, liquid metals, which are contacted with the reduced iron samples, penetrate into these pores by capillary force to cause the unusual wetting behavior. It has been already confirmed that liquid Ag, Sn, In and Bi show this phenomenon onto surface-porous iron samples as well as liquid Cu. This unusual wetting behavior of a liquid metal has been correlated to the normal contact angle of the liquid metal on a flat iron substrate.

Abstract: Oxidation behavior of Cu nanoparticles in the formation process of hollow Cu2O spheres was investigated by TEM. The thickness of Cu2O layers on Cu nanoparticles oxidized at 323 K in air was measured as a function of oxidation time. At the initial stage of oxidation until the oxide film with 2.5 nm in thickness is formed, the thickness of oxide films on Cu nanoparticles with the diameter of 10, 20 and 35 nm shows a nearly equal value regardless of diameter of Cu. After the formation of 2.5 nm layer, however, the growth rate of the oxide films on smaller nanoparticles becomes slower than that on larger nanoparticles. This result suggests that the voids formed at the Cu/Cu2O interface prevent Cu atoms from diffusing outward across the interface because the volume ratio of voids to inner Cu in smaller nanoparticles is much larger than that in larger nanoparticles.