Materials Science Forum Vols. 534-536

Abstract: Nanostructured or partially amorphous Al- and Zr-based alloys are attractive candidates for advanced high-strength lightweight materials. The strength of such materials is often 2 – 3 times higher than the strength of commercial crystalline alloys. Further property improvements are achievable by designing multi-phase composite materials with optimized length scale and intrinsic properties of the constituent phases. Such alloys can be prepared by quenching from the melt or by powder metallurgy using mechanical attrition techniques. This paper focuses on mechanically attrited powders containing amorphous or nano-(quasi)crystalline phases and on their consolidation into bulk specimens. Selected examples of mechanical deformation behavior are presented, revealing that the properties can be tuned within a wide range of strength and ductility as a function of size and volume fraction of the different phases.

Abstract: In this paper, less than 50nm nano-sized W-Ni-Fe composite powder was prepared by Sol-Spray-Drying Process. The preparing processes such as sol formation condition, reduction process of the procusor were studied in detail. The powder characteristics and sintering behavior of the powders were also studied. The powder is nearly spherical and it has good sinterability. In comparison with traditional micron mixed powder consisting of same composition, the melting temperature of the second phase lowered about 70°C, and the sintering temperature for nearly full densification was about 1390°C, which lowered more than 100°C.The effects of rare earth on sintering of the alloy were discussed.

Abstract: TiAl and TiAlN thin films are deposited on glassy carbon and Si substrates by the pulsed cathodic arc deposition process. In our pulsed cathodic arc system, because the spatial position of plasma on the surface of the evaporation source can be controlled by pulsed arc discharge, the thickness of the TiAl and TiAlN films can be controlled at nanometer scale. Amorphous stoichiometric Ti-Al films are synthesized from one Ti-Al alloy target at room temperature by changing the number of pulses of the arc discharge.

Abstract: We report on the mechanical and structural properties of nanocrystalline 8% and 10% mol yttria stabilized zirconia (YSZ) obtained using mechanical alloying (MA). For milling times above 32 hours the complete alloying of the starting Y2O3 and ZrO2 powders is achieved. The asmilled powders show a body-centered cubic crystalline structure with grain sizes in the order of 10 nm. After uniaxial pressing and sintering the compacts exhibit densities of more than 93% of the theoretical value. The microhardness of the compacts increases with sintering temperature, reaching a maximum value of 913 HV0.5 after sintering at 1220º C for 6 hours. The correlation of these enhanced mechanical properties with the microstructural changes induced by heat treatment of the nanocrystalline MA powders is discussed.

Abstract: Structure investigation results for MASHS powder Fe40Al/Al2O3 are presented. The powder structure formation proceeds via two stages. On the first step (mechanical activation) aluminothermal reaction takes plays in the system Fe+Al+Fe2O3, leading to formation of nanocomposite precursor Fe-Al-Al2O3. On the second step (SHS), iron and aluminum reacts, forming intermetallic FeAl. As-synthesized composite powder completely inherits the precursor structural morphology in spite of the phase transformations taking place during the production process. Such a production route provides the formation of intergrowth nanocomposite material structure with improved interfacial strength.

Abstract: Undoped CoSb3 powders were synthesized by mechanical alloying of elemental powders using a nominal stoichiometric composition. Nanostructured, single-phase skutterudite CoSb3 was successfully produced by vacuum hot pressing using as-milled powders without subsequent annealing. Phase transformations during synthesis were investigated using XRD, and microstructure was observed using SEM and TEM. Thermoelectric properties in terms of Seebeck coefficient, electrical conductivity, thermal conductivity and figure of merit were systematically measured and compared with the results of analogous studies. Lattice thermal conductivity was reduced owing to increasing phonon scattering in nanostructured CoSb3, leading to enhancement in the thermoelectric figure of merit. Mechanical Alloying associated with vacuum hot pressing technique offers an alternative potential processing route for the production of skutterudites.

Abstract: Amorphous diamond can emit electrons in vacuum when applied with an electrical field of only a few volts per micron. It is also extremely thermionic so the emitted current can increase millions times when heated to only a few hundreds degrees centigrade. As a result, amorphous diamond can be a thermal generator or a solar cell. The energy conversion efficiency can have much higher (e.g. 50%) than that (e.g. 15%) of silicon based solar cells that can absorb only a narrow spectrum of sun light. As a solar cell, amorphous diamond has another advantage that its radiation hardness is the highest of all materials, hence, its thermal electricity efficiency will not attenuate as does the solar cell based on photo electric semiconductorls. An immediate application of amorphous diamond is to coat it on electron emitting electrodes, such as that used as cold cathode fluorescence lamps (CCFL) that illuminate liquid crystal displays (LCD) for fornote books and television sets. Amorphous diamond can dramatically reduce the turn-on voltage to lit CCFL so the lamp life can be greatly extended. Moreover, the electrical current can be increased to enhance the brightness of the light.

Abstract: A resin paint dispersed with carbon particles was developed for the purpose of good conductivity and corrosion resistance for metal separators used in Polymer Electrolyte Fuel Cells (PEFC). The use of metal separators, especially Ti separators is a technology which has received much attention for its practical application as it allows for much greater compact stacking, since it is superior in both productivity and strength, in comparison with that of carbon molding separators. However, if pure Ti separators are used in a severe reaction of electricity generation, there is a
deterioration in conductivity, because of the formation of a passive film, which subsequently causes electricity generation difficulties after a few hours of use. Through examining the type of resin used for the purpose of controlling the passive state, the grain size of the conductive filler (graphite (Gr) + carbon black (CB)) and the composition combination for the purpose of secure conductivity, it was found that the combination of a scaly graphite-furnaced black mixed powder with a median particle diameter of 4μm and VDF-10%HFP copolymer resin was optimal. As a result of performing a single cell electricity generation evaluation of the Ti separator, which had the above mentioned coating material, the life of 22,000 or more hours was confirmed and an electricity generation evaluation is now being undertaken.

Abstract: Recently, in spindle motors for hard disk drive (HDD) devices, fluid dynamic bearings (FDB) with herringbone grooves have come to be used instead of ball bearings due to the demand for high density recording of the devices, improvement in the speed of data transfer, and the quietness of the motor. In this study, a 5-lobe bearing with high bearing stiffness using a sintered material, as a new trial, was developed, and the bearing performance was examined by simulated calculations and experiments. As a result, it was clarified that the 5-lobe bearing had the required performance for practical use in the spindle motor for HDD by means of optimizing the bearing’s dimensions. In addition, bearing loss of the 5-lobe bearing is lower than that of the herringbone bearing, and the 5-lobe bearing showed clearly that it is effective in being used in the miniaturization of HDD spindle motors.

Abstract: The electromechanical properties of a newly proposed 3-dimensional piezoelectric actuator have been investigated. Especially, the effects of 3-dimensional geometry on the maximum tip displacement were carefully investigated. As a result, it was found that the maximum strain of the 3-dimensional piezoelectric device was significantly enhanced up to 4.5 times higher than that of a disk shape device. This data was in good agreement with the finite element model analysis of strains and vibration modes. Moreover, the field -induced displacement stability of dome-shaped 3- dimensional piezoelectric actuator at various ac freguencies was superior to Rainbow actuator.