Current Nanoscience - Volume 10, Issue 1, 2014

Sintered bodies of TiO2nanopowders were fabricated by magnetic pulsed compaction (MPC) and subsequent 2 step sintering in this study and then, the effect of micro Ti and P-25 nano TiO2 powder addition on density, shrinkage and hardness was investigated. The optimum processing conditions in order to find the maximum density, hardness and shrinkage were found to be 0.5~0.7 GPa MPC pressure and 1450°C sintering temperature, with addition of 6% of TiO2 P-25 nanopowder. High pressure and rapid compaction using magnetic pulsed compaction (MPC) enhanced the density with increasing MPC pressure up to 0.7 GPa without the formation of surface or edge cracks on the consolidates bulks, and significantly reduced the shrinkage rate (about 15 in this case) of the sintered bulks compared to the general process (about 18%). Overall, most of the properties showed gradual changes with increasing MPC-ed pressure and TiO2 P-25 nanopowder content.

Polycrystalline copper nanoparticles were synthesized from copper chloride dihydrate solution using the liquid phase plasma reduction method. A bipolar pulsed power supply with tungsten electrodes was used to generate discharge in the aqueous solutions. While large size of dendrite-shaped copper nanoparticles were mostly observed in the initial stage and particle size decreased with discharge time. The particles were dispersed with less and less small particles by the addition of CTAB and anisotropic shapes nanoparticles were mostly observed at long time plasma-treated with high concentration of surfactant. Many spots could be seen in the selected area diffraction pattern (SADP) for polycrystalline particles.

Used mainly in industries manufacturing secondary battery, display, LED, semiconductor, and other state-of-the-art products, rare earth elements (REEs) remain in demand and such demand has increased drastically. However, due to the lack of alternative materials and recycling technologies for REEs, many countries encounter difficulty in acquiring REEs. As part of the efforts to secure REEs, some studies have developed wet process techniques in the material flow process, particularly for scrapping and disposal. Among these techniques, the agent extraction method and liquid/solid separation process allow the separation/ refinement of high-quality REEs due to their excellent selection and separation performance. This study used Amberite XAD-7HP as the solvent impregnated resin and D2EHPA as the agent to separate each REE from the standard sample solution, mixed with heavy REEs (e.g., La and Ba) and light REEs (e.g., Eu, Tb, and Y). Each one of these REEs was about 100ppm in DI water using extraction chromatography. In this study, the pH change of HCI was between 0.1 and 3N, and the flow speed of the solution was between 0.9 and 2.6ml/min. The extraction result was analyzed by ICP-AES. Each REE was successfully separated with the HCI pH between 0.1N and 1N, and with the speed of the HCI between 1.6 and 2.6m/min.

Using an a–InGaZnO(amorphous-IGZO) n-type semiconducting material, a vertically operating two terminal metalsemiconductor- ferroelectric-metal (MSFM) non-volatile resistive switching structure has been investigated. PZT thin film of composition Zr/Ti=10/90 was prepared by spin-coating method on the Pt/Ti/SiO2/p-Si(100) substrate, and 50 nm thick a-IGZO semiconductor thin film was deposited as a resistive layer by RF sputtering on the PZT layer. The fabricated coupling structure exhibited three orders of switching margin. At a reading voltage of 2.5V, the “on” current and “off” current were 2.6x10-9A and 4.5x10-12A respectively. Such an extremely low pico-level “on” current is determined by the leakage current of PZT and is attributed to the charge accumulation at the a- IGZO/PZT interface layer. On the other hand, the current level of the "off" state is associated with the charge depletion in the a-IGZO layer due to the polarity reversal in the PZT layer. The “on” current level observed in this study is about three orders of magnitude lower than the reported values of typical ferroelectric devices, indicating read speed might be slow. However, the potential for low power operating applications is still promising in the aspect that the increase the leakage current in PZT material is not as much difficult as decrease it.

This study attempted to manufacture Cu-In and Cu-Ga coating materials via the cold spray deposition (as a new process for sputtering target material) and to investigate the material’s applicability as a sputtering target for solar cell. To examine the microstructural and property changes made to those coating layers, purity, density, hardness, and porosity were measured. The results showed that coating layers of Cu-In and Cu-Ga could be well manufactured via cold spraying under optimal process conditions. With the Cu-In coating layer, the pure Cu and intermetallic compounds of Cu7In3 and Cu4In were found to exist inside the layer. In the case of Cu-Ga layer, pure Cu, and Cu3Ga were detected. The value of porosity significantly decreased after annealing heat treatment. A Sputtering test was actually conducted using the cold sprayed Cu based materials and it was confirmed that those coating materials could be applied as a sputtering target material for solar cell.

The need for composites has been growing in various industries because it has high mechanical properties for weight as well as superior stiffness and strength. The composites addressed in this study are multi-pore aluminum foam and honeycomb whose have excellent impact energy-absorption capability. In this study, impact tests of aluminum foam and honey core sandwich composite with porous core are conducted in a bid to examine its mechanical properties. Different impact energies such as 50J, 70J, and 100J are applied to these specimens. The greater the impact energy is the shorter the duration of the maximum load. Maximum load is higher at foam than at honeycomb sandwich. At 50J test, the striker damages on the lower face at honeycomb but it does not damage at foam. At 70J test, it penetrates the specimen of composite at honeycomb but it does not penetrate at foam. On comparative study between impact behavior results of aluminum foam and honey core sandwich composite with porous core, stiffness at aluminum foam sandwich is superior than at aluminum honeycomb sandwich. The stabilities on aluminum foam and honeycomb core composite structure can be predicted by use of this experimental result.

In order to find an effective method to increase the coercivity of Nd-Fe-B magnets using less Dy quantity, an ordinaryprocessed Nd-Fe-B sintered magnet was treated with dysprosium-compound suspension in this work. After the magnets were prepared by a normal sintering process, they were cut into the size of 10 x10 x5 mm3 pieces and then dipped in 20 % DyF3 suspension and 20 % DyHx suspension, for one minute, so that the magnets were coated with the dysprosium compounds. These coated magnets were further subjected to heat treatment. As a consequence, coercivity of the coated magnets was increased over 255 kAm-1 for Dy-contained one and 243 kAm-1 for Dy-free one without noticeable reduction in the remanence and the energy product values. This improvement of the coercivity was due to Dy diffusion along grain boundaries from the surface of the coated magnets, thereafter, a partial replacement of Nd in Nd2Fe14B main grain by Dy had taken place near the grain boundaries, resulting in the increase of anisotropy field in that area.

Microwave sintering has emerged in recent years as a promising technology for faster, cheaper and eco-friendlier processing of a wide variety of materials, which are regarded as significant advantages against conventional sintering procedures. The present investigation describes a technique for sintering two different ceramic materials by microwave heating: alumina-15vol.% zirconia and hydroxyapatite nanopowders. The results show that microwave sintering achieves higher density values, excellent mechanical properties and a homogeneous microstructure at lower sintering temperatures. The densities of microwave processed samples were close to the theoretical densities, and the near-net-shape of the green body was preserved without significant dimensional changes. The main advantages of microwave heating can be summarized as follows: a more flexible process, reduced processing times and production costs, and environmental benefits. Thus, microwaves are a clear alternative to conventional heating methods, using up to 70% less energy throughout the whole sintering process.

The oxidation behaviors of Ti powder thermal spray coated Mo-Si-B alloys have been investigated in order to identify the underlying mechanism for the effect of precursor Ti coatings on Mo-Si-B alloys. Pure Ti powder was thermally sprayed on the two phase alloy (Mo (solid solution)+T2(Mo5SiB2)) fabricated by arc melting elemental components. The oxidation tests performed at 1100 °C show that the Ti powder was tightly bonded and reacted with the surface of the substrate, and TiO2 layer was formed at the outer surface of the coated Ti layer as a result of oxidation exposure. The oxidation behaviors of pure elemental component coated Mo-Si-B alloys have been discussed in terms of microstructural observations during oxidation tests.

In this study, the mechanical properties and hot extrusion of carbon nanotubes (CNTs)-reinforced Mg (AZ31) based composite were investigated. To improve the mechanical properties of the Mg alloy AZ31, CNT /AZ31 composites were fabricated by mechanical alloying and hot extrusion process. Then, extrudates of these composites were formed using a hot-extrusion process. The conditions to prepare the extruded specimens, which were rod-shaped, were at extrusion temperature of 400 °C, an extrusion ratio of 19:1, and a ram speed of 1.0 mm/s. A 300 ton press was used for an extrusion. The microstructures were observed using by optical microscopy (OM), scanning electron microscopy (SEM), and energy-dispersive spectroscopy (EDS). The characterization of the hot-extruded CNT/AZ31 rods was performed through electrical resistance and tensile test. The extruded CNT/ AZ31 rods showed significantly improved ultimate tensile strengths and yield strengths, but the exhibited an elongation of less than 2%. These results were due to the formation MgO between Mg and the CNTs during themechanical alloying process.

The core structures of future nuclear systems require tolerance to extreme irradiation, and some critical components, for example, the fuel cladding in Sodium-cooled Fast Reactors (SFRs), have to maintain mechanical integrity to very high doses of 200 – 400 dpa at high temperatures up to 700 °C. The high Cr nanostructured ferritic alloys (NFAs) are under intense research worldwide as a candidate core material. Although the NFAs have some admirable characteristics for high-temperature applications, their crack sensitivity is very high at high temperatures. The fracture toughness of high strength NFAs is unacceptably low above 300 °C. The objective of this study is to develop processes and microstructures with improved high temperature fracture toughness and ductility. To optimize the afterextrusion heat treatment condition, both the computational simulation technique on phase equilibrium and the basic microstructural and mechanical characterization have been carried out. 9 Cr-NFA was produced by the mechanical alloying of pre-alloyed Fe-9Cr base metallic powder and yttria particles, and subsequent extrusion. The post-extrusion heat-treatments of various conditions were applied to the asextruded NFA. The tensile and fracture toughness tests were conducted for as-extruded and heat-treated samples at up to 700°C. Fracture toughness of the NFA has increased by more than 40% at every testing temperature after heat-treatment in the inter-critical temperature range. The increment of fracture toughness of the NFA after post-extrusion heat-treatment is attributed to the increased strength at below 500°C, and an increased ductility at 700°C.

The mechanical properties of automobile bevel gears manufactured by carburized guenching process are evaluated by using a finite elements method. The gear teeth of super, helical, and warm gears typically have constant cross sections and the same module, which results in the same frictional wear and stress. Bevel gears, however, are of a cone shape and thus the cross section of gear tooth change in the axial direction. As a result, the hardness after heat treatment depends on the axial location of the gear tooth. Hardness distribution due to the non-constant cross section causes gear abrasion or damage. It is difficult, however, to measure the hardness in threedimensional way through experiments. Hence, this study attempts to simulate the carburized quenching process through a numerical analysis in order to interpret the three-dimensional heat-treatment effects. The analytical results include variations of each structure, hardness, temperature over various sections depending on the cooling rate. To verify this approach, the gear used in the experiment was designed based on the same condition of the analytical model. The Carburized Quenching was carried out and the characteristics of carbon penetration, micro-structure and surface hardness were investigated by means of SEM and EPS. The theoretical results were compared with experimental results to verify the analytical method for Carburized Quenching.

Wire cutting Electrical Discharge Machine (Wire EDM) uses the electrical discharge heat to cut the work piece with thin wire of brass or stainless steel by electrical discharge heat. The electrode and the work piece are melted and the cutting fluid between the electro and the piece is evaporated with burst. The melted material is spread in the shape of very tiny sphere chip. By the abrupt heating and cooling on the cutting surface, residual stress and void occurs on the surface. However the surface of oil Electrical Discharge machining is in hardening due to carbon in oil, the surface of water-wire cutting is in ductile structure. This ductile phenomenon might be due to the penetration of the melted wire brass into surface of the work piece. This paper presents the variation of micro structure and hardness of cemented carbide, cut by Wire cutting EDM with discharge energy. Brass is used for electrode. The mechanical property of the structure is changed by cemented copper from brass. The hardness distribution of the machined surface is evaluated. It is found that the cutting surface layer is in ductile structure. Considering the penetration phenomenon of the melting brass wire into Cu and Zn surface layer, the variation of hardness is investigated. The thickness of WC-Co (material of cemented carbide) ductile layer, generated by WEDM, is evaluated. This result will be useful information for a fine machining.

This study aims to understand the changes in mechanical properties when WC-7.5wt%Co and WC-12wt%Co compositions were experimented with various mixing, milling and consolidation conditions. Magnetic Pulsed Compaction (MPC) was used to consolidate WC-7.5wt% Co and WC-12wt% Co powders where high-energy ball milling with varying duration was employed for generating various powder types. Mechanically alloyed milled, non milled and mixed (non milled and milled powders in certain proportions) powders with compositions of WC-7.5wt% Co and WC-12wt% Co were used, followed by applying high pressures to form the bulks. Maximum densities for mixed milled and non milled WC-Co bulks after MPC were found to be around 60-65%, which later turned nearly 82% after sintering, while a maximum hardness of over 1400 Hv was observed in WC-12wt%Co sample. Density, hardness and other microstructural behavior did not illustrate substantial changes when mixed powders were used, although variations were noticed when non-milled powders were used for consolidation.

In this study, eco-friendly Pb-free Bi2O3–B2O3–ZnO glass frits were selected as an inorganic additive for the Al paste used in Si solar cells. The effect of glass chemistry on the electrical property of the Al electrode and on the cell performance was investigated. The results showed that as the molar ratio of ZnO to B2O3 increased, the glass transition temperature and softening temperature decreased because of the reduced glass viscosity. In Al screen-printed Si solar cells, as the molar ratio of ZnO to B2O3 increased, the sheet electrical resistance of the Al electrode decreased and the cell efficiency increased. The microstructure of the sintered electrodes showed that the uniformity and thickness of the back-surface field were significantly influenced by the glass chemistry.

We synthesized Y2O3-reinforced nickel-matrix nanocomposites by mechanical alloying of Ni-Y2O3, Ni-Y, Ni-NiO-Y and Ni- CuO-Y powders. Mechanical alloying of constituent powder mixtures resulted in a complete dissolution of yttrium or Y2O3 in nickel, forming a nickel-based solid solution. However, undesirable oxides such as NiO easily formed after a prolonged milling and subsequent sintering even in an inert gas atmosphere. The oxidation occurred almost inevitably due to oxygen impurity which remained at the surface of starting powders and in an inert gas used for milling or sintering. To solve this problem, we added a small amount of carbon or polyethylene as process control agents during ball milling. The result showed that the addition of these carbon-based additives, in particular polyethylene, was very effective to reduce a harmful formation of over-grown oxides during annealing and sintering, while promoting the oxidation of yttrium to form fine and uniformly distributed Y2O3 from the nickel-based solid solution.

This work was conducted to establish an electrochemical noise (EN) measurement combined with a direct-current potential drop (DCPD) method, namely, an EN-DCPD technique, for in-situ monitoring of nano- or micro-scale crack initiation and the propagation of primary water stress corrosion cracking (PWSCC) of nickel based alloys, and to investigate its underlying mechanism. The EN signals of the potential and current were measured under various conditions of a simulated primary water chemistry of a pressurized water reactor (PWR), and the amplitude and frequency of the EN signals were analyzed in both the time and frequency domains. From the spectral and stochastic analyses, the effects of such experimental factors as the current application in DCPD, loading condition, temperature, and pressure of the primary water environment were found to be effectively excluded from the EN signals generated from the PWSCC propagation. From a stochastic analysis based on the shot-noise theory, the PWSCC propagation could be distinguished from the general corrosion, by considering the Weibull shape parameter.