Current Nanoscience - Volume 10, Issue 1, 2014

A model gas that mimics the syngas produced from gasification of municipal solid waste was applied, for the first time, to the synthesis of methanol over commercial Cu-based catalysts. The effects of various operation conditions, such as temperature, pressure, contact time (W/F), and H2/COx and CO/COx ratios, on the methanol yield were investigated. The Cu-based catalysts containing a small amount of Mg showed a higher activity than that without Mg. The catalysts were characterized by Brunaure-Emmett-Teller, X-ray diffraction, and temperature-programmed reduction. The methanol yield was the highest under the following operational condition: temperature 230-250oC, pressure 60 bar, W/F 0.05 g·l/min, H2/COx ratio 2, and CO/COx ratio 0.8.

Stress corrosion cracking (SCC) is one of major threats against the integrity of the structural materials composing a nuclear power plant (NPP). Lead is deleterious element accelerating the SCC of Alloy 600 used as a steam generator tubing material in an NPP. In the present work, the oxide property grown on an Alloy 600 surface was evaluated as a function of PbO content using electrochemical impedance spectroscopy (EIS) and a field-emission transmission electron microscopy, equipped with an energy dispersive X-ray spectroscopy (TEM –EDS) for the specimen prepared using a focused ion beam (FIB). Alloy 600 was immersed in 0.1M NaOH containing PbO in the range of 0-5,000 ppm at 315oC for 2 weeks. The oxide property was compared with SCC susceptibility obtained from a slow strain rate tension (SSRT) test for Alloy 600 in 0.1M NaOH containing PbO. The impedance value was greatly decreased by adding PbO into the solution indicating a decrease in passivity. The composition of the oxide was also changed by Pb in an aqueous solution. The duplex oxide layer consisting of outer porous nickel-rich oxide and inner dense chromium-rich oxide is modified to a Pb incorporated nickel-rich oxide layer. Modification of the oxide property induced by lead incorporation caused an obvious increase in SCC susceptibility. It is expected that lead observed at the crack tip of an early cracked pulled Alloy 600TT tube was considerably responsible for SCC acceleration of this tube among numerous sound tubes in an NPP.

A new fabrication process for generating open-cell metallic and alloyed foams was developed by combining electrical explosion of wire (EEW) and electrospray (ESP) techniques. Fe-Cr-Al alloy nano-powders prepared by EEW in ethanol were used as a starting material, and commercial polyurethane (PU) sponges were used as templates. Fe-Cr-Al foams were successfully fabricated with porosities greater than 90%. The porosity of the fabricated foams was controlled by spraying time during the ESP process. As spraying time increased from 1 to 5 h, porosity decreased from 97 to 90%. The sintered foam possessed a continuous open-cell structure, which was dependent on the structure of the PU template. The proposed method may be useful in the future as a simple means to fabricate open-cell porous materials.

In Pressurized Water Reactor (PWR) power plants, Alloy 600, a Ni-Cr-Fe alloy, is used for steam generator tube materials. However, these tubes have experienced a lot of corrosion problems during their operation times. Several chemicals have been investigated for inhibitors of stress corrosion cracking (SCC) in the tube materials. Titanium dioxide (TiO2), which is sonochemically treated in water, was tested to evaluate the inhibition effect of SCC using a reverse U-bend specimen of alloy 600 under the condition of a 10% NaOH solution and a temperature of 315°C. The TiO2 particle size was reduced by ultrasonic power, and the morphology of the TiO2 powder was observed by a transmission electron microscope (TEM) based on the ultrasonic processing times. The particle size of TiO2 affects the SCC rate of the tube materials, and shows an improvement in resistance on SCC when decreasing the particle size of TiO2. When the nano-sized inhibitor is applied, the property of the oxide layer is changed to a more dense composition. The chemical composition in the oxide layer was analyzed using an X-ray photoelectron spectroscope (XPS) with a variation in the ultrasonic process times, and the difference in the Ti-compound structure between the oxide surface and inner layer of oxide was compared using XPS data.

ODS steel normally contains an exceptionally high oxygen concentration owing to oxygen adsorption on the metal powder surfaces, as well as to the contamination during mechanical alloying and consolidation. In this study, the effect of oxygen concentration on the size distribution of oxide particles in ODS steel has been investigated. The oxygen concentration in one ODS steel sample was about 7000 ppm (sample A), while that in the other was controlled to be about 2500 ppm (sample B) by a hydrogen reduction process prior to consolidation. Sample A revealed a much smaller mean grain size (~10 μm) than sample B (~25 μm). Two types of oxide particles, fine YTiO4 (< 30 nm) and coarse Cr-O (>100 nm), were mainly found in both samples. The fine YTiO4 particles in sample A showed a larger mean particle size (15 nm) than those (9 nm) in sample B, while their number density was nearly the same. Coarse Cr-O particles in sample A revealed a much higher number density than sample B. It is thus concluded that the size distribution and grain size of ODS steel can be controlled by a control of the excess oxygen concentration.

Ti3Al0.3Si0.7C2 compounds were synthesized via the powder metallurgical process, and oxidized isothermally and cyclically between 900 and 1200 °C in air. They had good thermal shock resistance, forming adherent oxide scales during cyclic oxidation. They oxidized to rutile-TiO2, α-Al2O3, and amorphous SiO2, together with gaseous escape of carbon. The oxide scales that formed during isothermal and cyclical oxidation were similar in that an outer (TiO2, Al2O3)-mixed scale, a thin intermediate Al2O3 layer, and an inner (TiO2, SiO2)-mixed scale formed. The outer scale formed by the outward diffusion of Ti4+ and Al3+ ions. The intermediate scale formed by the outward diffusion of Al3+ ions. The inner scale formed by the inward diffusion of O2- ions. No selective oxidation occurred from the early oxidation period.

The formation of hollow Cu oxide nanoparticles through the oxidation process has been studied with Cu nanoparticles produced by the plasma arc discharge method. The initial copper nanoparticles had a size range of 40 - 60 nm and a thin Cu2O layer on the surface. After oxidation at 100°C, there was no significant change in the particles. After oxidation at 200°C, however, the particles consisted of Cu2O and CuO instead of metallic Cu. By further increasing the temperature up to 300°C, only CuO particles remained. The obtained CuO particles had a hollow structure which resulted from the Kirkendall effect.

Waste PCBs (Printed Circuit Boards) from LCDs (Liquid Crystal Displays) contain valuable raw materials, Sn, Cu, and other noble metals. Among these, high purity tin was obtained by electro-winning after appropriate acid leaching of tin. For this, leaching and electro-winning process parameters were optimized for solution stability, dissolution and electrolytic efficiency. Two aqueous procedures were developed and tested for electro-winning at mass-production capable basis: (1) HCl leaching after preliminary HNO3 dissolution was quite effective in leaching high proportion of Sn as analyzed by ICP (Inductively Coupled Plasma) method. (2) Direct one-step leaching in H2SiF6 + H2SO4 + H2O2 was also tested and the solution was finally selected as electrolytes in electro-winning due to massproduction capability. The most efficient recovery of tin was observed after 7h electro-winning in aqueous H2SiF5 + H2SO4 + H2O2 maintained at 40°C: 96%-pure tin was obtained with 93.2% recovery rate.

The effects of aluminum and strontium content on an Mg-Al-Ca-Sr alloy were investigated in terms of microstructural and mechanical properties. The addition of up to 2 wt% of strontium to the Mg-5Al-2Ca alloy caused the major interdendritic intermetallic phase to change from Al2Ca to a combination of Mg2Ca, Al4Sr, and Mg17Sr2. Moreover, the addition of 2 wt% of aluminum to the Mg- 5Al-2Ca-xSr alloys caused the formation of β-Mg17Al12 phase suppressing the formation of Mg17Sr2. The creep resistance was significantly improved by the addition of strontium due to the formation of a thermally stable Al4Sr phase in the interdendritic region. However, the ductility deteriorated as the amount of second phases along the interdendritic boundary increased. Furthermore, an increase in aluminum content resulted in a drop of ductility and creep resistance at elevated temperature due to the poor metallurgical stability by the formation of β-Mg17Al12 phase.

Refining silicon and silicon nanoparticles are an important research area in solar energy research. These are related with solar cell efficiency. Silicon nanoparticles are also used for thin film solar cell and screen printing technology. We review the synthesis of silicon nanoparticles and refining of silicon. Refined silicon nanoparticles with minimal boron content were fabricated with transferred arc plasma equipped with impactor system. To control the chemical reactions leading to the formation of these compounds, H2O was injected into the shield gas line of the plasma torch. The plasma reactive time for boron removal was evaluated. The reactive time was altered by changing the flow rate of the shield gas of the plasma torch, fixing the powder feed concentration. The removal ratio of boron was also evaluated to conform if the boron content in the silicon particles was in the range required for standard solar grade silicon. The sizes and compositions of the particles were analyzed by Scanning Electron Microscope (SEM), Transmission Electron Microscope (TEM), Inductively Coupled Plasma (ICP), and X-Ray Diffraction(XRD) analysis.

Nanocrystalline n-type 95%Bi2Te3-5%Bi2Se3 bulk with prepared by the combined application of high energy milling and hot extrusion processes in this study. The effect of milling time on the microstructure of powders was characterized by scanning electron microscopy (SEM). With increasing milling time, powder size quickly decreased, and finally yielded nanocrystalline Bi2Te3 of less than 200nm grain size and of spherical morphology, at 90min. Microstructure and thermoelectric properties of the samples after hot-extrusion were also investigated. The hot-extruded samples observed fine-grains and improved mechanical properties. The maximum power factor of the nanaocrystalline Bi2Te3 bulk sample reaches 4.6x10-5W/mK2 at room temperature, which is lower than the zone-melted one due to high impurity and oxidization.

In this work, we implemented a chemical rate theory model for the growth of nano-sized point defect clusters (PDCs) and copper- rich precipitates (CRPs) which can change the mechanical properties in the reactor pressure vessel material of a nuclear power plant. For the calculation of irradiation defect evolution, a number of time-dependent differential equations were established and numerically integrated. The concentration of mono-size vacancies and interstitials was saturated at an early stage of irradiation, and it was found that the vacancy concentration was higher than the interstitial concentration. The high concentration of vacancies induced a growth of the CRPs at the later stage. The concentration of PDCs and the size of CRPs were used to estimate the mechanical changes, and the calculation results were compared with the measured changes in yield strength and Charpy V-notch transition temperature shift obtained from the surveillance test data of Korean light water reactors (LWRs). It was observed that the estimated values were in fair agreement with the experimental results in spite of the uncertainty regarding the material property parameters and modeling method.

The effects of Nd-Fe-B magnet scrap size on extraction behavior were investigated by liquid metal extraction using molten Mg. The magnet scraps with Mg were placed into a stainless steel crucible and then heated to 1,073K for 10 to 50min. The amount of extracted Nd after liquid metal extraction process was increased with an increasing with holding time and scrap size, and the maximum contents of Nd in Mg were observed to be about 24.2 wt.% in the conditions of the 5mm sized scrap heated for 50min. It was revealed that Nd oxides existing in the magnet scraps prevent the dissolution of Nd into Mg.

The niobium capacitor shows somewhat more unstable characteristics than the commercial tantalum capacitors, but it will be nonetheless considered as an excellent substitute of tantalum capacitors in the future. In this study, niobium powder is fabricated by metallothermic reduction process using K2NbF7 as a raw material, KCl and KF as diluents, and Na as a reducing agent. The niobium particle size greatly decreases from 0.7μm to 0.2μm as the amount of diluent increases. However, when a higher surface area of niobium powder is desired, more amounts of diluents are used in the said method. The niobium powder morphology and particle sizes are very sensitive to the amount of sodium excess, thus the particle size of niobium powder increases with increases in the amount of sodium excess. When more diluent and sodium are used, the niobium powder is contaminated further by impurities such as Fe, Cr, Ni, and others [1,2].

The CVD apparatus for the uniform coating of silicon carbide was suggested and realized into a 3-dimensional computer-aided design (CAD) model. An experimental condition is simulated with a computational fluid dynamic program to obtain temperature and flow distribution in the CVD chamber. The simulated temperature showed the very uniform distribution especially in the hot zone region and that is thought to be the result of the design of the CVD apparatus. The temperature measured with a thermocouple showed the good matching with the simulated one, which reflected the assumption and the boundary conditions during the simulation were plausible.

The effective recovering process that consists of air-jetting, centrifugation and air-classifying was developed for phosphor recycling from flat panel display devices. At the first processing stage of air-jetting, 95% phosphor could be recovered while the recovery rate was dependent on the dimension of CCFLs. After two subsequent stages of mechanical separation, Blue, Red and Green rich phosphor could be selectively collected. The recovery of Red phosphor was 61.2% and purity of (Y, Eu)2O3 was 95%.

We have developed a mass production process of Si quantum dots using photo-induced chemical etching method with oxidation and etching agents and have investigated its optical properties. Average size of the fabricated Si quantum dots was estimated to be 2 nm. Absorption peaks of the fabricated quantum dots were observed in the short wavelength regions, e.g., 200 - 350 nm. On the other hand, in the case of raw sludge, absorption was not observed in the UV-visible wavelength regions due to the narrow energy band gap (e.g., 1.12 eV). The calculated energy band gap of fabricated Si quantum dots was calculated to be 3.5 eV by the modified Kubelka- Munk function. Blue emission peaks around 475 nm wavelengths were observed due to the quantum confinement effect. When the emission peak was fixed, two excitation peaks were observed in 340 nm and 380 nm, respectively, which seemed to be due to the energy band gap widening and/or surface coating with an ultrathin layer.

In this study, the effect of Sc addition on the microstructure and mechanical properties of Al-20Si alloys fabricated by extrusion of powders was investigated. The Al-Si-(Sc) powders produced by high pressure gas atomization were used in starting materials. The microstructure and structural characterization were performed by optical microscopy (OM), X-ray diffractometry (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) with an energy dispersive X-ray spectroscopy (EDS). Mechanical properties of extruded bars were investigated by micro hardness test, tensile test and wear test. With increasing the Sc contents in Al-Si alloys, mechanical properties were significantly increased. These enhanced mechanical properties can be explained based on the refinement of primary Si due to the suppression of growth of primary Si by the well-surrounded and uniformly distributed Sc.

The purpose of this study is to investigate the microstructure and mechanical properties in 9Cr-1Mo-V steel and its weld. A 220 mm thick forged plate with a typical composition of G91based on ASME A336 was used. Narrow gap welds were produced by submerged arc welding (SAW) with two different welding speeds. The microstructures in the base metal were typical tempered martensite at all locations, and the sizes of prior austenite grains were increased with the depth from the surface. The yield and tensile strengths tend to decrease with an increase in test temperature, especially at temperatures higher than 500°C. The upper shelf energy of the specimen from center was lower than that from the surface, and it also showed a higher index temperature. The toughness deterioration at the center might be caused by the larger size of the prior austenite grains and the existence of the delta ferrite. In the case of the weld, the larger sizes of the weld beads were observed in the upper region with a lower welding pass speed. There was no significant difference in the strength of the upper and lower welds, but the elongations of the upper weld were slightly larger than those of the lower weld. In the Charpy impact test, the lower weld showed a better impact toughness than the upper weld caused by the relatively smaller size of the weld beads and finer microstructures owing to a lower heat input by the increased welding speed.

The present work investigated the effects of MgO as a thermodynamic stabilizer and ZrO2 and mullite (3Al2O3·SiO2) as kinetic stabilizers on the mechanical properties of Al2TiO5 at high temperatures. Al2TiO5 was synthesized using reaction sintering at 1500, 1550, and 1600°C for 2 h. The mix-stabilized Al2TiO5 sintered at 1500°C showed the highest mechanical strength (138 MPa) at 1100°C, while the mullite-only Al2TiO5 sintered at 1550°C showed a mechanical strength of only 67 MPa at 1200°C. The strength improvement achieved at high temperatures was affected by not only secondary phase mullite to inhibit grain growth of Al2TiO5 and to improve strength but thermodynamic stabilizer to promote synthesis Al2TiO5. The coefficient of thermal expansion of mix-stabilized Al2TiO5 decreased with increase of sintering temperature due to its microcrack, however, that of the mullite-only Al2TiO5 was not changed because of large number of pores.