발간논문

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In this study, the effects of microstructural variations in heavy-section reactor pressure vessel (RPV) steels on tensile and Charpy impact properties were investigated. Two PRV blocks, OPR1000 and APR1400 (ORV, ARV) were taken from the archive materials used in Korea standard nuclear power plants. Test specimens were sampled from five different positions at intervals of 1/4 thickness from the inner surface to the outer surface. The chemical contents in the ORV and ARV were homogenous, and macro-segregation problems were not observed. In the ORV, the microstructure was fully composed of bainite structure. The bainite lath was fine at the surface but it coarsened toward the center. In the ARV, coarse-lath upper bainite was formed at the surfaces but polygonal ferrite was observed toward the center. Variations in cooling rate due to the material’s thickness caused changes in microstructure along the thickness during the water quenching heat treatment. The larger-sized ARV did not cool down quickly enough to create a fully bainitic microstructure. Tensile strength and Charpy impact properties tended to improve toward the surfaces. Compared to the ORV, the ARV showed higher strength and lower energy transition temperature (ETT). These variations in mechanical properties were related to the microstructure variations. Surface regions with the fine-lath bainite structure and small carbides showed better properties than the center region. The ARV, having smaller grain size and homogeneous carbide distribution, also showed better properties than the ORV. This is because the ARV was fabricated using an advanced steelmaking process, the VCD + Si + Al methods. The addition of Al resulted in the formation of AlN precipitates at high temperature, and suppressed the grain growth of austenite. (Received June 12, 2017; Accepted July 7, 2017)

keyword : reactor pressure vessels, Mn-Mo-Ni low alloy steels, SA508 Gr.3 Cl.1, microstructure, toughness

The material characterization of single crystalline Cu columns was numerically carried out at the submicroscopic level. A molecular dynamics (MD) simulation was employed using the embedded-atom method (EAM) interatomic potential between a pair of Cu atoms to describe the interactions among Cu atoms. First, the relationship between mechanical properties and factors affecting their behavior were numerically investigated using a crystal structure including several defects. The factors were specimen size, strain rate, and temperature. As the specimen size increased the normalized yield stress decreased, which was similar to results obtained at other length-scale. The yield stress tended to lead to exponential strain rate-hardening and a linear temperature-softening. Next, material characterization was conducted based on these results. These computational results can lead to the development of an in silico platform to characterize material properties and MD simulation can lay the groundwork for multi-scale modeling and simulation. (Received June 16, 2017; Accepted June 29, 2017)

keyword : molecular dynamics simulation, embedded-atom method, copper, material characterization, multiscale simulation

A thin, compact, and uniform cobalt layer was coated on ferritic stainless steel (Crofer 22 APU) by high frequency square pulsed electrodeposition in an aqueous solution containing cobalt chloride and boric acid. The effects of various electrodeposition parameters and post heat treatment on the morphology, thickness, roughness and adhesion of the coating layer were investigated. It was found that neutralizing with sodium hydroxide solution after acid washing of the polished sample surface prevented delamination of the sub-surface layer. The addition of boric acid had a strong influence on the nucleation and agglomeration behavior of the deposited Co, which consequently controlled both the thickness and the roughness of the coating layer. With increasing current density, the grain size decreased and, therefore, a more uniform microstructure could be achieved. The duty cycle of the square pulse strongly affected the morphology but not the roughness of the coating layer. Subsequent heat treatment at 800 ℃ in air greatly improved the adhesion of the Co layer to the substrate. By optimizing the aforementioned electrodeposition parameters and adopting the post heat treatment, it was possible to obtain a fine, uniform, compact, and thin (< 2 μm) Co layer with excellent adhesion properties. (Received April 19, 2017; Accepted June 17, 2017)

keyword : electrodeposition, Co coating, ferritic stainless steel, SOFC, interconnect

Numerical modeling of the self-propagating high temperature synthesis (SHS) of Cu_xNi_yZn_1-x-y Fe₂O₄ ferrites was carried out using the finite element method, to control the combustion synthesis behaviors of the ferrites. Additional pre-heating above 300 ℃ caused the combustion temperature at the inner surface of the reactant compact to completely propagate the SHS reaction. The porous Cu_xNi_yZn_1-x-yFe₂O₄ ferrites formed by the SHS were ball-milled and then magnetically separated and classified to obtain quasi-nano-sized powders. The reitveld refinement estimated that the SHS product formed at room temperature was about 64% ternary ferrites, while that formed with 598 K preheating was about 85% ternary ferrites. (Received May 12, 2017; Accepted July 19, 2017)

keyword : nano-powders, CuNiZn-ferrite, SHS method, FEM

In this study, planetary ball milled WC-5 wt%Co powders were sintered using the pulsed current activated sintering (PCAS) process. With this process, fully densified hard materials were obtained in a shorter time than conventional processes such as a HP and HIP. The particle sizes of the planetary ball milled WC and Co were 0.114 μm and 0.154 μm, respectively. The PCAS process was performed at temperatures up to 1300 ℃ with 60 MPa and electric current for 13 min without any significant change in the grain size. The WC-5 wt% Co hard materials were fully densified with a relative density of up to 99.8%. The density of WC-5 wt% Co hard materials increased with increasing shrinkage ratio. In addition, the Co particles penetrated into the WC particles by dissolving and re-precipitation, and finally hard materials were completely densified. The average grain size of the WC-5 wt%Co was 0.25 μm, and the hardness and fracture toughness were 2,386 kg/mm² and 6.0 MPa·m^1/2, respectively. The mechanical properties of the WC-5 wt% Co hard materials were excellent because of grain refinement and densification. (Received June 9, 2017; Accepted July 12, 2017)

keyword : pulsed current activated sintering process, rapid sintering, WC-.Co, planetary ball milling, mechanical property

A polycrystalline diamond compact (PDC) is a two-stage sintered body manufactured by placing diamond powders on a WC-Co hard material and then applying the high pressure high temperature (HPHT) sintering process. This study investigated the microstructure and wear properties of the PDC depending on initial diamond particle size. Three different sizes (12-22 μm, 10-20 μm, and 8-16 μm) of initial diamond powders were used to manufacture PDCs with the HPHT sintering process. Some Co and WC were observed along the boundaries between the diamond particles in the three manufactured PDCs. The diamond layer formed using small diamond particles showed finer and more even area distribution of Co along the diamond particles. VTL equipment was used to conduct a granite cutting wear test. The result confirmed that smaller initial diamond particle size leads to greater wear resistance properties. Observation of the PDC wear surface confirmed that the PDC made with larger diamond particles was more prone to be weak along the boundaries between the diamond particles, and there were instances where the diamond particles were displaced as a whole. For PDCs of smaller diamond particle size, abrasive wear occurred where the diamond particles were gradually worn away. (Received July 19, 2017; Accepted July 26, 2017)

keyword : polycrystalline diamond, high temperature high pressure sintering, wear, microstructure, diamond particle size

The effect of current densities on the electromigration (EM) failure mechanism of flip chip Cu/Ni/Sn-Ag/Cu solder bumps was investigated under stressing conditions at current densities ranging from 5.0~6.9 × 10³ A/cm² at 150 ℃. The EM failure times at 5.0 × 10³ A/cm² were around 11 times longer than at 6.9 × 10³ A/cm². A systematic failure analysis considering stressing time showed that a current density of 5.0 × 10³ A/cm² induced pancake void propagation near the Cu6Sn5 intermetallic compound/solder interface at the cathode, while a current density of 6.9 × 10³ A/cm² produced severe Joule heating due to high current crowding near the solder/Cu₆Sn₅ interface. This was due to electrons entering the location at the cathode, which led to local melting of the solder and fast Cu consumption. It was determined that the EM failure mechanisms of flip chip Sn-Ag solder strongly depend not only on the Ni barrier effect but also on current density, which drives the dominant failure mechanisms of pancake voiding and local Joule-heating melting. (Received May 11, 2017; Accepted August 16, 2017)

keyword : electrical/electron materials, soldering, diffusion, scanning electron microscopy(SEM), electromigration

Nanoscale Sn-Bi-Ag compound powders were successfully synthesized using the pulsed wire discharge (PWD) method. In PWD, when a high current is passed through high-density metal wires, the wires explode because of resistance heating, forming fine particles or metal vapor. In this study, we used Sn-Bi and Ag wires in order to obtain three-component nanopowders. A high current was applied to the wires between the electrodes in a N₂ atmosphere. We discussed the results based on the K factor, which is the ratio of the charging energy of the capacitor to the vaporization energy of the wire. The three-component (Sn-Bi-Ag) nanoparticles were synthesized under a N₂ atmosphere at 4 and 6 kV. From the particle-size distribution curves, it was found that the mean particle diameter (D₁) values of the Sn-Bi and Ag nanopowders were within the range of 16.32-42.37 nm under each condition. The melting point of the Sn-Bi-Ag nanoparticles was found to be within the range of 188.68-214.97 ℃, which is about 40 ℃ lower than that obtained from the phase diagram and computational thermodynamics of the Sn-Bi-Ag system. In this study, the nanopowders were obtained by subjecting the wires at extreme energies, to improve their solid solubility. (Received February 8, 2017; Accepted July 26, 2017)

keyword : alloy wire, nanoparticles, pulsed wire discharge, three-component

The silicon microlens is a key element in advanced optical packaging. However, the difficulty of its fabrication process and low yield have hindered the development of an silicon lens based optical device module. A silicon Fresnel lens was studied using ray tracing optical simulation, because its small sagittal height allows easy fabrication and high yield. The Fresnel lens was constructed from reference spherical lens with a controlled zone depth and partition number. The optimum zone depth was determined by target wavelength and the refractive index of the lens. The partition number was controlled for the highest coupling efficiency. The maximum coupling efficiency varied with the radius of curvature (ROC) of the lens and the coupling efficiency of the Fresnel lens was always higher than that of a reference lens. When the ROC was 700 μm, the Fresnel lens showed a coupling efficiency of -0.331 dB while the reference lens showed -0.674 dB. It was noted that the high coupling efficiency of the Fresnel lens could be obtained for a wide range of ROCs, which is huge advantage in the design of an optical module. The superior performance of the Fresnel lens was attributed to the smaller beam waist, as analyzed by the beam synthesis propagation method. The alignment tolerance of the Fresnel lens was similar to the reference lens in lateral offset, but it exhibited tighter tolerance with longitudinal offset. (Received May 16, 2017; Accepted July 31, 2017)

keyword : silicon lens, fresnel lens, optical packaging, coupling efficiency

A mixed potential-type NiO·YSZ (yttria stabilized zirconia) gas sensor employing 8YSZ solid electrolyte film was fabricated using a thermal spray coating method, and tested in a NO₂ atmosphere at various operating temperatures. The thermal spray coating method is useful for mass producing solid electrolytes without a forming, sintering or bonding process. In this study, the characteristics of the solid electrolyte made by thermal spray coating were examined and its potential application to a NO₂ gas sensor was evaluated with mixed sensing materials. The 8YSZ solid electrolyte film exhibited a single phase of ZrO₂ and had a constant conductivity in a wide range of oxygen partial pressures, even though the microstructure was rough-columnar type. In addition, it can be suggested that the lower ion conductivity and activation energy of the 8YSZ solid electrolyte film, which resulted from the rough microstructure, increased as much as that of the dense and smooth electrolyte layer prepared by the tape casting method. It produced a denser coating film after using finer granule powders. In particular, the NiO·YSZ mixed sensing material exhibited significantly improved response and recovery characteristics compared to pure NiO. This enhancement is attributed to the smaller NiO particles produced by the grain growth inhibitor YSZ, and the increased triple phase boundary between the sensing material, NO₂ gas and solid electrolyte. (Received May 31, 2017; Accepted July 21, 2017)

keyword : NO₂ Sensor, thermal spray coating, 8YSZ electrolyte, mixed potential type sensor, sensing material