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This article details how effectively dicing damage of silicon wafers can be mechanically minimized by appropriate laser-induced groove formation prior to wafer separation. Various laser dicing factors, such as the laser-control power, the scan rate and the scan number of the laser beam, were estimated to determine the optimum groove morphology for minimization of chipping damage. The experimental results show that repeated low-power laser beam scanning can be more effective for proper groove formation than single highpower laser beam scanning. In-situ scanning electron microscopic examinations show that the curvature of a laser-induced groove tip can be the most critical factor for minimizing chipping damage during silicon wafer separation. The suppression of dicing damage on a silicon wafer with a sharp laser-induced tip can be explained by atomic force microscopic examinations showing that cleavage fractures along a {011}-crystal plane can be more possible at smaller curvature radius of the laser-induced groove. (Received June 26, 2019; Accepted August 30, 2019)

keyword : semiconductor, silicon wafer, chip, fracture, reliability

In this study, the microstructure-controlled Zn interlayers were synthesized to improve the adhesion strength of Zn/Zn-Mg double layer coatings. The coating chamber temperature and working pressure were controlled based on the Structure Zone Model principles, to obtain a dense columnar microstructure in the Zn interlayer. By controlling the coating chamber temperature, however, porous nanowire structures resulted in an increasing chamber temperature over 100 °C, suggesting that it was not possible to obtain a columnar microstructure of the Zn interlayer. On the other hand, it was possible to synthesize a dense columnar structure in the Zn interlayer by controlling the working pressure. Furthermore, by decreasing the working pressure during deposition, the density of Zn interlayer increased from 79.1% to 93.6%. A lap shear test was performed to evaluate the adhesion strength, and the adhesion strength of Zn /Zn-Mg double layer coatings increased from 20.38 MPa to 24.48 MPa as the density of the Zn interlayer increased. These adhesion strength results were higher than those of the commercial galvanized steels, suggesting that an additional improvement in the adhesion strength would be possible by controlling the microstructure of the Zn interlayer in the Zn/Zn-Mg double layer coatings. (Received August 27, 2019; Accepted September 25, 2019)

keyword : Zn interlayer, structure zone model, columnar structure, adhesion, lap shear test

Al₂O₃ has a high Young’s modulus (380 GPa), a low density (3.98 g· cm^-3), good high-temperature mechanical properties, biocompatibility and excellent oxidation resistance. Al₂O₃ has been used for automotive, aerospace, bio-materials and various industrial applications. Despite its various merits, the low fracture toughness of the material below the brittle-ductile transition temperature has limited its use for wide application. To enhance its fracture toughness, the method commonly utilized has been to make a composite by the addition of a second phase to fabricate nanostructured materials. In the study, nano-powders of Al₂O₃ and Nb were synthesized during the ball milling according to the reaction(Nb₂O₅ + 10/3 Al → 2Nb + 5/3 Al₂O₃). The nanostructured Nb-Al₂O₃ composite was consolidated within a short time from the milled powders using high-frequency induction heated sintering. The average grain sizes of Al₂O₃ and Nb in composite sintered at 1400 °C were 63 and 250 nm, respectively. The relative density of the Nb-Al₂O₃ composite was about 99% under the simultaneous induced current and application of 80 Mpa pressure. The fracture toughness and hardness of the composite were about 8.7 MPa· m^1/2 and 1460 kg/mm², respectively. The fracture toughness of the nanostructured Nb-Al₂O₃ composite was higher than that of monolithic Al₂O₃. (Received June 7, 2019; Accepted September 10, 2019)

keyword : composite, Nb-Al₂O₃, nanomaterials, mechanical properties, sintering

Resistance Element Welding (REW) is a novel processing technology developed to join dissimilar materials such as aluminum and steel. It uses an auxiliary steel rivet (element) inserted into an aluminum sheet that forms a resistance spot weld with the steel sheet, and the aluminum work piece is mechanically joined by the interlocking rivet. The versatility and agility of REW is advantageous, especially in the automotive industry. However, the head of the rivet often extends out externally over the work piece, and the misalignment of the electrode with the head can be problematic since inserting the head and the weld are performed in separate stages. In the present paper, we performed REW using a headless rivet which has minimal to no overhanging part above the aluminum surface. The lap-shear strength of REW is higher than self-piercing rivets, which makes REW a promising solution for extending applications of multi-material structures. The microstructure of the joint involves a fusion zone which mainly consists of martensite, and the failure occurs near the heat affected zone. This study finds that the electrode-rivet alignment needs to be optimized to manage the optimal nugget size and to avoid current arcing through the aluminum work piece. (Received August 12, 2019; Accepted September 30, 2019)

keyword : welding, automotive, dissimilar metals, resistance spot welding, self piercing riveting

Evaluation of electrode degradation in resistance spot welding of 1180TRIP steel was carried out through an electrode life test, and weldability testing was performed by the measurement of the nugget size, the peak load analysis and the measurement of the electrode tip size at 30 intervals. Analysis of the change in the size of nuggets indicated a trend of rapid decrease. Moreover, the peak load showed that there was a rapid decrease until the 150th weld spot and then a slight increase until the 360th weld spot. The crack and cavity in the weld nugget and the protrusion of the weld pool were mostly observed after the 150th weld spot. Unlike the previous study, the electrode tip size steadily decreased during welding. The decreased electrode tip size could be explained by the narrow contact area between the electrode and sheet during welds, which would account for the continuous occurrence of sticking and expulsion between the electrode and the sheet. A geometric deformation occurred at the edge of the electrode owing to the deformation of the electrode without any alloying, whereas the region which was in contact with the sheet experienced Cu-Zn-Fe alloying. As a result, frequent electrode sticking onto the TRIP sheet caused the formation of protrusions and cavities on the electrode surface. Thus, the localized electrode degradation can be divided into three regions: region III where the electrode was undergoing wear and geometric deformation, region II where the protrusion occurred, and region I where the cavity was formed. Finally, the electrode degradation in the resistance spot welding of ultra-high strength steels is defined as “plateau formation” that is due to the geometric as well as metallurgical changes which contradict the existing “mushrooming effect”. (Received August 21, 2019; Accepted September 16, 2019)

keyword : resistance spot welding, ultra-high strength, trip steel, electrode degradation, plateau formation, electron probe micro analyzer

A large amount of waste nickel-cadmium batteries are being produced, and various studies have been conducted to recover the valuable metals contained in the batteries. Solvent extraction is mainly used to recover nickel and cadmium, but that method has economic and environmental problems. In this study, to solve these problems, we investigated the recovery of cadmium by a sulfide precipitation method. The electrode powders obtained from the waste nickel-cadmium battery were leached with sulfuric acid to make a nickel-cadmium mixed solution. Na₂S, (NH₄)₂S and FeS were used as a precipitant and added in a ratio of MeS/Cd = 0.5 - 2. Then, the precipitated powders and filtrate were analyzed to obtain CdS characteristics and cadmium recovery. The cadmium was separated by ion substitution which occurred when the sulfide was added to the nickel-cadmium solution. The recovery of cadmium increased with an increasing sulfide addition. Na₂S did not generate any secondary phase, and this study confirmed that 100% of the cadmium in the nickelcadmium solution was recovered. (Received April 30, 2019; Accepted September 16, 2019)

keyword : cadmium, NiCd secondary battery, recycling, sulfide precipitation

To attain high life standards, it is important to develop high-performance non-toxic gas sensors for public safety, environmental pollutant control, industrial processes, etc. Because reports on single element semiconductor-coated semiconducting metal oxides for sensing applications are rare, we synthesized SnO₂ nanowires and coated them with a 5 nm-thick or 10 nm-thick Si layer for H₂S gas sensing studies. SnO₂ nanowires were successfully synthesized using a highly pure metallic Sn powder at high temperature in a tube furnace by the vapor-liquid-solid method and Si was deposited on the nanowires by the sputtering technique. The desired morphology and composition of the synthesized nanowires were confirmed by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and energy-dispersive X-ray spectroscopy. Moreover, the gas sensing characteristics of pristine and Si-coated SnO₂ nanowires toward H₂S, CO, H₂, C₆H₆, C₂H₅OH and C₆H₇ gases were investigated. The sensing results revealed a good response to H₂S at the optimum operational temperature of 100 °C. Notably, Si-coated SnO₂ nanowire sensors showed a better response to H₂S than pristine SnO₂ nanowires. The mechanism of H₂S sensing is discussed in detail here. This study shows that the Si coating on the SnO₂ nanowire enhances its sensing performance and decreases the sensing temperature required for H₂S gas detection. (Received May 3, 2019; Accepted September 7, 2019)

keyword : SnO₂, nanowires, Si, coating, H₂S, gas sensor, semiconductor

With the increased use of diverse electronics in the high frequency spectrum, high-performance absorbing and shielding materials are needed to ensure electromagnetic compatibility. Microwave absorbance of polymer composites of carbonyl iron powders with frequency selective surface (FSS) is investigated, focusing on the frequency of 5.8 GHz. Composite sheets were prepared by hot pressing a mixture of carbonyl iron powders and silicone rubber. FSS with a square-loop geometry and surface resistance close to the optimum value was fabricated by a screen printing method using carbon black paste as conductive ink. The microwave absorbance for both normal and oblique incidence angles was determined using the commercial computational tool (ANSYS-HFSS). The particulate composites show a reflection loss of -21 dB at 5.8 GHz, with a small thickness of 2.5 mm. Further enhancement of microwave absorbance (less than -40 dB reflection loss) can be realized by attaching a resistive squareloop FSS on the grounded magnetic composite, which is attributed to the enhanced impedance matching driven by the additional resistive component of the attached FSS. The experimental results, obtained with a test sample of optimized composite structure with FSS, are in good agreement with the simulation results. In addition, it was found that the angular stability of the carbonyl iron absorber improved by attachment of FSS, particularly for TM polarization. (Received June 20, 2019; Accepted September 4, 2019)

keyword : carbonyl iron, microwave absorbers, frequency selective surfaces

A porous material can be regarded as a mixture of pores and solid phases, with effective thermal and mechanical properties that significantly differ from those of its base material. Material effective properties depend greatly on the structure and porosity. The effective properties of materials with different pore structures can vary greatly even for materials that exhibit the same porosity. Therefore, understanding the structure-property relationships is beneficial for designing optimum porous structures. The effective (thermal or electrical) conductivity value lies between the upper and lower bounds, corresponding to the horizontal and vertical lamella structures, respectively. The values of these bounds depend upon the porosity. However, many studies have simply treated these effective properties as only a function of the porosity, even though properties can differ at the same porosity. Herein, finite element simulations were performed to estimate the effective thermal and mechanical properties of spherical and cylindrical pore structures with different arrangements. More specifically, FCC- and HCP-type arrangements were considered for spherical pores, while parallel and perpendicular arrangements were considered for cylindrical pores. The results obtained were compared to the theoretical bounds and it was observed that the effective mechanical and thermal properties followed the power-law, whereas the effective yield strength did not. In summary, the results presented herein highlight the importance of pore structure in determining effective properties. (Received July 26, 2019; Accepted September 4, 2019)

keyword : porous material, pore structure, effective property, finite element simulation