In this study, a two-stage electrostatic precipitator (ESP) was developed using a novel automatic dry cleaning device to reduce the ultrafine particles in subway stations. Collection efficiency was evaluated with a pilot scale ESP (1.2m× 1.2m) and the scale of the test duct was half of the subway air handling unit. The maximum collection efficiency for 0.3 μm particles was 96.9%. In addition, we studied a method of automatic dry cleaning for maintenance of the ESP. The cleaning efficiency was analyzed according to the cleaning flow rate for each particle loading amount to achieve a recovery rate over 90%. In addition, we derived the equation to estimate the reduction in collection efficiency according to the particle loading amount. It was confirmed that the performance of the contaminated ESP was restored to the initial state by the automatic dry cleaning in this study and that the electrical energy consumption was 5 times lower compared to utilizing conventional water cleaning.

Indoor air quality management is essential for a healthy life. However, it is difficult to perceive, detect, and monitor the level of indoor air pollution and this means that it is possible to be exposed to more pollution indoors than outdoors. In this study, in order to derive effective indoor air quality management measures, public perceptions and behavioral characteristics regarding indoor particulate matter and air quality management methods were investigated through a survey of 1,000 people. Based on the survey, it was found that most of the respondents had a negative perception of the indoor air quality of their residence, and natural ventilation was the most used method for indoor air quality management. Although the frequency of use of air quality management devices such as air purifiers and mechanical ventilation systems was relatively low, their effect regarding air quality management was positively perceived. In particular, the results of survey indicated that respondents of families which included members with fragile health engaged in more active behavior regarding in indoor air quality management than those respondents whose family members had no health issues and that the former have used air quality management devices more frequently. Therefore, it is necessary to develop proper guidelines to encourage more people to actively participate in improving indoor air quality.

High concentrations of fine particles are increasingly being detected due to inflows from abroad and local emission sources in Korea. As most people spend about 90% of their time indoors, the use of indoor air cleaners has grown significantly as they are now thought to be essential items. In this study, the noise, power consumption, and clean air delivery rate (CADR) of commercial air cleaners were analyzed according to the structural shape of the air cleaners. Analyses were performed based on the experimental results of 249 cases for air cleaners certified by Korea Air Cleaning Association. The air cleaners with front inlet and upper outlet air flow direction, which currently account for the highest market share, were found to have the highest noise per CADR (dB(A)/(m3/min)). On the other hand, the air cleaners with the inlet and outlet air flow in the same horizontal direction were found to have lowest noise per CADR than other structures.

We investigate the effect of phosphorous content on the microstructure and magnetic properties of Fe83.2Si5.33-0.33xB10.67-0.67xPxCu0.8 (x = 1–4 at.%) nanocrystalline soft magnetic alloys. The simultaneous addition of Cu and P to nanocrystalline alloys reportedly decreases the nanocrystalline size significantly, to 10–20 nm. In the P-containing nanocrystalline alloy, P atoms are distributed in an amorphous residual matrix, which suppresses grain growth, increases permeability, and decreases coercivity. In this study, nanocrystalline ribbons with a composition of Fe83.2Si5.33-0.33xB10.67- 0.67xPxCu0.8 (x = 1–4 at.%) are fabricated by rapid quenching melt-spinning and thermal annealing. It is demonstrated that the addition of a small amount of P to the alloy improves the glass-forming ability and increases the resistance to undesirable Fex(B,P) crystallization. Among the alloys investigated in this work, an Fe83.2Si5B10P1Cu0.8 nanocrystalline ribbon annealed at 460oC exhibits excellent soft-magnetic properties including low coercivity, low core loss, and high saturation magnetization. The uniform nanocrystallization of the Fe83.2Si5B10P1Cu0.8 alloy is confirmed by high-resolution transmission electron microscopy analysis.

Iron-based amorphous powder attracts increasing attention because of its excellent soft magnetic properties and low iron loss at high frequencies. The development of an insulating layer on the surface of the amorphous soft magnetic powder is important for minimizing the eddy current loss and enhancing the energy efficiency of highfrequency devices by further increasing the electrical resistivity of the cores. In this study, a hybrid insulating coating layer is investigated to compensate for the limitations of monolithic organic or inorganic coating layers. Fe2O3 nanoparticles are added to the flexible silicon-based epoxy layer to prevent magnetic dilution; in addition TiO2 nanoparticles are added to enhance the mechanical durability of the coating layer. In the hybrid coating layer with optimal composition, the decrease in magnetic permeability and saturation magnetization is suppressed.

The enamel powders used traditionally in Korea are produced by a ball-milling process. Because of their irregular shapes, enamel powders exhibit poor flowability. Therefore, polygonal enamel powders are only used for handmade cloisonné crafts. In order to industrialize or automate the process of cloisonné crafts, it is essential to control the size and shape of the powder. In this study, the flowability of the enamel powders was improved using the spheroidization process, which employs the RF plasma treatment. In addition, a simple grid structure and logo were successfully produced using the additive manufacturing process (powder bed fusion), which utilizes spherical enamel powders. The additive manufacturing technology of spherical enamel powders is expected to be widely used in the field of cloisonné crafting in the future.

In this study, two types of SKD61 tool-steel samples are built by a selective laser melting (SLM) process using the different laser scan speeds. The characteristics of two kinds of SKD61 tool-steel powders used in the SLM process are evaluated. Commercial SKD61 tool-steel power has a flowability of 16.68 sec/50 g and its Hausner ratio is calculated to be 1.25 by apparent and tapped density. Also, the fabricated SKD61 tool steel powder fabricated by a gas atomization process has a flowability of 21.3 sec/50 g and its Hausner ratio is calculated to be 1.18. Therefore, we confirmed that the two powders used in this study have excellent flowability. Samples are fabricated to measure mechanical properties. The highest densities of the SKD61 tool-steel samples, fabricated under the same conditions, are 7.734 g/cm³ (using commercial SKD61 powder) and 7.652 g/cm3 (using fabricated SKD61 powder), measured with Archimedes method. Hardness is measured by Rockwell hardness testing equipment 5 times and the highest hardnesses of the samples are 54.56 HRC (commercial powder) and 52.62 HRC (fabricated powder). Also, the measured tensile strengths are approximately 1,721 MPa (commercial SKD61 powder) and 1,552 MPa (fabricated SKD61 powder), respectively.

The friction characteristics of Al-Fe alloy powders are investigated in order to develop an eco-friendly friction material to replace Cu fiber, a constituent of brake-pad friction materials. Irregularly shaped Al-Fe alloy powders, prepared by gas atomization, are more uniformly dispersed than conventional Cu fiber on the brake pad matrix. The wear rate of the friction material using Al-8Fe alloy powder is lower than that of the Cu fiber material. The change in friction coefficient according to the friction lap times is 7.2% for the Cu fiber, but within 3.8% for the Al- Fe alloy material, which also shows excellent judder characteristics. The Al-Fe alloy powders are uniformly distributed in the brake pad matrix and oxide films of Al and Fe are homogeneously formed at the friction interface between the disc and pad, thus exhibiting excellent friction and lubrication characteristics. The brake pad containing Al-Fe powders avoids contamination by Cu dust, which is generated during braking, by replacing the Cu fiber while maintaining the friction and lubrication performance.

A cold-work tool steel powder is used to fabricate 3-dimensional objects by selective laser melting using a high-pressure gas atomization process. The spherical powder particles form continuous carbide networks among the austenite matrix and its decomposition products. The carbides comprise Nb-rich MC and Mo-rich M2C. In the SLM process, the process parameters such as the laser power (90 W), layer thickness (25 μm), and hatch spacing (80 μm) are kept fixed, while the scan speed is changed from 50 mm/s to 4000 mm/s. At a low scan speed of 50 mm/s, spherical cavities develop due to over melting, while they are substantially reduced on increasing the speed to 2000 mm/s. The carbide network spacing decreases with increasing speed. At an excessively high speed of 4000 mm/s, long and irregularly shaped cavities are developed due to incomplete melting. The influence of the scan pattern is examined, for which 1 × 1 mm2 blocks constituting a processing layer are irradiated in a random sequence. This island-type pattern exhibits the same effect as that of a low scan speed. Post processing of an object using hot isostatic pressing leads to a great reduction in the porosity but causes coarsening of the microstructure.

In this study, H13 tool steel sculptures are built by a metal 3D printing process at various laser scan speeds. The properties of commercial H13 tool steel powders are confirmed for the metal 3D printing process used: powder bed fusion (PBF), which is a selective laser melting (SLM) process. Commercial H13 powder has an excellent flowability of 16.68 s/50 g with a Hausner ratio of 1.25 and a density of 7.68 g/cm3. The sculptures are built with dimensions of 10 × 10 × 10 mm3 in size using commercial H13 tool steel powder. The density measured by the Archimedes method is 7.64 g/cm3, similar to the powder density of 7.68 g/cm3. The hardness is measured by Rockwell hardness equipment 5 times to obtain a mean value of 54.28 HRC. The optimum process conditions in order to build the sculptures are a laser power of 90 W, a layer thickness of 25 μm, an overlap of 30%, and a laser scan speed of 200 mm/s.

Selective laser melting (SLM) can produce a layer of a metal powder and then fabricate a three-dimensional structure by a layer-by-layer method. Each layer consists of several lines of molten metal. Laser parameters and thermal properties of the materials affect the geometric characteristics of the melt pool such as its height, depth, and width. The geometrical characteristics of the melt pool are determined herein by optical microscopy and three-dimensional bulk structures are fabricated to investigate the relationship between them. Powders of the commercially available Fe-based tool steel AISI H13 and Ni-based superalloy Inconel 738LC are used to investigate the effect of material properties. Only the scan speed is controlled to change the laser parameters. The laser power and hatch space are maintained throughout the study. Laser of a higher energy density is seen to melt a wider and deeper range of powder and substrate; however, it does not correspond with the most highly densified three-dimensional structure. H13 shows the highest density at a laser scan speed of 200 mm/s whereas Inconel 738LC shows the highest density at 600 mm/s.

In this study, ultra-fine soft-magnetic micro-powders are prepared by high-pressure gas atomization of an Fe-based alloy, Fe-Hf-B-Nb-P-C. Spherical powders are successfully obtained by disintegration of the alloy melts under high-pressure He or N2 gas. The mean particle diameter of the obtained powders is 25.7 μm and 42.1 μm for He and N2 gas, respectively. Their crystallographic structure is confirmed to be amorphous throughout the interior when the particle diameter is less than 45 μm. The prepared powders show excellent soft magnetic properties with a saturation magnetization of 164.5 emu/g and a coercivity of 9.0 Oe. Finally, a toroidal core is fabricated for measuring the magnetic permeability, and a μr of up to 78.5 is obtained. It is strongly believed that soft magnetic powders prepared by gas atomization will be beneficial in the fabrication of high-performance devices, including inductors and motors.

Al-Si-SiC composite powders with intra-granular SiC particles were prepared by a gas atomization process. The composite powders were mixed with Al-Zn-Mg alloy powders as a function of weight percent. Those mixture powders were compacted with the pressure of 700 MPa and then sintered at the temperature of 565-585˚C. T6 heat treatment was conducted to increase their mechanical properties by solid-solution precipitates. Each relative density according to the optimized sintering temperature of those powders were determined as 96% at 580˚C for Al-Zn-Mg powders (composition A), 97.9% at 575˚C for Al-Zn-Mg powders with 5 wt.% of Al-Si-SiC powders (composition B), and 98.2% at 570˚C for Al-Zn-Mg powders with 10 wt.% of Al-Si-SiC powders (composition C), respectively. Each hardness, tensile strength, and wear resistance test of those sintered samples was conducted. As the content of Al-Si-SiC powders increased, both hardness and tensile strength were decreased. However, wear resistance was increased by the increase of Al-Si-SiC powders. From these results, it was confirmed that Al-Si-SiC/Al-Zn-Mg composite could be highly densified by the sintering process, and thus the composite could have high wear resistance and tensile strength when the content of Al-Si-SiC composite powders were optimized.

This research presents a preparation method of dental components by metal injection molding process (MIM process) using titanium scrap. About 20 μm sized spherical titanium powders for MIM process were successfully prepared by a novel dehydrogenation and spheroidization method using in-situ radio frequency thermal plasma treatment. The effects of MIM process parameters on the mechanical and biological properties of dental components were investigated and the optimum condition was obtained. After sintering at 1250oC for 1 hour in vacuum, the hardness and the tensile strength of MIMed titanium components were 289 Hv and 584 MPa, respectively. Prepared titanium dental components were not cytotoxic and they showed a good cell proliferation property.