• Journal of Powder Materials
• /
• v.26 no.1
• /
• pp.1-5
• /
• 2019

In this study, porous Mo-5 wt% Cu with unidirectionally aligned pores is prepared by freeze drying of camphene slurry with $MoO_3-CuO$ powders. Unidirectional freezing of camphene slurry with dispersion stability is conducted at $-25^{\circ}C$, and pores in the frozen specimens are generated by sublimation of the camphene crystals. The green bodies are hydrogen-reduced at $750^{\circ}C$ and sintered at $1000^{\circ}C$ for 1 h. X-ray diffraction analysis reveals that $MoO_3-CuO$ composite powders are completely converted to a Mo-and-Cu phase without any reaction phases by hydrogen reduction. The sintered bodies with the Mo-Cu phase show large and aligned parallel pores to the camphene growth direction as well as small pores in the internal walls of large pores. The pore size and porosity decrease with increasing composite powder content from 5 to 10 vol%. The change of pore characteristics is explained by the degree of powder rearrangement in slurry and the accumulation behavior of powders in the interdendritic spaces of solidified camphene.

• Tribology and Lubricants
• /
• v.34 no.6
• /
• pp.270-278
• /
• 2018

STS 316L prepared by additive manufacturing (AM) exhibits deterioration of mechanical properties and wear resistance due to the presence of defects such as black-of-fusion defects, internal porosity, residual stress, and anisotropy. In addition, high surface roughness (integrity) of AM products remains an issue. This study aimed to apply ultrasonic nanocrystal surface modification (UNSM) technology to STS 316L prepared by AM to increase the surface hardness, to reduce the surface roughness, and to improve the friction and wear behavior to the level achieved by bulk material manufactured using traditional processes. Herein, the as-received and polished specimens were treated by UNSM technology and their resulting properties were compared and discussed. The results showed that UNSM technology increased the surface hardness and reduced the surface roughness of the as-received and polished specimens. These results can be attributed to grain size refinement and pore elimination from the surface. Moreover, the friction of the as-received and polished specimens after UNSM technology was lower compared to those of the as-received and polished specimens, but no significant differences in wear resistance were found.

• Tribology and Lubricants
• /
• v.37 no.3
• /
• pp.83-90
• /
• 2021

In this study, an ultrasonic nanocrystal surface modification (UNSM) was used to improve the mechanical properties, scratch resistance and tribological performance of Cu-based bimetals, which are usually used to manufacture sliding bearings and bushings for internal combustion engines (ICEs). Two different Cu-based bimetals, namely CuPb10Sn10 and CuSn10Bi7, were sintered onto a low carbon steel substrate. The mechanical properties and dry tribological performance using a tensile tester and micro-tribo tester were evaluated, respectively. The scratch resistance was assessed using a micro-scratch tester at an incremental load. The tensile test results showed that the yield strength (YS) and ultimate tensile strength (UTS) of both Cu-based bimetals increased after UNSM. Furthermore, the scratch and tribological tests results revealed that the scratch resistance and tribological performance of both Cu-based bimetals were improved by the application of UNSM. These improvements were mainly attributed to the eliminated pores, increased hardness and reduced roughness after UNSM. CuSn10Bi7 demonstrated better mechanical properties, scratch resistance and tribological performance than CuPb10Sn10. It was found that the presence of Bi in CuSn10Bi7 formed a Cu₁₁Bi₇ intermetallic phase, which is harder than Cu₃Sn. Hence, CuSn10Bi7 demonstrated higher strength and wear resistance than CuPb10Sn10. In addition, a CuSn10Bi7 formed both SnO₂ and Bi₂O₃ that prevented adhesion and improved the tribological performance. It can be expected that under dry tribological conditions, ICEs can utilize UNSM bearings and bushings made of CuSn10Bi7 instead of CuPb10Sn10 under oil-lubricated conditions.

• Journal of the Korean Society of Manufacturing Process Engineers
• /
• v.20 no.8
• /
• pp.42-51
• /
• 2021

Investigations of process parameters are essential when fabricating high-quality parts using additive manufacturing. This study investigates the change in the mechanical characteristics of a SUS316L specimen fabricated using selective laser melting based on the energy density and bead overlap ratio. The SUS316L powder particles were spherical and 35 ㎛ in size. Single-bead and hexahedral shape deposition experiments were performed sequentially. A single bead experiment was performed to obtain the bead overlap ratios for different laser parameters utilizing laser power and scan speed as experimental parameters. A hexahedral shape deposition experiment was also performed to observe the difference in mechanical properties, such as the internal porosity, surface roughness, and hardness, based on the energy density and bead overlap ratio of the three-dimensional printed part. Laser power, scan speed, overlap ratio, and layer thickness were chosen as parameters for the hexahedral shape deposition experiment. Accordingly, the energy density applied for three-dimensional printing, and the experimental parameters were calculated, and the energy density and bead overlap ratio for fabricating parts with good properties have been suggested.

• Korean Journal of Chemical Engineering
• /
• v.35 no.11
• /
• pp.2256-2268
• /
• 2018

Hydrophilic and hydrophobic polyethersulfone (PES)-zinc oxide (ZnO) sublayers were prepared by loading of ZnO nanoparticles into PES matrix. Both porosity and hydrophilicity of the hydrophilic sublayer were increased upon addition of hydrophilic ZnO, while these were decreased for the hydrophobic sublayer. In addition, the results demonstrated that the hydrophilic membrane exhibited smaller structural parameter (S value or S parameter or S), which is beneficial for improving pure water permeability and decreasing mass transfer resistance. In contrast, a higher S parameter was obtained for the hydrophobic membrane. With a 2 M NaCl as DS and DI water as FS, the pure water flux of hydrophilic TFN0.5 membrane was increased from $21.02L/m^2h$ to $30.06L/m^2h$ and decreased for hydrophobic TFN0.5 membrane to $14.98L/m^2h$, while the salt flux of hydrophilic membrane increased from $10.12g/m^2h$ to $17.31g/m^2h$ and decreased for hydrophobic TFN0.5 membrane to $3.12g/m^2h$. The increment in pure water permeability can be ascribed to reduction in S parameter, which resulted in reduced internal concentration polarization (ICP). The current study provides a feasible and low cost procedure to decrease the ICP in FO processes.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
• /
• v.35 no.4
• /
• pp.377-385
• /
• 2022

Thermal batteries are specialized as primary reserve batteries that operate when the internal heat source is ignited and the produced heat (450~550℃) melts the initially insulating salt into highly conductive eutectic electrolyte. The heat source is composed of Fe powder and KClO₄ with different mass ratios and is inserted in-between the cells (stacks) to allow homogeneous heat transfer and ensure complete melting of the electrolyte. An ideal heat source has following criteria to satisfy: sufficient mechanical durability for stacking, appropriate heat calories, ease of combustion by an igniter, stable combustion rate, and modest peak temperature. To satisfy the aforementioned requirements, Fe powder must have high surface area and porosity to increase the reaction rate. Herein, the hydrothermal and spray drying synthesis techniques for Fe powder samples are employed to investigate the physicochemical properties of Fe powder samples and their applicability as a heat source constituent. The direct comparison with the state-of-the-art Fe powder is made to confirm the validity of synthesized products. Finally, the actual batteries were made with the synthesized iron powder samples to examine their performances during the battery operation.

• Journal of Electrochemical Science and Technology
• /
• v.13 no.3
• /
• pp.390-397
• /
• 2022

The use of microwaves as the energy source for synthesis and sintering of ceramics offers substantial advantages compared to conventional gas-fired and electric resistance furnaces. Benefits include much shorter processing times and reaching the sintering temperature more quickly, resulting in superior final product quality. Most oxide ceramics poorly interact with microwave irradiation at low temperatures; thus, a more complex setup including a susceptor is needed, which makes the whole process very complicated. This investigation pursued a new approach, which enabled us to use microwave irradiation directly in poorly coupled oxides. In many solid-state electrochemical devices, the support is either metal or can be reduced to metal. Metal powders in the support can act as an internal susceptor and heat the entire cell. Then sufficient interaction of microwave irradiation and ceramic material can occur as the sample temperature increases. This microwave heating and exothermic reaction of oxidation of the support can sinter the ceramic very efficiently without any external susceptor. In this study, yttria stabilized zirconia (YSZ) and a Ni-YSZ cermet support were used as an example. The cermet was used as the support, and a YSZ electrolyte was coated and sintered directly using microwave irradiation without the use of any susceptor. The results were compared to a similar cell prepared using a conventional electric furnace. The leakage test and full cell power measurement results revealed a fully leak-free electrolyte. Scanning electron microscopy and density measurements show that microwave sintered samples have lower open porosity in the electrode support than conventional heat treatment. This technique offers an efficient way to directly use microwave irradiation to sinter thin film ceramics without a susceptor.

• Composites Research
• /
• v.20 no.4
• /
• pp.9-17
• /
• 2007

Powder metallurgy has been employed for the development of SiC particle reinforced aluminum metal matrix composites by means of hot isotropic pressing and vacuum hot pressing. A material model based on micro-mechanical approach then has been presented for the processes. Densification occurs by the inelastic flow of matrix materials during the consolidation, and consequently it depends on many process conditions such as applied pressure, temperature and volume fraction of reinforcement. The model is implemented into finite element software so that the process simulation can be performed enabling the predicted relative density to be compared with experimental data. In order to determine the performance of finished products, further tensile test has been conducted using the developed specimens. The effect of internal void of the materials on mechanical properties therefore can be investigated.

• Journal of the Korea Institute of Building Construction
• /
• v.24 no.3
• /
• pp.297-308
• /
• 2024

This study presents a novel approach for evaluating fire-induced damage depth in concrete. The methodology leverages the principle that exposure to high temperatures causes internal expansion within concrete, leading to increased voids and microcracks in the damaged zones. This heightened porosity results in greater absorption rates compared to undamaged areas. By immersing fire-damaged concrete samples in water and subsequently monitoring the drying process, the depth of damage can be assessed. Differences in drying rates and color variations between damaged and undamaged areas serve as visual indicators for determining the extent of the damage. Experimental results from this water immersion method revealed damage depths of 38.7mm and 37.5mm for two different concrete mixtures. These measurements notably surpass the damage depths estimated using traditional phenolphthalein-based methods. This discrepancy suggests that utilizing the absorption rate principle, which is directly linked to the physical changes caused by thermal expansion, offers a more accurate and sensitive assessment of fire damage depth compared to methods relying solely on the presence of Portlandite for colorimetric indication.

• Geomechanics and Engineering
• /
• v.38 no.2
• /
• pp.125-137
• /
• 2024

In paste backfill mining, cemented coal gangue-fly ash backfill (CGFB) can effectively utilize coal-based solid waste, such as gangue, to control surface subsidence. However, given the pressurized water accumulation environment in goafs, CGFB is subject to coupling effects from water pressure and chloride ions. Therefore, studying the influence of pressurized water on the chlorine salt erosion of CGFB to ensure green mining safety is important. In this study, CGFB samples were soaked in a chloride salt solution at different pressures (0, 0.5, 1.5, and 3.0 MPa) to investigate the chloride ion transport characteristics, hydration products, micromorphology, pore characteristics, and mechanical properties of CGFB. Water pressure was found to promote chloride ion transfer to the CGFB interior and the material hydration reaction; enhance the internal CGFB pore structure, penetration depth, and chloride ion content; and fill the pores between the material to reduce its porosity. Furthermore, the CGFB peak uniaxial compression strain gradually decreased with increasing soaking pressure, whereas the uniaxial compressive strength first increased and then decreased. The resulting effects on the stability of the CGFB solid-phase hydration products can change the overall CGFB mechanical properties. These findings are significant for further improving the adaptability of CGFB for coal mine engineering.