• Journal of the Korean Ceramic Society
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• v.54 no.2
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• pp.108-113
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• 2017

The use of aluminum-based coagulants in water pretreatment is being carefully considered because aluminum exposure is a risk factor for the onset of Alzheimer's disease. Lightly burned-dolomite kiln dust (LB-DKD) was evaluated as an alternative coagulant because it contains high levels of the healthful minerals calcium and magnesium. An organic pore forming agent (OPFA) was incorporated to prepare porous granules after OPFA removal through a thermal decomposition process. A spray drying method was used to produce uniform and reproducible spherical granules with low density, since fine dolomite particles have irregular agglomeration behavior in the hydration reaction. The use of fine dolomite powder and different porosity granules led to a visible color change in raw algae (RA) containing water, from dark green to transparent colorlessness. Also, dolomite powders and granules exhibited a mean removal efficiency of 48.3% in total nitrogen (T-N), a gradual increase in the removal efficiency of total phosphorus (T-P) as granule porosity increased. We demonstrate that porous dolomite granules can improve the settling time and water quality in summer seasons for the emergent treatment of excessive algal blooms in eutrophic water.

• Korean Journal of Materials Research
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• v.19 no.10
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• pp.517-521
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• 2009

Si-C composite with hollow spherical structure was synthesized using ultrasonic treatment of organosilica powder formed by hydrolysis of phenyltrimethoxysilane. The prepared powder was pyrolyzed at various temperatures ranging from 900 to 1300 $^{\circ}C$ under nitrogen atmosphere to obtain optimum conditions for Li-ion battery anode materials with high capacity and cyclability. The XRD and elemental analysis results show that the pyrolyzed Si/C composite at 1100 $^{\circ}C$ has low oxygen and nitrogen levels, which is desirable for increasing the electrochemical capacity and reducing the irreversible capacity of the first discharge. The solid Si-C composite electrode shows a first charge capacity of $\sim$500 mAhg$^{-1}$ and a capacity fade within 30 cycles of 0.93% per cycle. On the other hand, the electrochemical performance of the hollow Si-C composite electrode exhibits a reversible charge capacity of $\sim$540 mAhg$^{-1}$ with an excellent capacity retention of capacity loss 0.43% per cycle up to 30 cycles. The improved electrochemical properties are attributed to facile diffusion of Li ions into the hollow shell with nanoscale thickness. In addition, the empty core space provides a buffer zone to relieve the mechanical stresses incurred during Li insertion.

• Journal of Powder Materials
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• v.23 no.5
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• pp.391-396
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• 2016

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 $N_2$ gas. The mean particle diameter of the obtained powders is $25.7{\mu}m$ and $42.1{\mu}m$ for He and $N_2$ gas, respectively. Their crystallographic structure is confirmed to be amorphous throughout the interior when the particle diameter is less than $45{\mu}m$. 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 ${\mu}_r$ 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.

• Journal of Korea Foundry Society
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• v.17 no.2
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• pp.158-163
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• 1997

Properties of $Al_{86}Ni_6Ce_4Mg_4$ alloy powder produced by helium gas atomization process were investigated by using DSC, XRD, SEM and TEM. The powders below 32 ${\mu}m$ in diameter were identified as an amorphous phase mixed with a ${\alpha}-Al$ phase. $Al_{86}Ni_6Ce_4Mg_4$ bulk alloy was manufactured by hot extruding the alloy powders at various temperatures, and the estimation of its mechanical properties was carried out subsequently. As a result, the bulk alloy extruded at the temperature below $450^{\circ}C$ exhibited the microstructure in which the near-spherical shape of some powders below 20 ${\mu}m$ were nearly unchanged and fine voids between matrix and powders were formed during extrusion process. On the other hand, the tensile strength and elongation at room temperature for $Al_{86}Ni_6Ce_4Mg_4$ bulk alloy extruded at $450^{\circ}C$ were 750 MPa and 7.5%, respectively.

• Journal of Korea Foundry Society
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• v.12 no.3
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• pp.210-219
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• 1992

The measurement of the apparent and tap density for Al-Pb-X(Sn,Sn-Si) powders produced by centrifugal atomizer showed that the larger theoretically calculated densities the larger those densities. And tap densities were not over 50% of the theoretical densities. The nip angle of Al-5wt%Pb alloy powders produced with 38000 r.p.m. of disk rotation was $3^{\circ}$ degree larger than that of Al-8.5wt%Pb-3wt%Sn(-4wt%Si, 8wt%Si) with 50000 r.p.m. The effects of roll gap and rolling speed on thickness and density of the single strips by rolling were that rolling speed increasing the thickness and density of strip decreased and roll gap increasing, the thickness of strip increased but the density decresed. The compactibility of Al-Pb-X with Al by rerolling showed that the coarse powder-strips were better than fine powder-strips. From the SEM study with EDX analysis on the sintered strips, it was found that Pb and Sn were segregated with maximum size $5{\mu}m$, and Si existed surrounding the segregation zone. After sintering the clad strips at $500^{\circ}C$, the pores, which were spherical with $5{\mu}m$ of mean diameter, partly remained around the particles of alloy powders area, while completely disappeared at clad interface. The hardness of strips of alloy powders decreased linearly with increasing sintering temperature.

• Progress in Superconductivity and Cryogenics
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• v.19 no.2
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• pp.19-24
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• 2017

Cryogenic milling which is a combined process of low-temperature treatment and mechanical milling was applied to fabricate high critical current density $(J_c)MgB_2$ bulk superconductors. Liquid nitrogen was used as a coolant, and no solvent or lubricant was used. Spherical Mg ($6-12{\mu}m$, 99.9 % purity) and plate-like B powder (${\sim}1{\mu}m$, 97 % purity) were milled simultaneously for various time periods (0, 2, 4, 6 h) at a rotating speed of 500 rpm using $ZrO_2$ balls. The (Mg+2B) powders milled were pressed into pellets and heat-treated at $700^{\circ}C$ for 1 h in flowing argon. The use of cryomilled powders as raw materials promoted the formation reaction of superconducting $MgB_2$, reduced the grain size of $MgB_2$, and suppressed the formation of impurity MgO. The superconducting critical temperature ($T_c$) of $MgB_2$ was not influenced as the milling time (t) increased up to 6 h. Meanwhile, the critical current density ($J_c$) of $MgB_2$ increased significantly when t increased to 4 h. When t increased further to 6 h, however, $J_c$ decreased. The $J_c$ enhancement of $MgB_2$ by cryogenic milling is attributed to the formation of the fine grain $MgB_2$ and a suppression of the MgO formation.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 2006.05a
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• pp.197-198
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• 2006

In the solid freeform fabrication (SFF) system using selective laser sintering (SLS), polyamide-12 powder is currently recognized as general material. In this study, some kinds of polyamide-6 powders with different shape and particlesize were fabricated to investigate the formability, the microstructure and mechanical properties. Also, to develop a more elaborate and rapid system, this study employs a new SLS device with a 3-axis dynamic focusing scanner system instead of the existing fe lens used in commercial SLS. Polyamide-6 powders having the average size of 100 m were treated thermally in order to keep the spherical symmetry in shape. These polyamide-6 powders were mixed with polyamide-12 powders having the average size of 50 m to give the bimodal distribution of size. These mixed powders showed the better fabrication in the selective laser sintering process because the smaller particles of polyamide-11 played an important role in the compact packing of powders by filling the void space between the large particles of polyamide-6. Also, Experiments have performed to evaluate the effect of a scanning path and sintering parameters by fabricating the various 3D objects.

• Korean Journal of Materials Research
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• v.20 no.3
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• pp.142-147
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• 2010

Nano-sized $BaNd_2Ti_5O_{14}$ powders were prepared by the spray pyrolysis process. Sucrose used as the organic additive enabled the formation of nano-sized $BaNd_2Ti_5O_{14}$ powders. The powders prepared from the spray solution without sucrose had a spherical shape, dense structure and micron size before and after calcination. However, the precursor powders prepared from the spray solution with sucrose had a large size, and hollow and porous morphology. The precursor powders had an amorphous crystal structure because of the short residence time of the powders inside the hot wall reactor. The complete decomposition of sucrose did not occur inside the hot wall reactor. Therefore, the precursor powders obtained from the spray solution with sucrose of 0.5M had a carbon content of 39.2wt.%. The powders obtained from the spray solution with sucrose of 0.5M had a slightly aggregated structure of nano-sized primary powders of $BaNd_2Ti_5O_{14}$ crystalline phase after calcination at $1000^{\circ}C$. The calcined powders turned into nano-sized $BaNd_2Ti_5O_{14}$ powders after milling. The mean size of the $BaNd_2Ti_5O_{14}$ powders was 125 nm.