• Journal of Powder Materials
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• v.19 no.6
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• pp.435-441
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• 2012

A new manufacturing process of Fe-Cr-Al powder porous metal was attempted. First, ultra-fine fecralloy powders were produced by using the submerged electric wire explosion process. Evenly distributed colloid (0.05~0.5% powders) was dispersed on PU (Polyurethane) foam through the electrospray process. And then degreasing and sintering processes were conduced. In order to examine the effect of sintering temperature in process, pre-samples were sintered for two hours at temperatures of $1350^{\circ}C$, $1400^{\circ}C$, $1450^{\circ}C$, and $1500^{\circ}C$, respectively, in $H_2$ atmospheres. A 24-hour TGA (thermo gravimetric analysis) test was conducted at $1000^{\circ}C$ in a 79% $N_2$+21% $O_2$ to investigate the high temperature oxidation behavior of powder porous metal. The results of the high temperature oxidation tests showed that oxidation resistance increased with increasing sintering temperature (2.57% oxidation weight gain at $1500^{\circ}C$ sintered specimen). The high temperature oxidation mechanism of newly manufactured Fe-Cr-Al powder porous metal was also discussed.

• Journal of Powder Materials
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• v.21 no.1
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• pp.55-61
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• 2014

Fe-Cr-Al powder porous metal was manufactured by using new electro-spray process. First, ultra-fine fecralloy powders were produced by using the submerged electric wire explosion process. Evenly distributed colloid (0.05~0.5% powders) was dispersed on Polyurethane foam through the electro-spray process. And then degreasing and sintering processes were conduced. In order to examine the effect of cell size ($200{\mu}m$, $450{\mu}m$, $500{\mu}m$) in process, pre-samples were sintered for two hours at temperature of $1450^{\circ}C$, in $H_2$ atmospheres. A 24-hour thermo gravimetric analysis test was conducted at $1000^{\circ}C$ in a 79% $N_2$ + 21% $O_2$ to investigate the high temperature oxidation behavior of powder porous metal. The results of the high temperature oxidation tests showed that oxidation resistance increased with increasing cell size. In the $200{\mu}m$ porous metal with a thinner strut and larger specific surface area, the depletion of the stabilizing elements such as Al and Cr occurred more quickly during the high-temperature oxidation compared with the 450, $500{\mu}m$ porous metals.

• Journal of Powder Materials
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• v.23 no.3
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• pp.247-253
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• 2016

In this study, solid solution heat treatment of consolidated nickel-based superalloy powders is carried out by hot isotactic pressing. The effects of the cooling rate of salt quenching, and air cooling on the microstructures and the mechanical properties of the specimens are analyzed. The specimen that is air cooled shows the formation of serrated grain boundaries due to their obstruction by the carbide particles. Moreover, the specimen that is salt quenched shows higher strength than the one that is air cooled due to the presence of fine and close-packed tertiary gamma prime phase. The tensile elongation at high temperatures improves due to the presence of grain boundary serrations in the specimen that is air cooled. On the contrary, the specimen that is salt quenched and consists of unserrated grain boundaries shows better creep properties than the air cooled specimen with the serrated grain boundaries, due to the negative creep phenomenon.

• Proceedings of the Materials Research Society of Korea Conference
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• 2011.05a
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• pp.58.1-58.1
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• 2011

WC-Co and other similar cemented carbides have been widely used as hard materials in industrial cutting tools and as mould metals; and a number of techniques have been applied to improve its microstructural characteristics, hardness and ear resistance. Cobalt is used primarily to facilitate liquid phase sintering and acts as a matrix, i.e. a cementing phase between WC grains. A uniform distribution of metal phase in a ceramic is beneficial for improved mechanical properties of the composite. WC-Co, starting from initial powders, is vastly used for a variety of machining, cutting, drilling, and other applications because of its unique combination of high strength, high hardness, high toughness, and moderate modulus of elasticity, especially with fine grained WC and finely distributed cobalt. In this study, that started with two different compositions of initial powders, WC-7.5wt%Co and WC-12wt%Co with initial powder size being 1~3 ${\mu}m$, magnetic pulsed compaction followed by subsequent vacuum sintering were carried out to produce consolidated preforms. Magnetic Pulsed Compaction (MPC), a very short duration (~600 ${\mu}s$), high pressure (~4 Gpa), high-density preform molding method was used with varied pressure between 0.5 and 3.0 Gpa, in order to reach an initial high density that would help improve the sintering behavior. For both compositions and varied MPC pressure, before and after sintering, changes in microstructural behavior and mechanical properties were analyzed. With proper combination of MPC pressure and sintering, samples were obtained with better mechanical properties, densification and microstructural behavior, and considerably improved than other conventional processes.

• Korean Journal of Materials Research
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• v.26 no.10
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• pp.561-569
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• 2016

$SrAl_2O_4$: $Eu^{2+}$ and $Dy^{3+}$ phosphorescent phosphors were synthesized using the polymerized complex method. Generally, phosphorescent phosphors synthesized by conventional solid state reaction show a micro-sized particle diameter; thus, this process is restricted to applications such as phosphorescent ink and paint. However, it is possible to synthesize homogeneous multi-component powders with fine particle diameter by wet process such as the polymerized complex method. The characteristics of $SrAl_2O_4$: $Eu^{2+}$ and $Dy^{3+}$ powders prepared by polymerized complex method with one and two step calcination processes were comparatively analyzed. Temperatures of organic material removal and crystallization were observed through TG-DTA analysis. The crystalline phase and crystallite size of the $SrAl_2O_4$: $Eu^{2+}$ and $Dy^{3+}$ phosphorescent phosphors were analyzed by XRD. Microstructures and afterglow characteristics of the $SrAl_2O_4$: $Eu^{2+}$ and $Dy^{3+}$ phosphors were measured by SEM and spectrofluorometry, respectively.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.12 no.4
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• pp.172-177
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• 2002

The CuZnAl alloys have some advantages against other shape memory alloys, such as the widely variable transformation temperature, the low cost and easy fabrication. The alloys have been produced mostly by metallurgical methods. Thereby a tendency to large grain sizes is observed, which causes brittle properties of the materials. In order to avoid these deficiencies a special powder metallurgical process, SPS(spark plasma sintering), is applied in the present investigation. The starting materials were the pure (99.9 %) Cu, Zn and Al element powders with different particle size. The relatively fine grained and homogeneous Cu-24.78Zn-9.11Al (at.%) and Cu-13.22Zn-17.24Al (at.%) shape memory alloys were obtained using the powders with size of 75-150 $\mu$m. The average grain size is about 70 $\mu$m and the phases at room temperature are the austenitic and martensitic phase respectively.

• 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.

• Progress in Superconductivity
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• v.10 no.1
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• pp.12-16
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• 2008

In this work, nano size $BaCeO_3$, which is a possible flux pinning medium of melt processed $YBa_{2}Cu_{3}O_x$ superconductor, was synthesized by the conventional solid state reaction method using powders. $BaCeO_3$ and $CeO_2$ were mixed thoroughly using a ball milling for 24 hours and calcined at $1200^{\circ}C$ for 5 hours for the formation $BaCeO_3$ powder. The obtained $BaCeO_3$ powder was attrition milled at various milling times of 60 min, 120 min and 240 min. The $BaCeO_3$ powders of various milling times were mixed with $YBa_{2}Cu_{3}O_x$ powder. Seed melt processed $YBa_{2}Cu_{3}O_x$-$BaCeO_3$ (15wt.%) superconductors were prepared and the superconducting properties were investigated. It was found that $T_c$ of $Y_{1.5}Ba_{2}Cu_{3}O_x$ samples was not significantly affected by $BaCeO_3$ addition, but $J_c$ of samples was increased by $BaCeO_3$ addition. The $J_c$ improvement by fine $BaCeO_3$ powder (120 min attrition-milled) was effective at low magnetic fields less than 2 T.

• Journal of the Korean Ceramic Society
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• v.34 no.10
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• pp.1083-1091
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• 1997

As one of the chemical powder fabrication methods, the powder preparation method by using oxalate has the following advantages; (1) easy to control the chemical stoichiometry, (2) easy to fabricate homogeneous and fine particles, and (3) easy to be thermaly decomposed at low temperature. In the present study, the initial morphology and size distribution of the powder were controlled and the homogeniaty was improved. By carefully controlling the pH with NH4OH, the Bi(Pb)-Sr-Ca-Cu-O superconducting powders were prepared and investigated for their properties. The microstructures and the superconducting properties of the pelletized samples were investigated. Also, the microstructures and electrical properties of the samples prepared by tape casting method were investigated. The fabricated powders were spherical with less than 400 nm, but most of them were agglomerated to be 1~3 ${\mu}{\textrm}{m}$ in size. The critical temperature of the pelletized sample annealed at 84$0^{\circ}C$ for 72 hours in air was 110K. And the critical currents of annealed samples in air prepared by tape casting process for 24 hours and 72 hours were 0.6 A (Jc=600A/$\textrm{cm}^2$) and 1.9A (Jc=1, 900A/$\textrm{cm}^2$) respectively.

• Korean Journal of Materials Research
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• v.28 no.2
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• pp.108-112
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• 2018

The present study demonstrates the effect of raw powder on the pore structure of porous W-Ni prepared by freeze drying of camphene-based slurries and sintering process. The reduction behavior of $WO_3$ and $WO_3-NiO$ powders is analyzed by a temperature programmed reduction method in Ar-10% H2 atmosphere. After heat treatment in hydrogen atmosphere, $WO_3-NiO$ powder mixture is completely converted to metallic W without any reaction phases. Camphene slurries with oxide powders are frozen at $-30^{\circ}C$, and pores in the frozen specimens are generated by sublimation of the camphene during drying in air. The green bodies are hydrogen-reduced at $800^{\circ}C$ and sintered at $1000^{\circ}C$ for 1 h. The sintered samples show large and aligned parallel pores to the camphene growth direction, and small pores in the internal wall of large pores. The strut between large pores, prepared from pure $WO_3$ powder, consists of very fine particles with partially necking between the particles. In contrast, the strut densification is clearly observed in the Ni-added W sample due to the enhanced mass transport in activation sintering.