• Journal of Powder Materials
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• v.21 no.6
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• pp.467-472
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• 2014

RBSC (reaction-bonded silicon carbide) represents a family of composite ceramics processed by infiltrating with molten silicon into a skeleton of SiC particles and carbon in order to fabricate a fully dense body of silicon carbide. RBSC has been commercially used and widely studied for many years, because of its advantages, such as relatively low temperature for fabrication and easier to form components with near-net-shape and high relative density, compared with other sintering methods. In this study, RBSC was fabricated with different size of SiC in the raw material. Microstructure, thermal and mechanical properties were characterized with the reaction-sintered samples in order to examine the effect of SiC size on the thermal and mechanical properties of RBSC ceramics. Especially, phase volume fraction of each component phase, such as Si, SiC, and C, was evaluated by using an image analyzer. The relationship between microstructures and physical properties was also discussed.

• 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.

• Journal of Welding and Joining
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• v.31 no.3
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• pp.76-83
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• 2013

This study investigated the effect of bonding temperature and holding time on microstructures and mechanical properties of diffusion bonded joint of Haynes230. The diffusion bonds were performed at the temperature of 950, 1050, and $1150^{\circ}C$ for holding times of 30, 60, 120 and 240 minutes at a pressure of 4MPa under high vacuum condition. The amount of non-bonded area and void observed in the bonded interface decreased with increasing bonding temperature and holding time. Cr-rich precipitates at the linear interface region restrained grain migration at $950^{\circ}C$ and $1050^{\circ}C$. However, the grain migration was observed in spite of short holding time due to the dissolution of precipitates to base metal in the interface region at $1150^{\circ}C$. Three types of the fracture surface were observed after tensile test. The region where the coalesce and migration of grain occurred much showed high fracture load because of base metal fracture whereas the region where those did less due to the precipitates demonstrated low fracture load because of interface fracture. The expected fracture load could be derived with the value of fracture area of base metal ($A_{BF}$) and interface ($A_{IF}$), $Load=201A_{BF}+153A_{IF}$. Based on this equation, strength of base metal and interface fracture were calculated as 201MPa and 153MPa, respectively.

• Applied Chemistry for Engineering
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• v.20 no.5
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• pp.505-510
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• 2009

It is well known that the properties of concrete such as the compressive strength, water permeability, water tightness and durability are affected by micro-structure in hardened cement paste. Especially, for durability of concrete, watertightness of cementitious materials is the most critical property among various properties. Recently, many types of material such as organic and/or inorganic materials are used for watertightness of concrete. In this study, we examined the effect of fluorine-based emulsion on watertightness property. And we also discussed the change of microstructure and formation of hydrates in hardened cement paste by the addition of fluorine-based emulsion. Cement paste with fluorine-based emulsion showed improvement of watertightness by the surface activation of cement paste and by formation of $CaF_2$ fine crystals in cement pore structure.

• Journal of the Korean Magnetics Society
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• v.7 no.1
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• pp.49-54
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• 1997

The effect of Co and annealing temperature on the magnetic properties, phase change and microstructure of melt-spun $(Nd_{14.73}Fe_{78.67}B_{6.60})_{100-x}Co_x$ (X=0, 1, 2, 3) ribbons has been studied. The Co containing ribbons were found to have higher coercivity ($_iH_c$) than the ribbons without Co. Intrinsic coercivity of 20.3 kOe has been obtained by addition of 2 at%Co. This effect by Co addition is also represented in the case of hot pressed and die-upseted magnets. The maximum intrinsic coercivities of hot press and die-upset $(Nd_{14.73}Fe_{78.67}B_{6.60})_{100- x}Co_x$ (X=0, 1, 2, 3) magnets are 16.9 kOe and 15.2 kOe when X=2.

• Transactions of Materials Processing
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• v.8 no.3
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• pp.247-247
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• 1999

The segregation (distribution) of nickel and the composition of its constituents influence the low thermal expansion characteristics (Invar effect) in Fe-30 wt.% Ni-12.5 wt.% Co-xC Invar alloy. The change of coefficient of the thermal expansion and magnetic properties were studied as an aspect of carbon addition causing the segregation of Ni in primary austenite of as-cast Fe-30 wt.% Ni-12.5 wt.% Co Invar alloy. The coefficient of thermal expansion of Fe-30 wt.% Ni-12.5 wt.% Co-xC Invar alloy showed its lowest value at 0.08 wt.% carbon, increased with increasing carbon content in the range of 0.08-1.0 wt.%C, kept constant at 1.0-2.0 wt.%C and decreased at carbon higher than 2.0 wt.%. The effective distribution of the coefficient of nickel in as-cast Fe-30 wt.% Ni-12.5 wt.% Co-xC Invar alloy increased with increasing carbon content. The volume fraction of they phase of Fe-30 wt.% Ni-12.5 wt.% Co-xC alloy increased with increasing carbon content. The microstructure of Fe-30 wt.% Ni-12.5 wt.% Co-xC alloy changed with the carbon content was independent of the coefficient of thermal expansion. The Curie temperature changed linearly with the carbon content and was similar to the change of the coefficient of thermal expansion. Moreover, the coefficient of thermal expansion decreased when the ratio of saturation magnetization to Curie temperature ($\sigma_s/T_c$) increased, decreasing the Curie temperature and showed a specific relationship with the magnetic properties of the Fe-30 wt.% Ni-12.5 wt.% Co-xCInvar alloy.

• Korean Journal of Materials Research
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• v.10 no.6
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• pp.430-436
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• 2000

In this paper, we have studied the effect of constituent ratio NiO, CuO and doped with B-Bi-Zn on proper-ties(microstructure, density, shrinkage, permeability as a function of frequency, etc.) of hexagonal-ferrite for high fre- quency chip-inductor material about several GHz. The permeability were analyzed by impedance analyzer(100 kHz∼ 40 MHz) and network analyzed(30 MHz∼3 GHz). As a result of the characteristics. the B-Bi-Zn glass ceramic was used to lower the sintering temperature for additive as function of frequency from 100kHz to 1.8 GHz showed con-stant tends. The maximum imaginary value of complex permeability was observed near the resonance frequency of 2 GHz.

• Korean Journal of Materials Research
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• v.4 no.3
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• pp.364-375
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• 1994

The microstructure of medium-carbon non-heat treated steels by air-cooling shows ferritepearlite structure. Compared to the conventional Q/T steels, the strength of these steels containing V, Nb and Ti are similar to Q/T steels. However, their toughness are inferior. In this study, the mechanical properties and microstructures of low-carbon Mo bearing steels produced by direct-quenching process were investigated. Especially, the effect of Mo on the direct-quenched steels was examined. The direct-quenched steels containing Mo were found to have higher strength and toughness. Mo seemed to be effective in producing the fine structure and dense precipitates because of decreasing transformation temperature. The best results were 1211 MPa in UTS and 127.5 J in toughness in the oil quenched 0.15C+O.llV+ 0.035Nb+ 1.81Mo steel. Compared to 0.4% C microalloyed steels, this results showed that UTS increased by 45% and toughness by 2 times.

• Korean Journal of Materials Research
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• v.25 no.5
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• pp.215-220
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• 2015

The effect of Al content on the processing of reaction-bonded $Al_2O_3$ (RBAO) ceramics using 40v/o ~ 80v/o Al-Zn-Mg alloy powder was studied in order to improve traditional RBAO ceramic processes that use ~ 40v/o pure Al powder. The influence of high Al content in starting $Al_2O_3$-Al alloy powder mixtures on its particulate characteristics, reaction-bonding, microstructure, physical and mechanical properties was revealed. Starting $Al_2O_3$-Al alloy powder mixtures with 40v/o ~ 80v/o Al alloy powder were milled, reaction-bonded, post-sintered, and characterized. With an increasing Al alloy content, the milling efficiency of Al alloy powder was lowered, resulting in a larger particle size after milling. However, in spite of the larger particle size of Al alloy powder, the oxidation, i.e., reaction-bonding, of the Al alloy was successfully completed via solid and liquid state oxidation, in which the activation energy of the oxidation was nearly the same regardless of Al alloy content. After reaction-bonding and post-sintering at $1600^{\circ}C$, RBAO ceramics from 80v/o Al alloy content showed a relative density of ~97% and a flexural strength of 251 MPa compared to ~ 96% and 353 MPa for RBAO ceramics from 40v/o Al alloy content, respectively. The lower flexural strength at 80v/o Al alloy content was due to the weak spinel phase that formed from Zn, Mg alloying elements in Al.

• Korean Journal of Materials Research
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• v.25 no.4
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• pp.165-170
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• 2015

Ceramics biomaterials are useful as implant materials in orthopedic surgery. In this study, porous HA(hydroxyapatite)/${\beta}$-TCP(tricalcium phosphate) composite biomaterials were successfully fabricated using HA/${\beta}$-TCP powders with 10-30 wt% $NH_4HCO_3$ as a space holder(SH) and $TiH_2$ as a foaming agent, and MgO powder as a binder. The HA/${\beta}$-TCP powders were consolidated by spark plasma sintering(SPS) process at $1000^{\circ}C$ under 20 MPa conditions. The effect of SH content on the pore size and distribution of the HA/${\beta}$-TCP composite was observed by scanning electron microscopy(SEM) and a microfocus X-ray computer tomography system(SMX-225CT). These microstructure observations revealed that the volume fraction of the pores increased with increasing SH content. The pore size of the HA/${\beta}$-TCP composites is about $400-500{\mu}m$. The relative density of the porous HA/${\beta}$-TCP composite increased with decreasing SH content. The porous HA/${\beta}$-TCP composite fabricated with 30%SH exhibited an elastic modulus similar to that of cortical bone; however, the compression strength of this composite is higher than that of cortical bone.