• Journal of the Korean Ceramic Society
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• v.34 no.8
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• pp.854-860
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• 1997

The tribological property of ceramics is very important for use in seal rings, pump parts, thread guides and mechanical seal, etc. In the present study, which RBSC/graphite composites were manufactured by adding graphite powders with different particle sizes to mixtures of SiC powder, metallic silicon, carbon black and alumina, effects on the tribological property of each RBSC/graphite composite was investigated in accordance with the particle size of the added graphite powder. The water absorption, the bending strength and the resistance for the friction and wear were measured, and the crystalline phase and the microstructure were respectively examined by using XRD and SEM. In case that the particle size of the graphite powder was fine(2${\mu}{\textrm}{m}$), the formation of $\beta$-SiC was accelerated, thereby making the increase of the bending strength and the decrease of the water absorption, but no improvement for the tribological properties. Furthermore, in case that the particle size of the graphite powder was some large(88~149${\mu}{\textrm}{m}$), the formation of $\beta$-SiC was not accelerated, to thereby make the decrease of the bending strength and the increase of the water absorption, but the improvement for the tribological property of only the composite having the graphite powder of 20 vol%. In addition, in case that the particle size distribution of the graphite powder was large (under 53 ${\mu}{\textrm}{m}$), there was no improvement for every properties. However, the composites, which the graphite powder with the particle size of 53~88 ${\mu}{\textrm}{m}$ was added in 10~15 vol%, had the most increased resistance for the friction and wear which show the worn out amount of 0.4~0.6$\times$10-3 $\textrm{cm}^2$, and the value of the bending strength is 380~520 kg/$\textrm{cm}^2$.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.3 no.2
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• pp.176-184
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• 1993

The glasses, which the $\beta$-spodumene as the principal crystalline phase could be precipitated, were melted by adding >, $P_2O_5, TiO_2, ZrO_2 in the Li_2O-Al_2O_3-SiO_2$ system. In order to achieve the glass-ceramic body of near-theoritical density by sintering method, the optimum condition of heat treatment, the effect of glass powder size and the properties were investigated by DTA, XRD, bulk density, thermal expansion and SEM. Addition of $P_20_5$ imProved the tendency of sintering and the sample with 9wt% $P_20_5$ content was the most dense OOdy by sintering method. The optimum condition of heat treatmemt was sintered for densitification at $740^{\circ}C$ and crystallized at $950^{\circ}C$. In the optimum condition, the relative density was above 90% and the thermal expansion was negative about $-1{\times}10^{-7}/^{\circ}C$.

• Korean Journal of Materials Research
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• v.22 no.11
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• pp.575-580
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• 2012

The purpose of this study is to investigate the crystalline structure and optical properties of (GaZn)(NO) powders prepared by solid-state reaction between GaOOH and ZnO mixture under $NH_3$ gas flow. While ammoniation of the GaOOH and ZnO mixture successfully produces the single phase of (GaZn)(NO) solid solution within a GaOOH rich composition of under 50 mol% of ZnO content, this process also produces a powder with coexisting (GaZn)(NO) and ZnO in a ZnO rich composition over 50 mol%. The GaOOH in the starting material was phase-transformed to ${\alpha}$-, ${\beta}-Ga_2O_3$ in the $NH_3$ environment; it was then reacted with ZnO to produce $ZnGa_2O_4$. Finally, the exchange reaction between nitrogen and oxygen atoms at the $ZnGa_2O_4$ powder surface forms a (GaZn)(NO) solid solution. Photoluminescence spectra from the (GaZn)(NO) solid solution consisted of oxygen-related red-emission bands and yellow-, green- and blue-emission bands from the Zn acceptor energy levels in the energy bandgap of the (GaZn)(NO) solid solutions.

• Journal of Magnetics
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• v.18 no.4
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• pp.400-404
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• 2013

Grain boundary diffusion technique with $DyH_3$ nanoparticles was applied to fabricate Dy-less sintered Nd-Fe-B permanent magnets with high coercivity. The magnetic properties and microstructure of magnets were systematically studied. The coercivity and remanence of grain boundary diffusion magnet were improved by 60% and reduced by 7% compared with those of the original magnet, respectively. Meanwhile, both the remanence temperature coefficient (${\alpha}$) and the coercivity temperature coefficient (${\beta}$) of the magnets were improved after diffusion treatment. Investigation shows that Dy is preferentially enriched as (Nd, Dy)$_2Fe_{14}B$ phase in the surface region of the $Nd_2Fe_{14}B$ matrix grains indicated by the remarkable enhancement of the magneto-crystalline anisotropy field of the magnet. As a result, the magnet diffused with a small amount of Dy nanoparticles possesses enhanced coercivity without remarkably sacrificing its magnetization.

• Journal of the Korea Institute of Military Science and Technology
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• v.21 no.6
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• pp.760-766
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• 2018

The present study investigates the effect of PMMA and BN content on microstructure, mechanical and dielectric properties of silicon nitride($Si_3N_4$) ceramics in $Y_2O_3-Al_2O_3$ additive system. The total additive content was fixed at 8 wt.% and the amount of PMMA varies from 0 to 40 wt.% and BN varies from 0 to 36 wt.%, respectively. The crystalline phases of the samples were determined by X-ray diffraction analysis. All the sintered sample shows complete transformation of ${\alpha}$ to ${\beta}-Si_3N_4$ during the sintering process indicated that the phase transformation was unaffected by the PMMA or BN content. However, the microstructure shows that the residual porosity increased with increasing PMMA and BN content. In addition, the flexural strength and the dielectric constant decrease with addition of PMMA and BN due to the residual porosity. This article provides empirical study of design parameters for $Si_3N_4$-based radome materials.

• Journal of Powder Materials
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• v.9 no.6
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• pp.394-408
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• 2002

Nanostructured high strength metastable Al-, Mg- and Ti-based alloys containing different amorphous, quasicrystalline and nanocrystalline phases are synthesized by non-equilibrium processing techniques. Such alloys can be prepared by quenching from the melt or by powder metallurgy techniques. This paper focuses on one hand on mechanically alloyed and ball milled powders containing different volume fractions of amorphous or nano-(quasi)crystalline phases, consolidated bulk specimens and, on the other hand. on cast specimens containing different constituent phases with different length-scale. As one example. $Mg_{55}Y_{15}Cu_{30}$- based metallic glass matrix composites are produced by mechanical alloying of elemental powder mixtures containing up to 30 vol.% $Y_2O_3$ particles. The comparison with the particle-free metallic glass reveals that the nanosized second phase oxide particles do not significantly affect the glass-forming ability upon mechanical alloying despite some limited particle dissolution. A supercooled liquid region with an extension of about 50 K can be maintained in the presence of the oxides. The distinct viscosity decrease in the supercooled liquid regime allows to consolidate the powders into bulk samples by uniaxial hot pressing. The $Y_2O_3$ additions increase the mechanical strength of the composites compared to the $Mg_{55}Y_{15}Cu_{30}$ metallic glass. The second example deals with Al-Mn-Ce and Al-Cu-Fe composites with quasicrystalline particles as reinforcements, which are prepared by quenching from the melt and by powder metallurgy. $Al_{98-x}Mn_xCe_2$ (x =5,6,7) melt-spun ribbons containing a major quasicrystalline phase coexisting with an Al-matrix on a nanometer scale are pulverized by ball milling. The powders are consolidated by hot extrusion. Grain growth during consolidation causes the formation of a micrometer-scale microstructure. Mechanical alloying of $Al_{63}Cu_{25}Fe_{12}$ leads to single-phase quasicrystalline powders. which are blended with different volume fractions of pure Al-powder and hot extruded forming $Al_{100-x}$$(Al_{0.63}Cu_{0.25}Fe_{0.12})_x$ (x = 40,50,60,80) micrometer-scale composites. Compression test data reveal a high yield strength of ${\sigma}_y{\geq}$700 MPa and a ductility of ${\varepsilon}_{pl}{\geq}$5% for than the Al-Mn-Ce bulk samples. The strength level of the Al-Cu-Fe alloys is ${\sigma}_y{\leq}$550 MPa significantly lower. By the addition of different amounts of aluminum, the mechanical properties can be tuned to a wide range. Finally, a bulk metallic glass-forming Ti-Cu-Ni-Sn alloy with in situ formed composite microstructure prepared by both centrifugal and injection casting presents more than 6% plastic strain under compressive stress at room temperature. The in situ formed composite contains dendritic hcp Ti solid solution precipitates and a few $Ti_3Sn,\;{\beta}$-(Cu, Sn) grains dispersed in a glassy matrix. The composite micro- structure can avoid the development of the highly localized shear bands typical for the room temperature defor-mation of monolithic glasses. Instead, widely developed shear bands with evident protuberance are observed. resulting in significant yielding and homogeneous plastic deformation over the entire sample.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.22 no.2
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• pp.92-98
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• 2012

The co-precipitation technique has been applied to synthesize biphasic calcium phosphate (BCP). $Ca(NO_3)_2{\cdot}4H_2O$, $(NH_4)_2HPO_4$, TEOS and $Mg(NO_3)_2{\cdot}6H_2O$ as the starting materials was used. After the heat treatment of powder crystalline phases HAp and ${\beta}$-TCP analysis showed a mixed phase. The overall spectra appear to have mainly two modes corresponding to characteristic $PO^{3-}_4$ and $OH^-$ groups. After immersion in Hanks' Balanced Salt Solution (HBSS) for 1 week a precipitation started to be formed with individual small granules on the specimen surface. An MTT assay indicated that ionic substituted BCP powders had no cytotoxic effects on MG-63 cells, and that they have good biocompatibility.