• Korean Journal of Materials Research
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• v.20 no.7
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• pp.374-378
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• 2010

$MgTiO_3$ thin films were prepared by r.f. magnetron sputtering in order to prepare miniaturized NPO type MLCCs. $MgTiO_3$ films showed a polycrystalline structure of ilmenite characterized by the appearance of (110) and (202) peaks. The intensity of the peaks decreased with an increase in the chamber pressure due to the decrease of crystallinity which resulted from the decrease of kinetic energy of the sputtered atoms. The films annealed at $600^{\circ}C$ for 60min. showed a fine grained microstructure without micro-cracks. The grain size and roughness of the $MgTiO_3$ films decreased with the increase of chamber pressure. The average surface roughness was 1.425~0.313 nm for $MgTiO_3$ films prepared at 10~70 mTorr. $MgTiO_3$ films showed a dielectric constant of 17~19.7 and a dissipation factor of 2.1~4.9% at 1MHz. The dielectric constant of the films is similar to that of bulk ceramics. The dielectric constant and the dissipation factor decreased with the increase of the chamber pressure due to the decrease of grain size and crystallinity. The leakage current density was $10^{-5}\sim10^{-7}A/cm^2$ at 200kV/cm and this value decreased with the increase of the chamber pressure. The small grain size and smooth surface microstructure of the films deposited at high chamber pressure resulted in a low leakage current density. $MgTiO_3$ films showed a near zero temperature coefficient and satisfied the specifications for NPO type materials. The dielectric properties of the $MgTiO_3$ thin films prepared by sputtering suggest the feasibility of their application for MLCCs.

• Corrosion Science and Technology
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• v.17 no.3
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• pp.101-108
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• 2018

AA7010 is an Al-Zn-Mg-Cu alloy containing Zr, developed as an alternate to traditional AA7075 alloy owing to their high strength combined with better fracture toughness. It is necessary to improve the corrosion resistance and surface properties of the alloy by incorporating plasma electrolytic oxidation (PEO) method. AA7010-T7452 aluminum alloy has been processed through the forging route with multi-stage working operations, and was coated with $10{\mu}m$ thick $Al_2O_3$ ceramic aluminina coating using the plasma electrolytic oxidation (PEO) method. The corrosion, stress corrosion cracking (SCC) and nano-mechanical behaviours were examined by means of potentiodynamic polarization, slow strain rate test (SSRT) and nano-indentation tests. The results indicated that the additional thermomechanical treatment during the forging process caused a fully recrystallized microstructure, which lead to the poor environmental cracking resistance of the alloy in 3.5% NaCl solution, despite the overaging treatment. Although the fabricated PEO coating improved general corrosion resistance, the brittle nature of the coating did not provide any improvement in SCC resistance of the alloy. However, the hardness and elastic modulus of the coating were significantly higher than the base alloy.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.27 no.5
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• pp.229-234
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• 2017

In this study, TiAl alloy was fabricated by a centrifugal casting method for turbo charge of automotive. Optimum conditions for defectless morphology using various ceramic mold were investigated. The crystal structure, microstructure, and chemical composition of the TiAl prepared by centrifugal casting were studied by X-ray diffractometer (XRD), optical microscopy (OM), field emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS), microvickers hardness analyzer (HV). Two kinds of dendrite structures were observed with 4-fold and 6-fold symmetries. The FE-SEM, EDS and HV examinations of the as-cast TiAl showed that the thickness of the oxide layer (${\alpha}$-case) was typically less than $50{\mu}m$.

• The Korean Journal of Ceramics
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• v.5 no.4
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• pp.324-330
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• 1999

$Si_3N_4$-$TiB_2$ with 2 wt% $Al_2O_3$ and 4 wt% $Y_2O_3$ additives was hot pressed in a flowing $N_2$ environment with varying $TiB_2$ content from 10 to 50 vol%. Variations of mechanical (hardness, fracture toughness, and flexual strength), and tribological properties as a function of $TiB_2$ content were investigated. As the content of $TiB_2$ increased, relative density decreased due to the chemical reaction of $TiB_2$in $N_2$ environment. The reduction of density causes mechanical properties to be degraded with an increase of $TiB_2$ in $Si_3N_4$. Tribological properties were dependent of microstructure as well as mechanical properties, however, they were degraded strongly by the chemical reaction of $Si_3N_4$-$TiB_2$ during hot pressing in $N_2$ environment. SEM and TEM observations, and X-ray diffraction analysis that the chemical reaction products at the interface are TiCN, Si, and $SiO_2$. Also, the comparison of XRD patterns of the $Si_3N_4$-40 vol% $TiB_2$ composites hot pressed at $1,750^{\circ}C$ for 1 hour between in $N_2$ and in Ar gas was made. The XRD peaks of Si and $SiO_2$ were not found in Ar, but still a weak peak of TiCN was presented.

• Proceedings of the Materials Research Society of Korea Conference
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• 2010.05a
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• pp.48.2-48.2
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• 2010

The effect of additives as rare-earth in dielectric materials has been studied to meet the development trend in electronics on the miniaturization with increasing the capacitance of MLCCs (multi-layered ceramic capacitors). It was reported that the addition of rare-earth oxides in dielectrics would contribute to enhance dielectric properties and high temperature stability. Especially, dysprosium and thulium are well known to the representative elements functioned as selective substitution in barium titanate with perovskite structure. The effects of these additives on microstructure and electric properties were studied. The 0.8 mol% Dy doped $BaTiO_3$ and the 1.0 mol% Tm doped $BaTiO_3$ had the highest electric properties as optimized composition, respectively. According to the increase of rare-earth contents, the growth of abnormal grains was suppressed and pyrochlore phase was formed in more than solubility limits. Furthermore, the effect of two rare-earth elements co-doped $BaTiO_3$ on the dielectric properties and insulation resistance was investigated with different concentration. The dielectric specimens with $BaTiO_3-Dy_2O_3-Tm2O_3$ system were prepared by design of experiment for improving the electric properties and sintered at $1320^{\circ}C$ for 2h in a reducing atmosphere. The dielectric properties were evaluated from -55 to $125^{\circ}C$ (at $1KHz{\pm}10%$ and $1.0{\pm}0.2V$) and the insulation resistance was examined at 16V for 2 min. The morphology and crystallinity of the specimens were determined by microstructural and phase analysis.

• Korean Journal of Materials Research
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• v.30 no.2
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• pp.61-67
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• 2020

BiFeO₃ with perovskite structure is a well-known material that has both ferroelectric and antiferromagnetic properties called multiferroics. However, leaky electrical properties and difficulty of controlling stoichiometry due to Bi volatility and difficulty of obtaining high relative density due to high dependency on the ceramic process are issues for BiFeO₃ applications. In this work we investigated the sintering behavior of samples with different stoichiometries and sintering conditions. To understand the optimum sintering conditions, nonstoichiometric Bi_1±xFeO_3±δ ceramics and Ti-doped Bi_1.03Fe_1-4x/3Ti_xO₃ ceramics were synthesized by a conventional solid-state route. Dense single phase BiFeO₃ ceramics were successfully fabricated using a two-step sintering and quenching process. The effects of Bi volatility on microstructure were determined by Bi-excess and Ti doping. Bi-excess increased grain size, and Ti doping increased sintering temperature and decreased grain size. It should be noted that Ti-doping suppressed Bi volatility and stabilized the BiFeO₃ phase.

• Journal of Powder Materials
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• v.25 no.5
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• pp.408-414
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• 2018

This study investigates the microstructure and wear properties of cermet (ceramic + metal) coating materials manufactured using high velocity oxygen fuel (HVOF) process. Three types of HVOF coating layers are formed by depositing WC-12Co, WC-20Cr-7Ni, and Cr3C2-20NiCr (wt.%) powders on S45C steel substrate. The porosities of the coating layers are $1{\pm}0.5%$ for all three specimens. Microstructural analysis confirms the formation of second carbide phases of $W_2C$, $Co_6W_6C$, and $Cr_7C_3$ owing to decarburizing of WC phases on WC-based coating layers. In the case of WC-12Co coating, which has a high ratio of $W_2C$ phase with high brittleness, the interface property between the carbide and the metal binder slightly decreases. In the $Cr_3C_2-20CrNi$ coating layer, decarburizing almost does not occur, but fine cavities exist between the splats. The wear loss occurs in the descending order of $Cr_3C_2-20NiCr$, WC-12Co, and WC-20Cr-7Ni, where WC-20Cr-7Ni achieves the highest wear resistance property. It can be inferred that the ratio of the carbide and the binding properties between carbide-binder and binder-binder in a cermet coating material manufactured with HVOF as the primary factors determine the wear properties of the cermet coating material.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.26 no.1
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• pp.22-26
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• 2013

PTCR ceramics of $(Ba_{0.998}Sm_{0.002})TiO_3+0.001MnCO_3+xSiO_2$ (x=1, 2, 3, 4, 5, 6 mol%) were fabricated by solid state method. Disk samples of diameter 5 mm and thickness about 1mm were sintered at $1,290^{\circ}C$ for 2 h in reduced atmosphere of $5%H_2-95%N_2$ followed by re-oxidation at $600^{\circ}C$ for 30 min. in $20%O_2-80%N_2$.and their microstructures and electrical properties were investigated with SEM and Multimeter. The color of sintered samples was strongly dependent on $SiO_2$ content showing that the color of samples with $SiO_2$ of 1~2 mol% was gray but that of samples with $SiO_2$ of 4~6 mol% was changed from gray to blue, which seems to be related with the reduction of samples due to the oxygen vacancies created during the sintering in reduced atmosphere. $SiO_2$ content had a great influence on the microstructure and the electrical properties. With increasing $SiO_2$ content, the grain size of samples increased and the resistivity as well as the resistivity jump ($R_{285}/R_{min}$) decreased, which is considered to be attributed to the resistivity change at grain interior and grain boundary due to the fast mass transfer through $SiO_2$ liquide phase during the sintering. Samples with 2 mol% $SiO_2$ has the resistivity of $202{\Omega}cm$ and the resistivity jump of 3.28. It is expected that $SiO_2$ doped $BaTiO_3$ based PTC ceramics can be used for multilayered PTC thermistor due to the resistance to the sintering in reduced atmosphere.

• Korean Journal of Materials Research
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• v.24 no.5
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• pp.236-242
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• 2014

Fabrication of reaction-bonded $Al_2O_3$ (RBAO) ceramics using Al-Zn-Mg alloy powder was studied in order to improve traditional RBAO ceramic processing using Al powder. The influence on reaction-bonding and microstructure, as well as on physical and mechanical properties, of the particulate characteristics of the $Al_2O_3$-Al alloy powder mixtures after milling, was revealed. Variation of the particulate characteristics of this $Al_2O_3$-Al alloy powder mixture with milling time was reported previously. To start, the $Al_2O_3$-Al alloy powder mixture was milled, reaction-bonded, post-sintered, and characterized. During reaction-bonding of the $Al_2O_3$-Al alloy powder mixture compacts, oxidation of the Al alloy took place in two stages, that is, there was solid- and liquid-state oxidation of the Al alloy. The solid-state oxidation exhibited strong dependence on the density of surface defects on the Al-alloy particles formed during milling. Higher milling efficiency resulted in less participation of the Al alloy in reaction-bonding. This was because of its consumption by chemical reactions during milling, and subsequent powder handling, and could be rather harmful in the case of over-milling. In contrast to very little dependence of oxidation of the Al alloy on its particle size after milling, the relative density, microstructure, and flexural strength were strongly dependent on particle size after milling (i.e., on milling efficiency). The relative density and 4-point flexural strength of the RBAO ceramics in this study were ~98% and ~365 MPa, respectively, after post-sintering at $1,600^{\circ}C$.

• Korean Journal of Materials Research
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• v.29 no.1
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• pp.43-51
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• 2019

Nacre of abalone shell features a "brick-and-mortar" microstructure, in which micro-plates of calcium carbonate are bonded by nanometers-thick layers of chitin and proteins. Due to the microstructure and its unique toughening mechanisms, nacre possesses an excellent combination of specific strength, stiffness and toughness. This study deals with the possibility of using nacre fragments obtained from abalone shell for making a bulletproof armor system. A composite plate laminated with abalone shell fragments is made and compression and bend tests are carried out. In addition, a bulletproof test is performed with hybrid armor systems which are composed of an alumina plate, a composite plate, and aramid woven fabric to verify the ballistic performance of nacre. The compressive strength of the composite plate is around 258.3 MPa. The bend strength and modulus of the composite plate decrease according to the plate thickness and are about 149.2 MPa and 50.3 GPa, respectively, for a 4.85 mm thick plate. The hybrid armor system with a planar density of $45.2kg/m^2$, which is composed of an 8 mm thick alumina plate, a 2.4 mm thick composite plate, and 18 layers of aramid woven fabric, satisfy the NIJ Standard 0101.06 : 2008 Armor Type IV. These results show that a composite plate laminated with abalone shell fragments can be used for a bulletproof armor system as an interlayer between ceramic and fabric to decrease the armor system's weight.