• Journal of the Korean Ceramic Society
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• v.38 no.9
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• pp.799-805
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• 2001

Two commercial alumina powders having different particle size of $0.5{\mu}m$ and 3${\mu}$m were presintered at 1120$^{\circ}$C for 2h and then lanthanum aluminosilicate glass was infiltrated at 1100$^{\circ}$C for up to 4h to obtain the densified glass-alumina composites. The effect of alumina particle size on packing factor, microstructure, wetting, porosity and pore size, and mechanical properties of the composite was investigated. The optimum mechanical properties and compaction behavior were observed for the 3${\mu}$m alumina particle dispersed composite. The 3${\mu}$m alumina particle size and distribution for he preform were within 0.1 to 48${\mu}$m and bimodal and random orientation. The strength and the fracture toughness of the composite having 3${\mu}$m alumina particles were 519MPa and $4.5MPa{\cdot}m^{1/2}$, respectively.

• Journal of the Korean Ceramic Society
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• v.40 no.10
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• pp.991-997
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• 2003

The effect of A1$_4$C$_3$ additive on the thermoelectric properties of SiC ceramics were studied. Porous SiC ceramics with 47∼59% relative density were fabricated by sintering the pressed $\alpha$-SiC powder compacts with A1$_4$C$_3$at 2100∼220$0^{\circ}C$ for 3 h in Ar atmosphere. Crystalline phases of the sintered bodies were identified by powder X-Ray Diffraction (XRD) and their microstructures were observed with a Scanning Electron Microscope (SEM). In the case of A1$_4$C$_3$ addition, the phase transformation of 6H-SiC to 4H-SiC could be observed during sintering. The Seebeck coefficient and electrical conductivity were measured at 550∼95$0^{\circ}C$ in Ar atmosphere. In the case of undoped specimens, the Seebeck coefficients were positive (p-type semiconducting) possibly due to a dominant effect of the acceptor impurities (Al, Fe) contained in the starting powder and electrical conductivity increased as increasing sintering temperature. Electrical conductivity of A1$_4$C$_3$doped specimen is larger than that of undoped specimen under the same condition, which might be due to the reverse phase transformation and increasing of carrier density. And the Seebeck coefficient of A1$_4$C$_3$ doped specimen is also larger than that of undoped specimen. The density of specimen, the amount of addition and sintering atmosphere had significant effects on the thermoelectric property.

• Korean Journal of Materials Research
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• v.8 no.12
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• pp.1099-1109
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• 1998

To investigate the effect of V and Sb on the corrosion behavior of Zr- based alloys, corrosion tests were performed on 6 kinds of Zr alloys in an autoclave at $360^{\circ}C$ for 100 days. The transition of the corrosion rate occurred in the sample containing 0.1wt.%V after 10 days but did not occur in the samples containing 0.2wt.%V and 0.4wt.%V. The corrosion resistance of V containing alloys increased with increasing V contents from 0.1 to 0.4wt.% and the alloys containing 0.4wt.%V showed the best corrosion resistance. In the ternary alloys containing 0.1wt.%Sb and 0.4wt.%Sb, the corrosion rate increased significantly from the short exposure time. It was observed that the optimal Sb content for corrosion resistance was 0.2wt.%. The size and volume fraction of precipitates increased with increasing V and Sb contents. The superior corrosion resistance was observed in the Zr alloy having precipitate size of 0.11-0.13$\mu\textrm{m}$. From the result of corrosion behavior and the obserbation of precipitates, the optimal size of the precipitate appear to control the electron conduction in the cathodic reaction and play an important role in maintaining a stable oxide microstructure.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.52-52
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• 2008

The positive temperature coefficient of resistivity (PTCR) effect in (1-x)$BaTiO_3$ - $x(Bi_{0.5}K_{0.5})TiO_3$ doped with $Nb_2O_5$ was investigated. $(Bi_{1/2}K_{1/2})TiO_3$ (BKT) is more environment-friendly than $PbTiO_3$ in order to use in PTC thermistors. The incorporation of 1 mol% BKT to $BaTiO_3$ increased the Curie temperature (Tc) to $148^{\circ}C$. Doping of $Nb_2O_5$ to $Ba_{0.99}(Bi_{0.5}K_{0.5})_{0.01}TiO_3$ (BaBKT) ceramic has enhanced its PTCR effects. For the sample containing 0.025 mol% $Nb_2O_5$, it showed good PTCR properties; low resistivity at room temperature (${\rho}_r$) of 30 $\Omega{\cdot}cm$, a high PTCR intensity of approximately $3.3\times10^3$, implying the ratio of maximum resistivity to minimum resistivity (${\rho}_{max}/{\rho}_{min}$) in the measured temperature range, and a large resistivity temperature factor (a) of 13.7%/$^{\circ}C$ along with a high Curie temperature (Tc) of $167^{\circ}C$. In addition, the cooling rate of the samples during the sintering process had an influence on their PTCR behavior. All the samples showed the best ${\rho}_{max}/{\rho}_{min}$ ratio when they have cooled down at a rate of $600^{\circ}C$/min.

• The Transactions of the Korean Institute of Electrical Engineers C
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• v.55 no.10
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• pp.467-474
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• 2006

The effect of pressureless-sintered temperature on the densification behavior, mechanical and electrical properties of the $SiC-TiB_2$ electroconductive ceramic composites was investigated. The $SiC-TiB_2$ electroconductive ceramic composites were pressureless-sintered for 2 hours at temperatures in the range of $1,750{\sim}1,900[^{\circ}C]$, with an addition of 12[wt%] $Al_2O_3+Y_2O_3(6:4\;mixture\;of\;Al_2O_3\;and\;Y_2O_3)$ as a sintering aid. The relative density, flexural strength, vicker's hardness and fracture toughness showed the highest value of 84.92[%], 140[MPa], 4.07[GPa] and $3.13[MPa{\cdot}m^{1/2}]$ for $SiC-TiB_2$ composites of $1,900[^{\circ}C]$ sintering temperature at room temperature respectively. The electrical resistivity was measured by the Pauw method in the temperature ranges from $25[^{\circ}C]\;to\;700[^{\circ}C]$. The electrical resistivity showed the value of $5.51{\times}10^{-4},\;2.11{\times}10^{-3},\;7.91{\times}10^{-4}\;and\;6.91{\times}10^{-4}[\Omega{\cdot}cm]$ for ST1750, ST1800, ST1850 and ST1900 respectively at room temperature. The electrical resistivity of the composites was all PTCR(Positive Temperature Coefficient Resistivity). The resistance temperature coefficient showed the value of $3.116{\times}10^{-3},\;2.717{\times}10^{-3},\;2.939{\times}10^{-3},\;3.342{\times}10^{-3}/[^{\circ}C]$ for ST1750, ST1800, ST1850 and ST1900 respectively in the temperature ranges from $25[^{\circ}C]\;to\;700[^{\circ}C]$. It is assumed that because polycrystallines, such as recrystallized $SiC-TiB_2$ electroconductive ceramic composites, contain of porosity and In Situ $YAG(Al_5Y_3O_{12})$ crystal grain boundaries, their electrical conduction mechanism are complicated. In addition, because the condition of such grain boundaries due to $Al_2O_3+Y_2O_3$ additives widely varies with sintering temperature, electrical resistivity of the $SiC-TiB_2$ electroconductive ceramic composites with sintering temperature also varies with sintering condition. It is convinced that ${\beta}-SiC$ based electroconductive ceramic composites for heaters or ignitors can be manufactured by pressureless sintering.

• Journal of the Korean Magnetics Society
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• v.5 no.4
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• pp.281-286
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• 1995

In order to study the effect of additives $SiO_{2}$ on the magnetic properties of hexaferrite sheet magnet, we used X-ray diffractometer, Mossbauer spectrometer, and VSM magnetometer. We have prepared $Ba_{0.25}Sr_{0.75}Fe_{12}O_{19}$ green sheets by the Dr. Blade method. Most of samples have a magnetoplurnbite crystal structure of typical M-type hexaferrite. The lattice parameters are found not to be affected by the addition of $SiO_{2}$. ${\alpha}-Fe_{2}O_{3}$ phase develops above $SiO_{2}$ 2.0 wt.%. Isomer shifts indicate that the valence of Fe ions is trivalent. Curie temperatures decrease slightly with increasing $SiO_{2}$ concentrations. It means that the $Si^{4+}$ subsitution for 12k-site $Fe^{3+}$ has an effect on the superexchange interactions Fe-O-Fe, which change the distance and the angle between cations and anions. It was suggested that ${\alpha}-Fe_{2}O_{3}$ phase results from the excessive Fe produced by subsituting $Si^{4+}$ for $Fe^{3+}$. Based upon the results of $Ba_{0.25}Sr_{0.75}Fe_{12}O_{19}$ added with $SiO_{2}$, we concluded that $H_{c}$, $M_{s}$ and $M_{r}$ depend more strongly on the microstructure chracteristics than on the cation substitution.

• Korean Journal of Materials Research
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• v.11 no.6
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• pp.502-507
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• 2001

The effect of solidification rate on the microstructure of directionally solidified IN792+ Hf superalloy has been studied. Solidification sequence and precipitation behavior of the alloy have been analysed by microstructural observation. The script carbide transformed to faceted carbide with decreasing solidification rates. The incorporation of ${\gamma}$ phase into the faceted carbide was due to dendritic growth of carbides. Some elongated carbide bars formed along the grain boundaries at a solidification rate of 0.5$\mu\textrm{m}$/s. Two zones, ${\gamma}$' forming elements enriched zone and depleted zone, were found in the residual liquid area. Eutectic ${\gamma}$/${\gamma}$' nucleated in the f forming elements enriched zone. Formation of eutectic ${\gamma}$/${\gamma}$' increased the ratio of (Ti+Hf+Ta+W)/Al and induced η phase precipitation. The ratio of (Ti+Hf+Ta+W)/Al decreased at lower solidification rates due to sufficient back diffusion in the residual liquid area. Hence, the Precipitation of the η Phase efficiently suppressed at the lower solidification rate.

• Korean Journal of Materials Research
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• v.9 no.12
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• pp.1245-1251
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• 1999

Phase-change optical disk very rapid recording, high densification of data, resulting in high feedback rate and good C/N(carrier to noise) ratio of a feedback signal. However, repetitive thermal energy may cause the deformation of a disk or the lowering of an eliminability and a cyclability of the recording. The lowering of the cyclability can be reduced by insertion of thin layer of ZnS-$SiO_2$ dielectric thin film in appropriate disk structure between the upper and lower part of the recording film. Using the Taguchi method, optimum conditions satisfying both the optimized quality characteristic values and the scattering values for film formation were found to be the target R.F. power of 200W, the substrate R.F. power of 20W, the Ar pressure of 6mTorr, and the electrode distance of 6cm. From the refractive index data, the existence of the strong interaction between the electrode distance and Ar pressure was confirmed, and so was the large effect of the electrode distance on transmittance. According to the analysis of TEM and XRD, the closer the electrode distance was, the finer was the grain size due to the high deposition rate. However, the closer electrode distance brought the negative effect on the morphology of the film and caused the reduction of transmittance. AFM and SEM analyses showed that the closer the electrode distance was, the worse was the morphology due to the high rate of the deposition. Under optimum condition, the deposited thin film showed a good morphology and dense microstructure with less defects.

• Nuclear Engineering and Technology
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• v.23 no.1
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• pp.1-11
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• 1991

The objective of this study is to investigate the effect of weld line on the mechanical strengths and the process of weld line elimination in the Zircaloy-4 resistance upset welding for the fabrication of heavy water reactor fuel rods. The weld current and the amount of upset increased linearly with the main heat, in which two relations between them were derived. It was found that the threshold to obtain sound weld was 50% of main heat in terms of weld upset size, mechanical strengths and weld line elimination. The weld microstructure of resistance upset welds of Zircaloy-4 comprsied basketweave, Widmanstatten and martensite respectively by changing the main heats. Dimples on uniaxially fractured surface at weld line in the Zr-4 welds were larger and deeper compared with those on biaxially fractured surface. It was also found that the process of the weld line elimination in the resistance upset weld of Zircaloy-4 could be divided into three stages in terms of the presence of many pores, their shrinkage and elimination, and the shrinkage of the original weld interface with increasing weld currents.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.31 no.5
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• pp.218-227
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• 2021

A thick film NTC thermistor for low temperature co-firing was manufactured by printing and sintering a paste prepared using NTC powder of Mn_1.5Ni_0.4Co_0.9Cu_0.4O₄ composition, lead free frit, and RuO₂ on a 96 % alumina substrate. The effect of frit and RuO₂ on the electrical properties of thick film NTC thermistor was studied. The resistance of the thick film NTC thermistor was higher than that of the bulk phase sintered at the same temperature, but it was found that the negative resistance temperature characteristic appeared more clearly and linearly in the resistance - temperature characteristic. On the other hand, the area resistance decreased as the sintering temperature increased, and the area resistance increased as the amount of frit added increased. The B constant of the thick film NTC thermistor was 3000 K or higher. Among them, it was found that the B constant of the thick film NTC thermistor made of paste with 5 wt% of frit added and sintered at 900℃ showed the highest B constant. Also, it can be seen that the area resistance decreased with the addition of RuO₂, and the change in the area resistance decrease of the thick film NTC thermistor obtained by sintering the paste containing 5 wt% of RuO₂ at 900℃ is the most obvious.