• Journal of the Korean Ceramic Society
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• v.45 no.11
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• pp.701-706
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• 2008

High frequency dielectric ceramics have potential for applications in mobile and satellite communications systems at frequencies higher than 10GHz. The Ba$(Zn_{1/3}Ta_{2/3})O_3$ ceramics are known to have a high quality factor, a small temperature coefficient of the resonance frequency and a high dielectric constant. On the other hands, sintering at high temperature for extended time is required to obtain the ordered structure for high quality factor. In this study, the microwave dielectric properties of $ZrO_2$ and NiO-added Ba$(Zn_{1/3}Ta_{2/3})O_3$ ceramics prepared by solid-state reaction have been investigated. Adding $ZrO_2$ and NiO could effectively promote the densification even the case of decreasing the sintering time. At the surface of samples, secondary phase of Ba-Ta compounds was formed possibly due to the evaporation of ZnO, however, the interior of the samples remained as pure Ba$(Zn_{1/3}Ta_{2/3})O_3$. The samples sintered at $1600^{\circ}C$ for 2h exhibited 1:2 ordering of Zn and Ta cations. Excellent microwave dielectric properties of $Q{\cdot}f$(>96,000 GHz) and ${\varepsilon}_r$=30 has been obtained.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.25 no.4
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• pp.127-134
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• 2015

Aluminum nitride (AlN) single crystals have attracted much attention for a next-generation semiconductor application because of wide bandgap (6.2 eV), high thermal conductivity ($285W/m{\cdot}K$), high electrical resistivity (${\geq}10^{14}{\Omega}{\cdot}cm$), and high mechanical strength. The bulk AlN single crystals or thin film templates have been mainly grown by PVT (sublimation) method, flux method, solution growth method, and hydride vapor phase epitaxy (HVPE) method. Since AlN suffers difficulty in commercialization due to the defects that occur during single crystal growth, crystalline quality improvement via defects analyses is necessary. Etch pit density (EPD) analysis showed that the growth misorientations and the defects in the AlN surface exist. Transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD) analyses were employed to investigate the overall crystalline quality and various kinds of defects. TEM studies show that the morphology of the AlN is clearly influenced by stacking fault, dislocation, second phase, etc. In addition EBSD analysis also showed that the zinc blende polymorph of AlN exists as a growth defects resulting in dislocation initiator.

• 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 Heritage: History & Science
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• v.46 no.1
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• pp.208-227
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• 2013

Twenty-seven bronze bowls excavated from the Goryo burial complex at Deobu-gol were examined for their microstructure and chemical composition to characterize the bronze technology practiced by commoners at the time. Results showed that the objects examined can be classified into four groups: 1) objects forged out of Cu-near 22%Sn alloys and then quenched; 2) objects cast from Cu-below 10% Sn alloys containing lead; 3) objects cast from Cu-10%~20% Sn alloys containing lead and then quenched; 4) objects forged out of Cu-10~20% Sn alloys containing lead and then quenched. This study revealed that the fabrication technique as determined by alloy compositions plays an important role in bronze technology. The use of lead was clearly associated with the selection of quenching temperatures, the character of inclusions and the color characteristics of bronze surfaces. It was found that the objects containing lead were quenched at temperatures of $520^{\circ}{\sim}586^{\circ}C$ while those without lead were quenched at the range of $586^{\circ}{\sim}799^{\circ}C$. The presence of selenium in impurity inclusions was detected only in alloys containing lead, suggesting that the raw materials, Cu and Sn, used in making the lead-free alloys for the first group were carefully selected from those smelted using ores without lead contamination. Furthermore, the addition of lead was found to have significant effects on the color characteristics of the surface of bronze alloys when they are subjected to corrosion during interment. In leaded alloys, corrosion turns the surface light green or dark green while in unleaded alloys, corrosion turns the surface dark brown or black. It was found that in fabrication, the wall thickness of the bronze bowls varies depending on the application of quenching; most of the quenched objects have walls 1mm thick or below while those without quenching have walls 1mm thick or above. Fabrication techniques in bronze making usually reflect social environments of a community. It is likely that in the late Goryo period, experiencing lack of skilled bronze workers, the increased demand for bronze was met in two ways; by the use of chief lead instead of expensive tin and by the use of casting suitable for mass production. The above results show that the Goryo bronze workers tried to overcome such a resource-limited environment through technological innovations as apparent in the use of varying fabrication techniques for different alloys. Recently, numerous bronze objects are excavated and available for investigation. This study shows that with the use of proper analytical techniques they can serve as a valuable source of information required for the characterization of the associated technology as well as the social environment leading to the establishment of such technology.