• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 2006.09a
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• pp.407-408
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• 2006

Dimensional precision is a critical parameter in net shape processing of ferrous PM components. Sinter-hardening alloys undergo a transformation from austenite to martensite. Martensite formation expands the sintered compact, while tempering hardened steels results in shrinkage. In addition, martensitic regions with high Cu and C contents may contain large amounts of retained austenite. The presence of martensite and retained austenite, in addition to the tempering step, all play a role in the final dimensions of a component. This paper investigates the dimensional and microstructural changes to two sinter-hardening grades through different post-sintering thermal treatments.

• Journal of the Korean Ceramic Society
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• v.28 no.11
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• pp.935-941
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• 1991

Effects of calcination process and ZrO2 addition on the electrical properties of [(Ba0.82Sr0.08Ca0.1)O]m(Ti1-$\chi$Zr$\chi$)O2 ceramics have been investigated. With the variation of A/B-site ratio m of the dielectric formulations, sintering behavior and the resistivity after sintering in a reducing atmosphere have been affected by the calcination process. When the dielectric formulations of m=1.01 were sintered in a reducing atmosphere, the room-temperature resitivity of 109 {{{{ OMEGA }}.cm was obtained for samples processed with two-step calcination, which was much lower than 1012 {{{{ OMEGA }}.cm of samples calcined once. It was confirmed that high resistivity of Ca-doped BaTiO3 ceramics, after sintering in a reducing atmosphere, is maintained by acceptor-like behavior of CaTi" which is formed by Ca substitution to Ti-site. It was also found out that the critical amount of B-site Ca substitution for reduction inhibition of BaTiO3 is around 0.005 mol. With the increasing amount of ZrO2 addition to dielectric formulations, Curie peak was depressed and Curie temperature was lowered due to the enhanced diffuse phase transition.tion.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 1992.11a
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• pp.38-41
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• 1992

Pb(Zr,Ti)$O_3$ powders were synthesized by the partial oxalate method and the modified partial oxalate method, where the difference between the two is the use of pre-reacted (Zr,Ti)$O_2$ in the former method. When compared with conventional mixed oxide method, calcination temperature can be reduced to less than $700^{\circ}C$ by both partial oxalate methods, and the resulting particle size was finer and more uniform. Using partial oxalate-derived PZT powders, sintering temperatures can also be reduced as low as $950^{\circ}C$ without sacrificing desired dielectric and piezoelectric properties, such as relative permittivity, electromechanical coupling factor, and piezoelectric coefficient. Two partial oxalate methods yield ceramics with almost the same physical and electrical properties, so that the step of producing ZTO powder does not seem to be necessary.

• Journal of the Korean Solar Energy Society
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• v.33 no.2
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• pp.93-100
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• 2013

Two-step thermochemical cycle using ferrite-oxide($Fe_2O_4$) device was investigated. The $H_2O$(g) was converted into $H_2$ in the first experiment which was performed using a dish type solar thermal system. However the experiment was lasted only for 2 cycles because the metal oxide device was sintered and broken down. Another problem was that the reaction was taken place mainly on a side of the metal oxide device. The m-$ZrO_2$, which was widely known as a material preventing sintering, was applied on the metal oxide device. The ferrite loading rate and the thickness of the metal oxide device were increased from 10.67wt% to 20wt% and from 10mm to 15mm, respectively. The chemical reactor having two inlets was designed in order to supply the reactants uniformly to the metal oxide device. The second-experiment was lasted for 5 cycles, which was for 6 hours. The total amount of the $H_2$ production was 861.30ml. And cerium oxide($CeO_2$) device was used for increasing $H_2$ production rate. $CeO_2$ device had low thermal resistance, however, more $H_2$ production rate than $Fe_2O_4$ device.

• Journal of Powder Materials
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• v.13 no.3 s.56
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• pp.172-177
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• 2006

Aging characteristics and mechanical properties of commercial 7xxx series Al composites were investigated from viewpoint of ceramic contents. After sintering process, sintered densities of blended and composite powder were 95 and 97%, respectively. Each part was solution-treated at $475^{\circ}C$ for 60 min and aged $175^{\circ}C$. And two-step aging was also performed form $120^{\circ}C$ to $175^{\circ}C$. The aging behavior of the sintered composite pow-der was different from that of sintered blended powder. The peak aging time of the composite was rapid as well due to strain. Before aging, mechanical properties of sintered composite powder was significantly higher than that of sintered blended powder. These increments of properties were directly affected by ceramic particles. However, after aging, incremental rate of mechanical properties was slowed in the composite.

• Journal of the Korean Institute of Electrical and Electronic Material Engineers
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• v.13 no.6
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• pp.485-492
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• 2000

The sintering characteristics and the effects of density on dielectric properties in 0.2Ba(Mg$_{1}$3(Nb$_{2}$3/)O$_3$-0.8Ba(Mg$_{1}$3//Ta$_{2}$3/)O$_3$ceramics were investigated. The samples were made by the powder mixing techniques with the two step calcining conditions. When the 1st and the 2nd calcining temperatures were 120$0^{\circ}C$ and the sintering temperature was 155$0^{\circ}C$the density of samples showed the highest value (7.45 g/cm$^3$, 98.5% of theoretical density) among them. The dielectric constant of samples was nearly independent of density but the tan $\delta$ and the temperature coefficient of dielectric constant decreased linearly with increasing of the density. The quality factor(Q$\times$f), the temperature coefficient of resonance frequency and the dielectric constant of Ba[Mg$_{1}$3(Nb$_{0.2}$/Ta$_{0.8}$)sub 2/3/]O$_3$ceramic were 79,548 GHz, +1.5 ppm/$^{\circ}C$ and 26 in the microwave range, respectively.ely.

• Journal of the Korean Ceramic Society
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• v.31 no.2
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• pp.129-136
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• 1994

Si3N4 with 6w/o Y2O3 and 1.5w/o Al2O3 has been gas pressure sintered and its densification behavior and the effect of the sintering variables on the microstructure and mechanical properties were investigated. Densification rate was higher at temperature below 1775$^{\circ}C$ and between 187$0^{\circ}C$ and 195$0^{\circ}C$ than between 1775$^{\circ}C$ and 187$0^{\circ}C$. The faster densification at temperature between 187$0^{\circ}C$ and 195$0^{\circ}C$ was thought to be due to the increased amount of liquid phase resulting from the increased amount of Si3N4 dissolving in the liquid. $\beta$-Si3N4 and Y-disilicate at temperatures below 1775$^{\circ}C$, and only $\beta$-Si3N4 at 187$0^{\circ}C$ and above were detected by XRD analysis. Three different two-step schedules were employed to obtain sintered body with above 99% theoretical density and to investigate the effect of the sintering variables on the density, the microstructure and the mechanical properties of the sintered body. The sintered density did not change with the heating rate, and the microstructure became coarser as the temperature increased. The strength decreased with the width of $\beta$-Si3N4 grain, while the fracture toughness increased with the square root of it. A ceramic cutting tool made of the sintered body showed an uniform flank wear after the cutting test.

• Journal of Hydrogen and New Energy
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• v.18 no.4
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• pp.356-364
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• 2007

The partial reduction and water splitting properties of metal substituted ferrite mediums for two-step thermochemical hydrogen production, were carried out by TPR/O(Temperature programmed reduction/oxidation) method at a temperature of below 1173 K and under atmospheric pressure. $ZrO_2$ was added to the ferrite as a binder to prevent the sintering. As the results, the reactivity of the metal species added to the ferrite mediums decreased in the order of Cu>Co>Ni>Mn, on the basis of water-splitting temperature. It was also found that the produced hydrogen amounts in the water-splitting step on partial reduced mediums were corresponding to the consumed hydrogen amounts in the previously partial reduction step.

• Journal of the Korean Ceramic Society
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• v.42 no.6 s.277
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• pp.405-410
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• 2005

In the hard oxidizing fire iron red glaze which colorated red exposed magnesioferrite and whitlockite-type crystals. And whitlockite-type crystals has formed before magnesioferrite forming in the step of sintering. This study tries to identify the coloration mechanism of hard oxidizing fire iron red glaze by the experiment of substitution of whitlockite-like crystals and to confirm such substitution be able to farm magnesioferrite. As the results of experiment, magnesioferrite was appeared during substitution of two kinds of whitlockite-type. It can be considered that the magnesioferrite colors the iron glaze to red with covering the glaze surface.

• Journal of the Korean Solar Energy Society
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• v.31 no.1
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• pp.107-114
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• 2011

Two-step thermochemical cycle using ferrite-oxide($Fe_3O_4$) device was investigated. The $H_2O$(g) was converted into $H_2$ in the first experiment which was performed using a dish type solar thermal system. However the experiment was lasted only for 2 cycles because the metal oxide device was sintered and broken down. Another problem was that the reaction was taken place mainly on a side of the metal oxide device. The $m-ZrO_2$, which was widely known as a material preventing sintering, was applied on the metal oxide device. The ferrite loading rate and the thickness of the metal oxide device were increased from 10.67wt% to 20wt% and from 10mm to 15mm, respectively. The chemical reactor having two inlets was designed in order to supply the reactants uniformly to the metal oxide device. The second-experiment was lasted for 5 cycles, which was for 6 hours. The total amount of the $H_2$ production was 861.30mL.