• The Journal of Korean Academy of Prosthodontics
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• v.35 no.1
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• pp.221-243
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• 1997

The objective of this study was to characterize the mechanical properties of $Y_{2}O_{3}$-containing glass infiltrated ceramic core material, which was made by pressureless powder packing method. A pure alumina powder with a grain size of about $4{\mu}m$ was packed without pressure is silicon mold to form a bar shaped sample, and applied PVA solution as a binder. Samples were sinterd at $1350^{\circ}C$ for 1 hour. After cooling, $Y_{2}O_{3}$-containing glass($SiO_{2},\;Y_{2}O_{3},\;B_{2}O_{3},\;Al_{2}O_{3}$, ect) was infiltrated to the sinterd samples at $1300^{\circ}C$ for 2 hours and cooled. Six different proportions $Y_{2}O_{3}$ of were used to know the effect of the mismatch of the thermal expansion coefficient between alumina powder and glass. The samples were ground to $3{\times}3{\times}30$ mm size and polished with $1{\mu}m$ diamond paste. Flexural strength, fracture toughness, hardness and other physical properties were obtained, and the fractured surface was examined with SEM and EPMA. Ten samples of each group were tested and compared with In-Ceram(tm) core materials of same size made in dental laboratory. The results were as follows : 1. The flexural strengths of group 1 and 3 were significantly not different with that of In-Ceram, but other experimental groups were lower than In-Ceram. 2. The shrinkage rate of samples was 0.42% after first firing, and 0.45% after glass infiltration. Total shrinkage rate was 0.87%. 3. After first firing, porosity rate of experimental groups was 50%, compared with 22.25% of In-Ceram. After glass infiltration, porosity rate of experimental groups was 2%, and 1% in In-Ceram. 4. There was no statistical difference in hardness between two materials tested, but in fracture toughness, group 2 and 3 were higher than In-Ceram. 5. The thermal expansion coefficients of experimental groups were varied to $4.51-5.35{\times}10^{-6}/^{\circ}C$ according to glass composition, also the flexural strengths of samples were varied. 6. In a view of SEM, many microparticles about $0.5{\mu}m$ diameter and $4{\mu}m$ diameter were observed in In-Ceram. But in experimental group, the size of most particles was about $4{\mu}m$, and a little microparticles was observed. The results obtained in this study showed that the mismatch of the thermal expansion coefficients between alumina powder and infiltrated glass affect the flexural strength of alumin/glass composite. The $Y_{2}O_{3}$-containing glass infiltrated ceramic core made by powder packing method will takes less time and cost with sufficient flexural strength similar to all ceramic crown made with slip casting technique.

• Journal of Korea Foundry Society
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• v.18 no.5
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• pp.431-438
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• 1998

Microstructure of A356 aluminum alloys cast in the permanent mold was investigated by optical microscope and image analyzer, with particular respect to the shape and size distribution of iron intermetallics known as ${\beta}-phase$ ($Al_5FeSi$). Morphologies of the ${\beta}-phase$ was found to change gradually with the Be:Fe ratio like these. In Be-free alloys, ${\beta}-phase$ with needlelike morphology was well developed, but script phase was appeared when the Be:Fe ratio is above 0.2:1. With the Be:Fe ratios of 0.4:1-1:1, script phase as well as Be-rich phase was also observed. In case of higher Be addition, above 1:1, Be-rich phase was observed on all regions of the specimens, and increasing of the Be:Fe ratios gradually make the Be-rich phase coarse. It was also observed that the ${\beta}-phase$ with needlelike morphology was coarsened with increase of the Fe content in Be-free alloys. However, in Be-added alloys, length and number of these ${\beta}-phases$ were considerably decreased with the increased Be:Fe ratio. Beryllium addition improved tensile properties and impact toughness of the A356 aluminium alloy, due to the formation of a script phase or a Be-rich phase instead of a needlelike ${\beta}-phase$. The DSC tests indicated that the presence of Be could increase the amount of Mg which is available for $Mg_2Si$ precipitate hardening, and enhance the precipitation kinetics by lowering the ternary eutectic temperature.

• Restorative Dentistry and Endodontics
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• v.8 no.1
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• pp.89-96
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• 1982

The purpose of this study is to identify the phases of four different types, low-copper lathe cut (Type II, class 1) and spherical (Type II, class 2) amalgam alloys which are made by Caulk company and high copper Dispersalloy (Type II, class 3) made by Johnson & Johnson and Tytin (Type I, class 2) made by S.S. White and to determine the Vickers hardness number on the individual phase and four different types of dental amalgam. After each amalgam alloy and Hg measured exactly by the balance was triturated by the mechanical amalgamator (De Trey), the triturated mass was inserted into the cylindrical metal mold which was 4 mm in diameter and 12mm in height and was pressed by the Instron Universal Testing machine (Model 1125) at the speed of 1mm/minute with 143$kg/cm^2$ according to the A.D.A. Specification No. 1. The Specimen removed from the mold, mounted and stored in the room temperature for 7 days. The speciman was polished with the emery paper from #220 to #1200 and finally on the polishing cloth with 0.3 and 0.05 um $Al_2O_3$ powder suspended in water. And then each specimen was etched by Allan's method and washed with Sodium Bisulfinite for 30 seconds. Finally differentiation and metallography on each phase were obtained by using metallographical microscope (Versamet, Union) and microhardness was obtained by using microhardness tester (MVH-2, Torsee). The results were as follows: 1. In the low-copper amalgam, the ${\gamma}$, ${\gamma}_1$ and ${\gamma}_2$ phase were observed and in the high-copper amalgam, the ${\gamma}$, ${\gamma}_1$. ${\epsilon}$ and ${\eta}$ phases were observed but ${\gamma}_2$ phase was not observed. 2. Among the microhardness of each amalgam phase measured under pressing a vickers diamond indenter with 2.0gm load for 30 seconds, e phase has the highest V.H.N (314 ${\pm}$ 20), and in low-copper amalgam 12 phase has the lowest V.H.N. (29${\pm}$1) and ${\eta}$ phase which was observed in high-copper amalgam has 230${\pm}$13 V.H.N and this phase is considerd to contribute to strengthen the handness in amalgam. 3. The V.H.N. measured under pressing a Vickers diamond indenter with 300.0gm load for 30 seconds in low-copper amalgam was lower than that of high-copper amalgam.

• Restorative Dentistry and Endodontics
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• v.16 no.2
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• pp.18-32
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• 1991

The purpose of this study was to observe characteristic properties of amalgam through the polarization curves and SEM images from 4 type amalgams (Amalcap, Shofu spherical. Dispersalloy and Tytin) with 3 different surface finish procedures (polishing, burnishing and carving) by using the potentiostats (EG & GPARC) and SEM (Jeol JSM-35). After each amalgam alloy and Hg was triturated as the direction of the manufacturer by means of mechanical amalgamator (Samki), the triturated mass was inserted into the cylndrical metal mold which was 12 mm in diameter and 10 mm in height and was pressed with $100kg/cm^2$. 4 specimens of each type amalgam were burnished with egg burnisher and another 4 specimens of each type amalgam were carved with Hollenback carver. Above 8 specimens and remaining untreated 4 specimens were stored at room temperature for about 7 days. Untreated 4 specimens of each type amalgam were polished with abrasive papers (Deer) from #400 to #1200 and finally on the polishing cloth with $0.5{\mu}m$ and $0.06{\mu}m$ $Al_2O_3 $ powder suspended water. Anodic polarization measurements was employed to compare the corrosion behaviours of the amalgams in 0.9% saline solution at $37^{\circ}C$. The open circuit potential was determined after 30 minutes immersion of specimen in electrolyte. The scan rate was 1 mV/sec and the surface area of amalgam exposed to the solution was $0.64cm^2$ for each specimen. All the potentials reported are with respect to a saturated calomel electrode (SCE). SEM images of each specimen were taken after + 800 mV (SCE) polarization. The results were as follows: 1. The corrosion potential of high copper amalgam was more anodic than that of low copper amalgam. 2. The polished amalgam were more resistant to corrosion than any other burnished and carved amalgam. 3. In the case of polishing, current density of high copper amalgam was lower than that of low copper amalgam.

• Korean Journal of Materials Research
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• v.11 no.10
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• pp.869-878
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• 2001

Alumina $(Al_2O_3)$ green bodies were fabricated by gel-casting using three kinds of alumina with different particle size (mean particle size: 4.6 $\mu\textrm{m}$, 0.32 $\mu\textrm{m}$, 10nm). The effects of particle size on gel-casting process and green microstructure were investigated. The optimum dispersion conditions using ammonium salt (D-3019) as dispersant were 0.2 wt% (4.63 $\mu\textrm{m}$), 0.5 wt% (0.32 $\mu\textrm{m}$), and 5.0 wt% (10 nm), in high solid loading. The optimum solid loading of each starting material for gel-casting was obtained as 59 vol% (4.63 $\mu\textrm{m}$), 57 vol% (0.32 $\mu\textrm{m}$), 15 vol% (10 nm), depending on particle size, indicating that nano-size particle (10 nm) represent lower solid loading as high specific surface area than those of other two starting materials. The drying at ambient conditions (humidity; $\thickapprox$90%) was performed more than 48hrs to enable ejection of the part from the mold and then at $120^{\circ}C$ for 2hrs in an air oven, showing no crack and flaw in the dried green bodies. The pore size and distribution of the gelcast green bodies showed the significant decrease with decreasing particle size. Green microstructure was dependent on the pore size and distribution due to the particle size, and on the deairing step. The green density maximum obtained was 58.9% (4.63 $\mu\textrm{m}$), 60% (0.32 $\mu\textrm{m}$), 47% (10 nm) theoretical density (TD), and the deairing step applied before gel-casting did not affect green density.

• The Journal of Korean Academy of Prosthodontics
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• v.35 no.4
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• pp.719-741
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• 1997

This study was undertaken to compare by Atomic Force Microscope the effects of various finishing and polishing instruments on surface roughness of filling and veneering composite resins. Seven composite resins were studied : Silux Plus (3M Dental Products, U.S.A.), Charisma (Heraeus Kulzer, Germany), Prisma THP (L.D.Caulk, Dentsply, U.S.A.), Photoclearfil (Kuraray, Japan), Cesead (Kuraray, Japan), Thermoresin LC (GC, Japan), Artglass (Heraeus Kulzer, Germany). Samples were placed and polymerized in holes (2mm thick and 8.5mm in diameter) machined in Teflon mold under glass plate, ensuring excess of material and moulded to shape with polyester matrix strip. Except control group (Polyester matrix strip), all experimental groups were finished and polishied under manufacturer's instructions. The finishing and polishing procedure were : carbide bur (E.T carbide set 4159, Komet, Germany), diamond bur (composite resin polishing bur set, GC, Japan), aluminum-oxide disc (Sof-Lex Pop-On, 3M Dental Products, U.S.A.), diamond-particle disc (Dia-Finish, Renfert Germany), white stone bur & rubber point( composite finishing kit, EDENTA, Swiss), respectively. Each specimens were evaluated for the surface roughness with Atomic Force Microscope (AutoProbe CP, Park Scientific Instruments, U.S.A.) under contact mode and constant height mode. The results as follows : 1. Except Thermoresin LC, all experimental composite resin groups showed more rougher than control group after finishing and polishing(p<0.1). 2. A surface as smooth as control group was obtained by $Al_{2}O_{3}$ disc all filling composite resin groups except Charisma and all veneering composite resin groups except Thermoresin LC(p<0.05). 3. In case of Thermoresin LC, there were no statistically significant differences before and after finishing and polishing(p>0.1). 4. Carbide bur, diamond bur showed rough surfaces in all composite resin groups, so these were inappropriate for the final polishing instruments.

• Korean Journal of Materials Research
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• v.8 no.12
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• pp.1165-1169
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• 1998

A single crystal cast blade was manufactured by CMSX 6, one of the first generarion nickel based single crystal superalloys by the selector method in a vacuum furnace. The single crystal has been grown with cooling rate of 2.5 mm/min, after pouring the molten alloy of 163$0^{\circ}C$ to the mold heated to 150$0^{\circ}C$. The cast structure could be classified into matrix (dendrite) and eutectic regions in ${\gamma}$'shape and size. The eutectic region showed higher Ti content. As the additional results of ${\gamma}$'precipitates by EPMA and CBED analysis the ${\gamma}$'size was less than 0.5~0.7$\mu\textrm{m}$, showing the chemical composition close to Ni$_3$Al of Ll$_2$ lattice structure. But ${\gamma}$'size has increased to bigger than 1.0$\mu\textrm{m}$, being near to eutectic region, changing its shape to bar or huge block types. These showed the chemical structure near to Ni$_3$Ti of D $O_{24}$ lattice structure. Therefore, ${\gamma}$'morphology of dendrite and eutectic regions depends absolutely on its chemical composition and lattice structure.