• Proceedings of the Materials Research Society of Korea Conference
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• 2009.11a
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• pp.31.1-31.1
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• 2009

In an approach to acclimate ourselves torecent ecological consciousness trend, a lead-free piezoelectric material, bismuth sodium titanate (abbreviated as BNT) based bismuth sodium barium titanate (abbreviated as BNT-BT), was considered as an environment-friendly alternative for a lead based piezoelectric system. Ceramic specimens of0.94[(BixNa0.5)TiO3]-0.06[BaTiO3] (x = 0.500~0.515) compositions were prepared by a modified mixed oxide method. To increase the chemical homogeneity andre action activity, high energy mechanical milling machine and pre-milled nanosized powder has been used. In this method (BixNa0.5)TiO3 (x=0.500~0.515) andBaTiO3 were prepared separately from pre-milled constituent materials at low calcination temperature and then separately prepared BNTX (X=1, 2, 3 and 4) and BT were mixed by high energy mechanical milling machine. Without further calcination step the mixed powders were pressed into disk shape and sintered at $1110^{\circ}C$. Microstructures, phase structures and electrical properties of the ceramic specimens were systematically investigated. Highly dense ceramic specimens with homogenous grains were prepared in spite of relatively low sintering temperature. Phase structures were not significantly influenced by the excess amount Bi. Large variation on the piezoelectric and dielectric properties was detected at relative high excess Bi amounts. When $x{\leq}0.505$, the specimens exhibit insignificant variation in piezoelectric and dielectric constant though depolarization temperature is found to be decreased. Considerable amount of decrease in piezoelectric and dielectric properties are observed with higher excess of Bi amounts ($x{\geq}0.505$). This research indicates the advantages of high energy mechanical milling and importance of proper maintenance of Bi stoichiometry.

• Journal of the Microelectronics and Packaging Society
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• v.7 no.4
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• pp.1-5
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• 2000

Properties of AC applied hot pressed ($Pb_{1-x}Sn_{x}$) Te thermoelectrics were investigated. Mechanical alloying process used to produce alloyed powder to reduce the inhomogeneity and to avoid vaporization of constituents. It showed an increase in the mechanical alloying time with increasing of Sn contents in ($Pb_{1-x}Sn_{x}$)Te. ($Pb_{1-x}Sn_{x}$)Te were sintered at 873 to 923K for 1-4 minutes, under 150 kgf/$\textrm{cm}^2$ by AC applied hot pressng method. The short sintering time of AC applied hot pressing process could reduce the vaporization of Te. The density of ($Pb_{1-x}Sn_{x}$) Te was more dependent on the sintering temperature than the sintering time. The p-n transition was observed at x=0.1 but only p type conduction behavior was observed at more than 20 mol% of Sn compositions. The maximum value of Seebeck coefficient is 250 $\mu$V/K for x=0.2 at 500K. As the amount of Sn increases, the peak value of Seebeck coefficient drops and shifts to higher temperature and the peak value of electrical conductivity decreased with increasing temperature.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.10 no.2
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• pp.145-151
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• 2000

The sintering behaviour of monolithic zirconium diboride (ZrB$_{2 }$) and ZrB$_{2 }$-based composite ZrB$_{2 }$-ZrC were studied using a pressureless sintering technique. The specimens were prepared using commercially available ZrB$_{2 }$ and ZrC powder which were pressed and subjected to pressureless sintering. The effects of lanthanum and neodymium used as sintering aids in the sintering processes were investigated. The sintered specimens were characterized using X-ray diffraction analysis and scanning electron microscopy. The ZrB$_{2 }$ specimen prepared using and addition of 1 wt% lanthanum and pressurelessly sintered at $2200^{\circ}C$ showed the maximum relative density of 96%. The ZrB$_{2 }$-ZrC composite specimen without the addition of any sintering aids exhibits the maximum sintered density but contains significantly detectable amount of secondary phase.

• Restorative Dentistry and Endodontics
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• v.9 no.1
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• pp.101-106
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• 1983

It was the purpose of this study to investigate the fracture mode of dental amalgam by observing the crack propagation, and to relate this to the microstructure of the amalgam. Caulk 20th Century Regular, Caulk Spherical, Dispersalloy, and Tytin amalgam alloys were used for this study. After each amalgam alloy and Hg measured exactly by the balance was triturated by the mechanical amalgamator (Capmaster, S.S. White), the triturated mass was inserted into the cylindrical metal mold which was 4 mm in diameter and 12 mm in height and was pressed by the Instron Universal Testing Machine at the speed of 1mm/min with 120Kg. The specimen removed from the mold was stored in the room temperature for a week. This specimen was polished with the emery papers from #100 to #200 and finally on the polishing cloth with 0.06${\mu}Al_2O_3$ powder suspended in water. The specimen was placed on the Instron testing machine in the method similar to the diametral tensile test and loaded at the crosshead speed of 0.05mm/min. The load was stopped short of fracture. The cracks on the polished surface of specimen was examined with scanning electron microscope (JSM-35) and analyzed by EPMA (Electron probe microanalyzer). The following results were obtained. 1. In low copper lathe-cut amalgam, the crack went through the voids and ${\gamma}_2$ phase, through the ${\gamma}_1$ phase around the ${\gamma}$ particles. 2. In low copper spherical amalgam, it was observed that the crack passed through the ${\gamma}_2$ and ${\gamma}_1$ phase, and through the boundary between the ${\gamma}_1$ and ${\gamma}$ phase. 3. In high copper dispersant (Dispersalloy) amalgam, the crack was found to propagate at the interface between the ${\gamma}_1$ matrix and reaction ring around the dispersant (Ag-Cu) particles, and to pass through the Ag-Sn particles. 4. In high copper single composition (Tytin) amalgam, the crack went through the ${\gamma}_1$ matrix between ${\eta}$ crystals, and through the unreacted alloy particle (core).

• 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.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2000.04b
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• pp.72-75
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• 2000

The effect of $Al_2O_3+Y_2O_3$ additives on fracture toughness of ${\beta}-SiC-TiB_2$ composites by hot-pressed sintering were investigated, The ${\beta}-SiC-TiB_2$ ceramic composites were hot-presse sintered and annealed by adding 4, 8, 12wt% $Al_2O_3+Y_2O_3$(6 : 4wt%) powder as a liquid forming additives at low temperature($1800^{\circ}C$) for 4h. In this microstructures, the relative density is over 97% of the theoretical density and the porosity increased with increasing $Al_2O_3+Y_2O_3$ contents because of the increasing tendency of pore formation. But the fracture toughness showed the highest of $7.0MPa{\cdot}m^{1/2}$ for composites added with 12wt% $Al_2O_3+Y_2O_3$ additives at room temperature. The electrical resistivity showed the lowest of $1.59\times10^{-3}\Omega{\cdot}cm$ for composite added with 8wt% $Al_2O_3+Y_2O_3$ additives at room temperature and is all positive temperature coefficient resistance(PTCR} against temperature up to $700^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.35 no.4
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• pp.311-318
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• 1998

The reaction bonded alumina ceramics with reinforced particles which have low shrinkage were pro-duced by blending of SiC or TiC or ZrO2 powders to the mixture of Al metal and Al2O3 powder. The powd-ers were attrition milled isostantically pressed and preheated tio 110$0^{\circ}C$ with a heating rate of $1.5^{\circ}C$/min The specimens were then sintered at the temperature range 1500 to 1$600^{\circ}C$ for 5 hours with a heating rate of 5$^{\circ}C$/min. The specimens showed 5-9% weight gain and 2-9% dimensional expansion through the complete oxidation of Al after preheating up to 11--$^{\circ}C$ the overall dimensional change of the specimens after the reaction sintering at 1500-1$600^{\circ}C$ was 6-12% The maximum densities were 92% theoretical. The fine grain-ed(average grain size :0.4 ${\mu}{\textrm}{m}$) microstructure were observed in the specimen with ZrO2 and SiC. But the microstructure of specimen with TiC was relatively coarse.(average grain size : 2.1 ${\mu}{\textrm}{m}$) The mullite phase was formed by the reaction of Al2O3 and SiO2 in a specimen with SiC. In the TiC contained specimen TiC was oxidized into TiO2 and finally reacted with Al2O3 to form Al2TiO5 during sintering.

• Journal of Hydrogen and New Energy
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• v.7 no.1
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• pp.29-37
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• 1996

Alkaline water electrolysis has been commercialized as the only large-scale method for a long time to produce hydrogen and the technology is superior to other methods such as photochemical, thermochemical water splitting, and thermal decomposition method in view of efficiency and related technical problem. However, such conventional electrolyzer do not have high electric efficiency and productivity to apply to large scale hydrogen production for energy or chemical feedstocks. Solid polymer electrolyte water electrolysis using a perfluorocation exchange membrane as an $H^+$ ion conductor is considered to be a promising method, because of capability for operating at high current densities and low cell voltages. So, this is a good technology for the storage of electricity generated by photovoltaic power plants, wind generators and other energy conversion systems. One of the most important R&D topics in electrolyser is how to minimize cell voltage and maximize current density in order to increase the productivity of the electrolyzer. A commercialized technology is the hot press method which the film type electrocatalyst is hot-pressed to soild polymer membrane in order to eliminate the contact resistance. Various technologies, electrocatalyst formed over Nafion membrane surface by means of nonelectrolytic plating process, porous sintered metal(titanium powder) or titanium mesh coated with electrocatalyst, have been studied for preparation of membrane-electrocatalyst composites. In this study some experiments have been conducted at a solid polymer electrolyte water electrolyzer, which consisted of single cell stack with an electrode area of $25cm^2$ in a unipolar arrangement using titanium mesh coated with electrocatalyst.

• Progress in Superconductivity and Cryogenics
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• v.19 no.2
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• pp.19-24
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• 2017

Cryogenic milling which is a combined process of low-temperature treatment and mechanical milling was applied to fabricate high critical current density $(J_c)MgB_2$ bulk superconductors. Liquid nitrogen was used as a coolant, and no solvent or lubricant was used. Spherical Mg ($6-12{\mu}m$, 99.9 % purity) and plate-like B powder (${\sim}1{\mu}m$, 97 % purity) were milled simultaneously for various time periods (0, 2, 4, 6 h) at a rotating speed of 500 rpm using $ZrO_2$ balls. The (Mg+2B) powders milled were pressed into pellets and heat-treated at $700^{\circ}C$ for 1 h in flowing argon. The use of cryomilled powders as raw materials promoted the formation reaction of superconducting $MgB_2$, reduced the grain size of $MgB_2$, and suppressed the formation of impurity MgO. The superconducting critical temperature ($T_c$) of $MgB_2$ was not influenced as the milling time (t) increased up to 6 h. Meanwhile, the critical current density ($J_c$) of $MgB_2$ increased significantly when t increased to 4 h. When t increased further to 6 h, however, $J_c$ decreased. The $J_c$ enhancement of $MgB_2$ by cryogenic milling is attributed to the formation of the fine grain $MgB_2$ and a suppression of the MgO formation.

• The Journal of Advanced Prosthodontics
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• v.7 no.5
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• pp.349-357
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• 2015

PURPOSE. The purpose of this study was to assess the impact of the core materials, thickness and fabrication methods of veneering porcelain on prosthesis fracture in the porcelain fused to metal and the porcelain veneered zirconia. MATERIALS AND METHODS. Forty nickel-chrome alloy cores and 40 zirconia cores were made. Half of each core group was 0.5 mm-in thickness and the other half was 1.0 mm-in thickness. Thus, there were four groups with 20 cores/group. Each group was divided into two subgroups with two different veneering methods (conventional powder/liquid layering technique and the heat-pressing technique). Tensile strength was measured using the biaxial flexural strength test based on the ISO standard 6872:2008 and Weibull analysis was conducted. Factors influencing fracture strength were analyzed through three-way ANOVA (${\alpha}{\leq}.05$) and the influence of core thickness and veneering method in each core materials was assessed using two-way ANOVA (${\alpha}{\leq}.05$). RESULTS. The biaxial flexural strength test showed that the fabrication method of veneering porcelain has the largest impact on the fracture strength followed by the core thickness and the core material. In the metal groups, both the core thickness and the fabrication method of the veneering porcelain significantly influenced on the fracture strength, while only the fabrication method affected the fracture strength in the zirconia groups. CONCLUSION. The fabrication method is more influential to the strength of a prosthesis compared to the core character determined by material and thickness of the core.