• The Journal of Korean Academy of Prosthodontics
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• v.35 no.4
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• pp.781-792
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• 1997

This study was peformed to evaluate the dynamic fatigue characteristics of the dental ceramics for all-ceramic crown. A feldspathic porcelain of VMK68. glass ceramic of IPS-Empress, and glass infiltrated alumina ceramic of In-Ceram were used. Disc specimens were prepared to the final dimensions of 12 mm in diameter and 1 mm in thickness. The biaxial flexure test was conducted using a ball-on-three-ball method. 240 specimens were tested in $37^{\circ}C$ water by testing 20 samples at each of four loading rates:0.05, 0.2, 1, and 5mm/min. 60 specimens were tested in a moisture-free environment by testing 20 samples at 5mm/min. The inert strength of VMK68 was 80.25MPa, and the fatigue parameters were n=29.1, ${\sigma}_{fo}=52.90MPa$. The inert strength of IPS-Empress was 104.76MPa, and the fatigue parameters were n=32.46, ${\sigma}_{fo}=67.52MPa$. The inert strength of In-Ceram was 429.33MPa, and the fatigue parameters were n=31.46, ${\sigma}_{fo}=258.36MPa$. 10-year failure stresses of VMK68, IPS-Empress, and In-Ceram were 20.3MPa, 24.8MPa, and 93.6MPa, respectively. Failure strength and fatigue life showed the highest value in In-Ceram, and then, IPS-Empress and VMK68.

• Analytical Science and Technology
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• v.6 no.2
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• pp.217-223
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• 1993

Thin films of the Au/Cr, Au/Ni/Cr and Au/Pd/Cr systems were deposited on alumina substrates at ambient temperature and $250^{\circ}C$ in a high-vacuum resistance heating evaporator and annealed at $300^{\circ}C$, $450^{\circ}C$ and $600^{\circ}C$ for 1 hour in air, respectively. The film thicknesses of Au, Ni(or pd), and Cr were $1000{\AA}$, $300{\AA}$, and $50{\AA}$, respectively. The substrate temperature during deposition and the post-deposition annealing temperature affected the sheet resistance of thin-films due to the inter-diffusion of each layer. As a result of Auger depth profile analysis, in the Au/Cr system Cr already diffused out to Au surface during deposition at the substrate temperature of $250^{\circ}C$ and Au distribution changed after heat treatment. In the Au/Ni/Cr and Au/Pd/Cr systems, diffusion phenomena of Ni and Pd were found and especially Ni (approximately 45 at.%) diffused out to Au surface and oxidized.

• Journal of the Korea Institute of Building Construction
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• v.19 no.5
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• pp.401-409
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• 2019

This paper aims to recycle red mud from a byproduct in the alumina industry as an alternative raw material for depleted natural resources. In order to apply red mud as a ceramic material, red mud ceramics were prepared according to mixing and temperature in a laboratory environment. Compared with KS L 4201 in terms of compression ratios and absorptions, it is found that two kinds of conditions for one type and three conditions for two types meet the standard. When red mud is used as a clay brick raw material, the substitution ratio of red mud is 10% or less, and the firing temperature is considered to be appropriate at $1200^{\circ}C$. In order to apply red mud to clay brick raw material in actual field, various samples and firing temperature should be considered in the future.

• Korean Journal of Crystallography
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• v.11 no.2
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• pp.102-107
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• 2000

20vol% Al2O3-dispersed 2Y-TZP ceramics was prepared by mixing of 2Y-TZP and Al2O3 powder with different particle sizes, and investigated the influence of Al2O3 particle size and sintering condition on the microstructure and fracture toughness. Sintering conditions of the Al2O3-dispersed 2Y-TZP specimens showed high density at sintering condition of 1350℃ and 1500℃ for 1∼5h, and homogeneous microstructure. The grain size of tetragonal zirconia and the fracture toughness increased with the size of dispersed Al2O3 particle. The highest fracture toughness (∼17.2MPa·m1/2) of all specimens was obtained in the specimens with dispersed Al2O3 particle size of 1.0㎛ and sintered at 1500℃ for 2h.

• Korean Journal of Materials Research
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• v.23 no.9
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• pp.531-536
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• 2013

This paper investigates the effect of prolonged high temperature exposure on concentric laminated $Al_2O_3-ZrO_2$ composites. An ultrafine scale microstructure with a cellular 7 layer concentric lamination with unidirectional alignment was fabricated by a multi-pass extrusion method. Each laminate in the microstructure was $2-3{\mu}m$ thick. An alternate lamina was composed of 75%$Al_2O_3$-(25%m-$ZrO_2$) and t-$ZrO_2$ ceramics. The composite was sintered at $1500^{\circ}C$ and subjected to $1450^{\circ}C$ temperature for 24 hours to 72 hours. We investigated the effect of long time high temperature exposure on the generation of residual stress and grain growth and their effect on the overall stability of the composites. The residual stress development and its subsequent effect on the microstructure with the edge cracking behavior mechanism were investigated. The residual stress in the concentric laminated microstructure causes extensive micro cracks in the t-$ZrO_2$ layer, despite the very thin laminate thickness. The material properties like Vickers hardness and fracture toughness were measured and evaluated along with the microstructure of the composites with prolonged high temperature exposure.

• Journal of Sensor Science and Technology
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• v.17 no.3
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• pp.203-209
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• 2008

Thick film formaldehyde (HCHO) gas sensors were fabricated by using $La_1_{-x}Sr_xMO_3$ (M= Fe, Co, Mn) ceramics. The powders of $La_1_{-x}Sr_xMO_3$ (M=Fe, Co, Mn) were synthesized by conventional solid-state reaction method. By using the $La_1_{-x}Sr_xMO_3$ (M=Fe, Co, Mn) paste, the thick-film formaldehyde sensors were prepared on the alumina substrate by silkscreen printing method. The experimental results revealed that $La_1_{-x}Sr_xMO_3$ (M= Fe, Co, Mn) ceramic powder has a perovskite structure and the thick-film sensor shows excellent gas-sensing characteristics to formaldehyde gas (sensitivity of $La_{0.8}Sr_{0.2}FeO_3$, S= 14.7 at operating temperature of $150^{\circ}C$ in 50 ppm HCHO ambient).

• The Korean Journal of Ceramics
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• v.1 no.3
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• pp.137-142
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• 1995

Monodispersed $Al_2O_3-TiO_2$ Powder was prepared by metal-alkoxide hydrolsis. A homogeneous nucleation/growth occurred in the solutions containing ethanol, butanol and acetonitrile, and resulted in spherical, submicrometer-sized powder. The titania and the alumina crystals were formed at $800^{\circ}C$ and $1000^{\circ}C$, respectively. These crystala were subsequently reacted each other beyond $1320^{\circ}C$ and formed $Al_2TiO_5$. The relative densities of sintered bodies prepared with as-received powder were examined at the temperature range of 1300-$1500^{\circ}C$ and they were about 79% at $1300^{\circ}C$. The formation of aluminum titanata decreased the relative density at the temperature range of 1300-$1450^{\circ}C$, and at above $1450^{\circ}C$, the relative density started to increase again. It was observed that $\alpha-Al_2O_3$-doped aluminum titanate was more stable than pure aluminum titante at $1200^{\circ}C$.

• The Korean Journal of Ceramics
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• v.2 no.1
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• pp.33-38
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• 1996

In thick film resistors, the characteristics of the frit and the reaction between glass frit and conductor material play an important role for their electrical properties. In this study, various glass frits in the system of $60RO{\cdot}20SiO_2$ $15B_2O_3{\cdot}5Al_2O_3$(RO=PbO, ZnO, CdO; mole%) were mixed with $RuO_2$ and coated on 96% alumina substrate. Only the glass frit containing PbO was reacted with $RuO_2$in$RuO_{2+}$-thick film resistor and produced the new crystalline phase of $Pb_2Ru_2O_{65}$. Their electrical resistivities strongly depend on the amount of $Pb_2Ru_2O_{65}$ crystalline phase obtained, which varied with firing temperature. The sheet resistivities of these resistors were varied from $10^3\; to\; 10^6\;{\Omega}/{\Box}$ depending on heat treatment, and the absolute value of TCR was decreased as the heat treatment temperature increaed. However, $RuO_2$ did not reacted with the glass frits containing ZnO nor CdO, and the resulting showed very high sheet resistivities.

• Journal of the Korean Ceramic Society
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• v.34 no.5
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• pp.483-490
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• 1997

Al2O3/metal composites were fabricated by oxidation and reaction of molten Al-alloy into two types of commercial Al2O3-SiO2 fibrous insulation board. The growth rate, composition and microstructure of these materials were described. An AlZnMg(7075) alloy was selected as a parent alloy. Mixed polycrystalline fiber and glass phase fiber were used as a filler. The growth surface of an alloy was covered with and without SiO2. SiO2 powder was employed as a surface dopant to aid initial oxidation of Al-alloy. Al-alloy, SiO2, fiber block and growth inhibitor CaSiO3 were packed sequentially in a alumina crucible and oxidized in air at temperature range 90$0^{\circ}C$ to 120$0^{\circ}C$. The growth rate of composite layer was calculated by measuring the mass increasement(g) per unit surface($\textrm{cm}^2$). XRD and optical microscope were used to investigate the composition and phase of composites. The composite grown at 120$0^{\circ}C$ and with SiO2 dopant showed rapid growth rate. The growth behavior differed a little depending on the types of fiber used. The composites consist of $\alpha$-Al2O3, Al, Si and pore. The composite grown at 100$0^{\circ}C$ exhibited better microstructure compared to that grown at 120$0^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.45 no.10
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• pp.584-588
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• 2008

Porous hydroxyapatite(HA) and HA-coated porous $Al_2O_3$ possessing pore characteristics required for bone substitutes were prepared by a slurry foaming method combined with gelcasting. The HA coating was deposited by heating porous $Al_2O_3$ substrates in an aqueous solution containing $Ca^{2+}$ and ${PO_4}^{3-}$ ions at $65{\sim}95^{\circ}C$ under ambient pressure. The pore characteristic, microstructure, and compressive strength were investigated and compared for the two kinds of samples. The porosity of the samples was about 81% and 80% for HA and $Al_2O_3$, respectively with a highly interconnected network of spherical pores with size ranging from 50 to $250{\mu}m$. The porous $Al_2O_3$ sample showed much higher compressive strength(25 MPa) than the porous HA sample(10 MPa). Fairly dense and uniform HA coating(about $2{\mu}m$ thick) was deposited on the porous $Al_2O_3$ sample. Since the compressive strength of cancellous bone is $2{\sim}12$ MPa, both the porous HA and HA-coated porous $Al_2O_3$ samples could be successfully utilized as scaffolds for bone repair. Especially the latter is expected suitable for load bearing bone substitutes due to its excellent strength.