• 연구논문집
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• s.22
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• pp.107-113
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• 1992

The $\alpha$-alumina/fused alumina/$\beta$"-alumina specimens were fabricated by addition of fused alumina to $\alpha$-alumina/$\beta$"-alumina system. The sintered densities with and without fused alumina were almost same. But, the bending strength was decreased due to the presense of defects caused by the inhomogeneous dispersion of large fused alumina particles. And the fracture toughness was also decreased because the crack propagated through the fused alumina particles due to the imperfection of $\beta$"-alumina coating around them.

• The Korean Journal of Ceramics
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• v.6 no.1
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• pp.78-82
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• 2000

Although alumina aggregates have been used as refractory aggregates due to the improved mechanical properties of refractories as a result of the low contraction of alumina aggregates, the aggregates have a difficulty in fabrication due to its low sinterability. Two types of alumina aggregates and a fused alumina aggregate containing transient liquid forming additives are prepared to investigate the sintering characteristics of aggregates. $Al_2O_3$rich composition in the $Al_2O_3$-MgO-$SiO_2$(-$TiO_2$) system is chosen for the transient liquid phase sintering and the final recrystallized bonding phase between grains inside the fused alumina aggregates is found to be a needle-like mullite phase. The flexural strength of alumina bars, reaction-bonded using the paste having a composition of $Al_2O_3$-MgO-$SiO_2$-$TiO_2$, is about 78 MPa, which is one half value of that of pure alumina.

• Journal of the Korean Ceramic Society
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• v.35 no.9
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• pp.923-932
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• 1998

Al2O3/Al composites were produced by displacement reaction method which was carried out by imm-ersing the sintered silica preform which was prepared form fused silica powder in molten aluminu. an ac-tivation energy of 94kJ/mole was calculated from Al-SiO2 reaction data in 1000-130$0^{\circ}C$ temperature range With increase of reaction temperature the alumina particle in the Al2O3/Al composites produced with pur metal Al showed grain growth and the growth of alumina particle in Al2O3/Al composite produced by using of Mg contained Al alloy was inhibited. The flexural strength of Al2O3/Al composites produced at 100$0^{\circ}C$ showed the highest value as 393 MPa. Flexural strength of the composite fabricated at 85$0^{\circ}C$ showed higher deviation than that of the composite produced at above 100$0^{\circ}C$ Low flexural strength of the composite fa-bricated at 120$0^{\circ}C$ due to the growth of pore and alumina particle size. The hardness of composites de-pended on alumina content in Al2O3/Al composite decreased with increasing of aluminium content in case the same alumina content and increased with increasing of silicon content in composite.

• Journal of the Korean Society for Heat Treatment
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• v.31 no.3
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• pp.120-125
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• 2018

The PMMA composited material mixed with alumina studied to find the optimal condition, the adequate ratio of mixing, for the associated mechanical properties such as anti-corrosive, weatherproof performance. The 80% of hardness and 52% of scratch coefficient improved according to increasing ratio of alumina, which is mostly 3 times higher than that of pure PMMA, on other hand 16% of tensile strength and 35% of flexural strength has lost while alumina was adding in. The most proper ratio, having the best availability in substantial production, of composite between pure PMMA and alumina is determined as 93 wt. % vs 7 wt. %. Results of related five properties had estimated by using of a pentagram.

• Proceedings of the Korean Society of Precision Engineering Conference
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• 1997.10a
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• pp.879-882
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• 1997

This paper aims to clarify the effect of the sol-gel sintered alumina grain and the PbO content of binder on the performance of the vitrified alumina stone for superfinishing the ball bearing race with the vitrified alumina superfinishing stone. The main conclusions obtained were as follows. The stone that contain 8- 11% PbO in binder shows the highest grinding ratio, and increasing or decreasing the PbO contents causes lower grinding ratio. When superfinishing with the stone using the 45% fused alumina and 55% sol-gel sintered alumina grain, the grinding ratio becomes the best, the other ratio of grain contents shows lower performance. With increasing the bending strength of stone, the grinding ratio becomes larger. The roughness of the finished surface is the lowest in case of using the stone of the above mentioned conditions.

• Korean Journal of Materials Research
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• v.8 no.7
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• pp.596-600
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• 1998

Reaction bonded $AI_2O_3$(RBAO)-SiC ceramics were prepared from the Al/$AI_2O_3$/SiC powder mixture. The calcined alumina and fused alumina were used as the raw $AI_2O_3$ powder. The same sized (3mm) and differently sized (3mm + Smm) $Zr0_2$ balls were used in attrition milling of starting powders. The effect of $AI_2O_3$ powder shapes on the milling efficiency and reaction sintering was investigated. After attrition milling, isopressed compacts were preheated to 1l00$^{\circ}$C with a heating rate l.5$^{\circ}C$/min and then sintered at the temperature range 1500 to 1$600^{\circ}C$ for S hours with a heating rate 5$^{\circ}C$/min. The powder mixture with fused alumina was effectively comminuted than the mixtures with calcined alumina. And the same sized balls milled more effctive than the differently sized balls. The reaction sintering behavior differed not depending on the shapes of $AI_2O_3$ powder.

• Applied Chemistry for Engineering
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• v.20 no.6
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• pp.638-645
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• 2009

High alumina refractory used in a coal gasifier was analyzed and the degradation mechanism by molten slag was investigated. The depth of refractory severely damaged by slag varied between 12~40 mm, including the adhered slag layer. The sample also showed the cracks formed in parallel to the slag/refractory interface. The degree of degradation varied with the micro-structures in the refractory. Fused alumina grains showed the uneven boundary and pore formation just along the edges, while the tablet alumina showed the slag penetrated between sintered alumina around which the formation of Al-Fe phase was observed. Calcium aluminate cements were not observed at the high temperature zone near the slag/refractory interface, probably due to dissolution into molten slag. Around large grains of alumina, rod shape alumina, which appeared to be recrystallized during cooling, were observed, and large pores were also formed around those grains. Therefore, in high alumina refractories, hot molten slag dissolves the bonding phase and rod-shape alumina phase is recrystallized upon cooling. During this process, cracks are developed due to structural change, and the degradation occurs by physical causes such as structural spalling.

• Journal of the Korean Ceramic Society
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• v.31 no.4
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• pp.448-456
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• 1994

The reaction-sintered alumina and zirconia-alumina ceramics were fabricated from the Al/Al2O3 or Zl/ZrO2(Ca-PSZ) powder mixtures via the attrition milling. And the effects of the milling characteristics of used raw powders on reaction sintering were investigated. After attrition milling and isopressing at 400 MPa the Al/Al2O3 specimen was oxidated at 1200℃ for 8 hours followed by sintering at 1550℃ for 3 hours. Because mixed powders of flake-type Al with coarse alumina was much more effectively comminuted than the globular-type Al with coarse alumina powders, it's sintered body of more than 97% theoretical density was achived, but low contents of Al leads to relatively higher shrinkage of about 8%. And because coarse alumina particles was much more beneficial in cutting and reducing the ductile Al particles, using the coarse alumina powder was much more effective in reaction sintering. Fused Ca-PSZ powder was reaction sintered with Al at 1550℃ for 3 hours and low shrinkage ZrO2-Al2O3 composites were fabricated. But because Al/Ca-PSZ powder mixtures were not effectively milled the reaction sintering and densification was difficult. And the Ca ion in Ca-PSZ grains diffused into alumina grains during sintering so that the unstabilization of Ca-PSZ body was occured which gave the microcracks in the specimens.

• Journal of the Korean Academy of Esthetic Dentistry
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• v.5 no.1
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• pp.34-43
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• 1996

In nowdays many dental CAD/CAM system were developed. Among those only Cerec and Celay were used globally as clinical application. Celay is a machinable ceramic system that is capable of milling inlays, onlays, and veneers from prefabricated industrial ceramic blocks(Vita Celay Blanks). The advatages of Celay are to simplify the manufactures and to save the processing time. For esthetics In-Ceram Alumina bridges were introduced into maxillary anterior restoration. They have a high strength, a high translucency and an excellent marginal adapation. But the laboratory processes are very difficult and complicated. So the construction of In-Ceram Alumina bridge combined with celay system was desgined. The patient is a 28 year old age male. The chief complain is missing of maxillary left central incisor. He wants to restore anterior bridge for esthetically. The Alumina bridge framework was constructed easily by celay system. Glass ilfiltration was occurred. After that, vitadura-${\alpha}$porcelain build up was occurred by conventional method. The translucency of In-Ceram Alumina 3 unit bridge revealed to be superior to that of porcelain fused to metal bridge. So we report it with clincal case and literature reviews.

• Journal of the Korean Ceramic Society
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• v.38 no.2
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• pp.238-238
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• 2001

15% Syn M, sintered mullite(SM) and fused mullite(FM) was added to produce high alumina refractory materials. Mullite content of SM, FM and Syn M was 93.17%, 86.79%, 97.57% repectively. Mechanical properties of specimens (added 15wt% mullite) ware investigated to be flexural strength 338.60MPa, compressive stcngth 9,427kgf/㎠. But the specimen which imported mullite was added showed 5~10%l lower stregth than syn M added.