• Journal of the Korean Ceramic Society
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• 제31권1호
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• pp.31-38
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• 1994

To overcome the demerit of conventional forming method, new forming method, pressureless powder packing forming method, was investigated. This technique is performed by powder packing followed by the infiltration of binder solution. Various alumina powders were used as starting materials and the powders showing good packing condition through powder packing experiment were chosen. The green densities prepared by this new forming method with these powders were lower than those of specimens by pressing method, but, nearly same density was obtained in case of green body prepared with the powders having high packing density. The distribution of binder in a green body was homogeneous and it was possible to a complex shape form by this forming method.

• Journal of the Korean Ceramic Society
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• 제33권9호
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• pp.1045-1049
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• 1996

Pressureless-sintered polycrystalline alumina and carbon steel were joined with Ag-33.5Cu-1.5Ti (wt%) brazing alloy. SEM observation revealed that two reaction layers with different thicknesses were continuously formed between the alumina and the brazing alloy. A thick layer formed on the brazing alloy side was identified as Ti3(Cu0.93Al0.07)3O phase with diamond cubic structure. Another thin layer adjacent to the alumina was revealed as $\delta$-TiO phase of which the crystal structure was HCP with a lattice parameter of a0=0.419 nm and c0=0.284 nm. It was confirmed using XPS analysis that $\delta$-TiO was formed directly by a redox reaction of alumina with titanium ir, molten brazing alloy.

• Journal of the Korean Ceramic Society
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• 제43권12호
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• pp.859-864
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• 2006

For the pressure compaction process of the ceramic powder, the green density is very different with both the ceramic body shape and the processing conditions. The density difference cause non-uniform shrinkages and deformations, and make cracks in the sintered ceramics. In this paper, Material properties of the alumina powder mixed with binder and the friction coefficient between the powder and the tool set were determined through the simple compaction experiments: Also the powder flow characteristics were simulated and the green density was analyzed during the powder compaction process with Finite Element Method (FEM). The results show that the density distributions of the green body were improved at the optimized processing condition and both the possibility of the farming crack generation and rho deformation of the sintered Alumina body were reduced.

• Journal of the Korean Ceramic Society
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• 제15권4호
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• pp.199-204
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• 1978

In the present investigation, refined alumina obtained from alunite locally available was used as a aluminous source to synthesize monocalcium aluminate $(CA)_3$ the major mineral constituent of alumina cement. The influence of $SiO_2$ and $Fe_2O_3$ contents on the formation of monocalcium aluminate was studied by X-ray diffraction analysis mainly. About 0.8-1.0 of $Al_2O_3/C_3O$ mol ratio and less than 4 percent of $SiO_2$ were desirable for the effective formation of CA. The small amount of alkali and sulphur contents contained in refined alumina from alunite as the impurities were affected to form $C_4A_3S$ and $C_3S_2$, disadvantageous compounds for the alumina cement, therefore the impurities should be restricted in minimum content as possible.

• Journal of the Korean Ceramic Society
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• 제25권6호
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• pp.585-592
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• 1988

Aluminum sec-butoxide was synthesized from aluminum foil and alcohol. The characteristics of the alumina powder prepared from the synthesized alkoxide by the Yoldas method were studied. As a result of measuring the synthesized aluminum sec-butoxide by GC, the purity of the synthesized alkoxide was 98.7% on the basis of commercial alkoxide and the yield was 78.5%. A127-MASNMR analysis shows that powders dried at 9$0^{\circ}C$ and $\alpha$-Al2O3 have 6-cordinated structure and ${\gamma}$-Al2O3 has 4-coordinated structure. The characteristics of alumina powder prepared from the synthisized alkoxide showed the same characteristics with the alumina powder prepared from the commercial alkoxide.

• Journal of the Korean Ceramic Society
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• 제28권12호
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• pp.1019-1025
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• 1991

The effect of dissolved Ca ion from plaster mold during slip casting on the microstructure and fracture toughness of high-purity sintered alumina were investigated. When the alumina slip containing 1000 ppm MgO was casted on a calcined alumina mold, the sintered compact had a homogeneous microstructure with equiaxed grains. However, when the same slip was casted on a plaster mold, the sintered compact consisted of the mixture of equiaxed and elongated grains. This inhomogeneous microstructure was also observed in the sintered alumina doped with 100o ppm MgO and 100 ppm CaO whose compact was prepared on the calcined alumina mold indicating that the inhomogeneity was caused by CaO. It was found that the specimen containing both MgO and CaO had higher fracture toughness than that containing MgO only. The enhanced fracture toughness by CaO is probably due to the crack deflection along the boundaries of the elongated grains.

• Journal of the Korean Ceramic Society
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• 제27권8호
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• pp.1011-1019
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• 1990

Pyrophyllite, which has low impurities, was used in the synthesis of mullite to decrease the glass phase, which can be formed from impurities such as alkali and alkali earth elemetns present in raw materials. But, as pyrophyllite has less alumina content than other aluminosilicate materials such as kaolin, more alumina sources were needed in the synthesis of mullite. In other to investigate the effect of particle size of alumina sources on the mullitization of pyrophyllite, aluminum hydrate gel and activated alumina were used. When activated alumina, which has large particle size, was added to pyrophyllite, mullitization was not fully accomplished regardless of temprature. In the case of aluminum hydrate gel, which has small particle size, the maximum yield of mullite was about 90.3% at 1700$^{\circ}C$, and grain size of mullite was larter than the case of activated alumina was added.

• Journal of the Korean Ceramic Society
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• 제26권2호
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• pp.235-241
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• 1989

A study on the APCVD(atmospheric pressure chemical vapor deposition) Al2O3 was done by using the aluminum-tri-isopropoxide/N2 reaction system at 40$0^{\circ}C$. When the flow rate of the carrier gas(N2) was over 2SLPM, heterogeneous reaction was observed. However, when the flow rate of the carrier gas was below 2SLPM, a porously deposited film or powder formation was observed. The film formed by a heterogeneous reaction was optically dense. The dense film is thought to be a kind of a hydrated alumina. After a thermal treatment of the film in the range of temperature from $600^{\circ}C$ to 1, 20$0^{\circ}C$, properties of the film seems to be changed due to dehydration and densification process. In the case of the powder on heat treatment(600~1, 20$0^{\circ}C$), both a phase transformation and the change of OH peak was observed.

• Journal of the Korean Ceramic Society
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• 제50권6호
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• pp.528-532
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• 2013

Well-dispersed slurries of submicron-sized alumina powders were pressure-infiltrated in 3D preforms of mullite fibers and the effects of the particle size and infiltration pressure on the particle packing characteristics were investigated. Infiltration without pressure showed that the packing density increased as the particle size decreased due to the reduction of the friction between the particles and the fibers. The infiltrated preforms contained large pores in the large voids between the fiber tows and small pores in the narrow voids between the individual fibers. Pressure infiltration resulted in a packing density of 77% regardless of the particle size or the infiltration pressure(210 ~ 620 kPa). Pressure infiltration shortened the infiltration time and eliminated the large pores in preforms infiltrated with the slurries of smaller particles. The slurry pressure-infiltration process is thus an efficient method for the packing of matrix materials in various preforms.

• Journal of the Korean Ceramic Society
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• 제29권8호
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• pp.651-659
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• 1992

Fracture toughness of alumina was determined by IF, IS, DT, SEPB methods, and the data were inter-compared. Round robin test on IF and IS methods was also conducted under the participitation of 4∼5 domestic institutes. Fracture toughness data determined by IF, IS, DT methods were similar, while those by SEPB method were smaller. Variation of toughness data determined by IS method using 98N of indentation load was significantly small compared to those determined by any other methods. Round robin test results showed that toughness data determined by IF method at various institutes do not coincide each other, while those by IS method do well coincide. Thus, it was concluded that inter-confidence on fracture toughness data, if determined by IS method at all institute, can be established between institutes.