• Proceedings of the Korean Powder Metallurgy Institute Conference
• /
• 2006.09a
• /
• pp.525-526
• /
• 2006

Calcium-hexaluminate phase $(CA_6)$ is known to be effective for the crack shielding due to the spinel block crystal structure. In this study, we focused to the control of $CA_6$ morphology for good damage tolerance behavior in alumina and zirconia/calcium-hexaluminate $(CA_6)$ composites. Calcium-hexaluminate $(CA_6)$ composites were prepared from zirconia, alumina and calcium carbornate powders. Calcium-hexaluminate $(CA_6)$ phase was obtained by the solid reaction through the formation of intermediate phase $(CA_2)$. $CA_6$ phase showed the column type abnormal grain grown behavior composed of small blocks. Due to the typical microstructure of $CA_6$, alumina and zirconia/calcium-hexaluminate composites provide a well controlled crack propagation behavior.

• Journal of Surface Science and Engineering
• /
• v.25 no.3
• /
• pp.150-157
• /
• 1992

A recovery process of pure alumina powder from the wasted aluminum etching solution of electrolytic condenser works was studied. The possibility of this process was considered in the basis of thermodynamic data nad physico-chemical properties for the recovered materials were tested. In order to obtain pure alumina, Fe3+ and Cu2+ in the solution as impurities were solvent-extracted, respectively, and then, Al3+ was precipitated by changing the pH of the solution. As the results, more than 99.9% of Al3+ in the solution was recovered by the precipitation method. The weight of the precipitate was reduced to about 65 wt.% of the original one by calcination and the sizes of the recovered powders were in order of 3-5$\mu\textrm{m}$. The precipitates were transformed to $\alpha$-Al2O3 at the calcination temperature about 120$0^{\circ}C$.

• Journal of the Korean Ceramic Society
• /
• v.29 no.1
• /
• pp.23-28
• /
• 1992

Alumina ceramics was reinforced by in-situ formation of La-${\beta}$-aluminate in ${\alpha}$-alumina matrix. The powder mixture of which composition is (100-12x)Al2O3+x(La2O3+11Al2O3) was prepared for the formation of La-${\beta}$-aluminate in ${\alpha}$-alumina matrix. The amount of La-${\beta}$-aluminate in the matrix was controlled by varing x which is number of moles. The dense composite was produced by sintering at 1600$^{\circ}C$ in air or hot-pressing at 1550$^{\circ}C$ under 30 MPa. Bending strength and fracture toughness were increased, resulting from the grain growth inhibition and the crack deflection and crack bridging mechanism when La-${\beta}$-aluminate was produced in ${\alpha}$-alumina matrix.

• Journal of the Korean Ceramic Society
• /
• v.29 no.3
• /
• pp.167-176
• /
• 1992

Zirconia-toughnened alumina(ZTA) powders that were uniformly coated with zirconia and yttria on the surface of alumina particles were prepared in order to inhibit the grain growth of alumina. Alumina particles were ultrasonically dispersed in the ethanol solution of Zr-n-propoxide, and then the Zr-alkoxide was hydrolyzed. Hydrated zironia as thin film was stabilized to tetragonal crystalline form by doping yttria as a stabilizer. The prepared ZTA powders had the good sinterability even at the lower temperature. As a result, the sintered bodies showed the enhanced fracture toughness compared with pure alumina. The relative density and fracture toughness(KIC) of the ZTA bodies sintered at 1550$^{\circ}C$ were 98% and 5 MPa$.$m1/2 respectively.

• Journal of the Korean Ceramic Society
• /
• v.36 no.11
• /
• pp.1243-1251
• /
• 1999

Densification behavior and microstructural evolution during the liquid-phase sintering of alumina-anorhite system were investigated as a function of MgO addition. When MgO component was added in either alumina or anorthite glass powder the aspect ratio of alumina grains decreased and concurrently the are of flat interface which was formed as a result of contact solid alumina gains rather increased. Consequently addition of MgO component in the Al2O3-amorthite system brought about suppression of the rearrangement of solid grains during the liquid phase sintering and then densification of specimens was also retarded.

• Journal of the Korean Ceramic Society
• /
• v.50 no.6
• /
• pp.396-401
• /
• 2013

The size of various alumina ceramics used in the semiconductor and display industries must be increased to increase the size of wafers and panels. In this research, large alumina ceramics were fabricated by pressure-vacuum hybrid slip casting (PVHSC) employing a commercial powder, followed by sintering in a furnace. In the framework of the PVHSC method, the consolidation occurs not only by compression of the slip in the casting room but also by suction of the dispersion medium from the casting room. When sintered at $1650^{\circ}C$ for 4 h, the fabricated large-size alumina ($1,550{\times}300{\times}30mm^3$) exhibited a dense microstructure corresponding to more than 99.2% of the theoretical density and a high purity of 99.79%. The flexural and compressive strengths of the alumina plate were greater than 340 MPa and 2,600 MPa, respectively.

• Journal of Technologic Dentistry
• /
• v.30 no.2
• /
• pp.39-45
• /
• 2008

Titanium and its alloys are widely used as dental implants materials because of their excellent mechanical properties. However, the alumina and zirconia ceramics are preferred to use as the substitute of Ti implants because there is a problems in esthetics and biocompatibility in Ti implant. The the glass infiltrated alumina ceramics are studied to increase the toughness and biocompatibility. The 45S5 and soda-lime glass powder was mixed with ethanol at ratio of 1:1 and brushed on the surface of alumina. Then it was heat treated in the electric furnace at $1400^{\circ}C$ from 30 min. to 5 hours. The glass powder was controlled from 200 to $350{\mu}m$ using ball milling. After heat treatment, the glass infiltrated specimen was tested in universal testing machine to measure the bending strength. The surface microstructure of each specimen was observed with SEM. The biocompatibility of 45S5 and soda-lime glass coated alumina was investigated using PBS at $36.5^{\circ}C$ incubator. The specimen was immersed in PBS for 3, 5, 7, 10 days. After that, the surface morphology was investigated with SEM. As the results of experiment, the 45S5 bioglass infiltrated alumina show the increase of bending strength according to the increasing of heat treatment time from 30 min. to 5 hours at $1400^{\circ}C$ Finally the 1370N bending strength of alumina increased to 1958N at 5 hours heat treatment, which shows 1.4 times higher. In contrast to this, the soda lime glass infiltrated alumina ceramics shows the convex curve according to heat treatment time. Thus it shows maximum bending strength of 1820N at 1 hour heat treatment of $1400^{\circ}C$ It gives 1.3 times higher. However, the bending strength of soda lime glass infiltrated alumina is decreasing with increasing heat treatment time after 1 hour. The precipitation on the surface of 45S5 glass infiltrated alumina was revealed as a sodium phosphate ($Na_{6}P_{6}O_{24}6H_{2}O$) and the amount of precipitation is increasing with increasing of immersion time in PBS. In contrast to this, there is no precipitation are observed on the surface of soda lime glass infiltrated alumina. This implies that 45S5 glass infiltrated alumina brings more biocompatible when it is implanted in human body.

• Journal of environmental and Sanitary engineering
• /
• v.15 no.1
• /
• pp.95-101
• /
• 2000

This study was investigated to the reaction of alumina sintering with alkaline. The soluble $NaAlO_2$ was made after the commercial ${\alpha}-Al_2O_3$ was calcinated with NaOH. The reaction of alumina was carried out to be based on the effects of calcination temperature, time, and the mixing ratio of ${\alpha}-Al_2O_3/NaOH$. The alumina was calcined over $500^{\circ}C$ with NaOH powder after it was sieved with 170/270 mesh. The calcined alumina with NaOH powder was dissolved into $25^{\circ}C$ distilled water and filtrated, and HCI was added to adapt pH 6.5~7.5. The residue was separated with vacuum pump for filtration after it was adapted to proper pH, and aluminum compound was precipitated with $Al(OH)_3$. The investigation was carried out with the variables; the calcination temperature($500-900^{\circ}C$), the calcination time (30~90 min), and the concentration of HCI when leaching(0.5~3.0N) respectively. In this investigation, the main product of ${\alpha}-Al_2O_3$ and NaOH was $NaAlO_2$ and the maximum conversion ratio was 91.4% under the optimum conditions as followed ; the ratio of NaOH/${\alpha}-Al_2O_3$ was 1.5 and the calcination conditions were $800^{\circ}C$ and 90 min.

• Journal of the Korean Ceramic Society
• /
• v.26 no.2
• /
• pp.235-241
• /
• 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.

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