• Journal of the Korean Ceramic Society
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• v.34 no.12
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• pp.1213-1220
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• 1997

Effect of particle size of alumina and amount of talc on tape casting and densification behaviors of alumina-talc system were investigated. The pseudoplastic behaviors of slurries increased with increase in amount of talc addition and decrease in alumina particle size. In case of using coarse alumina powder, densification of specimens were accelerated with increase of sintering temperature and amount of talc addition. On the contrary, fine alumina powder retarded of rearrangement of alumina particle during liquid phase sintering due to premature densification of alumina matrix region before formation of liquid phase and then densification of specimens were suppressed with increase of sintering temperature and amount of talc addition.

• Journal of the Korean Ceramic Society
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• v.35 no.10
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• pp.1085-1093
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• 1998

Amorphous alumina powder prepared by the fast calcination of aluminum trihydroxide(Al(OH)3 gibbsite) for 0.5 second at 580$^{\circ}C$ was investigated rehydration propeties. Phase composition crystal size and mor-phology surface area pore volume and pore size distribution of pesudo-boehmite and bayerite crystals changed with temperature time water/alumina ratio and particle size when amorphous alumina rehydrated with water. Phase compositions were examined with XRD and DTA and crystal sized morphologies were investigaed with SEM and TEM. Also rehydration properties of amorphous alumina were in-vestigated by measuring the surface area pore volume and pore size distribution.

• Journal of the Korean Ceramic Society
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• v.27 no.8
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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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• v.36 no.10
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• pp.1040-1047
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• 1999

After alumina sol was prepared by Yoldas process supported membranes were fabricated by adding ce and Re solution and SiO2 sol into alumina sol. The particle size of alumina sol was 11 nm and it was monodispersed transparent and stable for long time. The pore size of un-doped membrane started to increase to about 7,5nm at 1000$^{\circ}C$ and it was grown to twice (about 15nm) at 1100$^{\circ}C$ However the pore size of doped alumina was uniform to 1100$^{\circ}C$. The effect of retardation of grain growth was superior in SiO2 addition to that of Ce and Ru Because SiO2 doped samples transformed to needed-like phase and densified at 1200$^{\circ}C$ their application in membranes was limited. Ce and Ru doped sample showed vermicular structure identical to the un-doped ones at 1200$^{\circ}C$ But the particle size was smaller than that of un-doped ones.

• Journal of the Korean Ceramic Society
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• v.34 no.11
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• pp.1173-1181
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• 1997

Effects of particle size distribution of alumina ceramics on behaviors of tape casting were investigated with emphases on the rheological characteristic of slurry, green density, green sheet strength, and sintering density. For the control of particle size distribution of alumina, the commercial grade low soda alumina, which had different mean particle size of 3.58 ${\mu}{\textrm}{m}$ and 0.42 ${\mu}{\textrm}{m}$, were chosen and blended together. As results, the mixing of 80 wt% fine powder and 20 wt% coarse powder(designated to FC20) led to the increase of packing density and strength of green sheet, and made it easy to handle during processing without lowering of sintering density. Besides, the pseudoplastic behavior of slurry decreased with increase of the fraction of coarse alumina powder.

• Journal of the Korean Ceramic Society
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• v.37 no.7
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• pp.638-644
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• 2000

The sintering behaviors of the glass-alumina composites for low firing temperature were investigated as functiions of the volume fraction of alumina powder and the particle size with respect to porosity and pore shape. As the volume fraction of alumina powder was increased or the particle size of it was decreased, the sintering temperature of open pore-closing was raised. When the volume fractions of alumina which had 2.19$\mu\textrm{m}$ median diameter were increased with 20, 30, 40, and 50%, the sintering temperatures of open pore-closing were 425, 450, 475, and 500$^{\circ}C$. And when the median particle size of alumina was diminished from 2.19$\mu\textrm{m}$ to 0.38$\mu\textrm{m}$, the sintering temperature of open pore-closing was increased from 450$^{\circ}C$ to 475$^{\circ}C$. Especially, the sintering temperature, which showed maximum density, was corresponded with the stage of open pore-closing and after achieving maximum density over heating resulted in dedensification of specimen, so called, over-firing behavior.

• Journal of the Korean Ceramic Society
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• v.29 no.12
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• pp.956-962
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• 1992

The predominant sintering mechanisms of low firing temperature ceramic substrate which consists of borosilicate glass containing alumina as a filler are the rearrangement of alumina particles and the viscous flow of glass powders. In this system, sintering condition depends on the volume ratio of alumina to glass and on the particle size. When the substrate contains about 35 vol% alumina filler and the average alumina particle size is 4 $\mu\textrm{m}$, the best firing condition is obtained at the temperature range of 900∼1000$^{\circ}C$. The extensive rearrangement behavior occurs at these conditions, and the optimum sintering condition is attained by smaller size of glass particles, too. The formation of cristobalite during sintering causes the difference of thermal expansion coefficient between the substrate and Si chip. This phenomenon degradates the capacity of Si chip. Therefore, the crystallization should be prevented. In the alumina filled borosilicate glass system, the crystallization does not occur. This effect may have some relation with aluminum ions in alumina. For aluminum ions diffuse into glass matrix during sintering, functiong as network former.

• Journal of the Korean Ceramic Society
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• v.35 no.12
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• pp.1308-1315
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• 1998

Effects of particle size of alumina on densification behavior during liquid-phase sintering of alumina-talc system were investigated with emphasis on particle rearrangement process. In the case of using coarse alu-mina powder densiication of specimens was rapidly accelerated after formation of liquid phase due to easy particle rearrangement process with addition of talc and increase of sintering temperature. On the contrary when fine alumina powder was used premature densification of alumina matrix region formed before for-mation of liquid phase rigid skeleton structure and then it seemed to inhibit rearrangement process during crease of sintering temperature. As results the densification of specimens using coarse alumina powder was higher than that of the case of using fine one.

• Journal of the Korean Ceramic Society
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• v.35 no.6
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• pp.551-558
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• 1998

Detection of microcrack in {{{{ {Al }_{2 } {O }_{3 } }} ceramics were studided by AE(acoustic emission) technique with 4-point bending test in order to evaluate the fracture process and formation of microcrack. Fully-dense alu-mina ceramics having a different grain size were fabricated by varing the hot-pressing temperature. The grain size of alumina increased with increasing the hot-pressing temperature whereas the bending strength decreasd. The microcracks were observed by SEM and TEM. The generation of AE event increased with increasing the applied load and many AE event was generated at maximum applied load. Alumina with smaller grain size shows the generation of many AE event resulting in an increase of microcrack formation. An intergranular fracture is predominantly observed in fine-grained alumina whereas intragranular fracture occurs predominantly in coarse-grained alumina,. Analysis of micorstructure and AE prove that primary mi-crocracks occur within grain-boundaries of alumina. The larger microcracking were formed by the growth and/or coalesence of primary microcracks. Then the materials become to fracuture by main crack gen-eration at the maximum applied load.

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