• Journal of Powder Materials
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• v.19 no.1
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• pp.25-31
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• 2012

In this study, a high energy ball milling process was employed in order to improve the densification of direct nitrided AlN powder. The densification behavior and the sintered microstructure of the milled AlN powder were investigated. Mixture of AlN powder doped with 5 wt.% $Y_2O_3$ as a sintering additive was pulverized and dispersed up to 50 min in a bead mill with very small $ZrO_2$ beads. Ultrafine AlN powder with a particle size of 600 nm and a specific surface area of 9.54 $m^2/g$ was prepared after milling for 50 min. The milled powders were pressureless-sintered at $1700^{\circ}C-1800^{\circ}C$ for 4 h under $N_2$ atmosphere. This powder showed excellent sinterability leading to full densification after sintering at $1700^{\circ}C$ for 4 h. However, the sintered microstructure revealed that the fraction of yitttium aluminate increased with milling time and sintering temperature and the newly-secondary phase of ZrN was observed due to the reaction of AlN with the $ZrO_2$ impurity.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.12 no.12
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• pp.5438-5442
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• 2011

In order to use as weight-reduced sintered parts for automobiles, mechanical properties of sintered compacts of 2712 Al-alloys containing 0, 10, 20, and 33% ceramics, respectively, were investigated in this study. (1)Regardless of ceramic contents, all sintered compacts possessed 4~7% pores. (2)After aging the compacts sintered for 12 minutes, tensile strength of the 2712 Al-alloys containing 0, 10, 20, and 33% ceramics was 165, 260, 256, and 166 N/$mm^2$, respectively, whereas those aged after sintering for 30 minutes exhibited the tensile strength of 186, 229, 219, and 202 N/$mm^2$, respectively. In addition, the 2712 sintered compact containing 10% ceramics showed the maximum elongation of about 3.6%, (3)After aging the 10% ceramic-containing sintered compacts reached a maximum apparent hardness ($H_RF$) of 97. (4)The minimum wear volume ($174{\times}10^{-3}mm^3$) was shown in the 10% ceramic-containing 2712 sintered compact after aging. From these results, the 2712 sintered compact containing 10% ceramics thought to be the most suitable one for the fabrication of aluminum engine part.

• Journal of the Korean Ceramic Society
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• v.48 no.5
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• pp.418-425
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• 2011

In this study, a high energy ball milling process was introduced in order to improve the densification of direct nitrided AlN powder. The sintering behavior and thermal conductivity of the AlN milled powder was investigated. The mixture of AlN powder and 5 wt% $Y_2O_3$ as a sintering additive was pulverized and dispersed by a bead mill with very small $ZrO_2$ bead media. The milled powders were sintered at $1700^{\circ}C-1800^{\circ}C$ for 4 h under $N_2$ atmosphere. The results showed that the sintered density was enhanced with increasing milling time due to the particle refinement as well as the increase in oxygen contents. Appropriate milling time was effective for the improvement of thermal conductivity, but the extensive millied powder formed more fractions of secondary phase during sintering, resulted in the decrease in thermal conductivity. The AlN powder milled for 10min after sintering at $1800^{\circ}C$ revealed the highest thermal conductivity, of 164W/$m{\cdot}K$ in tne densified AlN sintered at $1800^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.28 no.3
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• pp.189-196
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• 1991

Si(OC2H5)4, commercial AlN and Y2O3 powder were used as the precusor of Si3N4, AlN, Y2O3, respectively. After Si3N4 powder was synthesized by carbothermal reduction and nitridation at 135$0^{\circ}C$ for 13h in N2 atmosphere, characteristics of synthesized powder and the ceramics sintered at 178$0^{\circ}C$ for 1h under 30MPa were investigated. In order to evaluate the reliability of sintered body, Weibull modulus was investigated. Premixing of carbon black as a reduction agent had no effect on Si(OH)4 formation, and Si3N4 powder synthesized from Si(OC2H5)4 was $\alpha$-Si3N4 single phase. Mechanical properties of sintered body were measured as follows : flexural strength ; 750MPa, fracture toughness ; 3.71Mn/3/2, hardness : 17.4GPa, thermal shock resistence temperature ; $600^{\circ}C$. Flexural strength at room temperature was 750MPa and was retained up to 110$0^{\circ}C$. The Weibull modulus of sintered body was 10.7.

• Journal of the Korean Ceramic Society
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• v.33 no.2
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• pp.169-176
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• 1996

The specimens which were prepared from $\alpha$-Si3N4 with additions of YAG(3Y2O3.5Al2O3)-10 wt% and various AlN contents were sintered in N2 atmosphere at 1$700^{\circ}C$ The effect of $\alpha$,$\beta$-solid solution contents and sintering time on mechanical properties were investigated. As the content of $\beta$-solid solution and sintering time increased the hardness is reduced but the hardness of specimen sintered over 10 hours is constant irrespective of sintering time. While the fracture toughness increased with increasing of $\beta$-solid solution and sintering time. The fracture toughness of specimen with 80% $\beta$-solid solution content increased from 3.89 to 6.66 MPam1/2 with sintering sintering up to 20 hours/ But the amount of increased fracture toughness of specimen with below 20% $\beta$-solid solution content is not significant.

• Journal of Powder Materials
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• v.18 no.4
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• pp.365-371
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• 2011

In this study, gradient porous Al-Cu sintered body was fabricated by powder metallurgy processing. Al-Cu powder mixtures were prepared by low energy ball milling with various milling time. After ball milling for 3h, the shape of powder mixtures changed to spherical type with size of 100~500 ${\mu}m$. Subsequently, Al-Cu powder mixtures were classified (under 150, 150~300 and over 300 ${\mu}m$) and compacted (20, 50 and 100 MPa). Then, they were sintered at $600^{\circ}C$ for various holding time (10, 30, 60 and 120 min) in $N_2$ atmosphere. The sintered bodies had 32~45% of porosity. As a result, the optimum holding time was determined to be 60 min at $600^{\circ}C$ and sintered bodies with various porosity were obtained by controlling the compacting pressure.

• Analytical Science and Technology
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• v.6 no.2
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• pp.207-215
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• 1993

$Si_3N_4$ powders were mixed with sintering additives($3Y_2O_3{\cdot}5Al_2O_3$, YAG) by coprecipitation method. Mixing homogeneity, sintered and mechanical properties of coprecipitation-mixed powder compacts were compared with those of mechanically-mixed ones. SIMS-analysis for composition on the surface and in the matrix of prepared powder mixtures showed that the added YAG exists mainly on the $Si_3N_4$ powder surfaces in a form of coating. From this result it could be concluded that coprecipitation method is superior to mechanical mixing in the mixing homogeneity. This mixing homogeneity can accompany an improvement in sintered density, microstructure and consequently the mechanical properties of sintered bodys.

• Journal of the Korean Ceramic Society
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• v.53 no.6
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• pp.635-640
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• 2016

Electrical behavior of AlN ceramics sintered with $Y_2O_3$ as a sintering aid has been investigated with respect to additional $TiO_2$ dopant. From the impedance spectroscopy, it was found that the grain and grain boundary conductivities have greatly decreased with addition of $TiO_2$ dopant. The $TiO_2$ dopant also increased the activation energy of the grain conductivity by about 0.37 eV; this increase was attributed to the formation of an associate between Al vacancies and Ti ions at the Al sites. Similarly, the electronic conductivity was reduced by $TiO_2$ addition. However, $TiO_2$ solubility in AlN grains was below the detection limit of typical EDX analysis. Grain boundary was clean, without liquid films, but did show yttrium segregation. The transference number of ions was close to 1, showing that AlN is a predominantly ionic conductor. Based on the observed results, the implications of using AlN applications as insulators have been discussed.

• Journal of the Korean Ceramic Society
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• v.36 no.2
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• pp.193-202
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• 1999

Al powder, AlN powder, SiC whisker and sintering aids were wet-mixed, and then the specimens prepared with mixed powder were reacted by nitridation at 600∼1400$^{\circ}C$ for 5 hrs. It was cleat that the higher nitridation and the more SiC whisker content were, the better bending strength was. The specimen of Al50/AlN50 reacted at 1400$^{\circ}C$ for 5hrs had the nitridation percent of 97%, the shrinkage under 2%, and the relative density of 78%. And the maximum bending strength of reaction-bonded specimen was 250 MPa. The specimens completely nitrided were post-sintered at 1700, 1800 and 1900$^{\circ}C$ for 2hrs. The post-sintered body had the shrinkage under 6% and the relative density of 86%. Because of the formation of solid solution between AlN and SiC whisker over 1800$^{\circ}C$, the promotion of mechanical properties according to SiC whisker addition was not observed. The post-sintered body had the maximum bending strength of 195 MPa.

• Journal of the Korean Ceramic Society
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• v.28 no.6
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• pp.457-464
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• 1991

Aluminum nitride (AlN) has excellent properties such as high thermal conductivity and electrical resistivity, whereas it has some disadvantages such as low sinterability and tendency to be hydrolyzed by moisture at room temperature. In the present work, the relative density, modulus of rupture and microhardness were examined for pressure-less-sintered AlN (synthetic and commercial) bodies which were prepared under the conditions of various sintering temperatures, holding times and additions of CaCO3 which showed the best effect on sinterability among the various sintering aids. As a result, the AlN bodies with 1.0 wt% CaCO3 (0.56wt% CaO) which were sintered at 1800$^{\circ}C$ for 20 min showed good densification. In this case, the relative densities were 95.9% and 95.2%, and microhardnesses were 10.3 GPa and 9.8 GPa for synthetic and commercial AlN respectively. And as the holding time at 1800$^{\circ}C$ was increased from 10 min to 60 min, the relative density was increased from 91.9% to 96.5%. It was considered that impurities of metals and oxygen promoted the densification of AlN at low temperature (1600$^{\circ}C$).