• Nuclear Engineering and Technology
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• v.28 no.5
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• pp.458-466
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• 1996

The effects of dopants on the modification of microstructure of UO$_2$ and UO$_2$-4wt%CeO$_2$ sintered pellets have been studied in hydrogen and $CO_2$/CO mixture atmospheres by using $Ta_2O_5$, TiO$_2$ and $Nb_2O_5$ as sintering additives. The dopant were added as oxide powders and homogenized by attrition milling. The mixed powders were pressed, and then sintered in hydrogen at 1$700^{\circ}C$ , or in oxidizing atmosphere using Controlled $CO_2$/CO mixtures at 125$0^{\circ}C$. Both density and microstructure of UO$_2$ are modified by the addition of dopants in reducing atmosphere. The sintered density is increased with $Ta_2O_5$ addition up to 0.33wt% and subsequently decreased with higher content of the additive. The effect on the densification and the gain growth are apparent with the addition of 0.24wt% $Nb_2O_5$. With 0.lwt% titania and 0.6wt% $Ta_2O_5$, the sintered density is decreased, but the grain size is increased. In oxidizing atmosphere, the grain sizes for UO$_2$ doped with the above additives are smaller than that for pure UO$_2$. The grain size of Ta or Nb-doped UO$_2$ is decreased with increasing $CO_2$/CO ratio, but that of pure UO$_2$or T-doped UO$_2$ is increased. A large portion of second phases is observed in UO$_2$ doped with 0.lwt% TiO$_2$ sintered in hydrogen atmosphere, while, in $CO_2$/CO atmospheres, the second phases or dopant agglomerates are not observed. For UO$_2$-4wt%CeO$_2$ mixed oxide, the effect of additives on the gain growth is not so much as that for the pure UO$_2$. This is attributed to the formation of clusters by dopant cations and Ce ions, so that the additives contribute to a lesser exent to the grain growth for the mixed oxide.

• Resources Recycling
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• v.19 no.5
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• pp.31-43
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• 2010

To fabricate porous SiC candle filter for filtration facility of the IGCC system, the candle type filter preforms were fabricated by ramming and vacuum extrusion process. A commercially available ${\alpha}$-SiC powders with various particle size were used as starting raw materials, and $44\;{\mu}m$ mullite, $CaCO_3$ powder were used as non-clay based inorganic sintering additive. The candle typed preforms by ramming process and vacuum extrusion were sintered at $1400^{\circ}C$ for 2h in air atmosphere. The effect of forming method and particle size of filter matrix on porosity, density, strength (flexural and compressive strength) and microstructure of the sintered porous SiC candle tilters were investigated. The sintered porous SiC filters which were fabricated by ramming process have more higher density and strength than extruded filter in same particle size of the matrix, and its maximum density and 3-point bending strength were $2.00\;g/cm^3$ and 45 MPa, respectively. Also, corrosion test of the sintered candle filter specimens by different forming method was performed at $600^{\circ}C$ for 2400h using IGCC syngas atmosphere for estimation of long-term reliability of the candle filter matrix.

• Korean Journal of Materials Research
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• v.2 no.6
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• pp.428-435
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• 1992

AISI 4600 Iron powder was mixed with 0~1.0% phosphor as F$e_2$P powder and/or 0~0.8% carbon as graphite powder in rotating mixer. Mixed powder was pressed 800MPa in double-punch mould. Compacts were sintered at 115$0^{\circ}C$for 30 min. in vacuum or mixed hydrogen and nitrogen gas. Sintered compacts were ground and polished, and etched by 2% nital etchant. The microstructure was observed by image analyzer and optical microscope. Density and microhardness were tested by ASTM B3l2 and Microvickers hardness tester. The results obtained were as follows : (1) As the amount of F$e_2$P powder increased, sintered microstructure showed more densified effect and the grain size was larger. (2) The shape of pore was rounded and the number of pore was decreased by F$e_2$P addition. But mean pore size was larger with F$e_2$P content. (3) Simultaneous alloying addition of F$e_2$P and graphite brought about larger grain growth than respective addition. (4) Sintering atmosphere did not affect the microstructure. (5) Hardness of sintered compact increased with phosphrous and carbon content.

• Journal of the Korean Ceramic Society
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• v.33 no.3
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• pp.285-292
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• 1996

The effect of AlN on the thermal conductivity of aluminum oxide pressurelessly sintered at nitrogen atmos-phere was investigated. Increasing aluminium nitride content up to 1~10 mol% the thermal conductivity of $Al_{2}O_{3}$-AlN system was singnificantly decreased and was constant with adding 20 and 25 mol% aluminium nitride. The thermal conuctivity of $Al_{2}O_{3}$ containing 1~10 mol% the thermal conductivity of $Al_{2}O_{3}$-AlN system was singificantly decreased and was constant with adding 20 and 25mol% aluminum nitride. The thermal conctivity of $Al_{2}O_{3}$ containing 1~10 mol% AlN showed a maximum at $1700^{\circ}C$ and decrea-sed with increasing sintering tempertures. This phenomenon was attributed to $\alpha$-$Al_{2}O_{3}$ and ALON formed by reacting $Al_{2}O_{3}$ with AlN up to $1700^{\circ}C$ and the secondary phases such as ${\gamma}$-ALON ($9Al_{2}O_{3}$.AlN)and $\Phi$($5Al_{2}O_{3}$.AlN) phase above $1750^{\circ}C$ The thermal conductivity of $Al_{2}O_{3}$ containing 20 and 25 mol% AlN showed maximum value at $1800^{\circ}C$ Both $\alpha$-$Al_{2}O_{3}$ and ALON existed up to $1600^{\circ}C$ value at $1800^{\circ}C$ Both $\alpha$-$Al_{2}O_{3}$ and ALON existed up to $1600^{\circ}C$ while only AlON phase existed above $1650^{\circ}C$.

• Journal of the Korean Ceramic Society
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• v.41 no.2
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• pp.125-130
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• 2004

The effect of SiC particle size on the microstructures and mechanical properties of A1$_2$O$_3$-SiC composite was investigated. Two types of SiC powders having average particle sizes of 0.15 ${\mu}{\textrm}{m}$ and 3 ${\mu}{\textrm}{m}$ were used. The grain growth in the specimen containing 0.15 ${\mu}{\textrm}{m}$ SiC was effectively inhibited due to the fine SiC particles. However, after the formation of some abnormal grains, fast and exaggerated grain growth occurred which led to the microstructure of large grains with irregular shape. Fracture strength decreased due to the abnormal large grains. On the other hand, for specimen containing 3 ${\mu}{\textrm}{m}$ SiC showed normal grain growth behavior from initial sintering stage. Large SiC particles, however, effectively inhibited exaggerated grain growth after nucleation of a few abnormal grains. As a consequence, microstructure consisted of homogeneous elongated grains. In the A1$_2$O$_3$-2.5SiC(0.15 ${\mu}{\textrm}{m}$)-2.5SIC(3 ${\mu}{\textrm}{m}$) composite fabricated by mixing the two types of SiC powder, abnormal grain growth occurred. However, the good fracture strength was maintained regardless of microstructural changes in this specimen.

• Journal of the Korean Ceramic Society
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• v.53 no.2
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• pp.258-262
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• 2016

The effect of zirconium dioxide ($ZrO_2$) on the properties of color conversion glasses was examined in the $BaO-ZnO-B_2O_3-SiO_2$ system. The difference in refractive index between glass and phosphor affect the optical properties of the color conversion glass because of light scattering. Reducing the difference in refractive index is a method to improve the luminous efficacy of color conversion glasses. As a reference, a type of glass that contains 25 mol% of each component was used. To increase the refractive index of the glass samples, the BaO content was increased from 25 to 40 mol%, and $ZrO_2$ was added at levels of 1, 3, and 5 mol%. Color conversion glasses were prepared by sintering a mixture of glass and 5 wt% $YAG:Ce^{3+}$ phosphor. As a result, the refractive index of the glass was found to be dependent on the BaO and $ZrO_2$ contents in the BaO-ZnO-$B_2O_3-SiO_2$ system. As the BaO and $ZrO_2$ contents were increased, the luminous efficacy of the color conversion glass was improved because the refractive index difference between the glass and the $YAG:Ce^{3+}$ phosphor decreased.

• Korean Journal of Materials Research
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• v.15 no.6
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• pp.413-418
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• 2005

Perovskite $Pb(Ni_{1/3}Nb_{2/3})O_3-Pb(Zr,Ti)O_3[PNN-PZT]$ ceramics were synthesized by conventional ceramic processing technique. In order to modify piezoelectric properties for sensor application in this system, NiO addition was considered to provide $Ni^{+2}$ as an acceptor, which was known to occupy with B site in the structure. The effect of NiO addition up to $8\;mol\%$ on the following piezoelectric properties as well as sintering properties was investigated. When NiO added more than $1\;mol\%$, average grain size was decreased and second phase was found to form. Moreover, the second phase caused decrease in relative dielectric constant $(\varepsilon_{33}T/\varepsilon0)$, electro-mechanical coupling factor $(k_p)$, and piezoelectric charge constant $(d_{33})$, while increasing mechanical quality factor $(Q_m)$. When $1\;mol\%$ NiO was added, density, dielectric properties and piezoelectric properties were abruptly increased.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.20 no.1
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• pp.25-29
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• 2010

Thulium oxide-doped barium titanate ceramics for MLCCs with perovskite structure were prepared by a sintering process at $1320^{\circ}C$ for 2 h in a reduced atmosphere. The effect of $Tm_2O_3$ addition on dielectric property of barium titanate ceramics has been studied in terms of their microstructures. Moreover, the phase identification of the dielectric specimens was conducted to define the secondary phase (pyrochlore). The specimen doped with 1 mol% $Tm_2O_3$ exhibited the highest dielectric constant. However, the dielectric constants of specimens with more than 2 mol% $Tm_2O_3$ to $BaTiO_3$ were the lower values than that of 1 mol% doped one. The grain size and the formation of pyrochlore phase associated with the dielectric properties were examined through morphology development and the structural analysis. Furthermore, these data were compared with the property of the dielectric material doped with $Er_2O_3$. It could be concluded that the dielectric property of ceramic capacitors were attributed to the change of pyrochlore phase and the tetragonality of $BaTiO_3$ with doping.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2008.06a
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• pp.52-52
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• 2008

The positive temperature coefficient of resistivity (PTCR) effect in (1-x)$BaTiO_3$ - $x(Bi_{0.5}K_{0.5})TiO_3$ doped with $Nb_2O_5$ was investigated. $(Bi_{1/2}K_{1/2})TiO_3$ (BKT) is more environment-friendly than $PbTiO_3$ in order to use in PTC thermistors. The incorporation of 1 mol% BKT to $BaTiO_3$ increased the Curie temperature (Tc) to $148^{\circ}C$. Doping of $Nb_2O_5$ to $Ba_{0.99}(Bi_{0.5}K_{0.5})_{0.01}TiO_3$ (BaBKT) ceramic has enhanced its PTCR effects. For the sample containing 0.025 mol% $Nb_2O_5$, it showed good PTCR properties; low resistivity at room temperature (${\rho}_r$) of 30 $\Omega{\cdot}cm$, a high PTCR intensity of approximately $3.3\times10^3$, implying the ratio of maximum resistivity to minimum resistivity (${\rho}_{max}/{\rho}_{min}$) in the measured temperature range, and a large resistivity temperature factor (a) of 13.7%/$^{\circ}C$ along with a high Curie temperature (Tc) of $167^{\circ}C$. In addition, the cooling rate of the samples during the sintering process had an influence on their PTCR behavior. All the samples showed the best ${\rho}_{max}/{\rho}_{min}$ ratio when they have cooled down at a rate of $600^{\circ}C$/min.

• Korean Journal of Materials Research
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• v.10 no.10
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• pp.671-676
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• 2000

$BaTiO_3$-based ceramics with partially oxidized Ti powders were prepared by sintering at $1350^{\circ}C$ for 1 h in v vacuum, and then heated in air. In this study, the effect of partially oxidized Ti powders on electrical properties and microstructures of $BaTiO_3$-based ceramics was investigated. It was found out that the semiconductive $BaTiO_3$-based ceramics beζame to show excellent PTCR (more than $10^5$) characteristic by adding 5~7 vol% of partially oxidized Ti powder. Also, it was found out that the sintered compact had extremely porous and fine-grained microstructure. The relative density and grain size of sintered compact with 5 vol% of partially oxidized Ti powders were 54% and $1.3\;{\mu\textrm{m}}$, respectively. The mechanism for the development of PTCR characteristic in $BaTiO_3$-based ceramics with partially oxidized Ti powders due to the adsorption of oxygen at grain boundaries, and could be explained, based on Heywang model.