• Archives of Metallurgy and Materials
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• v.63 no.3
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• pp.1485-1488
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• 2018

The effect of additives on the densification behavior and mechanical properties of pure and additive (Zr, B and Mg)-added silica ceramics were investigated for their application to the matrix phase of a silica fiber reinforced silica (SiO₂/SiO_2f) composite. The additives affected the rate of densification and crystallization (or transformation) of the amorphous silica. Among the compositions, pure silica ceramics sintered at 900℃ for 1 h showed the maximum flexural strength. Based on the results, SiO₂/SiO_2f was fabricated by a repeated vacuum-assisted infiltration method followed by the heat treatment at 900℃ for 1 h. The relative density of the composite was 78.2% with a flexural strength of 22.4 MPa. Fractography revealed that the composite was damaged by strong bonding at the fiber/matrix interface and the fracture of fiber.

• Journal of Ceramic Processing Research
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• v.21
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• pp.16-22
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• 2020

We fabricated SiC-TiC composites by hot-press sintering with aluminum and yttrium nitrate additive and evaluated crystal phase, relative density, microstructure, electrical resistivity and mechanical properties of the sintered body. And the effect of nitrate additive on the densification of SiC-TiC composites was compared with that of Al₂O₃ and Y₂O₃ additives. Because nitrate additives were uniformly dispersed in SiC and TiC mixture, it inhibited the growth of crystal grain between each other and formed fine and uniform microstructure, thereby improving mechanical properties and electrical resistivity. The electrical resistivity and flexural strength of the SiC-TiC composite with aluminum and yttrium nitrate additive were 2.3 Ω·cm and 652.3 MPa respectively.

• Journal of the Korean Ceramic Society
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• v.40 no.11
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• pp.1067-1072
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• 2003

The electrical stability of the varistor ceramics composed of Zn-Pr-Co-Cr-Dy oxides-based varistors was investigated at 0.0∼2.0 mol% Dy$_2$O$_3$ content under DC accelerated aging stress. The ceramic density was increased up to 0.5 mol% Dy$_2$O$_3$ whereas further addition of Dy$_2$O$_3$ decreased sintered ceramic density. The density sailently affected the stability due to the variation of conduction path. The nonlinearity of varistor ceramics was greatly improved above 45 in the nonlinear exponent and below nearly 1.0 ${\mu}$A by incorporating Dy$_2$O$_3$. Under 0.95 V$\_$1mA/150$^{\circ}C$/24 h stress state, the varistor ceramics doped with 0.5 mol% Dy$_2$O$_3$ exhibited the highest electrical stability, in which the variation rates of varistor voltage, nonlinear exponent, and leakage current were -0.9%, -14.4%, and +483.3%, respectively. The variation rates of relative permittivity and dissipation factor were +7.1% and +315.4%, respectively. The varistors with further addition of Dy$_2$O$_3$ exhibited very unstable state resulting in the thermal runaway due to low density.

• Journal of the Korean Ceramic Society
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• v.38 no.12
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• pp.1123-1131
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• 2001

The sintering behaviors of the renewed $Al_2$O$_3$ceramics were investigated as functions of the addition amount and particle size of recycling $Al_2$O$_3$powder, such as crushed powder of structural $Al_2$O$_3$ceramics and waste $Al_2$O$_3$adsorbent, were investigated. Pure $Al_2$O$_3$sample was fabricated by sintered at 1,$650^{\circ}C$ for 5h and it was crushed into powder (-40${\mu}{\textrm}{m}$and +40${\mu}{\textrm}{m}$ in particle size) by thermal shock treatment and crushing. Then, 10~50wt% of crushed $Al_2$O$_3$powder and waste $Al_2$O$_3$adsorbent were mixed with pure $Al_2$O$_3$powder and were subjected to re-sintering to renewed $Al_2$O$_3$sample. The density and the 3-point bending strength increased with increasing the sintering temperature without regard to the addition amount and particle size of recycling $Al_2$O$_3$powder, and that of the samples at the same sintering temperature decreased with increasing the addition amount and particle size of recycling $Al_2$O$_3$powder. Samples over 200 Mpa of 3-point bending strength were obtained by mixing ~30wt% of crushed $Al_2$O$_3$powder(-40${\mu}{\textrm}{m}$), ~20wt% of crushed $Al_2$O$_3$powder (+40${\mu}{\textrm}{m}$) and 10wt% of waste $Al_2$O$_3$adsorbent. 5~20wt% of waste glass powder containing renewed $Al_2$O$_3$samples for densification were fabricated by sintered at 1200~1$650^{\circ}C$ for 5h. The temperature of maximum density and 3-point bending strength decreased with increasing the addition amount of waste glass powder, however, these samples at above 140$0^{\circ}C$ showed lower density and bending strength than renewed $Al_2$O$_3$samples. The addition of waste glass powder did not improved the densification of renewed $Al_2$O$_3$sample.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.06a
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• pp.176-176
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• 2009

Influences of dose and particle size of $Al_2O_3$ filler and sintering time on the dielectric properties of $Al_2O_3$ filler/CaO-$Al_2O_3-SiO_2$ (CAS) glass composites were investigated with a view to applying the composites to the substrate material in low temperature co-firing ceramic (LTCC) technology. The increased addition of $Al_2O_3$ filler with the particle size of 1 ${\mu}m$ monotonically decreased the density of the sintered specimen at a given temperature, while sintering of the 10 wt% $Al_2O_3$ added specimen at $925^{\circ}C$ for 2 h demonstrated 96.0 % of the relative density, dielectric constant of 6.34, and quality factor of 2,760 GHz. As for the influence of the particle size of the $Al_2O_3$ filler, there existed an optimum particle size (30 ${\mu}m$) to ensure successful densification (96.5 %) of the 10 wt% $Al_2O_3$/CAS composites at $925^{\circ}C$ for 2 h, at which condition the specimen demonstrated dielectric constant of 5.45 and quality factor of 3,740 GHz. When the influence of the sintering time of the 10 wt% $Al_2O_3$) (30 ${\mu}m$) added specimen was investigated at the sintering temperature of $925^{\circ}C$, an overly long sintering time degraded dielectric properties due to the over-sintering and the significant growth of the second phase such as anorthite, while the sintering for 4 h demonstrated 96.58 % of the relative density, dielectric constant of 5.4, and quality factor of 4,050 GHz. These results demonstrate the feasibility of the investigated material as the substrate material in LTCC technology.

• Journal of Powder Materials
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• v.17 no.4
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• pp.312-318
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• 2010

High speed steels (HSS) were used as cutting tools and wear parts, because of high strength, wear resistance, and hardness together with an appreciable toughness and fatigue resistance. Conventional manufacturing process for production of components with HSS was used by casting. The powder metallurgy techniques were currently developed due to second phase segregation of conventional process. The powder injection molding method (PIM) was received attention owing to shape without additional processes. The experimental specimens were manufactured with T42 HSS powders (59 vol%) and polymer (41 vol%). The metal powders were prealloyed water-atomised T42 HSS. The green parts were solvent debinded in normal n-Hexane at $60^{\circ}C$ for 24 hours and thermal debinded at $N_2-H_2$ mixed gas atmosphere for 14 hours. Specimens were sintered in $N_2$, $H_2$ gas atmosphere and vacuum condition between 1200 and $1320^{\circ}C$. In result, polymer degradation temperatures about optimum conditions were found at $250^{\circ}C$ and $480^{\circ}C$. After sintering at $N_2$ gas atmosphere, maximum hardness of 310Hv was observed at $1280^{\circ}C$. Fine and well dispersed carbide were observed at this condition. But relative density was under 90%. When sintering at $H_2$ gas atmosphere, relative density was observed to 94.5% at $1200^{\circ}C$. However, the low hardness was obtained due to decarbonization by hydrogen. In case of sintering at the vacuum of $10^{-5}$ torr at temperature of $1240^{\circ}C$, full density and 550Hv hardness were obtained without precipitation of MC and $M_6C$ in grain boundary.

• Journal of the Korean Ceramic Society
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• v.40 no.10
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• pp.991-997
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• 2003

The effect of A1$_4$C$_3$ additive on the thermoelectric properties of SiC ceramics were studied. Porous SiC ceramics with 47∼59% relative density were fabricated by sintering the pressed $\alpha$-SiC powder compacts with A1$_4$C$_3$at 2100∼220$0^{\circ}C$ for 3 h in Ar atmosphere. Crystalline phases of the sintered bodies were identified by powder X-Ray Diffraction (XRD) and their microstructures were observed with a Scanning Electron Microscope (SEM). In the case of A1$_4$C$_3$ addition, the phase transformation of 6H-SiC to 4H-SiC could be observed during sintering. The Seebeck coefficient and electrical conductivity were measured at 550∼95$0^{\circ}C$ in Ar atmosphere. In the case of undoped specimens, the Seebeck coefficients were positive (p-type semiconducting) possibly due to a dominant effect of the acceptor impurities (Al, Fe) contained in the starting powder and electrical conductivity increased as increasing sintering temperature. Electrical conductivity of A1$_4$C$_3$doped specimen is larger than that of undoped specimen under the same condition, which might be due to the reverse phase transformation and increasing of carrier density. And the Seebeck coefficient of A1$_4$C$_3$ doped specimen is also larger than that of undoped specimen. The density of specimen, the amount of addition and sintering atmosphere had significant effects on the thermoelectric property.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.22 no.6
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• pp.305-311
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• 2012

In order to recycle the coal bottom ashes (denoted as BA) produced from a thermal power plant, the artificial aggregates (denoted as AAs) containing BA and red clay were manufactured, and the physical properties of AAs were studied as a function of particle size of BA and batch compositions. As-received BA had 38 wt% coarse particles of above 2 mm and many unburned carbon mass and porous slag particles were co-existed. So the two particle sizes of BA, the fine (< 100 ${\mu}m$) and coarse (< 2 mm), were prepared by milling and screening process. The AAs containing fine BA sintered at $1100{\sim}1200^{\circ}C$ had the higher bulk density and lower water absorption compared to the specimen made of coarse BA. The inside core of AAs manufactured by using coarse BA showed nonuniform and porous microstructure, while the AAs made of fine BA had a uniform and dense microstructure. In this research, the AAs containing BA and red clay with various bulk density (1.2~1.7) and water absorption (13~21 %) could be manufactured by controlling the particle size of BA and batch compositions, so the AAs of various physical properties could be applied to the wide fields such as construction/building materials in near future.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.16 no.7
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• pp.4835-4841
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• 2015

The effect of particle size and compaction pressure on the thermoelectric properties of n-type $FeSi_2$ was investigated. The starting powders with various particle size were pressed into a compact (compaction pressure; $70{\sim}220kg/cm^2$). The compact specimens were sintered at 1473 K for 7 h and annealed at 1103 K for 100 h under Ar atmosphere to transform to the semiconducting ${\beta}$-phase. The microstructure and phases of the specimens were observed by SEM, XRD and EDS. The electrical conductivity and Seebeck coefficient were measured simultaneously for the same specimen at r.t.~1023 K in Ar atmosphere. The electrical conductivity increased with decreasing particle size and hence the increases of relative density of the sintered body and the amount of residual metallic phase ${\varepsilon}$-FeSi due to a increase of the electrical conductivity. The Seebeck coefficient exhibited the maximum value at about 700~800 K and decreased with decreasing particle size. This must be due to a increase of residual metallic phase ${\varepsilon}$-FeSi. On the other hand, the change of compaction pressure appeared to have little effect on the thermoelectric properties. Consequently, the power factor would be affected more by particle size than compaction pressure.

• Journal of the Korean Ceramic Society
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• v.39 no.1
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• pp.68-73
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• 2002

The dielectric properties and synthesis of $LaAlO_3$ ceramics from mixtures of $La_2O_3-Al_2O_3$ (LAO) and $La_2O_3-Al(OH)_3$(LAH) via grinding process were investigated. The single phase $LaA1O_3$of LAO and LAH powders were formed at $1300^{\circ}C$ and $1000^{\circ}C$, respectively. A non-reacted $La_2O_3$ existing in calcined powder was changed to La(OH)$_3$by moisture in the air, and their samples were worse than those of the samples made from a $LaA1O_3$single phase powder. The densities of LAO samples sintered at 150$0^{\circ}C$ for 4 h and LAH samples sintered at $1400^{\circ}C$ for 4 h were 97.3% and 98.3% of theory density, respectively. Grains of LAH sample showed uniformity and their sizes were 0.75 ${\mu}{\textrm}{m}$, and LAO samples showed non-uniformity and their sizes were 4-5 ${\mu}{\textrm}{m}$. Dielectric constant of LAO and LAH samples were the same value (≒22), however dielectric loss of LAH sample (0.0003) were lower than that of LAO sample(0.001)due to grain size.