• 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.

• 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.

• Journal of the Korean Ceramic Society
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• v.36 no.2
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• pp.186-192
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• 1999

Dang-Chin feldspar powder with the mean particle size of 9.1 $\mu\textrm{m}$ was added to the synthesized mullite powder with the particle size of +325∼-200 mesh and the powder compact was prepared by PLPP(pressureless powder packing method). Densification behaviors were observed in sintering temperature range of 1200∼1400$^{\circ}C$. The binder solution of 4% PVA was infilterated into packed powder to the suitable strength. The PLPP method makes it possible to form compacts without clay as plasticizer. Therfore there was no defect caused by phase transition after sintering. Additionally, we observed the dense microstructure by the melting of feldspar. When the mullite compacts with feldspar of 30% were sintered at 1300$^{\circ}C$-4 hrs, we obtained the dense microstructure with zero water absorption and porosity <1%. When these compacts were sintered longer than 4 hrs at 1300$^{\circ}C$ or higher than 1400$^{\circ}C$, the examggerated grain growth of mullite was observed.

• Proceedings of the Korean Powder Metallurgy Institute Conference
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• 1994.04c
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• pp.6-6
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• 1994

Har dmaterials such as cemented carbides with or without coated layer, cermets, ceramics and diamond or c-BN high pressure sintered compact are used for cutting tools, wear -resistant parts, rock drilling bits and/or high pressure vessels. These hardmaterials contain not only hard phase, but also second consituent as the element for forming ductile phase and/or sintering aid, and the mechanical properties of each material depend on (1) the amount of the second constituent as well as (2) the grain size of the hard phase. The hardness of each material mainly depends on these two factors. The fracture strength, however, largely depends on other microstructur a1 factors as well as the above two factors. For all hardmaterials, the fracture strength is consider ably affected by (3) the size of microstructur a1 defect which acts as the fracture source. In cemented carbides, the following factors which are generated mainly due to the addition of the second constituent are also important; (4) the variation of the carbon content in the normal phase region free from V-phase and graphite phase, (5) the precipitation of $Co_3$ during heating at about $800^{\circ}C$,(6) the domain size of binder phase, and (7) the formation of ${\beta}$-free layer or Co-rich layer near the surface of sintered compacts. For cemented carbides coated with thin hard substance, the important factors are as follows; (8) the kind of coated substance, (9) the formation of ${\eta}$-phase layer at the interface between coated layer and substrate, (10) the type of residual stress (tension or compression) in the coated layer which depends on the kind of coating method (CVD or PVD), and (11) the properties of the substrate, and (12) the combination, coherency and periodicity of multi-layers. In the lecture, the details of these factors and their effect on the strength will be explained.

• Korean Journal of Materials Research
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• v.20 no.3
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• pp.125-131
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• 2010

In this study, Ti powders were fabricated from Ti scrap by the Hydrogenation-Dehydrogenation (HDH) method. The Ti powders were prepared from the spark plasma sintering (SPS) and their microstructure was investigated. Hydrogenation reactions of Ti scrap occurred at near $450^{\circ}C$ with a sudden increase in the reaction temperature and the decreasing pressure of hydrogen gas during the hydrogenation process in the furnace. The dehydrogenation process was also carried out at $750^{\circ}C$ for 2 hrs in a vacuum of $10^{-4}$ torr. After the HDH process, deoxidation treatment was carried out with the Ca (purity: 99.5%) at $700^{\circ}C$ for 2 hrs in the vacuum system. It was found that the oxidation content of Ti powder that was deoxidized with Ca showed noticeably lower values, compared to the content obtained by the HDH process. In order to fabricate the Ti compacts, Ti powder was sintered under an applied uniaxial punch pressure of 40 MPa in the range of $900-1200^{\circ}C$ for 5 min under a vacuum of $10^{-4}$ torr. The relative density of the compact was 99.5% at $1100^{\circ}C$ and the tensile strength decreased with increasing sintering temperature. After sintering, all of the Ti compacts showed brittle fracture behavior, which occurred in an elastic range with short plastic yielding up to a peak stress. Ti improved the corrosion resistance of the Ti compacts, and the Pd powders were mixed with the HDH Ti powders.

• Journal of Powder Materials
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• v.22 no.2
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• pp.111-115
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• 2015

This study investigates the microstructure and thermal shock properties of polycrystalline diamond compact (PDC) produced by the high-temperature, high-pressure (HPHT) process. The diamond used for the investigation features a $12{\sim}22{\mu}m$- and $8{\sim}16{\mu}m$-sized main particles, and $1{\sim}2{\mu}m$-sized filler particles. The filler particle ratio is adjusted up to 5~31% to produce a mixed particle, and then the tap density is measured. The measurement finds that as the filler particle ratio increases, the tap density value continuously increases, but at 23% or greater, it reduces by a small margin. The mixed particle described above undergoes an HPHT sintering process. Observation of PDC microstructures reveals that the filler particle ratio with high tap density value increases direct bonding among diamond particles, Co distribution becomes even, and the Co and W fraction also decreases. The produced PDC undergoes thermal shock tests with two temperature conditions of 820 and 830, and the results reveals that PDC with smaller filler particle ratio and low tap density value easily produces cracks, while PDC with high tap density value that contributes in increased direct bonding along with the higher diamond content results in improved thermal shock properties.

• Korean Journal of Materials Research
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• v.19 no.2
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• pp.55-60
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• 2009

In this study, Ti powder was fabricated from Ti scrap by a hydrogenation-dehydrogenation (HDH) method. The Ti powders were compacted by Spark plasma sintering (SPS) and the microstructure and mechanical properties of the powders were investigated. A hydrogenation reaction of Ti scrap occurred at temperatures near $450^{\circ}C$ with a sudden increase in the reaction temperature and a decrease in the pressure of the hydrogen gas as measured in a furnace during the hydrogenation process. In addition, a dehydrogenation process was carried out at $750^{\circ}C$ for 2hrs in a vacuum of $10^{-4}torr$. The Ti powder sizes obtained by hydrogenation-dehydrogenation and mechanical milling processes were in the range of $1{\sim}90{\mu}m$ and $1{\sim}100{\mu}m$, respectively. To fabricate Ti compacts, Ti powders were sintered under an applied uniaxial punch pressure of 40 MPa at in a range of $900{\sim}1200^{\circ}C$ for 5 min. The relative density of a SPSed compact was 99.6% at $1100^{\circ}C$, and the tensile strength decreased with an increase in the sintering temperature. However, the hardness increased as the sintering temperature increased.

• Journal of Powder Materials
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• v.18 no.6
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• pp.488-495
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• 2011

This study was performed to fabricate the porous titanium foam by space holder method using NaCl powder, and to evaluate the effect of NaCl volume fractions (33.3~66.6 vol.%) on the porosities, compressive strength, Young's modulus and permeability. For controlling pore size, CP titanium and NaCl particles were sieved to different size range of 70~150 ${\mu}m$ and 300~425 ${\mu}m$ respectively. NaCl of green Ti compact was removed in water followed by sintered at $1200^{\circ}C$ for 2 hours. Total porosities of titanium foam were in the range of 38-70%. Pore shape was a regular hexahedron similar that of NaCl shape. Porous Ti body showed that Young's modulus and compressive strength were in the range of 0.6-6 GPa and 8-127 MPa respectively. It showed that pore size and mechanical properties of Ti foams was controllable by NaCl size and volume fractions.

• Journal of the Korean Ceramic Society
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• v.29 no.6
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• pp.419-430
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• 1992

Fabrication possibility of low-shrinkage alumina without oxidation and wetting agent was presented on the basis of observation about oxidation behavior, microstructure and physical characteristics of such reaction agents free Al2O3-Al system. The composition less than Al 10w/o where Al can act as a sintering agent for Al2O3 was excluded. Under the condition of present experiments oxidation of Al2O3-Al system was dependent not on holding time but mainly on oxidation temperature. In thes case of Al powder not comminuted effectively during powder mixing of Al2O3-Al, columnar structure which would act as a hindrance to the densification during sintering developed more during oxidation with higher Al contents, and which made the fabrication of low-shrinkage Al2O3 ceramics impossible. If Al powder was comminuted effectively due to co-mixed Al2O3 characteristics, densification was improved because of no columnar structure and made the fabrication of sintered body with -2.7% dimensional change and 81% relative density possible. As a result, it is possible to fabricate dense low-shrinkage Al2O3 ceramics without oxidation and wetting agent under conditions such as smaller particle size of Al, Al contents below 50v/o, higher green density of Al2O3-Al compact and the use of Al2O3 powder used for high-density ceramics.

• Journal of the Korean Ceramic Society
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• v.41 no.2
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• pp.131-135
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• 2004

The effect of Co additive on the microstructural evolusion of WC was investigated. A small amount of Co powder was placed on the top-center of the pure WC powder compact and then sintered at 1950$^{\circ}C$. During sintering some abnormally large WC grains of different size and shape observed depending on the distance from the liquid source. However, in the region far away from Co liquid source, it showed low densification and the grains of WC were very small and uniform in size. A small amount of Co liquid phase has a remarkable influence on the AGG of WC and it has been explained in terms of 2-D nucleation and growth mechanism.