Search

The effects of fabrication method on the microstructure and sinterability of W-1 wt% Y2O3 were analyzed. W composite powders dispersed with Y2O3 particles were synthesized by the ultrasonic spray pyrolysis process or the ultrasonic spray pyrolysis/polymer solution process. A dense composite was fabricated by a combination of spark plasma sintering and final hot isostatic pressing. The powder synthesized by the ultrasonic spray pyrolysis had fine dispersed particles on the surface of the cubic powder and was composed of W, Y2O3 and W-oxides. On the other hand, in the case of the ultrasonic spray pyrolysis/polymer solution process, the nano-sized particles formed agglomerates and existed only as pure W and Y2O3 phases. All the sintered compacts treated with HIP showed an increase in relative density, and the sintered compacts of the powder synthesized by the ultrasonic spray pyrolysis/polymer solution process showed a maximum relative density of 97~99% and a fine grain size. The change in microstructure with different powder processing was explained by the presence of W-oxide and the size and distribution of Y2O3 particles. The Vickers hardness of the sintered compact reached the largest value of about 5 GPa in the powder synthesized by the ultrasonic spray pyrolysis/polymer solution process, which was interpreted to be a result of the relatively high density and decreased grain size.

The nanostructured rutile TiO 2 that have large specific surface area was formed on Ti powder through thermal oxidation. Thermal oxidation of Ti powder by using an external oxygen source was shown to be a simple and facile method to synthesize nanostructured rutile TiO 2 . Raw Ti powder was etched by using 30% HCl solution to control the surface morphology of Ti and to increase the surface area for effective diffusion of oxygen source. Then, etched powders were thermally oxidized at 750 and 850 °C with ethanol as the oxygen source. Coral-like nanostructured rutile TiO 2 of size 500 nm was formed at 850 °C from Ti powder etched for more than 20 min. The specific surface area of the resulting powder, which consisted of intermediate oxide (Ti 2 O) cores covered with nanostructured rutile TiO 2 , was 48 times that of the raw Ti powder.

The fabrication of the intermetallic phase T2-Mo3Si with continuous matrix of α-Mo was attempted with the combination process of high energy ball milling, pulverization of arc-melted ingot, addition of Mo by hydrogen reduction of MoO3 and spark plasma sintering processes. High energy ball milling or arc melting of Mo-16.7Si-16.7B (at %) powders were performed to obtain to intermetallic phase T2 and Mo3Si. The Mo phase of 57 vol% distributed intermetallic compound powders were prepared by hydrogen reduction of MoO3 and further mixing of elemental Mo powders. X-ray diffractometry analysis revealed that the intermetallic phase T2-Mo3Si can be produced by the pulverization process of arc-melted ingot. Hydrogen reduction of 1 vol% MoO3 mixed intermetallic powder followed by further addition of Mo powders was a more adequate method enabling the homogeneous distribution of the Mo phase than that of added MoO3 powders with total amount. The powder mixture was successfully consolidated by spark plasma sintering yielding a sound microstructure comprising the intermetallic phase T2-Mo3Si uniformly distributed in a continuous matrix of α-Mo.

In order to examine how the solid-liquid interface responds to temperature variation depending on the materials characteristics, i.e. faceted phase or nonfaceted phase, the moving solid-liquid interface of transparent organic material, as a model substance for metallic materials (pivalic acid, camphene, salol, and camphor-50wt% naphthalene) was observed in-situ. Plots of the interface movement distance against time were obtained. The solid-liquid interface of the nonfaceted phase is atomically rough; it migrates in continuous mode, giving smooth curves of the distance-time plot. This is the case for pivalic acid and camphene. It was expected that the faceted phases would show different types of curves of the distance-time plot because of the atomically smooth solid-liquid interface. However, salol (faceted phase) shows a curve of the distance-time plot as smooth as that of the nonfaceted phases. This indicates that the solid-liquid interface of salol migrates as continuously as that of the nonfaceted phases. This is in contrast with the case of naphthalene, one of the faceted phases, for which the solidliquid interface migrates in “stop and go” mode, giving a stepwise curve of the distance-time plot.

이 논문은 2015년도 정부 (미래창조과학부)의 재원으로 한국 연구 재단의 지원을 받아 수행된 연구임(No.2012R1A2A2A02046179). 본 연구는 2015년도 산업통상자원부의 재원으로 한국에너지기술평가원(KETEP)의 지원을 받아 수행한 연구 과제입니다(No. 20154030200900).

Microstructural examination of the Nb-Si-B alloys at Nb-rich compositions is performed. The Nb-rich corner of the Nb-Si-B system is favorable in that the constituent phases are Nb (ductile and tough phase with high melting temperature) and T2 phase (very hard intermetallic compound with favorable oxidation resistance) which are good combination for high temperature structural materials. The samples containing compositions near Nb-rich corner of the Nb- Si-B ternary system are prepared by spark plasma sintering (SPS) process using T2 and Nb powders. T2 bulk phase is made in arc furnace by melting the Nb slug and the Si-B powder compact. The T2 bulk phase was subsequently ballmilled to powders. SPS is performed at 1300oC and 1400oC, depending on the composition, under 30 MPa for 600s, to produce disc-shaped specimen with 15 mm in diameter and 3 mm high. Hardness tests (Rockwell A-scale and micro Vickers) are carried out to estimate the mechanical property.

In general, size, shape and dispersion of phases in alloys significantly affect mechanical properties. In this study, the mechanical properties of Mo-Si-B alloys were experimentally investigated with regards to the refinement of intermetallic compound. To confirm the size effect of the intermetallic compound phases on mechanical properties, two differently sized intermetallic compound powders consisting Mo5SiB2 and Mo3Si were fabricated by mechano-chemical process and high-energy ball milling. A modified powder metallurgy method was used with core-shell intermetallic powders where the intermetallic compound particles were the core and nano-sized Mo particles which formed by the hydrogen reduction of Mo oxide were the shells, leading to the microstructures with uniformly distributed intermetallic compound phases within a continuous α-Mo matrix phase. Vickers hardness and fracture toughness were measured to examine the mechanical properties of sintered bodies. Vickers hardness was 472 Hv for the fine intermetallic compound powder and 415 Hv for the coarse intermetallic compound powder. The fracture toughness was 12.4 MPa·√m for the fine IMC powders and 13.5 MPa·√m for the coarse intermetallic compound powder.

Porous Mo–30 wt.% W alloy with controlled pore structure was produced by a freeze drying and sintering process. Unidirectional freezing of MoO 3 –WO 3 /camphene slurry with dispersion stability was done at − 25 °C. Pores were generated by sublimation of the camphene vehicle crystals, and subsequent sintering in hydrogen atmosphere was performed to reduce the oxide powders and consolidate the walls, leading to a porous body with unidirectionally aligned channels. The sintered bodies with Mo–W alloy phase showed macroscopic aligned pore with a size of 200 μm. Microstructural observation revealed that the dendrite structure with small pores having several microns in size was presented in the internal wall. The strut between dendrite pores consisted of very fine particles with the size of about 100 nm and showed that the degree of necking between the particles was not extensive. The formation of small pores was discussed in terms of solidification behavior of the liquid camphene with solid particles.

The Mo–Si–B alloy has been in the spotlight as a promising alternative to Ni-based superalloys due to its superior thermal resistance and mechanical properties. In particular, the mechanical properties of the Mo–Si–B alloy are sensitively influenced by the mutual distribution of the ductile α-Mo phase and brittle intermetallic compound phases such as Mo 5 SiB 2 (T2) and Mo 3 Si (A15). For the Mo–Si–B alloy to possess an excellent fracture toughness, ductile α-Mo phase should form a continuous phase with the uniformly distributed intermetallic compound phases. In this study, a core–shell intermetallic powder composed of nano-sized α-Mo particles surrounding intermetallic compound particles was fabricated using Mo–Si–B intermetallic compound powders prepared through the mechano-chemical method and the chemical vapor transport (CVT) process of Mo oxide powder. Also, the Mo–Si–B alloy, which has a continuous α-Mo phase uniformly embedded by intermetallic compound phases, was fabricated by the pressureless sintering of the core–shell intermetallic powder.

The effect of sublimable vehicle composition in the camphor-naphthalene system on the pore structure of porous Cu-Ni alloy is investigated. The CuO-NiO mixed slurries with hypoeutectic, eutectic and hypereutectic compositions are frozen into a mold at . Pores are generated by sublimation of the vehicles at room temperature. After hydrogen reduction at and sintering at for 1 h, the green body of CuO-NiO is completely converted to porous Cu-Ni alloy with various pore structures. The sintered samples show large pores which are aligned parallel to the sublimable vehicle growth direction. The pore size and porosity decrease with increase in powder content due to the degree of powder rearrangement in slurry. In the hypoeutectic composition slurry, small pores with dendritic morphology are observed in the sintered Cu-Ni, whereas the specimen of hypereutectic composition shows pore structure of plate shape. The change of pore structure is explained by growth behavior of primary camphor and naphthalene crystals during solidification of camphor-naphthalene alloys.

【In this study, titanium(Ti) meshes and porous bodies are employed to synthesize carbon nanotubes(CNTs) using methane( $CH_4$ ) gas and camphene solution, respectively, by chemical vapor deposition. Camphene is impregnated into Ti porous bodies prior to heating in a furnace. Various microscopic and spectroscopic techniques are utilized to analyze CNTs. It is found that CNTs are more densely and homogeneously populated on the camphene impregnated Ti-porous bodies as compared to CNTs synthesized with methane on Ti-porous bodies. It is elucidated that, when synthesized with methane, few CNTs are formed inside of Ti porous bodies due to methane supply limited by internal structures of Ti porous bodies. Ti-meshes and porous bodies are found to be multi-walled with high degree of structural disorders. These CNTs are expected to be utilized as catalyst supports in catalytic filters and purification systems.】

【Porous W with controlled pore structure was fabricated by thermal decomposition and hydrogen reduction process of PMMA beads and $WO_3$ powder compacts. The PMMA sizes of 8 and $50{\mu}m$ were used as pore forming agent for fabricating the porous W. The $WO_3$ powder compacts with 20 and 70 vol% PMMA were prepared by uniaxial pressing and sintered for 2 h at $1200^{\circ}C$ in hydrogen atmosphere. TGA analysis revealed that the PMMA was decomposed at about $400^{\circ}C$ and $WO_3$ was reduced to metallic W at $800^{\circ}C$ . Large pores in the sintered specimens were formed by thermal decomposition of spherical PMMA, and their size was increased with increase in PMMA size and the amount of PMMA addition. Also the pore shape was changed from spherical to irregular form with increasing PMMA contents due to the agglomeration of PMMA in the powder mixing process.】

【The powder injection molding process having advantages in manufacturing three-dimensional precision parts essentially requires a debinding process before sintering to remove the binders used for preparing feedstock. In this study, powder injection molding of translucent alumina was performed, and carbon dioxide ( $CO_2$ ) is used as a supercritical fluid that makes it possible to remove a large amount of binder, which is paraffin wax. The relationship between the optical property of translucent alumina and the debinding condition (temperature and pressure) of supercritical $CO_2$ was investigated. As temperature and pressure increased, extraction rate of the binder showed rising tendency and average grain size after sintering process was relatively fine. On the other hand, optical transmittance was reduced. As a result, the debinding condition at $50^{\circ}C$ and 20 MPa that represents the lowest extraction rate, $8.19{\times}10^{-3}m^2/sec$ , corresponds to the largest grain size of $14.7{\mu}m$ and the highest optical transmittance of 45.2%.】

Porous Cu–Sn with controlled pore characteristics was synthesized by a freeze-drying and sintering process. CuO and SnO2 powders were selected as the source material, which are hydrogen-reduced to metallic Cu–Sn in the sintering stage. Camphene-based CuO–SnO2 slurries were prepared by milling at 60 °C with a small amount of dispersant. Freezing of a slurry was done at −40 °C with unidirectional control of the growth direction of the camphene. Pores were generated by sublimation of the camphene. The green bodies were sintered at 650 °C under a hydrogen atmosphere. The sintered bodies with Cu3Sn, Cu6Sn5 and β-Sn phases showed macroscopic aligned pores with an average size of 200 μm. The internal wall of the macroscopic pores is also porous, and there are a number of many small pores in it. The formation of macroscopic and microscopic pores was discussed in terms of solidification behavior of the liquid with foreign particles.

The 304 stainless steel powders were prepared by high energy ball milling and subsequently sintered byspark plasma sintering, and the microstructural characteristics and micro-hardness were investigated. The initial size ofthe irregular shaped 304 stainless steel powders was approximately 42 µm. After high energy ball milling at 800 rpmfor 5h, the powders became spherical with a size of approximately 2 µm, and without formation of reaction compounds.From TEM analysis, it was confirmed that the as-milled powders consisted of the aggregates of the nano-sized particles.As the sintering temperature increased from 1073K to 1573K, the relative density and micro-hardness of sintered sampleincreased. The sample sintered at 1573K showed the highest relative density of approximately 95% and a micro-hard-ness of 550 Hv.Keywords: 304 stainless steel powder, High energy ball milling, Spark plasma sintering, Micro-hardness, Microstructure ······························································································································· ·································································································

This paper describes the effect of the solidification behavior of camphene on the pore structure of porous W fabricated via a freeze-drying process. Camphene slurries with 10 and 15 vol% WO3 were prepared by milling at 50 °C with a small amount of dispersant. Porous W with unidirectionally aligned pore channels was prepared by freeze-drying of slurry and heat treatment in a hydrogen atmosphere. The sintered specimen with 10 vol% WO3 showed large pores with a size of about 100 μm which were aligned parallel to the growth direction of camphene. The internal wall of large pores had relatively small pores. In the slurry with 15 vol% WO3, large pores were not observed in the sintered body, and only small pores existed. The difference in the pore size in the two specimens is explained on the basis of the growth characteristics of the camphene dendrite depending on the amount of particles in the slurry. †(Received June 27, 2013)

Mo–Si–B alloys have attracted attention as alternatives to Ni-based superalloys for gas turbine blade due to their high melting point and excellent oxidation resistance, as well as mechanical properties including a higher service temperature than that of Ni-based superalloys. In previous research, most Mo–Si–B alloys have been fabricated by casting and mechanical alloying method, which have some problems concerning the presence of non-equilibrium phase, difficulty in controlling the microstructure, impurities during the process, and so on. In this work, mechano-chemical process to fabricate two phases intermetallic powders (Mo5SiB2 and Mo3Si) was carried out as an initial stage of research for controlling the microstructure of Mo–Si–B alloys. This method uses starting powders of MoO3, Si3N4 and BN and includes the stages of ball-milling for mechanical mixing and refinement and chemical reactions during reduction and continuative heat treatment under hydrogen atmosphere.

Freeze drying for porous Mo was accomplished by using powder as the source and camphor-naphthalene eutectic system as the sublimable material. Eutectic composition of camphor-naphthalene slurries with the initial content of 5 vol%, prepared by milling at with a small amount of oligomeric dispersant, was frozen at . The addition of dispersant showed improvement of dispersion stability in slurries. Pores were generated subsequently by sublimation of the camphor-naphthalene during drying in air for 48 h. To convert the to metallic Mo, the green body was hydrogen-reduced at , and sintered at for 2 h. The sintered samples, frozen by heated Teflon cylinder, showed large pores with the size of about 40 which were aligned parallel to the sublimable vehicles growth direction. The formation of unidirectionally aligned pores is explained by the rejection and accumulation of solid particles in the serrated solid-liquid interface.

Catalytic syntheses of CNTs on the pristine Ti mesh, the pristine Ti plate and the etched Ti plate have been conducted using thermal chemical vapor deposition (CVD) with Fe catalysts. Surface of the pristine Ti plate was etched in a sulfuric acid (H₂SO₄) solution to facilitate the uniform dispersion of Fe catalysts. The surface of Ti substrates, the distribution and the composition of catalysts, and the structure and dispersion of the CNTs were examined using Scanning electron microscope (SEM), transmission electron microscope (TEM), atomic force microscope (AFM), electron probe micro-analysis (EPMA) and Micro-Raman spectroscopy. Fe catalysts were dispersed uniformly on the surface of the etched Ti plate indicating that Surface modification by etching was effective. CNTs on the pristine Ti mesh and the etched Ti plate are more densely populated and have smaller diameters than CNTs on the pristine Ti plate. These results can be attributed to smaller Fe catalysts more homogeneously distributed on the pristine Ti mesh and the etched Ti plate. The calculated I(G)/I(D) ratios of 1.02 and 0.97 for CNTs on the pristine Ti mesh and the etched Ti plate, respectively, indicate a high degree of structural disorders on CNTs.

Porous Cu with macroscopically aligned channels was synthesized using a freeze-drying process. Camphene-based CuO slurry was prepared by milling at 60 °C with a small amount of dispersant. Freezing of a slurry was done at −25 °C while unidirectionally controlling the growth direction of the camphene. Pores were generated subsequently by sublimation of the camphene during drying. The green body was hydrogen-reduced at 300 °C for 30 min, and sintered in the furnace at 700 °C for 1 h under a hydrogen atmosphere. Microstructural observation reveals that all of the sintered samples are composed of only Cu phase and show macroscopic open pores with an average size of 100 μm which are aligned along its macroscopic growth direction. The internal wall of the macroscopic aligned pore shows relatively small pores due to the traces of the camphene left between the concentrated Cu particles on the internal wall. Increase in the porosity and pore size with increasing camphene content was explained by the change of the growth behavior of the camphene crystals.

Transparent organic materials have been frequently used as an analog of the solidifying metallic materials, because their transparency permits an in-situ observation of the microstructural development during solidification through optical microscopy. Pivalic acid (PVA)-ethanol system showing an anisotropic property in solid-liquid interfacial energy and interface kinetics was adopted in the present experiment, and the detailed experiments performed are as follows: (1) variation of dendrite tip temperature with growth velocity, (2) correlation between primary dendrite arm spacing () and the growth orientation away from the heat flow direction (tilt angle: ), (3) variation of dendrite tip radius (R) with growth velocity (V), (4) dendrite tip stability parameter () and its dependence on the concentration. Concerning the correlation between the dendrite tip temperature and growth velocity the present result is well suited to Hunt-Lu equation. As the tilt angle increases, the average primary dendrite spacing tends to increase.

In order to fabricate the porous metal with controlled pore characteristics, unique processing by using metal oxide powder as the source and camphene as the sublimable material is introduced. CuO powder was selected as the source for the formation of Cu metal via hydrogen reduction. Camphene-based CuO slurry, prepared by milling at with a small amount of dispersant, was frozen at . Pores were generated subsequently by sublimation of the camphene. The green body was hydrogen-reduced at for 30 min, and sintered at for 1 h. Microstructural analysis revealed that the sintered Cu showed aligned large pore channels parallel to the camphene growth direction, and fine pores are formed around the large pore. Also, it showed that the pore size was controllable by the slurry concentration.