The effect of the mixing method on the characteristics of hybrid-structure W powder with nano and micro sizes is investigated. Fine WO3 powders with sizes of ~0.6 μm, prepared by ball milling for 10 h, are mixed with pure W powder with sizes of 12 μm by various mixing process. In the case of simple mixing with ball-milled WO3 and micro sized W powders, WO3 particles are locally present in the form of agglomerates in the surface of large W powders, but in the case of ball milling, a relatively uniform distribution of WO3 particles is exhibited. The microstructural observation reveals that the ball milled WO3 powder, heat-treated at 750oC for 1 h in a hydrogen atmosphere, is fine W particles of ~200 nm or less. The powder mixture prepared by simple mixing and hydrogen reduction exhibits the formation of coarse W particles with agglomeration of the micro sized W powder on the surface. Conversely, in the powder mixture fabricated by ball milling and hydrogen reduction, a uniform distribution of fine W particles forming nano-micro sized hybrid structure is observed.

The effects of the heat treatment temperature and of the atmosphere on the dehydrogenation and hydrogen reduction of ball-milled TiH2-WO3 powder mixtures are investigated for the synthesis of Ti-W powders with controlled microstructure. Homogeneously mixed powders with refined TiH2 particles are successfully prepared by ball milling for 24 h. X-ray diffraction (XRD) analyses show that the powder mixture heat-treated in Ar atmosphere is composed of Ti, Ti2O, and W phases, regardless of the heat treatment temperature. However, XRD results for the powder mixture, heat-treated at 600oC in a hydrogen atmosphere, show TiH2 and TiH peaks as well as reaction phase peaks of Ti oxides and W, while the powder mixture heat-treated at 900oC exhibits only XRD peaks attributed to Ti oxides and W. The formation behavior of the reaction phases that are dependent on the heat treatment temperature and on the atmosphere is explained by thermodynamic considerations for the dehydrogenation reaction of TiH2, the hydrogen reduction of WO3 and the partial oxidation of dehydrogenated Ti.

An optimum route to synthesize Ti-Mo system powders is investigated by analyzing the effect of the heat treatment atmosphere on the formation of the reaction phase by dehydrogenation and hydrogen reduction of ball-milled TiH2-MoO3 powder mixtures. Homogeneous powder mixtures with refined particles are prepared by ball milling for 24 h. XRD analysis of the heat-treated powder in a hydrogen atmosphere shows TiH2 and MoO3 peaks in the initial powders as well as the peaks corresponding to the reaction phase species, such as TiH0.7, TiO, MoO2, Mo. In contrast, powder mixtures heated in an argon atmosphere are composed of Ti, TiO, Mo and MoO3 phases. The formation of reaction phases dependent on the atmosphere is explained by the partial pressure of H2 and the reaction temperature, based on thermodynamic considerations for the dehydrogenation reaction of TiH2 and the reduction behavior of MoO3.

This study is performed to fabricate a Ti porous body by freeze drying process using titanium hydride (TiH2) powder and camphene. Then, the Ti porous body is employed to synthesize carbon nanotubes (CNTs) using thermal catalytic chemical vapor deposition (CCVD) with Fe catalyst and methane (CH4) gas to increase the specific surface area. The synthesized Ti porous body has 100 μm-sized macropores and 10-30 μm-sized micropores. The synthesized CNTs have random directions and are entangled with adjacent CNTs. The CNTs have a bamboo-like structure, and their average diameter is about 50 nm. The Fe nano-particles observed at the tip of the CNTs indicate that the tip growth model is applicable. The specific surface area of the CNT-coated Ti porous body is about 20 times larger than that of the raw Ti porous body. These CNT-coated Ti porous bodies are expected to be used as filters or catalyst supports.

Cu-30 vol% SiC composites with relatively densified microstructure and a sound interface between the Cu and SiC phases were obtained by pressureless sintering of PCS-coated SiC and Cu powders. The coated SiC powders were prepared by thermal curing and pyrolysis of PCS. Thermal curing at 200 oC was performed to fabricate infusible materials prior to pyrolysis. The cured powders were heated treated up to 1600 oC for the pyrolysis process and for the formation of SiC crystals on the surface of the SiC powders. XRD analysis revealed that the main peaks corresponded to the α-SiC phase; peaks for β-SiC were newly appeared. The formation of β-SiC is explained by the transformation of thermally-cured PCS on the surface of the initial α-SiC powders. Using powder mixtures of coated SiC powder, hydrogen-reduced Cu-nitrate, and elemental Cu powders, Cu-SiC composites were fabricated by pressureless sintering at 1000 oC. Microstructural observation for the sintered composites showed that the powder mixture of PCS-coated SiC and Cu exhibited a relatively dense and homogeneous microstructure. Conversely, large pores and separated interfaces between Cu and SiC were observed in the sintered composite using uncoated SiC powders. These results suggest that Cu-SiC composites with sound microstructure can be prepared using a PCS coated SiC powder mixture.

The present study demonstrates the effect of freezing conditions on the pore structure of porous Cu-10 wt.% Sn prepared by freeze drying of CuO-SnO2/camphene slurry. Mixtures of CuO and SnO2 powders are prepared by ball milling for 10 h. Camphene slurries with 10 vol.% of CuO-SnO2 are unidirectionally frozen in a mold maintained at a temperature of -30oC for 1 and 24 h, respectively. Pores are generated by the sublimation of camphene at room temperature. After hydrogen reduction and sintering at 650oC for 2 h, the green body of the CuO-SnO2 is completely converted into porous Cu-Sn alloy. Microstructural observation reveals that the sintered samples have large pores which are aligned parallel to the camphene growth direction. The size of the large pores increases from 150 to 300 μm with an increase in the holding time. Also, the internal walls of the large pores contain relatively small pores whose size increases with the holding time. The change in pore structure is explained by the growth behavior of the camphene crystals and rearrangement of the solid particles during the freezing process.

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.

Porous W with spherical and directionally aligned pores was fabricated by the combination of sacrificial fugitives and a freeze-drying process. Camphene slurries with powder mixtures of WO3 and spherical PMMA of 20 vol% were frozen at −25 oC and dried for the sublimation of the camphene. The green bodies were heat-treated at 400 oC for 2 h to decompose the PMMA; then, sintering was carried out at 1200 oC in a hydrogen atmosphere for 2 h. TGA and XRD analysis showed that the PMMA decomposed at about 400 oC, and WO3 was reduced to metallic W at 800 oC without any reaction phases. The sintered bodies with WO3-PMMA contents of 15 and 20 vol% showed large pores with aligned direction and small pores in the internal walls of the large pores. The pore formation was discussed in terms of the solidication behavior of liquid camphene with solid particles. Spherical pores, formed by decomposition of PMMA, were observed in the sintered specimens. Also, microstructural observation revealed that struts between the small pores consisted of very fine particles with size of about 300 nm.

The effect of sublimable vehicle composition in the camphor-naphthalene system on the pore structure ofporous Cu-Ni alloy is investigated. The CuO-NiO mixed slurries with hypoeutectic, eutectic and hypereutectic compo-sitions are frozen into a mold at -25oC. Pores are generated by sublimation of the vehicles at room temperature. Afterhydrogen reduction at 300oC and sintering at 850oC for 1 h, the green body of CuO-NiO is completely converted toporous Cu-Ni alloy with various pore structures. The sintered samples show large pores which are aligned parallel to thesublimable vehicle growth direction. The pore size and porosity decrease with increase in powder content due to thedegree of powder rearrangement in slurry. In the hypoeutectic composition slurry, small pores with dendritic morphologyare observed in the sintered Cu-Ni, whereas the specimen of hypereutectic composition shows pore structure of plateshape. The change of pore structure is explained by growth behavior of primary camphor and naphthalene crystals dur-ing solidification of camphor-naphthalene alloys.

Fe-base superalloy powders with Y2O3 dispersion were prepared by high energy ball milling, followed by sparkplasma sintering for consolidation. High-purity elemental powders with different Fe powder sizes of 24 and 50mm were usedfor the preparation of Fe-20Cr-4.5Al-0.5Ti-O.5Y2O3 powder mixtures (wt%). The milling process of the powders was carriedout in a horizontal rotary ball mill using a stainless steel vial and balls. The milling times of 1 to 5 h by constant operation(350 rpm, ball-to-powder ratio of 30:1 in weight) or cycle operation (1300 rpm for 4 min and 900 rpm for 1 min, 15:1) wereapplied. Microstructural observation revealed that the crystalline size of Fe decreased with an increase in milling time by cyclicoperation and was about 15nm after 3 h, forming a FeCr alloy phase. The cyclic operation had an advantage over constantmilling in that a smaller-agglomerated structure was obtained. The milled powders were sintered at 1100oC for 30 min invacuum. With an increase in milling time, the sintered specimen showed a more homogeneous microstructure. In addition, ahomogenous distribution of Y-compound particles in the grain boundary was confirmed by EDX analysis.

The synthesis of NiTi alloy powders by hydrogen reduction and dehydrogenation process of NiO and TiH2 powder mixtures is investigated. Mixtures of NiO and TiH2 powders are prepared by simple mixing for 1 h or ball milling for 24 h. Simple-mixed mixture shows that fine NiO particles are homogeneously coated on the surface of TiH2 powders, whereas ball milled one exhibits the morphology with mixing of fine NiO and TiH2 particles. Thermogravimetric analysis in hydrogen atmosphere reveals that the NiO and TiH2 phase are changed to metallic Ni and Ti in the temperature range of 260 to 290oC and 553 to 639oC, respectively. In the simple-mixed powders by heat-up to 700oC, agglomerates with solid particles and solidified liquid phase are observed, and the size of agglomerates is increased at 1000oC. From the XRD analysis, the presence of liquid phase is explained by the formation and melting of NiTi2 intermetallic compound due to an exothermic reaction between Ni and Ti. The simple-mixed powders, heated to 1000oC, lead to the formation of NiTi phase but additional Ni-, Ti-rich and Ti-oxide phases. In contrast, the microstructure of ball-milled powders is characterized by the neck-grown particles, forming Ni3Ti, Ti-oxide and unreacted Ni phase.

Porous W with controlled pore structure was fabricated by thermal decomposition and hydrogen reduction process of PMMA beads and WO3 powder compacts. The PMMA sizes of 8 and 50 μm were used as pore forming agent for fabricating the porous W. The WO3 powder compacts with 20 and 70 vol% PMMA were prepared by uniaxial pressing and sintered for 2 h at 1200oC in hydrogen atmosphere. TGA analysis revealed that the PMMA was decomposed at about 400oC and WO3 was reduced to metallic W at 800oC. 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.

In this study, titanium(Ti) meshes and porous bodies are employed to synthesize carbon nanotubes(CNTs) using methane(CH4) 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.

This paper proposes a novel way of fabricating aligned porous Sn by freeze-drying of camphene slurry with stannic oxide (SnO2) coated Sn powders. The SnO2 coated Sn powders were prepared by surface oxidation of the initial and ball-milled Sn powders, as well as heat treatment of tin chloride coated Cu powders. Camphene slurries with 10 vol% solid powders were prepared by mixing at 50 oC with a small amount of oligomeric polyester dispersant. Freezing the slurry was done in a Teflon cylinder attached to a copper bottom plate cooled at −25 oC. Improved dispersion stability of camphene slurry and the homogeneous frozen body was achieved using the oxidized Sn powder at 670 oC in air after ball milling. The porous Sn specimen, prepared by freeze-drying of the camphene slurry with oxidized Sn powder from the heat-treated Sn/tin chloride mixture and sintering at 1100 oC for 1 h in a hydrogen atmosphere, showed large pores of about 200 μm, which were aligned parallel to the camphene growth direction, and small pores in their internal walls. However, 100 μm spherical particles were observed in the bottom part of the specimen due to the melting of the Sn powder during sintering of the green compact.

In this study, modified catalytic chemical vapor deposition (CCVD) method was applied to control the CNTs (carbon nanotubes) growth. Since titanium (Ti) substrate and iron (Fe) catalysts react one another and form a new phase (Fe2TiO5) above 700℃, the decrease of CNT yield above 800℃ where methane gas decomposes is inevitable under common CCVD method. Therefore, we synthesized CNTs on the Ti substrate by dividing the tube furnace into two sections (left and right) and heating them to different temperatures each. The reactant gas flew through from the end of the right tube furnace while the Ti substrate was placed in the center of the left tube furnace. When the CNT growth temperature was set 700/950℃ (left/right), CNTs with high yield were observed. Also, by examining the micro-structure of CNTs of 700/950℃, it was confirmed that CNTs show the bamboo-like structure.

The present study demonstrates the effect of solidification condition on the pore structure in freeze drying process using the slurries of CuOlimable vehicles. Camphene and Camphor-45 wt% naphthalene based slurries with 14 vol% CuO powder were frozen into a mold at -25℃<, followed by sublimation at room temperature. The green bodies were hydrogen-reduced and sintered at 500℃ for 1 h. The porous Cu specimen, frozen the CuO/camphene slurry into the heated mold of the upper part, showed large pores with unidirectional pore channels and small pores in their internal wall. Also, it was observed that the size of large pores was decreasing near the bottom part of specimen. The change of pore structure depending on the freezing condition was explained by the nucleation behavior of camphene crystals and rearrangement of solid powders during solidification. In case of porous Cu prepared from CuO/Camphor-naphthalene system, the pore structure exhibited plate shape as a replica of the original structure of crystallized vehicles with hypereutectic composition.

This study reports a simple way of fabricating the porous Cu with unidirectional pore channels by freezedrying camphene slurry with Cu oxide coated Cu powders. The coated powders were prepared by calcination of ball-milled powder mixture of Cu and Cu-nitrate. Improved dispersion stability of camphene slurry could be achieved usingthe Cu oxide coated Cu powders instead of pure Cu powders. Pores in the frozen specimen at -25oC were generated bysublimation of the camphene during drying in air, and the green bodies were sintered at 750oC for 1 h in H2 atmo-sphere. XRD analysis revealed that the coated layer of Cu oxide was completely converted to Cu phase without anyreaction phases by hydrogen heat treatment. The porous Cu specimen prepared from pure Cu powders showed partlylarge pores with unidirectional pore channels, but most of pores were randomly distributed. In contrast, large andaligned parallel pores to the camphene growth direction were clearly observed in the sample using Cu oxide coated Cupowders. Pore formation behavior depending on the initial powders was discussed based on the degree of powder rear-rangement and dispersion stability in slurry.

Cu-Ni alloys with unidirectionally aligned pores were prepared by freeze-drying process of CuO-NiO/cam-phene slurry. Camphene slurries with dispersion stability by the addition of oligomeric polyester were frozen at -25˚C,and pores in the frozen specimens were generated by sublimation of the camphene during drying in air. The green bod-ies were hydrogen-reduced at 300˚C and sintered at 850˚C for 1h. X-ray diffraction analysis revealed that CuO-NiOcomposite powders were completely converted to Cu-Ni alloy without any reaction phases by hydrogen reduction. Thesintered samples showed large and aligned parallel pores to the camphene growth direction, and small pores in the inter-nal wall of large pores. The pore size and porosity decreased with increase in CuO-NiO content from 5 to 10 vol%.The change of pore characteristics was explained by the degree of powder rearrangement in slurry and the accumulationbehavior of powders in the interdendritic spaces of solidified camphene.

This paper describes the surface modification effect of a Ti substrate for improved dispersibility of the cat-alytic metal. Etching of a pure titanium substrate was conducted in 50% H₂SO₄, 50˚C for 1h-12h to observe the sur-face roughness as a function of the etching time. At 1h, the grain boundaries were obvious and the crystal grains weredistinguishable. The grain surface showed micro-porosities owing to the formation of micro-pits less than 1 µm in diam-eter. The depths of the grain boundary and micro-pits appear to increase with etching time. After synthesizing the cat-alytic metal and growing the carbon nano tube (CNT) on Ti substrate with varying surface roughness, the distributiontrends of the catalytic metal and grown CNT on Ti substrate are discussed from a micro-structural perspective.

In order to fabricate the porous Al₂O₃ with dispersion of nano-sized Cu particles, freeze-drying of cam-phene/Al₂O₃ slurry and solution chemistry process using Cu-nitrate are introduced. Camphene slurries with 10vol% Al₂O₃ was frozen at -25˚C. Pores were generated by sublimation of the camphene during drying in air. The sinteredsamples at 1400 and 1500oC showed the same size of large pores which were aligned parallel to the sublimable vehiclesgrowth direction. However, the size of fine pores in the internal walls of large pores decreased with increase in sinteringtemperature. It was shown that Cu particles with the size of 100 nm were homogeneously dispersed on the surfaces ofthe large pores. Antibacterial test using fungus revealed that the porous Al₂O₃/1vol% Cu composite showed antifungalproperty due to the dispersion of Cu particles. The results are suggested that the porous composites with required porecharacteristics and functional property can be fabricated by freeze-drying process and addition of functional nano par-ticles by chemical method.