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.

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 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.