Catalysis of Organic..

164 Raney® Metastable Precursorsvacuum. An example of consolidated disks of skeletal Ni-Ag by SPS at 673K under32MPa is shown in Fig.14. The apparent density of the disk was about 3.7g/cm 3 .This is less than 50% lower than the density of a non-skeletal Ni-Ag mixture. Thismeans many pores remain in the disk. The specific surface area of disks consolidatedat various temperatures is shown in Fig. 15. The specific surface area is firstincreased and then decreased. The specific surface area of the disk consolidated at673K was almost the same as that of the as-leached specimen. Thus it was possibleto form bulk material by SPS without decreasing the specific surface area.The reason why the specific surface area increased or was kept at high value afterconsolidation is considered as follows. The sintering may occur at some primaryparticle boundaries and not at the boundary of nano-size skeletal particles boundaries.So the skeletal structure was kept after consolidation. The mechanical strength of thedisk was supported by sintering each primary boundary. The contribution of theprimary boundary to the specific surface area was negligibly small in comparisonwith the skeletal boundaries. Therefore, the decrease of the specific surface area withthe SPS temperatures was minimal. Theincrease of the specific area at 573K maycome from the separation of skeletalparticles during SPS. The skeletal metalcontains some hydrogen in the particles.When the particles are heated in vacuum,then the hydrogen gas may be generated.The large volume of rapidly expandinghydrogen gas may cause separation atskeletal particle boundaries. New surfacearea may be formed by the separation atthe boundaries. One point of evidencefor this was shown in Fig.16. In this case,a primary particle was mounted on acarbon mesh and directly observed byFigure 16. Dispersion of ultra-fineparticles by heating a primary leachedparticle in electron beam in vacuumTEM. When the particle is irradiated by the electron beam in vacuum, it is heatedsuddenly. The generated hydrogen gas forms fine particles as shown in Fig.16. Inthis figure, the top left shows generated skeletal size particles on a carbon mesh atlow magnification. The enlargement shows very fine particles of less than 10 nm.10nm2nmFigure 17. High resolution TEM image of skeletal structure. Each particle isFig. 17 High resolution image of skeletal structure. Each particle iscomposed of single crystal (left: low magnification; right: high magnification)