Processing salt-encapsulated tantalum nanoparticles for high purity, ultra high surface area applications

In this study a process for removing the protective encapsulate from salt encapsulated nanopowders has been developed and tested for tantalum nanopowder produced by sodium/halide gas-phase combustion synthesis. A sodium/halide flame can be used to produce salt-encapsulated nanoparticles for many metal and nonoxide ceramic materials. The salt encapsulate allows for control of size and morphology, and protects the core particles from oxygen contamination. Without a protective coating, non-oxide nano-powders can be pyrophoric or form a surface oxide. Despite the beneficial attributes of the NaCl encapsulate, it can also be a source of contamination. Thus, a method by which the encapsulate can be removed and the exposed particles subsequently processed without exposure to environmental contamination was developed. A two step process was developed, where the bulk of the salt is sublimed from the core particles at 880°C and directed out of the system with an inert gas flow followed by a vacuum sublimation at temperatures at or greater than 1200°C for 120 min. These conditions reduced Na and Cl concentrations to below detectable limits of their respective analyses (5 ppm Na and 20 ppm Cl). Heat treating the core particles for 120 min at 1200°C, 1400°C and 1600°C coarsened the particles from 30 nm to approximately 250 nm, 400 nm and 3μm, respectively. At 1600°C a fully dense Ta consolidate was produced by applying a uniaxial load of 45 MPa pressure for 210 min.