Effect of hydrogen annealing on the composition and particle size of molybdenum nanopowders

We have studied the behavior of molybdenum nanopowder consisting of particles with an average size of 23.7 nm. The powder was prepared by hydrogen plasma reduction of molybdenum trioxide and contained considerable amounts of oxygen in the form of amorphous molybdenum oxides and hydroxides. Annealing in hydrogen for 1 h at temperatures from 300 to 1000A degrees C is shown to cause crystallization of oxides, which then vaporize to form the volatile hydroxide MoO(2)(OH)(2). Most of the oxygen is removed by annealing below 700A degrees C. Starting at this temperature, particle growth is observed. The main mechanism behind the coagulation of molybdenum particles is vapor transport. Annealing at 1000A degrees C for 30-50 min in hydrogen with a dew point of -5A degrees C increases the particle size by about one order of magnitude.