Gravimetric investigation of passivation of compact samples made of nickel pyrophoric nanopowders
Pyrophoric nickel nanopowders obtained by a chemical-metallurgical method were used in investigations. The average nanoparticle size was 67 nm. Compact samples with diameters of 5, 7 and 10 mm of different densities were made of nonpassivated nickel nanopowder in a glove box in argon atmosphere. In preliminary experiments, in which closed weighing bottles with samples made of nickel pyrophoric nanopowder after extraction from the box and until extraction of samples from the weighing bottles were in an argon atmosphere, the optimal value of the relative density 0.2 of samples was determined, at which they maintained pyrophoric properties, since they self-ignition occurred at heating to ~550 - 600°C. Then, for the samples with the densities <0.2, the time spent with the closed weighing-bottle in the air as well as the relative weight gain of the samples sufficient for their passivation were determined. The amount of adsorbed monoatomic oxygen layers and the active surface fraction of compact nickel pyrophoric nanopowder samples were calculated based on the obtained relative weight gain information and literature data. It was established that during the exposure of the weighing boxes to the air the passivation of the samples with the preservation of their high chemical activity occurred, since when the oxidation reaction was initiated by a high-temperature source, a combustion wave propagated within the sample with the velocity about 0.3 mm / s. Analysis of the fracture of passivated samples showed no sintering of nickel nanoparticles; EDA showed an almost uniform distribution of oxygen over the cross section of all passivated samples, which makes it possible to conclude that passivation has superficial nature.