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Ageing of powders

High surface area powders are required in a number of technology areas. For example, the efficiency of catalysts is intimately linked to the surface area of the powder. In our context we are interested in explosives that are in the form of a powder. High surface area, small diameter powders tend to have a high Gibbs surface energy and tend to coarsen to reduce it.

The main mechanisms by which powders may change their surface area are believed to be:

  • evaporation-condensation, in which molecules detach from a particle surface, diffuse through the gas phase, and then condense on the surface of a different particle;
  • surface diffusion, in which the molecule undergoes long-term diffusion on (solid) particle surfaces without detaching into the gas phase.

To date the mathematical modelling of the coarsening process, sometimes referred to as ‘Ostwald Ripening’, has usually made the assumptions that the explosives comprise a collection of detached spherical particles of differing radii and that mass transfer occurs via the Gibbs-Thompson effect. While the assumption of spherical shapes is a reasonable starting point, it is known that crystal growth is highly dependent on local surface curvature. It may be that postulation of a distribution of shapes might be a better way to improve the mathematical description of the process. In previous work correction factors are employed to account for particle shapes, surface roughness and the diffusion process. The first highly transient phase of the coarsening process in particular is still not very well understood.

AWE would like to explore the coarsening process with the Study Group. In particular, they would like to relax the assumption of discrete particles, and examine the consequences of allowing contact and coalescence between nearby particles.

 

Problem presented by:

Janella Mansell (AWE)
John Curtiss (AWE)
Rod Drake (AWE)

 

Related resources:

Problem brief
Problem presentation (slides)
Problem presentation (video) *
Online discussion
Final presentation (slides)
Final presentation (video)

 

* For some unknown reason, the slides are not showing in this video.

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