A4 Thermal Coalescence (Premature Loss of Properties)

Topic

The high surface energy of these nanoparticles—a characteristic also established by another model in this set—results, via the Gibbs-Thomson effect, in a lowering of the sintering temperature to a level far below that of the same material in bulk form. For these nanoparticles, this high surface energy represents an excess of free energy that the material itself can reduce through sintering; the greater this available excess free energy, the lower the temperature at which sintering becomes energetically favorable. The fact that surface energy increases as nanoparticle size decreases means—again, according to the Gibbs-Thomson effect—that the reduction in sintering temperature is more pronounced, resulting in a significantly lower effective sintering temperature for these nanoparticles.

This substantial reduction in sintering temperature relative to the bulk material causes thermal coalescence between the nanoparticles to occur prematurely—at service temperatures that would never induce sintering in the bulk material. This thermal coalescence proceeds through the growth of inter-particle necks—a mechanism also described by a separate model concerning the sintering of metal powders. As this thermal coalescence progresses through neck growth, the previously distinct nanoparticles eventually fuse into much larger particles, thereby losing their original individual identities. Consequently, in this model—characterized at the outset by high surface energy and thermal coalescence—the very process of thermal coalescence causes the neck between particles to grow until they fuse into much larger particles. This destroys the quantum confinement described by another model in the set—a phenomenon that relies precisely on the nanoparticles remaining below a certain size. As thermal coalescence pushes the effective size of these nanoparticles beyond that threshold, quantum confinement is lost, and the nanoparticles—along with their individual identity—lose the properties that depended on that confinement. The fact that this premature loss of properties occurs at a temperature far lower than expected for the equivalent bulk material renders these nanoparticles much more vulnerable to thermal coalescence than their chemical composition alone would suggest.

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