(291e) Aggregate Sintering Dynamics of Crystalline Ceramic and Metal Nanoparticles

Aggregate
Sintering Dynamics of Crystalline Ceramic and Metal Nanoparticles

Fractal-like aggregates consist of multiple particles that are connected by chemical (e.g.
sintering) bonds. Such aggregates form by natural and man-made processes,
typically at high temperatures, like fly ash from volcano eruption and coal
combustion as well as aerosol synthesis of ceramics (titania,
fumed silica, alumina) and metals (Ni, Fe, Ag etc.). The morphology of such
particles has critical implications in their performance. In design of nanoparticle synthesis by aerosol processes the variation of
aggregate structure during particle formation hardly affects the primary
particle diameter. In contrast, it profoundly affects the collision and
mobility diameter that determines the transport, mechanical and optical
properties of such aggregates.

Here a recently developed model for viscous sintering of
amorphous aggregates¹ is extended to
simulate solid state sintering
mechanisms that describe coalescence of two differently sized
particles and multiparticle aggregates of crystalline particles (e.g. TiO₂ or Ag).
This model reproduces the initial neck growth and evolution of particles center-to-center distance for
equally sized pairs of particles and compared to the classic models²
and characteristic sintering times³. So the evolution of the
detailed morphology, radius of gyration and effective fractal dimension of ensembles
of irregular particles is presented as they asymptotically approach full
compactness by coalescence or sintering and compared to experimental data. Figure 1 shows the evolution of aggregate morphology
during grain boundary diffusion sintering with normalized time t/t₀ for initially 512 primary particles. At the beginning
the driving force for sintering is largest with the highest curvatures in the
particle necks (curvature = 1/radius: large = blue, small = red).

Figure
1: Snapshots of aggregates undergoing grain boundary diffusion sintering and
consisting of initially 512 monodisperse primary
particles generated by diffusion limited cluster-cluster (DLCA, D_f =
1.79). The colors correspond to the aggregate curvature (= 1/radius): large =
blue, small = red.

1. Eggersdorfer, M.L., Kadau,
D., Herrmann, H.J., Pratsinis, S.E., Multi-Particle
Sintering Dynamics: from Fractal-like Aggregates to Compact Structures. Langmuir in press (2011).

2. Coblenz, W.S., Dynys J.M., Cannon R.M., Coble, R.L., Initial Stage Solid State Sintering Models: a Critical Analysis and
Assessment. Mat. Sci. Res. 13 (1980) 141-157.

3. Seto,
T., Shimada, M., Okuyama, K., Evaluation of Sintering of Nanometer-Sized Titania
Using Aerosol Method. Aerosol Sci. Technol. 23 (1995) 183-200.