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.