Silica Sintering Rate and Mechanism by Molecular Dynamics

Silica Sintering Rate and Mechanism by Molecular Dynamics

S. Conrady, M.L. Eggersdorfer, B. Buesser and S.E. Pratsinis

Particle Technology Laboratory, Institute of Process Engineering,

Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zürich, Switzerland

Fumed silica is the third largest industrial aerosol commodity by value (after carbon black and TiO₂ and forth by volume after ZnO¹). Its main application is as a flowing aid including paints, microelectronics, pharmaceutics, cosmetics etc. It is typically manufactured by combustion of SiCl₄ resulting in mostly amorphous SiO₂ particles due to the high temperature cooling rates in the process. Silica (SiO₂) particles grow by simultaneous coagulation and sintering where their relative rates determine if aggregates (primary particles connected by sinter necks) or agglomerates (particles connected by van der Waals forces) are formed. The particle morphology has a major influence on the product performance, e.g. mobility, scattering, mechanical strength etc. and even possible health effects.

In order to control the structure it is important to control the dynamics of particle growth, most importantly the coagulation and sintering rate as a function of particle diameter and temperature. The sintering model of Koch and Friedlander ² describes the surface area evolution using characteristic times for sintering, which is the time needed to reduce the excess surface area of an aggregate over that of an equal mass sphere by 63% ³. Amorphous silica is usually sintering by viscous flow. For this characteristic sintering times have been proposed in literature accounting for a nanoeffect on sintering for the smallest particles by modifying the viscosity to account for the reduced melting temperature of silica⁴.

Here a molecular dynamics simulation on graphic processing units (GPU) is used to investigate amorphous silica nanoparticles undergoing sintering. The Si and O ions interact by a pairwise potential presented in the literature that has been originally developed for crystalline SiO₂.

After equilibrating each particle separately, the influence of particle size and temperature on the evolution of the surface area is discussed. Moreover, the differences and similarities between the sinter mechanism of amorphous silica and crystalline TiO₂ are investigated⁵ and a new sintering rate is proposed and compared to literature.^3,4

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2.         Koch W, Friedlander SK. The effect of particle coalescence on the surface area of a coagulating aerosol. Journal of Colloid and Interface Science. 1990;140(2):419-427.

3.         Xiong Y, Pratsinis SE. Formation of agglomerate particles by coagulation and sintering--Part I. A two-dimensional solution of the population balance equation. Journal of Aerosol Science. 1993;24(3):283-300.

4.         Tsantilis S, Briesen H, Pratsinis SE. Sintering time for silica particle growth. Aerosol Sci. Technol. 2001;34(3):237-246.

5.         Buesser B, Gröhn AJ, Pratsinis SE. Sintering Rate and Mechanism of TiO₂ Nanoparticles by Molecular Dynamics. J. Phys. Chem. C. 2011 (accepted).