(418m) 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

1.         Wegner K, Pratsinis SE. Scale-up
of nanoparticle synthesis in diffusion flame reactors. Chem. Eng. Sci. 2003;58(20):4581-4589.

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).