(429a) Particle Breakage in the Nanometer Range

Nanoparticles are increasingly used in many areas of the chemical and pharmaceutical as well as the ceramic and microelectronic industry. Besides the direct synthesis of these materials by chemical methods, wet grinding in stirred media mills is a suitable technique for the production of nanoparticles in the liquid phase with high solid concentrations. The manufacturing of fine particles is influenced by particle breakage and interparticle interactions. These interactions become relevant especially for particles smaller than 1 µm because of an increasing collision rate of the particles due to their Brownian motion. Thus, the particles have to be stabilized against agglomeration to advance the grinding progress. Whereas different stabilization mechanisms have been a subject of much research, the breakage behavior of particles in the nanometer range is not well understood until now. Especially, the question of the existence of a grinding limit and its influencing parameters are of great interest for nanoparticle production.

In this paper we will present results of long term grinding experiments of different materials in a stirred media mill. In all experiments a plateau in particle size was found in the lower nanometer range. This grinding limit as well as the breakage kinetic of the particles strongly depends on material properties like hardness, fracture toughness, and Young's Modulus. In order to control and optimize the grinding process, a correlation between those material parameters and the breakage behavior have been identified.

Beside the influencing parameters (material and process parameters) on the breakage behaviour, the mechanisms which cause particle breakage will also be presented. Due to mechanical stressing of the particles during the milling process, defects in the crystal lattice are generated which lead to a loss in crystallinity and therefore result in a reduction of crystallite size. By means of X-ray diffraction analysis and evaluation of the measured patterns it is possible to observe the development of the internal microstructure by determination of crystallite size and microstrain. The development of crystallite size with milling time is slower in comparison with the decrease of particle size. Nevertheless, the same final plateau size is found, i.e. the ultimate grinding limit has been reached. Thus, the microstructure development influences the breakage behavior and therefore determines the grinding limit, since the particle breakage is initiated at lattice defects being the weakest points in the particles. In addition, lattice imperfections provide the necessary energy for the creation of new surfaces at a breakage event. For tin oxide we found by high resolution TEM that shear bands and twins occur which are the main responsible mechanisms for the particle breakage. These experimental findings are completed by MD-simulation which reveals structure formation phenomena inside of stressed nanoparticles.