Within the scope of this work, a novel process (patent pending) to improve the flowability of fine powders is investigated. Therefore, nanoparticles are generated by favoring homogenous gas-phase reactions in a plasma enhanced chemical vapor deposition (PECVD) process and are simultaneously attached to the surface of the substrate particles. These nanoparticles act as spacers between the substrate particles and thus increase the distance between their surfaces. According to Hamaker's law, this leads to a reduction of the Van der Waals forces [1]. Additionally, under certain process conditions like low pressures, the substrate particles are coated by heterogeneous gas-phase reactions. The roughness of these coatings has a comparable effect on the Van der Waals force like the attached nanoparticles.

The novel process shows some significant advantages compared to alternative methods [2,3] for increasing the flowability of bulk solids. The process is a combination of the two process stages of nanoparticle formation and attachment in a single process. Thus, extra handling of nanoparticles, associated with additional adhesion effects, is not required. Compared to alternative treatments, like nanoparticle attachment by mixing [1], remarkable time and cost savings can be obtained. Moreover, the application of a non-equilibrium plasma provides the opportunity to treat temperature sensitive materials.

To investigate the nanoparticle formation and attachment in the PECVD process, glass beads (120 µm) as a model substrate were treated in a circulating fluidized bed reactor. The substrate particles were conveyed through the plasma zone of the reactor by the process gas mixture, consisting of O2, Ar and Hexamethyldisiloxane (HMDSO). The organosilicon monomer was used as a reactant for the formation of SiOx nanoparticles. The influences of varying process parameters like system pressure, gas composition and number of circulations on nanoparticle size distributions were investigated. Therefore substrate surfaces were analyzed by means of scanning electron microscopy (SEM).

The study demonstrates that the process enables the nanoparticle formation by PECVD and subsequent attachment to substrate particles in one single step. Theoretical considerations show that the nanoparticle sizes lie in a range, where the van der Waals forces between the substrate particles can be reduced. In a further step of this work, the flowability of treated powders will be investigated by means of ring shear tester measurements.

[1] I. Zimmermann, M. Eber, and K. Meyer, Z. Phys. Chem. 218 (2004), 51-102

[2] S. Jonat, S. Hasenzahl, A. Gray, P.C. Schmidt, J. Pharm. Sci. 93 (2004), 2635-2644

[3] J.H. Werth, M. Linsenbühler, S.M. Dammer, Z. Farkas, H. Hinrichsen, K.-E. Wirth, D.E. Wolf, Powder Technol. 133 (2003), 106-112