Daria Martin, Schering Stiftung Projektraum


Schering Prize 2000

Dr. Jörg Schmidt

Process Technology & Engineering
Degussa AG, Marl

Investigations of the reaction engineering for production of nanoparticles in microemulsions

Under the term - N a n o t e c h n o l o g y - a variety of scientific and technological activities are summarized for the production and use of nanocristalline materials. As nanocristalline materials have a high potential for different applications a series of methods for producing these materials have been developed within the last few years. They are based on very different physical and chemical principles. One simple, low-cost and variable method is the use of microemulsions as medium for precipitation reactions.

Hereby, the precipitation takes place inside the drops of an extremely fine emulsion. By means of this, more than 100 different materials have already been produced as powders with particle sizes in a range of only a few nanometers. Nevertheless, at the beginning of Jörg Schmidt's work there were no systematic investigations from which we could derive how to transfer these synthesises from laboratory to production.

At this point, the project for Mr. Schmidt's Ph.D. thesis started. It was his aim to develop the fundamentals for a scale-up of a nanoparticle synthesis in microemulsions from lab scale to production scale. Therefore, he chose the production of palladium nanoparticles by reducing palladium salts with sodium hypophosphite as model reaction. As reaction medium Mr. Schmidt used a water-in-oil microemulsion consisting of cyclohexane, water and a non-ionic surfactant which was characterized in detail in a parallel submitted Ph.D. thesis.

For his investigations, he set up a laboratory reactor in which he was able to adjust exactly reproducable conditions concerning addition and mixing of reacting partners. The characterization of nanoparticles was done by means of electron microscopy and X-ray diffractometer that were available for Mr. Schmidt at other TU institutes. The accuracy of his method allowed him to discuss differences of 0.5 nanometers in average particel size. Based on this method he could finally start his work on the actual subject of his PhD thesis.

Mr. Schmidt determined in his investigations concerning the reaction engineering of precipitation in a microemulsion both the influence of different processes and the influence of drop sizes of the microemulsion on particle size and particle size distribution. He showed that the reactants have to be mixed very quickly (with a Reynold number higher than 6000) in order to obtain reproducable results. Under these conditions one can control the particle size in the range of 5 up to 20 nanometers via feed rate or drop size as well as drop concentration. The precipitation reaction is controlled by the exchange process between the drops of microemulsion. The formation of nuclei showed to be the crucial step of particle formation. Mr. Schmidt was able to show the exact relation between all variables and the particle size with a kinetic model of the particle formation.

In an application oriented part of his thesis Mr. Schmidt showed that it is most important for the use of nanoparticles, for example in catalysis, to know the field of application already before production starts. Fully aware of this, the isolation of the product out of the reaction mixture can be created in such a way that undesired and for the use advers agglomerations of particles can be avoided. His results show abundantly clear that nanopowder cannot be distributed in bottles like a fine chemical to the user as by such a handling many important properties of these materials get lost.



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