Pergamon J. Aerosol Sci. Vol. 28, Suppl. I, pp. S685-S688, 1997 ©1997 Elsevier Science Ltd. All fights reserved Printed in Great Britain PlhSO021-8502(97)O0391-1 0021-8502/97 $17.00+0.00 MICROWAVE PLASMA SYNTHESIS OF CERAMIC NANAPOWDERS D. Vollath, D. V. Szab6 Forschungszentrum Karlsruhe, Institut ~r Materialforschung III P.O.Box 3640, D-76021 Karlsruhe, Germany E-mail: dieter.vollath@imf, fzk.de KEYWORDS Nanomaterials, microwaves, plasma, coated particles INTRODUCTION Ceramic nanoparticles and solids made of ceramic nanopowders often have very special proper- ties caused by the small size of the particles (Gleiter, 1989). In the case of an application of these materials as bulk solid one faces two crucial problems: If the powder is pressed and sintered, usually during sintering one observes a dramatic grain growth leading to a loss of the special properties. Even, if there is no grain growth observed, the close vicinity of the small particles may lead to an interaction of these particles changing their properties. A characteristic example for such a behaviour is superparamagnetism This property may be observed in small isolated magnetic particles, but bringing these particles together leads to an interaction of the particles. These interacting panicles behave like a large one. As a conse- quence, the property of superparamagnetism is lost. To solve these problems it is advised to coat the particles with a second layer. This layer should act as a diffusion barrier during sintering and additionally it may keep the particles in a certain distance to avoid interaction. To obtain a significant amount of the nanomaterials chemical reactions in liquids and in gases are in use. Generally, it is difficult to obtain isolated panicles by reactions in liquids. Gas phase reactions seem to have more advantages. Therefore, this route is often selected to synthesise nanoparticles. Synthesising large quantities of nanopowders by gas phase reactions one has to overcome a few contradicting problems. To obtain high production rates one needs a high particle density in the carrier gas. But: A high particle density leads to a high probability of particle agglomeration. One does not want to obtain the particles in ,,hard" agglomerates. To obtain fast formation of the particles one needs a high reaction temperature. But: A high reaction temperature leads to hard agglomerates. SYNTHESIS The only way to avoid agglomeration in the gas phase is to charge the particles electrically. To overcome kinetics obstacles responsible for low reaction rates at moderate temperatures it is of advantage to have the reactants dissociated or even ionised in the gas phase. Both conditions can be fulfilled if the reaction is performed in the plasma state. Usually, in a de or rf plasma the temperatures are in the range of a few thousand degrees centigrade. In a microwave plasma the temperatures are significantly lower. This is, because the energy transfer in an oscillating electri- $685