| A topic area which moves increasingly into the focus of nanotechnology-relevant microgravity research, is the formation and the production of nanoparticles in gaseous phase reactions. Research objectives in this context are a better understanding of particle-particle and particle-gas interactions within particle aggregation, as well as obtaining accurate data for the characterisation of the flow conditions in gaseous phase reactors. Microgravity allows here, among other things, the investigation of the influence of thermal convection on the agglomeration process, the size and the morphology of the nascent particles. Likewise, sedimentation effects are excluded, which play a role however only for larger particle aggregates. Using the Inert Gas Condensation Technique to Produce Nanopowders The inert gas condensation process is one of the established procedures for the production of nanopowders, e.g. for nanoporous metal powders. These metal powders are technically utilized for electrically conductive adhesives and polymers, which find application, among other things, for the surface mounting technique in electronics. Experiments under microgravity permit a detailed investigation of the agglomeration process, for example: • The determination of convection influence and inhomogeneities of the particle density on the morphology of the particle aggregates (form, porosity). • The assignment of parameter changes to powder morphology and thus, possibly, an improved control of the aggregates formation of sintering active nanoparticles in gases. Figure 1 gives a schematic overview of the inert gas condensation procedure from evaporation over particle formation and aggregation up to separation. | | | Figure 1. Powder formation in the Inlet Guide Vane (IGV) procedure. | Measuring Devices to Investigate Particle Aggregation Processes The Fraunhofer-Institute for Applied Material Research (IFAM) and the BTU Cottbus developed, in a DLR (Zentrum für Luft und Raumfahrt - the German Aerospace Centre) supported project, a measuring device for investigation of particle aggregation processes, which is applicable in parabolic flight experiments. As measuring methods here, a PIV/LDA procedure (Particle Image Velocimetry/ Laser Doppler Anemometry), optical microscopy and an in-situ sampling device were used. The method has an analytical resolution within the micrometer range. Figure 2 (see below) shows the microscopy system, PATRICIA, developed by the University of Jena, and an image of a measurement of silver aggregates. | 
| | Figure 2. top: image of the measuring device, PATRICIA, bottom: image analysis of a measurement of silver particles. | More Research Needed for the Laser Measuring Technique for Sub-Micron Particles and Agglomerates First, microgravity experiments were accomplished in parabolic flights. Here, it became obvious that the evaporation technique had to be modified for microgravity experiments, in order to obtain a steady particle density. Further research needs exist regarding the supplementing employment of a laser measuring technique for sub-micron particles and agglomerates. To what extent the results can be used for the optimization of IGV processes under terrestrial conditions, cannot to be assessed at present. Using the Flame Synthesis Process to Form Nanoparticles The formation of nanoparticles in flames is a further current topic in microgravity research. In the frame of a European Space Agency MAP project, for example, the LII (Laser Induced Incandescence) procedure was applied, by which the formation of nanoscale carbon particles in a flame can be examined online with high resolution. Here, the soot particles are heated with a laser beam and the thermal radiation is recorded, time-resolved, with a CCD (Charge Coupled Device) camera system. From the signal, both the volume concentration and the aggregate size of the soot particles can be determined. Figure 3 shows a schematic experimental set-up. The procedure was already applied in microgravity in the frame of parabolic flights and drop tower experiments. | 
| | Figure 3. Schematic experimental set-up for investigations of soot particles by means of the LII procedure. | Below, figure 4 shows the measuring data of a laminar ethene diffusion flame. | 
| | Figure 4. Measurements of a laminar ethene diffusion flame by means of the LII procedure. | Controlling the Retention Time of Particles during Flame Synthesis An exclusion of the gravity and buoyant force makes it possible to control the retention time of particles in the flame. Microgravity has a significant influence on the particle concentration and sizes, as can be seen in the figure 5 (below). First, microgravity investigations were accomplished in parabolic flight and drop tower experiments. As a goal of the investigations, a broad database for the modelling of flame synthesis processes and approaches for the production of new material configurations, is envisaged. | 
| | Figure 5. Measurements of the particle concentration in a flame under terrestrial and microgravity conditions. | Using the Laser Induced Incandescence (LII) Method to Characterize Other Types of Nanoparticles Besides soot-particle formation in flames, the LII method, in principle, can be used for the in-situ characterization of other types of nanoparticles, with a high temporal and spatial resolution. For this, however, first intensive investigations of the laser/particle interactions have to be accomplished with respect to the material classes involved. |