Space As Research Instrument For Nanotechnology

The application of microgravity as research instrument for nanotechnolo-gy, e.g. in the context of a possible industrial utilization of the ISS by nanotechnology companies, is a further aspect, which was examined in the frame of the ANTARES study. In the following some approaches, chances and barriers for the application of microgravity research for na-notechnolgy are pointed out.

6.1 Microgravity research for nanotechnology

The research in space offers the possibility for investigations under conditions, which can not or only partly be simulated on earth like the nearly complete absence of the gravity force (microgravity) and the cosmic radiation. From experiments in space thus realizations can be derived, which would not be accessible under terrestrial conditions. The historical development of the microgravity research in space reaches from the American Skylab at the beginning of the 70's to the International Space Station, which is installed since 1998 in the earth orbit. Current topics of microgravity research are among other things (see Seibert et al. 2001):

• Changes of the human physiology in space and space medicine

• Biological processes and biotechnology (cell and molecular biology, plant development, protein crystallization etc.)

• Basic and applied research in physics (crystal growth, fluid physics, plasma and combustion processes etc.)

The following phenomena, processes and procedures investigated in the context of microgravity research are also relevant here for nanotechnolo-gical developments (see Meier 2000):

• Obtaining exact data for the optimization of process technologies in gas phase synthesis of nanopowders and particles (among other things CVD and flame synthesis)

• Investigation of particle-particle/gas interactions concerning the aggregation in high vacuum, in sprays, in flames and in plasmas

• Investigation of the formation and stability of nanoemulsions

• Investigation of thermal transportation phenomena in magnetic liquids

• Self organization phenomena

• Advancement of analytical devices (nano-/micro system engineering, e.g. STM or AFM devices, lab-on-a-chip systems or laser-optical procedures)

Nanotechnology relevant microgravity experiments

Concrete research projects in this areas have been accomplished for some years e.g. by NASA in the frame of the PSRD (Physical Sciences Research division) and the MRD (Microgravity Research Division) programmes as well as ESA in the frame of the MAP (Microgravity Application Project) programme. In the following some of the most relevant topics of microgravity research relating to nanotechnology developments are summarized.

6.1.1 Formation of nanoparticles in gaseous phase

A topic area, which moves increasingly into the focus of nanotechnology Investigation of for- relevant microgravity research, is the formation and the production of mation and aggrega- nanoparticles in gaseous phase reactions. Research objectives in this con-

tion of nanoParticles text are a better understanding of particle-particle and particle-gas inte ractions 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.

without convection and sedimentation Inert gas condensation

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 electrical conductive adhesives and polymers, which find application among other things for the surface mounting technique in electronics. Experiments under mic-rogravity permit a detailed investigation of the agglomeration process (Meier 2000) e.g.:

• 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

The illustration 25 gives a schematic overview of the inert gas condensation procedure from evaporation over particle formation and aggregation up to separation.

Modification Filter deposition

Aggregation Nanoparticles Nucleation

Carrier gas

+ Reactive gas + Heat

W-platen ^ Metal vapor

Illustration 25: Powder formation in the IGV procedure (source: Guenther et al. 2002)

The Fraunhofer-Institute for Applied Material Research (IFAM) and the BTU Cottbus developed in a DLR supported project60 a measuring device for investigation of particle aggregation processes, which is applicable Investigations of the in parabolic flight experiments. As measuring methods here a PIV/LDA formation of metal na-procedure, optical microscopy and an in-situ sampling device were used. noparticles under mic-The method has an analytical resolution within the micrometer range. rogravity The illustration 26 shows the microscopy system PATRICIA developed by the University of Jena and an image of a measurement of silver aggregates. First microgravity experiments were accomplished in parabolic flights. Here it became obvious, that the evaporation technique had to be modified for |ig-experiments, in order to obtain a steady particle density. Further research need exists regarding the supplementing employment of a laser measuring technique for sub-^m particles and agglomerates. To what extent the results can be used for the optimization of IGV processes under terrestrial conditions, can not to be assessed at present.

60 DLR joint project MENAPA, FKZ 50 WM 0053/-54

Illustration 26: Above: Image of the measuring device PATRICIA, below: image analysis of a measurement of silver particles (source: Guenther et al. 2002) Flame synthesis

The formation of nanoparticles in flames is a further current topic in mic-Investigation of nano- rogravity research. In the frame of an ESA MAP project for example the scale carbon particles LII (Laser Induced Incandescence) procedure was applied, by which the by means of L||-proce- formation of nanoscale carbon particles in a flame can be examined on-dures line with high resolution. Here the soot particles are heated with a laser beam and the thermal radiation is recorded time-resolved with a CCD camera system. From the signal both the volume concentration and the aggregate size of the soot particles can be determined (Will 2002). The illustration 27 shows a schematic experimental setup. The procedure was already applied in microgravity in the frame of parabolic flights and drop tower experiments.

Investigated flame

Spectral filter Intensified CCD System

Illustration 27: Schematic experimental set-up for investigations of soot particles by means of LII procedure (source: Will 2002)

The illustration 28 shows the measuring data of a laminar ethene diffusion flame:

Illustration 28: Measurements of a laminar ethene diffusion flame by means of LII procedure (source: Will 2002)

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 illustration 29. 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.

Illustration 29: Measurements of the particle concentration in a flame under terrestrial and microgravity conditions (source: Will 2002)

Microgravity experiments for the optimization of aerogels

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.

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