Formation of nanoparticles in liquids

The most important procedures for the production of nanopowders and/or deposition of thin layers from the liquid phase include the sol gel procedure and the electro-chemical deposition. Both procedures are further suitable for the building of nanoporous materials. Nanophased systems in liquids (nano-suspensions and emulsions) were already investigated in different microgravity research projects. Topics of interest are for example the adsorption dynamics and the mass transfer on individual liquid/liquid boundary surfaces, droplet/droplet interactions as well as stability and phase inversion of model emulsions. Apart from realizations within the basic research range also approaches for the optimization of wet-chemical procedures for terrestrial applications are expected to be realized by means of microgravity experiments.

Although the gravimetric sedimentation of very small particles is to be neglected in relation to the random Brownian movement (see Roessler et al. 2001), gravitaty effects can play a role in wet-chemical processes in view of the long duration of particle aggregation as well as the impact of gas bubbles on EPD processes, which arise under the influence of the gravity force. The investigation of the influence of gravitation effects on the formation and characteristics of nanomaterials by Sol gel processes (e.g. of aerogels) could serve a better understanding of the gel aggregation with the gelling process, which could be used in principle for the optimization of the appropriate process technologies and materials. In a NASA research project an appropriate measuring device is currently developed, helping to examine the gelling process of aerogel formation by means of optical measuring procedures under microgravity conditions. Furthermore there are indications that the production of aerogels under microgravity can lead to improved material properties. Thus microgravity experiments are conducted by the University of Wisconsin with the aim of reducing the pore sizes of aerogels to obtain transparent, colorless aerogels which would be better suited for technical applications.61 Conventional aerogels with pore sizes of up to 200 nm appear frequently bluish and/or transluzent due to light scattering effects, which limit the technical applicability.

Also the electrophoretic deposition for the production of nanomaterials

‚ÄěReduced Gravity Aerogel Formation" (http://www.cae.wisc.edu/~aerogel/)

offers starting points for microgravity research. Although structures with an unusually high degree of order can be produced under terrestrial conditions by means of EPD, however a more or less strong deviation from the ideal course of the nanoparticles along the electrical field occurs by gravitational effects. This could lead to disturbances in the microstructure of the deposited nanomaterials. For fundamental investigation with the purpose of understanding the mechanisms of the EPD process this means that an accurate empirical analysis is not possible due to gravitational influence, so that a verification of the postulated models and simulations is likewise not accurate.

In principle both electrophoretic deposition and impregnation can be accomplished in aqueous as well as organic solvents. Although these two basically different procedures exhibit specific advantages, the use of organic solvents usually is not economical due to the clearly higher process times and the environmental incompatibility, so that aqueous dispersions have to be used for technically relevant processes. However the electrolytic decomposition of water, applying voltages above approx. 2 V within EPD/EPI processes, leads to the formation of gas bubbles on the electrodes, which disturbs the particle movement and causes defects in the mic-

r Results from micro gra-

rostructure of the deposited materials. While defects in the microstructure vjty research could be can mostly be avoided by applying ion permeable membranes, an inves- relevant for the com-tigation of the movement of the dispersed nano-particles in the electrical mercial application of field is however influenced through gas bubbles. Thus the investigation electrophoretic nanopar-of the electrophoretic caused movement and deposition of nanoparticles ticle deposition under microgravity could lead to a crucial contribution to understand the relevant mechanisms and thus accelerate the conversion into industrial development significantly.62

A further relevant topic field is the self-organization of molecules in liquids, which will significantly gain importance in the future for bottom-up strategies for the production of nanomaterials. Experiments under microgravity showed that gravitational effects had a crucial influence on self-organization processes of biological molecules. Although the influence of gravity on a single particle is only small, effects could arise in systems of a multiplicity of particles, which lead to a macroscopic self-organization into so-called dissipative structures. Thus in microgravity experiments it was observed, that molecules of the protein tubulin arrange themselves in a completely irregular form, while under the same conditions ordered structures arise in a terrestrial laboratory (Papaseit et al. 2000). The investigation of such self-organization phenomena under mic-rogravity could be relevant for the development of nanostructured materials by self-organization processes. Within the nanotechnology scene however, no concrete approaches are at present recognizable to take up this topic in the context of own research activities.

62 Jan Tabellion, Institute for powder technology of glass and ceramics, Saarland University, personal communication from 04.09.2002

on phenomena in ferrofluids

For the investigation of the physical properties of ferrofluids micrograviy Investigation of experiments are utilized to examine thermal transportation phenomena the_rma! transportati- and magnetic effects without distortion through gravity influences. Fer-

rofluids, which consist of a suspension of magnetic nanoparticles (ap-prox. 10 nm diameters) in a carrier liquid (e.g. oil or water), offer potential e.g. for the employment in thermal control elements, since their physical properties (e.g. the viscosity, thermal conductivity) can be controlled by exterior magnetic fields. Experiments under microgravity are expected to lead to a better understanding and controllabibity of mass and heat transport processes, which is necessary for potential technical applications of ferrofluids. Appropriate investigations are accomplished e.g. by the Center for Applied Space Technology and Microgravity (ZARM) in Bremen with parabolic flights and drop tower experiments in the frame of an ESA Map project. At a later time also experiments in the space shuttle and on ISS are planned (see chapter 6.2).

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