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A low pressure impactor is used to measure the mechanical stability of nanoparticle agglomerates. The stability at different impaction velocities is quantified as changes in the projection area of agglomerate fragments deposited on TEM grids mounted on the impaction plate. Platinum particles produced in a spark discharge generator are used as model particles. The mechanical stability of unmodified Pt-agglomerates is compared with the ones of thermally pretreated and coated agglomerates. The coatings are produced in a plasma assisted chemical vapor deposition process using the post-discharge environment of a dielectric barrier discharge. The coating process allows the coating of particles with silica at ambient temperature and so minimizes structural changes of the agglomerates before impaction. The precursors utilized are tetraethyl orthosilicate for silica and hexamethyldisiloxane for silica-organic coatings. Both coatings improve the stability of the agglomerates as indicated by reduced structural changes during impaction. The silica-organic coating, which is applied at 200 °C, seems to prevent sinter-related changes to the particles, which is observed for the uncoated particles at this temperature. This is further corroborated by the deposition behavior of these coated particles, which resembles more that of the original particles at 24 °C. Furthermore, the rebound behavior of coated agglomerates impacting on the TEM grid is shown. It is found that a minority of the particles rebound back from the TEM grid into the gas, which occurs in parallel to the structural changes.

Mit Hilfe papierchromatographischer, kinetischer und rontgenographischer Methoden wird gezeigt, das das Tetramethylammoniumsilicat der Zusammensetzung 1 N(CH3)4OH · 1 SiO2 · ∼8 H2O nicht – wie bisher angenommen – ein Poly- oder Phyllosilicat, sondern ein Tetra-anhydrido-bis-cyclotetrasilicat (Doppelviererringsilicat) der Formel [(CH3)4N]8[Si8O20] · ∼69 H2O ist. Durch Umsatz mit Hexamethyldisiloxan wurde daraus der entsprechende Trimethylsilylester ((CH3)3Si)8[Si8O20] in kristallisierter Form erhalten. By means of paperchromatographic, kinetic and X-ray methods it is shown that the tetramethylammonium silicate of the composition 1 N(CH3)4OH · 1 SiO2 · ∼8 H2O has not – as hitherto supposed – the constitution of a poly- or phyllosilicate, but that of a tetra-anhydrido-bis-cyclotetrasilicate (double-fourringsilicate), [(CH3)4N]8[Si8O20] · ∼69 H2O. Its trimethylsilylation by means of hexamethyldisiloxane gives the corresponding, crystalline trimethylsilyl ester [(CH3)3Si]8[Si8O20], which has been characterised by mass spectroscopy.

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