On Polymer Nanocomposites - Progress in Organic and Physical Chemistry: Structures and Mechanisms - page 225

determines the properties of UV-protectors [13]) by changing the characteristics of the
silicon powder.
17.2 OBJECTS OF RESEARCH
In this study, the possibilities of using polymers containing silicon nanoparticles as
effective UV protectors are considered. First, the structure of nc-Si obtained under
different conditions and its aggregates, their adsorption and optical properties was
studied in order to find ways of control the UV spectral characteristics of multiphase
polymer composites containing nanocrystalline silicon. Also, the purpose of this work
was to investigate the effect of the concentration of silicon nanoparticles embedded in
polymer matrix and the methods of preparation of these nanoparticles on the spectral
characteristics of such nanocomposites. On the basis of the data obtained, recommen-
dations for designing UV protectors based on these nanocomposites were formulated.
The nc-Si consists of core shell nanoparticles in which the core is crystalline sili-
con coated with a shell formed in the course of passivation of nc-Si with oxygen and/
or nitrogen. The nc-Si samples were synthesized by an original procedure in argon
plasma in a closed gas loop. To do this, we used a plasma vaporizer/condenser op-
erating in a low frequency arc discharge. A special consideration was given to the
formation of a nanocrystalline core of speci¿ ed size. The initial reagent was a silicon
powder, which was fed into a reactor with a gas À ow from a dosing pump. In the reac-
tor, the powder vaporized at 7,000-10,000°C. At the outlet of the high temperature
plasma zone, the resulting gas vapor mixture was sharply cooled by gas jets, which
resulted in condensation of silicon vapor to form an aerosol. The synthesis of nc-Si in
a low frequency arc discharge was described [3].
The microstructure of nc-Si was studied by transmission electron microscopy
(TEM) on a Philips NED microscope. X-ray powder diffraction analysis was carried
out on a Shimadzu Lab XRD-6000 diffractometer. The degree of crystallinity of nc-Si
was calculated from the integrated intensity of the most characteristic peak at 20 = 28°.
Low temperature adsorption isotherms at 77.3K were measured with a Gravimat-4303
automated vacuum adsorption apparatus. The FTIR spectra were recorded on in the
region of 400-5,000 cm -1 with resolution of about 1 cm -1 .
Three samples of nc-Si powders with speci¿ c surfaces of 55, 60, and 110 m 2 /g
were studied. The D values for these samples calculated by Equation (2) are 1.71,
1.85, and 1.95 respectively, that is, they are lower than the limiting values for rough
objects. The corresponding D values calculated by Equation (3) are 2.57, 2.62, and
2.65 respectively. Hence, the adsorption of nitrogen on nc-Si at 77.3K is determined
by capillary forces acting at the liquid gas interface. Thus, in argon plasma with addi-
tion of oxygen or nitrogen, ultra disperse silicon particles are formed, which consist of
a crystalline core coated with a silicon oxide or oxynitride shell. This shell prevents the
degradation or uncontrollable transformation of the electronic properties of nc-Si upon
its integration into polymer media. Solid structural elements (threads or nanowires)
are structurally similar, which stimulates self-organization leading to fractal clusters.
The surface fractal dimension of the clusters determined from the nitrogen adsorption
isotherm at 77.3K is a structurally sensitive parameter, which characterizes both the
structure of clusters and the morphology of particles and aggregates of nc-Si.