Nanocrystalline powders of GaN and SiC with grain sizes ranging from 2
to 30 nm were examined under high external pressures by in situ
diffraction techniques in DAC (Station F3, HASYLAB at DESY). The
experiments, which simulated the process of densification of pure
powders under high pressures, were performed both with and without a
pressure medium. Mechanism of generation and relaxation of internal
strains and their distribution in nanoparticles was deduced from the
positions and broadenings of Bragg reflections measured in situ under
high pressures at room temperature: (i) macro-strain was calculated
from average compressibility of the crystal lattice, (ii) micro-strain
- from FWHM of Bragg lines [1]. Under external stress there are
observed several pressure ranges in which micro- and macro-strains
behave differently. Densification of nanopowders occurs in steps and
it involves different mechanisms in relatively soft GaN (bulk modulus
125 GPa and very hard SiC. The shapes of plots presenting generation
of micro- and macro-strains change with the size of nanograins. The
effects of size and agglomeration of powders on mechanism of
relaxation of local strains under increasing and decreasing external
stresses is discussed.
Acknowledgements
This work was supported by Polish Committee for Scientific Research-
grant KBN 7 T08D 02417; Polish-German Project POL-00/009; HASYLAB
Projects I-99-068 & II-99-053.
[1] B.Palosz, S.Gierlotka, S.Stel'makh, R.Pielaszek, P.Zinn&U.Bismayer
(1999): J.Alloys Comp. 286, 184-192.

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