NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

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annealed aluminium powder was placed in the standard Bruker probe
system. The measurements were recorded at temperatures in the
range 290-300 K using resonant frequencies of 15 and 16 MHz, but
the results obtained were insensitive to these variations.
Powders (a) and (b) were sieved so as to separate the specimen
particle sizes so: 37 um, 37-44 um, and 44-76 pin. A few drops of
pure acetone were added to the largest grain specimens in order to
reduce distortion of the detected signals by mechanical vibrations
set up in individual crystallites. The problem could not be cured
by the powders being encapsulated in resin because the widths of
the resonance spectra of the cured slugs were found to be increased
by this treatment. (That this broadening was due to a quadrupole
interaction rather than any stress induced distribution of Knight
shifts was made evident by its partial removal upon rapid rotation
of the specimens about the magic angle. ) Owing to the small skin
depth (ti 20 um) the signal intensities obtained from the coarsest
powders were less than those from the finer powders. However, with-
in the limits of experimental error, the transformed lineshapes
were found to be unaffected by particle size.
The observed characteristic decay shapes from powder samples,
(a) and (c), were quite distinguishable from each other. (Powder
(c) was filtered to a particle size of less than 44 pm. ) Typical
FIDs obtained from these two samples are shown in Figure 7.1 together
with the corresponding lineshapes in the frequency domain. The
FID and absorption spectrum of powder (b) were very similar in appear-
ance to those obtained for (c), and are therefore not illustrated
separately.
The tails of nuclear spectra in the frequency domain are deter-
mined largely by the supposition made about the behaviour of the FID