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

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not prevent magnetic splinters from entering the filed powder
initially. A detailed chemical analysis of filed powders and
cold rolled foils would help in this respect.
In conclusion we note that there was a marked difference in
appearance between the individual grains of the filed powder speci-
mens and those prepared by the metal spraying technique. Under
a microscope the filed particles were very irregular in shape where-
as the sprayed particles tended to be elongated spheroids. How-
ever any surface effect would presumably have to be exceptionally
long-ranged in order to account for the observed amount of broaden-
ing in the nuclear resonance spectra.
7.4 VARIATION OF LINESHAPE WITH ROTATION SPEED
7.4.1 INTRODUCTION
The aluminium resonance line may be narrowed by rapid
rotation at the magic angle. No scalar broadening interaction of
the type discussed in Section 2.1.4 is present in aluminium. There-
fore it is predicted that the theoretical limit to the reduced
central spectrum will be determined only by the homogeneity of the
external magnetic field and the relaxation time of the aluminium
nuclei. Kessemeier and Norberg(34) have spun aluminium at speeds
up to 7.5 kHz and reported that the resonance signal was narrowed
by a factor of 10. Above 4 kHz they found that the FID was made
up of a Gaussian initial part and an exponential tail, the time
constant of which exhibited a roughly linear dependence on the rotat-
ion rate. The linewidth of static altnºinium powder is approximately
8 kHz, so it is expected that rotation rates considerably in excess