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

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A fourth type of aluminium powder (d) was used unannealed in the
Knight shift measurements. This was obtained commercially from
Koch-Light Laboratories and had a quoted purity of 99% with a
particle size of 8-15 um.
7.2 THE ACCURATE DETERMINATION OF THE ALUMINIUM KNIGHT SHIFT
7.2.1 INTRODUCTION
a
Knight shift values provide a direct comparison of the
magnitudes of the internal magnetic fields present at the sites of
a particular nuclear species in the metallic state and in a chosen
reference material. Most information about the metallic state
would be obtained by using an isolated atom as the reference, but
this is obviously not practical. Consequently measured Knight
shifts depend to some extent on the chemical shift interaction
present in the reference sample. For many metals the experimental
isotropic Knight shift is largely attributable to the direct
contact term given by equation (2.10). However, very often further
terms - as discussed briefly in Section 2.2.3(b) - also have to
be considered in order to obtain absolute agreement between theory
and experiment.
Previous experimental estimates of the Knight shift of alumin-
ium at room temperature
(76,91-95)
have all been in the range 1.50
to 1.68 x 10-3 , the generally accepted value being 1.61 x 10-3.
In most cases the reference sample was an aqueous solution of Aid 3.
Ultimately any Knight shift determination is dependent upon the
accuracy of the measurements of frequency or magnetic field and
the precision with which the centre of the nuclear resonance spectra
of the metal and reference can be defined. The width of the