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

a° P/'rvu
- 72 -
p is the resistivity, V is the frequency of radiation and p the
permeability of the sample.
Because of the small skin depths at rf frequencies, NMR measure-
ments are usually performed on samples of fine powder or thin foil.
In the CW method the resonance lineshapes from bulk metallic samples
are distorted by a linear mixing of the absorption and dispersion
modes and similar phase errors become apparent with the pulse tech-
nique(75). Measurements have been reported on single crystal
specimens including aluminium
(76)
, but the interpretation of the
results involves some empirical assumptions being made about the
degree of lineshape distortion. In this work good rf penetration
was ensured by using fine powder specimens with dimensions approxi-
mately equal to the skin depth.
Most of the metal powders- were obtained commercially, but the
pure aluminium powders were prepared in the laboratory. In one
process 2 mm diameter aluminium wire was fed through a Metallisation
Mark 16 metal spraying pistol at a rate of 4 ft s-1 and the result-
ing powder collected in a large container of water. The pressures
of the gas supplies to the pistol were found to be critical to the
attainment of the fine powder required and were optimized to the
following values:
Propane 40 psi Oxygen 38 psi Air 70 psi.
After repeated washes with analar grade methanol the collected powder
was rinsed with acetone and dried. It was then passed through a
series of filters. Typical yields were as shown below:
(74)