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

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7.5.3 VARIATION IN 'TAD' WITH ROTATION SPEED
As an interesting exercise the dipolar relaxation time
of the sprayed pure powder (a) was measured as a function of rotat-
ion speed. It was found that with the rotors spinning about the
magic axis, the modulation beats generated on the decay shapes dist-
orted the signal amplitude detected after the third pulse of the
Jeener and Brockaert sequence. However, by using the foil bearing
rotor system it proved possible to spin the samples about an axis
perpendicular to the external magnetic field. With this alignment
of the rotors the detected decay signals remained relatively undist-
orted and results could then be obtained from the Jeener and Brockaert
pulse sequence. As has been stated previously, use of the foil
bearing system meant that the aluminium sample was at a somewhat
higher temperature than the room, probably in the range 315-330 K.
The recorded variation in T1D against spinning rate is shown in
Figure 7.4. 'Rotation rates below 1.5 kHz could not be achieved with
the foil bearing rotor system. However, it was found that in the
range 1.5-2.7 kHz the relaxation times were too short to measure
with the techniques employed anyway. Because of the need to keep
the rotors spinning for quite a long period of time during each deter-
mination of T1D, measurements were limited to a maximum spinning
frequency of about 5 kHz. Compressed air could then be used as the
driving gas. No detailed theoretical consideration of the form of
the results has yet been undertaken.