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

- 110 -
figures may arise partly from a bulk susceptibility correction due
to the geometry of the samples used in each case. However a calc-
ulation similar to that performed for aluminium in Section 7.2.3(c)
puts the possible contribution to the experimental Knight shift
measured here of no more than 5 ppm. Certainly there was no measur-
able difference in the resonance frequencies of 51V
powder specimens and the solid araldite slugs.
8.3 NIOBIUM
in the loose
99.9% pure niobium powder with a particle size of less than
40 dun was obtained from Metals Research Limited. The static measure-
ments were recorded on specimens of the loosely packed unannealed
powder at room temperature using a resonant frequency of 15 MHz.
The spin-lattice relaxation time was measured using a 90-T-90
pulse sequence, but the recovery envelope was not found to obey a
simple exponential relationship. A large proportion of the magnet-
ization recovered in a few microseconds, but a slow component had a
time constant of about 1.1 ms. This form of recovery indicates
that there was a significant second-order quadrupole broadening
present in the powder specimens, a result which is in agreement with
that recorded by y Butterworth in similar powdered metal samples.
The FIDs recorded from these samples were found to be almost
exponential with a time constant of 11.5 ± 1.5 is. This value is
considerably shorter than that found by Butterworth from a pure foil
sample, but is consistent with his measured T2 values from powdered
specimens and the large variation in linewidths reported elsewhere.
The width of the Lorentzian lineshape defined by the exponential decay
of the transverse magnetization is given by
(127)