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

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- 96 - near the time origin. <strong>By</strong> using the solid echo (and Jeener and Brockaert) pulse sequence it was shown that the decay curves for sample (a) were Gaussian right back to the time origin. However this was not found to be the case for the other samples. Both the FIDs and the solid echo responses became more exponential than Gaussian close to zero time. The anomalously high values of second moment measured by other workers in samples similar to (c) are regarded generally as indicating the presence of a nuclear quadrupole interaction. If this is so, the validity of the data points predicted by the solid echo sequence is brought into quest- ion. The response following the 90-T-9090 sequence has been calc- ulated for those cases where the internal spin interactions arise either-from the dipolar coupling between spins (71) or from the first order quadrupole broadening in an imperfect cubic crystal but not a combination of the two. Nevertheless for times less than T2 the observed solid echoes were found to follow the shape of the FIDs. ' Consequently the FID dead time points were estimated so that the preliminary part of the decay envelope resembled the outline of the solid echo maximum. Experimental values of the second and fourth moments are recorded in Table 7.1. The large variation in the second moment values of the different specimens is indicative of the relative Lorentzian character of the experimental lineshapes. The errors quoted refer to the spread of measured values about the mean. The large error values given for samples (b) and (c) arise out of the variation in the tails of the transformed lineshapes caused by the uncertainty in fitting the dead time points. These estimates do not however include any consideration of the contribution to the second moment
- 97 - TABLE 7.1. COMPUTED SECOND AND FOURTH MOMENTS OF ALUM<strong>IN</strong>IUM POWDERS POWDER SAMPLE SECOND MOMENT FOURTH MOMENT kHz2 kHz4 99.995% pure sprayed (a) 9.4 ± 0.4 225 t 80 99.999% pure filed (c) 15 ± 2.5 99.5% pure sprayed (b) 18 ±4 summation from those for wings of the lineshapes which are indistin- guishable from the base-line. With CW spectra the moments are measured from the absorption curve with some assumption made about the shape of the wings. The absolute accuracy of the computed values depends therefore upon the validity of this assumption. In this work the resolution of the computer output describing the transformed lineshape was in each case better than 1 part in 2000 so the summation procedure was extended right out to a point where the signal intensity was 1/2000 of that at vo. It was then truncated. Of course if the wings of a lineshape are truly Lorentzian then the second moment is. infinite. Measurements of fourth moment were confined to sample (a) bec- ause the long wings of the transformed lineshapes of the other samples made any estimate meaningless. The ratio M4/M2 for sample (a) is equal to 2.55 whereas for a Gaussian curve the expression is identi- cally equal to three. The error limits, arising out of the uncertainty in the lineshape tails are large, but this result does serve to illustrate that the nuclear spectrum of the sprayed powder shown in Figure 7.1 can best be described as a flat topped Gaussian. This form of lineshape is further evident from the rather unusual shape - -
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NNR IN RAPIDLY ROTATED METALS By R.
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I wish to express my thanks to: ACK
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quadrupole relaxation. However theo
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2.3.5 Quadrupole Relaxation 26 (i)
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5.9 Sample Preparation 71 5.9.1 Cup
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1.1 BASIC THEORY -i- CHAPTER 1 INTR
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-3- Transforming to the rotating fr
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-5- lattice and T2 is the spin-spin
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and -7- Vd(t) f(w1)cos w't dw Sao -
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-9- where the terms represent the Z
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- 11 - If rii = nrij = (Xli + X2j +
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- 13 - In powdered samples a weak i
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- 15 - This contact term manifests
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- 17 - where 0 is the angle between
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- 19 - termed pseudo-dipolar. It is
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- 21 - the non-secular terms of the
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- 23 - to the lattice directly via
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- 25 - (2.25), can then be assumed
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- 27 - quadrupole moments to produc
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- 29 - where En are the Zeeman ener
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3.1 INTRODUCTION - 31 - CHAPTER 3 N
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Aý T. B a A"T"B " - 33 - It is a g
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(-Ur Ho FIGURE 3.1. : Co-ordinate s
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D, (t) = 331 2 + 3YI2 sin 2a 4 - 36
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- 38 - X (t) = cos a cos ßp + sin
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- 40 - 3.7 THE EFFECT OF ROTATION O
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- 42 - observations are inconsisten
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- 44 - that it apparently offered n
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- 46 - bearings suffer from instabi
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SI FIGURE 4.1 SCALE il cm : The con
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N to 9 . ýC 8 Ü7 Z w D 06 w ax U.
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- 50 - The rotors used in this work
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Ip IJýi O 1 t - 1 - 1 p OI ý I ý
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- 52 - at rotation speeds around 4
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- 53 - the manufacture of which was
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PTFE Tape_ FIGURE 4.7 rotors SCALE
Page 73 and 74: 4.4.4 COMMENTS - 55 - For completen
Page 75 and 76: Clamp Pf-£-- SECTION II SECTION 11
Page 77 and 78: 5.1 EQUIPMENT - 58 - CHAPTER 5 EXPE
Page 79 and 80: - 59 - the range ±1 V are digitise
Page 81 and 82: SCALE 3 cm j Cooling Gas ; tainiess
Page 83 and 84: Trensmitter EXL, FIGURE 5.3 : The s
Page 85 and 86: FIGURE $. 4 s The spectrometer prob
Page 87 and 88: Gas Tuning ransmi tter Input Receiv
Page 89 and 90: - 64 - phase can then be set so tha
Page 91 and 92: -66- The transient nuclear signals
Page 93 and 94: - 68 - 5.7.1 THE SOLID ECHO PULSE S
Page 95 and 96: - 70 - sequence may be repeated. In
Page 97 and 98: a° P/'rvu - 72 - p is the resistiv
Page 99 and 100: - 74 - combined with a low viscosit
Page 101 and 102: 6.2 RESULTS AND DISCUSSION - 76 - A
Page 103 and 104: IC 1 I. FIGURE 6.2 : Measured T1 va
Page 105 and 106: - 78 - TABLE 6.1. DEBYE TEMPERATURE
Page 107 and 108: - 80 - TABLE 6.2. RATIO OF TI VALUE
Page 109 and 110: - 82 - TABLE 6.3. DEPENDENCE OF T1
Page 111 and 112: - 84 - provides further evidence th
Page 113 and 114: - 86 - CHAPTER 7 EXPERIMENTAL RESUL
Page 115 and 116: - 88 - Lorentzian NMR lineshape of
Page 117 and 118: - 90 - at the top with epoxy glue.
Page 119 and 120: - 92 - (c) Bulk Susceptibility Effe
Page 121 and 122: - 94 - aluminium nuclei in the meta
Page 123: FIGURE 7.1 : 27A1 F. I. Ds and corr
Page 127 and 128: - 99 - and Slichter(104) reported t
Page 129 and 130: - 101 - of this value would be requ
Page 131 and 132: N JL I F- 0 z J I0 0123456789 ROTAT
Page 133 and 134: -103- results obtained at a tempera
Page 135 and 136: T10 ms 3. O 2.5 2'C H 1"C o.. v 234
Page 137 and 138: 0 - 106 - CHAPTER 8 MEASUREMENTS ON
Page 139 and 140: - 108 - process for vanadium the me
Page 141 and 142: - 109 - in the same way to the prec
Page 143 and 144: Qv == , 2 28 kHz -111- which is sli
Page 145 and 146: - 113 - from Cerac Inc. (USA). The
Page 147 and 148: - 115 - 8.4.3 SPIN-LATTICE RELAXATI
Page 149 and 150: tv D Xm =o N O Ul- X- z N O FIGURE
Page 151 and 152: which gives K=0.048 an ± 0.002 - 1
Page 153 and 154: - 118 - Lifetime broadening alone g
Page 155 and 156: - 120 - cannot be spun at low tempe
Page 157 and 158: - 122 - 11. L. E. Orgel, J. Phys. C
Page 159 and 160: - 124 - 59. P. J. Randall, Ph. D. T
Page 161 and 162: - 126 - 101. H. R. Khan, J. M. Reyn
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NNR IN RAPIDLY ROTATED METALS By - Nottingham eTheses ...

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