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

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4.1 <strong>IN</strong>TRODUCTION - 43 - CHAPTER 4 EXPERIMENTAL ROTATION SYSTEMS The technique of magic angle sample rotation has been used to observe fine structure in the NMR spectra of materials, which is otherwise obscured by dipolar broadening(47). The rotation rates employed have to be approximately the same as the static line- width, and complete removal of the tensor broadening interactions requires rates considerably in excess of this value. To this end it has been necessary to use systems of rotation which are capable of extremely high spinning frequencies. The highest rotation rate ever recorded is 1.5 MHz, using a fine metallic wire supported magnetically(48). Obviously similar forms of suspen- sion are excluded by the limitations imposed by NMR, so small gas turbine systems are normally employed. In the course of the work reported here three different such turbine systems were constructed and used, which we shall call the conical, foil bearing and Prague systems. Although the conical rotors (19) have been developed in the laboratory over a period of some years, the foil bearing rotors represented a new approach to the high speed rotation technique. Some time was spent on the development and successful adaptation of these rotors for use in conjunction with NMR measurements. The third type of gas turbine was constructed following the design used by a group of experi- mentalists in Prague(50). A study of this type of turbine showed
- 44 - that it apparently offered no advantages over the other rotor systems. Because of the non-trivial nature of the problem we include below a short discussion of some of the criteria relevant to the design of gas turbine systems for use in conjunction with NMR. 4.2 ASPECTS OF ROTOR DESIGN 4.2.1 MATERIALS The materials from which the rotors and the surrounding rig are constructed must not upset the NMR measurements either by affecting the homogeneity of the magnetic field or by causing stray pick-up in the detection coil. The rotor itself cannot be made from any metal because the speed of rotation in the external field is then severely restricted by induced eddy currents. 4.2.2 SIZE The size of the complete rotor system is. limited by the magnet gap. However the sample volume must be such that a reasonable signal-to-noise figure can be attained with the NMR equipment avail- able. Although this condition varies greatly depending upon the NMR sensitivity of the particular nuclei under investigation most turbine systems used are capable of carrying a sample of volume between 0.1 and 1.0 cc. The sensitivity of the system is considerably-enhanced by winding the detection coil co-axially about the rotor. The fill- ing factor is maximized in this way but it should be remembered that, if the same coil is used in the transmitter circuit, the Hl field produced is not perpendicular to Ho.
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NNR IN RAPIDLY ROTATED METALS By R.
Page 3 and 4: I wish to express my thanks to: ACK
Page 5 and 6: quadrupole relaxation. However theo
Page 7 and 8: 2.3.5 Quadrupole Relaxation 26 (i)
Page 9 and 10: 5.9 Sample Preparation 71 5.9.1 Cup
Page 11 and 12: 1.1 BASIC THEORY -i- CHAPTER 1 INTR
Page 13 and 14: -3- Transforming to the rotating fr
Page 15 and 16: -5- lattice and T2 is the spin-spin
Page 17 and 18: and -7- Vd(t) f(w1)cos w't dw Sao -
Page 19 and 20: -9- where the terms represent the Z
Page 21 and 22: - 11 - If rii = nrij = (Xli + X2j +
Page 23 and 24: - 13 - In powdered samples a weak i
Page 25 and 26: - 15 - This contact term manifests
Page 27 and 28: - 17 - where 0 is the angle between
Page 29 and 30: - 19 - termed pseudo-dipolar. It is
Page 31 and 32: - 21 - the non-secular terms of the
Page 33 and 34: - 23 - to the lattice directly via
Page 35 and 36: - 25 - (2.25), can then be assumed
Page 37 and 38: - 27 - quadrupole moments to produc
Page 39 and 40: - 29 - where En are the Zeeman ener
Page 41 and 42: 3.1 INTRODUCTION - 31 - CHAPTER 3 N
Page 43 and 44: Aý T. B a A"T"B " - 33 - It is a g
Page 45 and 46: (-Ur Ho FIGURE 3.1. : Co-ordinate s
Page 47 and 48: D, (t) = 331 2 + 3YI2 sin 2a 4 - 36
Page 49 and 50: - 38 - X (t) = cos a cos ßp + sin
Page 51 and 52: - 40 - 3.7 THE EFFECT OF ROTATION O
Page 53: - 42 - observations are inconsisten
Page 57 and 58: - 46 - bearings suffer from instabi
Page 59 and 60: SI FIGURE 4.1 SCALE il cm : The con
Page 61 and 62: N to 9 . ýC 8 Ü7 Z w D 06 w ax U.
Page 63 and 64: - 50 - The rotors used in this work
Page 65 and 66: Ip IJýi O 1 t - 1 - 1 p OI ý I ý
Page 67 and 68: - 52 - at rotation speeds around 4
Page 69 and 70: - 53 - the manufacture of which was
Page 71 and 72: 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
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- 78 - TABLE 6.1. DEBYE TEMPERATURE
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- 80 - TABLE 6.2. RATIO OF TI VALUE
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- 82 - TABLE 6.3. DEPENDENCE OF T1
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- 84 - provides further evidence th
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- 86 - CHAPTER 7 EXPERIMENTAL RESUL
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- 88 - Lorentzian NMR lineshape of
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- 90 - at the top with epoxy glue.
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- 92 - (c) Bulk Susceptibility Effe
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- 94 - aluminium nuclei in the meta
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FIGURE 7.1 : 27A1 F. I. Ds and corr
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- 97 - TABLE 7.1. COMPUTED SECOND A
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- 99 - and Slichter(104) reported t
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- 101 - of this value would be requ
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N JL I F- 0 z J I0 0123456789 ROTAT
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-103- results obtained at a tempera
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T10 ms 3. O 2.5 2'C H 1"C o.. v 234
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0 - 106 - CHAPTER 8 MEASUREMENTS ON
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- 108 - process for vanadium the me
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- 109 - in the same way to the prec
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Qv == , 2 28 kHz -111- which is sli
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- 113 - from Cerac Inc. (USA). The
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- 115 - 8.4.3 SPIN-LATTICE RELAXATI
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tv D Xm =o N O Ul- X- z N O FIGURE
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which gives K=0.048 an ± 0.002 - 1
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- 118 - Lifetime broadening alone g
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- 120 - cannot be spun at low tempe
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- 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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