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1.05 Fluctuation Electron Microscopy FEM uses spatial fluctuations in diffraction from nanoscale volumes to detect MRO in amorphous materials 66,74 using a TEM or STEM. In amorphous materials, homogeneous atomic structure means homogeneously random or maximally disordered and heterogeneous means more ordered. If the structure is homogeneous, the spatial fluctuations in diffraction will be small. If the structure is heterogeneous, the spatial fluctuations in diffraction will be large. At the other extreme of order, a homogeneous single crystal will show essentially zero fluctuations. To date, FEM has shown fundamentally new medium-range structure information in several different amorphous systems. Initially FEM was used to study amorphous semiconductors 72,73,75 , but recently it has been used to study amorphous oxides 76,77 , amorphous carbon films 78 , and amorphous metals 40,79 . FEM is done by systematically varying k to find conditions that excite Bragg-like diffraction from regions with ordered atomic structure. The diffracted intensity from an ordered region scales as the square of the number of atoms in the region, leading to a particularly bright spot in the image. If the beam falls between strong diffracting conditions, the ordered regions will be particularly dark. All of the images, however, consist of speckle - random patterns of black and white dots, the intensity and position of which change with k - so a quantitative measurement of “bright” and “dark” is required. In order to distinguish meaningful MRO from random structural fluctuations, Treacy and Gibson 74 defined the normalized image variance, 14
( , ) V k Q = ( r, , ) − ( r, , ) 2 I( r, k, Q) 2 I k Q I k Q 2 , (1.1) where I(r, k, Q) is the diffracted intensity as a function of position r on the sample, scattering vector magnitude k, and the resolution of the experiment R = 0.61 / Q. Q is the radius of the objective aperture in reciprocal space. The brackets 〈〉 indicate averaging over r. Qualitatively, V(k, Q) is a measure of the structural heterogeneity of the sample at the length scale of the resolution R: if the sample structure is homogeneous, V will be small. If it is heterogeneous, V will be large. If V is large for some k, there must be some pseudo-planar arrangements of atoms causing diffraction in the direction defined by k, and those pseudo-planes must have spatial extent limited to 1-3 nm. Formally, V, through 〈I 2 〉, is connected to the three- and four-body atom position distribution functions 80 . Figure 1.4 shows that V(k) measures the degree of speckle in dark-field images: the dark-field image at k = 0.45 Å -1 , the peak in the V(k), has more speckle (brighter bright features and darker dark features) than the image at the minimum of V(k), at k = 0.60 Å -1 . V(k) gives information on the type and degree of MRO based on the peak location and magnitude respectively 66 . Since changes in V with k come from changes in diffraction from ordered regions, the peak position tells us about the pseudo-interplanar spacings within those regions, and since the intensity of the diffraction depends on the region’s size, the magnitude of V(k) peaks tells us something about their size and density. Other factors, including orientational anisotropy and non-spherical regions can also influence V(k) 81 . 15
Page 1 and 2: MEDIUM RANGE ORDER IN HIGH ALUMINUM
Page 3 and 4: To my wife Elisabeth i
Page 5 and 6: Abstract High Al-content metallic a
Page 7 and 8: Table of Contents List of Figures
Page 9 and 10: List of Figures Figure Page Figure
Page 11 and 12: Figure 2.8: V(Φ) at varying values
Page 13 and 14: electron beam exposure. The sharp f
Page 15 and 16: Figure 5.7: FEM data for Al88Y7Fe5
Page 17 and 18: Chapter 1. Introduction Amorphous m
Page 19 and 20: Material Type Tg ( o C) Reference P
Page 21 and 22: dH / dt (arb. units) time Figure 1.
Page 23 and 24: enough to avoid any nucleation onse
Page 25 and 26: nanocrystals are thermally stable u
Page 27 and 28: The structural difference between t
Page 29: Structures for various metallic gla
Page 33 and 34: of 1-3 nm is readily achievable in
Page 35 and 36: than the STEM FEM V 66,67,84 . This
Page 37 and 38: interesting behavior upon devitrifi
Page 39 and 40: Λ is a characteristic ordering len
Page 41 and 42: amorphous matrix crystal l R d R bi
Page 43 and 44: For a crystal oriented to a Bragg c
Page 45 and 46: number of crystals in a column will
Page 47 and 48: 3 d π 2 Rt max Zmax Φ = . (2.21)
Page 49 and 50: eciprocal lattice point 1 d hkl awa
Page 51 and 52: A hkl 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
Page 53 and 54: V(C hklΦ,d) 0.4 0.3 0.2 0.1 0.4 0.
Page 55 and 56: V(C 111 Φ) 0.12 0.10 0.08 0.06 0.0
Page 57 and 58: Φ = 6π d ( 72 − 6π + π ρ) m
Page 59 and 60: V(t) 0.16 0.14 0.12 0.10 0.08 0.06
Page 61 and 62: ⎛ 3 ⎛ 4 6tΩ⎞⎞ d = ⎜ Chkl
Page 63 and 64: Chapter 3. Experimental Set-up Prep
Page 65 and 66: A B 0.25 1/Å 0.25 1/Å Figure 3.1:
Page 67 and 68: 〈IBF〉, to the bright field inte
Page 69 and 70: Once the relative thickness is know
Page 71 and 72: Given that FEM is a quantitative el
Page 73 and 74: eam at a uniform intensity across t
Page 75 and 76: Low Filtering Limit ω l (Å -1 ) 0
Page 77 and 78: sample 101 . Much of the influence
Page 79 and 80: 3.07 Thickness Dependant Thinning A
Page 81 and 82:
Residual XRD Intensity (arb. units)
Page 83 and 84:
Intensity (arb. units) 0.30 Al 88 Y
Page 85 and 86:
3.08 Beam Induced Crystallization o
Page 87 and 88:
Fraction of Atoms 0 120 keV 100 200
Page 89 and 90:
V(k) x10 -3 10 8 6 4 2 Al 92 Sm 8 P
Page 91 and 92:
to produce an accurate FEM data set
Page 93 and 94:
Deformation Al 92Sm 8 sample: Rapid
Page 95 and 96:
intensity (arb. units) 3 4 {111} {2
Page 97 and 98:
Al2O3 Al hkl d (1/nm) hkl d (1/nm)
Page 99 and 100:
easons. First, the as-spun V(k) for
Page 101 and 102:
atoms locally organized into an FCC
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in Figure 4.6. Still, this begs the
Page 105 and 106:
Φ 0.20 0.15 0.10 0.05 0.00 10 21 A
Page 107 and 108:
Chapter 5. Al88Y7Fe5 and Al88Y7Fe4C
Page 109 and 110:
diffracting condition through the m
Page 111 and 112:
increases the number of Al nanocrys
Page 113 and 114:
Fraction of nanocrystals x10 -3 Fra
Page 115 and 116:
Al 88 Y 7 Fe 5 Enthalpy (J/g) 0 -1
Page 117 and 118:
Al 88Y 7Fe 5 100 nm Al 88Y 7Fe 4Cu
Page 119 and 120:
5.04 Al88Y7Fe4Cu1 Discussion The de
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FEM is sensitive to both critical a
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devitrified structure has the possi
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fraction between the as quenched ma
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number, etc.) can give insight to t
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24. Liquid Metals Website, http://w
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66. Voyles, P. M., Gibson, J. M. &
Page 133:
105. Zuo, J. M. Electron detection
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William Stratton Ph.D. Thesis - MINDS@UW Home

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