Thuc tap ve phan tich TEM
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Thuc tap ve phan tich TEM

Thuc tap ve phan tich TEM

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Thuc tap ve phan tich TEM Presentation Transcript

  • 1. Exercise: Indexing of the electron diffraction patterns Louisa Meshi
  • 2. Formation of electron diffraction and HRTEM image
  • 3. Ewald sphere construction:
    • Bragg’s conditions are satisfied when the Ewald sphere cuts a reciprocal lattice point specified by the indices of the reflecting plane.
    sin  = = = g/2 1/  1/  1/d hkl * 1/2 =  /2d hkl Bragg’s law g hkl O P hkl Origin of the reciprocal lattice 2  specimen 1/  Points of reciprocal lattice (hkl) plane
  • 4. For diffraction in electron microscope:
    • The single crystal electron diffraction pattern is a series of spots equivalent to a magnified view of a planar section through the reciprocal lattice normal to the incident beam .
    specimen Ewald sphere (1/  >>g) 1/  Camera Length (L) r L r = 1  g ; rd hkl =L  , L  - camera constant r
  • 5. Types of electron diffraction patterns:
    • Ring pattern – from polysrystalline specimen. Major use:
        • Identification of the phases;
        • Analysis of texture;
        • Determination of the camera constant L  .
    • Spot pattern – from single-crystal region of the specimen. Major use:
        • The foil orientation can be determined;
        • Identification of phases;
        • The orientation relationship between structures can be determined.
  • 6. Ring pattern:
    • The reciprocal lattice becomes a series of sphere concentric with the origin of the reciprocal lattice.
    • The main steps of indexing ring patterns:
    • Measuring ring diameters D 1 , D 2 , D 3 …….
    • Calculation of the d hkl (using the expression: rd hkl =L  )
    • Use some structure database to index each ring.
    beam O hkl sphere D
  • 7. Spot pattern
    • All diffraction spots are obtained from planes belonging to one zone .
    O g 1 g 2 g 3 Crystal beam Ewald sphere Reciprocal lattice plane h 1 k 1 l 1 h 2 k 2 l 2 beam Zone of reflecting planes B – is a zone axis B Schematic representation of diffraction pattern: Real diffraction pattern: h 1 k 1 l 1 h 2 k 2 l 2
  • 8. Indexing the SAED pattern (spot pattern):
    • Choose a parallelogram with smallest R 1 , R 2 , R 3 .
    • Measure distances R 1 , R 2 , R 3 and angles  1 ,  2 .
    • Calculate d 1 ,d 2 ,d 3 (using the rule rd=L  ).
    • Correlate the measured d-values with d hkl taken from the list of standard interplanar distances for the given structure and ascribe h 1 k 1 l 1 and h 2 k 2 l 2 and h 3 k 3 l 3 indices for the chosen three spots.
    • Check the condition that h 1 +h 2 =h 3 ; k 1 +k 2 =k 3 ; l 1 +l 2 =l 3 .
    • Compare the measured angles (both  1 and  2 ) with the calculated angles.
    h 1 k 1 l 1 h 2 k 2 l 2 h 3 k 3 l 3  1  2 R 3 R 1 R 2 Zone axis of the ED pattern = (h 1 k 1 l 1 ) (h 2 k 2 l 2 )
  • 9. Practice time:
    • In the tutorial of the school you will find three electron diffraction patterns.
    • These patterns are taken from Cu and Al. (Crystallographic data and L  of the microscope - are given).
    • Index the SAED patterns and calculate the Zone Axis (ZA).