Group Meeting

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Group Meeting

• slides by Paul Matter
• presented by John Kuhn
• 4-13-05

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Our Group
Helen (PhD student)
Jean (visiting student)
Jean-Marc
Marlene (technician)
Quyen (PhD student)
Stephane (post-doc)
Bettie (secretary)
Laurie (visiting student)
Aurélien (visiting student)
Not shown Benoit (PhD student) and Lala (Asst.
Prof.)
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Mössbauer Spectroscopy

• Easier to use than Raman spectroscopy.
• Easy to understand the theory when Jean-Marc
  explains it.
• No one here hardly ever uses it.
• Can take a long time to obtain a quality
  spectrum (2 to 3 days).

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Mössbauer Theory

• Works like any other transmission spectroscopy,
  except it uses high energy gamma rays released
  by radioactive cobalt.
• To vary the energy of the rays the sample is
  vibrated at varying velocity, creating a doppler
  effect
• E Eo(1v/c) where c is the speed of light
• Most spectroscopy has E on the x-axis, however,
  in this case E varies from 19.4000001 to
  19.4000002 eV, therefore we just report v
  (velocity) instead.

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Why so slow?
Source is several years old, but can still be
used for another year
Gamma ray intensity decreases as the sample decays
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Analayzing Results
Typical spectra
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Analayzing Results

• Deconvolution done using a Fortran program (with
  French prompts), can take a while if the spectrum
  is complicated

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Mössbauer Theory - Analysis
6N coordinated Fe species with symmetrical
electron clouds will give a single adsorption
band. The energy can shift compared to other
species.
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Mössbauer Theory - Analysis
Magnetic Fe species have splitting of the energy
levels in the nucleus and will therefore give a
sextet.
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Mössbauer Theory - Analysis
Non-symmetrically coordinated species give you a
doublet
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Mössbauer Theory - Analysis

• Isomer shift (d mm/s) is the shift in the median
  energy compared to the reference, a-Feo.
• Quadrapole splitting (D mm/s) is the gap in
  energy between the doublets.
• Magnetic Dipole Splitting (H kOe) indicates the
  strength of the magnetic field in the sample.
• These values are unique for different phases of
  Fe.
• Observing changes in the magnetic field with
  temperature can be used to determine the particle
  size.
• Peak areas can be used to quantify the phases of
  Fe in a sample.

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Initial Data

• Lingzhi has one doublet at 25oC ? all Fe(III)
  oxide
• Johns LSCF 6428 and 2828 - have a doublet with a
  small D.
• Johns LSCF 8228 - has the same doublet and a
  sextet.

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Initial Data
Spectrum for Fe / Al2O3 treated in CH3CN at 900oC
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Initial Data
Fe / Al2O3 treated in CH3CN at 900oC

• The singlet and one doublet are from g-Feo, and
  g-Feo / C nearest neighbor respectively.
• g-Feo is the stable high T Fe phase that becomes
  encased in carbon during the treatment (Baker
  87).
• Sextet is q-Fe3C (no doubt at all).
• Other doublet could be partially oxidized FexO
• Carbide phase increases with treatment time, FexO
  decreases with time

Fe / Vulcan Carbon treated in CH3CN at 900oC

• The iron carbide sextet is much smaller

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Suggestions for Experiments

• Exhaustive survey of all our samples will take
  too long
• Jean-Marc built an in situ quartz cell to allow
  for treatments up to 900oC, and subsequent
  testing without exposure to the atmosphere.
• We can design novel experiments to analyze the
  Fe phases after various treatments.
• What to dilute the samples with is the major
  concern for in situ testing.