Electron Paramagnetic Resonance at Hunter College

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Electron Paramagnetic Resonance at Hunter College

• Makia Hughes
• John F. Kennedy High SchoolPhysics Department,
  Hunter College of Solid State NMR/EPR Lab EPR
  measurements of Lithiated Silver Vanadium
  Oxide Dr. John Flowers, Professor of Chemistry
  at Medgar Evers College of CUNY

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Our Goal

• To use EPR to examine and confirm the mechanisms
  of discharge in a Sulfur Vanadium Oxide (SVO)
  battery in order to develop higher quality
  batteries.

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What is EPR?

• Electron Paramagnetic Resonance also known as
  Electron Spin Resonance is a spectroscopic
  technique which detect species that have unpaired
  electrons.
• A surprisingly large number of materials have
  unpaired electrons. (examples free radicals and
  many transition metal ions)
• EPR alone yields inconvertible evidence of free
  radicals and has the unique power of identifying
  the paramagnetic species.

Emblem of the Society of Free Radical Research
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EPR Instruments

• The EPR Spectrometer consists of many different
  components.
• Microwave Bridge Electromagnetic
  Radiation Source
• Microwave Cavity Amplifies weak
  signals from the sample
• Magnet Splits the
  electronic spin energy levels
• Console Contains
  signal processing and control electronics and a
  
  units for
  acquiring a spectrum
• Computer Used to
  analyze data as well as coordinating all the
  units for acquiring a spectrum.

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How does EPR Work?

• Like a proton, an electron has a spin, which
  gives it a magnetic property known as a magnetic
  moment.
• When an external magnetic field is supplied, the
  paramagnetic electrons can either orient in a
  direction parallel or antiparallel to the
  direction of the magnetic field .
• This creates two distinct energy levels for the
  unpaired electrons and measurements are taken as
  they are driven between the two levels.

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EPR Spectroscopy

• EPR spectroscopy is the measurement and
  interpretation of the energy differences between
  the atomic or molecular states.
• These measurements are obtained because the
  relationship between the energy differences and
  the absorption of electro-magnetic radiation.
• To acquire a spectrum, the frequency of the
  electromagnetic radiation is changed and the
  amount of radiation which passes through the
  sample with a detector is measured to observe the
  spectroscopic absorptions

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EPR Results

• 4 Samples Tested
• 1) Pure SVO- Gives EPR signal due to the
  paramagnetism of SVO High Q value
• 2) Li0.72Ag2V4O11 Weak or No EPR signal
  Lower Q value
• 3) Li2.13Ag2V4O11 Weak or No EPR signal
  Lower Q value
• 4) Li5.59Ag2V4O11 Definite EPR Signal
  Lower Q value
• Supports our Hypothesis