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Transcript

  • 1. Atomic Structure and Properties
    • Today we will discuss:
    • The structure of metals, insulators and semi-conductors and how this affects:
        • Electrical properties
        • Appearance
        • Mechanical properties
    29 May 2009
  • 2. Metals
    • Metals - Because the free electrons in metals:
      • conduct well - are mobile and carry current
      • are shiny - oscillate in light, scattering light photons
      • are stiff - ‘glue’ ions together strongly
      • are ductile - provide non-directional glue, letting ions slip
    All atoms are ionised and the electrons are free to move…
  • 3. Insulators - Ceramics
    • Ceramics - Because the ionic bonds holding them together:
    • are insulators - lock electrons to ions or atoms, so none are free to move
    • are stiff - are hard to stretch strong bonds
    • are brittle - are directional bonds, so that atoms or ions cannot slip
    Electrons lock to ions so none are free to move…
  • 4. Insulators - Polymers
    • Polymers - Because the covalent bonds stringing monomers in long chains:
      • are insulators - lock electrons to atoms, with none free to move
      • are often flexible- can rotate, letting chains stretch or fold
      • are often plastic – make chains which can slip past one another
    The covalent bonds, stringing monomers into long chains…
  • 5. Semi-Conductors
    • These have properties in-between:
      • Metals and
      • Insulators
      • These have far fewer ionised atoms (and hence fee electrons)
  • 6. Semiconductors and Conductivity
  • 7. But what happens…
    • When things get hot!!!?!?!?!?
    • Using a beaker, tripod, multimeter, a piece of wire and a semiconductor sample (thermistor), test:
    • The effect of temperature on conductivity (measure resistance) of metals and semiconductors…
  • 8. What is happening…?
  • 9. Doping
    • Obviously it is not practical to heat up semiconductors – to increase conductivity we ‘dope’ them with other atoms.
    • Using page 124 summarise, with pictures, the 2 types of doping