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    5 5 Presentation Transcript

    • 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
    • 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…
    • 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…
    • 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…
    • Semi-Conductors
      • These have properties in-between:
        • Metals and
        • Insulators
        • These have far fewer ionised atoms (and hence fee electrons)
    • Semiconductors and Conductivity
    • 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…
    • What is happening…?
    • 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