How Bonding

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How Bonding Structure Affect the Observable
Properties of Materials Defects in Solids The
Relationship Between Structure Properties Chem
124 Fall 2008 Dr. Neff
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Defects

• Difficult/impossible to find or make perfect
  crystals
• Nearly all solids contain defects of one sort or
  another
• Defects can be missing, misplaced, or extra
  atoms
• Schottky defect -- vacancy in lattice

• Interstitial defect -- extra atom in holes in
  lattice

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Defects

• Dislocation -- missing row of atoms (or extra
  row of atoms)

• Frenkel defect -- atoms have moved to nonstandard
  positions

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Defects

• Extra atoms can be same as bulk or different
• When different -- Impurities --
• Intrinsic -- present naturally
• Extrinsic -- added intentionally -- ALLOYS
• Substitutional -- In regular lattice site
• Examples -- Cr in Steel -- Stainless Steel
• Zn in Cu -- Brass
• Sn in Cu -- Bronze

• Interstitial -- In-between lattice sites
• Example -- C in Steel

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How Defects Affect Observable Properties

• Presence of Defects causes/changes properties of
• metals
• Conductivity
• Presence of different atoms in lattice hinders
• electron mobility, lowering conductivity
• Melting Point
• Presence of different atoms in lattice changes
• bond strengths
• Example Larger Sn atoms in Cu lattice
• in bronze results in weaker bond strengths,
• so mp lower, bronze easier to work with
• and cast than pure Cu

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How Defects Affect Observable Properties

• Presence of Defects responsible for some
  properties of
• metals
• DISLOCATIONS responsible for Malleability/Ductil
  ity of Metals
• Really the ability to withstand lots of plastic
  deformation
• This is why metals are strong

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How Defects Affect Observable Properties

• Plastic deformation is really just movement of
  atoms
• Such movement costs energy -- requires bonds to
  be broken
• In metals, this energy cost is less than whats
  expected/calculated, so its easier to move atoms
  than it should be
• WHY ? -- dislocations

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How Defects Affect Observable Properties

• Presence of dislocations causes an energy
  imbalance or a tension in the crystal structure
• This tension is removed by atoms moving to fill
  in or correct the defect (remember movement of
  atoms means deformation)

• The energy cost associated with breaking bonds so
  this movement can occur is somewhat compensated
  for by the energy you get back upon relief of the
  tension.

SO -- it costs less to move atoms than expect --
metals easy to deform!
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How Defects Affect Observable Properties

• In perfect crystal with no such defects, more
  bonds need to be broken
• AND dont get back any energy as theres no
  tension to relieve
• -- so it costs more to move atoms

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How Defects Affect Observable Properties

• Increase Strength by adding in even MORE
  dislocations
• Cold Working or Work-Hardening
• Repeated hammering of metals introduces more
  dislocations
• Tempering
• Gentle heating to evenly distribute dislocations
• Quenching
• Rapid cooling to trap evenly distributed
  dislocations

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How Defects Affect Observable Properties

• Presence of Defects causes/changes properties of
  metals
• Presence of Impurities can also increase
  STRENGTH
• Presence of different atoms in lattice stresses
  the material
• The energy field associated with this stress
  repels that
• associated with moving dislocations, making
  such
• movement prohibitive
• Basically why C in steel makes it stronger