Chapter 12 Modern Materials

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Chapter 12Modern Materials
Chemistry, The Central Science, 10th
edition Theodore L. Brown H. Eugene LeMay, Jr.
and Bruce E. Bursten

• John D. Bookstaver
• St. Charles Community College
• St. Peters, MO
• ? 2006, Prentice Hall, Inc.

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Types of Materials

• Rather than molecular orbitals separated by an
  energy gap, these substances have energy bands.

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Types of Materials

• The gap between bands determines whether a
  substance is a metal, a semiconductor, or an
  insulator.

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Types of Materials
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Metals

• Valence electrons are in a partially filled band.

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Metals

• There is virtually no energy needed for an
  electron to go from the lower, occupied part of
  the band to the higher, unoccupied part.
• This is how a metal conducts electricity.

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Semiconductors

• Semiconductors have a gap between the valence
  band and conduction band of 50 to 300 J/mol

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Semiconductors

• Among elements, only silicon, germanium, and
  graphite (carbon), all of which have 4 valence
  electrons, are semiconductors.
• Inorganic semiconductors (like GaAs) tend to have
  an average of 4 valence electrons (3 for Ga, 5
  for As).

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Doping

• By introducing very small amounts of impurities
  that have more (n-type) or fewer (p-type) valence
  electrons, one can increase the conductivity of a
  semiconductor.

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Insulators

• The energy band gap in insulating materials is
  generally greater than 350 kJ/mol.
• They are not conductive.

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Ceramics

• They are inorganic solids, usually hard and
  brittle.
• Highly resistant to heat, corrosion, and wear.
• Ceramics do not deform under stress.
• They are much less dense than metals, and so are
  used in their place in many high-temperature
  applications.

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Superconductors

• At very low temperatures, some substances lose
  virtually all resistance to the flow of electrons.

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Superconductors

• Much research has been done recently into the
  development of high-temperature superconductors.

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Superconductors

• The development of higher and higher temperature
  superconductors will have a tremendous impact on
  modern culture.

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Polymers

• Molecules of high molecular mass made by
  sequentially bonding repeating units called
  monomers.

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Some Common Polymers
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Addition Polymers

• Made by coupling the monomers by converting
  ?-bonds within each monomer to ?-bonds between
  monomers.

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Condensation Polymers

• Made by joining two subunits through a reaction
  in which a smaller molecule (often water) is also
  formed as a by-product.
• These are also called copolymers.

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Synthesis of Nylon

• Nylon is one example of a condensation polymer.

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Properties of Polymers

• Interactions between chains of a polymer lend
  elements of order to the structure of polymers.

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Properties of Polymers

• Stretching the polymer chains as they form can
  increase the amount of order, leading to a degree
  of crystallinity of the polymer.

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Properties of Polymers

• Such differences in crystallinity can lead to
  polymers of the same substance that have very
  different physical properties.

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Cross-Linking

• Chemically bonding chains of polymers to each
  other can stiffen and strengthen the substance.

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Cross-Linking

• Naturally occurring rubber is too soft and
  pliable for many applications.

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Cross-Linking

• In vulcanization, chains are cross-linked by
  short chains of sulfur atoms, making the rubber
  stronger and less susceptible to degradation.

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Ceramics

• Made from a suspension of metal hydroxides
  (called a sol)

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Ceramics

• These can undergo condensation to form a
  gelatinous solid (gel), that is heated to form a
  metal oxide, like the SiO2 shown here.

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Biomaterials

• Materials must
• Be biocompatible.
• Have certain physical requirements.
• Have certain chemical requirements.

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Biomaterials

• Biocompatibility
• Materials cannot cause inflammatory responses.

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Biomaterials

• Physical Requirements
• Properties must mimic the properties of the
  real body part (e.g., flexibility, hardness,
  etc.).

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Biomaterials

• Chemical Requirements
• Cannot contain even small amounts of hazardous
  impurities.
• Cannot degrade into harmful substances over a
  long period of time in the body.

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Biomaterials

• These substances are used to make
• Heart valves

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Biomaterials

• These substances are used to make
• Heart valves
• Vascular grafts

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Biomaterials

• These substances are used to make
• Heart valves
• Vascular grafts
• Artificial skin grafts

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Biomaterials

• These substances are used to make
• Heart valves
• Vascular grafts
• Artificial skin grafts
• Smart sutures

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Electronics

• Silicon is very abundant, and is a natural
  semiconductor.
• This makes it a perfect substrate for
  transistors, integrated circuits, and chips.

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Electronics

• In 2000, Alan J. Heeger, Alan G. MacDiarmid, and
  Hideki Shirakawa won a Nobel Prize for the
  discovery of organic semiconductors like the
  polyacetylene below.

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Electronics

• Noncrystalline silicon panels can convert
  visible light into electrical energy.

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Liquid Crystals

• Some substances do not go directly from the solid
  state to the liquid state.
• In this intermediate state, liquid crystals have
  some traits of solids and some of liquids.

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Liquid Crystals

• Unlike liquids, molecules in liquid crystals
  have some degree of order.

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Liquid Crystals

• In nematic liquid crystals, molecules are only
  ordered in one dimension, along the long axis.

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Liquid Crystals

• In smectic liquid crystals, molecules are
  ordered in two dimensions, along the long axis
  and in layers.

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Liquid Crystals

• In cholesteric liquid crystals, nematic-like
  crystals are layered at angles to each other.

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Liquid Crystals

• These crystals can exhibit color changes with
  changes in temperature.

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Light-Emitting Diodes

• In another type of semiconductor, light can be
  caused to be emitted (LEDs).

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Light-Emitting Diodes (LEDs)

• Organic light-emitting diodes (OLEDs) are lighter
  and more flexible, and can be brighter and more
  energy efficient.
• Soon OLEDs may replace incandescent lights in
  some applications.

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Nanoparticles

• Different-sized particles of a semiconductor
  (like Cd3P2) can emit different wavelengths of
  light depending on the size of the energy gap
  between bands.

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Nanoparticles

• Finely divided metals can have quite different
  properties than larger samples of metals.

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Carbon Nanotubes

• Carbon nanotubes can be made with metallic or
  semiconducting properties without doping.