Chapter 12 Modern Materials
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classes of materials
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materials for structure
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materials for medicine
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materials for electronics
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materials for optics
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materials for nanotechnology
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12.1 Classes of Materials
A goal for modern chemistry and chemists is to design materials with specific
properties. We can better understand the physical and chemical properties of
materials by considering their atomic- and molecular-level structural features.
Metals and semiconductors
To explain the bonding in metals (section 23.5) and semiconductors, we must first
understand some molecular orbital theory (section 9.7).
9.7 Molecular Orbitals
Some aspects of bonding are not explained by Lewis structures, VSEPR theory,
and hybridization, e.g. why does O2 interact with a magnetic field? Why are some
molecules coloured? How do we explain metallic bonding?
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Molecular orbitals have some characteristics are similar to those of atomic orbitals.
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The hydrogen molecule
When n AOs overlap, n MOs form. For H2, 1s (H) + 1s (H) must result in 2 MOs:
Both bonding and antibonding molecular orbitals have electron density centered
around the internuclear axis
Note similarity to mixing of atomic orbitals to make hybrid atomic orbitals: the
difference here is that the orbitals used are on two different nuclei.
Bond order
Bond order
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… back to: Metallic Bonding (section 23.5)
So how does MO theory help us explain the bonding in metals? Recall
that n AOs are used to make n MOs.
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The energy differences between orbitals are small, so promotion of
electrons requires little energy: electrons readily move through the metal.
… back to: Metals and semiconductors (section 12.1)
Materials may be classified according to their band structure.
Metals
Good electrical
conductors.
Semiconductors
Band structure has an energy gap separating totally filled bands and empty bands.
Inorganic compounds that semiconduct tend to have an average of 4 valence
electrons:
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Electrical conductivity of semiconductors may be increased by doping.
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Doping yields different kinds of semiconductors:
Insulators and ceramics
Insulators have a band structure similar to semiconductors, but they have a much
larger band gap.
Ceramics are inorganic ionic solids that are hard, brittle, less dense than metals,
stable at high temperatures, and resistant to corrosion and wear.
12.2 Materials for Structure
Soft Materials: Polymers and Plastics
Polymers are molecules of high molecular weight that are made by polymerization
(joining together) of smaller molecules of low molecular mass.
Plastics are materials that can be formed into various shapes, usually with heat
and pressure.
Elastomers are materials that exhibit elastic or rubbery behavior. If a moderate
amount of a deforming force is added, the elastomer will return to its original shape.
Making polymers
Many synthetic polymers have a backbone of C–C bonds.
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Condensation polymerization: two molecules are joined to form a larger
molecule by the elimination of a small molecule.
Structure and physical properties of polymers
Synthetic and natural polymers commonly consist
of a collection of macromolecules of different
molecular weights.
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Stretching or extruding a polymer can increase crystallinity.
Crystallinity is also strongly influenced by average molecular mass.
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Polymeric properties may be modified by additives with lower molecular mass.
Cross-linked polymers are more rigid
than straight-chain polymers:
Hard Materials: Metals and Ceramics
Ductile, malleable, and highly conductive, metals are extraordinarily useful.
Ceramics are brittle but similarly have many applications - cutting tools, abrasives,
structural support, piezoelectric materials, tiles for the space shuttle, etc.
12.3 Materials for Medicine
A biomaterial is any material that has a biomedical application.
Characteristics of Biomaterials
Choice of biomaterial for an application is influenced by its chemical characteristics.
Polymeric Biomaterials
Our bodies are composed of many biopolymers,
e.g. proteins, polysaccharides (sugar polymers),
and nucleic acids (DNA, RNA).
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Man-made polymers are usually simpler; 1 or 2 different repeat units only, e.g.
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12.4 Materials for Electronics
Many modern devices rely on silicon wafers or “chips” containing complex patterns
of semiconductors, insulators, and metal wires.
Some polymers with delocalized electrons can act as semiconductors, but these are
generally not as robust as silicon.
Semiconductors are also used in the production of
solar energy cells. If you shine light with an
appropriate wavelength on a semiconductor,
electrons are promoted to the conduction band,
making the material more conductive.
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12.5 Materials for Optics
Light-emitting diodes (LEDs) are used in many types of displays. The mechanism
of action is the opposite of that involved in solar cells.
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Organic LEDs (OLEDs) have some advantages
over traditional LEDs.
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