ModernMaterials

Chapter 12Modern Materials

John D. Bookstaver

St. Charles Community College

St. Peters, MO

2006, Prentice Hall, Inc.

Chemistry, The Central Science, 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.;

and Bruce E. Bursten

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Types of Materials

Recall that atomic orbitals mix to give rise to molecular orbitals.

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Types of Materials

As the number of atoms grows, so does the number of molecular orbitals.

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Types of Materials

In such compounds, the energy gap between molecular orbitals essentially disappears, and continuous bands of energy states result.

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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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SuperconductorsThe 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.

Ethylene Polyethylene

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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 mustBe biocompatible.Have certain physical

requirements.Have certain chemical

requirements.

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Biomaterials

• BiocompatibilityMaterials cannot cause

inflammatory responses.

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Biomaterials• Physical

RequirementsProperties must mimic

the properties of the “real” body part (e.g., flexibility, hardness, etc.).

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Biomaterials• Chemical

RequirementsCannot 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 valvesVascular grafts

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Biomaterials

• These substances are used to make:Heart valvesVascular graftsArtificial skin grafts

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Biomaterials

• These substances are used to make:Heart valvesVascular graftsArtificial 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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ElectronicsIn 2000, Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa won a Nobel Prize for the discovery of “organic semiconductors” like the polyacetylene below.

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

H H H H H H H H H

HHHHHHHHH

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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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NanoparticlesFinely 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.

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