Chapter 12Modern Materials

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Types of MaterialsRather than molecular orbitals separated by an energy gap, these substances have energy bands.

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Types of MaterialsThe 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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MetalsValence electrons are in a partially filled band.

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MetalsThere 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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SemiconductorsSemiconductors have a gap between the valence band and conduction band of ~50 to 300 J/mol

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SemiconductorsAmong 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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DopingBy 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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InsulatorsThe energy band gap in insulating materials is generally greater than ~350 kJ/mol.They are not conductive.

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

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SuperconductorsMuch 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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PolymersMolecules of high molecular mass made by sequentially bonding repeating units called monomers.

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Some Common Polymers

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Addition PolymersMade 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 NylonNylon is one example of a condensation polymer.

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Properties of PolymersInteractions between chains of a polymer lend elements of order to the structure of polymers.

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Properties of PolymersStretching 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 PolymersSuch differences in crystallinity can lead to polymers of the same substance that have very different physical properties.

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Cross-LinkingChemically bonding chains of polymers to each other can stiffen and strengthen the substance.

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Cross-LinkingNaturally occurring rubber is too soft and pliable for many applications.

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

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CeramicsMade from a suspension of metal hydroxides (called a sol)

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CeramicsThese 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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BiomaterialsMaterials must

Be biocompatible.Have certain physical requirements.Have certain chemical requirements.

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BiomaterialsBiocompatibility

Materials cannot cause inflammatory responses.

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BiomaterialsPhysical Requirements

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

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BiomaterialsChemical 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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BiomaterialsThese substances are used to make:

Heart valves

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BiomaterialsThese substances are used to make:

Heart valvesVascular grafts

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BiomaterialsThese substances are used to make:

Heart valvesVascular graftsArtificial skin grafts

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BiomaterialsThese substances are used to make:

Heart valvesVascular graftsArtificial skin graftsSmart sutures

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ElectronicsSilicon 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.

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ElectronicsNoncrystalline silicon panels can convert visible light into electrical energy.

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Liquid CrystalsSome 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 CrystalsUnlike liquids, molecules in liquid crystals have some degree of order.

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

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

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Liquid CrystalsIn cholesteric liquid crystals, nematic-like crystals are layered at angles to each other.

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Liquid CrystalsThese crystals can exhibit color changes with changes in temperature.

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Light-Emitting DiodesIn 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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NanoparticlesDifferent-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 NanotubesCarbon nanotubes can be made with metallic or semiconducting properties without doping.

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