Modern Materials © 2009, Prentice-Hall, Inc. Chapter 12 Modern Materials John D. Bookstaver St. Charles Community College Cottleville, MO Chemistry, The Central Science, 11th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten

Modern Materials © 2009, Prentice-Hall, Inc. Types of Materials Recall that atomic orbitals mix to give rise to molecular orbitals.

Modern Materials © 2009, Prentice-Hall, Inc. Types of Materials In such compounds, the energy gap between molecular orbitals essentially disappears, and continuous bands of energy states result.

Modern Materials © 2009, Prentice-Hall, Inc. Types of Materials Rather than having molecular orbitals separated by an energy gap, these substances have energy bands.

Modern Materials © 2009, Prentice-Hall, Inc. Types of Materials The gap between bands determines whether a substance is a metal, a semiconductor, or an insulator.

Modern Materials © 2009, Prentice-Hall, Inc. Types of Materials

Modern Materials © 2009, Prentice-Hall, Inc. Metals Valence electrons are in a partially- filled band.

Modern Materials © 2009, Prentice-Hall, Inc. 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.

Modern Materials © 2009, Prentice-Hall, Inc. Semiconductors Semiconductors have a gap between the valence band and conduction band of ~ kJ/mol.

Modern Materials © 2009, Prentice-Hall, Inc. 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).

Modern Materials © 2009, Prentice-Hall, Inc. 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.

Modern Materials © 2009, Prentice-Hall, Inc. Insulators The energy band gap in insulating materials is generally greater than ~350 kJ/mol. They are not conductive.

Modern Materials © 2009, Prentice-Hall, Inc. Ceramics These are inorganic solids, usually hard and brittle. They are 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 place of metals in many high-temperature applications.

Modern Materials © 2009, Prentice-Hall, Inc. Superconductors At very low temperatures, some substances lose virtually all resistance to the flow of electrons.

Modern Materials © 2009, Prentice-Hall, Inc. Superconductors Much research has been done recently into the development of high-temperature superconductors.

Modern Materials © 2009, Prentice-Hall, Inc. Superconductors The development of higher and higher temperature superconductors will have a tremendous impact on modern culture.

Modern Materials © 2009, Prentice-Hall, Inc. Polymers Polymers are molecules of high molecular mass made by sequentially bonding repeating units called monomers.

Modern Materials © 2009, Prentice-Hall, Inc. Some Common Polymers

Modern Materials © 2009, Prentice-Hall, Inc. Addition Polymers Addition polymers are made by coupling the monomers by converting  -bonds within each monomer to  -bonds between monomers. Ethylene Polyethylene

Modern Materials © 2009, Prentice-Hall, Inc. Condensation Polymers Condensation polymers are 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.

Modern Materials © 2009, Prentice-Hall, Inc. Synthesis of Nylon Nylon is one example of a condensation polymer.

Modern Materials © 2009, Prentice-Hall, Inc. Properties of Polymers Interactions between chains of a polymer lend elements of order to the structure of polymers.

Modern Materials © 2009, Prentice-Hall, Inc. 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.

Modern Materials © 2009, Prentice-Hall, Inc. Properties of Polymers Such differences in crystallinity can lead to polymers of the same substance that have very different physical properties.

Modern Materials © 2009, Prentice-Hall, Inc. Cross-Linking Chemically bonding chains of polymers to each other can stiffen and strengthen the substance.

Modern Materials © 2009, Prentice-Hall, Inc. Cross-Linking Naturally-occurring rubber is too soft and pliable for many applications.

Modern Materials © 2009, Prentice-Hall, Inc. Cross-Linking In vulcanization, chains are cross-linked by short chains of sulfur atoms, making the rubber stronger and less susceptible to degradation.

Modern Materials © 2009, Prentice-Hall, Inc. Ceramics Ceramics are made from a suspension of metal hydroxides (called a sol).

Modern Materials © 2009, Prentice-Hall, Inc. Ceramics These can undergo condensation to form a gelatinous solid (gel), that is heated to form a metal oxide, like the SiO 2 shown here.

Modern Materials © 2009, Prentice-Hall, Inc. Biomaterials Materials used in the body must –be biocompatible, –have certain physical requirements, and –have certain chemical requirements.

Modern Materials © 2009, Prentice-Hall, Inc. Biomaterials Biocompatibility –The materials used cannot cause inflammatory responses.

Modern Materials © 2009, Prentice-Hall, Inc. Biomaterials Physical Requirements –The properties of the material must mimic the properties of the “real” body part (i.e., flexibility, hardness, etc.).

Modern Materials © 2009, Prentice-Hall, Inc. Biomaterials Chemical Requirements –It cannot contain even small amounts of hazardous impurities. –Also it must not degrade into harmful substances over a long period of time in the body.

Modern Materials © 2009, Prentice-Hall, Inc. Biomaterials These substances are used to make: –Heart valves

Modern Materials © 2009, Prentice-Hall, Inc. Biomaterials These substances are used to make: –Heart valves –Vascular grafts

Modern Materials © 2009, Prentice-Hall, Inc. Biomaterials These substances are used to make: –Heart valves –Vascular grafts –Artificial skin grafts

Modern Materials © 2009, Prentice-Hall, Inc. Electronics Silicon is very abundant, and is a natural semiconductor. This makes it a perfect substrate for transistors, integrated circuits, and chips.

Modern Materials © 2009, Prentice-Hall, Inc. 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.

Modern Materials © 2009, Prentice-Hall, Inc. Electronics Noncrystalline silicon panels can convert visible light into electrical energy.

Modern Materials © 2009, Prentice-Hall, Inc. 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.

Modern Materials © 2009, Prentice-Hall, Inc. Liquid Crystals Unlike liquids, molecules in liquid crystals have some degree of order.

Modern Materials © 2009, Prentice-Hall, Inc. Liquid Crystals In nematic liquid crystals, molecules are only ordered in one dimension, along the long axis.

Modern Materials © 2009, Prentice-Hall, Inc. Liquid Crystals In smectic liquid crystals, molecules are ordered in two dimensions, along the long axis and in layers.

Modern Materials © 2009, Prentice-Hall, Inc. Liquid Crystals In cholesteryl liquid crystals, nematic- like crystals are layered at angles to each other.

Modern Materials © 2009, Prentice-Hall, Inc. Liquid Crystals These crystals can exhibit color changes with changes in temperature.

Modern Materials © 2009, Prentice-Hall, Inc. Light-Emitting Diodes In another type of semiconductor, light can be caused to be emitted (LEDs).

Modern Materials © 2009, Prentice-Hall, Inc. Nanoparticles Different sized particles of a semiconductor (like Cd 3 P 2 ) can emit different wavelengths of light depending on the size of the energy gap between bands.

Modern Materials © 2009, Prentice-Hall, Inc. Nanoparticles Finely divided metals can have quite different properties than larger samples of metals.

Modern Materials © 2009, Prentice-Hall, Inc. Carbon Nanotubes Carbon nanotubes can be made with metallic or semiconducting properties without doping.