Chapter 12. Modern Materials. Chapter 12 Problems 7/3/2012. Problems 1, 4, 7, 9, 11, 13, 23, 29, 3143, 53, 55

Transcription

1 hemistry, The entral Science, 11th edition Theodore L. Brown, H. Eugene LeMay, Jr., and Bruce E. Bursten hapter 12 John D. Bookstaver St. harles ommunity ollege ottleville, MO hapter 12 Problems Problems 1, 4, 7, 9, 11, 13, 23, 29, 3143, 53, 55 onductors, Semiconductors, and Insulators 1

2 Delocalized Bonding in Sodium Metal Bonding in Metals Band theory. Extension of MO theory. N atoms give N orbitals that are closely spaced in energy. N/2 are filled. The valence band. N/2 are empty. The conduction band. Band Theory 2

3 Types of Recall that atomic orbitals mix to give rise to molecular orbitals. Types of In such compounds, the energy gap between molecular orbitals essentially disappears, and continuous bands of energy states result. Types of Rather than having molecular orbitals separated by an energy gap, these substances have energy bands. 3

4 Types of The gap between bands determines whether a substance is a metal, a semiconductor, or an insulator. Types of Metals Valence electrons are in a partiallyfilled band. 4

5 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. Semiconductors Semiconductors have a gap between the valence band and conduction band of ~ kj/mol. 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). 5

6 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. Insulators The energy band gap in insulating materials is generally greater than ~350 kj/mol. They are not conductive. eramics 6

7 eramics These are inorganic solids, usually hard and brittle. They are highly resistant to heat, corrosion and wear. eramics 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. Superconductors At very low temperatures, some substances lose virtually all resistance to the flow of electrons. Superconductors Much research has been done recently into the development of high-temperature superconductors. 7

8 Superconductors The development of higher and higher temperature superconductors will have a tremendous impact on modern culture. Polymers Polymers are molecules of high molecular mass made by sequentially bonding repeating units called monomers. Some ommon Polymers 8

9 Addition Polymers Addition polymers are made by coupling the monomers by converting -bonds within each monomer to -bonds between monomers. Ethylene Polyethylene ondensation Polymers ondensation 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. Synthesis of Nylon Nylon is one example of a condensation polymer. 9

10 Properties of Polymers Interactions between chains of a polymer lend elements of order to the structure of polymers. 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. Properties of Polymers Such differences in crystallinity can lead to polymers of the same substance that have very different physical properties. 10

11 ross-linking hemically bonding chains of polymers to each other can stiffen and strengthen the substance. ross-linking Naturally-occurring rubber is too soft and pliable for many applications. ross-linking In vulcanization, chains are cross-linked by short chains of sulfur atoms, making the rubber stronger and less susceptible to degradation. 11

12 eramics eramics are made from a suspension of metal hydroxides (called a sol). eramics These can undergo condensation to form a gelatinous solid (gel), that is heated to form a metal oxide, like the SiO 2 shown here. Biomaterials used in the body must be biocompatible, have certain physical requirements, and have certain chemical requirements. 12

13 Biomaterials Biocompatibility The materials used cannot cause inflammatory responses. Biomaterials Physical Requirements The properties of the material must mimic the properties of the real body part (i.e., flexibility, hardness, etc.). Biomaterials hemical 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. 13

14 Biomaterials These substances are used to make: Heart valves Biomaterials These substances are used to make: Heart valves Vascular grafts Biomaterials These substances are used to make: Heart valves Vascular grafts Artificial skin grafts 14

15 Electronics Silicon is very abundant, and is a natural semiconductor. This makes it a perfect substrate for transistors, integrated circuits, and chips. 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. H H H H H H H H H H H H H H H H H H Electronics Noncrystalline silicon panels can convert visible light into electrical energy. 15

16 Liquid rystals 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. Liquid rystals Unlike liquids, molecules in liquid crystals have some degree of order. Liquid rystals In nematic liquid crystals, molecules are only ordered in one dimension, along the long axis. 16

17 Liquid rystals In smectic liquid crystals, molecules are ordered in two dimensions, along the long axis and in layers. In cholesteryl liquid crystals, nematiclike crystals are layered at angles to each other. Liquid rystals Liquid rystals These crystals can exhibit color changes with changes in temperature. 17

18 Light-Emitting Diodes In another type of semiconductor, light can be caused to be emitted (LEDs). Nanoparticles Different sized particles of a semiconductor (like d 3 P 2 ) can emit different wavelengths of light depending on the size of the energy gap between bands. Nanoparticles Finely divided metals can have quite different properties than larger samples of metals. 18

19 arbon Nanotubes arbon nanotubes can be made with metallic or semiconducting properties without doping. 19