Chapter 12 Modern Materials

1. Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Chapter 12Modern Materials John D. Bookstaver St. Charles Community College St. Peters, MO  2006, Prentice Hall, Inc.

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

3. Types of Materials The gap between bands determines whether a substance is a metal, a semiconductor, or an insulator.

4. Types of Materials

5. Metals Valence electrons are in a partially filled band.

6. 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.

7. Semiconductors Semiconductors have a gap between the valence band and conduction band of ~50 to 300 J/mol

8. 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).

9. 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.

10. Insulators • The energy band gap in insulating materials is generally greater than ~350 kJ/mol. • They are not conductive.

11. 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.

12. Superconductors At very low temperatures, some substances lose virtually all resistance to the flow of electrons.

13. Superconductors Much research has been done recently into the development of high-temperature superconductors.

14. Superconductors The development of higher and higher temperature superconductors will have a tremendous impact on modern culture.

15. Polymers Molecules of high molecular mass made by sequentially bonding repeating units called monomers.

16. Some Common Polymers

17. Ethylene Polyethylene Addition Polymers Made by coupling the monomers by converting -bonds within each monomer to -bonds between monomers.

18. 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.

19. Synthesis of Nylon Nylon is one example of a condensation polymer.

20. Properties of Polymers Interactions between chains of a polymer lend elements of order to the structure of polymers.

21. 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.

22. Properties of Polymers Such differences in crystallinity can lead to polymers of the same substance that have very different physical properties.

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

24. Cross-Linking Naturally occurring rubber is too soft and pliable for many applications.

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

26. Ceramics Made from a suspension of metal hydroxides (called a sol)

27. Ceramics These can undergo condensation to form a gelatinous solid (gel), that is heated to form a metal oxide, like the SiO2 shown here.

28. Biomaterials • Materials must • Be biocompatible. • Have certain physical requirements. • Have certain chemical requirements.

29. Biomaterials • Biocompatibility • Materials cannot cause inflammatory responses.

30. Biomaterials • Physical Requirements • Properties must mimic the properties of the “real” body part (e.g., flexibility, hardness, etc.).

31. Biomaterials • Chemical Requirements • Cannot contain even small amounts of hazardous impurities. • Cannot degrade into harmful substances over a long period of time in the body.

32. Biomaterials • These substances are used to make: • Heart valves

33. Biomaterials • These substances are used to make: • Heart valves • Vascular grafts

34. Biomaterials • These substances are used to make: • Heart valves • Vascular grafts • Artificial skin grafts

35. Biomaterials • These substances are used to make: • Heart valves • Vascular grafts • Artificial skin grafts • “Smart” sutures

36. Electronics • Silicon is very abundant, and is a natural semiconductor. • This makes it a perfect substrate for transistors, integrated circuits, and chips.

37. 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.

38. Electronics Noncrystalline silicon panels can convert visible light into electrical energy.

39. 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.

40. Liquid Crystals Unlike liquids, molecules in liquid crystals have some degree of order.

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

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

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

44. Liquid Crystals These crystals can exhibit color changes with changes in temperature.

45. Light-Emitting Diodes In another type of semiconductor, light can be caused to be emitted (LEDs).

46. 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.

47. 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.

48. Nanoparticles Finely divided metals can have quite different properties than larger samples of metals.

49. Carbon Nanotubes Carbon nanotubes can be made with metallic or semiconducting properties without doping.