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

2. Modern Materials Types of Materials Recall that atomic orbitals mix to give rise to molecular orbitals.

3. Modern Materials Types of Materials As the number of atoms grows, so does the number of molecular orbitals.

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

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

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

7. Modern Materials Types of Materials

8. Modern Materials Metals Valence electrons are in a partially filled band.

9. Modern Materials 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.

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

11. Modern Materials 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).

12. Modern Materials 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.

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

14. Modern Materials 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.

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

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

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

18. Modern Materials Polymers Molecules of high molecular mass made by sequentially bonding repeating units called monomers.

19. Modern Materials Some Common Polymers

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

21. Modern Materials 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.

22. Modern Materials Synthesis of Nylon Nylon is one example of a condensation polymer.

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

24. Modern Materials 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.

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

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

27. Modern Materials Cross-Linking Naturally occurring rubber is too soft and pliable for many applications.

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

29. Modern Materials Ceramics Made from a suspension of metal hydroxides (called a sol)

30. Modern Materials 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.

31. Modern Materials Biomaterials Materials must  Be biocompatible.  Have certain physical requirements.  Have certain chemical requirements.

32. Modern Materials Biomaterials Biocompatibility  Materials cannot cause inflammatory responses.

33. Modern Materials Biomaterials Physical Requirements  Properties must mimic the properties of the “real” body part (e.g., flexibility, hardness, etc.).

34. Modern Materials 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.

35. Modern Materials Biomaterials These substances are used to make:  Heart valves

36. Modern Materials Biomaterials These substances are used to make:  Heart valves  Vascular grafts

37. Modern Materials Biomaterials These substances are used to make:  Heart valves  Vascular grafts  Artificial skin grafts

38. Modern Materials Biomaterials These substances are used to make:  Heart valves  Vascular grafts  Artificial skin grafts  “Smart” sutures

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

40. Modern Materials 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.

41. Modern Materials Electronics Noncrystalline silicon panels can convert visible light into electrical energy.

42. Modern Materials 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.

43. Modern Materials Liquid Crystals Unlike liquids, molecules in liquid crystals have some degree of order.

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

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

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

47. Modern Materials Liquid Crystals These crystals can exhibit color changes with changes in temperature.

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

49. Modern Materials 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.

50. Modern Materials 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.

51. Modern Materials Nanoparticles Finely divided metals can have quite different properties than larger samples of metals.

52. Modern Materials Carbon Nanotubes Carbon nanotubes can be made with metallic or semiconducting properties without doping.

53. Modern Materials This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by the U.S. Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal opportunity employer and does not discriminate on the following basis: against any individual in the United States, on the basis of race, color, religion, sex, national origin, age disability, political affiliation or belief; and against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998 (WIA), on the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in the United States, or his or her participation in any WIA Title I-financially assisted program or activity. This product was funded by a grant awarded under the President’s High Growth Job Training Initiative, as implemented by the U.S. Department of Labor’s Employment & Training Administration. The information contained in this product was created by a grantee organization and does not necessarily reflect the official position of the U.S. Department of Labor. All references to non-governmental companies or organizations, their services, products, or resources are offered for informational purposes and should not be construed as an endorsement by the Department of Labor. This product is copyrighted by the institution that created it and is intended for individual organizational, non-commercial use only.