1. 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

2. Modern Materials Modern Materials & Material Revolution Old: Stone, Wood, Copper, Bronze, Iron New: Plastics and other synthetic materials Polymer: Various Plastics Biomaterials (heart valve, atificial tissues, …) Semiconductors: Computer Chips, Light-emiting Diode (LED), Solar Cells Ceramic and Superconductors: Liquid Crystals: LCD (PC & TV Screen) Nanomaterials © 2009, Prentice-Hall, Inc.

3. Modern Materials © 2009, Prentice-Hall, Inc. Types of Materials: Metals, Semiconductors, Conductors Review: Molecular Orbital Theory: p 369, 375 Recall that atomic orbitals mix to give rise to molecular orbitals.

4. Modern Materials Silicon Crystal Lattice: Tetrahedral base

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

6. 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.

7. Modern Materials Formation of Bands As the number of atoms grows, so does the number of molecular orbitals, then merge to form bands many MOs a BAND

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

9. Modern Materials © 2009, Prentice-Hall, Inc. Semiconductors and Insuators Rather than having molecular orbitals separated by an energy gap, these substances have energy bands. Conduction Band: from antibonding MOs Valence Band: from bonding MOs

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

11. Modern Materials © 2009, Prentice-Hall, Inc. Classification of Materials: Based on electrical conductivity The gap between bands determines whether a substance is a metal, a semiconductor, or an insulator.

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

13. Modern Materials © 2009, Prentice-Hall, Inc. Metals No energy separation between Valence Band and Conduction Band Two bands merge to One Valence electrons are in a partially-filled band.

14. 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.

15. Modern Materials © 2009, Prentice-Hall, Inc. Semiconductors Semiconductors have a gap between the valence band and conduction band of ~50-300 kJ/mol (0.5~3 eV). 1eV=1.602x10 -19 J

16. Modern Materials Figure 12.04

17. Modern Materials Band Gap Energies Diamond: 5.5 eV, (Insulator) 155pm, larger overlap larger splitting Silicon: 1.1eV (Semicond.) 235pm smaller overlap smaller splitting

18. Modern Materials Figure 12.05b

19. Modern Materials Table 12.2 (p486) Band gap decreases as atomic sizes increase (Group IV elements) C 5.5 eV largest Si 1.11 Ge 0.68 Sn 0.08 Pb zero © 2009, Prentice-Hall, Inc.

20. Modern Materials Figure 12.06

21. Modern Materials Figure 12.03

22. Modern Materials Figure 12.07

23. 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).

24. 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. n-Type : Group IV & V p-Type : Group IV & III Transistor

25. 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.

26. Modern Materials Figure 12.03

27. 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.

28. Modern Materials © 2009, Prentice-Hall, Inc. Integrated Circuit (IC) (microcircuit, microchip, silicon chip, or chip) a miniaturized electronic circuit (consisting mainly of semiconductor devices) that is built in the surface of a thin semiconductor material. in almost all electronic equipment in use today and have revolutionized the world of electronics.

29. Modern Materials Semiconductor: Intrinsic : Pure semiconductor (Si) Extrinsic: Simeconductor with a dopant (P, Ga, etc) n-type or p-type

30. Modern Materials p-n Diode allows unidirectional flow of electrical current Current flows only if p-type is positive, and N type is negative No current flows, if a voltage is applied the other way → Rectifier

31. Modern Materials Transistor: basic unit of integrated circuit a semiconductor device commonly used to amplify or switch electronic signals.

32. Modern Materials Transistor: basic unit of integrated circuit a semiconductor device commonly used to amplify or switch electronic signals.

33. Modern Materials Figure 12.11

34. Modern Materials © 2009, Prentice-Hall, Inc. Photovoltaic Effect & Solar Cell conversion of solar energy, producing electricity from light (1)Light hits electrons in the VB (of n-type) excited it to CB (pumping e’s to higher energy level) (2)The promoted electrons at the upper energy level of the CB can cross the junction to CB of p-type, then to it’s VB, to complete a cycle.

35. Modern Materials Conversion Efficiency of Solar Cell light energy to electrical energy Most efficient with 1.3eV of band Gap Theoretical maximmm: 31% Lab: 24% Commercial: 15% Cost Comparison: $0.25~$0.65/kWh Coal-based Power plant $0.04~$0.06/kWh ($0.045, Ga Power, 2003) © 2009, Prentice-Hall, Inc.

36. Modern Materials Photovoltaic Effect & Solar Cells : n-p Diodes → produce electricity from light (1)Light hits electrons in in the valence bands exciting it to conduction bands (pumping e’s to higher energy level) (2)The excited electrons at the upper energy level of the CB moves to the lowest level of the CB. This process is revered in LED to generate light from electricity

37. Modern Materials Light Emitting Diode: a n-p diode - an opposite of photo voltaic process (1)Small voltage is applied across the n-p junction (2)Electrons in the CB of n-side are forced to the junction where they meets the holes at the p-side. (3) Electron falls into the holes that is at a lower energy level, thus generating a light. (4) Color (wavelength) light emitted depends on the band gap energy

38. Modern Materials © 2009, Prentice-Hall, Inc. Designing Light Emitting Diodes (LEDs) For LED above 550nm GaP : 2.26 eV Green (549nm) GaAs: 1.43 eV Infrared(IR, 867nm) Mix of GaP & GaAs: Red and others For LED below 550nm GaN: 3.4 eV Violet (~360nm) InN 2.4 eV Green (520nm) Mix of GaN and InN : Blue (~450nm) varying the composition of the elements in LEDs, lights of various color can be generated (Sample Exercise 12.3)

39. Modern Materials Find the wavelength & color of the LED made of GaP (2.26 eV). © 2009, Prentice-Hall, Inc. Solution: E = h x f = h x c / wavelength Wavelength = hc / E =(6.626E-34Js)(3.00E+8m/s) / 2.26eV x (1/1.602E-19eV/J) x (1E9nm/m) = 549 nm (Green)

40. Modern Materials Sample Exercise 12.3: GaP: 2.26eV(549nm), AlP: 2.43eV(510nm): Desired: 520nm. What’s the composition of GaAlP ? © 2009, Prentice-Hall, Inc. Solution: E = hc / wavelength = =(6.626E-34Js)(3.00E+8m/s) / 520nm = 2.38eV 2.38eV = E(GaP) + E (AlP) = 2.26x eV + 2.43(1-x) eV x: fraction of GaP (1-x): fraction of AlP 2.38 = 2.26x + 2.43(1-x) x= 0.294, for fraction of GaP 1-x= 0.706, for fraction of AlP. Ga 0.29 Al 0.71 P

41. Modern Materials © 2009, Prentice-Hall, Inc. Polymers Polymers are molecules of high molecular mass made by sequentially bonding repeating units called monomers. Natural: Rubber, Protein DNA, RNA, Cellulose, Starch Artificial:

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

43. 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

44. 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.

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

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

47. 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.

48. 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.

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

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

51. 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.

52. 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.

53. Modern Materials © 2009, Prentice-Hall, Inc. Biomaterials Natural Biopolymers: - Proteins: amino acids (monomers) Polypeptide bonds Enzymes, Structural material (Keratins) - Polysaccharides (sugar polymers): Glycogens,Cellulose - Polynucleotides: RNA, DNA Synthetic Biopolymers:

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

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

56. 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.).

57. 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.

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

59. Modern Materials © 2009, Prentice-Hall, Inc. Biomaterials Dacron: Polyethylene terephthalate Condensation polymer ethylene glycol and terephthalic acid - Heart valves, Vascular grafts

60. Modern Materials Artificial Tissue: Skin graft suitable scaffolds on which cells can grow - lactic acid / glycolic acid copolymer (p509) has many C-O bonds along the chain providing many opportunities for H-bonding with cell © 2009, Prentice-Hall, Inc.

61. Modern Materials Dental Fillings Old Method: Amalgam fillings (Hg with Ag, Cu) New Method: Composite fillings (white), - has been around for last two to three decades - composed of an organic polymer, bisphenolaglycidyl methacrylate (BIS-GMA), & inorganic particles such as quartz, borosilicate glass, and lithium aluminum silicate. (apply composite mixture to make a thin layer over a tooth; then polymerize them with UV) © 2009, Prentice-Hall, Inc.

62. Modern Materials Polymer Adhesives: Super Glue, Krazy Glue Polycyanoacrylate - consists of monomers of cyanoacrylate molecules. - Methyl-2-cyanoacrylate (monomer) CH 2 =C(CN)COOCH 3 (or C 5 H 5 NO 2 )C - polymerization can be initiated by H 2 O OH (from H 2 O) helps breaking the double bond. - requires air-tight container for storage © 2009, Prentice-Hall, Inc.

63. 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. pottery, china, cement, spark-plug insulators, etc.

64. Modern Materials Hardness - Ability to scratch Talc = 1 Diamond = 10 Hardness < 5 ½ : “soft” Corundum = 9 Topaz = 8 Fluorite = 4 Apatite = 5

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

66. 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.

67. Modern Materials © 2009, Prentice-Hall, Inc. Superconductors Substances that lose virtually all resistance to the flow of electrons below certain temperature, - Transition Temperature - a special property of excluding magnetic lines, levitating a magnet. (not for non-magnetic materials) - can save much of electrical energy - can make much stronger magnet levitated train with high speed (~250 MPH), MRI Meissner Effect

68. Modern Materials © 2009, Prentice-Hall, Inc. Superconductors (SC) 1911 : Hg (Metal): 1987, High Temp SC BP of liq. Nitrogen: 77K Much research has been done recently into the development of high-temperature superconductors.

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

70. Modern Materials © 2009, Prentice-Hall, Inc. Applications of Superconductors superconducting wire can generate very strong magnets, whivh id s not possible with Cu wires - Magnetically levitated high-speed train (~200 MPH) - MRI(Magnetic Resonance Imaging) - Superconducting supercolliders - Nuclear Fusion Power Plant Not practical yet

71. 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. High (179) Low (145) Cholesteryl benzoate

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

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

74. 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.

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

76. Modern Materials Liquid Crystal Displays Liquid Crystal can rotate a plane-polarized light.

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

78. 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.

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

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