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Watkins Chapter 12 CHAPTER 12 Modern Materials • Liquid Crystals • Polymers • Ceramics • Thin Films Chemistry 1201 1 Brown & Lemay Watkins Chapter 12 Liquid Crystals - Cholesterol Benzoate Melting Point: 145C Above 179C Clear Normal Liquid Chemistry 1201 Between 145C and 179C Milky Liquid Crystalline 3 Brown & Lemay Watkins Chapter 12 Liquid Crystals Three types • Nematic – 1-D order • Smectic – 2-D order • Cholesteric – 3-D order Chemistry 1201 4 Brown & Lemay Watkins Chapter 12 Liquid Crystals Nematic and Smectic molecules are Rigid or Stiff, Long and shaped like Rods Cholesteric molecules are Flatter and have Flexible Tails Chemistry 1201 5 Brown & Lemay Watkins Chapter 12 Liquid Crystals Cholesteric liquid crystals •Molecules are flat and sausage shaped with flexible tails •Changes in temperature, pressure or voltage cause the ordering between layers to change. •This results in color changes. – Liquid Crystal Displays – Liquid Crystal Thermometers Chemistry 1201 6 Brown & Lemay Watkins Chapter 12 Polymers Very Large Molecules Composed of many smaller molecules (monomers). Most familiar polymers (both natural and synthetic) are organic compounds Examples: plastics, DNA, proteins, rubber, starch, cotton, wool, nylon Other elements form inorganic polymers Examples: Silicones, poly-phosphate minerals, polyphosphazines Chemistry 1201 7 Brown & Lemay Watkins Chapter 12 Polymers • The monomer ethylene, C2H4, polymerizes to form polyethylene n C2H4 [C2H4]n • This is called addition polymerization because ethylene molecules are added to each other. Chemistry 1201 8 Brown & Lemay Watkins Chapter 12 Polymers When some monomer molecules join together, a small molecule (e.g. water) is also produced O H O N H + H O C N C H + H O H This is called condensation polymerization Examples: nylon, proteins Chemistry 1201 9 Brown & Lemay Watkins Chapter 12 Polymers Types of Synthetic Polymers • Plastic: materials that can be formed into shapes. • Thermoplastic: materials that can be shaped more than once using heat. • Thermoset: materials that can only be shaped once using heat. • Elastomer: material that is elastic in some way (rubber). – Also useful for fibers. Chemistry 1201 10 Brown & Lemay Watkins Chapter 12 Polymers Chemistry 1201 11 Brown & Lemay Watkins Chapter 12 Polymers Physical Properties of Polymers In general, polymer chains are flexible and easily entangled or folded Each individual chain contains a different number of monomer units low density polyethylene (LDPE) has an average molecular mass of 104 amu (~ 360 monomer units, used in plastic wrap) high density polyethylene (HDPE) has an average molecular mass of 106 amu (~ 36,000 monomer units, used in milk cartons) Chemistry 1201 12 Brown & Lemay Watkins Chapter 12 Polymers Physical Properties of Polymers Crystallinity (the amount of ordering in a polymer) produces less flexibility stiffer material, higher melting point Stretching or extruding a polymer can increase crystallinity Degree of crystallinity generally increases with average molecular mass LDPE is more flexible and lower melting than HDPE Chemistry 1201 13 Brown & Lemay Watkins Chapter 12 Polymers Crosslinking Polymers • Natural rubber – too soft, too chemically reactive, not useful • Crosslinking - bonds formed between polymer chains – stiffer, less reactive • Cross linking of rubber is called vulcanization Chemistry 1201 14 Brown & Lemay Watkins Chapter 12 Polymers Crosslinking Polymers • Rubber is usually cross-linked with sulfur. • Vulcanized rubber is stiffer, more elastic and less susceptible to chemical reaction. Chemistry 1201 15 Brown & Lemay Watkins Chapter 12 Ceramics Inorganic (no C-H bonds) Mostly metal oxides Very high melting, hard, brittle Less dense and more elastic than metals Nonmetallic (insulator) Crystalline or amorphous (glass-like) solids Chemistry 1201 16 Brown & Lemay Watkins Chapter 12 Ceramics Common ceramics Al2O3 (alumina, corundum) SiO2 (sand, quartz, silica) SiC (silicon carbide) ZrO2 (zirconia) BeO (beryllia) YBa2Cu3O7 (YBCO or 123) Chemistry 1201 17 Brown & Lemay Watkins Chapter 12 Ceramics Chemistry 1201 18 Brown & Lemay Watkins Chapter 12 Ceramics • Ceramic Composite: two or more different ceramics mixed together • Result: tougher ceramic. • Most effective method: add fibers to ceramic material. For example: – SiC fibers added to aluminosilicate glass. – Fiber must have a length 100 times its diameter. Chemistry 1201 19 Brown & Lemay Watkins Chapter 12 Ceramics Some Applications of Ceramics • Cutting tools – SiC drill bits, corundum sandpaper • Electronics – integrated circuits use alumina as insulator • Piezoelectrics (generate a voltage during mechanical stress) – quartz watches, ultrasonic generators • Thermal Tiles for the space shuttle. Chemistry 1201 20 Brown & Lemay Watkins Chapter 12 Ceramics YBCO - A Hi-Tc Superconductor • No electrical resistance • Tc above LN2 • Meissner effect: superconductor excludes all magnetic field lines, so the magnet floats in space (maglev) • Too brittle for wires, but can make thin films Chemistry 1201 21 Brown & Lemay Watkins Chapter 12 Thin Films • Thin films generally have a thickness between 0.1 m and 300 m. • Useful thin films must – – – – – Chemistry 1201 be chemically stable, adhere well to the surface, be uniform, be pure, have low density of imperfections. 22 Brown & Lemay Watkins Chapter 12 Thin Films Uses of Thin Films • Microelectronics (conductors, resistors and capacitors) • Optical coatings – reduce reflection from a lens – Reduce scratching on lens • Protective coatings for metals • Increase hardness of tools Chemistry 1201 23 Brown & Lemay Watkins Chapter 12 Thin Films Formation of Thin Film by Vacuum Deposition Thin film material is placed in one vacuum chamber and the objects to be coated (substrate) in another. Examples MgF2 Al2O3 SiO2 The material condenses on the substrate Chemistry 1201 24 Brown & Lemay Watkins Chapter 12 Thin Films Formation of Thin Film by Sputtering Thin Film material (M) is placed on the negative electrode Substrate is the positive electrode Ar atoms (in the chamber) are ionized to Ar+ Ar+ ions strike the negative electrode and eject M. Some M eventually hits the substrate and condenses. Chemistry 1201 25 Brown & Lemay Watkins Chapter 12 Thin Films Formation of Thin Film by CVD Chemical Vapor Deposition A volatile (gas phase) compound containing the thin film metal reacts with another gas at the surface of the substrate The thin film material thus produced is very pure TiBr4(g) + 2H2(g) Ti(s) + 4HBr(g) SiCl4(g) + 2H2(g) Si(s) + 4HCl(g) SiCl4(g) + 2H2(g) + 2CO2(g) SiO2(s) + 4HCl(g) + 2CO(g) 3SiH4(g) + 4NH3(g) Si3N4(s) + 12H2(g) Chemistry 1201 26 Brown & Lemay