Chapter 12 - Modern Materials 12.1 Semiconductors Inorganic compounds that semiconduct tend to have chemical formulas related to Si and Ge valence electron count of four. Semiconductor conductivity can be influenced by small amounts of impurity atoms (parts-per-million level). Process known as doping. Insulators and Ceramics Insulators have a band structure similar to semiconductors, but have a much larger band gap. Ceramics are inorganic ionic solids that are hard, brittle, stable at high temperatures, and less dense than metals Resistant to corrosion, heat and wear. 1
12.2 Materials for Structure Soft Materials: Polymers and Plastics Polymers are molecules of high molecular weight that are made by polymerization (joining together) of smaller molecules of low molecular mass. Plastics are materials that can be formed into various shapes, usually with heat and pressure. Elastomers are materials that exhibit elastic or rubbery behavior. If a moderate amount of a deforming force is added, the elastomer will return to its original shape. Making polymers Many synthetic polymers have a backbone of C C bonds. Addition polymerization: 2
Chemical equation for ethylene polymerization reaction: Condensation polymerization: two molecules are joined to form a larger molecule by the elimination of a small molecule. 3
Structure and Physical Properties of Polymers Synthetic and natural polymers commonly consist of a collection of macromolecules (large molecules) of different molecular weights. Stretching or extruding a polymer can increase crystallinity. Crystallinity is also strongly influenced by average molecular mass. Low density polymers have low average molecular mass and are highly branched. High-density polymers have much higher average molecular weights with less branching. Polymeric properties may be modified by additives with lower molecular mass called plasticizers. 4
Straight-chain polymers can be made more rigid by cross-linking. Vulcanization of natural rubber is an example of cross-linking. Hard Materials: Metals and Ceramics Metals are ductile, malleable, and highly conductive. Ceramics are brittle but similarly have many applications - cutting tools, abrasives, structural support, piezoelectric materials, tiles for the space shuttle, etc. 5
Making Ceramics Many ceramic are naturally occurring. Sol-gel process is an important method for making small particles with an uniform size. A metal alkoxide reacts with water to form metal-oh groups: Adding acid or base causes condensation of the metal-oh bonds: 12.3 Materials for Medicine A biomaterial is any material that has a biomedical application. Characteristics of Biomaterials Choice of biomaterial for an application is influenced by its chemical characteristics. 6
Polymeric Biomaterials Our bodies are composed of many biopolymers, e.g. proteins, polysaccharides (sugar polymers), and nucleic acids (DNA, RNA). Man-made polymers are usually simpler; 1 or 2 different repeat units only, e.g. 12.4 Materials for Electronics The Silicon Chip Many modern devices rely on silicon wafers or chips containing complex patterns of semiconductors, insulators, and metal wires. Plastic Electronics Some polymers with delocalized electrons can act as semiconductors, but these are generally not as robust as silicon. 7
Solar Energy Conversion Semiconductors are also used in the production of solar energy cells. Light of an appropriate wavelength can promote electrons in a semiconductor to the conduction band, making the material more conductive. 12.5 Materials for Optics Liquid Crystals Solids are characterized by their order, and liquids by their lack of order The first systematic report of a liquid crystal was cholesteryl benzoate (in 1888). Cholesteryl benzoate passes through an intermediate liquid crystalline phase (possesses some properties of liquids and some of solids). 8
Types of Liquid Crystalline Phases Liquid crystal molecules are usually long and rodlike. In normal liquid phases they are randomly oriented. Nematic liquid crystalline phases - Smectic liquid crystalline phases - Molecules are long compared to their width. Double bonds and benzene rings add rigidity. Benzene rings also aid in molecular stacking. 9
Semiconductor Light-Emitting Diodes Light-emitting diodes (LEDs) are used in many types of displays. The mechanism of action is the opposite of that involved in solar cells. Organic LEDs (OLEDs) have some advantages over traditional LEDs. 10
12.6 Materials for Nanotechnology Semiconductors on the Nanoscale Semiconductor particles with 1-10 nm diameters are called quantum dots. Band gaps change substantially with size in this range. Band gap is tunable with particle size many colors can be obtained from a single material. 11
Metals on the Nanoscale Metals in the 1-100 nm size range also exhibit different properties when compared to their bulk counterparts. 2 and 3 nm gold nanoparticles are no longer a noble unreactive metal 12
Carbon Nanotubes Sheets of graphite rolled up and capped at one or both ends. 13