Chapter 12 Modern Materials 12.1 Semiconductors Inorganic compounds that semiconduct tend to have an average of 4 valence electrons, and their conductivity may be increased by doping. Doping yields different kinds of semiconductors: Insulators and ceramics Insulators have a much larger band gap than semiconductors. Ceramics are inorganic ionic solids that are hard, brittle, less dense than metals, stable at high temperatures, and resistant to corrosion and wear. 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.
Condensation polymerization: two molecules are joined to form a larger molecule by the elimination of a small molecule. Structure and physical properties of polymers Synthetic and natural polymers commonly consist of a collection of macromolecules of different molecular weights.
Stretching or extruding a polymer can increase crystallinity. Crystallinity is also strongly influenced by average molecular mass. Polymeric properties may be modified by additives with lower molecular mass. Cross-linked polymers are more rigid than straight-chain polymers: Hard Materials: Metals and Ceramics Ductile, malleable, and highly conductive, metals are extraordinarily useful. Ceramics are brittle but similarly have many applications - cutting tools, abrasives, structural support, piezoelectric materials, tiles for the space shuttle, etc. 12.4 Materials for Electronics Many modern devices rely on silicon wafers or chips containing complex patterns of semiconductors, insulators, and metal wires. Some polymers with delocalized electrons can act as semiconductors, but these are generally not as robust as silicon. Semiconductors are also used in the production of solar energy cells. If you shine light with an appropriate wavelength on a semiconductor, electrons are promoted to the conduction band, making the material more conductive.
12.5 Materials for Optics 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. 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. It has some properties of liquids and some of solids.
Liquid crystal molecules are usually long and rodlike. In normal liquid phases they are randomly oriented. normal liquid liquid crystal liquid crystal liquid crystal 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. 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. 12.6 Materials for Nanotechnology Nanoscale semiconductors Semiconductor particles with 1-10 nm diameters are called quantum dots. Band gaps change substantially with size in this range. Nanoscale metals Metals in the 1-100 nm size range are interesting nanomaterials.
Carbon nanotubes Sheets of graphite rolled up and capped at one or both ends. An award-winning artist's impression of a nano-louse immunizing a single red blood cell. Looks real, doesn t it but is it any more so than the cartoon below? Is a picture worth a 1,000 words? http://www.nature.com/nature/journal/v421/n6922/full/421474a.html