Difference Between Advanced Polymers and Ceramics and Advanced Metals
Materials that are created using the modern understanding of structure-property relationships have come to be called advanced materials. There is actually no precise definition for advanced materials; in essence, they provide the properties that make it possible to meet requirements that could not be met without them.
Plastics from their beginnings have been light, relatively inexpensive, and easily moldable. But for many applications where these properties would be valuable, the common plastics are lacking in the necessary stiffness, strength, or resistance to high temperatures and chemical attack. Advanced polymers are overcoming all of these deficiencies.
In recent years, hundreds of new materials described as engineering plastics have been introduced. These plastics are designed primarily to replace metals and glass in applications that require strength and temperature resistance. They also offer improved flame retardation, weather resistance, transparency, lower moisture absorption, and various other properties.
Alloying or blending—intimately mixing two or more polymers—is an important strategy in property improvement for plastics as well as for metals. For example, polycarbonate is familiar as the tough, transparent, heat-resistant material used in windows (Lexan), lighting fixtures, and appliances. A blend of polycarbonate with polybutylene terephthalate (Xenoy) also is resistant to gasoline, allowing its use in automobile bumpers and other parts. Xenoy is also resistant to gamma radiation, so it can be used in medical applications where radiation is used for sterilization.
Another important strategy for advanced polymers is their preparation from liquid crystals. Liquid crystals are like liquids in that they flow and like crystals in that their molecules are arranged in structures within the liquid. The molecular rigidity and order characteristic of liquid crystals is retained in polymers made from them, reinforcing the solid polymer and causing it to be stiff and dimensionally stable. Liquid crystal polymers shrink very little after molding and are excellent insulators, making them ideal for electrical parts with precise dimensions.
The class of fibers referred to as aramids also is made from liquid crystals. Aramids have extraordinary tensile strength because of the alignment of the molecules along the fiber axis. Nomex fiber is highly heat and chemical resistant, is self-extinguishing if ignited, and is used in fire-resistant clothing. Kevlar fiber, which has very high tensile strength, is used in tire cord, industrial rope, and bulletproof vests.
In the design of advanced ceramics, the major challenge is to make the materials less brittle, allowing greater advantage to be taken of their superior resistance to heat, abrasion, chemical attack, and oxidation. Ceramic parts are eventually expected to allow for a more than 1,800° F (1,000° C) increase in the operating temperatures of jet engines, which will improve fuel efficiency. Because of their resistance to wear, ceramics might be used in engines that run without cooling systems. Advanced ceramics are also desirable because they are made from readily available and inexpensive natural raw materials.
A ceramic breaks when the tip of a crack penetrates far enough into the solid to split it. The spread of a crack often occurs along microstructural defects. One way to eliminate such defects is by forming the ceramic from a very fine powder. After firing, virtually no open space remains between the grains, making it difficult for a crack to move through the material. Another remedy for ceramic brittleness is to distribute microcracks uniformly in the material during processing; a crack entering the solid is stopped when it hits one of these microcracks.
Ceramics find many uses because of their electrical and magnetic properties. They can be insulators, conductors, semiconductors, or superconductors. Certain ceramics that are mixed oxides of two different metals display the fascinating property of piezoelectricity. A mechanical pressure on a piezoelectric material causes an electrical potential; conversely, such a material changes shape when exposed to an electrical field. Bending a piezoelectric crystal generates a spark, an effect that has been applied in igniters for gas stoves. Applications for advanced miniature piezoelectric motors and actuators include the read/write head in computer disc-burning drives; the auto-focus lenses on cell phones and digital cameras; and aircraft and automobile hydraulics.
Advanced metals already have begun to make a difference in automobiles. The owner of a new car will save 300 gallons (1,100 liters) of fuel over the car’s lifetime because of the replacement of carbon steel with new high-strength, lightweight alloys. New processing methods are overcoming metal fatigue, a primary cause of metal failure (when you bend a paper clip back and forth until it breaks, the break is the result of fatigue). For example, fatigue resistance is improved by directional solidification, a technique used for jet-engine turbine blades. A newly formed blade is slowly cooled from end to end, causing the microcrystalline grains in the metal to line up in the same direction. In another new processing method, porous aluminum is created by heating a lightly compacted powder. A sheet of porous aluminum 0.1 inch (3 millimeters) thick is as good a sound insulator as 2 inches (50 millimeters) of glass wool, and it is more durable.