Layperson's Guide to Materials

For solids:

Plastics hold there shape, after being deformed.
Not just most polymers are plastic, most metals are plastic. It just takes more force to make them deform.

Elastics return to their original shape, after being deformed.
Everything is elastic to some degree. Materials undergo elastic deformation, plastic deformation, and failure upon compression (squeezing) or tension (pulling).

Brittle materials can withstand little deformation, but are usually hard.

Ductile and malleable materials can withstand more deformation.

Tough materials can withstand both force and deformation.

Strong materials can withstand force under deformation.

Resilient materials can withstand elastic-deformation.

Hard materials require force to deface, scratch, or indent.

Metals freely conduct electrons with little or no applied voltage.
Examples are gold, aluminum, brass, and steel. Gold and aluminum are elements that in quantity form as metals when they are liquid or solid. At room temperature, mercury is a liquid metal. Brass and steel are a combination of elements (alloys) which form into a metal. Hey!, hydrogen (H) would form into a metal (the best) if you could get it cold enough.

Insulators do not conduct electrons.

Semiconductors conduct electrons when a moderate voltage is applied.
Examples are silicon and germanium.

Glasses contain lots of short-range order, and very little long-range order. Huh? In a gas their is no order, and in a crystal their is perfect order. The crystal has all of the atoms, molecules, or subunits lined-up perfectly in a row, like a three-dimensional tic-tac-toe board. All of the subunits are the same distance from each other, the next subunits are twice that distance, the next-next subunits are thrice that distance, etc.... In a glass, all of the subunits are the same distance apart, but the next or next-next subunits are a different distance. This usually occurs because all of the subunits are not lined up is the same direction. Also, there can be many voids in a glass. Finally, being a glass has nothing to do with being transparent or brittle. You can get normally metallic materials to form in a glass state. Beam me up, Scotty. You can get most "plastics/polymers" to form in a glass state.

Crystals, I just told you what they are; they not glasses.
You can get single-crystal quartz and diamonds, they're cheap. Try getting a single-crystal of Rhenium or Osium. You can actually get single-crystal of most of the elements and tons of different molecules. Those semiconducting chips in your watch and computer are actually formed ontop of Silicon single-crystal wafers.

Polycrystalline materials are made up of lots of little crystals.
Most common metals are in a polycrystalline form when you get them.

Amorphous materials have no order or very little order. The atoms, molecules, or subunits of the material are arranged in a completely random manner.

Liquid Crystals are a liquid, because they can flow and not support shear forces. However, they are crystals, because they have order.

Polymers are long-chain molecules. Each chain is made of small sub-units called mers. The chain is just lots of mers; hence, POLY-MERS. The chains usually don't stay straight (its difficult to be straight). Instead, the chains 'ball-up' and intertwine. Interestingly, many of your everyday polymers are plastics in a glass state.

Ceramics are any materials which are not organic and not metallic. They are not necessarily pottery or insulating. Infact, many high-temperature superconductors are actually ceramic, go figure.

Solids support shear forces.........i.e., solid!

Liquids do not support shear forces.........i.e., solid!

There are cases of materials which exhibit properties of being solid and liquid. There is not enough space here to discuss creep, flow, viscoelasticity, viscoplasticity, Newtonian Liquid, non-Newtonian Liquids, supercooling (yes it is real), polymorphism, and quasicrystals. There are shades of gray in this world. However, the primary definitions of materials are as listed above.

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07/22/2009 JQ