• Main phases (``states'') of matter are: solid, liquid, gas, plasma, but there are materials which can exist in several different solid or liquid phases.
• transition from denser to less dense phase (e.g. solid to liquid, liquid to gaseous) needs energy (heat), to break bonds, overcome cohesive forces,...
  e.g. heat of fusion'', ``latent heat of evaporation''
• phase (``state'') in which given material is depends on temperature and pressure
  

• solid:
  • has definite size and shape;
  • chemical bonds, intermolecular forces sufficiently strong and directional to preserve large-scale external form;
  • kinds of solids:
    crystalline, amorphous (glasses), polymers (plastics), and newer kinds of materials that don't quite fit into scheme:
    liquid crystals, fullerines, aerogels, quasicrystals

• liquid:
  • has definite size, but no definite shape - assumes shape of container;
  • held together by Van der Waals forces.

• gas:
  • has no definite size or shape - assumes size and shape of container
  • molecules in random thermal motion gas pressure
  • very little interaction between molecules (``ideal gas'': no interaction)

• plasma:
  • ionized gas, mixture of charged particles (positive and negative), thermal motion violent enough to overcome electric attraction between charged particles;
  • 99.9% of visible mass in universe is plasma;
  • conducts electricity.

SOLIDS:

• crystalline solids:
  atoms or molecules arranged in orderly, repeated fashion - ``lattice'';
  short- and long-range order;
  e.g. grains of salt, sand, gemstones, metals, ceramics, most rocks and minerals;
  have well-defined melting point = temperature at which intermolecular bonds break;
• amorphous materials (glasses):
  only short-range order, no long-range order;
  have no well-defined melting point - gradual softening;
• plastics
  composed of intertwined chains of polymers;
  can be molded into any shape;
  huge spread in properties to fit almost any application.

• liquid crystals:
  • consist of long molecules that can align themselves in a common direction;
  • molecular arrangement of liquid crystals: between liquid (disordered) and crystal (highly ordered);
  • optical characteristics depend on state of ordering, which can be affected by changes in temperature or electric field
  • LCD (liquid crystal display):
    every pixel = thin layer of liquid crystal between conducting surfaces;
    electric field crystals line up in such a way that they appear opaque.
• Fullerine:
  • is third form of pure carbon (others are graphite and diamond);
    hollow sphere made of 60, 70, or other even number of C atoms;
  • = ``buckyball'', short for buckminsterfullerine (named after R. Buckminster Fuller, because of similarity with his ``geodesic domes'');
    
  • buckyball is soccerball shaped arrangement of C's in hexagons surrounding a pentagon (total 12 pentagons and 20 hexagons);
  • when combined with other atoms, can behave as conductor, insulator, semiconductor, superconductor
  • can be used as ``cage'' for single atom or molecule
• Aerogel:
  • rigid network of atoms enclosing open spaces (pores); pores filled with air (or other gas)
  • ``solid smoke'', ``frozen mist''
  • can support more than 1000 times its weight,
  • can be made in wide range of over-all densities (e.g. silicagel made in density about 3 times that of air);
  • have been used by particle physicists in Cherenkov counters;
  • many applications envisaged: thermal insulators, cushions on satellites to catch meteorids without breaking them, carriers for catalysts to speed up chemical reactions,...
• Quasicrystals:
  • certain metals (e.g. Cu, Fe, Al) melted together, then cooled quickly alloy solidifies into tiny crystal-like grains that cannot fit together in repeated pattern cannot form large-scale crystal lattice;
    
  • example: dodecahedron - cannot arrange many of them in a way that does not leave space between them;
    
  • quasicrystal glasses synthesized so far are very light and strong;
  • applications uncertain.

strength, electrical conductivity, magnetic properties
semiconductors, microchips, transistors

properties depend on:

• kind of atoms
• how atoms are arranged
• how atoms are bonded

  STRENGTH
  = ability to resist changes of shape
  directly related to type of chemical bonding
  Van der Waals forces weakest

• compressive strength = abilty to withstand crushing
• tensile strength = ability to withstand pulling apart
• shear strength = ability to withstand twisting

the three kinds of strength often independent

• composite materials
  = combination of several materials, strength of one of the constituents offsets weakness of another
  e.g.
  • plywood: thin wood layers with alternating grain directions
  • reinforced steel: steel rods (tensile strength) in concrete (compressive strength)
  • fiberglass: cemented mat of glass fibers
  • carbon fiber composites: strong + light structural materials
  • automobile wind shields: layered to resist shattering
  • tires: rubber + steel

ELECTRICAL CONDUCTIVITY
in order of conductivity:
superconductors, conductors, semiconductors, insulators

• conductors: material capable of carrying electric current, i.e. material which has ``mobile charge carriers'' (e.g. electrons, ions,..)
  e.g. metals, liquids with ions (water, molten ionic compounds), plasma
• insulators: materials with no or very few free charge carriers;
  e.g. quartz, most covalent and ionic solids, plastics
• semiconductors: materials with conductivity between that of conductors and insulators;
  e.g. germanium Ge, silicon Si, GaAs, GaP, InP
• superconductors: certain materials have zero resisistivity at very low temperature