Page 6 - Curriculum Visions Dynamic Book

In some ceramics, such as diamond, all of the bonds are of the types described above. That makes diamond an exceptionally hard material. But in many ceramics layers of crystals are held together by weaker bonds. In the case
of graphite sheets of carbon atoms bonded strongly together are held sheet to sheet by much weaker bonds.
That is what allows graphite
to flake away and be used as
a lubricant, even though it is made of carbon atoms just as diamond is (which is used as an abrasive).
How ceramic crystals are
organized
Some ceramics have their atoms
packed extremely tightly. This applies
to diamonds and compounds such as magnesium oxide, which is made of large metal ions (which are positive ions, or cations) each surrounded by six smaller oxygen ions (or anions). Since the oxygen ions pack into the spaces between the metal ions very effectively, the compound is very stable. This pattern can only be broken up at very high temperatures, which is why many ceramics can be
used in ovens and kilns (and are called refractory, or high melting, materials).
Some metal ions are surrounded
by oxygen ions that are not so tightly bonded. It is also possible to replace some of the oxygens with other ions. This introduction of impurities to a structure is called doping, and it is very important in making some ceramics conduct electricity – it is the basis of the transistor used in silicon chips.
(Left and below) The structure of diamond produces an extremely hard material. Its hardness makes
it useful for such extreme applications as diamond-tipped saws and drills.
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