Crystal Structures
Ceramic bonds are mixed , ionic and covalent , with a proportion that depends on the particular ceramics . The ionic character is given by the difference of electronegativity between the cations (+) and anions (-) . Covalent bonds involve sharing of valence electrons. Very ionic crystals usually involve cations which are alkalis or alkaline-earths (first two columns of the periodic table) and oxygen or halogens as anions .
The building criteria for the crystal structure are two :
- maintain neutrality
- charge balance dictates chemical formula
- achieve closest packing
the condition for minimum energy implies maximum attraction and minimum repulsion . This leads to contact , configurations where anions have the highest number of cation neighbors and viceversa .
The parameter that is important in determining contact is the ratio of cation to anion radii , rC/rA. Table 13.2 gives the coordination number and geometry as a function of rC/rA. For example , in the NaCl structure (Fig. 13.2) , rC = rNa = 0.102 nm, rA =rCl. = 0.181 nm , so rC/rA.= 0.56 . From table 13.2 this implies coordination number = 6 , as observed for this rock-salt structure .
Other structures were shown in class, but will not be included in the test .
Silicate Ceramics
Oxygen and Silicon are the most abundant elements in Earth’s crust . Their combination (silicates) occur in rocks , soils , clays and sand . The bond is weekly ionic , with Si4+ as the cation and O2- as the anion . rSi = 0.04 nm , rO.= 0.14 nm , so rC/rA = 0.286. From table 13.2 this implies coordination number = 4, that is tetrahedral coordination .
The tetrahedron is charged : Si4+ + 4 O2- Þ (Si O4) 4- . Silicates differ on how the tetrahedra are arranged. In silica , ( SiO2 ) , every oxygen atom is shared by adjacent tetrahedra . Silica can be crystalline (e.g., quartz) or amorphous , as in glass .
Soda glasses melt at lower temperature than amorphous SiO2 because the addition of Na2O (soda) breaks the tetrahedral network . A lower melting point makes it easy to form glass to make , for instance , bottles .
