Download Ceramic Crystal Structures

Why Study Structures And Properties Of Ceramics?
Some of the properties of ceramics may be explained by their structures. For example:
(a) The optical transparency of inorganic glass materials is due, in part, to their
noncrystallinity (b) the hydroplasticity of clays( i.e., development of plasticity upon
the addition of water) is related to interactions between water molecules and the clay
structures (c) the permanent magnetic behaviors of some ceramic materials are
explained by their crystal structures.
Ceramic Structures:
Because ceramics are composed of at least two elements, and often more, their crystal
structures are generally more complex than those for metals. The atomic bonding in
these materials ranges from purely ionic to totally covalent; many ceramics exhibit a
combination of these two bonding types, the degree of ionic character being dependent
on the electronegativities of the atoms. Table 12.1 presents the percent ionic character
for several common ceramic materials; these values were determined using Equation
below and the electronegativities in Figure 2.7.
Electronegativity a measure of how willing atoms are to accept electrons (subshells
with one electron low electronegativity; subshells with one missing electron high
electronegativity). Electronegativity increases from left to right. The atomic bonding
in ceramics is mixed, ionic and covalent, the degree of ionic character depends on the
difference of electronegativity between the cations (+) and anions (-).
Ceramic Crystal Structures:
ceramics that are predominantly ionic in nature have crystal structures comprised of
charged ions, where positively charged (metal) ions are called cations, and negatively
charged (non-metal) ions are called anions. The crystal structure for ceramics depends
upon two characteristics:
1. the magnitude of electrical charge on each component ion, recognizing that the over
all structure must be electrically neutral.
2. the relative size of the cation(s) and anion(s),which determines the type of
interstitial site(s) for the cation(s) in an anion lattice.
Stable ceramic crystal structures: anions surrounding a cation are all in contact with
that cation. For a specific coordination number there is a critical or minimum cationanion radius ratio rC/rA for which this contact can be maintained.
The structures of ceramics fall into two main groups:
1- Simple crystal structures:
containing ionic or covalent bonds, or a mixture of the two. Examples are
magnesium oxide (used for furnace linings) which is an ionic compound with
cubic structure, and silicon carbide, with covalent bonds and a tetrahedral
structure similar to that of diamond. Alumina has a close packed hexagonal
structure, with a mixture of covalent and ionic bonds, with one-third of the
potential aluminum sites vacant in order to satisfy the valency requirements of
the two elements.
2- Complex silicate structures:
The majority of ceramic materials, in particular those derived from clay, sand
or cement, contain the element silicon in the form of silicates. The
arrangements are many, involving both chains of silicate ions (SiO4)-2, double
chains and links in sheet form. With the last of these, found in clays, crosslinking between adjacent sheets occurs when the clay is baked.