Chapter 12. Ceramics - Structures and Properties

12.1 Introduction
Ceramics are inorganic and non-metallic materials that are commonly electrical and thermal insulators, brittle and composed of more than one element (e.g., two in Al2O3)
Ceramic Structures

12.2 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.

12.3 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 (Fig. 13.9).
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.

12.4 Carbon
Carbon is not really a ceramic, but an allotropic form, diamond, may be thought as a type of ceramic. Diamond has very interesting and even unusual properties:
diamond-cubic structure (like Si, Ge)
covalent C-C bonds
highest hardness of any material known
very high thermal conductivity (unlike ceramics)
transparent in the visible and infrared, with high index of refraction
semiconductor (can be doped to make electronic devices)
metastable (transforms to carbon when heated)
Synthetic diamonds are made by application of high temperatures and pressures or by chemical vapor deposition. Future applications of this latter, cheaper production method include hard coatings for metal tools, ultra-low friction coatings for space applications, and microelectronics.
Graphite has a layered structure with very strong hexagonal bonding within the planar layers (using 3 of the 3 bonding electrons) and weak, van der Waals bonding between layers using the fourth electron. This leads to easy interplanar cleavage and applications as a lubricant and for writing (pencils). Graphite is a good electrical conductor and chemically stable even at high temperatures. Applications include furnaces, rocket nozzles, electrodes in batteries.
A recently (1985) discovered formed of carbon is the C60 molecule, also known as fullerene or bucky-ball (after the architect Buckminster Fuller who designed the geodesic structure that C60 resembles.) Fullerenes and related structures like bucky-onions amd nanotubes are exceptionally strong. Future applications are as a structural material and possibly in microelectronics, due to the unusual properties that result when fullerenes are doped with other atoms.