Chapter 13. Ceramics - Structures and Properties


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13.1 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

13.2 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. 

13.3 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- (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. 

13.4 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. 

13.5 Imperfections in Ceramics

Imperfections include point defects and impurities.  Their formation is strongly affected by the condition of charge neutrality (creation of unbalanced charges requires the expenditure of a large amount of energy. 

Non-stoichiometry refers to a change in composition so that the elements in the ceramic are not in the proportion appropriate for the compound (condition known as stoichiometry).  To minimize energy, the effect of non-stoichiometry is a redistribution of the atomic charges (Fig. 13.1).

Charge neutral defects include the Frenkel and Schottky defects.  A Frenkel-defect is a vacancy- interstitial pair of cations (placing large anions in an interstitial position requires a lot of energy in lattice distortion). A Schottky-defect is the a pair of nearby cation and anion vacancies. 

Introduction of impurity atoms in the lattice is likely in conditions where the charge is maintained.  This is the case of electronegative impurities that substitute a lattice anions or electropositive substitutional impurities. This is more likely for similar ionic radii since this minimizes the energy required for lattice distortion.  Defects will appear if the charge of the impurities is not balanced. 

13.6 Ceramic Phase Diagrams (not covered)

13.7 Brittle Fracture of Ceramics

The brittle fracture of ceramics limits applications.  It occurs due to the unavoidable presence of microscopic flaws (micro-cracks, internal pores, and atmospheric contaminants) that result during cooling from the melt.  The flaws need to crack formation, and crack propagation (perpendicular to the applied stress) is usually transgranular, along cleavage planes. The flaws cannot be closely controlled in manufacturing; this leads to a large variability (scatter) in the fracture strength of ceramic materials. 

The compressive strength is typically ten times the tensile strength.  This makes ceramics good structural materials under compression (e.g., bricks in houses, stone blocks in the pyramids), but not in conditions of tensile stress, such as under flexure. 

Plastic deformation in crystalline ceramics is by slip, which is difficult due to the structure and the strong local (electrostatic) potentials.  There is very little plastic deformation before fracture. 

Non-crystalline ceramics, like common glass deform by viscous flow (like very high-density liquids).  Viscosity decreases strongly with increases temperature. 

13.8 Stress-Strain Behavior (not covered)

13.9 Mechanisms of Plastic Deformation (not covered)

13.10 Miscellaneous Mechanical Considerations (not covered) 

Terms:

Anion
Cation 
Defect structure 
Frenkel defect
Electroneutrality
Octahedral position
Schottky defect
Stoichiometry
Tetrahedral position
Viscosity