crystalline ceramic

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CRYSTALLINE CERAMIC
BY ANJU CHANDAK DEEPSHIKHA
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Crystalline Ceramics

• As with plastics the amorphous ceramics tend to
  be transparent
• The structural ceramics tend to be crystalline
  and show greater strength as well as stability
  at high temperature

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AX Structure - CsCl

• Cl-

Note This is not a BCC structure.
Cs
Simple Cubic Crystal
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AX Structure - NaCl
2- FCC interpenetrating lattices.
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AX Structure - ZnS
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Silicate Ceramics

• Silica SiO2
• Many polymorphsQuartz CrystobaliteTridymite
• Low density Quartz 2.65g/cm3

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Crystalline Crystabolite
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Carbon

• Pure carbon has many polymorphs with vastly
  varying properties. It also exists in the
  amorphous state.
• Diamond Is similar to ZnS in structure
• Graphite is considered to be a crystalline
  ceramic
• Fullerenes C60 are a newly discovered polymorph
  - with interesting properties.

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Diamond

• AX type crystal structure similar
• to that of ZnS.
• Each carbon atom is covalently bonded to four
  other C atoms in a diamond-cubic crystal
  structure.
• The material is optically transparent and
  extremely hard (hardest natural material known)
  and durable.
• In engineering applications cruder or industrial
  forms of diamond that are much less expensive
  than the gemstone forms are used as abrasives
  indentors and coatings (especially thin films)
  for a variety of applications.

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Graphite

• Layers of hexagonally arranged and
• covalently bonded C atoms.
• Between layers weaker Van der Walls
• bonds are active giving easy slip
• on the 0001 crystallographic planes.
• Excellent as a dry lubricant relatively high
  strength at elevated temperatures high thermal
  and electrical conductivity low thermal
  expansion resistance to thermal shock and good
  machinability.
• Usage electrodes heating elements crucibles
  casting molds rocket nozzles and other
  applications.

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Fullerenes C60

• Molecular form of carbon with a
• hollow spherical structure resembling
• a geodesic dome (soccer ball.)
• Called buckyballs after R. Buckminister
• Fuller who pioneered the geodesic dome.
• Discovered in 1985 and have since been found to
  occur naturally in several sources.
• In the solid crystalline state C60 molecules
  pack together in a FCC unit cell arrangement with
  a lattice parameter a1.41 nm.
• The pure solid material density is about 1.65
  g/cm3 and it is relatively soft and is
  non-conducting since it has no free electrons.

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Try It!

• Calcualte the theoretical density of pure C60
  based on a FCC unit cell as shown

a1.41 nm
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Defects in Crystalline Ceramics

• Vacancy
• Interstitial
• Dislocation
• Grain Boundary

Cation Interstitial Anion Vacancy Cation
Vacancy Schotky Defect Frenkel Defect
Electro- neutrality
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Mechanical Properties

• Brittle Materials hard to perform a Tension
  Test.
• Flexural Test (Bend) is often substituted.
• Obtain Flexural Strength (Modulus of Rupture)
  Stiffness (Modulus of Elasticity) and Ductility.
• Strength is often good Stiffness my be high but
  Ductility and affected properties are poor.
• In crystalline ceramics dislocation motion is
  difficult because of the need to maintain
  electro-neutrality. Consequently plastic
  deformation is restricted.

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Stress-Strain Behaviour
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Hardness of Ceramics
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Effect of Porosity on Stiffness
Where Eo is the theoretical modulus of elasticity
with no porosity and P is the volume fraction of
porosity.
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Effect of Porosity on Strength

• Where so is the theoretical modulus of rupture
  with no porosity P is the volume fraction of
  porosity and n is an empirical material constant

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Fracture Toughness
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Amorphous Ceramics - Glasses (Na20 Ca0 K2O etc)

• The viscosity of the material at ambient
  temperature is relatively high but as the
  temperature increases there is a continuous
  decrease in viscosity.
• When the viscosity has decreased to the point
  that the ceramic is a fluid it is considered to
  have melted.
• At ambient temperature while it is still solid
  it is said to be in the glassy condition.
• There is no distinct melting temperature (Tm) for
  these materials as there is with the crystalline
  materials.
• The glass transition temperature Tg is used to
  define the temperature below which the material
  is a solid and defines a practical upper limit
  on service temperature.

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Specific volume of amorphous and crystalline
ceramics.
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Viscous Behaviour in Amorphous Ceramics
t shear stress h viscosity of material

• Plastic deformation does not occur by
  dislocation motion in amorphous or
  non-crystalline ceramics such as glass.
• Deformation is by viscous flow rate of
  deformation proportional to applied stress.

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Ceramic Phase Diagrams

• Note They are similar to metal alloy systems -
  except the temperatures are generally higher.

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ANY QUERY
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THANKS FOR YOUR ATTENTION