f12_01_pg416

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• Chapter 12 Ceramics

• a compound composed of 2 or more elements where
  at least one is a
• non-metal
• Thus, it is usually a compound compound of
  metallic (electropositive) non-metallic
  (electronegative) elements
• C allotropes are considered ceramics.

• Generally exhibit ionic bonding

• coulombic/electrostatic interatomic bond between
  2 oppositely charged ions
• electrostatic attraction

• ions arise from differences in electronegativity

- electronegativity ability of an atom in a
molecule to draw electrons to itself
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Ceramics
Table 12.1
Electronegativity
Ca 1.0, F 4.0
Si 1.8, C 2.5 (both non-metals)
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Ceramics

• Structure
• Consists of ions (rather than neutral atoms)
• cations and anions pack into crystal structures

• Factors Which Effect Crystal Structure
• Magnitude of charge for each ion type (charge
  neutrality)
• - total crystal structure must be electrically
  neutral

Ex. CaF2
so
Must be 2X F-to balance charge
CaF2
Ca2 and F-
2s22p5
4s2
Ex. Al2O3
so
Must be 23 to balance charge
Al2O3
Al3 and O2-
3s23p1
2s22p4
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Ceramics

• Factors Which Effect Crystal Structure
• Relative sizes of anions and cations

Ionic radius of cation (rC) Ionic radius of
anion (rA)
- usually, rC

• Recall rC protons pull in fewer electrons closer)
• rA r (of same atom) more
  electrons causes increase in charge-charge r
• repulsion causing them to spread out

Maximum of opposite charged nearest neighbors
Each cation wants a maximum of anion
neighbors Each anion wants a maximum of cation
neighbors
So, a stable ceramic crystal structure anions
surrounding cation are touching cation
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Table 12.2
Table 12.3
(for CN 6)
Anion orange/big Cation blue/small
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What Factors Determine Ionic Radius (Table 12.3) ?

• Ionic charge
• rC
• Increase charge ? decrease rC

Na (0.095 nm)
Fe3 (0.069)

• 2. Coordination Number (CN) of atomic (or
  ionic) nearest neighbors
• Increase CN ? tend to increase rionic
• Thus, rionic often reported for a specific CN
  Table 12.3 for CN 6

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How many anions can be arranged around a
cation so that anions touch cations ?
The CNcation ( of anion nearest neighbors)
Related to
rC/rA
For a specific CN, there is a minimum rC/RA which
permits anions to touch cation
Table 12.2 and next slide
As CN increases ? increase rC/rA
Cation must get bigger (if anion stays the same
size)
As CN increases ? coordination
geometry changes
CN 2 (linear) CN 8 (cubic)
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• Anions surrounding a cation are all
• in contact with that cation.
• Minimum rC/rA ratio establishes this

Below the minimum rC/rA ratio
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How many anions can you arrange around a cation ?
linear
Structure Name
Coordination Geometry
triangular
C.N.
ZnS
linear
2

(zinc blend)
triangular
0.155 - 0.225
3
NaCl
TD
0.225 - 0.414
4
(sodium
chloride)
OH
0.414 - 0.732
6
CsCl
cubic
(cesium
0.732 - 1.0
8
chloride)
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Common Types of Ceramic Crystal Structures
AX- Type Crystal Structures AX Compounds A
cation and X anion

• Equal of cations and anions
• Several crystal structures for AX ceramics
• Each names after a common material with the
  particular crystal structure

AmXp- Type Crystal Structures A cation and X
anion m /or p does not 1 (unequal of
cations/anions)
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(Oh coordination geometry)
(cubic coordination geometry)
(Td coordination geometry)
highly covalent bonding
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Can Predict Crystal Structure of Ceramic Based on
Ionic Radii
Predict crystal structure of KBr See Table 12.3
rc/rA rK/rBr- 0.138/0.196 0.704 ? CN 6 ?
OH coordination geometry ? NaCl structure
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• Silicate Ceramics (Sec. 12.3)
• Comprised of Si and O
• - most common elements on earth
• - soil, rocks, sands are silicates
• Structures of silicates classified based on
  arrangement of SiO44- tetrahedron rather than
  based on unit cells.
• Si-O bonds IC 51
• Silica (SiO2)
• Silica glasses
• - non-crystalline or amorphous form of silica

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What is the difference between quartz and glass?
Quartz crystalline SiO2
Glass amorphous SiO2
Long-range order
Only short-range order
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• Bioactive Glasses
• Bioactive Of or relating to a substance that
  has an effect on living tissue
• Glass
• Glass is a non-crystalline material that
  exhibits a glass transition which is the
  temperature or range of temperatures that define
  the region where the properties
• of the material change continuously from those
  of a solid to those of a liquid."
•  
• Lack long-range order (amorphous/non-crystalline)
  transparent
• Exhibits a glass transition glass transition
  temperature (Tg)
• the temperature at which, upon cooling, a
  non-crystalline ceramic transforms from a liquid
  to a rigid glass
• or, upon heating, properties switch from more
  solid-like to "liquid-like."
• For crystals, this change occurs abruptly at a
  certain temperature (Tmelt or Tm), usually a
  single temperature
• For glasses, this change occurs over several
  degrees (20 ?C) (Tg)
• Glasses are typically produced from the liquid
  state by cooling at a high rate.

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• Glass-Ceramics
• A polycrystalline ceramic materials that was
  formed as a glass and subsequently crystallized
  with heat treatment."
• polycrystalline refers to crystalline materials
  that are composed of more than one crystal or
  grains (collection of small crystals) see pp
  64-65. Two grains meet along a grain boundary
  (d).

• Bioactive Glasses and Glass-Ceramics
• In the early 1970s, Larry Hench (U of Florida)
  demonstrated that certain glass compositions
  bonded to bone.
• Original Bioglass 45S5
• 45 wt SiO2, 24.5 wt CaO, 24.5
  wt Na2O and 6 wt P2O5

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• Bioactive Glasses and Glass-Ceramics
• Generally, 3 key compositional features for
  bioactivity
• Less than 60 mol SiO2,
• High CaO and Na2O content
• High CaO/P2O5

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What Happens?
Following implantation of a bioactive implant,
certain ions leach out of the implant (e.g. Ca,
Si, Magnesium). These ions react with the ions
present in body fluids and forms carbonated
hydroxyapatite Ca5(PO4)3OH HCA which is the
mineral found in natural bone. An HCA layer
forms between the existing bone and
implant. After osteoblasts migrate from existing
bone into the HCA matrix, they differentiate and
proliferate and generate new ECM to produce new
bone at the interface between the implant and
existing bone. So, can be used for bone tissue
engineering or bone regeneration
HCA
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Bone Grafts
Cemented Hip Replacement

• Alternative to auto- or allografts and
• metallic grafts for
• Repair broken or injured bones
• Fuse joints to prevent movement

• Prevent hip replacement loosening
• Bioactive Glass/PMMA Cement

steel
hip
foot
Bioactive Glass/Polymer Graft (Scaffold)