Mullite 3Al2O3'2SiO2

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Mullite (3Al2O3.2SiO2)
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INTRODUCTION

• Mullite is one of the most important minerals
  and materials in crystallography and in
  ceramic science and technology.
• Mullite is strong candidate material for
  advanced structural applications at both low
  and high temperatures, because it has high
  strength at high temperature, low thermal
  expansion and good chemical stability.
• Owing to its high temperature-low pressure
  formation conditions mullite occurs only very
  rarely in nature, although it sometimes can be
  found in high temperature metamorphosed rocks.
• Mullite formation is also observed at the contact
  of alumina-rich sedimentary rocks with
  basaltic melts.

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Composition
Background

• Mullite is the mineralogical name given to the
  only chemically stable intermediate phase in the
  SiO2 - Al2O3 system. The natural mineral is
  rare, occurring on the Isle of Mull off the west
  coast of Scotland.

• Mullite is commonly denoted as 3Al2O3 .2SiO2
  (i.e. 60 mol Al2O3). However it is actually a
  solid solution with the equilibrium composition
  limits of 60 63 mol Al2O3 below 1600?C.

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Mullite

• Mullite is a good, low cost refractory material.
  The raw materials are easily obtainable and are
  reasonably priced. It has excellent high
  temperature properties with improved thermal
  shock and thermal stress owing to the low thermal
  expansion, good strength and interlocking grain
  structure.

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Key properties

• Mullite has long been used as a refractory
  material. Its properties include
•          Good high temperature strength
•          Good thermal shock resistance
•          Excellent thermal stability
•          Resistance to most chemical attack it
  has excellent stability in acid metal slags and
  is insoluble in most acids
•          Resistance to oxidation and attack by
  furnace atmospheres
•          Resistance to abrasion
•          Good electrical resistivity
• The approximate limiting temperatures of use are
  1800?C in air and 1600?C in vacuum.

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Table-Typical physical and mechanical properties
of mullite

• Density (g/cm3) 3.03
• Youngs Modulus (GPa) 130
• Fracture Toughness (MPa.m-1/2) 2-4
• Modulus of Rupture (MPa) 160
• Thermal Expansion Coefficient (x10-6 /C)
  4.5-5.6
• Thermal Conductivity (W/m.K) 4.0-6.0
  (100-1400C)
• Maximum Operating Temperature (C) 1725C in air

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Mullite Ceramics By varying the chemical and
mineralogical composition of the Al2O3SiO2
system, specific modifications to the properties
of the mullite ceramic can be achieved. Pure
mullite (3Al2O3 . 2SiO2) consists of 82.7
Al2O3 by mass and 17.3 SiO2. Dense sintering
of pure mullite cannot be achieved with
conventional sintering techniques.Mullite
ceramics have a microstructure whose mineral
phases consist of mullite, corundum (Al2O3) and
glass (SiO2). Sintered mullite usually contains
up to 10 glass phase material. The overall
porosity, however, is still around 10 by
volume.
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Porous mullite ceramics containing little glass
phase material have a relatively high strength,
comparatively low thermal expansion, and are
therefore highly resistant to thermal shock.
Resistance to creep at high temperatures is
superior to that of pure aluminium oxide
ceramic.Applications include kiln furniture for
temperatures up to 1,700C even in oxidising
atmospheres and carrier rollers in
high-temperature furnaces. Because of its low
thermal conductivity and high resistance to
corrosion, porous mullite is also used as an
industrial refractory material. The porosity can
be significantly reduced by increasing the
proportion of glass phase material (gt 10 ).
Densely sintered mullite ceramic combines
high-strength, good thermal shock resistance and
a useful resistance to creep. One important
application, for instance, is protective tubes
for thermocouples.
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Synthetic Mullite

• Various starting materials and preparation
  methods are used to make synthetic mullite
  ceramics. For example a mixture of solids, a
  mixture of sols, or a mixture of sol and salt can
  each be used as the starting materials.
  Similarly, a variety of preparation methods
  exist, for example reaction sintering of
  mechanically mixed powders, hydrothermal
  treatment of mixtures of sols and chemical vapour
  deposition.
• The starting materials and preparation method
  influence the properties of the mullite.
  Reaction sintered mullite made from mechanically
  mixed powders is usually characterised by low
  strength (lt200 MPa) and low fracture toughness (1
  2 MPa m-1/2) due to amorphous grain boundary
  phases. In contrast, gelation routes produce
  intimately mixed sub-micrometer particles that
  can be sintered or hot pressed to produce single
  phase materials with superior mechanical
  properties.
• Mechanical properties can be improved further by
  producing composites. Additions of ZrO2 and SiC
  have produced fracture toughness at room
  temperature close to 7 MPam-1/2.

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Crystal Chemistry of Mullite
Average structure of mullite is designated as
Al2Al22xSi2-2xO10-x
O Atoms
Al Atoms
Si Atoms
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Phase Diagram
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Mullite Synthesis

• Various preparation methods have been
  investigated for mullite synthesis
• They were classified into three different
  preparation routes as follows
• Sinter-mullite
• Fused-mullite
• Chemical-Mullite (High-purity mullite)

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Mullite Synthesis Sinter Mullite
Mullite prepared by a conventional process, i.e.
mixing powdered raw materials, was designated
sinter mullite. Oxides, hydroxides, salts and
silicates were used as starting
materials. Mullitization takes place by
solid-solid or transient liquid phase reactions
of the starting materials by aluminium, silicon
and oxygen atom interdiffusion. The mullitization
temperature is controlled by the particle size of
starting powders. The starting materials of SiO2
and Al2O3 are usually of micrometer size, and are
not small enough to accomplish complete
mullitization at relatively low firing
temperatures within a reasonable duration of
time. For example, the mullitization temperature
was reported to be higher than 1400C when quartz
particles below 2 µm in size and a-Al2O3
particles with the average particle size of
0.3-0.5 µm were used. (Hamato et al., 1986)
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Mullite Synthesis Sinter Mullite
Transformation of kaolinite to mullite
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Mullite Synthesis Sinter Mullite
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Mullite formation in a floor tile body
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Mullite formation in a porcelain tableware body
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Mullite Synthesis Fused Mullite
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Mullite Synthesis Chemical Mullite

• Sol gel method
• Precipitation method
• Hydrolysis method
• Spray pyrolysis method
• CVD method

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Processing of Monolithic Mullite Ceramics

• Important processing routes are
• Sintering of mullite powder compacts
• Reaction sintering of Al2O3- and SiO2-
  containing reactants
• Reaction bonding of mullite
• Reaction sintering of chemically produced
  mullite precursors
• Transient viscous sintering of composite powders

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Mullite Matrix Composites
Various mullite based composites have been
extensively investigated in recent years in order
to enhance the mechanical properties of mullite
ceramics. Most attention has been paid to mullite
matrix composites toughened by the addition of
ZrO2 particles and to fibers or whisker
reinforced materials.
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Mullite Matrix Composites
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Properties of mullite ceramics
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Properties of mullite ceramics
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Properties of mullite ceramics
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Properties of mullite ceramics
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Properties of mullite ceramics
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Industrial Uses of Mullite Ceramics

• Refractory Materials
• Heat Resisting Materials
• Engineering Materials for Heat Exchangers
• Electronic Packaging Materials
• Optical applications

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Industrial Uses of Mullite Ceramics
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Industrial Uses of Mullite Ceramics
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Industrial Uses of Mullite Ceramics
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Industrial Uses of Mullite Ceramics
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Industrial Uses of Mullite Ceramics
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Industrial Uses of Mullite Ceramics
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Ceramics based on mullite-corundum

• Metal moulds for metal casting
• Fixing plates for melting crucibles
• Dies for pressing articles from titanium alloys
• Stoppers for metallurgical furnaces
• Bottom plates for the installations intended for
  isostatic pressing titanium alloys

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Zirconia Mullite Grain/ Powder 

• Designations
• Chemical Name Zirconia-mullite
• Chemical Formula Al2O3 ZrO2 SiO2
• General Description
• A fused, high purity product produced in an
  electric arc furnace from Bayer alumina and
  zircon sand. It is composed of large needle-like
  mullite crystals containing co-precipitated
  monoclinic ZrO2.
• Typical Chemical Purities Available
• Al2O3 45.80, ZrO2 36.0 and SiO2 17.50
• Typical Applications
• Used in specialty product applications like
  ceramic pressure casting tubes, refractory shapes
  that require resistance to molten slag and molten
  glass.