Self-propagting High-temperature Synthesis _SHS_ of Silicon by dfhdhdhdhjr

VIEWS: 5 PAGES: 49

									  MET 421 & MET 521

Ceramics and Refractories

  Structure of Ceramics
       (Lecture 2)
The crystalline state



A solid is said to be a crystal if the atoms
are arranged in such a way that their positions
are exactly periodic.
A perfect crystal maintains its periodicity in the
three spatial dimensions. It is also called long
range order.

If the atoms in a solid are randomly arranged
without any crystalline structure or if they
just have a short-range order, then this solid
is called non crystalline or amorphous.
 Crystal lattice


In crystallography there are seven crystal systems:

triclinic
monoclinic

orthorhombic
tetragonal
cubic

trigonal (rhombohedral)
hexagonal
Unit cell   triclinic         monoclinic

bases       abc             abc

angels            90    =  = 90  
Unit cell   orthorhombic       tetragonal       cubic

bases          abc           a=bc            a=b=c

angels       =  =  = 90  =  =  = 90    =  =  = 90
Unit cell   rhombohedral      hexagonal

bases       abc             a=bc

angels       =  =   90    =  = 90  =120
Why do we have different crystal structures?

….because there are atoms / ions of different
sizes, charges and bonding types….
Structure of ceramics

Ionic bonding (SiO2, Al2O3, spinel)

Ionically bonded solids are made up of charged
particles - positively charged ions are called cations,
and negatively charged ions are called anions.
Their mutual attraction holds the solid together.

The underlying principle of this kind of bonding is
the transfer of electrons between the atoms of
different species.

The radii of ions differs from the radii of the atoms
(Goldsmith radius).
Structure of ceramics

Covalent bonding (SiC, TiC, TiN)

Whereas ionic bonds involve electron transfer
to produce oppositely charged species covalent
bonds arise as a result of electron sharing.

In principle, the energetics of the covalent bond
can be understood if it is recognized that electrons
spend more time in the area between the nuclei.
That arrangement of the electrons generates an
attractive interatomic force.

Covalent bonded crystals tends to be very hard,
brittle and have high melting points.
 Common Ceramic Crystal Structures


 Most Metals:         face centered cubic (fcc),
                      box centered cubic (bcc),
                      hexagonal close-packed (hcp)
Ceramic structures:

AX-type:          Rock salt, CsCl, zinc blende
                  and wurtzite


AX2-type:         CaF2, TiO2


AmBnXp-type:      Spinel and perovskites
Typical example of AX type



Rock salt (NaCl)

anion packing:
cubic close-packed

close neighbors: 6
Typical example of AX type



CsCl

anion packing:
simple cubic

close neighbors: 8
Typical example of AX type



zinc blende (ZnS)

anion packing:
face centered cubic

close neighbors: 4
Typical example of AX type



Wurtzite (ZnS,SiC)

anion packing:
hexagonal close-packed

close neighbors: 4
Typical example of AX2 type

Calcium fluorite

anion packing: simple cubic,

Anti-fluorite structure
(in comparison with the
fluorite structure, the
cations and anions have
switched places)

cubic close-packed

coordination number: 8
Typical example of AX2 type



 TiO2 Rutile

 anion packing:
 disordered cubic
 close-packed

 Stacking of TiO6
 octahedra
Typical example of AmBnXp-type



  Perovskite

  CaTiO3,BaTiO3,

  anion packing:
  cubic closed packed
Typical example of AmBnXp-type


Spinel
MgAl2O4

normal spinel
anion packing:
cubic close-packed

inverse spinel
anion packing:
cubic close-packed
Glass structure



Glass -an inorganic product that has cooled to
a rigid condition without crystallizing. It can be
considered as frozen liquid.

           Crystal structure (a)
           and
           glass structure (b)
           built from
           [SiO4]4- tetrahedra


(a)                          (b)
 Homework # 1


1. Find three definition of ceramics from different
sources. Deliver also the reference.

2. Determine the position of the basis atoms
for the following structures:
CsCl, zinc blende, wurtzite, CaF2, TiO2,
perovskites.
  MET 421 & MET 521

Ceramics and Refractories

  Structure of Ceramics
       addendum
       (Lecture 3)
Announcement



Homepage:
www.hpcnet.org/sdsmt/2002fa/met42101
www.hpcnet.org/Met421/notes
Structure of silicates


Based on [SiO4]-4 tetrahedra

There two types of oxygen that exist in silicate
structures:

Non bridging ( just bound to the one silicon atom)
Bridging (bound to two silicon atoms)


                                   Center Silicon
                                   Corner Oxygen
Non bridging oxygen (NBO)

NBO are negatively charged
and can be locally neutalized
by having cations such as
Na+, K+, Ca+, Al3+,
                                O2- Si4+   Na+
Underlying principles about NBOs

1. Number of NBO is proportional to the numbers
of moles of alkali or alkali earth metal oxide added.

2. The addition of of alkali or alkali earth metal
oxide to silica must increase the overall O/Si ratio
of the silicate.

3. Increasing number of NBOs results in the
progressive breakdown of silicate structure into
smaller units.
Silicate Structures


Island silicates

Chain silicates

Sheet silicates

Aluminosilicates

Network
Silicate Structures

Island Isolated tetrahedra [SiO4]4-
O/Si ratio   4.0
NBO          4.0
BO           0.0
Examples: Orthosilicates Mg2SiO4, Li4SiO4,
Silicon oxygen groups
Silicate Structures

Chains or rings [SiO3] n 2n-
O/Si ratio   3.0
NBO          2.0
BO           2.0
Examples: Pyroxenes Na2SiO3, MgSiO3, beryl
(rings)
Silicon oxygen groups
Silicate Structures

Double Chains [Si4O11] 6-
O/Si ratio   2.75
NBO          1.5
BO           2.5
Examples: asbestos minerals,
Silicon oxygen groups
Silicate structures

Sheets [Si4O10] 4-
O/Si ratio    2.5
NBO           1.0
BO            3.0
Examples: Talc Mg3(OH)(Si2O5)2 , mica, kaolinite
Silicon oxygen groups
Silicate structures

Alumosilicates
Sheets [Si4O10] 4-

O/Si ratio      2.5
NBO             1.0
BO              3.0




                      mica
    kaolinite
Silicate structures

Three dimensional networks
complete interconnected tetrahedra
O/Si ratio   2.0
NBO          0.0
BO           4.0
Examples: Silica polymorphs, quartz, tridymite,
          cristobalite
Silicon oxygen groups
high cristobalite
 Working example 1


Derive a generalized expression relating the
number of non bridging oxygen per Si atom present
in a silicate structure to the mole fraction of the
metal oxide added.
Working example 1 solution



 The number of NBOs has to be equal to the
 total cationic charge.

 Starting with a basis of y = moles of SiO2
 the additon of p moles of MqO results in
 the formation of z (pq) NBOs, where z is the
 charge on the modifying cation

      NBO = z(pq)/y
 The corresponding O/Si ratio, denoted by R is

       R = 2 + p/y
Working example 2


Calculate the number of bridging and
non bridging oxygens per Si for Na2O . 2 SiO2 .
What is the most likely structure of this compound?
Working example 2 solution



   NBO = z(pq)/y         R = 2 + p/y


For Na2O . 2 SiO2 , p=1, q=2, z=1, y=2


NBO = 1(2.1)/2 = 1, so the number of BO is 4-1=3

Furthermore, since O/Si ratio R= 2.5, it follows
that the most likely structure of this silicate is
a sheet structure.
amorphous ( glass, fused quartz)



Glass formers

intermediates

modifiers
Structure of silicates   O/Si ratio   Si-bridges

amorphous                2.00

sheet silicates          2.50

chain silicates          3.0

island silicates         4.0

aluminosilicates          -
A phase is defined as a region in a system
in which properties and composition are
spatially uniform.

A system is said to be in equilibrium when
there is no observable changes
in either properties or microstructure with
the passing of time, provided that no changes
in the external conditions occur at that time.
A phase diagram is a roadmap to interpret
and predict the microstructure distribution
and evolution, which can have a profound
effect on the properties of the materials.

A phase diagram provide the following information:

I) The phases present at equilibrium.
II) The composition of the phases present at any
time of during the heating or cooling
III) The fraction of each phase present
IV) The range of solid solubility of one element
or compound in another
In principle, phase diagrams provide the
following information:

I)     The phases present at equilibrium;

II)    The composition of the phases present at
       any time of during the heating or cooling;

III)   The fraction of each phase present;

IV)    The range of solid solubility of one
       element or compound in another;
One component system
Binary system
ternary system
Quaternary system

								
To top