Download follow up solids

Composition
Structure
Performance
Properties
Design and
construction
1
Solid Materials Package of Properties
Properties  f(composition)
Choice of composition +
Construction
Properties of Materials
2
• Solid properties = f(Composition)
• Solid properties = f(Atomic arrangement)
Ex:
diamond
vs.
graphite
Composition alone can’t give the properties of a material, they
are dependent on the atomic arrangement.
pictures from wikipedia
3
Solid State Chemistry
• Electronic structure of the elements holds the key to the
understanding of the long range atomic order in solids;
• Electronic structure of the atomic constituents and symmetry
arguments are the criteria for the material selection process
4
1. What atoms are involved and their electronic configuration?
2. What types of chemical bonds are formed?
3. How are the atoms arranged in the crystal structure?
4. What is the symmetry of the crystal?
5. Do these arrangements promote certain mechanisms for
electronic or atomic motions?
6. How do these mechanisms give rise to the observed properties?
5
Identifying solid materials by use of
 Atomic radii
 Chemical bond strengths
 Anisotropic atomic groupings
 Symmetry arguments
 Electronic band structure
6
Classification of solids based on
bonding type
1. Ionic solids (e- transfer)
2. Covalent solids (e- sharing)
3. Metals (delocalized e-)
4. Molecular (Hydrogen bonds or Van der Waals forces)
7
1. Ionic Solids
Electrostatic attractions between oppositely charged ions which is the
same in all directions
1ei.e. NaCl
Na
+ Cl  [Na]+ + [Cl]-
Ionic interactions are
omni-directional and
non-saturated
The resulting low energy configuration:
An ordered 3-dimensional network,
a "crystalline" solid.
8
1. Ionic solids
Have high degree of stability due to achievement of valence shell
octets through bonding
Non directional forces in such solids will result in highly ordered
macroscopic bodies
because of the repulsive
forces in the cleavage
plane  ionic crystals
are hard and brittle
9
Basic properties of ionic solids
(1) Non–directionality of the electrostatic forces leads to an ordered
solid (crystalline).
(2) Octet stabilization makes the solid non–conducting (electrical
insulator).
(3) Stability will also most likely make the material transparent (high
Eg) in the visible.
(4) The magnitude of the attractive forces makes the solid melt at
elevated temperature only. (NaCl-801ᵒC, MgO-2800ᵒC, CaF2-1418ᵒC)
10
2. Covalent Solids


The electrons are shared between two interacting atoms in a molecular
orbital (electron pairs);
Electron-pair bonds are positioned to maximize the orbital overlap;
Ex:
sp3carbon in diamond
sp2 carbon in graphite
Bond directionality because the atomic and hybrid orbitals are
quite definite and point in fixed directions
11
Properties of Covalent solids
The directionality of the bond makes the properties to vary widely
depending on the arrangement of the bonds
diamond
graphite
sp3 hybridization of carbon
Cubic structure
The electrons are very tightly
bound  very strong bonds
sp2 hybridization of carbon
Hexagonal structure
Strong covalent bonds in plane
weak delocalized bonds normal to the planes
 the planes can slide over one another
•Hard ( used as cutting tool)
•Insulator (Eg=5.4eV)
•Transparent to visible light
•High refractive index
•Lubricity ( used as lubricant)
•Decent electronic conduction
•Absorbs the photons and appears black
12
Diamond is the metastable form of carbon. The stable form is graphite!
Diamond can be synthesized from graphite at high pressure
Memorial diamonds from carbonized human remains by companies such as
GemLife (US), Algordanza (CH), HeartIn diamond (Ru)
13
Diamond vs. Silicon
Diamond – insulator
Silicon – semiconductor (Band gap, Eg = 1.1eV)
In Silicon, the Si atom is larger and the bond is longer (dSi-Si=2.35Å
vs. dC-C=1.54Å; weaker bonds  the e- are more easily liberated)
 smaller band gap and therefore semiconductor
Graphite vs. Boron Nitride
Graphite - conductor
Boron Nitride – insulator
In BN – two e- are only on N; the N atoms are
separated by an B atom in the plane  poor
orbital overlap and therefore insulator
dB-N=1.45Å
dC-C=1.42Å
In graphite – one unpaired e- is on each C
atom  good orbital overlap and therefore
good conductor
14
3. Metals
In metallic bond, the atoms loose their outer electrons to a common
electron band that runs throughout the solid (e- delocalization).
ion cores
We may therefore visualize metals
as a lattice of ion cores being held
together by a gas of free electrons.
gas of free electrons
15
Properties of Metals
•The atoms pack together like spheres and the bonding is non-directional
Planes slip is a common
phenomenon in metals
• Displacement of neighboring planes does not lead to charge effects
and therefore malleability and ductility
•High conductivity because the valence electrons never remain near any
particular atom very long
16
4. Molecular solids
1. Van der Waals or secondary bonding results from the coulombic
attraction between induced dipoles
+
-
+
-
solid He(1-1.5K and
2.5MPa)
Atomic or molecular dipoles
Van der Waals solids have extremely low melting points
2. Hydrogen bonding – a special type of secondary bonding exists
between some molecules that have hydrogen as one of the
constituents
ice
17
Directional vs. non-directional bonds
In solids with directional bonds, the atoms cannot pack together in
a dense manner, yielding to a low mass density [ (g/cm3)]
density of covalently solids < than metallic or ionic ones;
Density of solids -  (g/cm3):
•Ionic solids: NaCl (2.6 )
•Metals*: K (0.86); Ca (1.55); Sc (2.98); Ti (4.5)
•Covalent solids: Diamond (3.53); graphite (2.23); SiC (3.21 )
* The fewer electrons lost to get full outer shells, the lower density
18
Classification of solids based on atomic
arrangements:
Classes
Atomic arrangement
Ordering type
Ordered
Regular
Long range
Disordered
Random*
Short range
Name
Crystalline
“crystal”
Amorphous
“glass”
*Not totally random
19
LRO vs. SRO
Eordered < Eirregular 
crystalline form is favored
•Long-range order (LRO) - A regular repetitive arrangement of atoms in
a solid which extends over a very large distance.
•Short-range order (SRO) - The regular and predictable arrangement of
the atoms over a short distance - usually one or two atom spacings.
20
Transparent crystal vs. Opaque glass
Diamond (Eg=5.4eV)
Obsidian (volcanic glass: 70–75% SiO2,
plus MgO, Fe3O4)
Transparency: Eg>>3eV;
Transparency is not a property linked to the long range atomic
ordering of a solid; it is a function of the band gap of the solid;
pictures form Wikipedia
21
Solids classification summary
A. Based on bond type
1. Ionic solids
2. Covalent solids
3. Metals
4. Molecular solids
B. Based on atomic arrangements
I. Ordered solids = Crystalline
II. Disordered solids = Amorphous
22
Crystalline solids
• have characteristic geometric shapes
Particles vibrate about
their equilibrium
positions
NaCl
diamond
• have sharp melting points
• are anisotropic by nature
23
Crystalline forms
•Single crystal = the regularity of the pattern extends throughout a
certain piece of solid
Silicon crystal
When cut or hammered gently it
shows a clean fracture along a
smooth surface.
•Polycrystalline = the regularity or periodicity is interrupted at
the grain boundaries*
*grain boundary = where different
aligned grains meet with each other
24
Single vs. Polycrystals
• Single Crystals are anisotropic = properties vary with direction
E (diagonal) = 273 GPa
the degree of anisotropy increases with
decreasing structural symmetry
Fe
E (edge) = 125 GPa
• Polycrystals are isotropic = properties may/may not vary with direction
200 mm
the measured property represents
some average of the directional values
Epoly Fe = 210 GPa
From Callister 5e
25
Polycrystals
Most engineering materials are polycrystals.
Different color stands for
different grain orientation
• Each "grain” is a single crystal.
• crystals are randomly oriented,  overall component
properties are not directional.
• Crystal sizes varies (from 1 nm to 2 cm)
26
Crystals forms
• Isomorphism = crystals of different composition with the same form
Ex:
halite (NaCl)
fluorite (CaF2)
sylvite (KCl)
• Polymorphism = different crystal forms with the same chemical
composition; allotropes
Ex:
r .t .
  Fe( BCC )
1183K

r .t .
  Fe( FCC )

high pressure


C graphite( hexagonal)
1663K
  Fe( BCC )



Cdiamant( cubic)
27
Next
•
•
•
•
•
Introduction to Crystallography
Bravais lattices
Unit cell
Crystal structure
Characteristic of cubic systems
28