Download Topic 3: Structure of Materials

Topic 3: Structure of Materials
• Five levels of structure: electronic structure, atomic
structure, crystal structure, microstructure and
macrostructure
• Bonding between atoms (electronic & atomic structure)
determines structure at higher levels (crystal, micro- and
macro-structure)
• Structure & defects in structure ultimately determine
material properties
• Outline:
– Types of bonding (Ch. 5)
– Crystal structures, non-crystallinity (Ch. 6 & 7)
– Defects in crystals (Ch. 8)
• Reading assignment: Ch. 9 & 10
Bonding (Electronic & Atomic Structure)
Example:
Sodium (Na)
atomic number 11
11 electrons
electron
Shell (indexed by n)
Valence electron
Nucleus
(protons + neutrons)
• Each shell can hold a total of 2n2 electrons.
• Most atoms strive to achieve 8 electrons in their outermost shell.
• In the case of Na, it is much easier for Na to lose the 1 electron (resulting
in 8 electrons in the n = 2 shell) rather than grab 7 electrons for the n = 3
shell
• Electrons in outermost shell are called valence electrons
Types of bonding
• Depending on the type of each atom and
the environment each atom sees, several
bonding types arise
• Primary bonds (strong): metallic, ionic,
covalent
• Secondary bonds (weak): hydrogen & van
der Waals
• Generally, combination of more than one
type of bonding exists in a solid
Metallic bonds
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Na+
Positive ion cores
electrons
Na+
•
•
•
•
•
•
•
•
Na+
Na+
Na+
Metallic elements have only one, two, or at most three valence electrons.
In a metallic solid, the atoms give up all valence electrons to a “common pool”
or “sea” of electrons.
The remaining non-valence electrons and nuclei form positive ion “cores”.
The valence electrons are free to drift throughout the entire lattice and act as
electron glue holding the material together
Metallic bonds have no directionality and are strong in all directions.
They are good electrical and thermal conductors because of the free electrons
Atoms can pack closely, as a result, metals are heavy and dense
Atoms can move over each other and change position relatively easily (more
about this later). So, metals can be formed by pressing, forging, rolling, etc.
Ionic bonds
•
•
•
•
•
Na+
Cl-
Na+
Cl-
Cl-
Na+
Cl-
Na+
Na+
Cl-
Na+
Cl-
Cl-
Na+
Cl-
Na+
Examples: NaCl, Al2O3, MgO, CaO, H2O
In ionic materials, atoms achieve their eight electrons
in the outer shells by giving up valence electrons (e.g.,
Na) or by accepting electrons (e.g., Cl).
The resulting positively and negatively charged ions
attract each other and produce an ionic bond.
Bonds very strong and ions difficult to move
As no free electrons, poor electrical and poor thermal
conductors
Covalent bonds
Carbon: 4 valence electrons
•
•
•
•
•
•
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
C
Examples: C, Si, GaAs, SiC, H2
While metal atoms share their electrons throughout the piece of metal, in covalent
materials, atoms share electrons with specific nearby atoms to achieve 8 electrons in
their outer shells.
For example, atomic number of carbon is 6  4 valence electrons; in the diamond
form of carbon, each atom gets its 8 electrons by sharing its 4 valence electrons with
4 neighboring atoms
Bonds are directional, because electrons at one bond repel electrons at other bonds.
In diamond, there is a 109.5 degree angle between neighboring bonds
Covalent bonds are strong: hardest material is diamond
Poor electrical and thermal conductor because of no free electrons
Hydrogen bonds
Oxygen:
6 valence electrons
Hydrogen:
1 valence electrons
O
+
•
•
•
•
•
Bond responsible for holding water molecules in ice
In water, H atoms are covalently bonded to O, but the
electrons are somewhat shifted towards the O atoms
creating polarity (combination of ionic and covalent)
Hydrogen bond: Water molecules will arrange in a
hexagonal symmetry in ice; this accounts for the shape of
snowflakes that have the form of a complex 6-pointed
star
Hydrogen bond is weak as charges involved are small
Loose packing in ice which is why density of ice lower
than water
H
H
+
Hydrogen
bond
van der Walls bonds
•
•
•
Weakest of all chemical bonds
Result from weak electrostatic interaction; polarity
or dipole of molecules is not permanent or fixed in
direction
Closed shell atoms interact through van der Walls
bonding; example: solids of inert gas atoms,
graphite, polymers
Materials with both primary and secondary bonds: graphite (covalent in plane,
van der Waals between planes), polymers (covalent along chain, van der Waals
between chains)
In summary, chemical bonds affect: strength, ductility, melting point, electrical
and thermal conductivity, density, crystal structure, shape of solids, etc.
Close packed crystals
A plane
B plane
C plane
A plane
…ABCABCABC… packing
…ABABAB… packing
Close packed structures
Cubic close packed (CCP) or
Face centered cubic (FCC)
Hexagonal close packed (HCP)
Crystal Structures of Metals
• Three common crystal structures in metals:
– Face centered cubic (fcc): ABCABC… packing: Ni, Cu,
Ag, Al, Au
– Hexagonal close packed (hcp): ABABAB … packing: Mg,
Zn, Co, Ti
– Body centered cubic (bcc): Fe, Cr, W, Ta, Mo
• Easy for close packed planes to slide over each other:
slip planes (plays an important role in determining
deformation & strength)
shear
BCC unit cell
Crystal Structure
• Larger scale packing of atoms, molecules or other
building blocks; primarily determined by the bonding
between atoms and molecules
• Lattice: infinite array of points in space; all points have
identical surroundings; mathematical construct
• Crystal structure: Associate each lattice point with one or
more atoms
Crystal Lattices
Other Crystal Structures
• Diamond (carbon, silicon) lattice: FCC
lattice, with 2 atoms per lattice point
• Graphite: hexagonal lattice
• Sodium Chloride (NaCl): FCC lattice with 2
atoms (1 sodium & 1 chlorine) per lattice
point
• Polyethylene:
Amorphous Materials
•
•
•
•
No long range order, only short range order
Created by heating (atoms break away bonds and get dislodged from their
equilibrium positions), followed by quenching – rapid cooling so that atoms do not
have time to return to their equilibrium crystal structures
Amorphous glass (SiO2) occurs naturally
Sometimes we intentionally create amorphous metals (although very difficult) to
create very strong (but brittle) materials [remember, no close-packed slip planes in
amorphous materials], or novel magnetic materials
Defects in Crystals
• Dislocations (line defects: 1-dimensional)
• Point defects (0-dimensional)
• Grain boundaries (planar defects: 2dimensional)
Dislocation: line defect (1-dimensional)
mediates
•
•
•
Extra “half-plane” of atoms in a crystal
Dislocations make slip 1000 times easier, which is why metals deform easily
Slip of atom planes over each other due to deformation occurs one atom
row at a time, analogous to caterpillar motion or moving a pile of bricks one
at a time
Point Defects (0-dimensional)
In semiconductors, substitutional
impurities are called dopants, and
control the amount of charge carriers
An avenue for atomic
motion within the lattice,
in response to an
external mechanical or
electrical load
• Intrinsic (vacancies)
• Extrinsic (interstitial and
substitutional impurity atoms)
• Alter the mechanical properties (by
affecting slip and dislocation motion),
electronic properties (doping in
semiconductors), etc.
In stainless steel, carbon, which makes it a steel, is an
interstitial impurity in the iron lattice (and chromium,
which makes it stainless, is a substitutional impurity)
Grain Boundaries, Microstructure &
Macrostructure: planar defects (2-dimensional)
During solidification …
• Boundary between two different phases or materials, or between
two crystallites of the same material but oriented wrt each other
• Affects mechanical properties by affecting point and line defect
motion
• Other defects: voids, porosity, precipitates, secondary inclusions
Topic 3: Structure of Materials
• Five levels of structure: electronic structure, atomic
structure, crystal structure, microstructure and
macrostructure
• Bonding between atoms (electronic & atomic structure)
determines structure at higher levels (crystal, micro- and
macro-structure)
• Structure & defects in structure ultimately determine
material properties
• Outline:
– Types of bonding (Ch. 5)
– Crystal structures, non-crystallinity (Ch. 6 & 7)
– Defects in crystals (Ch. 8)
• Reading assignment: Ch. 9 & 10