Material Atomic structure and Crestallography

1. MATERIALS SCIENCE AND
ENGINEERING
An Introduction
Naman Kumar Gandhi
Mechanical Engineering
Indore institute of technology &
Science
Naman.gandhi@indoreinstitute.com

2. Introduction to Material science and
Engineering
• Atomic structure in Materials
• Atomic Bonding in Materials
• Types of material
• Crystal structure of materials
• Crystalline solids and their role in influencing
various properties

3. What is an atom?
• Atom: the smallest unit of matter that retains the identity of
the substance.
• Consists of a central nucleus surrounded by one or more
electrons
WHAT IS THE NUCLEUS
• The central part of an atom.
• Composed of protons and neutrons.
• Contains most of an atom's mass.

4. Atomic Structure
• Protons and neutrons are heavier than electrons and reside in
the center of the atom, which is called the nucleus. Electrons
are extremely lightweight and exist in a cloud orbiting the
nucleus
• Protons and neutrons have approximately the same mass.
However, one proton weighs more than 1,800 electrons.
Atoms always have an equal number of protons and electrons,
and the number of protons and neutrons is usually the same
as well

5. WHAT IS AN ELECTRON?
oNegatively charged
particle.
oLocated in shells that
surround an atom's
nucleus.

6. WHAT IS A NEUTRON?
o Uncharged particle.
oFound within an atomic
nucleus.

7. WHAT IS A PROTON?
oPositively charged
particle.
oFound within an
atomic nucleus.

8. Atomic Structure
• Atoms are composed of 2 regions:
– Nucleus: the center of the atom that
contains the mass of the atom
– Electron cloud: region that surrounds the
nucleus that contains most of the space in
the atom

9. Particle Charge Mass (g) Mass (amu)
Proton +1 1.6727 x 10-24 g 1.007316
Neutron 0 1.6750 x 10-24 g 1.008701
Electron -1 9.110 x 10-28 g 0.000549

10. Atomic Bonding in
Materials

11. Bonding in Materials
• Primary Bonding
There are three types of primary bonds
1. Ionic Bonding
2. Covalent Bonding
3. Metal Bonding

12. Ionic Bonding
• Ionic bonding occurs between charged particles. Ionic bonding occurs
between metal atoms and nonmetal atoms.
• Metals usually have 1, 2, or 3 electrons in their outermost shell. Nonmetals
have 5, 6, or 7 electrons in their outer shell. Atoms with outer shells that are
only partially filled are unstable. To become stable, the metal atom wants to
get rid of one or more electrons in its outer shell. Losing electrons will
either result in an empty outer shell or get it closer to having an empty
outer shell. It would like to have an empty outer shell because the next
lower energy shell is a stable shell with eight electrons.

13. Some Common Features of Materials
with Ionic Bonds:
• Strong and high energy bonds
• High melting point
• Non directional
• Hard and brittle or cleave rather than deform
Transparent
• Insulator

14. Covalent Bonding
• Where a compound only contains nonmetal atoms, a covalent bond is
formed by atoms sharing two or more electrons. Nonmetals have 4 or more
electrons in their outer shells (except boron). With this many electrons in
the outer shell, it would require more energy to remove the electrons than
would be gained by making new bonds. Therefore, both the atoms involved
share a pair of electrons. Each atom gives one of its outer electrons to the
electron pair, which then spends some time with each atom. Consequently,
both atoms are held near each other since both atoms have a share in the
electrons.

15. Some Common Features of Materials
with Covalent Bonds:
1. Often hard
2. Inorganic covalent compounds usually have
high melting point
3. Can be strong (Diamond)
4. Can be weak (Bi)
5. Directional

16. Metallic Bonding
The outermost electron (valance electron) of metal atoms are weakly bound and when
such atoms interact to become a solid, their valence electrons form a “gas of electrons
known as electron gas. Thus metallic bond results from the attraction between the
positive metal ions and the electron gas. The electron gas acts to hold the positive metal
ions to form metallic bond which provides the high electric and thermal conductivitities
and other characteristic properties of metals.

17. Some Common Features of Materials
with Metallic Bonds
• Good electrical and thermal conductors due to their free
valence electrons
• Opaque
• Relatively ductile
• High packing density
• Metallic bonds are weaker than ionic and covalent bonds.
• They are crystalline in nature.

18. Secondary Bonding
Van der Waals Bond
The van der Waal bonds occur to some extent in all materials but are
particularly important in plastics and polymers. These materials are made
up of a long string molecules consisting of carbon atoms covalently bonded
with other atoms, such as hydrogen, nitrogen, oxygen, fluorine. The
covalent bonds within the molecules are very strong and rupture only under
extreme conditions. The bonds between the molecules that allow sliding
and rupture to occur are called van der Waal forces.

19. Hydrogen Bonds
• Thus when water molecules are close together, their positive and negative
regions are attracted to the oppositely-charged regions of nearby molecules.
The force of attraction, shown here as a dotted line, is called a hydrogen
bond. Each water molecule is hydrogen bonded to four others.

20. Types of material
1. Metals
2. Ceramics
3. Polymers
4. Composites
5. Semiconductors
6. Bio Materials

21. Metals & Alloys
• Mechanical behavior of metals and alloys
• Tensile & compressive stress-strain relations
• Fracture toughness, fatigue, creep, wear and
abrasion
• Microstructure properties and applications of
ferrous and non-ferrous alloys
• Alloys

22. Atomic arrangement in Solids
• Crystalline – periodic arrangement of atoms:
definite repetitive pattern
• Non-crystalline or Amorphous – Random
arrangement of atoms.

23. Structure of Crystalline Solids
• Face Centered
• Body Centered
• Hexagonal closed crystal structure

24. Crystallography
Structure and Geometry of crystals
• Space lattice
A space lattice is an infinite array of points in three-dimensional Space
in which each point is identically located with respect to the other. It is
a periodic arrangement of points arranged in regular manner and having
repeat distance in three directions which are termed as fundamental
lattice vectors. At these lattice positions we place atom, or group of
two, three etc. atoms we obtain a crystalline solid. The crystal structure
is real while the lattice is imaginary

25. Unit Cell
the smallest group of atoms which has the overall symmetry of a crystal, and from
which the entire lattice can be built up by repetition in three dimensions.
Types of units cells
• A units cell is obtained by joining the lattice points. The choice of lattice points to
draw a unit cell is made on the basis of the external geometry of the crystal, and
symmetry of the lattice. There are four different types of unit cells. These are,
• (1) Primitive or simple cubic(sc) : Atoms are arranged only at the corners of the
unit cell.
• (2) Body centered cubic (bcc) : Atoms are arranged at the corners and at the center
of the unit cell.
• (3) Face centered cubic(fcc) : Atoms are arranged at the corners and at the center
of each faces of the unit cell.
• Lattice points are the positions where you can place an atom. If there are two
lattice point in a unit cell it means two atom will stay in single unit cell

26. 3
• tend to be densely packed.
• have several reasons for dense packing:
-Typically, only one element is present, so all atomic
radii are the same.
• have the simplest crystal structures. 74 elements
have the simplest crystal structures – BCC, FCC and HCP
We will look at three such structures...
METALLIC CRYSTALS

27. 4
• Rare due to poor packing (only Po has this structure)
• Close-packed directions are cube edges.
• Coordination # = 6
(# nearest neighbors)
(Courtesy P.M. Anderson)
SIMPLE CUBIC STRUCTURE (SC)
Click on image to animate

28. 5
• APF for a simple cubic structure = 0.52
Adapted from Fig. 3.19,
Callister 6e.
ATOMIC PACKING FACTOR

29. • Coordination # = 8
8
Adapted from Fig. 3.2,
Callister 6e.
(Courtesy P.M. Anderson)
• Close packed directions are cube diagonals.
--Note: All atoms are identical; the center atom is shaded
differently only for ease of viewing.
BODY CENTERED CUBIC STRUCTURE
(BCC)
Click on image to animate

30. a
R
9
• APF for a body-centered cubic structure = 0.68
Unit cell c ontains:
1 + 8 x 1/8
= 2 atoms/unit cell
Adapted from
Fig. 3.2,
Callister 6e.
ATOMIC PACKING FACTOR: BCC

31. 6
• Coordination # = 12
Adapted from Fig. 3.1(a),
Callister 6e.
(Courtesy P.M. Anderson)
• Close packed directions are face diagonals.
--Note: All atoms are identical; the face-centered atoms are shaded
differently only for ease of viewing.
FACE CENTERED CUBIC STRUCTURE
(FCC)
Click on image to animate

32. Unit cell c ontains:
6 x 1/2 + 8 x 1/8
= 4 atoms/unit cell
a
7
• APF for a body-centered cubic structure = 0.74
Adapted from
Fig. 3.1(a),
Callister 6e.
ATOMIC PACKING FACTOR: FCC

33. 10
• Coordination # = 12
• ABAB... Stacking Sequence
• APF = 0.74
• 3D Projection • 2D Projection
A sites
B sites
A sites
Adapted from Fig. 3.3,
Callister 6e.
HEXAGONAL CLOSE-PACKED
STRUCTURE (HCP)

35. fcc

38. • Atoms may assemble into crystalline or
amorphous structures.
• We can predict the density of a material,
provided we know the atomic weight, atomic
radius, and crystal geometry (e.g., FCC,
BCC, HCP).
• Material properties generally vary with single
crystal orientation (i.e., they are anisotropic),
but properties are generally non-directional
(i.e., they are isotropic) in polycrystals with
randomly oriented grains.
27
SUMMARY