Materials Engineering

Front 1

Crystalline Material

Back 1

A crystalline material is one in which atoms are arranged in a repeating fashion, or a periodic array over large atomic distances.

Hint 1

Periodic Array

Front 2

Crystal Structure

Back 2

A crystal structure is one in which atoms are arranged in a continuous and repeating fashion as found in crystal structures

Front 3

Unit Cell

Back 3

A unit cell is the basic structural block of a crystalline material which defines the entire lattice by the virtue of its geometry and the positions of the atoms within.

Hint 3

Virtue of Geometry; Positions of atoms within.

Front 4

Lattice Points

Back 4

The corners of the unit cell therefore serve as points which are repeated to form a lattice array.

Hint 4

Points to form lattice array

Front 5

Crystallographic Axes

Back 5

The vectors which connect a straight line of equivalent lattice points and delineate the edges of the unit cell are known as the crystallographic axes.

Front 6

Translational Symmetry

Back 6

Describes the periodic repetition of a structural feature across a length or through an area or volume.

Hint 6

Translational- Length/Area/Volume

Front 7

Point Symmetry

Back 7

Point symmetry describes the periodic repetition of a structural feature around a point. Reflection, Rotation, Inversion are point symmetries.

Hint 7

Around a point (Rotation, Reflection, Inversion)

Front 8

Bravais Lattice

Back 8

Is an infinite array of discrete points with an arrangement and orientation that appears exactly the same viewed from any point of the array.

Hint 8

Array of discrete points that looks the same from everywhere.

Front 9

Dense Packing of Metallic Crystals

Back 9

1. Typically, made of heavy element.

2.Metallic bonding is not directional, no restriction as to the number and position of nearest neighbour atoms

3.Nearest neighbour tend to be small in order to lower potential energy.

4.Have the simplest crystal structures.

Front 10

Atomic Radius

Back 10

Half of the distance between the centre of two adjacent atoms.

Front 11

Coordination Number

Back 11

The no. Of atoms directly surrounding the particular atom.

Front 12

Atomic Packing Fraction

Back 12

Close packing of atoms in a unit cell of the crystal structure is known as APF.

APF : No. Of effective atoms * (Volume of atomic per unit cell)/(Volume of unit cell)

Front 13

FCC

Back 13

a=2R√2 , coordination number = 12, APF= 0.74, Effective no. Of atoms = 4

Front 14

BCC

Back 14

a=4R/(√3), coordination number = 8, APF= 0.68, Effective no. Of atoms = 2

Front 15

HCP

Back 15

a=2R, Coordination Number= 12, no. Of atoms=6, APF=0.74

Front 16

Theoretical Density

Back 16

Front 17

Polymorphism

Back 17

Polymorphism is when a compound occurs in more than one crystal structure, depends on both temperature and pressure.

Hint 17

Calcite, Aragonite, Vaterite ( Calcium Carbonate)

Front 18

Allotropy

Back 18

Allotropy is nothing but, polymorphism in elemental compounds.

Front 19

Hexagonal Crystals Index System

Back 19

Front 20

Linear Density

Back 20

Linear Density is defined as the number of atoms per unit length whose centres lie on the direction vector for a specific crystallographic direction

LD=[ Number of atoms centred on direction vector]/[Length of Direction Vector]

Front 21

Planar Density

Back 21

Planar density is defined as the number of atoms per unit area that are centred on a particular crystallographic plane

PD= Number of atoms centred on plane/ Area of a plane

Front 22

Single Crystal

Back 22

For a crystalline solid when the periodic or repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption, the result is a single crystal. A noncrystalline material shows anisotropy, which means the measurement of physical properties in a monocrystalline material is direction dependent, and shows long range order.

Front 23

Polycrystalline Material

Back 23

Crystalline materials which do not have a regular or repeated pattern, or have a regular and systematic pattern only for a short distance and are a collection of many small crystals and grains.

Shows Isotropy, which implies that the measurement of physical properties is direction independent and has short range order.

Front 24

Nucleation

Back 24

Refers to the formation of the first nanocrystallites from molten material.

The physical process by which a new phase is produced in a material.

Nuclei act as templates on which crystals grow.

In order for form,rate of addition of atoms to nucleus must be faster than rate of loss.

Growth continues till equilibrium.

Front 25

Critical Radius

Back 25

The minimum radius that must be achieved by atoms clustering together in the liquid before the solid particle is stable and begins to grow.

The solid is an embryo if its radius is less than the critical radius, otherwise a nucleus.

Hint 25

Minimum radius before formation of nucleus

Front 26

Surface Energy

Back 26

A material is expected to solidify when the liquid cools to just below its freezing temperatures, because the energy associated with crystalline is solid is less than that of the liquid.

When the solid forms a liquid surface interface is created and a surface free energy is associated with it

The energy difference between the liquid and the solid is the free energy per unit volume and is the driving force for solidification.

Hint 26

Solid-Liquid interface below T freeze.

Front 27

Undercooling

Back 27

At the thermodynamic melting or freezing temperatures, the probability of forming stable and sustainable nuclei is extremely small. Therefore, solidification does not begin at the thermodynamic melting or freezing temperatures.

As the temperature continues to decrease below the equilibrium freezing temperature, the liquid phase that should have transformed into a solid becomes increasingly thermodynamically unstable.

Because the temperature of the liquid is below the equilibrium freezing temperature, the liquid is considered undercooled.

The undercooling is the difference between the equilibrium freezing temperature, and the actual temperature of the liquid.

As the extent of undercooling increases, the thermodynamic driving force for the formation of a solid phase from the liquid overtakes the resistance to create a solid-liquid interface.

Front 28

Types of Nucleation

Back 28

Homogenous Nucleation and Heterogenous Nucleation.

Front 29

Homogenous Nucleation

Back 29

Formation of a critically sized solid from the liquid by the clustering of a large number of atoms under high undercooling without an external surface.

Front 30

Factors Favouring Homogenous Nucleation

Back 30

Since atoms are losing thermal energy, probability of forming larger embryos increases.

The larger volume free energy difference between the liquid and the solid reduces the critical size of the nucleus.

Homogenous Nucleation occurs when the undercooling becomes large enough to cause the formation of a stable nucleus.

Hint 30

Thermal energy goes down, Embryo size increases. Larger volume free energy, crit. Rad falls.

Front 31

Critical Radius Formula

Back 31