# Engineering Materials Semester 1.1

## What is a Covalant bond

A bond where 2 or more atoms share a vacant electron to complete the outershell. It is a strong primary bond

## what is an ionic bond

when one electron donates one of its electrons to the other atom to complete its outer shell. the difference in charge attracts the atoms together. it is a strong primary bond

## what is a metallic bond

a bond between two or more metal ions whereby the atoms are in a 'sea' of delocalised electrons. a strong primary bond

## what are van der vaals forces

secondary bond so considerably weaker, arise from dipoles between 2+ atoms or molecules

## what is a hydrogen bond

special type of hydrogen bond whereby hydrogen is one of the constitutes.

## what is bonding energy

The energy required to seperate 2 atoms by an infinite seperation distance.

## what are crystalline materials

periodic arrangement of atoms over long distance calle long range order

## what are non-crystalline structures

also called amorphous. short range order only and they are complex structures with rapid cooling

## what is a lattice

three dimesional array of points coinciding with atom positions

## what is a unit cell

the smallest repiting entities in a structure.

## what is the hard sphere model

atoms are thought as solid spheres with well defined parameters

## lattice parameters: what is a cubic structure

a=b=c. all angles = 90 degrees. volume = a^3

## lattice parameters: what is a tetragonal structure

a=b/=c. all angles = 90 volume= a^2c

## lattice parameters: what is a orthorhombic structure

a/=b/=c angles all=90 volume = abc

## lattice parameters: what is a hexagonal structure

a=b/=c angles 2=90 and the angle between ab =120

volume = 0.866a^2c

## lattice parameters: what is a rhombohedreal / trigonal structure

a=b=c all angles are equal but none are 90. volume =

a^3 sqrt(1-3cos^2(α)+ 2cos^3(α)

## what is a monoclinic structure

a/=b/=c angles two equal 90. one angle β /= 90. volume =

abcsin(β)

## what is a triclinic structure?

a/=b/=c angles all angles different and none =90. volume=

abc sqrt(1-cos^2(α) - cos^2(β)- cos^2(γ) +

2cos(α) cos(β) cos(γ) )

## what is a single crystal

perfectly periodic. all unit cells interlock at the same orientation

## what is a polycrystalline material

collection of many small grains. during solidification many crystals form at various locations. grow from the surronding liquid. graains impinge on one another all at different orientations. may be some mismatch and areas between the grains is called grain boundary.

## what is anisotropy?

the directionality of properties

## number of atoms per unit cell

N=Ni+Nf/2+Ne/4+Nc/8

## what is coordination number?

number of atoms touching a particular atom or the number of nearest neighbours for an atom

## what is atomic packing factor?

sum of volume of atoms in a unit cell divided by the unit cell

## what is theoretical density?

ρ= NAw/VcNa

## what is a simple cubic structure

consists of 8 atoms in the corners of the cell. nothing else

## what is a body centred cubic?

8 quaters in the edges and then one other atom in the middle (interior)

## what is face centred cubic

8 quaters in the edges and then one half atom per face

## what is polymorphism

when a material has more than one structure

## what is allotropy

polymorphism found in a solid structure