Lecture 3 and 4 Flashcards
Density is linked to
atomic weight - but is not always the case
As increase temperature density does what and why
the density decreases as atoms;
amplitude of vibration increases
atoms gets further apart as it would require more energy to stay at the same distance and vibrate with greater amplitude see lennord jones
same mass for larger volume therefore lower density
thermal expansion
At certain temperature some materials undergo
a phase change ie FCC to BCC different structure
what can happen in a phase change
change in density
magnetic properties alter
a lot of mechanical properties
at certain temperature property you are relying on may disappear due to phase change
BCC
body centered cubic atoms touch across internal diagonal not a close packed system atoms all corners one in the middle not a stable structure
FCC
face centered cubic close packed structure based on layers of atoms, atoms touch across diagonal of face packing sequence ACB ACB ACB
Most materials are
crystalline structure (not are but focus on crystalline atm)
crystalline structures can be descirbed as
regular array of atoms can be described as unit cells which is a basic building block to make the whole structure
lattice spacing
a - length width and depth of unit cell (volume of unit cell = a^3)
sharing of atoms between unit cells meaning
if you had a unit cell of simple cubic structure at each corner was an atom then each atom would be shared with 8 other unit cells therefore an eighth of that atom would be inside the unit cell
total atoms in unit cell = 1/8 * 8 = 1
What can we tell from unit cell of an atom
everything dont care about the number of unit cells as it just makes the crystal larger repeat of the same unit cell can characterize the crystal by characterizing the unit cell
atomic packing fraction
number of particles x volume of particle (4/3pi r^3) / volume of unit cell
method for finding APF
find lattice structure in terms of radius of atoms (see what plane atoms touching on)
find volume of unit cell in terms of radius of atom
find number of atoms in unit cell (remember some shared)
insert into APF equation, radius cancels giving %
simple cubic structure
one atom at each corner of cube
What does the APF tell you
% of unit cell filled with atomic volume
HCP
hexagonal close packed close packed structure based on layers of atoms packing structure AB AB AB
number of atoms in FCC unit cell
1/2 * 6 + 1/8 * 8 = 4
types of materials FCC
aluminium copper
FCC close packed plane
(1 1 1)
FCC a in terms of r equation
a^2 + a^2 = (4R)^2
a^2 = 8R^2
a= 2root2 * R
perfect dislocation is where
atom crystal moved but maintained the structure of the crystal - hoped from one to the other
HCP materials
cobalt alpha titanium
HCP structure unit cell
hexagonal prism each face is a place A and A with the B layer in the middle unit cell is a third of this structure
HCP unit cell
2 atoms per unit cell V = (root3 * a^2 *c)/2
where c is the distance between two layers of the same structure ie A and A
