Structure & Crystalline Solids Flashcards
what are the two types of main structures of solid materials?
amorphous and crystalline
what is the structure of a crystalline material?
they have atoms situated in a periodic array over large atomic distances
give some examples of materials that have crystalline structures?
metals and most ceramics
what are amorphous materials?
materials that have no long range order, they are a non-repeating pattern (non crystalline) - these include glasses like silica, plastics and rapidly cooled metals
how do crystalline structures form?
if we cool something down or if we precipitate something out of something
how do ice crystals form on lakes? (can be applied to any formation of crystal structure)
- ‘nuclei’ form during solidification, each of which grows into crystals, nucleation - when the molecules/atoms are initially coming together
- crystals grow and meet
- eventually they cannot grow anymore and we form out solid crystal
- these grains are separated by an amorphous grain boundary because the orientation of the crystals are all mismatched
how can we reveal the grain structure of a crystal?
use polishing and etching (using acids)
what type of atomic packing does a crystalline structure have?
dense, regular packing (tend to have lower bond energy than amorphous structures)
what type of atomic packing does an amorphous structure have?
non-dense, random packing (varying bonds lengths) - they tend to have larger bond energy and bond lengths compared to crystalline structures) - more reactive, less energy needs to be put in
how do we describe the arrangement of crystalline solids?
using the unit cell - the smallest repeating arrangement (basic building block)
what is the difference between the hard sphere model and the reduced sphere representation?
hard sphere model - atoms donated by hard, touching spheres
reduced sphere representation - reduced spheres
what are the 4 main unit cells we find in metallic crystal structures?
SC - simple cubic
FCC - face centred cubic
BCC - body centred cubic
HCP - hexagonal close packed
why do metallic crystalline structures tend to have dense packing?
due to non-directional bonding in metals
what is the simple cubic structure? (SC)
- cubic unit cell
- very rare, only Po has this structure (polonium)
what does the coordination number refer to?
the no. of nearest neighbours or touching atoms
what is the coordination number of simple cubic structures?
6
how do we describe the packing?
- using the atomic packing factor
- we can expect values less than 1 because it is a ratio
- APF = volume of atoms in unit cell / volume of unit cell (assuming hard sphere model)
what is the equation for the atomic packing factor?
(no. of atoms per unit cell ) x (4/3π(r)^3) / a^3
where r is written in terms of a (lattice parameter of a cubic cell)
what is the atomic packing factor (APF) for a simple cubic structure?
0.52 (has 8 x 1/8 atoms = 1 atom per unit cell)
r = 0.5a
what is the face centred cubic structure (FCC)?
- atoms located at each corner and the centre of each cube faces
- some examples include copper, aluminium, silver, gold (ductile metals)
- coordination number = 12
- APF = 0.74 (has 8 x 1/8 atoms + 6 x 1/2 = 4 atoms per unit cell)
- r = 2^0.5a / 4 (because then face diagonal of the cube is equal to 4r which is equal to root2xa)
what does ductility relate to in terms of crystal structure?
ductility (ease of plastic deformation) is linked to crystal structure and close packed planes
how do slip systems work?
slip occurs on specific atomic planes and in specific crystallographic slip directions i.e slip systems
what are the slip planes?
they are the most close packed planes (note: only count different ones, ignore parallel)
for the unit cell what is the order of directions?
(x, y, z) - helps us define a plane or a direction
what is the equation for no. of slip systems?
no. of slip systems = slip directions x slip planes
what is the body centred cubic structure (BCC)?
- atoms located at each of the corners and the cube centre
- some examples include chromium, α-iron, tungsten
- coordination number = 8
- APF = 0.68 (has 8 x 1/8 atoms + 1 = 2 atoms per unit cell)
- r = 3^0.5a / 4 (because the central diagonal of the cube is equal to 4r which is equal to root3xa)
what is the direction of the slip plane in FCC structure?
(1, 1, 1)
how does the slip direction relate to the slip plane?
the slip plane is perpendicular to the slip direction
how many slip systems does a FCC structure have and why?
- 4 slip planes
- 3 slip directions
= 12 slip systems
(found in ductile metas as there are many opportunities for planes to slide over each other)
how many slip systems does a BCC structure have and why?
- 6 slip planes
- 2 slip directions
= 12 slip systems
what is the direction of the slip plane in BCC structure?
(1, 1, 0)
what is the hexagonal close packed structure (HCP)?
- hexagonal unit cell
- top and bottom faces of hexagonal cell consist of 6 atoms (a hexagon) with an atom in the centre
- the middle plane consists of 3 atoms
- some examples include magnesium, titanium, cadmium and zinc
- least ductile metals (only 1 slip plane)
- coordination number = 12 and APF = 0.74 like FCC
how many slip systems does a HCP structure have and why?
- 1 slip plane
- 3 slip directions
= 3 slip systems
(most brittle of the metals, but subtle difference between FCC structure)
what is the direction of the slip plane in HCP structure?
(0, 0, 1)
what type of structure is NaCl?
simple cubic
what are some applications of perfect crystalline structures?
- electronic and optical materials (Silicon wafers/chips)
- high performance single crystal turbine blades
- abrasive materials (synthetic diamond)
what does anisotropic mean?
different in different directions - some crystals have direction properties, different properties in different directions
what is an issue with manufacturing a silicon wafer (crystal)?
high embodied energy and cost of production (however we have a large elemental abundance of silicon) - silicon wafers are used in semi conductor devices
what does the size of the crystal relate to?
the cooling rate:
large crystals = cooled slowly
small crystals = cooled quickly
what are some similarities/differences between single VS poly-crystalline structures?
SINGLE:
- properties vary with direction (anisotropic)
- example: the stiffness of BCC iron
POLYCRYSTALS
- has some regions of crystalline areas
- properties may/may not vary with direction
- if grains are randomly orientated: isotropic
- if grains are textured: anisotropic
general summary of crystalline materials…
- atoms packed in periodic 3D arrays
- typical of metals, many ceramics, some polymers
- slow cooling
general summary of non-crystalline (amorphous) materials…
- atoms have no periodic packing
- occurs for complex structures
- rapid cooling
- examples include glasses and some polymers
what are defects in crystals?
areas in a crystal which aren’t perfect
why are defects/imperfections in crystals important?
many of the important properties of materials are due to the presence of imperfections
what are the 4 main types of defects?
- point defects (0D)
- line defects (1D)
- area defects (2D)
- volume defects (3D)
what are some examples of point defects?
- vacancy atoms
- self interstitial atoms
- substitutional atoms
what are some examples of line defects?
- dislocations
what are some examples of area defects?
- grain boundaries
what are some examples of volume defects?
- precipitates
- cracks
- porosity
what is a vacancy atom in terms of point defects?
where an atom is missing creating a distortion of planes
what are self interstitial atoms in terms of point defects?
where an extra atom is positioned between atomic sites creating a distortion of planes
what are substitutional atoms in terms of point defects?
when one of the main atoms in the crystal is substituted with something else (note: you can get interstitial substitutional atoms such as small atoms that get added in which don’t disrupt the lattice)
what are the conditions that make it likely for substitutions in the crystal to work?
- the difference in atomic radii must be less than 15%
- they must have similar electronegativities (close on the periodic table)
- same crystal structure for pure metals (e.g gold and silver)
what is a frenkel defect?
a special type of point defect where there is both a vacancy and an interstitial
why are metals ductile?
the movement of dislocations
what type of defect are dislocations?
line defects
what are dislocations?
dislocations are one-dimensional defects around which atoms are misaligned
what are the two types of dislocations?
- edge dislocations
- screw dislocations
what is an edge dislocation?
when an extra plane of atoms is inserted in a crystal structure
what is a screw dislocation?
a spiral planar ramp resulting from shear deformation
what is the burger’s vector?
a measure of lattice distortion (the amount the dislocation has shifted)
how do dislocations move?
- dislocations are mobile under an applied stress
- incrementally breaking/re-making bonds
- large numbers of dislocations move for plastic deformation to occur in metals and alloys (or will just fracture like a ceramic)
in what material do dislocations move smoothly (think caterpillar)?
in metals because there is non-directional bonding, movement is hard in covalent and ionic ceramics and so there is no dislocation movement
what is dislocation density?
total dislocation length per unit volume of material or the number of dislocations that intersect a unit area of a random section
what does the dislocation density determine?
the strength of the material
what does deformation do to the number of dislocations?
creates defects which increases the number of dislocations
what is a grain boundary in terms of area defects?
they are 2D defects that are regions between crystals, they are the transitions from a lattice of one region to that of the other
what does a high angle of misalignment mean for a dislocation?
harder to move compared to moving between a grain boundary with a smaller angle of misalignment
some general info on grain boundaries…
- they can vary considerably in size
for example grains can be large in diamond or silicon but very small (mm or less) in other things
what does the size of the grains define?
the number of grain boundaries - if there are little grains there are going to be more grain boundaries than if you had larger grains, therefore depending on the solidification and the number of grain boundaries, this will effect the deformation
what are precipitates in terms of volume defects?
you can grow 3D precipitates by heating up certain alumnium alloys (deliberate heat treatment) - they can help restrict the movement of dislocations
what type of materials do cracks and damage effect?
brittle materials
where do you come across porosity defects?
in ceramics and metals
what do the defects in a solid effect?
the mechanical properties of that solid and defects are inevitable
what is an example of a substitutional defect?
Cu in Ni
what is an example of an interstitial defect?
C in Fe
what is the difference between body centred cubic and body centred tetragonal?
in BCC, all the angles in the cube are 90 degrees and all the lengths of the cube are equal - in BCT, all the angles are 90 degrees but one out of the 3 lengths are slightly longer than the other (rectangular shape)
why do more grain boundaries mean for the strength of a material and why?
more grain boundaries = harder for dislocations to move and harder to deform the material
how many whole atoms are in a simple cubic unit cell?
1
how many whole atoms are in a FCC unit cell?
4
how many whole atoms are in a BCC unit cell?
2
