Fundamentals Flashcards
Define stress and strain
Stress = F/A Strain = Change in X/X
What is the UTS, Yield point, Failure point, plastic and elastic regions ?
UTS - most strain sample can endure
Yield point - when plastic deformation begins
Failure point - stress at which sample fails
Elastic region - follows Young’s modulus, all deformation recoverable
Plastic region - non-linear and permanent deformation
Draw and describe the features of an atom
Nucleus - where most mass centred
Surrounded by discrete e- shells, whose energy increases as distance increases
How does an electron move between shells?
e- absorbs photon and is excited to new shell (if energy high enough)
Then deexcites to lower shell which is more stable and emits a photon with energy equivalent to difference in shell energies - each atom has unique set of shells
What 4 things are used to define an e- energy?
Shell number (1 -> 3)
Sub shell number (s,p,d,f)
Orbital (x, y, z)
Spin number (-2 -> 2)
Which e- shells fill first?
Lowest energy ones - closest to nucleus
How many sub shells, orbitals and electrons can the 4 shells have?
Each orbital can hold 2 e- K - S sub shell, 1 orbital L - S and P s-shell, 1 and 3 orbitals M - S,P,D - 1,3,5 orbitals N - S,P,D,F - 1,3,5,7 orbitals
How many orbitals do the P and F sub shells contain?
P = 3 F = 7
What does 2p6 mean? (in e- shell terms)
2nd shell, P subshell, contains 6 e-
What are valence e-?
Electrons on the outer most shell of the atom - the ones that react
What is the atomic number and atomic mass number?
Atomic number = protons in nucleus
Mass number = average mass of all isotopes of atom
What is an isotope?
Atom which has more neutrons that another one of the same type
What is relative atomic mass?
1 RAM = 1/12 of mass of carbon 12 = 1U
Calculate RAM of chlorine which is 75% 35/17 chlorine and 25% 37/17 chlorine
0.75 x 35u + 0.25 x 37u
Why aren’t noble gasses reactive?
Because they have full outer shell so are most stable
What is e- affinity?
Energy released when an e- is added to a neutral atom - atoms with lots of valence e- have high affinity
What is ionisation energy?
Energy required to remove an e- from a neutral atom
What is electronegativity? And which atoms have high electronegativity?
A measure of how likely an atom is to gain an e- during a reaction
Atoms with high e- affinity and high ionisation energy (those with lots of valence e-)
What is an ionic bond?
Transfer of e- between atoms to complete both outer shells -> only done with different elements
Creates oppositely charged atoms which strongly bond together and have direction
What is coulombs equation and what does each symbol stand for?
Bond energy = 1/4πε . Q1.Q2. 1/r . B/r^n
ε = universal constant, B = empirical constant, Q1/2 = charge on ions, r = separation distance, n = constant
What is a covalent bond?
Both atoms donate an e- and share between them to complete outer shells, can have more than one covalent bond at a time (max = 8) & can happen between similar or dissimilar atoms
Bonds are very strong and directional
Can a bond be purely ionic?
No - all atoms have some level of electron negativity so there will always be some element of covalent bonding with in the ionic bonds
What is a metallic bond?
Metallic bonding causes metals to form a structure, each are donates delocalised e- to form an e- cloud in material - this cloud stops the +ve charged atoms from repelling and e- density changes throughout material
Bonds are non-directional
Rank them primary bonds in order of strength
Covalent bonds
Ionic bonds
Metallic bonds
Describe a Van der Waals bond
Secondary bond (no e- movement) caused by ionic attractions between atoms Weak bond
Sketch the lennard-Jones potential energy curve and what does it show?
Separation distance vs potential energy
X^2 curve which drops below x axis and then Asymptotes with x axis
Minimum of curve = minimum energy distance = equilibrium distance
Define a unit cell
Must have translational symmetry, contains all information of the cell, microstructure can be used to describe material properties, generally 3D shapes
Define a motif and primitive unit cell
Motif - a group of atoms that are repeated at every point in the lattice (usually 2/3 atoms)
Primitive unit cell - smallest possible unit cell that still represents the entire structure
How many crystal systems are there? And what are they?
Triclinic Monoclinic Orthorhombic Tetragonal Hexagonal Cubic Trigonal
How many bravais lattices are there?
14, all others are repeats
Draw the four possible unit cells
BCC
FCC
HCP
Primitive cubic
How many atoms are in each of the four unit cells?
BCC - 1 in each corner (shared by 8), 1 in centre (not shared) = 2
FCC - 1 in each corner, 1 in each face (shared by 2) = 4
HCP - 7 on both faces (6 shared by 6, 1 shared by 2), 3 in centre = 6
Primitive cubic - 1 in each corner = 1
What are the 3 lattice characteristics?
Lattice parameter (vector from one atom to another) Angle between translation vectors Coordination number (atoms in each lattice)
What do each of the following mean?
[101]
<101>
(101)
{101}[101] - lattice point or directon vector
<101> - family of coordinates - 100,001,1-10
(101) - 101 plane
{101} - family of planes
How are -ves written in lattice parameters?
Bar over the negative number
What’s happens to ions when temperature increases?
Lattice vibrates more, when T is high enough atom moves enough to break bonds (happens in melting) and sometimes form new ones if they’re energetically favourable (changing lattice structure)
Why does an element have a higher Tm than another
Stronger bonds/more bonds per area means more energy required to break intermolecular bonds = higher T needed = higher Tm
What are used to determine points on planes?
Miller indices
They are the reciprocals of lattice coordinates (E.g. 2 = 1/2)
What is the Miller indices of a plane that crosses the x axis at 1, and is parallel to the z and y axis
(1,0,0) as parallel = 1/infinity
What’s the Miller indices of a plane that crosses the x at 1/3, the y at 1 and is parallel to the z axis?
(3,1,0)
What is the Miller indices of a plane that intersects at (0,0,0), and how are planes grouped?
1/0 is impossible meaning plane can’t occur, must translate the plane and then take the Miller indices
E.g. (1,1,1) (0,0,0)
The same as vectors, {1,0,0} = (0,0,1) = (0,-1,0) = (0,1,0)
When are [] + <> used?
When are () + {} used?[] = direction, <> = group direction
() = plane, {} = group of planesHow are planes and plane spacing related?
Bigger density of planes = reduced space between them
d = 1/ square root (h2/a2 + k2/b2 + l2/c2)
What can the dot and cross product show?
Dot = finds angle between planes Cross = direction normal to both planes
What’s an interstitial site ?
Gaps between atoms in a crystal structure that can accommodate a smaller atom
What is the atomic packing factor? And what are the unit cell volumes for BCC and FCC structures?
% of the lattice which is filled, atom volume (atoms.1.25.πr3) unit cell volume
BCC = 4r/root 3
FCC = 2r.root 2
Calculate the interstitial sizes for BCC
In BCC interstitial size = a = 4r/root 3 = 2(R + r)
r = R (-1 + 2/root 3)
Where r = atom that could fit in interstitial site
What are the atomic packing factors of BCC, FCC and HCP?
BCC = 68% FCC = 74% HCP = 74%
What is meant by coordination number?
Number of nearest neighbours that an atom has (ie how many atoms are close to it)
What are the interstitial sites in an FCC lattice?
6 tetrahedra sites
13 octahedral sites
FCC has fewer, but much bigger, interstitials than BCC
What’s the difference between cation and anion?
Cation +ve charged and smaller
Anion -ve charged and bigger
What structure do ionic compounds have?
Always FCC
Describe coulombs force
Electrostatic potential energy causing atoms to repel or attract = energy of bond
Ebond = Eattract + Erepuls
= Q1.Q2/4π.εo.r2 + B/r^n
Where Q = valence.e, r=radius, B = imperial constant
When is the drop in electrostatic potential energy minimised?
When a structure is not close packed so atoms can be the equilibrium distance between each other
What is the loss in electrostatic energy? And what is meant by the mandelung constant?
Elost = M/a . e2/4π.εo
Where a= lattice parameter, e = e- charge, M = mandelung constant = measure of a structures efficiency at reducing electrostatic energy
What things dictate an ionic compounds structure?
Charge - all ionic compounds must have 0 charge
Relative size - stable if ions can can fit into interstitials or successfully strain lattice
Mandelung constant - if more than 1 structure can form, lowest mandelung will form as lowers energy
Describe the structure of a silicon crystal
Each atom is covalently bonded with 4 other Si, all bonds have the same length and strength, diamond structure is formed with atoms every [1/4,1/4,1/4]
Some ionic bonds are formed
Diamond is same structure but with C instead of Si
How does x-ray diffraction work out lattice spacing?
X-rays onto lattice, reflection at interface, time between reflections = distance between interfaces = distance between atoms/layers
What is Bragg’s law and what does it show?
Nλ = 2d sinθ
Where d = a/root(h2 + k2 + l2)
Shows at which θ a constructive interference is observed
Define a constructive and destructive interface
Constructive - waves are in phase = an integer λ apart = maxima and minima
Destructive - waves in antiphase = 1/2 integer λ apart = only minima
How can reflections be used to identify a crystal structure?
When many waves interfere = bigger chance of destructive interface
More planes = more waves = more destructive interference so closer packed structure = less waves
(FCC has less than BCC which has less than cubic primitive)
What are forbidden reflections in primitive cubic, FCC, BCC and bass centre lattices?
Forbidden reflections = when destructive is set up
Primitive = no forbidden
BCC = when h + k + l = odd
FCC = h, k, l aren’t all odd/even (E.g. (1,2,3)
Base centre = all reflection forbidden
How does a diffractometer work?
Sample in centre, x-ray source in with λ calculate using Bragg’s law = constructive interface, moving x-ray detector move around sample to detect x-rays
Amount of reflections etc will give idea about structure
