WK13 - Intro to Material Physics Flashcards
What is the nature of Ionic, Covalent, Hydrogen and Metallic Bonding?
Ionic Bonding
• lattice structure that consists of a regular arrangement of oppositely-charged ions held together by strong electrostatic forces (typically metal-nonmetal)
• transfer of electrons
• can conduct when molten/aqueous (not when solid) and high melting points
Covalent Bonding
• pair of electrons are shared between two atoms (typically nonmetals)
Hydrogen bonding
• interaction involving hydrogen atoms located between a pair of other atoms having a high affinity for electrons
• weaker than ionic or covalent but stronger than van der Waals forces
•molecules are bound together by weaker intermolecular forces, hence simple molecules have low boiling points
Metallic Bonding
• positive metal ions held together by a sea of negative delocalised electrons from the outer shells of the metal atoms.
• metals are very malleable as the layers of atoms can slide over one another, also great conductors
Crystalline vs amorphous structures
Crystalline Materials
• atoms arranged in periodically repeating arrays which are termed crystal or lattice structures
• metals have simple crystal structures (FCC, BCC, HCP)
• two main types: single and polycrystalline
Amorphous (Non-crystalline)
• atoms arranged in an irregular manner, without any short or long range order in atomic arrangements
• plastics, glass etc
What is the main difference between random packing and ordered packing structures?
Energy
• dense, ordered packing structures are typically packed closer together (smaller r) and sit at a lower bond energy, hence are more stable
What is the coordination number and Atomic Packing Factor (APF)? How to calculate?
Coordination Number = number of nearest neighbour or touching atoms
APF = (volume of atoms in unit cell)/(volume of unit cell)
*assume hard spheres for atoms
What is the APF for simple Cubic? How many atoms are in the unit cell? Coordination number?
= 8 x 1/8 = 1 atom/unit cell
APF = (1 x 4/3pi(R)^3)/a^3 = 0.52
Where R = 0.5a
Coordination = 6
How many atoms are in BCC? What is the APF? Coordination?
= 1 (Center) + 8 (corners)*1/8 = 2 atoms
APF = (2 x 4/3piR^3)/a^3 = 0.68
R = SQRT(3)*a/4
Coordination = 8
FCC - Number of atoms, coordination, APF?
= 6(face)1/2 + 8(corners)1/8 = 4 atoms
APF = 0.74 where R = SQRT(2)*a/2
Coordination = 12
What is the APF and what is it telling you?
APF is a dimensionless quantity that indicates how closely atoms are packed in a unit cell.
Tells you what percent of an object is made of atoms vs empty space
What is the FCC and HPC stacking sequence?
FCC - ABCABCABC…
HPC - ABABAB…
What is the theoretical density of a unit cell, rho
Density = rho = (Mass of atoms in Unit cell)/(Total volume of unit cell)
= (nA/N_A)/V_c = nA/Vc*NA
n = number of atoms per unit cell
A = atomic weight (of element)
Vc = Volume of unit cell = a^3 (for cubic)
NA = Avogadros number = 6.022x10^23
What is a point Coordinate?
A lattice position in a unit cell
Determined as fractional multiples of a,b and c unit cell edge lengths
What are Miller Indices?
Miller indices = h k l where these are the reciprocals of the intercepts of the plane on the unit cell.
e.g. a surface has intercepts of a/2, a and infinity. The reciprocal Miller indices are then 2 1 0
How do u calculate the spacing, d, between adjacent (hkl) lattice planes?
d(hkl) = a/SQRT(h^2 + k^2 + l^2)
Where a is the size of the unit cell
*only true of crystals where the primitive lattice vectors are orthogonal
How does XRD use Bragg’s Law to analyse materials?
The inter-planar spacing, d, of a crystal is used for characterisation and identification purposes
Angle incoming must be v similar to diffracted angle hence, only elastic interactions (those conserving energy) contribute to the diffracted signal used in XRD.
•in elastic scattering (Compton scattering an fluorescence) are also picked up by the detectors and account for the background signal
What are the two main types of defects?
Point defects - vacancies, interstitials and substitutional atoms
Large scale - dislocations, grain boundaries, twins and surfaces.
How to calculate the number of vacancies?
Nv = Nsexp(-Qv/KbT)
Nv = number of vacancies
Ns = number of regular lattice sites
Qv = Energy needed to form a vacant lattice site
Kb = boltzmann’s constant
T = Temperature
What is an Interstitial vacancy? What are the two types?
Vacancy - Atom has lefter its site and has to go somewhere
• it can move to the surface
• atom can stay in the crystal but not at a lattice point (interstitial)
Self-interstitial - Atom same as others
Interstitial impurity - misplaced atom different from atoms in lattice
What are substitutional impurities?
Impurity atoms can substitute into the lattice, taking the lattice site of one of the atoms from the original material.
There is a size limit to an impurity atom compared to the lattice atoms for it to be incorporated substitutionally
What are the two main types of dislocations?
Edge dislocations - a termination of a plane of atoms in the middle of the crystal and can be interpreted as an additional half-plane of atoms in between two planes. (2D only)
Screw dislocations - shifted out of the plane of the material (3D)
Most dislocations are a combination of the two
What are grains, grain boundaries? Single vs polycrystalline
Grains - different regions of the same crystal structure at different orientations
Single crystal/monocrystalline - A material with a single grain
Polycrystalline - A material with lots of grains
Grain boundaries - interface between two regions of different crystalline order or orientation
High angle grain boundaries are where vacancies etc are more likely to form and are more vulnerable to chemical attack, stress etc
Describe phases, phase boundaries, precipitates vs inclusions
Phase - a region of material that has a defined crystal structure and composition
A material with 2+ phases is known as multiphase
• dominant phase is called the matrix phase, whilst others are called phase precipitates or inclusions.
•• precipitates - coherency to the matrix
•• inclusions - a rejection of elements not happy in the matrix phase
Phase boundary is not a grain boundary
• a material can be multiphase but still a single crystal
What is the Lennard-Jones potential?
V = 4e[(x/r)^12 - (x/r)^6]
V = potential energy
e = depth of the potential well
x = position of zero potential
r = distance between the particles
Describes the potential energy of the interaction between two non-bonding atoms or molecules based in their distance of separation
Why is it useful to model atoms connected via springs?
These ‘springs’ follow the Lennard Jones potential
Useful for understanding the transfer of vibrational/thermal energy through longitudinal waves — called phonons
Difference between high stiffness and high strength?
High stiffness - has a high youngs modulas, requires high loads to elasticity deform it
- slope of stress strain curve in elastic region
High strength - which requires high loads to plastically deform it
- yield strength is maximum stress before plastic deformation occurs
