Materials- Structure of Materials

Question 1

Name 8 types of material properties and give 2 examples of each

Answer 1

Optical (colour, refractive index). Magnetic (curie point, magnetisation susceptibility). Electrical (resistivity, dielectric constant). Mechanical (hardness, ductility). Thermal (boiling point, thermal expansion coefficient). Chemical (pH, corrosion resistance). Acoustic (acoustical absorption, speed of sound). Radiological (specific activity, neutron cross section).

Question 2

How does temperature affect the vibrations of atoms?

Answer 2

Higher temperature means they have more energy so the amplitude of vibration becomes greater. So the object expands.

Question 3

What is a microstructurally sensitive property?

Answer 3

A property that is influenced by the presence of imperfections in the crystal structure. E.g ductility

Question 4

What is a microstructurally insensitive property?

Answer 4

A property that arises from binding energy, arrangement and packing of atoms. E.g density

Question 5

What is a composite?

Answer 5

Two or more different materials combined in one to achieve a combination of properties that is not displayed by any single material or incorporate the best characteristics of each component material.

Question 6

What is the atomic weight of an element?

Answer 6

A weighted average of the masses of each naturally occurring isotope of that element. Different to atomic mass which is just the number of nucleons.

Question 7

What is an orbital electron’s position described by?

Answer 7

A probability distribution of the distance it is likely to be away from the nucleus.

Question 8

What are the 4 quantum numbers every electron in an atom is characterised by?

Answer 8

Size of its probability density (electron cloud)
Shape of electron cloud
Spatial orientation of electron cloud
Number of states (energies) within each subshell

Question 9

What is true about the spins of each electron in a pair in an orbital?

Answer 9

They have opposite spins

Question 10

What are the shell names that correspond to the subshell numbers?

Answer 10

1 is K.
2 is L.
3 is M.
4 is N

Question 11

When is an atom in its ground state?

Answer 11

When all the electrons occupy the lowest possible energies in that atom

Question 12

What are valence electrons?

Answer 12

The electrons that occupy the outermost shell in an atom

Question 13

What does the term structure relate to?

Answer 13

The arrangement of a material’s internal components.

Question 14

What does it mean to say an element is electropositive?

Answer 14

It readily gives up some valence electrons and becomes a positively charged ion.

Question 15

What does it mean to say an element is electronegative?

Answer 15

It readily accepts electrons to form negatively charged ions, or share electrons with other atoms

Question 16

What causes attractive and repulsive forces between atoms?

Answer 16

Attractive forces are due to the various types of bonding between the atoms. Repulsive forces arise from interactions between the negatively charged electron clouds of the two atoms and are only important when the atoms are so close their outer electron shells begin to overlap.

Question 17

Graph of force between two atoms against their separation.

Answer 17

Attractive force is above origin, repulsive force is below origin. From right to left, net force is slightly attractive and curves up as separation decreases. Just before maximum net attractive force, repulsive forces arise. Net force line reaches maximum and curves down rapidly. The x intercept is r0 and is where the forces cancel each other out and is the most likely separation of the atoms (inter-atomic separation).

Question 18

Graph of potential energy against separation of two atoms

Answer 18

Repulsion is above origin, attraction is below. From right to left, net energy is slightly attractive and curves down as separation decreases. Just before minimum, repulsive energy starts. Net energy line reaches minimum and curves up rapidly. Distance from x axis to minimum is the bonding energy (E0) of the atoms and corresponds to r0.

Question 19

What is the bonding energy of two atoms bonded together?

Answer 19

The energy required to separate the two atoms to infinity.

Question 20

What would a large E0 mean for a material’s melting point?

Answer 20

Its is high

Question 21

How can a force against separation of two atoms curve tell you about the material’s Young’s modulus?

Answer 21

The slope of the net force curve is steeper at r0 for stiffer materials

Question 22

How can a potential energy against separation of two atoms curve tell you about a material’s linear coefficient of thermal expansion?

Answer 22

A deep and narrow trough normally correlates with a low coefficient of thermal expansion so relatively small dimensional alterations for changes in temperature.

Question 23

What are coulombic attractive bonding forces?

Answer 23

The attractive force between positive and negative ions.

Question 24

Describe ionic bonding and its properties

Answer 24

The metallic atoms give up their valence electrons to the non-metallic atoms. All atoms then acquire a stable or noble gas electron structure and an electrical charge. The magnitude of an ionic bond is equal in all directions around an ion (non-directional). All positive ions ante surrounded by negative ions in 3D and vice versa. Very large bonding energy.

Question 25

What is a directional bond?

Answer 25

A bond between specific atoms and may only exist in the direction between one atom and another that participates in the electron sharing.

Question 26

Describe covalent bonding

Answer 26

Atoms obtain stable electron structures by sharing electrons between adjacent atoms. The shared electrons belong to both atoms. The bonding is directional. The bonds can be very strong or very weak.

Question 27

Describe metallic bonding

Answer 27

The valence electrons are not bound to any particular atom in a metal and instead are free to drift throughout it and form a sea of electrons. The remaining non-valence electrons and nuclei for positively charged ion cores. The free electrons shield the iron cores from mutually repulsive electrostatic forces they would otherwise exert upon one another. It is non-directional. Bonding may be weak or strong.

Question 28

How does secondary bonding work?

Answer 28

The forces arise from atomic or molecular dipoles which exist whenever there is some separation of positive and negative portions of an atom or molecule. The bonding results from the coulombic attraction between the positive end of one dipole and the negative region of an adjacent one.

Question 29

What is a crystalline material?

Answer 29

One in which the atoms are situated in a repeating or periodic array over large atomic distances.

Question 30

What does the crystal structure of a material refer to?

Answer 30

The manner in which the atoms, ions or molecules are spatially arranged.

Question 31

What is an amorphous material?

Answer 31

One in which the atoms are situated in no long range order

Question 32

Describe the atomic hard sphere model

Answer 32

All the atom (ions) are thought of as being solid spheres having well-defined diameters. Spheres representing nearest neighbour atoms touch in another.

Question 33

What does the term lattice mean?

Answer 33

A three-dimensional array of points coinciding with atom positions (or sphere centres).

Question 34

What is a unit cell of a crystalline material?

Answer 34

The basic structural unit or building block of the crystal structure and defines the crystal structure by virtue of its geometry and the atom positions within.

Question 35

Describe the face-centered cubic unit cell

Answer 35

Has eight atoms at vertices of a cube shape. Has 6 atoms at the centre of the square faces created by the vertices. Has the equivalent of 4 atoms actually inside the cube. The atoms touch each other across the face diagonal. Coordination number of 12.

Question 36

What is coordination number?

Answer 36

The number of atoms that each atom is touching

Question 37

What is atomic packing factor?

Answer 37

The sum of the sphere volumes of all atoms within a unit cell divided by the unit cell volume.

Question 38

Describe the body-centred cubic unit cell

Answer 38

Has eight atoms at the vertices of a cube shape. One atom at the centre of the cube. Atoms touch along the body diagonal. There is the equivalent of 2 atoms inside the cube. The coordination number is 8. Less dense than FCC

Question 39

Describe the hexagonal close-packed unit cell

Answer 39

Is a hexagonal prism. Top face and bottom face has 6 atoms to form a hexagon which surround a single atom at the centre of each face. There are 3 full atoms in a triangle in the midplane. There is the equivalent of 6 atoms inside the unit cell. The coordination number is 12. The 3 midplane atoms touch some of the atoms in the top and bottom hexagons. Same density as FCC. Length greater than width of one side of hexagon.

Question 40

Formula for theoretical density of a metal

Answer 40

ρ=(nA)/(VcNa)
n is no. of atoms in each unit cell
A is atomic weight
Vc is volume of unit cell
N subscript A is Avogadro’s number (6.022x10^23)

Question 41

What does polymorphism mean?

Answer 41

Where a material can have more than one crystal structure

Question 42

What is allotropy?

Answer 42

Polymorphism found in elemental solids

Question 43

What are the lattice parameters of a crystal structure?

Answer 43

The 3 edge lengths a, b and c

The 3 interaxial angles α, β and γ

Question 44

What is a crystal system?

Answer 44

A group of structures that have the same unit cell configurations. Depends on the lattice parameters.

Question 45

What are the 7 crystal systems and how are their angles and lengths related?

Answer 45

Cubic: a=b=c, α=β=γ=90
Hexagonal: a=b not c, α=β=90 γ=120
Tetragonal: a=b not c, α=β=γ=90
Rhombohedral (trigonal): a=b=c, α=β=γ not 90
Orthorhombic: a not b not c, α=β=γ=90
Monoclinic: a not b not c, α=γ=90 not β
Triclinic: a not b not c, α not β not γ

Question 46

What is a close packed structure?

Answer 46

A crystal structure where the planes of atoms are arranged such that there is maximum atom packing density.

Question 47

What is a single crystal?

Answer 47

The result of when the periodic and repeated arrangement of atoms is perfect or extends throughout the entirety of the specimen without interruption.

Question 48

What is a polycrystalline material?

Answer 48

Crystalline solids composed of a collection of many small crystals (or grains)

Question 49

What is a grain boundary?

Answer 49

The region where two grains meet.

Question 50

How does a polycrystalline material form by solidification?

Answer 50

Initially, small crystallite nuclei form at various positions and have random crystallographic orientations. The small grains grow by the successive addition of atoms from the surrounding liquid to the structure of each. The extremities of adjacent grains impinge on one another as the solidification process approaches completion.