Materials

Question 1

Ductile meaning?

Answer 1

The property of a material that allows it to be stretched into fine wires or other shapes without fracturing. It is the material’s property that allows it to deform under any stress, can be tensile, compressive or shear. It is a measure of how much a material deforms under various types of stress.

It is a measure of the amount of plastic deformation a material can undergo before it fails.

Question 2

Why are metals ductile?

Answer 2

Their metallic bonding allows the positively charged ions to move over/slide over each other which allows the metal to deform under various types of stress. The delocalised electrons allow the positive ions to move over one another, in way acting like ball bearings.

Question 3

Why are metals electrically conductive?

Answer 3

The delocalised electrons are free to move around and carry the charge. Due to these available charge carriers metals are electrically conductive.

Question 4

Why are metals thermally conductive?

Answer 4

Metals are thermally conductive because the delocalised electrons gain the thermal energy and begin to vibrate more quickly, transferring the thermal energy into kinetic energy. They then collide more often and with more energy with other electrons and the positive ions in the metallic lattice. This means they transfer some of their kinetic energy to the other electrons and ions in the lattice so there is an even distribution of thermal energy in the lattice and this occurs quickly.

Question 5

Why are melting points and boiling points key indicators of bond strength?

Answer 5

The stronger the bonding in Giant Ionic Lattices, simple molecular, macromolecular structures or metals the more energy is required to break these bonds therefore the higher the bond energy. This energy can be supplied in the form of thermal energy.

The energy supplied is transferred to the particles in the bond and they vibrate more quickly, if enough energy is supplied they vibrate quickly enough to overcome the attractive forces between them and the bond breaks.

Question 6

How do you make an alloy?

Answer 6

You melt both elements (one must be a metal, the other can be a metal or a non-metal) and you mix them by pouring them into a mould or basin together. They then solidify and the resulting substance is harder and more durable.

Question 7

What are alloys made of?

Answer 7

Alloys can be a metal-metal mix or a metal non-metal mix. There MUST be a metal in the mix to create an alloy to ensure metallic bonding takes place.

Question 8

How do Alloys work?

Answer 8

The two material sin the alloy are bonded together (via melting into liquid form and then combining them) with metallic bonds.

If a metal and a metal make up the alloy then the resulting structure is a metallic structure. If a metal and a non-metal make up the alloy then the metallic bonds are still kept and metallic lattice in the metal is still retained. You may get a heterogenous microstructure if you mix a metal and a non-metal together which is where the two substances in the alloy exist in different phases but the metallic lattice structure is always kept.

The positive ions are different shapes and sizes so do not slide over each other as easily making the alloy less ductile and have a higher Young’s Modulus in general. An Alloy may not necessarily have a greater Young’s Modulus than pure metals. This depends on other factors as well.

Question 9

What are the properties of metals?

Answer 9

Ductile
Electrically Conductive
Thermally Conductive
Magnetic

Question 10

What are the types of magnetism exhibited by metals and other elements?

Answer 10

Ferromagnetism
Paramagnetism
Diamagnetism
Antiferromagnetism
Ferrimagnetism

Question 11

What are Magnetic moments, what are they caused by?

Answer 11

The atomic magnetic moment (the magnetic moment of an electron) is determined by two things, the spin or the electron and the orbital motion.

The spin of an electron is an intrinsic property of electrons. It is the intrinsic angular momentum which all elementary particles have. It has been quantised and can take two forms, spin-up and spin-down. This property of electrons means that they are spinning and have an angular momentum, this means charge is moving so there is a current produced and therefore a magnetic field produced. This magnetic field is very weak. The reason it is called the magnetic moment is because when this magnetic field that is produced around the electron due to it’s spin interacts with another external magnetic field there is a force which acts on the electron. This is because the two magnetic flux lines cannot cross so a force is produced which acts as a torque for the electrons, either in the same direction as it’s spin or in the opposite direction. When the torque acts in the opposite direction to the angular momentum the electron is said to “precess” and this process could lead to a change in the spin-state of the electron. This force that acts as a torque due to the interaction of the magnetic field of the electron due to it’s spin and the external magnetic field acts as a moment on the electron and it is produced by magnetic fields so is a magnetic moment.

Orbital Motion also produces a magnetic moment. When an electron orbits around another object about the axis of the object there is also charge moving so a current is produced and therefore also an magnetic field. This results in a orbital magnetic dipole moment. The direction of this magnetic moment is perpendicular to the plane of the electrons orbit (right hand rule.)

The total magnetic dipole moment/the atomic magnetic dipole moment produced results from both the spin and the orbital motion of the electron.

Question 12

How does a Ferromagnetic material work?

Answer 12

The atomic magnetic dipole moments in a material align when an external magnetic field is applied. Even when the external magnetic field is removed the atomic magnetic dipole moments in the material remain aligned so the material becomes magnetic. This process can occur without the presence of an external magnetic field in certain regions called magnetic domains. The individual magnetic moments of each electron align with the external magnetic field. If the magneitc moment is already aligned with the external field (due to spin state in the complementary direction to the external field) then an attraction is produced between the two. If the magnetic moment is in a different direction due to the opposite spin state then the electron precesses and it’s spin state changes so that there is an attraction between the electron and the external magnetic field. It is important to note that when ferromagnetic materials have been permanently magnetised their electrons are NOT in the same spin state. The electron are aligned in regions called magnetic domains so that their magnetic moments align. These magnetic domains align causing a net magnetic moment in the material.

Materials such as Iron, Nickel and Cobalt exhibit Ferromagnetism.
This fact that these materials exhibit ferromagnetism allow them to become permanently magnetised.
Every Ferromagnetic material has a specific temperature at which they lose their ferromagnetic properties and behave like paramagnets where the magnetic moments become disordered. This temperature is known as the Curie Temperature.

Question 13

What is paramagnetism?

Answer 13

The unpaired electrons (which have magnetic moments due to their intrinsic spin and orbital motion) have their magnetic moments align with the external magnetic field that is applied. This creates an attraction between the external magnetic field and the material. Unlike Ferromagnetic materials the paramagnetic materials do not retain their magnetisation after the external field has been removed, the magnetic moments de-align and go back to being random once again when the external magnetic field is removed.

Question 14

What are some of the properties of Polymers?

Answer 14

They are electrical and thermal insulators. This is because they have no free (delocalised electrons) to carry the charge. Each Polymer chain is made up of monomers which form the polymer via Addition or Condensation Polymerisation. This means that each polymer chain is made up of covalent bonds which are strong.

However, between the polymer chains there are weak Van Der Waals intermolecular forces and there may be hydrogen bonding if the polymer was formed via condensation polymerisation.
Both VdW and Hydrogen Bonding intermolecular forces are relatively weak compared to metals so the melting and boiling points of polymers are usually quite low.

Low density -> depends on how the molecules(polymer chains) are packed together.

Flexible: Due to the long chains and weak intermolecular forces present between the polymer chains they can slide over each other easily so the polymer is flexible. You can add a plasticiser to increase flexibility.

Question 15

What are Ceramics?

Answer 15

By definition Ceramics are inorganic, non-metallic polycrystalline materials.

Monocrystalline materials are sometimes also called ceramics even though they should not be. Monocrystalline materials are grown as one 3 dimensional crystal whereas Polycrystalline materials are formed via multiple crystal grains joined together in the production process.

Question 16

What are crystal grains?

Answer 16

Crystal grains are small, hard seeds of a material that form during the solidification process of the crystalline structure. They are essentially the many individual small crystals that make up the main crystalline structure. Each crystal grain has it’s atoms in a well-defined ordered lattice structure (opposite of amorphous). How these grains are arranged within crystalline material affect the material’s properties such as it’s strength, durability, ductility and conductivity. The grain are separated by grain boundaries which are regions where adjacent grains differ in orientation. This affects the material’s mechanical properties.

Question 17

Ceramics all properties?

Answer 17

Electrical and thermal insulators- because they have no free electrons or ions to carry the charge or transfer the thermal energy to kinetic energy and pass this on via collisions.

High melting points- due to strong ionic and covalent bonds within the ceramic structure. Unsure as to whether the crystals are made up of the atoms which have ionic and covalent bonding or something else? Research and ask about this.

Hard and Brittle-dislocation motion not allowed. Little or no plastic deformation, the reason why is due to low dislocation density (something to do with the microstructure of ceramics.) Don’t worry about this too much.

Often Ceramics are oxides so they are environmentally resistant because they resist oxidation from the environment.

Ceramics are always compounds. They can have solely ionic bonds or covalent bonds or they can have both. The type of bonding and crystal structure depends on the elements that make up the ceramic.

Question 18

What is dislocation motion?

Answer 18

Dislocation motion is a type of mechanical motion.

Question 19

Hydrogen bonding definition?

Answer 19

An electron deficient hydrogen atom is attracted to the lone pair on a highly electronegative atom. In order for the Hydrogen to be electron deficient it must be bonded to a highly electronegative atom such as N, O or F.

Question 20

Solids which do not have their atoms arranged in an ordered structure are called?

Answer 20

Amorphous. The atoms are randomly distributed leading to a disordered structure. This gives amorphous solids a very different structure to crystalline solids which have a well-defined lattice structure.

Question 21

What is a Unit cell?

Answer 21

A Unit Cell is the smallest number of atoms in 3D space which can describe the lattice structure in the crystal. It is the smallest number of atoms after which the pattern becomes visible.

Question 22

Atom stacking, stacking sequences look in book.

Question 23

What is it called when materials can have different crystal structures in different physical states?

What does this mean for the physical size and appearance of the structure?

Answer 23

Polymorphism/Allotropy. This is what it is called when a material has an FCC structure at 300 kelvin (example) and a BCC structure at 600 Kelvin. This change in crystal structure means that the atoms pack in a different way so the volume of the overall structure changes. A material can change it’s crystal structure as the temperature increases and then change back to the original crystal structure again if the temperature keeps increasing.

Question 24

What are some examples of materials with a BCC structure?

What is the Unit Cell of the BCC structure?

Answer 24

Iron, Chromium, Tungsten and Titanium. There are more…
The structure is an atom at each corner of a cube and an atom at the centre. This is the Unit Cell of the BCC structure. Eight atoms (one at each corner of the cube) and one atom in the middle. This arrangement is considered to be relatively close packing and it is stable.

The high stability of the BCC structure is due to the high number of nearest and next-nearest neighbours. Essentially the nearest neighbour to the middle atom is the atoms in the corners and the next nearest neighbours are the atoms in the centres of the surrounding cubes. Since the atoms at these centres are not that much further away from the centre atom of the central cube than the atoms at the corners of the central cube this means the structure is highly stable.

Question 25

What factors must we consider that can affect the states of materials and their crystal structure?

Answer 25

Temperature and Pressure Boiling point of water is 100 degrees C In a nuclear reactor the temperature of the water is 300 degrees due to a different pressure being applied.