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AS Physics (Unit 1: G491) > Looking inside materials > Flashcards

Flashcards in Looking inside materials Deck (22)
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What is are the structures of ceramics?

Ceramics can be crystalline, polycrystalline or amorphous.


What is the structure of glass?

Glass is usually amorphous, a random array of atoms bonded together by covalent bonds without any clear sense of order.


Why is glass brittle?

- Glass consists of atoms bonded together in an amorphous structure by directional covalent bonds.
- There are no slip planes, so atoms in glass are unable to slide across each other easily to cause plastic flow.
- Stress is concentrated at the tips of cracks and cannot be relieved by plastic flow, so the crack propagates through the material, causing failure.


Why does glass not require much energy to break?

- When stress is applied, some directional covalent bonds in glass break and the atoms move out of place slightly, releasing energy.
- The energy released is not used up to form new bonds because the atoms are out of place and covalent bonds are highly directional.
- Energy is used to break more covalent bonds which subsequently releases more energy.
- Due to this process, only an initial input of energy is required to break the covalent bonds required to break glass.


What is the structure of metal?

- Metals are either arranged crystalline structures or polycrystalline structures.
- Metallic cations are held together by electrostatic force of attraction towards electrons in the sea of delocalised electrons.
- 3D arrangement of metallic cations in hexagonal or cubic close-pack structure.
- In polycrystalline structure, atoms in each grain are arranged in crystalline structure but differ from other grains.


Why are metals tough?

- Metallic cations in metals are held together by non-directional metallic bonds. This means that the layers of metal ions are able to slide past each other and settle in new positions, there are slip planes to allow for plastic flow.
- When a crack occurs on surface of metal, plastic flow of atoms occur at tip of crack, smoothing the crack and spreading the stress so the crack doesn't propagate as easily as it would in a brittle material.


Why do metals require lots of energy to break?

- When stress is applied, metallic bonds break and layers of metallic cations slide past each other to new positions, releasing energy.
- Metallic bonds are non-directional, so when layers settle in new positions, metallic bonds form again using the energy from initial bond breaking.
- The result of this is that the metal suffers some plastic deformation, but all energy is used up in the system.
- To deform the metal further, energy needs to be constantly applied in order to reach the metal's breaking point.


Why are metals malleable and ductile?

- Metallic lattices are held together by non-directional metallic bonds. There are slip planes between layers of metallic cations in lattice which allows for plastic flow.
- Between layers of metallic cations, there are small one atom gaps called dislocations.
- These dislocations move through the metal one atom at a time without much stress until they reach a grain boundary or edge of crystal. This is equivalent to one layer of cations sliding across the other.
- This leaves the metal deformed plastically with relatively small amounts of stress, making metals malleable and ductile.


How can dislocations be pinned to make the metal stiffer/ more brittle?

- Lots of grain boundaries can pin dislocations.
- When lots of dislocations are present, they tangle with each other, pinning themselves.
- Smaller or bigger metal atoms are able to fit into these dislocations and prevent them from moving, pinning them.


What are the structures of polymers?

- Polymers consist of lots of polymer chains entangled randomly to make up the polymer material.
- Polymer chains are held together by non-directional intermolecular forces, so are able to slip past each other easily.


Why are polymers elastic?

- Some polymers, like polythene, are long chained molecules that fold up, and their bonds can twist freely.
- When stress is applied, bonds in polymer chains twist and the molecules becomes longer, so the material stretches, but polymer chains do not move.
- However, once the stress is removed, the polymers fold up again and return to their original shape.


How do we make polymers stiffer?

- Polymer chains can be cross linked with disulfide bridges. These bridges make it harder for polymer chains to unfold when stress is applied, making elastic deformation harder, making the plastic stiffer.


How does plastic deformation occur in polymers?

- Polymers consist of a random entanglement of polymer chains in an amorphous structure.
- Polymer chains are held together by non-directional intermolecular forces.
- When stress is applied, polymer chains slide past each other, untangle, and settles in their new positions; forming orderly crystalline regions.


How does elastic deformation occur in metals?

- There are slip planes between layers of metallic cations in the metallic crystal that allow for plastic flow.
- When stress is applied, layers of metallic cations slightly slide across each other, extending the metal.
- However, if on layer doesn't exceed the tipping point, then it will fall back to its original position after stress is removed.


Why do metals conduct electricity?

Delocalised electrons in the sea of delocalised electrons are able to move freely through the material, acting as charge carriers for an electric current.


Why are ceramics/ polymers insulators?

There are no free charge carriers (such as ions or delocalised electrons) within the structure of these materials, meaning that an electric current is unable to move through them.


Why does the resistance of metals increase with temperature?

As temperature increases, electrons gain more kinetic energy and scatter while moving through the lattice. This makes them slightly less mobile and less able to carry a current, so conductance decreases and resistance increases.


Why does the resistance of semiconductors decrease with temperature?

As temperature increases, electrons gain kinetic energy and more break free from the cations to become delocalised. Due to the increase in charge carrier density, the material is better at carrying a current, so conductivity increases, and resistance decreases.


How can the conductivity of semiconductors be improved without increasing temperature?

Doping can be used to increase conductivity of semiconductors. Phosphorus atoms can be added to the semiconductor which contributes one more delocalised electron to the sea. This increases charge carrier density and increases conductivity (n-type doping). Alternatively, boron atoms can be added to the semiconductor. Boron actually takes an electron from sea of delocalised electron, leaving a positive hole. This hole acts as a charge carrier and increases charge carrier density.


What are the overall properties of metals?

- Strong.
- Malleable & ductile.
- Tough.
- Stiff.
- Electrical conductors.


What are the overall properties of ceramics?

- Strong.
- Brittle.
- Stiff.
- Electrical insulators.


What are the overall properties of polymers?

- Weak.
- Tough.
- Ductile.
- Flexible
- Electrical insulators.