Chapter 5: Looking Inside Materials

Question 1

What is an amorphous structure?

Answer 1

A disordered arrangement of molecules. No long range order.

Question 2

How are amorphous structures obtained?

Answer 2

By the rapid cooling of a liquid into a solid, without time for the molecules to form a crystalline structure.

Question 3

What is a crystalline structure?

Answer 3

A highly ordered array of atoms. Regular layers of atoms and molecules.

Question 4

What is a polymer structure?

Answer 4

molecular chains

Question 5

What is an ionic bond?

Answer 5

electrostatic attraction between oppositely charged ions. rigid, directional and stiff. usually brittle

Question 6

what is a covalent bond?

Answer 6

A shared pair of electrons. rigid, directional, stiff. usually brittle.

Question 7

what are the mechanical properties of polymer chains?

Answer 7

very variable - if bonds can rotate, chains straighten out giving very large strains. if cross-linked, chains are rigid and inflexible.

Question 8

Are metals normally crystalline, polycrystalline or amorphous?

Answer 8

Polycrystalline - consisting of many grains surrounded by irregular grain boundaries

Question 9

What does each grain consist of?

Answer 9

A lattice of positive metal ions in a sea of delocalised electrons.

Question 10

Describe the mechanical properties of metallic bonds

Answer 10

bonds are stiff, strong bit NON-directional, so that layers can slip over one-another.

Question 11

Describe elastic deformations considering metallic bonding

Answer 11

The bonds are stretched so that the ions move apart. The sea of delocalised electrons acts as a glue and re-attracts the metal ions to their original positions

Question 12

Describe the two methods of plastic deformation in metals.

Answer 12

1. Layers of atoms slip over one-another.

2. Dislocations can easily move across a crystal grain - very little stress required.

Question 13

How can you make a metal stiffer, harder and more brittle (and why do they become so)?

Answer 13

Alloying metals, impurity atoms can lodge within the lattice and obstruct the movement of layers or dislocations making the material stiffer, harder and more brittle.

Question 14

How do cracks propagate in brittle materials?

Answer 14

Very quickly and easily - due to the high stress concentration at the tip of the crack.

Question 15

How do cracks propagate in tough materials (such as metals)?

Answer 15

In tough (and ductile) materials, the plastic flow around the crack tip reduces the stress concentration and “blunts” the tip.

Question 16

What are the electrical properties of metals?

Answer 16

Metals conduct electricity because they contain a large number of delocalised electrons which can move easily through the lattice carrying current.

Question 17

What are the electrical properties of ceramics and plastics?

Answer 17

They are electrical insulators as the electrons are bound to the atoms and no charged particles are free to move.

Question 18

Describe the electrical properties of semiconductors and name a semiconducting material.

Answer 18

Silicon is a semiconductor and has mainly bound electrons, but very few have enough energy to become conduction electrons. More thermal energy releases electrons, so CONDUCTIVITY INCREASES WITH TEMPERATURE INCREASE.

Question 19

What is N-type silicon?

Answer 19

a small impurity with a -ve charge is added (doped) to the silicon, which donates an extra electron to each silicon atom, providing greater conduction.

Question 20

What is P-type silicon?

Answer 20

a small impurity with a +ve charge is added (doped) to the silicon, which creates “electron holes”, which act as positively charged particles, providing greater conduction.