Deck 15 - Looking inside materials 3

Question 1

What sort of structures do ceramics have?

Answer 1

Ceramics are crystal structures built from ions of two or more elements.

Question 2

What is the second flaw about ceramics(apart from that they are brittle)?

Answer 2

Ceramics objects are often made by shaping the raw materials and then heating them.

Question 3

Explain how a process called sintering can be used to turn silicon nitrade into a turbine blade.

Answer 3

Silicon nitride powder is placed in a mould and heat under high pressure. The surfaces of the particles melt and flow so they become glued together.

Question 4

What is a cermet?

Answer 4

A “Cermet” is a combination of ceramic particles in a metal matrix. Such a material can be hard tough and strong.

Question 5

Explain how silicons atomic structure relates to its properties.

Answer 5

Atoms share electrons with neighbouring atoms to form covalent bonds. - These bonds are directional: they lock atoms in place. - The atoms cant slip due to the covalent bonds so silicon is hard and brittle. - The atoms are linked in a rigid structure.

Question 6

Explain the structure of NaCl and explain how its structure relates to its properties.

Answer 6

-Some atoms give electrons to other atoms to form an ionic bond. - Because oppositely charged ions attract ions hold each other in place. - The bonds are strong: salt crystals are stiff - The ions cannot slip: salt crystals are hard and brittle

Question 7

Explain the structure of metal and explain it links to its properties.

Answer 7

-Atoms in metals are ionised. The free electrons move between the ions. - The negative charge on the electrons “glues” the ions together, but the ions can easily change places. - The bonds are strong: metals are stiff - The ions can slip: Metals are ductile and tough.

Question 8

What happens when you stretch a metal?

Answer 8

Stretching a metal stretches bonds- but not by much.

Question 9

Explain how polyethene’s bonds react when stretched.

Answer 9

• Polythene is a long flexible chain molecule that folds up. - Stretching polythene rotates bonds, making it longer.

Question 10

Name and explain how two different polymers make chain rotations difficult.

Answer 10

-Polystyrene has benzene rings sticking out sideways which make chain rotations difficult. - Bakelite- Has many cross-linked chains. These cross-links stop the chains from unfolding.

Question 11

What is polythene flexible but strong?

Answer 11

Its free to rotate its bonds by strong covalent bonds.

Question 12

Show how the structure of rubber changes when stretched.

Answer 12

Question 13

Explain what happens to polythene’s structure when stretched.

Answer 13

Polythene is semicrystalline. When stretched plastically, the chains slip past each other. More of the material becomes crystalline.

Question 14

What is plastic deformation in polymers due to?

Answer 14

Plastic deformation in polymers is due to chains slipping past one another.

Question 15

What is the vulcanisation of rubber?

Answer 15

Rubber is heated with sulfur. Cross-links are formed between the polymer chains by the sulfur atoms.

Question 16

Name to different semi-conducting materials.p

Answer 16

Silicon and germanium

Question 17

Give the uses of semiconductors for electronic devices.

Answer 17

Transistors are the devices made from semi-conductors. Transistors are used to control and amplify electric signals.

Question 18

Draw a conductivity vs temperature/k graph for metals and explain what you would see.

Answer 18

• The conductivity of metals decreases a little as temperature increases.
• No more electrons become free to move. Moving electrons scatter from the wibrating lattice.
• Metals do not conduct well when they are hot because charge carriers become less mobile, but the number stays the same.

Question 19

Draw the conductivity vs temperature graph for silicon and explain the observations one would see.

Answer 19

• The conductivity of a pure semiconductor increases dramatically as temperature increases.
• At higher temperatures, more atoms become ionised. The conductivity increases increase because there are more charge carriers free to move. Effects of extra lattice vibrations are much smaller.

Question 20

Explain silicons conductivity.

Answer 20

• Silicon has a poor conductivity because it is made from silicon which forms 4 covalent bonds, all the electrons are used in these bonds so there are no delocalised electrons.
• However, less than one a million million silicon atoms are ionised giving a very small fraction of electrons that are free to move.

Question 21

Explain how silicon is doped with phosphorous to get more electrons.

Answer 21

• The phosphorous atoms ionise, giving electrons freedom to move throughout the material.
• Phosphorous has five electrons in its outer shell. Four are shared with silicon atoms. One becomes free to move and conduct, leaving positive phosphorus ions.
• Silicon combined with phosphorus is known as n-type silicon.

Question 22

Explain how silicon is doped with boron to make p-type silicon.

Answer 22

-Boron has three electrons in its outer shell. One more is stolen to give four to share with silicon atoms. The “electron hole” left behaves like a mobile charge positive charge.